Iec Ts 60871-3-2015.Pdf

IEC TS 60871 3 Edition 2 0 201 5 06 TECHNICAL SPECIFICATION Shunt capacitors for AC power systems having a rated voltage above 1 000 V – Part 3 Protection of shunt capacitors and shunt capacitor banks[.]

IEC TS 60871 - 3

Edit io 2.0 2 15-0

Shunt capacit ors for A C pow er sy stems hav ing a rated v oltage abov e 1 000 V –

Part 3: Prot ect ion of shunt capacitors and shunt capacit or bank s

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IEC TS 60871- 3

Edit io 2.0 2 15-0

Shunt capacit ors for A C pow er sy stems hav ing a rated v oltage abov e 1 00 V –

Part 3: Protect ion of shunt capacit ors and shunt capacit or bank s

CONTENTS

FOREWORD 5

1 Sco e 7

2 Normative ref eren es 7

3 Terms an def i ition 7

4 Internal f uses 7

4.1 General 7

4.2 Fu e c aracteristic 8

4.2.1 Rated c r ent 8

4.2.2 Rated dis harge ca a i ty 8

4.2.3 Dis on ectin ca a i ty 8

4.2.4 Voltage with tan ca a i ty af ter o eration 8

4.3 In uen e of ca acitor element con g ration on ca acitor l f e 8

4.3.1 Ca acitor with al elements con ected in p ral el 8

4.3.2 Ca acitor with elements con ected in series an p ral el 8

5 External f uses 8

5.1 General 8

5.2 Fu e c aracteristic 9

5.2.1 Rated c r ent 9

5.2.2 Rated voltage 9

5.2.3 Time-c r ent c aracteristic 9

5.2.4 Dis harge ca a i ty 10 5.3 Fu e typ s 10 5.3.1 General 10 5.3.2 Expulsion f uses 10 5.3.3 Cur ent-l mitin f uses 11 5.3.4 Combination c r ent-lmitin /expulsion f uses 1

5.4 In uen e of ca acitor b n con g ration on f use selection 11 5.4.1 Sin le series section grou ded star an delta b n s 1

5.4.2 Sin le series section u grou ded star b n s 11

5.4.3 Multiple series section b n s 11

5.5 Co rdination with case rupture c rves 11

6 Unb lan e detection 12

6.1 Op ration 12

6.2 Typ s of u b lan e protection 12

6.2.1 Neutral c r ent (Fig re 3) 12

6.2.2 Neutral voltage (Fig re 4) 12

6.2.3 Cur ent u b lan e b twe n neutrals (Fig re 5) 13

6.2.4 Phase voltage u b lan e (Fig re 6) 13

6.2.5 Voltage dif f eren e (Fig re 7) 13

6.2.6 Cur ent u b lan e in brid e con ection (Fig re 8) 13

6.3 Cur ent an voltage tran f ormers 13

6.3.1 Cur ent tran f ormers 13

6.3.2 Voltage tran f ormers 14

6.4 Relay an protection set in s 14

6.5 Sen itivity 14

6.6 Initial u b lan e 15

7 Overlo d c r ent 15

7.1 Op ration 15

7.2 Protective ar an ement 15

7.3 Cur ent tran f ormers 15

7.4 Relay 15

7.5 Protective setin s 16

8 Over an u dervoltage 16

8.1 Op ration 16

8.2 Overvoltage protection 16

8.3 Un ervoltage protection 16

8.4 Reclosin 16

9 Other protection 17

9.1 Surge ar esters 17

9.1.1 General 17

9.1.2 Op ration 17

9.1.3 Lig tnin tran ients 17

9.1.4 Switc in tran ients 17

9.1.5 Temp rary overvoltages 17

9.1.6 Rated voltage 17

9.1.7 Energ a sorption 18

9.2 Dampin devices 18

9.2.1 Ca acitor switc in 18

9.2.2 Inru h c r ents 18

9.2.3 Voltage tran ients 19

9.2.4 Ratin s 19

9.3 Sy c ronized switc in 19

9.3.1 Op ration 19

9.3.2 Bre ker contacts delay 19

10 Saf ety 19

10.1 Dis hargin devices 19

10.1.1 General 19

10.1.2 Internal resistors 20

10.1.3 External dis harge devices 2

10.1.4 Dis hargin af ter dis on ection 2

10.2 De d metal c p rts 2

Bibl ogra h 2

Fig re 1 – Fu e typ s 10 Fig re 2 – Typical case rupture c rves f or a proximately 3 0 0 cm³ case volume 21

