EL-5036-V2-Power-Transformers

The unit transformer in a generating station nects the electric power output of the generating unit to the high-voltage electric transmission gridi the unit auxiliaries transformer, stat

Power nansformers

Written by Stone & Webster Engineering Corporation

For further information on EPRI's technical grams contact the EPRI Thchnical Information Divi- sion at (415) 855-2411, or write directly to EPRI's Thchnical Information Center at P.O Box 10412, Palo Alto, CA 94303

Taps and connections

Station auxiliary systems

Installation and maintenance

Voltage regulation

Copyright© 1987 Electric Power Research Institute, Inc All rights reserved

Reprinted in 1998 by Energy Conversion Division,

Steam-Turbine, Generator, Balance-of-Plant Target

Electric Power Research Institute and EPRI are registered service marks of Electric Power Research Institute, Inc Notice

This series was prepared by Stone & Webster Engineering Corporation as an account of work sponsored by the Elec- tric Power Research Institute, Inc (EPRI) Neither EPRI, members of EPRI, Stone &, Webster Engineering Corpora- tion, nor any person acting on behalf of any of them: (a) makes any warranty, express or implied, with respect to the use of any information, apparatus, method, or process disclosed in this series or that such use may not infringe privately owned rights, or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of, any information, apparatus, method, or process dis- closed in this series

In the past, several electrical equipment

manufac-turers published reference books dealing with

specific technical areas Many utilities have stated

that these reference books have been very useful

to them in dealing with plant emergencies and in

making decisions on design, system planning, and

preventive maintenance

Unfortunately, manufacturers today seldom

publish or update reference books on electric

power apparatus, mainly because of tighter

bud-get constraints Until now, utilities have had no

up-to-date industrywide practical reference manual

covering the various electric power apparatus and

electrical phenomena commonly encountered in

power plants The Power Plant Electrical

Refer-ence Series was planned to fill this need

EPRI believes that the series will save utilities

time and money It will aid plant engineers in

• Prevention of forced outages through proper

installation, application, and protection of

station auxiliary equipment

• Recognition of potential problems and their

prevention

• Selection of appropriate methods of

main-tenance to ensure trouble-free equipment

This volume deals with power transformers A

power transformer connects the generator to the

high-voltage transmission system Another power

transformer connects the generator to the plant

medium-voltage auxiliary power system

'Irans-former impedance is the major factor in the

volt-age regulation of the auxiliary power system, as

well as in the short-circuit duty of the switchgear

Selection of transformers for use in power stations

requires knowledge of the power system and

var-ious parameters

A wealth of information about transformers is

available in the transactions of the IEEE and in the

ANSI/IEEE standards and applications guides EPRI has also published a great deal of information on transformers, including studies of transformer life characteristics (EL-2622), dielectrics, accessories, and monitoring equipment The purpose of this book is

to bring out the concepts that are most useful to power plant personnel, without requiring an under-standing of the rigorous engineering analysis necessary for the basic design transformers

D K Sharma

Electrical Systems Division Electric Power Research Institute

The unit transformer in a generating station nects the electric power output of the generating unit to the high-voltage electric transmission gridi the unit auxiliaries transformer, station service transformer, and secondary-unit substation trans-formers supply the electric auxiliaries required for operation of the power plant In the lower range

con-of sizes, power transformers may be con-of standard design types, but many of the transformers used

in power plants and all of the larger ones are custom-designed-similar, but seldom identical, to others built previously This volume covers the practical aspects of the selection, specification, in-stallation, operation, testing, and maintenance of these power transformers

lransformer designs of particular interest to power plant operators include liquid-immersed, dry-type, and vapor-cooled transformers ranging

in size from 500 kVA to 1200 MVA The function and application of each design are described in detail, from load considerations to noise criteria Photographs show the various types of oil-preservation systems, transformer connections, and bushings A variety of gages, monitors, and indicators may be provided for liquid-immersed transformersi these accessories are also discussed

The authors wish to acknowledge the help they received from many technical publications pre-pared by people in the industry They also express their appreciation to the following people for their reviews, suggestions, and guidance in general

Electric Power Research Institute

D K Sharma, Project Manager

R Steiner, Associate Director, Electrical Systems Division

J C White, Program Manager

G Addis, Project Manager

Stone & Webster Engineering

EPRI Review Committee

J R Boyle, Thnnessee Valley Authority

L E Brothers, Southern Company Services

J Erlingsson, Pacific Gas and Electric Company

R G Farmer, Arizona Public Service Company

R G Hodgson, Los Angeles Department of Water

& Power

J A Maxwell, Georgia Power Company

W L Nail, Jr., Mississippi Power& Light Company

D G Owen, Duke Power Company

B K Patel, Southern Company Services

R A Schaefer, Public Service Company of Oklahoma

J E Stoner, Jr., Duke Power Company

D M Van Thssell, Jr., Florida Power& Light Company

J E White, Thmpa Electric Company

The authors owe special thanks to W J McNutt, General Electric Company, member of the Trans-formers Committee of IEEE, who reviewed the final manuscript

Application of Loss Values 2·8

2.8 Oil Preservation Systems 2-8

Sealed.:nmk System 2-8

Inert Gas System 2-9

Modified Conservator System 2-9

2.10 Taps 2-14 No-Load Thp Changers 2-14 Load Thp Changers (LTCs) 2-14

2.11 Bushings 2-15

2.12 Accessories 2-18 Liquid Level Gage 2-18 Thmperature Indicators 2-18 Flow Indicator 2-20 Bushing Current 'Iransformers 2-20 Resistance Thmperature

Detectors 2-20 Sudden Pressure Relay 2-20 Gas Detector Relay 2-21 Fault Gas Monitor 2 21 Pressure Relief Device 2 21 Lifting Eyes and Jack Bosses 2-·22-Lightning Arresters 2-22-

2.13 Application Considerations 2 22-Maximum Sustained Load 2 2-2-Altitude 2-25 Ambient Thmperature 2 25 Number of Windings 2-2-5 Voltage Ratings and

Overexcitation 2 25 'Iransient Overvoltage 2·26 Load Current Waveform 2-26 Harmonic Current Derating 2-27 Impedance Voltage and

Regulation 2-28 Impedance and Through-Faults 2-29 Phasing Out Three-Phase

Circuits 2-29 Loss Evaluation 2-30 Noise Criteria 2-30

2.14 Shipping Considerations 2-32

Shop Thsting 2-48 Field Thsting 2-49

FIGURE PAGE

Oil Preservation System 2·9

2-2 Transformer With Inert Gas Oil

Preservation System 2·10

2-3 Transformer With Modified

Con-servator Oil Preservation System 2-11

2-4 Transformer Terminal Designation

in Accordance With ANSI Standard

C57.12.70-1978 2·12

2-5 Typical Transformer Phase

Relationships , 2-13

2-6 Power Transformer With LTC

2-7 Apparatus Bushing of the

Paper-Oil Capacitor (POC) Type 2·17

2-8 EVH Bushing 2·18

2-9 High-Current Type-A Bushing

25-kV, Class-4500 A and Above 2·19

Relay 2·21

2-15 General Guide for Permissible

Short-Time Overexcitation of

Power Transformers (Rated Volts

per Hertz = 100% Excitation) 2·26

Measure-merits of Large General Electric

Power Transformers (Early 1970s) 2·31

Steam Turbine Generator Unit 2·34

Power transformers are used in generating

sta-tions to connect the main generator to the

high-voltage transmission system and to connect

sources of electric power to distribution

sub-systems for operation of plant auxiliary electrical

equipment at medium- and low-voltage levels The

proper selection of transformers for each

appli-cation requires a knowledge of the types available

and their range of applicability It also requires a

knowledge of terms, conventions, tolerances, and

factory tests as established in industry standards

Background

Power plant electrical equipment operating at ac

voltages of 120, 460, 575, 4000, 6600, or 13,200 V

receives its power from higher-voltage sources: the

main generator and the switchyard 'fransformers,

which are located near the load (where possible),

interconnect the voltage levels Although the

smaller sizes of power transformers may be of

standard design types, the larger ones are

custom-designed and similar, but seldom identical, to

others This book provides practical guidance in

the selection of this equipment

Objectives

This volume will provide power station engineers

with a background of transformer knowledge that

will enable them to apply transformers correctly,

assist in understanding existing standards and the

various options required for power transformer

se-lection, and provide guidance to power plant

per-sonnel in planning inspection and testing programs

Approach

A national survey of utility requirements yielded

pertinent information, and a search of available

literature on power transformers identified

spe-cific information pertaining to power plant

applications The EPRI Review Committee, with

members from 11 utilities throughout the United

States, and other industry experts reviewed the

material for accuracy and completeness The

resulting information was the basis for this volume

of the Power Plant Electrical Reference Series

Spe-ABBREVIATIONS

AA transformer cooling method: ventilated

ac alternating current

ANSI · American National Standards Institute

BIL basic lightning impulse insulation level

Btu British thermal unit(s)

