IEEE Transactions on Power Delivery, Vol. 11, No. 2, April 1996

A New IEEE Application Guide for Shunt Reactor Switching

D.F. Peelo, Senior Member IEEE BC Hydro Canada

Abstract - The paper gives a brief overview of the content of the new IEEE Application Guide for Shunt Reactor Switching, C37.015. The interruption of shunt rector currents involves a com- plex interaction between the circuit breaker and the source and re- actor circuits resulting in several transient overvoltage phenomena. The derivation of the overvoltage levels for the three cases of di- rectly grounded, ungrounded and reactor grounded shunt reactors is explained. For the overvoltage calculation, the so-called chopping number concept is used. Methods of limiting the overvoltages are also discussed.

1. INTRODUCTION

Shunt reactor switching is probably the only switching duty that has not been treated to this point in IEEE and IEC stan- dards. The fundamental work on the switching of small induc- tive currents, the generic description that includes shunt reac- tor switching, was done by CIGRE Working Group 13.02. The results of the work were published in Electra Nos. 72, 101 and 113. It was recognized that a rating structure ap- proach to shunt reactor switching was not appropriate and that there was, therefore, a need to express the CIGRE work in practical circuit breaker application terms. The work of writ- ing an application guide was undertaken by the Working Group on Shunt Reactor Switching of the IEEE High Voltage Circuit Breaker Subcommittee. The Working Group included representation from IEC and thus reflected an excellent exam- ple of cooperation between the two organisations in the prep- aration of a power electrical engineering document. The Guide prepared by the Working Group is published as an IEEE Application Guide (C37.015) and as an IEC Technical Report (Publication 1233).

95 SM 607-2 PWRD by the IEEE Switchgear Committee of the IEEE Power Engineering Society for presentation at the 1995 IEEE/PES Summer Meeting, July 23-27, 1995, Portland, OR. Manuscript submitted November 28, 1994; made available for printing April 27, 1995.

A paper recommended and approved

E.M. Ruoss, Life Fellow IEEE ABB High Voltage Technologies Ltd. Zurich, Switzerland

881

Shunt reactor switching imposes a unique and severe duty on circuit breakers and shunt reactors. Each interruption involves a complex interaction between the circuit breaker and the source and the reactor (load side) circuits. This interaction re- sults in overvoltages dependent on system parameters and characteristics of the circuit breaker. The application guide details the derivation of the overvoltage levels for the three cases of directly grounded, ungrounded and neutral reactor grounded shunt reactors and methods of limiting the overvolt- ages. The guide details further the application of circuit break- ers for the duty using the so-called chopping number approach based on laboratory test results. The guide includes, as an an- nex, a detailed case study to illustrate the use of the guide. The purpose of this paper, presented on behalf of the Working Group (refer to Acknowledgment), is to give a brief overview of the guide and its content. Those readers, who wish to pur- sue the application of circuit breakers for shunt reactor switching in detail, are referred to the guide and its extensive list of reference material.

2. TYPES OF TRANSIENT PHENOMENA

When a circuit breaker interrupts shunt reactor current several transient phenomena can be observed, but not all of them oc- cur necessarily during each interruption. These transients and their associated oscillation modes are dependent on the circuit breaker characteristics and on the system configuration where the reactor is installed. An overview of these transient is given below and will be explained in more detail in the section on directly grounded reactors.

Fig. 1 shows a general single phase diagram of a reactor in- stallation, which can be used to explain the four possible tran- sient oscillation modes in a simplified way.

1. Load side oscillation: A successful shunt reactor current in- terruption results in the decaying load oscillation with the trapped energy oscillating between the inductance L and the capacitance CL of the load side circuit. The frequency range of this oscillation is in the order of 1 - 5 kHz. Overvoltages due to current interruption prior to its natural zero (current chopping) may be observed with the load side oscillation. This overvoltage and consequently the TRV across the break- er depends on the magnitude of the chopped current.

0885-8977/96/$0S.00 0 1995 IEEE

882

Lb CB

LS - -- I - 1 1 - 1 - - I I -* I ' V

I

maturely to zero due to arc instability, a phenomenon referred to as current chopping.

