transformer trouble , test , drying , parallel operation

8/11/2019 Transformer Trouble , Test , Drying , Parallel operation

1/35

Transformer Troubles

a)

Star-siar

transformers.

-

It should

first

be

noted

that

in

the

case

of three-phase

transformers,

ofnecesiit),.

in

ph4ge

in

alllblgg_lh3 9s

at

the same

time,

and

hence

in

the

case ofstar

connected

windings

no third

harmonic

voltage

can

exist

between

lines,

although

they

are

present between

each terminal

and

the

neutral

point.

Similarly,

the

third

harmonic

components

of

current

can

only

exist

if

a ieturn

path

is

providdvia

the

neutral

point. Thus

if a star-star

connected

transformer

has

both

the

primary

and

secondary

neutral

points insulated,

no

third

harmonic

currqntf

san

flaf

L

rr:sultirrg

in

the

presence

of

triple

frequency

voltage

between

the

secondary

terminals

and

the

secondary

neutral

point. The

line voltages

will

be sinusoidal,

but

the

neutral

point

wi11, instead

of

being

at zero

potential, osciilate

by

an amount

equal

to

the

third

harmonic

component.

If,

however,

the

primary neutral

point is connected

to

the

generator

neutral,

a

path

for

the

flow of

the third

harmonic

component

of the

magnetizing

current

will

be

provided

and

the wave

of

flux

and induced

voltages

on

the

secondary

side

will

be

fiee

from

harmonics.

Similarly,

if the

secondary

side is

used

as

a

three-phase

four-wir::

s,rzstem,

a path

for

the

flow of

a

third

harmonic current

will

be

provided

via

the

load,

and

in

this case

both

third

harmonic

currents

and

voltages

exist

on

the seconciary

side.

Because

ofthis

objective

feature

star-star

connected

transformers

are seldom

used.

If,

however,

connected

in

a

tertiary,

or

stabilizin.g

winding

is

provided on

the

delta,

a

local circuit

i

Such an

auxiliary winding

can also be

used

to

supply power

for

local

putposes

at

a

voitage other

than that

of the

primary

or

secondary

windings.

The

minimum

capacity

of the stabilishing

winding,

apart

liom

any

consideration

of

the supply

of

external

load

currents,

is determined

by

the third

harmonic

compQnqnl-oflhe

-qlaglslzi]u

current required.

A stabilishing

winding

also introduces

additional

reactance

eflect

between

the

incclming

and

outgoing

lines,

and serves

as a leactor

on

the

system.

Star-interconnected

star

transformers.

-

This

is a

useful

type

of

winding

as,

due

to

the

interconnection

between

the

two

halves

of

each

phase winding

on

the

secondary

side,

the

third

harmonics

ofthe

voltage

cancel

out,

so

that

no

third

harmonic

components

of

(b)

{"'dgx-:*


2/35

(c)

voltage

can

exist between

the

terminals

and

the

secondary

neutral

point. This

allows

three-phase,

four-wire

unbalanced loads to be supplied

without the

objectionable

presence

of

circulating

currents. In spite

of this, however,

the

possibility

of

neutral

inversion still

exists

with

this type of

comection.

Delta

connected

transformers.

-

Transformers

which

have

a delta connected

winding

on

either the

primary or the secondary side or both,

provide

a

path

for

the

flovv

of

the

requisite

third

harmonic

component of

the magnetizing

current,

thus relieving

the

supply no-load

current of

the

necessity

of supplying

this.

Delta-star

connected

transformers are the

most common,

as

the secondary

will

provide

a three

phase, four-

wire system

free

from third

harmopic

voltages

between

line

and neutral.

Star-delta

transformers

are

only

suitable

for balanced

loads.

The

primary

current

is sinusoidal,

and a third harmonic

circulating current

flows in the secondary

delta

connection

without

appearing

in

the secondary

lines.

The

presence

of

this

third

harmonic

circulating

current

coffects

the flux

curve

to an approximately

sine shape,

and thus

suppresses

the

third

harmonic

induced

e.m.f.

A condition

of

balance

between

third

harmonic magnetizing

current, and an

approximately

sine

shaped flux

wave

is

thus

established.

In the

case

of

a

delta-delta connected

transformer,

the

third

harmonic

component

of

the no-load magnetizing current

is

shared between

the

two

windings

depending

on

their

respective impedances.

If

third

harmonic

currents

are

present

in an

abnormal degree,

they

may

prove

objectionable,

due

to the

increase

in

transformer

or cable

losses. W}'ere

third

harmonic voltage exist

on

the

secondary side,

they may result

in

heavy cable

charging

currents,

due

to

the reduced

triple

frequency

capacitive reactance

of

the

cables,

while

in the

case

of condensers

for

power

factor improvement

excessive

currents

may

be

taken

with

resultant

overheating,

which

has been

known to

cause eventual

breakdown

of

the

insulation.

Some

telephonic

disturbances are also traceable

to

third

harmonic

pressures

in transformers. It

is also interesting

to

note that

third

harmonic

components

will be

prominent in switching in current rushes, where

paths

for their:

flow

are

provided,

due

to

the

saturation

of

the

iron

circuits

during this transient

period.

t.

dfu*--=y


3/35

Sglg

lntp15[4

rlspects

of

-

:,-:

,,..,

Il44lformers

.

,,,.,.

What

is

transformer

Noise?

'i.he

main

source

of

sound in

a

transformcr

is

a

magneto-strictiorr. Tliis

is thc

properly

of

eiectr.ical sheet steel

whicir oi lrses

it to

elongate and

contract

in

the

preselce

ol

the

n.ragnetic

field. These

periodic

mechanjcal nlovements produce

sound

vibrations

in the

core,

having

a

ftindamental freciuency

equu) to

twice the

flux

frequency.

l)ue

to

a non-linear

relationship

between n.ragneto-striction

and

llux

ciensity a

number 1b harmonically

related

irigher

frequency

are also

produc.':d.

This results

in

a

noise

spectrum,

composed

of fr"tndamental

frequencies

of

120 cycles

per

second and

harmonics

of

240,360,480,600 etc.

The

pulsations

of

the

core also

travel

through other

parts

of

the transformer,

which have a

tendency

to vibrate

at one or other

ofthese

same

frequencies.

These

add

to

the

overall

sound

level

in

the forrr:

of

interlaminar core and

coil

vibrations,

enclosure,

and

sub-asscmbly

vibrations.

The

amount

of

thi'r

rLddition

is dependent

upon the surfaoe

area and resonant

frcquency

of

eac,h componeni

part.

Holir to control

transformer

sound and

minimize

its transmission

-

As

outUned

above, there

are four main sources

of

sound

in

a

transformer:

i.

Core

vibrations

2.

Interlarninar

core anci

gap

vibrati,rns

and coil vibrations

3.

Subassembly

vibrations

4.

Enclosurevibrations

I,Iever

Make Megger

Tests

uuder Vacuum

or Without

Oil

The

great

the vacuum

(r'nless

it's

perfect)

the

Iower the spark-over level. At

tlre vacuum

values

normally

reached in

li:r: fi.:ld,

it

is

very likely the

megger

learlings

.,vi11

L;e

zero;

in

other u,urds,

rreeping

or

flashitg to

gouncl

will occur.

Breakdown strength

under vacuuur is ir

function

of both

pressure

and distance.

It

is not

our intent at

this poirt

to

go

into

a

long discourse

on the

phenomenon

of

voltage

Lreakdown

but

simply to

CAUT-IO|'I

yott

not

Io

megger

a

unit

while

under

vac;uum.

It

is

also

recommended

that you

do

nct

rnegger

a

unit in

air

because

the values

may

lead

to

wrong

conclusions and will not

agree

with

readings

taken

at

a later

date

when they

are

taken

under

oil.

Readings taken

in oil

are

about ll2to

ll4

that of the insulation resistance in

air.

Evaiuation

of

Insulation

Megger

readings,

power

factor

readings,

and

oil dielectric

breakdown values are

inlpofiant

indicators

as to

whether

or not

a

traxsformer

is wet.

The

dielectric

strength o{

oil

for high

voltage

oorver transfonrers musi

be 30kv

or belter before it

is

put

into

a

new

unit

in

the ficid.

The megger

readings on high voltage power

tlansformers,

when corrected

Io

20"C, should be

1

megohm

per

kv

or higher.

.'

Whal is traflsfor

er Noise.doc


4/35

o

On

EHV

transformers

a

minimum value

of 3 megohms

per kv is

required.

