BEE-001 block 2

BEE-001POWER

DISTRIBUTION SECTOR

Indira Gandhi

National Open UniversitySchool of Engineering and Technology

Block

2OPERATION AND MAINTENANCE

UNIT 4

Introduction to the Power Distribution System 7

UNIT 5

Substation Equipment and Distribution Lines 49

UNIT 6

Distribution Transformer 77

Course Design Committee

Sh. R.K. ChaudhryNPTI, Delhi

Ms. Indu MaheshwariNPTI, Delhi

Shri R.V. ShahiFormer Secretary, Ministry of PowerGovt. of India

Shri Arvind JadhavFormer Joint Secretary, DistributionMinistry of Power, Govt. of India

Shri V.S. SaxenaDirector, Power Finance Corporation

Shri Gaurav BhatianiProject Manager, USAID, India

Shri Vinod BehariGM(HRD), Power Finance Corporation

Dr. D. RayAddl. GM (HRD)Power Finance Corporation

Shri Sudhir VadehraChief, DRUM Project Secretariat

Project Coordinator: Prof. S.C. Garg

Programme Coordinator: Mrs. Rakhi Sharma

Prof. S.C. GargSchool of SciencesIGNOU

Prof. Gayatri KansalSchool of Engineering and Technology IGNOU

Prof. VijayshriSchool of Sciences IGNOU

Dr. Ajit KumarSchool of Engineering and TechnologyIGNOU

Block Preparation Team

Course Coordinator: Mrs. Rakhi Sharma

Shri Pankaj Prakash, Editor

Director (Finance)

Uttaranchal Electricity Regulatory CommissionDehradun

Mr. N. R. HalderNPTI, Delhi

Mrs. Rakhi SharmaSchool of Engineering and Technology, IGNOU

Ms. Anjuli ChandraDirector, CEA

Prof. VijayshriSchool of Sciences, IGNOU

Acknowledgements: Prof. S.C. Garg for valuable comments and suggestions on the units.

The contribution of Shri Pankaj Batra, Director, CEA to the section on Grid Management, LoadScheduling and Load Balancing is thankfully acknowledged. Some material contained in theunits has been sourced from the courses developed under the DRUM project, and is thankfullyacknowledged.

Production

Shri Y.N.Sharma, SO(P)School of Engineering and Technology, IGNOU

Shri Aditya Gupta

This programme has been developed by the School of Engineering and Technology, IGNOU incollaboration with the Ministry of Power, USAID-India and the Power Finance Corporation underthe Distribution Reform Upgrades and Management (DRUM) Project.

March, 2007

© Indira Gandhi National Open University, 2007

ISBN:

All rights reserved. No part of this work may be reproduced in any form, by mimeograph or any other means, without permission in

writing from the Indira Gandhi National Open University.

Further information on the Indira Gandhi National Open University courses may be obtained from the University’s office atMaidan Garhi, New Delhi-110 068.

Printed and published on behalf of Indira Gandhi National Open University, New Delhi by Director, School of Engineering &Technology.

Printed at

Unit 4 Introduction to the Power Distribution System

Contents

4.1 Introduction 84.2 Description of the Power Distribution System 8

Voltage Levels 10

Conductors 10

High Voltage Distribution System (HVDS) 11

4.3 Components of the Distribution System 13Substation 13

Transformer 14

Feeders 16

Meters for Measurement of Energy and OtherElectrical Quantities 19

4.4 Distribution System Planning 20Planning Horizon 21

Principal Areas of Activity 22

4.5 Operation and Maintenance Objectivesand Activities 26Operation and Maintenance Objectives 26

Activities Involved in Operation and Maintenance 28

Renovation and Modernisation (R&M) and LifeExtension Schemes 28

4.6 Grid Management, Load Scheduling andLoad Balancing 30Grid Management 31

Load Scheduling and Dispatch 35

Load Balancing 39

4.7 Summary 414.8 Terminal Questions 42

7

Operation and Maintenance 5

5.1 Introduction 505.2 66-33/11 kV Substation Equipment 505.3 11/0.4 kV Substation Equipment 575.4 Distribution Line Equipment 60

Overhead Lines 60

Underground Power Cables 65

5.5 O&M Practices for Substation Equipmentand Distribution Lines 68General Maintenance Practices 68

Maintenance of Lines 70

Operation and Maintenance of Capacitors 71

Hot Line Maintenance 72

Unit 5 Substation Equipment and Distribution Lines49

Appendix 1 Reactive Power Control in Distribution Systems 43

Appendix 2 Functions of O&M 45

5.6 Length of LT Lines, HT:LT Ratio andImpact on Losses and Voltage 73Impact of Increasing HT Lines 74


Unit 6 Distribution Transformer77

6.1 Introduction 786.2 Distribution Transformers: Selection and

Placement 78Classification of Transformers 78

Criteria for Transformer Selection 80

Placement of Transformers 81

6.3 Reasons for Transformer Failures 81Ageing 82

Manufacturing Defects 83

Improper Structure of Distribution Transformer 85

Impact of Natural Calamities 85

Improper Operation and Maintenance (O&M) 86

6.4 Transformer Testing 87Testing of Windings−Insulation and Mechanical

Strength 87

Testing of Insulating Transformer Oil 88

Other Tests 90

6.5 Enhancing Transformer Life and Efficiency 90Transformer Operation 91

Maintenance Methods 93


Appendix 1 Case Studies on Averting Distribution Transformer Failure 98

Appendix 2 A Checklist for Preventive Maintenance of Distribution Transformers 103

OPERATION AND MAINTENANCE

In Block 1, you have learnt about the challenges that the power distribution sector in

India is faced with such as huge transmission and distribution losses, unreliable

power supply, poor quality, lack of concern for consumers and a highly skewed tariff

structure. You have also studied about the salient features of the Energy

Conservation Act, 2001, Electricity Act, 2003, the National Electricity Policy and the

National Tariff Policy. We have discussed the distribution reforms being ushered in

the otherwise monopolistic service sector. The overarching aim of these reforms is

to help the power sector overcome its weaknesses. You will agree that distribution is

the cutting-edge of the power industry and it needs to get back on the right track.

This Block is dedicated to the most critical part of the power system viz. the Power

Distribution System or simply the Distribution System. The Units in the block

describe the components of the Distribution System and the philosophy and

practices of their operation and maintenance.

Unit 4 (entitled Introduction to the Power Distribution System) is devoted to the

general description of the Distribution System and spells out the operation and

maintenance philosophy, objectives and activities. It also touches upon an overview

of the planning process and the concept of grid management, load scheduling and

load balancing in the context of the Distribution System.

Units 5 and 6 go deeper into the components of the Distribution System describing

their operation and maintenance practices. Unit 5 as is evident from its title,

Substation Equipment and Distribution Lines, deals with the O&M of the

substation equipment and distribution lines and Unit 6 (entitled DistributionTransformer) covers the working principle, operation and maintenance of

Distribution Transformers in detail.

We hope that the information about the power distribution system and the O&M

practices presented here would help you in improving the performance of the power

distribution system and deliver reliable quality power to the consumer within optimum

fixed and operating costs. We wish you all the very best!

Learning Objectives

Unit 4

Introduction tothe PowerDistributionSystemAfter studying this unit, you should be able to:

describe the important features of the power

distribution system; outline the advantages of high voltage

distribution system (HVDS); describe various components of the power

distribution system; explain various activities involved in distribution

system planning; discuss the operation and maintenance

principles and practices for the power distribution system; and

explain the fundamental features of grid management, load scheduling and load balancing.

Operation andMaintenance

8

In Unit 1 of Block 1, you have been very briefly introduced to the power supply

system. You have also learnt in Unit 1 that the demand for electrical power in

India is enormous and growing steadily. Units 2 and 3 have provided you an

overview of the power sector with a special focus on the power distribution

sector, which is responsible for covering the last mile in reaching power to the

consumers.

In this Unit, we give a description of the power distribution system and its

components. We acquaint you with the concept of distribution system

planning, which forms the basis for the smooth operation of the power

distribution system. We also present the general principles and practices

underlying the operation and maintenance of the system. In the next Unit, we

deal specifically with the operation of substation equipment, distribution lines

and their maintenance requirements.

You are familiar with the power supply system. You know that electricity is

generated at 11 kV by electrical generators which utilise the energy from

thermal, hydro, nuclear, and renewable energy resources. To transmit

electricity over long distances, the supply voltage is stepped up to 132/220/

400/800 kV, as required. Electricity is carried through a transmissionnetwork of high voltage lines. Usually, these lines run into hundreds of

kilometres and deliver the power into a common power pool called the grid.

The grid is connected to load centres (cities) through a sub-transmissionnetwork of usually 33 kV (or sometimes 66 kV) lines. These lines terminate

into a 33 kV (or 66 kV) substation, where the voltage is stepped-down to 11 kV

for power distribution to load points through a distribution network of lines at

11 kV and lower.

The power network of concern to the end-user is the distribution network of

11 kV lines or feeders downstream of the 33 kV substations. Each 11 kV

feeder which emanates from the 33 kV substation branches further into

several subsidiary 11 kV feeders to carry power close to the load points

(localities, industrial areas, villages, etc.). At these load points, a transformer

further reduces the voltage from 11 kV to 415 V to provide the last-mile

connection through 415 V feeders (also called Low Tension (LT) feeders) to

individual customers, either at 240 V (as single-phase supply) or at 415 V (as

three-phase supply). The utility voltage of 415 V, 3-phase is used for running

the motors for industry and agricultural pump sets and 240 V, single phase is

used for lighting in houses, schools, hospitals and for running industries,

commercial establishments, etc.

A feeder could be either an overhead line or an underground cable. In urban

areas, owing to the density of customers, the length of an 11 kV feeder is

generally up to 3 km. On the other hand, in rural areas, the feeder length is

4.1 INTRODUCTION

4.2 DESCRIPTION OF THE POWER DISTRIBUTION SYSTEM

Introduction tothe Power

DistributionSystem

9

much larger (up to 20 km). A 415 V feeder should normally be restricted to

about 0.5 −1.0 km. Unduly long feeders lead to low voltage at the consumer

end. The power supply system, including the distribution network, is depicted

in Fig. 4.1.

Fig. 4.1: Typical Electric Power Supply System with Distribution Network

The main components of the power distribution system and their brief

descriptions are given in Table 4.1.

Table 4.1: Components of the Power Distribution System

Component Description

Grid Substation (GSS) Power from transmission network is deliveredto sub-transmission network after steppingdown the voltage to 66 kV or 33 kV through220/132/66/33kV Grid substations.

Sub-transmission Network Power is carried at 66 or 33 kV by overheadlines or underground cables.

Power Sub-Transmission(PSS)

Power is stepped down by 66-33/11 kV to 11 kVfor distribution.

Primary DistributionFeeders

Power is delivered from PSS through primaryfeeders at 11 or 6.6 kV to various distributiontransformers.

Distribution Substation(DSS)

Power is further stepped down by 11/0.4 kVtransformers to utilisation voltage of 415 V.

Secondary DistributionNetwork

It carries power from DSS at 415 V (240 Vsingle phase) to various consumers throughservice lines and cables.


10

4.2.1 Voltage Levels

You have just learnt that the voltage range varies widely in various parts of the

power supply system. We give these voltages in Table 4.2.

Table 4.2: Voltages at Different Segments in the Power Distribution System

Higher voltages are used for 3-phase, 3-wire supply to large consumers. Low

voltage distribution of generally 415 V, 3-phase 4-wire system and 240 V

single phase, two wire, phase to neutral system is used for small and medium

consumers. The size and, hence, voltage of supply to a consumer is decided

by the load of the consumer.

4.2.2 Conductors

The 11 kV feeders carry comparatively bulk power from secondary substation

(33/11 kV) to distribution substation transformers (DTRs). Distributors (or

secondary network) carry power from DTRs through service lines (or LT

feeders) which deliver power from the supplier’s nearest support to

consumer’s premises up to the energy meter, through a weather-proof

service wire.

All lines have inherent resistances, inductances and capacitances, resulting in

a voltage drop in the line. Thus, to minimise voltage drop in a line, the values

of these parameters should be carefully selected. For LT supply, the declared

voltages at the consumer premises are 415/240 V. All appliances andmotors give good performance for long duration if this voltage ismaintained.

The following factors should be considered for the proper selection of

conductor size:

• current carrying capacity; and

• tensile strength of the conductor.

The size of conductor for a distributor is determined in the following manner:

• The current that the distributor has to carry is calculated on the basis of

the load incident on the conductor (including anticipated load growth).

• The conductor size capable of carrying this current at the ambient

temperature of the area is selected from standard tables.

Power System Segment Voltages

Generation voltages 415 V, 6.6 kV, 10.5 kV, 11 kV 13.8 kV,

15.75 kV, 21 kV and 33 kV

Transmission voltages 33 kV, 66 kV, 132 kV, 220 kV, 400 kV

High voltage primary distribution or

sub-transmission

3.3 kV, 6.6 kV, 11 kV, 22 kV, 33 kV,

66 kV

Low voltage distribution phase 415 V (3 phase) and 240 V (1 phase)


DistributionSystem

11

• The voltage drop is calculated taking products of loads and their

distances.

The following types of conductors are available:

• All Aluminium (Standard) Conductor (AAC);

• Aluminium Conductor Steel Reinforced (ACSR Conductors);

• All Aluminium Alloy Conductors (AAAC).

ACSR and AAAC conductors are used for secondary distribution systems.

ACSR conductors are preferred to AAC conductors for long spans owing to

their greater tensile strength. The current carrying capacity of ACSR

conductors is as follows:

Squirrel (7/2.11) 115 A

Weasel (7/2.59) 150 A

Rabbit (7/3.35) 208 A

The numbers in bracket indicate the number of strands/diameter in mm.

4.2.3 High Voltage Distribution System (HVDS)

You have learnt in Unit 1 that significantly high losses take place in the

secondary distribution system. This is due to higher current densities and

ease of pilferage at low voltages. One of the latest innovations in efforts to

reduce technical and commercial losses is the use of High VoltageDistribution System (HVDS) or LT-less system.

Fig. 4.2: Typical High Voltage Distribution System

In this system, the secondary distribution system with long LT feeders running

up to consumer premises from the distribution substation is totally absent.

The primary distribution system at HT level (11 or 33 kV) is used toreach the nearest point for a group of small number of consumers. Theconsumers are then connected to the HT Distribution System at thesepoints through small pole mounted transformers used for supplyingpower to them through LT service lines.


12

We now describe the advantages of HT distribution compared to conventional

LT distribution system.

v Low Losses and Improved Voltage Profile

The comparison of current, losses and voltage drop for the distribution of

the same power through HT and LT systems is presented in Table 4.3.

We have considered 100 as the base value for LT system. From the table,

you can see that for the distribution of the same power, technical losses

and voltage drop are much less in HT distribution system when compared

to LT distribution systems.

Table 4.3: Comparison of Current, Voltage Drop and Power Losses for PowerDistribution through HT and LT Distribution Systems

LT distribution systems are easily accessible and prone to pilferage and

the use of HVDS reduces the chances of theft of electricity to a very low

level. Now-a-days, utilities are installing meters at the HT transformer itself

to ascertain commercial losses on that particular transformer. In sum, the

HT distribution system has the following advantages:

• use of small size ACSR or aluminium alloy conductor or high

conductivity steel wire;

• better voltage profile;

• reduced line losses; and

• reduced commercial losses.

v Improved Reliability and Security of Supply

The use of HT distribution leads to improved reliability and security of

supply for the following reasons:

• The faults on HT lines are far less compared to those of LT lines.

• In order to avoid theft in LT lines from transformer to consumer

premises, usually Aerial Bunched Cables (AB Cables) are used to

supply power at LT to consumer from the distribution transformer.

With AB Cables, the faults on LT lines are eliminated. This, in turn,

Single phase

6.35 kV

HT distribution

system

3 phase 4 wire

415 V

LT Distribution

system

Current (Amps)

Losses (kW)

Voltage drop

11.0

8.5

12.7

100.0

100.0

100.0


DistributionSystem

13

reduces the failure of distribution transformers and enhances reliability

of supply.

• Since the number of small distribution transformers is high in HVDS,

the failure of one transformer does not affect supply to other

consumers connected to other transformers. In the event of failure of

distribution transformers, only a small number of consumers (2 to 3

power consumers or 10 to 15 domestic consumers) would be

affected. On the other hand, a large distribution transformer supplies

power through LV distribution lines to even remotely located

consumers in LVDS. Hence, the failure of an existing large size

distribution transformer would affect a group of 40 to 50 power

consumers and/or 100 to 200 domestic consumers.

You may like to consolidate these ideas before studying further.

In this section, we describe various components of the power distribution

system, viz. substations, transformers, feeders, lines and meteringarrangements.

4.3.1 Substation

A substation is the meeting point between the transmission grid and the

distribution feeder system. This is where a fundamental change takes place

within most T&D systems. The transmission and sub-transmission systems

above the substation level usually form a network (about which you will study

in the next section). But arranging a network configuration from the substation

to the customer would simply be prohibitively expensive. Hence, most

distribution systems are radial (also described in the next section), i.e., there

is only one path through the other levels of the system.

Typically, a substation consists of high and low voltage racks and buses for

4.3 COMPONENTS OF THE DISTRIBUTION SYSTEM

a) Compare the distribution system of your utility with another utility in

respect of voltage levels and conductors used for each component of

the distribution system.

……………………………………………………………………………….

……………………………………………………………………………….

b) What is HVDS? Outline its advantages over the LT system.

……………………………………………………………………………….

……………………………………………………………………………….

SAQ 1: Power distribution system


14

power flow, circuit breakers at the transmission and distribution level,

metering equipment and the control house, where the relaying,

measurement and control equipment is located. But the most important

piece of equipment that gives the substation its capacity rating is the

substation transformer. It converts the incoming power from transmission

voltage levels to the lower primary voltage for distribution. Very often, a

substation has more than one transformer.

Apart from the transformer, a substation has other equipment such as

lightning arrestors, isolators, etc. You will learn about the substation

equipment in detail in Unit 5 and the distribution transformers in Unit 6. Here

we give a brief introduction of the most critical component of a substation,

the transformer.

4.3.2 Transformer

A transformer is an electrical device that transfers power from one circuit to

another without change in frequency. The purpose of a transformer is to

convert one AC voltage to another AC voltage. A transformer comprises

two or more coupled conducting coils (windings), which are wound on

common laminated core of a magnetic material such as iron or iron-nickel

alloy (Fig. 4.4). These are called primary and secondary windings.

The alternating current in the primary winding creates an alternating magnetic

field in the core just as it would in an electromagnet. The secondary winding

is wrapped around the same core. The changing magnetic flux (magnetic

field per unit area per unit time) in the primary winding induces alternating

current of the same frequency in the secondary winding. The voltage in the

secondary winding is controlled by the ratio of the number of turns in the two

windings.

If the primary and secondary windings have the same number of turns, the

primary and secondary voltages will be the same. For step-down

Fig. 4.3: Power Distribution Substations


DistributionSystem

15

transformers, the secondary winding has lesser number of turns than the

primary. For example, to step-down voltages from 240 V at the mains to 6 V,

there needs to be 40 times more turns in the primary than in the secondary. In

case of step-up transformers, the number of turns in the secondary winding

is more than those in the primary winding.

The transformer is one of the simplest of electrical devices, yet transformer

designs and materials continue to be improved every day.

Fig. 4.4: Principle Underlying a Transformer

For an ideal transformer, it is assumed that the entire magnetic flux linked with

the primary winding is also linked to the secondary winding. However, in

practice it is impossible to realize this condition. While a large portion of the

flux called common or mutual magnetic flux links with both the coils, a small

portion called the leakage flux links only with the primary winding. This

leakage flux is responsible for the inductive reactance of a transformer.

Specifications of Transformer

A transformer should be provided with more than one primary winding or with

taps on the winding if it is to be used for several nominal voltages. The Rated

Power of the transformer is the sum of the VA (Volts x Amps) for all the

secondary windings. The important specifications for a transformer are:

primary frequency of incoming voltage (50 Hz), maximum primaryvoltage rating, maximum secondary voltage rating, maximum secondarycurrent rating, maximum power rating, efficiency, voltage regulation and

output type (3 wire or 4 wire).

Transformers in a distribution system can be configured as either

single-phase primary configuration (with three single-phase transformers) or a

three-phase configuration (one three-phase transformer). Three-phase

transformers are connected in delta (∆) or wye (Y) configurations. While delta

configuration is used for three wire transmission and sub-transmission

system, wye (or star) configuration is suitable for 4 wire distribution systems.

A wye-delta (Y - ∆) transformer has its primary winding connected in a wye

and its secondary winding connected in a delta. A delta-wye transformer has

Fig. 4.5: 11 kV/415 V - 240VPole MountedTransformer

The relation between the voltages, currents and number of turns in the primary and secondary coils is given by

2 1 2

1 2 1

V I N

V I N= =

Here V1, I1 and N1 represent the voltage, current and the number of turns, respectively, in the primary coil and V2, I2 and N2 represent the voltage, current and the number of turns, respectively, in the secondary coil.

NOTE


16

its primary winding connected in delta and its secondary winding connected

in a wye.

Types of Transformer

Transformers can be categorised based on the type of core used, type of

cooling used, the method of mounting the transformer or the intended use for

which it is designed. We shall deal with the first three categories of

transformers in Unit 6. Here we give a brief introduction to the categorisation

of transformers on the basis of their use as power transformers and

distribution transformers. Power substations use power transformers

while the distribution substations employ distribution transformers. While the

underlying principle of operation is the same for both the transformers, they

differ in their design since they are required to operate under different

conditions at power and distribution substations. Table 4.4 gives a

comparison of these two types of transformers.

Table 4.4: Comparison of Power Transformers and Distribution Transformers

4.3.3 Feeders

Feeders route the power from the substation throughout the service area.

They are typically either overhead distribution lines mounted on wooden

poles, or underground buried or ducted cable sets. Feeders operate at the

primary distribution voltage in primary distribution system and secondary

distribution voltage in the secondary distribution system.

Power Transformers Distribution Transformers

Convert power-level voltages fromone level to another in a GridSubstation (GSS) or PowerSubstation (PSS) with voltagesabove 33 kV.

Step down the primary distributionvoltage of 11 kV or 22 kV tosecondary distribution of 400 Vbetween phases and 230 V betweenphase and neutral through delta-starwinding.

Since it is fed through the gridnetwork, a power transformer isusually not a critical component forsupply to consumers as alternativepaths for flow of power areavailable through the grid.Accordingly, it is generally possibleto cut it out of circuit withoutaffecting supply to consumers.

Distribution transformers, beingconnected to the consumersthrough radial feeders, which haveonly one path, have to becontinuously energised formaintaining uninterrupted supply toconsumers.

