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Modeling of Vacuum Circuit Breaker and its use forStudying Medium Voltage Reactor Switching

Prashant V Navalkar, Gopal Gajjar

Abstract—A Vacuum circuit breaker (VCB) model is developedin Alternate Transients Program (ATP). This model simulates theinterruption of high frequency currents and multiple reignitionsunder certain network conditions like interrupting low inductivecurrents. The model is used to study the requirement of surgesuppressors for reactor switching at two substations of TataPower.

Index Terms—Vacuum circuit breakers, high frequency cur-rents, reignition, surge suppressor

I. INTRODUCTION

THE circuit breakers are used in transmission and distri-bution networks for normal as well as fault switching

duties. Circuit breakers are designed to interrupt at naturalcurrent zero and also withstand the resulting thermal anddielectric stresses. Depending on the medium used, for arcextinction, circuit breakers are classified as oil, SF6, vacuumetc. In medium voltage networks, both SF6 and vacuum circuitbreakers are used due to their excellent dielectric properties.

For modelling VCBs, different breaker models have beenused in literature. The following properties are generally takeninto account whilst developing VCB models [1].

This paper aims at developing a model of VCB in AlternateTransients Program (ATP/ EMTP) which takes into account theovervoltages and reignition behaviour of VCB. The model isthen applied to study the overvoltages arising out of reactorswitching.

This paper is organised as follows. In section II, we lookat the reasons for occurrence of overvoltages in VCBs andsubsequently develop the VCB model in section III. Section IVdiscusses the issues associated with shunt reactor switching.The model is used in a practical application in Section V.Section VI concludes the paper.

II. OVER VOLTAGES IN VACUUM CIRCUIT BREAKERS

When currents in highly inductive circuits e.g., reactors,transformers on no-load, unloaded motors etc., are interruptedby VCBs, overvoltages are likely to result. The opening ofVCB contacts before current zero can lead to high frequencycurrent transients. The four main reasons for over voltages in aVCB are current chopping, multiple reignitions, virtual currentchopping and pre strikes [2]. We now discuss the phenomenaof current chopping and multiple reignition in greater detailas they are more relevant to reactor switching.

Prashant V Navalkar is with Tata Consulting Engineers Ltd., Mumbai. e-mail: pvnavalkar@tce.co.in

Gopal Gajjar is a research scholar with Dept. of Elect. Engg. IIT Bombay,Mumbai. e-mail: gopal@ee.iitb.ac.in

A. Current Chopping

A premature interruption of alternating current before itsnatural current zero is known as chopping. This phenomenonhappens due to the fact that just before current zero, whenthe instantaneous current is a few amperes, the arc becomesunstable, causing an abrupt interruption of the current. Themagnitude of the chopping current depends on the moment ofcontact separation, nature of contact material and the nature ofcurrent to be interrupted. For an explanation of overvoltagesdue to current chopping refer Fig. 1.

Fig. 1. Simplified circuit representation for current chopping phenomenon

Let Ich be the chopped current, hence the stored electro-magnetic energy is 1/2LI2

ch. This energy is transferred toelectrostatic energy in the capacitance 1/2CV 2. The resultingvoltage increase on the load side stresses the contact gap. Thisovervoltage is proportional to the surge impedance of the loadas well as the magnitude of the chopped current. It should alsobe noted that after current chopping, the transient recoveryvoltage (TRV) across the gap increases. If the resulting rateof rise of recovery voltage (RRRV) is higher than the rate ofrise of dielectric strength (RRDS), the arc will be reestablishedi.e., the gap will reignite.

B. Voltage Escalation due to Multiple Reignitions

The possibility of reignition has been discussed above. Ifreignition does occur, then the arc will be reestablished. Thiswill lead to a flow of high frequency current due to the storedcharges in the stray capacitance on either side of the contactgap. This high frequency current is superimposed on the powerfrequency current, as shown in Fig. 2.

This high frequency current can probably be extinguishedat one of its current zeros. If this interruption does happen, theprocess described in II-A repeats, i.e., the voltage escalationcontinues till either:

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Ipf

Ihf

Fig. 2. High frequency current superimposed on power frequency current

• The contact separation is enough to withstand the TRV.In this case there is a successful current interruption.

• The power frequency current magnitude is high enoughto prevent zero crossing of the sum of power frequencyand high frequency current. In this case there is nointerruption and current continues for one more halfcycle.

III. DEVELOPMENT OF VACUUM CIRCUIT BREAKERMODEL

The breaker model developed in ATP/ EMTP incorporatesthe following properties discussed in the previous section [3],[4]• Ability to chop current before natural current zero• Recovery of dielectric strength between contacts during

opening• Capability of quenching high frequency current at its zero

crossing

A. Current chopping model

The mean chopping current is calculated using the equation(1) [1], [3], [4].

Ich = (ωiαβ)q (1)

Where, ω is the angular frequency (2πf ), i is the amplitudeof the power frequency current and α, β and q are parametersdependent of the contact material. Typical values of theseparameters are, α = 6.2 × 10−16 sec, β = 14.3 andq = (1− β)−1.