Several

one

minutes

readings

should be taken

with

a

motor

driven

megger

and

the

megohm

results

oornpared

rvith factory

data. Power

factor

should be

less than

2% on

oil-filled

transformers

q,itl.r

two

windings

and

less lhan

2-1l2lo

on

units

with load

tap

changers when

corrected

to

20"c

In

sul ation

There

is not

much

that

can be done

in

the

way of

maintenance

of

the solid insulation

of oil-

insulated

transformers

and regulators,

but

correct

load and

temperature

control can

do much

to keep insulation

aging

within normal limits. Aging of insulation

is

affected

by

the operating

temperature

of the

windings and

the

condition

of the oil

in which

it

is submerged.

Oil

of high

acidity

will

accelerate

again

of

the

solid

insulation.

The

aging

rate

of

organic

insulation

doublre.for approximately

cach

8"C increase in its

temperaturejthe

lower

the

operating

temperature,

the longer

the insulation life.

Since

aging

is

so much

more

rapid at

the higher range

of temperature,

excessively

high

temperatures

should

be

avoiJed as

much as

possible.

On

the

other

hand,

little

is

gained

by

having

insulation

life

exceed the obsolescence life ofthe

apparatus.

Maximum

life is assured

only

under

uniform,

normal loading. Therefore, loading

should

be controlled

as much

as

possible to

obtain

that

objcctivc.

1'he Secondary

of

a

current transformer (CT)

should not

be

open-circuited

under:rny

circumstances.

Why?

If

the

CT secondary

becomes

open

circuited,

a

high voltage

will

exit

across the

secondary

because

the large

ratio

of se,,ondary to

primary

turn causes the

transforrter

to act

as a step-up

transformer.

Also,

since thc

counter ampere-tums

ofthe

secondary

no longer

exist,

the flux in

the

core, instead

of

being

due

to the

difference of the

primary and

secondary

ampere-turns,

will

now

be due

to

the total

primary

ampere-turns

acting

alone.

This

causes a large

increase

in

the

flux,

producing

excessive core

loss

and heating,

as well as

a

high

voltage across

the

secondary

termjnals.

Ovei-excitatioir

Capacity

and

Inrush Current

The

extent of

which

the transformer can be overexcited

is

determine

by the highest

permissible

flux

density.

With

the

increase of flux

density

the

no-load

current and

its

harmonics

rise,

and

also

the

losses

in

the

in

the

iron

cor:e,

the

windings

and

the

structural

parts.

The

flux density

is

proporlional to

the Voltage

applied

and

inversely

proportion

to the

frequency

of

that voltage.

The applied Voltage may

deviate

permanently by

J:5%

from

the

rated

voltage

and

the frequency

may deviate permanently by

+2.5'y.

from

the

rated frequency.

When

transformers

are

switched

on, an increased

current of

exponentially

decreasing

amplitude

may

appear.

The magnitude

of

this inrush

current

depends

on the

instantaneous

value

of

the

voltage

and

the

residual

magnetism

of

the

transformer

at

the

moment

of

switching

on.

what is transformer Noise-doc


5/35

*a-

#l

In

the

500

to

1,000

kvA range,

the

amplitudes

of

the

inrush

curents

decay

to

half

their

original

vaiue

after

8

to

10

cycles.

With

units

of

1,000

to 10.000

kVA

this

decay

takes

10 to

60

cycles

and

with

larger

powers 60

to

3,600

cycles

Overload

Capacity

Transformers

can

be

temporarily

or

pelm',nently

overload,

depending

on

the

initial

ioading,

the temperatures

of

the

cooling

media

and

the

permissib:e

maximum

values

of the

winding

temperatures.

If

the

permissible

windings

temperatures

are exceedecl,

the

normal

lile

of

the

transformer

is

shofiened,

unless

the oveiloading

is

compensated

by

periods

of

lower

load.

If

transformers

are

to

be

overload,

it is

important

to

check in

advance

whether

all

parts

of

the

transformer

and

the

equipment

are capable

of

meeting

the

demands.

made

on

them

(for

instance,

bushings,

tap-changers

and

cable end-boxes).

Table

below

contains

representative

values

for

the period

of

overloading

of

oil-immersed

transformers

in relalion

to

the

previous load

and

the

oil temperature

at

the

beginning

of

overloading.

The

averago

winding

temperatures

do not

exceed

105"c

if

the

average

temperaturE

of the

cooling

medium

during

the

period

in

question

is

25oC.

If

transformers

with

F-cooling

are

operat-d without

fans,

the

overloads

and

the

previous

permanent

loads

should

be relatedlo

600/o

ofthe rated

power

ofthe

transformer.

Transformer

Noise

The transformer

noise

is

majnly

genelated by the

magnetostriction

qf

the

iron

core.

It is

dependent

on

flux

density.

changes

of 10%

in

flux

density

cause

of

change

of2_to

3

dB

(A)

in

the noise

level within

the

12

to

16

kGs

range.

The radiated

sound

spectrum

is between

100

and

500H2.

Table:

Duration

of Overloading

of

Oil-immersed

Transformers

Previous

continuous

loading

as

Yo of

rxled

power

Oil t

nr

p

c ralLr re at the

L,.ginning

of the overload

in

'C

for

cooling

medlod

tlrrratirin ofovcrload

lor an overload,as

oZ

of

raied

Po\Yer,

ol

SC, FC, WC

10%

200/. 30%

40%

5OVr

h

h

mtn

mln

mln

50

55

49 3

1.5

60 30

l5

75

68

60 2

1.0

l0

t5

90 78

68 I

0.5

15

8

4

I

VN|L"'

V{% Vt t

Y

{r:t

what

is

transforner Noise.doc

(f


6/35

No

-Load

Losses of

Transformers

The noload

losses

is the

loss measured

by a wattmeter

when rated

voltage at rated

frequen^cy

is applied

to either

winding, the

other winding

being open-circuited.

It includes core

loss, I2R

loss

in

the

winding

due

to exciting current,

and

the dielectric

loss in

the

insulation. TlTis

no-

load loss

appears in

the transflormer,

independent

olthe

load and

for

all

practical

purposes

is

independent of

temperature.

The

core

loss

consists

of hysteresis

loss and

eddy-current

loss.

The hysteresis

loss

is thc

energy

required to

magnetize

the core

first in one

direction

and then

in

the

other.

It increase

directly

with

the

frequency

and

depends on

the maximum

flux

density

and

the

quality

ofthe

core

steel.

The

eddy-current

losses

are

produced by eddy

currents

which are

set-up

in the core

in that

iron

is

a

conductor

ol

current as-well-as

magnetic

flux.

The change offlux

in the core

induces

a

voltage

in

the core

which

causes

eddy current

to flow.

This

loss is

proportional

to

the

square

of

the

eddy current

or

the

square of the

induced

voltage and

is

therefore

proportional

to the

square

of the

flux

density

and

the frequency.

Exciting

Current and

Flux

of

Transformer

The current

which flows

under no-load

conditions

is called

the exciting

current.

'fhe

exciting

current

is

actually

made

up

of

two

components.

One component

supplies the

losses

in

the

core caused

by hysteresis

and

eddy

curents,

and

the other

component

produces

the

magnetizing

force

in

the core.

The

alternating

current voltage

in the

primary winding

of

a

transformer

produces

a

quantity

termed

"FIux"

in

the

core.

This oscillating

llux

is

what

causes the

voltage to

be

produced

on

the secondary

winding

ofthe transformer.

Load

Losses oI

Transformer

'fhe

load losses are

the

losses measured

by

a

wattmeter

when sufhcient

voltage

is

applied to

one

winding to

produce full-load

current

in

the

other winding

which

is short circuited.

The

load loss consists

of

the

I2R

loss

in

the

winding

due

to full-load

current

passing through

the

windings

and a

stray

loss

which

is produced by

stray

flux

inducing

currents in

the winding.

The

stray

losses can

be reduced

by

)aminating

and

transposing

the copper

in the

windings.

The stray loss

may vary

lrom 10 to

20

percent of the

I'R

loss at

full-load

current.

Both losses

are affected

by

changes

in

temperature.

The I2R

loss increases

with

temperature,

while

the

stray

loss decrease

with temperature.

The load

losses of a transl'ormer

ate

guaranteed

on the

basis of

an average

winding

temperature.

All losses

are dissipated

in

the

form of

heat and

must

be

controlled

if

the

transformer

is

to

be efficient

and

practical

in

design.