Fig re 3 – Star con ection with the neutral grou ded throu h a c r ent tran f ormer 21

Fig re 4 – Star con ection with voltage tran f ormer b twe n neutral an grou d 21

Fig re 5 – Star con ection with u grou ded neutral an voltage tran f ormers con ected in an o en delta 22 Fig re 6 – Double-star con ection with u grou ded neutral 2

Fig re 7 – Star con ection with grou ded neutral an voltage tran f ormers con ected in dif f erential me s rement 2

Fig re 8 – Brid e con ection 2

Fig re 9 – Line overc r ent relay f or ca acitor b n , grou ded 22

Fig re 10 – Line overc r ent relay f or ca acitor b n , u grou ded 2

Ta le 1 – Meltin c r ents f or typ -K (f ast f use l n s, in amp res 2

Ta le 2 – Meltin c r ents f or typ -T (slow) f use l n s, in amp res 2

INTERNATIONAL ELECTROTECHNICAL COMMISSION

Part 3: Protection of shunt capacitors and

shunt capacitor banks

1 Th Intern tio al Ele trote h ic l Commis io (IEC) is a worldwid org niz tio for sta d rdiz tio c mprisin

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• the req ired s p ort can ot b o tained f or the publ cation of an International Stan ard,

despite re e ted ef f orts, or

• the s bject is sti u der tec nical develo ment or where, f or an other re son, there is

the f uture but no immediate p s ibi ty of an agre ment on an International Stan ard

International Stan ard IEC 6 8 1-3, whic is a tec nical sp cif i ation, has b en pre ared by

IEC tec nical commit e 3 : Power ca acitors an their a pl cation

This secon edition can els an re laces the f irst edition publ s ed in 19 6 This edition

con titutes a tec nical revision

This edition in lu es the f ol owin sig if i ant tec nical c an es with resp ct to the previou

edition:

a) Cle rer writin of f ormulas on energ l mitation f or expulsion f uses;

b) Updated normative ref eren es an bibl ogra h ;

c) A new clau e f or s n hronized switc in has b en ad ed

The text of this tec nical sp cif i ation is b sed on the f ol owin doc ments:

En uiry draft Re ort o v tin

Ful inf ormation on the votin f or the a proval of this tec nical sp cif i ation can b f ou d in

the re ort on votin in icated in the a ove ta le

This publ cation has b en draf ted in ac ordan e with the ISO/IEC Directives, Part 2

A l st of al p rts in the IEC 6 8 1, publ s ed u der the general title Sh nt c p citors for a.c

p wer s stems h vin a rated v lta e a o e 1 0 0 V, can b f ou d on the IEC we site

The commite has decided that the contents of this publ cation wi remain u c an ed u ti

the sta i ty date in icated on the IEC we site u der "ht p:/we store.iec.c " in the data

related to the sp cif i publcation At this date, the publcation wi b

• tran f ormed into an International stan ard,

SHUNT CAPACITORS FOR AC POWER SYSTEMS HAVING

Part 3: Protection of shunt capacitors and

shunt capacitor banks

This p rt of IEC 6 8 1, whic is a tec nical sp cif i ation, gives g idan e on the protection of

s u t ca acitors an s u t ca acitor b n s it a pl es to ca acitors ac ordin to IEC 6 8 1

-1

2 Normative ref erences

The f ol owin doc ments, in whole or in p rt, are normatively ref eren ed in this doc ment

an are in isp n a le f or its a pl cation For dated ref eren es, only the edition cited a pl es

For u dated ref eren es, the latest edition of the ref eren ed doc ment (in lu in an

amen ments) a pl es

IEC 6 5 9, H igh-v lta e fu e for the ex tern l prote tion of s u t c p citors

IEC 6 8 1-1, Sh nt c p citors for a.c p wer s stems h ving a rated v lta e a o e 1 0 0 V

