Transformer protection 20.ppt

8/20/2019 Transformer protection 20.ppt

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1 > Transformer Protection

TRANSFORMER PROTECTION

Target Audiance

•Electrical Engineers

•Protection Engineers

•Test & Commissioning Engineers

•Power System Engineers

•Utility Engineers

•Electrical Technicians


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7/1217 > Transformer Protection


Introduction

Transformers are acritical and expensive

component of thepower system !ue tolong lead time forrepair and replacementof transformers" ama#or goal oftransformer protectionis limiting the damageto a faultedtransformer Thecomprehensivetransformer protectionprovided $y multiplefunction protectiverelays is appropriate forcritical transformers ofall applications




The type of protectionfor the transformersvaries depending onthe application and

importance of thetransformerTransformers areprotected primarilyagainst faults andoverloads The type ofprotection used shouldminimi%e the time ofdisconnection for faultswithin the transformerand to reduce the risof catastrophic failureto simplify eventualrepair




Any extendedoperation of thetransformer under

a$normal conditionsuch as faults oroverloadscompromises the lifeof the transformer"which means

ade'uate protectionshould $e providedfor 'uicer isolationof the transformerunder such condition



The various possible transformer faults are,

1. Overheating

2. Winding faults

3. Open circuits

4. External faults

. Over fluxing

TYPES OF FAULTS



The! are due to

"ustained overloads and short circuits

#ailure of cooling s!stem

$rotection

The thermal overload rela!s and temperature rela!s, sounding

the alarm are used

The thermocouples or resistance temperature indicators arealso provided near the %inding.

These are connected in a bridge circuits.

When the bridge balance gets disturbed for more than the

permissible duration circuit brea&er trips.

OVERHEATING



The %inding faults are called internal faults. The overheating or

mechanical shoc&s deteriorates the %inding insulation. 'f the%inding insulation(is %ea&, there is a possibilit! of

$hase to phase faults

Earth faults

'nter turn faults

$ersistent fault %ould have a possibilit! of oil(fire.

$)OTE*T'O+

ifferential protection

Over current protection is also used as a bac&up protection

#or earth fault protection, the restricted earth fault protection

s!stem, neutral current rela!s or lea&age to frame protection

s!stem is used.

WINDING FAULTS



The open circuit in one of the three phases causes the

undesirable heating of the transformer.

Open circuits are much harmless compared to other faults.

Transformers can be manuall! disconnected from the s!stem

during such faults.

OPEN CIRCUIT



Through faults are the external faults %hich occur outside the

protected -one.

These are not detected b! the differential protection.

'f faults persists the transformer gets subected to thermal and

mechanical stresses %hich can damage the transformer.

$)OTE*T'O+

The overcurrent rela!s %ith undervoltage bloc&ing, -erose/uence protection and negative se/uence protection are

used to give protection

The setting of the overcurrent protection not onl! protects

transformer but also covers the station busbar and portion of a

transmission line.

THROUGH FAULTS



The flux densit! in the transformer core is proportional to the

ratio of the voltage to fre/uenc! i.e. 0f

Transformers are designed to %or& %ith certain value of fluxdensit! in the core. 'n the generator transformer unit, if full

excitation is applied before generator reaches its s!nchronous

speed then due to high 0f the overfluxing of core ma! occur.

$)OTE*T'O+

0f rela! called voltshert- rela! is provided to give the

protection against overfluxing operation.

OVERFLUXING



Other faults

Tap changer faults

igh voltage surges due to lightning

"%itching surges

'ncipient faults



Importance of Protecton

etect and isolate fault

aintain s!stem stabilit!

To limit the damage in adacent e/uipment

inimi-e fire ris&

inimi-e ris& to personnel



Tran!former Connecton!



5

* c

b

aa2a1

b1 b25152

2 1

*2 *1 c1 c2

5*

*2

*1 2

1

5251

a

b

cc2

c1

a2

a1

b1 b2

6*loc& face7 numbers refer to

position of lo% voltage phase (

neutral vector %ith respect to high

voltage phase ( neutral vector.

8ine connections made to highest

numbered %inding terminal available.

8ine phase designation is same as

%inding.




