White Paper for Current Transformer Applications for Protective Relaying_Pending TOS

8/9/2019 White Paper for Current Transformer Applications for Protective Relaying_Pending TOS

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Document name Relaying Current Transformer Application

Category ( ) Regional Reliability Standard

( ) Regional Criteria

( ) Policy

( ) Guideline

(X) Report or other

( ) Charter

Document date July 10, 01!

"dopted#appro$ed by %perating Committee

Date adopted#appro$ed

Cu&todian (entity

re&pon&ible 'or

maintenance and

upeep)

Relay or Group

Stored#'iled Phy&ical location*

eb +R*

Pre$iou& name#number (i' any)

Statu& (X) in e''ect

( ) u&able, minor 'ormatting#editing re-uired

( ) modi'ication needed

( ) &uper&eded by .....................

( ) other .............................( ) ob&olete#archi$ed)

W E S T E R N E L E C T R I C I T Y C O O R D I N A T I N G C O U N C I L W W W . W E C C . B I Z

1 5 5 N O R T H 4 0 0 W E S T S U I T E 2 0 0 S A L T L A K E C I T Y U T A H 8 4 1 0 3 - 1 1 1 4 P H 8 0 1 . 5 8 2 . 0 3 5 3


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White Paper:Relaying Current Transformer Application

y

Relay Work Group

Western Electricity Coordinating Council

July !" #!$

Relaying Current Transformer Application

Ta%le of Contents

&ntroduction''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''(




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July 10, 01! o' /1

&' C)RRE*T TRA*+,-R.ER C/ARACTER&+T&C+''''''''''''''''''''''''''''''''''''''''''''''''''(

"2 Core Con&truction and "ccuracy Cla&&e&222222222222222222222222222222222222222222222222222222222222/

2 34citing Current22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222225

C2 Remanence22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222226

D2 7hermal Rating&2222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222228

&&' C)RRE*T TRA*+,-R.ER 0)RDE*'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''

"2 General22222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222222211

2 3''ect o' C7 Connection&22222222222222222222222222222222222222222222222222222222222222222222222222222222222211

C2 3''ect o' "u4iliary C7&222222222222222222222222222222222222222222222222222222222222222222222222222222222222222221

&&&' E+T&.ATE+ -, CT TRA*+&E*T PER,-R.A*CE'''''''''''''''''''''''''''''''''''''''''''''''1

"2 9&: Saturation 'actor2222222222222222222222222222222222222222222222222222222222222222222222222222222222222222221!

2 71 : primary &y&tem time con&tant222222222222222222222222222222222222222222222222222222222222222222221!

C2 7 : Current tran&'ormer &econdary time con&tant22222222222222222222222222222222222222222221/

&2' PER,-R.A*CE RE3)&RE.E*T+ -, C)RRE*T TRA*+,-R.ER+ ,-R RE4A5&*G

P)RP-+E+''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''6

"2 Di&tribution ;eeder&, Pha&e and Ground %$ercurrent222222222222222222222222222222222222221? 7ran&mi&&ion ine Sy&tem&222222222222222222222222222222222222222222222222222222222222222222222220

2' E7A.P4E+ -, CA4C)4AT&-*+'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''1

"2 Ratio and Pha&e="ngle 3rror o' C7*22222222222222222222222222222222222222222222222222222222222222222221

2 3&timation o' Current 7ran&'ormer Per'ormance*22222222222222222222222222222222222222222222225

C2 "@SA C7 Relaying "ccuracy Cla&&e&*2222222222222222222222222222222222222222222222222222222222222222oBe$er, the energy di&&ipated in each di& i& a 'unction o' the current through thedi& and the $oltage drop acro&& it &o it i& u&ually nece&&ary to add a &erie& pair o'

tBo di&& in parallel Bith the 'ir&t pair to handle the e4tra energy2

Since the in&tantaneou& or high=current tripping relay i& in &erie& Bith the $oltage=

dependent re&i&tor di&, it& picup increa&e& dramatically2 " typical change in picup