Internal f uses f or s u t ca acitors are selective c r ent-l mitin f uses ar an ed in ide a

ca acitor As def i ed in IEC 6 8 1-4, they are desig ed to isolate f aulted ca acitor elements

or ca acitor u it, to al ow o eration of the remainin p rts of that ca acitor u it an the b n

in whic the ca acitor u it is con ected

The o eration of an internal f use is initiated by the bre k own of a ca acitor element The

af f ected element is in tantane u ly dis on ected by the o eration of the element f use

without inter uption in the o eration of the ca acitor

The n mb r of external y p ral el con ected ca acitors an the avai a le s ort-circ it c r ent

of the s p ly s stem s ould not af f ect the c r ent-l mitin of internal f uses

It s ould b noted that internal f uses do not provide protection again t a s ort circ it b twe n

internal con ection or a s ort circ it b twe n active p rts an casin , b th of whic may

le d to case rupture

4.2 Fus c ara teristic

There is no def i ition or test method existin f or element f uses

Element f uses are, in general, desig ed f or mu h hig er c r ents than the maximum

p rmis ible element c r ent They are me nt to dis on ect only f aulty elements The f aulty

elements an their f uses are not inten ed to b re laced

4.2.2 Rate dis harge c pabi ty

IEC 6 8 1-4 an IEC 6 871-1 sp cif y that the ca acitor b s bject to f i e u damp d

dis harges f rom a d.c c arge level of 2,5 U

N For sp cial a pl cation , where inru h c r ents

an /or p ak voltages are l mited, lower dis harge req irements are a pl ca le

4.2.3 Dis onne ting c pabi ity

Req irements an test proced res are given in IEC 6 8 1-4 These tests verif y that the f use

has a c r ent-l mitin action

4.2.4 Volta e withsta d c pabi ity af ter operation

Req irements an test proced res are given in IEC 6 8 1-4

4.3 Influe c of c pa itor eleme t configuration on c pa itor l f e

4.3.1 Capa itor with al eleme ts conn cte in paral el

Af ter the bre k own of an element, the resp ctive f use wi melt in les than a mi isecon

owin to the dis harge c r ent f rom the p ral el con ected elements an ca acitors an the

p wer f req en y c r ent f rom the s p ly The ca acitor may, however, contin e o eratin

with a cor esp n in ly red ced output

If the ca acitor is op rated at a f i ed bu voltage, no variation in o eratin voltage on the

remainin he lth elements wi oc ur

4.3.2 Capa itor with eleme ts conne te in s rie a d paral el

Af ter the bre k own of an element, al p ral el con ected elements dis harge their stored

energ or p rt of it into the f aulty element The p wer f req en y c r ent is l mited by the

remainin he lth elements con ected in series

Af ter the dis on ection of the f aulty element, the ca acitor contin es o eratin with a

cor esp n in ly red ced output The remainin he lth elements of the group are then

stres ed with a voltage a proximately m × n /[m (n – 1) + 1] times the initial voltage, where n

is the n mb r of p ral el con ected elements p r group an m the n mb r of seri

es-connected sections p r u it In certain cases the voltage may b hig er, f or example d e to

neutral s if t with an u grou ded star con g ration

5 External f uses

External f uses f or s u t ca acitors are def i ed in IEC 6 5 9 as inten ed to cle r f aults in ide

a ca acitor u it an to p rmit contin ed o eration of the remainin p rts of the b n in whic

the u it is con ected They wi also cle r an external ca acitor bu hin f las over

The o eration of an external f use is general y determined by the p wer f req en y f ault

c r ent an by the dis harge energ f rom ca acitors con ected in p ral el with the f aulty

ca acitor

The initial bre k own is u ual y of an in ivid al element within a ca acitor This in aria ly

b comes a s ort circ it whic removes al elements in p ral el with it an el minates one

series section f rom the ca acitor Should the cau e of the initial f ai ure contin e, f ai ure of

s c es ive series section (whic se an in re sin voltage with e c series section

removed) wi oc ur This cau es an in re se in the c r ent throu h the ca acitor to the p int

where the f use o erates removin the f ai ed ca acitor f rom the circ it

It s ould b noted, p rtic larly in the case of p p r or p p r/f ilm dielectric ca acitors, that

the ca acitor case may oc asional y rupture in the event of f ai ure This oc urs when the

initial element f ai ure has hig resistan e b twe n the s orted electrodes d e to the

presen e of p p r an s stained arcin generates gas whic swel s the case to the p int

where it may rupture b f ore the protectin f use can dis on ect the ca acitor

Ca acitors with al -f ilm dielectric have a lower in iden e of case rupture b cau e the f ilm

melts an general y al ows a low resistan e s ort b twe n the electrodes However, case

rupture may sti oc ur d e to arcin when there is a broken internal con ection an when

there is ex es ive stored energ avai a le in p ral el ca acitors an /or hig p wer f req en y

f ault c r ent

5.2 Fus c ara teristic

The rated c r ent of the selected f use s ould b con istent with the criteria u ed f or the

selection of a switc or circ it-bre ker f or the same b n From the variou national

stan ard the minimum ac e ted ratin is 1,3 times the rated ca acitor c r ent

In a ste d -state b sis, there is no ne d f or the f use ca a i ty to ex e d that f or the switc

or circ it-bre ker However, tran ient con ition s c as c r ents as ociated with s stem or

b n switc in s ould b con idered It is common to u e a f use with a c r ent ratin of 1,6

times the rated ca acitor c r ent

IEC 6 5 9 sp cif ies that the f use rated c r ent b at le st 1,4 times the ca acitor ratin

