Teleprotecio CIGRE

PROTECTION USING

TELECOMMUNICATIONS Joint Working Group 34/35.11 December 2000

Cigré JWG 34/35.11

PROTECTION USING TELECOMMUNICATIONS CIGRE JWG 34/35.11

Protection using Telecommunications Cigré Joint Working Group 34/35.11 - Final Report -

Regular members of JWG34/35.11:

Per Odd GJERDE (Convenor)

(Norway)

Hermann SPIESS (Secretary)

(Switzerland)

Alastair ADAMSON

(United Kingdom)

Ken BEHRENDT

(United States)

Michael CLAUS

(Germany)

Alouis W. H. GEERLING

(Netherlands)

José Angel GONZALES VIOSCA

(Spain)

Christopher HUNTLEY

(Canada)

Carlos SAMITIER OTERO

(Spain)

Yoshizumi SERIZAWA

(Japan)

Kent WIKSTROM

(Sweden) Corresponding members:

Ricardo de AZEVEDO DUTRA

(Brazil)

Stephen HUGHES

(Australia)

David C. SMITH

(South Africa) Comments and contributions received from:

Hervé HOUKE

(France)

Trygve JORDAN

(Norway)

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Contents 1

FOREWORD, SCOPE, OBJECTIVE ..............................................................................................................7

2

POWER SYSTEMS AND FAULT CLEARING .............................................................................................9 2.1 ELECTRIC POWER SYSTEMS ..............................................................................................................................9 2.2 ELECTRIC POWER SYSTEM FAULTS AND CLEARING ........................................................................................12 2.2.1 Electric Power System Faults ................................................................................................................12 2.2.2 Fault Clearing .......................................................................................................................................13 2.3 WHY DOES PROTECTION NEED TELECOMMUNICATION? ..................................................................................15 2.4 INTRODUCTION TO POWER SYSTEM PROTECTION ...........................................................................................15 2.4.1 Fault clearing system.............................................................................................................................17 2.5 HOW IS TELECOMMUNICATION USED..............................................................................................................19

3

PROTECTION USING TELECOMMUNICATIONS .................................................................................21 3.1 LINE PROTECTION ...........................................................................................................................................21 3.1.1 Analog Comparison Schemes ................................................................................................................21 3.1.1.1 Current differential protection ........................................................................................................................... 22 3.1.1.2 Phase comparison protection ............................................................................................................................. 28 3.1.1.3 Charge comparison protection ........................................................................................................................... 31

3.1.2

State Comparison Schemes....................................................................................................................33

3.1.2.1 3.1.2.2 3.1.2.3 3.1.2.4 3.1.2.5 3.1.2.6

Intertripping Underreach Distance Protection ................................................................................................... 34 Permissive Underreach Distance Protection ...................................................................................................... 36 Permissive Overreach Distance Protection ........................................................................................................ 37 Accelerated Underreach Distance Protection..................................................................................................... 38 Blocking Overreach Distance Protection ........................................................................................................... 39 Deblocking Overreach Distance Protection ....................................................................................................... 40

3.2 BUSBAR PROTECTION ......................................................................................................................................42 3.2.1 Two-breaker busbar configuration........................................................................................................42 3.2.1.1 Normal fault clearing ......................................................................................................................................... 42 3.2.1.2 Breaker failure ................................................................................................................................................... 43

3.2.2

One- and a half breaker busbar configuration ......................................................................................43

3.2.2.1 Normal fault clearing ......................................................................................................................................... 44 3.2.2.2 Breaker failure ................................................................................................................................................... 45

3.2.3

Two zones / one breaker configuration..................................................................................................46

3.2.3.1 Normal fault clearing ......................................................................................................................................... 46

3.3 OTHER PROTECTION SCHEMES ........................................................................................................................47 3.3.1 Generator protection .............................................................................................................................47 3.3.2 Transformer protection..........................................................................................................................47 3.3.3 Reactor protection .................................................................................................................................48 3.4 SYSTEM PROTECTION ......................................................................................................................................48 3.4.1 Back-up protection ................................................................................................................................49 3.4.2 System-wide protection..........................................................................................................................53 4

TELECOMMUNICATION SYSTEMS FOR PROTECTION ....................................................................55 4.1 TELECOMMUNICATION CIRCUITS ....................................................................................................................56 4.1.1 Private and rented circuits ....................................................................................................................56 4.1.2 Analogue and digital circuits.................................................................................................................56 4.2 TELECOMMUNICATION NETWORKS .................................................................................................................57 4.3 TRANSMISSION MEDIA ....................................................................................................................................58 4.3.1 Pilot wires / Copper wires .....................................................................................................................58 4.3.2 Power Line Carrier (PLC).....................................................................................................................60 4.3.3 Microwave Radio...................................................................................................................................62 4.3.3.1 Multichannel radio............................................................................................................................................. 63 4.3.3.2 Single channel radio .......................................................................................................................................... 64

4.3.4 4.3.5

Optical fibres .........................................................................................................................................65 Satellites ................................................................................................................................................67

4.3.5.1 GEO - Geosynchronous Earth Orbit Satellites................................................................................................... 67

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4.3.5.2 MEO - Medium Earth Orbit Satellites ............................................................................................................... 68 4.3.5.3 LEO - Low Earth Orbit Satellites ...................................................................................................................... 68

4.4 MULTIPLEXING TECHNIQUES AND DIGITAL HIERARCHIES ..............................................................................69 4.4.1 Multiplexing Techniques........................................................................................................................69 4.4.1.1 Frequency Division Multiplexing (FDM).......................................................................................................... 69 4.4.1.2 Time Division Multiplexing (TDM) .................................................................................................................. 70 4.4.1.3 Code Division Multiplexing (CDM).................................................................................................................. 71

4.4.2

Digital Hierarchies................................................................................................................................72

4.4.2.1 PDH - Plesiochronous Digital Hierarchy........................................................................................................... 72 4.4.2.2 SDH - Synchronous Digital Hierarchy .............................................................................................................. 73

4.5 NETWORK TECHNOLOGIES ..............................................................................................................................75 4.5.1 Transport Networks ...............................................................................................................................77 4.5.2 Service Networks ...................................................................................................................................78 4.5.2.1 4.5.2.2 4.5.2.3 4.5.2.4

4.5.3

Circuit Switched Networks (POTS, ISDN) ....................................................................................................... 79 Packet Switched Networks (X.25, Frame Relay)............................................................................................... 80 Cell Switched Networks (ATM) ........................................................................................................................ 80 Datagram Networks (IP)................................................................................................................................... 81

Local Area Networks .............................................................................................................................82

4.5.3.1 Topology............................................................................................................................................................ 83 4.5.3.2 Media Contention and Protocols........................................................................................................................ 84 4.5.3.3 Advanced topologies ......................................................................................................................................... 85

4.6 NETWORK DESIGN AND OPERATION ...............................................................................................................86 4.6.1 Introduction ...........................................................................................................................................86 4.6.2 Technological considerations................................................................................................................88 4.6.2.1 PDH/SDH Networks.......................................................................................................................................... 88 4.6.2.2 ATM Networks.................................................................................................................................................. 89 4.6.2.3 IP Networks ....................................................................................................................................................... 91

5

TELEPROTECTION INTERFACES ............................................................................................................93 5.1 CONTACT INTERFACES ....................................................................................................................................93 5.2 ANALOG INTERFACES ......................................................................................................................................94 5.2.1 Pilot-wires (50/60Hz) ............................................................................................................................94 5.2.2 Voice frequency circuits (2-wire/4-wire) ...............................................................................................94 5.3 DIGITAL DATA INTERFACES.............................................................................................................................94 5.3.1 Electrical interfaces...............................................................................................................................94 5.3.2 Optical fibre interfaces ..........................................................................................................................95 5.3.3 LAN / Ethernet interfaces ......................................................................................................................96

6

PERFORMANCE AND RELIABILITY REQUIREMENTS ......................................................................99 6.1 REQUIREMENTS ON TELECOMMUNICATION SYSTEM ........................................................................................99 6.1.1 Introduction ...........................................................................................................................................99 6.1.1.1 Terminology and General Requirements ......................................................................................................... 100 6.1.1.2 Definitions ....................................................................................................................................................... 103

6.1.2

Requirement from analog comparison protection ...............................................................................108

6.1.2.1 Time synchronization through GPS................................................................................................................. 108 6.1.2.2 Time synchronization through communication network.................................................................................. 108

6.1.3

Requirements from state comparison protection .................................................................................109

6.1.3.1 Propagation Time............................................................................................................................................. 109

6.1.4 Requirements from intertripping .........................................................................................................109 6.1.5 Requirements from system protection..................................................................................................110 6.2 REQUIREMENTS ON TELEPROTECTION ...........................................................................................................111 6.2.1 Requirements on interface compatibility .............................................................................................112 6.2.2 Functional requirements......................................................................................................................112 6.2.2.1 6.2.2.2 6.2.2.3 6.2.2.4 6.2.2.5

Analog comparison protection control and monitoring ................................................................................... 113 State comparison protection control and monitoring ....................................................................................... 113 Erroneous signal detection............................................................................................................................... 114 Loop-back and misconnect detection............................................................................................................... 114 Actions on alarm conditions ............................................................................................................................ 114

6.3 REQUIREMENTS ON PROTECTION ...................................................................................................................115

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6.3.1

Requirements on analog comparison protection .................................................................................115

6.3.1.1 Need for delay compensation .......................................................................................................................... 115

6.3.2

Requirements on state comparison protection.....................................................................................116

6.3.2.1 Interface co-ordination..................................................................................................................................... 116 6.3.2.2 Delay Compensation........................................................................................................................................ 116

6.3.3 Requirements on other protections......................................................................................................116 6.4 CONSIDERATIONS ON INTERFACES AND INSTALLATION PRACTICES ...............................................................116 7

PROTECTION SYSTEM CONFIGURATIONS AND DESIGN ..............................................................119 7.1 PROTECTION SCHEMES AND TELECOMMUNICATION SYSTEMS COMPATIBILITY ............................................119 7.2 DESIGN CHECKLIST .......................................................................................................................................124 7.2.1 Application ..........................................................................................................................................124 7.2.2 Interfaces .............................................................................................................................................124 7.2.3 Contractual..........................................................................................................................................125

8

FUTURE TRENDS AND PROBLEMS TO BE SOLVED..........................................................................128 8.1 TRENDS IN COMMUNICATION ........................................................................................................................128 8.1.1 General Network Development............................................................................................................128 8.1.2 Transport Technologies.......................................................................................................................128 8.1.3 Networking Technologies ....................................................................................................................129 8.1.4 Service Access/Provisioning Technologies..........................................................................................129 8.1.5 Integration of Technologies.................................................................................................................129 8.1.6 New Technologies for QoS provision ..................................................................................................130 8.1.7 Intra- and inter-substation communication .........................................................................................131 8.1.7.1 Intra-substation communication....................................................................................................................... 131 8.1.7.2 Inter-substation communication....................................................................................................................... 132

8.2 TRENDS IN PROTECTION ................................................................................................................................134 8.2.1 Considerations on new protection philosophies ..................................................................................134 8.3 OPEN ISSUES AND PROBLEMS TO BE SOLVED.................................................................................................137 8.3.1 Protection relay interoperability .........................................................................................................137 9

CONCLUSIONS .............................................................................................................................................139

ANNEX A1

TELEPROTECTION SYSTEM CONFIGURATIONS ............................................................141

ANNEX A2

TELECOMMUNICATION SYSTEMS CHARACTERISTICS ..............................................144

ANNEX A3

QUALITY OF SERVICE .............................................................................................................146

A3.1 INTRODUCTION TO QOS............................................................................................................................146 A3.2 QOS DEFINITION IN ATM NETWORKS.......................................................................................................147 A3.2.1 ATM Service Categories..................................................................................................................149 A3.2.2 ATM over SDH/SONET...................................................................................................................151 A3.2.3 Applications Summary.....................................................................................................................152 A3.3 QOS DEFINITION IN IP NETWORKS ............................................................................................................152 A3.4 IP TO ATM SERVICE MAPPING..................................................................................................................155 A3.5 QUALITY OF SERVICE STANDARDS ...........................................................................................................156 ANNEX A4

PROTECTION SYSTEM TIME SYNCHRONIZATION TECHNIQUES.............................157

A4.1 TIME SYNCHRONISATION FOR SIMULTANEOUS SAMPLING.........................................................................157 A4.1.1 Internal timing synchronization ......................................................................................................157 A4.1.2 External timing synchronization .....................................................................................................159 LIST OF FIGURES.................................................................................................................................................161 LIST OF TABLES...................................................................................................................................................163 BIBLIOGRAPHY ...................................................................................................................................................164

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ABBREVIATIONS..................................................................................................................................................166 INDEX ......................................................................................................................................................................169

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1 FOREWORD, SCOPE, OBJECTIVE Deregulation in both the telecommunication and electric power industry, together with new telecommunication network technologies and advances in numerical protection, has resulted in the need to reconsider traditional methods of delivering teleprotection schemes and their associated bearer services. Fibre-optic technology is commonly deployed in new telecommunication networks for inter-station communication, and utility-owned and public telecommunication networks from third parties are available for protection purposes. Trends in substation automation move towards the use of bus- and LAN technologies within substations and interchange of information in numerical form. Numerical protection has been state-of-the-art for protection relaying for some years. In September 1996, Cigre SC34 "Power System Protection and Local Control", and SC35 "Power System Communication and Telecontrol", decided to form the joint working group Cigré JWG 34/35.11, with the following scope of work: -

Assess the state of development of advanced protection using inter-site communications Analyze the relevance and opportunities of newly released telecommunication technologies (referring to the work of WG 35.07) Identify and promulgate opportunities for future advances in the joint discipline of teleprotection Examine the need for, and if necessary compile, a lexicon of terminology to suit the new environment Develop a new report to update the Technical Brochure Ref. No. 13, 1987.

JWG 34/35.11 met for a kick-off meeting in Oslo in September 1997. The working group agreed that a new version of the former Technical Brochure "Protection systems using telecommunication" (Ref. No. 13, 1987) should be produced. The document should create awareness for the requirements, opportunities and risks of protection systems using telecommunications, and guide protection and telecommunication engineers towards a common understanding for the design and operation of reliable teleprotection schemes that meet performance requirements in the most economical way. This Technical Brochure has the following content: Chapter 2 describes power systems from a teleprotection point of view, with focus on power system faults, their reasons and characteristics, and fault clearing requirements. It continues with the definition of fault clearing systems, protection systems, protection schemes, and ends up with explanations why teleprotection is needed, and how protection can use telecommunication to meet fault clearing requirements. Chapter 3 describes protection relaying principles and protection schemes using telecommunications, and deals - from a power system point of view - with various aspects around the need of teleprotection, its benefits and adverse implications if the teleprotection service would fail. Chapter 4 gives an overview of telecommunication systems, with focus on capabilities and limitations related to protection signal transmission. Problems and risks that may arise with different types of telecommunication technologies are addressed, and functional and reliability aspects are dealt with, both under normal conditions and - most important - under power system

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fault conditions. Chapter 5 deals with interfaces. Requirements on interfaces between protection, teleprotection and telecommunication devices are given. Chapter 6 focuses on performance requirements on protection, teleprotection and telecommunication functions. Chapter 7 deals with protection system configuration and design. Compatibility issues between protection schemes and telecom technologies are addressed to provide a guide for protection and telecommunication specialists to design teleprotection systems that will meet fault clearing requirements. Chapter 8 gives an outlook on future trends and addresses some problems to be solved. In Chapter 9 the document is summarized some conclusions are drawn. Annexes A1 to A4 contain some related topics and additional information, which the JWG has found valuable for the better understanding of the subject.

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2 POWER SYSTEMS AND FAULT CLEARING 2.1 ELECTRIC POWER SYSTEMS Electric power systems consist of three principal components: generating stations, transmission systems, and distribution systems. Generating stations convert mechanical or thermal energy to electric energy, typically in the form of 50 or 60 Hz alternating current. Transmission systems transmit electric energy from the generating stations to the distribution system. To transmit electric energy efficiently over long distances, transmission system power lines are typically operated at 200 kV to 800 kV. The operating voltage of generators and distribution systems is typically in the range of 2.4 kV to 25 kV. Electric power systems deliver electric energy to power consuming equipment owned by residential, commercial, industrial, and governmental customers. Consumer products typically operate at several hundred volts. Power transformers are required to step the power system voltage up and down to connect various power system segments having different system operating voltages. Power lines designed to transmit electric energy, called transmission lines, are often networked to improve service capability and reliability. This permits lines to be taken out of service for planned maintenance, or forced out of service by fault clearing, without disrupting the delivery of electric energy from the generating source to the customer. Branches of the network are connected at nodes, called busbars or buses. Power systems are almost always three phase systems, including conductors for 3 phases and ground wires. Throughout this report only single line diagrams are shown. Some simple busbar configurations are shown in Figure 2.1-1 and Figure 2.1-2.

Node in network = busbar Breakers Generator and transformer unit 1

Overhead power line Overhead power line

Generator and transformer unit 2

Underground or submarine cable

Breakers Figure 2.1-1:

Single-line diagram of a typical power station

Nodes at different voltage levels are connected by transformers. These connection points, transformers and other units are made within a limited geographical area, called a station.

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Configuration of a typical power generating station is shown on Figure 2.1-1, and Figure 2.1-2 shows a typical transformer station.

Node in network = busbar Breakers Power lines/cables

Transformer 1

Overhead power line

Transformer 2

Overhead power line

Underground or submarine cable

Load Breakers Figure 2.1-2:

Single line diagram of a typical transformer station

Power flows through all healthy transmission lines in the electric power system network as it moves from generation sources to consuming equipment owned by customers. Electric power system networks operated by more than one electric power utility are often tied together to form a large grid that supports the transmission of power over a very large area, sometimes spanning several countries. Figure 2.1-3 shows the routes of major power lines connected in the Scandinavian power grid.

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TROMSØ ALTA

ISLAND NARVIK

REYKJAVIK

OFOTEN

TORNEHAMN

FINNLAND PETÄJÄSKOSKI

SVARTISEN

PIRTTIKOSKI

LETSI

SCHWEDEN AJAURE

PIKKARALA

GRUNDF.

TUNNSJØDAL LINNVASSELV.

VUOLIJOKI

STORNORRFORS

STORFINNF.

TRONDHEIM

ALAPITKÄ JÄRPSTRÖMMEN

NEA

ALAJÄR VI

HJÄLTA

HUUTOKOSKI

NORWEGEN

PETÄJÄVESI KANGASALA ULVILA

(132)

TOIVILA

OLKILUOT O

HIKIÄ HYVINKÄÄ

BERGEN

LOVIISA

RJUKAN

HELSINKI

OSLO

INKOO

BORGVIK

ENKÖPING

HASLE

V

STOCKHOLM

STAVANGER LISTA

NORRKÖPING

KRISTIANSAND

DC

DC

GÖTEBORG NÄSSJÖ

OSKARSHAMN

RINGHALS

TJELE

HELSINGBORG

KARLSHAMN

[04/ \

MALMÖ KøBENHAVN

KASSØ

/

50 100 150 km

FLENSBURG

HAMBURG

DEUTSCHLAND

Figure 2.1-3:

The Scandinavian Power Grid

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2.2 ELECTRIC POWER SYSTEM FAULTS AND CLEARING 2.2.1 Electric Power System Faults Power system conductors energized to extremely high voltages in three phase systems must be properly insulated from each other and from ground. This insulation is achieved by special insulation materials covering each conductor and/or by air insulation. Air is a very inexpensive insulator, but requires very large spacing. Special non-conducting materials typically insulate the energized conductors in generators, transformers, capacitors, reactors and cables where compact design is essential. Overhead power lines are insulated by air, except at the point they are attached to the supporting poles and towers. Special insulators made of porcelain, glass, or insulating plastic with special surface design and shape achieve the combination of strength and electric insulation to make this attachment. Overhead line design principles are shown in Figure 2.2-1 which also indicates possible arc fault tracks. The three phase conductors

Phase - Ground fault Ground wires

Insulator Phase - Ground fault

Tower

Ground Phase - Phase fault

Figure 2.2-1:

Power line with examples of fault types and fault positions

All power system components are exposed to faults due to insulation breakdowns. The Scandinavian power system shown in Figure 2.1-3, for instance, typically experiences approximately 3000 faults per year. Voltage stresses caused by lightning and switching transients, and contamination due to polluted air are major sources of insulation breakdowns. Mechanical stresses caused by wind, vibration, ice, and snow-loading are major sources of insulator and supporting structure damage that also leads to insulation breakdown. For power lines, most insulation breakdowns are in air between phases and/or phases and ground. Most frequently, insulation breakdowns are along the surface of insulators due to excessive voltage stresses. An example of insulator flashover is shown in Figure 2.2-2

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A flashover in air influences a few rather narrow ‘corridors' where the air loses electrical insulation capabilities. The air does not recover its insulation capability as long as the current is flowing. Therefore it is very important to interrupt the fault current as soon as possible to recover insulation capability. When the current is interrupted, air recovers it’s full insulation capability within a fraction of a second. If the fault current is interrupted rather fast, normally no damage is caused to conductors, insulators or towers. Then the power line can be re-energized within a short time so it can again carry power in the grid. One of the most important design and operational criteria for a power transmission system is that the power system should withstand tripping of at least one power line (unit) without any unnecessary interruption of consumers or power producing units. Wide area weather disturbances, like lightning storms, severe wind storms, and ice storms, expose multiple transmission lines to the risk of faults within the same time period. Consequently, high speed tripping and fast reclosing of tripped transmission lines may be very important to avoid power system collapse due to two or more power lines out of service at the same time. Faults on power apparatus like breakers and units like generators, transformers and cables are most probably breakdowns and damage of special insulation materials. This causes damage that must be repaired before the unit can be re-energized to carry power. This may take a considerable length of time, depending on the availability of spare parts, and trained service personnel. Sometimes units are completely destroyed and must be replaced before normal operation can be achieved.

Figure 2.2-2:

Insulator flashover

Faults on both power lines and other power units can also be caused by misoperation of earth switches and "forgotten" security ground connections. Power system faults caused by weather, animals, high trees, humans, or equipment failure disrupt normal power flow by diverting current through a short-circuited connection and collapsing power system voltage. In addition to equipment damage, power system faults cause transients that adversely affect sources of generation and customer loads. Consequently, faults must be detected and isolated very quickly. Electric power system generators, transformers, busbars, and power lines are therefore monitored by protective relays designed to detect power system faults and operate isolating devices designed to interrupt damaging fault current.

2.2.2 Fault Clearing Power system fault clearing requirements are very important design and operational criteria for power systems. Faults can cause damage that requires expensive repair work or investments for equipment replacement. Faults also cause severe operational disturbances. Generators

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accelerate, motors retard and severe voltage drops can easily result in tripping of complex industrial plants. For the power system itself, severe disturbances can result in collapses and blackout for regions, and, in severe cases, even for several countries. Today’s society does not accept frequent severe disturbances and blackouts because of their heavy reliance on electric power consuming devices for business activities, safety, lighting, heating, cooking, communication and many other conveniences. Therefore, to avoid severe disturbances and blackouts, sound protection practices are used to provide rapid fault clearing. In some cases, formal requirements for fault clearing are provided to assure consistent levels of reliability levels throughout the power system. Formal requirements may be grouped in external requirements and utility requirements as already done in [3]. External requirements may encompass: - Customer power quality and interruption requirements. - Requirements from insurance companies who underwrite equipment failures. - Legal requirements to meet ‘prudent utility practice and industry standards’ in case primary equipment failures result in personal injury or property damage and legal actions are taken against the utility by the parties incurring damage. - International and national safety regulations, imposed by governmental and other agencies. - Requirements imposed by manufacturers of primary equipment in order to validate equipment warranties. - Requirements from occupational safety and hazard prevention. Utility requirements The power system must be designed and operated to avoid instability, loss of synchronism, voltage collapse, undesired load shedding, and unacceptable frequency or voltage. Good protection practices help meet these objectives by detecting and clearing faults rapidly. Rapid fault clearing helps: - Prevent severe power swings or system instability - Minimize disruption of system power transfer capability - Prevent unreliable services - Limit or prevent equipment damage It is very important to clear the fault within specified ‘limits’ to ensure that the healthy remainder of the power system can continue to serve it’s customers with acceptable quality and reliability. Requirements on protection Protection performance requirements are issued to satisfy external and utility requirements. These requirements specify how protective schemes must perform on specific contingencies to fulfill external and utility requirements. They typically provide a balance between the conflicting goals of dependability and security. Dependability goals require maximum sensitivity and fast response time to detect and clear all faults quickly with very low probability of a failure to trip. Security goals require maximum selectivity and slow response time to minimize the probability of an unwanted trip on an unfaulted circuit. Security is an issue during fault conditions as well as during normal, unfaulted conditions. Simply stated, the implementation of these protection requirements should result in dependable operation of only those relays protecting the faulted unit, and secure non-operation of the relays during non-fault conditions and when faults occur on adjacent power system units. This balance is met only through proper protection scheme design, proper relay and equipment selection,

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and proper connection and setting of these relays and equipment to achieve appropriate sensitivity and coordination. When protection schemes detect a fault on the equipment or line they protect, they signal isolating devices, called circuit breakers, to open, isolating the faulty segment of the system, and restoring normal system voltage and current flow in the power system. Protection schemes command circuit breakers to isolate faults with no intentional time delay. When the protection scheme and circuit breakers operate properly, the fault is isolated within the required fault-clearing time. Protection applied on high voltage systems, where fault-clearing times are most critical, typically detect faults and operate in about one to two cycles. Some schemes operate in less than one cycle. Circuit breakers operate in one to three cycles. The combination of high-speed protection schemes and fast circuit breakers can interrupt a fault in about two cycles, although more common fault-clearing times range from three to six cycles.

2.3 WHY DOES PROTECTION NEED TELECOMMUNICATION? Protection systems must meet sensitivity, time response, selectivity and reliability requirements in order to meet fault clearing requirements. Fault clearing systems (see 2.4.1) for generators, busses, transformers or other units within a substation can normally meet these requirements without using telecommunication. Telecommunication may be needed for the protection of these substation units only if a breaker is missing or fails to interrupt fault-currents. Protection schemes for extremely high voltage transmission lines, however, very seldom meet all these requirements without using telecommunications. Some protection schemes, such as stand-alone step-distance schemes, provide very reliable and sensitive protection capable of clearing all power system faults without using telecommunications, but time response and/or selectivity requirements can only be met by using telecommunications. Telecommunications are therefore needed to ensure that time response and selectivity requirements are met for all power system fault conditions! Telecommunications is also essential for some types of protection schemes, like analogue comparison schemes, to operate. If telecommunication fails, backup protection schemes ensure that power system faults will be cleared, but they may not be cleared within specified performance requirements. Then the probability of uncontrollable power swings and partial or complete system blackout increases significantly. Alternative methods for reducing the probability of fault-induced blackouts is to build additional generating stations and transmission lines, or add redundant telecommunications. In virtually all cases, it will be far less expensive to add redundant telecommunications. Telecommunications is therefore vital to the reliability and economy of modern electric power systems.

2.4 INTRODUCTION TO POWER SYSTEM PROTECTION Power system protection schemes are designed to detect and clear faults, in accordance with requirements on protection, as discussed in the previous sub-chapter, to: -

Minimize adverse affects on customer loads Minimize disruption of system power transfer capability Coordinate tripping with protective relays in other protection zones Prevent severe power swings or system instability Limit or prevent equipment damage

Power units and lines are protected in zones to coordinate fault detection and clearing. A

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protection zone is defined as all high voltage power system equipment (and all necessary control, supervision and protection equipment) between two or more circuit breakers. Selective fault clearing (selectivity) is to trip the breakers for the faulty zone, and not trip any additional breakers for non-faulty zones. Five basic zones of protection are shown in Figure 2.4-1. These zones of protection are identified as Generators (1), Transformers (2), Busses (3), Lines (4), and Loads, such as Motors (5). 5

M

5

M

3

2

Station C 3

Station A 1

G

3

3

Station B 2 1

4

4

1

2 4

G

4

G

3

3

Station D 4

Figure 2.4-1:

4

Typical power system and its zones of protection

The boundaries of each zone of protection, as it applies to protective relays, are determined by the location of the current transformers that provide the representation of primary system currents to the protective relays. Other parameters, such as voltage, are also used by some protective relays to perform their protection function, but the current transformer location determines the protection zone boundary. Overlapping zones of protection is an established protection concept represented by Figure 2.4-2. As shown, the current transformers are typically located on opposite sides of the circuit breaker, or on one side and as close as possible to the circuit breaker that is tripped to clear faults in the respective protection zones. Protection zone boundaries for power units such as generators, transformers, busses, and motors are typically within the same substation, permitting one relay to monitor currents at the boundary of its protection zone. Likewise, the same relay can easily be connected to issue trip signals to all circuit breakers at the boundary of its protection zone. The boundary for line protection, however, is typically located at two different stations that may be separated by a considerable distance. This separation makes it impossible for one relay to sense currents at both ends of the line, or control breakers at both ends of the line. It is therefore common practice to install at least one relay and circuit breaker at each end of the

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line. These relays may operate independently, or they may share information to improve their operating speed, or they may require communication between them to operate.1

CT for Zone B Zone B

Zone A CT for Zone A a. CTs on opposite sides of breaker

CT for Zone B Zone B

Zone A CT for Zone A

b. both CTs on same side of breaker Figure 2.4-2:

Overlapping protection zones established by current transformer location

2.4.1 Fault clearing system A Fault Clearing System is defined in this report according to Figure 2.4-3. Fault currents must be interrupted from both (all) sides. The fault clearing system therefore includes : - Protection system - Mechanisms of circuit breakers Fault Clearing System includes one or more protection systems and the circuit breakers required to clear (interrupt) a fault and isolate the faulted portion of the circuit. Protection System includes a complete arrangement of protection equipment and other devices required to achieve a specified function based on one protection principle. A protection system is all embracing and includes protection functions as well as auxiliary power systems, sensors for detecting measured quantities, controls and circuitry for closing/opening circuit breakers, teleprotection and telecommunications for interchange of information between protective functions and all necessary connections between these functions and units. (Example: A phase comparison protection system, or a line current differential protection system.) Sensors include voltage transformers and current transformers that scale primary system voltages and currents down to secondary values compatible with the protective device design. The Teleprotection Function converts the signals and messages from the protection function into signals and messages compatible with the telecommunication system, and vice versa. The teleprotection function may be integrated with the protective device, or the telecommunication equipment, or it may be in a stand-alone device. The Telecommunication System provides a communication link between ends of a protected 1

Protection schemes that share information to improve operating speed are sometimes referred to as “non-unit” protection schemes. Protection schemes that require communication to operate are sometimes referred to as “unit” protection schemes.

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circuit, permitting the exchange of information (analogue data and/or status) or transmission of commands. In Figure 2.4-3, the telecommunication system may be dedicated point-to-point, shared point-to-point, or a network. High Voltage Equipment

Fault Clearing System Protection System Auxiliary power

Protection Functions

Control Circuit Breaker

Mechanism

Sensors Teleprotection Function

Protection Scheme Protection Zone

Telecommunication System

Teleprotection Function Sensors Circuit Breaker Protection Functions

Control

Mechanism

Auxiliary power

Figure 2.4-3:

Fault clearing system

Protection Function(s) may be performed by multiple protective relays working together, or more commonly in modern protection systems, by one or more multi-function protective relays.

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Protection functions in one station interchange information with protection functions in a remote station via teleprotection and telecommunications. This sub-total functionality forms a Protection Scheme.

2.5 HOW IS TELECOMMUNICATION USED Telecommunication is essential for analog comparison protection schemes (see 3.1.1) to share data between relays at each end of the protected line. Telecommunication is needed for direct intertripping schemes to pass tripping commands from the protection and control scheme at one line terminal to the power circuit breaker at the other line terminal. Telecommunication is used with state comparison protection schemes (see 3.1.2) to reduce the overall tripping time for faults on the protected line section. Analog comparison protection schemes typically share data, such as line current magnitudes and phase angles, to differentiate between power system faults within the protected zone or outside the protected zone. Communication between relays at each line terminal is essential to the operation of analog comparison protection schemes. State comparison protection schemes share the logical status of relay elements to determine if the fault is internal or external. These schemes are generally built by adding and interfacing communication to stand-alone relays to improve tripping speed for faults in the end-zone areas not protected by direct tripping relays. Schemes that use communication to improve tripping speed are referred to as communication assisted schemes. Telecommunication is also used for intertripping schemes that must communicate a trip command to a remote substation circuit breaker to isolate a fault within the local station, block and control schemes, and wide area protection schemes. All of these schemes are described in greater detail in Chapter 3. Telecommunication systems used for protection are described in Chapter 4. Protection using telecommunication provides consistent relay tripping times in the order of 2 to 3 cycles for faults over the entire length of a protected transmission line. Stand-alone protection schemes may take upwards of 20 to 30 cycles to trip both line terminals of a faulted line. Protection schemes using telecommunication can thereby reduce the tripping and clearing time for line faults by as much as 18 to 28 cycles compared with stand-alone protection schemes. This reduced tripping time greatly reduces the affect of faults on generators, power transfer, and customer loads, and reduces the damage to faulted and unfaulted equipment. The faster fault clearing speeds are essential to the efficient and economic operation of modern power systems. As described in this document, protective relays are interfaced with telecommunication systems through the teleprotection function. The teleprotection function may be performed by a standalone device, or it may be integrated with the protective relay or with the telecommunication equipment. Interfaces between protection relays, teleprotection, and telecommunication systems are described in Chapter 5. The following chart is excerpted from the IEC 60834-1 standard to help show the relationship between protection, teleprotection, and telecommunication. From the teleprotection point of view, the relatively selective protection schemes shown in Figure 2.5-1 are typically communication-aided state comparison schemes (see 3.1.2), and the absolutely selective protection schemes in Figure 2.5-1 are typically communication dependent analog comparison-schemes (see 3.1.1.).

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Figure 2.5-1:

Fundamental terms on protection and teleprotection (From IEC60834-1)

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3 PROTECTION USING TELECOMMUNICATIONS 3.1 LINE PROTECTION 3.1.1 Analog Comparison Schemes Analog comparison protection is based on the transmission and comparison of electrical parameters such as primary currents (amplitude and/or phase) between the ends of a protected line. Each end sends its registered values to each other and compares them with the remote ones. When an internal fault occurs, the result of the comparison will be a differential value, so that, if it is higher than a threshold, the relay will initiate the trip. These systems are called analogue comparison protection systems because they exchange analogue quantities such as amplitude and/or phase with the other ends. They are sometimes also referred to as "unit protection" or "closed" schemes. The term “unit” refers to the clear interdependence between the ends for operation and to the closed and absolutely selective characteristic of this protection. Obviously, the comparison must be made between magnitudes at the same instant, which implies a transmission and comparison system as fast as possible. A delay must be provided for the local signal to compensate for the transmission time of the remote value. Unlike the time-grade protection such as distance and time overcurrent relays, the trip of the analog comparison protection is instantaneous for every fault on the protected line. It is applicable to any overhead line or cable at all voltage levels and for any type of system neutral arrangement. It is particularly suitable where: -

-

Step distance relays (without acceleration schemes) have limitations, for example: à Very short lines and cables due to their low impedance, which makes it difficult to find an adequate setting to get a instantaneous trip for faults on the main part of the line. à Multi-terminal lines, since the intermediate infeeds modify the impedance seen by the distance relays, which depends not only on the distance to the fault, but also on the infeed from the remote terminals, making impossible an accurate measure of the impedance. No potential transformers and only current transformers are installed at each end of the line.

We can distinguish two types analog comparison protection systems: longitudinal current differential protection and phase comparison protection. The current differential protection compares the power frequency signals proportional to the primary power system currents (amplitude and phase angle), while the phase comparison one is based on comparison of the phase angle (or sign) between currents of each end of the protected line. Since both of them use only current information, in comparison with the distance or other system protections, analog comparison protections have the following advantages: -

Not responsive to system swings and out-of-step conditions Unaffected by inadvertent loss-of-potential (i.e., due to a blown potential fuse)

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-

No mutual coupling problems from parallel lines. This may cause the line-to-ground fault current reverses and flows into a weak source terminal, causing faulty directional discrimination if other protection systems are used Not subject to transient problems associated with coupling capacitor potential devices With segregated current differential there are no problems of phase selection for single pole auto-reclosing at simultaneous faults on different circuits and phases close to one line end, because it operates only for faults between current transformers in each phase. Some relaying problems in EHV transmission lines due to applying series capacitors are also overcome, e.g. voltage reversal, current inversion or phase imbalance.

When phase selection is required for single phase tripping, especially at simultaneous faults on different circuits and phases or in a faulty line when handling heavily loaded EHV lines, the phase-segregated technique is used. The analogue information is transmitted separately for each phase. In cases where the complete information about the polyphase conditions is not essential and single-phase tripping is not needed, the non-segregated technique is used. It reduces the threephase system of currents to a single-phase one by means of a mixing device. The communication link needs therefore to only accommodate the transmission of this single phase information. Some mixing techniques are described in [1]. 3.1.1.1 Current differential protection Operating principles As mentioned above the current differential protection is an absolutely selective protection system for transmission lines, tripping instantaneously for faults in the protected zone defined by the current transformers of each end of the line. It is based in the principle of current comparison. The Figure 3.1-1 shows a basic scheme of the differential protection. In each terminal, an evaluation circuit compares the sum of the local and remote current values, i.e. the differential current, with an operation threshold value Iop. In normal operation conditions or external faults, the current entering at one end is practically the same as one leaving at the other end, so the differential current value is practically zero and the protection will remain stable. For a fault on the protected power line the differential current value will exceed the operation value and the protection will trip. When very large currents flow through the protected zone for a fault external to the zone a differential current appears due to the different ratio error and saturation characteristic of the current transformers, which could exceed the operation level. Such a maloperation of the protection is prevented by the stabilizing. The stabilizing characteristic uses a bias current, which is usually proportional to the sum of the absolute values of the currents at each terminal, i.e. |iA| + |iB|, in order to make the protection less sensitive for higher through currents. This technique is also called percentage restraint.

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A

IA

B

IB

SA

TX

iA

Telecommunication system

TX

SA

iB

DEL

DEL

iB

+

RX

Id

RX

TPF

Id> Iop

TPF

iA

+ Id Id>Iop

SA = Signal adapter (filtering, mixing circuit, A/D conversion, etc.) TX = Transmitter RX = Receiver Iop = Operation threshold according to stabilizing characteristic DEL = Delay compensation TPF = Teleprotection Function

Figure 3.1-1:

Principle of differential protection

Figure 3.1-2 shows an example of percentage restraint characteristic with two slopes: the lower slope ensures good sensitivity to resistive faults under heavy load conditions, while the higher slope is used to improve relay stability against saturation of the current transformers and other distortion effects under heavy through fault conditions. The selection of the minimal operation current Is1 is based upon the magnitude of line capacitance current and switching transients expected on the protected line. The capacitance of the three conductors to earth and, except in single core cable, also between each other, makes that under undisturbed conditions the current at both ends differs in angle and magnitude. Particularly in cables, the capacitive charging current can attain significant values. Nevertheless, usually the necessary rise of the Is1 does not involve an important loss of sensitivity. Idiff Idiff > k2xIbias - (k2-k1)Is2 + Is1

TRIP slope k2

NO TRIP

Idiff > k1xIbias + Is1

slope k1

Is1

Is2

Ibias

Idiff = iA + iB Ibias = |iA| + |iB|

Figure 3.1-2:

Differential protection: Example of percentage restraint characteristic

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The differential principle may be applied to multi-terminal lines. The protection relies on the sum of the inflowing currents, which are added geometrically. For this purpose, the measuring circuits have to be so arranged that at each end of the line, the local current and the currents from each of the others ends of the line are available for comparison. Generally, the most recent designs allow up to three terminals applications. For a multi-terminal system, the master/slave or centralized configuration is also used. In this case, the current values are sent to a specific terminal for evaluation of the differential current. This terminal will henceforth be noted as a master, while the terminal sending information about currents will be denoted as a slave terminal. For a two-terminal system, the master/slave configuration can, of course, also be used, but a master/master or distributed configuration, where the current information is exchanged between both terminals and evaluated at both ends is normally preferred, since this gives a shorter operating time than that in a master/slave configuration. See Figure 3.1-4 and Figure 3.1-5 for more details about centralized and distributed configurations. The saturation of the current transformers for heavy through currents normally requires the selection of a higher slope setting which involves a loss of sensitivity for internal faults. Recent protections include some techniques to detect the saturation, so in only such conditions is the protection desensitized increasing the restraint slope. To avoid the maloperation of the remote protections, the terminal that detects the saturation includes a code in the message transmitted to the other ends, so that all terminals increase the degree of stabilization. Time delay compensation As mentioned, the current values used in the differential protection must be taken at the same instant at all ends of the power line for comparison, so a delay circuit is needed to compensate the transmission time for the remote values. Classical designs incorporate an adjustable delay for aligning the current values. However, when digital communication systems with automatic route switch are used, the time delay can change and the protection must continuously adjust the time alignment. For this purpose, digital devices incorporate different techniques in which the messages of current values sent through the communication channel are tagged with the sampling time. The principles of some synchronization techniques are described in more detail in A4.1. An error in delay compensation results in a differential current that - according to Figure 3.1-2 - increases the risk of unwanted tripping. For more information see 6.1.2.2 and 6.3.1.1. Additional functions Generally, differential protections use intertrip functions, i.e. the sending of trip commands to the remote ends. Intertrip commands are sent through the same communication channels used to transmit the current values (switching the channel frequency to a specific intertrip frequency when analogue links are used, or flagging the corresponding command bits in the out-going data messages in digital links). The intertrip function is activated either when the relay reaches a trip decision, or by closing an external contact connected to an input of the relay. The intertrip function can be used for: - Breaker failure protection - Stub protection: this is applied in switchyards with 1½ circuit breaker configuration. Operating an input by external contact when the line isolator opens allows to protect the line between the circuit breakers and the line isolator.

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Telecommunication systems used for differential protection Differential protection systems using pilot wires for 50/60Hz signals Pilot wires connect both ends electrically and establish a differential circuit where the secondary quantities may be in the form of current signals or voltage signals, which are proportional to the primary current. Accordingly, there are two basic methods of creating a differential circuit, current balance or voltage balance. Figure 3.1-3 shows a basic scheme of a current balanced system using three pilot wires.

