energies

Article Field Experiments on 10 kV Switching Shunt Banks Using Ordinary and Phase-Controlled Vacuum Circuit Breakers

Wenxia Sima 1, Mi Zou 1,*, Qing Yang 1, Ming Yang 1 and Licheng Li 2

1 State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Chongqing 400044, China; [email protected] (W.S.); [email protected] (Q.Y.); [email protected] (M.Y.) 2 School of Electric Power, South China University of Technology, Guangdong 510640, China; [email protected] * Correspondence: [email protected]; Tel.: +86-23-6511-1795 (ext. 8307); Fax: +86-23-6510-2442

Academic Editor: Ying-Yi Hong Received: 17 December 2015; Accepted: 27 January 2016; Published: 30 January 2016

Abstract: During the switching on/off of shunt capacitor banks in substations, vacuum circuit breakers (VCBs) are required to switch off or to switch on the capacitive current. Therefore, the VCBs have to be operated under a harsh condition to ensure the reliability of the equipment. This study presents a complete comparison study of ordinary and phase-controlled VCBs on switching 10 kV shunt capacitor banks. An analytical analysis for switching 10 kV shunt capacitor banks is presented on the basis of a reduced circuit with an ungrounded neutral. A phase selection strategy for VCBs to switch 10 kV shunt capacitor banks is proposed. Switching on current waveforms and switching off overvoltage waveforms with, and without, phase selection were measured and discussed by field experiments in a 110 kV substation in Chongqing, China. Results show that the operation of phase-controlled VCBs for 10 kV switching shunt capacitor banks is stable, and phase-controlled VCBs can be used to implement the 10 kV switching on/off shunt capacitor banks to limit the transient overvoltage and overcurrent. The values of overvoltage and inrush current using phase-controlled VCBs are all below those with ordinary VCBs.

Keywords: vacuum circuit breakers (VCBs); phase selection; shunt capacitor banks; field tests

1. Introduction Switching shunt capacitor banks is the most widespread method for the compensation of reactive power or for the stabilization of for power quality reasons [1–6]. Circuit breakers are usually switched to adjust the reactive power capacity according to the load change. Capacitive switching is conducted with an almost daily frequency according to an inquiry performed by the CIGRE (International Council on Large Electric systems) Working Group 13.04 in 1999 [7]. About 60% of all capacitor banks are switched up to 300 times a year, and an additional 30% are switched up to 700 times a year. Capacitor-switching transients have caused many problems for the past few years [8–14]. Among these problems, the bearing switching overvoltage may be the most direct and severe one. Thus, studying the effect of capacitor-switching transients on capacitors is essential. In China, some 220 kV power substations (especially in Hainan, an island in Southern China) or most 500 kV series compensation station do not directly offer power supply to the users, and their 10 kV busbars are not usually connected to the industry loads or the other loads. Substation and reactive compensation equipment are connected to 10 kV busbars in most of these substations. According to the fault statistics report from Chongqing electric power corporation, the faults caused by 10 kV switching transients are always under the condition with no load or light loads. Thus,

Energies 2016, 9, 88; doi:10.3390/en9020088 www.mdpi.com/journal/energies Energies 2016, 9, 88 2 of 14 the condition of no load or light loads will be more severe to the safety of power system and equipment Energies 2016, 9, 88 during the switching transients. Inloads. a 10 kV Thus, ungrounded the condition neutralof no load system, or light loads switching will be shuntmore severe capacitor to the safety banks of often power caused system severe insulationand faults equipment of shunt during capacitor the switching banks transients. or dry type in China in recent years [15,16]. For example,In ina 10 our kV previousungrounded work neutral on system, switching switching shunt shunt capacitor capacitor banks banks and often shunt caused reactors severe [15,16], insulation faults of shunt capacitor banks or dry type transformers in China in recent years [15,16]. 12 kV vacuum circuit breakers (VCBs) were used, and capacitor explosion occurred during our field For example, in our previous work on switching shunt capacitor banks and shunt reactors [15,16], 12 experimentskV vacuum on switching circuit breakers 10 kV (VCBs) shunt were capacitor used, and banks capacitor using explosion ordinary occurred VCBs induring Chongqing, our field China, as shownexperiments in Figure on1. switching After checking 10 kV shunt the capacitor burnt capacitor banks using body, ordinary we found VCBs in that Chongqing, the failure China, was as caused by a breakdownshown in Figure between 1. After the checking two terminals the burnt of capacitor the capacitors. body, we found Overvoltage that the failure was deemedwas caused as by the main reason ofa breakdown this accident. between Therefore, the two based terminals on ourof the previous capacitors. study Overvoltage and experience was deemed in this as field,the main in a 10 kV ungroundedreason system, of this accident. we proposed Therefore, that based the on VCBs our previous with phase study selectionand experience was in a potentialthis field, in and a 10 alternative kV ungrounded system, we proposed that the VCBs with phase selection was a potential and alternative methodmethod to limit to the limit overvoltage the overvoltage and and overcurrent overcurrent causedcaused by by switching switching shunt shunt capacitor capacitor banks. banks.

