Presentation made during the IEEE Transformers …grouper.ieee.org/groups/transformers/meetings/s2001_amsterdam/... · 1 Shunt Reactor Switching Dielectric stresses produced by circuit-breakers

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Shunt Reactor Switching

Dielectric stresses produced bycircuit-breakers to shunt reactors.

Presentation made during the IEEE Transformers Committee meeting,Amsterdam, Netherlands, April 2001

Presented by:

Pierre Riffon P. Eng.

Hydro-Québec

Test Specialist

Quality Control Department

01/04/11

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Circuit-breaker testing

1- Circuit- breaker tests for shunt reactorswitching are covered by the following guides :

A- Technical Report IEC 1233, 1994 "High-voltagealternating current circuit-breaker - Inductive loadswitching".

B- IEEE Std. C37.015-1993 "IEEE Application Guide forShunt Reactor Switching".

These two documents are technically identical since they are theresult of a cooperative effort between a WG of the IEEE HighVoltage Circuit-breaker Subcommittee and a Task Force of theIEC Technical Committee SC17A.

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2- Purpose of the circuit-breaker tests

a) To determine the interrupting window without re-ignition.

b) To determine the current chopping characteristics of thecircuit-breaker.

c) To determine the worst probable overvoltage that couldappear across the reactor and across the circuit-breaker.

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5

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Tests results:

1- Re-ignition performance vs arcing time ;

2- Chopping number (overvoltage) vs arcing time ;

3- Re-ignition voltage vs arcing time.

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Graph no.1; I = 250 A; Minimum gas pressure

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0

Prospective arcing time (ms)

Wit

hst

and

/ R

e-ig

nit

ion

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Graph no.2: I = 250 A; Minimum gas pressure

0,00E+00

2,00E+04

4,00E+04

6,00E+04

8,00E+04

1,00E+05

1,20E+05

1,40E+05

0 2 4 6 8 10 12

Arcing time (ms)

Ch

op

pin

g n

um

ber

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Graph no. 3: I = 250 A; Minimum gas pressure

0,0

50,0

100,0

150,0

200,0

250,0

0,0 1,0 2,0 3,0 4,0 5,0

Prospective arcing time (ms)

Rei

gn

itio

n v

olt

age

(kV

pea

k)

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Dielectric stresses imposedon shunt reactors :

Important aspect :

Shunt reactor switching is a daily event (in some cases,shunt reactors may be switched on and off several timesduring the same day)

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1- Ideal case, no re-ignition and no current chopping

- Type of stresses: Switching surge type of waveshape (dampedoscillatory overvoltage). Could be close to lightning impulsewaveshape for dry-type reactors.

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- Frequency :

fLCtransient

L

= 1

2π

Typical frequencies :

- ≈ 1,0 to 41 kHz for oil-immersed reactor (stray capacitance ≈ 0,8to 4 nF);

- >100 kHz for dry-type shunt reactors (stray capacitance ≈ 100 to500 pF)



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REACTOR CURRENT

-150

-130

-110

-90

-70

-50

-30

-10

0,075 0,076 0,077 0,078 0,079 0,08 0,081 0,082 0,083 0,084 0,085

Time (s)

Cu

rren

t (p

.u.)

REACTOR VOLTAGE

-700000-600000-500000-400000-300000-200000-100000

0100000200000300000400000500000600000700000

0,075 0,076 0,077 0,078 0,079 0,08 0,081 0,082 0,083 0,084 0,085

Time (s)

Vo

ltag

e (V

)

IDEAL CASE

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- Reactor transient voltage amplitude (grounded neutral):

UU

peakn= ×23

This transient voltage is not severe since it is the phase toground peak voltage.



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Ideal circuit-breakers do not exist, some are more sensitive tore-ignitions while others may chopped considerably thecurrent.


1- Ideal case, no re-ignition and no current chopping(continued)

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2- Real case, no re-ignition and current chopping

Current chopping is circuit-breaker dependent.

For example :

- For air-blast circuit-breakers : Typical chopping number :15 - 25 x 104;

- For SF6 circuit-breakers : Typical chopping number : 4 -17 x 104;

- For minimum oil circuit-breakers : Typical choppingnumber : 7 - 10 x 104;

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Dielectric stresses imposedon shunt reactors:

2- Real case, no re-ignition and current chopping (continued)

Where:

ich = Copping current;Ct = Total capacitance seen by the circuit-breaker (straycapacitance on the shunt reactor + parallel capacitance acrossthe circuit-breaker pole).

Chopping number (λ) = ich / √Ct

Chopping results in an overvoltage across the shunt reactor andacross the circuit-breaker.

