Power System Protection Fundamentals
Dr. Youssef A. Mobarak
[email protected] 2014
Topic_1
AGENDA Why protection is needed Principles and elements of the protection system system Basic protection protection schemes sc hemes Digital relay advantages advantages and enhancements
DISTURBANCES: DISTURB ANCES: LIGHT OR SEVERE The power power system must m ust maintain acceptable operation 24 hours a day Voltage
and frequency must mus t stay within certain limits
Small disturbances The
control system can handle these Example: variation in transformer or generator load
Severe Severe disturbances require a protection protection system They
can jeopardize the entire power power system They cannot be overcome by a control system
POWER SYSTEM PROTECTION Operation during severe disturbances: System
element protection System protection Automatic reclosing Automatic transfer to alternate power supplies Automatic synchronization
TYPICAL BULK POWER SYSTEM Generation-typically at 4-35kV
Transmission-typically at 230-765kV Receives power from transmission system and transforms into subtransmission level
Subtransmission-typically at 69-161kV Receives power from subtransmission system and transforms into primary feeder voltage
Distribution network-typically 2.4-69kV
Low voltage (service)-typically 120-600V
PROTECTION ZONES 1.
Generator or Generator-Transformer Units
2.
Transformers
3.
Buses
4.
Lines (transmission and distribution)
5.
Utilization equipment (motors, static loads, etc.)
6.
Capacitor or reactor (when separately protected) Bus zone
Unit Generator-Tx zone
Bus zone Line zone
Bus zone Motor zone
Transformer zone
Transformer zone
~ Generator
XFMR
Bus
Line
Bus
XFMR
Bus
Motor
WHAT INFO IS REQUIRED TO APPLY PROTECTION 1. One-line diagram of the system or area involved 2.
Impedances and connections of power equipment, system frequency, voltage level and phase sequence
3.
Existing schemes
4.
Operating procedures and practices affecting protection
5.
Importance of protection required and maximum allowed clearance times
6.
System fault studies
7. Maximum load and system swing limits 8.
CTs and VTs locations, connections and ratios
9. Future expansion expectance
10. Any special considerations for application.
C37.2: DEVICE NUMBERS
Partial listing
ONE LINE DIAGRAM Non-dimensioned diagram showing how how pieces of electrical equipment are connected Simplification of actual system Equipment is shown as boxes, circles and other simple graphic symbols Symbols should follow ANSI or IEC conventions
LINE SYMBOLS [1]
LINE SYMBOLS [2]
LINE SYMBOLS [3]
LINE SYMBOLS [4]
1-LINE [1]
PROTECTION SYSTEM A series of devices whose main purpose is to protect persons and primary electric power equipment from the effects of faults
BLACKOUTS
Characteristics Loss of service in a large area or population region Hazard to human life May result in enormous economic losses
Main Causes Overreaction of the protection system Bad design of the protection system
SHORT CIRCUITS PRODUCE HIGH CURRENTS
Three-Phase Line a b c I
Fault
Substation Thousands of Amps
I
Wire
ELECTRICAL EQUIPMENT THERMAL DAMAGE t
Damage Curve
Damage Time
I
Rated Value
I n I md
Short-Circuit Current
MECHANICAL DAMAGE DURING SHORT CIRCUITS Very destructive in busbars, isolators, supports, transformers, and machines Damage is instantaneous Mechanical Forces
f 1
f 2
i1 i2 Rigid Conductors
f 1(t ) = k i1(t ) i2(t )
ELECTRIC POWER SYSTEM EXPOSURE TO EXTERNAL AGENTS
DAMAGE TO MAIN EQUIPMENT
THE FUSE
Fuse
Transformer
PROTECTION SYSTEM ELEMENTS Protective relays Circuit breakers Current and voltage transducers Communications channels DC supply system Control cables
THREE-PHASE DIAGRAM OF THE PROTECTION TEAM CB
CTs
Protected Equipment
Control
Relay
VTs
DC TRIPPING CIRCUIT +
SI DC Station Battery
Relay Contact