CI present worth of outlay in the year of first

commercial operation (Eq A-1)

EHV extra-high voltage

f annual inflation rate (decimal) (Eq A-1)

FA transformer cooling method: oil immersed,

forced-air cooled

FOA transformer cooling method: oil immersed,

forced-oil cooled with forced-air cooler

FOB free on board

FOW transformer cooling method: forced-water

internal rate of return

internal rate of return expressed as a

decimal rather than as a percentage (Eq A-1)

number of years between the price year

and the year of tiTSt commercial operation

R resistance

REG transformer regulation RIV radio influence voltage rms root-mean-square SCR short-circuit ratio SST station service transformer UAT unit auxiliaries transformer

UT unit transformer

V volt(s)

Z transformer impedance voltage

A W Goldman and C G Pebler

2.1 INTRODUCTION

Power-transformers are used in power plants to

connect the main generator to the high-voltage

(HV) transmission system and to connect sources

of auxiliary power to distribution subsystems for

plant auxiliary electrical equipment at lower

volt-age levels Since they are basically static devices,

they require less maintenance than most of the

other apparatus It is important, however, (1) that

each transformer be selected properly for the

in-tended application; (2) that it be protected from

voltage surges, external short circuits, and

prolonged overload; and (3) that it be inspected,

maintained, and tested on a routine basis

The power transformers of particular interest

to the designers and operators of power plants

range in size from 500 kVA to 1200 MVA in

three-phase designs and from 500 kVA to 550 MVA in

single-phase designs 'fransformers installed inside

a building may be dry-type, resin encapsulated,

or liquid immersed in high-fire point or low-heat

release insulating fluids 'fransformers installed

outdoors are generally mineral oil immersed

In the lower size range the transformers may

be of repetitive design, but many of the

transform-ers used in power plants and all of the larger ones

are custom designed-similar, but seldom

identi-cal, to others built previously

'fransformer power and energy losses, though

relatively small, are of interest to the user for two

reasons: They cause increased fuel consumption,

and they result in heat release The fuel consumed

in generating the loss of energy is an important

item in operating cost The heat must be removed

and dissipated by some combination of

conduc-tion, convecconduc-tion, and radiation "Self-cooled"

trans-formers do not require any power-driven cooling

auxiliaries Forced-cooled transformers employ

forced-water or forced-air cooling and may also

use pumps to circulate the insulating fluid The

addition of rotating machinery to an otherwise

static device reduces the physical size and initial

cost of the transformer for a specific output

rating, but it may also reduce reliability and

in-crease maintenance cost and losses

Oil-immersed transformers require oil

preser-vation systems to exclude oxygen and water vapor;

this retards sludging and deterioration of tric properties Gas formation under oil may indi-cate local hot spots or decomposition of solid insulating materials For this reason gas monitors are often installed to detect and collect generated gases for laboratory analysis 'fransformer oil should be sampled and tested at regular intervals The analysis of both the collected gas and the oil samples provides warning of abnormal conditions Power transformers are factory tested to ensure quality of design and manufacture and to demon-strate their ability to meet performance require-ments Data obtained during such tests may also provide benchmarks for later field tests

dielec-A large transformer may be damaged by proper handling during loading, shipment, on-site storage, testing, or installation These operations warrant meticulous attention

im-The application of the above material to unit transformers (U'Th), unit auxiliaries transformers (UA'Th), station service transformers (SS'Th), and secondary unit substation transformers is covered under appropriate headings in this volume

2.2 DEFINITION OF TERMS

Basic lightning impulse insulation level (BIL) A specific insulation level, expressed in kilovolts, of the crest value of a standard lightning impulse

Basic switching impulse insulation level A specific insulation level, expressed in kilovolts, of the crest value of a standard switching impulse

Chopped-wave impulse A voltage impulse that is terminated intentionally by sparkover of a gap Decibel (dB) See Sound pressure level

Demand factor The ratio of the maximum demand

of a system to the total connected load of the system Diversity factor The ratio of the sum of the individ- ual maximum demands of the various subdivisions of

a system to the maximum demand of the whole system Eddy-current loss Power dissipated due to eddy cur- rents This includes the eddy-current losses of the core, windings, case, and associated hardware

Front-of-wave lightning impulse test A voltage impulse with a specified rate of rise that is terminated intentionally by sparkover of a gap that occurs on the

rising front of the voltage wave with a specified time

to sparkover and a minimum crest voltage Complete

front-of-wave tests involve application of the following

sequence of impulse waves: (1) one reduced full wave;

(2) two front of waves; (3) two chopped waves; (4) one

full wave

Graded insulation The selective arrangement of the

insulation components of a composite insulation system

to equalize more nearly the voltage stresses

through-ou! the insulation system

Harmonic factor The ratio of the root-mean-square

(rms) value of all the harmonics to the rms value of the

Hot spot temperature The highest temperature

in-side the transformer winding It is greater than the

aver-age temperature (measured using the resistance change

method) of the coil conductors

Hysteresis loss The energy loss in magnetic material

that results from an alternating magnetic field as the

elementary magnets within the material seek to align

themselves with the reversing magnetic field

Impedance voltage The voltage required to circulate

rated current through one of two specified windings

of a transformer when the other winding is

short-circuited, with the windings connected as for rated

volt-age operation It is usually expressed in per unit, or

per-cent, of the rated voltage of the winding in which the

voltage is measured

Insulation level An insulated strength expressed in

terms of a withstand voltage

Insulation power factor The ratio of the power

dis-sipated in the insulation, in watts, to the product of

effective voltage and current, in voltamperes, when

tested under a sinusoidal voltage and prescribed

conditions

Lightning impulse insulation level An insulation

level, expressed in kilovolts, of the crest value of a

light-ning impulse withstand voltage

Liquid-immersed transformer A transformer in

which the core and coils are immersed in an insulating

liquid

Load tap changer (LTC) A selector switch device,

which may include current-interrupting contactors,

used to change transformer taps with the transformer

energized and carrying full load

No-load tap changer A selector switch device used

to change transformer taps with the transformer

deenergized

Oil-immersed transformer A transformer in which

the core and coils are immersed in an insulating oil

Overload Output of current, power, or torque by a

device in excess of the rated output of the device on

a specified rating basis

Overvoltage A voltage above the normal rated

volt-age or the maximum operating voltvolt-age of a device or circuit

Primary winding The winding on the energy input

side

Partial discharge An electric discharge that only tially bridges the insulation between conductors and that may or may not occur adjacent to a conductor Par- tial discharges occur when the local electric field inten- sity exceeds the dielectric strength of the dielectric involved, resulting in local ionization and breakdown Depending on intensity, partial discharges are often accompanied by emission of light, heat, sound, and radio influence voltage (with a wide frequency range)

par-Radio influence voltage A radio frequency voltage

generally produced by partial discharge and measured

at the equipment terminals for the purpose of mining the electromagnetic interference effect of the discharges

deter-Secondary unit substation A unit substation in which the low-voltage (LV) section is rated 1000 V or below