(a) (b)

Figure 1. An explosion involving switching 10 kV shunt capacitor banks with vacuum circuit breakers Figure 1. An explosion involving switching 10 kV shunt capacitor banks with vacuum circuit breakers (VCBs). (a) Testing site; and (b) burnt capacitors. (VCBs). (a) Testing site; and (b) burnt capacitors. Even though the transient duration is usually very short, switching shunt capacitor banks Evenproduces though harmful the transient transients durationwhich can isreduce usually the verylife of short, the devices switching [17,18]. shunt Conventionally, capacitor banks pre‐insertion or inductors, and R‐C circuits, have been applied to limit the switching produces harmful transients which can reduce the life of the devices [17,18]. Conventionally, transients during opening or closing shunt capacitor banks [19,20]. These methods may have reduced pre-insertionor limited resistors the switching or inductors, overvoltage and and R-C transients snubber to circuits, some extent. have However, been applied pre‐insertion to limit resistors the switching transientswill during dissipate opening energy and or closingrelease a shunt large amount capacitor of heat, banks and [ the19, 20operation]. These of methodspre‐insertion may inductors have reduced or limitedmay the cause switching electric sparks overvoltage and bring and about transients resonance. to somePre‐insertion extent. resistors However, or inductors pre-insertion will also resistors will dissipateresult in energy increasing and equipment release a costs large and amount may not of provide heat, appreciable and the operation transient reduction of pre-insertion [18,21]. The inductors may causemethod electric of R‐ sparksC snubber and circuits bring may about bring resonance. about severe Pre-insertion harmonic distortion resistors [21,22]. or inductors Thus, an will also alternative, more economical, and effective approach to reduce and limit the switching overvoltage result in increasing equipment costs and may not provide appreciable transient reduction [18,21]. and transients is phase‐controlled switching. The significant advantages of phase‐controlled The methodswitching of include R-C snubber limitation circuits of closing may inrush bring currents, about suppression severe harmonic of switching distortion overvoltages, [21 lower,22]. Thus, an alternative,rate of power more equipment economical, failures and and effective less maintenance approach of to frequently reduce and used limit circuit the breakers switching [17,19,23]. overvoltage and transientsCIGRE Working is phase-controlled Group A3‐07 switching.presented a Theseries significant of studies on advantages the development of phase-controlled and application switchingof includeVCBs limitation with phase of closing selection inrush [17,24]. currents, According suppression to their statistical of switching data, circuit overvoltages, breakers with lowerphase rate of power equipmentselection have failures been widely and lessused maintenance in 26.4–800 kV ofpower frequently systems, usedand about circuit 64% breakers of all circuit [17 breakers,19,23]. CIGRE with phase selection are applied to switch shunt capacitor banks. Many theoretical and simulation Workingstudies Group on A3-07the overvoltage presented and ainsulation series of failures studies caused on the by switching development shunt capacitor and application banks have of VCBs with phasebeen selectionreported [25–28]. [17,24]. However, According only to theira few statistical studies have data, been circuit done breakers on the comparison with phase and selection have beenapplication widely of used ordinary in 26.4–800 and phase kV‐controlled power VCBs systems, in substation and about by field 64% tests. of For all this circuit reason, breakers field with phase selectionmeasurements are applied of switching to switch 10 kV shuntshunt capacitor capacitor banks banks. have Many been conducted theoretical with and the simulation use of both studies on the overvoltageordinary VCBs and and phase insulation‐controlled failures VCBs causedto compare by the switching opening overvoltage shunt capacitor and closing banks current have been caused by 10 kV switching shunt capacitor banks. reported [25–28]. However, only a few studies have been done on the comparison and application of ordinary and phase-controlled VCBs in substation by field tests. For this reason, field measurements of switching 10 kV shunt capacitor banks have been conducted with the use of both ordinary VCBs and phase-controlled VCBs to compare the opening overvoltage and closing current caused by 10 kV switching shunt capacitor banks. Energies 2016, 9, 88 3 of 14