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The trapped energy in the reactor at the instant of currentchopping (½ x L (ich)

2) has to be transferred to the parallelcapacitance. Thus,

Where "∆∆∆∆V" is the overvoltage produced by current chopping.

½ x L x (ich)2 = ½ x CT x (∆V)2

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For a given shunt reactor "L", the overvoltage produced isproportional to the current chopped. Moreover, a reactor with alarge stray capacitance (e.g. oil-immersed) will be less affectedby the current chopping (larger CT). And,

∆V = ich x √(L/CT)



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The amplitude of the current chopping is proportional to thearcing time of the circuit breaker. For a longer arcing time, the"interrupting strength" of the circuit-breaker is larger and thebreaker can chop more current.

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- Type of stresses: Switching surge type of waveshape(damped oscillatory overvoltage). Could be close to lightningimpulse waveshape for dry-type reactors.

- Frequency :

fLCtransient

L

= 1

2π

23



Typical frequencies:

- ≈1,0 to 41 kHz for oil-immersed reactors (stray capacitance≈ 0,8 to 4 nF);

- >100 kHz for dry-type shunt reactors (stray capacitance ≈ 100to 500 pF) ;

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- Reactor transient amplitude (grounded neutral):

Where:

ka: Overshoot factor due to current chopping

U k Upeak a

n= × ×23

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This transient voltage can be severe depending of the circuit-breaker chopping characteristics and on the shunt reactor valueand its stray capacitance.

The Hydro-Québec's technical approach on this problem consists,for each specific case, to evaluate the worst probableovervoltage that will be imposed on a specific combination"reactor-circuit-breaker".

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The test results (chopping number vs arcing time) are evaluatedand an equation of the chopping number vs arcing time is derivedfrom the test results (linear regression of the results obtained).Since this has also a statistical behavior (that it is why a largenumber of tests are required), the worst probable overvoltageis calculated considering the longest possible arcing time(tarc max.) without re-ignition (i.e. 8,3 ms for 60 Hz application)and by using "3 x Se" (99,9% of the cases) where "Se" is the"standard error of estimate".



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The equations used are:

and

λmax = λo + B x tarc max. + 3 x Se

kN

Qa = +× ×

× ×13

2

2λω

max

W h e r e :

N : N u m b e r o f i n t e r r u p t i n g u n i t s i n s e r i e s ;λ m a x : W o r s t p r o b a b l e c h o p p i n g n u m b e r d e r i v e d f r o m t e s t r e s u l ta n a l y s i s ;ω : 2 x π x f ;Q : T h r e e - p h a s e r e a c t i v e p o w e r o f t h e s h u n t r e a c t o r ;B a n d λ 0 : E q u a t i o n c o n s t a n t s .

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N o t e : IE C t e c h n ic a l r e p o r t a n d IE E E g u id e s u g g e s t t o u s e " 2 xS e " ( 9 7 ,7 % o f t h e c a s e s ) . W it h in H y d r o - Q u é b e c , s h u n t r e a c to r sa r e n o r m a l ly u s e d o n 8 0 0 k V s y s te m s ( d i r e c t ly c o n n e c t e d ) a n dth e m a r g in b e t w e e n t h e p r o t e c t iv e le v e l o f t h e s u r g e a r r e s te r a n dth e S IW L o f t h e r e a c to r is m u c h lo w e r t h a n f o r lo w e r s y s t e mv o lt a g e s . B e c a u s e 8 0 0 k V s h u n t r e a c t o r s a r e s t r a t e g ice q u i p m e n t a n d b e c a u s e o f t h e i r r e p la c e m e n t c o s t , a " 3 x S e "c r i t e r ia is u s e d .



and

U k Upeak a

n= × ×23

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As a typical example, a 800 kV SF6 puffer type circuit-breakerused to switch a 330 MVAr reactor (3 single phase units rated110 MVAr) may produced overvoltages "ka" in the range of1,32 p.u.

The same circuit-breaker used to switch a 165 MVAr reactor (3single phase units rated 55 MVAr) may produced overvoltages"ka" in the range of 1,55 p.u.

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For extra-high voltage applications (550 kV and 800 kV) ,because of the low margin between the SIPL and SIWL(typically 25%) and because that reactor switching is a dailyevent, we do recommend that the calculated overvoltage peakto be less than 0,68 x SIWL (0,8 (ageing) x 0,85 (margin) =0,68). For these cases, the arrester can not give a properprotection, mainly if we consider a certain dielectric strengthreduction of the reactor due to ageing.