&etting might be 'rom " to /0"2

Solution

7hi& approach i& to match the higher C7 ratio and the loBer one Bith a &pecial

au4iliary C72 7hi& au4iliary C7 mu&t ha$e di&tributed Binding& on a toroidal core (abu&hing C7)2 (@e$er try to u&e an ordinary au4iliary C7 in a current or $oltage bu&

di''erential circuit2)

" $ariation o' thi& method i& to u&e one o' the higher ratio C7& on one o' the PC&

'or double duty (au4iliary C7 and bu& di''erential C7)2 7he di&ad$antage o' thi&

&cheme i& that Bhen the PC Bith the higher ratio i& remo$ed 'rom &er$ice, the bu&

protection mu&t be remo$ed 'rom &er$ice2

Solution !




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7hi& &olution eliminate& the concern about remo$ing the critical matching C7 'rom

&er$ice, but the di''iculty o' determining the C7 &econdary current di&tribution in the

C7 connection& o'the 100*/ C7&HBhich i& re-uired to calculate a proper relay

&ettingHlimit& it& application2

Solution /

7hi& &olution pre&ent& a haard to e-uipment and per&onnel becau&e o' the high

$oltage that may be generated2 At re-uire& the calculation o' the $oltage de$eloped

by highe&t ratio C7 Bhen connected a& de&cribed abo$e2 7he pea $oltage

de$eloped mu&t be le&& than the circuit in&ulation rating (1/00 $olt& root=mean

&-uare (rm&)) by an appropriate &a'ety 'actor, and the tapped $oltage mu&t be

enough to &ati&'y the relay manu'acturerM& &etting in&truction2 34tra thyri&tor& or

$ari&tor& may be re-uired in the relay, or acro&& the unu&ed portion o' the C7(&)2

4o9 &mpedance DifferentiallyConnected 0us Protection Relays

%$ercurrent relay& ha$e long been connected in bu& di''erential circuit& Bith $arying

degree& o' &ucce&&2 " 'eB general rule& 'or their application are*

a2 +&e the highe&t a$ailable C7 ratio&2 @ote* the ma4imum through='ault

current &hould not e4ceed 0 time& the C7 rating2 ()

b2 @e$er u&e an ordinary au4iliary C72 A' an au4iliary C7 mu&t be u&ed, u&e a

toroidal type (bu&hing C7) Bith a di&tributed Binding2

c2 @e$er u&e an o$ercurrent di''erentially=connected relay near a generating

&tation or Bhere high X#R ratio& e4i&t &ince the time con&tant at the&e

location& Bill produce $ery &e$ere C7 &aturation problem&2

d2 @e$er u&e plunger=type or hinged=armature in&tantaneou& relay& Bithouttime delay in thi& type o' circuit unle&& they are &et $ery high2 7hey operate

too 'a&t and operate e-ually Bell on DC2 >oBe$er, they may be u&e'ul Bhen

u&ed in conKunction Bith induction di& o$ercurrent relay&2 A' the

in&tantaneou& unit i& &et 'or the &ame picup a& the time unit Bith the

contact& connected in &erie&, by dropping out 'a&ter it pre$ent& the time unit

'rom 'al&e tripping by coa&ting clo&ed a'ter the error current i& gone2 7hi&




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a&&ume& that the drop out o' the in&tantaneou& de$ice ha& not been e4tended

by the DC component o' an o''&et Ba$e2

e2 @e$er u&e on bu&e& that ha$e more than three or 'our &ource& o' 'ault

current2 7he current tran&'ormer on the 'aulted line Bill probably &aturate &o

&e$erely that the error current Bill cau&e the relay to trip 'or through='ault&2

'2 " &tabiliing re&i&tor can be $ery u&e'ul in impro$ing the &ecurity o' an

o$ercurrent relay in a di''erential circuit (&ee e4ample in calculation

&ection (),(5))2

.edium/igh&mpedance Percentage RestraintDifferential Relay

7hi& relay Ba& introduced in the +2S2 about 1oBe$er it i& trouble&ome and e4pen&i$e to bring the &econdarie& o' all o' the PC