Secondary winding The winding on the energy

output side

Sound level A weighted sound pressure level obtained

by the use of metering characteristics and the ings A, B, or C specified in American National Standards Institute (ANSI) Standard S1.4

weight-Sound pressure level The sound pressure level, in

decibels, is 20 times the logarithm to the base 10 of the ratio of the pressure of the sound to the reference pres- sure of 2 times w-s N/m 2 (0.00002 microbar), also written 20 N/m 2

Station service transformer (SST) A transformer

that supplies power from a station high-voltage (HV) bus

to the station auxiliaries It also supplies power to the unit auxiliaries during unit startup and shutdown and/or when the VAT is not available

Surge arrester, lightning arrester A protective

device for limiting surge voltages on equipment by charging or passing surge current; it prevents continued flow of follow current to ground and is capable of repeating these functions as specified

dis-Switching impulse Ideally, an aperiodic transient

voltage that rises rapidly to a maximum value and falls, usually less rapidly, to zero

Switching surge A transient wave at overvoltage in

an electrical circuit caused by a switching operation

Thp changer See No-load tap changer

Temperature rise The difference between the

tem-perature of the part under consideration (commonly the

"average winding rise'' or the "hottest spot winding rise'')

and the ambient temperature

'Iransient overvoltage The peak voltage during the

transient conditions resulting from the operation of a

switching device

Unit auxiliaries transformer (UAT) A transformer

intended primarily to supply all or a portion of the unit

auxiliaries

Unit transformer (UT) A power system supply

trans-former that transforms all or a portion of the unit power

from the unit to the power system

Withstand voltage The voltage that electrical

equip-ment is capable of withstanding without failure or

dis-ruptive discharge when tested under specified

conditions

The industry recognizes two general types of

power transformers: liquid-immersed

transform-ers and dry-type transformtransform-ers

LIQUID-IMMERSED TRANSFORMERS

A liquid-immersed transformer consists of a

mag-netic core-and-coils assembly, either single-phase

or polyphase, immersed in fluid having good heat

transfer and insulating properties The

liquid-immersed transformer permits compact design,

and at this time transformers with ratings above

10,000 kVA or 34.5 kV are always liquid immersed

Initially, the fluid was always a highly refined

mineral oil Since such oils are flammable,

liquid-immersed transformers located within buildings

were installed in fireproof vaults Later,

nonflam-mable fluids were developed for this application,

the most common being an askarel,

polychlori-nated biphenyl (PCB) These fluids have high

specific inductive capacitance (also called relative

dielectric constant or relative capacity) and good

heat transfer properties but are more expensive

and have lower dielectric strength than mineral

oil The Toxic Substances Control Act of 1976 (1)

and the Code of Federal Regulations (2) now

pro-hibit the manufacture of PCBs and limit the use

of PCB-bearing equipment The federal regulation

specifies rigid rules and requirements for marking

PCB-bearing equipment in service and for

dispos-ing of PCB-beardispos-ing equipment and contaminated

materials resulting _from liquid spills (3)

More recently other fluids having high fire points and low rates of heat release, though more expensive than askarels, have been introduced to replace it (for example, silicone, tetrachloroethy-lene, trichlorotrifluoroethane, and highly refined paraffinic oil)

Another recent development, the vapor-cooled transformer, is classified as liquid immersed and

is suitable for indoor installation This design employs a low-boiling point organic fluid for heat transfer The latent heat of vaporization absorbs the heat produced by transformer losses That latent heat is then released in a heat exchanger external to the transformer tank, which condenses the vapor and returns it to the transformer tank

in liquid form Vapor-cooled transformers may be equipped with cooling fans to increase kilovoltam-pere rating up to 50%

The application of high-fire point, low-heat release liquid-insulated transformers versus mineral oil-insulated transformers involves eco-nomic and fire hazard considerations The former are somewhat less hazardous, but they are more expensive than the latter, with silicone liquid-filled being the most expensive

Provisions for containing oil spills, should the tank rupture, are covered in this volume in Sec-tion 2.18

DRY-TYPE TRANSFORMERS Dry-type transformers are generally more expen-sive than oil-immersed transformers and depend

on solid insulation-film coatings, paper tape, or

a combination of the two-for most of their electric strength Single-phase and polyphase dry-type transformers are available in ventilated designs, totally enclosed nonventilated designs, sealed-tank designs, and gas-filled designs, the ventilated type being least expensive Their abil-ity to withstand lightning and switching surge impulse voltages is less than that of liquid-immersed designs It may therefore be prudent to protect their HV terminals with surge arresters, even when the external leads to these terminals are not directly exposed to lightning

di-Ventilated dry-type transformers are suitable for most applications inside buildings In atmospheres heavily loaded with dust or fibers, however, they must be cleaned at regular intervals to keep their ventilation passages clear This type may be equipped with fans to increase their kilovoltam-pere rating by 33%% They have the lowest initial cost of any in the family of dry-type transformers

Totally enclosed, nonventilated dry-type

trans-formers are suitable for use in moderately

con-taminated industrial environments Because they

are nonventilated, they are designed to have low

heat losses-that is, very high efficiencies

Sealed-tank transformers have the ability to

function in the severest environments They have

their own sealed atmosphere and can function in

misty, oil-laden, dusty, highly contaminated areas

Tnese transformers also have high efficiencies

be-cause of the necessity of having low heat losses

Dry-type transformers are currently available in

self-cooled ratings up to 10,000 kVA and at voltages

up to 34.5 kV

A variant of the dry-type transformer that is resin

encapsulated has been introduced recently In one

form of this design, "cast-coil;' the coil is placed

in a mold and the resin coating is cast around it

These transformers are available in sizes up to

5,000 kVA and voltages up to 34.5 kV In another

form the coils are dipped in resin The

resin-encapsulated design may be used in harsh

environ-ments where ventilated dry-types may not be

suitable Although their initial cost is higher than

other dry-types, they may nevertheless be

econom-ical in high-load factor applications because of

their lower load losses (Volume 7, Au;te.iliary

Elec-trical Equipment)

Some of the resins used in earlier

resin-encapsulated transformers gave off vapors at high

temperatures that were found to be flammable,

toxic, or both In more recent designs these

con-cerns have been resolved by tests and analysis of

the vapors showing them not to be harmful (4)

The application of a ventilated dry-type versus

a nonventilated type or a sealed, gas-filled

dry-type transformer involves economic and

environ-mental considerations (clean, dust-laden, wet, or

highly contaminated atmosphere) The gas-filled

transformer has the highest initial cost

The application of a ventilated dry-type versus

a ventilated, encapsulated dry-type transformer

also involves these considerations

Volume 7, Section 7.5 gives a comparison of the

relative equipment costs of the various dry-type

transformers

2.4 RATING BASIS AND

TEMPERATURE RISE

Power transformers are output rated They are rated

to deliver specified kilovoltamperes continuously

at a specified secondary voltage and frequency under "usual" operating conditions and with a standard temperature rise When operated within their ratings they have "normal" life expectancies They may be operated beyond their ratings under certain conditions without loss of life expectancy

or under other conditions with a somewhat dictable sacrifice of life expectancy 1tansformers

pre-in power plants generally are selected to operate within their ratings except for brief transient periods, such as during motor starting or during the time required for relay operations to clear through-faults

Usual and unusual operating conditions for liquid-immersed transformers are defmed in ANSI Standard C57.12.00-1980 (5); those for dry-type transformers are defined in ANSI Standard C57.12.01-1979 (6) Some unusual operating con-ditions are:

• Ambient temperature above 40°C or with 24-h average above 30°C

• Altitude above 3300 ft

• Sustained operation at more than 110% (no load) or 105% (loaded) of rated secondary volts or volts per hertz

• Load current waveform distortion (harmonic factor greater than 0.05)

• Primary phase voltage unbalance

• Secondary phase current unbalance

• Damaging fumes or vapors, excessive or abrasive dust, salt spray, or excessive moisture