$hase displacement9

:roup 1

$hase displacement1;9

:roup 2

8ag phase displacement39

:roup 3

8ead phase displacement39

:roup 4

d9

=d9

d>->



6*loc& #ace7 numbers refer to position of lo% voltage

phase(neutral vector %ith respect to high voltage phase

neutral vector

8ine connections made to highest numbered %inding

terminal available

8ine phase designation is same as %inding

E#amp$e % & D' %% Tran!former

B1B2 b2b1

a2a1A1A2

C1C2 c2c1

?uestion @ o% to connect %indings A

HighVoltage

Windings

LowVoltage

Windings

A PhaseWinding

s

B PhaseWindings

C PhaseWinding

s




O(erc"rrent Protecton


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)e/uirements

#ast operation for primar! short circuits

iscrimination %ith do%nstream protections

Operation %ithin transformer %ithstand

+on(operation for short or long term overloads

+on(operation for magnetising inrush

Tran!former O(erc"rrent Protecton


24/121


B0

80

9

1

9 set to 1.2 ( 1.3 x through fault level

U!e of In!tantaneo"! O(erc"rrent Protecton


25/121


*oncerns rela! response to offset %aveforms C*

transientD

efinition

I( ) I*

I* x (++

I1 "tead! state

rms pic& up

current

I2 #ull! offset

rms

pic&up

current

I(

I*

!C

Tran!ent O(erreac)


26/121


HV2

51

51

51

LVHV1

Time LV

HV1HV2

CurrentI!LV"

1#2I!LV"

I!HV"

In!tantaneo"! H*) Set O(erc"rrent Re$a' App$e+ to a Tran!former


27/121


I,- I,-

I,-

I,-

+.//

C"rrent D!tr,"ton


28/121


I,-9.;>> I,-

9.4 sec

80 rela!

0 rela!


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31/121



32/121


P $$ $ T f


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:rid suppl!

#eeders

1

1

>F

>F

1

1

Para$$e$ Tran!former!Drectona$ Re$a'! -%.

P $$ $ T f


34/121


11

1

1

5us"ection

Para$$e$ Tran!former!Drectona$ Re$a'! -/.

:rid suppl!

#eeders

1


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36/121


Eart) Fa"$t Protecton

Tran!former Eart) Fa"$t!


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1 p.u. turns3 p.u. turns

x'$

$)

) '#$rotective)ela!

:3 ratioturnsEffective

. 3

. currentFault : atfaultaFor

. x3

current,primaryThus, valuesloadfulltocurrentE/FlimitsResistor

F.L.2

F.L.

F.L.

χ

χ χ χ

χ χ

=

Ι=Ι=

Ι=Ι

secondary!.T.thenr,transformeof currentload

fullon"asedisside#primary$onratio!.T.%f

3

circuit2

χ =

Tran!former Eart) Fa"$t!

O(erc"rrent Re$a' Sen!t(t'


38/121


χI

&'

(vercurrent

Relay

I$ as multi%le

o$ I#L#

1#&

'

(

)

*

+

,

-tar-ide

.elta-ide

, + * ) ( ' 1#&χ

%#u##/0ercurrent ela -etting 3

I#L#

O(erc"rrent Re$a' Sen!t(t' to Eart) Fa"$t! -%.



39/121


1&

'

(

)

*

5

+

,

2

1

, + * ) ( ' 1#&χ

%#u##

I$ as multi%le

o$ I#L#

-tar-ide

.elta-ide

O(erc"rrent Re$a' Sen!t(t' to Eart) Fa"$t! -/.

χI

&'

(vercurrent

Relay



40/121


χ

I

IP I4

1&

'

(

) *

5

+

,

2 1

, + * ) ( ' 1#&

χ

%#u##

I4

IP

I

O(erc"rrent Re$a' Sen!t(t' to Eart) Fa"$t! -0.

I$ as multi%le

o$ I#L#

Eart) Fa"$t on Tran!former Wn+n*


41/121


ifferential )ela! "etting G of "tar Winding $rotected19G ;G

29G 41G

39G 2;G

49G 1FG

9G FG

Eart) Fa"$t on Tran!former Wn+n*

I

ifferential)ela!

"etting I"

χ

#or rela! operation, I H I"

e.g. 'f I" 29G, then H 29G for

operation

i.e. χ H IG

Thus IG of %inding is not protected

χ

2

√ 3I

χ2

√ 3

Unre!trcte+ Eart)fa"$t Protecton -%.


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$rovides bac&(up protection for s!stem

Time dela! re/uired for co(ordination

Unre!trcte+ Eart)fa"$t Protecton -%.