Power transformers are most ofthe time loaded to levels justbelow the rated power and,accordingly, they are designed tooperate at maximum possible fluxdensity level with maximumefficiency at near full load.

Distribution transformers are most ofthe time lightly loaded and in orderto have maximum all day efficiency,they are designed to work at lowflux levels with maximum efficiencyoccurring at lower loading.

Efficiency is theratio of outputpower (kW or MW)and input power(kW or MW),whereas energyefficiency is theratio of energydelivered (kWh orMWh) and energyinjected (kWh orMWh) in a systemexpressed inpercentage terms.

Efficiency oftransformer ismaximum at aloading (as afraction of full loadcurrent) when itsiron losses equalohmic or copperlosses (due tocurrent flowing inthe windings). Thevoltage regulationof transformer isthe ratio of voltagedrop in atransformer fromno-load to full loadand the no-loadvoltage expressedin percentageterms.


DistributionSystem

17

Fig. 4.6: Distribution Feeders

The most common primary distribution voltages in use are 11 kV, 22 kV and

33 kV. The main feeder, which consists of three phases, may branch into

several main routes.

Fig. 4.7: Typical layout of feeders in a primary distribution system (numbers

indicate transformer capacities)

The main branches end at open points where the feeder meets the ends of

other feeders – points at which a normally open switch serves as an

emergency tie between two feeders.

By definition, the feeder consists of all primary or secondaryvoltage level segments of distribution lines between twosubstations or between a substation and an open point (switch).

Definition of a feeder


18

Feeders are connected in a configuration, which depends on the type of

network required in the distribution system. Three types of network are

normally available in the electrical distribution system:

• radial;

• loop; and

• cross-loop network.

Since the radial feeder emanates from one point and ends at the other in the

radial network, load transfer in the case of breakdown is not possible.

Although a radial feeder can be loaded to its maximum capacity, in the case of

breakdown, quite a large area may remain in dark until the fault is detected

and repaired.

In loop arrangement, two feeders are connected to each other so that in the

case of breakdown, the faulty section can be isolated and the rest of the

portion can be switched on. In this type of system, the feeder is normally

loaded to 70% of its capacity so that in the event of breakdown it can share

the load of other feeders also.

A cross-loop network provides multiple paths and the flexibility further

increases. In case of breakdown in any line, the faulty system can be isolated

and supply can be resumed very quickly. In this type of network, feeders

should normally be loaded to 70% of their current carrying capacity. This

system is highly reliable, but very expensive.

Fig. 4.8: Alternative Layouts for Primary and Secondary Network, 33 and 11 kV

In big cities, the concept of 33 kV ring main is very popular and two ringmains are laid: one outer and one inner. The outer ring main is laid using the

panther conductor and the inner ring main is laid using the dog conductor.

The use of these two types of ring mains provides excellent flexibility to the

system and at the time of breakdown, supply can be immediately switched on

from another 132 kV substation. While making any distribution planning

(discussed in Sec. 4.4) for metros, the aspect of outer and inner 33 kV ring


DistributionSystem

19

mains is extremely essential and should be included for providing

uninterrupted supply.

Table 4.5 gives a comparison of the three types of network configurations.

Table 4.5: Comparison of radial, loop and cross-loop network

4.3.4 Meters for Measurement of Energy and Other ElectricalQuantities

Meters are required to be installed at various points of the Distribution System

including the substation equipment and the consumer end. They are required

for correct recording of electrical quantities for operational and safety

purposes as well as energy accounting. The meters installed at the interface

points of generation-transmission and transmission-distribution are called

interface meters. Meters installed at consumer premises by the utility are

called consumer meters.

The Central Electricity Authority regulations on Installation and Operation of

Meters provide for the type, standards, ownership, location, accuracy class,

installation, operation, testing and maintenance, access, sealing, safety,

meter reading and recording, meter failure or discrepancies, anti tampering

features, quality assurance, calibration and periodical testing of meters,

additional meters and adoption of new technologies in respect of the following

meters for correct accounting, billing and audit of electricity:

§ Interface meter,

§ Consumer meter, and

§ Energy accounting and audit meter.

It is important to note that these regulations make the use of static meters

mandatory for new consumers.

Radial DistributionSystem

Loop DistributionSystem

Cross-loop NetworkDistribution System

• Single path to eachgroup of customers

• Lowest constructioncost system

• Simple to plan, designand operate

• No reserve – loss offeeder implies loss of

supply

USED IN RURAL AREAS

• Double path to eachgroup of customers

• Medium cost system

• Moderately simple toplan, design andoperate

• Loss of feeder resultsonly in temporary lossof supply

USED IN URBAN AREAS

• Multiple paths to eachgroup of customers

• High cost system

• Complex to plan,design and operate

• Highest reliability

• Used in large cities

and for critical loads

USED IN URBAN AREAS


20

The need for electrical power is growing at a rapid pace on account of rapid

growth of population, industrialisation and urbanisation resulting in high load

density pockets with multi storied complexes. This is coupled with manifold

increase of deep tube wells on account of low ground water level and huge

number of electric pumps connected to the system during the agricultural

season in rural areas. In order to meet the future power needs of the nation,

it is essential to upgrade the existing distribution system and increase its

efficiency and at the same time reduce the technical losses. This requires

proper planning: Utilities have to plan much ahead to meet the presentas well as the projected future demand for quality power supply.

In the context of the current chronic power shortage, the shooting prices of

fuel and the need for conservation of available fossil fuel resources, you can

well understand the urgency of eliminating high losses in the transmission and

distribution system. The high percentage of losses in our country is a matter

of national concern. The main cause of these high losses is laying of

unplanned distribution system in the country. Proper distribution systemplanning, financial support and implementation of the plans should be

able to bring down the losses and provide uninterrupted quality supply to the

consumers.

Distribution Planning requires an analysis of various factors such as load

growth, funds, ecological consideration, availability of land, etc. Distribution

planning in a utility involves

• ascertaining the time horizon for which it is envisaged,

• spelling out the specific activities required in the planning process, and

• implementation of plans.

We take up each of these in the following sections.

4.4 DISTRIBUTION SYSTEM PLANNING

a) Make a list of the substation equipment.

……………………………………………………………………………….

……………………………………………………………………………….

b) What is a feeder? Compare the radial, loop and cross-loop network

configurations in an electrical distribution system.

………………………………………………………………………………

………………………………………………………………………………

SAQ 2: Components of power distribution system


DistributionSystem

21

4.4.1 Planning Horizon

Distribution planning studies can be carried out in different manners, each

with different objectives and requirements. Planning can be done for different

time horizons and accordingly it is called medium/long-term planning or

short-term planning.

v Medium/Long-term Planning

Medium/long-term planning is normally carried out as a part of a master

plan for the distribution system as a whole. It normally considers a 5 to 15years time frame and is based on the state/national as well as local load

forecasts, industrialisation plan and agricultural load forecasts. The main

objectives of this type of plan are to:

• verify the present capacity of lines and substations;

• verify additional capacity and investments required to meet the load

growth for putting up new 33/11 kV substations, new 33 kV lines,

11 kV feeders, etc.;

• arrange tie-ups for additional power-purchase agreements;

• upgrade existing transmission capacity;

• upgrade existing networks;

• develop strategies for reduction of technical losses;

• estimate the funds required; and

• arrange tie-ups with the financial agencies for funds.

v Short-term Planning

For proper distribution planning, we first need to study the existing system,

ascertain loss level and decide on immediate action to be taken to meet

the requirement of consumers and provide them uninterrupted quality

power supply. In the present scenario, it has been found that 11 kV and

33 kV feeders are loaded more than 100% of their rated current carrying

capacity. This results in very high technical losses and needs to be

immediately relieved through short-term planning.

Thus, the objectives of short-term planning are to:

• develop specific case studies and projects in a systematic manner;

• adjust capacity of 33 kV,11 kV feeders, power transformers,

distribution transformers and LT lines;

• take immediate action to bifurcate heavily loaded 33 kV, 11 kV feeders;

• augment conductor with the properly sized conductors;

• reduce length of LT lines (maximum 0.5 km per transformer);

• implement projects for proper maintenance;

• calculate required investments; and

• arrange tie-ups with financial institutions for funds.


22

4.4.2 Principal Areas of Activity

The principal areas of activities associated with distribution planning are as

follows:

• Existing Load Data Study: The study of the existing system forms a

critical input to distribution planning and includes activities such as

− updating all distribution system statistics;

− evaluating changes in technic and economic planning criteria; and

− evaluating and updating load forecasts, voltages and consumers

category-wise with a time horizon of 10 to 15 years.

• Future Load Growth Study: Load forecasts are extremely important in

Distribution Planning. These are mainly used for:

− power purchases;

− reinforcement of distribution system expansion planning;

− demand side management;

− tariff application; and

− monitoring of loss reduction programme.

The forecasts may be done on a short-term, medium-term or long-term

basis. These have to be carried out systematically and rigorously to be of

any help in distribution planning. Otherwise, they can lead to wrong

estimates and the planning can go awry.

The steps involved in the load forecasting process are:

− data collection;

− data validation;

− selection of methodologies;

− development of assumptions;

− development of energy and demand forecasts; and

− comparison with the historical load growth data.

Identify the short-term, medium/long-term planning objectives for yourutility.

……………………………………………………………………………….......

……………………………………………………………………………….......

……………………………………………………………………………….......

……………………………………………………………………………….......

……………………………………………………………………………….......

SAQ 3: Planning horizon


DistributionSystem

23

• Power Factor/Reactive Load Study: A study of loading pattern and

voltage drops needs to be carried out to ascertain the reactive powercompensation, which is required to be provided at different points of the

distribution system so as to maintain voltages within specified limits. You

can study about power factor and reactive compensation in Appendix 1given at the end of this unit.

• Study of Thermal Capability of Conductors (Capacity of Feeders/Circuits): The thermal capacity of line circuit is dependent on the size of

the conductor and type of environmental factors, i.e., ambient

temperature, wind speed and solar radiation. A study of peak loadings for

the conductors needs to be carried out to figure out overloaded feeders

with the help of standard tables giving rated currents for each type of

conductor. Accordingly, a proper action plan for bifurcation or replacement

of feeder needs to be chalked out.

• Economic Impact Study: A study of the economic impact of the

implementation of the distribution system plan needs to be carried out.

Cost-benefit analysis needs to be done to ascertain whether the

investments in implementation of distribution plan lead to long term

savings and improvement in supply quality.

We now present a case study to illustrate how utilities can take advantage of

distribution planning.

DISTRIBUTION SYSTEM PLANNING: A CASE STUDY OF MP MADHYAKSHETRA VIDYUT VITARAN COMPANY LIMITED

MP Madhya Kshetra Vidyut Vitaran Company Limited, Bhopal (the Central

Discom) is one of the leading utilities, to have established a Distribution

System Planning Cell. In the first phase, the Central Discom took up short

term system planning and established the equipment and network data base.

Field data such as the length of feeders, size of conductors, configuration of

poles, present loading, annual input and single line diagrams was collected for

all 33 kV and 11 kV feeders. Load forecasts were made for the next five years

on the basis of the historical load growth data. The following information was

also collected:

• details of existing capacitors installed at 33/11 kV substations; and

• details of existing 33/11 kV substations along with data on the capacity of

transformers, number of feeders, loading, etc.

The number of 33 kV rural feeders in the Discom is 282. These have been

strung with Racoon conductors having current carrying capacity of 200 A.

The Discom selected all the 33 kV feeders having loading more than150 A for

study and analysis.

As the National Tariff Policy has made it mandatory for power utilities to

segregate technical and commercial losses within one year, a detailed study


24

was conducted through CYMDIST software, which is a proven tool for finding

technical losses at each voltage level of the distribution system.

This software provides for two types of studies:

• voltage drop analysis; and

• short circuit analysis.

On the basis of the study and the data acquired, the DISCOM took measures

such as:

• capacitor placement;

• augmentation of the conductor of feeder; and

• bifurcation of the feeder.

These were followed by further systemic analysis. It was concluded that

losses could be reduced substantially by adopting all the three measures or

combinations of two or only one of these, depending upon loading conditions

and voltage regulation.

The study was initially conducted on 20 select feeders but later it was

extended to the heavily loaded 224 rural feeders. It was found that 49 feeders

had losses amounting to more than 10% and 52 feeders had losses between

5 -10%. An analysis of the data revealed that for some feeders, voltage

regulation could not be brought within permissible limits even after the

placement of Capacitor Bank, proposing a new 33 kV feeder and augmenting

the conductor size. Further studies were carried out and locations were

identified for putting up 132/33 kV substations. Through short term studies,

the Discom identified 5 such locations.

A similar study conducted for 11 kV feeders led to the following conclusions:

i) additional power transformer would need to be put up in the existing

substation;

ii) a new 33/11 kV substation would need to be constructed, thereby

reducing the length of 11 kV lines;

iii) the existing 11 kV feeders would need to be bifurcated; and

iv) the conductors of existing feeders would need to be augmented.

The details of planning activities undertaken by the Central Discom are

presented below.

• Existing Load Data Study: The number of 33 kV feeders in the Central

Discom is 347, of which 65 are urban feeders and 282 rural. The

CYMDIST software was used to study all the parameters of existing

feeders in urban areas. Various steps, such as augmentation of existing

Racoon conductor by Dog conductor, bifurcation of feeder and

installation of Capacitor Bank in 33/11 kV substations, were taken. After

the implementation of these measures, voltage regulation was found

to be within permissible limits. The loading of the 33 kV rural feeders is

given ahead.


DistributionSystem

25

Table 4.6: Loading of 33 kV Rural Feeders

The number of 11 kV feeders in the Central Discom is 1749, of which 358 are

urban feeders and 1391 rural. In the first phase, a study of all 11 kV urban

feeders was conducted through the CYMDIST software and action was taken

for bifurcation of feeders, augmentation of conductor capacity and putting up

new 33/11 kV substations. All 11 kV feeders of urban areas now have Racoon

conductor and the load of each feeder is within 100 A. The voltage regulations

are also within permissible limits.

In the second phase, a study of 11 KV rural feeders is being carried out. All the

existing feeders are laid on ACSR weasel conductor. The length of the feeders

ranges from 4 km − 100 km and voltage regulation varies from 3% to 24%.

The break-up of the rural feeders on the basis of load is given in Table 4.7.

Table 4.7: Break-up of the Rural Feeders

The number of 11 kV feeders having load more than 200 A is 180 and a study

is being conducted on them in the first phase through the CYMDIST software.

• Future Load Growth Study: Historical load growth data was the major

basis in anticipating the future load growth.

• Power Factor and Reactive Load Study: A thorough study was made on

the existing power factor and existing load on the system and as per the

data obtained from CYMDIST software study, necessary compensation

was provided by installation of 11 kV capacitor bank on 33/11 kV

substations.

Load Number of 11 kV Feeders

More than 200A

Between 150−−−−−200 A

Between 100−−−−−150 A

Between 75−−−−−100 A

Less than 75 A

180

170

790

172

79

Load Number of 33 kV Feeders

More than 300A

Between 250−−−−−300 A

Between 200−−−−−250 A

Between 150−−−−−200 A

Less than 150 A

31

25

56

28

142


26

• Thermal Capability of Conductor: Operating data was used to check

whether the conductors were being operated within thermal capability

limits.

• Economic Impact: On analysis of the data derived from the CYMDIST

study, it was concluded that the payback period is between 24 to 30

months. The results obtained from CYMDIST software have been used for

preparing the loss reduction model of the company.

You may like to review this information in your own context. Attempt the

following SAQ!

The distribution system constitutes the interface of a utility with consumers

who judge the performance of the utility by the performance of its distribution

system. Therefore, proper operation and maintenance of the power

distribution system is essential. Any failure on this account may deprive the

user of electric supply and lead to chaotic conditions. There are two types ofmaintenance: Preventive Maintenance and Breakdown Maintenance.

• Preventive Maintenance is maintenance done prior to the onset of

Monsoon and after the end of Monsoon.

• Breakdown Maintenance is done on breakdown in the installation.

In this section, we discuss the general O&M objectives and activities for the

power distribution system.

4.5.1 Operation and Maintenance Objectives

You will agree that the prime goal of a power utility, like any other business, is

to achieve consumer satisfaction with optimum effort and costs while

maintaining reasonable profit levels. The operation and maintenance (O&M)

practices of a utility contribute significantly in attaining this goal. These

activities should help in improving the reliability and maintenance of plant and

equipment, maximising capacity utilisation, increasing operating efficiency,

and reducing operating and maintenance costs.

4.5 OPERATION AND MAINTENANCE OBJECTIVES AND ACTIVITIES

Suggest ways in which your utility can benefit from distribution system

planning.

………………………………………………………………………………......

………………………………………………………………………………......

SAQ 4: Distribution planning


DistributionSystem

27

The objectives of O&M for distribution systems may thus be spelt out as

follows.

The O&M strategy adopted by a utility can be evaluated in terms of certain

parameters, which are given below.

The specific functions of the O&M System are described in detail in

Appendix 2 to this unit.

OBJECTIVES OF OPERATION AND MAINTENANCE

v Ensuring quality and reliability of supply to consumers.

v Reducing equipment operating and maintenance costs througheffective utilisation of capacity and resources.

v Increasing the availability and reliability of plant and equipment witheffective maintenance planning.

v Improving spares planning and reducing spares inventories.

v Standardising work procedures.

v Ensuring the safety of maintenance personnel.

v Providing a mechanism for making estimates and controllingmaintenance expenses.

v Generating MIS reports for better decision-making and control.

v Bringing down technical and commercial losses to an optimumminimum level.

v Avoiding any bottleneck in capacity by matching expansion with thegrowing demand.

PARAMETERS FOR EVALUATION OF O&M STRATEGY

v Reduction in

− T&D losses,

− overloading of feeders and transformers,

− consumer interruption

− cost per consumer

− number of trippings due to overloading.

v Degree of improvement in voltage profile vis-à-vis voltage regulation.

v Increase in revenue.

v Enhancement of peak demand and energy supplied.

v Number of consumers supplied.

v Improvement in level of service and collections.

NOTE

Source: Special reporton CEA website“Guidelines for ProjectManagement andPerformance Evaluationof Sub-transmission andDistribution Project”.


28

4.5.2 Activities Involved in Operation and Maintenance

The following activities are involved in the operation and maintenance of the

Distribution System:

• continuity of service;

• technical operation and maintenance;

• training and retaining of operational staff;

• renewal of maintenance contract;

• upkeep of spare parts inventory;

• record keeping of faults in the network/equipment problems, solutions,

modifications and enhancements;

• close monitoring of budgeted expenditure;

• preparation, continuous updating and proper maintenance of operational

and network data;

• record of protective and isolating devices installed and their relay settings;

• record of schedule of maintenance and preventive and routine

maintenance of network elements;

• development of spare parts;

• development of maintenance practices, tools and procedures for trouble

free operation; and

• record of transformer/switchgear oil testing and its parameters.

Utilities should have manuals for O&M to ensure efficient and trouble free

operation of the system/equipment. These manuals should contain the

following information:

• factory and site test certificates for each item of the system with reference

to relevant design calculation and quality assurance standards;

• maintenance instructions for all plants and other preventive and corrective

maintenance procedures;

• maintenance and inspection schedules for all items/equipment giving type

of works required on a weekly, monthly, annual basis; and

• proforma of the required maintenance record sheets for all the

component/equipment.

We now outline the modern approach to operation and maintenance of power

systems.

4.5.3 Renovation and Modernisation (R&M) and Life ExtensionSchemes

Basically, the deterioration of electrical components in the distribution system

is related to electric, thermal, mechanical, chemical, environmental and

combined stresses. Hence, failure of equipment could be due to insulation

failure, thermal failure, mechanical failure or any combinations thereof.


DistributionSystem

29

The concept of simple replacement of power equipment in the system,considering it as weak or a potential source of trouble, is no longervalid in the present scenario of financial constraints.

Renovation, modernization and life extension of existing substations,

sub-transmission and distribution network and field equipment outside the

substations is one of the cost effective options for maintaining continuity and

reliability of the power supply to the consumers. Renovation andmodernisation (R&M) is primarily needed to arrest the poor performance of

the substation equipment (mainly transformers and switchgears), which are

under severe stress due to poor grid conditions, poor and inadequate

maintenance and polluting environment.

In this changed paradigm, efforts today are being directed to explore new

approaches/techniques of monitoring, diagnosis, life assessment and

condition evaluation, and possibility of extending the life of existingassets, i.e., generator, circuit breaker, surge arrestor, oil filled equipment like

transformers, load tap changer, etc., which constitute a significant portion of

assets for generation, transmission and distribution system.

Assessing the condition of the equipment is the key to improvingreliability. The knowledge of equipment condition helps to target the

maintenance efforts to reduce equipment failures. Reduction of failures of

equipment improves reliability and effectively extends the life of equipment.

Hence, utilities are continuously in search of ways and means other than

conventional methods/techniques to assess the condition of equipment in

service. Thus, remedial measures can be taken in advance to avoid

disastrous consequences thereby saving valuable resources.

For assessing normal operation, strategic planning and scheduling,three major tasks need to be identified:

• incipient failure detection and prevention − supervisory function,

monitoring;

• identification of malfunction or fault state − offered by diagnostic

techniques; and

• planning for repair, replacement and upgrading − life assessment and

condition evaluation techniques.

Researchers and manufactures have come out with various conditionassessment, diagnostic monitoring, preventive maintenance, predictivemaintenance (PDM) techniques for the equipment to reduce the risk of

failure and extend their effective life and thereby help utilities overcome the

challenges they face. Various condition assessment tools are used to

establish the health of equipment using latest on line and off line diagnostic

testing techniques/technologies.

Predictive maintenance is gaining popularity as it helps eliminate

unscheduled downtime of expensive equipment and reduces the overall cost

of maintenance. This approach, sometimes called ‘condition-based

There are no established guidelines for the time interval during which R&M and life extension studies must be carried out. The R&M and life extension studies must be done when the performance of the equipment is noticed as deteriorating but not later than two years from the previous such study.

NOTE


30

maintenance’, relies on planned inspections, testing, analysing and trending

of the relevant equipment parameters. In most cases, these parameters can

determine the equipment’s health and must be followed up by proactive

actions that change the way the equipment is operated to reach the goals set

out. In other words, the performance of the equipment is analysed to

determine its condition and predict when it will need attention.

The techniques so developed are grouped under Residual Life Assessment(RLA) techniques. The potential of such techniques is tremendous and their

benefits are so many that utilities cannot ignore their importance in the

present scenario.