The value of the chopped current depends on the capacitance Ct seen from the circuit breaker terminals, the number N of intempters in series per pole and the socalled chopping num- ber h for a single interrupter, which is a characteristic value of the circuit breaker. The chopping current level is given by the following equation.

with

Fig. 1. General single line diagram of a directly grounded shunt re- actor installation Ls source side inductance Cs source side capacitance CB circuit breaker Lp, Cl, inductance and capacitance of the breaker Lb connection series inductance CL L shunt reactor inductance

load side capacitance (mainly shunt reactor capacitance)

2. Reignition oscillations: Three different oscillation circuits are involved if a reignition occurs in the circuit breaker due to the transient recovery voltage across the circuit breaker. A "first parallel oscillation" occurs when C discharges through the circuit breaker and its inherent inductance L . The frequency of this oscillation is in the order of 1 to f0 MHz. The circuit breaker will not interrupt the current associated with this oscillation and therefore has no signifi- cance with respect to overvoltages.

In the reignition voltage oscillation ("second parallel oscilla- tion"), which follows the first parallel oscillation, the capaci- tances C and CL as well as the inductance Lb are involved and res& in equalization of the voltages across Cs and Cc The frequency of the reignition voltage oscillation is in the or- der of 50 to 1000 kHz. Its magnitude depends on the magni- tude of the load side oscillation and chopping overvoltage and consequently also on the chopped current.

Under certain system configurations a "main oscillation I ' wilt be developed in which the parameters of the total circuit are involved (L, Ls Cp CL). This oscillation will also depend on the load side oscillation.

Since these transients all depend on the load side oscillation and load side overvoltage (chopping overvoltage) determined by the magnitude of the chopped current, it is essential to know the value of the chopped current level.

P

3. DIRECTLY GROUNDED REACTORS

The switching of directly grounded reactors can be analysed using the equivalent single line circuit shown in Fig. I . ChoDDing current level and choping overvoltage. Basically, circuit breakers have no difficulty intempting shunt reactor currents. In fact the current may be forced pre-

The maximum value of Ct leading to the highest chopping level and the worst case for the chopping overvoltage occurs when Cs >> CL in which case C, is given by

e; =Cp+CL .

This chopping number concept can be applied to all circuit breaker types except for vacuum circuit breakers. Ranges for typical chopping numbers are given in Table 1.

TABLE 1. CIRCUIT BREAKER CHOPPING NUMBERS

I Circuit breaker type I Chopping number h I Minimum oil

Air blast

SF6 I 4-17. lo4 (AF-0.5) I I I I

The energy trapped in the load side inductance and capaci- tance at the instant of current chopping will oscillate between the inductance and the parallel capacitance. The first peak of the oscillation has the same polarity as the system voltage at the time of interruption, leading to the chopping overvoltage (commonly referred to as the suppression peak overvoltage).

The chopping overvoltage can be calculated by observing the energy balance. The energy stored in the reactor inductance at the moment of current interruption will be fully transferred to the effective load side capacitance at the instant of the maxi- mum overvoltage peak. This is illustrated by Fig. 2 and the following formula with reference to the circuit elements shown in Fig. 1 and the quantities defined in Fig. 2.

energy at energy at current inter- = cho ping ruption pea[ voltage

Current instability leading to current chopping r

/ Recovery voltage peak overvoltage

I , Load side oscillation

Source side power frequency voltage

Suppression peak overvoltage

Fig. 2. Current chopping phenomena and chopping overvoltage a) current through breaker b) voltage across shunt reactor

The chopping overvoltage factor k, can then be calculated as follows

Vo is the power frequency crest voltage across the shunt re- actor at the instant of current interruption and is usually prac- tically the peak voltage of the power frequency system volt- age to ground. This formula can be re-written by introducing the three phase MVA rating of the reactor and taking Vo as the crest voltage of the rated system voltage to ground and assuming Cs >> CL and C is negligible. P

where Q h

0 N

The chopping overvoltage depends in this case only on the chopping number and the reactive power of the reactor. In a majority of cases the above mentioned criteria are practically

is the three phase reactor ratin (VA) is the chopping number (AF- . ) for a single inter- rupter is the angular rated power frequency is the number of interrupting units in series per pole.

04

883

fulfilled and the formula leads to conservative results giving the highest possible chopping overvoltage factor.

Reignition overvoltages When a reignition in the circuit breaker occurs, the load side voltage rapidly tends to the source side voltage, but over- shoots producing a reignition overvoltage as shown in Fig. 3. The reignition transient oscillates around V'o as axis which is the case when C is not dominating over C,.The value of VIo is determined i y the following equation

In such a case a main circuit oscillation will occur as shown in Fig. 3. In most cases at HV and EHV, the system side capacitance is considerably greater than the load side capacitance, V', be- ing equal to V . That means the reignition overvoltage tran- sient will oscifiate around V,, which is the conservative ap- proach for the practical application.

Fig. 4 shows two examples of shunt reactor installations. In Fig. 4a) the shunt reactor is installed close to a power sta- tion or substation. In such a case is Cs >> C , whereas in Fig. 4b), where the reactor is installed in the mifidle of a line, C, may be only 3 to 4 times CL and, as explained above, re- ignition transients behave differently.