The main contribution of this study is to investigate and demonstrate the potential effects and advantages of phase-controlled VCBs in 10 kV switching transients instead of 110 kV system, when Energies 2016, 9, 88 compared with the ordinary VCBs. The rest of this paper is organized as follows. In Section2, the overvoltageThe main and contribution the overcurrent of this of study switching is to investigate 10 kV Y-connected and demonstrate shunt the capacitor potential banks effects are and analyzed by analyticaladvantages method of phase based‐controlled on a simplified VCBs in 10 circuit. kV switching In Section transients3, a phase-selectinginstead of 110 kV system, control when strategy for VCBs tocompared switch onwith or the to ordinary switch off VCBs. capacitor The rest banks of this is paper proposed. is organized In Section as follows.4, six In typical Section cases2, the of field experimentsovervoltage are compared and the overcurrent and discussed. of switching Lastly, 10 kV the Y‐connected conclusion shunt is presented capacitor banks in Section are analyzed5. by analytical method based on a simplified circuit. In Section 3, a phase‐selecting control strategy for 2. AnalyticalVCBs to Analysisswitch on or to switch off capacitor banks is proposed. In Section 4, six typical cases of field experiments are compared and discussed. Lastly, the conclusion is presented in Section 5. The analytical analysis of switching shunt capacitor banks is studied. Figure2a illustrates the switching2. Analytical shunt Analysis capacitor bank circuit for an ungrounded power system with Y-connected shunt capacitorThe analytical banks. analysisUsa( tof), switchingUsb(t), shunt and Ucapacitorsc(t) are banks the is powerstudied. sources.Figure 2a illustratesR is the the internal switching shunt capacitor bank circuit for an ungrounded power system with Y‐connected shunt resistance of source. Ka, Kb, and Kc are the circuit breakers. L is the series inductance, and capacitor banks. Usa(t), Usb(t), and Usc(t) are the power sources. R is the internal resistance of source. C are the shunt capacitor banks. In a 10 kV ungrounded neutral system, the typical values Ka, Kb, and Kc are the circuit breakers. L is the series inductance, and C are the shunt capacitor banks. ˝ of theseIn parametersa 10 kV ungrounded are as follows:neutral system,Usa(t )the = 8.165sin(100typical valuesπ tof) kV, theseU sbparameters(t) = 8.165sin(100 are as follows:πt + 120 ) kV, ˝ Usc(t) =U 8.165sin(100sa(t) = 8.165sin(100πt ´π120t) kV,) kV,Usb(tR) == 8.165sin(100 0.2 Ω, C =πt 27.14+ 120°)µ kV,F, L U=sc(t) 9.90 = 8.165sin(100 mH. Theπt reduced − 120°) kV, circuit in Figure2Rb = can 0.2 Ω be, obtainedC = 27.14 μ withF, L = the 9.90 assumption mH. The reduced of simultaneous circuit in Figure switching 2b can on/offbe obtained for thewith three-phase the circuit breakers.assumption of simultaneous switching on/off for the three‐phase circuit breakers.

(a) (b)

Figure 2. Circuit of switching shunt capacitor banks. (a) Symmetrical three‐phase circuit; and Figure 2. Circuit of switching shunt capacitor banks. (a) Symmetrical three-phase circuit; (b) reduced circuit. and (b) reduced circuit. 2.1. Closing Operation 2.1. ClosingAccording Operation to the principle of Kirchhoff‘s voltage law and initial conditions, Equation (1) can be Accordingobtained as: to the principle of Kirchhoff‘s voltage law and initial conditions, Equation (1) can be 2 obtained as: d()d()Utcc Ut 2LC RC Ucs() t U () t (1) d U ptdqt 2 dU dpttq LC c ` RC c ` U ptq “ U ptq (1) dt2 dt c s where Utssmc() U sin(ωθ t ) , ω is the angular frequency of source, θ c is the closing phase angle, where U ptq “ U sinpωt ` θ q, ω is the angular frequency of source, θ is the closing phase angle, ands U sm issm the source voltagec amplitude. c o and Usm isWith the source θ c  90 voltage, the solutions amplitude. for Equation (1) are given as follows: o With θc “ 90 , the solutions for Equation (1) are given as follows: Utccm0cm0() U cos(ω t)( U U )cos(ω t) (2) d()Ut ( U U ) Ucptq “ Uc0cmcmcospωtq ` pU0 ´ Ucmqcospω0tq (2) itccm0() C Uω Csin(ω t)sin(ω t) (3) dt LC/ dUcptq pU0 ´ Ucmq where U0 is thei cpvaluetq “ Cof the initial“ ´capacitorUcmωC sinvoltage,pωtq ´Ucm is the steadysin pvoltageω0tq of capacitor (3) dt L{C Usm Ucm  , ω  1/ (LC ) , and ω is the oscillation frequency. The amplitude 220 0 where U0 isωCR the value(ω L1/ ofω theC) initial capacitor voltage, Ucmais the steady voltage of capacitor of the closingU currentsm and voltage are given as: Ucm “ , ω0 “ 1{ pLCq, and ω0 is the oscillation frequency. The amplitude 2 2 ωC R ` pωL ´ 1{ωCq UU2U a cmax cm 0 (4) of the closingb current and voltage are given as: iIcmax cm(1 ff 0 / ) (5)

|Ucmax| “ |2Ucm ´ U0| (4) Energies 2016, 9, 88 4 of 14

icmax “ Icmp1 ` f0{ f q (5)

