190850039 Overhead Power Line Manual 111 PDF

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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

Page No.

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

12 12

Courtesy of National Grid - UK Fig.1.9 Bipolar HVDC 1.5 Summary

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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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT

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

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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.

Proprietary Document Overhead Power Lines Module1 - Power System Overview

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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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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT

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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OVERHEAD POWER LINES MODULE 1 - POWER SYSTEM OVERVIEW PROPRIETARY DOCUMENT

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

R1

R2

N

I3

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 I2 ) + (-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 180210 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...