7/35

DIELEC

TRIC

ABSO

*--li;

?i

i,.(haoral

A2LI

'fhllo

an

lnsuLatlon

noggor hae

boon

used

for

many

yoars,

thero aro

stlII

nany uE-

bqn

faotore

1n ltg

use

for

moasurlng

the

quallty

of

lnsulation,

espaolally

that

@

h.rgo

genorators.

Thoro

aro

somo

quostlong as to tho

bsst voltage

to

bo usod;

tho

offoot

of

tho

tomperature

of

tho

wlndlngs,

tho

longth

of tlne

roadlnge

should

bo

takon

and

tho froquonoy of

tho

roadlngs,

to what

oitant

duat aBd

dirt

on

the

vladtags

ntght

affoot

tho roadlngs,

and

flnally,

Yhat

should

bs

coasldorod

tho

lcnror ihfts

of usofulness

rond.o .lng

tho replaco:nont

of insulrtlon

advtsablo"

Yftrilo

thors

le Do

cLear-cut

agroomont

on how

to interprot spot

neggior roadlngs,

tho ln-

dratry ic la

egreement

that

perlodlc

roadlngs,

tekon ovsr

a courso

of

yoars

uuder

:

rldlar

couditions,

will

give

a

truo

plcture of tho dotorlorablon

of the lneulatlon

;f;.ud

oau

bs used

to forotell

whsn rewlnding

or

re-lnsulatlon

w111

be noaded.

Ths

t,l*ro

o" loso

aocepted

nsthod

of

rooording

thls

porlodio

Lrformattou

1g

in tho fono

,'lr.o

a dlolectrlc

absorption

curveo

Jlhs rords

rdielectric

aloorptionrt rofor

tq-t-h9,.abq-o1p o3

o

f shargo

ls.

tho.lnsula--

ho rords

"d1e

Ioctric

a_b

s_o_ fllo-n-

_"

ral ar

-l_q_l-_hg,

-aD

q-ofPllofl

-9-r

snar8o I'u.

troe

-

r.nEurn--

193.

bu_t__ sjLeloc.ldq.gb-s-o-qp-t-l.o-n-o-urve

is

u

aually-plcbted..wlth

lnsulatlon

valuea,

ia..Sad

fion

a

mo,ggor,-.p1otto

d

aga{,11g --tlne.-

T-h-l-s-is-.rqq,-b1- ,i1.s--Lrye.r

se-qt-c-{l-

..road

fron

a_ms-Bgorr_

Plo_ttod_aga1:l1s -_ 1nlg.-

:t-t1-1_8_r8-

{oa,r_ly

_ ,n-_Llry9.r

80-ql-q-ql:-

fLt-qqtr:g_a

sof

p-t

io:e.-curvelt-n

th;t it

will

have

a

gradually lacroaslng charaoterlstlo

lOotrlo

abi-orpt

1o]1 curve.r, -n

that it

wlll

have

a

Bradually

lncroaslnB

charaotgrl.s

,

Gh-".rp";f

to

trru r""toh*

3f

u.

gradually

decroaslng.lrrlact3rrltlo

which

would

bo

tho

case

lf

oharging

or'rrabsorbodrr

current

were

plottod

agalnst

tlme"

tAbrgllggl-gg5rglt

lg-y,g

curront

whlch

flows

whon

a constant

poJenttal

ilLtnpr9ls

B9l

u

tho

Instran

or Yolra

Eqr\aPPl

lcallou

at a-varfabG-

ra{e

,

-Adiffi[.fae--on=._

ffd-"66rAEE

eE

o?6F a

c

on

siadtatfe-Ten

ati on

i-varlab1e

factors" Ij_:_q_lr*P_91."d

-

Although,

as

statod

ProviouslY,

roDt

ney

peroist

for

hours,

tho

a

nuoh

ehortar

tims.

Tho sloPe

tbo

roslatanco

at 10

nlnutos

to

fitraffi

ti-mila

c

to

r s

r

-cE-pa

cf tanc e

cur re

nt

.

die

I

o

ot

r I

c

ab

s

orp-

t@apaElEin6i--curro{rstnstrgouirej-t-o-ShaTEo

fEiipliltanoe

of iho

maETno

r,rfnding

to tho

core.

Dloloctrlo

absorptlon

or

polari;lng

current

ls

a curront

caused

-by

tho absorption.of

e]

ectr

lcal.

-

charge

tn

ibo

dinl"it"t",

in

thc

forrns

of

tho oI{igning

of dipolos

or

charged

polar

molocules

throughout

the lnsulatlnS.rnatorial and

the

ptllng of

froe

oloctrons

on

the

lnter-

facosi Loakago

ourront

is oompoood

of

the currant

that

pasies

lnternally

through

tho lnsuLatloi

and

that whioh

passeo

or

loaks

along

the outsldo

.of

ths

insulatLon.

0f

these

dlfforont curronts,

the capacitanco

curront

ls

of

appreoiablo

valus

for

.

oEly

a

fsw

seconds

after

the voltags

ls

appllod,

and

tho Loakago

curreDt

remalns

.oou;tant

as

long

as the voltage

is applled"

Tho

shapo

of

tho

dioloctrlc

absorptlon

./our"g"

thorsfore. ia Indicative

of

tho rats

of flov

of diolectrlo

absorptlon our-

Y

ront.

whloh

hae a

docroasi.ng

oharacterlstic

but with

a much lonor

rats

of

chango

than

has

tlio

capacltancs

current"

In

faot,

in a

good,

dry lnsulatlon

thls

currsnt

nay

docroaso

gradua).Iy

over

a

por.lod

of

ssvsral

hours"

The

rate

of

docreaso

of thls

ourrent

anC,

Joo"uqrr"r,tly,

tho

shapa

of

the

disloctrlc

absorptlon

curvo

ls

usuolly

nors

lndlcatlvo

of

tho

quallty

of the

lnsulatlon

thau

tho

actual valus

of

oirmrg

aoacurod

for

th6 varlous

polnte on

tho ourvo.

For

th18

roason

lt

18

excoedingLy

lnportant

for

the

data

for

tho

curve

to

be obtatnod

under

alnllar

oonditlons

oach

tlno

a

partlcular

pleoe

of

equlprnont

ls tosted.

tho

gradual

decrsaso

of

dloleotrlc

absorptlon

cur-

charactoristic

of

ths curvo

becomeg

eetabllshod

in

of tho

ourve, orprossed

1:r

ths

form

of

a ratlo of

.

..

tho

rasistanco

at oD

rainute,

is

called the

polariza-

tloqlndox

(P.

,-)-

and

ls ueod

ln

comparing

and

aualyzlng

tho

curvos"


8/35

fl

I

LZ-2

P"I"

=

lnsulation

rssistanco

8.t I0

minutos

Insulatlon

rosistanco

at

1

ninute

A do-pclarizsrtion

or

diachargo

cur.ve

for

a

liivon

pioco

of

oqutpment

would

rese

tho aLoorptlon curve

in

revorseo so

to

speak"

Consoquontly

an,oqual-

amount

of

1e

roqulrid

to d18char6o

a

glvon

pleco of

equipmont

&ftor

a

voltago

has

booa

appl

a"

*a,

corsu*sd

in

building

up

tho charge.

Thls

is

an

oxooodlEgly

lmPortant

oo[.

sldsratlon

ln

tostlng

oqui-pnent.

llfhan

severaf

tosts

qrs

mado

at

dlf,feront

ture.s

e.nd voltagoo

as requlrod

for

our standard

testa it-ig

aliolutely

osasntlal

that

the

wi.ndln[s

bolng

testod

bo

grounded

botwoon.runs

for

F

at loa

I

Slnoe

ths

dogroo

of

polarizatton

of

the ineul.atlon

of

equipnent

imodiatoly'aftei

ehut-down

and

Just

procedlng

a tost

cannot

bo

known,

it

18

advlsabls

that

to

bo

teetod ohould

be

grounded aa

soon

as

praotloabls

and

tho

ground.

bo

loft

u.

touohod

througilout

the

porlod

of

proparation"

If

thts

porlod

18

ahort,

lt

1o

to

ths timo

through

whioh tho volta6o

wa; apPllsd

in

the

provious ruu'

procautlon

io

tak6n,

the

fo1).owlng

tost

will

havo

no valuo,

beoau6s tho

dls

,rLtt

lo

partlally polarlzod st

ths

boginnlng

of

the toot"

advlgable

to

oheck the

wlndlng

to dstormlne

whsthsr

the

groundlng

timo

hae

bsoa

sufflc 6nt

to drain ths

lnsulatlon

oomplotely.

