On the Problem of Shunt Reactor Tripping duringSingle- and Three-Phase Auto-Reclosing

Daniil A. Panasetsky1,2, Alexey B. Osak11Melentiev Energy Systems Institute of Russian Academy of Sciences

2Irkutsk National Research Technical UniversityIrkutsk, Russia

Email:panasetsky@gmail.com

Abstract—The application of shunt reactors (both controlledand uncontrolled) on HVAC overhead transmission lines im-proves operational characteristics, but at the same time it bringsthe line closer to the resonance. Operation of the shunt reactor-compensated transmission line that is close to the resonance,causes a large number of problems which in particular canbe solved by temporary tripping of the reactor(s). The articlefocuses on some aspects of the SF6 shunt reactor circuit breakeroperations during one-phase and three-phase auto-reclosing. Inparticular, the problem of accident-free shunt reactor tripping,as well as possible ways to solve it are discussed.

Index Terms—circuit breakers, energy dissipation, power sys-tem simulation, short-circuit currents, transmission lines, zerocurrent switching.

I. INTRODUCTION

Shunt reactors (SR) (both controlled and uncontrolled) inHVAC transmission lines are applied to compensate for theexcess reactive power. Being very important components ofelectric power systems, they have to be protected against ab-normal conditions. From this perspective, the most crucial ele-ment is a reactor circuit breaker (CB) whose failure may leadto serious consequences not only for the nearby equipment,but for the power system as a whole. The SR circuit breakerswitching characteristics depend on the installation method.In [1] two most typical cases were studied and compared– installation at line terminals and at a substation busbar.Reactors compensating the reactive power of a long HVACtransmission line are usually switched on and off simultane-ously with the line, their breakers are connected directly to theline and not to the substation bus bars (Fig.1). On the one hand,the application of SR improves operational characteristics, buton the other hand, it brings the line closer to the resonance.Operation of the line that is close to the resonance, causes alarge number of problems associated with overvoltages, andwith the necessity for small aperiodic current tripping. Theproblem of small aperiodic current tripping is particularlyrelevant for the modern SF6 circuit breakers [2], especially ofauto-compression type. The problem of small aperiodic currenttripping may occur during single-phase (SPAR) or three-phase

This work was supported by the Russian Scientific Foundation under project“Development of an intelligent system for preventing large-scale emergenciesin power systems,” Grant No.14-19-00054.

(TPAR) auto-reclosing. The resonance overvoltage may occuronly during SPAR.

One of the most effective solutions to both problems isthe resonance detuning by shutting down the shunt reactor(s)during TPAR and SPAR. However, as is shown below, thetripping of the shunt reactor(s) in this situation may also befraught with difficulties, related to the possible occurrenceof not-breaking currents with a high content of aperiodiccomponents flowing through the CB of the reactor. And, aswill be shown later, apparently, none of the standard technicalsolutions for linear SF6 CB can be directly applied to theidentified problem for reactor SF6 CB. Thus, the objective ofthe paper can be summarized as follows:

1) Attract the attention of technicians and equipmentmanufacturers. The final answer to the question aboutthe need for special measures can be provided only byfull-scale experiments, but not by mathematical models.If necessary it is proposed to introduce a new CBparameter that would characterize the maximum amountof energy that can be safely dissipated by one pole ofthe breaker.

2) Offer an alternative solution and discuss its advan-tages and disadvantages. It is proposed to solve theproblem by introducing a power resistor unit into thecircuit of the SR.

II. OPERATING FEATURES OF COMPENSATED LINES

Let us briefly explain the necessity for tripping the SR ofthe compensated transmission line during SPAR and TPAR.The degree of compensation for capacitive reactance of thetransmission line using the SR is defined by the followingformula [3]:

Fig. 1. The test system. 500 kV transmission line

K =XTL

XSRT(1)

where XSRT = XSR/N is the total inductive reactance ofN reactors, connected to the transmission line; XTL - totalcapacitive reactance of the transmission line. If K is close to1 then the transmission line is close to the resonance.

Further analysis will be performed using a 500 kV transmis-sion line, presented in Figure 1. Studies were carried out usingMATLAB/Simulink. In our studies we used the simplifiedgrounded wye connected model of the 160 MVAR shunt re-actor with the following parameters [4]: R = 12 MΩ/phase,L = 5.3 H/phase, r = 30 Ω/phase, C = 14 nF/phase.