The main objective of RLA is to determine the condition of a set of

equipment (e.g., transformers) in order to identify the most vulnerable

component/equipment. Based on the evaluation, utilities can develop a

strategic replacement plan for a particular population of equipment. The aim

should be to maximize the availability and utilization by avoiding unexpected

failures and at the same time minimizing risk. Strategies for life assessment

are quite complex and involve many aspects (both user-oriented and

manufacturer-oriented). Their details are beyond the scope of this course.

In the next section, we introduce the concepts of grid management, load

scheduling and load balancing. However, you may first like to revise the ideas

presented in this section.

4.6 GRID MANAGEMENT, LOAD SCHEDULING AND LOAD BALANCING

In this section, we consider the aspects related to grid management, load

scheduling and load balancing in a power distribution system.

Outline the O&M objectives of your distribution utility. What activities are

undertaken by it to fulfil these objectives?

………………………………………………………………………………

………………………………………………………………………………

………………………………………………………………………………

………………………………………………………………………………

………………………………………………………………………………

………………………………………………………………………………

………………………………………………………………………………

SAQ 5: O&M objectives and activities


DistributionSystem

31

4.6.1 Grid Management

Let us consider the following questions:

q What is a grid?

q What is grid management?

q What does grid management involve?

q What is a grid?

You know that a power system has a generating unit to generate electrical

energy, which is consumed at the load. This energy cannot be stored and has

to be consumed at the same instant. But since the load is not concentrated at

one place and it is not possible to have a generator very close to the load

centre at all times, we go for transmission lines, which facilitate transmission

of power from generator to load. Thus all generation units and load centres are

connected and a grid is formed (Fig. 4.9).

The grid is basically a connection of generating stations, substationsand loads through transmission lines, at a voltage level above thedistribution voltage. The distribution voltage, however, is not strictly defined.

It is different for different areas. In some distribution systems, power is taken

from the grid at 33 kV, in some it is taken at 66 kV and in some, it may even be

taken at 220 kV. Therefore, the grid covers the above mentioned high voltage

system down to the level of connection point of the distribution system.

There are many advantages of having a grid.

Definition of a grid

Grid is defined in the Electricity Act, 2003, as : “the high voltagebackbone system of inter-connected transmission lines,substations and generating plants.”

ADVANTAGES OF A GRID

v RELIABILITY: The system is more reliable since we can serve the

load in more than one ways. As a result, even if one generation

unit fails the rest can share its load.

v STABILITY: The system becomes more stable as the chances of a

fault disturbing the whole system become less.

v ECONOMY: In a grid, the cost required is lesser than a dedicated

system since lesser installed capacity is required as well as lesser

spinning reserve is involved.

GENERATION (POWER PLANTS)

TRANSMISSION NETWORKS (GRID)

LOCAL DISTRIBUTION SYSTEM

CUSTOMERS

Fig 4.9: Grid as anIntermediaryBetweenGenerationand Load


32

Regional and State Grid in India

In the 1960s, India was demarcated into 5 electrical regions (NR, SR, ER,

WR, NER) for planning, development and operation of the power system with

regional self sufficiency. As on date, we have three synchronous power

systems: Northern, Central (WR-ER-NER) and Southern. Bulk power transfer

is possible among the regional grids through the inter-regional links. The

Northern and Southern Systems are connected to Central System through

separate HVDC links and, hence, each of the three systems can operate at

different frequencies. The State Grid of each State is connected to Regional

Grid for inter-State power exchanges.

q What is Grid Management?

Grid management, as the term implies, is managing the grid. This

consists of on-line real-time operation of the grid as well as off-lineoperational planning.

The real time operation of the grid is looked after by the Load DispatchCentre, which is basically a round-the-clock control room manned by grid

operators or load dispatchers, who operate the grid by giving instructions to

the personnel of the concerned generators and substations.

Load Dispatching, as the name implies, involves dispatching of load(or power) from the generator to the load. This is done through the

transmission system. Load Dispatch Centre constantly observes the grid

parameters and tries to ensure good grid operation.

Operational planning is the planning done in advance, in order to ensure that

♦ generation matches the load at all points of time;

♦ the voltage profile at all points of the grid remains within acceptable limits;

♦ none of the transmission lines or inter-connecting transformers get

over-loaded; and

♦ the grid operates in a stable manner, i.e., there are no power swings.

Operational Planning also involves coordination of protection of the grid so

that only the faulted element gets isolated and the remaining grid continues to

operate in a satisfactory manner.

The Grid Management in our country is done by the Regional Load Dispatch

Centre (RLDC) at the Regional Level and by the State Load Dispatch Centre

(SLDC) at the State Level. Each State, Central Generating Stations and

Independent Power Producers (IPPs) are treated as constituents of the

Region.

Fig. 4.10: Two Key Players in Grid Management

REGIONAL LOAD DISPATCH CENTRE

STATE LOAD DISPATCH CENTRE


DistributionSystem

33

Let us explain further the quality parameters of electric supply.

Frequency is a global phenomenon, i.e., it is the same at all points of agrid which is operating in synchronous operation. Frequency is an

indication of the balance between generation and load in a grid. If the

generation exactly matches the load, the frequency would be the nominal

frequency, i.e., 50 Hz. If generation is more than the load in a grid as a whole,

the system frequency would be greater than 50 Hz. If generation is less than

the load, the system frequency would be less than 50 Hz.

Voltage is a local phenomenon, i.e., it can be different at differentpoints of the grid. Therefore, the grid operator has to ensure that the proper

voltage profile is maintained at all points of the grid. For ensuring proper

voltage profile, capacitors or reactors are installed at different points in the

grid. If it is observed that the voltage is low at a particular point in the grid,

then capacitors are installed at that point. Similarly, if voltage is observed to

be high, as per the studies, then reactors are installed at that point. The basic

OBJECTIVES OF GRID MANAGEMENT

v Reliability,

v Grid security,

v Economy, and

v Quality in electric supply.

v RELIABILITY comes with integrated grid operation for smooth

evacuation of power from generating stations and its delivery at

the states’ periphery.

v SECURITY comes by maintaining the system parameters like

frequency, bus voltages, line loadings and transformer loadings

within permissible limits. It involves stable and smooth operation

of the grid, i.e. minimum interruptions of power, either through

tripping of single grid elements (like generator, transmission line,

interconnecting transformer, HVDC back-to-back pole) or grid

disturbances involving tripping of a large number of grid elements

simultaneously or even a total blackout.

v ECONOMY comes by merit order generation, optimization of hydro

resources, minimization of losses and judicious inter-regional

exchanges. It envisages getting the cheapest power to the

customers through minimization of transmission losses and

ensuring that the cheapest generation is used first, then the next

costly generation and so on.

v QUALITY in electric supply is now gaining importance. The

parameters of quality are frequency, voltage and harmonics.


34

purpose of these elements is to ensure that the reactive power requirement of

the load or transmission lines is met.

Besides this, there are also other voltage phenomena like unbalanced voltage

in the three phases, voltage dip, etc. Voltage unbalance in the grid could be

caused due to the tripping of one of the phases of a transmission line or due

to unbalanced load in the three phases emanating from the distribution

systems or bulk loads. Voltage dip, on the other hand, is a transient

phenomenon caused by a transient fault or tripping of an element at a remote

location of the grid. Stormy weather could also cause flashover between

arcing horns, resulting in voltage dip.

Harmonics is recently becoming an issue in the modern world, due to a

number of electronic devices connected in the grid as well as in the

distribution system, which converts AC to DC through rectifiers or which chop

an AC wave for voltage or current control. In the grid, harmonics are caused

by HVDC stations, which convert AC to DC and back from DC to AC.

In the distribution system, harmonics are caused by power supplies and

inverters through which power is supplied to computers and all household

appliances using digital technology, which have permeated our lives. For this,

standards have been laid down in the Regulations for Technical Standards for

Connectivity to the Grid. As per the provisions of these Standards, the limits

for individual and total harmonics distortion have been given.

q What Does Grid Management Involve?

Grid management involves

• forecasting (demand pattern);

• planning (outages, unit commitment, resource scheduling);

• coordination (between stakeholders);

• supervision (grid parameters);

• real time operation and control for optimal utilization of available resources

in the grid, which involves

− scheduling,

− monitoring, and

− restoration of grid;

• off-line operational planning involving grid security issues, restoration of

grid and commercial issues or billing.

We shall talk about these aspects in detail in the next section.

The load dispatch centre is primarily responsible for management of the grid.

Its various functions: ex-ante (a Latin term meaning before-hand), real-time

and post-facto (meaning after the fact) are given in Table 4.8.


DistributionSystem

35

Table 4.8: Grid Management Functions

4.6.2 Load Scheduling and Dispatch

Load scheduling means fixing the schedules of generation of power for

generating stations and the schedules for drawal of power by the States taking

into account drawal schedules from shared power sector projects and

schedules of power purchased from buyers to sellers. Scheduling is donefor the day ahead by the Regional Load Dispatch Centre to ensure

balance between load and generation in the grid with the aim of achieving an

operating grid frequency of 50 Hz. Since power cannot be stored to a large

extent, power generated has to be used at that instant of time. Therefore, it

has to be ensured that the generation matches the load at each point of time.

Schedules are prepared on a 15 minute basis, to see to it that the average

generation of electrical energy over 15 minutes matches the load over those

15 minutes. Scheduling is done one day before for the day ahead, as per a

time schedule specified in the Indian Electricity Grid Code (named simply as

EX-ANTE REAL-TIME POST-FACTO

• Forecasting demandfor the forthcomingperiod

• Scheduling ofresources atdisposal

• Planning the gridelement outageslike generatormaintenance, etc.

• Providing for openaccess transmissioncorridors.

• Resourcere-scheduling as andwhen required

• Implementation ofproper contract ofservice as entitled

• Supervising andcontrolling gridparameters

• Ensuring real-timebalancing ofresources

• Ensuring gridsecurity andreliability

• Coordinating theoutages and loadshedding

• Ensuring properpower quality

• Minimizing lossesand optimizingresource utilization

• Tacklingemergencieseffectively andefficiently.

• Reportingevents occurring ina grid operation

• Analyzing theevents thatoccurred

• Collecting theenergy meter data

• Processing thedata collected

• Energy accounting

• Operating the poolaccount,unscheduledinterchangeaccount and thereactive energyaccount.


36

“Grid Code” as per the Electricity Act, 2003), so that the State Power Utilities

plan for load management for the next day.

For example, suppose a State finds that after taking into account its own

expected generation for the next day and the net drawal schedule for the next

day, it would fall short of meeting its anticipated requirement for the next day

by 200 MW during peak time. Then it would have to plan a load shedding of

200 MW during peak time. Schedules can also be changed on the same day

due to major load variations experienced by a State due to abnormal weather

conditions. For example, rains in summer could cause reduction of

agricultural load and AC load; heavy rains could cause disruptions in the

transmission system and hence loss of load. The rescheduling would be valid

after a time gap of about one and a half hours so as to enable implementation

of the new schedules. Rescheduling can also be done by the Regional Load

Dispatcher in cases of transmission bottleneck and grid disturbance.

Scheduling is important because it is meant to ensure the desired

operating frequency of the grid. There are financial penalties for violating

these schedules if these violations burden the grid and financial incentivesif the violations help the grid, through a component of the tariff, known as

unscheduled interchanges.

Monitoring the parameters of the grid is the prime real time function ofthe Load Dispatch Centre. These parameters include operating frequency,

voltage levels at all points of the grid, status of line and transformer loading

throughout the grid, especially at crucial points. In order to help the grid

operator monitor these parameters over the large number of points in the grid,

the Load Dispatch Centre is equipped with SCADA (Supervisory Control and

Data Acquisition) System. You will study about it in Block 3 of the course

BEE-002.

Based on the alarms generated by the SCADA System, action is taken by the

grid operator by giving instructions to all concerned. All instructions of thegrid operator have to be followed. All instructions are also recorded on a

sound recorder, to be replayed at the time of analysis of a grid incident or any

other contingency. The grid needs to be constantly monitored to observe

whether it is operating within its limit. This work is being done in the RLDCs

and SLDCs. The LDCs coordinate between the Central Generating Stations

and States (through SLDCs).

Restoration of grid involves restoring the grid after a grid disturbance.

Grid disturbance normally takes place in a matter of milliseconds and there is

no time for the grid operator to react. Therefore, the operational procedures

for restoration of a grid are planned well in advance and come under the

scope of off-line operational planning. The grid operator just has to follow

the procedure for restoring the grid.

We now describe the load scheduling process as it takes place.

The Load Scheduling Process

The process starts with the Central Generating Stations (CGS) in the region

declaring their expected output capability (in MW) for 96 slots of 15 minutes

duration during the next day to the Regional Load Dispatch Centre (RLDC).


DistributionSystem

37

The RLDC breaks up and tabulates these output capability declarations as per

the beneficiaries’ plant-wise shares and conveys their entitlements to State

Load Dispatch Centres (SLDCs). The latter then carry out an exercise to see

how best they can meet the load of their consumers over the day, from their

own generating stations, along with their entitlement in the Central stations.

They also take into account the irrigation release requirements, distribution

utilities’ load schedules for next day and load curtailment, etc. that they

propose in their respective areas.

The SLDCs then convey to the RLDC their schedule of power drawal from the

Central stations (limited to their entitlement for the day). The RLDC

aggregates these requisitions and determines the dispatch schedules for the

Central generating stations and the drawal schedules for the beneficiaries

(State as a whole) duly incorporating any bilateral agreements and adjusting

for transmission losses. These schedules are then issued by the RLDC to all

concerned and become the operational as well as commercial datum for

inter-State and CGS transactions.

However, in case of contingencies, Central stations can prospectively revise

the output capability declaration, beneficiaries can prospectively revise

requisitions, and the schedules are correspondingly revised by RLDC. It is for

the SLDCs to further break-up these State entitlements into Discom

entitlements and State Generation Schedules.

While the schedules so finalized become the operational datum, and the

regional constituents are expected to regulate their generation and consumer

load in a way that the actual generation and drawals generally follow these

schedules, deviations are allowed as long as they do not endanger the system

security.

Load Shedding

During the normal operation of a grid, it is possible that the load exceeds the

generation. If this happens the frequency of the system goes down. The

standard frequency is 50 Hz. But the frequency can go down to about 49.0 Hz.

After this value, it is not advisable to allow it to reduce it any further since it can

cause the system to lose synchronism and lead to ultimate collapse of the

system. As a result, we go for purposeful shedding of load, known as loadshedding. The load shedding is a process of reducing load on the grid so as

to save the grid as a whole.

Load shedding can be done in two ways:

1. Automatic: For this purpose automatic under-frequency relays are

installed. These relays carry out automatic shedding of load if the

frequency falls below a certain level.

2. Manual: Special guidelines have been provided by RLDCs/SLDCs for

the load shedding at different frequency levels. These guidelines

depend upon the grid parameters at the particular instance as well as


38

some fixed guidelines for frequency falling below a particular limit or

area-wise/consumer category-wise shedding.

Off-Line Operational Planning

You have studied in Sec. 4.6.1 that off-line operational planning involves

• grid security issues;

• restoration of grid; and

• commercial issues or billing.

We discuss these briefly.

Grid security issues involve

• load generation balance planning in respect of active power for the next

year, which is reviewed on a quarterly and then monthly basis, in order to

ensure that the frequency stays at the nominal level;

• installation of capacitors or reactors to obtain a proper voltage profile in

the grid;

• line and transformer loading;

• protection coordination;

• monitoring of the grid; and

• proper analysis of tripping of lines as well as of grid disturbance and taking

corrective measures thereof.

Under-frequency and rate-of-change of frequency relays are installed as

security measures to cut off load in case of gradual or sudden drop in

generation, respectively, to ensure nominal frequency in the grid. Islanding

schemes of important generators and loads are also planned as a last resort

to isolate or island them in case of a blackout so that the important power

stations and loads keep functional. Therefore, these islands are made in

such a way that the generation and load in these islands approximately

match.

Under operational planning, procedures are also formed for restoration ofgrid in case of tripping of some or more elements of the grid or for total

blackout. This is done by the Regional Load Dispatch Centre responsible for

real time operation of the regional grids, in consultation with all the players

involved in grid operation. One of the points involved in grid restoration is the

“black start”, which means starting of a generating unit after a blackout.

Since hydro generators require the least power for starting, they are normally

started first or, in other words, used for black start.

Commercial billing by the various generators is done in accordance with the

Availability Based Tariff approved by the Central Electricity Regulatory

Commission. Under the Availability Based Tariff (ABT), the beneficiaries are

required to pay charges in three components, viz., annual fixed charges,

energy charges and Unsheduled interchanges (UI) charges.


DistributionSystem

39

Annual fixed charges are required to be paid by the beneficiaries irrespective

of actual drawals or schedules. The implemented schedules, as described

earlier, are used for determination of the amounts payable as energy charges.

Deviations from schedules are determined in 15-minute time blocks through

special metering, and these deviations are priced depending on frequency.

These deviations are called unscheduled interchanges (UI).

The pricing for UI is linked to system frequency such that the constituent

causing the grid frequency to improve/worsen in worst conditions gets

rewarded/penalised at higher price and vice versa. Further, the UI pool account

is zero sum account, i.e., the amounts received from constituents are

distributed amongst the other constituents. As long as the actual generation/

drawal is equal to the given schedule, UI is zero and the payment on account

of the third component of Availability Tariff is zero. In case of under-drawal, a

beneficiary is paid back to that extent according to the frequency dependent

rate specified for deviations from schedule.

4.6.3 Load Balancing

Load Balancing is the process of achieving and maintaining equal load on

each phase of a distribution transformer. The loadings on primary and

secondary side of a DTR are shown in Fig. 4.11.

Fig. 4.11: Balancing of Load in a DTR

If load on each phase of the distribution transformer is not equal, it iscalled unbalanced loading of transformer. Practically speaking, balanced

load cannot be maintained on the transformer due to the inherently varying

nature of load. Each transformer supplies power to resistive loads (bulbs,

heaters, etc.) and inductive loads (motors, etc.). These loads can be either

single phase, distributed separately on the three phases, or three-phase in


40

nature. If the distribution transformer is supplying power to only three phase

loads, then achieving and maintaining balanced load on transformer could be

an easier task. But in practice, this happens very rarely, because each

installation possesses either three phase or single phase or both the loads,

which keep changing at different points of time.

Apart from natural unbalancing, unbalanced load may also result from loadshedding of one phase in each of the LT feeders emanating from a

distribution substation. Even though the system may have been balanced

initially, it is difficult to have similarly loaded outgoing feeders and achieve

equal load shedding in the three phases. In some cases, due to constraints on

availability of proper switching facility on each feeder, it is difficult to shed

equal load from each phase.

Thus, it is really a difficult task for a distribution utility to maintain balanced

load on the distribution transformer. However, it is important for many

reasons.

Difficulties in Maintaining Balanced Load on a Transformer

The distribution transformer supplies power to the domestic and/or

commercial consumers. In practice, it is seen that all consumers will not

switch on their entire connected load at the same instant of time. The

switching of load will vary with time and also with the requirement of the

consumer. This is expressed in terms of the diversity factor. If it is equal to

1, it means that all the consumers are in need of their entire connected load at

the same instant of time. In practice, the diversity factor ranges between 2

and 3. If diversity factor is more, then there is a greater possibility that the load

on each phase of the distribution transformer will vary and not be equal.

Now, consider an Electric Utility which is supplying power to small industrial

consumers. In this case, the transformer will be supplying to more three

phase loads than single phase loads. So, the diversity factor for small

industrial consumers will be less compared to the domestic/commercial

consumers. For large industrial consumers, the diversity factor will be even

less. But some degree of unbalanced loading will still remain on the

transformer. The task of distribution utility is to reduce this degree of

unbalanced loading.

IMPORTANCE OF LOAD BALANCING

• Extended life of the transformer, which remains in service for longer

period of time.

• Improved quality of supply.

• Lesser maintenance cost on distribution/power transformer leading to

increase in Utility’s Operational Profit.

• Consumer Satisfaction.

NOTE

Diversity factor isdefined as the ratio ofthe sum of the individualmaximum demands ofvarious parts of a powerdistribution system to

the maximum demand

of the whole system. Itmeasures thestaggering of differenthours of the day andindicates flatness ofload curve. That is, itdenotes MVA vs hoursof the day curve.


DistributionSystem

41

We offer some tips in this regard.

Some Tips to Operate Transformers Near Balanced Loading

• Connect single phase load on each phase of distribution transformer, so

that at the end, current in each phase of transformer will be almost equal. It

has been seen that, linemen or wiremen connect the single phase load to

the lower phase of a pole. It may be due to illiteracy and/or hesitation to

connect the single phase load on the top phase of pole. The distribution

utility must ensure supervision of the job at the time of connecting new

single phase load to avoid such practices.

• In some distribution utilities, transformers do not have current measuring

instruments and, hence, continuous surveillance cannot be done to check

whether distribution transformer is equally loaded (Balanced). In this case,

the current must be measured for all phases by using Clamp-on-Meters, at

least once during peak hours and a record should be kept. By analysing

the past trend, the average current can be calculated. But due

consideration must be given to changing weather conditions and/or extra

loads due to festivals, etc.

• Providing Solid Earthing to the neutral of transformer.

• Using proper size of Blow-out-Fuses.

On this note, we bring the discussion in this unit to an end. In this unit, you

have learnt about the Power Distribution System, and the general goals and

practices for its maintenance. We now summarise the contents of the unit.

• The Distribution System contains:

− Sub-transmission system in voltage ranges from 33 kV to 220 kV.

The energy goes from power substations to distribution substations

through primary system and then from distribution substations to

secondary distribution system for local voltage distribution.

− Primary circuits of feeders, usually operating in the range of 11 kV to

33 kV, supply the load in well defined geographical areas.

− The distribution transformers, usually installed on poles or near the

consumer sites, transform the primary voltage to the secondary

voltage, which is usually 240/415 V.

− Secondary circuits at service voltage which carry energy from the

distribution transformers along the streets, etc.

• The components of Distribution System include substations,

transformers, feeders and metering system, etc.

• The O&M objectives and general practices for distribution system focus on

improving the reliability and maintenance of plant and equipment,

maximising capacity utilisation, increasing operating efficiency, and

4.7 SUMMARY


42

reducing operating and maintenance costs.

• Grid management, load scheduling and load balancing are important

for the smooth functioning of the power distribution system.

1. Which component of the distribution system can be a critical

bottleneck in supplying uninterrupted power to consumers and why?

2. What configurations of feeder networks can be used in a distribution

system? Discuss their suitability in different circumstances.

3. Compare the distribution system planning criteria used in your utility

with those given in the unit and describe them.

4. What do you understand by load scheduling and Unscheduled

Interchanges?

5. Does your utility have written O&M practices? If yes, study those and

suggest improvements in the same. If not, what practices are followed

in the field?

