Overhead Power Line Manual 111

OVERHEAD POWER LINES MANUAL CONTENTS

Module No

Description

1

POWER SYSTEM OVERVIEW

2

ROUTE PLANNING, SURVEYS & WAYLEAVES

3

SUPPORT STRUCTURE FOUNDATIONS

4

STRUCTURES

5

CONDUCTORS NETWORK & ENVIRONMENTAL CONSTRAINTS

6

CLEARANCES, FITTINGS & MAINTENANCE

Appendix 1

ADVICE TO THIRD PARTIES FOR WORKING SAFELY HEALTH & SAFETY EXECUTIVE, UK

Appendix 2

ADVICE TO THIRD PARTIES FOR WORKING SAFELY CALIFORNIA (OSHA)

Appendix 3

HEALTH EFFECTS OF ELECTROMAGNETIC FIELDS ASSOCIATED WITH HIGH VOLTAGE LINES

Appendix 4

IMPEDANCES & FAULT LEVEL CALCULATIONS

Appendix 5

WORKED EXAMPLE SAG AND TENSION

Overhead Power Lines Manual – Contents Warning Notice: This manual is a Proprietary Document - no part of it may be reproduced or used in any way other than for the purposes of this seminar and its practical sessions

OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW INDEX

Section No

Description Figure or Table

1.1 Relation between Electricity Demand and Commercial Activity 1.1.1 Relation between electricity demand and commercial activity 1.1.2 Generating Station Sites 1.2 Choice of Voltage Levels for Transmission and Distribution Networks 1.2.1 Historical significance Fig.1.1 Generation, Transmission and Distribution in the UK 1.3 Overhead Lines versus Underground Cables Fig.1.2 Overhead Lines are Insulated by Natural Air Fig.1.3 Insulation Requirements and Heat Produced in Underground Cables 1.4. Balanced Transmission and Distribution (3 Conductors) Fig.1.4 Single Line Diagram : 3-phase Alternator feeds a 3phase load Fig.1.5 Wye-connected alternator feeds a Wye-connected Resistive load Fig.1.6 Three-phase Balanced Currents Fig.1.7 Illustrating Two Voltage Level Circuits on the same pole Table 1.1 Author’s Experience of Support Structures versus Voltage Level Fig.1.8 400 kV Transmission Line

Courtesy of National Grid - UK

Fig.1.9 Bipolar HVDC 1.5 Summary

Page No.

1 1 1 2 3 3 5 6 7 8 9 9 10 11 12 12 12 12

Overhead Power Lines Page 1 of 1 Module1 - Power System Overview – Contents Warning Notice: This manual is a Proprietary Document - no part of it may be reproduced or used in any way other than for the purposes of this seminar

OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT

1.1 The Reason for Transmission and Distribution Power Networks 1.1.1 Relation between electricity demand and commercial activity The demand for electrical energy in any country, and within any geographical area of that country, is directly related to the density of industrial and/or agricultural activity. There are two main components to electricity demand: •

Industrial and commercial component

•

domestic component

In general the utilisation of electrical energy by industry, commerce and even in homes to some extent depends upon the availability, or otherwise, of alternative economic energy sources which are suitable for the application. Electrical energy is high grade and hence its employment, either domestically or industrially, is often governed as much by technical constraints as by economic constraints. For example electrical energy is almost always employed for motive power and lighting. For heating applications its employment depends upon availability of other fuels and the relative costs involved. 1.1.2 Generating Station Sites A major factor that leads to the development of transmission and distribution networks is that modern generating stations are rarely sited close to centres of industry, commerce and population. The constraints that lead to this remoteness are as follows: • technical • economic • environmental For example, hydro power stations offer a cheap running cost but have to be sited where the water resources exist. Such sites are rarely close to the main centres of industry and population. Large thermal stations require huge quantities of cooling water and are thus sited on the coast or on rivers. The cost of transportation of indigenous fuels such as coal and oil often governs the siting of thermal power stations leading to their construction close to the source of fuel. Economies of size in the design of turbines and alternators result in the wish to construct large power stations. Environmental and social constraints are imposed on those engaged in the development of large generating stations. There is thus the inevitable need for transmission and distribution of the generated electrical energy to the centres of industry and population. Recent advances in the employment of combined cycle gas turbine generation, notably in Europe, have reduced to some extent the need for extensive transmission networks. However, new transmission and distribution networks continue to be built or extended throughout the world for the reasons stated above. Proprietary Document Overhead Power Lines Module1 - Power System Overview

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1.2

Choice of Voltage Levels for Transmission and Distribution Networks

In general voltage levels for power networks are chosen in accordance with: •

level of power to be transmitted

•

distances of transmission or distribution

Although it is an over simplification, the current rating of the conductors governs the power transfer capacity of a circuit. The higher the voltage the higher is the power transfer for the same conductor size. Power transfer given by √3 VL IL where Vl = Line voltage, IL = Line current There is no internationally recognised distinction between what is a transmission system and what is a distribution system. The definition varies from country to country and from utility to utility. Here are some of the nominal line to line voltage levels currently employed in AC transmission and sub-transmission networks. • 1000 kV - EHV Transmission • 750 kV - EHV Transmission • 400 kV - EHV Transmission • 380 kV - EHV Transmission • 275 kV - HV Transmission • 230 kV - HV Transmission • 132 kV - HV Sub-transmission • 110 kV - HV Sub-transmission • 66 kV

- MV Sub-transmission

Here are some of the nominal voltage levels normally employed in utility and large industrial distribution networks. •

66 kV

• 33 kV • 13.8 kV • 11 kV • 6.6 kV • 3.3 kV

Proprietary Document Overhead Power Lines Module1 - Power System Overview

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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT 1.2.1 Historical significance When considering the reasons for the choice of the various voltage levels it is useful to consider the history of electricity production and we shall use the United Kingdom for our example. Early production of electricity in the United Kingdom from the late 1880s until the 1930s centred on local towns and villages. Relatively small generating stations were constructed with sufficient capacity for the local needs at the time. The de-facto ‘transmission/distribution’ voltage standards were 3.3 kV, 6.6 kV and 11 kV, rarely exceeding the latter since the level of power to be transmitted was in the tens of MW and the distances usually less than 15 km. The actual value of the nominal ‘transmission’ voltage chosen stemmed from the choice of normal consumer 3-phase line to line voltage of 415 V, 3.3 kV being approximately eight times that voltage. The major consideration, however, was available switchgear technology relating to both short-circuit levels and surge withstand available at the time of design and construction. Some localised interconnection of these separate networks in order to share spare generating capacity and effect economies of production began to develop and again a de-facto standard of 33 kV was often adopted. This has, therefore, been retained in many networks as one of the standard voltage levels in the UK and adopted by its switchgear manufacturers. Some circuits appeared at 66 kV when switchgear technology permitted and distances and levels of power to be transmitted increased. 66 kV networks remain in service in many parts of the world but the voltage level is rarely adopted for newly designed networks. Interconnection of these local undertakings began in earnest, towards the end of the 1930s and the ‘transmission’ voltage chosen for the developing grid system was 132 kV. This matched the larger transmission power levels/distances required (40-50 km 50100 MW) and again matched the available switchgear technology, originally bulk oil circuit breakers and towards the end of the development air blast breakers. At the same time in North America 110 kV was chosen almost arbitrarily as the best compromise available. The constraints on generating station size and siting, previously referred to, required a higher transmission voltage to be selected for further expansion of the UK grid from the mid 1950s onwards. The voltage level chosen was 275 kV. The switchgear employed was almost entirely air-blast. At the same time in North America, however, 220/230 kV became a de-facto transmission voltage standard.


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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT The choice of voltage levels for transmission and distribution in developing countries excluding those within the USSR was largely influenced by from where the expertise was drawn to design, build and operate the networks. Thus, developing networks influenced by UK engineers employed 415 V as the standard consumer voltage with 3.3 kV, 6.6 kV, 11 kV, 33 kV, 66 kV, 132 kV and 275 kV for distribution/ transmission. On the other hand networks influenced by North America employed 220V as the standard consumer voltage and for example 110 kV and 230 kV for transmission and distribution. The EHV Transmission voltage in the UK is now 400kV. Clearly manufacturers of transmission and distribution components who had invested in research and development at the appropriate voltage standards wished to export to countries without power networks or with a rapid expansion in electricity utilisation. See figure 1.1

Fig.1.1 Generation,Transmission & Distribution in the UK - Courtesy National Grid Company This brief introduction has been included to give a broad picture as to why Transmission and Distribution Networks exist widely right across the globe. Proprietary Document Overhead Power Lines Module1 - Power System Overview

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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT On occasions it may be possible to reinforce a transmission system without installing new lines or cables, through technological improvements which permit the existing transmission system to be worked harder, but there will also be times when additions to the system are required. New power stations have to be connected to the country’s existing transmission system Industrial and urban development may also lead to the need for additional lines and/ or substations.

1.3 Overhead Lines versus Underground Cables Main High and Extra High Transmission Circuits are predominantly Overhead Lines, the subject of this seminar. However it is useful in this introductory Module to address the reasons why Overhead Lines are favoured over Underground Cables. Conductors transmitting electricity need to be insulated from the ground. The major differences between overhead lines and underground cables arise from the different ways in which they are insulated. Overhead lines use air whereas underground cable conductors are wrapped in layers of insulating material. Air is the simplest and cheapest insulation, and the heat produced by the electricity flowing through the bare overhead conductors is also removed naturally and efficiently by the air. The live conductors are kept away from the earth by hanging them from porcelain or glass insulators which are suspended from the structure, steel latticed towers, wooden poles etc. For Transmission networks these are always steel towers (Pylons). Wooden poles and other structures are usually employed at the lower (Distribution) Voltages. (See figure 1.2). When conductors are buried underground, however, high quality insulation is needed to withstand the very high voltage, so layers of insulating material are used. Insulation is wrapped layer upon layer around the central copper conductor. A sheath of lead or aluminium covers this and there is an outer covering of plastic to prevent corrosion (See figure 1.3) Unfortunately the insulation also retains the heat generated in the copper conductors. Heat is also generated in the metal sheath and in the insulation, and the earth does not cool conductors as well as air. The result is that underground conductors would tend to run much hotter than overhead ones. So, the underground conductor has to be bigger than its overhead counterpart to reduce its electrical resistance and hence the heat produced. This leads to a conductor up to four times bigger for the same amount of electricity transmitted. As many as 12 separate cables may be needed for an EHV underground transmission circuit. Each cable needs to be well-spaced from others for good heat dissipation and installed at a depth of about a metre to ensure safety. Four separate trenches, each containing three cables, may be needed to match an overhead line.