There are two main problems associated with the discon-nection of compensated lines: the problem of aperiodic currenttripping and the problem of resonance overvoltage duringSPAR.

A. Aperiodic current tripping

This problem is particularly relevant for the modern SF6circuit breakers, especially of auto-compression type. It iscaused by the fact that the tripped currents of a compensatedline do not cross zero for a long time because of the higheraperiodic component as compared to the periodic one [5], [6].The following conditions should be met [7]:

1) there is at least one unfaulted phase in the compensatedline which is switched on only from one side;

2) the phase is switched on at the point closer to voltagezero crossing;

3) the phase is switched off without time delay.Such conditions may occur during unsuccessful TPAR.

Figure 2 shows the transmission line currents in the case oftwo-phase-to-ground fault with unsuccessful TPAR. SR is inoperation during the transient. In the course of reclosing thecurrent of the unfaulted phase C does not cross zero for morethan 0.2s, which may result in damages of the circuit breaker.

This problem does not exist, for example, for air circuitbreakers. An analysis carried out by independent experts fromABB Group [2] and from St. Petersburg [8], shows that thereasons why tripping of SF6 CB may lead to accident can besummarized as follows:

1) SF6 CB creates an emergency situation due to thenature of the switch-on process. The probability ofcontact closure for air CB is close to 1 near the voltagemaximum, i.e. the closing results from the air gapbreakdown . In this case the DC component of thecurrent is minimal. Unlike air CB, the contacts of SF6breaker can be closed near zero voltage. This leads to asignificant DC component. This behavior is due to highinsulation properties of sulfur hexafluoride.

2) SF6 CB creates an emergency situation due to thenature of the switch-off process. SF6 CB has closed arcchutes wherein the intensity of arc extinction depends onthe energy of the arc defined by the breaking current.SF6 switch cannot turn off low currents until they

cross zero. This feature is evaluated as positive whendisconnecting small inductive currents, because this willcause the minimum overvoltages (due to the absence ofcurrent chopping). Sometimes this effect is enhanced bymeans of synchronizers [9]. However, with respect tothe considered aperiodic current tripping problem, thispositive characteristic is negative, since it leads to aprolonged arcing. Long arcing leads to the burning ofthe contacts, to the overheating of the sulfur hexafluorideand to the extreme increase of the pressure inside arcchute. In the absence of a valve that relieves pressure,there is an explosive destruction of the breaker poleswith damage of the surrounding equipment. Air circuitbreaker has a surplus of air under constant pressure; itextinguishes the arc with intensity that does not dependon the current strength, without waiting when the currentgoes through zero.

To solve this problem the following methods can be used:1) Application of a synchronization device that controls

the moment when each phase is reclosed close to thevoltage maximum [10], [11]. However, shunt reactor-compensated transmission lines present an additionalcomplexity on the voltage waveshape across the openCB [12], [13], and when 0.7 < K < 1.3, the stan-dard controlled switching devices may not always beefficient [14].

2) Installation of closing resistors that will accelerate thedecay of aperiodic components. This technical solutionis associated with high financial cost [15], [16].

3) Tripping of the reactor (reactors) during TPAR todetune the resonance. This solution is effective from atechnical viewpoint and has a low price.

Thus, the shutdown of the reactor is the most appropriatecontrol action to deal with the problem. Figure 3 again shows

Fig. 2. Line currents. Two-phase-to-ground fault with unsuccessfulTPAR. Without shunt reactor tripping.

the transmission line currents in the case of two-phase-to-ground fault with unsuccessful TPAR, but this time the reactorCB is switched off in the dead time.

B. Resonance overvoltage during SPAR

The tuning of the line close to the resonance causes over-voltage that occurs after the extinction of the arc in the deadtime. The overvoltage leads to the restriking of the arc, which,in turn, prevents the successful implementation of SPAR. Adetailed analysis of this phenomenon is given in [17]. Thus,for the compensated lines the detuning of the resonance is theonly countermeasure against re-ignition of the arc. To this endone or all three phases of the reactor should be switched offin the dead time. Figure 4 shows the line voltages in SPARwith and without tripping the reactor.

The analysis suggests that in practice there can be situationsthat will need the reactor tripping during TPAR and SPAR.