51

51

51

514

Unre!trcte+ Eart) fa"$t Protecton


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*an provide better sensitivit!

C*.T. ratio not related to full load currentD

C'mproved 6effective7 settingD

$rovides bac& up protection for transformer and s!stem

51

51

51

51451

4

Unre!trcte+ Eart) fa"$t Protecton

Star Wn+n* REF


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$rotected

=one

)E

#

)ela! onl! operates for earth faults %ithin protected -one.

Jses high impedance principle.

"tabilit! level @ usuall! maximum through fault level of

transformer

Star Wn+n* REF

Re!trcte+ E1F Protecton


45/121


80 restricted E#

protection trips

both 0 and 80 brea&er

)ecommended setting @ 19G

rated


5 * +



46/121


5 * +

80 restricted E# protection trips both 0 and 80 brea&er

)ecommended setting @ 19G rated


De$ta Wn+n* Re!trcte+ Eart) Fa"$t


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$rotected -one

)E#

"ource

elta %inding cannot suppl! -ero se/uence current tos!stem

"tabilit! @ *onsider max 80 fault level

)ecommended setting @ less than 39G minimum earth fault

level

De$ta Wn+n* Re!trcte+ Eart) Fa"$t

H*) Impe+ance Prncp$e


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ProtectedCircuit

6

7

CT

L

6

7

CT

LI

I-

-

T

V-

L L

H*) Impe+ance Prncp$e

0oltage cross )ela! *ircuit 0" I# C)*T K 2)8D

"tabilising resistor )"T limits spill current to I" Crela! settingD

∴ )"T ( )) %here )) rela! burden

*T &nee point

0L$

20"

2I#

C)*T

K 2)8

D

0"

I"

T)e U!e Of Non2Lnear Re!!tor! -3etro!$!.


49/121


uring internal faults the high impedance rela! circuit

constitutes an excessive burden to the *TMs.

ver! high voltage develops across the rela! circuit and

the *TMs.

*auses damage to insulation of *T, secondar!%inding and rela!.

agnitude of pea& voltage 0$ is given b! an approximate

formula Cbased on experimental resultsD

0$ 2 √ 20L C0# ( 0LD

Where 0# "%gr. #ault )ating in amps x = of rela!

circuit *T ratio

N setting

etrosil re/uired if 0$ H 3&0

T)e U!e Of Non Lnear Re!!tor! -3etro!$!.

Non2Lnear Re!!tor! -3etro!$!.


50/121


I/P

-T

V-V7

etrosil *haracteristic

0 * ' β

"uitable values of * β

chosen based on @

ax secondar! current under fault conditions

)ela! setting voltage

Non Lnear Re!!tor! -3etro!$!.

REF Protecton E#amp$e


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17VA!58"11&&&V+15V

1*&&91CT

+#'Ω

(&7VA

-1*&&91

CT

+#(Ω

7CA:1+

I-

Am%2 Core )9*)mm!)#+1Ω9;m"1&&m Long

*alculate @

1D "etting voltage C0"D

2D 0alue of stabilising

resistor re/uired

3D Effective setting

4D $ea& voltage

developed b! *TMs

for internal fault

REF Protecton E#amp$e

So$"ton -%.


52/121


Earth fault calculation @(

Jsing ;90 base

"ource impedance 1 p.u.

Transformer impedance 9.9 x ;9 4 p.u.

1

Total impedance 14

p.u.

∴

'1 1 9.9F14 p.u. 14

5ase current ;9 x 19>

√ 3 x 41

1112I> mps

∴ '# 3 x 9.9F14 x 1112I>

23;49 mps Cprimar!D

14.I mps Csecondar!D

1 $.J.1 4

I1

4

I2

4

I9

"e/uence iagram

So$"ton -%.

So$"ton -/.


53/121


C1D "etting voltage0" I# C)*T K 2)8D

ssuming 6earth7 *T saturates,

)*T 4.; ohms

2)8 2 x 199 x F.41 x 19(3 1.4;2 ohms

∴ "etting voltage 14.I C4.; K 1.4;2D

I3.> 0olts

C2D "tabilising )esistor C)"D

)" 0" ( 1

I" I"2 Where '" rela! current setting

∴ )" I3.> ( 1 ;3> ohms

9.1 9.12

So$"ton -/.