ST Pilot wires

MCT

MCT TR

TR

ST

Evaluation circuit

Evaluation circuit

MCT = Mixing current transformer TR = Transformer for tripping ST = Transformer for stabilizing effect

Figure 3.1-3:

Basic scheme of a current balanced system using three pilot wires

In this case, the three-phase system is converted into a single AC current in the mixing transformer MCT (non-segregated). One differential system for each power phase (segregated) of the protected circuit can also be provided. If high resistance faults are expected or faults on which the value of earth fault current is relatively low, a fourth measuring system for the zero sequence component can be introduced. This however, increases the number of pilot wires and therefore the communication cost of the comparison information. In both methods, a replica of the vector difference is formed at each line end by means of a transformer ST for the stabilizing effect and a replica of the vector sum of the currents flowing at each end by means of a further transformer TR for the tripping effect. These values are evaluated separately at each line end in a measuring module and a tripping command is issued to the circuit-breaker when the fault current has exceeded a permanently adjusted threshold value. Where the voltage induced into the pilot cables during earth faults may exceed the rated values, the protective relays should be isolated from the pilot wires by isolating transformers, which can also be used to subdivide the total length of the pilot wires into two or three sections. This prevents the equipment from being subjected to excessive longitudinal voltage due to

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interference. In any case, the grounding conditions should be considered. The application of differential protection using pilot wires is restricted on lines up to 10-25 km depending upon the scheme used. So for longer lines, modulation techniques over other transmission media should be used. More details about differential protection using pilot wires and their limitations can be found in [1] and in chapter 4.3.1. Differential protection systems using modulation or coding techniques Modulation or coding techniques that are compatible with analog and digital telecommunication circuits are used to overcome some of the shortfalls experienced with direct pilot wire coupling.1 Typical techniques that are used: -

-

-

Frequency modulation (FM) for analog voice frequency (VF) channels. The instantaneous current values at each terminal are transmitted as analogue quantities to the other terminals in a voice frequency band (0.3 to 3.4 kHz) using frequency modulation. Whatever transmission media for analogue voice channels may be applied. Numerical coding for digital telecommunication systems The instantaneous current values at each end of the power line are sampled, converted to digital data and transmitted towards the other terminals through a digital telecommunication system. Sample rates ranging from 12 to 60 samples per cycle have been used. Normally, the telecommunication system is shared with other services like voice, telecontrol, etc. using Time Division Multiplexing techniques (see 4.4.1.2). The protection system is connected to the PCM) multiplexer through standard interfaces. The most commonly used electrical interfaces are those contained within the ITU-T or EIA recommendation and are described in 5.3.1 and in [2]. Dedicated optical fibres. Direct optical fibre links between protection terminals are also used. A higher reliability is achieved because intermediate devices are eliminated. However, when using dedicated fibres over long distances, the cost can be prohibitive beyond 10-20 km. See 4.3.4 for more information on optical fibres.

Multi-terminal configuration Transmission line protection based on a current differential scheme detects zone faults by using each terminal current and transmits the detection results of the zone fault to the other terminals. There are two types of multi-terminal current differential protection configurations; centralized and distributed configurations. As these configurations are applied to a single zone protection, they may be also applied to multi-zone and wide-area protections. 1

Note on pilot-wire replacement: The corrosion problems of buried copper wires, with the trend of telcos to replace copper-pair cables with fibre communication links, have put pressure on utilities to consider alternate means of connecting their extensive infrastructure of pilot-wire relays; this has created a market for specialized interface units which emulate these copper wires. The accuracy requirements of such interfaces depend on the accuracy requirements of the relay settings, the main parameters of concern are: The interfaces’ dynamic range. This should not limit on fault currents, whilst providing the required signal integrity during low line-current conditions. The end-to-end propagation delay. Since a 10% fault current error would be caused by the 5 degrees phase error accruing from 230µs on a 60Hz grid (280µs on a 50Hz grid), this delay is critical (this teleprotection application has the most stringent delay requirements of all teleprotection applications). In practice, up to 1ms may be manageable for the protection of 2-ended lines, but 500us or less may be required for 3-ended lines.

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Centralized configuration Figure 3.1-4 shows an example of line protection for a five-terminal EHV line [7]. Each terminal has a terminal unit that detects the current and transmits the data to the main unit terminal via a communication channel. This configuration simplifies the unit of each terminal and communication channel. Since the main unit has current data of all terminals, the fault locator function can be easily implemented by using these data.

Figure 3.1-4:

Centralized configuration

Distributed configuration Figure 3.1-5 shows a distributed configuration of five-terminal current differential line protection system. Each terminal has the current differential protection function as well as the signal transmitting function that multiplexes current data at each terminal into one communication signal. Master station A sends its own current data to slave station B. Slave stations B, C, D and E multiplex their own current data over communication signal. Slave station E turns back this signal toward slave station D. Now current data of all terminals are on the communication bus and available for protection. In addition, this system contains sampling synchronization function which enables the simultaneous sampling of current data at each terminal with high accuracy. Many installations were conducted using a 1.544-Mbit/s fiber-optic communications channel for HV double-circuit multi-terminal (up to ten terminals) or tapped lines [8]. In this network configuration where current differential calculation is usually carried out at each terminal, a centralized scheme where only master station conducts the calculation and sends the transfer trip signal to all slave stations is also available.

Figure 3.1-5:

Distributed configuration

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3.1.1.2 Phase comparison protection Operating principles Phase comparison protection is based on the comparison of the phase angle between currents of each end of the protected power line. Under normal load conditions or in case of an external fault, the angle measured between the local current and the current at the remote ends will be small. If the angle is large, it is due to an internal fault. The basic principle of all phase comparison systems is to measure the angle as above mentioned. However, the method of doing so can differ from manufacturer to manufacturer. A phase comparison system can be characterised by the following features: -

Comparison is made for each phase separately. A zero sequence circuit may also be included => Segregated protection. The currents of the three phases are mixed into one quantity for comparison => Nonsegregated protection. The measurement is made twice every period => Full-wave phase comparison. The measurement is made once every period => Half-wave phase comparison. The phase angle signal is transmitted to the remote end only when a starter has picked up. Measuring is carried out continuously and the signals are permanently transmitted. A phase comparison scheme can be designed for a blocking mode or for an unblocking mode of operation, similar to a distance protection system using telecommunication.

The current which is used in the comparison is converted into a square wave signal, so that the positive portion corresponds to the positive half-cycle and the zero portion corresponds to the negative half cycle. The square wave from the remote terminal is compared with the local square wave as shown in Figure 3.1-6.

A

SA

SQ

TX

iA

DEL

∆ϕ>θ

B

IB

IA

& iB


TX

iB

RX

RX

TPF

TPF

SA = Signal adapter (mixing circuit, filtering, etc.) SQ = Squarer TX = Transmitter RX = Receiver DEL = Delay compensation ∆ϕ = Coincidence angle θ = Stabilizing angle & = Logical AND TPF = Teleprotection Function

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SA

SQ DEL

& iA

∆ϕ>θ


a) External fault or normal load

b) Internal fault

IA

IA

IB

IB

iA

iA

iB

iB

iA & iB

iA

& iB

∆ϕ>θ

∆ϕ<θ Half-wave phase comparison

Figure 3.1-6:

Phase comparison operating principles

In normal line conditions, there is a (small) phase current difference between line ends due to: - The capacitance of the power line - Errors due to the equipment, e.g. current transformers, sequencer, filters, squarer, etc. - The time delay due to the signal propagation time between the terminals To prevent false trips, a critical angle is defined, commonly called stabilizing angle, which limits the maximum phase difference between currents, which would correspond to a boundary between tripping and stabilizing. In a non-segregated phase comparison protection, the three currents are mixed into one quantity by means of a composite sequence network. The half-wave system use starters, normally based on overcurrent detectors, to determine whether a fault has occurred, to initiate signal transmission to the remote end and to permit local tripping. In the full-wave system, the comparison is made for each semi-period and normally is therefore faster than the half-wave type. Phase comparison information is transmitted all the time to the remote equipment, and no starter is required. The comparison in the segregated protection system is similar to the non-segregated protection but, the comparison is made separately for each phase. It is very suitable for single pole reclosing when handling heavily loaded EHV lines and parallel circuits on the same towers. Segregated protection is more sensitive for earth faults than non-segregated protection, but it is more costly and the requirements on communication are higher. More details about non-segregated and segregated techniques can be found in [1].

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Some recent designs use dynamic principles based on the variation of the instantaneous current values in for example two periods, i.e. ∆i = i(t) - i(t-2T), so that the signal to compare is the sign of this variation value ∆i. This principle, normally operating in combination with a conventional phase comparison with starter, gives a higher sensitivity for high resistance ground faults. Time delay compensation As described for differential protection system, time delay compensation must be also provided in phase comparison protection in order that phase current values can be compared at the same instant. Depending on the technology, the channel delay can be compensated either by dynamic measuring techniques, or by a fixed delay setting in the protection relay. This later case is only useful when there is no possibility for time delay variations. Additional functions A number of complementary functions may be included in the protection relay. Intertrip functions are used to trip the remote breakers by means of sending a command through the same communication channel used for comparison signals. An overcurrent criterion normally supervises the remote trip to prevent tripping under normal conditions. The same additional functions mentioned for current differential protection are also applicable for phase comparison protection. Telecommunication systems used for phase comparison protection Non-phase-segregated technique In a half-wave comparison scheme it is very common to use power line carrier as communication medium, with the same carrier frequency used for both directions. The carrier is amplitude modulated i.e. switched “on” during positive half-cycles, and “off” during negative halfcycles, or vice-versa. This system operates as a blocking scheme. For an internal fault, if the blocking signal from the other end is not received, the output of the comparator circuit sends a trip command when the starters have picked up. This system might behave incorrectly in some situations due to the noise generated during a fault, i.e. blocking the operation for internal faults (=> delayed tripping) or deblocking for external faults (=> unwanted tripping). In a full-wave comparison different frequencies for the two directions must be used. A FSK (frequency shift keying) signal is used, which can be transferred over pilot wires, power line, radio or fibre-optic link. The communication equipment continuously monitors itself and when a fault occurs, the local signal is compared with the remote for both positive and negative halfcycle in the protection relay. Phase-segregated technique In this case, the values of each phase are transmitted separately via independent channels. Most recent phase comparison systems usually operate in segregated mode and use digital communication systems. The square signals to compare are sampled and converted to digital data, which are transmitted serially to the opposite terminal by the telecommunication system. Data rates and electrical or optic interfaces are the same as those mentioned for differential protection. When starters are used to initiate the comparison, a sequence of “guard” bits is transmitted in

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normal state of operation, in order to monitor the channel availability and performance by the receiver. Some designs optionally include a modem to interconnect two terminals through a 4-wire audio channel. In this case, a data rate of 9'600 or 19'200 bit/s may be used. 3.1.1.3 Charge comparison protection Charge comparison is based on the principle of conservation of charge at a node. The charge entering one line terminal must be approximately the same as the charge leaving the other line terminal(s) of a healthy transmission line. This is also the principle from which Kirchoff’s Current Law (the theoretical basis of current differential relaying) is derived. To perform charge comparison, the waveform of each line terminal’s phase and residual current is sampled every ½ millisecond. The half-cycle area under each wave is measured by integrating current samples between zero-crossings. For each phase and ground, the resulting ampere-second area (i.e., coulombs of charge) is stored in local memory, along with polarity and start/finish time-tags. This storage operation occurs only if the magnitude exceeds 0.5 ampere r.m.s. equivalent and the half-cycle pulse width is equal to 6 ms or more.1 Every positive (negative 3Io) magnitude is also transmitted to the remote terminal, along with phase identification and some timing information related to pulse width and queuing time (if any) at the transmitting terminal. When the message is received at the remote terminal, it is immediately assigned a received time-tag. A time interval is then subtracted from the received time-tag. This interval represents the channel delay compensation (which does not have to be precisely equal to the actual channel delay time) and the timing information contained in the received message. The adjusted received time-tag (after subtraction) is then compared with the local start and finish time-tags, looking for a “nest”, per Figure 3.1-7 (shown for an external fault).

Remote current

Actual channel delay time Time adjusted in received message

Channel delay compensation

Time interval subtracted Local current

Start time-tag

Finish time-tag

Received time-tag

Adjusted received time-tag

Figure 3.1-7: 1

Operation of charge comparison, external fault

Magnitude is actually measured in terms of ampere-seconds (i.e., coulombs). However, all values are converted to amperes rms equivalent, based on a perfect 60 Hz (or 50 Hz) sine wave, without offset.

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A nest is achieved when the adjusted received time-tag is greater than the local start time-tag and smaller than the local finish time-tag, for a given half-cycle stored in memory. When the nesting operation is successful, the local and remote current magnitudes (actually charges converted to equivalent currents) are then added to create the scalar sum (sum of absolute magnitudes). The scalar sum becomes the effective restraint quantity and the arithmetic sum becomes the effective operate quantity, per the bias characteristic shown in Figure 3.1-8.

BIAS LEVEL

ARITHMETIC SUM

(TRIP)

(RESTRAINT) SCALAR SUM

Figure 3.1-8:

Bias characteristic of charge comparison

The bias level is an operate threshold which provides security in the presence of spurious operate current due to line charging current, current transformer mismatch, analog-to-digital conversion quantizing errors, etc. As shown in Figure 3.1-8, the bias level rises sharply after the scalar sum reaches a high value. This provides security for unequal CT saturation during high current external faults. At lower currents, the bias level has a slight upward slope. This takes care of the relatively minor non-communications-related errors that increase with current level, such as CT ratio errors. The operating characteristic of charge comparison, when plotted on a polar diagram, is the “ideal” rainbow-shape of Figure 3.1-9. Referring to Figure 3.1-7, if the adjusted received timetag nests with a local negative half-cycle, this is equivalent to the upper half of Figure 3.1-9. If the adjusted received time-tag nests with a local positive half-cycle, then the arithmetic sum and scalar sum are equal to each other, which describes a 45 degree line on the bias characteristic (well above the bias threshold for all except very small values of current). This is equivalent to the lower half of Figure 3.1-9.

IR

IL

RESTRAINT REGION OF IR

Protected line

IL

Figure 3.1-9:

Ideal polar diagram characteristic

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The bias level of charge comparison is significantly more sensitive than that of conventional current differential relays for line protection. The conventional relay requires a gradually increasing bias to take care of increasing spurious operate current for a given assumed error in channel delay compensation (the biggest single source of spurious operate current). In contrast, charge comparison introduces no additional communications-related error as the currents get bigger, for a given error in channel delay compensation. Furthermore, for a given magnitude of through current, no operate error current is introduced, at all, for increasing channel delay compensation error (up to + 4 ms, at which point a total relay misoperation occurs – typical of a digital system). The + 4 ms misoperation threshold for charge comparison is almost three times the + 1.5 ms (approximately + 30 degrees on 60 Hz systems) misoperation threshold which is typical of conventional current differential schemes with circular polar diagram characteristics. Lit: [38]

3.1.2 State Comparison Schemes State comparison protection schemes use communication channels to share logical status information between protective relay schemes located at each end of a transmission line. This shared information permits high speed tripping for faults occurring on 100 percent of the protected line. The logical status information shared between the relay terminals typically relates to the direction of the fault, so the information content is very basic and requires very little communication bandwidth. Additional information may also be sent to provide additional control, such as transfer tripping and reclose blocking. For instance, breaker failure protection in ring bus and breaker and one-half bus configurations must transfer trip the remote terminal breaker(s) to isolate the failed breaker. Refer to chapter 3.2.2.2 for Bus Bar Protection/Breaker Failure Protection for more information on this subject. Overall, the communication requirements for state comparison protection schemes are considerably less stringent than for analog comparison protection schemes. Communication speed, or minimum delay, is always of utmost importance because the purpose for using communication is to improve the tripping speed of the scheme. Also, variations in communication speed are better tolerated in state comparison schemes than in the analog comparison protection schemes discussed in an earlier section. Communication channel security is essential to avoid false signals that could cause incorrect tripping, and communication channel dependability is important to ensure that the proper signals are communicated during power system faults, the most critical time during which the protection schemes must perform their tasks flawlessly. Comparing the direction to the fault at one terminal with the direction to the fault at the other terminal permits each relay scheme to determine if the fault is within the protected line section, requiring the scheme to trip, or external to the protected line section, requiring the scheme to block tripping. Directional distance and/or directional overcurrent relays are typically used at each line terminal to determine the fault direction. The relays used at each line terminal operate independent of the relays at other line terminals; some may even be set to provide time delayed tripping for faults outside the protected line section, hence the term “non-unit” protection, or “open system” protection is sometimes given to these types of schemes. If it were possible to set relays to see all faults on their protected line section, and to ignore faults outside of their protected line section, then there would be no need for communication schemes to assist the relays. However, distance and directional overcurrent relays cannot be set to “see” faults within a precise electrical distance from their line terminal. They are imprecise because of many factors, including voltage and current transformer errors, relay operating tolerance, line impedance measurement errors and calculation tolerance, and source

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impedance variations. The primary relay elements used to detect line faults are therefore set to see or reach either short of the remote line terminal (this is called under reaching), or to see or reach past the remote line terminal (this is called over reaching). Communication between line terminals at different electric power substations could be accomplished by simply extending a number of wires between the substations. Connecting a relay contact output from a relay scheme at one terminal to a relay scheme control input at the other line terminal with a pair of copper wires provides the communication necessary for one relay scheme to tell the other relay scheme that it has, or has not, seen a fault. Unfortunately, connecting communication wires directly between substations is not that simple and can even be hazardous. Voltage drop, induced voltages, and ground potential rise between substations during a fault make direct metallic wire connection between relay schemes unreliable, insecure, and hazardous. Communication for state comparison protection schemes must therefore be designed to provide safe, reliable, secure, and fast information transfer from one relay scheme to another. The communication scheme must also be able to transmit information in both directions at the same time. The amount of information required to transfer between relay schemes depends on the relay scheme logic. The basic and most common state comparison protection schemes are described in the following subsections. Their communication requirements are discussed within these subsections. The order in which they are presented does not imply their priority, relative importance, or usage. Other schemes and combinations of schemes may be designed to meet specific protection needs, however, they are typically all based on the basic schemes described in this document. The terminology used to describe these state comparison protection schemes may differ from utility to utility and country to country. State comparison schemes are basically defined according to the impedance zone which sends the protection signal to the remote end of the line. The following Table 3.1-1 shows the preferred CIGRE scheme names and alternate scheme names used elsewhere. CIGRE scheme names will be used throughout this document. CIGRE State Comparison Protection Scheme Name

Alternate State Comparison Protection Scheme Name

Intertripping underreach distance protection

Direct underreach transfer tripping

Permissive underreach distance protection

Permissive underreach transfer tripping

Permissive overreach distance protection

Permissive overreach transfer tripping

Accelerated underreach distance protection

Zone acceleration

Deblocking overreach distance protection

Directional comparison unblocking

Blocking overreach distance protection

Directional comparison blocking

Table 3.1-1:

State Comparison Protection Schemes

3.1.2.1 Intertripping Underreach Distance Protection The basic logic for a Intertripping Underreach Distance Protection scheme is shown in Figure 3.1-10. This scheme requires underreaching functions (RU) only, which are usually provided by phase and ground distance relay elements. The scheme is usually applied with an active channel that transmits a GUARD signal during quiescent, or unfaulted, conditions. The transmitter is keyed to a TRIP signal when the associated underreaching relay element detects a fault within its reach. The underreaching functions (RU) must overlap in reach to prevent a

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gap between the protection zones where faults would not be detected. RU RU

Bkr 1

Bkr 2 Protected Line Teleprotection Equipment

RU

OR

TX

TX

RX

RX

TRIP Bkr 2

TRIP Bkr 1

RU

OR

Protection Equipment RU - underreaching trip function, must be set to reach short of remote terminal and must overlap in reach with RU at remote terminal

Figure 3.1-10:

Intertripping Underreach Distance Protection Scheme Logic

For internal faults within the overlap zone, the underreaching functions at each end of the line operate and trip their associated line breaker directly. At the same time, the RU function keys its respective transmitter to send a direct transfer trip signal to the relay scheme at the remote line terminal. Receipt of the trip signal from the remote line terminal also initiates line breaker tripping. This scheme provides high speed tripping at both line terminals for all faults within the protected line section under most conditions. However, it will not provide tripping for faults beyond the reach of one of the RU functions if the remote breaker is open or if the remote channel is inoperative. If only one communications channel is used at each terminal, security may be jeopardized because any erroneous output from the channel initiates an instantaneous breaker trip. For this reason, this scheme is often applied with dual channels where both outputs must provide a TRIP signal to initiate a breaker trip. Or a slight delay may be added to a single channel output to ensure that the remote trip signal is valid before tripping the breaker. Time-delayed overreaching back-up tripping functions that do not interface with the communication scheme are usually added to trip the associated line breaker for faults beyond the reach of the RU functions when the remote breaker is open, or when the communication channel is inoperative. This scheme may use virtually any communication media that is not adversely affected by electrical interference from fault generated noise or by electrical phenomena, such as lightning, that cause faults. Communication media that use a metallic path are particularly subject to this type of interference, and must, therefore, be properly shielded, or otherwise designed to provide an adequate communication signal during power system faults.

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3.1.2.2 Permissive Underreach Distance Protection The Permissive Underreach Distance Protection scheme requires both overreaching (RO) and underreaching (RU) relay functions at both line terminals. This scheme is similar to the Intertripping Underreach Distance Protection scheme except that all communication assisted tripping is supervised by overreaching relay elements having what is often called a zone 2 reach. The scheme is usually applied with an active channel that transmits a GUARD signal during quiescent, or unfaulted, conditions. The transmitter is keyed to a TRIP signal when the associated underreaching relay element detects a fault within its reach. The underreaching functions (RU) must overlap in reach to prevent a gap between the protection zones where faults would not be detected. Basic logic for the Permissive Underreach Distance Protection scheme is shown in Figure 3.1-11. The relay functions and logic are easily performed with modern multi-zone phase and ground protective relays. Distance type relay elements are most often used for the underreaching functions (RU), and distance relay elements or directional overcurrent relay elements are used for the overreaching functions (RO). RO RU RU RO

Bkr 1


RU

TX

RO

RX

&

OR

Duplex Communication Link

TRIP Bkr 1

TX

RU

RX

RO

TRIP Bkr 2

OR

&

Protection Equipment RU - underreaching trip function, must be set to reach short of remote terminal and must overlap in reach with RU at remote terminal RO - overreaching trip function, must be set to reach beyond remote end of line

Figure 3.1-11:

Permissive Underreach Distance Protection Scheme Logic

When the underreaching relay elements detect a fault, they trip the local breaker directly and key a TRIP signal to the remote line terminal. Unlike the Intertripping Underreach Distance Protection Scheme, the Permissive Underreach Distance Protection Scheme supervises the received trip signal with an overreaching relay element. Communication assisted tripping occurs only if the overreaching relay element detects a fault during the time that a trip signal is received from the remote line terminal via the communication channel. Because the received communication signal is supervised by the output from an overreaching relay element, there is less concern about a false signal causing an incorrect trip. This scheme is therefore typically applied with a single duplex communication channel. This scheme may use virtually any communication media that is not adversely affected by electrical interference from

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fault generated noise or by electrical phenomena, such as lightning, that cause faults. Communication media that use a metallic path are particularly subject to this type of interference, and must, therefore, be properly shielded, or otherwise designed to provide an adequate communication signal during power system faults. The overreaching (RO) relay elements often start a zone 2 timer to provide time delayed tripping for faults outside the reach of the underreaching (RU) relays elements if the communication channel is inoperative. 3.1.2.3 Permissive Overreach Distance Protection The Permissive Overreach Distance Protection scheme requires only overreaching relay functions. Phase distance functions are used almost exclusively for detection of multi-phase faults, whereas ground distance functions or directional ground overcurrent functions can be used for the detection of ground faults. The scheme is usually applied with an active duplex communication channel that transmits a GUARD signal during quiescent, or unfaulted, conditions. The transmitter is keyed to a TRIP signal when the associated overreaching relay element detects a fault within its reach. Basic logic for the Permissive Overreach Distance Protection scheme is shown in Figure 3.1-12. RO RO

Bkr 1


RO

TX


RX

&

RO

TX RX

TRIP Bkr 1

TRIP Bkr 2

&

Protection Equipment RO - overreaching trip function, must be set to reach beyond remote end teminal

Figure 3.1-12:

Permissive Overreach Distance Protection Scheme Logic

For a fault anywhere on the protected line, both of the RO functions operate and assert one of the inputs to the logic AND (&) gate. At the same time, RO also keys the transmitter TRIP signal. Receipt of the TRIP signal at each terminal, and an output from the RO function, satisfies the logic AND (&) gate to produce a TRIP output to the breaker. For external faults, the RO functions at only one end of the line will operate, so communication assisted breaker tripping is not initiated at either terminal. The scheme is very secure in that it does not trip for any external fault if the channel is inoperative. Conversely, the scheme is lacking in dependability because it will not trip for any internal faults if the channel is inoperative. The scheme also will not trip for any fault if the fault is not detected at all terminals of the line. The scheme may not trip at high speed for close-in

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faults at the strong terminals because the fastest tripping time that can be expected is dependent on the slowest function to operate for an internal fault. Some means must be used to key the transmitter at an open breaker if tripping is to be initiated for faults seen at the other terminals. Breaker auxiliary contact switch keying with echo logic is commonly used to provide this requirement. Time-delayed back-up tripping can be provided because the scheme uses overreaching functions. Because the GUARD signal is transmitted continuously, the channel can be monitored on a continuous basis. This scheme may use virtually any communication media that is not adversely affected by electrical interference from fault generated noise or by electrical phenomena, such as lightning, that cause faults. Communication media that use a metallic path are particularly subject to this type of interference, and must, therefore, be properly shielded, or otherwise designed to provide an adequate communication signal during power system faults. 3.1.2.4 Accelerated Underreach Distance Protection Basic logic for the Accelerated Underreach Distance Protection scheme is shown in Figure 3.1-13. This scheme requires the use of underreaching relay element functions (RU) that can be extended in reach by the receipt of a TRIP signal from the relay scheme at the remote line terminal. The RU functions must be set to overlap in reach to avoid a gap in their fault detection. This generally requires the use of ground distance functions for the detection of ground faults, whereas phase distance functions are used for the detection of multi-phase faults. The scheme is often applied with an active communication channel that transmits a GUARD signal during quiescent, unfaulted conditions, and is keyed to a TRIP signal when the associated RU function detects a fault within its reach. Extended RU

RU RU

Extended RU

Bkr 1


RU


TX

TX

RX

RU

RX

Extend RU

Extend RU

TRIP Bkr 2

TRIP Bkr 1 Protection Equipment

RU - underreaching trip function, must be set to reach short of remote terminal and must overlap in reach with RU at remote terminal. It must be capable of being switched in reach.

Figure 3.1-13:

Accelerated Underreach Distance Protection Scheme Logic

For an internal fault within the overlap zone of the RU functions, breaker tripping is initiated directly at both line terminals and each communication channel is keyed to the TRIP signal. Receipt of the TRIP signal extends (accelerates) the reach of the RU functions to beyond the remote line terminal. This reach extension has no further affect because breaker tripping has

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already occurred at each line terminal. For an internal fault near one terminal, the RU function at that terminal operates, tripping the breaker and keying its transmitter to the TRIP signal. Receipt of the TRIP signal at the other terminal extends the reach of that terminal’s RU function, which then detects the fault to initiate tripping. For external faults, none of the RU functions operate, therefore tripping does not occur at either line terminal. The scheme is more secure than the Direct Underreach Distance Protection scheme because it does not trip directly on receipt of a trip signal. Conversely, it is slower than the Permissive Underreach and Overreach Distance Protection schemes because it must wait for the extended RU function to detect the fault before tripping. As mentioned before, it also requires a special relay with zone extension capability. This scheme may use virtually any communication media that is not adversely affected by electrical interference from fault generated noise or by electrical phenomena, such as lightning, that cause faults. Communication media that use a metallic path are particularly subject to this type of interference, and must, therefore, be properly shielded, or otherwise designed to provide an adequate communication signal during power system faults. 3.1.2.5 Blocking Overreach Distance Protection Basic logic for a Blocking Overreach Distance Protection scheme is shown in Figure 3.1-14. The scheme requires overreaching tripping functions (RO) and blocking functions (B) as shown. Distance functions are used almost exclusively for multi-phase fault protection, but either ground distance functions or ground directional overcurrent functions are used for ground fault detection. A quiescent, or OFF/ON, communications channel is typically used with this type of scheme. The power line itself is often used as the communications medium because the communication channel is not required when the fault is on the protected line. The communication channel is only used to transmit a block trip signal when the fault is external to the protected line. Audio tone over leased phone lines, microwave radio, and fibre-optic media are also used. The transmitter is normally in the OFF state for quiescent conditions and is keyed to the ON state by operation of any one of the blocking functions. Receipt of a signal from the remote terminal applies the NOT or inverted input to BLOCK the trip output. The overreaching tripping functions (RO) must be set to reach beyond the remote terminal of the transmission line with margin so they will be able to detect a fault anywhere on the transmission line. The blocking functions (B) are used to detect any fault not on the protected line that the remote tripping functions are capable of detecting; so they must be set to reach further behind the terminal than the tripping function at the remote terminal. For a fault external to the protected line, one or more of the blocking functions operate to key its respective transmitter to send a blocking signal to the remote terminal. Receipt of the blocking signal blocks tripping in the event one of the tripping functions has operated for the remote fault. The coordinating timer, TL1, is required to allow time for a blocking signal to be received from the remote terminal. It is set to compensate for channel time, signal propagation time and for any difference in operating time that might result if the remote blocking function is slower than the local tripping function. For a fault anywhere on the transmission line, one or more of the tripping functions (RO) at each terminal will operate and apply an input to its respective AND gate (&). The blocking functions will not operate for an internal fault, therefore neither transmitter is keyed, so that there is no output from either receiver. The logic at each terminal produces an output that starts the TL1 timer. When the TL1 timer expires, the scheme produces an output to trip the breaker.

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RO B B RO

Bkr 1


B

TX

RO

RX

TL1 C 0.0

&

Simplex or Duplex Communication Link

TRIP Bkr 1

TX

B

RX

RO

TRIP Bkr 2

&

TL1 C 0.0

Protection Equipment

RO - overreaching trip function, must be set to reach beyond remote end of line B - blocking function, must be set to reach beyond overreaching trip function at remote end of line C - Coordinating time, required to allow time for blocking signal to be received (set equal to channel time plus propogation time plus margin)

Figure 3.1-14:

Blocking Overreach Distance Scheme Logic

The scheme is very dependable because it will operate for faults anywhere on the protected line even if the communication channel is out of service. Conversely, it is less secure than permissive schemes because it will trip for external faults within reach of the tripping functions (RO) if the channel is out of service. This scheme does not require breaker auxiliary contact or echo logic keying when the remote breaker is open to permit tripping for faults anywhere on the line. It provides relatively fast tripping (dependent on coordinating time delay) for most source and line conditions. However, it may not trip weak terminals of the transmission line, if fault levels are below the sensitivity of the tripping relays. If quiescent (OFF/ON) communication channels are used there is no way to monitor the channel continuously because the channel is only keyed on during external faults. A communication channel check-back scheme is often used to periodically key a momentary block signal to check the channel status. Some check-back schemes echo a signal back to verify that the channel is operational in both directions. Other schemes must receive a signal within a preset time period to declare the channel in service. The overreaching functions can be used to drive timers so that time-delayed back-up tripping can be provided for faults within reach of the overreaching functions. 3.1.2.6 Deblocking Overreach Distance Protection As mentioned in some previous sections, metallic communication paths adversely affected by fault generated noise may not be suitable for some teleprotection schemes that rely on a signal transmitted during a protected line fault. With power line carrier, for example, the communication

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signal may be attenuated by the fault, especially when the fault is close to a line terminal, thereby disabling the communication channel. Multi-phase power line carrier coupling schemes can be used to minimize this problem. The Deblocking Overreach Distance Protection scheme includes logic specifically designed to accommodate a loss of communication signal during the protected line fault. The Deblocking Overreach Distance Protection scheme, like the Permissive Overreach Distance Protection scheme, uses overreaching phase distance functions almost exclusively for multi-phase fault detection, and ground distance or directional ground overcurrent functions for ground fault detection. The logic requires the use of an active communication channel that transmits a GUARD signal during quiescent, or unfaulted, conditions, and is keyed to a TRIP signal when the associated overreaching relay element detects a fault within its reach. To overcome the loss of signal caused by the internal line fault, deblocking logic permits a TRIP output if the loss of signal occurs at nearly the same time the overreaching relay function(s) detect a fault. A tripping period is controlled by a timer that is typically set between 150 and 300 milliseconds. Basic logic for the Deblocking Overreach Distance Protection scheme is shown in Figure 3.1-15. RO RO

Bkr 1

Bkr 2 Protected Line Frequency Shift Power Line Carrier Communication Link

GUARD OR TRIP TX

RO

RX TRIP

LOG

GUARD OR TRIP

}

{

&

OR

TRIP Bkr 1

TRIP Bkr 2

OR

T

& T

0.0

&

&

TX

RO

RX TRIP

LOG

0.0

RO - overreaching trip function, must be set to reach beyond remote end teminal LOG - Loss of GUARD detection from receiver, RX T - deblocking time delay, typically set for 150 to 300 milliseconds.

Figure 3.1-15:

Deblocking Overreach Distance Protection Scheme Logic

If the signal loss is due to a fault on the protected line, at least one of the overreaching trip functions (RO) will be picked up. Thus, tripping will be initiated when the deblocking output is produced. If none of the permissive trip functions are picked up, the channel will lock itself out 150 - 300 milliseconds after the signal is lost and will stay locked out until the GUARD signal returns for a pre-set amount of time. It is important to understand that this logic requires that the loss of signal associated with the operation of an overreaching relay element must only be caused by a fault on the protected line. Loss of signal due to external line faults will cause false trips. Therefore, the Deblocking Overreach Distance Protection Scheme Logic is used almost exclusively with power line carrier communication.

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3.2 BUSBAR PROTECTION Very often fault clearing criteria for a power system specify that busbar faults must be cleared in the order of 5 cycles, and that only a few feeders are allowed to be tripped. This may be the maximum allowed disturbance for a power system, in order to maintain stability of the remaining power system after fault clearing. Therefore phase to phase faults and phase to ground faults should be cleared within 5 cycles. Typical power system busbar configurations are shown on Figure 3.2-1, Figure 3.2-2, and Figure 3.2-3. Busbar protection is typically based on differential current principles. Busbar protections are mostly configured with zones, one or more zones for bus A and one or more zones for bus B. The busbar protection very often includes breaker failure protection, time delayed typically 5 to 9 power frequency cycles.

3.2.1 Two-breaker busbar configuration Two-breaker power system busbar configuration is shown on Figure 3.2-1. With two current transformers in each bay, busbar protection functions (measuring and trip actions) are independent of isolator positions. Breaker failure protection is started from busbar protection, line protection and transformer protection. Bus B

Bus A

BP-A Id-A

CBFP -A-L1

BP-B CB-A-L1

d

CB-B-L1

CBFP -B-L1

Id-B

Line 1

Notation : Bus-A is section A of the bus.

c

CB-A-L1 is circuit breaker A for line 1. CBFP -A-L2

CB-A-L2

CB-B-L2

CBFP -B-L2

Line 2 b

a

CBFP -A-T

CB-A-T

CB-B-T

BP-A is bus protetion for bus zone A. CBFP-A-F1 is circuit breaker failure protection for breaker A on feeder 1. Feeders may be lines, transformers or any other feeder. Id-A is current differential protection for bus zone A.

CBFP -B-T

Transf c

Figure 3.2-1:

Two breaker busbar configuration

3.2.1.1 Normal fault clearing For improving dependability or security, combinations of protection systems may be applied. The protection system has to detect faults and initiate actions on following faults:

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Fault location a (b) : CBFP-A (CBFP-B) trips bus A (B), and the fault is cleared. There is no need for telecommunication. Fault location c : This is a fault for line protection or transformer protection, see Chapter 3.1 and 3.3.2. Fault location between CB and CT, exemplified with fault location d : Busbar protection zone A trips bus A. But the fault is not yet cleared - there is still infeed from bus B and Line 1. To obtain fast fault clearing, the breaker failure protection 'CBFP-A-L1' trips breaker B on Line 1 and must initiate tripping of the remote breaker(s) on Line 1. This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1 to trip the line at least at the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection executes the tripping, it should be three phase without initiation of automatic reclosing. Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this fault is very seldom. If the line protection is performed by distance relays, transmitting a carrier signal to accelerate the 2nd zone of the line protection, at the remote line end, would provide a good solution. 3.2.1.2 Breaker failure The following fault clearing procedures apply in case of a breaker failure. Fault location a (b): For fault location a, if breaker CB-A-L1 is stuck, CBFP-A has to trip CB-B-L1 and initiate tripping of remote breaker(s) on Line 1. This can only be done by means of telecommunication as described in chapter 3.2.1.1 for fault location d. Fault location c: If breaker CB-A-L1 (CB-B-L1) is stuck, CBFP-A-L1 (CBFP-B-L1) has to trip bus A (B). There is no need for telecommunication in this case. Fault location between CB and CT, exemplified with fault location d : The scenario is the same as described in 3.2.1.1.

3.2.2 One- and a half breaker busbar configuration One- and a half breaker busbar configuration is shown on Figure 3.2-2. Busbar protection functions (measuring and trip actions) are independent of isolator positions. Breaker failure protection is started from busbar protection, line protection and transformer protection.

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Bus A

Bus B

BP-A Id-A

CBFP -A-L1

CB-A-L1 e

g

h

Line 1

CBFP -A-L2

CBFP -A-T1

CBFP -B-L1

CB-L3-L4

CB-A-L3

CB-B-L4

Line 4

a

b

CB-T1-T2

CB-B-T2

T2 d

Bus-A is section A of the bus.

CB-A-L1 is circuit breaker A for line 1.

Line 3

CB-A-T1

Id-B

T1 is transformer 1.

c

T1

Figure 3.2-2:

CB-B-L2

Line 2 c

Notation :

BP-B

CB-L1-L2 f

d

CBFP -B-L2

CB_L1-L2 is circuit breaker between line 1 and line 2. BP-A is bus protetion for bus zone A.

CBFP -B-T2

CBFP-A-F1 is circuit breaker failure protection for breaker A on feeder 1. Feeders may be lines, transformers or any other feeder. Id-A is current differential protection for bus zone A.

1½ breaker busbar configuration

3.2.2.1 Normal fault clearing For improving dependability or security, combinations of protection systems may be applied. The protection system has to detect faults and initiate actions on following faults : Fault location a (b) : CBFP-A (CBFP-B) trips Bus A (B), and the fault is cleared. There is no need for telecommunication. Fault location c and d : This is a fault for line protection or transformer protection, see Chapters 3.1 and 3.3.2. Fault location between CB and CT, exemplified with fault location e (h) : Busbar protection zone A trips Bus A. But the fault is not yet cleared - there is still infeed from bus B and Line 1. To obtain fast fault clearing, the breaker failure protection 'CBFP-A-L1' trips breaker CB-L1-L2 and must initiate tripping of remote breaker(s) on Line 1. This remote tripping can be executed either by direct intertripping of breakers, or by ’commanding’ or helping line protection systems on Line 1 to trip the line at least at the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection execute the trip, it should be three phase without initiation of automatic reclosing. Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this type of fault is rare in practice. If the line protection is performed by distance relays, transmitting a carrier signal to accelerate the 2nd zone of the line protection, at the remote line end, would provide a good solution.

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Fault location f and g: If current measurement for line protection of Line 1 and Line 2 crosses, this is a fault for line protection or transformer protection. See ’Fault location c and d’ above. 3.2.2.2 Breaker failure The following fault clearing procedures apply in case of a breaker failure. Fault location a (b): For fault location a, if breaker CB-A-L1 is stuck, CBFP-A-L1 has to trip CB-L1-L2 and initiate tripping of remote breaker(s) on Line 1. This can only be done by means of telecommunication as described in Chapter 3.2.1.1 for fault location d. Fault location c : If breaker CB-A-L1 (CB-B-L1) is stuck, CBFP-A (B) has to trip Bus A (B). There is no need for telecommunication in this case. If breaker CB-L1-L2 is stuck, the breaker failure protection of that breaker has to initiate tripping of remote breaker(s) of Line 1 (2). This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1 (2) to trip the line at least in the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection execute the trip, it should be three phase without initiation of automatic reclosing. Fault clearing time will normally exceed 5 cycles. As the current transformer and circuit breaker are very close, this type of fault is rare in practice. Fault location f or g : If current measurement for line protection of Line 1 and Line 2 ’crosses’, this is similar to ’Fault location c’ above. Fault location between CB and CT, exemplified with fault location e or h : The probability of this fault location in combination with stuck breakers is very low. Normally no breaker failure protection is applied.

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3.2.3 Two zones / one breaker configuration Bus B

Bus A

BP-A

BP-B I-A-L1

Id-A

I-B-L1

CB-L1

Id-B

Line 1

Notation : Bus-A is section A of the bus.

c

T1 is transformer 1. CB-L1 is circuit breaker for line 1.

Coupler-A-B

CB-T1 is circuit breaker for transformer 1. Coupler is coupler between bus section A and B.

d

b

a

CB-T1

Figure 3.2-3:

I-A-L1 is isolator A for line 1. Id-A is current differential protection for bus zone A.

T1

Two protection zones / one breaker busbar configuration

3.2.3.1 Normal fault clearing Fault location a and b: The busbar protection trips the bus, and the fault is cleared. If a line breaker fails, the second zone of the line protection ( Z< ) at the opposite line end serves as back-up protection. Fault location c: Busbar protection zone A and/or B trips bus A and/or B dependent of isolator positions. But the fault is on the line side of the breaker. Therefore, the fault is not cleared. To achieve fast fault clearing, trip command from busbar protection - dependent of isolator position - must initiate tripping of remote breaker(s) of Line 1(n). This remote tripping can be executed either by direct intertripping or by ’commanding’ or helping line protection systems on Line 1(n) to trip the line at least in the remote end. Telecommunication is needed. Automatic reclosing is not wanted on busbar faults, so if line protection executes the trip, it should be three phase without initiation of automatic reclosing. Fault clearing time will not necessarily exceed 5 cycles. Fault location d: The protection initiates a trip command, but the fault is not yet cleared. In order to clear the fault busbar protection zone B is designed to trip bus B if receiving a signal from zone A for more than 5 cycles.

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3.3 OTHER PROTECTION SCHEMES The following protection schemes may require telecommunication for intertripping.

3.3.1 Generator protection Generator and step-up transformer protection is normally designed to detect all faults and abnormal conditions dangerous for generators and step-up transformers. If action is needed, a stop signal is issued to the generator and a trip command is issued to a breaker interfacing the power grid. Telecommunication is normally not needed. As indicated on Figure 3.3-1, telecommunication is needed to trip a remote breaker if, for instance, the breaker interfacing the power grid is stuck is stuck or has not been installed to reduce capital expenditure. Substation Intertripping (telecommunication)

Short or long overhead power line

Step-up transformer Stop

Generator

Figure 3.3-1:

Transformer protection & Generator protection

~ Generator protection

3.3.2 Transformer protection The transformer protection normally consists of differential protection, overpressure protection and residual current protection. Overcurrent and impedance protection are often used as backup protection. The absence of a circuit breaker on the high voltage side in order to economize on circuit breakers requires an intertripping system to the adjacent station. In the event of an internal fault a lock out signal is recommended in order to block the closing of the connected circuit breakers.

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Lockout Intertripping


Power line protection

Power line protection Transformer protection

Trip Trip

Figure 3.3-2:

Transformer protection

3.3.3 Reactor protection Reactors are used to regulate the network voltage. These reactors are placed on the high voltage line to compensate capacitive generation. Normally the reactors have no circuit breakers, hence the reactor protection must send a trip and intertrip signal to the circuit breakers to both ends of the power line.