III. THE PROBLEM

Let us turn analyse the process inside the SR CB. Considerthe following situation (Fig. 5):

1) At some time point there is a single-phase short circuit atthe beginning of the line close to the reactor (Fig.5, A).

2) Linear breakers disconnect the faulted phase from bothsides (SPAR procedure). The arc continues to burn, onephase of the reactor is discharging to the point of fault(Fig.5, B).

3) Directly after the disconnection of the faulted phase, thephase of the reactor must also be switched off (Fig.5, C).As stated above, this should be done in order to detunethe resonance and thus ensure the extinction of the arc(Fig.4). In the case of TPAR this should be done toprevent the damage of the line breaker during reclosing(Fig.2).

Fig. 3. Line currents. Two-phase-to-ground fault with unsuccessfulTPAR. SR tripping in the dead time.

Fig. 4. Line voltages during SPAR with and without tripping of thereactor.

Fig. 5. Explanatory diagram. Tripping of the SR during SPAR.

The oscillograms of phase currents and voltages of thereactor in the considered situation are shown in Figure 6.

The closer the short circuit to the reactor, the more aperiodicthe reactor current characteristic, the larger the time constantvalue, and the worse the conditions for current tripping.

The main question of the paper can be summarized asfollows. In the considered situation, is it possible to switch offthe current (with high aperiodic and low periodic components)which flows through the reactor? In other words, is it possibleto perform this operation without the reactor SF6 CB beingdamaged?

Fig. 6. Phase currents and voltages of the SR.

This question was asked to a number of equipment manu-facturers. All of them replied that the final answer will needa series of expensive full-scale experiments. Finally we wereasked to consider one of the following solutions:

1) Closing resistors. This option has the following disad-vantages. First, currently closing resistors are made onlyfor linear, but not for the SR CB. Second, as a rule, theclosing resistor is put into operation only when CB isswitched on, but in our case, the resistor must be putinto operation when CB is switched off. In addition, asnoted above, the price of the resistor is quite significant,but the need for its use in our case is not fully justified.Also, it should be noted that it is unacceptable to installboth a closing resistor and a synchronization device inCB. However, the latter would be useful to improve thetransients during normal operation or emergency controlswitching.

2) Introduction of a delay before the tripping shuntreactor CB. This measure will reduce the aperiodiccurrent, and in the delay time most of the energy, storedin the SR, will dissipate. This solution has the followingdisadvantage. If the delay is too long, it may be difficultto guarantee that SPAR procedure will be successful,due to the resonance overvoltage to occur (Fig.4). Thus,the delay should be combined with a means acceleratingthe dissipation of the energy stored in SR.

None of the proposed solutions can answer all the questions.The problem remains unsolved.

IV. WAYS OF SOLVING THE PROBLEM

The following solutions to the problem can be proposed:

1) Full-scale experiments for the identification of a rangeof acceptable impacts on the SR CB.

2) Temporary introduction of resistors into the SR neutral.

A. Full-scale experiments

To determine the range of acceptable impacts on a shuntreactor SF6 CB, the problem can be reformulated as follows.What is the minimum amount of energy that can be dissipatedby one pole of the CB without the breaker being damaged?The maximum energy stored in a single phase of a SR can becalculated using the formula:

Wmax =U2max

3 · ω2 · L(2)

where Umax - maximum operating phase-to-phase voltage(RMS); ω = 2πf0; L - phase inductance of the reactor.

If the experiment confirms the existence of the problem, itis possible to introduce a new parameter (in addition to theconventional technical characteristics of the HVAC CB) thatwould characterize the maximum amount of energy which canbe dissipated by one phase of the CB without the breakerbeing damaged. When selecting a CB, this parameter shouldbe compared with the value of (2). For instance, for theconsidered test system (Fig.1), Wmax ≈ 0.2 MJ , and only theexperiment can answer the question of whether this amountof energy can damage the SR CB.

B. Temporary introduction of resistors into the neutral of SR

Initially, the introduction of resistors into the circuit ofSR was proposed in [7] to solve the problem of aperiodiccurrent tripping (Fig.2). In the normal state the resistors arebypassed by low voltage CB installed in the neutral (Fig.7).If necessary, the resistors are temporarily put into operationthereby improving the quality of transients. Let us showthat the same approach can be used to improve SR trippingconditions during TPAR and SPAR.