So$"ton


54/121


C4D $ea& voltage 2√20L C0# ( 0LD

0# 14.I x 0" 14.I x I3> 13I4> 0olts

I"

#or PEarthM *T, 0L 1.4 x 23> 339 0olts Cfrom graphD

∴ 0$EL 2√2 x 339 x C13I4> ( 339D

>&0Thus, etrosil voltage limiter %ill be re/uired.

So$"ton

Tra+tona$ Lar*e Tran!former


55/121


Tra+tona$ Lar*e Tran!former Protecton Pac4a*e

33L

0

9

1

299

1903311L0 1+ ;F

>99

>4

>99


56/121

Dfferenta$ Protecton


57/121


Overall differential protection ma! be ustified for

larger transformers Cgenerall! H 0D. $rovides fast operation on an! %inding

easuring principle @

5ased on the same circulating current principle as the

restricted earth fault protection

o%ever, it emplo!s the biasing techni/ue, to maintain

stabilit! for heav! through fault current

5iasing allo%s mismatch bet%een *T outputs.

't is essential for transformers %ith tap changing

facilit!.

nother important re/uirement of transformer

differential protection is immunit! to magnetising in

rush current.

5a!e+ Dfferenta$ Sc)eme


58/121


5'"5'" I*I(

O$E)TE

I( )

I*

ifferential

*urrent

O$E)TE

)E"T)'+

ean Through*urrent

I( 0

I*

*

I( )

I*



59/121


80

)

0

$)OTE*TE

=O+E

*orrect application of differential protection re/uires *T ratio

and %inding connections to match those of transformer.

*T secondar! circuit should be a 6replica7 of primar! s!stem.

*onsider @

C1D ifference in current magnitude

C2D $hase shift

C3D =ero se/uence currents

Dfferenta$ Connecton! An+ po$art' mar4n*!


60/121


$1 $2 2 1 a1 a2 $2 $1

5 *

p ' *

U!e of Interpo!n* CT


61/121


))

)

$1 $2 2 1 a1 a2 $2 $1

"2 "1 $1 $2

'nterposing *T provides @ 0ector correction

)atio correction

=ero se/uence compensation

"1 "2 "2 "1

p *


62/121


Interpo!n* c"rrent tran!former!



63/121




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75/121



76/121



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80/121



81/121


"2"1

$1$212 a2a1$2

10

>>&0 11&019 ;99

$1

!1

"1"2

:iven above@ +eed to consider (

C1D Winding full load current C2D Effect of tap changer Cif an!D

C3D *.T. polarities

ssuming no tap changer

#ull load currents@( >>&0@ 131 mp 4.3F mps secondar!

11&0@ F;F mp 4.I2 mps secondar!

o%ever, re/uire 11&0 *.T.Ms to be connected in

Thus, secondar! current √3 x 4.I2 ;.2

∴ 1ATI2 C211ECTI23 IS 1E4UI1E!


19 ;99


82/121


))

)

$1 $2 2 1 a1 a2 $2 $1

"1 "2 $1 $2

4.I2

"1 "2 "2 "1

4.3F

C2.>D CD

19 ;99

't is usual to connect 11&0 *.T.Ms in and utilise a interposing *.T. Cthis

method reduces lead 0 burden on the line *.T.MsD

*urrent from >>&0 side 4.3F mpThus, current re/uired from %inding of int. *.T. 4.3F mp

*urrent input to %inding of int. *.T. 4.I2 mp

∴)e/uired int *.T. ratio 4.I2 4.3F 4.I2 2.2

√3

a! also be expressed as @ 2.>

Effect of Tap C)an*er


83/121


e.g. ssume >>&0 KG, (1G

'nterposing *.T. ratio should be based on mid tap position

id Tap C(GD >2.F &0

$rimar! current C1 0D 13; mp

"econdar! current 4.> mp

∴ 'nterposing *.T. ratio re/uired 4.I2 4.>

√3

C D

4.I2 2.>>

a! also be expressed as @ 2.F

*ompared %ith 2.> based on nominal voltage

Com,ne+ Dfferenta$ an+ Re!trcte+ Eart) Fa"$t Protecton


84/121


To differential rela!