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For high-voltage (< 362 kV) and for medium voltageapplications, the SIPL of the shunt reactor surge arrester isrelatively low and the margin between the SIPL and the SIWLis larger than 32%. For these cases, the parallel surgearrester is normally giving a proper shunt reactor switchingovervoltage protection even for aged shunt reactors.

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3- Real case, re-ignition and current chopping

Re-ignition of a circuit-breaker occurs when the TRV appliedto the circuit-breaker is higher than the "dynamic" voltagewithstand capability of the circuit-breaker during anopening operation.

Re-ignition occurs normally with small arcing times. For highvoltage SF6 circuit-breakers, it is typical that arcing times lowerthan 4 ms lead to re-ignitions.

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W h e r e :

L B : I n d u c t a n c e o f t h e r e - ig n i t io n c i r c u i t ( b e t w e e n t h e s o u r c e s id ec a p a c i t o r a n d t h e s h u n t r e a c t o r ) .C S : S o u r c e s id e c a p a c i t a n c e ;C L : L o a d s id e c a p a c i t a n c e .


3- Real case, re-ignition and current chopping (continued)

Type of stresses: Fast transient type of waveshape (dampedoscillatory overvoltage, lightning impulse or chopped wave).

- Frequency :

fC C

L C CtransientL s

B L s

= × +× ×

1

2π

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35



Typical frequencies:

- From 50 kHz to 1,0 MHz. This frequency is mainly dependentof the stray capacitance of the shunt reactor (CS >>> CL) andthe length of re-ignition circuit (LB) between the source sidecapacitance and the shunt reactor.

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- Reactor transient voltage amplitude (grounded neutral) :

a) Before the re-ignition, worst case :

U k Upeak to ground a

n⋅ ⋅ = × ×2

3 (same as for current chopping case)

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W h e r e :

β : D a m p in g f a c t o r o f t h e r e - i g n i t i o n t r a n s ie n t , n o r m a l l y β ≤ 0 , 5 ;k a : S u p p r e s s io n p e a k o v e r v o l t a g e c a u s e d b y c u r r e n t c h o p p i n g ;C S : S o u r c e s id e c a p a c i t a n c e ;C L : L o a d s id e c a p a c i t a n c e .

A n d g e n e r a l l y , C S > > > C L



b) After re-ignition, worst case (re-ignition at the peak of therecovery voltage):

U kC

C Ck U

peak to ground aS

S L

an

⋅ ⋅ = + × + ×−

−

× ×( ) ( )1 1 2

3β

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As for the case of current chopping, for high voltage andmedium voltage shunt reactors (≤ 345 kV), the surge arresterwill protect adequately the reactor against the maximumphase-to-ground peak voltage. Nevertheless, for extra highvoltage shunt reactors (500 and 765 kV), the arrester maynot adequately protect the reactor if we consider this type ofstress as a daily event (margin of 20% for ageing and 15 % onthe LIWL, 0,68 p.u. of the LIWL).

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The phase-to-ground overvoltage produced by a re-ignition isnot the worst dielectric stress generated during this event. Thedv/dt applied to the shunt reactor during a re-ignition canbe extremely severe and the surge arrester connectedacross the shunt reactor will not protect it.

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D i e l e c r i c s t r e s s e s o n s h u n t r e a c t o r s d u r i n g r e - i g n i t i o n s

N o m i n a l s y s t e m v o l t a g e

B I L C h o p p e d w a v e

L I P L ( 2 , 5 x p h a s e - t o -

g r o u n d v o l t a g e p e a k )

2 x L I P L ( u l t i m a t e

m a x i m u m d v / d t d u r i n g r e - i g n i t i o n )

d v / d t s t r e s s e s

( % o f t h e c h o p p e d

w a v e l e v e l )

P h a s e - t o -g r o u n d

o v e r v o l t a g e ( % o f t h e

L I W L ) *( k V ) ( k V ) ( k V ) ( k V ) ( k V ) ( % )

1 5 1 1 0 1 2 0 3 0 , 6 6 1 , 2 5 1 , 0 2 3 , 9

2 5 1 5 0 1 6 5 5 1 , 0 1 0 2 , 1 6 1 , 9 2 9 , 3

3 4 , 5 2 0 0 2 2 0 7 0 , 4 1 4 0 , 8 6 4 , 0 3 0 , 3

4 5 2 5 0 2 7 5 9 1 , 9 1 8 3 , 7 6 6 , 8 3 1 , 6

6 9 2 5 0 2 7 5 1 4 0 , 8 2 8 1 , 7 1 0 2 , 4 4 8 , 56 9 3 5 0 3 8 5 1 4 0 , 8 2 8 1 , 7 7 3 , 2 3 4 , 6