C7& into the relay hou&e in&tead o' paralleling them in the 'ield2 Since the relay

operate& on one ampere, au4iliary C7& are u&ually re-uired 'or all o' the C7&2 7hi& i&

al&o e4pen&i$e and con&ume& much &pace2

Transformer Differential Relaying

Current tran&'ormer& u&ed 'or tran&'ormer di''erential relaying are &ubKect to &e$eral

'actor& that are not ordinarily pre&ent Bith other 'orm& o' di''erential protection2 7he

'olloBing are application principle& pertaining to electromechanical relay&, Bith

comment& appropriate to microproce&&or relay& incorporated2

a2 ecau&e o' the current tran&'ormation by the poBer ban, the C7 ratio&

may be di''erent to compen&ate 'or the di''erent primary current&2 hile many

C7 tap& and relay tap& are a$ailable, they &eldom mae a per'ect match2 7hi&

re&ult& in error current in the relay2 A' the tran&'ormer ha& a load tap changer,

thi& error Bill change Bith the tap po&ition2




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tran&'ormer impedance Bhen it 'loB& through the&e PC&2 A' the C7& are merely

paralleled, they can &aturate une-ually and produce error current& that may cau&e

an incorrect operation2 A' a ring bu& or breaer=and=a=hal' &cheme i& in$ol$ed, the

chance o' thi& type o' through='ault occurring i& high enough that each C7 &hould be

connected to it& oBn re&traint Binding2 A' the PC& are part o' a double=bu&=&cheme,

the chance& o' thi& through=current 'loBing are remote2

7Bo PC& Bith paralleled C7& al&o may pre&ent problem& Bith &ecurity 'or through=

'ault& on the loB &ide o' the tran&'ormer becau&e the error current o' the e4i&ting C7&

i& doubled2

Current tran&'ormer& connected in delta al&o may cau&e problem& Bith &ecurity 'or

through='ault& becau&e the current tran&'ormer mu&t circulate current through tBo

relay& and lead burden& 'or &ome 'ault& (&ee &ample problem)2

E' Directional -


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compared Ba$e'orm& may le&&en the operating tor-ue 'or 'ault& in the protected

one2 +nder e4treme ca&e&, pha&e &hi't& may e$en cau&e 'al&e operation 'or

'ault& in the re$er&e direction2

"& in electromechanical relay&, microproce&&or relay de&ign& al&o may u&e a

polariing &ource in addition to an operating current2 Some de&ign& incorporateBa$e'orm ero=cro&&ing point& 'or pha&e re'erence and may be &u&ceptible to

pha&e &hi't& cau&ed by C7 &aturation e''ect2 Some microproce&&or relay& may be

higher &peed (le&& than 02/ cycle& operating time) and loBer burden than their

electromechanical counterpart&2 7here'ore, depending upon the C7M& time to

&aturation (&ee Section AAA = N3&timate& o' C7 7ran&ient Per'ormanceN)

microproce&&or relay& may be le&& liely than electromechanical relay& to

operate (or not operate) incorrectly, becau&e the microproce&&or relay& may

ha$e completed their mea&urement& prior to C7 &aturation2 Iany microproce&&or

de&ign& 'ilter the input Ba$e'orm& and con&e-uently are 're-uency=dependent,

a& Bell2

2 %$ercurrent 3lement&

7he common electromechanical induction di& de&ign& u&ed 'or time=o$ercurrent

(7%C) characteri&tic& mea&ure the rm& operating current2 +nder C7 &aturation

condition&, the actual rm& $alue o' the relay current Bill be le&& than under non=