"2

"1

$1

$2)E#

"2

"1$1

$2

"2"1$1 $2

a2a112

Re!trcte+ Eart) Fa"$t Protecton

T)ree Wn+n* Tran!former


85/121


20

11L0

>30

132L0

1>99399

90

33L0

1999

4.I

2.;;

19.33

2.;;

.1

All interposing CT ratio5s referto common 67A $ase 8/,67A9

Tra+tona$ U!e of Interpo!n* CT


86/121


!1C(39QD

))

)


87/121


)ifferentialelement

*umeric Relay

+irtual interposin !T

+ectorialcorrection

Ratio

correction


Compen!aton

o-er transformer

Tran!former 3a*net!n* C)aracter!tc


88/121


T%ice+ormal

#lux

+ormal

#lux

+ormal+o 8oad*urrent

+o 8oad *urrentat T%ice +ormal

#lux

C)aracter!tc

3a*net!n* Inr"!) C"rrent Stea+' State


89/121


m

m

0

)

Φ

7

mI

Stea+' State

3a*net!n* Inr"!) C"rrent S6tc) on at Vo$ta*e 7ero 2 No re!+"a$ f$"#


90/121


m

Φ

Φm*

7

I

S6tc) on at Vo$ta*e 7ero No re!+"a$ f$"#

Tran!former Dfferenta$ ProtectonEffect of 3a*net!n* C"rrent


91/121


Effect of 3a*net!n* C"rrent

ppears on one side of transformer onl!

"een as fault b! differential rela!

+ormal stead! state magnetising current is less thanrela! setting

Transient magnetising inrush could cause rela! to

operate

Tran!former Dfferenta$ ProtectonEffect of 3a*net!n* Inr"!)


92/121


Effect of 3a*net!n* Inr"!)

"O8JT'O+ 1 @ T'E E8<

llo%s magnetising current to die a%a! before rela! canoperate

"lo% operation for genuine transformer faults

Tran!former Dfferenta$ ProtectonEffect of 3a*net!n* Inr"!)


93/121


Effect of 3a*net!n* Inr"!)

"O8JT'O+ 2 @ 2+ Cand TD )O+'* )E"T)'+T

a&es rela! immune to magnetising inrush

"lo%er operation ma! result for genuine transformer faults

if *T saturation occurs

T'pca$ 3a*net!n* Inr"!)Wa(eform!


94/121


A

C

:

Wa(eform!


95/121


Inter2T"rn Fa"$t Protecton

Inter2T"rn Fa"$t


96/121


+ominal turns ratio

#ault turns ratio

*urrent ratio

( 11,999 249

( 11,999 1

( 1 11,999

)e/uires 5uchhol- rela!

*T

E

"horted

turn

8oad

Intert"rn Fa"$t C"rrent 1 N"m,er of T"rn! S)ort Crc"te+


97/121


199

;9

>9

49

29

9 19 1 29 2

19

;

>

4

2

#ault current in short

circuited turns

$rimar! input current

#ault current

Cmultiples of rated currentD

$rimar! current

Cmultiples of rated currentD

Turn short(circuited Cpercentage of %indingD

of T"rn! S)ort Crc"te+

Non2e$ectrca$ Protecton


98/121


5uchhol- $rotection

$ressure )elief $rotection

"udden $ressure $rotection

Winding Temperature $rotection

Oil Temperature $rotection

Oil 8evel bnormal $rotection

5"c))o$8 Protecton


99/121


The function of the rela! is to detect an abnormal condition

%ithin the tan& and send an alarm or trip signal.

5"c))o$8 operate 6)en&


100/121


:as produced from the transformer due to fault

n oil surge from the tan& to the conservator

complete loss of oil from the conservator Cver! lo% oil levelD

Pre!!"re Re$ef De(ce


101/121


TransformersM last line of defense against internal pressure


102/121

O$ Temperat"re Protecton


103/121


T!picall! sealed spiral(bourdon(tube dial indicators %ith li/uid(

filled bulb sensor

5"c))o$8 Re$a' In!ta$$aton


104/121


x internal pipe

diameter CminimumD

3 x internal pipe

diameter CminimumD

Transformer

3 minimum

Oil conservator

*onservator

5"c))o$8 Re$a'


105/121


$etcoc&

*ounter balance%eight

Oil level

#rom transformer

perture aduster

eflector platerain plug

Trip buc&et

To oilconservator

ercur! s%itch

larm buc&et


106/121


O(erf$"#n* Protecton

Tran!former protecton


107/121


Over fluxing protection

Over fluxing arises principall! from the

follo%ing s!stem conditions.

igh s!stem voltage

8o% s!stem fre/uenc!

:eomagnetic disturbances

The latter result in lo% fre/uenc! earthcurrents circulating through a transmission

s!stem.