Usm where Icm is the steady current of capacitor banks Icm “ , f0 “ ω0{2π, and R2 ` pωL ´ 1{ωCq2 f0 ě 500 Hz in the most serious case. Thus, the closing overvoltageb is below 2.0Ucm, and the closing current may reach 11 Icm in the most serious case. o With θc “ 0 , the amplitude of the closing current and voltage are given in the same way as:

Ucmax “ Usmp1 ` f { f0q (6)

|icmax| “ |2Icm ´ I0| (7) o where I0 is the value of the initial capacitor current. Therefore, closing at θc “ 0 significantly decreases the closing current by comparing Equation (7) with Equation (5).

2.2. Opening Operation

When the moving and static contacts are separated, Ucptq is almost equal to Usptq because the arc voltage and inductance voltage are very low (jωL “ 1{jωC ˆ 5%). The capacitor voltage will be maintained at Usm after arc extinguishing if capacitor the discharge is ignored. The transient recovery voltage of the contacts will be as follows:

Urptq “ Usptq ´ Usm “ Usm rsinpωt ` π{2q ´ 1s (8)

Urptq is equal to ´2Usm after half a cycle (ωt “ π). If the dielectric recovery voltage is less than 2Usm at this moment, the re-striking and subsequent high-frequency oscillation will occur. The high-frequency voltage and current of the capacitor are given as follows:

Ucptq “ Usm ´ 2Usmcospω0tq dUcptq (9) $ icptq “ C “ 2ω0CUsmsinpω0tq & dt The overvoltage of the% capacitor is equal to 3Usm at ic(t) initially crossing zero during high-frequency oscillation. In the same moment, the overvoltage of capacitor will be maintained at 3Usm if the arc extinguishes. After a half cycle, the transient recovery voltage of the contacts reaches 4Usm. If re-striking occurs again, the maximum value of Uc(t) will reach 5Usm. With this analogy, the overvoltage of the capacitor becomes 3 p.u., 5 p.u., 7 p.u., and so on. It should be noted that, however, in practical switching operation, the switching operation time cannot be completely consistent because of the switching time scatter of three-phase vacuum breakers. In a 10 kV ungrounded system, shunt capacitor banks are always in Y-type ungrounded neutral in China; random non-simultaneously switching generally will result in a shift of the voltage of the neutral point of the capacitor banks, higher overvoltage and overcurrent on capacitors and circuit breakers, and more complicated oscillation [29,30]. In particular, the overvoltage and overcurrent of the last switching pole will be affected by the switching angle, delay time and line-to-line voltage of the other two phases [29,30]. Thus, in 10 kV ungrounded system, the phase-controlled switching method is proposed in this study to limit the overvoltage and transients.

3. Phase-Selecting Control Strategy for 10 kV Ungrounded Capacitor Banks The structure of phase-controlled VCBs used in this experiment (MDS2B-12) is presented. As depicted in Figure3, the MDS2B-12 mainly consists of a separate three-pole permanent magnetic actuator VCB, a three-pole phase selection intelligent controller, a power supply, a potential transformer testing capacitance residual voltage, and a telecommunications system. The strategies and processes of switching shunt capacitor banks based on the phase-controlled methodology are analyzed, as shown in Figures4 and5. To reduce the closing inrush current for Energies 2016, 9, 88 5 of 14

Energies 2016, 9, 88 ungroundedEnergies 2016 shunt, 9, 88 capacitor banks, the strategy for closing capacitor banks is to close the two phases whilezero their (5Energies phasems later 2016 , 9than, 88 the are above equal two and phases). then Figure to close 4 shows the third the sequence phase while of closing its phase capacitor voltage banks crosses zero (5withzero ms phase(5 later ms laterselection. than than the Takingthe above above the two two line phases). phases).voltage Figureof Figure phase4 4 A showsshows and phase the the sequence sequence B as reference, of of closing closing the breakercapacitor capacitor receives banks banks zero (5 ms later than the above two phases). Figure 4 shows the sequence of closing capacitor banks with phasethewith closing phase selection. selection.signal Takingat t Taking0, the the breakers the line line voltage ofvoltage phase of of A phase phase and phase A and and B phase phaseclose B at Bas t as abreference,, reference,and the thebreaker thebreaker breaker of phasereceives receives C with phase selection. Taking the line voltage of phase A and phase B as reference, the breaker receives the closingclosesthe closing at signal tc. tsignalAd, attBdt, atand, thet0, ttheCd breakers are breakers the closing of of phase phase delay A A times, andand phase respectively. B B close close at at tabt, and, and the the breaker breaker of phase of phase C C the closing signal0 at t0, the breakers of phase A and phase B close at tab, andab the breaker of phase C closes at tc. tAd, tBd, and tCd are the closing delay times, respectively. closes at tccloses. tAd ,att Bdtc. ,tAd and, tBd,t andCd aretCd are the the closing closing delay times, times, respectively. respectively.