Intertrip


Intertrip

Reactor

Reactor

Reactor protection

Reactor protection

Figure 3.3-3:

Reactor protection

3.4 SYSTEM PROTECTION Figure 3.4-1 shows the relationship between protected zones/areas and operate times for various protection schemes. Main protection systems operate to clear faults at the very

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beginning in power transmission lines, busbars, transformers, and so on. If faults cannot be cleared by a main protection, however, successive operations are needed in the forms of backup, multi-zone and/or system-wide protections. Time after fault occurrence

Local backup protection

System stabilising protection Remote backup protection

Main protection #2 (Redundant or backup)

Wide-area or system-wide protection Multi-zone protection

Main protection #1 Protected zones/area

Figure 3.4-1:

Relationship between protected area and operate time with respect to protection schemes

3.4.1 Back-up protection Back-up protection [3] is a protection system that operates independently of specified components in the fault clearing system. It may duplicate the main protection system or may have the task to operate only when the main protection fails to operate or when the main protection is temporarily out of service. Back-up protections are usually categorized into circuit local back-up protections, substation local back-up protections, and remote back-up protections. On EHV networks it is common practice to use duplicated line protections as circuit local backup protection; a main protection (#1) and another redundant main protection (#2), taking account of maintenance or failures of one of the two main protections. A substation local back-up protection including a circuit-breaker failure protection is energized from instrument transformers located within the same substation as the corresponding main protection and is not associated with the same primary circuit. For example, when a circuit breaker failure occurs after a power line fault and a main protection operation, the breaker failure protection trips all the circuit breakers connected to the same busbar in the substation, if it is confirmed that the main protection has operated and the fault is not cleared. A remote back-up protection is located in a substation remote from that substation in which the corresponding main protection is located. The conventional remote back-up protections employ distance relays and utilize local electrical data for operating in zone 2 or wider zones. Figure 3.4-2 shows a network protected by distance protections without telecommunications. The distance protection uses current and voltage measured at one end of the power line. The protection uses these measurements to decide if the fault lies within the zones of the distance protection. A zone of the distance protection is open at the remote end. Zone-1 of the distance protection covers only about 85% of the power line. Zone-2 of the distance protection at A reaches beyond the remote terminal B. Zone-1 of the distance protections at B and zone-2 of the distance protection at A both detect fault close to B on the power line from B to C. To obtain rapid fault clearing, distance protections operate instantaneously when the fault occurs within zone-1. To obtain selectivity we have to delay the tripping for faults within zones-2 and 3. This

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co-ordination delay is usually about 0.4 seconds.

Figure 3.4-2:

Distance protection providing remote backup

Zone-2 of the distance protection at A must cover the entire power line from A to B. Zone-2 of the distance protection at A must not reach beyond zone-1 of the distance protection at B. Zone-2 of the distance protection at A backs up the distance protection at B. However, this is true for only one part of the power line from B to C. Zone-3 of the distance protection at A provides back-up for the rest of the power line from B to C. We have to delay the tripping from zone-3 of the distance protection at A more than the tripping from zone-2 of the distance protection at B, direction C. Splitting protection for busbar using communication for multi-circuit multi-terminal line For the configuration of double busbar and double circuit transmission lines, if a fault persists due to a CB failure or main protection failure, separation of the busbar by using splitting protection before remote back-up operation is effective to prevent interruption. However, for multi-terminal lines the splitting protection is done by sequential tripping and the operation time may not be coordinated with remote back-up operation. Figure 3.4-3 gives a sample application for three-terminal transmission lines.

Figure 3.4-3:

Splitting protection (BD) using telecommunications for multi-circuit and multiterminal line. Ry, CB and Td denote operating times of relay (30 ms) and CB (40 ms) and time delay for coordination, respectively.

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Splitting protections operate sequentially from (1) to (2), and then to (3). Therefore, it cannot be coordinated with the operation time (340 ms) of remote back-up protection (zone 2 of distance relay) of substation A, and remote back-up tripping occurs at the substation A end on both lines, which results in the isolation of substation C. To prevent such isolation, splitting protection at substation C performs transfer tripping of busbar CBs at substations A and B using communication channels. Busbars at substations A and B are separated within 260 ms which allows coordinating of the remote back-up protection. Lit.: [4]. Coordination time control using communication For the configuration where long distance transmission lines adjoin short distance transmission lines, coordination between remote back-up protections may not be achieved. Figure 3.4-4 gives an example where there is a long distance transmission line between substations A and B and a short distance transmission line between substations B and C. If fault F1 occurs at the busbar in substation B, zone 2 of distance relay of substation A may operate. Zone 2 of substation A cannot be coordinated in the standard zone-2 time setting of 270 ms. In this case, the time setting needs to be changed from 270 ms to 370 ms, which is equivalent to the operation time of zone 3. However, there is another problem that remote back-up operation (zone 2) of the substation B is delayed for the busbar fault F2 at substation B. In order to accelerate the operate time, the splitting protection operation signal is sent from substation B to substation A by a communication channel, and the operation time of zone 2 in substation A is shortened to 270 ms. Lit.: [4].

Figure 3.4-4:

Coordination time control using telecommunications. Ry, CB and Td denote operating times of relay (30 ms) and CB (40 ms) and time delay for coordination, respectively.

Wide-area current differential back-up protection To cope with such complexity of coordinating operate times and reaches and obtaining necessary selectivity in remote back-up protection employing distance relays, wide-area backup protection based on a current differential algorithm which utilize electrical data at remote stations employing wide-area telecommunication networks among substations is proposed as shown in Figure 3.4-5. The wide-area back-up protection system covering multi-zones consists of central equipment (CE) and terminal equipment (TE) which are connected by telecommunication networks. The terminal equipment samples all the currents from instrument transformers installed at a busbar and at power transmission lines and power transformers

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connected to that busbar. The data are transmitted by the terminal equipment to the central equipment through data communication channels. As this protection scheme has to be installed for every busbar, one central equipment may cover two or more protected areas or busbars. For the double-busbar and double-circuit configuration shown in the figure, the conventional backup protection firstly performs busbar splitting protection to prevent the interruption of a sound circuit or isolation of transmission lines, which leads to longer operate time, wider outage area or isolated transmission lines. When a fault occurs at F in the busbar in substation B, for example, and if the busbar protection fails to operate, the conventional remote back-up protections or distance relays of substation A and C operate in zone-2 after the bus-tie splitting protection operates to prevent disruption of the sound circuits. The wide-area back-up protection operates to minimize the outage area, which is the same as the main protection in this case. Therefore, the operate time is 140 ms shorter than the conventional protection and the outage area is smaller. Lit.: [6].

Figure 3.4-5:

Wide-area current differential back-up protection employing telecommunications

The wide-area current differential protection system requires wide-area timing synchronization for simultaneous current sampling. As some current differential multi-terminal line protections employ centralized timing synchronization scheme in their telecommunication circuits, a similar scheme may be applied to such wide-area protections. More terminals, however, lead to the complexity of achieving total synchronization among the terminals using telecommunication circuits. Satellite-based wide-area timing synchronization such as GPS may be an alternative solution. Since back-up protections are initiated after a main protection operated, delays for transmitting current data and tripping signals are not necessarily crucial, while timing synchronization and data integrity and reliability are still important.

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Since this kind of wide-area protection system employing telecommunications can collect various kinds of power system data simultaneously sampled throughout the area, further sophisticated power system monitoring, control, protection, and restoration could be achieved.

3.4.2 System-wide protection System stabilizing protection operates in a wider area than that for power line protections or in a system-wide area to prevent power system disturbance. For example, when severe faults such as double faults in a double-circuit transmission line occur, and even if main and back-up protections operate properly, it may result in a power system disturbance such as overload, power swing, abnormal frequency or voltage. Operations of such protection are load shedding, generator shedding or system separation, which in many cases requires wide-area telecommunications. Some adaptive protections, defined as a philosophy that permits and makes adjustments to protection functions automatically for making the protection more attuned to the prevailing power system conditions, require wide-area on-line telecommunication channels [9]. A predictive out-of-step protection [10] operates for preventing total system collapse caused by step-out between large-capacity generator groups due to a serious fault in the trunk power transmission line as shown in Figure 3.4-6. When a double-fault occurs along both circuits of a double-circuit line forming one route, the substations at both ends of the line are disconnected and power transmission capability is interrupted. If a successive fault occurs after reclosing, a slow cyclic power swing develops between the western generator group and the bulk power system. The same situation occurs in case of failure of a busbar protection to operate during a busbar fault. Over time, the phase difference of the generator groups thus undergoes oscillating divergence. If this condition is not corrected, an out-of-step situation will begin to occur in various parts of the power system and may lead to total collapse of the power system. Taking account of this characteristic of the power system, the western area can be isolated from the bulk power system before an out-of-step situation occurs and then be operated independently. This eliminates power swing between the generator groups of the two systems and restores stability. This separation of the western area is performed in a manner to preserve the power supply and demand. The separation point is selected based on the power flow at pre-determined points for separation before the fault. Adjustment of the supply/demand balance of each area after separation is performed by governor control of the corresponding generator groups. The western generator group, however, may under certain conditions becomes overloaded. In this case, load shedding via under-frequency relays is relied upon to correct the unbalance. This protection is accomplished by using on-line voltage data, or busbar voltage waveforms, collected from the generator group by central equipment to predict step-out based on the measured voltage phases and then issuing a system separation command. This system consists of central equipment and RTUs, and requires sampling synchronization for the voltage phase measurement. The telecommunication requirements from this protection are almost the same as the wide-area current differential protection described above. The required overall operating time is less than a few hundreds of milliseconds, where transmission time including initiation and processing of frame-formatted cyclic data transmission (Tac in Figure A1-5) should be less than several tens of milliseconds, and propagation delay requirement including media and equipment delay be at most several milliseconds.

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Figure 3.4-6:

A system-wide protection; predictive out-of-step protection

Another system stabilizing protection, which consists of central processing unit, fault detecting unit, and transfer trip unit, operates for stabilizing power frequency or transient instability [11]. The central processing unit collects data on generated energy and load as controllable quantities from power stations and substations, automatically recognizes electric power system connections, performs calculations in advance to prepare for faults, and automatically determines control quantity and objects for each pattern of separation. If a fault occurs, the central processing unit sends a trip signal to the transfer trip unit based on the calculation results. The fault detecting unit detects a route disconnection fault in any of the EHV lines, and calculates power flow through the main lines, frequency, and voltage drops, and transmits these data to the central equipment. If faults occur, the transfer trip unit receives a transfer trip signal from the central processing unit, and sheds the generators and/or loads as controllable quantities based on the received information to stabilize the frequency of each separated part. A means of high-speed multiplexed data transmission of large volume of information is essential to a power stabilizing system that provides adaptive approach at high speed. A dedicated transmission unit is used for the important information such as fault or route-off detection and transfer trip signals to ensure high-speed and reliable transmissions, while relatively large volumes of information that do not necessarily require high speed are transmitted by an EMS/SCADA communication network. The overall operate time and transmission time requirements are similar to the previous system stabilizing protection.

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4 TELECOMMUNICATION SYSTEMS FOR PROTECTION The purpose of a telecommunication system in conjunction with protection systems is to transfer a protection signal in due time from the protection equipment at one station to a similar equipment at the remote station. A secure and dependable point-to-point communication is normally required for this purpose. Possible transmission media are: -

Pilot wires / copper wires Power line carrier (PLC) links Microwave radio links Fibre optic links Satellite links

The telecommunication link should have a high degree of availability and should transmit the protection signal as fast as necessary to the remote station with the highest possible reliability. The actual requirements on transmission speed, dependability for wanted operation and security against unwanted operation may vary for different protection schemes and line configurations. Practical and economical reasons may define which type of transmission medium has to be used. All communication systems are subject in varying degrees to interference and noise of various forms. These can corrupt the information arriving at the receiver, either by simulating a signal when no such signal has been transmitted, or by delaying or blocking a true signal. In analog systems, there are many ways in which transmission can be degraded. For example, the signalto-noise ratio may be poor, or the signal may suffer distortion or crosstalk from one user to another, or the system may clip the input signal. In comparison, a digital system has the parameters: bit rate, error rate, delay, and delay variation. Channel impairments may result in bad messages, no messages, excessive message delay, excessive message delay variation and/or excessive delay difference in the transmit and receive direction. The quantity of information per unit time (bits per second) which a communication channel can transfer depends on its bandwidth and on the received signal quality1 (normally expressed as Signal-to-Noise Ratio, SNR). The signal transfer delay introduced by the medium is normally low for terrestrial links, since in most media the signals propagate typically at speeds between 60% and almost 100% of the speed of light in vacuum. The propagation delay is for example about 3.3µs/km for open-wire (e.g. Power Line Carrier) and microwave radio links, about 5µs/km for optical fibres and 5 .... 10µs/km for pilot wires. The significant part of the overall operating time of a teleprotection system is normally introduced by the terminal equipment including their interfaces to the protection, by intermediate repeater stations and network node devices with channel routing functions. See also Figure A1-5 in ANNEX A1. Transmission time delay, bandwidth and signal quality are important parameters when considering the design of a telecommunication system used for protection. The criteria apply equally for both analogue and digital communication systems. For digital systems it is however 1

The maximum information flow that a communication channel can transfer without errors is called its capacity. According to Shannon's law the channel capacity is given by the formula C = B x ld(1 + SNR), with C = Capacity in bits/s, B = Channel bandwidth in Hertz, ld = logarithm to the base of 2, and SNR = Signal-to-Noise Ratio of the received signal. The channel capacity is a theoretical value that can only be approximated at the cost of excessive signal transfer delay due to infinite coding- and decoding efforts. The formula also indicates that a bandwidth related data rate increase is compromized by the bandwidth related SNR deterioration (=> the wider the bandwidth is, the more noise is captured).

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more convenient to use the data rate instead of the bandwidth and to express the transmission quality in terms of bit errors (e.g. bit error rate, errored seconds etc.) rather than signal-to-noise ratio.

4.1 TELECOMMUNICATION CIRCUITS The term "circuit" may be used to characterize legal aspects or physical properties of a communication service. Some examples of circuits are described below.

4.1.1 Private and rented circuits Communication circuits may be utility owned or rented from third parties. Security, dependability and availability of rented circuits do not always satisfy the requirements from protection. Some typical threats and risks are: -

Rented circuits are beyond the control of the power utility Rented circuits may be re-routed for operational reasons. This can change the transmission characteristic, e.g. the signal transfer delay, which may cause problems to the protection function Signals may be injected into the circuit for routine tests or maintenance reasons which may prevent protection from operating or may cause unwanted operation The medium (wires, fibres, radio etc.) and hence its associated typical risks may not be known to the user

Circuit or service providers however may offer circuits or services with guaranteed performance, which seem to be applicable to protection.

4.1.2 Analogue and digital circuits All physical transmission media are analogue by nature. The distinction between analogue circuits and digital circuits is defined solely by the communication equipment technology. The term "analogue" or "digital" circuits thus mainly relates to the physical properties of the communication interface, see also interfaces (a) and (b) in Figures A1-1 to A1-5 in 9. If an interface accommodates waveforms that vary continuously with time and amplitude, that interface provides an analogue circuit. If an interface accommodates signals that may change between few (normally 2 or 3) amplitudes at certain instants of time only, that interface provides a digital circuit. Analogue communication systems have enhanced protection systems for many years. Their advantage is their efficient use of bandwidth, especially for the transmission of analogue signals such as voice. Historically, analogue communication systems provided analogue circuits to the user. The situation has changed with the advances in digital electronics and signal processing, with the development of bandwidth efficient digital modulation principles and with the breakthrough in optical fibre technology. Due to the availability and the advances in digital communications, it is increasingly being used for the protection of power systems. Digital communication systems may provide both analogue circuits (e.g. for voice, telefax and modems) and digital circuits (for data) to the user. The relatively simple characterization of a digital communication system is an important advantage over analog systems, where there are many parameters and ways in which a transmission can be degraded.

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Important parameters for analogue circuits are the bandwidth, the frequency response ( = attenuation and group delay) and the signal-to-noise ratio at the receiver input. Analogue circuits are characterized by their “graceful degradation” under disturbed channel conditions, i.e. the quality of the received signal deteriorates gradually with increasing disturbance and noise. Important parameters for digital circuits are the bit rate, the data transfer delay and delay variation (timing jitter) and the error rate of the received data. The impairments of the communication medium, which may be quite severe, are hidden from the user. Error rates can be bounded to very low values by placing regenerative repeaters at periodic intervals (intermediate stations) along the physical medium. Digital circuits are characterized by their “threshold behavior” under disturbed channel conditions. Simply speaking, they are either very good or not available. The protection of power systems normally imposes very stringent demands on the communication system regarding its real-time properties. The signal transfer time and transfer time variation is for example much more critical for protection signal transmission than for general data or voice communication. Voice frequency circuits The term voice frequency (VF) circuit is used for analogue circuits that pass frequencies between approximately 300 Hz and 3400 Hz and block frequencies outside this range. Historically this frequency range has been defined for the transmission of speech signals. Today, analogue voice frequency circuits are provided by both analogue and digital telecommunication systems and may be used by voiceband modems for data transmission up to approximately 33 kbit/s. Voice frequency circuits may further be characterized according to the number of wires that are required: 2-wire circuits employ the same wire pair for transmitting and receiving, whilst with 4wire circuits one wire pair is used for transmitting and the other wire pair is used for receiving.

4.2 TELECOMMUNICATION NETWORKS The requirements from protection on communication have traditionally been met with simple point-to-point links. The introduction of high capacity digital networks is therefore hardly justified by its exclusive use for power system protection. The deployment of digital networks is primarily motivated by the need for enhanced power network control and increasing data traffic in distributed systems, and particularly by new telecom business opportunities in deregulated markets. Protection may however technically and economically benefit from modern communication networks if some inherent network problems and their impact on protection operating performance are carefully analyzed. Finally, properly designed networks are a prerequisite for the emerging wide-area protection systems that will require the exchange of information between many sites rather than isolated point-to-point links. Networks can enhance the availability of a protection system when the network inherent redundancy and route diversity is exploited. Measures have however to be taken to ensure that automatic re-routing is prevented from re-using the same bearer (e.g. the same fibre cable) when attempting to re-direct a channel which has failed, thus destroying the diversity concept. Pre-definition of a primary and an alternate path with ensured diversity and guaranteed signal transfer delay is suggested. Special attention has to be paid to networks where the protection information may pass through network nodes with switching, routing and loop-back facilities, or when the protection signal

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shares an aggregate with other services. Switching, routing and multiplexing techniques bear a certain risk that a transmitted signal is directed to a receiver which it was not originally intended for (channel cross-over or signal loop-back). The consequences of signal misdirections on protection are different for analog comparison and state comparison protection schemes. For example, signal misdirection is typically less critical for state comparison protection systems that are normally in the guard (non-operate) state, than for analog comparison protection systems, which depend on continuous exchange of information between line ends and may immediately trip the line if signals from the wrong transmitter are received.1 In addition to inadvertent signal misdirection there exist some other network-related risks that are new or have a different impact when compared to traditional ‘hard-wired’ point-to-point links, for example: -

Automatic re-route to some non-defined alternate path with inadequate performance for protection Automatic re-route to a pre-defined non-preemptible (dedicated) path Excessive outage time until re-route completed Different propagation time delays between the various paths selected Possibility of different go and return propagation time delays Protection circuits may be bumped at the expense of others when re-routing after a link failure, unless prevented by adequate circuit priority rating mechanisms Channel may not revert back to its original path unless manually optimized, eg on a least cost basis Unacceptable signal transfer delays due to queuing mechanisms in networks with dynamic bandwidth allocation Availability may be less than expected due to the particular definition of "Available Time" for telecom ISDN circuits according to ITU-T G.821

Power system protection performance may be unacceptably jeopardized unless appropriate measures are taken regarding the control and management of the network, and unless the protection system is designed to cope with typical network related risks. More on networks is found in Chapters 4.4, 4.5 and 4.6.

4.3 TRANSMISSION MEDIA 4.3.1 Pilot wires / Copper wires Pilot wires consist of a pair of metallic wires normally embedded in an aerial or underground cable. They have historically been widely used for transmitting protection signals. Although the 1

State comparison schemes and command-based protection systems: When an inadvertent channel mix-up or loop-back occurs in a command based protection system, normally only the guard signal (‘do not trip’) is misdirected or looped back, as the system is normally in the guard state. A residual risk for a missed tripping or unwanted tripping exists for the unlikely case when the channel-misdirection would coincide with protection operation. Analog comparison schemes: Signal misdirection is more critical for analog comparison schemes like current differential protection. A channel cross-over or signal loop-back would simulate a differential signal, which may immediately produce an unwanted tripping. Should a channel cross-over coincide with a line fault, an unwanted tripping for the wrong line may be produced. Terminal equipment addressing with address validation times of less than the protection relay’s operating time is therefore a prerequisite. Any measure for improving the security has however to be weighted against its adverse impact on dependability.

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tendency is to replace pilot wires by optical fibres which are free from electromagnetic interference, their use may still be justified for economical reasons. Pilot wire circuits may be utility owned or rented from telecommunication companies. Utility owned pilot wires often follow the same route as power cables. Since pilot wires may be subject to dangerous induced voltages during faults in the primary system, appropriate installation precautions must be taken in order to prevent maloperation, hazards to personnel and damage to equipment. The electrical parameters of pilot wires such as signal attenuation and signal delay per unit length depend on their mechanical parameters like wire diameter, insulation material and cable construction, as well as on the signal frequency. Values for the attenuation coefficient between 0.5 dB/km and 3 dB/km in the audio frequency range are typical. The signals, which are transmitted over pilot wires are historically sometimes DC signals or signals at power frequency (50 or 60 Hz) e.g. from pilot wire differential relays. Transmission of DC or AC signals at power frequency is hardly used any more due to the pronounced susceptibility to interference from the primary system. Normally the information is modulated onto a carrier which “shifts” the information from the power frequency range into the audio frequency range for transmission. At the receiving end, the information signal can be separated from the power frequency by means of filtering. This function is usually performed by means of a teleprotection equipment operating over a 2- or 4-wire circuit. 2-wire circuits use the same pair of wires for transmitting and receiving. Transmit and receive signals are normally separated by their respective frequencies. With 4-wire circuits, a pair of wires is allocated to the transmitter and a pair of wires is allocated to the receiver. The same frequency is normally used for transmitting and receiving. A typical application for pilot wires is the transmission of binary on/off protection commands using dedicated teleprotection equipment in conjunction with distance or directional comparison relays. The protection command is modulated onto an audio frequency carrier somewhere in the 0.3 kHz to 3.4 kHz range, which makes the transmission less susceptible to power frequency interference and high frequency noise. The teleprotection equipment may also multiplex several commands from different relays onto the same wires. Internet access and multimedia services had a tremendous impact on the development of new high speed transmission principles for copper wires. High-speed modems would allow the use of pilot wires for higher data rates, ranging from several tens of kbit/s for voiceband modems to up to 10 Mbit/s over short distances for wideband (xDSL) modems. However, due to their inherent high signal transfer delay (latency) and their sensitivity to channel disturbances, the use of high-speed modems is not recommended for the transmission of protection signals. Electromagnetic interference, power frequency harmonics and wideband noise produced by faults in the power system are likely to block the modem receiver just in that moment when the communication is truly needed.

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Summary Advantages and disadvantages of Pilot Wires as related to protection signal transmission are: Advantages

Disadvantages

•

High availability and reliability with MTBF in the order of 200’000 to 500’000 hours

•

Wide deployment

•

Low cost, especially when also used for other purposes

•

Little interference from power lines if separate routing is used

Table 4.3-1:

•

High sensitivity to induced voltages in the event of power line faults and lightning strokes

•

Problems with potential barriers at the station entrance

•

Crosstalk between circuits in the same cable deteriorates performance and reduces link lengths

•

Buried cables my be damaged or broken by civil works

•

Short to medium link lengths only, due to the attenuation, bandwidth, crosstalk and interference constraints

•

High cost of new cables, e.g. when civil works are required.

Advantages and disadvantages of pilot wires

4.3.2 Power Line Carrier (PLC) A PLC system uses the high voltage power line as a transmission medium. Both overhead lines and buried high voltage cables can be used. Lines with mixed overhead line sections and cable sections are also possible, but each case has to be carefully investigated. PLC systems have been extensively used for more than 60 years on HV and EHV lines for the transmission of voice, control data and protection signals. PLC links are entirely under the control of the power utility. They normally provide the shortest and most direct connection between line ends, power stations and substations and are in many cases justified by the transmission of protection signals, where PLC links have proven to perform very effectively. Continued operation has even been reported for power lines, which were broken down after an earthquake. Due to their reliability PLC links are often the preferred back-up medium for selected important channels of wideband communication systems. This is especially true for protection signals. The carrier frequency range which can be used by PLC systems is normally between 40 kHz and 500 kHz. It is sometimes subject to national regulations to prevent interference with other systems operating in the same frequency band. The carrier frequency range between 40 kHz and 500 kHz is subdivided into slots of 4 kHz bandwidth. A PLC link may typically use one to four such slots for transmitting and receiving, depending on the number of channels and on the technology used. Traditional Analogue PLC transmitters translate a voice frequency band of 4 kHz gross bandwidth into one of the 4 kHz carrier frequency slots using single sideband (SSB) modulation. The voice frequency baseband may contain speech, superimposed data and protection signals which share the channel by means of frequency division multiplexing (FDM, see 4.4.1.1). During

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the transmission of protection commands the speech and data signals may be switched off such that the maximum transmit power is available to the protection signal. This "boosting" of the protection signals compensates the additional signal attenuation, which is introduced by the fault on the protected power line when signalling over faulty lines. As voice and data are interrupted during protection signalling, boosting is not recommended for the transmission of persistent commands, as might be the case for reactor protection for example. Emerging Digital PLC systems translate serial digital data into one or several 4 kHz slots at carrier frequency range using bandwidth efficient digital modulation techniques, such as quadrature amplitude modulation (QAM) or multicarrier (MCM) modulation. The serial aggregate data may accommodate digitized speech and digital data by time-division-multiplexing (TDM, see 4.4.1.2). Protection command transmission is usually accomplished by means of a dedicated subsystem in order to achieve the required dependability and minimum signal transfer delays under faulty line conditions. For both analogue and digital PLC systems the signal at carrier frequency is amplified to typically 5 to 100 Watts output power (PEP, peak envelope power) and coupled to the power line via an impedance matching device and a high voltage coupling capacitor. For optimum transmission performance under faulty line conditions coupling onto two phases in “push-pull” mode is normally preferred. Line traps in series to the power line prevent the carrier signals from being shunted by the local busbar and prevent signal leakage to adjacent lines. Several PLC terminals may share a common coupling equipment. The propagation of the signal along a multi-conductor power line may be explained by the combined transmission of independent modes whose number is equal to the number of nonearthed conductors above ground. Each mode propagates with its specific attenuation and velocity. The signal attenuation depends on the construction of the power line, the line condition and on the carrier frequency used. It is typically in the range of 0.02 dB/km to 0.2 dB/km, increasing with frequency. The signal quality may be impaired by various noise sources. Corona noise results from electric impulse discharges along the surface of the phase conductors. Its spectrum extends well into the carrier frequency range. Corona noise is always present on an energised line and is perceived as background noise in a PLC receiver. Its level depends on the power system voltage and design, the climatic conditions and the altitude above sea level. It normally does not constitute a problem to protection signal transmission since its level is less than other channel impairments caused by line faults for example. Isolator operation creates high frequency noise of high amplitudes which cause poor signal-tonoise ratios in the PLC receiver. Its duration may last some seconds depending on the isolator design. The signal quality degradation depends largely on the method of coupling and on the characteristics of the equipment. The interference produced by isolator noise is most severe in comparison with other noise sources. Because it occurs under healthy line conditions it may cause unwanted operation of the protection system. Operation of breakers produces disturbance similar to isolator operation. Its duration is however limited to the operating period of the circuit breaker which is typically less than 20 ms. During line faults the PLC channel is subject to strong transient noise at the onset of the line fault until the arc has established, followed by an immediate increase in signal attenuation due

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to the short circuit of the faulty phase(s). During the interruption of the fault current, noise is produced again by the operation of the circuit breakers. Interference produced by power system faults occurs during the time when the protection is in active operation. It may therefore prevent operation of the protection system. Channel impairments during line faults are the primary reason why PLC links have so far been restricted for two applications for protection signal transmission, where they have however proven to perform most effectively: - Transmission of binary status information in conjunction with distance protection or directional comparison relays in state comparison protection schemes - Transmission of phase comparison signals in conjunction with phase comparison relays in analog comparison protection schemes. Summary Advantages and disadvantages of PLC links as related to protection signal transmission are: Advantages

Disadvantages

•

The overhead power line constitutes a very reliable transmission medium

•

The power line is normally the shortest and "fastest" link between line ends, power stations and substations

•

PLC teleprotection links are normally “hard-wired” point-to-point links with little risk of unwanted re-routing, switching or tampering

•

Channel is subject to increased disturbance during faults in the primary system

•

Application for protection signal transmission is limited to the transmission of binary commands and non-segregated phase comparison signals

•

Not applicable for current differential protection

•

The equipment is situated at the power station, giving easy access for control and maintenance

•

The narrow bandwidth (few kHz) constrains the number of signals that can be transferred and the signal transfer time

•

The medium (power line) and terminal equipment are under the full control of the utility

•

Limited frequency band available, limiting the number of PLC links that can work in a given network (frequency congestion)

•

Very long distances of many hundred kilometers may be covered without intermediate repeaters.

•

No earth potential rise problems since the transmitter and receiver as well as coupling equipment are normally situated within the station earth network

Table 4.3-2:

Advantages and disadvantages of power line carrier links

4.3.3 Microwave Radio Microwave radio links have been extensively used by many electric power utilities mainly to satisfy the increasing demand for more communication capacity. Until the introduction of fibreoptic links they represented the only true wideband medium which could accommodate a large number of voice channels. Another reason for selecting microwave radio links is their relative immunity against electromagnetic interference produced by the high-voltage power network.

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From a legal point of view terrestrial microwave radio systems may be broadly categorized into licensed radio and unlicensed radio systems: Licensed radio systems operate in a "protected" frequency range that has been licensed to the utility by national authorities. Licensed radio systems typically constitute point-to-point multichannel links that are used in the backbone of the communication network. Unlicensed radio systems operate in an "unprotected" frequency range that is open to the public. Unlicensed radio systems usually share a common frequency band and support point-tomultipoint communication. Access to the shared medium (i.e. the common frequency band) is accomplished either through TDMA (Time Division Multiplex Access) or CDMA (Code Division Multiple Access), or a combination of the two, to prevent mutual interference between users. See also chapters 4.4.1.2 and 4.4.1.3.1 4.3.3.1 Multichannel radio Although the microwave equipment may be owned and operated by the power utility, the frequency bands for its use have to be licensed from national authorities. The frequency bands for microwave radio systems are typically between 400 MHz and 40 GHz. In legacy analog microwave systems a number of voice baseband channels with 4 kHz bandwidth each are combined onto a single aggregate signal by frequency division multiplexing (FDM, see 4.4.1.1). One or several of the 4 kHz baseband channels may be used individually or collectively for the transmission of protection signals. Earlier analogue microwave systems used frequency modulation (FM) where the analogue aggregate FDM signal varies the frequency of the emitted carrier. Analogue microwave systems are mainly of historical interest since they have been gradually replaced by digital systems. In digital microwave systems a number of digital data channels of typically 64 kbit/s each are combined onto an aggregate data stream using time division multiplexing (TDM, see 4.4.1.2). Analogue signals such as speech are converted into digital data prior to multiplexing. One or several of the digital 64 kbit/s channels may be used either individually or collectively for the transmission of protection signals. In digital microwave systems frequency modulation has been replaced by phase shift keying (PSK) modulation or combined phase-amplitude shift keying, which is also called quadrature amplitude modulation (QAM), with 16-QAM being widely used today. Higher level QAM like 64QAM or 128-QAM provide a higher bandwidth efficiency, i.e. they allow to transmit more bits per second in a given bandwidth, however at the expense of an increasing susceptibility against interference and noise. Licensed microwave radio links are normally point-to-point with maximum distances between 40 and 100 km. The distances that can be covered depend on the transmitter output power, on the frequency band used, on atmospheric conditions, on the topography and on obstacles, which may impede signal propagation or cause signal reflections. Signal reflections may lead to multipath propagation which causes a certain additional attenuation or signal extinction when the direct wave and the reflected wave are opposite in phase at the receiver. Waves reflected by the ionized part of the atmosphere or by a changing refractive index due to temperature or humidity variations have the same effect, but since the degree of reflection is subject to a random process, the received signal varies statistically with time. This phenomenon is called “fading” as the received signal can fade or disappear on a statistical basis. 1

TDMA and CDMA are general media access technologies that are for example typically used in point-to-multipoint radio systems where many users share a common frequency band.

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These drawbacks can be overcome by careful link planning, by the positioning of intermediate repeaters, by the selection of the transmitted power and by the antenna design. Diversity is commonly used to improve the availability of microwave links by adding some degree of redundancy. Space diversity is obtained if different antennas are located in different positions on the antenna tower. Frequency diversity is used when the same signal is transmitted using different frequencies. At the receiving side the signals coming from different antennas are combined to achieve the best possible signal at the output, permitting to limit the outage time of the link during the worst period of the year. 4.3.3.2 Single channel radio Unlicensed radio systems normally constitute single-channel point-to-multipoint short-haul links that can be set up on the fly at moderate cost. Single-channel point-to-multipoint microwave radio links have been used at MV level with less stringent demand on signal transfer times. Access to the common medium - i.e. the shared radio frequency band - is accomplished by means of TDMA (Time Division Multiple Access) or CDMA (Code Division Multiple Access) to prevent mutual interference between transmitters operating simultaneously, see also 4.4.1.3. Successful operation of such systems has been reported from South Africa. Both intertripping as well as differential protection signals are transferred over point-to-point TDMA-based singlechannel radio links between outstations, with typical signal propagation delays (outstation to outstation) in the range of 14ms to 22ms1 at data rates of 19.2kbit/s and 64kbit/s. The use of unlicensed spread spectrum radio for the transmission of state indication in a state comparison scheme for a 138 kV line has been reported from the USA. Summary Advantages and disadvantages of microwave radio links as related to protection signal transmission are: Advantages

1

Disadvantages

•

Wideband medium, with scalable capacity (number of channels)

•

Little interference from the primary system

•

No earth potential rise problems when the transmitter and receiver are situated within the station earth network

•

Fast setting up, especially when towers are existing or when roof top installations are possible, or when unlicensed radio systems can be used

•

Frequency bands constitute a limited resource and may not be available as desired

•

Influence of atmospheric conditions such as rain, fog, snowfall, sandstorms. Unless a high signal margin is provided, the link may be temporarily lost due to fading

•

Correlation / coincidence between poor weather conditions, line faults and poor link performance exists

•

Problem of getting line-of-sight both for single-hop and multi-hop links

•

Multiple hops introduce extra cost, reduce reliability and cause additional signal transfer delays

Caution: It is most important to mention that the “upstream” and “downstream” time delays between masterstation and outstation are different with certain types of TDMA equipment – this can lead to difficulties with differential protection systems: An additional “dummy” outstation may have to be installed at the master-station site for the sole purpose of equalizing the go and return time delays!

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Table 4.3-3:

•

Microwave antenna towers are subject to lightning strokes

•

Potential barrier problems when the transmitter and receiver are located outside the station earth network

•

Many channels are lost when a high capacity microwave link fails

•

Unequal upstream and downstream signal propagation delays of certain TDMA or CDMA radio systems may cause serious problems for differential current protection relays

Advantages and disadvantages of radio links

4.3.4 Optical fibres The deployment of optical fibres for signal transmission started in the seventies with a few shorthaul links and has made tremendous progress since then regarding fibre and terminal equipment technology. The unique advantage of optical fibres is their immunity to electromagnetic interference, their isolating quality and their extremely wide bandwidth, all making the introduction of optical fibre links very attractive for electric power utilities. Optical fibres are normally used in pairs, i.e. one fibre is used for transmitting and one for receiving. Communication over one fibre in both directions is technically possible, for example using time-shared multiplexing or wavelength division multiplexing (WDM, see chapter 4.4.1.1) techniques. It has however been rarely used for long distance telecommunication systems so far. A number of optical fibres (10 … 50 … 100) are normally embedded in an underground or aerial cable. The immunity against electromagnetic disturbance allows installing fibre-optic cables along the same route as power cables. They may also be integrated into power cables or ground wires of HV power lines. The latter design which is called OPGW (OPtical Ground Wire) is preferably used by electric power utilities. Other popular techniques are the mounting of ADSS (All Dielectric Self-Supporting) Cables along the towers, or the Helical Wrapping of a fiberoptic cable around the ground wire or phase wire, which may be advantageous for refurbishing existing lines. Lashed aerial cable techniques are also used whereby an alldielectric cable is lashed to a messenger (e.g. earth wire) by means of a tape or cords. In all cases the mechanical strength of the towers has to be examined regarding the additional load introduced by the optical cables, especially when extra loads due to snow and ice are to be expected. Care must be taken with ADSS and Helical Wrap cables to avoid surface erosion caused by dry-band arcing in high field strength locations. For long distance links, Multimode Step-Index fibres and multimode Graded Index fibres are of historical interest only. They have been almost completely superseded by Single Mode fibres which provide a very large bandwidth over a long distance. The transmission properties of optical fibres are characterised by their attenuation per unit length (dB/km) and by their chromatic dispersion (ps/nm∗km). Chromatic dispersion means that

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lightwaves of different wavelengths (= "colours") propagate with different velocities. An injected light impulse, which is for technical and physical reasons composed of several wavelengths, thus tends to “broaden” when it propagates. The impulse broadening limits the useful bandwidth of the link because the individual impulses can no longer be discriminated by the receiver when they overlap significantly. Using laser emitters with a narrow emission spectrum is therefore mandatory for long-haul high bitrate links. At 1300 nm optical fibres naturally exhibit minimum dispersion which introduces minimum pulse distortion at high data rates. At 1550 nm the attenuation is lowest, however the dispersion is higher than at 1300 nm. Special fibre designs called Dispersion Shifted Fibres minimize the chromatic dispersion at 1550 nm, however at the expense of a higher attenuation due to mechanical stress combined with certain penalties when used in WDM systems [12]. The maximum length of an optical fibre link may therefore be either attenuation limited or dispersion (bandwidth) limited which is an important system planning issue for high capacity long-haul links. Very long distances can be overcome by means of optical boosters and amplifiers which inject more light power into the fibre at the transmit side and amplify the received signal on an optical basis at the receiving end. Laser Diodes (LD) or Light Emitting Diodes (LED) may be used as optical transmitters. Laser diodes are required for long repeater spans (up to about 100 to 200 kilometres, depending on the bit rate) and high bitrates (up to some Gigabits per second), whereas LEDs are cost efficient for shorter distances and lower data rates. The optical power injected by a LD into a single mode fibre is in the order of 1 Milliwatt, that of an LED is around 10 to 20 Microwatts. The emitted wavelength of both LDs and LEDs is in the infrared range at either around 850nm, 1300 nm or 1550 nm. Special optical transmit- and receive devices such as Optical Boosters and Erbium Doped Fibre Amplifiers (EDFA) may be used for bridging extra long distances of several hundred kilometers without intermediate repeater stations. Wavelength Division Multiplexing (WDM) may be used to further exploit the huge transmission capacity of optical fibres, or simply to use the same fibre for different communication systems by “stacking” their optical transmitters onto the same fibre, each transmitter using a different wavelength. More on the subject is found in chapter 4.4.1.1 and in [12]. As applied to protection signal transmission, either dedicated optical fibres from relay to relay may be used, or the protection signal may be electrically or optically multiplexed with other services, as shown in ANNEX A1, Figures A1-2 and A1-3. Whilst the installation of dedicated optical fibre cables for the transmission of protection information would match the traditional point-to-point approach and guarantee minimum signal transfer delays, it might not be easily justified for cost reasons. However, the use of dedicated fibres is facilitated when the incremental cost of extra fibres in a cable are low, or when “spare” fibres can be used. A more economical means to achieve a certain isolation of the protection from other services and/or systems is to perform the multiplexing at the optical level using WDM (Wavelength Division Multiplexing), where only the optical fibre but not the terminal equipment is shared between individual systems. More on the subject of WDM is found in chapter 4.4.1.1 and in [12]. Fibre optic communication systems are - with very few exceptions - realised as digital systems. Since the optical fibre represents a wideband medium, a large number of channels and services are usually combined into an aggregate by some form of time-division-multiplexing (TDM, see Chapter 4.4.1.2). The aggregate digital bitstream finally modulates the optical transmitter (Laser diode or LED) by switching it on and off in accordance with the data to be transmitted.

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Summary Advantages and disadvantages of optical fibre links as related to protection signal transmission are: Advantages

Disadvantages

•

Wideband medium, supports extremely high data rates

•

Many channels are lost when a high capacity fiberoptic link fails

•

Immune against electromagnetic interference from the primary system (at the optical level)

•

Repair is difficult when fibres are integrated into high voltage cables or OPGWs

•

Immune against athmospheric interference

•

•

Perfect electrical isolation between link ends and between high-voltage euipment and telecom equipment

High installation cost when only moderate data rates are needed.

•

•

No crosstalk between fibres

Dedicated fibres for protection signal transmission may not be justified for cost reasons

•

Normally extraordinarily low bit error rate

•

•

No earth potential rise problems

For long distances (> 200km) repeater stations have to be used

•

Little influence by atmospheric conditions

•

Fairly long repeaterless distances possible (…..200km)

Table 4.3-4:

Advantages and disadvantages of optical fibre links

4.3.5 Satellites The race for satellite communication has been on ever since the announcement of pocket-sized ground terminals to provide a truly global mobile telephone service. At present, there are many different satellite systems that have been proposed to complement terrestrial communication networks, all at varying developmental stages. Narrowband satellite systems which carry many voice or low speed data channels - up to 9'600 bits per second - are more advanced in terms of development than wideband systems supporting SDH and ATM (see Chapters 4.4.2.2, 4.5.1 and 4.6.2.2 on SDH and ATM). The reason is mainly due to new or more acute issues related to creating broadband satellite links with QoS (Quality of Service) guarantees (ANNEX A3). Projects have been launched worldwide to investigate the integration of terrestrial wideband networks with satellite networks. Satellites are usually classified according to the type of orbit they are in. 4.3.5.1 GEO - Geosynchronous Earth Orbit Satellites GEO satellites are placed in the orbit such that their period of rotation exactly matches the Earth’s rotation, i.e. they appear stationary from earth. Earth station antennas do therefore not need to move once they have been properly aimed at a target satellite in the sky. Today, the majority of satellites in orbit around the earth are positioned in GEO at 36’000 km orbital height. It is at the precise distance of 36’000 km that a satellite can maintain an orbit with a period of rotation exactly equal to 24 hours. Due to the long distance of 36’000 km GEO satellites experience long up-down signal propagation delays of about 250 ... 280 ms which normally excludes them from being used as a communication medium for protection signal transmission, with perhaps few exceptions for wide-area protection applications.

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4.3.5.2 MEO - Medium Earth Orbit Satellites Technological innovations in space communications have led to new satellite system designs over the past few years. MEO satellite systems have been proposed that will orbit at distances of about 10’350 km. The lower distance as compared to GEO systems means improved signal strength at the receiving antenna, which allows for smaller receiving terminals. The lower distance also translates into less signal transmission delay of about 120 ms which leads to a significant performance improvement for certain real-time applications such as voice communication. As applied to protection signal transmission the delay appears however still unacceptable for most applications, with perhaps the exception of wide-area protection where requirements on signal transfer times to remote locations that are distributed over a geographically widespread area may be less stringent. Another problem is that signal interruptions of approximately 25 milliseconds duration are expected about every 2 hours when the signal is switched from one satellite to the next: As the satellite descends towards the horizon, the traffic being serviced by that satellite must be handed over to the satellite just ascending from the opposing horizon. 4.3.5.3 LEO - Low Earth Orbit Satellites Proposed LEO satellite systems are divided into three categories: Little LEOs operating in the 800 MHz range, big LEOs operating in the 2 GHz or above range, and mega LEOs operating in the 20 - 30 GHz range. The higher frequencies associated with mega LEOs translate into more communication capacity and better performance for real-time applications. Present systems support moderate data rates of up to 9’600 bit/s yet, with much higher data rates being targeted for the near future. The orbital distance of LEO satellite systems is between 750 and 1500 km, giving rise to signal up-down propagation delays of about 20 to 30 ms. As applied to protection signal transmission the delay introduced by a single LEO satellite updown link may be acceptable for certain protection applications, provided that the extra delay possibly introduced by relaying the signal between satellites plus the delay introduced by the terrestrial section can be kept sufficiently low. It is noted that signal interruptions of 3 to 9 milliseconds duration are expected about every 8 to 12 minutes when the signal is switched from one satellite to the next (roaming): as the satellite descends towards the horizon, the traffic being serviced by that satellite must be handed over to the satellite just ascending from the opposing horizon. Moreover, the signal propagation delay variation as the signals are routed dynamically from satellite to satellite before reaching the terrestrial destination will require further detailed investigation, before MEO and LEO satellite channels may eventually be used for conveying protection signals. LEO satellite systems may eventually become a communication alternative for certain protection applications when signal transfer delay and reliability requirements are not very demanding. Little experience seems to exist today in this area. There are still many open research issues that need to be addressed before such systems can be used.