This f als b twe n the two values mentioned a ove of 1,3 an 1,6 For some b n s, the

fuse ratin may b hig er than 1,6 times the ca acitor rated c r ent to avoid spuriou f use

o eration d e to switc in tran ients an f or mec anical re son

NOT Th c ntin o s ratin of th fu e is n t n c s ariy its n me late ratin For e ample, a e p lsio fu e

ln with a ratin mu h smaler th n th ratin of th fu e h ld r c n c ry 15 % of its n me late ratin o a

c ntin o s b sis It is e tremely imp rta t th t th a tu l c re t ratin of th fu e ln b k own Ty ic ly, fu e

h ld rs are a aia le in two c re t ratin s, o e for u to 5 A a d th oth r for u to 10 A, wh re s fu e ln s

u e in th s h ld rs are rate fom 5 A to 10 A Th s h ld rs als v ry in v lta e ratin , e.g u to 9 kV, 9 kV

to 16 kV a d 16 kV to 2 kV

5.2.2 Rate volta e

The rated voltage of the f use s ould b not les than 1,1 times the rated voltage of the

ca acitor with whic it is as ociated in order to me t the req irements of IEC 6 5 9

5.2.3 Time-c r e t c ara teristic

Time-c r ent c aracteristic are avai a le f rom most f use man f acturers to as ist in

co rdination

This inf ormation is sometimes avai a le in ta le f orm

5.2.4 Dis harg c pabi ty

The external f use s ould b ca a le of with tan in inru h tran ients an c r ents d e to

external s ort circ its IEC 6 5 9 sp cif ies tests to verif y the I

The f ol owin inf ormation on expulsion f uses s ould b noted:

a) Expulsion f uses are normal y u ed in outdoor a pl cation d e to noise an gases

rele sed d rin f use o eration

b) Expulsion f uses have l mited p wer f requen y f ault c r ent ca a i ty Theref ore con ult

with the f use man f acturer when f ault c r ent at the f use ex e d 1 8 0 A, or u e

c r ent-l mitin f uses

Floatin star con ected b n s an those with multiple series section minimize the

imp rtan e of the p wer f req en y inter uptin cap bi ty of the f use

c) Expulsion f uses have l mited a i ty to cle r again t the dis harge energ of ca acitors

con ected in p ral el with a s orted ca acitor Stan ard f uses are general y rated at

15 kJ or les ; con ult the f use man f acturer

Both f use tub s an ca acitor cases may rupture d e to energ avai a le in the event of a

ca acitor f ai ure The pro a i ty of case rupture in the event of ca acitor f ai ure is

general y con idered ac e ta le with al -f ilm ca acitors when the p ral el energ has

general y b en l mited to 15 kJ This l mit is calc lated on the b sis that the ca acitor

voltage is at 1,1 times the pe k value of rated voltage (when hig er p wer f req en y

overvoltages are anticip ted, the p ral el energ s ould b red ced ac ordin ly) At rated

voltage, this l mit is eq ivalent to 4 6 0 k ar of p ral el con ected ca acitors at 6 Hz an

3 9 0 k ar at 5 Hz For al -p p r an f ilm/p p r ca acitors, the energ is typical y l mited

to 10 kJ

From energ in W.s (J): Energ = C × (U

rms)2

Substitutin ca acitan e: C (µF) =

2

NN2

d) Expulsion f use l n s are avai a le in ANSI Typ T an Typ K (se bibl ogra h )

The dif f eren e in p rf orman e is in the time f or meltin of the l n , as s own in Ta les