6. What is load balancing? How can it be achieved?

7. What is the significance of Grid Management? Who has the

responsibility of Grid Management in your State and your utility?

8. What are the activities involved in Distribution System Planning?

4.8 TERMINAL QUESTIONS


DistributionSystem

43

APPENDIX 1: REACTIVE POWER CONTROL IN DISTRIBUTION SYSTEMS

Reactive power (kVAR) control represents an efficient method of reducing

the cost of utility operation. The savings brought about by kVAR/voltage

control are not confined to the monetary value of the energy saved; the

released system capacity can serve to delay a costly expansion and reduce

the ageing of components. kVAR control provides appropriate placementof compensation devices to ensure a satisfactory voltage profile whileminimizing the power losses and the cost of compensation.

Other ancillary benefits gained by correcting the power factor are:

• lower energy losses,

• better voltage regulation, and

• released system capacity.

All electric equipment requires “vars”, a term used by electric powerengineers to describe the reactive or magnetizing power required bythe inductive characteristics of electrical equipment. These inductive

characteristics are more pronounced in motors and transformers, and

therefore, can be quite significant in industrial facilities.

The flow of vars, or reactive power, through a watt-hour meter will not affect

the meter reading, but the flow of vars through the power system willresult in energy losses on both the utility and the industrial facility.

Some utilities charge for these vars in the form of a penalty, or kVA demand

charge, to justify the cost for lost energy and the additional conductor and

transformer capacity required to carry the vars. In addition to energy losses,var flow can also cause excessive voltage drop, which may have to becorrected by either the application of shunt capacitors, or other moreexpensive equipment, such as load-tap changing transformers,synchronous motors, and synchronous condensers.

The power factor triangle shown in Fig. 1 is the simplest way to understand

the effects of reactive power. The longest leg of the triangle (on the upper or

lower triangle), labelled total power, represents the vector sum of the reactive

power and real power components. Mathematically,

The angle Φ in the power triangle is called the power factor angle and is

mathematically equal to:

TOTAL POWER = (REAL POWER)2 + (REACTIVE POWER)2

cos Φ =

REAL POWER (kW)

TOTAL POWER (kVA)


44

Fig. 1: Power Factor Triangle

The ratio of the real power to the total power in the equation above (or the cos

of Φ) is called the power factor.

The advantages of PF improvement by capacitor addition are as follows:

a) Reactive component of the network and total current in the system

from the source end are reduced.

b) I2R power losses are reduced in the system because of reduction

in current.

c) Voltage level at the load end is increased.

d) kVA loading on the source generators as also on the transformers

and line up to the capacitors reduces giving relief. A high powerfactor can help in utilizing the full capacity of your electricalsystem.

Cost Benefits of PF Improvement

While cost of PF improvement is in terms of investment needs for capacitor

addition, the benefits to be quantified for feasibility analysis are:

a) Reduced kVA (maximum demand) charges in utility bill,

b) Reduced distribution losses (kWh) within the plant network,

c) Better voltage at motor terminals and improved performance of

motors.

A high power factor eliminates penalty charges imposed when operating with a

low power factor. Investment on system facilities such as transformers,

cables, switchgears, etc. for delivering load is reduced.

It is the power distribution engineer’s responsibility to manage theoperating system at an optimum power factor.


DistributionSystem

45

Material and Equipment Information

− Classification of maintenance material (class / sub-class).

− Material identification with a material code.

− Equipment identification and details.

− Updating equipment details.

− Maintaining commercial details.

− Maintaining equipment hierarchy for all equipment and assemblies.

− Maintenance planning and scheduling.

− Maintaining bills of material / sub-assembly / component.

Operational Location Information

− Identification of the operational location.

− Maintaining corresponding location of equipment.

− Maintaining details of locations.

Work Specification

− Standardising work specifications.

− Maintaining notes on work specifications.

Preventive Maintenance and Overhaul

− Generate preventive maintenance plans.

− Listing of locations, equipment and tasks to be included in the plan.

− Allocation of tasks to specific groups.

− Recording the material required.

− Generating work orders.

− Overhauling equipment as per requirement.

Breakdown Maintenance

− Noting the time of fault and the priority status.

− Processing the request.

− Enabling maintenance work.

Condition-based Maintenance

− Predicting potential machine failures.

− Recording data about malfunctioning equipment.

Signature Analysis

− Identifying parameters to be monitored for each class of equipment.

APPENDIX 2: FUNCTIONS OF O&M


46

− Identifying test point for measuring the above.

− Entering a target or optimum value, and limits for the parameter.

− Determining the frequency of measurements to be made at each test

point.

− Generating schedules for taking readings.

− Capturing readings at each test point.

− Analysing readings and generating alarm / warning signals.

− Displaying message to generate work order if required.

Maintenance Requests

− Issuing maintenance requests.

− Closing maintenance requests.

Maintenance Work Orders

− Generating work orders.

− Informing the concerned departments.

− Assigning tasks to maintenance personnel or vendors.

− Issuing material for doing the same.

− Storing the details of material issued / purchased.

− Monitoring status of tasks.

Safety Procedures and Permit To Work

− Sending permit-to-work (PTW) requests to operations.

− Receiving the necessary permits from operations.

Spares Planning

− Identifying spares required for equipment.

− Viewing the updated stock positions.

− Generating lists of spares featuring quantity required and criticality.

Maintenance History

− Recording detailed maintenance history.

− Recording breakdown details / cause of failure / action taken /

downtime.

− Generating reports for equipment failures.

Document Management

− Maintaining document master.

− Generating document register.

Maintaining Contract Details.

Maintenance Personnel and Workgroup


DistributionSystem

47

− Maintaining work group details.

− Viewing work schedules.

Capturing Operations Data

− Maintaining operations logs.

− Maintaining generation and transmission schedules.

− Maintaining parameter values.

− Maintaining fuel details.

Estimates and Expenditure

− Generating estimates.

− Capturing estimates.

MIS Reports

− Generation reports.

− Plant-availability reports.

− Outage reports.

− Fuel reports.

− Generation schedules.

− Equipment registers.

− Equipment-performance reports.

− Equipment-history cards.

− Fault-analysis reports.

− Maintenance plans.

− Maintenance requests.

− Permit to work.

− Work-order permits.

− Condition-analysis reports.

− Expenditure reports.

− Maintenance schedule miss reports.

− Material-consumption reports.

− Document register.

− Accident reports.


48

49

SubstationEquipment and

DistributionLines

Unit 5

SubstationEquipment andDistribution Lines

Learning Objectives

After studying this unit, you should be able to:

describe the main equipment required for the construction of a 66-33/11kV substation;

classify the distribution line equipment; describe the main equipment for overhead

lines; discuss the important features of underground

power cables in the distribution system network; enunciate the general operation and

maintenance practices for substation equipment, distribution lines and capacitors;

explain hotline maintenance techniques and

tools; and explain the effect of HT/LT ratio on line losses

and voltage.

50


In Unit 4, you have studied about the power distribution system and its

components. You have also learnt about distribution system planning and

the general O & M objectives and practices. You will agree that the smooth

operation of the power distribution system depends on how well it is

maintained. This includes the operation and maintenance of all its

components.

We begin this unit with a discussion of the substation equipment and

distribution lines so that you know the standards prescribed for the equipment.

Adhering to these standards would ensure the smooth operation of the

equipment. We next discuss the operation and maintenance of equipment

used in the 66-33/11 kV substations, 11/0.4 kV substations, overhead lines,

underground cables and capacitors. Finally, we take up hot line maintenance

tools and techniques and the impact of LT/HT ratio on losses. In the next unit,

we deal with the O & M of distribution transformers separately.

Equipment in a substation can broadly be categorized as follows:

• structures;

• power transformers;

• bus-bars;

• circuit breakers (33 kV and 11 kV);

• isolators or isolating switches (33 kV and 11 kV);

• earthing switches;

• insulators;

• power and control cables;

• control panel;

• lightning protection − surge arrestors;

• instrument transformers (current and power transformers, i.e., CTs and

PTs);

• earthing arrangements;

• reactive compensation;

• DC supply arrangement;

• auxiliary supply transformer; and

• fire-fighting system.

The design of the substation equipment must comply with the requirement of

relevant Indian Standards.

5.1 INTRODUCTION

5.2 66 - 33/11 kV SUBSTATION EQUIPMENT

51


DistributionLines

We now briefly describe each one of these.

• Structures

Structures are required to provide entry from the overhead line to the

substation and to extend out required number of feeders. The numbers of

structures should be kept to a minimum as large number of structures

would not only be uneconomical but give an ugly look to the substation and

may prove to be obstructions in extending bus-bar, lines, etc. The mainstructures required for 33/11 kV substations are:

− incoming and outgoing gantries;

− support structures for breaker, isolators, fuses, insulators, CTs

and PTs; and

− bus-bars.

Switchyard structures can be made of fabricated steel, RCC or PSCC,

Rail or RS Joist.

• Power Transformers

You have learnt about the underlying principle and design of a power

transformer in Unit 4. The general operation and maintenance practices of

power transformers are similar to those of distribution transformers, which

are discussed in detail in Unit 6.

••••• Bus-bars

A bus-bar in electrical power distribution refers to thick strips of copper or

aluminum that conduct electricity within the substation (Fig. 5.1). The size

of the bus-bar is important in determining the maximum amount of current

that can be safely carried. The bus-bar should be able to carry the

expected maximum load current without exceeding the temperature limit.

The capacity of bus should also be checked for maximum temperature

under short circuit conditions.

Different types of bus-bars, namely, single bus-bar, single bus-bar with

bus sectionalizer, main and transfer bus, double bus-bar, double bus-bar

with double breaker scheme and mesh scheme are used in a substation

in accordance with the safety and reliability considerations.

• Circuit Breakers

A circuit breaker is a switching device built ruggedly to enable it tointerrupt/ make not only the load current but also the much largerfault current, which may occur on a circuit.

A circuit breaker contains both fixed contacts and moving contacts. The

purpose of circuit breakers is to eliminate a short-circuit that occurs on a

line. Circuit breakers are found at the arrivals and departures of all lines

incident on a substation. When the circuit breaker is closed these

contacts are held together. The mode of action of all circuit breakers

consists in the breaking of the fault current by the separation of the moving

contacts away from the fixed ones. An arc is immediately established on

Fig. 5.1: Bus-bars

52

Operation andMaintenance separation of the contacts. Interruption of the current occurs after the arc

at these contacts is extinguished and current becomes zero.

Elements of a Circuit Breaker

Circuit breakers contain the following elements, irrespective of the

medium for arc quenching and insulation:

− main contact at system voltage;

− insulation, such as porcelain, oil or gas, between the main

contacts and ground potential;

− operating and supervisory accessories, of which tripping

facilities are most important.

A wide variety of closing and tripping arrangements (using relays with

variable time delay) and a number of operating mechanisms (based on

solenoids, charged springs or pneumatic arrangements) are available

now-a-days.

The types of breakers used in a distribution system are:

− air break type;

− oil break type;

− vacuum type; and

− SF6 gas breaker.

Fig. 5.2: Circuit Breakers: a) Oil Break Type Breaker; and b) SF6 Gas Breaker

The rated voltage of circuit breakers for 33 kV level is 36 kV, and for 11 kV,

it is 12 kV. The short circuit current rating is 25 kA. The 11 kV switchgear

is generally metal enclosed indoor type.

(a) (b)

53


DistributionLines

• Isolators

Isolators are mechanical switching devices capable of opening or closing

a circuit

− when a negligible current is broken or made, or

− only a small charging current is to be interrupted, or

− when no significant voltage difference exists across the

terminals of each pole.

Fig. 5.3: Isolators

Isolators are capable of carrying current under normal conditions and

short circuit currents for a specified time. In open position, the isolator

should provide an isolating distance between the terminals. The standard

value of rated duration of short time current capacity withstand for isolator

and earthing switch is normally 1 second. A value of 3 seconds is also

sometimes specified. For 33 kV, horizontal type isolating switches are

used. The rated normal current is 630 A at 36 kV. For 11 kV, both

horizontal and vertical mounting isolating switches of 400 Amps at 12 kV

are used.

• Earthing Switches

Earthing switches are provided at various locations to facilitate

maintenance. Main blades and earth blades are interlocked with both

electrical and mechanical means. The earthing switch has to be capable

of withstanding short circuit current for short duration as applicable to the

isolator.

• Insulators

An electrical insulator resists the flow of electricity. Application of

voltage difference across a good insulator results in negligible electrical

current. Adequate insulation is of prime importance for obvious reasons of

reliability of supply, safety of personnel and equipment, etc.

The insulators in use at substations are post insulators of pedestaltype. The station design should be such that the number of insulators is

kept at a minimum at the same time ensuring security of supply. In the

areas where the problem of insulator pollution is expected (such as near

the sea or thermal station, railway station, industrial area, etc.) special

insulators with higher leakage resistance should be used.

54

Operation andMaintenance ••••• Power and Control Cables

Power and control cables of adequate current carrying capacity and

voltage rating are provided at the substation. Power cables are used for33kV,11 kV or LT system to carry load current. The control cables are

required for operating and protection system connections. The cables are

segregated by running in separate trenches or on separate racks.

••••• Control Panels

Control panels installed within the control building of a switchyardprovide mounting for mimic bus, relays, meters, indicatinginstruments, indicating lights, control switches, test switches andother control devices. The panel contains compartments for incoming

lines, outgoing lines, bus-bars with provision for sectionalizing, relays,

measuring instruments, etc.

The panel is provided with:

− suitable over-current and earth fault relays to protect the

equipment against short circuit and earth faults; and

− measuring instruments such as ammeter, voltmeter and energy

meter for 33kV and 11 kV systems.

• Lightning Protection−−−−−Surge Arrestors

Large over voltages that develop suddenly on electric transmission and

distribution system are referred to as “surges” or “transients”. These are

caused by lightning strikes or by circuit switching operations. Surge

arrestor is a protective device for limiting surge voltages on equipment by

discharging or bypassing surge current.

The surge arrestor which responds to over-voltages without any time

delay is installed for protection of 33 kV switchgear, transformers,

associated equipment and 11 kV and 33 kV lines.

The rated voltage of arrestors for 33 kV should be 30 kV for use on 33 kV

systems and with nominal discharge current rating of 10 kA. The rated

voltage of lightning arrestors should be 9 kV (r.m.s.) for effectively earthed

11 kV system (coefficient of earth not exceeding 80 % as per IS: 4004)

with all the transformer neutrals directly earthed. The nominal discharge

current rating should be 5 kA.

• Instrument Transformers (Current and Voltage Transformers)

The substations have current and voltage transformers designed toisolate electrically the high voltage primary circuit from the lowvoltage secondary circuit and, thus, provide a safe means of supplyfor indicating instruments, meters and relays.

Ø Current Transformer (CT)

Current transformers are used in power installations forsupplying the current circuits of indicating instruments

Fig. 5.4: Surge Arrestors

55


DistributionLines

(ammeter, wattmeter, etc.), meters (energy meter, etc.) andprotective relays. These transformers are designed to provide a

standard secondary current output of 1 or 5 A, when rated current

flows through the primary. A fundamental characteristic of CT is its

transformation ratio, expressed as the ratio of the rated primary to

rated secondary current. Current transformers have two inherenterrors: the current ratio and phase displacement. These two

errors serve as a basis on which current transformers are classified

for accuracy.

Fig. 5.5: a) Current Transformers; and b) Voltage Transformer

Ø Voltage Transformer or Potential Transformer (PT)

These instrument transformers are used for supplying the voltage

circuit of indicating instruments, integrating meters, other measuring

apparatus and protective relays or trip coils. These may be of single

phase or three phase design and of the dry or oil immersed types. A

voltage transformer or PT is rated in terms of the maximum burden

(VA output) it will deliver without exceeding specified limits of error. On

the other hand, a power transformer is rated by the secondary output it

will deliver without exceeding specified temperature rise. All voltage

transformers are designed for a standard secondary voltage of 110 V

or 110 / 3 V.

• Earthing Arrangements

Earthing has to be provided for

− safety of personnel,

− prevention of and minimizing damage to equipment as a result

of flow of heavy fault currents, and

− improved reliability of power supply.

The basic grounding system is in the form of an earth mat with risers.

A current transformer is an instrument transformer in which the current ratio is within the specified limit. The primary

winding is connected

in series with the load

and carries the load

current to be

measured. The

secondary winding is

connected to the

measuring instrument

or relay, which

together with the

winding impedance of

the transformer and

lead resistance

constitute the burden

of the transformer.

NOTE

Voltage transformer is an instrument transformer in which the secondary voltage is substantially proportional to the primary voltage and phase angle near to zero for an appropriate direction of connection.

NOTE

(b)(a) (b)

56

Operation andMaintenance Risers of MS flat are generally provided. Earth mat is provided within the

substation area. The earth rods are connected to the station earth mat.

The earthing must be designed so as to keep the earth resistance as low

as possible. Earthing practices have been discussed in Unit 6 of the

course BEE-002.

• Reactive Compensation

Reactive compensation (as indicated by system studies of the network)

has to be provided. It is always a good idea to ensure a power factor

correction for transformers, since even when they are operating on low

load (e.g., during the night) they absorb reactive power, which must be

compensated to avoid unnecessary loadings and losses. You can recall

this aspect from Appendix 1 to Unit 4. Shunt capacitors (Fig. 5.6) are

connected on the secondary side (11 kV side) of the 33/11 kV power

transformers. The capacitors are generally of automatic switched type.

The automatic system of the capacitor bank has the task of switching in

the necessary capacitance according to the load requirements at each

given moment.

• Station Battery/DC Supply Arrangement

Station batteries supply energy to operate protection equipment such as

breakers and other control, alarm and indicating equipment (Fig. 5.7). The

station batteries are a source for operating DC control system equipment

during system disturbances and outages. During normal conditions the

rectifier provides the required DC supply. However, to take care of rectifier

failure, a storage battery of adequate capacity is provided to meet the DC

requirements.

Normally, in a 33/11 kV substation, the DC system is of 30 cells consisting

of 15 lead acid storage batteries or Nickel-Cadmium batteries. The battery

is connected in parallel with a constant voltage charger and critical load

circuits. The charger maintains the required voltage at battery terminal and

supplies the normally connected loads. This sustains the battery in fully

charged condition. The correct size battery charger has to be selected for

the intended application.

• Auxiliary Supply Transformer

An Auxiliary Supply Transformer of adequate capacity is required to be

provided for internal use for lighting loads, battery charging, oil filtration

plant, etc. The supply should be reliable. In a substation it is normally

provided from a station transformer connected on 33 or 11 kV bus bar.

• Fire Fighting System

In view of the presence of oil filled equipment in a substation, it is

important that proper attention is given to isolation, limitation and

extinguishing of fire so as to avoid damage to costly equipment and

reduce chances of serious dislocation of power supply as well as ensure

safety of personnel. The layout of the substation itself should be such that

the fire should not spread to other equipment as far as possible. Fire

Fig. 5.6: Shunt Capacitors

Fig. 5.7: Battery Bank

57


DistributionLines

extinguishers of the following type must be provided:

− Carbon dioxide extinguisher, and

− Dry chemical powder extinguisher.

Carbon dioxide (CO2 type) extinguisher and Dry chemical powder type

extinguisher should conform to IS: 2878 and IS:2171, respectively. For oil

fire, foam type extinguishers are used (see Unit 7, BEE-002 also). The fire

fighting equipment should be maintained and kept in top condition for

instant use as per IS: 1948-1961 “Fire Fighting Equipment and its

Maintenance including Construction and Installation of Fire Proof Doors-

Fire Safety of Buildings (General)”.

So far we have described the equipment in a 66-33kV/11kV substation.

You may like to review the information before studying further.

The main equipment at an 11/0.4 kV distribution substation comprises:

• distribution transformers;

• transformer mounting structure;

• protection system;

• earthing system;

• lightning arrestors;

• LT distribution box; and

• reactive compensation.

We shall be discussing the distribution transformers in detail in the next unit.

Here, we briefly describe the remaining components.

• Transformer Mounting Structure

Transformers can be mounted outdoors (Figs. 5.8 and 5.9) in one of the

List the equipment being used in your utility for the construction of

33/11 kV substation along with their typical ratings.

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

5.3 11/0.4 kV SUBSTATION EQUIPMENT

SAQ 1: Equipment at 66-33/11kV substation

58

Operation andMaintenance following ways: Plinth mounting, H-pole mounting and direct

mounting. We describe these mountings, in brief.

Ø Plinth mounting: The transformer is mounted on a plinth made of

concrete. The plinth has to be higher than the surroundings. The

method can be used for all sizes of transformers. Where the

distribution substations are plinth mounted, they are efficiently

protected by fencing so as to prevent access to the apparatus by

unauthorized persons.

Ø H-pole mounting: The transformer can be mounted on cross-

arms, fixed between two poles, which are rigidly fastened to the

poles. The transformer has two base channels, which rest on the

transformer mounting structure.

Ø Direct mounting: The transformer is clamped directly to the pole by

suitable clamps and bolts. This method is used for transformers up

to 25 kVA only.

• Protection System

The HT side of all transformers is normally protected by drop out

expulsion type fuse. Three 11 kV drop out fuse units comprising aset are installed on mounting cross-arm. The fuse element is

soldered on both ends between woven wires, which are sufficiently

strong to withstand tension when fixed to the terminals on both ends.

The element is housed in an insulated tube of paper or insulating

material. Horn gap fuses are also used on distribution transformers on

HT side. The fuse wire is fixed between arcing horns. The advantage is

that ordinary fuse wire of rated capacity can be used for replacement

while for drop out fuses, fuse elements are required to be stocked for

replacement.

Fig. 5.8: Transformer Mountings: a) Plinth Mounting; and b) H-Pole Mounting

Fig.5.9: Direct Mounting

(a) (b)

59


DistributionLines

• Earthing

Pipe earthing or rod earthing is provided for the distribution substation.

Three electrodes forming an equilateral triangle are provided so that

adequate earth buffer is available.

• Lightning Arrestors

11 kV lightning arrestors 9 (kV) of outdoor type are used for diverting the

lightning surges to earth resistance of earth. The lightning arrestor should

be installed on the HT side and its lead should be kept at a minimum.

• LT Distribution Box

For transformers of 100 kVA and above, sheet metal LT distribution box

consisting of LT breaker and fuse cut-outs is provided from where

distribution feeders are to be taken out. The size of the box has to be

suitable for accommodating MCCB, fuse cut-outs, cable connectors,

bus-bars, etc.

• Reactive Compensation

The load incident on the distribution system is mostly inductive, requiring

large reactive power. The best method is to compensate the reactive

power at the load end itself but it is difficult to implement in practice.

Hence, providing compensation on the distribution system is essential. So

wherever the power factor is low, reactive compensation may be provided

on the distribution transformers.