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Summary Advantages and disadvantages of satellite links as related to protection signal transmission are: Advantages

Disadvantages

•

Coverage of geographically widely spread areas

•

Easy and fast deployment of ground terminal stations

•

Electrical isolation between terminals

Table 4.3-5:

•

High signal propagation delay

•

Availability and reliability may not be adequate for protection

•

Subject to adverse atmospheric influence, including lightning strokes and snow and ice covering satellite dishes

•

prohibitive costs for permanent connections and/or high bandwidth

Advantages and disadvantages of satellite links

4.4 MULTIPLEXING TECHNIQUES AND DIGITAL HIERARCHIES 4.4.1 Multiplexing Techniques Because of the installation cost of telecommunication systems, such as microwave radio or optical fibre links, it is desirable to share the communication medium among multiple users or multiple services. Multiplexing is the sharing of a communications medium through local combining of signals at a common point. Multiplexing is thus a technique that is used to transmit two or more signals over a shared medium. The reverse action of extracting the individual signals from the aggregate at the receiving end is called demultiplexing. Three basic types of multiplexing are commonly employed: frequency-division multiplexing (FDM), time-division multiplexing (TDM) and code-division multiplexing (CDM).1 As there is a certain - although low - risk of accidental channel cross-over in multiplexed systems, it is recommended that precautions are taken at the teleprotection side to prevent unwanted operation of the protection. Robust synchronization procedures and/or terminal equipment addressing2 may be used. The benefits of measures for improving the security have however to be carefully balanced against their adverse influence on dependability. 4.4.1.1 Frequency Division Multiplexing (FDM) With FDM, multiple channels or multiple services are combined onto a single aggregate by frequency translating, or modulating, each of the individual signals onto a different carrier frequency for transmission. The individual channels are thus separated in the aggregate by their frequencies, i.e. each channel has its dedicated frequency slot. At the receiving end, the reverse action of extracting the individual signals is accomplished by filtering. While each user's 1

Note on 'Multiplexing' and 'Multiple Access': Both techniques deal with the sharing of a communication channel or a transmission medium among communication users. The term 'multiplexing' is relevant for the sharing of a communication channel or medium through the local combining of signals at a common point (signal aggregation or signal concentration). The three main multiplexing techniques are FDM, TDM, CDM. 'Multiple access' deals with the sharing of a common medium among terminal stations that are located at physically different locations by mastering the medium access procedures. Similar to multiplexing, the three multiple access technologies are FDMA, TDMA and CDMA respectively, which are widely used in radio communications.

2

Terminal addressing will also protect against protection maloperation when signals are (inadvertently) looped back for testing or maintenance reasons.

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information signal may be either analog or digital, the combined FDM signal is inherently an analog waveform. FDM is therefore primarily used with analogue transmission systems. Wavelength Division Multiplexing (WDM) With optical fibre systems, a special form of FDM called WDM (Wavelength Division Multiplexing) is increasingly being introduced to further exploit the huge capacity of optical fibres. Several transmission systems, each using a different wavelength or 'colour', may be stacked onto the same fibre using WDM. In its simplest form, WDM uses different optical windows for the multiplexing, e.g. the windows centred around 1300 nm and 1550 nm wavelength. More sophisticated systems multiplex a number of optical channels (e.g. 4, 16, 32 or 64) within the same optical window centred around 1550 nm wavelength. As the spacing between the different wavelengths becomes very narrow in this case, the technology is called Dense Wavelength Division Multiplexing (DWDM). An in-depth treatment of WDM technology is found in [12]. As WDM actually creates 'virtual fibres' it may also be employed for the de-coupling of transmission systems from each other. Dedicated teleprotection links that operate quasi-isolated from other telecom services could be realized using WDM for example: In Figure 4.4-1, system 1 consists of a protection relay with internal or external teleprotection function plus a fibre-optic transmitter/receiver operating at wavelength λ1. System 2 could be any other fibre-optic communication system operating at wavelength λ2 and carrying other services such as data and voice. A failure or maloperation of System 2 should not adversely affect System 1, as the only common parts of the two systems are the optical fibre and the passive optical wave-division multiplexer / demultiplexer. Although the isolation of the teleprotection from other services by means of WDM appears attractive from an operational point of view, it may not be easily justified for cost reasons.

System 1

λ1

λ1 System 1

λ1+λ2

System 2

Figure 4.4-1:

λ2

Optical fibre

λ2

WDM

WDM

System 2

Principle of Wavelength Division Multiplexing for 2 wavelengths, 1

4.4.1.2 Time Division Multiplexing (TDM) Multiplexing may also be conducted through the interleaving of time segments from different signals onto a single shared transmission path. With TDM, multiple channels thus share the common aggregate based on time. While TDM may be applied to either analog or digital signals, in practice it is applied almost always to digital signals. The digital signals may be interleaved bit-by-bit (bit interleaving), byte-by-byte (byte interleaving) or cell-based where data is broken up into “cells” consisting of a number of bytes. 1

Figure 4.4-1 shows a simplex (i.e. unidirectional) communication for simplicity reasons. Full-duplex (i.e. bi-directional) operation would require either a second fibre, or a 3rd and a 4th wavelength (λ3 and λ4 respectively) on the same fibre.

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Most modern telecommunication systems employ some form of TDM for transmission over longdistance routes. The multiplexed signal may be sent 'directly' (called 'baseband' transmission) over optical fibres, or it may be modulated onto a carrier signal for transmission over analogue media, such as microwave radio or coaxial cables for example. TDM can be split into various subclasses. The most important are Fixed TDM and Statistical TDM. Fixed TDM In fixed TDM - sometimes also called synchronous TDM - each channel has its assigned timeslot which sustains a fixed data rate and uses aggregate bandwidth irrespective of actual user data being transmitted or not. The number of channels is normally equal to the number of timeslots in a frame. Due to the fixed allocation of channels and timeslots, data can always be transmitted. Buffering and flow control are not required. Continuous data flow at a fixed bit rate without delay variations is ensured, a condition which is a prerequisite for protection signal transmission. Statistical TDM Statistical - sometimes also called asynchronous TDM - multiplexers rely on the ‘bursty’ traffic characteristics of certain information sources. Data may be transmitted in any timeslot as long as there are free slots available. Relying on the statistics of the data, the number of channels or the peak data rate which is supported by the statistical multiplexer may be larger than the total number of timeslots or the aggregate data rate in a frame. Data buffering and flow control is employed to store and withhold data until a free timeslot or free cells become available. Buffering and flow control introduce extra delay as well as delay variations, and data may be discarded in case of overload. Loss of information is normally not acceptable for protection signal transmission. Statistical multiplexing has therefore to be avoided unless the required quality of service is explicitly guaranteed. A multiplexing technology which was originally intimately bound up with the emerging SDH (Synchronous Digital Hierarchy, see 4.4.2.2) standards is ATM (Asynchronous Transfer Mode, see also 4.5.2.3 and 4.6.2.2) which was conceived as a way in which arbitrary-bandwidth communication channels could be provided within a multiplexing hierarchy consisting of a defined set of fixed bandwidth channels. ATM multiplexers support both constant bit rate (CBR) and variable bit rate (VBR) traffic, where CBR which basically emulates fixed TDM is a prerequisite for today’s protection systems using telecommunication. 4.4.1.3 Code Division Multiplexing (CDM) In CDM, several signals share a common medium (copper wires or radiowaves) using the same frequency band simultaneously. Multiplexing of different channels is achieved by utilizing different pseudorandom binary sequence codes that modulate a carrier. The process of modulating the signal by the code sequence causes the power of the transmitted signal to be spread over a larger bandwidth. Systems based on CDM are therefore sometimes also referred to as 'Spread Spectrum' (SS) systems. The spreading of the spectrum enhances the noise immunity of such systems. CDM and in particular CDMA (code division multiple access) is mainly used with unlicensed spread spectrum radio where many simultaneous users have to share the same frequency band. CDM/SS techniques may also be used with wire-based systems to enhance the transmission capacity and noise immunity. Its application for inter-substation communication would however need to be further examined with respect to cost efficiency and transmission performance.

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The use of unlicensed single channel CDMA/SS radio for the transmission of protection commands for a 138 kV line has been reported from the USA. However, no practical installations of CDM/SS technology for the transmission of protection signals over copper wires have been reported. A final conclusion on CDM is not possible at the time of writing.

4.4.2 Digital Hierarchies Digital transport systems form the backbone of modern telecommunication networks or widearea networks (WAN). As the demand for information transmission increased and levels of traffic grew higher it became evident that larger number of channels need to be bundled in order to avoid having to use excessively large number of individual physical links. Thus, it was necessary to define further levels of multiplexing which are structured in Digital Hierarchies. 4.4.2.1 PDH - Plesiochronous Digital Hierarchy Multiplexing structure Digital telecommunication systems have historically been based on the plesiochronous digital hierarchy (PDH). PDH systems accommodate "almost synchronous" channels in multiples of 64 kbit/s. The base rate of 64 kbit/s represents the digital equivalent of an analogue telephone channel using traditional, uncompressed PCM speech coding techniques. The PDH hierarchy levels and transmission rates are given in Table 4.4-1 below. Hierarchical level

Europe

North America

Japan

0

64 kbit/s

64 kbit/s1

64 kbit/s1

1

2’048 kbit/s

1’544 kbit/s

1’544 kbit/s

2

8’448 kbit/s

6’312 kbit/s

6’312 kbit/s

3

34’368 kbit/s

44’736 kbit/s

32’064 kbit/s

4

139’264 kbit/s

139’264 kbit/s

97’728 kbit/s

Table 4.4-1:

PDH - Plesiochronous Digital Hierarchy levels

When multiplexing a number of digital signals with the same nominal bitrate they are likely to have been created by different pieces of equipment each generating a slightly different bitrate due to their independent internal clocks. A technique called “bit stuffing” is used for bringing the individual signals up to the same rate prior to multiplexing. Dummy bits or justification bits are inserted at the transmit side and discarded by the demultiplexer at the receiving end, leaving the original signal. The same problem with rate alignment occurs at every level of the multiplexing hierarchy. The process of multiplexing “almost synchronous” signals is called “plesiochronous”, from Greek. The use of plesiochronous operation throughout the hierarchy has led to the adoption of the term “Plesiochronous Digital Hierarchy”. Plesiochronous operation does not allow extracting and inserting individual channels from the aggregate without prior demultiplexing and subsequent re-multiplexing, leaving towers of multiplexers. With the exception of vendor specific solutions, network management and performance monitoring throughout the hierarchy is not adequately supported with PDH systems either, as PDH systems have developed over time with insufficient provision for standardized management. These disadvantages have - amongst others - finally led to the definition of a new digital transmission hierarchy: the Synchronous Digital Hierarchy. 1

Some (legacy) systems may provide only 56 kbit/s to the user.

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4.4.2.2 SDH - Synchronous Digital Hierarchy Multiplexing structure The rapid growth of digital networks and the convergence of telephone and high-speed data networks have enforced the development of new standards, which would facilitate the deployment of complex networks with new services and comprehensive network management options. Proposals in ITU-T for a Broadband Integrated Services Digital Network (B-ISDN) opened the door for a new synchronous multiplexing standard that would better support switched broadband services. The new standard appeared first as SONET (Synchronous Optical Network) in the United States. Initially, the objective of the SONET standard was to establish a North American standard that would permit interworking of equipment from multiple vendors (1985 …1987). Subsequently, the ITU-T (former CCITT) was approached with the goal of migrating this proposal to a worldwide standard. Despite the considerable difficulties arising from the historical differences between the North American and European digital hierarchies, this goal was achieved with the adoption of the SDH (Synchronous Digital Hierarchy) standards (1988). In synchronous networks, all multiplexing functions operate synchronously using clocks derived from a common source. SDH embraces most of SONET and is an international standard, but is often mistakenly regarded an European standard, because most of its suppliers carry only the European PDH bit rates specified by ETSI (European Telecommunication Standards Institute). While there are commonalities between SDH and SONET, particularly at the higher rates, there are significant differences at the lower multiplexing levels, in order to accommodate the requirement of interworking the differing regional digital hierarchies. Through an appropriate choice of options, a subset of SDH is compatible with a subset of SONET; therefore, traffic interworking is possible. Interworking for alarms and performance management is however generally not possible between SDH and SONET systems. The ITU-T recommendations define a number of basic transmission rates within the SDH and SONET, see table below, with further levels proposed for study. SDH Synchronous Transport Module level

STM-1 STM-4

SONET Aggregate Rate

Optical Carrier level

Synchronous Transport Signal level

Aggregate Rate

Max. number of simultaneous voice channels (informative)

OC-1

STS-1

51.840 Mbit/s

783

155.520 Mbit/s

OC-3

STS-3

155.520 Mbit/s

2'349

622.080 Mbit/s

OC-12

STS-12

622.080 Mbit/s

9'396

STM-16

2’488.320 Mbit/s

OC-48

STS-48

2’488.320 Mbit/s

37'584

STM-64

9’953.280 Mbit/s

OC-192

STS-192

9’953.280 Mbit/s

150'336

Table 4.4-2:

SDH - Synchronous Digital Hierarchy levels

The recommendations also define a multiplexing structure whereby an STM-N (Synchronous Transport Module level N) or STS-N (Synchronous Transport Signal level N) aggregate can carry a number of lower bitrate signals as payload, in order to facilitate the transport of legacy PDH tributaries. SDH / SONET are expected to dominate transmission for decades to come, as the multiplexing structure has been designed to carry not only current services but also emerging ones using ATM and/or IP framing structures for example.

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Although SONET and SDH were conceived originally for optical fibre transmission, SDH radio systems exist at rates compatible with both SONET and SDH. SDH/SONET network topologies and network resilience A synchronous network will be more reliable than PDH due to both the increased reliability of individual elements, and the more resilient structure of the whole network. SDH will allow development of network topologies which will be able to achieve 'network protection', that is to survive failures in the network by reconfiguring and maintaining service by alternate means. Network protection can be accomplished by the use of cross-connect functionality to achieve restoration, or through the use of self-healing ring architectures. Two main types of synchronous ring architectures have been defined: - The Dedicated Protection Ring - This is a dedicated path switched ring which sends traffic both ways around the ring, and uses a protection switch mechanism to select the alternate signal at the receive end upon failure detection. - The Shared Protection Ring - This is a shared switched ring which is able to provide 'shared' protection capacity which is reserved all the way around the ring. In the event of a failure, protection switches operate on both sides of the failure to route traffic through the reserved spare capacity. The ability to share protection capacity in shared protection rings can in many instances offer a significant capacity advantage over dedicated protection rings. This means, in economic terms, less equipment, lower cost and less operation efforts. However, this is at the expense of a slower restoration time than a path switched ring. Protection switching in a ring topology can be either "uni-directional" or "bi-directional". Unidirectional means that only the faulted path is reverted along the ring by selecting the healthy fibre at the receiving end, whilst the non-faulted path follows the original route. With bi-directional protection both the go and return path are switched to follow the opposite direction along the ring. It is noted that only bi-directional protection will maintain equal signal propagation delays for the go and return path, whilst uni-directional protection may introduce unequal propagation delays that may cause severe difficulties for current differential protection relays! The synchronous ring structure, with its inherent resilience, is a powerful building block from which survivable networks can be built: A typical power system control network has a radial (star) topology, with point-to-point links connecting a central control station with associated substations. SDH/SONET network implementations may connect the substations in rings. The logical star connection is achieved by configuring the channels within the SDH/SONET network in order to provide the required logical point-to-point links. In case of a path interruption, signal flow may be reversed along the ring such that communication is sustained. More about SDH network design and -operation is found in chapter 4.6.2.1 of this document. SDH/SONET for power system protection Since SDH/SONET networks provide a set of fixed bandwidth channels with a deterministic transmission characteristic, they are well suited for applications that rely on the transmission of a sustained fixed data rate and short signal transfer times, as needed by differential current protection for example. As SDH/SONET signals follow a fixed physical path through the network, SDH/SONET channels will exhibit a fixed transmission delay with low delay variations

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or "jitter" unless paths are re-routed automatically or manually due to network failures. Transmit and receive directions may however still experience different signal propagation times when the physical route does not follow the same path. Provisions to accommodate the non-equal signal transfer times have to be built into the protection relay in this case, see also Chapter 6.3.1.1. In conclusion, transport networks based on SDH/SONET technology can be designed to meet the stringent requirements of legacy and future protection systems regarding signal transfer times and error characteristics. Propagation velocity of the light pulses in optical fibres is around 200 km/ms, signal transfer delays between ports of an SDH/SONET node are typically well below 1ms, and networks are designed to produce very low error rates (<< 10-6) under normal operating conditions. Issues that are more critical to the operation of the protection scheme are related to network management and network security, e.g., the impact of path re-routing on transmission time variation and on circuit availability. These are however primarily matters of network planning and network operation. More about the subject on wideband transport technologies and networks can be found in [13] and [2].

4.5 NETWORK TECHNOLOGIES A Telecommunication Network is a set of communication and switching devices that work in a collaborating way to provide a telecommunication service on access points distributed over a wide area. Depending on the geographical extension of the network, it can be classified into three groups: - Local-Area Network (LAN). The users are geographically close together in the same building or area. - Metropolitan-Area Network (MAN). The users are located in a campus or a city that do not cover a wide extension. - Wide-Area Network (WAN). The users are located far apart over a region or one or several countries. Network components are connected by means of communication equipment. The largest WAN in operation today is the Internet which give service to hundreds of million users all over the world. WAN is the more complex type. LAN and MAN are simplifications of the general principles included in WAN. Therefore, the following introduction to the general concepts of the networks technologies will be focused on WAN. Although the main network components are communication equipment and switching nodes, which are the hardware devices of the network, software components that perform the algorithms are the elements that define the final service performance. Networking algorithms work in a distributed way in order to achieve a global service performance. Nevertheless, the actual implementation of these algorithms is local so that every switch performs its own task in a way that is collaborating with the task performed by the other network components. In order to achieve the best cost performance ratio, and so get a cost-effective approach, networks are designed following a non-regular architecture. Although there are no general rules that can apply to the architectural design, as it depends on the geographical distribution of the users and on the type of services offered by the network, a network can be formed by two main parts or layers: the transport network and the service network.

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S E R V IC E ACCESS P O IN T S

S E R V IC E LAY ER

TRA NS PO RT LAY ER

Figure 4.5-1:

Network Architecture

The transport layer takes care of the communication between geographically separated sites providing the transport service for the telecommunication service network and so connecting the different parts of the whole service network. Depending on the geographical distribution, the transport layer could form a mesh of channels connecting different sites, or be reduced to the actual links that connect the switching nodes of the service network. The service layer performs all the necessary functions to offer the final service to the user. It is formed by switching nodes connected by means of dedicated links or throug channels of the transport network. Service networks can be classified according to the technology on which they are implemented. They can also be classified by the service they provide; nevertheless, this classification has became obsolete due to fact that modern networks are designed to integrate a wide variety of services. Service networks could be structured, depending on the size and geographical distribution, in access and core networks. The access network connects end-users to the closest core node whereas the core network performs transit functions in order to set-up a connection between access nodes thus establishing the final end-to-end connection. Figure 4.5-2 shows the main components of a complete network. It can be seen that transport functions are placed in the bottom layer as they provide the basic interconnection functionality to the upper functions, the networking layer. This layer is formed by a set of functions that are responsible for the delivering of the final service to the end user.

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NETWORK SERVICES

SIGNALLING

ADDRESSING

NETWORKING SWITCHING

ROUTING

PDH/SDH

TRANSPORT

PHYSICAL MEDIA

Figure 4.5-2:

Network components

There are four main functions included in the networking layer: -

Switching. A set of hardware and software components whose function is to establish connections throughout a node. Routing. An algorithm and the related protocol whose function is to route calls or user’s information that allow a call or the user’s information to be routed to its final destination. Addressing. A set of rules that allow unique and well-known identification to be assigned to every user in the network. Signalling. A protocol that allows call-control and auxiliary information to be interchanged among the nodes of the service network. Signalling services are used by the other components of the networking layer in order to carry out a collaborating task and achieve a global performance of the network service.

It is important to comment that these components should interact among themselves to offer the final networking service so that every node of the service network has to be able to support those functions and work in a collaborating way so that a global performance could be achieved.

4.5.1 Transport Networks Transport Networks are basically formed by communication links connecting different sites and Cross-Connect equipment that establish fix connections across the nodes. These connections allow information flows to be transported through several stations to the final destination. These connections are established from the Management Centre and so cannot be controlled by the final user. These permanent connections are usually devoted to link nodes of the service network, though, they can also be used to interconnect any other device when networking functionality is not required. Out of the analogue transmission systems, which can form transport networks by means of analogue transit connections, two main digital transport technologies, PDH and SDH can be distinguished. Plesiochronous Digital Hierarchy PDH was the base of the former digital networks. PDH is a mature technology that has been relegated, in modern networks, to access functions. It is based on the Time Division Multiplexing TDM technique. This technique divides the capacity of a

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channel in equal shares among its users by assigning a part of the time to every user. Synchronous Digital Hierarchy SDH is the newest transport technology. It is used for high capacity transport applications, being the transport technology in which the Broadband ISDN (BISDN) is based. See also chapter 4.4.2.

4.5.2 Service Networks Service networks provide the final service to the user. The service that is offered by the transport layer is used to interconnect the nodes that form the service network. In 1978, the ISO (International Standards Organization) started the works to define an open communication architecture. After ten years of works, the OSI (Open System Interconnection) model was released. The OSI model defines a generic architecture for data communication networks that due to its global and wide perspective is normally used to explain the operation of communication networks. Two types of systems have been defined in the OSI reference model: End-Systems and Intermediate Systems. An End-System is a terminal equipment that delivers the final service to the user. An Intermediate System is a network device, which does not directly support users but forwards received data towards the final destination. Intermediate systems do not need to understand the information being sent between the users, but need to understand and possibly modify the information added by the network to provide the communication. End Systems may be directly connected, but more normally rely on the service provided by one or more Intermediate Systems. Examples of intermediate systems are routers or network switches. The communications process between End systems and Intermediate systems is usually defined in terms of the seven layers OSI reference model. In this reference model, shown on Figure 4.5-3, intermediate systems handle only protocol information at and below the network layer, whereas end systems use protocols at all the layers of the reference model.

Application Programs

Application Programs

7

Application Layer

Application Layer

6

Presentation Layer

Presentation Layer 6

5

Session Layer

Session Layer

5

4

Transport Layer

Transport Layer

4

3

Network Layer

3

3

Network Layer

3

2

Data Link Layer

2

2

Data Link Layer

2

1

Physical Layer

1

1

Physical Layer

1

Medium

Figure 4.5-3:

Medium

Seven layer OSI model

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7


Every layer has a well-defined functionality and provides a service to the upper layer in the model. The functions carried out by every one of the OSI model layers are: 1. Physical layer. The responsibility of the physical layer is to transmit unstructured bits of information across a link. It deals with the physical aspects such as the shape of connector, pin assignment, etc. 2. Data link layer. The responsibility of the data link layer is to transmit the information across a link. It deals with error detection and correction, information alignment and addressing when several system are reachable as in LANs or multipoint links. 3. Network Layer. The responsibility of the network layer is to enable the communication between any pair of end system in the network. The network layer deals with the route calculation function, congestion control, etc. 4. Transport layer. The responsibility of the transport layer is to establish a reliable communication stream between a pair of End systems. It deals with the detection and correction of the errors introduced by the network layer, such as packet lost or duplicated, reordering of out-of-order information, etc. 5. Session layer. The responsibility of the session layer is to co-ordinate the way data are transferred throughout the communication provided by the transport layer. 6. Presentation layer. The responsibility of the presentation layer is to adapt, when necessary, the different internal data representation format used by the End system that are transferring information. 7. Application layer. The responsibility of the application layer is to deliver the communication service to the application that is using the service provided by the network. Service networks can be implemented by using different technologies. Every technology in general could be more suitable to offer some services. Since the design of modern network is focused towards service integration, only those technologies that can be able to offer service integration are being considered for future designs. Nevertheless, we are going to mention not only future trends but also existing technologies owing to their capabilities to support the related applications. 4.5.2.1 Circuit Switched Networks (POTS, ISDN) Circuit switched networks are connection oriented networks. The establishment of a connection requires a call set-up that chooses a path in the network in which the necessary resources to support the connection are reserved. Resources are allocated to a connection whilst this connection is maintained, even though they were not used. Only when the connection is released will the resources be liberated. Circuit switched networks can be based on analogue transport technology, on digital transport, whether PDH or SDH, or on a mixed configuration. Due to the fact that a circuit of constant bitrate, usually 64 kbit/s, is used to support the connection, a deterministic delay performance is achieved in the final service offered. On the other hand, since every connection established in the network is based on the use of 64 kbit/s channels, when a connection is used for the transmission of information with a lower bit-rate poor resource efficiency is obtained, unless sub-multiplexing techniques are applied. Narrow-Band ISDN is the ITU-T standard for Digital Circuit Switched Networks. Switched and permanent connections can be established. The integration of services is limited to connectionoriented constant bit-rate service types.

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4.5.2.2 Packet Switched Networks (X.25, Frame Relay) Packet switched networks are connection-oriented networks. The establishment of a connection requires a call set-up that chooses a path in the network in which the necessary resources to support the connection are reserved. The difference with Circuit Switched Networks is that network resources are shared by the users, that is to say, resources are only used when needed. Thanks to this, a 64 kbit/s channel can be shared by several connections of lower bitrates. This mechanism allows resource optimisation to be achieved at network level but, on the other hand, a non-deterministic delay is obtained for every virtual connection due to the effect of statistical multiplexing used in the network. This type of networks offers packet-oriented services. It is not possible to obtain constant bitrate services due to the intrinsic non-deterministic delay of its transmission mode. They are mainly used to interconnect computers as they offer data communication services. X.25 and Frame-Relay networks are examples of this type of networks. They provide data packet communication service primary used for LAN or Mainframe interconnections and data network implementation. 4.5.2.3 Cell Switched Networks (ATM) Asynchronous Transfer Mode is a very efficient switching technology that has been adopted by the ITU-T as the base for the Broadband ISDN (B-ISDN) network. B-ISDN is a connection-oriented network. Thanks to the use of ATM, any type of service such as packet-oriented, circuit-oriented, constant bit-rate, variable bit-rate or even connectionless can be integrated on the same network. In ATM networks, the information is carried in cells. The cells follow the pre-established path in the network. Cells are generated depending on the amount of information the user wants to transmit. Resource optimisation is achieved as cells are generated only when some information has to be transmitted, so that the network capacity is shared by the users, the total amount of bandwidth required being lower due to the statistical multiplexing gain. One of the new concepts introduced by ATM is the flexible bandwidth and QoS service allocation, (see A3.2). It is possible for every user to set requirements on bandwidth, total Endto-End delay and delay variation. Thanks to this, connections with fixed bandwidth and bounded delay and delay variation can be defined in an ATM network. This possibility is used to offer the Circuit Emulation Service (CES), [14]. The performance of a circuit emulated by an ATM network is comparable to that experienced with current TDM technology. CES offers structured DS1/E1 Nx64 kbit/s (Fractional DS1/E1, where a selected subset of the 32 channels from the entire frame are transmitted, i.e. N = 1…32) services as well as unstructured DS1/E1 (2'048 kbit/s gross data rate, transparent bit-by-bit transmission)

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CBR

DTE

IWF

IWF

DTE

ATM

Figure 4.5-4:

Reference model of the Circuit Emulation Service (CES)

Figure 4.5-4 shows the reference model of the CES. We can distinguish three main components: The Data Terminal Equipment (DTE), the Internet-Working Function (IWF) and the ATM network. The DTEs are the actual users of the service, a protection relay or a teleprotection equipment providing 64 kbit/s or N times 64 kbit/s for instance. The IWF provides the conversion of the bit-stream generated by the users into cells and the reconstruction of the original bit-stream at the reception side including timing recovery and jitter removal. This function is usually embedded in the ATM access device. Finally, the ATM network should provide a Constant Bit-Rate (CBR) virtual channel that should have been dimensioned with the fixed bandwidth required to carry the bit-stream provided by the users. When Nx64 kbit/s working mode is selected, cross-connect (DXC, Digital Cross-Connect) functionality can also be provided, thereby being possible to deliver every single 64 kbit/s channel to different locations in a similar way as PDH cross-connect devices do. Since the CES has to offer quality performance similar to a PDH/SDH connection, it has to fulfil the requirement of the related standards. Therefore, a CES has to comply with ANSI T1.403 and ITU-T G.824 for jitter and wander performance of digital networks that are based on the 1544kbit/s hierarchy, and ITU-T G.823 for networks that are based on the 2048 kbit/s digital hierarchy, see also Table 4.4-1. Other facilities related with the data format and structure such as framing, alarm transmission, loops, etc, should comply with the relevant standards already applied to PDH/SDH connections. The Bit Error Rate (BER) of the emulated channel should comply with the ITU-T G.826 recommendation for E1 (2048 kbit/s) and the ANSI T1.510-1994 for DS1 (1544 kbit/s), or ITU-T G.821 for lower bit rates, e.g. 64 kbit/s. CES could find a direct application to connect existing protection relays or digital teleprotection equipment to ATM networks without the need of a specific implementation or external adaptation devices. Nevertheless, no practical experience of using this approach has been reported until the moment of writing this document. 4.5.2.4 Datagram Networks (IP) The traditional network concept we have discussed so far is based on the circuit-switched approach. Each connection is associated to a circuit that has resources allocated for its exclusive use along a path. There is no uncertainty about the bandwidth or delay along this path so the Quality of Service in terms of bandwidth and delay can be guaranteed. Datagram networks have introduced a very different mode of operation. Network resources (bandwidth, buffers, etc.) are statistically shared among their users. This presents many advantages for computer communication applications, since data traffic tends to be bursty so that resource reservation would lead to low utilisation levels. In datagram networks, data

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packets are delivered to the Network without any resource allocation, and the network exerts its “best effort” to serve the packets. Two different working modes can be distinguished in data networks, “Hop-by-Hop” control, used in Virtual Circuit Networks and “End-to-End” control used in Datagram Network. In the first approach, a connection is set up in the network, so that every intermediate system involved in this connection change its internal state. Every node takes care of every packet and guarantees its transmission towards its destination. This scheme suffers from a side effect known as “fate-sharing”: The End-to-End connection depends on the state of all the intermediate systems involved in this connection. If any of those systems fails, the connection will be lost. In the datagram approach, the End-to-End connection does not depend on the state of any one of the intermediate systems of the network. If one of the intermediate systems fails, the information will be routed using another path so that the final users will not be aware of this change. This scheme increases the overall availability of the network with this effect being more important for bigger networks. In the End-to-End scheme, the responsibilities are shared between the network and its users. The Network is responsible for the routing whereas the users are responsible for the control of the communication. Thanks to this approach, the Datagram networks present an unmatchable resilience level as well as the best resource optimisation. These characteristics make them suitable for mission-critical applications such as most of the applications that can be found in the Power Utility Control Network environment. Datagram networks using IP (Internet Protocol) cannot assure the QoS as the network presents a non-deterministic transmission delay. Then it cannot be applied to delay sensitive applications such as teleprotection, unless some specific Quality of Service mechanisms were added in order to guarantee bandwidth and/or delay. Refer to chapter A3.3 in ANNEX A3. The great flexibility of this type of networks makes them suitable for service integration. Although they cannot intrinsically offer a constant bit-rate service, thanks to a new application protocol that has been defined (Real-time protocol) it is possible to eliminate the delay variation at the application level. However this comes at the expense of an additional delay that may not be acceptable for protection.

4.5.3 Local Area Networks The Local Area Network (LAN), is by far the most common type of data network. As the name suggests, a LAN serves a local area (typically the area of a floor or a building). Typical installations are in industrial plants including substations, office buildings, college or university campuses, etc. In these locations, it is feasible for the owning organisation to install high quality, high-speed communication links inter-connecting nodes. Typical data transmission speeds ranges from 10 to 1000 Megabits per second. In summary, a LAN is a communication network that can be characterised by the following facts: -

It is local. Geographically limited to one or several buildings It has multiple systems attached to a shared medium It offers high total bandwidth that is shared by the users Limited number of users (hundreds)

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-

Low delay and low error rate Intrinsic Broadcast capabilities

The following characteristics differentiate existing LAN technologies: -

Topology: The way network devices are connected. Straight-line bus, ring, and star are the most common arrangements. Protocols and media contention: The rules and encoding specifications for data interchange and for the administration of the shared medium. Physical media: Devices can be connected by twisted-pair cable, coaxial cable, fibre or wireless.

4.5.3.1 Topology As shown in Figure 4.5-5, there are three basic topologies used in LANs: -

-

Bus topology. All devices are connected to a central cable, called the bus or backbone. Bus networks are relatively inexpensive and easy to install for small networks. Ring topology. All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it. Ring topologies are relatively expensive and difficult to install and maintain, but they offer higher bandwidth than bus topology and can span large distances. Star Topology. All devices are connected to a central hub. Star networks are relatively easy to install and manage, but bottlenecks can occur because all data must pass through the hub. S ta r

R in g

B us

Figure 4.5-5:

LAN Topologies

The market trend in terms of topology is going towards the star topology since both hubs and switches have enough capacity to cope with all the traffic that can be generated in a LAN. Star topology has been adopted to support internal substation communication in the new UCA architecture. Bus topology is a cost-effective solution for small LANs but due to the fact that is less reliable than star topology, it is not recommended for substation applications. Although ring topology is conceptually the best approach, its lack of flexibility and scalability has put it aside of the main innovative applications though still maintains a considerable market

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share. 4.5.3.2 Media Contention and Protocols LANs are based on the use of a shared medium to connect all the users. In this environment, only one user can transmit at a time. Some mechanism must therefore exist to allocate bandwidth among users in such a way that: - Each user gets a fair share of the bandwidth - Each user gains access to the medium within a reasonable amount of time - The bandwidth used for arbitration be minimized. The two most popular bandwidth arbitration mechanisms used on LANs are token schemes and contention schemes. In a token scheme, a user can send a piece of information when it has the token. The token is circulated from user to user. In a contention scheme, every user can send at will when it sees the channel in idle condition. When two or more users transmit at the same time a “collision” occurs. This situation is resolved by means of contention mechanisms that have a probabilistically fair behavior. The CSMA/CD (Carrier Sense Multiple Access with Collision Detection) working principle which is used by the Ethernet LANs has proved to be the most efficient and flexible contention scheme able to adapt to different speeds and physical media. Every different aspect of LAN networks, from physical specifications to protocols, has been developed by the IEEE 802 committee and later adopted by international standardization bodies. Protocols involved in the data interchange in LANs are confined to the OSI data link layer. Figure 4.5-6 shows the protocol stack of the Ethernet LAN and its relation with the OSI 7 layers reference model. The 100 Mbit/s Ethernet protocol stack has been shown since this is the standard adopted for the internal substation communication. OSI 7- Layer reference Model

IEEE CSMA/CD Model

Application Higher Layers Presentation Session

Logical Link Control LLC

Transport

Media Access Control MAC RS

Network

Media Independent Interface MII

Data Link Physical Media

Physical

100 Mbit/s

Figure 4.5-6:

LAN protocol layering

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The definition of an intermediate media independent interface simplifies the adaptation to different physical media. The most relevant physical interfaces used in the substation environment are: -

100BASE-TX that uses a 2 pairs UTP (Unshielded Twisted Pairs) category 5 cable in a point-to-point arrangement (Star topology) 100BASE-FX that uses an optical fibre (2 strands) in a point-to-point arrangement (Star topology)

4.5.3.3 Advanced topologies LANs offer a set of advantages that make them very attractive for local communication provision. They are a cost-effective solution that can carry any type of network protocol, even work without network layer protocol, providing high bandwidth to the users. Due to this, it is very appealing to extend its range of application. Nevertheless, this is not always possible since LANs have also certain limitations: - The number of users that can be connected in a LAN segment is limited. - The geographical expansion of a LAN is limited. - The amount of traffic that can be carried is limited. To overcome these limitations, devices like repeaters, bridges and Ethernet switches have been designed. They allow several LANs to be interconnected thereby becoming a single LAN from the point of view of the users without the above mentioned limitations. The typical functionality of the devices that allow LAN topology expansion is: -

-

-

Repeater. A network device used to regenerate or replicate a signal. Repeaters are used to regenerate the signal distorted by the physical media. A repeater can relay messages between subnetworks that use different protocols or cable types. A repeater cannot do any kind of intelligent routing performed by other devices like Switches, Bridges or routers. Hub. A common connection point for devices in a LAN. It is the centre of a LAN with star topology. A hub contains multiple ports. When a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets. Bridge. A device which connects two or more LANs. The two LANs being connected can be alike or dissimilar. For example, a bridge can connect an Ethernet with a TokenRing network. There are several type of bridges, the most common being the “learning bridges”. This type of bridge is able to learn where every user is placed and forward the packets to the port where the final user is connected. When they are connected in a meshed network with some loops on it, a very simple routing scheme, the spanning tree algorithm, allows loops to be avoided and therefore packets are forwarded to the final destination using the shortest path. It has to be mentioned that this very simple routing scheme function is carried out by the Data Link Layer without any kind of relation with the routing protocols that can be included in the network layer to cross the WAN. Thanks to the Data Link Layer Routing the information is forward to the final user inside our own network but it cannot be used to reach users of other networks. Ethernet Switch (Switched Hub). This is a device with a similar functionality to a Bridge but with much more capacity and a wider range of extra functions. Most Ethernet switches support the Virtual LAN functionality defined in the IEEE 802.1Q standard. One of the most relevant functions included in this specification is the virtual LAN definition,

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that is to say, a group of users connected in any physical LAN segment can be logically associated so that they work as if they were connected in a single LAN segment with broadcast functionality. Between different Virtual LAN segments attached to the same device, bridging functionality is provided. Most of the off-the-shelf LAN access/control devices include Hub, Bridging and Switching functions. These devices allow the network manager to define several virtual LANs that share the same physical infrastructure and set when hub-, bridging- or switching functions have to be used in order to optimize the LAN performance. When the LAN serves a site of a Wide Area Network, routing functionality can also be included in most of these devices, being then possible to forward packets to any other site of the same Wide Area Network.

4.6 NETWORK DESIGN AND OPERATION 4.6.1 Introduction Protection devices that operate over the same power line or subsystem require communication to improve their protection capabilities. The telecommunication service supporting protection communication should comply with a set of tough requirements. The main requirements that are considered in the definition of a Telecommunication service for Protection can be expressed with three concepts: Transmission time, Dependability and Security. Transmission time is the time required by a signal to propagate along the path that provides communication between the protection devices. This delay corresponds to the addition of the delays introduced by every component of the path, that is, the propagation delay of every link involved in the path and the transit delay of every cross-connect or switch that establish the communication path. Refer also to ANNEX A1, Figure A1-5. IEC 60834-1 states that the propagation delay introduced by the communication circuit should be less than 5 milliseconds, see Figure A1-5 in ANNEX A1. This delay is the addition of the delays introduced by every link or node involved in the communication path. It should be highlighted that those networks that introduce a variable delay in the information, such as IP datagram or ATM networks, should be engineered to assure the maximum delay instead of the typical engineering approach that works with the mean delay of the information. In order to design the network architecture, any possible communication path used to transport protection information should be analysed in order to verify if it complies with the End-to-End delay required by the protection scheme that is going to use this path. Some protection schemes, such as Differential Current Protection, require a symmetrical delay for the go- and return direction in the communication path. This requirement should be considered in the design of the network in order to avoid the use of asymmetrical paths as they present different propagation delay in each direction. Security performance of a protection scheme is generally related to the Bit Error Rate (BER) of the channel that communicates the protection systems. It is assumed that the design of the links used for any type of application is carried out in accordance to the ITU-T recommendations and so the expected performance in terms of BER will be always in the working limits under normal operating conditions. Dependability is related to the BER and the signal propagation delay of the channel. Assuming

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that the digital path supporting the protection communication complies with the ITU-T standards for the Quality of Service of digital connections the dependability will depend on the performance of the protocol used by the protection communication device (i.e. teleprotection function). Availability of a communication path can be defined as the ability of this path to perform its required function at any instant of time within a given time interval, assuming that the external resources, if required, are provided. It can be expressed as:

A=

MUT MDT + MUT

Where MDT is the Mean Down Time and MUT is the Mean Up Time. In general, it is widely accepted that the availability objective for a Telecommunication Service for protection should reach the 99,99%. This availability level, as can be seen from the abovementioned expression, is quite difficult to achieve with a single link so that a back-up link should be considered. When a network implements the communication path, other protection measures can be included in order to improve the availability of the service without devoting several links for every protection device. Among these protection measures such as recovery or self-healing mechanisms are the more relevant issues to be considered. When these protection procedures are implemented in a network, other functional components such as routing and addressing should also be analyzed in order to prevent collateral effects that could affect the protection service performance. Out of the above mentioned concepts directly related with the protection service, there are other requirements at the network level that should also be studied as they can influence the protection service operation. Service isolation and service prioritization facility is another important issue to be considered in the design of a network for protection communication: Modern digital networks are used to integrate different types of services. This is a common practice in transport networks as the aggregation of different traffic services allows a more efficient use of high capacity links such as fibre-optic cables or microwave radio links. Integration of services implies that different users are sharing the network resources. Under these conditions, the network should provide mechanisms in order to guarantee the isolation among different users. By providing this, every user has a guarantee of use of their assigned resources and so it can expect a guaranteed level of availability. On the other hand, networks integrating mission-critical services should provide a priority mechanism that allows the implementation of different availability levels. The priority mechanism should guarantee that in case of lack of resources due to an outage situation or network congestion, the service with a higher priority will always be able to use the remaining network resources and so maintain the expected service availability. Network Security is a very important aspect to be considered in the design of a network that offers communication service to protection applications: Network Security includes different aspects that all together provide the necessary means for

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secure and reliable operation of the network. The goal of the security functions is to assure that any other user of the network cannot interfere with, by any means, the proper operation of the protection system. That is to say, the network should withstand any security attack coming from any other user of the network whether on purpose or by mistake. The security aspects of the Network Management Centre are standardized by the ITU-T and the ISO. The operational details are discussed later in this chapter. Among others, it should be mentioned the implementation of checking procedures to assure that the parties connected by a path belong to the same “type of user” such as protection devices of the same electrical subsystem, etc. This procedure could prevent a wrong establishment of connections. The control of misconnection or misinsertion of information due to a failure in any network component is also a relevant functionality.