Th18

cal1 bo

dons

by

oonnocttaS

tho

wlnclLtg

under

to8t to

tho

nogger

Lnstrumont

wlth

the

motor ds-onorglzed

aud

notlng

if

iho

lnetrumont

pointor dsflocte

up-scalo

boyond

lnflnlty"

Another factor

which he,s eonslderablo

effoct

upon

tho

dteloctrlc

aboorptlon

chargo

on

the

windlng,i

to touch

the

wladln6;l

oircuits

whi1o.

tho

to

If

the

gonorator.

or

soon

aa

1t

ls

shut

and

any

spot

mogg;or

readlng

ls

tho

tomperature

rjf

the

dielectric"

Fornulas

havc

boen

dove-loped,

-iowever,

*it},

*tioh

fairlyaccurate

tamperaturo

coofflcients

of

reslstancc

can

be

developod

for

insulations

providing

tost data

aro avallable,

takon

at

two

differeni

tomporatures"

I

.Jtn

matrng

dlElectric

absorptlon

tosts

any

abnormality

tn readln6s

should

bs

ln-

-vestigatid,

for

a

serious

break-down

of

insulation

may

havo

takon

P1aco.

Safoty_

Pi

ocayt]ons

Tho

2,500-volt

moggor

builds

up

a relativoly

high etatic

this

chargs

should

bo dischargod

before

anyono

is

aLlowod

Likowise,

no

one

should

bo

allowod

to

work

oa

tho

excitor

is

in

pr ogr"o i; ri

"

Pre

l ini

rnar-y

Opcrations

Tho

follovring

Proli-r:lina::y

operations

should

be

performed"

othor

eqlripmJ,li

iu

i., sorvico

they

should

bo

porformod

as

1.

Havo

tirc

grounci

switch

closed

to

drain

off

any

-chargo

on

tho.

Btator

wlndlngi

It

is

goo d

prlctico

to

loavo

tho

noutral

OCB

cLosod

throughout.

the

ehut-down

poriod

and

to

maintain

a

Eround

on

all

windings

to

bo tostod

throughout

the

pror

P0r1(Ju

urru

parato.y

poriod

boforo

tefts

aro

mado.

hrrthormoro,.if.thore

t:."1{

P::ilti^-

ivhsther-

tho

dlainage

tj"ms

hus'boen

sufficient,

tho

winding

should

bo

tesl:9

{9tif

rosidual

ehargo.

This

is

dono

by

connecting

iho

windlng

to

tho

mogEor

rrlth

tho

^oiot

a"-or',u. trua.

If

the

inst;mont

polnier

deflocts

up-scalo

boyonil

i:cflnlty'

tho

winding

should

bo grounded

for

additiona]

tirno

'

2"

Remove

gonoro.tor.fieId

slip-ring

bruEhos

the

main

exciter

armatur

and

plIot

exciter

on

the

stator"

.

Jfti

and

ground

this

fleld.

AIeo

and

keop

all

g.roundod

during

ground

tho

to

)


9/35

_ _ .

.

,tktzhl/5)+)

r2-1

.

:

t

s,

J

;

f,

).

Ctuo"

tho

stator

and

its connecting

vrindin8s

of all lightning

arrostors and

:

potonti.al

transforners,

and

disco:rnoct

tho pc,ror cable

as near to

tho

generator

as

poso1b1e.

If

ar.y appreciabls omount

of cablo ls lofb

connocted

in

the circuitl

the

nogger

roadlnga

v"l"II

bo

influoncocl

by

thj-s cable,

r"..td thelr

5611

not

be trus

roslstivity

reaclings of

tho

stotor.

Tho

length,

size,

and

type of oable

should

be

rocorded

on

the data

sheets.

Also,

if

tho cablos

termlnato

outslde

ths

bu11d1n8,

rot-I'and

dry-bulb

temperahrres

must

be oltained.

tl.r

[.

flIpe

all

p]aces

n'hore

coblos

ter:rLj-nate

and cond.uctors

go

to open

rviring.

.t

:,i,

lhars

is

ahvays moro

or J,oss loaka5e

ovor the

end.s of

the

cables

to

the

shoaths

at

l,:,il

ruoir

placos,

and

wiping

the dust

cieposi.ts

will

reduce

tho nurnber

of

).oakago

patho.

[]['

Cto*ing

the

wlndings

befcro tho tost is

of

qusstionablo value,

but

it is

deslrablo

ili:

to

iroo. all

porcelain

insulators

wipud off

that aro

on

the circuit

being

moggered.

[it

fi,ft

includes

tho noutral onrl

as

weIl as

the

lino

ond

of

the

winding.

Tho

fact

$Ij:

tU"t tho

windings woro or wore not cleanod

should

be rocord.ed.'

If

the

vri:rdlngs

[l]:

aro

cleanea,

rnake

tests

al'i;or cleaning

only.

),.

i

Test

Procsd.rrro

.,/.

i,.,

---

\l1?-

t..

i...

0onorator Stator

h:'

--

iii'

.&to

negger

reacllngs

at

[O'

C.

or

as

near

this

tomperaturo

as

practicable.

Two

il.

rota of

ieadings

aio

to

bo

rnader

tho first

at

lOO

volts;'(62J

'6]-ta

if

that

is

the

Ni

rrtiug of

tho

megger),

and the

second.sot

ab

2 00 volts. Tho

:notor

on

tho mogger

[r':

rhould be

allowed

to c

on,:,

up

to

fuI]

spoed beforo

tho

ground

on

tho

st&tor

is rs-

li:ll

rcvoa.

Reaclings

should

i;o

rnads

at I -soconrl

intorva-]s

for

tho first

minuts

and

at

fii;

r;:;ir";; iilEi"ui.'r",

tho next r minutos

for

a iotal

tims

of

16

minutes. Thuo

i'

'

ttroro

are

two seriss

of

tests

and two curvos

should

be dra'an.

It

1s

essontial that

thr-,

rvindinns

irnder

test

bo

grounded

).on6

enough

botween

tosts

ii':

6-difrn-olr-TE6-iTdt1c

ti o.

-Sec

tho

f

j-rst

two

paragraphs

on

pag

Az-z

undor

i,r;,

"

rrorrllrtl.r.1r

J

uPtJ

a\"r

.|.,

l.i.

Ilolds

and

Exc

ltor

s

t'i,-

ll,

thko

suro thB.t the

vrindiugs

have

been

groundor).

for

a

len5th

ol

time suff:ciont

to

j

draln

all charp-e fron

t'J

dtelectrli.

Th.r

lcst

flivon

in Proliminary

Oporatic::s

;

(poragraph 1) should

bc rnadc if

there

is

any

rluestion.

llll(e

only

one

sot of

retldings, usin6

tho

625-?o1t

or

lOO-volt

mog6or

at

l4Oo

C'

lho

roading

intorvais

strould be

the

s8.rTro as

on

tho

stator,

but

they

should

oniy

be

ocotlnued. until

tho

curvc

flattens

out

vrhioh

is

usu811y

less

than.ten

minutos.

111

voltnoter

and

instrlmont

wiring

shou]J

be

disoonneoted.

while

megSering

ls

In

Progr88s.

,/ ,,

Y[6lviu

Bridgo

Tests

.-:-

.i

,

trdlo

bridgo

testo aro

not

. "i"'

Stator

or arnature

trind:ing

,,,i

: ftor

tho absorpti"on

tcsts.

part of

the

absorption

tosts,

the rosistanco

of

tho

ihould

be

rnoasured

with

a

Kolvin

bridge

immedlatoly

rations

I'or

.

indic

ati

ons and a

test

1or

prosence

or'&

c


10/35

A2-4

CaDacitanco

Bridge

Te

st

Uslng

a

capacitancc

bridge,

rloasure

in66

and

giound.

Alsol

record

t)re

the

capacitance

botive

en

dissipation

factor

vrhich

.

ij

the

stator

wind-

I

i-s

also

read

on

..

tilo

capacitance

bri"d

go

.

