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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
http://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-schemehttp://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-schemehttp://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-schemehttp://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-scheme
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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
http://electrical-engineering-portal.com/download-center/books-and-guides/power-
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
http://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-schemehttp://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-schemehttp://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/2012/11http://electrical-engineering-portal.com/author/edvardhttp://electrical-engineering-portal.com/category/protectionhttp://electrical-engineering-portal.com/category/transformers-2http://electrical-engineering-portal.com/transformer-differential-protection-principles#commentshttp://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-schemehttp://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-differential-protection-schemehttp://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/2012/11http://electrical-engineering-portal.com/author/edvardhttp://electrical-engineering-portal.com/category/protectionhttp://electrical-engineering-portal.com/category/transformers-2http://electrical-engineering-portal.com/transformer-differential-protection-principles#comments
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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*
http://electrical-engineering-portal.com/applications-and-forms-of-differential-relayshttp://electrical-engineering-portal.com/overcurrent-protection-transformer-nec-450-3http://electrical-engineering-portal.com/applications-and-forms-of-differential-relayshttp://electrical-engineering-portal.com/overcurrent-protection-transformer-nec-450-3
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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
http://electrical-engineering-portal.com/using-tap-changers-to-match-the-system-voltagehttp://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/false-differential-current-transformerhttp://electrical-engineering-portal.com/false-differential-current-transformerhttp://electrical-engineering-portal.com/2016/03http://electrical-engineering-portal.com/author/edvardhttp://electrical-engineering-portal.com/category/protectionhttp://electrical-engineering-portal.com/category/transformers-2http://electrical-engineering-portal.com/false-differential-current-transformer#commentshttp://electrical-engineering-portal.com/using-tap-changers-to-match-the-system-voltagehttp://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/false-differential-current-transformerhttp://electrical-engineering-portal.com/false-differential-current-transformerhttp://electrical-engineering-portal.com/2016/03http://electrical-engineering-portal.com/author/edvardhttp://electrical-engineering-portal.com/category/protectionhttp://electrical-engineering-portal.com/category/transformers-2http://electrical-engineering-portal.com/false-differential-current-transformer#comments
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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*
http://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/transformer-differential-protection-principleshttp://electrical-engineering-portal.com/transformer-differential-protection-principles
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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$• O%t'of'pase syncroni(in! of a connected !enerator
n e0ample of inrush current following a reclose operation meas%red at tedistri+%tion s%+station +rea3er*
http://electrical-engineering-portal.com/false-differential-current-transformer#inrush-currentshttp://electrical-engineering-portal.com/false-differential-current-transformer#overexcitation-conditionshttp://electrical-engineering-portal.com/false-differential-current-transformer#current-transformer-saturationhttp://electrical-engineering-portal.com/false-differential-current-transformer#inrush-currentshttp://electrical-engineering-portal.com/false-differential-current-transformer#overexcitation-conditionshttp://electrical-engineering-portal.com/false-differential-current-transformer#current-transformer-saturation
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"i!%re 8 – E0ample
inr%s c%rrent meas%red at a s%+station #many distri+%tion transformers to!eter$
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
http://electrical-engineering-portal.com/essential-basics-of-total-harmonic-distortion-thdhttp://electrical-engineering-portal.com/practical-considerations-of-transformer-inrush-currenthttp://electrical-engineering-portal.com/essential-basics-of-total-harmonic-distortion-thdhttp://electrical-engineering-portal.com/practical-considerations-of-transformer-inrush-current
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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*
http://electrical-engineering-portal.com/false-differential-current-transformer#situationshttp://electrical-engineering-portal.com/false-differential-current-transformer#situations
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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
http://electrical-engineering-portal.com/applications-and-characteristics-of-differential-relays-ansi-87http://electrical-engineering-portal.com/false-differential-current-transformer#situationshttp://electrical-engineering-portal.com/applications-and-characteristics-of-differential-relays-ansi-87http://electrical-engineering-portal.com/false-differential-current-transformer#situations