1 1 5 3 5 0 3 8 5 2 3 4 , 7 4 6 9 , 5 1 2 1 , 9 5 7 , 71 1 5 4 5 0 4 9 5 2 3 4 , 7 4 6 9 , 5 9 4 , 8 4 4 , 91 1 5 5 5 0 6 0 5 2 3 4 , 7 4 6 9 , 5 7 7 , 6 3 6 , 7

1 3 8 4 5 0 4 9 5 2 8 1 , 7 5 6 3 , 4 1 1 3 , 8 5 3 , 81 3 8 5 5 0 6 0 5 2 8 1 , 7 5 6 3 , 4 9 3 , 1 4 4 , 01 3 8 6 5 0 7 1 5 2 8 1 , 7 5 6 3 , 4 7 8 , 8 3 7 , 3

1 6 1 5 5 0 6 0 5 3 2 8 , 6 6 5 7 , 3 1 0 8 , 6 5 1 , 41 6 1 6 5 0 7 1 5 3 2 8 , 6 6 5 7 , 3 9 1 , 9 4 3 , 51 6 1 7 5 0 8 2 5 3 2 8 , 6 6 5 7 , 3 7 9 , 7 3 7 , 7

2 3 0 6 5 0 7 1 5 4 6 9 , 5 9 3 9 , 0 1 3 1 , 3 6 2 , 12 3 0 7 5 0 8 2 5 4 6 9 , 5 9 3 9 , 0 1 1 3 , 8 5 3 , 82 3 0 8 2 5 9 0 5 4 6 9 , 5 9 3 9 , 0 1 0 3 , 8 4 8 , 92 3 0 9 0 0 9 9 0 4 6 9 , 5 9 3 9 , 0 9 4 , 8 4 4 , 9

3 4 5 9 0 0 9 9 0 7 0 4 , 2 1 4 0 8 , 5 1 4 2 , 3 6 7 , 33 4 5 1 0 5 0 1 1 5 5 7 0 4 , 2 1 4 0 8 , 5 1 2 1 , 9 5 7 , 73 4 5 1 1 7 5 1 2 9 0 7 0 4 , 2 1 4 0 8 , 5 1 0 9 , 2 5 1 , 5

5 0 0 1 3 0 0 1 4 3 0 1 0 2 0 , 6 2 0 4 1 , 2 1 4 2 , 7 6 7 , 55 0 0 1 4 2 5 1 5 7 0 1 0 2 0 , 6 2 0 4 1 , 2 1 3 0 , 0 6 1 , 65 0 0 1 5 5 0 1 7 0 5 1 0 2 0 , 6 2 0 4 1 , 2 1 1 9 , 7 5 6 , 65 0 0 1 6 7 5 1 8 4 5 1 0 2 0 , 6 2 0 4 1 , 2 1 1 0 , 6 5 2 , 4

7 6 5 1 8 0 0 1 9 8 0 1 5 6 1 , 5 3 1 2 3 , 1 1 5 7 , 7 7 4 , 67 6 5 1 9 2 5 2 1 2 0 1 5 6 1 , 5 3 1 2 3 , 1 1 4 7 , 3 6 9 , 87 6 5 2 0 5 0 2 2 5 5 1 5 6 1 , 5 3 1 2 3 , 1 1 3 8 , 5 6 5 , 5

C a s e s w e r e t h e r e - g n i t i o n d v / d t i s h i g h e r t h a n t h e c h o p p e d w a v e l e v e lC a s e s w e r e t h e r e - g n i t i o n d v / d t i s h i g h e r t h a n 6 8 % o f t h e c h o p p e d w a v e l e v e l ( r e q u i r e d m a r g i n )

* : C o n s i d e r i n g a d a m p i n g f a c t o r o f 0 , 5 a n d a k a o f 1 , 3

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For reactors rated 69 kV to 161 kV, the dv/dt produced by are-ignition is in excess of the required margin for agedreactors (more than 68 % of the CWWL). For these cases, re-ignitions can be accepted as an accidental event.

It should be noted that for some cases (lower BIL ratings ofeach nominal voltage class), the dv/dt produced by a re-ignition will exceed the dv/dt produced by a chopped wave.For these cases, re-ignitions of circuit-breaker shall beavoided.