Main circuit oscillation

I I t I A

Reignition overvoltage

Load slde oscillation oscillation

Reignition

Fig. 3. Chopping and reignition overvoltage phenomena

V , power frequency crest voltage across shunt reactor at instant of current interruption (- crest of system line to ground voltage) suppression peak overvoltage in pu of V , reignition overvoltage peak to ground in pu of V , reignition overvoltage excursion in pu of V,

k, k,, ks

884

2E3b Power station

I 1

Fig. 4a) f

line Z line

Fig. 4b) CB

- -

Fig. 4. Examples of shunt reactor installation (For designations see Fig. 1)

With damping considered, the maximum magnitude of the re- ignition overvoltage to ground ( k pu) is given by the equa- tion k , = 1 + p(1+ k,)

where p is the damping factor whose value will not normally exceed 0.5. The voltage breakdown at a reignition creates a steep voltage transient that is imposed on the shunt reactor. Since the volt- age breakdown in the circuit breaker is practically instantane- ous, the steepness is solely determined by the frequency of the reignition transient (second parallel oscillation) which de- pends only on the system / station layout (Cp CL. L in Fig. 1, see also Fig. 3). The frequency of the second paralkl oscil- lation may be in the order of 50 kHz up to 1 MHz. This steep transient may be unevenly distributed across the shunt reactor winding and overstressing of the entrance turns of the wind- ing could occur. The maximum peak-to-peak excursion voltage of the reigni- tion transient (k, pu) is given by

P

k, = ( 1 + B ) ( 1 + k u )

Recoverv voltage across circuit breaker. The transient recovery voltage across the breaker following interruption is the difference between the source side and the load side voltages (see Fig. 2 and 3). If no reignition occurs, the per unit crest value of the transient recovery voltage (k,.,, pu) is

k," = 1 + k,

4. UNGROUNDED REACTORS AND REACTOR GROUNDED REACTORS

The overvoltage factors for ungrounded and reactor grounded reactors can be derived from a general diagram. This general schematic for chopping and reignition overvoltages is shown in Fig. 5 .

Prior to interruption of current in any phase the neutral of the reactor is at ground potential due to symmetry of the voltage and circuit. When the first pole clears the neutral potential shifts through a transient oscillation towards a bias voltage K (pu) dependent on the grounding arrangement. The load side oscillation of the first pole-to-clear will oscil- late around the shifted neutral voltage. This is illustrated by Fig. 6 showing the various chopping and reignition voltage factors. The derivation of the equation for the chopping overvoltage k can be achieve by using the diagrams of Fig. 7. Fig. 7a sffows the simplified scheme of Fig. 5 and Fig. 7b is the equivalent diagram for calculating k,. The energy balance - as explained for the solidly grounded shunt reactor - leads to the equation given below, where K is the neutral shift in pu (from Fig. 7a) and L* the equivalent load side shunt reactor inductance Fig. 7b. V , is kb.Vo.

CB

-+zh-r&

U I T C G I - -

Fig. 5. General diagram of a shunt reactor arrangement

CB circuit breaker phase to phase capacitance of connection and shunt reactor

CG phase to ground capacitance L shunt reactor inductance LN neutral reactor reactance

885

Compared to the solidly grounded shunt reactor, the highest chopping overvoltage to ground is not k , but - due to the neutral voltage shift - k, (refer to Fig. 6). For directly grounded reactors (K-1) the formula can be ap- plied to any phase. For ungrounded or reactor grounded re- actors, the equations apply to the first-pole-to clear. However it can be assumed that the chopping overvoltages are lower for the second and third pole to clear due to no or smaller neutral voltage shift and lower stored energy at current chop- ping.

Table 2 lists the various overvoltage factors for solidly grounded , ungrounded and reactor grounded shunt reactors. For the designation of these factors refer to Fig. 6.

a t

Time

Fig. 6. General schematic for chopping and reignition overvoltages as well as TRV for the general case of Fig. 5.