4.6.2 Technological considerations The design of a network that supports teleprotection service should be carried out taking into account the specific requirement of the protection scheme. The solution adopted will depend on the networking technology that supports this service. Different technologies are analyzed in order to identify advantages and specific considerations that should be considered in the design. 4.6.2.1 PDH/SDH Networks Transport Networks are used to transfer signal between different access points. These networks are based on permanent dedicated circuits multiplexed over higher capacity communication trunks. PDH and SDH are the basic technologies used in transport networks, see also Chapter 4.4.2. Both technologies are based on the Time Division Multiplexing technique, see 4.4.1.2. Thanks to this, they present a deterministic and relatively low transmission delay. Apart from changing signal transfer delays due to route switching, their use for most of the protection schemes do not pose any problem as the transmission delay is low in comparison with most of the protection requirements. The use of a fixed connection established over a PDH or SDH network for the communication of two protection devices does not present any type of drawback as the incremental delay compared with a direct link is very low. On the other hand, as we have seen before, in order to achieve the availability level requested for this type of service we have to implement some type of recovery mechanism in the network that allows the use of an alternative path when the main path fails. See also 4.4.2.2 on network resilience of SDH networks. The BER of a digital connection established in a PDH/SDH network is normally very low and so it will not have any effect on the Security and Dependability of the protection scheme that uses this path. Nevertheless, the quality of the path can be affected by the synchronization of the network or may be adversely affected by a power system fault due to EMI (Electromagnetic Interference). The implementation of a good network synchronization plan is very important to achieve the transmission quality levels expected in this type of networks. A poor synchronization scheme will lead to signal slips that produce error bursts that increment the BER of the digital path leading to a poor transmission quality or loss of signal. This effect can disturb the proper protection scheme operation. It can be relevant for Current Differential Protection schemes

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since a slip can be seen as a sudden phase change in an analogue signal. SDH transport networks uses a pointer mechanism to indicate the phase of the information inside the main information frame. Changes in phase leads to pointer adjustments that if mishandled can produce sudden phase changes in the transported signals that have a similar effect to the above mentioned slips. The implementation of recovery or Self-Healing mechanism in a PDH network is based on proprietary solutions. A careful analysis of these algorithms should be carried out in order to find out if it is possible to control the routing of the alternative path and so limit its length. A sudden increment in the number of hops of a path will present a considerable increase in the total delay of the transport path. Uncontrolled changes in delay can disturb the proper operation of certain types of protection schemes such as Differential Current Protection. In any case it should be analysed that any of the main or back-up path do not present an End-to-End propagation delay greater than 5 …8 ms or whatever the particular protection relay can tolerate. Recovery mechanisms in SDH network are fully standardised. There are two basic mechanisms that could be applied to improve the overall availability of the transport Network, the Multiplexer Section Protection and the Path Protection. Multiplexer Section Protection is a straightforward method that protects the connection between two nodes by adding back-up links. In order to achieve a full coverage in the protection both links should use physically diverse routes. Path Protection Mechanism protects the End-to-End connection of the final users over the transport network. This method has proved to be very efficient in small ring configurations, but it presents a serious drawback for certain types of protection schemes such as Current Differential due to the fact that the back-up path can be configured with an asymmetrical layout that leads to an asymmetrical delay. This effect together with a possible sudden change in delay due to a different back-up path length will drive to erroneous protection actuation. Both methods present a poor bandwidth utilisation as the amount of traffic that should be protected requires the same back-up capacity reserved in the network even though not all of them will fail at the same time. More about SDH network resilience is found in Chapter 4.4.2.2. PDH/SDH presents intrinsic service isolation and security. Due to the fact that these networks are based on the TDM technique and no signalling is available in a transport network, it is not possible for a user to attack another connection. The only point in which security measures should be considered is in the Control Centre. 4.6.2.2 ATM Networks ATM technology, which is based on statistical multiplexing technique, is a very efficient connection-oriented switching technology that can also offer a transport service with a guaranteed QoS (Quality of Service). Among the parameters that specify the quality of a connection are the End-to-End delay and the BER. ATM technology is an appealing solution for protection communications as long as it is possible for a user to specify the overall QoS requirements.

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The BER in an ATM link depends only on the characteristics of the physical media used as no error detection or correction techniques are applied to the user data ( = payload). Nevertheless, the BER could be considered as negligible due to the wide use of highly reliable links, mainly based on fibre-optics. However, the BER experienced by the protection service will be affected by the Cell Loss Rate CLR (a QoS parameter, see A3.2 in ANNEX A1), which depends on the scheduling, and Call Admission Control (CAC) algorithms used in the network. The use of ATM networks to communicate protection devices requires a careful analysis of the End-to-End delay of the service. The performance of an ATM connection in terms of delay depends on the cell scheduling policy in the switches as well as on the Call Admission Control algorithms implemented in the network. It is important to find out if the scheduling algorithms of the switches are able to isolate protection information from the rest of the traffic of the network, so that the possible interference among different traffic flows that could drive to an uncontrolled End-to-End delay can be prevented. Network topology should be designed taking into account the availability of low-delay communication paths between protection devices. Routing algorithms should also be analyzed in order to find out if they implement QoS routing. This facility is a necessary piece to guarantee an End-to-End bounded delay. The availability of the protection communication service can be improved if the network implements some recovery mechanism that can use the spare capacity of the network. Recovery mechanisms are under standardization process in the ATM-Forum. The great advantage of ATM networks is that they can choose the back-up route in function of the QoS requested by the service user. Thanks to this, the back-up path will always comply with the requested End-to-End delay. The design of the network should take into account that in order to protect the communication path used by the protection service the network must have some spare capacity, and the topology design must include some physically disjointed routes with low transmission delay. Synchronization is not a critical issue in ATM networks. As they work in asynchronous mode, the synchronization is carried out at link level and no synchronization plan is needed for the whole ATM network. Nevertheless, the transport of Constant Bit Rate (CBR) signals and the interconnection of PDH/SDH network throughout ATM network using the Circuit Emulation Service (CES) requires a clock synchronization plan to be deployed. Due to the fact that ATM networks are based on statistical multiplexing, the emulation of a transmission circuit will require the following aspects to be taken into account: -

Total end-to-end delay has to be limited Jitter and wander have to be limited according to transmission standards Clocking facilities have to be provided to support the network synchronization plan

The two former requirements are achieved by the proper ATM service specification whereas the last one is not under the control of the ATM network. Jitter and wander reduction as well as clocking facilities will have to be provided by the InterNetworking Function (IWF), see Figure 4.5-4.

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Two clocking modes can be used: -

Synchronous mode in which timing is supplied by the IWF Asynchronous mode in which timing is supplied by the user (DTE)

Since the ATM network cannot transmit timing information suitable for network synchronization, the IWF has to include the clock recovery functions. There are two methods to recover clock information: the Synchronous Residual Time Stamp (SRTP) and the Adaptive Clock. SRTP requires a common reference clock to be available at every access point of the ATM network providing a transport service, whereas the adaptive method does not require any external clock. Due to this fact, some wander may be introduced by the adaptive method thereby not being recommended to use it in emulation of circuits that support the communication of analog comparison protection devices. The ATM Forum and the ITU-T have standardized this service. In the network design phase, the synchronization plan should be carefully analyzed; taken into account that ATM network should not be used to transport timing references and therefore, a network-wide reference clock will have to be provided. ATM networks present intrinsic service isolation. Every different service integrated in an ATM network is based on a Virtual network; this network is formed by a set of Virtual Paths (VP). Cells belonging to a VP cannot be delivered by any means to another VP. Flow isolation in the same virtual network is also an intrinsic characteristic of ATM networks. Cells belonging to a due flow are identified by a particular Virtual Connection Identifier. These cells cannot be delivered by any means to another connection. Security aspects in ATM networks should be considered in the design phase in order to guaranteed the integrity of the Protection information transported by the network. Despite the tremendous complexity involved, the signalling network can be used to perform a malicious attack disturbing the proper operation of any service if no counterpart measures are implemented. The ATM Forum is defining a common framework of security in ATM networks. This work will produce a set of standards that will improve the security and offer robust protection to any external security attacks. 4.6.2.3 IP Networks IP networks are packet switched networks that work in Datagram mode. The End-to-End connection does not depend on the state of any one of the intermediate systems of the network. If one of the intermediate systems fail, the information will be routed using another path so that the final users will not be aware of this change. This scheme dramatically increases the overall availability of the network, however at the expense of a non-deterministic delay. Due to the fact that IP networks are based on statistical multiplexing and network access techniques, they present a non-deterministic delay. The quality of service, in terms of delay and packet loss, depends not only on the network working principles but also on the offered traffic load. In the actual network implementations, the End-to-End delay is not guaranteed. Nevertheless, if the network has been properly engineered and includes a priority scheme, the delay experienced by the information with higher priority can be limited to an acceptable value. In this way, the protection information flow is isolated from the

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rest of the users and is not affected by traffic overloads. The IETF (Internet Engineering Task Force, an international institution developing standards for the internet) is working on the standardization of a new signaling protocol - RSVP - that will allow the implementation of real-time services over IP networks. This new facility will open the possibility of offering services with guaranteed End-to-End bounded delays. IP networks present an intrinsic survivability. As far as a path exist between the users the communication will be maintained. There is no need for any specific recovery mechanism but the network should be designed with some spare capacity in order to maintain all the services in operation in the event of an outage situation. IP networks do not provide any type of synchronization of the flows that are being transported. Applications that require this functionality should use specific synchronization protocols such as Real-Time Protocol (RTP) that is embedded into the application and provides the timing recovery functionality. The application of such type of protocols for delay sensitive protection schemes should be very carefully analyzed in order to find out if the resolution and precision of this type of protocols fulfil the operational requirement of this type of protection scheme. An IP network presents intrinsic service isolation. In general, every application that uses the network services, such as protection, is associated to a port number of an IP address. The IP address range could identify the service whereas the port number identifies the flow. If the addressing scheme of the private IP network has been properly defined and static IP address allocation is implemented every service is unequivocally identified by a fixed IP address range. In this situation, there is little risk that information belonging to a due application could be delivered to another application in the network. IP networks present an intrinsic network security due to its working mode. In a Datagram network, it is not possible for a user to know the physical path that will be used by the information. In fact, every datagram can follow a different path in the network so no specific measures are needed to prevent security attacks at network level. Although the network is intrinsically secure, the applications using the network are not protected at all. It should be analyzed if some security measures have to be taken into account in the network design to protect certain types of critical applications such as the one that supports protection service.

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5 TELEPROTECTION INTERFACES Four types of interfaces are commonly used for protection relaying: - Contact based interfaces - Analogue interfaces - Electrical serial data interfaces - Optical fibre interfaces In the near future, Ethernet interfaces will likely be introduced. not only for intra-station communication using LANs, but also for inter-station communication across a WAN. The type of interface depends strongly on whether the teleprotection device is a separate equipment or whether it is an integrated function of the protection relay. Little has been done in the standardization area in respect of surge protection of copper-based Ethernet/LAN circuits that operate in the electromagnetic hostile environment of power stations and substations, as this technology has mainly been deployed in the office environment. Interface co-ordination has to be ensured regarding: - Type of interface (applicable standard) - Data rate (digital) or bandwidth (analog) - Signals to be used - Signal flow direction - Electrical insulation requirements - EMC requirements - Cable screening and signal ground connections - Connector design and pin/signal allocation And in particular for digital circuits: - Clock provisioning for synchronous operation - Low level data formats (asynchronous data format, synchronous operation) - Data flow control Unless all interface parameters are properly co-ordinated between devices, proper operation of the protection scheme cannot be expected. The checklist in chapter 7.2.2 may serve as a guideline.

5.1 CONTACT INTERFACES This is an interface of type (a) in Figure A1-1 to A1-5 in ANNEX A1. Contact interfaces are typically used to connect protection relays to teleprotection equipment in traditional state comparison or intertripping schemes. The sender closes a contact to initiate operation and applies the station battery voltage (110VDC to 250VDC) or an auxiliary voltage to the receiving input circuit (typically a relay coil, opto-coupler or transistor input). Contact based protection relays/schemes have so far been the only ones which are interoperable and support multi-vendor platforms. For example, distance relays of different manufacturers have been used to protect power lines using simple contact interfaces between the protective relay and the teleprotection equipment. The advantage of the contact interface is its simplicity and robustness. Its disadvantage is that its application is limited to binary trip/do not trip command transmission, and the need for a separate teleprotection equipment external to the protection relay. EMC and insulation requirements for contact interfaces are found in [27].

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5.2 ANALOG INTERFACES 5.2.1 Pilot-wires (50/60Hz) This is an interface of type (a) in Figure A1-1 to A1-5 in ANNEX A1. The connection is copper wires (2, 3 or 4) to the pilot-wire terminals on the pilot-wire relay. The signals are 50 or 60 Hz sinusoids, with transients during fault conditions. The levels and impedances do not conform to any standards, being proprietary to the various vendors of pilotwire relays. Peak voltages of 20 to 150 volts are typical. The pilot-wire interface must withstand fast transient surges as well as longitudinal induced voltages at power frequency. Insulation voltages of 5kVrms to 10kVrms at power frequency are typically required. As this type of interface won't be used for new designs, it is unlikely that standards will ever be developed for pilot-wire interfaces.

5.2.2 Voice frequency circuits (2-wire/4-wire) This is an interface of type (b) in Figure A1-1 to A1-5 in ANNEX A1. The connection is copper wire pairs to the VF (Voice Frequency) terminals on the teleprotection equipment. The signals comprise typically the sum of several 300 to 3400 Hz sinusoidal tones, each being (usually) frequency-modulated with the information being conveyed. The impedance is normally 600 Ohms balanced. The signal levels should be set as high as possible without overload or causing near-end crosstalk, generally 0 to –10 dBm (1mW to 0.1mW) per tone. When using rented circuits, maximum permitted levels may be subject to national regulations or to requirements from the circuit provider. EMC and insulation requirements are defined in IEC 60834-1 [27].

5.3 DIGITAL DATA INTERFACES 5.3.1 Electrical interfaces When the teleprotection function is integrated in the protection relay, the interface circuit to the telecommunication system is normally accomplished by means of a serial data interface of type (b) in the Figures of ANNEX A1. The serial interfaces shall comply with international standards for data communication. They have however to be enhanced with surge protective circuits to prevent damage. Commonly used interface types are shown in Table 5.3-1. An in-depth treatment of these interfaces regarding their application to protection is found in [2]. With the exception of the G.703 interface, serial interfaces are normally not electrically isolated from ground or from each other in case of multiple interface circuits. A special design to provide electrical isolation from ground and between interface circuits may be requested, as these circuits are installed in the electromagnetic hostile environment of power stations and substations with inherent risks of ground loops and strong EMI, in particular during fault incidents. EMC and insulation requirements are found in [27], [28].

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ITU-T

V.11

Operating mode

RS 232c/d/e

RS 422a

Data rate (typical)

up to 38.4kbit/s asynchronous

V.24/V.28

EIA

up to 38.4kbit/s

RS 485

up to 38.4kbit/s

------

RS232c/d/e

up to 38.4kbit/s

V.11/X.24

G.703 co-directional

Table 5.3-1:

RS 422a

synchronous

------

up to 64kbit/s or n*64kbit/s (n=1...32)

RS 485

up to 64kbit/s or n*64kbit/s (n=1...32)

-----

n*64kbit/s; n=1…32

Electrical isolation Not part of the standard; Electrical isolation requires special design Not part of the standard; Electrical isolation requires special design Not part of the standard; Electrical isolation requires special design Not part of the standard; Electrical isolation requires special design Not part of the standard; Electrical isolation requires special design Not part of the standard; Electrical isolation requires special design yes

Serial data interfaces

5.3.2 Optical fibre interfaces Optical fibre interfaces are normally of type (b) in Figures A1-1 to A1-5 in ANNEX A1. Optical fibres provide perfect electrical isolation between units and are immune against electromagnetic interference. Standards exist for optical fibre connectors and optical fibres, and also for the interfaces to optical LANs and high capacity SDH communication systems, see for example chapter 4.4.2.2. However, due to the lack of standards for protocols and low level data formats in the area of low-speed fibre-optic communication, optical fibre interfaces for interface (b) in ANNEX A1 have so far all been proprietary.1 Optical fibre interfaces may be used for: - Direct fibre connections between protective relays - Fibre connections between the protective relay and the telecommunication system (typically a multiplexer) - Fibre connections between the teleprotection equipment and the telecommunication system Electro-optical (E/O) converters may be required as intermediate devices between proprietary optical fibre interfaces and serial electrical interfaces in a multi-vendor environment. The development of integrated substation control and protection schemes has accelerated the need 1

The IEEE is presently (2000) developing a standard for nx64kbit/s relay-to-multiplexer communication. The German VDE is presently (2000) developing a DIN/VDE standard for protection relay communication protocols.

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to find suitable methods of providing noise immune methods of interconnecting services and providing power supplies to the interface converter units within the substation environment. Many electro-optical converters and DC power filters are available and offer advanced solutions to the well-known problems experienced by protection and telecommunication engineers. The majority of the units are proprietary and are only compliant when used in conjunction with the approved proprietary protection solutions. It is essential that the convergence of Substation control and Protection philosophies address the need to have common international standards for protocols and interfaces to meet the needs of standard substation network build.

5.3.3 LAN / Ethernet interfaces Local Area Networks are increasingly deployed in HV substation for intra-station communication. They have become a widely used technology for the implementation of the communications required by the local substation control and other auxiliary functions. With the advent of the new UCA (Utility Communications Architecture), being standardized by IEC 61850, new and more advanced functions have been added to this local communication interface being not only used for local control but also to support telecontrol and protection communications. Furthermore, its field of applications has been extended from the communication room or the control building to the bay level and switchyard. Although there are several LAN technologies as explained in Chapter 4.5.3, Ethernet is predominantly being used in the substation environment. Ethernet, which was developed in the 1970s, was the technological basis for IEEE 802.3 specification, which was initially released in 1980. The differences between Ethernet and IEEE 802.3 LANs are subtle. Ethernet provides services corresponding to Layers 1 and 2 of the OSI reference model (see Figure 4.5-3 and Figure 4.5-6), while IEEE 802.3 specifies Layer 1 and the Medium Access (MAC) portion of the Data Link Layer (Layer 2), but does not define a logical link control protocol (LLC). Today, the term "Ethernet" is often used to refer to all carrier sense multiple access/collision detection (CSMA/CD) LANs that generally conform to Ethernet specifications, including IEEE 802.3. The following paragraphs and Table 5.3-2 depict the most common physical interfaces used to implement LAN in substations. 10Base2 - 10 Mbit/s Thin Coaxial Ethernet interface This interface uses a thin and flexible coaxial cable that can be directly plugged into the Ethernet interface of the device or computer using a BNC type connector. The coaxial cable has to be laid from one computer to the next thereby interconnecting every device of the LAN. The distance between terminals can range from 0.5 m to 185 m. The flexibility and low cost of the thin coaxial system has made it a popular solution for networking clusters of computers. However, thin coaxial is limited to 10 Mbit/s and can pose some grounding problems due to the earth current in the shield of the coaxial cable. It is therefore not advisable to use it between buildings of the same substation or at the bay level in substations. 10BaseT - 10 Mbit/s Twisted-Pair Ethernet The 10BaseT interface operates over two pairs of wires, one pair is used for receive data signals and the other pair is used for transmit data signals. The two wires of each pair must be twisted together for the entire length of the segment. The two pairs are connected by means of an eight-pin RJ-45 type connector. This interface is a point to point interface intended for star

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topology and requires the use of a Hub equipped with a port for every device connected to the LAN. The length of the cable from the attached device to the Hub can range up to 100 m. Due to non-compliance with EMC requirements and test severity levels that have been developed for protection relay I/O circuits, like the Fast Transient Test for example, the 10BaseT interface should only be used with care for protection relaying, or when EMC requirements have been agreed between user and manufacturer. 10BaseFL. - 10 Mbit/s Fibre Optic Media Ethernet The 10BaseFL interface is a point to point connection that provides complete electrical isolation. As in the previous case, a Hub (see 4.5.3.3) is required to interconnect every device to the LAN segment. While Ethernet interfaces used in metallic media segments has protection circuit designed for medium level electrical hazards, fibre optical interface provides total immunity from electrical discharges including the effect of lightning strikes and ground current that can be found in substations and specially in connection between different buildings of the same substation. Another advantage of this type of interface is the distance it can span ranging up to 200 m from the host to the Hub. The interconnection with the optical fibre requires a specific Media Attachment Unit (MAU) that can be build into the Ethernet card or externally provided. In the latter case, the external unit is connected to the Ethernet interface using the Attachment Unit Interface (AUI) which is based on a 15 pin connector and can range up to 15 m. 100BaseTX - 100 Mbit/s Twisted-Pair Ethernet The 100BaseTX interface operates over two pairs of wires, one pair is used for receive data signals and the other pair is used for transmit data signals. The most popular wiring used today is the Unshielded Twisted-Pair (UTP) cable though shielded twisted-pair cable can also be used. The two wires of each pair must be twisted together for the entire length of the segment. The two pairs are connected by means of an eight-pin RJ-45 type connector. This interface is a point to point interface intended for star topology and requires the use of a Hub (see 4.5.3.3) equipped with a port for every device connected to the LAN. The length of the cable from the attached device to the Hub can range up to 100 m when a specific UTP Category 5 cable is used. As for previous similar metallic interfaces, this type of interface should only be use for wiring inside a building. Due to non-compliance with EMC tests and test severity levels that have been developed for protection relay I/O circuits, the 100BaseTX interface should only be used with care for protection relaying, or when more moderate EMC requirements have been agreed between user and manufacturer. 100BaseFX - 100 Mbit/s Fibre Optic Media Ethernet The 100BaseFX interface is a point to point connection that provides complete electrical isolation and immunity to EMI. As in the previous case, a Hub (see 4.5.3.3) is required to interconnect every device to the LAN segment. This interface uses two fibres, one for reception and other for transmission. The fibres are connected to the device using SC, ST or FDDI type fibre-optic connector.

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IEEE802.3

Ethernet Data rate (Mbit/s) Max. segment length (m) Medium Topology

10Base2

10BaseT

10BaseFL

100BaseTX

100BaseFX

10

10

10

10

100

100

500

185

100

100

50-Ohm coax (thick)

50-Ohm coax (thin)

Unshielded twisted-pair wire

Optical fibre pair

Bus

Bus

Star / Hub

Star /Hub

Table 5.3-2:

100 (UTP category 5 cable) Unshielded twisted-pair wire Star / Hub

100 Optical fibre pair Star / Hub

Common physical LAN interfaces

Warning: LANs with Bus/Star/Hub topologies exhibit the risk of "single point-of -failures", unless appropriate precautions like redundant LANs or some other form of protection against single point-of-failures are implemented.

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6 PERFORMANCE AND RELIABILITY REQUIREMENTS 6.1 REQUIREMENTS ON TELECOMMUNICATION SYSTEM 6.1.1 Introduction When setting up performance requirements, it is important to relate the same to a defined reference point or interface. The boundary between protection, teleprotection and telecommunication is not always well understood, particularly when some of the functions (e.g., teleprotection) are integrated into the protection relay or into the telecommunication terminal equipment. It is therefore important that a common understanding be achieved. The figures in ANNEX A1 should facilitate this. The arrangement of teleprotection schemes and their various interfaces will depend on whether the teleprotection is an integrated part of or separate from the protection equipment. The principal interfaces are referred to in Figures A1-1 to A1-4 in ANNEX A1. In an integrated arrangement the interface between the protection equipment and the teleprotection equipment will invariably be within the same equipment case or cubicle. Furthermore, the two parts will have been designed as one, so that interface levels, impedance matching etc. and other specialized requirements will be a function of the design and not a function of the external environment. As a result of this internal interface some of the requirements stated in this document will not apply to this category of equipment. However, the requirements of the interface between the telecommunication system and the teleprotection equipment / -function will, in most instances, still be applicable. In the separated arrangement, the teleprotection will most likely be physically divorced from the protection equipment , and, since it will not have been designed as a complete unit, all the requirements in this document apply. Correspondingly, the teleprotection equipment and the telecommunication equipment may form an integrated arrangement. In this case some of the requirements concerning interface (a) in Figures A1-1 to A1-5 (ANNEX A1) are still applicable. Other interfaces, not shown in the Figure in ANNEX A1, exist between the sensors (current and voltage transformers) and the protection equipment, as well as between the protection equipment and the tripping circuit of the breaker. Since the requirements in this paragraph apply mainly to the telecommunication and teleprotection functions, performance requirements associated with these interfaces are not defined. However, when overall performance has to be evaluated, it may be necessary to set up or simulate the power frequency quantities in a suitable test environment. Table 6.1-1 and Table 6.1-2 summarize basic requirements from protection and may serve as a guideline for assessing teleprotection systems. Particular systems are then dealt with in more detail in the following paragraphs. Obviously, requirements are different depending on which interface (a) or (b) in ANNEX A1 is considered. Actual requirements depend on many factors, such as line voltage level, protection system design, utility practice, degree of redundancy, power grid stability, etc. It is therefore neither possible nor practical to establish fixed requirements that would cover all cases. The figures in

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Table 6.1-1 and Table 6.1-2 merely permit the comparison of various systems and highlight some critical issues that need to be addressed when designing a protection system using telecommunications. The requirements in Table 6.1-1 and Table 6.1-2 focus on digital systems. In numerical relays, the teleprotection equipment - which caters for the data integrity - may be integrated as a function into the protection relay. This is mainly the case for analog comparison protection relays. Interface (a) is then not directly accessible for the user since it is relay internal. Thus, interface (b) in Figures A1-1 to A1-5 in ANNEX A1 is of particular interest in the context of this document. Requirements for the more traditional interface (a) are also given in the table, as these are relevant for dedicated teleprotection equipment, or when the teleprotection function is integrated into the telecommunication terminal equipment. Where possible, references to existing standards are made. 6.1.1.1 Terminology and General Requirements The basic requirements are summarized in Table 6.1-1 and Table 6.1-2. Terms and specific requirements related to these tables are explained below in some detail. Definitions are found in Chapter 6.1.1.2. Propagation time Signal propagation time across a telecommunication network is one of the most critical parameters, as it should be kept to a minimum to ensure that a circuit can be tripped as quickly as possible. Protection may therefore specify a maximum acceptable value measured at reference point (b), see the figures in ANNEX A1. Validation of the actual value may be performed for each link as part of the commissioning tests. The delay should also be measured with the circuit manually switched to its alternate route(s). Propagation time symmetry (differential delay) Propagation time symmetry - i.e. equal propagation times for transmit- and receive direction - is normally not required for state comparison schemes and command-based protection systems. However, propagation time symmetry between transmit- and receive paths is likely to be a critical issue for differential protection systems which measure the round trip delay and assume that the one-way delay is half of this value. If transmit and receive paths have different delays, this assumption is not valid. Non-equal propagation times may be caused by transmit and receive signals being switched to different paths through the network, or by data buffering and queuing in network nodes or traffic (over)load. Particular attention has to be paid to ring topologies where the signal may be sent in the opposite direction around the ring in case of a communication network fault. This may lead to unacceptable differential delays which must be prevented, for example by ensuring that independent switching of transmit and receive paths is disabled. Maximum allowable values for differential delays depend on the particular protection / teleprotection equipment design and should not be exceeded by the telecommunication system (measured at interfaces (b), see ANNEX A1). Propagation time variation While propagation time variation - i.e. static or transient changes in propagation time - is normally not an issue for state comparison schemes or command-based protection systems, it is crucial for current differential protection where synchronous samples of the power frequency waveform need to be compared at each end of the protected line. Static changes in propagation time may arise due to signals being re-routed to different paths through the network. Dynamic propagation time variation (jitter) may be the result of data

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buffering and queuing in network nodes or traffic (over)load. GPS based time stamping or samples indexing may be efficient solutions to overcome the problems due to propagation time variation for current differential protection, as this would ensure that synchronous pairs of samples are compared at each line end. Maximum allowable values for propagation time variation depend on the particular protection / teleprotection design and should not be exceeded by the telecommunication system (measured at interfaces (b), ANNEX A1). Availability Availability figures are not explicitly included in Table 6.1-1 and Table 6.1-2, as the definition of availability (or available time) according to ITU-T G.821 has little relevance for protection, or at least needs a closer examination. As defined in ITU-T G.821, a period of unavailable time begins when the bit error rate in each second exceeds 10-3 for ten consecutive seconds and terminates when the bit error rate in each second drops below 10-3 for ten consecutive seconds. Whilst a period of ten or more seconds during which a communication link is unavailable may be acceptable for non time-sensitive data or voice communication as messages may be repeated, it is not accepted for protection, because protection operation represents an emergency case where communication has to be unconditionally available. High-end protection systems are therefore normally designed as redundant systems. Nevertheless, the definition according to ITU-T G.821 suggests that a teleprotection system should remain operational up to bit error rates of 10-3, although performance may start to degrade at error rates that exceed 10-5. Re-routing Re-routing - and in particular automatic re-routing - is a salient feature of modern communication networks. It requires however special consideration if this function is applied to protection channels. For example, it is recommended that the BER and the delay at which a telecom network initiates a changeover be co-ordinated with the teleprotection dependability characteristic. There may be an unacceptable break in service of typically some tens of milliseconds if changeover is initiated due to a disturbed channel that is coincident with a power system fault. In many cases, there would be a good chance of signaling over the disturbed channel and successfully tripping the circuit, if a hasty route switching can be inhibited. As there is a real possibility - for example when re-routing is based on an equal delay or least cost strategy - that automatic re-routing could end up in transporting both Main 1 and Main 2 protection over the same bearer (e.g. same fibre cable or same radio link), special measures have to be taken to ensure that this eventuality cannot arise, e.g. by pre-defining two paths (main and alternate) with appropriate diversity and known signal transfer delays. Data integrity Although a telecommunication system is designed to produce low error rates (< 10-6) under normal operating conditions (measured at interface (b) in ANNEX A1), there may always be certain exceptional conditions that produce excessive error rates. Examples are fading microwave links, link failures, synchronization failures, bit slips or equipment defects. Error rates may then accept any value up to 50% before the corrupt data is eventually blocked by the telecommunication system. This requires that an error detection system be included in the teleprotection receive function in order to keep the residual error rate sufficiently low to prevent maloperation of the protection. Formats and protocols in accordance with IEC 60870-5-1 may be used.

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Channel squelching Although communication protocols for protection signal transmission (= teleprotection protocols) usually provide a high resilience against transmission errors, it is advisable that telecommunication receive circuits are muted or clamped to a predefined state (normally "all ones") when the error rate becomes excessive for an extended period. High error rates of up to 50% may for example be found in conjunction with a link failure, or data may be inverted due to an equipment failure. Data muting or clamping may be accomplished at the receive side of interfaces (b) or (a) in Figures A1-1 to A1-4 in ANNEX A1. Terminal equipment addressing Protection systems should incorporate some form of terminal equipment addressing in order to prevent maloperation when communication links are looped back or when a signal should be routed to the wrong terminal. Terminal addressing is of particular importance for differential current protection, as a misdirected signal would simulate a differential current, which would immediately trip the line. Address validation time should therefore be shorter than the protection relay’s signal processing/decision time. The address validation time is less critical for state comparison or command-based protection systems, as temporary signal misdirection would normally only exchange guard signals between teleprotection terminal units, thus not producing unwanted tripping. The addressing facility is normally implemented in the teleprotection function (Figures A1-1 to A1-4 in ANNEX A1). Network management and configuration Apart from precautions against random errors in the received data or against problems related to automatic re-routing, appropriate security measures to protect from inadvertent human-made errors need to be installed at the network management level. Password protection or channel locking may be used for that purpose. For example, in the case of Main 1 and Main 2 circuits being conveyed via the same network, it has to be ensured that there is no possibility of the circuits following the same route and causing a single contingency to simultaneously fail all the teleprotection associated with a particular line. Synchronization When analogue voice-frequency (VF) bearer services or asynchronous digital circuits are used, synchronization of network with the teleprotection signaling devices is not fundamental to the effective operation of the service. When synchronous digital services are required the need for secure and stable synchronization is imperative. In most configurations the teleprotection signaling device is provided as a Data Terminating Equipment (DTE). For the majority of schemes the bearer network is controlled from a master clock source, and all DTEs slave the timing from the master. On few other applications the DTEs may be required to take on the role of “master clock”. As the number of digital teleprotection bearer services increase, the need to ensure that the network providing the connectivity for these services is part of a well defined and secure synchronized network, becomes an essential part of the power utilities strategy. It is important that the telecommunications engineer and the protection engineer have a clear understanding of their equipment and its needs. Too often the service fails due to a lack of clear and knowledgeable decisions made at the interface. The difficulties likely to be encountered by synchronization problems are well documented in [30]. This document should be considered as essential reading as part of this report.

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Quality of Service (QoS) Parameters Today's teleprotection systems normally rely on channels that provide a deterministic signal transfer delay and sustain a constant bit rate (CBR) over time. PDH and SDH networks (see 4.4.2) using static multiplexing techniques comply with this requirement. The situation is however totally different when transmission technologies that employ statistical multiplexing (ATM), bandwidth-on-demand or "best effort" techniques (IP) are used. The impact of delay and delay variations on the performance of the protection scheme has to be analyzed with care in these cases, before these technologies can be considered for protection signal transmission purposes. A set of QoS parameters which confine for example minimum and maximum cell transfer delay (CTD) and cell delay variation (CDV) have been defined for ATM, see ANNEX A3. Similar efforts are under way to improve the real-time behavior of IP based communication. It is however not yet clear how far ATM and in particular IP based networks can meet the most stringent requirements for protection signal transmission, as the efforts in the ATM/IP-area regarding real-time performance improvements focus on multimedia service integration for public telecommunication networks, where signal transfer times are less critical and delays up to 200 ms are accepted in most cases. 6.1.1.2 Definitions The following definitions are related to Table 6.1-1, Table 6.1-2 and the Figures in ANNEX A1. More terms are explained in Chapter 6.1.1.1. Telecommunication system - telecommunication link System composed of telecommunication equipment and the associated physical link required to transmit information signals over a distance (IEC 60834-1, [27]). Teleprotection equipment - teleprotection function Equipment specially designed to be used in conjunction with a protection system. The teleprotection equipment, which is connected to a telecommunication link between both ends of the protected circuit, transforms the information given by the protection equipment into a form suitable for transmission. The functionality of the teleprotection equipment may be integrated as a function (algorithm) in modern numeric protective relays or in the telecommunication equipment. Teleprotection system System composed of teleprotection equipment and an associated telecommunication system between the ends of a protected circuit (IEC 60834-1, [27]) Propagation time The time elapsed between the instant of application to a telecommunication system (link, network), under stated conditions, of a specific value of the transmitted signal and the instant when the received signal assumes the corresponding value at the input of the teleprotection receiver (Signal transfer delay between interfaces (b) to (b) in Figures A1-1 to A1-5). Propagation time symmetry A measure for the similarity of the propagation time in transmit- and receive direction. Sometimes also referred to as differential delay. Propagation time variation The change in propagation time with time. Sometimes referred to as "jitter" or "wander".

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Propagation time may be measured and quoted if interfaces (b) in Figures A1-1 to A1-4 are accessible. Transmission time The transmission time of a teleprotection system is the time elapsed between the instant of change of state at the teleprotection transmit input and the instant of the corresponding change of state at the teleprotection receive output (Interfaces (a) in Figures A1-1 to A1-4), excluding propagation time. ([27], [28]). The nominal transmission time T0 is the transmission time measured under disturbancefree transmission conditions. The maximum actual transmission time Tac is the maximum transmission time encountered under disturbed transmission conditions for a defined dependability and signal-to-noise ratio (SNR) or bit error rate (BER). Overall operating time of a teleprotection system / Teleprotection operating time The overall operating time TA of a teleprotection system is the time elapsed between the instant of change of state at the teleprotection transmit input and the instant of the corresponding change of state at the teleprotection receive output (Interfaces (a) in Figures A1-1 to A1-5), including propagation time and additional delay due to interference and/or noise [27], [28]. Transmission time (T0, Tac) and teleprotection operating time (TA) may be measured and quoted if interfaces (a) in Figures A1-1 to A1-5 are accessible. Protection operating time The protection operating time TB is the time interval between the instant a specific set of values of the input energizing quantities is applied under specific conditions at the input of the protection system, including sensors or current and/or voltage transformers, and the instant when the protection relay output circuits are operated [28]. Bandwidth Analogue systems: The width of the frequency range used by the communication channel, expressed in Hertz (Hz). Digital systems: Sometimes used as a synonym for data rate. Data rate A measure for the information per unit time transferred across a certain reference point / interface of a system, including any overhead. Normally expressed in bits per second. Dependability Dependability relates to the ability to issue and receive a valid signal (command or message) in due time in the presence of interference and/or noise when a corresponding signal has been transmitted [27], [28].1 For practical reasons the probability of a missing signal is normally measured. The reference 1

Dependability as defined for teleprotection does not necessarily have the same meaning to the operation of the scheme. For example with blocking schemes, a missing blocking command is likely to occur in the presence of an external fault condition and may cause unwanted tripping (=> lack of security of the protection scheme).

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point for the measurement is interface (a) in Figures A1-1 to A1-5 (ANNEX A1). A poor dependability gives rise to a failure to trip or a delayed trip in an intertripping (direct tripping) or permissive tripping scheme. A poor dependability in a analog comparison protection scheme may give rise to a failure to trip or a delayed trip, or may produce unwanted tripping. Security Security relates to the ability to prevent interference and noise from generating an unwanted signal (command or message) at the receiving end when no corresponding signal has been transmitted, [27], [28].1 For practical reasons the probability of an unwanted signal is normally measured. The reference point for the measurement is interface (a) in Figures A1-1 to A1-5 (ANNEX A1). With permissive tripping schemes, the risk of an unwanted tripping action due to inadequate security of the teleprotection function is generally low, while in intertripping (direct tripping) schemes each unwanted command will lead to an unwanted tripping action. A poor security in an analog comparison protection scheme will typically produce unwanted tripping. Data integrity Data integrity relates to the probability that received data - that have passed certain error detecting and/or error correcting procedures - are correct. Data integrity can be expressed as residual error probability and may be categorized in classes, where each class is characterized by its upper bound of residual error rates (IEC 60870-5-1). Data integrity is improved by using protocols that detect, reject or discard data that have been corrupted due to bit errors introduced by the telecommunication channel.

1

Security as defined for teleprotection does not necessarily have the same meaning to the operation of the scheme. For example with blocking schemes, an unwanted command may lead, depending upon its duration, either to a delayed trip or to a failure to trip (=> lack of dependability of the protection scheme).

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Critical; (< 10ms)

Critical; (< 10ms)

Critical; (< 10ms)

Less critical; (< 30 ms)

Permissive Underreach Distance Protection

Permissive Overreach Distance Protection

Accelerated Underreach Distance Protection

Direct Intertripping Command Schemes

Table 6.1-1:

Critical; (< 10ms)

Blocking Overreach Distance Protection

Propagation Time

Not required

Not required

Not required

Not required

Not required

Propagation Time Symmetry

Analog: < 3kHz Digital: < 10kbit/s

Low 4


Low 4


Low 4


Low 4


Low 4

Bandwidth or Data Rate 1

< 10-5 normal; < 10-4 during power system fault





Bit Error Rate (BER)

Less critical; (< 50ms)

Critical; (< 25.. 30ms)



Critical; Depending on co-ordination times (< 25ms)

Overall Operating Time TA 1

High; IEC 60834-1

High; IEC 60834-1

High; IEC 60834-1

Medium to high; IEC 60834-1

High; IEC 60834-1

Dependability

1; 2

1, 3

High; IEC 60834-1

Medium; IEC 60834-1


Medium; IEC 60834-1

Low to medium; IEC 60834-1

Security

Requirements from protection on telecommunication and teleprotection: State Comparison Schemes. For terms and definitions refer to Chapters 6.1.1.1 and 6.1.1.2.

Not critical to the operation of the scheme





Propagation Time Variation

Interface (a) to (a) in Fig. A1-1 to A1-5

High; IEC 60870-2-1 Class I3

Medium; IEC 60870-2-1 Class I2




Data Integrity Misconnection critical when present during fault occurrence. Risk of nonselective tripping. Address validation time less critical, since system normally in guard state Misconnection critical when present during fault occurrence. Risk of delayed tripping. Address validation time less critical, since system normally in guard state Misconnection critical when present during fault occurrence. Risk of delayed tripping. Address validation time less critical, since system normally in guard state Misconnection critical when present during fault occurrence. Risk of missed tripping. Address validation time less critical, since system normally in guard state Misconnection critical when present during fault occurrence. Risk of not tripping and of tripping the wrong line. Address validation time less critical, since system normally in guard state

Impact of misconnection; Need for terminal addressing

General Recovery Time (nonredundant systems) and Availability

Transmission time, bandwidth, dependability and security are interrelated. One can for example improve one quantity at the expense of another. For example, for a fixed bandwidth, security can be improved at the expense of transmission time and/or dependability. The particular choice depends on the actual system requirements and on practical limitations. 2 The interdependence between dependability and communication quality parameters (SNR or BER) is explained in IEC 60834-1. The figures suggest that for a BER of <10-6 the teleprotection shall not suffer a noticeable dependability deterioriation. For a BER of 10-6 to 10-3 the teleprotection shall still be able to perform its function, although a certain loss in dependability is to be expected. 3 The interdependence between security and communication quality parameters (SNR or BER) is explained in IEC 60834-1. The figures suggest that the teleprotection shall guarantee a stated security against unwanted operation irrespective of the actual BER or SNR (worst case scenario). 4 Although the requirement on bandwidth or data rate is low for command based protection schemes, a channel with 64 kbit/s capacity (or a multiple thereof) may be requested in digital systems for standardization reasons.

1

STATE COMPARISON PROTECTION

Interface (b) to (b) in Fig. A1-1 to A1-5

Recovery time: After a random communication failure less critical for command-based non-unit protection schemes, as the system is normally in the guard state. Availability: The overall system design must ensure that proper operation is not adversely affected by the fault occurrence (electromagnetic interference)

4

3

2

1

Critical; (< 10ms)

Critical; (< 10ms)

Critical; (< 10ms)4


Phase Comparison with time stamp

Longitudinal Current Differential w/o time stamp

Longitudinal Current Differential with time stamp

Charge Comparison

Table 6.1-2:

Critical; (< 10ms)

Phase Comparison w/o time stamp

Propagation Time

Less critical

Less critical

Critical for systems based on loop roundtrip delay measurement

Less critical

Low to medium;

Critical for systems based on loop roundtrip delay measurement

7.2kbit/s to 64kbit/s

n*64kbit/s for multiterminal lines

n*64kbit/s for multiterminal lines 64kbit/s typical.

Analog: < 3kHz Digital: < 10kbit/s Typically 64kbit/s

Analog: < 3kHz Digital: < 10kbit/s Low to medium;

Bandwidth or Data Rate 1

Propagation Time Symmetry

High; IEC 60834-2

Less than 1/2 period of the power frequency for high end protection


High; IEC 60834-2


1 to 1.5 cycles


< 10-6 normal; < 10-5 during power system fault High

High; IEC 60834-2



High; IEC 60834-2

Dependability


Overall Operating Time TA 1


Bit Error Rate (BER) 1; 2

Medium to high



Medium; IEC 60834-2

Medium; IEC 60834-2

Security

1; 3

Requirements from protection on telecommunication and teleprotection: Analog Comparison Schemes. For terms and definitions refer to Chapters 6.1.1.1 and 6.1.1.2.



Critical; (< 0.1ms)


Critical; (< 0.3ms)

Propagation Time Variation

Interface (a) to (a) in Fig. A1-1 to A1-5

High

High IEC 60870-2-1 Class I3




Data Integrity

Address validation time needs to be shorter than relay decision time

Critical; Risk of unwanted tripping;









Impact of misconnection; Need for terminal addressing

General Recovery Time (nonredundant systems) and Availability

Transmission time, bandwidth, dependability and security are interrelated. One can for example improve one quantity at the expense of another. For example, for a fixed bandwidth, security can be improved at the expense of transmission time and/or dependability. The particular choice depends on the actual system requirements and on practical limitations. The relationship between dependability and communication quality parameters (SNR or BER) is explained in IEC 60834-2. It is suggested that for a BER of less than 10-6 the teleprotection shall not suffer a noticeable dependability deterioration. For a BER of 10-6 to 10-3 the teleprotection may still be able to perform its function, although a loss in dependability is to be expected. The relationship between security and communication quality parameters (SNR or BER) is explained in IEC 60834-2. The figures suggest that the teleprotection shall guarantee a stated security against unwanted operation irrespective of the actual BER or SNR (worst case scenario). For pilot wire replacement a propagation time of less than 1ms is required.