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As shown in the table, for medium voltage shunt reactors (≤45 kV), the dv/dt produced during single or multiple re-ignitionsevent (the later only applicable to vacuum circuit-breakers, max.value = 2 X LIPL) is generally well protected by the surgearrester (less than 68% of the CWWL) because of the hugemargin between the protective level of the surge arrester andthe chopped wave rated level of the shunt reactor.

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For reactors rated above 161 kV, the dv/dt produced by are-ignition is always higher than the chopped wave rating ofthe reactor. For these cases, the dv/dt produced by a re-ignition will exceed the dv/dt produced by a chopped waveand re-ignitions of the circuit-breaker shall be avoided.

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4- Ways to limit overvoltages

Advantage:

- Does limit the phase-to-ground overvoltage applied tothe reactor;

a) Shunt reactor arrester protection (shall always be used, normalpractice ) :

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4- Ways to limit overvoltages (continued)

a) Shunt reactor arrester protection (shall always be used, normalpractice ) :

Disadvantages:

- Does not give appropriate protection against thedv/dt produced during a re-ignition (mainly for reactorsrated 69 kV and above).

- Not effective to reduce the probability of a re-ignition ofthe circuit-breaker.

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b) Opening resistors (used in the past on air-blast circuit-breakers, normally not offered today):

Advantages:

- Does limit the amplitude of the current chopping;

- Effective to limit the chopping overvoltage;

- Effective to limit the probability of re-ignition andeffective to limit the overvoltage caused by a re-ignition.

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b) Opening resistors (used in the past on air-blast circuit-breakers, normally not offered today; continued) :

Disadvantages :

- Expensive ;

- Reduced reliability of the circuit-breaker ;

- Increase the need of circuit-breaker maintenance ;

- Thermal limitation.

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c) Arresters in parallel with the circuit-breaker (solutionused on some SF6 circuit-breakers) :

Advantages :

- Does limit the amplitude of the TRV across the circuit-breaker, effective to limit the re-ignition window;

- Effective to limit the resulting dv/dt in case of a re-ignition;

- Passive element.

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c) Arresters in parallel with the circuit-breaker (solutionused on some SF6 circuit-breakers; continued) :

Disadvantages :

- Expensive;

- Thermal limitation.

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d) Synchronous switching (today's preferred solution) :

Advantages :

- Does forbid opening operations within the re-ignitionwindow;

- Cost (not necessarily for MV circuit-breakers);

- Easy to implement for all types of circuit-breakers.

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d) Synchronous switching (today's preferred solution;continued) :

Disadvantages :

- Need to carry extensive circuit-breaker testing for theevaluation of the re-ignition window and current choppingcharacteristics;

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d) Synchronous switching (today's preferred solution;continued) :

Disadvantages :

- Need to carry special mechanical tests for thedetermination of the scatter in opening times in relationwith control voltage, gas pressure, available energy in theoperating mechanism, ambient temperature, etc...;

- Electronic component (sensitivity to EMC).

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Conclusions

- Switching of shunt reactors may produce severedielectric stresses to shunt reactors;

- Determination of circuit-breaker characteristics (testprocedure) in relation with shunt reactor switching(current chopping and re-ignition window) is well definedin IEC and IEEE;

- C57.21 (IEEE Standards Requirements Terminology, andTest Code for shunt reactors Rated Over 500 kVA) shallrefer to these documents for the evaluation of theovervoltages;

We do propose to use "3 x Se" instead of "2 X Se" for extrahigh voltage shunt reactors (500 and 765 kV) because ofthe reduced protective margin.

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Conclusions

- C57.21 shall prescribe maximum overvoltage levels thatshunt reactors are able to withstand as a daily event(considering ageing and safety margin). This conclusioncan be also applicable to power transformers.

a) We propose that the maximum overvoltage producedby current chopping shall not exceed 68% of therated SIWL rating of the reactor or 56% of the ratedBIL rating (68% x 83% x LIWL) if SIWL is not defined;

b) We propose that the maximum dv/dt producedduring re-ignition (if re-ignitions are allowed as apossible daily event) to be less than 68% of the ratedCWWL of the reactor;

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Conclusions

c) We propose to add in C57.21 that shunt reactorsrated 230 kV and above are not designed towithstand the stresses produced by a re-ignition ofthe circuit-breaker unless special measures havebeen taken in the reactor design or in the circuit-breaker design.

d) We propose to add in C57.21 that in no case, thedv/dt produced during a re-ignition should exceedthe rated CWWL of the shunt reactor.

Documents

Presentation made during the IEEE Transformers …grouper.ieee.org/groups/transformers/meetings/s2001_amsterdam/... · 1 Shunt Reactor Switching Dielectric stresses produced by circuit-breakers