:CLv; 1 =-CL[(l+K)vo]2 1 +-$& 1 2 * 2 2

1 3" d ( i + K ) 20Q k, = ( l + K ) I+---K

1 i o =system peak phase to - - ground voltage

CB

I I 1 Vd,.ive = Vo + neutral shift (K x V,) =

vo (I+K)

Fig. 7. Diagram for the calculation ofko a) General diagram for first pole to clear b) Equivalent diagram

5. RELATED PHENOMENA

Interaction between ohases. Due to mutual coupling between the individual phases, the in- teraction manifests itself in the form of beating of the recov- ery voltage oscillation. The degree of beating is dependent on the length of the connection between the circuit breaker and the reactor and on the type of the shunt reactor. When the cir- cuit breaker is located close to the shunt reactor, the interac- tion is minimal or nonexistent (see Fig. Sa). For long connec- tion length, beating of the recovery voltage will occur as

$- shown in Fig. Sb due to the phase to phase capacitance C

High freauencv current intenuotion and voltarre escalation. The occurrence of a reignition results in several modes of cur- rent oscillations superimposed on the reestablishing of the load current. If current zeros are produced the circuit breaker may attempt to interrupt this current.

In the event an interruption occurs during the second parallel or main oscillation, the oscillating energy may have changed. A new reignition may occur close to the recovery voltage peak, and if the energy has increased, the reignition voltage may be higher than at the first reignition. This procedure may be repeated several times with increasing voltage magnitudes. This is referred to as "voltage escalation" and is a significant characteristic of vacuum circuit breakers due to the ability to interrupt high frequency currents. Other circuit breaker types may interrupt high frequency currents especially when the frequency of the reignition current is less than approximately 70 kHz and normally only at lower amplitudes.

6. OVERVOLTAGE LIMITATION

Surge arresters at the shunt reactor terminals to ground will limit the chopping gvervoltages. They will operate only if the overvoltage is higher then the arrester protective level. The energy the arresters have to absorb, even for the highest pos- sible chopped current, is an order of magnitude less than the line discharge energy, and is of no significancc.

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TABLE 2. SUMMARY OF OVERVOLTAGE FACTORS FOR GROUNDED, UNGROUNDED AND REAC- TOR GROUNDED REACTORS

I grounded reactor ....................

k, j

................. : ...................................................................

K j0

kc 1 ak, ..... ...... -. .......

.....................................................................................

F I ' V ]+aka

ungrounded reactor i reactor grounded reactor (general formula) < ..............................................................................................................................

G 1 . 5 1 s - 0.5

.......................................................................................................... < .............................................................................................................................. I 1

1.5 L 2+-

LN

0.5+a(k, +0.5) j K + X ( ~ , + K ) .......................................................................................................... & ........................................................................................................

1.5+a(k, + O S ) f l + K ) + a ( k , + K )

1 + p11.5 + a(ka + 0.5)j

.........................................................................................................................................................................................................................................

. 1 +p[(1 + K ) + a ( k , + K)]

(1 + p)[ 1.5 + a( k , + 0.511

k, + 0.5 j k , + K

! ( l+P)[( l+K)+a(k, + K ) ] i ..............................................................

.........................................................................................................................................................................................................................................

cx is the damping factor associated with the chopping overvoltage oscillation and is in the order of 0.9. A conserva- tive approach is to assume a =I . p is the damping factor associated with the reignition overvoltage oscillation and can be assumed to be 0.5.

High chopping overvoltages can be limited by installing opening resistors and associated resistor switches across the main interrupters of the circuit breakers. Such opening resis- tors are seldom used for modem SF6 circuit breakers since the chopping overvoltages are much lower than for air-blast breakers.

Metal oxide arresters applied across the circuit breaker can be used to limit the reignition overvoltages. Such arresters limit the magnitude of the transient recovery voltage across the cir- cuit breaker and consequently also the reignition transients. Furthermore the probability of occurrence of reignition is re- duced.

Load side voltage

n - * / Shunt reactor current

Fig. Sa. Load side oscillation with circuit breaker located close to the shunt reactor

Reignitions in modern circuit breakers can be eliminated by controlled opening of the circuit breaker. Contact parting will take place at such an instant as to achieve arcing times suffi- ciently longer than the minimum arcing time. The circuit breaker will interrupt the current at the first current zero after contact parting but the contact gap will have sufficient dielec- tric strength to avoid reignitions.

Load side voltage 1 .

Shunt reactor current

Fig. 8b. Load side oscillation with circuit breaker located remote from the shunt reactor

7. TESTING ASPECTS

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9. CONCLUDING REMARKS

It is hardly possible to perform laboratory tests with real shunt reactors. The purpose of laboratory tests is mainly to determine the chopping number of a single interrupter of the circuit breaker. Further the probability of reignitions can be estimated by searching for the, minimum arcing time. The Ap- plication Guide presents in an annex how to apply laboratory tests to actual shunt reactor installations.