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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT What this means is that installing underground circuits entails construction activity amounting to the width of a dual carriageway road. The total width required ranges from 15 to 30 metres, depending mainly on the power to be transmitted, but also on local details like soil conditions and cable engineering. The amount of soil and rock excavated is more than 30 times greater than for the equivalent length of overhead line where only pylon foundations are required.

Fig.1.2 Overhead Lines are insulated by Natural Air


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Fig.1.3a Insulation Requirements and Heat Produced in Underground Cables Another reason why Overhead Lines are favoured is that the electrical characteristics of cables are very different than overhead lines. Their capacitance is extremely high compared to an equivalent overhead line. At Transmission level voltages this also results in increased capital and operating costs in the necessary provision of shunt reactors to absorb surplus VARS.


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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT In Summary here are the main issues: •

It costs between 15 and 25 times as much to install underground cable as to build an overhead line. For example this roughly means an extra £9,500,000 for every kilometre of overhead line that is put underground at 400 kV

•

Due principally to the long time to repair faults, underground cables are on average out of service for a period 25 times longer than that for overhead lines. The repair costs are also significantly greater.

•

Underground cables have advantages in minimising the visual impact of electricity transmission. Where overhead line pylons are impracticable, river and sea crossings Also in dense urban areas, underground cables have the advantage

•

During construction of underground cables, the volume of spoil excavated is over 30 times that required for the equivalent overhead line route. Disruption in both urban and rural environments is greater in extent and duration when laying these cables as compared to overhead lines.

Operational problems arise where underground cables are employed at any voltage and particularly in densely populated cities with a high level of construction activity. The author’s experience in the capital city of Riyadh where the medium distribution voltage is 13.8kV, cable faults due to the intrusion of digging machinery were widespread. When all these economic, operational and environmental factors are taken into account, overhead lines have significant advantages compared to underground cabling, particularly in the high voltage transmission of electricity. This has generally meant that underground cabling has been the exception for transmission in all countries around the world. There are cases where undergrounding at high voltage has been justified for reasons of visual amenity (for example, in special circumstances in areas nationally designated for their scenic beauty) and where transmission substations need to be placed in the centre of cities and towns. Short lengths of underground cabling are inevitably employed for some road and river crossings. In addition it is economically practical to terminate overhead transmission circuits using cables for modern compact SF6 substations. 1.4. Balanced Transmission and Distribution Transmission and Distribution overhead lines and cables are balanced when the load currents flowing in each of the three phases are equal in magnitude. Examining the single line diagram of figure 1.4 which illustrates a simple system with a single generator feeding a star connected load. Figure 1.5, a three-phase diagram of the same simple system shows a 4-wire feeder connecting the wye connected generator stator coils with the 3-phase wye connected resistive load.


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3-phase Resistive

3-phase Alternator 3-phase Circuit

LOAD

Fig.1.4 Single Line Diagram : 3-phase Alternator feeds a 3-phase load

3-phase Alternator Wye

I1

3-phase Circuit

3-phase Resistive Load

I2 IN I3

R1

R2

N R3

Fig.1.5 Wye-connected alternator feeds a Wye-connected Resistive load If in the three-phase system of figure 1.5, the resistive loads are identical, then the currents in each resistor will have the same magnitude but displaced by 120 degrees, (in phase with the voltages). The three currents in the resistors meet at the neutral point N. The sum of the three phase currents will therefore flow in the neutral return path (IN). However this current IN is the sum of I1 + I2 + I3 and is zero at any instant in time. For example at 90 degrees the sum is

I1 + (- 0.5 x I 2 ) + (-0.5 x I3 )


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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT I1, I2 and I3 represent the peak values of current shown on Figure 1.6 and are equal in magnitude because the value of the resistors R1, R2 and R3 are equal. At Peak Value of I1(Red) I2 (yellow) and I3 (Blue) at -0.5 of peak

I1

I2

I3

Current

+

30 60 90 120 150 180 210 240 270 300 330 360

R1 = R2 = R3 in figure 3.5 Fig.1.6 Three-phase Balanced Currents The conclusion is that provided the three loads are identical then there is no current in the neutral (return path). The “System” is said to be BALANCED when the load demand in each of the three phases is equal. Most large industrial three-phase loads, such as large motors, are inherently balanced. However single phase loads connected to three-phase systems are connected across one phase only which results in an imbalance. Thus, it is only necessary to provide a neutral path when the three-phase loads are not balanced (4-wire Distribution systems). Occasionally, situations develop on the power system that result in temporarily unbalanced conditions, short-circuits. This permits the use of only three load-current carrying conductors for main Transmission and Distribution overhead line circuits. At voltages above 132kV, these conductors are strung on steel-latticed towers of many different shapes and size (Refer to module 4 for more details). There are, however, many examples of 132kV circuits supported on steel-latticed towers. There are also examples of lower voltage circuits strung on steel towers, also referred to as pylons. One or two earth wires are strung from the top of the pylons. These are not load-current carrying and act as lightning protection. They also carry earth fault currents for very short durations during short-circuits


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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT In the case of Transmission circuits, more than one circuit is usually strung on the same tower. The most common arrangement is two and this is usually referred to as a “Double-Circuit”. (Refer to Module 4 for more details) Each phase of a circuit is often constructed of twin or quad conductors to achieve the maximum current carrying capacity in relation to tower size and strength. Each conductor is normally a bundle of say 7 stranded aluminium wires around a hollow centre, filled with a special grease to aid heat dissipation. The current carrying capacity is dependent upon the maximum operating temperature of the conductor bundle, typically 50-75 0C. (Refer to Module 5 for more details) The conductors are insulated from the grounded steel towers by means of strings of porcelain, toughened glass or a polymeric. These are known as “insulators” or “sheds”. At lower voltages other constructions usually wooden poles and occasionally reinforced concrete are employed. These are invariably single-circuit in many countries. Where the main distribution system (say 33kV, 11kV etc) is overhead, the lower consumer voltage conductors are strung on the same poles when it is economic to do so. (Figure 1.7 shows an 11kV circuit with the 415V conductors strung on the same pole) In North America and in a number of other countries, where steel poles are sometimes employed, it is not uncommon to have more than two voltage level circuits on the same structure.

11kV Circuit

415V Circuit Fig.1.7 Illustrating Two Voltage Level Circuits on the same pole Author’s note – The 415V circuit has 5 conductors. This is basically a four-wire circuit as single phase loads are tapped off to consumers along its length. The fifth wire is used to feed the street lighting. A Lamp is just visible in the centre of photograph towards the bottom. This is connected to a time or light intensity control contactor at the feeding 11KV/415V Transformer Proprietary Document Overhead Power Lines Module1 - Power System Overview

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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT Two Pylon photographs are included in figures 1.8 and 1.9 Table 1.1 gives the author’s experience of the types of support structure employed at the various voltage levels. Voltage Level (Line)

Steel Latticed Circuits (DC)

220V 415V 3.3kV 6.6kV 10kV 11kV 13.8kV 15kV 20kV 33kV 66kV 110kV 132kV 230kV 380/400k V 500kV 750kV

Steel Latticed Circuits (SC)

Steel Poles (DC)

Steel Poles (SC)

Reinforced Concrete Poles (SC)

Wooden Poles (SC)

X X X X X X

X X X X X X X X X X X X X

X X

X

X X X X X

X X X X

X

X

X X

X X

X X

Table 1.1 Author’s Experience of Support Structures versus Voltage Level

Fig.1.8 A 400 kV Transmission Line – Courtesy of National Grid UK

Fig.1.9 Bipolar HVDC Line

1.5 Summary The Module has introduced the need for High Voltage Overhead Transmission and Distribution Circuits, together with the reasons behind the choice of Overhead Lines rather than underground cables. It also addressed the issue of balanced systems, The module is an overview as a precursor to the detail in the modules that follow. Proprietary Document Overhead Power Lines Module1 - Power System Overview

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES CONTENTS Section No


2.1 Introduction - Statutory Requirements 2.1.1 The Electricity Act of 1989 2.2 . Physical Restraints on Routing 2.2.1 Route Security Fig.2.1 A 400 kV Quad Conductor Line – Houses built adjacent to the tower and underneath a span 2.2.2 Safety Fig.2.2 Typical Overhead Line Danger Notices 2.2.3 Vandalism 2.2.4. Airfields 2.2.4.1 Flight Paths 2.2.4.2 Contours 2.2.4.3 Special towers Fig.2.3(a) 400kV Quad Conductor -Suspension Tower (BES-L6) Fig.2.3(b) 400kV Quad Conductor - Suspension Tower (BES-L9) - LOW HEIGHT CONSTRUCTION 2.2.4.4 Service Lines 2.2.4.5 Communication and Technical Sites 2.2.5 Roads and Highways Table 2.1 Typical Road Crossing Clearance requirements for overhead lines 2.2.6 Armed Services Land 2.2.7 River Crossings 2.2.8 Railways 2.2.8.1 Crossings 2.2.8.2 Parallelism 2.2.8.3 Clearances 2.2.9 Mineral Workings 2.3 The Economics of Route Selection 2.3.1 Section Lengths Fig.2.4 Illustrating Tension Towers, Suspension Towers and Spans 2.3.2 Re-alignment of Route to Avoid Obstacles Fig.2.5 Illustrating Line Deviations for Minimum Additional Angle Towers

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Overhead Power Lines Page 1 of 3 Module 2 –Route Planning, Surveys & Wayleaves – Contents Warning Notice: This manual is a Proprietary Document - no part of it may be reproduced or used in any way other than for the purposes of this seminar

OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES CONTENTS

Section No


2.3.3 Underground Cable Sections – Incorporated in Overhead Transmission Lines 2.3.3.1 Additional Costs and Electrical Problems 2.3.3.2 Sealing End Compounds and Terminal Supports 2.3.3.3 Reinstatement 2.3.4 Building Sterilisation 2.3.5 Tree Cutting Fig.2.6 Illustration for Integration of a cleared area 2.3.6 Route Planning for Ease of Construction and Maintenance 2.3.7 Power Corridors 2.3.8 The investigation of Alternative Routes 2.3.8.1 Technical Feasibility 2.3.8.2 Cost 2.3.8.3 Amenity 2.3.8.4 Future Known Developments 2.3.8.5 Agriculture 2.4. Types of Objections to be Considered when Planning Routes 2.4.1 Towers and Insulators 2.4.2 Conductors 2.4.3 Sealing Ends Fig. 2.7 Circuits Cabled into Power Plant (Kilroot, UK) 2.4.4 Corona Discharge 2.4.5 Wind 2.4.6 Arcing 2.4.7 Radio and Television Interference (“active”) 2.4.8 Drips and Ice 2.4.9 Land Use Interference 2.4.10 Agricultural Interference 2.4.11 Damage to Trees 2.4.12 Damage to Land 2.4.13 Physiological – Effects on the Human Body 2.5 Surveying and Profiling 2.5.1 Introduction 2.5.2. Routing Objectives