It should be noted that chopping current depends on whenthe contacts separate, the closer the initial opening is to currentzero, higher is the chopping current.

B. Dielectric Strength

The dielectric strength is modeled as a function of contactdistance [1], [3], [4] as follows

U = A(t− topen) +B (2)

Where, topen is the moment of the contact separation, Ais the rate of rise of dielectric strength, B is the constantdielectric strength of the breaker.

The values of A and B for four typical dielectric strengthcharacteristics are given in Table I [4]

TABLE IDIELECTRIC PARAMETERS OF VCB

A Bin V/µs in V

2 020 030 100050 0

TABLE IIPARAMETERS FOR HIGH FREQUENCY QUENCHING CAPABILITY

C Din A/µs2 in A/µs-0.34x105 255

0 1000 600

0.31x106 155

As can be seen from (2), the dielectric strength between thebreaker contacts as a function of time. If the TRV exceedsthe dielectric strength, the breaker switch is closed, thussimulating a reignition.

C. High frequency quenching capability

As discussed, reignition occurs when TRV exceeds dielec-tric strength across breaker contacts. This reignition leads toflow of high frequency current which will superimpose onthe power frequency current. If the magnitude of the highfrequency current is higher than the power frequency current,the summation current can be forced to zero. VCBs canquench this current, the extinction can be successful if therate of change of current during zero crossing is within thehigh frequency quenching capability of the VCB. VCBs aregenerally capable of quenching current with a high di/dt,typically, 150 -1000 A/µsec.

The quenching capability of VCB is also modeled using alinear equation [1], [3], [4].

di

dt= C(t− topen) +D (3)

Where parameters C and D control the high frequencyquenching capability. The typical values of constants C andD are given in Table II.

Eqn. (3) defines the di/dt limit, if the actual rate of changeof high frequency current at zero crossing is greater than thislimit, the arc will not be extinguished.

IV. SHUNT REACTOR SWITCHING

In this section we present a brief overview of shunt reactorswitching. We also bring out the differences between applica-tion of vacuum and SF6 type circuit breakers for shunt reactorswitching. Our aim is to apply the developed VCB model tostudy switching of medium voltage shunt reactor.

Usually interrupting the shunt reactor current is not a bigchallenge to any circuit breaker [5], [6]. In fact, as wehave discussed earlier in II-A, the breaker interrupt currentprematurely. This property of current chopping is shared byboth vacuum and SF6 technologies. While, current interruption

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is not a problem, all high voltage breakers should prove thecapability of withstanding the abnormal TRV generated dueto current chopping through type tests. So with proper testprocedures the capability of the circuit breaker to handle theshunt reactor switching is ensured. But, this ignore of effects ofthe switching on the shunt reactor itself. None of the standards[5], [6] handles it.

All breakers may experience single reignitions duringswitching of shunt reactor currents. The shunt rector is sub-jected to sharp voltage rise due to secondary oscillationsduring each reignition incidences. Theoretically magnitude ofvoltage rise due to reignitions can be as high as 3 p.u. [6]even if current chopping is ignored. If suppression voltage isconsidered due to significant current chopping the reignitionimpulse voltage subjected to shunt reactor may be still higher.This sharp rise time impulse is detrimental to the shunt reactorinsulation. A surge arrester is applied across the shunt reactorto limit the overvoltage endured by the reactor to less than 2.5p.u. Surge arrester is always required irrespective of the typeof the breaker used, as single reignitions occur in all breakers.

If the circuit breaker is prone to multiple reignitions thenit will significantly reduce the life of the the shunt reactor.SF6 circuit breakers are better in this regards as they do notgive rise to multiple reignitions. In medium voltage systemthe shunt reactors are usually need to be switched quitefrequently. Hence, even though SF6 breakers are free ofproblems due to reignitions are some times avoided. Vacuumcircuit breakers are preferred due to their large mechanicalendurance property. Vacuum circuit breaker however, are knowto have multiple reignitions. So some application check isrequired before applying vacuum circuit breakers for shuntreactor switching. The phenomenon of multiple reignitionsdepends on the circuit arrangements and is unique for eachsite. So for every application a separate check is required.One solution to avoid multiple reignitions is to apply a seriescombination of resistor and capacitor (RC) surge suppressorsbetween the shunt reactor terminal and ground. We discussfurther on this in next section.

V. APPLICATION OF VACUUM CIRCUIT BREAKER MODE

The VCB model developed in Alternative Transients Pro-gram (ATP-EMTP) was validated using the circuit developedby J. Helmer [3]. After validation, the model was usedto simulate opening of the of 30 MVAR reactor circuit atBackbay Receiving Station of Tata Power. The cable lengthsand actual network data has been taken from the layouts andother data available with TCE. We now discuss the results ofthe simulations.