5.3.3 Cur e t l miting fus s

The f ol owin inf ormation on c r ent-l mitin f uses s ould b noted:

a) Cur ent-l mitin f uses may b u ed f or in o r an outdo r a pl cation

b) Cur ent-l mitin f uses wi l mit the p wer f req en y s ort-circ it c r ent to les than the

prosp ctive value an wi red ce the c r ent to zero b f ore the normal workin f req en y

c r ent zero

c) General y, c r ent-l mitin f uses imp se no up er l mit on the p ral el stored energ

avai a le to a s orted ca acitor However, some c r ent-l mitin f uses have a maximum

l mit f or p ralel energ The f use man f acturer s ould b con ulted regardin the

dis harge energ inter uptin ratin

d) It s ould b noted that some f uses wi not cle r on p wer f req en y c r ent The f use

man f acturer s ould b con ulted regardin the inter uptin ratin f or p wer f req en y

c r ent

5.3.4 Combination c r e t l miting/e pulsion f us s

As the name impl es, these f uses combine a total y en losed c r ent-l mitin f use with an

expulsion f use

a) As with expulsion f uses, combination f uses are normal y u ed in outdo r a pl cation d e

to noise an gases rele sed d rin f use o eration

b) As with c r ent-l mitin f uses, combination f uses wi l mit the p wer f req en y s o

rt-circ it c r ent to les than the prosp ctive value an wi red ce the c r ent to zero

b f ore the normal workin f req en y c r ent zero

c) As with c r ent-l mitin f uses, combination f uses general y have no up er l mit on the

p ral el stored energ avai a le to a s orted ca acitor However, some combination f uses

have a maximum l mit f or p ral el energ The f use man f acturer s ould b con ulted

regardin the dis harge energ inter uptin ratin

d) It s ould b noted that some f uses wi not cle r on p wer f req en y c r ent The f use

man f acturer s ould b con ulted regardin the inter uptin ratin f or p wer f req en y

c r ent

5.4 Influe c of c pa itor ba k conf iguration on f us s le tion

5.4.1 Single s rie s ction grou de star a d delta ba k

Cur ent-l mitin or combination f uses are normaly req ired b cau e a s orted ca acitor is

s bjected to hig f ault c r ents that may cau e the f use holder or ca acitor case to rupture

5.4.2 Single s rie s ction unground d star ba k

The avai a le energ f rom p ralel con ected ca acitors wi pro a ly b the determinin

f actor in selectin either expulsion or one of the c r ent-l mitin o tion

5.4.3 Multiple s rie s ction ba k

Avai a le s ort-circ it c r ent is not a f actor in these b n s sin e the multiple series section

wi l mit the f ault c r ent throu h a s orted ca acitor In large b n s havin man ca acitors

in p ral el p r series section, expulsion typ f uses may of ten b u ed if the b n

con g ration is c an ed, e.g to double star, to l mit the p ral el energ

5.5 Coordination with c s rupture c rv s

In ad ition to the con ideration in f use selection alre d de lt with, the f use s ould

co rdinate with the rupture c rves f or the b n ca acitor u its

These c rves are avai a le in some national stan ard an f rom man f acturers of ca acitors.

Fig re 2 i u trates examples of these c rves

To minimize ris of case rupture, selected f uses s ould provide co rdination in the "low

pro a i ty" region Ref er to 5.3.2, 5.3.3 an 5.3.4 f or comments on energ f rom ca acitors

con ected in p ral el

6 Unbalance detection

Eac time an internal ca acitor element f ai s, a sl g t c an e of voltage distribution an

c r ent f low within the ca acitor b n is en ou tered The mag itu e of these c an es

de en s up n the n mb r of f ai ed elements an their location within the b n If an

external y f used ca acitor is dis on ected by its f use, a larger voltage an c r ent c an e is

o tained than if sin le elements are dis on ected by internal f uses

By the u e of variou b n con ection an relayin s hemes, the voltage or c r ent

u b lan e may b me s red an uti zed f or protection The main purp se of the u b lan e

protection is to give an alarm or to dis on ect the entire ca acitor b n when overvoltages

acros he lth ca acitors, adjacent to a f ai ed ca acitor, are ex es ive Normal y not more

than 10 % overvoltage s ould b al owed (overvoltage lmit ac ordin to IEC 6 8 1-1)