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES CONTENTS

Section No


2.5.3 Preliminary Routing 2.5.3.1 Survey Equipment Requirements Fig. 2.8 Logical Sequence for Overhead Line Design, Planning and Routing 2.5.3.2 Aerial Survey 2.5.3.3 Ground Survey 2.5.3.4 Ground Soil Conditions 2.5.3.5 Wayleave, access and Terrain 2.5.3.6 Optimisation Fig.2.9 400kV single-circuit twin conductor overhead line crossing the Zagos Mountains in Iran Fig.2.10 Helicopter assisted conductor stringing – Hong Kong 2.5.4 Detailed Line Survey and Profile 2.5.4.1 Accuracy Requirements 2.5.5 Profile Requirements 2.5.5.1 Vertical and Horizontal Scales 2.5.5.2 Templates Fig.2.11 Typical Sag Template Details (Courtesy of EDP Limited) 2.5.5 Computer aided Techniques 2.6. Summary

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT

2.1 Introduction - Statutory Requirements. In almost all countries electricity transmission or distribution companies have obligations under law as far as the erection of overhead power lines at any voltage. The requirements of course vary widely across the globe. In the United Kingdom when the generation, transmission and distribution of electricity was vested with a number of state owned companies acts of parliament governed and restricted these activities. The electricity act of 1957 established the Central Electricity Generating board (CEGB) and vested in them the responsibility for developing and maintaining an efficient, coordinated and economical system of supply of electricity in bulk to all parts of England and Wales. Regional electricity companies were formed to distribute the bulk power to consumers It was a requirement in formulating any plans to construct new circuits, particularly overhead transmission lines or to modify existing routes, that due regard to the desirability of preserving natural beauty, of conserving flora, fauna, and the geological or features of special interest. In addition, they were required under the statute to protect buildings and other objects of architectural or historical interest. In other words they were charged with ensuring that the effect of overhead line construction on the environment was minimised. Later in 1968 when the Countryside Act was passed all government departments and public bodies were charged with this and other similar responsibilities. From the early days of electricity production in the UK following Edison’s first power station in New York (1878) the way things were done was a 3-step process as follows : •

The Government of the day enacted legislation to cover the functions of electricity supply i.e. The Electricity Supply Acts, 1926, 1947, 1957 etc

•

The appropriate Government Minister of the day, by virtue of the powers conferred upon him by the Act, produces technical requirements in the form of a Statutory Instrument - The Electricity Supply Regulations (See below )

•

The Generators and Distributors of the day interpreted the requirements of the Acts and Regulations into Engineering Standards

Privatisation and the formation of the National Grid Company along with Generating Companies and Distribution Companies in the early 1990’s, did not relieve electricity companies in the UK from these responsibilities as far as the construction of overhead lines was concerned. The conditions imposed in the lower voltage overhead distribution for reasons of amenity continued to be less stringent than those for the higher transmission voltages requiring the use of large steel towers (Pylons)

Proprietary Document Overhead Power Lines Module 2 –Route Planning, Surveys & Wayleaves

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.1.1 The Electricity Act of 1989 Continuing with the UK as an example. Section 37 of the 1989 Act states “An electric line shall not be installed or kept installed above ground except in accordance with a consent granted by the appropriate cabinet minister. There are two main exceptions to this rule: 1. Where a line does not exceed a voltage of 20kV and will supply only one consumer (a ‘service’ line) 2. Where the line is to be built on land within the direct control of the appropriate Electricity Company. The first exception was intended to cater for a single consumer on a long HV spur where it would not be technically satisfactory feed the load at 415/240V. All electricity supply lines, whether overhead or underground, also require consent from landowners. This is known as wayleave. This allows the constructor to install and maintain the line. However, the landowner can also tell the utility to remove its line at any time under most wayleave agreements. The exception to this is if a line follows a public highway. That’s one reason why roads are dug up so frequently. A wayleave is not required. So the cost of a new overhead line is not only calculated in terms of towers, poles, conductors, cross-arms and other hardware, but also in the time and effort required to obtain a wayleave. These costs are in addition to the total engineering capital cost of the project even before construction can begin and can be broken down as follows:1. The cost of obtaining the Section 37 consent. 2. Direct costs associated with adequate preparation for the public enquiry if required. 3.

Costs associated with a change to the economically designed line route.

4. Costs associated with changes to tower or pole design, conductors as a result of the public enquiry. The objectors are hardly ever satisfied with the original route and compromises are required. Such enquiries may result in a final consent but the terms are such that changes to the design e.g. tower heights, special towers in certain locations, conductor type are rendered necessary. This is particularly, relevant to distribution lines where the requirement for covered conductor rather than bare conductor on wooden-pole lines is required for safety reasons. This scenario exists in most developed countries.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT At the time of the Electricity Act of 1989 a number of regulations relevant to the production and distribution of electricity were issued by the government. These are listed below; those most relevant to the construction of overhead lines are underlined. •

The Electricity Supply Regulations 1988

•

The Electricity at Work Regulations 1989

•

The Provision and Use of Equipment Regulations 1992

•

Personal Protective Equipment at Work (PPE) Regulations

•

The Management of Health and Safety at Work Regulations 1992

•

The Construction (Design and Management) Regulations 1994

The Construction (Health, Safety and Welfare) Regulations 1996 Based on these regulations new design standards were drawn up by the electricity companies and many existing standards were amended.

•

Very significant changes have been made to the way the Electricity Supply Industry (ESI) operates in the United Kingdom as a direct result of the 1989 Electricity Act. This Act still maintains that a properly engineered design is used and all construction, maintenance and operation is in a safe and efficient mode, but changes the onus of responsibility onto the supply company. The company has to ensure that within current standards the line is fit for the purpose for which it is intended Obtaining and retaining consents for overhead lines in the UK and in other developed countries continues to become increasingly difficult. Landowners and local authorities in the UK are now generally more aware of their legal position under the 1989 Act. This ensures that they can influence line routes and designs selected by the electricity company alone. They also realise that following privatisation the industry is now a commercial enterprise and in some cases it becomes almost impossible to obtain consents for new routes. Although it is possible to seek compulsory powers under the Act, in practice this is very rarely applied to overhead lines below 132kV, where the costs of obtaining consent can be higher than the line construction itself. At the transmission voltages of 275 kV and 400 kV, where the costs per unit length for cable alternatives (See Module 1) are very much higher, compulsory powers are often enforced to obtain the necessary overhead route. As the voltage reduces, so does the cost differential between overhead and underground particularly at 11kV. If wayleaves for the required designed route are objected to and this would, therefore, lead to a Public Enquiry then the cable route is adopted due to overall economics.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT The process is roughly:1. Network designers identify that a new line is needed or a current line needs upgrading 2. A suitable design is obtained and a route specified (See Section 2.2) 3. A Wayleave then needs to be obtained from the landowners along the route. 4. The Local Authority and environmental groups must also be consulted. 5. Section 37 (Government) consent must be obtained. 6. If serious objections from landowners or other bodies e.g. English Heritage, Royal Society for Protection of Birds, etc.) with an interest in the route then a Public Enquiry may be needed. It can be seen from section 2.1.2 that in the UK it is almost certainly cheaper, to obtain wayleave and section 37 consent by a sensitive choice of line route. In most countries the major areas of concern for the public are generally visual impact, Health and Safety effects and financial depreciation of their property. These will be addressed in this section. Objections can be based on the actual route (too close, obscures view) and the line design (at the lower voltages low profile wood pole being preferred to towers). Landowners may also request undergrounding. Alternatives such as routing close to forest edges or keeping well below the skyline is often requested.

2.2. Physical Restraints on Routing 2.2.1 Route Security A problem when seeking routes for overhead lines in developing areas is security of tenure for the works after they have been erected. As the word suggests a wayleave for a structure base, whether for a steel tower or a wooden pole, does not mean that the land so utilized is purchased from the landowner, rather that it is a kind of lease, permitting its construction and future access to it for maintenance.