Figure 3. Structure chart of phase‐controlled VCBs (MDS2B‐12). Figure 3. Structure chart of phase-controlled VCBs (MDS2B-12). Figure 3. Structure chart of phase‐controlled VCBs (MDS2B‐12). Figure 3. Structure chart of phase‐controlled VCBs (MDS2B‐12).

Figure 4. Sequence of closing capacitor banks with phase selection.

Figure 4. Sequence of closing capacitor banks with phase selection. Figure 4. Figure Sequence4. Sequence of of closing closing capacitor capacitor banks banks with with phase phase selection. selection.

Figure 5. Sequence of opening capacitor banks with phase selection.

Figure 5. Sequence of opening capacitor banks with phase selection. Figure 5. Sequence of opening capacitor banks with phase selection. Figure 5. Sequence of opening capacitor banks with phase selection.

Energies 2016, 9, 88 6 of 14

The breakers should start to open a little earlier than the phase current crosses zero to ensure Energies 2016, 9, 88 sufficient circuit breaker fracture gap distance (∆tb as shown in Figure5). The strategy for opening capacitor banks is to open one phase and then to open the other two phases when the current of the The breakers should start to open a little earlier than the phase current crosses zero to ensure first-pole-to-clear reaches zero (5 ms later than the first phase). Figure5 shows the sequence of opening sufficient circuit breaker fracture gap distance (Δtb as shown in Figure 5). The strategy for opening capacitorcapacitor banks banks with is to phase open selection. one phase and The then breaker to open receives the other the two opening phases signalwhen the at tcurrent0, the breakerof the of phasefirst B opens‐pole‐to at‐cleartb, and reaches breakers zero of (5 phase ms later A and than phase the first C opens phase). at tFigureac. tAd ,5t Bdshows, and thetCd sequenceare the opening of delayopening times, respectively. capacitor bankstAo ,withtBo, phase and tCo selection.are the The times breaker between receives the moving the opening contact signal starting at t0, tothe move and thebreaker moment of phase the movingB opens at and tb, and static breakers contacts of arephase completely A and phase separated. C opens at tac. tAd, tBd, and tCd are the opening delay times, respectively. tAo, tBo, and tCo are the times between the moving contact 4. Fieldstarting Test to Results move and and the Discussion moment the moving and static contacts are completely separated.

The4. Field field Test test Results of 10 kV and switching Discussion shunt capacitor banks is conducted on the 10 kV side of a 110 kV substation in Chongqing, China. The short circuit capacity for the 10 kV system is 245 MVA, and The field test of 10 kV switching shunt capacitor banks is conducted on the 10 kV side of a 110 kV the system short circuit reactance is 0.45 Ω. Figure6 shows the site layout of the field experiment, substation in Chongqing, China. The short circuit capacity for the 10 kV system is 245 MVA, and the and Figure7 presents the corresponding test circuit. #653 represents the ordinary VCBs, and #65301, system short circuit reactance is 0.45 Ω. Figure 6 shows the site layout of the field experiment, and #65302,Figure and 7 #65303 presents represent the corresponding the three test groups circuit. of #653 phase-controlled represents the VCBs, ordinary respectively. VCBs, and #65301, The layout of the#65302, voltage and measurement #65303 represent points the three (VMPs) groups and of the phase current‐controlled measurement VCBs, respectively. points (CMPs) The layout in the of field test arethe also voltage shown measurement in Figure points7. The (VMPs) divider and ratio the current of the measurement capacitor voltage points dividers (CMPs) in is the 350:1, field andtest the frequencyare also bandwidth shown in ofFigure the capacitor7. The divider dividers ratio is of from the capacitor 50 Hz to voltage 50 MHz. dividers The current is 350:1, measurement and the rangefrequency and the frequency bandwidth bandwidth of the capacitor of the dividers Rogowski is from coils 50 Hz are to 0–6 50 kA MHz. and The 30 current Hz–1 MHz, measurement respectively. The parametersrange and the of frequency the capacitors bandwidth and inductorsof the Rogowski are shown coils are in 0–6 Table kA1 and. 30 Hz–1 MHz, respectively. DuringThe parameters the operation of the capacitors of ordinary and inductors VCBs and are phase-controlled shown in Table 1. VCBs switching on and off the During the operation of ordinary VCBs and phase‐controlled VCBs switching on and off the corresponding shunt capacitor banks, the signals from the Rogowski coils and capacitor voltage corresponding shunt capacitor banks, the signals from the Rogowski coils and capacitor voltage dividers are transferred to the signal processing unit. The signals are sampled and stored through dividers are transferred to the signal processing unit. The signals are sampled and stored through a a multichannelmultichannel high-speed high‐speed frequency frequency conversion conversion data data acquisition acquisition card card (maximum (maximum sampling sampling frequency frequency of upof to up 40 to MHz) 40 MHz) of a of computer. a computer.