&aturated C7 condition&2 7here'ore, the 7%C relay element Bill tae longer to

operate than de&ired2 o&& o' relay coordination may re&ult 'rom thi& delay,

e&pecially Bith the application& u&ing more in$er&e time=o$ercurrent

characteri&tic relay&2 7ime=o$ercurrent relay& typically are not $ery &u&ceptible toDC o''&et& &ince DC o''&et& u&ually ha$e died out Bithin the delay period o' the

relay2 An&tantaneou& o$ercurrent unit& (A%C) can be $ery &en&iti$e to DC o''&et&2

7he DC o''&et& may cau&e high=le$el tran&ient &pie& in the relay current that

could cau&e the A%C element to trip 'or 'ault current le$el& beloB it& &et point2

"dditionally, 'or C7 di&tortion o' 'ault current& near the in&tantaneou& picup

&etting, the in&tantaneou& relay may ha$e an unde&irable trip delay (e2g2,

additional 0=/ m& or more)2

Iicroproce&&or relay de&ign& u&ing analog or digital techni-ue& (&uch a&

microproce&&or ba&ed) Bill typically 'ilter the input current Ba$e'orm to eliminateDC o''&et&, harmonic&, etc2 7hi& may alle$iate &ome o' the problem& relating to

the in&tantaneou& element&2 >oBe$er, the time=delay element&M operating time&

may e4perience the &ame type o' unpredictability, loBer &en&iti$ity, and e4tra

delay time& a& e4perienced Bith the electromechanical de&ign&2




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2 Di&tance Relay&

An general, the reach o' di&tance relay& Bill be a''ected by C7 &aturation2 7hi& i&

true Bhether the relay de&ign i& an electromechanical de&ign u&ing rm& current

mea&urement, or a microproce&&or de&ign re'erencing pea in&tantaneou& or

rm& current le$el& or $ariou& combination& o' Ba$e'orm $alue& 'or mea&urement27he di&tance relay Bill be de&en&itied 'or 'ault& near the end o' it& one becau&e

the relay Bill &en&e the 'ault a& being 'urther aBay than it actually i&, ba&ed on

the reduced current& produced 'rom the C7 &econdary circuit&2 Short line and

particularly one 1 in&tantaneou&=relay application& Bith a 'ault $alue near the

relay operating deci&ion point &hould be con&idered care'ully i' C7 &aturation i&

po&&ible2 " guideline re'erred to by G2D2 Roce'eller 'rom Con&olidated 3di&on

Co2 o' @eB Qor i&* N7o a$oid delayed tripping 'or 'ault& near the one 1 deci&ion

point, time to C7 &aturation &hould e4ceed 12/ cycle&2N Iicroproce&&or relay&

Bith their high=&peed operation and loB burden may be pre'erred in the&e

application&, depending on the indi$idual &ituation& and &y&tem& in$ol$ed2

7he directional characteri&tic& o' di&tance relay& are generally ade-uate and

&electi$e 'or re$er&e direction 'ault& e$en Bith pha&e &hi't& in the current

Ba$e'orm mea&urement2 >oBe$er, clo&e=in, high=magnitude 'ault& in the re$er&e

direction may re&ult in 'al&e tripping by directional relay&2 7hi& may occur i' the

re'erence &tabiliing $oltage a& mea&ured by the relay i& beloB a de&ign

minimum $alue in the pre&ence o' ero=cro&&ing pha&e &hi't error in the current

Ba$e'orm cau&ed by &aturated C7&2 3lectromechanical relay& u&ing induction

unit& produce an operating tor-ue ba&ed on the product o' an integrated 'ull cycle

o' $oltage and the 'undamental current Ba$e'orm2 7hi& reduce& the po&&ibility o'

di&tortion=related 'al&e tripping2

An &ummary, current tran&'ormer &aturation e''ect& &hould be con&idered Bhen

applying directional o$ercurrent and di&tance relay&2 ;or relay directional

element&, C7 &aturation may cau&e 'al&e directional interpretation created by

pha&e &hi't& betBeen the relati$e mea&urement point& on the di&torted current

and undi&torted $oltage Ba$e'orm& (or po&&ibly di&torted $oltage Ba$e'orm& i'