Another f un tion of the u b lan e protection is to remove the b n f rom service f or a f ault not

isolated by a f use or to protect b n s that are not internal y or external y f used Unbalan e

protection is not a re lacement f or s ort-circ it protection

6.2 Type of unbala c prote tion

6.2.1 Neutral c r e t (Figure 3)

The ca acitors are star con ected (grou ded) with a c r ent tran f ormer b twe n neutral an

grou d An u b lan e in the b n wi cau e c r ent to f low f rom neutral to grou d

This protection s heme is sen itive to phase u b lan es in the network an de en s up n

the s stem b in ef f ectively grou ded The setin s s ould al ow f or normal variation an

the sen itivity of the protection may theref ore b imp ired Harmonic c r ents (3rd harmonic

in p rtic lar) wi p s throu h the c r ent tran f ormer; a f ilter may sometimes b neces ary

to avoid disturb n es

6.2.2 Neutral volta e (Figure 4)

The ca acitors are star connected (u grou ded) with a voltage transf ormer b twe n neutral

an grou d A voltage dif f eren e b twe n neutral and grou d wi b me s red at u b lan e

The sen itivity is relatively p or d e to in uen e by phase u b lan es an the s heme

de en s up n the s stem b in ef f ectively grou ded The set in s s ould alow f or normal

variation an the sen itivity of the protection may theref ore b imp ired The method is most

s ita le in combination with external f uses

Voltage tran f ormers u ed in this a pl cation s ould b rated f or f ul s stem voltage The

neutral voltage wi rise sig if i antly d rin switc in an the tran f ormer may b saturated if

not cor ectly rated Resistive dividers an static relay may b u ed in te d of voltage

tran f ormers to overcome the pro lems with saturation, tran ient overvoltages on switc in

an the high cost of a voltage tran f ormer rated f or f ul s stem voltage

6.2.3 Cur e t u bala c betwe n ne trals (Figure 5)

The ca acitors are ar an ed in two p ral el stars (u grou ded) with a c r ent tran f ormer

b twe n the neutrals The stars do not have to b eq al in size An u b lan e in the b n

wi cau e c r ent to f low in the neutral

This protection is not af f ected by u b lance in the network an it is not p rtic larly sen itive

to harmonic The s heme may b u ed f or b th internal an external f uses As the

sen itivity p rf orman e is go d the method is esp cial y u ef ul f or internal f uses The c r ent

tran f ormer s ould b rated f or f ul s stem voltage

6.2.4 Pha e volta e unbala c (Figure 6)

The ca acitors are star con ected (u grou ded) an thre l ne-to-neutral voltage

tran f ormers are u ed with their secon aries con ected in o en delta An u b lan e in the

b n wi cau e a neutral s if t voltage an th s an output sig al f rom the o en delta These

voltage tran f ormers s ould have in ulation rated f or primary to grou d an primary to

secon ary voltages

Due to thre -phase s mmation the output mag itu e is hig er than would b the case with

the neutral to grou d me s rement (Fig re 4) The sen itivity p rf orman e is theref ore

improved The in uen e by phase u b lan es is sti a con ern

6.2.5 Volta e diff ere c (Figure 7)

The ca acitors are con ected u grou ded or grou ded star The voltage s if t of e c phase

midp int (or close to midp int is me s red relative to its l ne-to-neutral voltage by me n of

voltage tran f ormers Sig als wi b o tained se arately f rom e c phase where ca acitor

f ai ures oc ur

This method is s ita le f or large ca acitor b n s sin e the total b n wi b divided into

thre se arate protection zones This may b of imp rtan e f or overal sen itivity The

method is not in uen ed by phase voltage u b lan e

For very large ca acitor b n s a double star connection may b used where comp rison are

made b twe n the midp ints of the two bran hes of e c phase

6.2.6 Cur e t u bala c in bridg conne tion (Figure 8)

The ca acitors in e c phase are ar an ed in two bran hes with a c r ent tran f ormer

con ected b twe n midp ints or close to midp ints of the two bran hes F i ures an where

in the bran hes wi cau e an u b lan e c r ent to f low throu h the c r ent tran f ormer

This method is s ita le f or large ca acitor b n s sin e the total b n wi b divided into

thre se arate protection zones The method is not in uen ed by phase voltage u b lan es

It may b u ed in delta- or star con ected b n s with the neutral grou ded or u grou ded

6.3 Cur e t a d volta e tra sf ormers

6.3.1 Cur e t tra sformers

Rated c r ent is b sed on the calc lated u b lan e at dif f erent f ai ure modes Harmonic

c r ent s ould b con idered in determinin the ratin For internal y f used ca acitors the

c r ent may b very low an c r ent tran f ormers s ould b c osen ac ordin ly

For u grou ded b n s the rated voltage s ould cor esp n to the s stem voltage L wer

ratin s may b u ed if the b n s are grou ded