A. Without Surge ProtectionThe plots of TRV are shown in Fig. 3 show that at first

current zero, the TRV developed is more than the RRDS acrossthe breaker, which leads to reignition. At second current zero,the dielectric strength is sufficient leading to a successful arcinterruption.

The VCB current plot given in Fig. 4 shows the highfrequency current through the breaker. Notice the multiplereignitions and the high frequency zero crossings.

Fig. 3. Plot showing TRV without surge protection

B. With RC Surge Suppressor

A surge capacitor reduces the steepness of an incomingsurge. The capacitance value is chosen such that it limitsno load chopping voltages to 2.5 times normal line crestvoltages [7]. The value of the capacitance chosen depends ontransformer / reactor rating, chopping current magnitude etc.as given in (4) [4].

Cs =89.1×MVA× Io

kV 2 × fµF (4)

Where, MVA is transformer / reactor rating in MVA, kVis line to line supply voltage in kV, f is supply frequency inHz. and Io is chopped current in % of rated current.

It should be noted that a capacitor cannot damp the am-plitude of incoming surges. A resistance in series with thesurge capacitor will help to absorb some of the energy of thesurge, thus serving to damp it. To critically damp the reignitioncurrent, the damping resistance value is given by (5)

Rs ≥√Lc

C(5)

Where, Lc is cable inductance in H and C is total equivalentcapacitance in F. C includes the capacitance Cs of surge ca-pacitor and any other capacitance of signifies like capacitanceadded for power factor correction etc.

C. With Surge Arresters

Even with the provision of RC surge suppressor as describedabove, virtual current chopping cannot be eliminated. This

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Fig. 4. Plot showing breaker current without surge protection

would cause an increase in the load side voltage whichcannot be eliminated by RC surge suppressors alone. Hence,to provide complete surge protection, a surge arrester isrequired.In some cases, even with a RC surge suppressor, thevirtual current chopping cannot be avoided. In event of currentchopping, the TRV increases and if it exceeds the breakerwithstand limit, reignition may occur. This cannot be preventedby the surge suppressor alone, making it essential to provide asurge arrester, which would provide complete surge protection.Surge arresters limit the magnitude of the voltage surge butcannot change its rate of rise.

D. Case Study with RC surge Suppressor

A surge capacitance value of 100 nF is sufficient to elim-inate reignition, however since a capacitor value of 400 nFhas already been provided at Backbay R/S, cases with C=300 nF and 400 nF were also studied. A range of dampingresistance values from 10 to 50 ohms were also used in thesimulation. The plots shown below are with R=20 ohms andC= 400 nF. Fig. 5 shows the TRV plot and Fig. 6 shows thebreaker current plot. It can be seen that high frequency zeroshave been eliminated. The study was repeated for Dharavireceiving station and similar results are obtained.

VI. CONCLUSION

A model of VCB was developed in ATP/ EMTP and usedto study the system overvoltages during reactor switchingresulting from current chopping, multiple reignitions etc. TheVCB model was included in reactor switching study forBackbay and Dharavi receiving stations of Tata Power. For

Fig. 5. Plot showing TRV with surge protection. Notice that the TRV is wellwithin the dielectric strength of the VCB.

Fig. 6. Plot showing breaker current with surge protection. Notice that highfrequency zeros have been eliminated and multiple reignitions are avoided.

eliminating multiple reignitions, a value of R = 20 ohmsand C = 400 nF for the RC surge suppression circuit wasrecommended. A 33 kV surge arrester with a MCOV of 22kV was recommended to be provided on the reactor circuit forcomplete surge protection.

ACKNOWLEDGEMENT

The authors would like to thank Tata Power for giving usan opportunity to work on this problem.

REFERENCES

[1] S.M. Wong, L.A. Snider and E.W.C. Lo, “Overvoltages and Reignitionbehaviour of Vacuum Circuit Breaker,” International Conference onPower System Transients - IPST, 2003.

[2] “Overvoltages - Measurement and Statistical Simulation,” IndustrialGroup, TAVRIDA ELECTRIC.

[3] J. Helmer and M. Lindmayer, “Mathematical Modeling of the HighFrequency Behaviour of Vacuum Interrupters and Comparison withMeasured Transients in Power systems,” IEEE XVIIth InternationalSymposium on Discharges and Electrical Insulation in Vacuum - Berke-ley, 1996.

[4] B. Kondala Rao, Gopal Gajjar, “Development and Application of Vac-uum Circuit Breaker Model in Electromagnetic Transient Simulation,”Power India Conference, IEEE, 2006

[5] “High-voltage switchgear and controlgear Part 110: Inductive loadswitching,” IEC 62271-110, Edition 2, 2009.

[6] “IEEE Guide for the Application of Shunt Reactor Switching,” IEEEStd C37.015-2009, 2009.

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[7] A.H. Moore, T.J. Blalock, “Extensive field measurements support newapproach to protection of arc furnace transformers against switchingtransients,” Power Apparatus and Systems, IEEE Transactions on,vol.94, no.2, pp. 473- 481, Mar 1975.