Fig.2.1 A 400 kV Quad Conductor Line – Houses built adjacent to the tower and underneath a span.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT As the development in the area proceeds, the electricity company concerned may be asked to move the line (this applies particularly to distribution lines and the building of new roads) or to raise the height of existing towers to give adequate clearance to new buildings being placed under the line. It is amazing that people actually build on open land adjacent and almost underneath a span of a transmission line. (The photograph in Figure 2.1 illustrates this). Apart from the problem of finding an alternative overhead route in the vicinity, there are operational difficulties in preserving network security or direct supplies to consumers in the case of distribution lines since an outage is required for the diversion. It is, of course, an engineering judgment based on the time scale of the development as to whether it is worthwhile financially to construct an overhead line in the first place and to place the circuits underground when the development requires it. Wayleaves have been in existence in one form or another for hundreds of years. It is simply an agreement between two parties whereby party B wishes to carry out some simple activity on party A’s land. The two come to an agreement about what the activity is, how long it should go on for, any payment to be made by B to A in respect of the use of his land and any other matter the two feel it is their mutual interest to agree upon. For overhead line structures, there is usually a small payment made by the electricity company to the landowner, depending on the type, for example in the UK, payment for a single wood pole on arable land is currently just over 11 sterling pounds or $15. Nobody gets rich on this; In fact the payment serves only to consolidate the legal position whereby the Electricity Undertaker could be forced to remove its plant from the Grantor’s property if it defaulted on payment. There is another problem in the UK that sometimes arises associated with wayleaves (usually for distribution pole mounted lines) is the fact that it only binds the person who signs it - it does not bind the land. The practical consequence of this is that the wayleave is only legally valid for as long as the person who granted it owns the land. If the letter of the law were to be followed, a new wayleave would need to be sought each time the land on which equipment is placed changes hands. If a new owner refuses to accept money in respect of an existing consent and requires that the equipment be removed, legally the electricity company is trespassing and must remove its equipment after the expiry of a three month period from the landowner. This all makes the use of roadside verges very attractive to electricity companies as access rights and wayleaves are not required.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT A much more secure way of obtaining a right to place equipment on private land in the UK is to secure an easement. An easement is an absolute right in law to carry out an activity on someone else’s land. This gives by far the greatest protection to any equipment placed on private land and is extensively used by other Statutory Undertakers to secure their water and gas pipelines etc. Its two greatest benefits as far as the electricity company is concerned are: • Unlike a wayleave, it cannot be rescinded by any person other than the beneficiary unless there is a clause included in the documentation to that effect. • It binds the land rather than the person who granted it, so again assuming there is no wording included to the contrary, it is attached to the title of the land and transfers with it. Consequently, whoever owns the land in the future is bound by it. An easement can also be drawn up to include other things that could be relevant - for example a “not to build” clause is usual as a way of protecting an underground cable from being built over. Similarly, a route can be reserved where a line passes over a potential gravel or other mineral rich area. In this particular situation, it may not be in the electricity company’s best interest to remain stoically fixed on its route - it may be better to divert the line, but only after the person or company who is gaining the benefit of mineral extraction has agreed to meet the cost of such a diversion. In this way, an electricity company can be protected from unexpected costs incurred by demands for the re-siting of its equipment. However, it is usually much more difficult to persuade a landowner to enter into an easement (or Deed of Grant) because it binds his land absolutely and it may become an encumbrance in the future. It can devalue potential development land, particularly if the easement carries a “not to build” clause. An easement can be very expensive depending upon the individual circumstances. 2.2.2 Safety Lines constructed overhead in built-up areas, both residential and commercial, pose certain problems of safety not generally encountered when passing over open country or desert. (a) Industrial The use of cranes adjacent to or under the line for working or constructional work by other than the electricity company whose safety rules dictate how such work should proceed requires particular vigilance. Electricity companies usually have procedures in place. In the UK the Health and Safety Executive give guidance to companies who need to work under overhead lines (Refer to Appendix 1). OSHA is responsible for such matters in the USA (Refer to Appendix 2). A standard warning notice employed in the UK is shown on figure 2.2 Proprietary Document Overhead Power Lines Module 2 –Route Planning, Surveys & Wayleaves

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT (b) Residential The erection of television aerials at roof level under or near to a line on domestic properties can lead to serious or fatal accidents. The flying of kites and wire-controlled model aircraft in school playing fields, recreational parks and domestic gardens under a line can be dangerous and the attention of the local education authorities should be drawn to this where such possibilities could arise. Notices of possible dangers need to be posted

Fig.2.2 Typical Overhead Line Danger Notices 2.2.3 Vandalism In the Urban areas, vandalism involving damage to overhead structures is common. This usually takes the form of tower climbing, stone throwing at equipment or gun shooting at the insulators. Porcelain insulators tend to crack when hit by such missiles without shattering and the damage cannot readily be identified from ground level. However, increased corona discharge will inevitably occur leading to losses and ultimate failure. (See Module 5 for more information on Corona Discharge and Module 6 for identification and repairs). In the case of glass insulators, however, there is a tendency for them to shatter and so the damage becomes visible. Apart from the costs that are involved, the need for frequent repairs to a line reduces network security 2.2.4. Airfields Special consultations are necessary about proposed line routes in the vicinity of operational military and civil airfields. In most countries, airfields have safeguarded areas to ensure that there are no obstructions placed which would endanger aircraft on take off or landing on the runways. It is important to ensure that all airfields are checked for safeguard areas even when there is no activity as requirements, particularly in military airfields are constantly changing. The details are usually held by the country’s Ministry of Defence and their Civil Aviation Authority.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.2.4.1 Flight Paths In general terms, the taller the proposed line and the supporting structures, the greater the separation distance required from the airfield runways. Details of the permissible heights for structures across and adjacent to flight paths, for a specific airfield, must be obtained from the safeguarding authorities who may specify conditions to ensure the safety of aircraft. This can take the form of illuminating the peaks of structures with red warning lights as a guide to pilots using the airfield. This warning equipment often involves additional costs and wayleaving requirements for the low voltage supplies associated with such installations and their maintenance. Brightly coloured balls to mark earthwires and/or conductors in the vicinity of flight paths in order to make them clearly visible can also be employed 2.2.4.2 Contours The separation distance of an overhead line of a particular height can vary very considerably within the safeguarded area if the ground contours in the vicinity of the flight-path are not level. The levels in the area should be studied by appropriate ground surveys to ascertain whether the proposed route is on the most satisfactory alignment having regard to the requirements of safety and construction (See also Module 6) 2.2.4.3 Special towers The author has included this item within this section on airfields but the employment of special towers is not uncommon in situations other than in the vicinity of such safeguarded areas. There are examples around the world where the steel towers for the majority of the route length are of lattice construction but with steel pole type structures on a few spans entering a substation close to urban development. In many countries, specially designed reduced-height towers are employed close to airfield runways. Figures 2.3 illustrate this with an example from the UK Supergrid system. Figure 2.3(a) is the standard suspension tower for an L6 double-circuit 400 kV line. Figure 2.3(b) shows the special lower height design. This design carries three phases of each circuit on two cross-arms instead of the normal three. Two phases of each circuit are carried on the lower cross-arm and one phase of each circuit on the upper cross-arm. This effectively reduces the height of the standard tower by about 18.5 metres. However, these towers are only employed where absolutely necessary as they can be more visually intrusive and they do detract from the general symmetry of the line


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Fig.2.3(a) 400kV Quad Conductor Suspension Tower (BES-L6)

Fig.2.3(b) 400kV Quad Conductor Suspension Tower (BES-L9) LOW HEIGHT CONSTRUCTION

2.2.4.4 Service Lines As previously addressed government consent in the UK is not required to construct a wooden-pole 11kV line in order to feed a single consumer. However in the safeguarded area in the vicinity of airfields there is an exception and consultations are required with the appropriate aviation authority. If airfield clearances are only affected marginally then it may be possible to obtain a relaxation from the authorities if an alternative route has major disadvantages 2.2.4.5 Communication and Technical Sites The clearances require for Communication stations and technical sites are based on electrical rather than physical criteria and detailed consultations are required for all voltages from 11kV upwards. Complexity of communication systems, satellite, UHF and so on, the problem of maintaining separation is on the increase throughout the world. Civil aviation authorities have operational navigation beacons and radar stations throughout most countries.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT The interference to radio signals resulting from the emission of radio waves or “radio noise” from an overhead line is known as “active interference”. Dirty or cracked insulators may cause this to occur. On newly commissioned lines, however, it can be caused by dirt on the conductors collected during the “running out” and erection of the line. Cleaning the conductors and replacement of damaged insulators can usually remedy this interference. The earliest consultations are advisable so that any adjustments to a proposed route can be made prior to detailed investigations into other constraining issues. Where a line would pass close to an existing or planned communications or technical site, the operational standards of that line will need to be maintained to the designed parameters at all times by high standards of maintenance to avoid any interference of an active nature. This increases the life-time costs of the line. 2.2.5 Roads and Highways There are usually government restrictions placed on electrical utilities when the route crosses existing and future planned streets and main roads. For example, In the UK a distinction is made between ordinary streets/roads and what is defined as special roads. These constitute the motorways and trunk roads having dual carriageways. Minimum clearances above ground and roads are specified at the conductor’s likely maximum temperature. Thus the maximum electrical load must be taken into account at the design stage. The restrictions require that “All supplier’s works shall be sufficient for the purposes for, and the circumstances in, which they are used and so constructed, installed, protected (both electrically and mechanically), used and maintained as to prevent danger or interruption of supply so far as is reasonably practicable”. The important part here is the “sufficient for purpose”! This means the onus is on the supplier to make sure everything is safe. Required typical road crossing clearances are shown in Table 2.1 Up to 33kV 33kV-66kV 66kV-132kV 132kV-275kV 275kV-400kV

5.8m 6.0m 6.7m 7.0m 7.3m

Table 2.1 Typical Road Crossing Clearance requirements for overhead lines


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT When planning a route that is likely to affect a major road, certain criteria should be observed 1. The crossing should be as near to a right-angle as possible to minimise the amount of scaffolding required. 2. Towers or poles adjacent to the road should be sited to give the maximum clearance over the road with the minimum values as given in table 2.1 but without increasing the total number of structures in that section of the line. 3. Where there are future proposals for lighting standards to be erected on the road, the crossing point of the line should be reconciled as far as possible with the future positions of any such lamp standards. 4. Where the overhead line is to be constructed before the road, arrangements for appropriate scaffold sockets in the central reservation should be made to facilitate future line maintenance requirements. Mobile scaffolding machines e.g. the Skycradle specially designed for wide road crossing sections that support the conductors help to reduce problems during the construction and future maintenance. Inevitably, however, scaffolding is often required for such crossings. Where appropriate a suitable lay-by on either side of the road to accommodate the skycradle should be considered to ease the problems of future line maintenance. 2.2.6 Armed Services Land In most countries it is difficult to gain access to land owned by the military, much of which has restricted access. The electricity company requires initial access to construct the line, followed by a permanent 24-hour availability of access for emergency repairs and maintenance to ensure continuity of supplies and overall network security. Often the military have security arrangements which make this unrestricted access totally impracticable, particularly at “safeguarded” establishments. Problems of identification can arise where the construction labour force, varying from day-to-day, requires access to such sites. Certain areas of land are set aside for the storage of explosive materials and others as firing ranges for exercises. An overhead line within these sites could be a hazard and have a marked effect on power system security. Where such restraints exist, the best approach is to avoid the area even if it means a considerable change and extension to the route length. Consultations with the appropriate party by the electricity company should take place at an early stage of the route planning to ensure that these areas are identified and where necessary avoided.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.2.7 River Crossings In most countries there are restrictions on waterway crossings, rivers, creeks, canals etc. Similar to road crossings they should be designed as near to a right angle to ease future maintenance. Often such crossings require raised towers if the necessary span to cross the waterway is greater than the average span for the tower design. Wooden pole circuits up to 132kV will generally be supported on steel towers at the point of crossing. 2.2.8 Railways It is appreciated that few railways exist in this part of the world. However, this section is included for interest. 2.2.8.1 Crossings Overhead line crossings of railway tracks should be planned to be as near to a right-angle as possible in order to minimise the amount of scaffolding required during the construction of the line and its subsequent maintenance. This will also reduce the amount of induction in the railway overhead telegraph circuits at the crossing to a minimum. 2.2.8.2 Parallelism Where railway lines have overhead signal telegraphs. The [lacing of high voltage overhead lines in parallel can cause induction in the signal circuits, interfering with their operation. The placing of these signal circuits underground in order to remedy this is costly and where possible such parallelism should be avoided by modifying the route of the power line. In some cases it will be cheaper to place the signal circuits underground rather than modify the route of the power line. Obviously early consultation with the railway authorities about the proposed route is desirable. Care must also be taken to ensure that any modification to the route of the lone does not have an adverse effect on the amenity of the surrounding area. 2.2.8.3 Clearances When calculating the clearances to be provided over railway tracks, account has to be taken of any track electrification, wither existing or proposed, using pantograph collection of motive power from overhead conductors carried on gantries. Where practicable, provision should be made in the planning of the overhead power line for a tower or pole to be sited close to the railway line in order to give maximum clearance over it without the need for excessively high towers or poles. When planning routes it should be borne in mind that access both for construction and future maintenance may well be constrained by the traffic needs of the railway company.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.2.9 Mineral Workings In mineral working regions, it may be prudent to plan for an increase in the height of the line supports at the time of construction to give extra clearances for jibs or tipper lorries working over known future extraction areas. This additional cost is usually found to be cheaper than paying compensation to the mining company for modifying their removal methods. Additional clearances for any operating plant such as washers and crushers may also be necessary. Maintaining support for a line across gravel crossings can lead to problems and, erosion of the ground around the line supports by the action of wind and water (or by the over-working of the operator) can lead to the need for expensive remedial works to maintain stability In addition, the working of certain minerals involves blasting operations. This can conflict with the presence of an overhead line and result in damage to the conductors or supporting structures. Where possible, these areas should be avoided. Supports near to gravel operating plants frequented by heavy lorries are prone to damage unless adequately protected at additional cost. In some areas of gravel workings, it may become necessary to deviate lines where clearance is insufficient for working and where sterilisation would lead to a greater cost than re-alignment. This can lead to problems of system security during the alterations.