ANALOG COMPARISON PROTECTION

Interface (b) to (b) in Fig. A1-1 to A1-5

Depending on backup scheme, recovery time after a random communication failure may be critical for analog comparison protection schemes, as most of these rely on continuous transmission of information. Availability: The overall system design must ensure that proper operation is not adversely affected by the fault occurrence (electromagnetic interference)


6.1.2 Requirement from analog comparison protection General requirements of analog comparison protection schemes are listed in Table 6.1-2. When setting demands on the communication system from line current differential protection using a digital telecommunications network, we must consider the two basic forms of protection systems: -

Protection using GPS systems to time tag the current values. Protection using the communication network for time synchronization between the measuring points

These two types of protection system place differing demands on the communication network. 6.1.2.1 Time synchronization through GPS Overall operating time For a high-end protection, the overall operating time TA for a teleprotection system should be less than 1/2 of a power frequency period. Any longer delay will adversely affect the total operating time and fast fault clearing time of the protection. Data integrity The protocol used for teleprotection must detect errors before any unwanted functions occur. Security against unwanted operation must normally be given priority over dependability. Route switching A protection using GPS as time base is not affected by the number and frequency of route switching operations. However, the requirement of a maximum acceptable transmission time must always be fulfilled to guarantee the stated performance for the protection. Propagation time symmetry A protection using GPS as time base is not affected if propagation times in the transmit and receive direction are not the same, provided that the maximum propagation time is bounded within useful limits. 6.1.2.2 Time synchronization through communication network Overall operating time For a high-end protection, the overall operating time TA for a teleprotection system should be less than 1/2 of a power frequency period. Any longer delay will adversely affect the total operating time and fast fault clearing time of the protection. Data integrity The protocol used for teleprotection must detect errors before any unwanted functions occur. Security against unwanted operation must normally be given priority over dependability. Route switching A protection using the communication system for time synchronization can not allow too many or too frequent route switching operations. The maximum number or frequency of routeswitches depends both on manufacturer's algorithm for time synchronization and on how much

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the transmitting and receiving times will change due to a route switch. If the frequency of route switching is high but the change in propagation times is very little, then the protection will not be affected at all. A fixed, dedicated connection would of course eliminate this problem. Propagation time symmetry A protection using the communication system for time synchronization can tolerate some difference between transmit and receive propagation times. Traditional current differential relays normally assume equal transmit and receive propagation times, as the round-trip time is normally measured and divided by 2. Any difference in the actual times will automatically end up in a differential current and will directly affect both dependability and security. An example on how security is affected is given below. If the minimum allowed differential current limit is set to 20% of rated current, and this limit is used up to 100% of rated current before the we start to stabilize for current through the line, we can allow up to 1.2 ms difference between transmit and receive times, because receive time = 4.2 ms, transmit time = 3 ms à differential time = 1.2 ms, and average time = 3.6 ms

This gives a time difference for the comparison of the current vectors of 0.6ms = 10.8 degrees in a 50 Hz system, resulting in a differential current of sin(10.8) = 0.19*Irated. To obtain a protection with high sensitivity the difference in receive and transmit time must be 5 to 10 times less than given in the example above. This results in a maximum time difference between the transmit and receive direction of around 0.1 - 0.25 ms only.

6.1.3 Requirements from state comparison protection General requirements of state comparison protection schemes are listed in Table 6.1-1. 6.1.3.1 Propagation Time Permissive and deblocking type state comparison protection schemes typically do not have any minimum or maximum time delay requirements; they simply wait for the tripping signal to arrive before issuing a tripping signal to the breaker for an internal line fault. One exception to this is that if the permissive tripping signal arrives too soon, it may be rejected by the relay as an erroneous signal caused by power system fault generated noise. The protective relay must make this determination by comparing the signal arrival time to the fault detection time. Blocking schemes, on the other hand, include a coordination delay timer setting to compensate for communication signal latency. This timer is set based on the longest expected blocking signal delay. Signal delays greater than the timer setting will cause the relay to incorrectly trip the line breaker for an external line fault. The teleprotection function should measure communication signal delay and alarm if the delay is above a predetermined percentage of the blocking timer setting.

6.1.4 Requirements from intertripping General requirements from intertripping are listed in Table 6.1-1. Intertripping schemes typically have rather high demands on both dependability and security,

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and may compromise on maximum permissible operating times. Requirements on propagation time depends on the overall operating time requirement which is typically in the order of one power system cycle. Dependable operation minimizes the risk of personal hazards and/or damage to power system components. Security requirements against unwanted tripping are mainly driven from an economical aspect and reliable customer service point of view.

6.1.5 Requirements from system protection To carry out load or generation shedding, system separation, or overload protection for severe power system faults, wide-area protections such as system stabilizing protection collect power system data such as power flows, currents, voltages, main protection operations, that is, upward information transmitted from terminal units installed at power apparatus to a central processing unit (CPU), and transmit commands (downward information) based on the calculation by the CPU. As this protection is secondary or subsequent to main protections, the requirements on telecommunications are of the same level as main protections except for overall operate times, transmission times and data rates. Overall operate times mostly required for recent sophisticated wide-area protections are 150 to 300 ms. Generally, dedicated transmissions are used for important information such as fault or route-off detection and transfer trip signals to ensure high-speed and reliable transmissions, while relatively large volumes of information that do not necessarily require high speed are transmitted by an EMS/SCADA communication network. For example, frame-formatted cyclic digital transmission, HDLC or EMS/SCADA communications with transmission speeds of 1200 or 2400 bit/s are usually applied for upward information transmissions. High-speed multiplexed digital transmissions or dedicated transfer trip signal transmissions are used for the downward information transmission. Transmission delays of 30 to 50 ms are assigned. Sophisticated system stabilizing protections such as predictive out-of-step protection which make use of numerical or microprocessor-based relays and utilize sampling synchronization technique require the same level of telecommunication system as numerical current differential teleprotection systems, especially with respect to propagation time and propagation time symmetry. Most system stabilizing protections are configured in reliable double redundant systems. Requirements such as BER and unavailability other than time constraints and data rates are based on and similar to the present power line teleprotection requirements. The BER requirement taken to satisfy field data for teleprotection unavailability, due to microwave channel disturbances, should be of the order of 10-5. The system unavailability requirement is significantly related to the sum of sustainable equipment availabilities for a microwave teleprotection system, but the availability of protective relay still does not match the system availability model. Double redundancy is indispensable when considering maintenance of teleprotection systems, particularly at EHV levels. From both network viewpoints of power systems and telecommunications, wide-area or network protections will increasingly become more important in future complex and enlarged power systems to meet the customers‘ demands of cost reduction and higher reliability. FACTS, for example, is taken into account for stabilizing and optimizing the large interconnected power systems of the future. As these system are situated in the grey zone between protection and control, the possibility of such centralized or decentralized protection or control systems would form what is likely to be a large secondary control system. Therefore, requirements and evaluations with regard to network aspects would be needed. For example, as most protection systems are constructed in a redundant manner, when there are two network protection systems, A and B, that are overlapping with each other on one component, c, if each of the corresponding redundant components of network protection systems A and B is out of service,

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and if the critical common component c gets into failure, both protection systems are required, possibly leading to a more extensive outage. Therefore, such cross dependability and security checks among network protection and control systems for complex power systems will become significantly important. A procedure to avoid the inappropriate overlapping of telecommunication channels is also needed for power system stabilizing protection systems as shown in Figure 6.1-1. A failure on the overlapped link may result in a halt or failure of the protection system operation.

Figure 6.1-1:

Inappropriate overlaping of relay communication links in a double redundant protection system

6.2 REQUIREMENTS ON TELEPROTECTION The teleprotection function constitutes the interface between a protection function and a telecommunication system. The teleprotection function converts the signals and messages from the protection function into signals and messages compatible with the telecommunication system, and vice versa. For instance, a protection device may have a contact output and a dc voltage driven control input, and the telecommunication system may require a modulated audiotone signal. The teleprotection function must convert the protection device output contact to one or more specific audio-tone transmitted frequencies, and also convert the received audio-tone frequency(ies) to a control voltage recognized by the protection device control input. In modern digital protection devices, the teleprotection function must convert a digital word representing the value of a sampled analog value to a series of light pulses compatible with an optical communication system. The teleprotection function may be built into the protection device, or it may be a device, separate from both the protection device and telecommunication system, or it may be built into the telecommunication system. Both the protection function and telecommunication system place requirements on the teleprotection function, which are summarized as follows: - Present a compatible interface, suitable for the application and environment - Control transmitted signals/messages and monitor received signals/messages to: à Guarantee the required signal/message integrity à Supervise the availability of the telecommunication circuit à Reject signals/messages that are not destined for the local protection device à Raise an alarm in case of abnormal conditions and take appropriate action Refer to IEC60834-1 [27] and IEC60834-2 [28] for more information about requirements on teleprotection functions.

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6.2.1 Requirements on interface compatibility The interface between protection function and teleprotection function, and between teleprotection function and telecommunication systems must pass signals and messages with minimal delay, attenuation, and disruption. To do this, the mated devices and systems must have compatible interfaces, and the interfaces must be suitable for the application and environment in which they are installed. Compatibility is usually guaranteed by using an interface that meets a recognized standard. The International Standards Organization (ISO) Open Systems Interconnection (OSI) Reference Model (see Chapter 4.5.2) describes a digital network communication architecture physical layer that can be related to the teleprotection interface. In this model, the communication network physical layer is responsible for transmitting raw bits over a communication channel. The physical layer design must ensure that when one side sends a 1 bit, it is received by the other side as a 1 bit, not as a 0 bit. The design issues include: - electrical or optical signal characteristics - electrical or optical connectivity - physical and mechanical characteristics - procedural rules Questions that must be addressed include: - how many volts should be used to represent a 1 and how many for a 0 - the time duration of each bit - whether transmission may proceed simultaneously in both directions - how is the initial connection established and how is it disconnected when both sides are finished - and how many pins the network connector has and what each pin does. Standard interfaces that address these issues should be used, where applicable and whenever possible. However, standards do not exist for all interfaces or leave room for mutual agreement between manufacturer and user, in which case the interface must be completely specified to ensure compatibility.

6.2.2 Functional requirements The following statements focus on digital systems, however, similar considerations apply to legacy analog teleprotection systems. Digital communication systems for analog comparison and state comparison protection functions carry messages between digital protective relays at each end of a power line. The relays continuously transmit and receive new messages as the relays continuously monitor power system parameters, watching for abnormal conditions that may require them to act swiftly and securely to interrupt and isolate the cause of the power system abnormality. The messages contain data that are critical to the proper operation of the relays. Each message includes a “frame” that may include a destination address, data formatting information, and error checking. Message timing may also be critical. Any data, framing, or timing errors introduced in the digital communication system can, and probably will cause relay misoperation if not detected and rejected or corrected. Most digital relays that send and receive digital messages therefore include message error checking schemes to verify that each received message is accurate and valid before using the data contained in the message. Bad messages may be the result of a single transient noise burst caused by an internal communication system problem, or by an external condition like lightning, or power system electrical switching transient. Bad messages may also be the result of signal attenuation caused

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by the declining health of communication circuits or equipment. Bad messages can also be good messages sent to the wrong location by faulty communication switching equipment, or intentionally misrouted, such as in a loop-back condition performed during communication circuit testing. No matter what the cause, the digital protective relay or associated digital communication system interface must detect and reject or correct bad messages (including misrouted good messages) to prevent misoperation. It is common practice to alarm or annunciate a communication problem to alert the appropriate communication system maintenance personnel about existing or impending communication system problems. Some relays log communication errors to help diagnose the nature and cause of communication system problems. At the very least, the relay should provide a time-delayed output to alarm for the continuous receipt of bad messages or complete loss of messages. In addition, modern digital relays can perform other logic functions to modify their performance, or the performance of the communication system when a problem occurs. While it is important that the teleprotection function and telecommunication system send and receive messages between protective relays as reliably and securely as possible, it is also important that these functions not resend old messages and data that error checking determined were not correctly received at the remote terminal. The teleprotection and telecommunication systems should continue to send only the newest messages and data, and discard old messages and data. Resending old messages may delay proper protection scheme operation, or cause protection scheme misoperation and incorrect breaker tripping. 6.2.2.1 Analog comparison protection control and monitoring Analog comparison protection schemes for digital communication circuits generally require protocols with error check procedures to avoid unwanted operations due to errors in the messages. Error checking may include parity check, check sum, and cyclic redundancy check (CRC) sequence. In addition, other checks can be included to increase the security: - Checking the length of the received messages - Checking the time tag sequence of two consecutive messages - Detecting changes in the delay time above a tolerance setting When a message error is detected, the message is rejected and the protection is reset. Below a predetermined message error rate, the protection remains stable but the operating time is delayed due to the loss of messages. If the percentage of messages rejected exceeds a value, e.g. 25%, during a defined period of time, the operating time becomes intolerable and the protection is blocked until the message error rate decreases to acceptable values. On blocking, a “channel failure” alarm is raised by the relay and sent to indicate a failure in the communication channel. If the protection uses only one channel, the “protection function failure” alarm is also raised. Some protection systems have a redundant channel through a different communication path so the channel with a better performance is selected. In case of failure in both channels, both channels and protection alarms are raised. 6.2.2.2 State comparison protection control and monitoring State comparison protection schemes designed for digital communication circuits generally include protocols with error checking to detect bad messages that can cause protection scheme relay misoperation. Because each bit in a state comparison protection scheme’s message may have a different function, and therefore a different priority, some schemes apply additional security measures for each individual bit. For instance, a bit used to communicate a permissive transfer trip does not need the same security as a bit used to communicate a direct, unsupervised transfer trip. The protective relay scheme may therefore require that two or three

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consecutive messages be received with the direct transfer trip bit asserted before taking action to perform the direct transfer trip output. This is analogous to using a time delayed output in an analog scheme to improve the security of an output. Likewise, if a bad message is detected, the protective relay should reject and ignore the bad message, and may assume the status of each bit in the expected message based on predefined criteria. For instance, the relay may be programmed to assume the bit should be in one of three states if the message is bad: - a logical zero (0) - a logical one (1) - or it should retain the status the bit had in the last good message. For additional security, the relay may require that several good messages be received after one or more bad messages to ensure that the communication system has returned to a satisfactory performance level before using the bits in the new messages. 6.2.2.3 Erroneous signal detection Protective relays may detect erroneous permissive, deblocking, or blocking signals based on signals arriving too soon after power system fault detection. Proper permissive, deblocking, and blocking signals always incur some delay due to natural latencies in the relays and telecommunication system. Premature signals can be generated by power system fault noise induced on the communication system. While the teleprotection device may have filtering to reject noise and pass only valid signals, filtering can be compromised by unique noise patterns. 6.2.2.4 Loop-back and misconnect detection The telecommunication service provider commonly checks telecommunication circuits by performing a loop-back test. During the loop back test, the signal sent by a relay, intended for the relay at the remote terminal, is returned to the originating relay. State comparison protection schemes can operate incorrectly when a power system fault occurs while the telecommunication circuit is in a loop-back configuration. The teleprotection device and/or relay should detect the communication circuit loop-back configuration and disable communication assisted tripping until proper communication circuit connectivity is restored. The relay and/or teleprotection device should also alarm during this condition to alert the appropriate operating, maintenance, or telecommunication service personnel. 6.2.2.5 Actions on alarm conditions Digital protective relays or communication schemes typically do not alarm on a single bad message, but should alarm on a string of consecutive bad messages that could indicate a permanent communication system failure. However, because individual bad messages, or small groups of bad messages may indicate an impending communication system failure, the digital protective relay or communication system should log the occurrence of bad messages, and track the performance of the communication system over time by comparing the number of bad messages to the number of good messages over a period of time. If the percentage of bad messages exceeds a predefined threshold appropriate for that communication system, the relay or communication system should alarm or annunciate the problem to alert communication system maintenance personnel. In addition to alarming, digital protective relays may also take additional action when communication system problems persist. For instance, the protection scheme could: - force the communication system to switch communication paths or activate a redundant communication channel.

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change relay settings to compensate for the loss of communication-aided high-speed protection. For instance, a state comparison protection scheme may enable RemoteEnd-Just-Opened logic, or Zone 1 Extension logic that provide faster (but less secure) tripping without communication assistance. - block the protection scheme from operating. - trip to take the protected system out of service to preclude the possibility of a fault occurring without high speed protection in service. This would only be done in extreme circumstances where time delayed tripping cannot be tolerated under any circumstances. In most cases, however, where communication assisted high-speed protection is required and critical to power system stability, a second protection scheme with its own independent communication scheme is used to ensure high speed protection for the loss of one communication scheme. Most state comparison protection schemes, such as zone distance relay schemes can provide stand-alone protection with high-speed tripping for faults on the majority (but not all) of the protected line segment, which reduces the reliance on communication-assisted tripping. Most analog comparison protection schemes are installed with supplemental step distance schemes, which provide secure, but slower, tripping without communication. -

6.3 REQUIREMENTS ON PROTECTION 6.3.1 Requirements on analog comparison protection 6.3.1.1 Need for delay compensation Comparison of measured quantities from differential protection relays must be based on pairs of samples that were taken at the same instant of time. As the samples are transferred to the opposite end for comparison, the delay which is introduced by the telecommunication link has to be taken care of by the protection end device that performs the comparison. Commonly used methods to accomplish time synchronization between the samples to be compared have been described in chapters A4.1 and 6.1.2 of this document. The most popular being: -

Propagation delay estimation based on measurements, e.g. round-trip propagation delay measurement Time tagging of samples, where each sample carries its unique time tag with it, either received as a time stamp from an external source such as GPS, or as an index derived from the relative sample position with respect to the power frequency zero-crossings.

The second method employing time tagging is preferred, as the comparison of the respective samples then does neither depend on equal signal propagation times for the 'go' and 'return' direction (propagation time symmetry), nor on stringent limitations for signal propagation time variations. See also chapters 6.1.1.1 and 6.1.1.2 for explanations and definitions. As propagation time symmetry may be jeopardized when communication channels are re-routed upon a failure in the communication network, or because signal propagation times may vary with time due to signal buffering and queuing in the network, time tagging is a prerequisite for protection using general communication networks, unless the network has been explicitly engineered to comply with the timing requirements that are imposed by the protection relay. Refer for example to Chapters 4.2, 4.6 and 6.1. In conclusion, whatever method for time synchronization is chosen, the protection relay - when used in conjunction with modern telecom networks - will have to cope with: - Static propagation times

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-

Propagation differential times between ‘go’ and ‘return’ direction Dynamic propagation time variations

6.3.2 Requirements on state comparison protection 6.3.2.1 Interface co-ordination The interface between protection and teleprotection devices must be coordinated to minimize delay, attenuation, and signal disruption. Likewise, the same coordination must be made between the teleprotection device and the telecommunication system. Physical connection and electrical and/or optical compatibility must also exist at each of these interfaces. 6.3.2.2 Delay Compensation Most state comparison protection schemes tolerate communication delay and delay variation very well. Blocking Overreach Distance Protection schemes, however, require a coordination delay timer setting based on the expected communication signal delay. Communication delays greater than the timer setting can cause a protection maloperation for an external fault. Conversely, unnecessary tripping delay occurs when the actual communication signal delay is significantly less than the timer setting. The protective relay should measure the communication signal delay and adjust the timer to minimize the time the relay waits before issuing a tripping signal for an internal line fault, yet assuring adequate time to receive a blocking signal for an external line fault.

6.3.3 Requirements on other protections Apart from interface co-ordination no special requirements apply.

6.4 CONSIDERATIONS ON INTERFACES AND INSTALLATION PRACTICES The design of protection and telecommunications devices to operate in an environment, complying with pre-defined standards for quality of service RFI interference) and EMC (electromagnetic compatibility), requires a consistent methods used to provision, install and deliver the telecommunications teleprotection signalling device.

electrical hostile (radio frequency approach in the service, to the

Many problems are caused by bad installation practices and by using inappropriate telecommunication services for the required teleprotection signalling function. This chapter attempts to capture a number of critical issues, and highlight the areas that may be the source of service failure if adequate standards of installation are not implemented. Fundamentally the protection requires a telecommunications bearer service that will provide perfect performance during electrical network faults. This may only be two or three times per year and requiring the allocated circuit traffic capacity for no more than a few seconds. The protection engineer also expects the telecommunications bearer service to be monitored and managed by the circuit provider. On the whole this can not be achieved. As the monitoring of the bearer services provided must, by default, be a task performed by the teleprotection function/device, manufactures and protection engineers require to develop alarm strategies that are more in keeping with the bearer services provided and the needs of the scheme. To achieve this requirement teleprotection signaling devices and bearer circuits require to be procured that will guarantee quality of service before, during and after any incident. The circuit shall be designed to provide a secure and resilient service that will continue to function, within specified limits, before, during and after an electrical fault or network disturbance. The total

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connectivity, including local connections and local power requirements, must not compromise the ability of the teleprotection function to perform its task. When designing and provisioning the required network bearer service the following should be considered: - Is the bearer service immune to electrical disturbance? - Can the bearer service provide the required operating characteristics? - Can the bearer service provide the managed resilience and diverse routing in compliance with service needs? - Does the teleprotection function raise an alarm if the bearer service does not comply with pre-defined characteristics? - Assuming that we can provide immune external bearer services, how do we ensure that we do not compromise the ability of the protection service to function by poor local installation practices? It is imperative that the methods used to interconnect and power the teleprotection signalling device and the bearer service addresses the needs to comply with standards for EMC, RFI, rise of earth potential and other electrical disturbances. If this is done the security and integrity of the service will not be compromised The existing arrangements developed over a number of years were introduced at the time teleprotection signaling schemes were reliant on physical copper connections and / or voice frequency bearer services. The PTO (Public Telephone Operator) Bearer Service Providers, have over the last 20 years significantly revised their networks and the services available on them. Copper with DC connectivity is no longer available and the analogue network infrastructure, used to deliver voice frequency services, has now been replaced by new digital network infrastructures. Most voice frequency services are now derived from digital technologies. Methods used to deliver the bearer service to the teleprotection device within the substation have evolved rather than been developed to meet the changing needs of the service. Protection services have become more sophisticated and in turn demands more from the bearer service provided. The migration of service provision from pure analogue services to analogue derived digital services has already caused problems with Differential Delay for analog comparison Protection schemes. Guaranteed and consistent circuit parameters can no longer be assumed on networks delivered by public service providers. Network and local considerations must be addressed and understood. In the typical electrical hostile environment found in substations it is necessary to ensure that the equipments are compliant and the methods used to house, power and interconnect these services will not compromise the effective operation of bearer or teleprotection device. If consideration is given to the above likely risks, existing configurations can be made secure and should provide trouble free service. Future developments and operational strategies should however identify the best methods that are now available to minimize the likelihood of teleprotection failure. Proposals are now being developed that use the increased availability of optical fibre service provisions and interface devices. If external services can be provided over alternatively routed fibre and interfaced with

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appropriate service equipments housed in EMC / RFI cubicles, the likelihood of bearer service failure, during an electrical disturbance, will be greatly reduced. If power requirements are designed to include appropriate filters and surge arrestors and if cables are run in appropriate fashion, the interference experience by noise on DC power supplies to equipment will be insignificant. The connection from the delivered bearer service to the teleprotection device must consider the local environmental risks and adopt the most appropriate interface. This connection from EMC / RFI cubicle to teleprotection device may be best achieved by using Optical Fibre cable and the appropriate converter units at each end. When copper cable is being used best practice for screening and route choice, to reduce the impact of induced voltage, may provide acceptable results. A comprehensive treatment of EMC including practical design-, cabling- and installation guides is found in [37] and in the IEC standards below. IEC 61000-5-1: IEC 61000-5-2:

Electromagnetic compatibility (EMC) - Part 5: Installation and mitigation guidelines - Section 1: General considerations - Basic EMC Publication Electromagnetic compatibility (EMC) - Part 5: Installation and mitigation guidelines - Section 2: Earthing and cabling

Table 6.4-1:

IEC publications for EMC and installation

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7 PROTECTION SYSTEM CONFIGURATIONS AND DESIGN 7.1 PROTECTION SCHEMES AND TELECOMMUNICATION SYSTEMS COMPATIBILITY The following tables provide combinations of protection scheme and telecommunication system technologies with comments regarding the acceptability of listed combinations. The tables are intended as an aid to protection engineers and telecommunication service providers in the selection of appropriate combinations of protection scheme and telecommunication system technologies. All of the tables list protection schemes outlined in Chapter 2.4.1, Protection Using Telecommunication, condensed into basic groups according to their general communication requirements, as outlined in Chapter 6 of this document. In general, all state comparison and intertripping schemes require less communication bandwidth and are virtually immune to communication delay variations. Communication system bandwidth and delay variation requirements for analog comparison protection schemes, on the other hand, vary widely and must be segregated to identify their differences. Notations are made, where appropriate, to explain unique conditions that impact the protection scheme and telecommunication technology selection. Three tables are used to segregate the telecommunication technologies into the basic types of communication service outlined in ANNEX A1: - Media for Dedicated Point-to-Point Service (Table 7.1-1) - Multiplexing Technologies for Shared Point-to-Point Service (Table 7.1-2) - Network Technologies (Table 7.1-3). In general, the characteristics of dedicated point-to-point service are determined by the installed media, which have a wide range of bandwidth and delay characteristics. Multiplexed point-topoint service is also heavily influenced by the installed media, however, the media selected for these systems generally have wide bandwidth capability. The overall performance is therefore most significantly influenced by the multiplexing technique. Consideration must also be given to the affect that intermediate drops, repeaters, communication “load”, and multiplexing steps will have on the overall end-to-end performance, especially delay characteristics. Network systems are comprised of communication branches, taps, and loops, where the physical media may be different on different parts of the network. Like multiplexed point-to-point systems, the media selected for network systems generally have wide bandwidth capability. Network system performance is therefore most significantly influenced by network transfer protocol. Consideration must also be given to the affect of network configuration, network communication loading, and operational switching. The following three tables present the expected protection scheme performance using individual communication technologies. It must be stressed that protection systems rely on end-to-end communication performance. Evaluation of end-to-end performance on telecommunication systems that include a mix of media, multiplexing technologies, and network technologies must consider the worst case sum of all technologies used in the communication path. The best performance can therefore be expected from simple, direct, homogeneous communication systems. The performance of complex networked communication systems may be very difficult if not impossible - to predict. Experimental field trials may provide the best indication of expected communication system performance.

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Metallic wire pairs (see 4.3.1)

Protection Scheme ê All State Comparison Schemes (see 3.1.2) Direct intertripping (see3.2.1.2) Digital Current Differential (see 3.1.1) Analog current differential (see 3.1.1) Pilot wire relays (50/60Hz) (see 3.1.1) Phase Comparison (see 3.1.1.2) Charge Comparison (see 3.1.1.3)

Power Line Carrier (see 4.3.2)

Licensed Radio (Microwave) (see 4.3.3.1)

Unlice nsed Radio (see 4.3.3.2 )

Optical fibre (see 4.3.4)

Pilot wires (see 4.3.1)

Voice Frequency Circuits (see 4.1.2)

Analog (SSB)

Digital (QAM, MCM)

Analog

Digital

Digital

Digital

(1), (2)

OK

OK

(6)

OK

OK

(8)

OK

(1)

OK

OK

(6)

OK

OK

(8)

OK

(1)

Not recommended

(3)

Not applicable

(3), up to 19.2kbit/s

(3)

(3), (8)

OK

OK

OK

(8)

Not recommended

(3)

(3)

(3), (8)

(3)

OK

Not used

Not used

Not used

(4)

(4)

Not used

(4)

(3), (5)

(3), (5)

(3), (5)

(3)

(3)

(3), (8)

(3)

(3)

OK

OK

OK

OK

(8)

OK

Table 7.1-1:

Not recommended (8) Not recommended (8)

Protection Schemes vs. Media

Notes: (1) (2) (3) (4) (5) (6) (7) (8)

Possible using audio tone communication. Direct Transfer Trip is possible using d.c. voltage scheme, typically applied with ac current differential protection scheme. OK if communication channel delay is kept within the relay’s delay compensation adjustment range Possible, if delay can be kept below 1ms Analogue non-segregated Phase Comparison only OK with (analog) sub-system for teleprotection command transmission, see 4.3.2 OK for command-based systems Might be possible, however not recommended for reliability reasons. No applications in HV grids reported

Protection schemes are described in Chapter 2.4.1. Media are described in Chapter 4.3.

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Protection Scheme ê All State Comparison Schemes (see 3.1.2) Direct intertripping (see3.2.1.2) Digital Current Differential (see 3.1.1) Analog Current Differential (see 3.1.1) Pilot wire relays (50/60Hz) (see 3.1.1) Phase Comparison (see 3.1.1.2) Charge Comparison (see 3.1.1.3)

Frequency Division Multiplexing (see 4.4.1.1)

Wavelength Division Multiplexing (see 4.4.1.1)

Fixed TDM (see 4.4.1.2)

Statistical TDM (see 4.4.1.2)

Code Division Multiplexing (see 4.4.1.3)

OK

OK

OK

(2)

(3)

OK

OK

OK

(2)

(3)

OK

OK

(1)

(1), (2)

(3)

OK

OK

(1)

(1), (2)

(3)

Not used

OK

(4)

Not recommended

Not recommended

OK

OK

(1)

(1), (2)

(3)

OK

OK

(1)

(1), (2)

(3)

Table 7.1-2:

Protection Schemes vs. Multiplexing Techniques

Notes: (1) (2) (3) (4)

OK if communication channel delay is kept within the relay’s delay compensation adjustment range. Dynamically allocated bandwidth is not recommended unless one has full control over delay No practical installations using Code Division Multiplexing for protection have been reported yet. Possible, if delay can be kept below 1ms

Protection schemes are described in Chapter 2.4.1. Multiplexing techniques are described in Chapter 4.4.

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Transport Networks (see 4.5.1) Protection Scheme ê All State Comparison Schemes (see 3.1.2) Direct Intertripping (see3.2.1.2) Digital Current Differential (see 3.1.1) Analog current differential (see 3.1.1) Pilot wire relays (50/60Hz) (see 3.1.1)

Service Networks (see 4.5.2)

PDH (see 4.4.2.1)

SDH / SONET (see 4.4.2.2)

Cell switched (i.e. ATM) (see 4.5.2.3)

Datagram (IP) (see 4.5.2.4)

Circuit switched (POTS, ISDN) (see 4.5.2.1)

Packet switched (X.25, Frame Relay) (see 4.5.2.2)

OK

OK

(1)

Not recommended (2), (4)

Not recommended (3)

Not recommended (4)

OK

OK

(1)

Not recommended (2)

Not recommended (3)

Not recommended (4)

(7)

(5), (7)

(1)

Not recommended (2), (6)

Not recommended (3)

Not possible (4)

(7)

(5), (7)

(1), (7)

Not possible (8)

Not recommended (3)

Not possible (8)

OK (9)

OK (9)

(1), (9)

Not possible (2)

Not recommended (3)

Not possible (4), (8)

Not recommended (3)

Not possible Analog: (4), (8) Digital: (4)

Not recommended (3)

Not recommended (4)

Phase Comparison (see 3.1.1.2)

(7)

(5), (7)

(1), (7)

Charge Comparison (see 3.1.1.3)

(7)

(5), (7)

(1), (7)

Table 7.1-3:

Analog: Not possible Digital: Not recommended (2), (6) Not recommended (2), (6)

Protection Schemes vs. Network Technologies

Notes: (1) (2) (3) (4) (5) (6) (7) (8) (9)

No experience yet, but no obvious reason why it will not work with proper network design and ATM QoS guarantees, see ANNEX A3. Might be applicable in future WAN with proper network design with QoS guarantees. Today (1999), QoS standards are still evolving. Not recommended unless the circuit is permanently established. Call setup is too slow or may be rejected. Connection may not be under the control of the service user or may be released inadvertently. Delay is non-deterministic and may be excessive under heavy traffic load OK, if requirement for propagation time symmetry is met, or protection terminals compensate for delay asymmetry Protection devices may have to cope with significant propagation time and propagation time variation OK if propagation time is within the relay's compensation range and delay variation is limited, see chapter 6, Table 6.1-2 Transparent transmission of analogue signals is not supported If propagation time of less than 1ms end-to-end is guaranteed by the network

Protection schemes are described in Chapter 2.4.1. Networks are described in Chapter 4.5.

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Table 7.1-4 broadly summarizes communication technologies and highlights some critical issues that have to be addressed even when a properly designed communication link/system/network is assumed. Type of communication

Critical issue

Dedicated point-topoint link

cost

Multiplexed PDH circuits (point-to-point)

propagation time

SDH / ATM based telecom networks

IP based WANs

propagation time

propagation time

Ethernet/IP based LANs

propagation time

Table 7.1-4:

Advantage for protection - ensures minimum propagation time - independent and deterministic - deterministic behavior - Network resilience - deterministic behavior with QoS guarantees

- Network resilience

- mainly for intrasubstation control & monitoring - little experience for protection

Configuration summary

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Disadvantage / Risk / Warning

Remedy

- single point-of-failure

- introduce redundancy

- single point-of-failure

- introduce redundancy - use terminal addressing - time stamping in protection relay - proper telecom network design and system engineering - use terminal addressing

- channel crossover - propagation time variation due to rerouting - channel crossover - propagation time variation - non deterministic behavior - missing QoS guarantees - propagation time variation - EMC if copper based LAN - single point of failure

- time stamping in protection relay - proper network design and system engineering - time stamping in protection relay - proper network design and system engineering


7.2 DESIGN CHECKLIST 7.2.1 Application 1. Are protection and communication scheme compatible? (Refer to tables in chapter 7.1) q What protection schemes are needed? q What communication schemes are available? q Are redundant protection and/or communication schemes required? 2. Does the selected teleprotection scheme provide the required performance? (Refer to chapter 6) q Teleprotection operating time (refer to Figure A1-5) q Dependability q Security q Addressing q Availability during power system fault conditions (EMC/RFI) q Telecommunication outage recovery time (sometimes referred to as 'protection switching time') 3. Redundant system evaluation? q Probability of telecommunication common mode failures - Power supplies and power source - Clocking scheme / architecture - Independent circuit routing - Telecom equipment and media

7.2.2 Interfaces (see Table 7.2-1) q q

q

q q

q

Type of interface Electrical / optical parameters - Connectivity - Electrical (voltages) - Optical (power budget) Physical / mechanical - Connectors - Pins / pinout Power requirements - Converters Cabling - Shielding / grounding - Type of cable / wire - Cable length - Who supplies Environment - EMC - Distance

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7.2.3 Contractual q q

q q

q

Performance monitoring Reporting - Communication system - Protection Guarantees Network policy - Priorities - Who decides - Disaster recovery plan Technical support - Hot line

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1

Pilot wire input

Voltage Current Signal dynamic range Impedance Insulation / Isolation

Voltage Current Signal dynamic range Impedance Insulation / Isolation

Impedance Receive level Isolation/Insulation

Impedance Transmit level Isolation voltage Insulation resistance

Max. voltage ON/OFF threshold voltage / current Load current

Wetting voltage Current rating (make, carry, break)

Bandwidth / Frequency spectrum

Bandwidth / Frequency spectrum Frequency restrictions? Level restrictions?

Pick-up time

contact position (N/O, N/C)

Crosstalk (Near-end, far-end)

max. leakage current

Electrical / optical parameters

Type of interface (a) or (b) refers to the definition in the Figures A1-1 to A1-4 in ANNEX A1.

n-wire (n = 2,3,4)?

output

4-wire

2-wire

4-wire

2-wire

Transistor input

Optocoupler input

Relay coil

Solid state contact (Transistor, SCR)

n-wire (n = 2,3,4)

input

output

input

output

Mechanical contact

Type of interface1

Table 7.2-1 Part 1: Contact Interfaces and Analogue Interfaces

Contact interface

Pilot wire circuit

Analog voice frequency circuit

Interface (a) in ANNEX A1

Interface (b) in ANNEX A1

Interface (a) in ANNEX A1

Electrical isolation

EMC generic standards

Signal flow directions

Type of terminals / wire termination

Wiring, wire diameters

Shielding


EMC standards



Wiring, wire diameters


EMC standards



Wiring, Wire diameters

Mechanical / cabling / other

1

Table 7.2-1:

input

output

(electrical)

Digital circuit

Fibre bandwidth (MHz * km)?

Multimode fibre

Optical signalling speed (pulse rate)

Optical wavelength

Optical signalling speed (pulse rate)

Spectral width of light source (LED, LASER)

Optical wavelength

Do pattern restrictions apply? (e.g. o/1 density)

Data rate?

Do data pattern restrictions apply? (e.g. 0/1 density)

Data rate and tolerance

Min. / Max. optical receive level?

Optical power launched into fibre?

Synchronous operation

Synchronous operation

Asynchronous operation

Electrical / optical parameters

Clock vs. data phasing

Clock master / slave?

Clock provisioning:

Data flow control?

Data format? (e.g. Start, Stop, Parity, Number of bits)

Checklist for Interface co-ordination between protection / teleprotection / telecommunication devices

Single mode fibre

Multimode fibre

Fibre bandwidth (MHz * km)?

Contra-directional? Co-directional? (preferred)

G.703

Single mode fibre

Device DTE or DCE designation?

Device DTE or DCE designation?

V.11/X.24

RS 485

RS 422

V24./V.28

RS232

Type of interface1

Type of interface (a) or (b) refers to the definitions in the Figures A1-1 to A1-4 in ANNEX A1.

Serial interface

Optical fibre interfaces

Table 7.2-1 Part 2: Digital Serial Interfaces and Optical Fibre Interfaces



Compatibility between F/O transceiver and fibre

Number of fibres needed

Fibre type, Standard

Connector type, Standard

Grounding and cable shielding


EMC standards


Which signals of the standard are being used?

Connector type (male / female ….) Connector pin-out

Mechanical / cabling / other


8 FUTURE TRENDS AND PROBLEMS TO BE SOLVED 8.1 TRENDS IN COMMUNICATION 8.1.1 General Network Development Traditional Teleprotection systems were designed for use on transmission mediums with very limited capacity. Teleprotection applications were limited both in terms of capacity and speed. Special communication networks were often designed to accommodate the peculiar requirements of Teleprotection. Two main factors have changed the traditional ‘legacy approach’: -

-

New transmission technologies, particularly in fibre, now provide abundant capacity for any utility application. Commercial transmission solutions already provide capacity in excess of 100 Gbit/s. Experimental work indicates that viable solutions with 10 times this capacity will be available soon. The ‘theoretical’ limit is still another order of magnitude above today’s experimental level. Integration of services is a main development in the telecommunications industry. This requires unified transport, switching and service-access systems. For this integration to take place mechanisms to guarantee a certain level of Quality is required. Quality of Service (QoS) -provisioning is now becoming an integral part of the network allowing any service to be connected/maintained with a guaranteed QoS-profile.

These new telecommunications and data -communications technologies will mean that utilities may be able to buy standard, off-the-shelf telecommunications products or even outsource their communications needs. Within the Communications/Protection area of the utilities, the focus is expected to shift from communication network design to a more facility managed approach.

8.1.2 Transport Technologies The ITU-T defined standard Synchronous Digital Hierarchy (SDH) or the similar (but by no means identical) North American standard SONET has evolved to a level of maturity where it’s stability is no longer questioned. SDH offers bit-rates from about 155 Mbit/s (STM-1) to 10 Gbit/s (STM-64). This technology allows traffic to be switched and routed through a network. High levels of availability may be achieved by exploring a certain (planned) degree of redundancy in the network topology. Through the Network Management, critical services may be allocated spare paths/containers in the network to give a desired degree of resilience. Thus a certain Quality of Service (QoS) may be planned for each individual application. For services transported over an SDH network, any desired Bit Error Rate (BER) in combination with Availability can in theory be reached depending on the resilience (and cost) of the network. The switching of a container through new network paths is highly undesirable for differential protection applications in that the receive and transmit paths may have different routes and thus different time-delays. Several solutions are being proposed to overcome this, such as adapting the differential protection schemes to independent reference clocks or to use ‘channel locking’ to fix the paths of the containers that hold the teleprotection information on a permanent basis. Dense Wavelength Division Multiplexing (DWDM) is a technology that is increasingly used in high capacity transport systems, utilizing, to a greater extent, the potential available bandwidth

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in the fibre and exploring optical technology to switch individual bandwidth-slots in each node of the network.

8.1.3 Networking Technologies Several networking technologies have evolved over the last few years. Of these, Asynchronous Transfer Mode (ATM) is expected to become a dominant technology. At present it offers many advantages to traditional networking and switching technologies: -

A possibility of integrating all services through an ubiquitous multiplexing and switching architecture. A possibility to give each service an individual QoS-profile at call-set-up or path-set-up. An efficient and fast switching architecture.

It has been shown that an ATM network may easily maintain and exceed the IEC Integrity level 3. Therefore security and, with a proper network topology, availability requirements may be met with ATM networks. Both absolute time delay and time delay variations are QoS-parameters that can be specified at call-set-up in an ATM-network - provided the ATM-nodes are designed for this. In practice this means that the Teleprotection messages are switched into high priority queues where no or little other traffic may be waiting, and that the interface access is sufficiently fast ( > 2 Mbit/s). However, the achieved time-domain granularity may not be sufficient in some networks to give the desired QoS-profile, unless special considerations are made. Several techniques may be employed to get around the time delay problems. They will need careful validation before they can be considered to be mature.

8.1.4 Service Access/Provisioning Technologies Internet Protocol technology (IP) will by many be associated with the global Internet that happens to be based on a set of protocols with the same name. The global Internet has a reputation for low performance. However, networks based on IP technology are exceptionally reliable when the network has been designed and dimensioned properly. Of special interest to the utility-applications is the fact that existing IP-Networks have been designed for the transmission of ‘delay-insensitive’ data. Such best effort technologies are not sufficient for power-utility applications. However, triggered by the onset of applications such as ‘multimedia’, new promising protocol-suits are emerging such as IPv6 (Latest Internet Protocol), RVSP (Resource Reservation Protocol) and RTP/RTCP (Real Time Protocol / associated Control Protocol). These new protocols will allow integration of diverse services with QoSspecific profiles for each application. Although security and dependability can be secured with the new protocols, transmission time is still a critical issue at present. It is still not clear whether this will be resolved with sufficient granularity on the IP-platform.

8.1.5 Integration of Technologies The division of the three technologies into three network levels above (transport, networking and service access) will not always be the case: -

ATM solutions are being implemented where the ATM technology accounts for a fair portion of the transport mechanism, only the lowest layer (multiplex section and downwards) is left to SDH components ATM has through its Adaptation Layers provisions for direct access to services

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-

IP is used as a networking technology especially for data-networks

A set of other and more detailed scenarios could be given. This, however, is outside the scope of this brochure. Essential for all these technologies is a Network Management system. Most systems on the market today are Element Management systems strongly coupled to a particular vendors equipment. There are some generic management platforms in existence today, but these generally require substantial work to be developed and maintained in a specific network environment consisting of multi-vendor equipment. Partly because of the complexity of the network management system, unifying and simplifying the network technologies and limiting the number of different equipment in a network is clearly an important task in smaller networks. Three Network Management technologies are under development at present: -

OSI-Management/Telecommunication Management Network (TMN) Simple Network Management Protocol (SNMP) Web-based Management

TMN is the only approach that gives a complete management view at present. The other two platforms may also be suited for a network view but this requires some extra (often) networkspecific development. It will still take time before any of the platforms have reached a stage where they can provide a multivendor/multitechnology network view without proprietary development.

