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Transformer Differential Protection

Scheme

SIPROTEC 4 7UT6 Differential Protection Relay for Transformers – Connection of

transformer differential protection wit i! impedance RE" #I7$ and ne%tral c%rrent

meas%rement at I&

Introd%ction

Differential protection is a %nit'type protection for a specified (one or piece of

e)%ipment* It is +ased on te fact tat it is only in te case of fa%lts internal to te

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(one tat te differential current  #difference between input and output currents$ will

+e high*

,owe-er. te differential c%rrent can sometimes +e s%+stantial even without an

internal fault *

Tis is d%e to certain caracteristics of current transformers #different saturation

levels, nonlinearities$ meas%rin! te inp%t and o%tp%t c%rrents. and of te power

transformer +ein! protected*

/it te e0ception of te inr%s and o-ere0citation c%rrents. most of te oter

pro+lems. can +e sol-ed +y means of te percent differential relay. wic adds to te

normal differential relay two restrainin! coils fed +y te (one'tro%! c%rrent. +y

proper coice of te res%ltin! percent differential caracteristic. and +y proper

connection of te c%rrent transformers on eac side of te power transformer*

Percenta!e restraint differential protecti-e relays a-e +een in ser-ice for many

years* Figure 1sows a typical differential relay connection dia!ram* Differential

elements compare an operatin! c%rrent wit a restrainin! c%rrent*

Te operating current  #also called differential current $. I d . can +e o+tained from

te phasor sumof te c%rrents enterin! te protected element1

"i!%re 2 – Simple

dia!ram connection for differential power transformer protection

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I d  is proportional to te fa%lt c%rrent for internal fa%lts and approaces (ero for any

oter operatin! #ideal $ conditions*

Tere are different alternati-es for o+tainin! te restraining current . I RT * Te most

common ones incl%de te followin!1

/ere k  is a compensation factor. %s%ally ta3en as 1 or 0,5 *

Te differential relay !enerates a tripping signal  if te differential c%rrent. I d . is

!reater tan a percenta!e of te restrainin! c%rrent. I RT  1

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substations/transformer-dierential-protection-scheme

Transformer Differential Protection

PrinciplesPOSTED !O" # $%#$ &' ED"()D *! P)OTE+T*O!,  T)(! S O) E )S *T0 #1 +OE!TS

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Transformer Differential Protection Principles #on poto Penelec Collins-ille transformer

s%+station 4 +y P Powerliner 5 "lic3r$

Inter'windin! fa%lts

Similar to +%s protections. transformers are protected +y differential relays*

Inter-winding faults #short circuits$ and !ro%nd fa%lts witin power transformers can

+e detected +y tis protection sceme*

"ail%re to detect tese fa%lts and )%ic3ly isolate te transformer may ca%se serious damage

to the device*

Remem+er tat a differential relay is +asically an instantaneo%s o-erc%rrent relay tat

operates on te difference of c%rrent flowin! into and o%t of te protected (one*

"or transformers te differential protection #Figure 1$ is +asically te same as tat for

a +%s +%t tere are certain differences tat we will loo3 more closely at*

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These differences are a direct result of three characteristics or a transformer

1.  transformer as a t%rns ratio so te c%rrent in is not really e)%al to te c%rrent

o%t* Te c%rrent transformers are not li3ely e0actly matced to te transformer t%rns

ratio so tere will always +e an %n+alance c%rrentin te operatin! coil of a transformer

differential relay*

2. Transformers re)%ire ma!netisin! c%rrent* Tere will +e a small c%rrent flow in te

transformer primary e-en if te secondary is open circ%ited*

3.  transformer as an inr%s c%rrent* Tere is a time period after a transformer is

ener!i(ed %ntil te ma!netic field in te core in alternatin! symmetrically* Te si(e and

te len!t of tis inr%s depends on te resid%al field in te core and te point in te

ac cycle te transformer is re'ener!i(ed*

In lar!e transformers in mi!t +e ten or twenty times te f%ll'load c%rrent initially and it