2.3 The Economics of Route Selection. The earlier sections for this module have addressed the consent requirements and the physical restrictions on the construction of an overhead power line. The technical constraints or the main items to consider are: •

The electrical voltage and current (load) requirements of the proposed overhead line

•

The mechanical strength capabilities of the line components (conductor, fittings, steelwork and structures) used to meet with the environmental loads (wind, ice and snow)

•

The components’ reliability and longevity in the field and the techniques or methodologies adopted for the construction and maintenance of the overhead line.

Each of these will be addressed in more detail in later modules. Most utilities will have standard tower arrangements for various voltage levels and conductor sizes. Figures 2.3 in section 2.2.4.3 on special towers were sketches of two 400 kV quad 0.4 sq inches Aluminium Conductor Steel Reinforced (ACSR) conductor tower support structures. Module 3 gives some further examples of steel support structures and wood-pole support structures at lower voltages. The examples in figures 2.3 are suspension towers. A different support structure is required called the tension tower. Proprietary Document Overhead Power Lines Module 2 –Route Planning, Surveys & Wayleaves

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.3.1 Section Lengths The length of a section in an overhead power line is that continuous part of the conductors in suspension between two tension structures. Figure 2.4 is a photograph denoting a tension tower, two suspension towers and the meaning of a span. Tension Tower

1 Span between Suspension Towers

Fig.2.4 Illustrating Tension Towers, Suspension Towers and Spans Tension structures are required for one or more of the following possible reasons: • •

Wherever a line changes direction Where “uplift” of the suspension insulators would occur due to the tower being sited on relatively low ground in a valley and where excessive extensions would otherwise be required to overcome the problem

• Where the length of a straight section of suspension structures would exceed the design parameters of the line for construction and maintenance e.g. the maximum continuous length of a conductor that can be handled during these operations • At points where the maintenance requirements necessitate the dropping of conductors where one line crosses over another and security for the lower circuits is required. Consequently, section lengths are determined by the engineering requirements of line design and operation, by changes in direction caused by physical obstructions, and the availability of suitable sites for the structures. It is one of the objectives of economic line planning to ensure that the minimum number of supporting structures is used. Each additional supporting structure adds to the cost of the line, may cause additional interference to the land on which it is sited, and can be considered an added visual intrusion into the area.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT The distance between line supports will vary with: The type of construction employed The profile of the ground over which the line will pass The cross-sectional area of the conductors being supported The need to maintain the electrical safety clearances required from the ground and from any buildings or other obstructions Additional factors in design limitation concerned with the “sum” of adjacent spans. The latter limits the total length of any two adjacent spans to the design parameters of the type of line being constructed, e.g. referring again to figure 2.3 the normal individual span of the L6 line type is 366 metres. Thus, by careful selection of the positions of tension towers, the use of extensions on steel tower lines where available and the avoidance of obstructions where practicable, a line can be designed where the sections provide the optimum run of conductor length supported by the minimum number of structures. Any larger steel tower lines, the stringing (erection) of conductors is a major operation and it is necessary to have the conductor drums at angle positions together with winches and other miscellaneous gear. The accessibility to those positions must be considered in an effort to reduce the problems of transporting heavy loads into a site. Additionally, the heavier angle towers are not as acceptable in the landscape as the lighter suspension towers and their siting should take into account the requirements of amenity as well as accessibility. Increasing the height of any particular support in a steel tower line generally has a minimal impact overall and so consideration should be given during the planning stages to the inclusion of extensions on certain towers if this would avoid adding an additional tower into the section with its attendant constructional and amenity problems The application for a tolerance provision when it is submitted to the appropriate government department enables the constructing electricity company to adjust the line slightly in order to meet any last-minute wishes of the landowner and occupier’s affected subsequent to the original consent being granted and after a detailed survey has been carried out.

Tolerance provision – this is the UK term – other countries have similar

procedures.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.3.2 Re-alignment of Route to Avoid Obstacles Care has to be taken when planning the route for a proposal, to ensure that a situation does not arise where it becomes necessary to use a series of angle structures to deviate around an obstacle. This is both costly and is usually objectionable on the grounds of amenity. In the case of wooden-pole lines, this inevitable leads to the need for stays which are an additional interference with uses of the land. It is preferable to re-align the route in the early stages of planning in order to avoid the obstacle (Figure 2.5 illustrates this) DEVIATIONS FOR OBSTACLES SMALL AMENITY AREAS OR THOSE OF SPECIAL INTEREST DEVIATION A -B -C-D

Involves the use of

Four additional angle structures

DEVIATION X -B -C -Z

Involves the use of

Two additional angle structures

DEVIATION X -Y -Z

Required

No additional angle structures

Y B A

C

Z

D

x

OBSTACLE

Fig.2.5 Illustrating Line Deviations for Minimum Additional Angle Towers 2.3.3 Underground Cable Sections – Incorporated in Overhead Transmission Lines The selection of a route where a section of proposed overhead line has to be placed underground, either because of a physical constraint like an airfield or because of the amenity considerations, will lead to an increase in the total expenditure on a scheme. The line planner needs to be aware of, and will usually be required to explain the reasons for this during the negotiations previously described in this module.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.3.3.1 Additional Costs and Electrical Problems When a section of underground cable is incorporated in an overhead line, terminal equipment is required at each end. These, together with the cable itself, raise problems of additional costs, electrical security and amenity. Until the development of XLPE cables when such cables were of the oil filled type, much larger spacing between the cable phases was necessary with additional oil tank reservoirs and in some cases water cooling. Additional land was utilised and maintenance levels were high. The advent of XLPE cables at transmission voltage has overcome the need for oil tanks, water cooling etc. However at the terminations, sealing end compounds and terminal supports are still required and if the section is of sufficient length it needs to be separately protected so involving even more cost. 2.3.3.2 Sealing End Compounds and Terminal Supports Sealing end equipment can vary from small boxes mounted on wooden pole supports at the lower voltages to large compounds of about half an acre containing terminal towers and other equipment for high voltage lines. This equipment is required at positions where underground cables are connected to overhead line conductors. On the high voltage distribution and transmission lines, the complexities of the sealing ends, either when mounted on platforms at the sides of the towers or in compounds on the ground adds to the amenity problem. Concealment is not always straightforward and the terminal structures required are the heavier types of tension tower with considerable “clutter” on the cross-arms added to by the “down leads” to the ends. At or near to substations, where underground cable entries may be necessary both for operational and amenity reasons, a multiplicity of sealing ends can occur in a small area. The overall effect of this can be improved if there is little or no cover, by carefully matching of types, either all mounted on platforms, if this is possible, or alternatively, all sited in compounds on the ground. 2.3.3.3 Reinstatement The passage of an overhead line across agricultural fields etc much of the surface is undisturbed except for the areas around supporting structures and along access routes. Cable excavations on the other hand, cover a more extensive area and the re-instatement of the surface along the entire route after laying has finished is a more complicated task.


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(a) Surface Soil The fertile top soil on agricultural fields needs to be removed and stacked separately from the subsoil so that it can be returned last, without mixing, upon completion of the excavations (b) Drainage Systems Interference with field drainage systems can be remedied by the use of suitable pipes laid to bridge the cable excavations. An adequate width of pipe is required so that it is supported on either side of the excavations by undisturbed ground to avoid any problems of subsidence (c) Walling, fences and hedges All will require reinstatement after the work is complete. Hedges require double protective fencing during the period of re-growth In areas of high amenity, where chalk, limestone or other light coloured materials are brought to the surface during cable laying, care should be taken to ensure that the “scar” is re-instated with a covering of top soil, and grassed if necessary. As a result of the above requirements, re-instatement required on cable routes far exceeds that necessary for overhead lines. 2.3.4 Building Sterilisation Whereas an overhead line, building can be permitted under line subject to preservation of the required electrical safety clearances from the conductors and the maintenance of access to the supporting structures, the restrictions on building works over or adjacent to underground cable routes are more severe. For obvious reasons, no development can take place over a cable route because of the danger that can arise from excavations and the need for access facilities to the installation at all times for emergency repairs or maintenance. In places where cable pass under or over permanent features such as railways, rivers and canals, the provision of tunnels, ducts and supporting bridges has to be made for the cables so that they can be easily withdrawn and repaired should this be necessary. Care must be taken to ensure that heat dissipation from the cable is not prejudiced otherwise the cable will be de-rated. A protective or support strip is required on either side of the cable trench to prevent interference with the circuits either by building operations or from invasion by tree roots. As a result, the compensation payable for an easement on a cable route is nearly always greater than that paid fro the equivalent length of overhead line. Proprietary Document Overhead Power Lines Module 2 –Route Planning, Surveys & Wayleaves

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.3.5 Tree Cutting Whilst this problem for line routing is seldom met in the Middle East and Gulf States Countries, this section is included for interest. The continued tree planting, however, in many places suggests that the author should anyway include this section. In a country with as many trees as the UK, it is inevitable that some tree felling and lopping will be required for any major overhead line proposal. The extent of the felling varies with the size of the line and the line planner should be concerned to reduce the amount necessary to an absolute minimum compatible with achieving the maximum screening for the line as it crosses the countryside. Tree felling and lopping is expensive. The provision of alternative replacements for trees that have been felled and compensation payments for the sterilisation of woodland, where applicable, adds considerably to this cost. In addition, the fencing of a line route where it passes through a new plantation to prevent destruction by rabbits, deer and other fauna all add to the cost. In small areas of woodland or copses where a major part of the whole would be felled or lopped or in the case where a route would pass through and close to the edge of a woodland area, consideration should be given, where the ownership permits, to clearing the area completely and returning it to the adjacent enclosures by an adjustment of the fencing and walling. This arrangement has some advantages as follows: The occupier adds to this land and an area of woodland remaining, which would have been difficult and uneconomic to manage, is removed The electricity company has no continuing maintenance problem with the cutting of any re-growth under the line or any further sterilisation payments to make. Figure 2.6 illustrates this. As it is generally necessary at high voltages to dissipate the heat from underground cables by a wider spacing of the circuits than when they are carried overhead, the overall effect of felling is greater for an underground cable than for the equivalent overhead line where it is sometimes possible to substitute lopping for complete felling.