• Abnormal vibration, shocks, or tilting

• Restricted air circulation These or other unusual operating conditions, if ap· plicable, should be stated in purchase specifications Although transformers are kilovoltampere rated, their true continuous load limits are determined

by secondary winding current ratings Note that the secondary may be either the HV or the LV winding If the secondary winding has taps, then the permissible continuous load is determined by the current rating of the tap in use, though

it is called a "full-kVA" tap

The kilovoltampere rating does limit permissible load at secondary voltages above tap voltage rating, but at voltages below tap voltage rating the tap cur-rent rating intervenes At 95% secondary voltage the maximum continuous kilovoltampere load is 95% of nameplate kilovoltamperes

Standard temperature rise is the average ing rise (by resistance) that, in "usual" ambient

wind-temperature and with suitable allowance for

hot-test spot difference, is within the long-time

with-stand capability of the insulating materials For

liquid-immersed transformers, that rise is 65°C

(15°C hot spot allowance) Liquid-immersed

formers are now rated for 65°C rise Many

trans-formers having 55/65°C-rise ratings, however, are

still in service Both designs are suitable for

con-tinuous operation at their 65°C-rise ratings The

difference between them is that the performance

characteristics, full-load losses, and impedance

voltage drop for the 55/65°C-rise transformer are

based on 55°C-rise loading Where a transformer

must operate in a higher-than-usual ambient

perature, it is customary to specify a reduced

tem-perature rise The result is a larger transformer

that under "usual operating conditions;' carries

more load For example, if the temperature rise

of a liquid-immersed transformer is specified as

55°C, the permissible load increase under 30°C

conditions that permit a 65°C rise will be 12%

The average temperature winding rise for

dry-type transformers, depending on the insulation

system, may be 80°C, l15°C, or 150°C (all with

30°C hot spot allowance) (6)

2.5 INSULATION LEVEL

'Iransformers must be insulated to withstand the

voltages to which their windings and terminals

may be subjected in service These include the

normal ranges of power-frequency voltages

pub-lished in ANSI Standard C84.1-1982, the impulse

overvoltages that may be produced by lightning

strikes on their terminals or on connected

trans-mission lines, and the transient overvoltages that

may be produced by operation of transmission line

circuit breakers Mineral oil-immersed

transform-ers can withstand very high crest voltages if the

duration of the transient is measured in

microseconds

The basic lightning impulse insulation level (BIL)

of a transformer is the crest value of the voltage

it can withstand if the impulse voltage has the

wave shape defined as "full wave" in ANSI

Stan-dards C57.12.00 and C57.12.90 That shape,

in-tended to be representative of a lightning impulse,

has a rise time of 1.2 J.LS and a decay time, or tail,

of 50 J.LS Crest values for other wave shapes are

keyed to the BIL For example, for 900-kV BIL the

associated crest values for front of wave, chopped

wave, switching surge, and low frequency are

1240, 1035, 745, and 395 kV, respectively The wave shapes of these other transients are also de-fined in the standards The front-of-wave shape is intended to be representative of a lightning im-pulse chopped before crest by a rod gap The chopped-wave shape is intended to be represen-tative of a lightning impulse chopped at crest or immediately thereafter The switching surge wave-form is intended to be representative of the tran-sient that may be produced by operation of a transmission line circuit breaker The low-frequency wave shape is sinusoidal at power fre-quency (or a low multiple of power frequency) to avoid core saturation during a factory test The transformer transient voltage strength re-quired in a particular application depends on the lightning arresters that can be installed at the transformer terminals to protect it If the arrester has too low a voltage rating, it may be destroyed

by follow current at power frequency following

a voltage surge Minimum safe arrester voltage ings must be determined by a transient network analysis of the transmission system The trans-former transient voltage strength should then ex-ceed the voltage rating of the arrester by an appropriate margin-usually in the range of 15 to25%

rat-'fransformer price is affected by BIL One facturer has published base price multipliers, show-ing that for 345-kV service the base price would apply without multiplier for a BIL of 1050 kV The multiplier would be less than 1 for 900-kV BIL and greater than 1 for 1175-kV BIL This information

manu-is not based on industry standards, but it does dicate the industry pricing practice

in-BILs for dry-type transformers are given in ANSI Standard C57.12.01-1979 (6), and the wave shapes are defined in ANSI Standard C57.12.91-1979 (7)

DUAL-, AND TRIPLE-RATED TRANSFORMERS

LIQUID-IMMERSED TRANSFORMERS Liquid-immersed transformers larger than 500 kVA may have both a self-cooled rating and one or two additional forced-cooled ratings The rating increase produced by forced cooling varies with transformer size, as shown in 'Th.ble 2-1 (8) At 20,000 kVA and above transformers may have a single forced-cooled rating and no self-cooled rating

Table 2.1 Forced-cooled Ratings

P-ercent of Self-cooled Self-cooled kVA kVA Wrth Auxiliary

2500-9999 2500-11 ,999 125 10,000 and up 12,000 and up 133%

OA/FA/FA

- OA/FA!FOA 10,000 and up 12,000 and up 133% 166'%

SOURCE: This material is reproduced by permission of the National

Electrical Manufacturers Association from NEMA Standards

Publi-cation No NEMA TR 1-1980, Transformers, Regulators, and Reactors

© 1980 by NEMA

The standard method of indicating these

multi-ple ratings is to list the rating(s), followed by the

corresponding cooling method(s) For example:

• 2000/2300 kVA, OAIFA indicates a

trans-former with a self-cooled (OA) rating of

2000 kVA and a forced-air-cooled (FA) rating

of 2300 kVA

• 12,000/16,000/20,000 kVA indicates a

trans-former with a self-cooled rating and two

stages of forced cooling Such transformers

have large radiators to produce

thermosi-phon circulation of the oil in the self-cooled

mode They have two banks of fans and oil

pumps These transformers are indicated as

follows:

and the second stage of forced cooling use

forced air The first stage uses half of the

available fans (one bank); the second stage

uses all available fans (both banks)

forced cooling uses forced air and the

sec-ond stage uses forced oil and forced air

o OA!FOAJFOA indicates that both the first

stage and the second stage of forced

cool-ing use forced oil and forced air The first

stage uses half of the available fans and oil

pumps (one bank); the second stage uses

all available fans and pumps (both banks)

• 25,000 kVA, FOA indicates a transformer

with no self-cooled rating It has compact

coolers in place of radiators and cannot

re-main energized, even at no load, without its

fans and pumps in operation Nevertheless,

most UTh and many UATh are of the FOA

type This type is used less frequently for SS'IS, which remain energized continuously but are heavily loaded infrequently In this type of service the triple-rated transformer

is advantageous, because its mechanical ing auxiliaries are required only during the periods of heavy load

cool-A Ucool-AT serving a maximum load of 20 MVcool-A could

be either 12/16/20 MVA, OAIFX!FX, or 20 MVA, FOA The triple-rated transformer can carry 12 MVA with no mechanical cooling auxiliaries in opera-tion In this application that capability may not be

an advantage, since half of the 20-MVA load may

be present when the machine is synchronized and the auxiliaries load is transferred to this trans-former; the 12-MVA self-cooled limit thus is ex-ceeded before the turbine generator reaches half load Although the triple-rated and FOA alterna-tives may have identical core-and-coil assemblies, the FOA transformer is less expensive and requires less space in an area where space is usually limited

On the other hand, if a generator breaker is stalled between the generator and the transform-ers, the triple-rated UAT can operate without mechanical cooling auxiliaries during unit shut-down Volume 7, Auxiliary Electrical Equipment,

in-covers the application of generator breakers, and Volume 8, Station Protection, covers transformer and generator protection

Large UTh are nearly always of FOA (or see below) design Again, this is primarily because