Coupling Capacitor ?oltage 7ran&'ormer& are u&ed)2 +npredictable or added time

delay and loBer &en&iti$ity may be e4perienced Bith o$ercurrent element&2

Anaccurate di&tance mea&urement&, ma4imum tor-ue angle characteri&tic

change&, and other 'actor&, may lead to 'al&e operation& in directional

o$ercurrent and di&tance relay&2 7he be&t Bay to a$oid the&e potential problem&

i& to tae precautionary &tep& through minimiing C7 &econdary burden, by u&ing

larger conductor &ie& in C7 lead&, and u&ing loBer burden relay& Bhen po&&ible,

combined Bith u&ing ade-uate accuracy cla&& C7&2 7he tran&ient re&pon&e

characteri&tic& o' the C7& &hould be con&idered (e2g2, time to &aturation) Bhen




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deciding on electromechanical or microproce&&or relay application&2 ith proper

planning mea&ure& and con&ideration o' the&e 'actor&, correct relay operation

&hould re&ult under the great maKority o' 'ault condition&2

,' E/2 Transmission 4ine +ystems

7he e''ect& on the&e relay &y&tem& due to C7 &aturation can be &eriou& becau&e o'

their operating &peed, the con'iguration o' current tran&'ormer &ource&, and the need

'or e4actly the &ame per'ormance at each terminal o' the relaying &y&tem&2

7he relay operating &peed i& typically eight=to=/ milli&econd&2 An many ca&e& the&e

may ha$e operated, be'ore any &aturation e''ect& tae place2 %n an e4ternal 'ault,

although &aturation i& le&& liely, the &y&tem Bill be dependent upon correct C7

operation throughout the 'ault period2

Io&t pha&e compari&on &y&tem& are de&igned to accept &igni'icant pha&e angle

error& Bithout undue e''ect&2 " more liely &ource o' problem& i& di&tortion o' themagnitude o' pha&e -uantitie& producing incorrect component& on Bhich proper

operation depend& primarily internal 'ault&2

Directional compari&on &y&tem& Bill ha$e per'ormance problem& &imilar to &tand=

alone directional in&tantaneou& element&2 %ne di''erence i& that in&tead o' Ku&t one

or tBo C7& a''ecting the de$ice&F per'ormance the C7& at both end& are in$ol$ed &o

there may be a& 'eB a& tBo or a& many a& &i4 C7& in$ol$ed2

7he con'iguration o' C7& i& an important con&ideration in the operation o' the&e

&cheme& &ince the primary current& in each C7 and each C7F& hi&tory determine&Bhat in'ormation i& &upplied to the relay2 7hi& e4po&e& the relay &y&tem to po&&ible

problem& 'rom any o' the connected C7&2 An many ca&e&, the 3>? &y&tem Ba&

de&igned 'or &hort circuit duty that ha& not and may not e$er be e4perienced2 7hi&

mean& that it may be many year& into the li'e o' the 3>? &y&tem be'ore trouble i&

e4perienced Bith C7 &aturation2 >oBe$er, the mi&matched C7& may produce

problem& long be'ore the problem& o' &aturation are e$ident2




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2' E7A.P4E+ -, CA4C)4AT&-*+

A Ratio and PhaseAngle Error of CT:

7he Ratio Correction ;actor (RC;) i& the 'actor by Bhich the mared ratio mu&t bemultiplied to obtain the true ratio2 7he true ratio e-ual& the mared ratio time& the

ratio correction 'actor2

ratiomarked

ratiotrueRCF=

here*

true ratio : the ratio o' rm& primary current to the rm& &econdary current, SP II

mared ratio : the ratio o' the rated primary current to the rated &econdary current

a& gi$en on the nameplate2

7he pha&e=angle o' a C7 i& de&ignated by the Gree letter 2 7he Pha&e="ngle

Correction ;actor (P"C;) i& the 'actor by Bhich the reading o' a Batt meter,

operated 'rom the &econdary o' a C7 mu&t be multiplied to correct 'or the e''ect o'

pha&e di&placement o' current due to the mea&uring apparatu&2 At i& the ratio o' the

true poBer 'actor to the mea&ured poBer 'actor2

( ) = coscosPACF

here*

: angle o' lag o' load current behind load $oltage

cos : poBer 'actor : co&ine o' the angle betBeen the $oltage and current

7he 'actor by Bhich the reading o' a Batt meter or the regi&tration o' a Batt hour

meter mu&t be multiplied to correct 'or the e''ect o' ratio error and pha&e angle i& the

7ran&'ormer Correction ;actor (7C;)2

PACFRCFTCF =




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E8ample :

A' a C7 ha& a pha&e angle error : O1/M and i& u&ed 'or mea&uring a load Bhere the

poBer 'actor i& 02/00 lagging, determine it& pha&e angle correction 'actor, P"C;2

12 7he primary current lag& the line $oltage by an angle Bho&e co&ine e-ual& thepoBer 'actor2

( ) == !"!!#!cos 1

or

"!!#!!coscos ==

2 7he &econdary current lead& the primary current by 1/M2 7here'ore, the

&econdary current actually lag& the primary $oltage by '$""% 2

'!"%! ==

( ) '$""%'1"!'!"% ==

7hu&, ( ) ( ) "!3!'$""%coscos ==

%%2"#!"!3!

"!!#!

)cos(

cos ==

=

PACF

A' the C7 ha& an RC; o' 12000, Bhat i& the 7C; at the &ame poBer 'actor

%%$"#!

%%2"#!!!2!#1

==

= PACFRCFTCF

E8ample #:

Calculation o' the ratio o' relaying C72

Con&ider a bu&hing C7 Bith the 'olloBing characteri&tic&*

==

=

!#2%&resistancesecondarCC1!!ationclassiicrelain*

"!!ratio




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12 7he rating o' C100 mean& that the ratio i& to be calculated on the ba&i& o' 100

$olt& at the &econdary terminal& Bith 100 amp& 'loBing through the burden,

Bhich in turn mean& a burden o' =12 Re'erring to 7able 1 'or a =1 burden*

inductancem+2#3L

resistance!#"R

==

==

2 7he inducti$e reactance, LX , i& then calculated*

( )=

==

!1!3#2!1$#32

23H

fLXL

here ' : 're-uency in >

2 7o obtain the induced or e4citation $oltage SEE

, the re&i&tance o' the &econdary

Binding mu&t be added to the impedance, , o' the burden2

22XRZ +=

( ) ( ) 22 !#&!#"2%! ++=Z

= 1

7he induced $oltage i& there'ore*

( ) ( ) VampsESE 11&1!! ==

Re'erring to ;igure 10, the e4citation cur$e 'or thi& tran&'ormer indicate& that an

e4citation current, Ae, o' 021/ amp& i& re-uired to produce thi& $oltage2

Since thi& e4citation current i& &hoBn a& the e-ui$alent &econdary current, it

&hould be compared to the burden &econdary current o' 100 amp&2

7he ratio o' e4citing current to burden current i&,

!!1"#!1!!1"#! =

7here'ore the Ratio Correction ;actor (RC;) i& 12001/2




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7able 1

Standard urden& 'or Current 7ran&'ormer&

Bith 'i$e=amp &econdarie&

urden

De&ignation

Re&i&tance

(T)

Anductance

(m>)

Ampedance

(T)

?olt

"mpere&

(at /")

PoBer

;actor

Ietering urden&

=021 020< 02115 021 2/ 02

White Paper for Current Transformer Applications for Protective Relaying_Pending TOS

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Transcript of White Paper for Current Transformer Applications for Protective Relaying_Pending TOS