*0uardini'.

of

Cablos

'

hiold

of

col'laj'n

Ssrorator'

r

'

It

has

bcen

founct

possiblo

to

Suard

the

tround,:

,,-

^^Lr^d

+n^h

*ha.,..

ih'ili."iirl;;";;.";;"uiiu3t

or

tno

insriration

on

the

cabros

fro4.tho

,,,,

megger

readings

on

the

;;';;-;;;

siator'

"llt'^l:

::::''nl:':::":'"tlllli*""1

Ii '[i"lll'';:li"]tt"""

;;";i';-;;";;;

shio-Iis,

ancr-:I:.::".:'::i.:::::il

*"-

tho,nesser

is

connecte


11/35

DRYING

F

TRANSF

.fhere

are

a

number

of

ipprbved

methods

for

dry'in

z

out

the transformer

core

and

coils'

Gencrally

,lr.

i;;;'oitn-.-*-urr.rurer

should

be

foLlorved.

If

the

drying

out

process

is

carelessty

or

***.4,

performei,

great

damage

may

-result.to

the.

transformff

insulation

through

over

hcaring_

Drying

out

of

pcrver

r-.fir

rrr

.tl,ruld

be carried

out

ur,der

the

personal

sup,'nision

;;;:-;;-;ed

officcr.

,re

following

gcnci.lr

insrrucrioos

are

grr'q

for

carr)'ing

out

rli'ing

opcrelic.:n.

.l

hc

transformer

must

b

suppled

during

the

drying

operatron

with

a

quantit]. o1'hcat

equivalenl

tothclatenlh.utofv,porisaionfor-tlremoisrurctot,cremol'cd.Atnormalaln]ospheric

prcssure

watcr

*.ill

onry,rfiir.-,,piary'

it"*

insulation

aI tempsrarurc

aho.c

E00C.

J'ct

the

insulation

is

litely

to

b"

badll:Lmag"4

ii

t"-'''-

part of

it

is

heatcd

to

more

than

100"C'

evcn

for

quite a sltort

timc.

U'the

on

it

o"ttt'"'i,

it

wi

also

bc

damaged'

The

orer-heading

nill

cause

Jrarti""

and

products

of

oxjdation

will

promote

fuiher

dcterioration

of

the

oil

evsn

at

lorvcr

;;il.

fi

.ry

,tro-inlure

thc

jnsulatjon.

i'hc

rarge

of

tempcr,arure

^within

n'hich

a

transformcr

can

be

effccti ,cly

Ji"d

i.

thus

vcrl'

narro \,

the

morc

so,

zs

ferv

of

thE

mcthods

by

*,hich

it

can

be heated

gr.,.

oJo*

distributitn

of

teinperature.

it

is

for

tlds reason

that

the

operation

is

so

critical.

a

.rrying

t

"n.r,rrmr:r

sloul,J

ne'er

be le{1

unattended.

I-l


12/35

The

temperature

limits

and

Ioads

should

be

strictly

adhered

in

ordcr

to

obtain

desired

results

*

ithout

dimage

to

translormers'

NOTE:

It

m-ay

be

nqled

that

highcr

the

prescribed

oil.

temperature'

the

smaller

the

load;

which

means

trrat

additional

blrrk.i;*g;f;."oJ

i"

required

for sim

ar

currsnts

th,n

for

higher

currents,

in

order

to

bring

the

;rr

",."rp;;;;p

to

rh.

varues

shown

in

rhc

above

table.

itisalsoessentialthatplentyofr.err(ilationbcprolide


13/35

/'z

I

)re

Megohms

v&1

dpproxima

tely

n

,ersl}ry

with

temp"raturc

and

for

a

l00C

change

of

tenrpcrature

the

Megohms

ttt-gt

'i"i"i"

ofipproximately

2

:

I

for

example'

:

Megohnis

at

"at

'"

at

(

.1

850C

750C

650C

-

-o^

))

(-

300

600

1200

2400

r

ror

oir

circur^rri"1i,

d?.:

,?J;ff,JI'l-Jj;TJffi1l;ilT.:"#F'J':T,l^:i

iffi

ti

hottom

of

rhe

ta*

and

wtthi

hcatcrs

llncd

to

filttt

app'oxil'la'iiiit:

"lrtt't"n

wilt

ut-'"'luirtd

fo'

C\tr'

1io0

g'rllons

of

oil. Local

r..rverltceling

or

'r'"""i"i*'u"

^"iita

so

'thrt

loaiing

on

lhc

h:rl'r'

sltoulJ

nut

crcced

3

\\,/Sq.

cm.

no.

rt

oula"fi.

tii""*tt

"rtr"

"lr

"t"tJJ

gi%

"t

the

ltcatcrs

or

750(l

ncar

rlre

rr

indurSs.

Prcc:tutiotrs:

-

i)

Vent Dipc

with

single

diapfuagrn

should

bc

rcmo'u'cd

and

thc

h


14/35

Hence

or

y lop

valve

should

bc

'iosed

leaving

the

botlom

damage

the

ratliators'

one

opened.

,'

tiii\

Normallv

the

dclivery

pipe

of the

frlter

should

be

connecled

to

lhe

bofiom

valve

i\r,'/

;r

th;i;

i";.aintaining

"'ironn

tcmperaturc

in

he

tansformer'

xi\)

ttire

extinguishers

mar

be

prorided

for

conibatirg

accidental

fir':s'

sr')

The

tap

should

b

so

set

for

passing

short

circuit

currsnt

tlut

all

the

wirdirg

is

in

':''l

;;,ri,",,

rlrat

the

entte

siilding

mly

bc

dried'

rri)

lf

mcrcwy

thermometers

are

used

special

care

should

be

taken

o

prerent

thc

bulb

l-,rcakhg

oa

.o*.qr*f

'ntt"a'"1

ltlcury

into

the

transformcr

winding'

\Iercury

rt

"r*ottttt]-'iottiJ

intUutt-'

be

used

rvith

thc

meul

shiclding

Ibr

pr.r."iittg

.r.fl

utcidtntt

lt

is normally

ricsirable

to

usc

dial

lwc

therlnometcrs

or

'\lcohol

thermometers

as

far

as

possible

r:rii)

Rciorc

rccording

insulation

rcsistancc

valucs'

thc suppl)

to

the

rrindurgs

slro'uld

,''\rr./

ir"."i.i,.a

"n-La,fr.

i..a.

discorur,.ctctl

sircc

[]rc

leads

mar

c,'ntrihutc

to

]orr

t*0.g.

obtaincd'

duc

to

their

inhcrcnt

characteristic

Drl

ing

undcr

\

acuunl:

:iismcthodcoruistsolheatingupthetranslbrrtsr.bl'shortcircuituntleroilandapplyinga

rcuum

10 removc

the

ntni't*?

;i+'it

^ttttoa

can

be

cnlplol

cd

lbr

translt-rnncrs

ntounted

in

:ong

steel

tanl:s

suirb)y

dtt;gt";';;;li:i

ttl

s:tc))'

xitir-srand

erlcmal

prcssure

rcsulting

cm

a

vacuum

of

27

inches

i' 'ott

lr4oJem

transformers

are

gcncral11'

strong

cnough

to

idrsland

this

Pressure

llowevJ'

i'

J"'U'f

i

cascs'

this

me

thod

shoulcl

r'-ot

bc

emplol

cd rvithout

-*iots

approvat

of

nlunuf"ru'io

;-"'*"

punrp

should

be

connectcd

to

thc

top

of

lilc

anslbrmer

ank.

..r:

should

bc

LrI:en

to

see

tllrt

thc

diaplLr:rgm

on

tl:c

c-rplosion

\cnt

of

thc

tlattslormer

is

mo'cd

and

thc

opsning

tt'rtilp

*itft

u'uf^iltg

platc'

lhc

transformcr

tan-L

sidcs

should

be

.soed

*'ith

some

covcring

"ti

"j'"tp'J"

fttt

oi

iimil-',nalcrial

to

prevent

unnecessary

loss

i

*;

'ii;;

;;;;,

;,;;"ili"g-;;

,adiarors

should

be

closcd

to obtair

the

deslred

tsmpsrature'

hetrarsformershouldbefieldwitlroilsuchthatthecoilsandirsulationarecompletelycovered

v

oil.

Onc

oi

thc

n'indings

ftl'V

lt f-

V

l

should

bc

short

circuited

and

3

phase

supply

ncithcr

rcater

nor

apprcciably

""'Utt'tft*

1fl"

;*fta'ntt

voltage

of

the

transformer applied

to

the'

ther

n inding

to

enable

" "t

f'flf"'i-"'*"nt

as

possible

but not

larger

than

full

load

currenf

obtaurcrj.