Delegates Notes


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Before

WOODLAND

Key

Overhead Line Roads

After

WOODLAND

INTEGRATION OF A CLEARED AREA OF WOODLAND WITH ADJOINUNG ENCLOSURES BY RE -ALIGNMENT OF BOUNDARIES TO INCREASE EASE OF MANAGEMENT

Fig.2.6 Illustration for Integration of a cleared area


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.3.6 Route Planning for Ease of Construction and Maintenance Although the main parameters for a line route will be resolved by a reconciliation of the many factors already addressed, care in the siting of main structures in accessible areas can go along way towards reducing the problems of construction and subsequent maintenance. This approach can also reduce, where appropriate, the compensation payable during either construction or maintenance. Therefore, the line planner will also be considering at the appropriate time the effect that access to the proposals will have on the land and its management during the construction period. The aim will be to ensure that the minimum damage is caused to third party assets and that the maximum use can continue to be made of the land by the third party as the construction proceeds. In general, the higher the voltage of the overhead line to be constructed then the longer is this construction period. At 400 kV it may sometimes continue intermittently over a period as long as three years. In order to do this effectively, the line planner needs to be experienced in the methods of working employed during the construction. In addition the planner must be aware of the likely loads of equipment that has to be transported into the various sites and the period of time that the work is likely to continue on the holding. The availability of existing access roads is critical in desert countries where the construction of temporary reinforced tracks as access roads can be time consuming and expensive. In extreme cases, it is possible, but somewhat expensive, to construct a line by carrying in all equipment and materials by helicopter. 2.3.7 Power Corridors Over the years, consideration has been given in a number of countries to the need for, and the acceptability of “power corridors”. The practice is much favoured in North America. Support for the use of power corridors, where overhead lines are kept parallel and close together, has been based on the supposition that this practice would limit the visual intrusion of those lines to specific areas rather than appear as a network throughout the entire landscape. The layout of many transmission networks around the world does not lend itself to power corridors. Grid Systems with their major switching and transforming substations site some thirty to 40 km apart at most, produces separated radial patterns rather than parallel routes for the interconnecting network and associated distribution lines.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT In many networks problems of parallelism do arise in the vicinity of substations, where numbers of transmission and distribution lines meet. Therefore, it is often the policy at entries by cabling the lower voltage distribution lines for considerable distances pf say 1.5 kM or more to obtain a reasonable separation distance between lines. At the same time at compact Transmission substations with more than one or two circuits and equipped with Gas Insulated Switchgear (GIS) it is necessary anyway to cable in circuits with short lengths. When constructing new overhead lines, double-circuit supports are best employed to provide for the maximum future capacity even if only one circuit is required initially. In addition, dual construction at medium voltage 11-13.8 kV helps to reduce the proliferation of poles over limited distances. Power line corridors are best suited to countries where the landscape is on a large scale and where the electricity companies purchase land and own “rightsof-way” for their overhead lines. In the UK, the majority of overhead lines cross properties under different ownership using either easement or wayleaves as previously addressed in the module. In this case the wishes of the owners and occupiers need to be considered when planning a route and their acceptable requirements for the siting of towers and poles on the land that they own has to be complied with wherever possible. If power corridors are to be an advantage for the visual impact, then the type of line construction in the main must be similar for symmetry. This would mean designing lines with similar tower heights, span lengths and therefore matching catenaries. This is particularly difficult in the UK when:

There are substantial variations in the ground profile.

There are scattered building developments along the route causing separation of the lines.

The requirements of the individual wayleave grantors have to be met

The visual impact of miss-matches with different types of tower and span lengths that can occur in the locality of a power corridor is infinitely worse than the aggregated impact of two well-separated lines. Even where lines of a similar type have been successfully matched the visual impact is still severe. Other problems can arise in farming areas with power corridors as a result of the increased interference caused to agricultural and forestry operations by lines close to and in parallel. The need for multiple supports and stays increases the difficulty in the farm enclosures and the wider swathes required prevent the growing of trees in large areas of any woodland that the corridor passes through. Suitable positions required for the several structures adjacent to each other are often difficult to find. There is also an impact on system security. In areas of high lightning incidence or conductor clashing due to high winds, the probability of multiple fault conditions occurring almost simultaneously increases dramatically. Proprietary Document Overhead Power Lines Module 2 –Route Planning, Surveys & Wayleaves

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.3.8 The investigation of Alternative Routes There are two main categories of alternative routes for overhead lines that require investigation during the planning stages:

Alternatives which are investigated by the electricity company in the normal course of selective route planning, using the same criteria for assessment as those used for the selection of the preferred route for which it is proposed to seek consent.

Alternatives suggested by objectors to the preferred route which have not necessarily been subjected to the same criteria as those applied to the preferred route. These must be investigated and assessed in the same manner as the electricity company’s preferred proposal and a reasoned statement made about their suitability or otherwise based on engineering, wayleaving (where appropriate) and amenity considerations

In the planning of routes for the larger transmission lines, where considerable distances can be involved, it is not generally feasible to obtain consents along both the preferred route and also any alternatives being considered. The case in the UK is that outline consent is usually sought from the individual landowners until they are all obtained and then final consent being obtained from the government. In the planning of short routes the situation is different in that it is more normal to obtain consents for all its length before requesting consent from the appropriate government department. 2.3.8.1 Technical Feasibility The alternatives(s) must achieve the same standards as the electricity company’s preferred proposal in respect of the quantity, quality and security of supply. 2.3.8.2 Cost If its is to be considered seriously, the cost of any alternative must follow the general level of costs incumbent on the electricity company’s preferred proposal. A thorough appraisal of the number and types of structures to be used on each route is required in order to compare costs. This requires a ground survey on the alternative routes unless an aerial survey has already been flown which covers this (Refer also to Module 3). When comparing the costs of alternatives, consideration has to be given to the vulnerability of such a route to undergrounding either because of a physical constraint such as an airfield or for amenity reasons. Any additional problems associated with access for construction and maintenance should be calculated and included, together with the extra construction costs caused by the need to pile drive foundations or excavate in rock where this is required.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.3.8.3 Amenity Any assessment of the effect on amenity will include the following comparisons: Visibility overall and prominence of the alternative routes 1. The values of the countryside along each route 2. The amount of tree felling if required 3. The effect on existing properties and their environment 2.3.8.4 Future Known Developments These will include considerations of the following 1. 2. 3. 4.

Urban development. Road proposals. Mineral working areas. Changes in land designation e.g. land set aside for a future new town or amenity park. 5. Future proposals in the area to be made by the electricity company

2.3.8.5 Agriculture The overall effect of the number of structures per kilometre of line upon agricultural operations should be considered particularly where aerial crop spraying is adopted. It may be useful to discuss the general merits of alternative routes with representatives of the appropriate government ministries when they are broadly similar in all other respects. Where alternative routes cover wide areas, it is sometimes convenient to fly along them in a helicopter to eliminate, where possible, those routes that have hidden physical constraints, such as properties in woodland, which are not visible from the surrounding roads. The actual selection of the final route should be done on the ground where the contours are more readily apparent and the effect of the proposal on the countryside as it will be seen by people can be assessed. Aerial and Ground surveys are addressed later in this module. (Refer to section 2.5)

Delegates Notes


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2.4. Types of Objections to be Considered when Planning Routes 2.4.1 Towers and Insulators The greatest visual impact of any overhead line is caused by the supports, whether steel or wood pole, and the associated insulators. The visual impact of wood pole lines is over a very limited area and they are generally capable of being assimilated into the landscape without becoming a major intrusion. The impact increases, however, when a number of wood pole lines are sited close together or when there is associated top-hamper such as pole-mounted transformers and cable sealing ends. The larger steel tower lines, however, are generally out of scale with the intimate countryside covering most of the landscape of countries in Europe and elsewhere where there is a large amount of fauna. The heights of indigenous trees are rarely as tall as main transmission towers. Unfortunately, the parts of the tower that cause the greatest visual impact are the cross-arms and the insulators that are by necessity located in the upper part of the tower. They cannot thus be screened effectively. The proportions and design of the upper sections of steel towers can only vary within the engineering standards prescribed and the line planner must therefore concentrate on reducing the number of towers with multiple insulators and heavy cross-arms to a minimum. (Refer also to Module 3) In Europe, experiments have been made by painting steel towers with other than “battleship grey” in an effort to reduce their effect on the landscape. However, the changing seasonal colours render this exercise somewhat futile and the general consensus is that the weathered galvanising or grey paint is the best compromise. In the UK the Electricity Supply Industry has discussed new tower designs with the Royal Fine Arts Commission and, over the years, this has helped to produce such improvements in design as waisted or eiffelised towers with a more pleasing appearance. Porcelain, toughened glass and polymeric/resin (composite) are the three insulating materials used on overhead lines. Variations in the colour have been employed in a number of countries. Porcelain insulators have been given a silvergrey finish instead of the more normal brown colour and insulators made of green instead of clear glass have been employed on some instances. There is a tendency, however, for glass insulators to reflect in sunshine and become very much more visible. The number of strings of insulators on some suspension towers is reduced from the normal two to one per phase by making a slight overall increase in the size and load bearing capacity of the remaining single string.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT The increase in the physical size of lines necessitated by the increases in operating capacity has been of considerable benefit. The multiplicity of smaller lines required to provide the same power transfers at lower voltages would have resulted in an unacceptable level of “wirescape” and interference with land use. Many objections to towers and their attendant insulators can usually be overcome by a minor re-positioning along the line and an intelligent appreciation of the surrounding environment. 2.4.2 Conductors To increase power flow through each route accomplished, both the line voltage and the number of conductors need to be increased. At 400 kV quadruple conductors in each phase are normally employed. The four conductors are joined at intervals along the span by spacers to maintain conductor separation in all conditions. Some 400 kV lines employ double conductors again with spacers. Both these arrangements keep corona discharge within acceptable limits it being inversely proportional to the effective emitting area. (Refer also to Module 5) at the same time as increasing the capacity. However, this quad and twin type construction also increases the prominence pf the line spans, although at a distance, the twin and quadruple conductor formations merge to become one entity visually. In order to reduce the visual impact of the quadruple conductor formations, conductors were developed with larger cross-sectional areas for use in twin formation per phase on the heavy duty 400 kV lines. When planning the route profile, the design engineer should, wherever possible, endeavour to provide a sympathetic match of the conductor catenary to the physiographic qualities of the area. 2.4.3 Sealing Ends Sealing ends are used at points where an overhead line is taken underground by means of a cable. This occurs where a line entry into a substation has to be cabled or where a section of line is placed underground for operational or amenity reasons. (Refer also Section 2.3.3.2). Fig. 2.7 Circuits Cabled into Power Plant (Kilroot, UK)