SI
52a 52 TC
–
Relay
Circuit Breaker
Red Lamp
CIRCUIT BREAKERS
CURRENT TRANSFORMERS
Very High Voltage CT
Medium-Voltage CT
VOLTAGE TRANSFORMERS
Medium Voltage
High Voltage
Note: Voltage transformers are also known as potential transformers
TYPICAL CT/VT CIRCUITS
Courtesy of Blackburn, Protective Relay: Principles and Applications
CT/VT CIRCUIT VS. CASING GROUND Case
Secondary Circuit
Case ground made at IT location
Secondary circuit ground made at first point of use Prevents
shock exposure of personnel Provides current carrying capability for the ground-fault current Grounding includes design and construction of substation
SUBSTATION TYPES •
Single Supply
•
Multiple Supply
•
Mobile Substations for emergencies
•
Types are defined by number of transformers, buses, breakers to provide adequate service for application
SWITCHGEAR DEFINED Assemblies containing electrical switching, protection, metering and management devices Used in three-phase, high-power industrial, commercial and utility applications Covers a variety of actual uses, including motor control, distribution panels and outdoor switchyards The term "switchgear" is plural, even when referring to a single switchgear assembly (never say, "switchgears") May be a described in terms of use: "the
generator switchgear" "the stamping line switchgear"
PROTECTIVE RELAYS
EXAMPLES OF RELAY PANELS
MicroprocessorBased Relay Old Electromechanical
HOW DO RELAYS DETECT FAULTS? When a fault takes place, the current, voltage, frequency, and other electrical variables behave in a peculiar way. For example: Current
suddenly increases Voltage suddenly decreases
Relays can measure the currents and the voltages and detect that there is an overcurrent , or an undervoltage, or a combination of both Many other detection principles determine the design of protective relays
MAIN PROTECTION REQUIREMENTS Reliability Dependability Security
Selectivity Speed System
stability Equipment damage Power quality
Sensitivity High-impedance
faults Dispersed generation
PRIMARY PROTECTION
PRIMARY PROTECTION ZONE OVERLAPPING Protection Zone A
52 To Zone A Relays
Protection Zone B To Zone B Relays
Protection Zone A 52 To Zone A Relays
Protection Zone B
To Zone B Relays
BACKUP PROTECTION
Breaker 5 Fails
C
D
A
E
1
2
5
6
11
12
T B
F
3
4
7
8
9
10
BALANCED VS. UNBALANCED CONDITIONS
I a
I c I c
I a
Balanced System I b
Unbalanced System I b
Typical Short-Circuit Type Distribution Single-Phase-Ground:
70 – 80%
Phase-Phase-Ground:
17 – 10%
Phase-Phase:
10 – 8%
Three-Phase:
3 – 2%
DECOMPOSITION OF AN UNBALANCED SYSTEM I a I c
I b I a1
I c1 I b 2
I a 0
I b 0 I c 0
I a 2
I b1
Zero-Sequence
Positive-Sequence
Single-Phase
Balanced
I c 2
Negative-Sequence
Balanced
CONTRIBUTION TO FAULTS
Z
A G
FAULT TYPES (SHUNT)
C
X
X
B Z
Z
Short Circuit Calculation Fault Types – Single Phase to Ground
Z
A G C
Z
A
X
G B
C Z
X
Z
Short Circuit Calculations Fault Types Line to Line
X
B Z
X
Z
Short Circuit Calculations Fault Types Three Phase
X
AC & DC CURRENT COMPONENTS OF FAULT CURRENT
VARIATION OF CURRENT WITH TIME DURING A FAULT
VARIATION OF GENERATOR REACTANCE DURING A FAULT
USEFUL CONVERSIONS
PER UNIT SYSTEM Establish two base quantities: Standard practice is to define Base power – 3 phase Base voltage – line to line
Other quantities derived
with basic power equations
SHORT CIRCUIT CALCULATIONS PER UNIT SYSTEM Per Unit Value =
Actual Quantity Base Quantity
Vpu = Vactual Vbase Ipu = Iactual Ibase Zpu = Zactual Zbase
I
Z
base
base
MVA base x 1000 = 3 x kV L-L base
=
Zpu2 =Zpu1 x
kV 2L-L base
MVA base kV 2base1 x MVA base2 kV 2base2 MVA base1
FAULT INTERRUPTION AND ARCING
POWER LINE PROTECTION PRINCIPLES Overcurrent (50, 51, 50N, 51N) Directional Overcurrent (67, 67N) Distance (21, 21N) Relay Operation Time
Differential (87)