mi!t ta3e se-eral min%tes to red%ce to ne!li!i+le -al%es*

"i!%re 2 –

Transformer Differential Protection

Transformer differential relays a-erestraint coils as indicated in Figure 1* Te -al%e

of te operate c%rrent as to +e a certain set percenta!e i!er tan te c%rrent

flowin! in te restraint coils* "or tis reason transformer differential relays are said to

percenta!e'differential relays*

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Referrin! a!ain to Figure 1. yo% will notice tat wen te transformer is first

ener!i(ed. tere will not +e any c%rrent flowin! in !T" * Te !T1 secondary

c%rrent I1s flows tro%! +ot te restraint and operate coils and pre-ents operation

%nless te c%rrent is -ery i!*

Te restraint coils also pre-ent relay operation d%e to tap'can!es . were te ratio

of transformer inp%t to o%tp%t c%rrent can contin%o%sly -ary*

One oter item incl%ded in transformer differential relays. +%t not sown in te dia!ram. is

second armonic restraint*

/en transformers are first ener!i(ed tere is o-er'fl%0in! #saturation$ of te core and

te lar!e inr%s ener!i(in! c%rrent as a distorted wa-eform* Tis wa-eform is

descri+ed as a-in! i! second armonic content*

Te transformer differential relays ma3e %se of tis 3nown fact and add in e0tra

restraint wen it detects tis second harmonic * Tis e0tra feat%re pre-ents te

transformer from trippin! d%e to ma!neti(in! c%rrent wen +ein! ener!i(ed. +%t does

not add any time delay*

eca%se te differential relay will not operate wit load c%rrent or fa%lts o%tside te

protected (ones #through faults$. it can +e set to operate at a low -al%e of c%rrent

tere+y !i-in! rapid operation wen a fa%lt occ%rs* Tere is no need to time delay te

operation of te relay and terefore a fast actin! type of relay can +e %sed*

Resource Science and Reactor Fundamentals – Electrical – CNSC Technical

Training Grouphttp://electrical-engineering-portal.com/transformer-dierential-protection-principles

3 Main causes of false differential

current in transformer POSTED () $# $%#2 &' ED"()D *! P)OTE+T*O!,  T)(!S O ) E) S *T0 $ +OE!TS

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8 9ain ca%ses of false differential c%rrent in transformer 

Differential c%rrent Certain penomena can ca%se a substantial differential current to flow. wen tereis no fa%lt. and tese differential c%rrents are !enerally s%fficient to ca%se apercenta!e differential relay to trip* ,owe-er. in tese sit%ations. te differentialprotection so%ld not disconnect te system +eca%se it is not a transformer internalfa%lt*

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Differential power

transformer protection sceme

Such phenomena can be due to the non-linearities in the transformer core.Some of these situations are considered below //

2*  Inr%s c%rrents:*  O-ere0citation conditions8*  C%rrent transformer sat%ration

2* Inr%s c%rrents

Magnetiing inrush current in transformers res%lts from any a+r%pt can!e of tema!neti(in! -olta!e* lto%! %s%ally considered as a res%lt of ener!i(in! atransformer. te ma!neti(in! inr%s may +e also ca%sed +y

• Occ%rrence of an e0ternal fa%lt

• ;olta!e reco-ery after clearin! an e0ternal fa%lt

• Can!e of te caracter of a fa%lt #for e0ample wen a pase'to'!ro%nd fa%lt e-ol-es

into a pase'to'pase'to'!ro%nd fa%lt$• O%t'of'pase syncroni(in! of a connected !enerator 

 n e0ample of inrush current following a reclose operation meas%red at tedistri+%tion s%+station +rea3er*