8.1.6 New Technologies for QoS provision A number of new networking architectures and technologies offering an easier and more accurate way of QoS implementation are currently under development. Most of them are basically focused on video and TV broadcast service delivery. Amongst the latest developments, Dynamic Time-slot Multiplexing (DTM) and Dynamic Packet Transport (DPT) are the approaches that can better suit the QoS requirements of Teleprotection. Both technologies are based on the same working principle, the transport of IP packets over SDH/Sonet networks, being the main differences between them the actual implementation and the scalability limit. In both cases the bandwidth allocated to transport a flow of IP packets can change dynamically during the life of the flow. Although these technologies are currently focused towards MAN applications, there is no reason that prevents its application in a WAN. It can be noticed that whilst DPT is more focused towards IP transport, DTM seams to be a more generic approach that will be able to transport virtually any type of traffic. The development of these technologies has not been fully completed although some field installations are offering basic services and a comprehensive range of equipment has been announced. The process to consider these technologies a candidate for service provision will require a standardization process to be successfully accomplished. None of the main standardization bodies, the ITU-T (International Telecommunication Union), the IEC (International Electrotechnical Commission) or the IETF (Internet Engineering Task Force), have already accepted to start-up such a process.

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8.1.7 Intra- and inter-substation communication Computer networks are being introduced in power systems for intra- and inter- substation and control centre communications using local and wide area networks. Since numerical relays are increasingly introduced and they consist of microprocessors and digital devices, computer communications can be inherently applied for protective relay communications within and between substations. Various attempts and practices for substation integrated or coordinated protection, control and data acquisition are being performed. 8.1.7.1 Intra-substation communication Figure 8.1-1 shows an example for a basic configuration of intra-substation communications network [22]. The IEEE 802.4 Token Bus technology using optical star couplers is employed for coordinated substation digital protection and control systems. As the cycle time of a token in such LANs is normally not constant, the LAN system can maintain a constant cycle time by sending variable length dummy data from its main station for achieving synchronized sampling of current and voltage at the data acquisition and control units. The IEEE 802.3 Ethernet technology is used for the intra-substation control communications.

Figure 8.1-1:

Local and wide area networks for protection

Figure 8.1-2 shows a concept of an integrated substation and the relationship between applications from the Utility Communications Architecture (UCA) proposed by EPRI [23]. UCA provides integrated utilities computer communication protocols which enable a wide variety of utility information and control systems to share data seamlessly including real-time communications among corporate headquarters, power plants, control centres, substations, distribution automation equipment, and customer sites. Since UCA covers not only SCADA systems, RTUs and substation automation devices, but also microprocessor-based intelligent electronics devices (IED) including numerical relays, protective relay communications are also accommodated by such networks. The siginificant work in the UCA proposal is to identify how equipment should interoperate, and how IEDs (Intelligent Electronic Devices) publish and subscribe to each other's information. Substation devices and functions are represented as standardized object models in GOMSFE (Generic Object Models for Substation and Feeder Equipment) in the UCA proposal. Modeling

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provides a way to standardize information exchange between other models and devices. As the IEEE 802.3 Ethernet technology is proposed for protection communications, its dynamic performance has been evaluated by simulating the LAN traffic that would be generated by a particularly severe incident in a typical substation. Under the reasonable assumption that messages generated by a power system fault are spread over 1 millisecond, it was found that [24], [29]: - A 10 Mbit/s shared hub Ethernet network has limited performance, as it can deliver less than 20 messages in 4ms. - Three types of LANs (10 Mbit/s switched hub, 100 Mbit/s shared hub, and 100 Mbit/s switched hub) all can deliver 100 messages within 4 ms if messages are spread over 1 ms. This intra-substation communication project is co-ordinated with IEC activities to establish a process bus standard where a serial unidirectional point-to-point link using Ethernet is employed as an interim step (see also Chapter 8.3.1). Substation Controller

EMS SCADA

Engineering Workstation

Digital Communications

Relays

Meters

Oscillography

Equipment Diagnostics

Sensors

Figure 8.1-2:

Integrated substation applications dealt in UCA architecture

These intra-substation networks can be used for transmitting protection signals among protective relays inside a substation and for supervising and controlling protective relays. As supervisory and control information can be also transmitted to other substations or control centres using wide-area networks (WAN) connected to the intra-substation networks (LAN), the issue is whether one can use the substation LAN for communicating between protection relays in different substations or not. 8.1.7.2 Inter-substation communication Between the LAN and the WAN a Gateway is normally needed for protocol conversion as shown in Figure 8.1-3, because the protocols and network technologies (Chapter 4.5) utilized with the LAN and WAN are typically not the same. Although the delay depends on the message length and transmission speed of the WAN, one can broadly say that the main delays are introduced by the substation LANs (random delay for accessing the shared medium), the receiving Gateway (WAN-LAN protocol conversion) and by the propagation delay that is introduced by the WAN. In general, networking protocols and the data traffic they support can be characterized as being either connection-oriented or connectionless. With a connectionless protocol, no interaction between the terminals takes place prior to the exchange of data, and no fixed path through the network is set up. Data packets are routed

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through the network individually on a hop-per-hop basis. Connection-less thus also means that data packets that belong to the same message may follow different routes with different signal propagation delays through the network, and that the message transfer delay is determined by the slowest packet. As data packets may not arrive in the same sequence as they have been sent, they have to be re-assembled in the correct order at the receiving end. Because of its nondeterministic latency characteristic, connection-less type of communication has so far not been utilized for the transmission of protection signals across a WAN. With a connection-oriented protocol between two network terminals, the source and destination terminals interact with one another prior to the transport of data in order to set up and secure a fixed connection between them. All data packets or cells will follow the same route once the path has been established. A connection may be requested by the terminals through a standardized signalling protocol or dial-up procedure (e.g. PSTN or ISDN), or a permanent connection may be established by the network operator on a contractual basis (leased lines), or the network may provide a fixed and permanent connection by design ('hard-wired' circuits). Since WANs that are based on PDH and SDH technology (see Chapters 4.4.2, 4.6.2.1) provide a set of permanent communication channels with fixed bandwidths and minimum signal transfer delays, they are today the first choice in the selection of network technologies for the fast and reliable transmission of protection signals. More on the subject of WANs and telecommunication networks is found in Section 4 of this document. In conclusion, LANs and Gateways can introduce substantial and unpredictable time delays which may not comply with the total available time budget for protection signal transmission. Considering the stringent requirements for bounded signal propagation times (upper limits, propagation time variation and -symmetry) and Quality of Service guarantees, it seems more advisable to link protection relays directly via PDH or SDH channels (and perhaps ATM with the most stringent AAL Class 1), thus bypassing the LAN and the Gateway by using a deterministic serial data link to access the WAN. A very careful bandwidth and latency study would need to be performed in any case.

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Figure 8.1-3:

Substation LANs connected by a WAN

8.2 TRENDS IN PROTECTION 8.2.1 Considerations on new protection philosophies Recent developments in Information Technology and powerful solutions in (distributed) real-time processing are already applied in Process Automation. By using these technologies in HV substations for protection and control, there can be opportunities for the development of new protection philosophies in HV networks. Because of the emerging competitive market in the utility area, cost-saving aspects will be become more and more a key issue. Future protection schemes should be reliable, more intelligent and above all cheaper. Therefore, the use of new information technology and data processing will be necessary. Protection Systems In the present situation HV networks are protected by schemes and philosophies that are mainly based on the following technologies: - Analog comparison protection - State comparison protection - Teleprotection Today, most applied protection schemes only use limited communication facilities (point to point connections via analog links or sometimes digital 64 kbit/s channels). The relays used in present schemes mostly get only information on bay level. The connections to the transformers and HV devices in that bay are hard wired. In the future, the use of communication networks enables the possibility to collect real-time information from a much wider area than only from a substation bay. Summarized can be stated:

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Today:

-

Future:

-

Challenge:

-

Analog comparison protection with point-to-point communication of analogue quantities State comparison protection with point-to-point communication of binary state indication signals Analog comparison-, state comparison- and new wide-area protection systems LAN technology in substations to interconnect all equipment inside substations WAN based on SDH/ATM technology with enhanced real-time IP to interconnect substations with high speed communication Integration of protection and control in one device with separate functions program modules Voltage, current, active and reactive power flow values available as real-time information on LAN level in a substation and on WAN level in HV networks To do more and to use more intelligent protection solutions based on advanced real-time (<5ms delay times) communication possibilities on LAN level in substations and on WAN level in a HV network

Comparable developments in Substation Automation In the area of Substation Automation there is a trend (EPRI with UCA2 architecture) for applying fast LAN Technology to interconnect all the protection, control and HV equipment in the substation. The data-communication is assumed to have a high transmission speed and low latency. This concept can also be used in future protection schemes. The following Figure 8.2-1 represents a substation LAN which is connected to all the relevant equipment within the substation. The circle represents a LAN connected to the various substation devices (dotted lines). The square boxes represent integrated protection and control units.

LAN P/C

P/C

P/C

Figure 8.2-1:

Substation with LAN configuration

It is assumed that in substations, equipped which such a LAN, all the relevant information for protection and control is available on the LAN and can be used by all protection and control devices. This means that every relay can protect not only one bay, but it can perform protection calculations with information from all other bays within the entire substation. Relays can even

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send trip commands to the other circuit breakers of the various bays via the LAN connection. In this situation, distributed processing of protection functions and distributed redundancy will be possible in a substation. Therefore, separate redundant hardware modules for every single bay can be avoided. The amount of hardware may even be reduced to two or three dedicated devices that protect the entire substation and connected lines and transformers. In such a system, substantial cost-savings in protection schemes will be obtained. Developments in high-voltage networks As mentioned before there is a future trend for building substations with LAN communications. When this is realised in several substations, these can be interconnected by a WAN with high transmission speed and high performance to suit the protection schemes. The following Figure 8.2-2 represents a WAN.

WAN

P

LAN - A

LAN - B

LAN - C

Figure 8.2-2:

LAN - D

HV (sub)network with WAN

In Figure 8.2-2 a high-voltage (sub-)network consists of various substation LAN's which are interconnected by a WAN. In such a system, all information is available on every protection/control device within this WAN network. Depending on the possibilities of the relays, distributed processing of protection functions for analog comparison-, state comparison, intertripping and wide-area protection can be carried out. This communication structure creates new opportunities for future protection schemes on network level. In the telecommunication industry, path protection and re-routing is an integrated function. May be in the HV environment future protection can also co-operate with network management like in the telecommunications discipline, because a lot of real time information is available that can be used for fault clearing and automatic re-routing. This means that in every substation bay information of all other bays in the network is available, so it may be used for more advanced protection functionality and even for re-routing, because the information about network topology and traffic load is also available. Possibilities of (wide) area protection In case of a failure in a HV network which is equipped with a area protection the following options will become feasible:

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-

intelligent fault clearing based on more available information; complicated HV failures can be (re)evaluated at network level; redundant and secured (2 out of 3) calculation for critical issues; trip signals can be send to the most effective breakers; load effects caused by fault clearing, can be estimated on HV network level; automatic and fast network restoration facilities; smart load scheduling based on active- and reactive power balance; adaptive protection systems will be possible.

Application examples For the various options mentioned in chapter 5 a few examples of area protection functionality can be described: -

-

-

Petersen earthed HV-networks mostly include overhead lines and to some extent cables. In case of an earth fault in the cable part, a persisting earthing fault condition is not acceptable. In that situation a distributed area protection can determine the fault location fast and accurately and trip the faulty cable. Cross-country fault clearing can be better carried out by a second protection level, also by an area protection. The first protection level trips the fault selective or if not, only one of the two faults places. Because of the complexity of a cross-country failure, the fault clearing can be more selective with the second protection level and if necessary, a remaining earth fault can be tripped. Apart from that, the determination of the fault, that has to be cleared (the first fault) can be based on the network topology. A future reactive power protection could be build by processing voltage on HV busses and reactive power flows in HV- (sub-) networks. A smart load scheduling application in combination with reactive power protection can be used to minimise the number of customers that will be switched off.

Conclusion Application of LAN in substations in combination with a HV network WAN, with SDH, ATM and real-time Internet Protocol communication challenges the design of future protection systems. The existence of an information WAN on HV network level is the main advantage. This gives the possibility of creating distributed processing and inherent redundancy. This means that the number of protection devices, including wiring to the HV components, can be reduced without loss of availability, dependability and security of the system, if the stringent real-time requirements can be met by the proposed concept.

8.3 OPEN ISSUES AND PROBLEMS TO BE SOLVED 8.3.1 Protection relay interoperability Protection relay interoperability relates to the ability of protection relays from different manufacturers to perform a stated function in a collaborating way. As for today, interoperability has only be possible between protection relays in state comparison protection schemes (see Chapter 3.1.2) that use simple contact interfaces (see Chapter 5.1) for exchanging binary state information between the local protection relay and the relay at the remote end. The reason for this favorable situation can be attributed to the fact that the communication between relays is reduced to the simple exchange of "Trip" or "Do not trip" commands, and that the protection relays can perform their impedance/distance algorithms at

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each line end independently from each other even when proprietary algorithms are used. Thus only a minimum of co-ordination - if any - between relays is needed. The situation is however less satisfactory for analog comparison protection schemes like current differential protection, or for state comparison protection schemes when the teleprotection function is integrated in the protection relay. Main reasons for incompatibility are proprietary protection algorithms and/or proprietary communication protocols due to missing standards or due to the choice of different options from existing standards. The standards to be developed for protection relay interoperability would have to encompass at least Layers 1 to 4 in the OSI reference model (Figure 4.5-3) when networking functionality shall be included in the (tele-)protection device. True interoperability between protection relays would require to develop standards for all layers of the OSI reference model, including the Application Layer. Whether interoperability of protection relays from different vendors would justify the development of a full set of complex standards is questionable. Nevertheless, it is becoming clear that for future designs consideration should also be given to emerging standards in related areas of utility communication, and that existing standards should be applied whenever possible and economically justified. Some ongoing - and for future teleprotection possibly relevant - standardization activities are: -

-

-

IEC TC57: Communication Networks and Systems in Substations. Presently developing IEC 61850 and companion standards, with one objective being to incorporate the UCA 2.0 (Universal Communication Architecture, Revision 2) proposal from EPRI. IEC TC57: Communication Networks and Systems in Substations. FDIS (Final Draft International Standard) of IEC 60870-5-104, specifying the transport of teleoperation data (IEC 60870-5-101) using TCP/IP. IEEE PES/PSRC: Presently developing a proposal for N x 64 kbit/s optical fibre interfaces between a teleprotection device and a multiplexer equipment. IEEE PES: Published a trial use recommended practice (IEEE P1379) for data communication that recommends both DNP (Distributed Network Protocol) from the DNP User Group Technical Committee and IEC 60870-5 protocols as originally developed for SCADA/EMS systems. May be considered to be used for teleprotection. DIN/VDE (Germany): Currently developing a national standard dealing with communication protocols and data formats for protective relaying, with focus on layers 1 and 2 of the OSI reference model.

As for today, protection systems using telecommunication are still some distance away from "plug-and-play" solutions, particularly for analog comparison schemes. This is one of the areas where forthcoming standardization may show promise.

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9 CONCLUSIONS The Telecommunications evolution and the IT revolution has resulted in the need to reconsider tried and tested methods of delivering teleprotection schemes and their associated bearer services. The roll out of new communication network technologies presents possible opportunities for cost efficient and advanced solutions for teleprotection systems. Wider bandwidths offer scope for the transmission of higher speed data for improved protection functionality and potentially better reliability, combined with rich analytical information. New network technologies can provide improvements in security, availability and resilience, as modern networks are configurable from a centralized network management system and support automatic re-routing in case of single link failures. Service availability may therefore be better than for legacy network technologies or simple point-to-point configurations, and proper network design can ensure that the risk of single point of failures is reduced. Network technologies that permit delivery of messages to multiple locations, or collection of data from multiple locations will be essential for novel schemes such as wide area protection or system wide protection, stabilizing protection and centralized protection. The continued development of transport modes that can offer this type of service must be recognized and suitable design concepts considered. Optical fibre offers the best solution in an electrical hostile environment and is generally accepted as the most appropriate communication medium for developing service network platforms for the Electricity Supply Industry. New communication network technologies have however also brought up new problems and new challenges to protection, mostly related to the non-deterministic signal transfer time behavior. Propagation time, its variation and symmetry are all critical characteristics that require careful consideration with respect to teleprotection requirements. The inherent problems associated with latency and signal transfer delay variation of “bandwidth on demand” or "best effort" techniques may be overcome with the availability of higher bandwidths in the core and access network, together with controlled Quality of Service mechanisms. Quality of Service guarantees and sufficiently low network latency may however only be available at significant additional cost. Measures to overcome problems related to signal transfer time variations introduced by the communication network can also be taken in the protection relay, e.g. by time stamping of measured quantities (samples). It is however obvious that some legacy protection and teleprotection equipment may not work with, or be able to take advantage of new Telecom / IT network technologies, because they have been designed for deterministic (with respect to bandwidth and latency) communication channels. The economic effectiveness of high capacity service connections with performance guarantees will always be subject to business justification. Protection is a niche application that has specific characteristic requirements associated with the bearer services provided by the core business Telecommunications infrastructure or by the Public Telephone Operator (PTO). Bearer services for Teleprotection often represent less than 0.001% of network services provided by third party Telecom Service Providers. It is becoming clear that the Electricity Supply Industry cannot drive the development of products and standards in the Telecommunications Industry. The unique requirements for stringent signal propagation delays, delay variation and delay

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symmetry are not common or even similar to any other customers needs. Future Telecom / IT platforms and services offered by third party network service providers may not be able to provide circuits that are compliant with existing and legacy teleprotection requirements. Protection engineers must understand the changes that will be introduced by third party providers, and develop solutions that are compatible with the new platforms being offered. If these changes can not be accommodated, the Electricity Supply Industry will require to develop its own Network for operational needs that can guarantee the long term availability of compliant and resilient bearer services for the teleprotection and other mission-critical services needs. Many utilities are already developing their own networks that are dedicated to their operational needs using optical fibre infrastructure solutions. With an optical fibre infrastructure in place, the availability of high capacity bandwidth at no additional cost is now implicit. Larger bandwidths (more information within the acceptable delay) may open the door for new and improved protection schemes such as multi terminal line protection, selectivity improvement for double circuit lines and wide-area protection, to name a few. The cost effectiveness and operational advantages of ensuring continued support of legacy teleprotection services and the ability to accommodate new schemes on a compliant and managed network are as yet unproven. Organizational changes that demand outsourcing and facility management of assets may jeopardize the integrity and security of the telecommunications platform that provides the essential elements of the electrical power systems protection schemes. The combined understanding of Electricity Supply Industry manufactures and the industries' Protection / Telecommunication Engineers is needed to ensure that risks are minimized and the most appropriate solutions are adopted.

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ANNEX A1 TELEPROTECTION SYSTEM CONFIGURATIONS (a) Protection equipment or Protection function

(b) Teleprotection equipment or Teleprotection function

(a)



Protection equipment or Protection function

Telecommunication system Teleprotection system

Figure A1-1:

General teleprotection system configuration

(a) Protection equipment or Protection function


Figure A1-2:

Teleprotection equipment or Teleprotection function

Dedicated medium or dedicated link (i.e. non-shared)


Multiplexer


Multiplexer

Other functions / services


Figure A1-4:


Teleprotection connected via multiplexed communication system (shared medium)


(a)

(b)

Other functions / services

Figure A1-3:


Teleprotection connected via dedicated medium or dedicated link (non-shared)


(a)

(b)

(a)

(b) Telecommunication network or WAN


Teleprotection connected via telecommunication network

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Legend to Figures A1-1 to A1-4: Protection equipment; Protection function:

Performs the protection function, excluding communication. Synonym for protection relay.

Teleprotection equipment; Teleprotection function:

Converts the information from the protection equipment (relay) into a signal suitable for reliable transmission over a telecommunication link /system/network. Includes all functions that are necessary to obtain the desired dependability, security, and data integrity. Performs intermediate function between protection and telecommunication. May also multiplex signals from several protection equipments/functions, status indications etc. onto a single telecommunication channel. Typical layouts: • Separate equipment for command-based state comparison protection schemes. Interfaces (a) and (b) are accessible. • May be integrated into the protection equipment, particularly for analog comparison protection schemes. Interface (b) is accessible. Interface (a) is equipment internal and normally not accessible. • May be integrated into telecom terminal equipment. Interface (a) is accessible. Interface (b) is equipment internal and normally not accessible.

Telecommunication system; Telecommunication link:

System composed of telecommunication equipment and the associated physical link / medium required to transmit information signals across a distance

Dedicated medium; Dedicated link:

A medium / link / circuit / channel that carries only the teleprotection service.

Multiplexer:

A device which combines several signals or services onto a common medium for transmission

Telecommunication network:

A set of communication and switching devices that work in a collaborating way to provide a telecommunication service between access points distributed over a wide area

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Fault inception

Fault clearing TC 42 - 210 ms Fault clearing time for a protection system TB 12 - 130 ms Protection operating time TA 2 - 70 ms Teleprotection operating time (Overall operating time for a teleprotection system)

(a)

(a)

Propagation time

(b)

Teleprotection transmit function

Fault recognition time

Time for initiating transmit action

Telecommunication system (-circuit / -link / -network) 1)

10 - 30ms

Teleprotection receive function

Protection equipment

Circuit breaker

Propagation time 2)

0 - 5ms

Protection equipment

(b)

Selection and decision time, incl. O/P circuits 1…10…40ms 3)

1 - 5ms

Additional delay due to disturbance

0 - 20ms

Relay decision time

0…10…30ms 4)

Operating time including arcing time

30 - 80ms

To 2 - 45ms Nominal transmission time (propagation time not included) Tac 2 - 65 ms Maximum actual transmission time under disturbed channel conditions for a defined dependability and signal to noise ratio or BER. (propagation time not included) Tpac 12 - 125 ms Maximum actual protection operating time time under disturbed channel conditions for a defined dependability and signal to noise ratio or BER. (propagation time not included) Notes: 1) 2)

3) 4)

Including the physical medium (cable, fibre) and the telecommunication intermediate- and terminal equipment 0 ms applies when interface (b) is connected back to back. The propagation delay depends on the route length and on the number and type of intermediate and terminal equipment involved. 5 ms may be difficult to achieve for networks. However, network latency will normally be offset by short times in the teleprotection receive function (typ. 2 … 10 ms) for digital systems operating at 64 kbit/s or higher, such that the requirement for the overall operating time can be met. See note 3) Typical values for analog (narrowband) teleprotection systems are in the range 10 to 30 ms Typical values for digital (wideband) teleprotection systems operating at 64kbit/s or higher are in the range 2 to 10 ms 0 ms can apply to intertripping (direct tripping equipment) 2 to 10 ms are typical for state comparison protection relays (time for phase selection and O/P circuits operation) 10 to 30 ms are typical for unit protection relays (time for signal comparison and O/P circuits operation)

Figure A1-5:

Typical operating times for protection systems incorporating teleprotection

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Signal propagation time 5-10µs/km

1

Signal round trip time 250-280ms (up + down) Signal round trip time ~100ms (up + down) Signal round trip time 10…30ms (up + down) Signal propagation time in cable ~5µs/km + max. 0.6ms per 64kbit/s to 2Mbit/s Multiplexer Signal propagation time in cable 5µs/km + max. 0.6ms per 64 kbit/s to 2Mbit/s Mux + 15µs per 2/8 Mbit/s Mux + 1µs per repeater Signal propagation time in cable ~5µs/km + 35µs for 2Mbit/s port to STM1 + 40µs STM1 aggregate + 110µs STM1 to 2 Mbit/s port Signal propagation time in cable ~5µs/km + 1 ms processing time ATM Edge Mux + 6 ms packetization delay for 64kbit data + 0.5ms for each ATM core switch Note: Normally, the QoS parameter for Maximum Cell Transfer Delay (MaxCTD) will be defined. E.g., ITU-T I.356 suggests an upper bound on the CTD of 400ms for an international ATM connection. Non-deterministic; No guarantee 5…. 15ms shared hub / 1… 2ms switched hub + 6 ….12ms WAN time 1….. 3ms shared hub / < 1ms switched hub + 6 …12ms WAN time

Signal propagation time in fibre cable ~5µs/km

Few ms; No guarantee Few ms; No guarantee

No guarantee

< 3ms cell delay variation (CDV) for QoS class1 (ITU-T I.356)

< 3ms typical

Not applicable Not applicable

No data available

Not applicable

Critical; No guarantee No data available

(Similar to SDH)

< 120µs per ADM or repeater

< 1ms with bidirectional protection

No data available

~0.6ms

Not applicable

Not applicable

Not applicable

Not applicable

Not applicable

Not applicable (Pt-Pt links) Not applicable (Pt-Pt links) Not applicable (Pt-Pt links)

Add / Drop time

< 1ms

< 0.1ms

< 1ms

< 1ms

>> 1ms

No data available

No data available >> 1ms

No data available

< 1ms if the same route < 1ms if the same route

< 1ms

Propagation time symmetry (Differential delay) < 1ms if the same route

No data available

<< 1ms

<< 1ms

<< 1ms

<< 1ms

Propagation time variation

< 10-6 < 10-6

exists (Sync. failure) exists (Sync. failure)

exists (Sync. failure)

~15min ~ 1ms for single link failure; depending on system vendor

Not applicable

Not applicable

Not applicable

same as SDH if SDH is transport layer

Not applicable

exists (Routing error) exists (addressing error) exists (addressing error)

> 64kbit/s

< 10

> 64kbit/s -5

> 64kbit/s on demand

> 64kbit/s

> 64kbit/s

< 10-5

< 10-5

Cell misinsertion ratio (CMR) < 1/day < 10-6 (ITU-T I.356)

< 10-6

> 64kbit/s

> 64kbit/s

> 64kbit/s

< 10

< 10-3

No data available No data available

No data available

No data available

> 64kbit/s

> 64kbit/s

< 10-6

> 64kbit/s

> 64kbit/s

few kHz; << 64 kbit/s 4 … 8kHz; < 64kbit/s1

Bandwidth or Capacity

< 10-3

< 10-3

Bit Error Rate (typical) Not applicable

-3

No data available

low (human error) low (human error) low (human error) low (human error)

Channel crossover probability

< 10-3

No data available

Not applicable

Not applicable

Not applicable

Not applicable

Routing recovery time / Network restoration time

For voice and data only. For protection signal transmission, the capacity is sufficient for binary protection commands (state comparison schemes) or for non-segregated phase comparison applications (unit schemes).

Ethernet 100MB LAN

Ethernet 10MB LAN

IP networks

ATM networks

SDH networks

PDH networks

PCM cable links

LEO Satellites

MEO Satellites

GEO Satellites

Fibre Optic Cables

Power Line Carrier links Signal propagation time on HV line 3.3µs/km (High Voltage Lines) (+ ~1.5ms for each PLC terminal equipment) Signal propagation time in air 3.3µs/km Microwave Links. (+ ~1…2ms for each terminal equipment)

Pilot Wires

Propagation time (from interfaces (b) to (b) in ANNEX A1)

Summary of transmission characteristics of various media and telecommunication systems. All values are approximate.

ANNEX A2 TELECOMMUNICATION SYSTEMS CHARACTERISTICS


ANNEX A3 QUALITY OF SERVICE A3.1 INTRODUCTION TO QOS Quality of Service becomes a hot topic for power network protection when the telecommunication service is rented from a service provider rather than using dedicated links that are under the control of the service user. Transmission media and circuits are typically unknown to the service user in this case, and traditional planning methods may no longer apply. Quality of Service is something that is often talked about as an important user requirement, but in the past little has been done in the standards area to give users any real influence over the QoS they may be able to obtain for the services they require. Typically, service characteristics are fixed when systems are built or when communications services are subscribed to, after which there is not much that users can do. OSI network and transport layer protocols allow limited signaling of QoS requirements, but in practice, they offer little more than the ability to choose throughput classes when X.25 is used. However, this situation is changing. Real-time applications can differ enormously in their requirements for throughput and transit delay. Power network protection has extremely stringent requirements for delivery within known time-windows, and will often need to use the same communication network as other traffic. Therefore, the demand is growing for power utility users to be able to state or negotiate the QoS they need. Much of the work on the dynamic treatment of QoS is still at the research stage. QoS mechanisms are being developed for time-critical communications, the Internet, multi-media communications and so on. ITU-T and ISO/IEC are attempting to help in all this by developing some common concepts and terminology (so that not everything is called a QoS parameter), and by providing a central place where QoS methods and mechanisms can be published. In the following paragraphs, a general introduction into the concept of QoS is given, followed by a more detailed discussion regarding its application for ATM, where the concept of QoS has been consistently adopted from the beginning. QoS Parameters In the common case, everything related to the desired quality of the traffic can be referred to as Quality of Service parameters. One may distinguish between user-level QoS, application-level QoS, system level QoS and at even lower levels. Parameters considered here relate to the network. Services may be broadly categorized in a qualitative manner into the following service classes or service categories: - Deterministic Typical use is for "hard" real-time applications - Statistical Typical use is for "soft" real-time applications - Best effort Everything else, no guarantees are made This crude approximation will probably not be sufficient in many cases. Thus, the following quantitative QoS parameters may be specified:

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-

Throughput can be based both on average data rate and/or peak data rate. The ratio of peak rate and average rate of data streams is known as burstiness. Reliability relates to a certain probability of data loss which can be tolerated Delay can be specified based on an absolute or probabilistic bound. Delay variation or Jitter is the (short-term) variation in delay a message experiences.

Parameters are static if they are valid for the entire duration of the connection. If they can be changed while a transmission is in progress, they are said to be dynamic.

A3.2 QOS DEFINITION IN ATM NETWORKS As the concept of QoS has been thoroughly adopted for the specification of ATM layer services, the ATM QoS concept is discussed in some detail below and follows [26]. ATM service classes The two main bodies that establish ATM specifications are the ATM-Forum and the ITU-T. A unified approach to the definition of ATM services in the ATM Forum and in ITU-T is presented in the Table below. The ATM-Forum uses Service Categories instead of ATM Tranfer Capabilities and QoS classes as defined by the ITU-T. Since different names are adopted to define concepts that are very similar in purpose, the differences are more apparent than real. The close relationships that have been established between the two bodies give a further chance to harmonize their documents in the course of their parallel development. An ATM Service Category (ATM Forum name) or ATM Transfer Capability (ITU-T name) is intended to represent a class of ATM connections that have homogeneous characteristics in terms of traffic pattern, QoS requirements and possible use of control mechanisms, making it suitable for a given type of network resource allocation. The ATM-Forum has split the Service Categories into real-time traffic (CBR and rt-VBR) and non-real-time traffic (nrt-VBR, ABR and UBR). In the ITU-T, real-time and non-real-time are included in the QoS classes rather than in the ATM Transfer Capabilities. The ATM-Forum on the other hand permits the use of the ITU-T QoS classes, thus the differences are more apparent than real. A first classification of these services/capabilities may be seen from a network resource allocation viewpoint. We can identify: - A category based on a constant (maximum) bandwidth allocation. This is called Constant Bit Rate (CBR) in the ATM Forum and Deterministic Bit Rate (DBR) in ITU-T. - A category based on a statistical (average) bandwidth allocation. This corresponds to the ATM Forum Variable Bit Rate (VBR) and ITU-T Statistical Bit Rate (SBR). The ATM Forum further divides VBR into real-time (rt-VBR) and non-real-time (nrt-VBR), depending on the QoS requirements. - A category based on "elastic" bandwidth allocation, where the amount of reserved resources varies with time, depending on network availability. This is the Available Bit Rate (ABR). The same name is used both in the ATM Forum and in ITU-T. - A category considered only in the ATM Forum is the Unspecified Bit Rate (UBR). No explicit resource allocation is performed; neither bandwidth nor QoS objectives are specified. In the rest of this document, the service categories are addressed based on the ATM Forum "Traffic Management Specification" only.

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Correlation of ATM Forum and ITU-T ATM services: ATM Forum TM 4.0:

ITU-T I.371:

ATM SERVICE CATEGORY

ATM TRANSFER CAPABILITY

Traffic characteristics, Purpose -

Constant Bit Rate (CBR)

Deterministic Bit Rate (DBR)

-

Real-Time Variable Bit Rate (rt-VBR)

(for further study) -

Non-Real-Time Variable Bit Rate (nrt-VBR)

Statistical Bit Rate (SBR)

-

Available Bit Rate (ABR)

Available Bit Rate (ABR)

-

Unspecified Bit Rate (UBR)

(no equivalent)

-

Real-time / time-delay sensitive data with QoS guarantees Low cell delay variation Circuit emulation Bursty real-time data Efficient use of network resources for delay sensitive data Statistical multiplexing

Bursty non-real-time data Efficient use of network resources for delay variation insensitive data Statistical multiplexing Dynamic bandwidth Flow control with feedback Network resource exploitation Best effort delivery, no guarantees Applications with vague throughput and delay requirements Low cost

Application examples

-

Videoconferencing Telephony Audio/Video Distribution Teleprotection

-

-

Voice communication with bandwidth compression and silence suppression Interactive compressed video LAN interconnection SCADA / EMS Future teleprotection? Response-time critical transaction processing applications (e.g., banking transactions) Multimedia E-mail No teleprotection LAN interconnection LAN emulation TCP/IP E-Mail File transfer No teleprotection

-

E-Mail File transfer Messaging TCP/IP No teleprotection

-

Traffic Parameters A source traffic parameter describes an inherent characteristic of a source. The following parameters are considered for the purpose of defining the Service Categories : - Peak Cell Rate (PCR) - Sustainable Cell Rate (SCR) - Maximum Burst Size (MBS) and Burst Tolerance (BT) - Minimum Cell Rate (MCR) - Cell Delay Variation Tolerance (CDVT) - QoS Parameters The traffic contract defines how the network should react when parameters that characterize the traffic are exceeded. A traffic contract may be established either when the network user first subscribes to a network service, or dynamically via user interface signaling or network management negotiation. QoS Parameters The QoS parameters selected to correspond to a network performance objective may be negotiated between the end-systems and the network, e.g., via signalling procedures, or can be taken as default. One or more values of the QoS parameters may be offered on a per

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connection basis: - Peak-to-peak Cell Delay Variation (CDV) - Maximum Cell Transfer Delay (maxCTD) - Cell Loss Ratio (CLR) A number of additional QoS parameters have been identified, but their negotiation is not foreseen but are assigned at call set-up, e.g.: - Cell Error Ratio (CER) - Severely Errored Cell Block Ratio (SECBR) - Cell Misinsertion Rate (CMR).

A3.2.1

ATM Service Categories

Constant Bit Rate (CBR) The CBR service category is used by connections that request a fixed (static) amount of bandwidth, characterized by a Peak Cell Rate (PCR) value that is continuously available during the connection lifetime. The source may emit cells at or below the PCR at any time, and for any duration. Thus, the only traffic parameter specified for the CBR service category is the Peak Cell Rate (PCR). The CBR service category is intended for real-time applications, i.e., those requiring tightly constrained Cell Transfer Delay (CTD) and Cell Delay Variation (CDV). It would be appropriate for protection signal transmission, provided that the signal propagation time requirements (see ANNEX A1) that are influenced by the CTD and CDV parameters can be met. The basic commitment made by the network is that once the connection is established, the negotiated QoS is assured to all cells conforming to the relevant conformance tests. It is the end station's responsibility to send only traffic that is compliant with the contract (PCR). The network checks the traffic against the contract, and noncompliant cells are discarded. Real-Time Variable Bit Rate (rt-VBR) Like CBR, VBR is a reserved bandwidth service. The real-time VBR service category is intended for time-sensitive applications, (i.e., those requiring tightly constrained delay and delay variation such as voice and video). Sources are expected to transmit at a rate which varies with time. Equivalently, the source can be described as "bursty". Traffic parameters are Peak Cell Rate (PCR), Sustainable Cell Rate (SCR) and Maximum Burst Size (MBS). Cells which are delayed beyond the value specified by CTD are assumed to be of significantly less value to the application. Real-time VBR service may support statistical multiplexing of realtime sources. Rt-VBR may be a candidate to be studied for future teleprotection implementations, as protection systems typically need little communication capacity during the guard state and request significant communication capacity in the operate state. Non-Real-Time Variable Bit Rate (nrt-VBR) The non-real time VBR service category is intended for applications which have bursty traffic characteristics and do not have tight constraints on delay and delay variation. As for rt-VBR, traffic parameters are PCR, SCR and MBS. For those cells which are transferred within the traffic contract, the application expects a low Cell Loss Ratio (CLR). For all cells, it expects a bound on the Cell Transfer Delay (CTD). Non-

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real time VBR service may support statistical multiplexing of connections. Since the transmission of protection signals is extremely time sensitive, non-real-time VBR is not applicable for teleprotection. Available Bit Rate (ABR) The Available Bit Rate (ABR) is a service category intended for sources having the ability to reduce or increase their information rate if the network requires them to do so. ABR service can be seen as a mix of reserved and non-reserved bandwidth service. Periodically, a connection polls the network and, based upon the feedback it receives, adjusts its transmission rate. Polling is done by Resource Management (RM) cells sent by the source and looped back at the destination so that the network elements and the destination can provide feedback information. It is recognized that there are many applications having vague requirements for throughput. They can be expressed as ranges of acceptable values, e.g., a maximum and a minimum, rather than as an average value (that is typical for the VBR category). Traffic parameters, which the end system may specify, are therefore a maximum required bandwidth and a minimum usable bandwidth. These are designated as the Peak Cell Rate (PCR) and the Minimum Cell Rate (MCR), respectively. The MCR may be specified as zero. Although no specific QoS parameter is negotiated with the ABR, it is expected that an endsystem that adapts its traffic in accordance with the feedback will experience a low Cell Loss Ratio (CLR) and obtain a fair share of the available bandwidth according to a network specific allocation policy. Cell Delay Variation (CDV) is not controlled in this service, although admitted cells are not delayed unnecessarily. Since ABR service is not (as specified at present) intended to support real-time applications and as no specific QoS parameters are negotiated, the ABR service category is not applicable to protection signal transmission. Unspecified Bit Rate (UBR) The Unspecified Bit Rate (UBR) service category is a "best effort" service intended for noncritical applications, which do not require tightly constrained delay and delay variation, nor a specified quality of service. UBR sources are expected to transmit non-continuous bursts of cells. UBR service supports a high degree of statistical multiplexing among sources. As UBR service does not specify traffic related service guarantees, it is not applicable for protection signal transmission. The Table below summarizes Service Category Attributes and QoS Guarantees.

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ATM Service Category ê

Traffic Parameters ê

QoS Parameters (Guarantees) Min. Cell Loss (CLR)

Delay and Delay Variance (maxCTD, CDV)

Bandwidth

Use of Feedback Control

CBR

PCR, CDVT

YES

YES

YES

NO

rt-VBR

PCR, SCR, MBS, CDVT

YES

YES

YES

NO

nrt-VBR

PRC, SCR, MBS, CDVT

YES

NO

YES

NO

ABR

PCR, MCR, CDVT (+ traffic behavior parameters)

YES

NO

YES

YES

UBR

PCR, CDVT

NO

NO

NO

NO

A3.2.2

ATM over SDH/SONET

ATM may use various technologies as its transport vehicle (physical layer). Frequently, SDH/SONET transport systems will be used. Errors introduced by the transport system as well as other impairments will have a negative impact on the QoS. The Table below shows some sources of degradation for the ATM QoS parameters. QoS Parameters

General network design

ATM specific

Link / network / medium parameters

CER Cell Error Ratio Propagation Time Propagation Time Variation Bit Errors / Media Errors Switch Architecture Buffer Capacity Traffic Load / Resource Allocation Number of Nodes / Hops Network / Hardware Failures

CLR Cell Loss Ratio

CMR Cell Misinsertion Ratio

maxCTD Maximum Cell Transfer Delay

CDV Cell Delay Variation

X

X

X

X

X

X

X

X

X

X

X

X

X

X (except CBR)

X (except CBR)

X (except CBR)

X (except CBR)

X

X

X

X

X

The following Table shows the expected impact of QoS parameters onto general communication performance parameters at interface (b) in the Figures in ANNEX A1, and their possible adverse impact onto the (tele)protection function. It is noted that the table is speculative as little experience yet exists with protection signal transmission over ATM.

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ATM QoS parameters

Communication performance parameters BER Jitter Bit Error Rate

A3.2.3

CER Cell Error Ratio CLR Cell Loss Ratio CMR Cell Misinsertion Ratio maxCTD Maximum Cell Transfer Delay CDV Cell Delay Variation

(Tele-)Protection parameters Dependability

Security

Transmission Time

X

X

X

X X

X

X

X

X X

(X)

X

X

X

(X)

(X)

Applications Summary

The Table below is an attempt to sum up the indications outlined in this section as related to typical power utility applications. It is not intended to create a restrictive correspondence between the identified application areas and ATM-layer services, and should therefore not be taken restrictively. Application areas for ATM service categories: APPLICATION AREA ê Critical Data, not delay sensitive Critical Data, delay sensitive Circuit Emulation LAN Interconnect LAN Emulation State Comparison Protection Signal Transmission Analog Comparison Protection Signal Transmission

CBR

rt-VBR

nrt-VBR

ABR

UBR

fair

fair

optimum

fair

not suitable

good

good

not suitable

not suitable

not suitable

optimum

good

not suitable

not suitable

not suitable

fair

fair

good

optimum

good

good

fair…optimum ?

not suitable

not suitable

not suitable

good

fair…optimum ?

not suitable

not suitable

not suitable

The ratings (optimum - good - fair - not suitable) refer to the efficiency/cost advantages the use of ATM may have for the stated application areas. The ratings given for protection signal transmission are speculative and apply subject to the condition that native ATM interfaces are available and that the requirements for the end-to-end signal propagation time can be fulfilled (see Figure A1-5).

A3.3 QOS DEFINITION IN IP NETWORKS Although IP networks have been traditionally considered as best-effort networks, the new QoS architecture has widened their applications. The Internet Engineering Task Force IETF has defined two models for providing QoS: the Integrated Services (Int-serv) and the Differentiated Services (Diff-serv). The Integrated Services model is based on the resource reservation paradigm. Before data

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are transmitted, the applications must set-up a path and reserve the resources. This working principle resembles the one used in circuit switched networks therefore being adequate for nonelastic applications such as voice telephony, or Protection communication in the future. The complete implementation of this architecture requires QoS aware applications or the addition of Gateways between the legacy applications and the Integrated Services core since the network has to be informed about the profile of the traffic offered and the QoS requirements of the user. Differentiated Services is based on the definition of different classes of services. The packets are marketed differently depending on the service class their application/flow belongs to. Therefore, Differentiated Services is essentially an evolution of the static-priority scheme. Whereas Int-serv architecture guarantee and End-to-End QoS, that is to say, application-toapplication, the Diff-serv architecture guarantees the QoS only in its own domain. Differentiated Services is an evolving architecture and in fact, still an immature technology under discussion by different working groups of the IETF. The provision of QoS for the Teleprotection application is a must, since both the bandwidth and the delay of the virtual channel established throughout the network to support the protection relay communication has to be guaranteed in a deterministic way. Existing protection relays do not include IP Ethernet interface; therefore, we have to focus on the new and evolving technologies. In this field, the new Utility Communication Architecture UCA is the most relevant example. The UCA architecture is based on the use of standard protocol stacks. Two tracks with different profiles have been defined including both the ISO and the TCP/IP approaches. Data interchange with devices in real-time networking environments is accomplished by adding the following specific components: -

Generic Object Models for Substation and Feeder Equipment, (GOMSFE) which defines a set of object models for use across a broad range of typical utility devices; Generic Object Oriented Substation Event, (GOOSE) which allows a device to broadcast status information on the local segment; Common Application Service Models, (CASM) which defines a standardized set of abstract services supporting the UCA object models, as well as the methods of mapping the services to the Manufacturing Message Services (MMS).