t
I
Radial Line
Fault APPLICATION OF INVERSE-TYPE RELAYS
Load
INVERSE-TIME RELAY COORDINATION I
Distance t
T Relay Operation Time
I
Radial Line
t
T
T Distance
DIRECTIONAL OVERCURRENT PROTECTION BASIC PRINCIPLE I
V
F 1
F2 Relay Reverse Fault (F2)
Forward Fault (F1)
I
I SETTING
Z S 1
(0.8) Z L1
V
Relay operates when the following condition holds: I FAULT
V
E
I a
I SETTING
As changes, the relay ’s “reach” will change, since setting is fixed I
E
I
DISTANCE RELAY PRINCIPLE d
L
I a I b I c ,
V a V b V c ,
,
2 1
,
Three-Phase Solid Fault
Suppose Relay Is Designed to Operate When:
| V a | (0.8) | Z L1 || I a | X
Plain Impedance Relay
Operation Zone Radius Z r1
Z Z r 1
Z r1 R
2
R X
2
Z
2
Radial Line
NEED FOR DIRECTIONALITY
F1
F2
2
1
3
4
RELAY 3 Operation Zone
5
6
X
F1 R
F2 Nonselective Relay Operation F1
F2 1
2
3
4
5
6
Operates when:
Z Z M cos MT
X
RELAY 3 Operation Zone
X
F1 F2 The Relay Will Not Operate for This Fault
Directional Impedance Relay Characteristic R
V I Z M cos MT
Z M Z
MT
R
THREE-ZONE DISTANCE PROTECTION Time Zone 3 Zone 2 Zone 1 1
2
3
4
5
6
X Time Zone 1 Is Instantaneous
C B
A
R D
DISTANCE PROTECTION SUMMARY Current and voltage information Phase elements: more sensitive than 67 elements Ground elements: less sensitive than 67N elements Application: looped and parallel lines L
I L
I R
T Relays
R
Communications Channel
Exchange of logic information
R Relays
T
R
PERMISSIVE OVERREACHING TRANSFER TRIP Bus A 1
2
Bus B
3
4
5
6
FWD
FWD
Bus A 1
RVS
2
Bus B
3
4
5
6
FWD
FWD
RVS
DIFFERENTIAL PROTECTION PRINCIPLE Balanced CT Ratio CT
CT Protected Equipment
50
External Fault
I DIF = 0
No Relay Operation if CTs Are Considered Ideal
CTR
CTR Protected Equipment Internal Fault
50
I DIF > I SETTING
PROBLEM OF UNEQUAL CT PERFORMANCE CT
CT Protected Equipment External Fault
50
I DIF 0
False differential current can occur if a CT saturates during a through-fault Use some measure of through-current to desensitize the relay when high currents are present
POSSIBLE SCHEME – PERCENTAGE DIFFERENTIAL PROTECTION PRINCIPLE CTR
Ī SP
Ī RP
CTR
Protected Equipment
Ī S
Ī R
Relay (87)
Compares:
I OP
k I RT
IS
k
I R
| I S | | I R | 2
DIFFERENTIAL PROTECTION APPLICATIONS Bus protection Transformer protection Generator protection Line protection Large motor protection Reactor protection Capacitor bank protection Compound equipment protection
DIFFERENTIAL PROTECTION SUMMARY The overcurrent differential scheme is simple and economical, but it does not respond well to unequal current transformer performance The percentage differential scheme responds better to CT saturation Percentage differential protection can be analyzed in the relay and the alpha plane Differential protection is the best alternative selectivity/speed with present technology
MULTIPLE INPUT DIFFERENTIAL SCHEMES EXAMPLES Differential Protection Zone Ī SP
Ī RP
Ī T
I 1
I 2
I 3
I 4
OP
Bus Differential: Several Inputs Three-Winding Transformer Differential: Three Inputs
ADVANTAGES OF DIGITAL RELAYS Multifunctional
Compatibility with digital integrated systems
Low maintenance (self-supervision)
Highly sensitive, secure, and selective
Adaptive
Highly reliable (self-supervision)
Reduced burden on CTs and VTs
Programmable Versatile
Low Cost
A GOOD DAY IN SYSTEM PROTECTION…… CTs
and VTs bring electrical info to relays Relays sense current and voltage and declare fault Relays send signals through control circuits to circuit breakers Circuit breaker(s) correctly trip
A BAD DAY IN SYSTEM PROTECTION …… CTs
or VTs are shorted, opened, or their wiring is Relays do not declare fault due to setting errors, faulty relay, CT saturation Control wires cut or batteries dead so no signal is sent from relay to circuit breaker Circuit breakers do not have power, burnt trip coil or otherwise fail to trip