'fl)c

tcrnperatuc

;;;;;;t'i;

shoultl

bc

ti"J

tio*ty

t"o

approximat"ly'550C

in

::

lir.

r

8 ho*'s;

ro

o:oc

;n

*',.

:';;J;;

,

;i

u..,

zs'c

i'"t'"

tt'il0

a

noun

'tJl.,

tre

7s0c

imperatlrlc

is

reached

tf"

t"*io"tt'

should

be

disconnected

from

the

source

of

power

and ilie

il

draincd

out

and

rr;u'rm

'rifi

tJ"i"t'^"tf

itte

ll

'lhc.racuum

should

bc

a

mjnimum

of

25" but

,referably

28'.

'1hc

tutu*''

should

be

maintained

until

rempclarurc

drops

to 50'C'

lhc

raruformer

should

again

bt

l'x;;;i;

r.I"t

();r

"

600C

atul

tlte

short

circuit

switchcd

on

and

the

ycle rcpcated.

Temperaure

-JLt'f"i""

resistance

of

the

windings

should

be

laken

evcry

tryo

rou.rs

and

the

drying

process

conthued

Lntil

ihe

mcgger:iadings

risc

ovgr

the

minimum

and

are

,tcadily

rising.


15/35

Thc

tabl,r

below

shows

the

relation

behl'een

vacuunl

and

volatilisatio[

tcmperatule

of the

water

contained

in

Lhe

oil:-

.

1''i:"

.Jil:H*

Y#*:*:"

degrees

Centigrade

oo ?a

'99.15

98.60

97.71

9s.85

e?.77

87.89

80.42

69.21

53.31

. ?I)O?'

0

75.54

7

5.35

7 5.01

71.26

i2.85

70.5

I

56.80

61.12

52.60

.10.54

23.50

0

0

5

10

20

30

40

50

60

70

80

90

100

From

the

dbo.",e

it

can

be

clearly

secn

thrt

the

eraporation

temperaturc

of

moisture

dcpends

^.iriv

,p""-,fie

degleo

of

,acuum.

If

l'acuum

is

mai,taincd

the

moisture

in

thc

core

and

lvinding

evaporatcs

even

at

very

low

lemporaturc-s.

the

tcmpcraturc

ol

thc

translbrmcr

-is

raised

t

l,

,Alfo.rour

short

circuit

oi

the

ri'i-ndings

and

oil

circulatign

tlrrough

strcant

linc

llltcr.

'l'hc

oil

medium

is

used

mainly

for

distributing

the

heat

uniforml'

throughout

thc

core-

and

rvinding'

It

l,

""..,'",v

tlrat

tlrc

oi]

should

be

pu,.

-d

hot

while

iI is

bcing

field

in.

For

this

purpose

it is

aduar..,tag"ous

to

usc

a

largc

tank

oi

suitrbl"

capacity

wlcre

the oil

is

circulared

tl,ough

strearn

line filrer

to

naintain

high

temperaturc

bc,forc

lilling

in.

Onl}'aftcr

thc

transfonncr

is

complctcll'

fillcd

n.ith oil

the

shortlircuii

should

bc sn'itchcd

on.

l-lnder

rto

circumstan::s

the

short

circuit

should

be ,*,it.h"d

on i;ither

during

thc

pcriod

rvhen

tlte

vacuum

is

being

appLcd

or

during

fillinginoperation


16/35

Raise

the

temperah-Ee

of

thc

core

and

windings

gradually'

Before'applying

vacuum

make

sure

that

the

tank is

designed

for this

purpose'

:

I)recautions:-

i)

ii)

iii)

i1)

Vent

lpe

diaphragrn

rnust

be

removed

and

plate.

the hole

blan-ked

off

with

a

sturdY

Ilrecautions

regarding

simultaneous

short

circuit

and

oi1

circulation

have

already

;;;;

unier

fialheadtng

a:rd

the

sarnc

m-ay

be

complied'

Drling

bY

Hot

Arr

Circulation:

ln

tiris

method

tle

hot

air from

elcctric

healcr

is

blorm

to utc

Lranslbrm0r

rvindings

tluough

art

;;;il;;

towcr

part of

the

tank

and

is

allowed

to

escape

ttubugh

an opel

ng

in

the

top

cover. The

oil from

tlte

core

-a

*inairg.

should

be completely

rcmoved

before

starting

the

a.yng*.toperation.Thtrarsformershoul-ldbeloggedwithtarpauLinorothersirni]armateri3lto

pr'",',:iito*

of

tcat

and

to

assist

in kcepi-ng

t1e

whole

of

the

intcrior

tanl

at s

ch

a

temprafure

;-;;;,;;

;ndensation.

Thermu*.teri

should

be

fixed

in

suitable

places

to i.dicate

the

".pri"*.

,f

ij

the

int.t

,it

iiij

tf"

outgoing

air'

'lhe

hot

air

should

bc

supplied

by

mcans

of

a

nf"*:*

-a

electric

heaters

*hi.'h

rrlry

be"controllcd

so

as lo

regulate

thc

temperaturc

'lhe

mouth

of

the blower

at

rvhich

the

cool

air

cnten

should

bc

cor''erod

rvith

onc

or two

lal

ers

o{

clea:r

.,"",.r".ntopleventdrrstbeingdraw,niltothcrvindings.Itisncccssan,toforccasmuchofthc

;;u.d;;;r;;rsible

through

th"e

ducrs

olrhc

rransformcr

rvinding

lor

accomplishing

d{ing

to

".u""",r-ri.,

LaJIles

should

be

placcd bernecn

core

and

tenk

closing

inlen'ening

spacc

as

Inu ll

as

pcrssible.

.Ihe

remDeraluc

of

thc

tncorning

air should

bc raiscd

gradualll'

from

55cC

to

850C

in

about

24

;il;ffi;;i.,.i

r,

ss'C

ut;r11ct

till

the

end

of

d4'ing

'l

o

ersure

uniform

drying

the

volume

,i

fra"r.a

air

should

be

such

that

the

outlet

temperaturc

an

inlct

temperalure

are

as

near

as

f".riur".

During

drl,ing

out

operation,

the

insulating

material

loses

the

nroisture

contcnt

prescnt

i,,

i1'

*a

Lpr"""".-

iJ

di"l""t

i"

characte

ristio.

Ar

thc

samc

time,

the

mechanical

strcngth

is

corsiderably

decrcase


17/35

-

.

.

jjifi;H"dilr;,G]fi'iio'

n**'"*;

of

the

airblast

exceeds

presetvalue'

nsliouldbemadeforfacilitalingautomaticcutoflofheatersincasethc

blorvcr

stoPs.

Suitable

baIflcs

may

be

p'rovided

1o

disperse

the

tsmPcrafure

inside

the

tank'

Trarsformer

may

be

propcrly

lagged

to

rnaintain

unifonn

inlct

and

outlet

temperature'

to

prev 'nt

condensation

undemiath

the

top

cover'

The

temDeranfe

of

the

incoming

air

may

be

gradually

raised

so

that

the

maximum

*d"*.;id

be

reached

in

about

24

hours'

OiI

should

5e

completely

drained

offbefore

commencing

the

drying

opcration'

6,

,,^

4,

'-

_

7

hot blast

lo

maintain

uniform


18/35

.'-4.,.,,

h-Y

PARALI,EL

OPERATIOI\I

OF

TRANSFORMERS

General

conditions

for

the

satlsfactory

operation

of

Transformers

in

Parallel.

SINGLE

trarsformcr

may

bc

said

to

opcrate

satisfactorily

if it

carries

its

rated

ioad

without

"it

Be

dr9P, but-where

two oi

more

tsansformers

are to

operate

in

parallel

on

the

prt".ry

id

secondary

bul bars,

several

othsr

conditions

must

also

be futfilled-

Under

this

saiisfactory

opcration

also

implies

equafity

ofload

sharing

in

proportion

b

rhe

rating

each trarsformer,

absence

of circulating

currents

and apgoximately

similar

internal

power

in

each

transilomrcr.

In

order

to

satiiff

these latter

requiremcnts,

the

following

conditions

bc met:

(a)

The

phase sequence

or order

of

voltage

rise

on

the

termirals

of

the

two tansformers

which

are

to operate

in

parallel

must

be the same.

(b)

The

potarity

of

thc two

tansformers

must be

thc

same.

Two

trarsformers

might

havc

'

.