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"i!%re 8 – E0ample

inr%s c%rrent meas%red at a s%+station #many distri+%tion transformers to!eter$

Since te magnetiing branch representin! te core appears as a s%nt element inte transformer e)%i-alent circ%it. te ma!neti(in! c%rrent %psets te +alance +etweente c%rrents at te transformer terminals. and is terefore e0perienced +y tedifferential relay as a !false" differential current*

Te relay. owe-er. m%st remain sta+le d%rin! inr%s conditions* In addition. from testandpoint of te transformer life'time. trippin!'o%t d%rin! inr%s conditions is a #er$

undesirable situation< rea3in! a c%rrent of a p%re ind%cti-e nat%re !enerates i!

o-er-olta!e tat may =eopardi(e te ins%lation of a transformer and +e an indirect ca%se of an

internal fa%lt$*

The following summaries the main characteristics of inrush currents //

• >enerally contain DC offset. odd armonics . and e-en armonics*

• Typically composed of %nipolar or +ipolar p%lses. separated +y inter-als of -ery low

c%rrent -al%es*• Pea3 -al%es of %nipolar inr%s c%rrent p%lses decrease -ery slowly*

• Time constant is typically m%c !reater tan tat of te e0ponentially decayin! dcoffset of fa%lt c%rrents*

• Second'armonic content starts wit a low -al%e and increases as te inr%s c%rrent

decreases*

Read more about practical considerations of transformer inrush current   

RE!" #$RE 

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>o +ac3 to Sit%ations ?

:* O-ere0citation conditionsO-ere0citation of a transformer co%ld ca%se unnecessar$ operation of transformerdifferential rela$s* Tis sit%ation may occ%r in !eneratin! plants wen a %nit'connected !enerator is separated wile e0portin! ;Rs* Te res%ltin! s%dden -olta!erise impressed on te %nit transformer windin!s from te loss of ;R load can ca%se ai!er tan nominal -olts per ert( condition and. terefore. an o#ere%citation e#ent*

This could also occur in transmission s%stems where large reactive load is tripped

from a transformer with the primar# winding remaining energi$ed &

/en te primary windin! of a transformer is o-ere0cited and dri-en into sat%ration.more power appears to +e flowin! into te primary of te transformer tan is flowin!o%t of te secondary* differential relay. wit its inp%ts s%pplied +y properly selectedCTs to accommodate ratio and pase sift. will percei-e tis as a c%rrent differential+etween te primary and secondary windin!s and. terefore. will operate*

Tis wo%ld +e an %ndesira+le operation. as no internal fault would e%ist. wit te c%rrent

im+alance +ein! created from te o-ere0citation condition*

Since o-ere0citation manifests itself wit te prod%ction of odd armonics. and sincete tird armonic #and oter triples$ may +e effecti-ely cancelled in @ transformerwindin!s. ten. te fift armonic can +e %sed as a restrainin! or a +loc3in! )%antity inte differential relay in order to discriminate +etween te o-er'e0citation and te fa%ltystate*

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SIPROTEC 4 7UT6 Differential Protection Relay for Transformers – Connection of transformer 

differential protection wit i! impedance RE" #I7$ and ne%tral c%rrent meas%rement at I&

>o +ac3 to Sit%ations ?

8* C%rrent transformer sat%ration

Te effect of CT sat%ration on transformer differential protection is do%+le'ed!ed* lto%!. te percenta!e restraint red%ce te effect of te %n+alanced differentialc%rrent. in te case of an e0ternal fa%lts. te res%ltin! differential c%rrent wic may +eof #er$ high magnitude can lead to a rela$ male-operation

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For internal faults, the harmonics resulting from CT saturation could dela%

the operation of differential rela%s having harmonic restraint&

>o +ac3 to Sit%ations ?

Reference %%  Transformer "ifferential 'rotection Scheme (ith )nternal Faults"etection !lgorithm *sing Second +armonics Restrain !nd Fifth +armonics loc-ing.ogic – $uahdi "ris, Farag& #& Elmareimi and Re-ina Fouad 

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