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT In urban areas, vandalism can be a problem at sealing ends which are fitted to platforms up a tower. To protect the equipment from stone throwing and guns, protective screens may have to be erected and these add to the equipment that has to be mounted on the tower, thus adding to the visual intrusion of the line. 2.4.4 Corona Discharge Among the factors that affect the choice of conductor in a transmission line, apart from the amount of current to be carried, is the phenomenon known as corona. As the voltage on a conductor is increased beyond a certain point, a hissing or “frying” noise will become audible. In the dark, a glow can be seen around the conductor. The voltage at which this occurs depends upon the following conditions:

The diameter of the conductor

The surface characteristics of the conductor

The distance between the conductors

The atmospheric Conditions

The noise from corona discharge can be very intrusive on residential properties situated in quiet urban or rural surroundings adjacent to the line. (Refer also to Module 5 for further details on corona discharge) (Refer also to Module 6 – Maintenance of Overhead Lines) 2.4.5 Wind In exposed areas or in conditions of high wind velocity, a hum occurs as the wind passes over the surface of a conductor that can be very penetrating and like corona cause a nuisance to residential properties. Attempts to reduce this noise by greasing have had little success. 2.4.6 Arcing This is usually associated with a defect on the line such as cracked or defective insulators. These faults should be remedied as soon as possible. (Refer also to Module 6 – Maintenance of Overhead Lines) 2.4.7 Radio and Television Interference (“active”) The interference to radio signals resulting from the emission of radio waves or “radio noise” from an overhead line is known as “active interference”. (Refer also to Section 2.2.4.5)


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.4.8 Drips and Ice The following sub-section clearly does not apply in hot climates but has been included for completeness and interest. Over open ground there is unlikely to be any serious physical damage caused under the line due to the action of rain or ice and consequently this factor has little effect on the selection of a line route. However the possibility of damage caused by ice falling from conductors over buildings and gall-houses must be taken into account. It is desirable in such climates to avoid any glasses areas on farms and market gardens since consequential damage can result. 2.4.9 Land Use Interference The presence of a tower or pole line and associated conductors places a restriction on the use of the land over which it passes. Development is prevented at tower and pole positions. Access must be maintained to them at all times and the height of any development under the line is restricted by the electrical safety clearances required between building and conductors. 2.4.10 Agricultural Interference Towers, poles and stays almost invariably interfere with farming operations. Even when sited in or astride hedges, they cause problems when these are cut mechanically. The main interference is to arable cultivation where ploughing, discing, harrowing and rolling operations have to avoid the bases of towers or poles/stays. Aerial crop spraying is inevitably interfered with particularly where there are many lines entering a substation. 2.4.11 Damage to Trees Again this is not relevant to any great extent in this part of the world but as previously described construction inevitably requires some felling and lopping of trees. 2.4.12 Damage to Land This is generally a construction problem caused by the passage of heavy loads and the line planner should always consider the effect of the proposed route and accesses to the support positions on the enclosures that would be affected, particularly for the heavy tension towers at the changes in direction.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.4.13 Physiological – Effects on the Human Body. Over the past two decades, there has been a considerable in public awareness and interest about the physiological effects of high-voltage electromagnetic and other radiation effects on the human body when it is subjected to various periods of exposure. To date there is no clausal evidence connecting brain cancer and other illnesses with persons who are subjected to the high-voltage electromagnetic fields from overhead lines at least up to 400kV, (Refer also to Appendix 3)

Delegates Notes


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2.5 Surveying and Profiling 2.5.1 Introduction Before any new line is built or any major changes made to an existing line it will need to be surveyed and profiled. Essentially, surveying means going along the line and making a detailed note of: •

Every existing support structure if upgrading an existing circuit age, condition),

(height,

•

Span length,

•

Conductor type,

•

Nearby tree growth,

•

Land heights relative to the conductor

•

Land height relative to mean sea level (normally to a local Ordnance Survey (OS) datum point)

•

Road, rail, river or footpath crossing

•

Fences, houses etc near to the line

Any obstacle that can affect, even remotely, the existence of the overhead line needs to be considered. Surveying is essential as new buildings are built and land changes use in ways that could affect safety or normal operation of the line. Large electrical utilities will have qualified surveyors whilst smaller utilities will employ specialist-surveying companies. It is often advisable anyway to employ a consultant who knows what to do and can almost certainly do it quicker and cheaper and more effectively. The output of all this will be a profile of the land within, say, 10 metres either side of the existing or planned line. It is important for overhead line planners to realise what is involved in a survey and the results that can be expected. It is also important to know what cannot be expected of the surveyor. Standard surveys and profiles are relatively inexpensive in comparison to line construction or refurbishment costs; however as the cost of the line includes surveying and profiling the costs should be kept to a minimum by deciding on the route in general terms with a preliminary survey. There are two situations that require surveys and profiling 1. A new line 2. Refurbishing an existing overhead line.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT The logical sequence for the design and planning of the routing of an overhead line is shown in figure 2.8. It is assumed in this example that the client, or his consultant, carries out the preliminary routing and includes this information in a tender specification such that competitive tenders may be received from a variety of design and construct contracting organisations. The contractor will then carry out the detailed line routing and profile work. By careful preliminary routing the effect on the environment may be minimised. In addition, the client is in a position to narrow the choice of tower and span design to the most economic. At the same time the client is able to take into account a strategy for minimum maintenance costs. 2.5.2. Routing Objectives The preliminary routing work determines the physical constraints involved as describe in section 2.2. and allows the establishment of the least-cost solution for the overhead line. Estimated quantities for the towers, foundations and conductors may then be included in tender documentation for the supply and erection of the overhead line. The detailed routing survey and profile allows the towers to be located in the most economic manner. It will take into account proximity restrictions and maintenance of specified design parameters such as electrical clearances, wind spans, angles of deviation and so on. 2.5.3 Preliminary Routing 2.5.3.1 Survey Equipment Requirements 1. Appropriate maps for the expected line route and adjacent areas to a suitable scale, typically 1:10,000 for cross-country work. Standard 1:50,000 scale maps are also useful to get a general idea of the route. A 1:2,500 map is generally used for precise pole position on medium voltage circuits. 2. Good survey quality compass and compass bearing monocular. These greatly assist orientation and road or river crossing locations together with the projection of sections. 3. Theodolite and level may be worthwhile but not essential for the preliminary survey 4. 100m tapes 5. Hammers, identification paint and pegs 6. Ranging rods to enable checks and recording of location relative to centres of proposed angle towers.


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ENGINEERING INFLUENCES

EXTERNAL INFLUENCES

Decision to build Transmission line From (A) to (B)

Climatic conditions

Select Only from Standard voltages

Electrical

NATIONAL REGULATIONS

Survey/ Geological/ geographical

(A) SOURCE OF POWER SUPPLY Voltage selection

Preliminary Route selection

Optimise Voltage selection

Environmental Impact studies

Route optimisation

Optimise Conductor selection

Select Only from Standard conductors

Ground profile survey Voltage drop and loss calculations

Statutory clearances

Civil/ Structural

Preliminary Conductor selection

Environmental Constraints

Loading Conditions

System Planning

Preliminary ruling span Insulation Selection and coordination

Finalise Conductor selection

Shield wire Selection and location

Operational and Maintenance constraints

Flashover Risk analysis

Try alterative conductors

Family of towers Tower top layout

Soil survey

Preliminary Tower configuration

Foundation designs

Ruling Span optimisation

Review Original Conductor Tension choice

Reconsider Type of Conductor Originally selected

Profile analysis

Try alterative voltages PWWR and Optimum Line casting

Final tower and foundation design Tower spotting optimisation

Power Demand location (B)

Fig. 2.8 Logical Sequence for Overhead Line Design, Planning and Routing Proprietary Document Overhead Power Lines Module 2 –Route Planning, Surveys & Wayleaves