The shunt capacitor supplies constant reactive power at its location,

independent of the load. Fixed or automatic switched type capacitors of

adequate rating are to be provided on the LT bus of the distribution

transformers. In the switched capacitor system, the capacitors are

switched on and off along with the load to avoid over-voltage during low

load operation.

List the equipment and their typical ratings, being used in the distribution

substations of your utility. Are all the protection equipment listed above

being used?

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

SAQ 2: Equipment at distribution substation

60


The distribution lines can be either overhead or underground. These are

usually overhead, though for higher load densities in cities or metropolitan

areas, these are underground. The choice between overhead and

underground depends upon a number of widely differing factors such as the

importance of service continuity, improvement in appearance of the area,

feasibility in congested areas, comparative annual maintenance cost, capital

cost and useful life of the system.

5.4.1 Overhead Lines

An overhead power line is intended for transmission of electric power by a

bare or covered overhead conductor supported by insulators, generally

mounted on cross-arms near the top of poles. The overhead line may be

66, 33, 11 kV or LT line. The basic equipment used for the line remains the

same. The main equipment required for an overhead line is as follows:

• supports,

• cross-arms,

• insulators,

• earthing knob,

• earthing coil,

• strain hardware set,

• conductors,

• line accessories,

• guard wires, and

• LT line spacers.

We describe each one of these, in brief.

• Supports

A support is a column of wood, concrete, steel or some other material

supporting overhead conductors by means of arms or brackets. The

supports used for overhead line construction vary in design and the

purpose they have to perform. The different types of supports for overhead

lines are: wood poles, concrete poles, steel poles and lattice typetowers.

Ø Wood poles: Chemically treated wood poles are used for distribution

lines. The advantage of using wood poles is that they are low in cost.

However, they are susceptible to decay. The specifications for wood

poles are covered by IS:876 and IS:5978. According to this standard,

the timber suitable for poles has been classified into three groups

5.4 DISTRIBUTION LINE EQUIPMENT

(a)

(b)

Fig. 5.10: a) OverheadLines Mountedon Cross-armPoles;b) Close-up

61


DistributionLines

depending upon its strength. For example, IS 6056 for jointed wood

poles for overhead lines specifies that sal, deodar, chir, kail, wood be

used. Jointed wood poles with wire bound lap joint are considerably

less expensive and found to be very suitable for LT and HT lines in

rural areas.

Ø Concrete poles: Concrete poles are more expensive than wood poles

but cheaper than steel tubular poles. Concrete poles are of three types:

− Pre-cast cement concrete poles (PCC) made of cement

concrete;

− Reinforced cement concrete poles (RCC);

− Pre-stressed cement concrete poles (PSCC).

The low maintenance, competitive price and aesthetic appearance of

PCC poles makes them superior to steel or wood for use in electric

lines. Ease and speed of installation means faster project completion

and lower installation cost. RCC poles have an extremely long life and

need little maintenance but they are bulky in size and comparatively

heavy. They have shattering tendency when hit by a vehicle. PSCC

poles take care of these shortcomings to some extent. However, the

handling, transportation and erection of these poles is more difficult

because of their heavy weight.

Ø Steel poles: The steel poles are of the following types:

− Steel tubular poles whose specifications are covered by

IS:2713-1967. Due to their light weight, high strength to weight

ratio and long life, they possess distinct advantages over other

types of poles. The use of a pole cap at the top, concrete muff

in the ground and regular coating of paint prolongs their life.

− Old and second hand rails and Rolled Steel (RS) joists are

frequently used as supports for overhead lines. The portion

embedded in the ground should be protected by concrete muff

and the remaining portion by regular paint unless galvanised

steel is used.

Ø Lattice type supports: These are fabricated from narrow base steel

structures. They are light in weight and economical and can be

assembled at site if bolted construction is used. Normally both welded

and bolted types are used.

• Cross-arms

The shape and length of the cross-arms depend upon the desired

configuration of conductors. The following types of cross-arms and

brackets are used:

− V cross-arms for tangent locations with clamps;

62

Operation andMaintenance − double channel cross-arms for tension or cut point locations where

double poles are used; and

− top clamps.

Cross-arms of hand wood (sisso, sal), or creosoted soft wood (chir) or

fibre glass are mostly used. Steel cross-arms are stronger and last much

longer. MS angle iron and channel iron sections are generally used for this

purpose. Smaller sections are used for communication circuits.

• Insulators

You have learnt that an electrical insulator resists the flow of electricity.

Application of a voltage difference across a good insulator results in

negligible electrical current. Insulators made of glazed porcelain, tough

glass and polymers are used for supporting the conductors. Porcelain

insulators prevent the electrical current from energizing the power pole.

The principal types of insulator are described below:

− Pin insulators are manufactured for voltages up to 33 kV and are

cheaper than the other types. IS:1445 and 731 cover detailed

specifications for these. The pins for the insulators are fixed in the

holes provided in the cross-arms and pole top brackets. The

insulators are mounted over the pins and tightened. The cost of pin

insulators increases very rapidly as the working voltage is increased.

For high voltages these insulators are uneconomical. Moreover,

replacements are expensive.

− Disc insulators are made of glazed porcelain or tough glass. They

are used as insulators on high voltage lines for suspension and dead

ending.The line conductor is suspended below the point of support by

means of the insulator or a string of insulators. A disc insulator

consists of a single disc-shaped piece of porcelain, grooved on the

under-surface to increase the surface leakage path between the metal

cap at the top and the metal pin underneath. The cap is recessed so

as to take the pin of another unit, and in this way a string of any

required number of units can be built up. The cap is secured to the

insulator by means of cement. Disc insulators are “ball and socket” or

“tongue and clevis” type. A suspension clamp is used to support the

conductor, if suspension configuration of the line is chosen.

− Shackle insulators are used for distribution lines dead ending

and supporting conductors laid in vertical formation. IS:1445-1977

covers shackle insulators for voltages below 1000 V. The two

standard sizes listed in this specification are 90 mm dia x 75 mm

height and 115 mm dia x 100 mm height. A shackle insulator is

supported by either two straps and two MS bolts or one U clamp or

D strap and two MS bolts as per IS:7935.

− Stay insulator/Guy strain insulators of egg type porcelain

are used for insulating stay wire, guard wires, etc. wherever it is not

Fig. 5.11: Pin Type Insulator

Fig. 5.12: Disc TypeInsulator 11 kV

63


DistributionLines

proposed to earth them. As per IS: 5300, two strength sizes (ultimate

tensile strength) are used: 44 kN and 88 kN, respectively, for LT and

HT lines.

−−−−− Stays/Guys and staying arrangement: Guys of stranded steel wire

are used on all terminal, angle and other such poles where the

conductors have a tendency to pull the pole away from its true vertical

position. The guys are fastened to the poles near the load centre point

with the help of pole clamps. The other end of the guy/stay is secured

to a stay rod embedded in the ground. The stay rod should be located

as far away as possible.

••••• Earthing Knob

The earthing knob is used for supporting the neutral-cum-earth wire used

for earthing of metal parts of supporting structures of low-tension lines,

i.e., 400/230 V lines. The knob is generally made of cast iron 52x42 mm

and its electrical resistance is not to exceed 200 mega ohms. Moreover,

the breaking strength at the neck of the knob is not to be less than

11,500 kg when force is applied.

• Earthing Coil

Two types of earthing arrangements are used. One is with GI pipe and the

other is with GI wire. In case of GI pipe earthing, 40 mm dia and 2500 mm

long pipe is used for earthing of supports and fittings. GI wire is used for

earthing of lines. Generally 8 SWG wire with 115 turns, 50 mm dia and

1500 mm length is used.

• Strain Hardware Set

The conductor is strung between sections through a strain hardware set. It

is fixed with the last disc of the string of disc insulator. It is made from

malleable iron or aluminium alloy. Alloy hardware is preferable as the

losses are less.

• Conductors

Conductor represents 30 − 50% of the installed cost of the line. All

aluminium conductors (AAC), all aluminium alloy conductors (AAAC) and

aluminium conductor steel reinforced (ACSR) are generally used.

Technical specifications of conductors are covered in IS: 398. These

conductors are of standard construction and the ultimate tensile strength

of the whole conductor is based on the total strand strength.

• Line Accessories

This is the associated equipment required for fastening the conductors to

supports and taking off the power or supply points such as joints material,

clamps and compounds. For lines up to 33 kV, the following fittings are

used:

♦ Conductor dead-end fittings

− LT conductor dead-end grips,

64

Operation andMaintenance − guy grips dead-end,

− service grips,

− full tension splices,

− distribution ties,

− side ties,

− spool ties,

− tee connectors,

− lashing rods, and

− line guards.

Preformed fittings made of aluminium alloy are used as it saves cost,

labour and time. It also eliminates chances of error of judgment. No

tools are required. These fittings are fast and simple to apply and

assure uniformity of application every time.

♦ Joints should conform to IE Rule 75. For conductors up to

50mm2, crimped joints are made with simple hand crimping tools

and for higher sizes, compression type or hydraulic type crimping

tools are used. Joints are of the following types:

− uni-joints/ compression joints,

− twisting joints,

− two part compression joints, and

− dead-end joints.

♦ Insulator ties secure the conductor to the insulator. In general, the

tie wire should be the same kind of wire as the line wire, i.e., for

tying aluminium conductors on insulators, aluminium wire should

be used. The tie should be made of soft annealed wire so that it is

not brittle and does not injure the line conductor.

♦ Taps and jumpers are made by various accessories, which are

not subjected to mechanical tension. Tapping should be taken off

only at a point of line support.

• Guard Wires

Guard wires are to be used at all points where a line crosses a street,

road or railway line, other power lines, telecommunication lines,

canals, rivers, along the road and public places. As per IE Rule 88,

guard wires of galvanized steel of minimum 4 mm dia having breaking

strength not less than 635 kg should be used.

• LT Line Spacers

Very often clashing of LT conductors in the mid span takes place due

to sag, wind and longer spans. This results in faults and interruptions.

Spacers are provided to overcome this problem.

You may like to revise the information given in this section before

studying further.

65


DistributionLines

5.4.2 Underground Power Cables

Due to the fast growth in load densities in major towns and cities, 33 kV, 11 kV

and LT underground cables are being used to meet the ever growing demand

of electric power. The underground cable system has attainedconsiderable importance in distribution networks. This is because intowns and cities, almost all roads are already occupied by LT, HT overhead

lines, telephone lines, street lights, advertising boards, etc., on either side of

the roads. Further, high-rise buildings make it difficult to go for overhead

systems for sub-transmission or distribution. Moreover, the overhead system

with bare conductors is prone to frequent breakdowns causing interruption in

power supply. Uninterrupted power supply can be maintained by employing

underground cable ring system. The underground cabling system is

particularly important for metropolitan cities, city centres, airports and defence

services.

Underground distribution costs are between 2 to 10 times that of the overhead

system. Yet, it is preferred due to elimination of outages caused by abnormal

weather conditions such as snow, rain, storms, lightning, fires, stress,

accidents, etc. Moreover, this system is environment-friendly and has a long

life. In addition, improved cable technology has reduced the maintenance cost

of the underground system compared to the overhead system. We

summarise the main reasons for underground cable systems in Box 5.1.

Box 5.1: Reasons for Having Underground Cables

Ø The right of way for erecting overhead systems is no longer available;

Ø It is possible to extend the supply from source to load centres on anyroute profile;

Ø Fairly uninterrupted and reliable power supply can be maintained;and

Ø Aesthetic beauty of the town/city as a whole can be ensured.

List the equipment being used for construction of overhead distribution

lines in your utility. Describe the types of supports and insulators being

used for construction of lines.

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

SAQ 3: Overhead lines

66

Operation andMaintenance We now describe, in brief, the selection criteria, sizing, jointing and

terminating of underground cables.

• Selection of Cable

The following factors influence the selection of cable:

− load;

− system voltage;

− type of insulation;

− short circuit rating;

− mode of installation; and

− economy and safety.

For the same conductor size, the maximum continuous current

carrying capacity depends on the depth of laying, ground temperature,

silicon oil resistivity, ambient temperature, proximity of other cables,

type of ducts used. Paper Insulated Lead Covered, PVC and XLPE

cables are being used. Depending upon the voltage at which the

power is transmitted or distributed, the cables are designed as

follows:

• Sizing of Cables

The sizing of cables depends on the following factors:

− current carrying capacity,

− short circuit current,

− voltage drop, and

− losses.

• Jointing and Terminations

Cables are laid in lengths supplied over reels. Cable extensions are made

through joints and terminated at the ends to connect them to the system

for use. Since the cable consists of many items right from the conductors

to the outer sheath, all joints are to be made as straight through jointsso that each joint has the same features/characteristics of the original

cable.

Straight jointing is ensured by providing:

− core continuity;

− stress controlling screens;

− insulation;

− continuity of earth potential parts of the cable by clamping and running

the earth lead;

1. EHV Cables 66 kV and above2. Medium and HV Cables 3.3 kV to 33 KV3. LT Cables up to 1100 V

67


DistributionLines

− mechanical protection by installing the brass or aluminium covers; and

− finishing over the mechanical protective cover.

Cable ends are terminated by providing:

− stress control screens;

− the earthing clamp lead, etc.;

− insulation;

− lugs; and

− rain sheds.

While making joints and terminations, it is essential to know the size and

type of the cable in order to select appropriate kits for joints and

terminations. The kits contain the accessories required along with

instruction sheets for step-by-step procedure for making joints and

terminations. The cable and end terminations should be prepared as per

the dimensional drawing and procedure given in the instruction sheet.

Types of Joints and Terminations

The joint is considered to be the weakest link in the system but the overall

reliability of a distribution system depends on it. Therefore, jointingaccessories and techniques have an important and critical roledespite their comparative low value in the overall investment.

The following types of joints and terminations are used:

− cast iron moulded,

− epoxy resin type,

− heat shrinkable,

− cold shrinkable, and

− ‘push on’ type.

The heat shrinkable, cold shrinkable and ‘push on’ type joints and

terminations do not need any setting time and can be taken into service

immediately.

So far, we have discussed the construction of the substation equipment and

distribution lines. In the next two sections, you will learn about the general

O&M practices for these components of the power distribution system.

List the reasons for using underground cables. State the selection

criteria, sizing, jointing and terminating of underground cables.

………………………………………………………………………………………

………………………………………………………………………………………

………………………………………………………………………………………

SAQ 4: Underground cables

68


Planned maintenance schedules for various components of the power

distribution systems are carefully drawn up by the power distribution utility

even before the installation work is completed. During plant shut-downs for

overall maintenance and before re-energisation, the sub-transmission and

distribution plants are subjected to certain inspection and testing procedures.

This also applies to the cable route that has been de-energised for a long

period of time. Such planned shut down of the plant to be tested and network

reconfiguration ensure continuity of supply to consumers while the testing

takes place.

The power distribution utility must formulate such planned outage schemes at

different times of the year (depending upon the load demands) for different

maintenance periods in such a fashion that consumer supply is least affected.

This also involves putting in place a system for handling customer complaints

about power supply breakdowns.

Customer Relationship Management System

A trouble call management facility should be provided to attend to the power

supply interruptions promptly and to improve the reliability of power supply as

well as minimise the down time. It should also attend to fuse off calls promptly

as well as the complaints of the customer on quality of supply. A computer

based facility provided in the substation/complaint attending centre would

certainly improve this aspect of O&M.

We now describe some general maintenance practices for the substation

equipment and distribution lines.

5.5.1 General Maintenance Practices

There are two aspects of general maintenance:

v Firstly, replacement of the parts that are worn out during thenormal operation must be carried out from time to time.

v Secondly, preventive maintenance should be carried out fordetecting deterioration and mal-operation of the systemcomponents.

In the daily operation of the substation it is the duty of the attendant to inspect

the equipment externally and remedy any abnormality that does not require

disconnection of the apparatus. During this inspection, a watch is required to

be kept for deposits of dust and dirt on the equipment, heating of contacts,

joint or some part, low oil level and oil leakages, etc. Checks should also be

made to ensure that

• the locks and doors of the switch house are in good condition,

• no leaks have developed in the roof,

5.5 O&M PRACTICES FOR SUBSTATIONEQUIPMENT AND DISTRIBUTION LINES

NOTE

Source: Special reporton CEA website“Guidelines for ProjectManagement andPerformance Evaluationof Sub-transmission andDistribution Project”.

69


DistributionLines

• the ventilating and heating systems are operating normally,

• the prescribed safety aids are in place and in good order,

• the earthing connections remain unbroken,

• the packing of the cables entering or leaving a cable trench or tunnel

within the premises are intact,

• the ventilating louvers are not damaged, and

• the access roads leading to the oil filled apparatus are unobstructed,

and will allow the approach of the fire engines in the event of an oil fire

during an emergency.

On-line inspection and testing is normally limited to visual, external andphysical examination in order to ensure that the plant is in a safecondition. Infra-red detectors must be used periodically for inspection of

overhead lines and open terminal, substation bus-bars for hot spots caused by

faulty terminals. In addition, live line washing techniques are also availablefor cleaning overhead lines or open terminal substation insulators.

Purified water with a high resistance value is used in a fine spray fitted from

well-earthed nozzle. Functional testing and trip schemes require special

switching arrangements initially to reconfigure the power system network.

Switchgear site tests during operational maintenance stage vary from utility

to utility depending upon the quality of upkeep of the equipment and

environmental conditions of the site. These generally involve the following

checks and tests:

• General checks include inspection and checking of

− the tightness of terminal connection, piping junctions and bolted joints;

− painting and corrosion protection;

− cleanliness;

− cracking and chipping of bushings;

− foundation bolts; and

− lubrication of contacts and moving parts of the circuit breakers.

• Electrical circuit checks include checking of

− insulation check;

− dielectric strength of the insulating oil;

− level of the oil;

− quality of SF6 gas/ insulating medium such as humidity content, filling

pressure or density except for sealed apparatus;

− leakage of oil, etc.

• Mechanical tests include

− inspection of operating circuits (hydraulic, pneumatic, spring charged)

and consumption during operation;

70

Operation andMaintenance − verification of correct rated operating sequence (recharging, etc).

• Time checks include checking and adjustment of

− track alignment and interlocking mechanism;

− closing and opening times;

− operation and control of auxiliary circuits;

− recharging time of operating mechanism after specified sequence;

and

− other specific operations.

• Electric tests include

− dielectric tests; and

− testing of the resistance of main circuit.

If the substation is constantly attended, the rounds of switchgear are usually

planned for each shift so that all the equipment will be looked at least once a

day. Equipment is also inspected immediately after a trip out.

Substation switchgear requires regular cleaning in accordance with its design,

type of insulation, the degree of pollution of the atmosphere or ambient air,

etc. The frequency of cleaning depends upon the type of layout of the

apparatus and insulators. However, cleaning must be done during eachpreventive maintenance activity.

Even though the vacuum switchgear does not require elaborate maintenance

like the oil insulated switchgear, it is still necessary to make periodic routine

inspection. The absence of ionized gas and carbon during interruption

removes the major source of insulation contamination.

5.5.2 Maintenance of Lines

Pre-monsoon inspection of all 33 kV lines should be completed between

January and March every year after obtaining due approvals for pre-arranged

shut downs for the entire programme.

The staff responsible for the pre-monsoon inspection should carry all the

necessary equipment such as ropes, petroleum jelly, cotton waste and

sufficient O&M materials like insulators, discs, nuts for the pins, binding wire,

etc.

In the routine maintenance practices, all the tree clearances are doneand all the minor defects like damaged insulators, improper pin binding,

loose jumpering and loose stays are rectified during the inspection itself. All

the insulators are cleaned, all AB switches are lubricated and defective blades

replaced. The defects that may take considerable time for rectification are

noted down and attended within the next one week. Examples are insertion of

poles, replacement of damaged conductors, replacement of damaged

supports, etc.

Periodical patrolling of 33 kV lines has to be done on a monthly basis. The

71


DistributionLines

patrolling is also done and suspected defects rectified, whenever the line trips

on fault. One of the major precautions to be kept in mind by the maintenance

staff is to take the permit to work or line clear to work on distribution lines.

Procedure for Permit to Work (Line Clear)

A line clear or a permit to work (PTW) on any electrical equipment or line is

issued by an authorised person to another authorised person. If there are

more than one gangs working under the same supervisor, each gang takes

sub-line clears from the supervisor who has taken the line clear. In case, the

line clear has to be issued for the supervisor, s/he takes self line clear. In this

case also, all the precautions that are to be followed in issue and return of line

clear are followed.

Line clear books are very important records. Pages in these books are serially

numbered and no paper from this book is used for any other purpose. If any

page is to be destroyed, the custodian specifically mentions the reasons for

doing so. It is attested by his/her dated signature. The line clear books are

reviewed periodically by the Competent Authority.

Line clear can be issued/received over telephone. It is desirable that the

issuer/receiver recognise each other’s voice. The requisition for line clear and

the line clear issue messages are repeated by both the parties to ensure that

line clears are issued/received on the equipment on which it is intended. A

secret code number is followed in such cases. You may like to revisit Units 6

and 7 of the course BEE-002 for the details.

5.5.3 Operation and Maintenance of Capacitors

A routine check of the capacitor performance is made by measuring current

with the help of Ammeter/Tong tester once in two months and the record is

maintained. If any reduction in current /failure of capacitor is noticed, supplier/

manufacturers must be contacted immediately and replacement of capacitor

initiated.

The status of the capacitor is determined by the voltage at the highest voltage

bus available at the substation. It is subject to the maximum permissible

voltage at the bus on which the capacitor bank is connected and the loading

factor. The loading factor is the ratio of the total MVA load on the bus at which

the capacitor is installed to the MVAR rating of the capacitor. Accordingly, the

switching on/off of the capacitor bank is done as per Table 5.1.

Table 5.1: Voltage of Highest Level at the Substation

Voltage of Highest Level at the Substation (kV)System Voltage

Above Between Below

For 220 kV level

For 132 kV level

For 66 kV level

For 33 kV level

230

140

70

30

230 - 220 220- 215 215- 205 205

122

60

30

140 - 130

70 - 68

35 - 34

132 - 128

68 - 65

34 - 32

128 - 122

65 - 60

32 - 30

72

Operation andMaintenance The loading factor and the status of capacitor switch are given in Table 5.2.

Table 5.2: Loading Factor and the Status of Capacitor Switch

LV bus voltage is controlled by changing transformer taps. Notwithstanding

the above, if the voltage at the bus on which capacitor is connected is 1.1 per

unit or higher, the capacitor is switched off.