FigureFigure 6. Site6. Site layout layout of of the the fieldfield tests in in a a 110 110 kV kV substation. substation.

Energies 2016, 9, 88 7 of 14 Energies 2016, 9, 88

FigureFigure 7. Test 7. circuitTest circuit of switching of switching 10 10 kV kV shunt shunt capacitor capacitor banks banks in a in 110 a kV 110 substation. kV substation.

Table 1. Parameters of the capacitors and reactors. Table 1. Parameters of the capacitors and reactors. Parameters Phase A Phase B Phase C Total Power frequency 50HZ 50HZ 50HZ ‐ ParametersRated power of capacitors Phase2004 kVAR A 2004 Phase kVAR B 2004 kVAR Phase 6012 C kVAR Total Power frequencyNumber of capacitors 50HZ6 50HZ6 6 50HZ18 - Rated powerNumber of capacitors of groups 2004 kVAR3 20043 kVAR3 2004 kVAR9 6012 kVAR NumberNumber of of capacitors capacitors in each group 62 2 62 66 18 NumberCapacitance of groups of each capacitor 27.14 3 μF 27.14 μ 3F 27.14 μF ‐ 3 9 Number of capacitorsRated voltage in eachof each group capacitor 11/ 2 3 kV 11/ 3 2 kV 11/ 3 kV 2‐ 6 Rated power of each capacitor 334 kVAR 334 kVAR 334 kVAR ‐ Capacitance of each capacitor 27.14? µF 27.14? µF 27.14? µF- RatedSeries voltage reactor of each rate of capacitor shunt capacitor bank 11 { 35% kV 115%{ 3 kV5% 11 { 3 kV5% - Rated power of reactors 100.2 kVAR 100.2 kVAR 100.2 kVAR 300.6 kVAR Rated power of each capacitor 334 kVAR 334 kVAR 334 kVAR - Number of reactors 3 3 3 9 Series reactor rate of shunt capacitor bank 5% 5% 5% 5% Number of groups 3 3 3 9 RatedNumber power of reactors reactors in each group 100.2 kVAR1 100.21 kVAR1 100.2 kVAR3 300.6 kVAR NumberInductance of reactors of each reactor 9.90 3 mH 9.90 mH 3 9.90 mH ‐ 3 9 NumberRated of voltage groups of each reactor 310 kV 10 kV 3 10 kV ‐ 3 9 Number of reactorsRated power in each of each group reactor 33.4 1 kVAR 33.4 kVAR 1 33.4 kVAR ‐ 1 3 InductanceQuality of each factor reactor (Q) of the reactor 9.90 mH50 9.9050 mH50 ‐ 9.90 mH - Rated voltage of each reactor 10 kV 10 kV 10 kV - RatedThe power switching of each of the reactor shunt capacitor 33.4 banks kVAR are conducted 33.4 10 kVAR times by using 33.4 kVAR phase‐controlled - QualityVCBs (#65301), factor (Q) and of are the conducted reactor 12 times by ordinary 50 VCBs (#653). 50 The operation 50 of opening shunt - capacitor banks is implemented after 5 min of closing the shunt capacitor banks. However, the operation of closing is conducted after 10 min of opening the shunt capacitor banks to fully discharge The switching of the shunt capacitor banks are conducted 10 times by using phase-controlled the capacitor banks. Six different typical cases are analyzed and discussed, as shown in Table 2 and VCBs (#65301),Figures 8–13. and It are should conducted be noted 12that, times in Cases by ordinary2 and 5, the VCBs three (#653). groups of The capacitor operation banks of are opening shunt capacitorsimultaneously banks switched. is implemented The main afterreason 5 of min these of two closing cases the is to shunt make a capacitor comparison banks. with Cases However, 1 the operationand of 4, closing in which is the conducted shunt capacitors after 10are minalso switched of opening by ordinary the shunt VCBs, capacitor to investigate banks the to impact fully dischargeof the capacitorcapacitor banks. compensation Six different capacity typicalon the switching cases are overvoltage, analyzed overcurrent, and discussed, and transient. as shown in Table2 and Figures8–13. It should be noted that, in Cases 2 and 5, the three groups of capacitor banks are Table 2. Typical cases of switching 10 kV shunt capacitor banks simultaneously switched. The main reason of these two cases is to make a comparison with Cases 1 No. Operation Breaker Group Voltage base value (kV) Current base value (A) and 4, inCase which 1 Switching the shunt on capacitorsWithout phase are selection also (#653) switched 1 by ordinary8.165 VCBs, to investigate155.56 the impact of capacitorCase compensation 2 Switching on capacity Without phase on the selection switching (#653) overvoltage,1–3 8.165 overcurrent, and transient.466.69 Case 3 Switching on With phase selection (#65301) 1 8.165 155.56 Case 4 Switching off Without phase selection (#653) 1 8.165 155.56 Case 5 SwitchingTable off Without 2. Typical phase cases selection of (#653) switching 1–3 10 kV shunt8.165 capacitor banks. 466.69 Case 6 Switching off With phase selection (#65301) 1 8.165 155.56 No. Operation Breaker Group Voltage Base Value (kV) Current Base Value (A)