POW-of space considerations In addition it may be more difficult to design a low-impedance transformer

of the triple-rated type, because the oil channels through the windings must be large enough to permit gravity circulation of oil before the oil

pumps are brought into operation Larger oil

channels tend to increase leakage reactance

WATER-COOLED TRANSFORMERS

Forced-water-cooled (FOW) transformers are

often used instead of FOA types at hydroelectric

plants because of the ready availability of cooling

water They are also often used at underground

hydro or pumped storage plants, where the

trans-formers must be underground to be near the

equipment they serve Large power transformers

have also been enclosed in masonry vaults for

noise control purposes In such cases water

cool-ing may be the only feasible method of heat

dissi-pation Because of concern for water leakage into

the oil, however, such transformers have specially

designed heat exchangers with double tube sheets

and concentric tubes to provide two metal

barri-ers between the two fluids In this design the

neutral space between the metal barriers can be

monitored and an alarm actuated if either barrier

begins to leak

DRY-TYPE TRANSFORMERS

All dry-type power transformers have self-cooled

ratings Those commonly used indoors in power

plants are ventilated (rated AA) Some are

equipped with fans to give them a dual rating

(AA!FA) A common size for LV secondary unit

sub-station transformers is 1000/1333 kVA, AAIFA Note

that the forced-cooled rating is one-third larger

than the self-cooled rating

'Iransformers are very efficient Large

liquid-immersed transformers may have efficiencies

higher than 99% Nevertheless, it may be

worth-while to pay an initial price premium for loss

reduction, which will result in still higher efficiency

'Iransformer losses can be divided into three

general categories: no-load losses, load losses, and,

for forced-cooled transformers, cooling-system

losses The no-load losses are mainly core hysteresis

and eddy-current losses, which are incurred as

long as the transformer is energized They remain

essentially constant The load losses are due to the

heating of winding conductors by the passage of

current and by other stray losses in conductors

and tank walls, which are load related These losses

increase as the square of load current The cooling

system losses are power used to drive the ical cooling auxiliaries-fans and oil pumps-where these auxiliaries are present

mechan-In medium and large power transformers the load losses are much greater than the no-load losses The ratio of load losses to no-load losses will be influenced by the loss evaluation figures

in the purchaser's bidding documents 1b simplify

a generalization of available data, one can pare values on the basis of core-and-coils rating

com-On this basis a 20-MVA FOA transformer, a 12/16-MVA OAIFA transformer, and a 12/16/2Q-MVA OAIFOAIFOA transformer are directly comparable

At 12 MVA such a transformer would have a ratio of load losses to no-load losses on the order

of 3.5:1 At 16 MVA this ratio would be greater by

a factor of 1 777; and at 20 MVA (if permissible) the factor would be 2.779

Very large pbwer transformers, nearly always

FOA, have loss ratios on the order of 7:1 Lower ratios might be economical in many cases, but such ratios may not be achievable within shipping limitations

EVALUATION METHOD Loss evaluation is the process of estimating the amount of initial outlay justified to avoid future costs Specifically, it answers the questions: "What price premium are we justified in paying to reduce transformer no-load loss by 1 kW? What premium for 1 kW of load loss?" When the initial cost pre-mium (a single payment amount) is compared with the future costs avoided thereby (a nonuniform series of annual amounts), it is convenient to use life-cycle cost methods, which convert all cash flows to present worth It is, for example, not justifiable to spend $100 today to avoid a $100 ex-pense ten years from today; a far smaller amount invested in some other aspect of the company's business would grow to $100 in ten years It is the smaller amount that is the present value of the future cost

Loss evaluation seeks to determine how much the purchaser would be justified in paying for the transformers to reduce no-load loss by 1 kW and how much per kilowatt for a similar reduction in load loss Since the premium would be a single payment on delivery and the savings that justify

it are a nonuniform series of future costs, their equivalence must be found by present-worth methods These methods, which involve the capi-tal structure of the company, the estimated load-ing schedule for the transformer, and the present

and anticipated future cost of the fuel used for

generation, are discussed in Appendix A

APPLICATION OF LOSS VAWES

With no guidance about how losses are to be

evalu-ated, each transformer bidder will offer the

de-sign that meets its temperature rise guarantee at

minimum initial cost For large power

transform-ers that are expected to operate at high load

fac-tors, this is not the most economical choice A

better design would have more iron, more copper,

and less cooling equipment Although this design

would increase initial cost, it would reduce losses

As was pointed out previously, transformer

losses are partially avoidable Estimating loss

values and including them in the invitations for

competitive bids effectively make the supplier and

the purchaser partners in determining what

frac-tion of the losses is economically avoidable In the

case of smaller transformers the cost per

kilovolt-ampere is so large that any significant fraction

added to it in order to reduce losses would

out-weigh the future savings attributable to the loss

For certain large transformers, notably SSTh, the

load factor is so low that load losses have small

economic value But SSTh are energized for

essen-tially the entire year, and their no-load losses are

incurred at full strength all of that time For these

transformers the no-load losses have significant

economic value Therefore, a design in which core

flux density is reduced below conventional levels

may well justify its higher cost, because a small

reduction in flux density produces a large

reduc-tion in hysteresis loss and a larger reducreduc-tion in

core eddy-current loss This reduction in flux

den-sity also significantly reduces magnetostriction

noise In the case of these medium power

trans-formers the large-volume market is in substation

transformers of fairly uniform design Not all

sup-pliers are in a position to tailor their basic designs

closely to the special needs of every purchaser For

that reason each manufacturer will make its own

decision on the design to be offered and the prices

For transformers installed indoors losses have

a significant indirect cost due to the fact that the

heat released by the transformer must be removed

by the ventilating system and may represent an

appreciable portion of the load on that system For

this reason some purchasers prefer 80°C-rise

dry-type transformers to the less expensive, but less

efficient, 115°C- and 150°C-rise designs

2.8 OIL PRESERVATION SYSTEMS

Mineral oils used in power transformers degrade

in prolonged exposure to oxygen or moisture Water suspended in the oil reduces its dielectric strength and that of cellulosic insulation to which the water may migrate Oxidation may affect di-electric properties and may cause sludge forma-tion Sludge, in turn, clogs small oil passages through the windings and impairs heat removal, allowing hot spots to develop Solid insulation may

be degraded rapidly in the hot spots, and such degradation reduces insulation life expectancy Oil preservation systems have been developed to pre-vent such degradation (8)

Mineral oil has a relatively large thermal cient of expansion, and therefore the oil level in

coeffi-a trcoeffi-ansformer tcoeffi-ank rises coeffi-and fcoeffi-alls with coeffi-ambient temperature and with load If the oil level becomes too low, the bottom portions of HV bushings and the current transformers that are often fitted around them are left without the oil immersion

on which they may depend for voltage gradient control and for cooling The oil level cannot rise

· above the top of the tank unless external sions are made for expansion

provi-The oil preservation system must allow for the oil expansion and contraction and must prevent moisture and oxygen from being drawn into the tank Three general types of oil preservation sys-tems are in common use: the sealed-tank system, the inert gas system, and the modified conservator system

One manufacturer provides, as standard, the oil preservation system for the following various volt-ages and ratings:

Three-phase,

Operating Voltage Class (kV) 650C MVA Rating Up to 138 161 to 230 Above 230

Up to 67.2 OA sealed-tank inert gas modified

Above 67.2 OA modified modified modified

or 112 FOA conservator conservator conservator

SEALED-TANK SYSTEM

In the sealed-tank system the interior of the tank

is sealed from the atmosphere The gas-plus-oil ume remains constant over the temperature

vol-range The transformer tank and lead entrance

bushings are tightly sealed Contamination of the

oil proceeds very slowly because of the careful

drying and vacuum filling done before the tank

is sealed

This system has one limitation: With time the

pressure tends to become negative whenever oil

temperature falls below the temperature at which

the tank was filled When this happens moisture

and a1r will be drawn into the transformer if a leak

does occur

Maintenance of this system is minimal The

pressure-vacuum gage can be obtained with alarm

contacts to alarm when overpressure or excessive

negative pressure occurs

Figure 2-1 shows a transformer with a

sealed-tank system

INERT GAS SYSTEM

In the inert gas system a blanket of dry nitrogen

is maintained over the oil in the transformer tank

at a pressure slightly higher than atmospheric

pressure Thus, any leakage is outward and does

not contaminate the oil

During cooling periods nitrogen is fed from

metal bottles near the transformer through a

regulating valve, which maintains a slight positive

gage pressure at the top of the tank During

heat-ing periods a discharge regulator releases surplus

gas to prevent overpressure There must be a

suffi-cient "dead-band" between the settings of the two

regulators to allow for drift and random variation

of set points and to ensure that in-feed and

dis-charge never occur at the same time If that were

to occur, the entire contents of the gas bottles

could be lost

The inert gas system requires regular

main-tenance: depleted gas bottles must be replaced,

nitrogen use must be recorded, and the settings

of the pressure regulators must be verified

Another possible disadvantage of the inert gas

system involves formation of bubbles in the oil

There is always a small but measurable quantity

of gas-nitrogen or other gases-dissolved in the

oil During a coolin_g period and resultant

depres-surization some of the gas comes out of solution

in the form of bubbles Migration of gas bubbles

into regions of high dielectric stress may cause

ionization of the voids within the bubbles because

the dielectric strength of the voids is lower than

that of the sUITOtmding oil A chain of ionized voids

can produce dielectric failure The seriousness of

Courtesy of McGraw-Edison Co., Pittsburgh, Pa

this threat is controversial; many transformer users continue to have satisfactory experience with inert gas systems