5.5.4 Hot Line Maintenance

Work performed on transmission and distribution lines while they are

energized and in service is called hot line maintenance. Hot line tools are

all types of tools mounted on insulated poles used to maintain energized high

voltage lines and other safety equipment. Insulated disconnect stick,

wire-holding stick, auxiliary arm, cross-arm mount, pole mount, wire tong,

saddles,flexible line hose and hoist link stick are some of the hot line toolsin use.

When working with energized power lines, linemen must use protection to

eliminate any contact with the energized line. Some distribution-level voltages

can be worked using rubber gloves. The limit of how high a voltage can be

worked using rubber gloves varies from company to company according to

different safety standards and local laws. You may like to refer to Units 6

and 7, Block 2 (BEE-002) for more information.

Fig. 5.13: Hot Line Maintenance

Loading Factor Status of Capacitor Switch

Above 2

Between 1 to 2

Below 1

Off

Off

Off

Off

Off Off

Status-Quo

Status-Quo

Status-Quo

On On

On

On

On

On

73


DistributionLines

Voltages higher than those (which can be worked using gloves) are worked

with special sticks known as hot-line tools, with which power lines can be

safely handled from a distance. Linemen must also wear special rubber

insulating gear when working with live wires to protect against any accidental

contact with the wire. The buckets from which linemen sometimes work are

also insulated using rubber.

For high voltage and extra-high voltage transmission lines, specially trained

personnel use so-called “live-line” techniques to allow hands-on contact with

energized equipment. In this case, the worker is electrically connected to the

high voltage line so that he is at the same electrical potential. The lineman

wears special conductive clothing which is connected to the live power line, at

an instant such that the line and the lineman are at the same potential allowing

the lineman to handle the wire safely. Since training for such operations is

lengthy, and still presents a danger to personnel, only very important

transmission lines are the objects of live-line maintenance practices.

The ratio of primary line length to its concerned secondary distribution line

length is one of the important factors that influence the performance of

primary distribution. Over the years, large scale expansion of the urban

system and rural electrification programme in the country has resulted in

considerable expansion of Low Tension (LT) distribution network. The size of

the distribution transformers has been constantly increasing to meet the

increasing demand due to load growth.

As a result, the length of LT lines/circuits is also increasing resulting in high

losses in LT lines, excessive voltage drops, frequent faults on LT network and

higher rate of failure of distribution transformers. This has also resulted in very

large length of LT lines as compared to High Tension (HT) lines resulting in

high LT/ HT ratios. The ratio of LT to HT lines in our country has been of the

order of 3. This results in high losses and low voltages at the consumer end.

a) At which line voltages do personnel in your company carry out the

maintenance work using

i) rubber gloves, and

ii) hot-line tools?

b) Explain the live-line maintenance technique.

…………………………………………………………………………………

…………………………………………………………………………………..

…………………………………………………………………………………..

SAQ 5: Hot line maintenance

5.6 LENGTH OF LT LINES, HT:LT RATIO AND IMPACT ON LOSSES AND VOLTAGE

74

Operation andMaintenance 5.6.1 Impact of Increasing HT Lines

Increasing HT lines can help in reducing both line losses and voltage drops.

Reduction in Line Losses

In the low voltage distribution system, supply at low voltages with long LT lines

using smaller conductor sizes causes high line losses. However, the loss in

HV system for the distribution of the same power is less than 1% of the LV

system. Hence, with HV system the total energy losses are considerablyreduced.

Reduction in Voltage Drops

The voltage drop in LV lines is very high as the lines are long and have smaller

conductor sizes. In HV distribution systems, the voltage drop for the

distribution of same quantum of power is less than 1% as against that in low

voltage distribution system. This ensures proper voltage profile at the

consumer end.

All other parameters, like load factor, power factor, etc., remaining the same,

the percentage losses in a system having higher LT/HT ratio will behigher than in a system having lower LT/HT ratio. A ratio of 1 to 1.2 would

be very beneficial to power distribution. As this measure is a must to improve

efficiency and voltage regulation of distribution, additional capital investment

should not come in the way.

With this discussion on the impact of increasing HT lines on reduction in line

losses and voltage drops, we now end the unit and summarise its contents.

• In the overall power development scenario, the Transmission and

Distribution system constitutes the essential link between power

generating sources and the ultimate consumers and substations and lines

have to be erected for providing quality power supply.

• The main equipment used in a substation comprises structures,

transformers, bus-bars, circuit breakers, isolators, earthing switches,

lightning arrestors, substation batteries, fire extinguishing equipment, etc.

Overhead distribution lines and underground cables (in urban areas) carry

power to the end-user.

• There are two aspects of general maintenance: replacement of partsthat are worn out from time to time and preventive maintenance for

detecting deterioration and mal-operation of the system components.

Periodic checks and tests should be carried as per specified procedures,

which may vary from utility to utility depending upon the site conditions.

• Special hot line maintenance techniques and tools are required for

maintaining live lines.

• Due to increasing LT lines in the distribution system, losses, excessive

voltage drops and frequent faults have resulted in the LT network leading to

5.7 SUMMARY

75


DistributionLines

a higher rate of failure of distribution transformers. The high LT/HT ratios

result in high losses and low voltages at the consumer end. An LT/HT ratio

of 1 or 1.2 is preferable.

1. Describe the equipment required for the construction of a 66-33/11 kV

substation.

2. Describe the equipment required for the construction of a 11/0.4 kV

distribution substation.

3. What equipment is required for the construction of an overhead

distribution line?

4. Distinguish between current and voltage transformers.

5. List the different types of underground cables in use today. What criteria

are used for the selection of these cables?

6. State the precautions that need to be taken in jointing and terminating

underground cables.

7. Give reasons why underground cabling is being opted for in urban areas.

What are its advantages?

8. Explain hot line maintenance techniques and tools.

9. Explain the impact of LT/HT ratio on losses and voltage.


76


77

DistributionTransformer

Learning Objectives

Unit 6


After studying this unit, you should be able to: classify distribution transformers; explain the criteria for distribution transformer

selection and placement; identify the causes for failure of distribution

transformers; discuss the various methods of testing a

transformer; and

describe the different ways of enhancing transformer life and efficiency.

78


In Unit 4, you have learnt that the transformer is an electrical device used for

stepping down or stepping up the supply voltage. You know that the

distribution transformer (DTR) steps down the primary distribution voltage of

11 kV or 33 kV to secondary distribution of 415V between phases and 240V

between phase and neutral.

In this unit, you will study about distribution transformers in detail covering

their selection criteria, causes of failure, tests on DTRs and ways of

improving their life and efficiency.

6.2 DISTRIBUTION TRANSFORMERS: SELECTIONAND PLACEMENT

Distribution transformers are used both in electrical power distribution and

transmission systems. The power rating of a transformer is normally

determined by the cooling method and the coolant used. Oil or some such

other heat conducting material is commonly used as coolant. Ampere rating is

increased in a distribution transformer by increasing the size of the primary

and secondary windings; voltage ratings are increased by increasing the

voltage rating of the insulation used in making the transformer.

The criteria for selection of a transformer and its technological detailsdepend on its intended use or purpose, working conditions andoperating requirements. For example, a power transformer is selected in

the sub-transmission level because it is consistently loaded up to full rating in

order to obtain maximum efficiency at full load. On the other hand, distribution

transformers (in the power distribution system) are under-loaded most of the

time in order to ensure maximum all day efficiency at the expense of lower

efficiency during peak hours. In order to understand these criteria, you need to

know about transformer classification.

6.2.1 Classification of Transformers

Apart from classification on the basis of purpose (as power transformers and

distribution transformers) transformers are also classified on the basis of:

v Type of Core Used; and

v Type of Cooling Used.

v Type of Core Used

In laminated-steel-core transformers, two main types of cores are used:

core type and shell type.

• Core type transformers have cores with a hollow square

through the centre (Fig. 6.1). Note that the core is made up of

many laminations of steel.

6.1 INTRODUCTION

6.2 DISTRIBUTION TRANSFORMERS: SELECTIONAND PLACEMENT

79


Fig. 6.1: Core Type Transformer

• Shell type transformers are the most popular and efficient

transformers and they have a shell core (Fig.6.2). Note that

each layer of the core consists of E- and I-shaped metallic

sections, which are butted together to form the laminations.

The laminations are insulated from each other before being

pressed together to form the core.

Fig. 6.2: Shell Type Transformer

v Type of Cooling Used

There are two types of transformers in this category: Dry type and

oil-filled.

• Dry type transformers use natural air cooling and are usually of very

small ratings. They are rugged and simple in construction and are not

plagued with the failures related with oil cooling.

• Oil filled transformers are of two types: One type uses

self-cooling and the other type uses forced cooling.

Self-cooling oil filled transformers have natural circulation of

insulating oil within which the entire transformer is immersed. These

are of moderate ratings and are suitable for outdoor duty as these

80


require no housing other than their own and thereby save on cost. For

higher ratings, either the smooth surface of tank is corrugated or is

provided with radiators/pipes to get greater heat radiation area.

Large transformers require forced oil/water cooling. The coolant

is circulated by a pump to radiate high quantity of heat generated and

also to minimise the size of the transformer.

6.2.2 Criteria for Transformer Selection

The criteria used for selection of proper rating/size of DTRs are described

below:

A . Voltage Rating

While the secondary side voltage rating of the transformer is fixed as

400 V, 3 phase with neutral available along with the three phase wires in star

configuration, the primary side voltage is decided by the voltage of incoming

feeder(s). If there are more than one incoming feeders, the feeder for fixing

the criterion for transformer selection can be decided on the basis of its

proximity with the substation, load delivering capability and availability of

suitable voltage transformer. The number, steps and type of tap changer, i.e.,

on load or off load, is decided by operating requirements of voltage and

current.

B. Size/Capacity/kVA Rating

kVA rating of transformer(s) is decided on the basis of the following factors:

• existing load to be catered;

• future load growth to be absorbed;

• diversity factor (DF) of load for different categories of consumers.

Diversity factor is greater than or equal to one and can be used to get the

required rating by dividing the sum of maximum demand of individual

consumer categories by DF;

• margin for future load growth;

• safety factor for avoiding overloading;

• level of all day efficiency to be achieved; and

• selection of an optimum loading of transformer(s), usually around 60%,

used to calculate the required capacity of the transformer(s).

C. Number of Transformers

Distribution substations are seldom provided with a single transformer of

required rating (except pole mounted transformers/substations). However,

reliability of supply can be severely affected if only one transformer is used

especially when it fails or when it is under maintenance. Reliability of supplyincreases with increase in the number of transformers.

It is important to note that capacity requirement and hence cost per

transformer reduces with increase in the number of transformers, as the load

gets divided, but the total cost increases as the relative benefit of reduction in

NOTE

Diversity factor isdefined as the ratio of

the sum of the individual

maximum demands of

various parts of a power

distribution system to

the maximum demand

of the whole system. It

measures the

staggering of different

hours of the day and

indicates flatness of

load curve. That is, it

denotes MVA vs hours

of the day curve.

81


the cost of each transformer is lower than the reduction in capacity.

Moreover, there is additional cost for associated equipment for each additional

unit of transformer. Thus, in order to optimise the reliability and the costinvolved, the number of transformers is usually kept between two tofour. Transformers are required to comply with the latest edition of IS 2026.

The size specifications for this purpose are given in Table 6.1.

Table 6.1: Standard Sizes of Transformers

6.2.3 Placement of Transformers

You have learnt in Unit 4 that in the High Voltage Distribution System (HVDS),

transformers are usually of very small rating (5 to 25 kVA) and provide supply

to 10 − 25 consumers. Such transformers, being large in number and very

small in size are mounted on a pole at an optimum location and are provided

with metering and isolating equipment. Similarly, medium sized self-oil cooled

transformers are also placed in open space and may be pole mounted. Large

sized transformers are fixed on the ground with proper foundation and are

enclosed within premises housing the associated equipment of transformers.

So far we have discussed the selection criteria and their placement. We will

now describe causes of transformer failures. But, before proceeding further,

you may like to answer an SAQ.

SAQ 1: Transformer selection and placement

6.3 REASONS FOR TRANSFORMER FAILURES

As you are aware, the distribution sector has a large number of distribution

transformers of various capacities. Any failure of these transformers is bound

to cause great inconvenience to the consumers and huge financial losses to

the utilities. It is therefore extremely important to avoid transformer failure.

11kV / 0.433 kV 160, 200, 250, 315, 500, 630, 1000, 1600, 2000 kVA

33kV / 0.433 kV 630, 1000, 1600, 2000 kVA

a) Describe the salient features of a distribution transformer.

………………………………………………………………………………….

………………………………………………………………………………….

b) Classify the different types of transformer in your utility. What

parameters are used in the selection of transformers in your utility?

………………………………………………………………………………….

………………………………………………………………………………….

SAQ 1: Transformer selection and placement

6.3 REASONS FOR TRANSFORMER FAILURES

82


We list below some important reasons for distribution transformer failure.

• Poor Performance

This could be due to

− low oil level;

− draining of oil due to leakage/theft of oil;

− improper earthing;

− frequent faults on LT lines due to loose spans leading to short circuit;

− mechanical failure of winding;

− improper tree clearance of LT lines;

− defective breather and consequent ingress of moisture;

− low electric strength of oil/winding insulation; and

− corrosion of core laminations.

• Improper Protection

This could be the result of

− using defective or over rated fuses;

− consistent overloading; and

− not providing Lightning Arrestors (LAs).

• Manufacturing Defects

These result from

− improper / inadequate design;

− poor quality of material;

− bad workmanship; and

− poor short circuit withstand capacity.

All these reasons for transformer failure can be classified under the following

heads (Fig. 6.4):

v Ageing,

v Manufacturing defects,

v Improper structure/erection of distribution transformer,

v Improper operation and maintenance, and

v Natural calamities.

We now discuss each one of these briefly.

6.3.1 Ageing

The expected life span of the distribution transformers above 100 kVA capacity

is about 35 years and that of up to 100 kVA capacity is about 25 years. But

experience shows that most of transformer failures begin to occur even

before 20 years of its life.

83


Until a few years ago, distribution transformer manufacturers incorporated

many more safety factors in design. In recent years, manufactures have

adopted the cost-benefit approach in the design of transformers, which just

about manages to satisfy the requirements of IS specification. The result is

that they compromise on both quality and reliability requirements of IS

specification. While the transformers so manufactured meet the requisite

standards when tests are conducted before and immediately after installation,

they fail to do so after a few years of being in operation due to ageing.

Thus, many transformers are unable to serve the expected full life period and

even if they are in service, they are quite likely to fail before the expected full

life due to lower reliability. Hence, giving top priority to the replacement of

those in-service transformers that have served their full-life period will reduce

transformer failure.

6.3.2 Manufacturing Defects

In the past, distribution transformers served for more than 60 years, which is

double the life expectancy. But now many distribution transformers fail after a

few years of service and have to be repaired twice or thrice during their life

time. Many reasons for pre-mature failure of the distribution transformer are

related to manufacturing defects. We describe them, in brief.

A. INADEQUATE/POOR DESIGN

The trend of design is now towards lowering the manufacturing costs per

unit even if it is at the expense of quality. Moreover, the tender appraisal is

mostly confined to the initial cost, with no consideration for maintenance of

the transformer up to the end of its fair life period. Consequently the safety

factor is affected adversely. The following aspects of transformer design

impact transformer failure:

• Transformer tank size: Inadequate clearance for free circulation of oil

can lead to abnormal temperature rise, causing great damage to the

HV winding insulation and, consequently, premature failure of

transformers.

• Percentage impedance (mechanical strength of coil): Most of the

distribution transformers are located in remote areas and many a

times it is not possible to give special attention to the operating

conditions. Harsh conditions can also lead to failure. The solution tothis problem lies in designing transformers with largeimpedance so as to increase theirshort circuit withstandcapacity.

Percentage impedance depends upon the following factors.

− Size of wire used in HV coils − Economical size of coil yields

lower size gauge wire, but this reduces the mechanical

capability of coils. As a result, the coils may not be able to

withstand higher current densities which occur during the short

circuit conditions.

NOTE

The cost of repairsduring the fair life periodplus the initial cost iscalled the estimatedcost of the transformer.It is at present a fewtimes more than theinitial cost at which thedistribution transformeris procured.

84


− Radial distance between HV and LV coils − Increasing the radial

distance between HV and LV coils increases the percentage

impedance. It also leads to better mechanical strength of the coil

to withstand higher short circuit stresses developed during short

circuit conditions. But this will lead to higher cost.

− Effect of impedance on the short circuit stresses −The short

circuit stresses are proportional to the square of the short circuit

current. If the impedance is increased from 4.5 % to 5 % - 5.5%,

the effect on the short circuit stresses developed in the

transformer is reduced considerably.

• Improper use of aluminium wires: Improper use of aluminium wires

leads to HV coil failure. The use of aluminium conductors has been

recommended for windings up to 200 kVA transformers. However, the

super enamel covering the aluminium wire tends to crack during

asymmetrical conditions and leads to coil failure. Hence, the use of

higher cross-section conductors with double paper covering would be

desirable.

• Improper use of interlayer papers: Coil failure is usually seen as an

electrical failure. This generally occurs when interlayer insulation

breaks down at the end of the turn and creeps to the next layer. This

type of insulation failure can be avoided by using folding papers and

reinforcing the end turn insulation with proper sleevings. Uniform

separation of HV coil along with the inner coil, using spacers helps to

avoid pressing of end turns as well as any further shrinkage during

service.

• Use of inferior quality materials: Use of inferior quality wires for

coils, poor quality of oil and other insulation material, etc., to bring

down the cost of the transformer also increases the probability of

failure of the transformer before full life of the transformer. The design

calculation of the tenderers should conform to the quantity and grade

of input materials of core and windings furnished in the tender. For

ensuring this, the transformer should be subjected to strip test. This

will make sure that the losses and impedance furnished in the tender

have been actually achieved by the transformer.

B. IMPROPER WORKMANSHIP

Apart from poor design, sub-standard execution of a good design also

becomes a reason for transformer failure. We now briefly describe some

such reasons.

• Improper alignment of HV windings: When a transformer is loaded,

the primary and secondary ampere-turns act in magnetic opposition

but are in complete alignment with respect to the core and coils. When

current flows through the coils, magnetic field is set up around them,

which has an associated magnetic flux. Even a small error in the

alignment of either coils, i.e., an asymmetrical ampere-turn balancing,

85


leads to production of cross magnetic fluxes. This results in lower

impedance and hence mechanical failure of the coils.

• Improper clamping arrangement: Inadequate clamping

arrangements of the HV coils lead to vibrations and movement of the

coils during short circuit conditions resulting in failure of HV Coils.

• Improper connections: In many cases, the connecting delta leads to

the bushing are not properly supported on the framework, resulting in

breakage during trans-shipment or at the time of the first charge of

transformer. Moreover, improper soldering of leads will result in open

circuit even at normal full load conditions. Also, such transformers may

fail while encountering the first fault or after a few faults.

• Inadequate tightening of core: Even with proper fuse protection on

the HV side, inadequate tightening will result in failure of transformer

due to collapse of the windings. The transformer can fail due to this

fault even under minor fault condition in the LT distribution due to

mechanical vibration in the core and windings.

6.3.3 Improper Structure of Distribution Transformer

IE Rules, 1956 specify various standard clearances to be maintained when

distribution transformers are to be erected. The standard clearances adopted

for transformer structures will avert its failure (Table 6.2).

Table 6.2: Clearances for Transformer Structures

Non-adherence to these standards makes DTRs prone to failures.

6.3.4 Impact of Natural Calamities

v Heavy lightning: If the HTLAs (HT Lightning Arrestors) fail to divert direct

lightning strikes or surges due to discontinuity in the earthing system, the

HV winding can fail due to surge voltage or the HT Lightning Arrestor itself

may burst.

v Bushing flashover: Dust and chemicals carried with air and deposited

on the bushings reduce the electric leakage distance and cause flashover.

To avoid this, the bushings (both HT and LT) should be cleaned properly at

regular intervals. However, cotton waste should not be used for cleaning,

as this may cause scratches in the bushing and subsequently lead to

flashover of bushing.

HT bushing to ground

ABS switch fixed contact to HG fuse

Guy Shackle to ground

If the length of the jumper is more

than 5 inches

13 feet

7 feet

10 feet

LT/HT pin insulators are used to fix

the jumper

Transformer Structure Clearance

86


v Failure due to contact with birds and other animals: To avoid failure of

the distribution transformer due to a squirrel crossing it or due to birds

sitting on it, the HT/LT bushing and HT/LT jumper leads from the bushing

should be covered with yellow tape insulation. This yellow tape insulation

will also indicate the overloaded operation of the transformer by the

change of colour of the tape from Yellow to Black.

6.3.5 Improper Operation and Maintenance (O&M)

Transformer failure can also stem from poor O&M practices. For example, in

addition to normal full load, continuous over-heating and higher no-loadlosses may reduce the life of the transformer due to reduction in the life of

insulating papers, oil, etc. Other improper O&M practices leading to

transformer failure are discussed later in the section on enhancing

transformer life and efficiency. In Table 6.3, we summarise some reasons for

transformer failure.

Table 6.3: Failure of Distribution Transformers

Collect data on the annual failure rate of DTRs in your utility for the past

three years. Identify the reasons for their failure and classify them among

the above mentioned five aspects described from Sec. 6.3.1 to Sec.

6.3.5.

………………………………………………………………………………….

………………………………………………………………………………….

SAQ 2: Failure of distribution transformers

Reason forDTR Failure

Failure Rate

Damage toHV Coils

Damage toLV Coils

Damage toBoth

OtherReasons

65 %

5 %

10 %

20 % • Poor construction of transformer tank;

• Defective Joints;

• Oil oozing out;

• Punctured radiators and bushing

gaskets;

• Damaged tap changers, etc.

Causes

• Compressed windings;

• Open Circuit Insulation failure;

• Dislodged spacers; and

• Broken support/inadequate bolts.

• Overloading;

• Defective termination of coils; and

• Inadequate size of fuses.

• Any of the above causes.

87


Thus far, you have studied about the selection criteria of transformers, their

placement and the reasons for transformer failure. If you are able to prevent

these causes of transformer failures, the battle is more than half won. The

rest is taken care of by transformer testing prior to its installation.

6.4 TRANSFORMER TESTING

Transformer testing needs to be carried out to ensure that the distribution

transformers are built with

• adequate electrical strength to withstand over voltage (due to switching

surges) impinging on the winding without causing flashover; and

• adequate mechanical strength to bear the mechanical stresses

developed on the winding during short circuits.

The following tests must ideally be conducted on the units before their

acceptance:

• testing of windings − insulation and mechanical strength;

• testing of insulating transformer oil; and

• other tests.

We now describe these tests in brief.

6.4.1 Testing of Windings −−−−− Insulation and Mechanical Strength

This involves four types of tests, which we have described briefly in Table 6.4.