O(erf$"#n*


108/121


:enerator transformers

:rid transformers

Jsuall! onl! a problem during run(up or shut do%n, but can be

caused b! loss of load load shedding etc.

#luxΦ ∝ 0

f

Effects of overfluxing @ 'ncrease in magnetising current

'ncrease in %inding temperature

'ncrease in noise and vibration

Overheating of laminations and metal parts Ccaused b! stra! fluxD

$rotective rela! responds to 0f ratio "tage 1 ( lo%er .0.).

"tage 2 ( Trip field

O(erf$"#n* 5a!c T)eor'


109/121


m2

'e

m

0 &fΦ

*J"E"

8o% fre/uenc!

igh voltage

:eomagnetic disturbances

E##E*T"Tripping of differential element CTransient overfluxingD

amage to transformers C$rolonged overfluxingD

V1H8 O(erf$"#n* Protecton


110/121


Φ

0 f α

L

Trip and alarm outputs for clearing prolonged overfluxing

larm @ efinite time characteristic to initiate corrective action

Trip @ 'T or T characteristic to clear overfluxing condition

"ettings

$ic&(up 1. to 3.9 i.e. 1190 x 1.9 2.31

9-

T setting range 9.1 to >9 seconds

V1H8 C)aracter!tc!


111/121


1999

199

19

11 1.1 1.2 1.3 1.4 1. 1.>

Operating

time CsD

0 -

"etting

L >3L 49

L 29

L

L 1

t 9.; K 9.1; x L

C ( 1D2

Enables co(ordination %ith plant %ithstand characteristics


112/121


T)erma$ O(er$oa+ Protecton

O(er temperat"re

: ll d d l d t ti l d l


113/121


:enerall! regarded as overload protection also deals

%ith failure of or interference %ith pumps and fans or

shutting of valves to pumps

Winding hot spot temperature is the main issue, but both

oil and %inding temperature are usuall! measured and

used to@

R initiate an alarm

R trip circuit brea&ers

R control fans and pumps

O(er temperat"re


114/121


T%o temperatures must be monitored@

SWinding temperature CPWT'MD (Cshort

thermal D this can rise rapidl!, %ithout

much of an increase in oil temperature

SOil temperature CPOT'MD (Clong thermal D

this can rise slo%l! to a critical point

%ithout an unacceptable %inding

temperature increase

T'pca$ a$arm an+ trp $e(e$!


115/121


%inding alarm ( I9U* to

119U*

V %inding trip ( 119U* to

13U*

V oil alarm ( ;9U* to IU*

V oil trip ( IU* to 11U*

O(er)eatn* Protecton


116/121


' load

Tsetting

' loadTop oil of po%er transformer

eater

Thermalreplica

Temperaturesensing resistor

)emote

Temp. indication8ocal

Off

On

Off

On

larm

Trip

$umpcontrol

#ancontrol


117/121


T'pca$ Tran!former Protecton to

Fee+er!

Protecton of Para$$e$Tran!former Fee+er!

iher


118/121


iher voltae"us"ar

(!0 (! 0

FT1 FT1

REF

)

h

REF

)

hT T

REF REF

2 ' 2'1EF2 stae 1EF2 stae

)(! (! (! )(!

LoadLoad

Lo-er voltae

"us"ar

Tran!former Fee+er!


119/121


#EEE

)

$'8OT

"$W$

W

#or use %here no brea&er separates the transformer from thefeeder.

Transformer inrush current must be considered.

'nrush is a transient condition %hich ma! occur at the instant of

transformer energisation.

ag. 'nrush current is not a fault condition

∴ $rotection must remain stable.

*T provides a bloc&ing signal in the presence of inrush

current and allo%s protection to be used on transformer feeders.

Tran!former Fee+er Protecton


120/121


)y''

2'

II

III

I

1' 12

0TW

924F5!

'6

%L(T1 4!%4!%

4!T4!T23

26

!

5

ii

iii

i'2

12 1'

!

5

2&

27

28

29

23

26

2&

27

28

29

27

29

'9

'

;

3

'

'7

'7

'7

'

'9

'

'3'6

'7

'

23

26

2&

27

28

29

23

262&

27

28

29

'9

'7

1'

Protectonof Tran!former Fee+er!


121/121

o-er transformer

&6<

1cheme


+ectorialcorrection

Ratiocorrection


Transformer protection 20.ppt

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