8. CIRCUIT BREAKER SPECIFICATION

The specification of circuit breakers for shunt reactor swit- ching is described in the guide. Given the interactive nature of the duty, the user must provide electrical details not only of the voltage, the current level to be switched and the shunt reactor grounding arrangement, but also of the characteristics of the complete load side circuit, e.g. the shunt reactor and the connection arrangement between the circuit breaker and the shunt reactor. Overvoltage limit specification is an impor- tant issue and warrants some discussion in this paper.

Shunt reactors can be described electrically in terms of their inductance and inherent parallel capacitance. Typical values for MV to EHV are given in an annex to the guide. Induc- tance values range from less than 100 mH at MV to 10 H at EHV. Corresponding natural frequencies range from 40 kHz or less to 5 kHz or less. The inherent capacitance for oil-filled reactors is fairly constant; for dry-type reactors, the capaci- tance is 3 to 4 times lower than for oil-filled reactors and the natural frequencies will be correspondingly higher.

The connection arrangement contributes electrically to the duty through its capacitance (the series inductance of the con- necting buswork also contributes, but only to the reignition circuit which is not a specification matter as discussed be- low). Typical capacitance values for buswork, supporting in- sulators, instrument transformers etc. are provided in an an- nex to the guide. The sum total of these capacitances, togeth- er with the inherent capacitance of the reactor, constitutes the effective capacitance to ground of the load side circuit.

The application guide recommends that the specified limita- tion for the suppression peak overvoltage be 2 pu for EHV and HV applications and 2.5 pu for MV applications. This is a legitimate specification requirement for the circuit breaker manufacturer to meet despite the interactive nature of the du- ty. Reason: for a given application, reactor rated Q. it has been shown that the suppression peak overvoltage is depen- dent only on the chopping number, which is a characteristic of the circuit breaker. In contrast, reignition overvoltages are dependent on the interaction between the circuit breaker and the user's system and load side circuit arrangement. No spe- cific limitation with respect to these overvoltages should, therefore, be stated. The calculation of reignition overvoltage levels is described in the guide.

This paper presents a summary of the main part of the Appli- cation Guide on Shunt Reactor Switching, which includes al- so a bibliography with published articles, and part of the an- nexes. Annexes A to F of the Application Guide contain addi- tional information on the subject, namely Annex A Shunt reactor characteristics Annex B: System and station characteristics Annex C: General derivation of chopping and reignition over- voltages Annex D: Oscillation modes Annex E: Application of laboratory test results to actual reac- tor installations Annex F: Statistical formulas for derivation of chopping and reignition reignition overvoltages.

10. REFERENCES

[ I ] IEEE Standard C37.015 - 1993 Application Guide for Shunt Reactor Switching [2] IEC Technical Report (Publication 1233). Application Guide for Shunt Reactor Switching The IEEE Standard C37.015 - 1993 contains a large number of ref- erences related to the switching of shunt reactors.

1 1. ACKNOWLEDGMENT

The authors acknowledge the dedicated effort of the Working Group on Shunt Reactor Switching that developed and wrote the applica- tion guide.The Working Group had the following membership: D.F. Peelo (chair), S.S. Bemeryd, J.A. Bonner, J.H. Brunke, R.D. Gar- zon, K.I. Gray, S.R. Lambert, A.K. McCabe, D.E. Parr, G.O. Per- kins, E.M. Ruoss, G. Seyrling, H.M. Smith, D.L. Swindler and T.J. Tobin.

12. BIOGRAPHIES

D.F. Peelo received his B.E. degree from University College Dublin in 1965. He joined ASEA after graduation, working in the High Voltage Laboratory of the Power Transmission Products division in Ludvika, Sweden. In 1973 he joined BC Hydro working first in the Quality control and Inspection Department and later in the Stations Engineering Division, where he is currently a Specialist engineer re- sponsible for the procurement and application of switchgear and surge arresters. He is a Fellow of the IEE and a Senior Member of the IEEE, and at present is the Vice-Chair, Standards Coordination of the IEEE PES Switchgear Committee. E. M. Ruoss received his diploma in Electrical Engineering from the Swiss Federal Institute of Technology, Zurich, Switzerland in 1952. He joined Brown Boveri, Baden, Switzerland, as development engineer in the High Power Laboratory. After working for 2 years in the US he became in 1963 manager of the High Power Laboratory engaged in the development of circuit breakers. From 1976 he was in charge of circuit breaker application and system studies and head of a study group. At present he is consulting engineer to ABB High Voltage Technologies Ltd., Switzerland. He is a Life Fellow of IEEE, member of the IEEE High Voltage Cir- cuit Breaker Subcommittee and active in IEEE Working Groups. From 1984 to 1990 he was chairman of CIGRE Study Committee 13, Switchgear, and is Honorary Member of CIGRE.