Table 6.4: Testing of Windings

6.4 TRANSFORMER TESTING

Test Description

High VoltageTest

This is done to check the dielectric strength of the insulationbetween the windings operating at different voltages (HVand LV) and between each of these windings, core andearthed parts of the transformer. It is also called the MajorInsulation Test for the transformers.

InducedVoltage Test

This is conducted to test the dielectric strength of theinter-turn, inter-layer, inter-disc and inter-phase insulation.This test is also called the Minor Insulation Test.

Short CircuitTest

Ratio Test This test can be carried out for testing the transformer ratio(for example, shorted turns can cause impropertransformer ratio). This ratio is measured with a highaccuracy portable digital turns ratio tester only afterensuring shutdown and complete isolation of thetransformer from the system.

This is conducted for testing the mechanical strength of thetransformer in terms of its impedance parameters.

NOTE

In practice, transformer

testing is usually

dispensed with just for

want of testing facility at

the utility’s laboratories.

88


We give below the findings of the Central Power Research Institute (CPRI)

about the short circuit test.

Box 6.1: Findings of Central Power Research Institute on Short Circuit Test

6.4.2 Testing of Insulating Transformer Oil

At present, transformer oil is subjected to Breakdown Voltage (BDV) test toensure its electrical strength. Other tests to confirm important transformer

characteristics such as acidity, resistivity, etc., are not carried out before

accepting bulk supply. Thus, there is every possibility that manufacturers use

inferior quality of oil. This can lead to poor insulation resistance between High

Voltage to Earth, Low Voltage to Earth and High and Low Voltages and reduced

cooling rate. Moreover, it can give rise to abnormal temperature increases even

before loading the transformer to its rated capacity.

Thus, it is important to check whether the oil used is a new one or areconditioned one or a reclaimed one before the transformer is installed.

Both water and water saturated oils are heavier than clean and dry oil, and sink

to the bottom of the container.

The following tests are usually conducted on the transformer oil:

• inspection of samples;

• acidity test;

• analysis of dissolved gases; and

• electric strength test.

We now briefly describe these tests.

• Inspection of Samples: Colour and odour of the oil provide useful

information on the quality of oil and its fitness for use. In Table 6.5, we list the

factors that should be watched out for during inspection.

Table 6.5: Inspection of Oil Samples

From the available data on short circuit tests, it seems that the failure rateof transformers manufactured by small and medium scale industries is atpar with those of large scale industries. But in practice the rate of failure isvery high. The reason could be that the materials used for bulk manufactureof transformers are not the same as those used for the transformer producedfor testing purposes. This is a distinct possibility because the materials costcontributes a major share to transformer cost. Manufacturers use inferiorquality materials to bring down the cost to compete in the highly competitivemarket. Hence, there is a need for proper quality assurance at themanufacturing stage even though the prototype has successfully passed

the Short Circuit Test.

Cloudiness

Presentation Reason

Muddy colour

Dark brown

Green colour

Suspended solid matter such as iron oxide/sludge

Moisture

Presence of dissolved asphaltenes

Presence of dissolved copper compounds

Acid smell Presence of volatile acids which can causecorrosion

89


• Acidity Test: Transformer oil deteriorates gradually while in service due to

oxidation. The acidity in the oil causes rusting of ironing work inside the

tank above the oil level and the attached varnish on the windings. The

recommended limits for acidity test and the action required are given in

Table 6.6.

Table 6.6: Action Required for Various Acidity Levels of Transformer Oil

• Analysis of Dissolved Gases: The permissible concentrations of

dissolved gases in the oil of a healthy transformer are given in Table 6.7.

Table 6.7: Permissible Concentrations of Dissolved Gases in Transformer Oil

G

• Electric Strength Test (Breakdown Voltage Test): This is the most

commonly known test applicable to mineral insulating oils, and it was

originally developed to test the breakdown voltage of the oil. We describe

the method of its measurement in Box 6.2.

Box 6.2: Method of Measurement of Breakdown Voltage

Acidity Level Action

Below 0.5 mg KOH/g

Between 0.5 and 1.0 mg KOH/g

Above 1.0 mg KOH/g

No action needs to be taken providedthe condition of oil is satisfactory in allother respects.

Oil should be kept under observation.

Oil should be treated or discharged.

Hydrogen

Methane

Acetylene

Ethylene

Ethane

Carbon monoxide

Carbon dioxide

100 / 150 ppm

50 / 70 ppm

20 / 30 ppm

100 / 150 ppm

30 / 50 ppm

200 / 300 ppm

3000 / 3500 ppm

200 / 300 ppm

100 / 150 ppm

30 / 50 ppm

150 / 200 ppm

100 / 150 ppm

400 / 500 ppm

400 / 500 ppm

200 / 300 ppm

200 / 300 ppm

100 / 150 ppm

200 / 400 ppm

800 / 1000 ppm

600 / 700 ppm

600 / 700 ppm

Gas Less than 4 Yearsin Service

More than 10Years in Service

4-10 Years inService

An oil test cell is used in which an alternating voltage is applied betweentwo metal spheres 12.5 mm in diameter with a gap of 2.5 mm betweenthem. The voltage is increased until breakdown occurs. The flashovermust be quickly stopped to allow six successive measurements of therupture voltage on the same sample. The value of the electric strength ofthe sample tested is the average of the six measurements.

90


6.4.3 Other Tests

We now briefly describe some other tests, which are usually conducted on

transformers. These include temperature rise test, drying out oftransformer, and open circuit and Sumpner’s test.

• Temperature rise test is performed to measure the temperature rise of

the main and conservator tanks. The transformer passes the test if the

rise is within the specified limits given by the manufacturer.

• Drying out of transformer is necessary if

− tests indicate the presence of moisture in transformer oil;

− the oil does not withstand the dielectric strength test; and

− the insulation resistance readings are not satisfactory.

Box 6.3: Method of Drying Out the Transformer

• Open circuit and Sumpner’s tests: The open circuit test is carried

out on a transformer for calculating no load parameters and core losses.

Sumpner’s back to back test is done for testing performance on full load

for two similar transformers.

You may like to review the information presented so far before studying

further.

In this section, we describe the O&M practices that can enhance the life of a

transformer.

Normally HOT OIL CIRCULATION method should be used for drying out thedistribution transformer. In special circumstances, where this method doesnot give satisfactory results, SHORT CIRCUIT WITH HOT OIL CIRCULATIONshould be used. In this method, both core and winding inside the tank aresimultaneously dried out/streamlined with filter. The moisture dries out fromthe windings into the oil and is removed from the oil by evaporation andfiltering.

State the purpose of tranformer testing. Draw a list of the tests to be

carried out ona transformer defore it is accepted by a utility

………………………………………………………………………………….

………………………………………………………………………………….

SAQ 3: Transformer testing

6.5 ENHANCING TRANSFORMER LIFE ANDEFFICIENCY

91


6.5.1 Transformer Operation

The following factors need to be kept in mind while operating a transformer:

A. Overloading of distribution transformer should be avoided.

• Inadvertent burden of extra load due to unauthorized

connections or loads should be identified by periodic testing of current

in the distribution transformer. The Tong Tester may be used for this

purpose at peak hours and other times of the day. Alternatively,

maximum demand ammeter may be connected to exactly determine

the maximum load current drawn from the transformer. In the event of

extra load, transformer failure can be prevented and its life enhanced

by

− transferring the load to the nearby transformer;

− enhancement of the existing transformer capacity; or

− installing a new transformer.

• Unequal loading in three phases may also cause overloading in one

phase. In such a case, redistribution of the loads, as far as possible

equally, among the 3 phases, will prevent transformer failure.

B. Fuse wires (HG fuse and feeder fuses) should be of proper size.

• HG fuse is the only reliable protection for distribution transformers

under conditions of fault in LT distribution. The LT fuses should be of a

heavy size since these are meant for high currents on the LT side.

Then the chances of LT fuses blowing in short time decrease.

• Higher size HG fuses are sometimes used in distribution transformers

due to non-availability of proper size HG fuse wire and also because

the seriousness of the consequences is not realized. THIS SHOULDNEVER BE DONE because if the HG fuse is of higher size, the fault

will sustain for a longer period until the heavy size fuse blows. This

can result in increased chances of transformer failure. Even if the

fault lies within the transformer, using HG fuse of proper size will

minimize the damage to the transformer.

• Notwithstanding the use of HG fuse protection, the use of proper size

of fuses on LT feeder depending upon the load has to be ensured to

avert transformer failure.

C. Two phasing in rural areas should be prevented.

• Often, two phase supply is maintained on rural distribution feeders to

prevent operation of three phase motors for staggering the peak hour

loads. However, agriculturists/consumers have invented many

methods to start the motor and run it under 2-phase conditions. The

total power intake of the motor under the 2-phase condition will be

approximately the same as under the 3-phase, contributingunbalanced

92


overload in 2 phases. In turn, the distribution transformer will also be

subjected to overload in these 2 phases and the core of the

transformer will have unbalanced magnetic field in the region of

saturation point. This will also cause transformer failure and needs to

be checked.

• Similarly, in all the areas covered by the distribution lines with 2-phase

arrangements, all the single phase lighting loads are dumped in these

2 phases so that supply is available under all conditions. This leads to

heavy unbalanced current through the neutral conductor and the

transformer is likely to be overloaded in these 2 phases. Further, the

flow of unbalanced current in the neutral conductor will raise the

potential of the neutral with respect to the earth which is dangerous to

the consumers.

In both these cases, adopting 3-phase balance load scheme should

enhance both the life and efficiency of the transformer as it avoids

unnecessary overloading of 2 phases.

D. Thin breakable diaphragm should be used in the explosion vent.

Use of metallic diaphragm will result in the explosion of the transformer

itself due to the development of high pressure. To avoid this, a thin

breakable diaphragm should be used in the explosion vent. This will

cause the explosion of only the vent under high pressure conditions. It will

avert transformer failure on this account.

E. The correct diversity factor for loads should be adopted.

The value of diversity factor (DF) is assumed for different categories of

load to decide the capacity of the transformers. Due to reduced hours of

supply in the hours of critical power generation, the actual DF is less than

the assumed value. This leads to overloading of the transformers and

results in their failure. Transferring load from an overloaded transformer to

another transformer or replacing it with a new higher capacity transformer

will avert such situations and enhance the life and efficiency of the

transformer.

F. Non-standard methods should be avoided.

Avoiding the use of non-standard methods can avert transformer failure.

We now describe some of these.

• Use of ACSR conductor, bare or enclosed in PVC pipe from the

transformer bushing as against insulated PVC cables.

• Use of open type fuse for the secondary control of the transformer

as against the standard porcelain fuses.

• Use of single fuse to control more than one feeder.

• Use of Aluminium strands of ACSR conductors as fuses in LT

open type fuses and HG fuses as against tinned copper.

Having learnt about the correct ways of transformer operation, you may like to

know: What methods are used for transformer maintenance?

93


6.5.2 Maintenance Methods

We describe these methods, in brief.

• Prevention of Tree Fouling on LT Lines: Sustained tree fouling with LT

conductors may result in conductor snapping or cracked LT Pin

Insulators. This causes heavy earth fault current, which may lead to

transformer failure. Regular tree cutting and trimming will avoid such

failures.

• Prevention of Tree Fouling on HT Lines: Tree fouling on HT lines may

cause failure of transformers due to flow of earth fault current. This is

because the primary of all the transformers are delta-connected and all

the 3 windings from cluster of transformers connected to this HT

distribution line will feed the fault apart from the source of EHT substation

(EHTSS). Monitoring and regular tree cutting is the solution to this

problem for enhancing the life of the transformer.

• Maintenance of Breather: Non-provision of fly-nuts for the breather

container will create a gap through which moisturised air will enter into the

transformer tank. To arrest this gap, neophrine gaskets should be

provided instead of rubber gaskets. Rubber gets damaged if it comes in

contact with transformer oil. If oil is not filled in the breather, then the dust

particles will not be absorbed from the air entering the transformer tank,

causing transformer failure. Hence, proper maintenance of breather will

prevent such failures.

• Removal of Water Condensate in the Transformer: Due to absorption

of moisture from atmosphere over a long period of time, a large quantity of

water may collect in the transformers. Since water has higher density it

gets collected at the bottom of the transformer. Indeed water level can

even reach the bottom level of the windings, resulting in failure of

transformers. Sometimes it leads to bursting as well. This is because of

silt formation at the bottom of oil, which prevents escape of gas formed,

resulting in bursting of the bottom of the tank. Occasional draining of oilfrom the bottom of the transformer will check collection of suchlarge quantity of water. The conservator tank also acts as the collector

of water-condensate of moisture entering through the breather. This

should be removed before it contaminates the oil and causes transformer

failure.

• Prevention of Oil Leakage in Bushings or Any Other Weak Part of

the Transformer: Oil leakage may be due to excessive heating or

pressure that may develop in the bushing. This will bring down the oil level

in the tank. Moreover, moisture will find its way into the tank through the

aperture from which oil is oozing. This can result in the contamination of

the oil in the tank and hence in the deterioration of HV/LV insulation and

ultimately lead to transformer failure. To avoid this, bimetallic clamps with

proper size of bolt and nuts connected to the LT bushing may be used,

which will reduce excessive heating and damage to bushing rods.

Reduction in heating will lead to higher life and efficiency of the

transformer.

94


• Avoiding Low Oil Level: Poor visibility of the oil level in the glass level

gauge due to accumulated dust, etc., may not show the exact level of oil in

the tank. Oil is likely to go below the core level and the jumper wire from

core winding assembly to the bushing rod will not be covered with oil. This

leads to excessive temperature rise and the failure of inter-turn insulation

as well as flashover of the windings. Transformer life and efficiency can be

improved by avoiding such excessive temperature rises.

• Avoiding High Oil Level: Oil should be filled upto the marking in the

conservator tank. There should be space in the conservator tank for

expansion of oil when the transformer is loaded. If the conservator tank is

completely filled with oil, the transformer may fail due to high pressure

resulting in explosion of vent pipe. Maintaining correct oil level will, thus,

enhance transformer life by avoiding such failure.

• Prevention of Low BDV of the Oil: The Breakdown Value/Voltage (BDV)

of the transformer oil may become very low due to oil contamination. This

results in the increase of the carbon content and decrease of resistance in

the oil. Since the temperature of the oil remains the same, acidity of the oil

increases resulting in deterioration of insulation of the windings and

transformer failure. To prevent this, oil has to be filtered in order to remove

the dust and processed through the reclamation plant to reduce the acidity

and improve the BDV value of the oil. The minimum BDVs for different

voltage ratings of transformers are given in Table 6.8.

Table 6.8: Minimum Breakdown Voltage Ratings for Transformers of

Different Voltage Ratings

Higher electric strength of oil will, therefore, reduce chances of failure

during abnormal conditions of operation such as lightning/surge voltages

in the system and increase the life of transformer.

• Preventing Low Insulation Resistance (IR) Value: The insulation

resistance may be lowered due to moisture content in the oil and in the

winding insulation, and may cause transformer failure. To prevent this, the

entire transformer core with windings should be placed in hot air chamber

until the moisture content is removed from the core and winding insulation

resistance can be measured with the help of a Megger. The minimum safe

insulation resistance for different voltage ratings of windings is given in

Table 6.9.

Rated Voltage of the Transformer Minimum Breakdown Voltage

Up to 66 kV

Above 66 kV and Up to 110 kV

Above 110 kV and Up to 230 kV

30 kV

40 kV

50 kV

95


Table 6.9: Minimum Safe Insulation Resistance in MΩΩΩΩΩ (Mega-ohm)

Ensuring appropriate insulation resistance enhances the life oftransformer.

• Preventing Loose LT Lines: If the LT lines are very loose or they sag,

shorting of LT lines could occur and cause frequent blowing of feeder

fuses. It could also cause conductor snapping if proper size of fuses are

not used or if the structure is not properly earthed. To prevent this from

happening, phase separators should be used or lines should berestaged. Loose LT lines should be checked to reduce short circuit

stresses on transformer and hence enhance its life.

• Proper Maintenance of Fuse Gaps in HT/LT Side of the

Transformer: Improper maintenance of the fuse gap setting results

either in frequent blowing of fuses or non-blowing of fuses when required.

The desirable fuse gap setting is given in Table 6.10. Adequate gap

setting will avert transformer failure and hence enhance its life.

Table 6.10: Appropriate Fuse Gap Setting

In this section, we have discussed major O&M measures required to

prevent transformer failure and enhance its life. It is your job to ensure that

these measures are implemented on a regular basis.

At this point, you may like to pause and review these measures.

SAQ 4: O&M of transformers

66 kV and above

22 kV and 33 kV

6.6 kV and 11 kV

Below 6.6 kV

30°°°°°C

600

500

400

200

40°°°°°C

300

250

200

100

50°°°°°C

150

125

100

50

60°°°°°C

75

65

50

25

Rated Voltage of theWinding

Minimum Safe Insulation Resistance in MΩΩΩΩΩ at

Different Temperatures

Type of Fuse Gap

11 kV HG fuse gap

22 kV HG fuse gap

LT open type feeder fuse gap

Fuse Gap Setting

8 inches

10 inches

6 inches

List the reasons for distribution transformer failure in your area. Which

O&M measures described above could have prevented these?

………………………………………………………………………………….

………………………………………………………………………………….

SAQ 4: O&M of transformers

96


With this discussion on various ways of enhancing transformer life by paying

proper attention to its operation and maintenance, we end this unit. In this unit

you have studied important aspects related to distribution transformer,

reasons of transformer failure, transformer testing and ways of enhancing life

and efficiency of transformer by paying attention to the operation and

maintenance aspects of these transformers. Let us now present the summary

of its contents.

6.6 SUMMARY

• Distribution transformers are used both in electrical power distribution and

transmission systems. Their power ratings as well as continuous voltage

rating are the highest. The criteria for selection of a transformer andits technological details depend on its intended use or purpose,working conditions and operating requirements.

• Transformers are classified on the basis of type of core used and typeof cooling used.

• The criteria used for selection of proper rating/size of DTRs are based on

voltage rating, size/capacity/kva rating, number of transformers, etc.

• Transformers may be mounted on a pole placed in open space or fixed on

the ground with proper foundation depending upon their sizes and uses.

• Some important reasons for distribution transformer failure are poorperformance due to low oil level, draining of oil due to leakage/theft of oil,

improper earthing, frequent faults on LT lines due to loose spans leading to

short circuit, mechanical failure of winding, improper protection,

manufacturing defects, etc. These reasons for transformer failure can be

classified under the heads of ageing, manufacturing defects, improperstructure/erection of distribution transformer, improper operationand maintenance and natural calamities.

• Transformer failure can also stem from poor O&M practices such as

continuous over-heating and higher no-load losses.

• Transformer testing needs to be carried out to ensure that the distribution

transformers are built with adequate electrical and mechanicalstrength.

• Tests such as testing of windings −−−−− insulation and mechanicalstrength, testing of insulating transformer oil, must ideally be

conducted on the units before their acceptance.

• Transformer life can be enhanced by following proper O&M practices.


1. Classify the distribution transformers being used in your utility as per the

categories given in Sec. 6.2.

2. Examine the parameters of at least two substations of your utility and write

6.6 SUMMARY


97


your assessment on whether the transformers installed are of correct

number and sizes. Give reasons for your answer.

3. Discuss measures to avoid DTR failures at the Operation and

Maintenance level.

4. Which tests can be performed on DTRs before and immediately after

installation?

5. How many consumer complaints in a year are related to DTR failure in

your utility?

6. Describe the kind of DTR failures that take place in your utility.

7. What is the normal correction time for complaints related to DTR failures

in your utility? How can this time be reduced to a level acceptable to

consumers? Explain giving reasons.

8. Analyse the reasons for transformer failure in your utility.

9. Which of the Operation and Maintenance Methods are applied in your

utility to ensure improved life and efficiency of transformers?

10. Suggest ways to reduce DTR failure rate in your utility.

98

Operation andMaintenance APPENDIX 1: CASE STUDIES ON

AVERT ING DISTRIBUTIONTRANSFORMER FAILURE

Classification of Failures

Care to be Exercised to Avoid Failure

1. Insulation Failure

2. Damage to HT Coil

3. Damage to LT Coil

4. Damage to Core and

Laminations

5. Failure of Tap switch and Tap

arrangement

1. Oil Sample not satisfactory

2. Lead connections cut off

3. Wornout bushing rods

4. Broken bushings

5. Gasket leakage

6. Welding leakage

7. Leakage through valves

8. Broken gauge glass

9. Broken vent diaphragm

10. Worn out breather

MinorMajor

1. Maintenance ofoil level

2. Maintenance ofbreather withsilica gel and oilseal

3. Periodicaltesting of IRvalues

4. Periodical testsin transformer

5. Earth resistancevalues and Earthmaintenance

6. Keepingstandard voltageand frequency atload terminals

7. Maintaining LAsto prevent

In Manufacturing Stage In Transport In Working Conditions

1. Safe handlingduring transportand erection

2. Adoption ofstandards forerection oftransformerstructure

3. Standardconstruction ofLT lines

1. Proper insulationarrangement

2. Mechanicalrigidity towithstand heavyforces

3. Adequatecoolingarrangement

4. Adequatequantity of oil forinsulation andcooling

5. Maintainingatmosphericpressure insidewith pure air

6. Rigid fixing ofcore-coil unitinside main tank

99


7. Pucca earthing of core and other metallic parts

damage due tosurges

8. MaintainingLT System

9. Keeping theloading withinthe limits

Examination of Failed Distribution Transformers

1. Oil level andquantity available

2. Places of oilleakage

3. Condition ofbreather andsilica gel

4. Condition ofbushing andbushing rods

5. Condition of ventdiaphragm

6. Condition ofvalves

7. IR value andcontinuity

8. BDV test on oil

1. Conditions of HTcoils in all the threephases

2. Checkup of leadconnection fromcoil (Delta and Starpoints)

3. Condition of core

4. Condition of tapswitch andconnections

5. Condition of coreearthing

6. Presence of sludge and moisture in oil and physical condition of oil

1. By injecting 15 Von LV side andmeasuring stackvoltage ofHT coil

2. By injecting15/Ö3 volt inbetween one LVphase and Neutraland measuringvoltage of stackon correspondingHT phase coilsand on otherphase HT coils

3. Short circuit testby injecting 400Von HV side

External Check up Internal PhysicalVerification

Transformer HealthTest

Case Studies on Averting Distribution Transformer Failures

Details ofDefectsNoticed atField

Observation at Lab Probable Causefor the Defects

RectificationDone andSuggestionsto AvoidRecurrence inFuture

Case 1

HT leads insulationnear top of bushingfound charred.

Case 1

Conservator oil levelbelow bushing rod.Absence of oilaround causedheating of leads.