O," r-"-"

pi,..

rrqu*""

voltagc

risc

but on tcsting

out

it

might

bc

fo,nd

that

zcro

voltage was not

obtained

between

similarly

marked

terminals

on

the

to

units

due

to

oe

pota;ty

being

incorrecg

which

would

cause

the- furstantaneous

terminal

PrSssures

to

be 18O

degrees

out

of

phase.

If

zero

voltagc is

obtained

between

ail

pairs

of

similarly

maried

termhals

on

the

two

transformers

then it

is

an indication

that

the

phase

sequencc

as

well

as the

polarity

are

identical

in

each case'

(c)

The

two

tarsformers

must

have

the s:rme

p(essure

ratio

so

that

on

ope,r

ckcuit

the

' '

s"cond".1,

'iottages

are

eqnal.

When

installing

npw

kansformcrs

the

position

of

the

tapping

swilches

should

be examined.

(d)

The

angle

of

phase displacemcnt

between

the

primary

aod

secondary

vectors

mlst

be

'

'

thc

samc

for

cach

transformcr.

This

phase angle

differerice

is

a fixed

inhef

snt

(e)

For satisfactory

sharing

of the

load

cunent

the impedance

voltage

drop

at firll

load

should

be

the same

for

both

transformsrs.

This

:necessitates

the

transformers

having

the same

values

of

pe.rcentage

impedance

voltage drop,

but

not

necessarily

the

same

valuc of

ohmic

impcdan""f

fiir

latter

value dcpending

on

thc

KVA

rating

of

the

tansformer,

(f;

In

addition

to

having

equal

pcrcentage

impedance

voltage

&ops

it

is advisable

that

the

two transformsrs

iaVc-

nearly

the

same

ratios

of

percentage resistance

to

psrcentage

reactance

pressure

drops,

which

necessitates

similar

ratios

of

resistance

to

icactance in

each

casc.

If

this condition

is

not

met

there will

be a

phase displacernent

between

the

tsansformer

and the

total

toad

currents

resulting

in

different

power

factors

for each

bansformer

and necessitatin

E

a

grcatet KVA

output

than

is

acnully

required,

with

accompanying

additional

heating losses

and

Poorer

voltage

regulation.

2.

Loa'd

Sharing

under

Normal

Conditions

The

noltage

chaacteristic

of

a tansforma,

connecling

terminal voltage

with

load

curreng

will

usual-ty

bc

a

&ooping

curvc somewhat

similar

to

that

of a dircct

current

shut-wound

gcnerator'

Howevcf,

the

actual

value of

the

terminal

voltage

at

any

Particular

load

wiil

depend not

only

on

the

value'of

the imlrcdancc

voltage &op

but

the

phase

relation

of

thie

&op

to

the

apptied

voltage,

*ft.t

io

tum

will

iepcnd

on

ttre

power

factor

of

the

load. Two

transformers

which

are

to

operate

irt

prt.U"f

shotld hlve

,i-rtu.f,"

'

s:uLp'd

crment-voltage characteristics,

so

that thc

*op

in

terminal

pressure betweefl no

load

an{

iiU

Ioarl is

the

sa.:ne in

each. |1i

otiier

words,

tiiey

shouici


19/35


20/35

ft

.i"

a-s

usua

howcver,

for

Ere

traosformer

rr-rne-a1gc

to

b

cxpressed

as

I'crcntage

drops

at

firll

load

-t

,g

*#frai"Itt

the

"qui*r*t

tnnedance

of

the

traosformcr'

l*t

,4:pcrcentage

impedance

of

transformer

No'

1

and

17;

perc*fiageimpedance

of

rransfomra

No'

2

Thenassumingforthcprese,rtthatlrand12arethefullloadcrrrerrtsofthctransformen,Zland

;J;;hti";;pedanLs

maY

bc

exPressed

as

.-

"Y

'r

-

Il'1oo'

Thus

from

which

11

then

bccomes

and

similarlY

--

zzY

.t7-

Ir.lffi'

Zz

-TT

lz=It

z2

VI1

+

z1VI2

zzVIt

lkVA

)

x

zz

r,=r,

@iTT;E6jT;

Theaboveisvalidforbottrsingle.phasearrdthree.phasetransformers.

The

correct

Pairing

of

Transformers

Thecorrectpairingoftraruformersislargely.


21/35

:#, .?iffih:

:ffi

I',*"Ty

;:g- r:

bue

bar,ettering

-h

ffi*T,:f**;

;;il

;,Ja;X

i:ffi*t

H,J:,,,ffi.ffi

,r,

Ph


22/35


23/35

R'

-

R'

=

iz

-

iir

= tt*icc

phase voltage,

Y'

-

Y'

=bz

-

br

=

twice'phasc

voltage,

B'

-

B'

=

cz

-

Gr

=

twice

phasc

voltage,

whilc

readings betwecn other

pairs

of

tcrminals

will

give

indications

of

phase

volts.

Similar results

are

obtained

for delta{elta connected

tansformer, this

case bcing

shown

in

Fig.l

.

The

secondaqr

of

the second transformer

is shown

with

each

phase

.

rwersed

with respect

to

the fst

kansformer.

the

result

is

:

complete

rewrsal

of

the

veotors

of

induced

terminal

e.m.f.s,

and

giving

180

degrees

phase

displaccrnent

between the similarly

te

nuedbus

b3rs-L',.Y', B',

as

in

the case of

ttre

star'star

,.

rl

^

x

r

t

n'

Ftc,

5

-Eff.4t

of intcrchanging

two

cxrcrnal conncctions

on

primary

sidc

of

della-star

transformcr

.

connectcd transformers.

If

voltagc ri,adings

behreefl

the

secondaries

are

to

be

made, one

phase must

first

bc

conncctd

as

thsre

is

no

cornmon neutral point

availablc. If

R',

R'

are

commoned,

thcn reading

between

Y'

-

Y'

and

B' -

B'

will

givc

twicc

normal

lino

volts, as

can

bc

seen

&om

tho vcctor

diagpms'

(c)

Phase

sequence,

and

the

interaction

between

this

and

polarity.

- Phase

scquence

is ody

a

iactor

of

importance

in

ths

parallel

opcration

of threo

phasc

tansformers'

* ofballk"

of

three single_ptnse

units

opsratod

as

a

three-pharc

group-.

It

has

besn shown

in

the

precrding scction

in

the case

of

tansformn

with

eimilarly

connected

primary

ina r""ondrry

windings

such

as star-star

tarsformers,

that

a

rgvcfsal

ofttr.

poturity on one

sidc,

although

it

will

rsnder

parallel-opcration

impossiblg

will

not

alier

ths

phasc sequance

of

thc

se&ndary

induccd

volages,

Conracrscly

a

pair oi star-star

or dclta{clta

transformers

which

haw

thc

sarnc

relative

polarifu,

but

whose

secondaries

have

oppositc

phase sequencel

'ay

F

operated

in

parauet by

rcconnecting

the

cxtamal

connections

oo

cither

thc

6

4


24/35

primrry or thc

socoodary

sides so

as

to

produce

similar

phase

5


25/35

/\

-,\

8r

R

Y

E

B,

B,

Y,

f

.....-E

4

1;";8,'1,:,"n"i\".11i,.lli

o1';''llEl'X'i"if

lil'

lLil3l""fl$"3i"o'""

this

property

pf

star-delta

and

delta-star

transformers

-also

makes

it

possible

to

cnrrect

rhs

cffect

of

*""*';";;;"[surtine

from

a

rcvcrsal

of

rtre

intcmal

connections

on

the

primary

lti"il'J;

ti;es'

by

a

change

in

the

ext'rnai

conncctions,

and

thus

'"'*";

;Ji;.iiiu"i

"pi'"ii"''

*ithout

alteration

to

the

intemal

connectiors'

Fig'

"

f,;;;t#J'1":'i*

ITj""#IL.I";3,*il'i

H:

#*x:6:l'g*;,15{i;13*ff"1**;e*,;1[

ffffi.j'HIffi-T"ffi'?,"J#;i

**",

"f

d'

X:

".,HlHf,'#:

;;;"

;h*"

,Jqt'*"'

o-f

the

secondary

t*-

IT

"I

I

made

the

sarne

as

o*.

,",

*".

iiitire

additional_

reversal

of

thc

secondary

ext

nal

comection

',

^"J:i"i"

tr"t

**naitr

tY

tff*1|,*

trJ";

ffiil;.",

then

be

paralleled'

Two

other

sets

io*".

t"

*otttd

out

in

a

similar

manner'

rhe

case

of

a

delta-star

transformcr

-*

*U'l

r:

::ffiLH"::"I"#t:l1l

i*.?tr^*

on

the

delta

side

of.one

ol

1"',:nu

..ftom

which

it

will

be

Iffi"i-th*'gi"ing

reverse

pol"rity'

is

shown

T-tl?