Case 1 Switching on Without phase selection (#653) 1 8.165 155.56 Case 2 Switching on Without phase selection (#653) 1–3 8.165 466.69 Case 3 Switching on With phase selection (#65301) 1 8.165 155.56 Case 4 Switching off Without phase selection (#653) 1 8.165 155.56 Case 5 Switching off Without phase selection (#653) 1–3 8.165 466.69 Case 6 Switching off With phase selection (#65301) 1 8.165 155.56 Energies 2016, 9, 88 8 of 14

Energies 2016, 9, 88 The peak line-to-neutral voltage of 10 kV power systems is as follows [16,31]: The peak line‐to‐neutral voltage of 10 kV power? ? systems is as follows [16,31]: Upeak “ 10 ˆ 2{ 3 “ 8.165 kV (10) Upeak = 10 2 / 3 8.165 kV (10) 4.1. Case 1 4.1. Case 1 Before the switching-on operations in Case 1, #65302 and #65303 are opened, and #65301 is closed. The circuitBefore breaker the ofswitching this switching‐on operations operation in Case is #653. 1, #65302 Figure and8a shows#65303 theare closingopened, currentand #65301 waveform is of Caseclosed. 1 without The circuit phase breaker selection. of this switching The maximum operation value is #653. of the Figure closing 8a shows current the inclosing this current case occurs in phasewaveform B, and of itsCase value 1 without is 0.63 phase kA (4.04selection. p.u.). The The maximum duration value of of the the closing closing shuntcurrent capacitor in this case banks’ occurs in phase B, and its value is 0.63 kA (4.04 p.u.). The duration of the closing shunt capacitor current transient is about 110–120 ms, and the high-frequency oscillation frequency of the closing banks’ current transient is about 110–120 ms, and the high‐frequency oscillation frequency of the shunt capacitor banks’ current is about 198 Hz. closing shunt capacitor banks’ current is about 198 Hz. FigureFigure8b illustrates 8b illustrates corresponding corresponding closing closing voltage voltage waveforms in in Case Case 1. The 1. The maximum maximum of the of the closingclosing voltage voltage is 10.08 is 10.08 kV kV (1.23 (1.23 p.u.). p.u.). The The durationduration of of the the closing closing shunt shunt capacitor capacitor banks’ banks’ voltage voltage transienttransient is only is only 20 ms.20 ms.

0.8 12 Phase A Phase A Phase B Phase B 0.4 Phase C 6 Phase C

0.0 0

Current I/kA Voltage U/kV -0.4 -6

-0.8 -12 0.00 0.04 0.08 0.12 0.00 0.05 0.10 0.15 Time/s Time/s 0.8 12

0.4 6

0.0 0

Current I/kA Voltage U/kV Voltage -0.4 -6

-0.8 -12 -0.01 0.00 0.01 0.02 -0.01 0.00 0.01 0.02 Time/s Time/s

Figure 8. Waveforms of the closing current and capacitor voltage in Case 1. (a) Current waveform; Figure 8. Waveforms of the closing current and capacitor voltage in Case 1. (a) Current waveform; and and (b) voltage waveform. (b) voltage waveform. 4.2. Case 2 4.2. Case 2 Before the switching‐on operations in Case 2, #65301, #65302, and #65303 are all closed. The Beforecircuit breaker the switching-on of this switching operations operation in Case is also 2, #653. #65301, Figure #65302, 9a shows and the #65303 closing are current all closed. waveform The circuit breakerin Case of this 2 without switching phase operation selection isbut also with #653. three Figure groups9 aof shows shunt capacitor the closing banks. current The maximum waveform in Casevalue 2 without of the phase closing selection current but in this with case three occurs groups in phase of shunt A, and capacitor its value banks. is − The2.09 maximumkA (4.49 p.u.). value of the closingThe duration current of in the this closing case occursshunt capacitor in phase banks’ A, and current its value transient is ´ 2.09 is about kA (4.49 90–100 p.u.). ms, The and duration the high‐frequency oscillation frequency of the closing shunt capacitor banks’ current is about 192 Hz in of the closing shunt capacitor banks’ current transient is about 90–100 ms, and the high-frequency this case. oscillation frequency of the closing shunt capacitor banks’ current is about 192 Hz in this case. Figure 9b illustrates the corresponding closing voltage waveforms in Case 2. The maximum of Figurethe closing9b illustrates voltage also the occurs corresponding in phase A, and closing its value voltage is 14.25 waveforms kV (1.75 p.u.). in Case The 2.voltage The maximum transient of the closingreaches voltage the steady also state occurs after inabout phase 20 ms. A, and its value is 14.25 kV (1.75 p.u.). The voltage transient reaches the steady state after about 20 ms.