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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.5.3.2 Aerial Survey Aerial survey photographs greatly aid the routing designer and reduce the time taken for the ground survey. The proposed route is indicated on the photographs if available in conjunction with maps of the area. 2.5.3.3 Ground Survey The ground survey is required since the exact route may differ very considerably from that proposed by studying maps and aerial photographs. This is as a result of all the difficulties of obtaining wayleaves that have been described earlier in this module. 2.5.3.4 Ground Soil Conditions It may be possible in exceptional cases to route the overhead line such that the chosen ground conditions favour low foundation costs. However, in practice large savings are unlikely to be reaped since considerable deviations are likely to be necessary to achieve this condition. The increase in overall materials and construction costs of the overhead line route will usually obviate the savings so accrued. 2.5.3.5 Wayleave, access and Terrain Figure 2.9 shows an example of a 400kV single circuit overhead line crossing extremely difficult terrain in the Zagros Mountains of Iran which connects the Shah Kabir Dam hydro-electric scheme to Arak. Overhead lines often cover areas without good communications access. In such cases the construction is greatly assisted by the use of helicopters. Figure 2.10 illustrates this 2.5.3.6 Optimisation (a) Practical Routing Considerations The sending and receiving ends of the transmission line from existing or future substations or tee-off points are first established and are usually well defined. The straight line between these two points must the n be investigated to see if this really represents the cheapest solution. In practice, wayleave availability, access, ground conditions, avoidance of populated or high atmospheric pollution or industrial areas, difficulties for support erection and maintenance almost always require deviations from the straight line option. (Refer again to section 2.3.5 figure 2.6) Further economic considerations involving parameters, which are extremely difficult to equate in financial terms, such as the impacts of the line on the environment, must be considered as described earlier in this module. If a 400 kV tower costs $100,000 to design and detail then the use of a limited number of standard designs may well prove cheaper than having a large number of special tower types necessary to achieve the more direct line route. Proprietary Document Overhead Power Lines Module 2 –Route Planning, Surveys & Wayleaves

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Fig.2.9 400kV single-circuit twin conductor overhead line crossing the Zagos Mountains in Iran (Courtesy of Balfour Beatty)


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Fig.2.10 Helicopter assisted conductor stringing – Hong Kong (Courtesy of Balfour Beatty)

Lines should not be routed parallel to pipelines or other similar services for long distances because of possible induced current effects. If this is not possible then there should be a minimum distance of around 10 metres between the vertical projection of the outer phase conductor for a 132/145 kV line and the pipeline. Similar precautions must be taken with regard to proximity to gas relief valves or hydrants. Gaz de France sets threshold values of maximum AC induced currents in pipelines at 100 a/M2. Corrosion effects from AC should be negligible because of the current reversal. However, research has shown that there is a small polarising effect which could lead to corrosion in the very longterm. Oil companies require a minimum clearance between overhead line counterpoise (if installed) and buried steel pipes of say, 3 metres for the first 5 kA of earth fault current plus 0.5 metres for each additional kA. (b) Methodology Once the terminal points for the line have been established they are linked on the maps avoiding the obstacles as previously described. Angle or section towers may be provided near the terminal points in order to allow some flexibility for substation entry and slack spans or changes to the future substation orientation and layout. The proposed route is then investigated by walking or driving along the whole of the route. The purpose of this thorough investigation is to endure that the route is feasible and what benefits could accrue from possible changes.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT The feasible preliminary route is then plotted on the maps to at least 1:10,000 scale. The approximate quantities of different tower types (suspension, 30O angle towers, 60O angle towers, terminal towers etc), conductor and earthwire(s) are established. Suspension towers will often account for more than 80% of the total number of towers required on the overhead line route and quantities must be optimised and accurately assessed. Ground conditions are recorded during the field trip in order to estimate the different tower foundations required (piled-and, if necessary, is access for a piling rig possible? – screw anchor, normal, rock etc). At the same time an estimate of the difficulties likely to be encountered in obtaining the required tower footing resistance and the need for counterpoise, tower earth rods etc is made. The cost of the line is proportional to the tower steel and foundation loads. The tower weight, W, may be approximated from Ryles formula: -

W = C x h x M …………………………………………………(2.1) where W is the tower weight of steel (kg)

C is a constant h is the tower overall height (m) M is the tower overturning moment under maximum loading conditions at ground level (kg) Tower heights and their overturning moment are established for a variety of basic spans. This then allows the engineer to concentrate on the total number of intermediate towers required. Such an iterative procedure is, of course, very suited to computer analysis. However, the computer algorithm will place towers in inconvenient or impossible locations without the knowledge resulting from the field survey described. The costs of suspension insulator strings, fittings and foundations are then added to the estimated number of towers in order to derive the basic span and the first approximation to the cheapest overhead line routing solution. A constant ratio is applied to each basic span in order to obtain the wind span. This constant (typically 1.1 times) is necessary to allow for some flexibility over uneven ground. A factor of 2 x over the basic span may be used as a guide to the weight span (which does not greatly affect tower design) and this will allow for tower spotting and wind spans to be optimised. The average span is the basic span multiplied by an efficiency factor which rakes into account the nature of the ground and varying span lengths envisages from flat to hilly terrain. The estimated quantities for materials may be derived from the average span. Finally, the technical specifications for the overhead line are drawn up for use in tender documentation.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.5.4 Detailed Line Survey and Profile 2.5.4.1 Accuracy Requirements The objective is to draw up a plan and section so that further refinement of the tower distribution may be made. The party carrying out the work will depend upon the type of contract being let by the electricity company in charge of the works. The required accuracy should be ± 0.3 m in the horizontal plane and ± 0.1 m in the vertical plane. Greater accuracy is possible from the survey data but in practice cannot be easily transferred to the profile. The location of angle and terminal towers is best specified in a contract document rather than allow a complete free hand to the overhead line contractor. This is because access may be an important parameter for the electricity company if maintenance costs for the line are to be minimised. The vertical profile ground line is surveyed from one angle or terminal tower to the next. When national maps of good quality are available the vertical survey data may be cross-referenced to bench marks of a known level. Horizontal survey dimensions to tower centrelines are checked against the 1:10,000 map and differences investigated until resolved on site. In hilly terrain side slopes in excess of ± 0.3 m must be recorded together with all major features (angles of deviation, other power overhead lines or cable routes, underground services, roads, rail, river and pipeline crossings, buildings adjacent to the wayleave etc) 2.5.5 Profile Requirements 2.5.5.1 Vertical and Horizontal Scales In order to keep the drawings to a manageable size the detailed survey drawings are scaled to typically 1:200 vertical and 1:2000 horizontal or as necessary in hilly terrain. On sloping sites it will be necessary to ensure that foundation depths are not compromised and individual tower legs may be adjusted to correct to the tower centre profile level. The profiles, whether computer generated or not, should be on graph type paper with a grid background. This greatly eases the reading of span lengths or clearances even when photocopy prints have slight distortions.


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT 2.5.5.2 Templates A typical sag template is shown in figure 2.11. Historically templates were prepared on Perspex ( ≈ 3mm thick) with all the engraving on the back using the same scales as the ground profile. The templates show:

The maximum sag condition (usually at maximum temperature but could be under extreme loading conditions)

The minimum sag condition (Usually at minimum temperature without the loading)

Basic span and cases up to about ± 20% above and below the basic span

Today modern computer tools permit the sag/tension relationship to be calculated using full catenary equations. Normally the parabolic approximation suffices unless special long spans or hilly terrain with slopes greater than 15 o are envisaged. Using the parabolic approximation for the tension for any equivalent span is then given by:

(

EA x α x (t 2 - t 1 ) + W1 x g2 x L2 x EA/ 24 T1 2

2

) - T = (W 1

2

2

where E = modulus of elasticity MN/m2 A = conductor cross-sectional area mm2 α = coefficient of linear expansion per OC t1 = initial temperature OC t2 = final temperature OC W = weight of conductor and may include wind and/or ice loadings W1 = initial conductor effective weight in kg/m W2 = final conductor effective weight in kg/m g = gravitational constant (1 kgf = 9.81 N) 9.81 m/sec2 L = span length m T = conductor tension N T1 = initial conductor tension N T2 = initial conductor tension N


)

x g 2 x L2 x EA /24 T2 - T2

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20mm Min

Approx 1.8 times Basic Span

Approx 1.2 times Basic Span 20mm Min

50 mm Minimum True Size

SAG at Max Temp.

Ground Clearance Inc. Creep

SAG at Min Temp.

SAG TEMPLATE

TEMPLATE CONSTRUCTION

THE TITLE BLOCK MUST BE LOCATED IN THE LOWER RIGHT HAND CORNER AND CONTAIN THE FOLLOWING INFORMATION: Equivalent Span in large printing Conductor Data such as Name and Stranding Sagging basis giving design parameters such as Tensions, Temperatures, Wind and Ice (if applicable); Scales as used on profile; Route Title including voltage; Client’s name; Contractor Name

Material to be Perspex between 2 and 3 mm thick, Stable in the country where line is constructed. All lines and curves to be clearly engraved on the reverse face as convenient. Curves are to be marked with relevant wording giving sag and temperature or ground clearance value. All Engraving to be permanently filled with black paint

Fig.2.11 Typical Sag Template Details (Courtesy of EDP Limited)


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OVERHEAD POWER LINES MODULE 2 – ROUTE PLANNING, SURVEYS & WAYLEAVES PROPRIETARY DOCUMENT The sag (S in metres) for different span lengths is then derived from the following expression: -

S=

W x g x L2 m …………………………………………………………………..(2.2) 8T

ice weight per unit length = By(y + x) kg/m Wind load =

p(2 y + x ) N/m ………………………………………………………(2.3) 1000

Effective conductor weight = =

⎡ (wind load)2 ⎤ 2 ( ) + + weight of conductor ice ⎢ ⎥ g2 ⎣ ⎦

Where x = conductor diameter in mm y = radial thickness of ice mm p = wind pressure N/m2 More than one technically feasible solution for tower locations is always available and therefore the final test of acceptability is based on cost. It is always essential that tower fittings, extensions and foundations are taken into account. In addition, clearances must not be infringed. 2.5.5 Computer aided Techniques It is now normal practice to use computer-aided techniques to prepare the overhead line profile. Chainage, level, vertical and horizontal angles may all be transferred directly from a modern theodolite via a portable computer to an office power line survey and computer aided drafting and design (CADD) facility. This eliminates the need for completion by the surveyor of a field record book and any transcription errors that may occur. In addition, some packages allow details of type of ground, ownership, etc., also to be recorded electronically during the survey. As a further feature such surveys may be linked into coordinates derived from geostationary geographical information system satellites (GIS). It has been estimated that power utilities using such systems can achieve survey and data transfer time savings of up to 50% and nearly 80% for line design involving poles and towers respectively. Once field data has been transferred to the CADD tool the ground line profile may be automatically produced with all the annotations that the surveyor has included in the field. Overhead line structures may be spotted at any point along the profile manually by the engineer or automatically by the computer and strung with any conductor type. The software library containing conductor, pole and tower information will then be used to calculate sag and tension for the given conductor, uplift forces on any structures and ensure ground clearances are not infringed at a user specified temperature.


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2.6. Summary This module has covered the physical and amenity constraints on ideal routing of overhead lines. It has covered the need for surveying and profiling at the preliminary stages of a project together with the detailed surveying and profiling required before construction commences.

Delegates Notes


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Overhead Power Line Manual 111

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