The resistance of the power resistor unit is determined bythe insulation level of the SR neutral:

Umax ·R√3 (R2 +X2)

≤ UN (3)

where Umax - maximum operating phase-to-phase voltage ofthe SR (RMS); R - resistance of power resistor unit; X - SRphase reactance; UN - insulation level of the neutral.

Fig. 7. Explanatory diagram. Connection of power resistor units tothe circuit of shunt reactor.

For the equipment of 500 kV the typical insulation levelof the neutral is 35 kV. Thus, for the considered test system(Fig.1), Rmax ≈ 200 Ω. Taking into account the possibility ofresonance overvoltage (if arc extinguished before the reactoris tripped off) we can finally take R = 150 Ω. Figure 8 showsthe current of the SR faulted phase during SPAR with andwithout temporary introduction of a 150 Ω power resistor.

Maximum energy consumed by the power resistor is deter-mined by the SR maximum current flow:

WRN =U2max ·R · t

3 (R2 +X2)(4)

where Umax - maximum operating phase-to-phase voltage ofthe SR (RMS); R - resistance of power resistor unit; X - SRphase reactance; t - operation time of the resistor.

Advantages. The proposed solution is universal. It allowssolving not only the possible problem of SR tripping duringSPAR and TPAR, but also the problem of aperiodic currenttripping. At the same time, its implementation cost is lower incomparison with the cost of preinsertion resistors. The reasonfor this is that the closing resistors are installed on the highvoltage side of the SR (built into the CB), and their priceis determined by the need to ensure the thermal stability incase of a close fault on the high voltage side. Maximumenergy consumed by the closing resistor is determined by thefollowing equation:

WR =U2max · tR

(5)

where Umax - maximum operating phase-to-phase voltage ofthe SR (RMS); R - resistance of closing resistor; t - operationtime of the closing resistor.

The price of power resistors and CB installed in the neutralis in turn determined by the thermal stability in the case of SRload current flow (4). The load current is much smaller thanthe short circuit current.

Compare maximum energies consumed by the closing re-sistor and by the resistor installed in the neutral. If Umax =525 kV , R = 150 Ω, X = 1570 Ω, t = 0.02 sec, thanWRN ≈ 0.11 MJ and WR ≈ 12 MJ , which is abouta hundred times more. Given such a big difference, it canbe assumed that the difference in the prices of the resistors(including installation and accessories) may be less than the

Fig. 8. SR faulted phase current with and without the introduction ofa 150 Ω power resistor.

price of the necessary 35kV circuit breaker installed in theneutral (Fig.7).

Disadvantages. Due to the introduction of resistors, trippingof the reactor should be delayed. For the considered test systemthe delay should be about 0.05-0.1 sec. Note that the minimumduration of the dead time during SPAR is about 0.8-0.9 sec.It is likely that even if the delay is taken into account, theSR will be switched off before the arc is extinguished and theovervoltage occurs. The minimum duration of the dead timeduring TPAR is about 0.3-0.4 sec. and there is no overvoltageafter the arc extinction. Hence, it is guaranteed that the reactorwill switch off before reclosing. Thus, despite the fact that thedisconnection process is slightly extended in time, the delaywill not affect the reliability of the relay protection operation.However, it should be noted that the delay alone without theintroduction of resistors cannot be considered as an adequatemeasure.

V. CONCLUDING REMARKS

The research has demonstrated that on the one hand switch-ing of shunt reactor during single and three-phase auto-reclosing is an effective measure for the resonance detuning,but on the other hand, this measure may be associated withsome difficulties. As the computer simulations prove, tempo-rary introduction of power resistors into the neutral of SRwill resolve not only the problem of shunt reactor trippingduring SPAR and TPAR, but also the aperiodic current trippingproblem. Only full-scale experiments but not mathematicalmodels can give the final answer to the questions posed inthe paper.

REFERENCES

[1] Ching-Yin Lee, Chang-Jhih Chen, Chao-Rong Chen, Yen-Feng Hsu,“Comparison of transient phenomena when switching shunt reactors onthe line’s two terminals and station busbar,” Power System Technology,2004. PowerCon 2004. 2004 International Conference on, vol.2, no.,pp.1255-1260, Vol.2, 21–24 Nov. 2004.