""##J'".

.i"'*"""a'"v

Hi#;imift

Poiarity

ancl

phase

sequonce

of

*-

sidc

bus

bars

by

suitabty

rlJi'ittttir'*

t"

"*t"-"r

connections

as

in

tho

Provrous

case

r:::11:1fii'::"'""ff'

T;ffi{;if

H;#i"q,Wrtr,*"n*ffi;.ffi

::

"*Tfi'..1a"^rlri"i6ov in

parallel

unless

m--ca"u

;;r;

-plmary-

ana

[:[r]m'

mex'

;:"[f"'

';r[+&l5'

ffi

'*m;:

tris ohase

displacrrnent

depends

primarily'

on

the

typ^

primary

and

secondary

i"riii"tt'*a

"

tl*

""p'bl"

of

variation

by

alteration

ot

ilti-.v

orth"

wioil[l:f#;.t'.f,,i.;

or

ssoondaf,v

sidss'

In

cadam

di--


26/35

q.pes

of

tansforf,cr

aho,

the

nhase

rlisnflmcnl-'

rraY

bc

altcred

by

a-

rErrmgcme'[t

of

trc

cxrerral

-c;;cctionr'

which

prowides

a

vahublc

means

ol

suirabty

pairing

trmsform;;ffi;;i

u" ioi-o--to

hrw

differcnt

phasc

disolaccmcnl

In

othcr

t**

;'o.J;;-.,d

Ph*t

displacernent

b"*alt"

"ri"-v

and

secondary

ioat'Ja

"tft'g"t

"'n

otity

bc

changed

by

'reconnechon

or

#;#;;.;;;;

;;

;

io"r

m"in

i*f

F

consider

sfrich

can

be

":*-"rfii

W

a""rr"g;t

"

t1p"

ot

tt*"rormer

applicable

to

each

case'

l

:

2.

Y

,

Y

I

'.''

F,6.

9

-srar.srar

rransrormer,.shorrhjjI?.ili;:,tJ'jl:::mcnt

tcr*ccr rri'r:::)

and s"ooaJr]

9

^1

L\ c,t

Jt"]

,J

al

cl

,'-Lfl

Y'--+_+

l--

rl

^

/Y

,,H,

.,'

R'

FlG. I

-D:a8ram

shoqing

how-

incortc'l

.Polarity

of

delta sidc

of

delta'star

''-'

ir"nsfo.m-cr

m"i'ii

ii'iiita

ui

iiicratibn

of

thc

crrcmal

conncctions

s

having

zero

phase

displacement

berween

primary

and

'-'

s."ond"ry

induced

lerminal

vologes'

Transformer

under

this

headings

consist^

of

slar-star'

delta-deh4

and

delta-

intsrconnected

"t"

""*fttrnt.'"fne

case

for

a

star-star

connected

unit

is

shown

in

Fig.

s

.

and

from

ilU;;t';Garns

it

will

be

noted

that

the

secondarv

vector

of induc'ed

tj'"

;L;;i;

*"t

prt""

does

not

differ

in

angular

position

&om the

.*."p*aiog

ffi""""-

--J

trt'

th:

enmary

and

secondary

line

;J;;";;'f'B,

*i;

-

b2

are

prallel

and

have

the

same

sense'

&

Y

D


27/35

(2)

Threc-phasc

hansformen

having

lg0 degrees

phase

dilplacemcat

betwecn

pdmary

and

secandry

incladed

tirminol

vitog"s.

-

,

The

tansfomrem

belonging

to

this

class

havc

similar

typcs

of

windings

to

those

considcred

in

the

abo.ne

section,

but

in

cach

case

'

\1;

FtO.

lO

-Sur.rcvcrsc

srar lransforocr-

showing

lg0 d.8rc.s

ph.se

disolaccmcnr

bcrwccn

primary

and

s


28/35

rcs

and

that

the

PrimarY

and

ffi;#*T.Hry,H"TH#:St#ffi1ffi

ffiI#:'*

Hcpcndent

'

mH :ffi#Hr,

::,fffi;":;x*s

e

phasc

dispracomcnt

bctwc

c'n

m*f:*r:****1

il",il'il".;;;ry::f

;ili::f

f; ,i',H.T:"HXffi

,;;-;-"","."i1a"r'"+

terminal

vollagc

on

eacn

and

ttrat

the

vectors

or

the

ilfi;;;;i;

I

u'

P9 ?

;"*

T*H:1"J.t#"iJi;

*:"#L'":iitrr*.*;i:ffi

. 1'lf"ffl;t""*a

6''r'

Figs

z

and

'I3

show

the

corrcsponoing

iiigr"

t

for

star-delta

and

star-interconnected

star

transformers

which

have

a

similar

phasc

displaccrnent

characteristic.

(4

)'11ree-phase

. IT*Tfi

^rHX?

"

"?,2iif;2""

r,?#;.*;Xff

':

"

phase

displacement

betweerl

['flmary

drrs

Jvvvri*r

z

D

.l

I

I

I

I

tl

l.

v

,

11


29/35


30/35

The

Bridsh

Stendard

Specilicetion

for Power

Transformer'

No'

171

(1959)

(a) Need

for

uni{ormity.

-

Transform.ers

which

1nay

have.

to

bo

paralleled

rc

not

;fi;)i-;,fi&ia

w

o"

,"*"

ir*

-a

o"rri"re

it

is

ar

obvious

advantage

if

il#n

'*

.gr*a

-"rt

od

"f-";G

i*'l"als

a1t

rnakine

internal

connections'

which

can

bc

artopted

tty

"U

ffi"f..t**.

B.S.

1?1

(1959)

providcs

the

;;;;;;,-darfiLtior,

;

futd

-q

*Tdi"qT-ections

this

information

bcing

given

in

section

14';;Iht

"tdification

lach

tlpe

of

taruformcr

has

dcsigrratcd

to

it

three

tV

U"f"

*i'i"f

chssiS

it

according

to

its

intcrnal

connections

and

phase

Otpi.."t*t

betwesn

pti*"r'

and

secondary

induced

voltages.

Thephase

di.pl"J;;;;;J;il

bv;"1""]-face

number

so

that

0u

is

indicated

by 0.0

o'clock

aJ;;-t

by;0

"ti*f.

etc'

In

this

chapter

the

wind-ing

and

vector

diagrams

frr""

U"*

ituJted

in

accordancc

with

B'S'

No'

171

(1959)'

For

threc-ptraso

ruosfor-o,

ther"

tt"

fo*

main

groups

arranged

according

to

the

phase

displacem"rrt

U"rw"*-tt'"-ptl"'*y

'1a

*gt"i'*

induced

voltages'

The

reference

Ddo,

for

"-"-pfi'-iiai""tit 'h-"t

F:

transformer'

is

delta-delta

".*""i"4

""ties

uflder

rn"'io'g*p

l,

and

tlut

it

hal

zero

phase

displacernent'

This

tansformer

may

ut-ti't"ti'a'i"

paralleJ

with

a transformo

whose

charactsristics

are

dcfioea

-Uy-ti,

reference'Yyo,

because

although

rhis-is a

star-

star

transformer

it

rru,

"tro

,lnaer

main

group

I

and

has zcro

phase displacernent

Similarly,

a

bansformcr

of

"ft"n"t

yJf

refcrs

to

a star-delta

unit

belonging

to

main

group 3

ancl

has

"

PhJ;;i;;"*ent

corresponding

to I'0

o'clock

or

-

30"'

The

enginecr

who

is

resporsible

for

the

commissioning

or

connecting

up

of

.t

"*i"il*,

ir

well

aalvised

to

obtain

a copy

of

the

abovc

specification'

Marking

of

terminals

and

windings

'

- Capitel

l"I"'=.

-"

used

to

indicatc

the

H'V'

and

small

lettsrs

thc

L.v.

#;;;;

*rutt'

i"

*tit

book

have

besn

refsrred

to

as

thc

pdmary

and

secondary

;;G'

The

corresPonding

primary

and

secondary

winding

of

each

phase

",,

A'o

fr"*

suffx

n''

inberg.as

Ar'

Az'

transformer trouble , test , drying , parallel operation

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