A transformer using the inert gas system is shown in Figure 2-2 The control cabinet and nitro-gen gas piping are visible

MODIFIED CONSERVATOR SYSTEM Because of the perceived disadvantages of the in-ert gas system, a competing system has been de-veloped in which the transformer tank is kept completely filled with oil from a conservator (tank) above the level of the transformer tank cover A

portion of the volume of the conservator is pied by air, which breathes in and out as the oil volume changes with temperature The air is prevented from contact with the oil by an imper-vious diaphragm or air cell

occu-This system has its own drawbacks The vator must be configured and located with respect

conser-to the HV bushing terminals conser-to maintain the quired air-strike distance from terminals to grounded metal Given the manholes, pressure

re- ·

.f I

- - - · >

· ··

Courtesy of Westinghouse Electric Corp., Pittsburgh, Pa

Figure 2-2 Transformer With Inert Gas Oil PreseNation System relief diaphragms, lightning arresters, and, in

some cases, isolated-phase bus enclosures on and

around the top of the transfonner, the proper

con-figuration and location of the conservator may be

difficult to achieve in some applications In

addi-tion the diaphragm or air cell may not remain

per-manently impervious The bottom of the air cell

rests on the surface of the oil The float of the

liq-uid level gage, also riding at the oil surface, rests

against the underside of the air cell If the air cell

develops a leak, it will gradually fill with oil and

sink below the surface of the oil, carrying the float

downward The liquid level gage alann will

oper-ate indicating either a damaged cell or low oil level

Access openings are provided at both ends of the

tank for tank cleaning or air cell inspection This

system has been widely accepted

A transformer using the modified conservator system is shown in Figure 2-3

2.9 TRANSFORMER CONNECTIONS

Any three-phase transfonner winding may be nected in delta, wye, or zigzag; it may even be con-nected in aT connection, which is sometimes used for grounding transformers The relative phasing between primary and secondary may be zero or any

con-multiple of 30 electrical degrees Few of the many possible combinations are used in power plants

· A UT, also called a generator step-up or main transformer, is a transfonner (or bank) used to connect the generator to the HV system

Courtesy of General Electric Co Bridgeport, Conn

A VAT, also called a normal station service

trans-former, is one (usually fed from the main

genera-tor leads) that supplies power to the unit

auxiliaries

An SST, also called a reserve station service

transformer or startup transformer, is one that

supplies power from a station HV bus to the plant

auxiliaries

The phasing relationship between primary and

secondary windings of a three-phase transformer

is expressed in terms of terminal designations, for

which the standard convention is as follows: If one

is facing the LV side of the transformer, the HV

terminals are Hl, H2, and H3 from left to right and

the LV terminals are Xl, X2, and X3 from left to

right, as shown in Figure 2-4 More extensive

in-formation may be obtained from Reference 9

'Iransformer winding phase relationships are

shown on the transformer nameplate

The terms primary (winding) and secondary

(winding) are necessary in discussing transformer ratings A transformer is fully loaded when its secondary winding is carrying full-load current The terms HV and LV are necessary in discussing phasing, because ANSI standard phasing requires the HV to lead the LV by 30 electrical degrees, regardless of whether the HV winding is the pri-mary or the secondary

'IJpical phasor diagrams of connections used for transformers in power plants are shown in Fig-ure 2-5

UTs Most UTh, whether three-phase units or banks of three single-phase units, are connected in delta on the primary (LV) side and in grounded wye on the secondary side In any wye-delta, delta-wye, or

wye-zigzag transformer, unless otherwise

speci-fied, the secondary voltages at terminals Hl, H2,

and H3 lead the primary voltages at terminals Xl,

X2, and X3, respectively, by 30 electrical degrees

(Figure 2-5)

The grounded wye connection of the HV

wind-ing permits gradwind-ing its insulation from specified

BIT at the terminals to a lower BIT at the grounded

neutral The delta connection of the LV windings

provides a low-impedance path for zero-sequence

and third-harmonic currents, thereby facilitating

selective relay tripping for single phase-to-ground

faults on the HV system and improving secondary

voltage waveform The UT primary is usually

im-pedance grounded at the generator neutral For

other types of transformer neutral grounding see

Volume 8, Station Protection

UATs

UA'IS are most frequently connected in delta on

the primary side and in wye on the secondary side

but with Hl, H2, and H3 voltages lagging Xl, X2,

and X3 voltages by 30 electrical degrees The

wye-connected LV windings permit some form of

neu-tral grounding to facilitate selective relay tripping

for single phase-to-ground faults on the

medium-voltage auxiliary power system The lagging phase

angle may be the simplest method of placing UAT secondary voltages in phase with SST secondary voltages in typical cases (Figure 2-5b)

SSTs SS'Th are usually connected in grounded wye on the HV side to permit the use of graded insula-tion The LV windings may also be wye connected

to provide for a three-phase, four-wire system or for neutral grounding If the source of the SST is the same HV bus as the one receiving the genera-tor output, the phasing shown in Figure 2-5c may

be used This connection results in a secondary voltage in phase with the output of a UAT phased

as shown in Figure 2-5b

A wye-wye transformer in this application does not necessarily require a delta tertiary to provide

a low-impedance path for zero-sequence currents

A three-legged core design, most frequently offered in this size range, provides a virtual ter-tiary sufficiently well coupled to the other wind-ings to present a low impedance as compared with the neutral grounding resistor usually applied on the secondary side

If the HV source is different from the one to which the UT is connected, it may be necessary

to use a delta-connected secondary for correct

H1 X1

H1

a Unit transformer b Unit axiliaries transformer

c Station service transformer d Station service transformer

e Station service transformer Ill f Secondary unit substation transformer I

g Secondary unit substation transformer II h T-connected grounding transformer

Figure 2-5 Typical Transformer Phase Relationships

phasing, in which case a separate grounding

trans-former is required to derive a neutral Alternatively,

a zigzag-connected secondary can provide the

same phasing as a delta and would provide the

neu-tral, but it may be the more expensive alternative

Figure 2-Sd) If the voltage of the other source is less than 230 kv; a delta-connected HV winding (which sacrifices the graded-insulation advantage) with a wye-connected secondary permits the same phasing at a lower cost than a wye-zigzag design

SECONDARY UNIT SUBSTATION TRANSFORMERS

Secondary unit substation transformers are nearly

always connected in delta on the primary side, the

source voltage being low enough to remove any

advantage in grading the insulation (that is, using

a lower voltage insulation at the end of each

wind-ing) The secondary may be either delta or wye

The wye connection is required if the LV neutral

is tQ be grounded, if a voltage regulator is to be

connected into the phase windings at the neutral

ends, if a four-wire system for phase-to-neutral

single-phase loads is required (Figures 2-5f and

2-5g) Relative phasing of LV networks in power

stations is seldom important, because they are

seldom interconnected

GROUNDING TRANSFORMERS

A zigzag-connected autotransformer may be used

on a three-wire system to derive a neutral for

grounding The T connection is sometimes

pre-ferred when there are no phase-to-neutral loads,

because it permits the use of a two-legged core

with a single winding on each core leg, resulting

in a less expensive design The neutral connection

is made to a tap on the stem of the T (Figure 2-5h)