[2] Report ABB Power Technologies, “Investigation of Breaker Failurewhen Disconnecting a Highly Compensated Line,” no. 1JNR100009-697Rev. 01.

[3] I. Naumkin, M. Balabin, N. Lavrushenko, R. Naumkin, “Simulation ofthe 500 kV SF6 circuit breaker cutoff process during the unsuccessfulthree-phase autoreclosing,” Proceedings of the IPST2011, Delft, TheNetherlands, June 14-17, 2011.

[4] L. Prikler, G. Ban, G. Banfai, “EMTP models for simulation of shuntreactor switching transients,” International Journal of Electrical Power &Energy Systems, Volume 19, Issue 4, May 1997, Pages 235-240.

[5] V. E. Kachesov , D. V. Kachesov, “Requirements for Switching Al-gorithms of EHV Shunt Compensated OHL by SF6 Circuit Breakers,”Proceedings of the IPST2011, Delft, The Netherlands, June 14-17, 2011.

[6] I. Naumkin. “Emergency failures of SF6 circuit breakers during thecommutations of the compensated 500-1150kV transmission lines,” (inRussian). Elektrichestvo, no.10, 2012, p.22-32.

[7] I. Naumkin, V. Pod’yachev, L. Sarin, D. Kochura, “Methods of perfor-mance assurance for SF6 circuit-breakers at switchings of compensated500-1150 kV overhead power lines,” Proceedings of the IPST2013,Vancouver, Canada, July 18-20, 2013.

[8] G. Evdokunin, M. Dmitriev, S. Goldstein, U. Ivanitskii, “High-voltageoverhead lines. Switching and their impact on circuit breakers,” (inRussian). Novosti Elektrotehniki, no.3, 2008.

[9] R. J. Rajotte, C. Charpentier, S. Breault, H. H. Le, Hieu Huynh,J. Desmarais, “Field tests of a circuit breaker synchronous control,” PowerDelivery, IEEE Transactions on, vol.10, no.3, pp.1301-1309, Jul 1995.

[10] CIGREWorking Group 13.07, “Controlled switching of HVAC circuitbreakers – planning, specification and testing of controlled switchingsystems,” ELECTRA, no.197 Aug.2001, pp.23-33.

[11] R. Alvinsson, “Controlled Switching of Circuit Breakers: A TechnologyStatus Review,” ABB Switchgear AB, BC 96-027, Sweden.

[12] K. Froehlich, A. C. Carvalho, B. L. Avent, C. Hoelzl, W. Hofbauer,D. F. Peelo, M. Stanek, P. Hoegg, and J. H. Sawada, “Controlled closingon shunt reactor compensated transmission lines – Part I: Closing controldevice development. Part II: Application of closing control device forhigh-speed autoreclosing on BC hydro 500 kV transmission line,” IEEETrans. Power Del., vol.12, no.2, pp. 734-746, Apr. 1997.

[13] P. M. Mestas, M. C. Tavares, A. M. Gole, “Implementation and Perfor-mance Evaluation of a Reclosing Method for Shunt Reactor-CompensatedTransmission Lines,” Power Delivery, IEEE Transactions on , vol.26, no.2,pp.954-962, April 2011.

[14] M. Dmitriev, “110-750kV SF6 circuit-breakers. Methodology for theselection of measures against aperiodic currents,” (in Russian). NovostiElektrotehniki, no.4, 2013.

[15] A. C. Legate, J. H. Brunke, J. J. Ray, and E. J. Yasuda, “Eliminationof closing resistors on EHV circuit breakers,” IEEE Trans. Power Del.,vol.3, no.1, pp.223-231, Jan.1988.

[16] A. C. Carvalho, M. Lacorte, and O. Knudsen, “Improved EHV lineswitching surge control by application of MO-Arrester and controlledswitching,” in Proc. Int. Conf. Energy Management and Power Delivery,Nov.1995, pp.292-297.

[17] M. Dmitriev, G. Evdokunin, “EMTP Simulation of the SecondaryArc Extinction at Overhead Transmission Lines under Single PhaseAutomatic Reclosing,” Proceedings of the IEEE St.Petersburg PowerTech,St.Petersburg, Russia, June 27-30, 2005.