None of these application elements have been designed to specify their QoS requirements to the network. Therefore, the provision of Teleprotection service will require static QoS requirements that will have to be introduced in the Network Management Centre. Three classes of services could be identified in IP QoS networks: Guaranteed Service, Controlled Load and Best-Effort. The first one is used to support those applications that are non-tolerant to uncontrolled delays and/or losses. The second is used to support elastic applications, that is to say, applications that can control the traffic flow that they are offering to the network and therefore can tolerate some changes in delay and throughput, whereas that the third one is used by those applications that do not require QoS. Teleprotection service is associated with the Guaranteed Service class. Nevertheless, special

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care has to be taken to verify that the network is providing deterministic guarantees. To achieve this, the network architecture requires a set of components to be provided. The principle of QoS assurance is based on an interaction between the user and the network. This interaction is expressed by means of a Traffic Contract or agreement that includes the commitments of both parties. The total delay has two parts: a fixed delay caused by the transmission delay of the path and a queuing delay caused by the intermediate nodes. The fixed delay depends only on the chosen path and is not determined by the QoS mechanism but by the setup mechanism. The queuing delay is determined and controlled by the QoS mechanism provided by the network. The queuing delay is primarily a function of two parameters the token bucket size and the data rate. These two values are under the application control, and therefore, the application can estimate the delay and if it is larger than required, the bucket size and the rate can be modified to achieve a lower delay. The end-to-end QoS is based on an assured bandwidth provided by the network that, when used by a shaped flow, produces a delay bound service with no queuing loss. This service scheme can only control the maximal queuing delay without providing any kind of control on the jitter since the delay will change from the transmission delay of the path to the transmission delay plus the queuing delay. The traffic profile is specified by means of the Traffic Specification or TSpec whereas that the QoS requirements are specified by means of the Service Request Specification or RSpec. Both specifications form the two parts of the Traffic Contract. TSpec defines the flow’s traffic pattern allowed in terms of the average packet rate, maximum burst rate and size, and the packet size. It has to be considered that these parameters define the maximum allowed not the actual traffic profile. Excess traffic could be directly discarded or transmitted as Best-effort depending on the traffic policy of the network. RSpec specifies the QoS a flow requires. That is to say, the bounded maximum End-to-End delay expressed in this case by means of the bandwidth reservation required by the flow defined by means of TSpec. The network should reserve enough resources to guarantee that no losses due to network congestion will ever occur. A detailed specification of the QoS parameters including formats and range of values can be found in the RFC 2212 “Specification for Guaranteed Quality of Service”. The same document also describes internal network architecture to achieve end-to-end QoS. TSpec and RSpec are calculated by a QoS reservation algorithm embedded in the application. The algorithm takes into account the actual QoS requirement and the propagation delay of the path to carry out the calculation of the TSpec and RSpec parameters That once accepted by the admission control will be delivered to the network elements. Other QoS aspects such as service availability are not defined in the Traffic Contract. They have to be achieved by means of the proper network design.

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Parameter Bucket depth “b” Bucket rate “r” Peak rate “p” Minimum policed unit “m” Datagrams shorter than accounted as length m Maximum datagram size “M”

Table 8.3-1:

Units Bytes/sec Bytes/sec Bytes/sec m

are Bytes

m
Bytes

M<=MTU

TSpec Parameters

Parameter Transmission rate “R”

Units

Range 1 to 40 Tbytes/sec R>r

Byte/sec

Slack term “s” Difference between the desired delay µsec and the delay obtained using “R”

Table 8.3-2:

Range 1 to 40 Tbytes/sec 1 to 40 Tbytes/sec 1 to 40 Tbytes/sec

0 to 232-1µsec

RSpec Parameters

Although IP networks can offer deterministic guarantees to those flows that require it, only the maximum end-to-end delay can be controlled. The difference between the propagation delay and the bounded delay that includes the queuing delay cannot be controlled, thereby obtaining a considerable delay jitter that can impair the global quality of the virtual connection. This jitter could be increased by the imprecision of the queue schedulers in the packet switches. That is to say, the algorithm that controls the process of packet switching can only offer the calculated performance for infinitesimal short packets. The longer the packets the bigger the imprecision in the delay control. It has to be considered that the packet length of any other flow switched in the same node will affect the delay jitter of our flow. Due to this, the use of IP networks to support Teleprotection services should be limited to well-controlled domains in which all the above mentioned factor fall into our control. ATM technology, which is based on a short and fix packet length called cell, can be envisioned as feasible solution to support the Teleprotection service since it can guarantee and effectively control both the delay and the delay variation. The combination of IP technology that offers cost-effective access interfaces and the ATM technology that offers network wide reliable delay control might probably be one future approach to support the Teleprotection service in the broadband environment. Lit:. [31], [32], [33], [34], [35], [36].

A3.4 IP TO ATM SERVICE MAPPING The service offered to the final user of the network, which could be defined by the service class and its QoS parameters, should not be affected by the network implementation. That is to say, services classes, traffic descriptors and QoS parameters of a QoS IP network have to be maintained even thought ATM backbones or any IP over ATM architecture had been chosen to implement the network. In order to achieve this goal, the devices that interconnect the IP and the ATM subnetworks have to include, among others, the capability of translating every service aspect as well as QoS parameters in order to assure an end-to-end QoS. Due to the different service definition in both networks the services are mapped according to the

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following association: -

Guaranteed Service Controlled Load Best Effort

à à à

CBR or rtVBR nrtVBR or ABR (with a minimum cell rate) UBR or ABR

The details of service mapping as well as the QoS and traffic parameters could be found in the RFC 2381. “Interoperation of Controlled-Load Service and Guaranteed Service with ATM”, [36]. Traffic and QoS parameters are defined in the IP environment in terms of bytes/s whereas that in ATM are defined in cells/s. The corresponding mathematical transformations that takes into account overhead introduced by the different size of IP and ATM headers can be found in RFC 2381, [36]. Since these functions are included in the standards, they are always included in the Gateways that interconnect IP and ATM networks.

A3.5 QUALITY OF SERVICE STANDARDS ITU-T RECOMMENDATION No. E.800 X.140 X.641 X.642 I.350

TITLE Terms and definitions related to quality of service and network performance including dependability General quality of service parameters for communication via public data networks Information technology - Quality of Service - Framework Information technology - Quality of Service – Guide to methods and mechanisms (presently at the stage of draft) General aspects of quality of service and network performance in digital networks, including ISDNs

ISO/IEC EQUIVALENT none none 13236 13243 none

I.356

B-ISDN ATM layer cell transfer performance

none

I.371

Traffic Control and Congestion Control in B-ISDN

none

I.731 Q.2723.1

Types and general characteristics of ATM equipment (Paragraph 7: Generic performance requirements) B-ISDN User Part - Support of additional traffic parameters for Sustainable Cell Rate and Quality of Service

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none none


ANNEX A4 PROTECTION SYSTEM TIME SYNCHRONIZATION TECHNIQUES A4.1 TIME SYNCHRONISATION FOR SIMULTANEOUS SAMPLING Two principal teleprotection functions for microprocessor-based current differential line protection, in which protection relays are comparing data with the same tag of time, is to provide the timing synchronization for the simultaneous sampling of current waveforms at all remote terminals of the line and the current data transmissions among the terminals. There are two ways to achieve timing synchronization; internal timing synchronization using its own teleprotection signaling channel and external timing synchronization using external timing source such as GPS as shown in Figure 8.3-1.

Figure 8.3-1:

A4.1.1

Two ways of timing synchronization for current differential teleprotection

Internal timing synchronization

The internal timing synchronization scheme between two terminals is implemented in the transmission or teleprotection equipment, and timing synchronization signals are transmitted in the teleprotection channel to self-adjust the internal clocks of the terminals by sending a message back and forth between the terminals. There are basically two types of message transmissions including timing pulses between master (or reference) and slave (or synchronizing) terminals: round-trip and mutual (or two-way) transmission methods. However, many variations exist for implementation. In the round-trip transmission method shown in Figure 8.3-2(a), a reference timing pulse at the master terminal is transmitted to the slave terminal. The transmitted reference pulse is returned to the master terminal. The returned reference pulse is delayed by round-trip transmission through the outgoing and incoming transmission lines (δ1 and δ2). Transmission delay is measured at the master terminal, and the data is transmitted to the slave node. The slave node executes delay compensation corresponding to a half of the round-trip delay, (δ1 + δ2)/2 to achieve timing synchronization.

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In the mutual or two-way transmission method, both terminals mutually transmit reference pulses. Each terminal measures the time difference between the transmission of its own pulse and the reception of the opposite terminal's pulse; TM at the master terminal and TS at the slave terminal. TM is transmitted to the slave terminal which executes delay compensation corresponding to a half of the difference between the two delays, (TM - TS)/2. The delay compensation is initiated upon reception of the master node's pulse by the slave terminal. A synchronization error of a few microseconds between two PDH-based synchronization devices was experimentally obtained in multi- (four or five) repeater microwave.

Figure 8.3-2:

Two-types of timing pulse transmission methods

Figure 8.3-3 shows another implementation. A terminal acts as the reference clock for the system. A numbered message is sent from the synchronizing terminal at time tA1. It is received at time tB1’ at the reference terminal and returned back at time tB2. It is received at the synchronizing terminal at time tA2’. The times tB1’ and tB2 are sent with the next message to the synchronizing terminal. The difference ∆t between the clocks in the synchronizing and the reference terminal can now be calculated by the synchronizing terminal as ∆t =

t A1 + t A2' t B1' + t B 2 − 2 2

The clock in the synchronizing terminal can now be adjusted by a fraction of ∆t until ∆t becomes zero. The synchronizing and reference terminal clocks are synchronized and the samples of current can be compared at the same sampling instant. Since the clocks are crystal controlled, they maintain synchronism for long times of transmission interruptions [16].

Figure 8.3-3:

Implementation of round-trip transmission method

Another implementation is shown in Figure 8.3-4. In this case the terminals also use the same data polling technique as described above for the measurement of the channel delay time. Every terminal calculates the time delay δ with regard to the other ones as

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δ ≡ δ1 = δ 2 =

(t A2' − t A1 ) − (t B 2 − t B1' ) 2

With the measurement of the time delay, the local sampling timing can be adjusted as t AB 2 = t A2 − δ .

Figure 8.3-4:

Another implementation of round-trip transmission method

Although this method can achieve synchronization between the terminals, an asynchronous operation where the internal clocks of the terminals don’t need to be synchronized can be also performed. The sampling time of the received current vector values from the remote terminal can be measured as tAB2. Since the current samples have not been taken at the same sampling instant, a vector transformation in software is required to rotate the remote vector by an angle corresponding to the time tAB2 - tA2, and then to compare with the local value sampled at tA2. These procedures assume the same time delay δ in both directions, so in systems where both directions can be switched via different routes (SDH and the difference in time delay for data transfer in both directions will introduce determination of the differential current [15], [5]. Requirements on differential solutions are discussed in chapter 6.

A4.1.2

communication ATM networks) an error in the time delay and

External timing synchronization

Internal terminal-to-terminal basis synchronization may not be available for wide-area applications, because the multiple-link synchronization mechanism is hard to be implemented in conventional telecommunication or teleprotection equipment. External time synchronous signal may be effective for that purpose. This eliminates the timing synchronization function from the telecommunication system used for teleprotection signaling channels, which makes the telecommunication system design much easier. There are many ways to provide precise timing signals externally. Form the viewpoint of technological maturity and the ease of availability, the satellite-based Global Positioning System (GPS) is a solution [17], [18]. Other satellites such as the Russian GLONASS (Global Navigation Satellite System) are also candidates for the wide-area time dissemination. Synchronization with GPS satellite signals is the preferred technique at the present time. Global Positioning System using on-board atomic clocks (cesium or rubidium) consists of 24 satellites in 12-hour orbits at an altitude of 20,183 km. There are six orbits used with 4 satellites in each orbit. Using the transmissions from these satellites, positions of objects can be determined with an accuracy of 10 meters in three dimensions, and in the common-view time transmission provided by these satellites, 1-pulse-per-second (1-pps) signals at any location in the world with an accuracy of about 1 µs (basic time synchronization accuracy is ±20 ns) are available when

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decoded by appropriate receiver clocks. Synchronous phasor measurement technique using GPS has been developed to measure power system phasors, transmit the data with time-tags, and then record or analyze them in real time [19]. The technique can be used for teleprotection signaling and wide-area adaptive protections. Prudent considerations should be taken with regard to unavailability and/or precision degradation due to intentional or unintentional radio interference, satellite and/or receiver failures and so on. Recently, however, highly accurate but low-cost timing sources which is usually synchronized with the GPS clock and maintains a microsecond-order accuracy for several days even when the GPS signal is lost are being developed. A variety of alternatives exist for time synchronization using a terrestrial signal from a central location. AM radio broadcasts are least expensive, but their accuracy is limited to a few milliseconds. An access to reference time server using UNIX-based NTP (network time protocol) via TCP/IP networks or Internet is less accurate. Utilities can use their own private telecommunication channels such as microwave or fiber-optic circuits where the solution may approach 1 µs, and custom or dedicated fiber-optic links may achieve better accuracy. In terms of future use by utilities of broadband digital communications, SDH networks, which are presently master-slave frequency synchronous networks, appear promising for a terrestrial time synchronous system. Terrestrial SDH-based time synchronous system of which accuracy is comparable with GPS is under study in several organizations [20], [21]. Current SDH networks are equipped with clock supplies to synchronize its operation clock frequency all over the network, making it easy to handle multiplexing and demultiplexing of digital signals. In order to time-synchronize frequency-synchronized networks of this type, externally additive time synchronizing equipment which transmits time signals to adjacent nodes, measures round-trip or two-way delays between the nodes, and compensates the two-way differential delay, was proposed as shown in Figure 8.3-5, [21]. To transmit a time signal, especially reference timing pulse in SDH networks, undefined bytes in the SDH frame overhead are used. In the system auxiliary time synchronizing equipment (TSE) is attached to existing SDH transmission equipment and clock supply equipment (CSE). In these systems frequency synchronization is conducted by CSEs which usually have a digital processing phase locked loop, and phase or time synchronization is carried out by TSE. Experimental results indicated that an accuracy of about 1 µs can be achieved. GPS Time Transfer GPS satellites

Terrestrial Time Transfer using digital (SDH) networks Clock Master station CSE-M M S

S

GPS receivers

Slave stations

Data TSE-M

SDH

Time signal CSE-S

Slave stations S Time signal

S

S

Power Control and Protection System

TSE-S

SDH Time signal

SDH: SDH Transmission Equipment (existing) CSE: Clock Supply Equipment (existing) TSE: Time Synchronous Equipment M: Master, S: Slave

Figure 8.3-5:

GPS and an SDH-based master-slave time transfer network

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LIST OF FIGURES FIGURE 2.1-1: FIGURE 2.1-2: FIGURE 2.1-3: FIGURE 2.2-1: FIGURE 2.4-1: FIGURE 2.4-2: FIGURE 2.4-3: FIGURE 2.5-1: FIGURE 3.1-1: FIGURE 3.1-2: FIGURE 3.1-3: FIGURE 3.1-4: FIGURE 3.1-5: FIGURE 3.1-6: FIGURE 3.1-7: FIGURE 3.1-8: FIGURE 3.1-9: FIGURE 3.1-10: FIGURE 3.1-11: FIGURE 3.1-12: FIGURE 3.1-13: FIGURE 3.1-14: FIGURE 3.1-15: FIGURE 3.2-1: FIGURE 3.2-2: FIGURE 3.2-3: FIGURE 3.3-1: FIGURE 3.3-2: FIGURE 3.3-3: FIGURE 3.4-1:

SINGLE-LINE DIAGRAM OF A TYPICAL POWER STATION ........................................................................9 SINGLE LINE DIAGRAM OF A TYPICAL TRANSFORMER STATION ..........................................................10 THE SCANDINAVIAN POWER GRID .....................................................................................................11 POWER LINE WITH EXAMPLES OF FAULT TYPES AND FAULT POSITIONS ..............................................12 TYPICAL POWER SYSTEM AND ITS ZONES OF PROTECTION ..................................................................16 OVERLAPPING PROTECTION ZONES ESTABLISHED BY CURRENT TRANSFORMER LOCATION ................17 FAULT CLEARING SYSTEM ..................................................................................................................18 FUNDAMENTAL TERMS ON PROTECTION AND TELEPROTECTION (FROM IEC60834-1) .......................20 PRINCIPLE OF DIFFERENTIAL PROTECTION..........................................................................................23 DIFFERENTIAL PROTECTION: EXAMPLE OF PERCENTAGE RESTRAINT CHARACTERISTIC .....................23 BASIC SCHEME OF A CURRENT BALANCED SYSTEM USING THREE PILOT WIRES ..................................25 CENTRALIZED CONFIGURATION .........................................................................................................27 DISTRIBUTED CONFIGURATION ..........................................................................................................27 PHASE COMPARISON OPERATING PRINCIPLES .....................................................................................29 OPERATION OF CHARGE COMPARISON, EXTERNAL FAULT ..................................................................31 BIAS CHARACTERISTIC OF CHARGE COMPARISON...............................................................................32 IDEAL POLAR DIAGRAM CHARACTERISTIC ..........................................................................................32 INTERTRIPPING UNDERREACH DISTANCE PROTECTION SCHEME LOGIC.............................................35 PERMISSIVE UNDERREACH DISTANCE PROTECTION SCHEME LOGIC .................................................36 PERMISSIVE OVERREACH DISTANCE PROTECTION SCHEME LOGIC ....................................................37 ACCELERATED UNDERREACH DISTANCE PROTECTION SCHEME LOGIC .............................................38 BLOCKING OVERREACH DISTANCE SCHEME LOGIC ...........................................................................40 DEBLOCKING OVERREACH DISTANCE PROTECTION SCHEME LOGIC .................................................41 TWO BREAKER BUSBAR CONFIGURATION ...........................................................................................42 1½ BREAKER BUSBAR CONFIGURATION .............................................................................................44 TWO PROTECTION ZONES / ONE BREAKER BUSBAR CONFIGURATION ..................................................46 GENERATOR PROTECTION ..................................................................................................................47 TRANSFORMER PROTECTION ..............................................................................................................48 REACTOR PROTECTION .......................................................................................................................48 RELATIONSHIP BETWEEN PROTECTED AREA AND OPERATE TIME WITH RESPECT TO PROTECTION SCHEMES 49 FIGURE 3.4-2: DISTANCE PROTECTION PROVIDING REMOTE BACKUP ........................................................................50 FIGURE 3.4-3: SPLITTING PROTECTION (BD) USING TELECOMMUNICATIONS FOR MULTI-CIRCUIT AND MULTITERMINAL LINE. RY, CB AND TD DENOTE OPERATING TIMES OF RELAY (30 MS) AND CB (40 ............................50 FIGURE 3.4-4: COORDINATION TIME CONTROL USING TELECOMMUNICATIONS. RY, CB AND TD DENOTE OPERATING TIMES OF RELAY (30 MS) AND CB (40 MS) AND TIME DELAY FOR COORDINATION, RESPECTIVELY. ....................51 FIGURE 3.4-5: WIDE-AREA CURRENT DIFFERENTIAL BACK-UP PROTECTION EMPLOYING TELECOMMUNICATIONS ....52 FIGURE 3.4-6: A SYSTEM-WIDE PROTECTION; PREDICTIVE OUT-OF-STEP PROTECTION .............................................54 FIGURE 4.4-1: PRINCIPLE OF WAVELENGTH DIVISION MULTIPLEXING FOR 2 WAVELENGTHS, ................................70 FIGURE 4.5-1: NETWORK ARCHITECTURE.................................................................................................................76 FIGURE 4.5-2: NETWORK COMPONENTS ....................................................................................................................77 FIGURE 4.5-3: SEVEN LAYER OSI MODEL .................................................................................................................78 FIGURE 4.5-4: REFERENCE MODEL OF THE CIRCUIT EMULATION SERVICE (CES).....................................................81 FIGURE 4.5-5: LAN TOPOLOGIES .............................................................................................................................83 FIGURE 4.5-6: LAN PROTOCOL LAYERING ................................................................................................................84 FIGURE 6.1-1: INAPPROPRIATE OVERLAPING OF RELAY COMMUNICATION LINKS IN A DOUBLE REDUNDANT PROTECTION SYSTEM .........................................................................................................................................111 FIGURE 8.1-1: LOCAL AND WIDE AREA NETWORKS FOR PROTECTION .....................................................................131 FIGURE 8.1-2: INTEGRATED SUBSTATION APPLICATIONS DEALT IN UCA ARCHITECTURE.......................................132 FIGURE 8.1-3: SUBSTATION LANS CONNECTED BY A WAN ...................................................................................134 FIGURE 8.2-1: SUBSTATION WITH LAN CONFIGURATION .......................................................................................135 FIGURE 8.2-2: HV (SUB)NETWORK WITH WAN......................................................................................................136 FIGURE 8.3-1: TWO WAYS OF TIMING SYNCHRONIZATION FOR CURRENT DIFFERENTIAL TELEPROTECTION ............157 FIGURE 8.3-2: TWO-TYPES OF TIMING PULSE TRANSMISSION METHODS ..................................................................158

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FIGURE 8.3-3: FIGURE 8.3-4: FIGURE 8.3-5:

IMPLEMENTATION OF ROUND-TRIP TRANSMISSION METHOD ............................................................158 ANOTHER IMPLEMENTATION OF ROUND-TRIP TRANSMISSION METHOD ............................................159 GPS AND AN SDH-BASED MASTER-SLAVE TIME TRANSFER NETWORK ............................................160

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LIST OF TABLES TABLE 3.1-1: STATE COMPARISON PROTECTION SCHEMES .....................................................................................34 TABLE 4.3-1: ADVANTAGES AND DISADVANTAGES OF PILOT WIRES .......................................................................60 TABLE 4.3-2: ADVANTAGES AND DISADVANTAGES OF POWER LINE CARRIER LINKS................................................62 TABLE 4.3-3: ADVANTAGES AND DISADVANTAGES OF RADIO LINKS .......................................................................65 TABLE 4.3-4: ADVANTAGES AND DISADVANTAGES OF OPTICAL FIBRE LINKS ..........................................................67 TABLE 4.3-5: ADVANTAGES AND DISADVANTAGES OF SATELLITE LINKS.................................................................69 TABLE 4.4-1: PDH - PLESIOCHRONOUS DIGITAL HIERARCHY LEVELS ....................................................................72 TABLE 4.4-2: SDH - SYNCHRONOUS DIGITAL HIERARCHY LEVELS ........................................................................73 TABLE 5.3-1: SERIAL DATA INTERFACES .................................................................................................................95 TABLE 5.3-2: COMMON PHYSICAL LAN INTERFACES ..............................................................................................98 TABLE 6.1-1: REQUIREMENTS FROM PROTECTION ON TELECOMMUNICATION AND TELEPROTECTION: STATE COMPARISON SCHEMES. FOR TERMS AND DEFINITIONS REFER TO CHAPTERS 6.1.1.1 AND 6.1.1.2. ...................106 TABLE 6.1-2: REQUIREMENTS FROM PROTECTION ON TELECOMMUNICATION AND TELEPROTECTION: ANALOG COMPARISON SCHEMES. FOR TERMS AND DEFINITIONS REFER TO CHAPTERS 6.1.1.1 AND 6.1.1.2. ...................107 TABLE 6.4-1: IEC PUBLICATIONS FOR EMC AND INSTALLATION ..........................................................................118 TABLE 7.1-1: PROTECTION SCHEMES VS. MEDIA...................................................................................................120 TABLE 7.1-2: PROTECTION SCHEMES VS. MULTIPLEXING TECHNIQUES ................................................................121 TABLE 7.1-3: PROTECTION SCHEMES VS. NETWORK TECHNOLOGIES ....................................................................122 TABLE 7.1-4: CONFIGURATION SUMMARY .............................................................................................................123 TABLE 7.2-1: CHECKLIST FOR INTERFACE CO-ORDINATION BETWEEN PROTECTION / TELEPROTECTION / TELECOMMUNICATION DEVICES ........................................................................................................................127 TABLE 8.3-1: TSPEC PARAMETERS ........................................................................................................................155 TABLE 8.3-2: RSPEC PARAMETERS ........................................................................................................................155

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BIBLIOGRAPHY [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

CIGRE SC34 WG34-35.05, "Protection systems using telecommunications”, TB 13, 1987. CIGRE SC34 WG34.05, "Application of wide-band communication circuits to protection prospects and benefits”, TB 84, 1991. CIGRE SC34 WG34.01, "Reliable Fault Clearance and Back-up Protection", TB 140, April 1999. T. Nagasawa and et al., "Present Situation and Experiences of Back-up Protection in Japanese EHV Networks", CIGRE SC34, South Africa, 1997. Y. Serizawa, et al., “Wide-band communication requirements for differential teleprotection signaling” 600-03, CIGRE Symposium Helsinki 1995. Y. Serizawa, et al., "Wide-Area Current Differential Backup Protection Employing Broadband Communications and Time Transfer Systems", IEEE PES 1998 Winter Meeting, PE-203-PWRD-0-11-1997, Tampa, 1998. T. Nagasawa, et al., "Present Status and Experiences in Grouping of Protection Functions in Integrated Systems", 1999 CIGRE SC34 Colloquium, 108, Florence, Italy, October 1999. J. Kobayashi, et al., "The State of the Art of Multi-circuit and Multi-terminal Overhead Transmission Line Protection Systems Associated with Telecommunication Systems", CIGRE, Paris, 34-203, 1990. CIGRE SC34 WG34.02, "Adaptive Protections and Control", 1995. Y. Ohura, et al., "A Predictive Out-of-Step Protection System Based on Observation of the Phase Difference between Substations", IEEE Trans. Power Delivery, Vol. 5, No. 4, 1990. M. Tsukada, et al., "New Stabilizing Protection Systems with an Adaptive Control Approach", 34-204, CIGRE SC34 Colloquium, Stockholm, 1995. "Wavelength Division Multiplexing for Electricity Utilities"; TB 131 to be published. CIGRE SC35 WG35.07, "Power System Communications in the High Speed Environment", TB 107, December 1996. ATM Forum af-saa-0032.000, “Circuit Emulation Service Interoperability Specification”, September 1995. C.G.A Koreman et al., "Requirements for SDH networks due to protection signalling” 400-02, Cigré Symposium Helsinki 1995. T. Einarsson et al., "Experiences of current differential protections for multi-terminal power lines using multiplexed data transmission systems” 34-203, Cigré Session 1994. W. Lewandowski and C. Thomas, "GPS Time Transfer", Proc. IEEE, Vol. 79, No. 7, 1991. R. E. Wilson, "Use of Precise Time and Frequency in Power Systems", Proc. IEEE, Vol. 79, No. 7, 1991. IEEE Std 1344-1995, "IEEE Standard for Synchrophasors for Power Systems", IEEE Power Engineering Society, 1996. M. Kihara and A. Imaoka, "System configuration for standardizing SDH-based time and frequency transfer", European Frequency and Time Forum, No.418, pp. 465-470, 1996. Y. Serizawa et al., "SDH-Based Time Synchronous System for Power System Communications", IEEE Trans. Power Delivery, Vol. 13, No.1, Jan. 1998. K. Yanagihashi et al., "Applications of co-ordinated control, protection and operation support system in EHV substations", CIGRE SC34, Paris, 1996. "Utility Communication Architecture: Substation Integrated Protection, Control and Data Acquisition: Requirements Specification", RP3599-01, EPRI, 1996. J. T. Tengdin, et al., "LAN Congestion Scenario and Performance Evaluation", IEEE

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[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]

PES Winter Meeting, New York, 1999. M. Tsukiyama et al., "Reliability of new digital type current differential carrier relaying system via microwave channel", CIGRE SC34/35 Colloquium, Tokyo, 1983. "ATM Service Categories: The Benefits to the User", The ATM Forum, White Paper EMAC, 1997. IEC 60834-1, Second edition 1999-10, "Teleprotection equipment of power systems Performance and testing. Part 1: Command systems IEC 60834-2, First edition 1993-06, "Performance and testing of teleprotection equipment of Power Systems - Part 2: Analogue comparison systems IEC 6061850-5, 1st CD February 1999, "Communication Networks and Systems in Substations - Part 5: Communication Requirements for Functions and Device Models" CIGRE 1996: WG34/35.03; "Experience in the use of digital communication links for Protection" R. Braden, D. Clark, S. Shenker, "Integrated Services in the Internet Architecture: an Overview", RFC 1633, June 1994 S. Blake, D. Black, M. Carlson, E. Davis, Z. Wang, W. Weiss, "An Architecture for Differentiated Services", RFC 2475, Dec. 1998 S. Shenker, C. Partridge, R. Guerin, "Specification of Guaranteed Quality of Service", RFC 2212, Sept. 1997 S. Shenker, J. Wroclawski, "General Characterization Parameters for Integrated Service Network Elements", RFC 2215, September 1997 S. Keshav, "An Engineering Approach to Computer Networking", Addison-Wesley M. Garret, M. Borden, "Interoperation of Controlled-Load Service and Guaranteed Service with ATM", RFC 2381, August 1998 Sten Benda, “Interference-free Electronics – Electromagnetic Compatibility”, ISBN 9144-00454-0 Studentlitteratur L. J. Ernst, W. L. Hinman, D. H. Quam, and J. S. Thorp, “Charge Comparison Protection of Transmission Lines – Relaying Concepts”, presented at the IEEE Power Engineering Society Winter Meeting, January 1992.

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ABBREVIATIONS AAL ABR ADSS AM ANSI ATM AUI BER BFP B-ISDN bit/s CAC CASM CB CBR CDM CDMA CDT CDV CDVT CER CES CLR CMR CPU CRC CSMA/CD CT CVT dB DCE DCS Diff-serv DSL DTE DTT DWDM DXC EDFA EHV EIA EMC EMI EMS EPRI FDDI FDM FDMA

ATM Adaptation Layer Available Bit Rate All Dielectric Self-Supporting (Cable) Amplitude Modulation American National Standards Institute Asynchronous Transfer Mode Attachment Unit Interface Bit Error Rate Braker Failure Protection Broadband Integrated Services Digital Network bits per second Call Admission Control Common Application Service Model Circuit Breaker Constant Bit Rate Code Division Multiplex Code Division Multiple Access Cell Transfer Delay Cell Delay Variation Cell Delay Variation Tolerance Cell Error Ratio Circuit Emulation Service Cell Loss Ratio Cell Misinsertion Ratio Central Processing Unit Cyclic Redundancy Check Carrier Sense Multiple Access with Collision Detection Current Transformer Capacitive Voltage Transformer Decibel Data Circuit terminating Equipment Digital Clock Supply Differentiated services Digital Subscriber Loop Data Terminal Equipment Direct Transfer Trip Dense Wavelength Division Multiplex Digital Cross-Connect (Multiplexer) Erbium-Doped Fibre Amplifier Extra High Voltage Electrical Industries Association Electro-Magnetic Compatibility Electro-Magnetic Interference Energy Management System Electrical Power Research Institute Fibre Distributed Data Interface Frequency Division Multiplex Frequency Division Multiple Access

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FM FR GEO GI GLONASS GOMSFE GOOSE GPS GSM HV Hz IEC IED IEEE IETF Int-serv IP ISDN ISO ITU kbit/s LAN LD LED LEO MAN MAU Mbit/s MBS MCM MCR MDT MEO MM MMS MODEM MUT NTP OPGW OSI PCM PCR PDH PEP PES PLC POTS PSK PTO QAM QoS

Frequency Modulation Frame Relay Geosynchronous Earth Orbit (Satellite) Graded Index (optical fibres) Global Navigation Satellite System Generic Object Models for Substation and Feeder Equipment Generic Object Oriented Substation Event Global Positioning System Global System Mobile High Voltage Hertz International Electrotechnical Commission Intelligent Electronic Device The Institute of Electrical and Electronics Engineers Internet Engineering Task Force Integrated services Internet Protocol Integrated Services Digital Network International Standards Organization International Telecommunications Union kilobits per second Local Area Network Laser Diode Light Emitting Diode Low Earth Orbit (Satellite) Metropolitan Area Network Media Attachment Unit Megabits per second Maximum Burst Size Multi-Carrier Modulation Minimum Cell Rate Mean Down Time Medium Earth Orbit (Satellite) Multi-Mode (optical fibres) Manufacturing Message Services Modulator - Demodulator Mean Up Time Network Time Protocol Optical Ground Wire Open Systems Interconnection Pulse Code Modulation Peak Cell Rate Plesiochronous Digital Hierarchy Peak Envelope Power Power Engineering Society (of IEEE) Power Line Carrier (equipment) Plain Old Telephone Service (System) Phase Shift Keying Public Telephone Operator Quadrature Amplitude Modulation Quality of Service

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RFC RFI RSpec RSVP RTCP RTP RTU SCADA SCR SDH SM SNMP SNR SOH SONET SS SSB STM(-N) TCP TDM TDMA TE TMN TSpec UBR UCA UDP UTP VAC VBR VBR-nrt VBR-rt VC VDC VF VP WAN WDM

Request For Comment Radio Frequency Interference Service Request Specification Resource Reservation Protocol Real Time Control Protocol Real Time Protocol Remote Terminal Unit Substation Control and Data Acquisition Sustainable Cell Rate Synchronous Digital Hierarchy Single-Mode (optical fibres) Simple Network Management Protocol Signal-to-Noise Ratio Section Overhead Synchronous Optical Network Spread Spectrum Single-Side-Band Synchronous Transport Module (- level N) Transmission Control Protocol Time Division Multiplex Time Division Multiple Access Terminal Equipment Telecommunication Network Management Traffic Specification Unspecified Bit Rate Utility Communication Architecture User Data Protocol Unshielded Twisted Pair Voltage Alternating Current Variable Bit Rate Variable Bit Rate - non real-time Variable Bit Rate - real-time Virtual Container Voltage Direct Current Voice Frequency Virtual Path Wide Area Network Wavelength Division Multiplex

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INDEX 2

D

2-wire circuit .....................................................57, 59

data integrity..........................................................101 definition ...........................................................105 datagram ..................................................................82 datagram networks...................................................81 deblocking .........................................................30, 41 deblocking overreach distance protection ...............41 dedicated protection ring SDH/SONET.......................................................74 delay ......................................... See propagation time delay compensationSee propagation time compensation dependability ...............................................33, 55, 86 definition ...........................................................104 differential delay...... See propagation time symmetry differential protection ........................................47, 58 digital circuits ..........................................................57 digital communication systems ...............................56 digital hierarchies PDH, SDH...........................................................72 directional distance relay.........................................33 directional overcurrent relay....................................33 diversity...................................................................57 space, frequency..................................................64 DWDM Dense Wavelength Division Multiplex ...............70

4

4-wire circuit .....................................................57, 59 A

accelerated underreach distance protection .............38 adaptive protections.................................................53 addressing............................................77, 87, 92, 106 of terminal equipment .................................69, 102 analog comparison protection..................................21 analogue circuits......................................................57 analogue communication systems ...........................56 ATM67, 71, 80, 86, 89, 103, 129, 133, 137, 146, 147, 151, 152, 155 ATM networks ........................................................89 attenuation ...............................................................59 auto-reclosing ..........................................................22 availability .................................................58, 87, 101 B

back-up protection......... 35, 38, 40, 46, 47, 49, 51, 52 bandwidth ................................................................55 definition ...........................................................104 bit error rate impact om availability.......................................101 blocking overreach distance protection ...................39 boosting ...................................................................61 bridge.......................................................................85 C

carrier frequency range............................................60 CDM code division multiplex .......................................71 CDMA code division multiple access..............................64 cell switched networks ............................................80 centralized timing synchronization..........................52 channel cross-over ...................................................58 checklist system design ....................................................124 chromatic dispersion optical fibres........................................................65 circuit.......................................................................56 circuit switched networks ........................................79 clock provisioning .................................................102 contact interface ......................................................93 coordinating timer ...................................................39 current differential protection21, 22, 74, 100, 101, 157

E

echo logic ................................................................38 EDFA erbium doped fibre amplifier...............................66 electric power system ................................................9 EMC Electromagnetic Compatibility............................93 installation practice ...........................................117 requirements on interfaces ..................................93 error detection........................................................101 Ethernet ...........................................................96, 131 external timing synchronization ............................159 F

fading.......................................................................63 fault clearing............................................................13 fault clearing system................................................17 FDM frequency division multiplex...............................69 frequency division multiplex...............................60 fibre-optic cables .....................................................65 frequency modulation........................................26, 63 FSK frequency shift keying.........................................30 full-wave phase comparison ....................................30

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G

Gateway.................................................132, 153, 156 generator shedding...................................................53 ground potential rise ................................................34 H

half-wave phase comparison ...................................30 hub.....................................................................85, 97 I

installation .............................................................116 interface ...................................................99, 112, 126 contact .................................................................93 EMC....................................................................94 fibre-optic............................................................95 LAN / Ethernet..............................................85, 93 protection / telecommunication... 26, 30, 56, 93, 99 serial....................................................................94 VF - voice frequency...........................................94 interference26, 35, 36, 38, 39, 55, 59, 60, 61, 62, 63, 64, 65, 67, 88, 90, 95, 104, 105, 116, 160 internal timing synchronization .............................157 interoperability of protection relays............................................137 intertripping underreach distance protection ...........34 intra-substation networks.......................................132 IP networks..............................................................91 ITU-T ......................................................................73 J

jitter ............................ See propagation time variation L

LAN local area network ...........................75, 82, 96, 132 topologies............................................................83 LD laser diode ...........................................................66 LED light emitting diode .............................................66 line traps ..................................................................61 load shedding...........................................................53 lock out signal .........................................................47 loop-back.................................................................57 M

maloperation................ 22, 59, 70, 101, 102, 105, 113 MCM multicarrier modulation.......................................61 microwave radio ......................................................62 modem high speed ...........................................................59 modulation.........................................................26, 59 multiplexer...............................................................95 multiplexer section protection .................................89 multiplexing.....................................................59, 119

CDM - Code Division Multiplex ........................71 FDM - Frequency Division Multiplex.....60, 63, 69 fixed, synchronous, PDH, SDH ........................103 PDH, SDH...........................................................88 plesiochronous ....................................................72 statistical, asynchronous..........................80, 89, 91 synchronous ........................................................73 TDM - Time Division Multiplex.......61, 63, 66, 70 WDM - Wavelength Division Multiplex.65, 66, 70 multiplexing, demultiplexing...................................69 multi-terminal lines .....................................24, 26, 50 N

network layers .........................................................75 network resilience SDH/SONET.......................................................74 network security ................................................87, 89 network synchronization..........................................88 networks general...........................................................57, 75 PDH, SDH...........................................................88 risks .....................................................................58 noise .................................................. See interference non-segregated protection........................................22 O

OPGW optical ground wire .............................................65 optical fibre interface...............................................95 optical fibres ............................................................65 optical transmitters ..................................................66 OSI reference model................................................78 over reaching ...........................................................34 overall operating time..............................................55 definition ...........................................................104 P

packet switched networks..................................80, 91 path protection.........................................................89 PCM pulse code modulation...................................26, 72 PDH.................................................................88, 133 plesiochronous digital hierarchy ...................72, 77 PDH/SDH networks ................................................88 peak envelope power ...............................................61 percentage restraint..................................................22 performance monitoring ........................................112 performance requirements on telecommunication / teleprotection106, 107, 108 permissive overreach distance protection................37 permissive underreach distance protection..............36 phase comparison segregated, non-segregated .................................29 phase comparison protection .............................21, 28 phase-segregated protection ....................................22 pilot wires ..........................................................25, 58

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interface...............................................................94 pilot-wire relay ........................................................94 PLC analog, digital......................................................60 channel impairments ...........................................61 coupling...............................................................61 modes, propagation modes..................................61 power line carrier ................................................60 reliability .............................................................60 plesiochronous.........................................................72 power line carrier...............................................40, 41 power system faults .................................................12 predictive out-of-step protection .............................53 propagation time29, 39, 53, 55, 58, 67, 68, 75, 86, 88, 89, 100, 104, 106, 107, 110, 115, 132, 144, 149, 152 compensation ........................ 24, 30, 115, 157, 158 definition ...........................................................103 difference ..........................................................133 symmetry74, 75, 86, 89, 100, 106, 107, 108, 109, 115 variation ...................... 68, 100, 101, 106, 107, 115 propagation time symmetry.....................................58 definition ...........................................................103 propagation time variation........... 75, 81, 90, 100, 155 definition ...........................................................103 protection communication dependent ..................................19 communication-aided..........................................19 protection function introduction .........................................................18 protection operating time definition ...........................................................104 protection scheme....................................................19 analog comparison ........................................19, 21 state comparison............................................19, 33 protection switching SDH/SONET.......................................................74 protection system.....................................................17 PSK phase shift keying................................................63 Q

QAM quadrature amplitude modulation..................61, 63 R

Radio licensed, unlicensed ............................................63 reactor protection.....................................................48 repeater ....................................................................85 requirements from wide-area protection.................................110 re-routing .................................................................57 time coordination ..............................................101 risks (for protection) delay related21, 29, 33, 52, 55, 56, 57, 58, 59, 61,

64, 67, 68, 71, 74, 75, 80, 82, 86, 89, 90, 91, 92, 100, 101, 103, 104, 108, 115, 119, 128, 129, 133 maloperation..............................................106, 107 multiplexing ........................................................69 network related............................................58, 133 PLC related .........................................................62 rented circuits......................................................56 re-routing.....................................................58, 101 security related ..................................................105 signal crossover...................................................58 signal loopback ...........................58, 102, 113, 119 S

satellites ...................................................................67 saturation ...........................................................22, 24 SDH.................................................................88, 133 synchronous digital hierarchy .......................73, 78 security ........................................................33, 55, 86 definition ...........................................................105 self-healing SDH/SONET.......................................................89 serial interface .........................................................94 service networks ......................................................78 shared protection ring SDH/SONET.......................................................74 signal quality ...........................................................55 signal transfer delay.................................................55 slips..........................................................................88 SNR signal-to-noise ratio.............................................55 SONET synchronous optical network...............................73 splitting protection...................................................50 spread spectrum.......................................................71 squelching..............................................................102 stabilizing angle.......................................................29 stabilizing protection ...............................................53 standardization.......................................................138 starters .....................................................................29 state comparison protection .....................................33 statistical multiplexing.............................................71 ATM....................................................................89 synchronization......................................................102 synchronous transport module.................................73 system-wide protection............................................49 T

TDM fixed, synchronous ..............................................71 statistical, asynchronous......................................71 time division multiplex .................................61, 70 TDMA time division multiplex access ............................64 telecommunication system definition ...........................................................103 introduction, purpose...........................................17

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usage of ...............................................................19 teleprotection equipment / function definition ...........................................................103 teleprotection function introduction, purpose ..........................................17 teleprotection system definition ...........................................................103 time coordination.....................................................51 time delay variation .................................................30 time stamping ........................................................101 times operating times..................................................143 timing synchronization for simultaneous sampling ..........................................................................157 transformer protection .............................................47 transmission time.....................................86, 106, 107 definition ...........................................................104 maximum actual................................................104 nominal .............................................................104 transport networks .............................................77, 88

U

under reaching .........................................................34 V

virtual paths .............................................................91 voice frequency circuit ............................................57 voiceband modem....................................................57 voice-frequency interface ........................................94 W

WDM Wavelength Division Multiplex ..............65, 66, 70 wide-area current differential protection .................52 wide-area protection ..........................................51, 57 requirements on telecommunication .................110 wide-area timing synchronization............................52 Z

zone ..... 15, 22, 35, 36, 38, 39, 42, 43, 46, 49, 51, 115

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Teleprotecio CIGRE

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