2.10 TAPS

A power transformer may have taps in either

pri-mary or secondary windings so that its effective

turns ratio may be changed In power plants such

changes are not usually required to establish a

new output voltage; instead they are needed to

reestablish the desired output voltage after a

departure due to a change in source voltage or

in load-related impedance voltage drop If tap

changing must be done while the transformer is

loaded, special switching equipment is required

to transfer load current from one tap to another

without an interruption of service This is called

tap changing under load

NO-LOAD TAP CHANGERS

No-load tap changers employ manually operated

switching equipment that changes the turns ratio

of the three phases simultaneously and by the

same amount In the case of single-phase

trans-formers, each has its own manually operated

no-load tap changing-switching device The no-no-load

tap changing-switching device is in the tank with the core and coils in both three-phase and single-phase transformers The no-load tap changer can

be operated only when the transformer is gized Conventionally, a transformer has two full-capacity 2%% taps above and two below rated volt-age In a step-down transformer the taps above rated primary voltage are less likely to be used than those below it For that reason some pur-chasers prefer to specify one tap above and three taps below rated voltage, an option available at no change in price The taps may also be ordered closer together than 2%%, an option usually avail-able without price premium The taps can be omit-ted altogether with a saving in the price of the transformer Both of these last two options are worth serious consideration in power plants The use of no-load taps in a UT (generator step-up transformer) is a special case, because the

deener-HV winding that nearly always contains the taps

is the secondary This case is discussed in greater detail in Section 2.13

LOAD TAP CHANGERS (LTCs) LTCs are often used in distribution substations but are seldom used in power plants (10) In the United States the conventional LTC has 32 taps at %% spac-ing, 16 above and 16 below rated voltage, to pro-duce a voltage range of ± 10% The transformer may have reduced capacity on taps below rated voltage Where an LTC is used on a power plant transformer, its purpose is not to alter the volt-age supplied to utilization equipment but to re-store that voltage after a change in load or in the source voltage supplied to the transformer wind-ing has occurred The tap changer should be on the transformer primary whenever possible If it

is on the secondary, the rated kilovoltamperes may not be available under heavy load conditions

As an illustration of this point consider a 12-MVA, 24- to 4.16-kV transformer connected to the leads of a 24-kV generator and fitted with a secondary LTC The (full-kVA) tap voltage and cur-rent ratings will be as shown in the following abbreviated table:

Assume, for simplicity, that generator voltage

remains at 24 kV and that the set point of the

contact-making voltmeter controlling the LTC is

4.16 kV

At no load the tap changer would remain in the

neutral position because secondary voltage would

match set point At full load the secondary voltage

at the 4-kV bus might be reduced 5% by voltage

drops in the transformer impedance and

second-ary leads impedance The LTC would compensate

by moving to Thp R8, which has a voltage rating

of 4.368 kV and a current rating of 1586 A The

actual load current is 1665 A, a 5% overload The

situation becomes worse if the generator is

oper-ating at 95% voltage

This problem does not arise if the taps are on

the primary The secondary voltage rating and the

voltmeter set point would both be 4160 V The LTC

tap required to produce rated secondary current

(assuming power factor 0.8 or higher) must be

within the tap rating, because the transformer is

output rated

LTCs are usually equipped with automatic

con-trol equipment to maintain a manually preset

secondary voltage This equipment usually

pro-vides for remote control and indication of tap

position The control typically includes an

auto/manual transfer switch, a raise/lower control

switch, a set-point adjuster, a tap position indicator,

and position limit-indicating lights The equipment

also provides maintenance adjustments for

dead-band, starting time delay, and time delay between

tap changes The dead-band and time delays

re-duce wear and tear from unnecessarily frequent

operation during brief voltage transients With

usual adjustments the dead-band is on the order

of 1 %; the starting time delay is about 30 s, and the

time between tap changes is 1 to 1 Yz s

Addition of an LTC to a power transformer

in-creases its cost by approximately 40% The

addi-tion of electromechanical switching equipment to

an otherwise essentially static device increases

maintenance cost In addition the moving parts

and the extra winding taps, which raise

mechani-cal and electrimechani-cal stress, may have a significant

im-pact on reliability

If an LTC is used on a power plant SST, the time

delays may have special significance, as discussed

in Section 2.15

The LTC switching equipment is located in a

sep-arate oil-filled compartment connected to the

transformer main tank

Figure 2-6 shows a power transformer with an

LTC The latter is located in a separate

compart-ment, throat connected to the transformer tank, below the top of the tank

2.11 BUSHINGS

Bushings are used on liquid-immersed ers to carry the winding terminal connections through the grounded metal cover or sidewall of the tank A porcelain rain shield over the exter-nal portion is skirted to provide a long surface creepage path from terminal to ground flange The internal portion below the ground flange is generally immersed in the transformer insulating fluid This portion may also be encased in porcelain

transform-HV bushings are of the condenser type, ated with layers of oil-impregnated kraft paper Copper or aluminum foil layers of graded axial length in the paper insulation structure distrib-ute electrical stresses and control voltage gra-dients The shell is filled with oil to keep the paper saturated, and the outer terminal is fitted with an oil level gage or sight glass A cushion of dry nitro-gen above the oil allows for thermal expansion and contraction of the oil This cushion is sealed at a pressure above atmospheric pressure to exclude air and moisture Bushings of this type must be shipped and stored in a nearly upright position

insul-to prevent dryout of any of the layers of paper

In bushings rated 115 kV and higher one of the foil layers is made available as a bushing potential tap through an insulated conductor just above the ground flange This tap must be impedance grounded through an external potential device or solidly grounded by a grounding cap whenever the bushing is energized Condenser-type bushings (Figure 2-7) rated below 115 kV, down to and including 15 kV, have a power factor tap The power factor tap connects to the ground layer of the capacitor core An aluminum cap covers the insulated power factor tap assembly and grounds the tap connection when it is not in use Bushings are of two types, depending on their provision for connection to the transformer wind-ings In a fixed-conductor type the central tube or rod conducts current from the top terminal to the bottom terminal The winding lead is connected

to the bottom terminal In a draw-lead type the winding lead is drawn upward through the cen-tral tube and connected to the top terminal Fig-ure 2-7 shows a bushing with a threaded copper tube that can be used with a fixed-conductor or

a draw-lead type connection Figure 2-8 is an

extra-high voltage (EHV) bushing of the draw-lead type

HV bushings are generally selected to have the

same BIL as that of the transformer HV winding

For situations in which the atmosphere is highly

contaminated with particulate matter or for

high-altitude installations it may be desirable to use

bushings having a longer porcelain rain shield If

this aim is achieved by using bushings with a

higher BIL than that of the winding, the lower

por-tion of the bushing will also be longer, requiring

a taller tank, which may exceed shipping

limita-tions The alternative is an extra-creep design, in

which the rain shield is taller but the portion

in-side the tank is not extended

Lower-voltage high-current bushings, which are

used on the primary terminals of UTh, are generally

fixed-conductor, bulk type, again with porcelain

rain shields and oil impregnated (Figure 2-9) Such

bushings are not usually equipped with oil level

Courtesy of McGraw-Edison Co., Pittsburgh, Pa

gages, but oil leakage has occasionally been a lem There could also be a heat dissipation problem

prob-if bushings with a lower temperature rating are connected to isolated phase bus conductors oper-ating at 105°C 'll'ansformer specifications should state terminal conditions

Secondary bushings on UATh and SSTh are of the porcelain type, at least 110 kV BIL, and are some-times mounted in the sidewalls of the tank below transformer oil level Faulty seals in such bushings have caused fires in a few cases when transformer oil leaked through a bushing seal into a cooler con-trol cabinet

Bushings are manufactured in accordance with the requirements of ANSI/IEEE Standard 24-1984

(11) and tested in accordance with requirements and test procedures of ANSI/IEEE Standard 21-1976 (12)