Case 1

All 3 HG Fusesblow out slowlyafter 1 hour(Examined atField).

Case 1

Insulationsleeveschanged. Higheroil level was

100


Case 4

L.T.voltagemeasured andfound alright.Butfound voltage dropwhen load isconnected.

Case 4

Neutral bushingconnectionloosened inside.

Case 4

Improper handlingof neutral bushingduring meggering.

Case 4

NeutralConnection setright and sent foruse. Advised tohandle bushingconnections

properly.

Case 5

LT voltage foundalright. But load in“R” Phase couldnot be loaded.

Case 5

LT & HT connectionfound alright. Novisual defect. SCtest revealed nocurrent in R Phase,full current inneutral.

Case 5

Examined thesolderedconnections in “R”phase and found“R” phase bottomdelta connectionimproper.(not completelycut).

Case 5

Defectiveconnectionsresoldered andfound alright.Impropersoldering gaveway during use.

Case 2

Oil spurt out(Similarhappening onprevioustransformersalso).

Case 2

B phase endwindings (Top andBottom foundshattered).

Case 2

Lightning surgesentered and causedshattering. (Areaprone to lightning).

Case 2

Coil rewound andsent. Theprevious failurewas also in same“B” Phase.Asked toexamine HT LASof “B” Phase. Allthree LASchanged and nosuch failurethereafter.

asked to bemaintained.This is amanufacturingdefect andcompany wasaddressed.

Case 3

Undue heating inLT Rod. “R”phase Rod wornout andInsulation tapecharred.

Case 3

“R” phase rod insideconnectionloosened.

Case 3

Connectionloosened due toimproper handling ofbushing duringjumperconnections.

Case 3

Connectionstightened andsent for use.Advised to usechecknuts andproper handling.

Case 6

HG Fuse in all 3phases blownout.

Case 6

Inter turn short inR Phase − 3rdstack.Y Phase − 4thstackB Phase − 2nd and3rd stack.

Case 6

Suspected heavyabsorption ofmoisture. Oilsample notsatisfactory.Crackle test provedpositive.

Case 6

All good stacksremoved. Corewith LT coilplaced in Hot AirChamber anddried. Failedcoils replaced.Put into use

101


Case 7

HG Fusefrequently blowsout in “B” phase.

Case 7

Insulation of leadinside “B” phasebushing foundcharred. All coils ingood condition.

Case 7

Insulation of leadinside “B” phasebushing foundcharred. All coils ingood condition.

Case 9

HG in “R” Phaseblown out.(Similar failure in5 months).

Case 9

“R” phase HT coilsfailed with symmetrygiving way. LT “R”phase also failed.

Case 9

Due to Heavy ShortCircuit forcebecause ofintermittent feedingof fault current.

Case 9

LT R Phaseconductorfrequentlytouched nearbyneutral. Fusenot blown. Earthvalue high :30 Ohms. Askedto rectify earthingsystem andadopt proper LTfuse.

Case 10

HG fuse blowingout on load.(attended atsite).

Case 10

Oil level found uptocore level only.Gauge glassshowing OK level.HT lead insulationcharred due to heat.Coil alright.

Case 10

Gauge glassindicationmisleading. Actuallyoil level is low.

Case 10

Gauge glasscleaned. Blockin air holeremoved.Insulation ofleadsstrengthened.Transformer putback in service.

Case 11

HG fuse blownout. LT IR Valuezero.

Case 11

All LT leadsremoved. Now LTMegger value is30 M Ohms.

Case 11

“Zero” IR value is onLT Leads and not intransformer.

Case 11

The LT lines andcables wereasked to beinspected anddefects rectified.

Case 12

Core and channelshort circuited with“C” phase coil topstack and leads.

Case 12

Core not keptearthed (notprovided afterrepairs). Stray

Case 12

Advised the fieldto revampearthing system.Transformer

Case 12

HG fuse isblown out.

Case 8

Informed thata) No oil could betaken fromsampling valve -oil not flowingout.b) Oil level inconservator is full(attended atfield).

Case 8

Examined at spotand observed thatthe Field Report iscorrect.

Case 8

Examined at spotand observed thatthe Field Report iscorrect.

Case 8

Air trapped underthe bottomportion andpushed the oilup. Advised torelease airfrequentlythrough air plugand also throughtop lid also.

Case 7

Advised the fieldto release airmonthly.

after circulationand test.

102


Case 13

No fault in the twotransformersbrought to Lab.

Case 13

Advised to inspectthe line for anyjumper cut.

Case 13

Reported thatone phase linecut on loadsidewith incomingthree phaseintact in pininsulator(Location in themidst of a lakefull of water).

Case 14

Removed asunequal voltage.IR value andcontinuity OK.

Case 14

Delta connection cutin bottom of “B”Phase.

Case 14

May be due toageing and wearand tear.

Case 14

Continuity test isOK since it isconnected inDelta. Solderedand sent for use.

Case 15

“B” phase coil inter-turn short in 2ndstack. No shatteringof coils, contactingwith Earth.

Case 15

Insulation failure dueto ageing. Since noearth contact ofwinding, I.R. valuesare OK.

Case 15

Failed stackreplaced andsent for use.Flexibility ofconnectionincreased andsoldered withrod.

Case 16

Unequal voltage.continuity notfound with the “B”phase HT.

Case 16

HT lead in “B”bushing rod, cameout.

Case 16

The lead was tight.Hence came outfrom rod.

Case 16

Flexibility ofconnectionincreased andsoldered withrod.

Case 17

Found heavysludges andmoisture absorption.IR value low. Foundvent pipe diaphragmbroken. Gaugeglass broken.Breather all right.

Case 17

Sludging due toageing, water entrythrough vent pipeand gauge glass.

Case 17

Oil completelydischarged. Coreand coil cleaned.Dried in chamber.Put hot oilcirculation withnew oil andtested OK.

Case 13

Transformerchanged for twotimes due tounequal voltage.

Case 15

HG fuse blownout. IR value andcontinuity OK.

Case 17

Oil sample notsatisfactory for 3consecutivetests.

Earthing of core notfound.

voltage caused shortcircuit.

condemned sincelaminations gotcharred.

Case 18

Casual examinationrevealed lowquantity of oil thanthat at name plate −260 litres. Available30 litres.

Case 18

Non availability ofsufficient oil − verysmall spacingbetween tank andcore and top cover,leads to failure.

Case 18

Frequent failure ofcertain maketransformer.

Case 18

Taken up withcompany byPurchase Wing.(IneffectiveCoolingSystem).

103


The ground and the spacejust immediately under thetransformer and thestructure should be freefrom bushes, grass andtemporary sheds.

Area wireman/LineInspector (or) Foreman

Transformer Yard

Sl. No. What to Inspect What Maintenance to Do?Why? And When?

Who has to do? Underwhose Supervision?

Earthpit The three earthpits providedfor the transformer shouldbe free from bushes andshould be visible from theground and should bewatered so as to have lowresistance value. Lowground resistance isimportant for satisfactorylightning arrestorsoperation.


Transformer Tank Aircooling is used almostexclusively for distributiontransformers, the surface ofthe tank being artificiallyenlarged by radiators, tubingand the like. The rate atwhich the transformer oildeteriorates increasesrapidly with rising tempera-ture. Hence due attentionmust be paid for the provi-sion of adequate ventilation.One immediate step is toclean the entire transformertank.


Oil level The transformer core andthe winding should becompletely immersed in theoil. Low level of oil will causeburning of H.T. coil. Hencethe oil level in the gaugeglass has to be checkedand topped up whenevernecessary.

Lineman/Line Inspector(or) Foreman


MONTHLY

4.

3.

2.

1.

Oil leakage Oil leakages from drainvalve, gaskets and tankhave to be checked andrectified at the earliest.

APPENDIX 2: A CHECKLIST FORPREVENTIVE MAINTENANCE OFDISTRIBUTION TRANSFORMERS

104


Safety to personnel can beassured by adopting anearthing system so designedthat under both normal andabnormal condition nodangerous voltage canappear on the equipment towhich personnel haveaccess. Damage to equip-ment can be minimised bythe provision of paths of lowimpedance from theequipment to the earthsystem. The earthingsystem will be of no valueunless the connections aretight, and there is continuityfrom the equipment to theearth and the earth exists.

Breather Moisture entering the oil asa result of the so calledbreathing action greatlyreduces its dielectricstrength so that break-downs from coils orterminal leads to tank orcore structure may takeplace. To prevent moistureentering into transformeroil, sillicagel dehydratingbreathers are fitted to thetransformer with a sightglass so that the colour ofthe sillicagel crystals maybe seen. The colourchanges from blue to pinkas the crystal absorbsmoisture. When thecrystals get saturated withmoisture they becomepredominantly pink andshould therefore bereactivated. For reactivationthe crystals should bebaked at a temperature ofabout 200oC until the wholemass is at thistemperature and the bluecolour has been restored.Alternatively change thesillicagel.


L.T. Fuses In the maintenance cardprovided to the areawireman the size of the fusewires for the L.T. maindifferent capacities of thedistribution transformers arefurnished. Only the correct

Area wireman/LineInspector (or)Foreman

7.

6.

Earth5. Lineman/Line Inspector(or) Foreman

105


H.G. Fuses In the maintenance cardprovided to the areawireman, the size of theH.G. fuses wires for thedifferent capacities of thedistribution transformer isfurnished. Only the correctsize of the fuse wire has tobe provided. In today’sdynamic situation, thedistribution transformers aresubjected to intermittentoverloads resulting in theglow H.G. fuses and subse-quent breakdown. Appropri-ate sizes of these fuseshave to be kept in readystock and replaced timely.

Lineman/Line Inspector(or) Foreman

Insulationresistance

Some of the materials usedin insulation are organic innature and are thereforesusceptible to deteriorationby heat, oxygen, moistureand corrosive liquids. Eventhe best are vulnerable ifincorrectly used or applied.Insulation breakdowns aremost often the result ofinternal or external heat andmoisture. The heatingcauses chemical changesin the insulation that areaggravated by the presenceof moisture. The insulationresistance value betweenH.V. to earth, L.V. to earthand H.V. to L.V. and mainL.T. leads have to berecorded and the trend hasto be observed so thatcorrective steps could betaken before the total failureof the insulation. Theminimum safe insulationresistance in meg ohms at30oC for 11 KV is 400 andfor L.V. it is 100. For every10oC increase intemperature the I.R. valuewill get halved for both11 kV and L.V.

Line Inspector (or)Foreman/JuniorEngineer (or)Assistant Engineer

Load current Sustained heavyoverloads produce hightemperatures throughout

Assistant Engineer orJunior Engineer/Ass.Exe. Engineer

3.

2.

1.

size of fuse wires have to beprovided.

QUARTERLY

106


the transformer. The oil

insulation becomes brittle

and in time probably flakes

off the conductors in places

on producing short circuits

between turns.

Transformers with a high

ratio of copper loss to iron

loss are less able to

withstand overload and are

therefore more liable to fail

on account of overloading.

The load current has to be

measured at different times

especially at peak load

hours to know the overload

situation and to enhance

the transformer capacity or

to bifurcate the loads at the

appropriate time.

Voltage The voltage should be

checked to make sure that

the transformer has the

proper tap position. Over

voltage produces

excessive noload loss. It is

also necessary to ensure

proper voltage to the

consumers as per the

Indian Electricity Rules,

1956. For this purpose the

voltages at thetransformer

end and tail end have to be

measured and necessary

improvements have to be

made either by changing

the taps provided or by

strengthening of

conductors, bifurcation of

loads, etc.

ANNUAL

AB Switch This component provided

on the transformer structure

helps to break or make the

electric circuit to the

transformer through a

system of contacts. These

have to be lubricated for

ease in operation and to

avoid accidents to depart-

mental persons due to

non-operation of the blades

in any of the phases or all.

Lineman/Lineinspector (or)Foreman

1.

4. Assistant Engineer orJunior Engineer/Ass.Exe. Engineer

107


Line and earthconnections

Lineman/Lineinspector (or)Foreman

2. The line and earthconnection of the ABswitches, H.T. and L.T.lightning arrestors and H.T.and L.T. bushings have tobe checked and any looseconnections have to bemade tight. When the lineconnections loose, thecurrent is allowed to pass inthe form of an arc whichcreates enormous heatresulting in melting ofcontacts or breakdown ofconnections andenergisation of thestructure. When the earthconnections are loose andwhen the fault occurs, theentire fault current cannotbe earthed properlyresulting in failure of

equipment or causingaccidents.

Transformer oil The dielectric strength ofthe transformer oil mainlyprovides an indication ofthe physical condition ofthe oil. The impuritiesmost likely to influencethe dielectric strength ofthe oil, in practice, aremoisture and fibres. Theoil is said to have passedthe test if two out of threesamples successfully

stand the test at 40 KVfor one minute.

Line inspector (or)Foreman/A.E. (or)J.E.

Earth resistance Safety to personnel canbe assured by adoptingan earthing system sodesigned that under bothnormal and abnormalconditions no dangerousvoltage can appear onthe equipment to whichpersonnel have access.Damage to equipmentcan be minimised by theprovision of paths of lowimpedance from theequipment to the earthsystem.

A.E. (or) J.E./Asst.Exe. Engineer

4.

3.

108


Sl. No. Work to be carried out Personresponsible for thework

Personresponsible for thecompletion of thework

I. Weekly / Fortnightly

1. Breather oil, silica gel,checking, replenishing

Area Wireman Line Inspector /Foreman

II. Monthly

1a Maintaining thetransformer yard and theearth-pits neat and tidy andwatering the earth-pits.

b Cleaning the entiretransformer including thebushings.

c. Checking that oil level isbelow the mark.

d. Checking for oil leaks andreporting if any noticed.

e. Checking the earthconnections.

f. Reconditioning the breather.(by reactive silica gel, orreplacing if necessary).

Area Wireman Line Inspector /Foreman

Checking the LT fuses andrenewing them if necessary.

g.

2. Topping up oil wherenecessary.

Line Inspector Foreman

1. Renewing the H.G. fuses. Line Inspector Foreman

2. Measuring the insulationresistance.

Line Inspector /Foreman

Foreman

3a. Measuring the load current.

Assistant ExecutiveEngineer

b. Measuring the voltage atthe transformer and atthe tailends of thefeeders.

Section Officer

III. Quarterly

MAINTENANCE SCHEDULE OF DISTRIBUTION TRANSFORMERS

109


1a. Lubricating line ABSwitches and checkingtheir operation.

b. Checking the line and earthconnections of the HT/LTlightning arrestors.

c. Checking the HV and LVbushing connections.

Line Inspector Foreman

2. Getting the oil samplestested for dielectricstrength.

Line Inspector /Foreman

Section Officer

3. Measuring the earthresistance.

Section Officer Assistant ExecutiveEngineer

Note 1.The Assistant Executive Engineer should inspect every distributiontransformer in the Sub-division once in every year and ensure thatmaintenance works are carried out as per this schedule.

2. The Section Officer should inspect every distribution transformer in thesection once every quarter and ensure that maintenance is carried outas per schedule.

3.The Line Inspector/Foreman should inspect every distributiontransformer in his/her jurisdiction once every month and ensure thatbmaintenance works are carried out as per schedule.

IV. Annual

110


MONTHLY

Sl. No. What to inspect What Maintenance to Do?Why? And When?

Who has to do?Under whose Super-vision?

1. Yard and EarthPits

Maintaining the Yard(underneath the Structures)and earth pits neat and tidyand watering the earth pits.Otherwise the site becomesuntidy and it becomesdifficult to carry outoperation on the structureespecially during break-down. By watering the earthpits, the earth resistancewill be less, facilitating easyand quick earthing of faultcurrent/voltage.

Wireman/Lineinspector, Foreman

2. EarthConnections

Checking earth connectionsboth at earth pits and alsothe metal parts. By keepingthe earth connectionsproper, fault current caneasily pass to earth duringfault condition, thusfacilitating quick tripping ofthe feeders.

Wireman/LineInspector, Foreman

QUARTERLY

1. Dividing/Termination

Checking earth condition ofdividing/termination to seethat no cracks are beingdeveloped and it is properlydivided/clamped. Alsochecking that connectionsfrom the termination to thebus are proper. Checkingonce in a quarter, even if acrack is developed, orcompound is leaking orfixing is getting loosened orthe connections arebecoming loose, it may befound out in time and timelyaction may be taken.Otherwise a breakdownmay happen and it isdifficult to work in astructure, as L.C. fromdifferent source of supply isnecessary.

Lineman/LineInspector, Foreman

PREVENTIVE MAINTENANCE SCHEDULE OF DISTRIBUTIONTRANSFORMER STRUCTURES

111

DistributionTransformerANNUAL

1. Concreting/Coping of theSupports

Checking the condition ofthe concreting/Coping ofthe supports of thestructures to see that thecoping and concreting isintact. If there are cracksor the coping or concretingis coming off, preventiveaction may be taken toconcrete or coping. Thesupports fixing to earthbecome weak and duringthe time of heavy rains,cyclone or flooding, thestructure may fall, leadingto major breakdown. Ifcoping is not done in thecase of metal they mayget corroded due tourination by dogs.


2. Supports Checking the condition ofthe supports and if cracksor damages are noticed,action can be taken toreplace or rectify thedefects. If not done, thestructure itself may fall dueto cyclone, floods, etc.


3. Earth Resistance Measuring the earthresistance to check if it iswithin permissible limits. Ifit is beyond permissiblelimits, action may be taken

to reduce the earthresistance so that earthfaults are cleared quicklyand accidents are avoided.

Junior Engineer orAsst. Engineer/Asst.Exec. Engineer

4. Earth Connectionof Metal Parts

Checking the earth connec-tions of metal parts toensure that the metal partsare properly connected tothe earth so that any earthfault of the metal parts arecleared quickly andefficiently. If not, accidentsmay happen.

Wireman/Line Inspec-tor, Foreman

5. Operation of ABSwitches

Lubricating the ABSwitches and checkingtheir operation so that theAB Switches be operatedat ease and correctly, attimes of emergency.

Wireman/Line Inspec-tor, Foreman

112


Otherwise duringemergencies, the ABSwitches cannot be openedleading to unavoidable delayin attending to a switchleading to accidents also.

6. Line and EarthConnections ofAB Switches

Checking that Line andEarth connections of ABSwitches are properlydone. If line connection isnot proper, it may lead toloose contact resulting inbreakdown. If earth connec-tion is not proper, it mayresult in high resistanceand faults may not becleared in time. It may leadto accidents also.


7. Line and earthConnections ofHT LightningArrestors

To check if line and earthconnections of HT lightningarrestors are properlymade; if line or earthconnection of the lightningarrestor is not madeproperly, lightning surgeswill not be cleared leadingto breakdown or accident.


8. ConnectionsFrom and ToBusbars

Checking the tightening theconnections from thebusbars and theconnections of the busbarsto the lines. If the connec-tions are not tight enoughand proper, it may lead toloose contact resulting in

breakdown.


9. Provision ofTubular Busbar

Checking the provision oftubular busbar andchanging the conductorstrung busbars to tubularbusbars, if not provided. Ifrigid tubular busbars arenot provided loose connec-tions may develop instrung busbars leading tobreakdowns.

10. Insulators Checking all insulators tosee that there are nocracks or damages aredeveloping. If they aredeveloping they may bechanged in time to avoidaccidents.


11. Jumpers Checking the condition andadequacy of all jumpers to

Foreman or LineInspector/Junior

Foreman or Lineman/Junior Engineer orAssistant Engineer

113


see that they are proper. Ifthe condition is bad, theyhave to be replaced. If theyare inadequate also, theyhave to be replaced. If notdone, it may lead to loosecontact resulting in break-down. If the jumper is notadequate, the currentcarrying capacity becomesless leading to overloading,etc.

Engineer or AssistantEngineer

12. Guides for ABSwitches

Check if the guides for ABSwitches are proper.Otherwise, it becomesdifficult to operate the ABSwitches leading todifficulties at times ofemergencies. Sometimesit leads to accidents also.


13. Provision ofDanger Boards

Checking the provision of“Danger Board”. If notprovided, provide the same.It may, to a certain extent,prevent unauthorisedpersons to meddle with thestructure. It may alsocaution authorised personswhen they have to work onthe structure especially atdouble and multiple feedlocations.


14. Provision ofCaution Board

Check provision of“Caution Board” tocaution that care shouldbe taken to work on thestructure. If not available,the same should beprovided.

Foreman or LineInspector/JuniorEngineer or Assistant

Engineer

15. Painting ofDetails

Painting the details of thesupply and if not avail-able, paint the same. Thedetails will help to identifythe feeder and to work. Ifnot painted, it may lead tomal-operation and maylead to accidents.

Foreman or LineInspector/JuniorEngineer or AssistantEngineer

Once in Three Years

1. Painting of MetalParts

Painting all metal parts willavoid corrosion.

Wireman/AssistantEngineer or JuniorEngineer

114


Sl. No. Details of the maintenancework to be carried out

Do the work Check andensure thecompletion of thework

MONTHLY

1. Maintaining the yard(underneath the structure)and the earth pits neatand tidy and wateringearth pits.

Wireman Line Inspector /Foreman

2. Checking the earthconnection.


QUARTERLY

1. Checking the condition ofdividing / termination.


ANNUAL

1 Checking the condition ofconcreting / coping of thesupports of the structures.


2. Checking the condition of thesupports of the structures.


3. Measuring the earthresistance.

Junior Engineer /Asst. Engineer

Asst. / ExecutiveEngineer

4. Checking the earthconnection of metal parts.


5. Lubricating the AB switchesand checking their operation.

Lineman Line Inspector /Foreman

6. Checking the line and earthconnection of AB switches.


7. Checking the line and earthconnection of HT lightningarrestors.


8. Checking and tightening theconnection from bus bars tothe lines.


9. Checking for provision oftubular bus bars andchanging the conductorstrung bus bars to tubularbus bars.

Foreman / LineInspector

Junior Engineer /Asst. Engineer

Person responsible to

MAINTENANCE SCHEDULE OF DISTRIBUTION TRANSFORMERSTRUCTURES

115


10. Checking all insulators. Lineman Line Inspector /Foreman

11. Checking the conditions andadequacy of all jumpers.


Foreman

12. Checking guides for ABswitches.

Wireman Asst. Engineer /Junior Engineer

13. Checking the provision of‘Danger’ boards and if notprovided, providing the same.

Wireman Foreman / LineInspector

14. Checking the provision ofcaution boards and if notprovided, providing the same.


Asst. Engineer /Junior Engineer

15. Checking the painting ofdetails of HT supply and ifnot provided, providing thesame.

Foreman Asst. Engineer /Junior Engineer

Once in 3 years

1. Painting of all metal parts. Wireman Asst. Engineer /Junior Engineer

Documents

BEE-001 block 2