See Section 2.22 for bushing maintenance

Figure 2-6 Power Transformer With LTC

* POC design

Clear/view oil reservoir

Nameplate

Mounting flange/ground sleeve assembly

Bottom coo assembly*

Courtesy of Lapp Insulator Co LeRoy, N.Y

Gaskets High compression coil springs *

housing

Bushing potential tap

.,_ _ Paper-foil capacitor core

+ Lower porcelain assembly *

Courtesy of Lapp Insulator Co., LeRoy, N.Y

Figure 2-8 EHV Bushing

2.12 ACCESSORIES

The accessories described individually in the lowing subsections are available for large liquid-immersed transformers Few of them are applica-ble to dry-type transformers

fol-LIQUID LEVEL GAGE

The typical liquid level indicator is a sealed ment body Inside, an indicating needle sweeping

instru-a cinstru-alibrinstru-ated scinstru-ale is minstru-agneticinstru-ally coupled to instru-an ternal pivoted float arm, with the float at the top surface of the insulating fluid The scale is marked

ex-to indicate high, low, and 25°C levels The dicator includes alarm switches

in-For a transformer with an inert gas oil vation system, the indicator is mounted at the top

preser-of the transformer tank wall For a transformer with a conservator or constant oil pressure sys-tem, the indicator is mounted on the conservator

One indicator (Figure 2-lOa) displays the top oil temperature In the other indicator, called a wind-ing temperature or hot spot temperature indicator (Figure 2-lOb), the well is heated electrically by current proportional to transformer load, supplied

by a current transformer The electric heating simulates the winding hot spot rise over top liq-uid temperature In some cases the heater leads are extended to an external terminal box for shunting by a calibrating resistor The initial value

of the resistor is calculated, but it may change ing the temperature rise test, if made There have been instances in which no temperature rise test was made on a particular transformer and the hot spot indicator gave false indications of overheat-ing in service until the calibrating resistor was replaced

dur-Oil filler cap

J

Switch-setting tabs

Indicating unit

reading pointer Credl Reset-shaft cap and gasket

Maximum-Temperature detector

Union connector

a Courtesy of Westinghouse Electric Corp., Pittsburgh, Pa.; b Courtesy

of General Electric Co., Bridgeport, Conn

Figure 2-10 Temperature Indicator Relay

FLOW INDICATOR

'Ii'ansformers employing forced-oil cooling may be

equipped with a flow indicator, including alarm

switches, for each pump 'JYpically, the indicator

is a vane-operated instrument mounted on the

pump discharge pipe The scale is not calibrated;

it merely shows whether there is oil flow from the

pump

BUSHING CURRENT TRANSFORMERS

A bushing current transformer consists of a short sleeve of magnetic material with a distributed toroidal secondary winding It is supported below the cover of the transformer tank at the bushing opening so that the bushing lead, passing through

it, acts as a single-turn primary Where required, two or three current transformers can be installed

at each bushing One of them is likely to be used for transformer differential relays Good relaying practice prohibits putting any other burden on such current transformers (Volume 8, Station Pro-

tection) Another might be used for other relays, and a third might be used for metering Most bushing current transformers are provided with taps for multiratio ratings

As with other current transformers, bushing current transformer secondary windings must be short-circuited when no burden is connected, be-cause their open-circuit voltages may be high enough to be dangerous to personnel They may also cause insulation failure

RESISTANCE TEMPERATURE DETECTORS Where remote indication, recording, or data log-ging of top oil or winding hot spot temperature

is desired, the local temperature indicators can be supplemented or replaced by 10-0 copper resis-tance temperature detectors In general it is not feasible to embed such detectors in the trans-former windings They should be located in the wells just below minimum oil level (13)

SUDDEN PRESSURE RELAY

A sudden pressure or fault pressure relay ure 2-11) responds by rapid closure of an electrical contact to sudden pressure rise in the liquid in which its sensing element is immersed Designed for mounting on the transformer tank wall near the base or on a valve body, it senses the pressure transient produced by an internal arc

(Fig-Because some of the early sudden pressure relays were prone to operate erroneously under other conditions, many users wired them for alarm only The modern relay has been made in-sensitive to mechanical shock and vibration, pump surges, and normal pressure variations caused by transformer temperature changes User confi-dence has been restored; some users now regard

it as a sensitive and reliable primary protective device and wire it for breaker tripping to isolate

a faulted transformer

Courtesy of General Electric Co Bridgeport Conn

GAS DETECTOR RELAY

A gas detector relay (Figure 2-12) collects bubbles

of gas generated below liquid level and closes an

electrical contact when a significant gas volume

has accumulated Since most of the combustible

gas is generated by the decomposition of oil or of

solid insulating materials, relay operation may

pro-vide warning of incipient dielectric failure Gas

bubbles that do not indicate decomposition may

form when there is a rapid change in temperature

Since the gas detector relay does not discriminate

between combustible and noncombustible gas, it

might operate in either case Determining whether

the gas evolution is a matter of concern requires

that a sample be collected for mass spectrometric

analysis in a laboratory

FAULT GAS MONITOR

A combustible gas monitor that continually

monitors the levels of dissolved hydrogen, carbon

monoxide, acetylene, and ethylene gas in oil is

available commercially The device mounts on the

transformer with the electrochemical sensor below

the oil level It is provided with dual-stage alarm

circuitry for early incipient fault warning The

monitor is shown in Figure 2-13 Sixty of these units

have been installed at a major American utility to

protect current transformers that have a history

of generating high quantities of hydrogen before

failure These monitors have operated flawlessly

to indicate sudden increases in hydrogen As a sult it was possible for the current transformers

re-to be removed from the circuit before failure Information on combustible gas analysis and in-terpretation is given in Kelley's article "'Transformer Fault Diagnosis by Dissolved Gas Analysis" and in ANSI Standard C57.104-1978 (14)

PRESSURE RELIEF DEVICE One or more pressure relief devices (Figure 2-14) may be installed in openings in the transformer cbver to relieve dangerous pressure that may build

up within the tank The device consists of a loaded diaphragm, automatically reset, with a mechanical semaphore to indicate that it has oper-ated, and alarm contacts Because these devices are of a standard size, with limited relieving ca-pacity, it may be advisable to install several on a very large transformer to prevent a rupture of the tank during a transformer fault

spring-Courtesy of General Electric Co Bridgeport Conn

_

\

Courtesy of Syprotec Corp., Rouses Point N.Y

LIFTING EYES AND JACK BOSSES

Lifting eyes and jack bosses facilitate the handling

of the transformer during manufacture, loading

for shipment, unloading at destination, and

instal-lation In some cases jack bosses have been

mounted so low on the assembly as to require

toe-jacks, which are less commonly available t1:J.an

con-ventional hydraulic jacks Large transformers can

be damaged seriously when conventional jacks are

applied under protrusions not designed for this

purpose

'Iransformer outline drawings should be

exam-ined carefully for these features In some cases the

manufacturer may be able to revise the design to

provide greater clearance under jack bosses if the

problem is identified before tank fabrication

LIGHTNING ARRESTERS

Lightning arresters are most effective in

protect-ing transformer insulation from surge voltages if

they are installed very close to the winding

ter-minals For this reason it is common practice to

purchase the arresters with the transformer and

to require mounting brackets for them on the transformer tank For selection of arrester ratings see Section 2.5

2.13 APPLICATION CONSIDERATIONS

When a source of electric power at one voltage level is required to serve utilization equipment designed for another (usually lower) voltage level,

a transformer is required between source and· load Selection of the proper transformer requires consideration of the following elements:

• Maximum sustained load

by chemical processes at a rate that is a function

of absolute temperature The relationship of time-to-end of life versus temperature is linear when plotted on appropriate scales This line is called an Arrhenius curve For a particular insulat-ing compound or system the slope of the Arrhen-ius curve is determined at two or more elevated temperatures The temperatures are selected to produce failure (end of life) in an acceptable time period, but the temperatures are not so high that they produce phase changes in the material An

end-of-life condition is usually defined in terms of mechanical properties of the insulation When the insulation becomes too brittle to remain in place during the vibration, shock, and thermal expan-sion that are charactistic of a normal load cycle,

a dielectric failure is imminent (13, 15)

When transformer windings are below the perature for which they were designed (because

tem-of low ambient temperature or because of a prior