Energies 2016, 9, 88 9 of 14 Energies 2016, 9, 88

2.4 14 Phase A Phase A Phase B Phase B Phase C Phase C 1.2 7

0.0 0

Voltage U/kV Current I/kA Current

-1.2 -7

-14 -2.4 0.00 0.05 0.10 0.15 0.00 0.05 0.10 0.15 Time/s Time/s 2.4 14

1.2 7

0.0 0

Current I/kA Voltage U/kV -1.2 -7

-14 -2.4 -0.01 0.00 0.01 0.02 -0.01 0.00 0.01 0.02 Time/s Time/s

FigureFigure 9. Waveforms 9. Waveforms of the of the closing closing current current and and capacitor capacitor voltage voltage in in Case Case 2. 2. ((aa)) CurrentCurrent waveform; and and (b) voltage waveform. (b) voltage waveform. 4.3. Case 3 4.3. Case 3 Before the switching‐on operations in Case 3, #653 is closed, and #65302 and #65303 are opened. BeforeThe circuit the switching-onbreaker of this operations switching operation in Case 3, is #653 #65301. is closed, Figure and 10a #65302 shows andthe closing #65303 current are opened. The circuitwaveform breaker in Case of this 3 with switching phase selection. operation The is strategy #65301. of Figure the closing 10a showsshunt capacitor the closing banks current with phase waveform in Caseselection 3 with is phase addressed selection. in Section The 2. strategy The maximum of the closing value of shunt the closing capacitor current banks in this with case phase occurs selection in is addressedphase A, and in Section its value2. is The 0.33 maximum kA (2.12 p.u.). value The ofduration the closing of the closing current shunt in this capacitor case occurs banks’ incurrent phase A, transient is about 100–110 ms, and the high‐frequency oscillation frequency of the closing shunt and its value is 0.33 kA (2.12 p.u.). The duration of the closing shunt capacitor banks’ current transient capacitor banks’ current is about 206 Hz in this case. is about 100–110 ms, and the high-frequency oscillation frequency of the closing shunt capacitor banks’ Figure 10b illustrates the corresponding closing voltage waveforms. As shown in Figure 10b, current is about 206 Hz in this case. phase A and phase B are closed at t1 when their line voltage Uab crosses zero, and then phase C is Figureclosed after 10b 5 illustrates ms at t2 when the its corresponding phase voltage crosses closing zero. voltage The transient waveforms. voltage As of shownphase C infrom Figure t1 to 10b, phaset2 Ais andequal phase to the B neutral are closed voltage at tof1 whenthe shunt their capacitor line voltage banks.U Thus,ab crosses the zero zero,‐crossing and then switching phase C is closedstrategy after 5 is ms achieved. at t2 when Almost its phase no high voltage‐frequency crosses voltage zero. The oscillation transient occurs voltage in ofthis phase case, C and from thet1 to t2 is equalcorresponding to the neutral maximum voltage of of the the high shunt‐frequency capacitor oscillation banks. voltage Thus, the is very zero-crossing low (about switching0.76 p.u.). The strategy is achieved.time error Almost of the phase no high-frequency‐controlled circuit voltage breaker oscillation for closing the occurs shunt in capacitor this case, banks and is thebelow corresponding ±0.2 ms. maximum of the high-frequency oscillation voltage is very low (about 0.76 p.u.). The time error of the 4.4. Case 4 phase-controlled circuit breaker for closing the shunt capacitor banks is below ˘0.2 ms. Before the switching‐off operations in Case 4, #65302 and #65301 are opened, and #65301 is 4.4. Caseclosed. 4 The circuit breaker of the switching operation is #653. Figure 11a shows the opening current waveform in Case 4 without phase selection. The breaker contacts of phase B start to separate at t3, Beforeand phase the B switching-off is the first‐pole operations‐to‐clear. The in transient Case 4, #65302 currents and of phase #65301 A and are opened,phase C are and always #65301 equal, is closed. The circuitbut in the breaker opposite of the direction, switching after operation the current is of #653. phase Figure B decreases 11a shows to zero the at the opening moment current of t4, which waveform in Caseis also 4 without the time phase of the selection.contacts of Thephase breaker A and phase contacts C start of phase to separate. B start The to breaking separate arc at durationt3, and phase of B is thephase first-pole-to-clear. B is Δt1 = 1.7 ms The (Δt transient1 = t4 − t3). currentsAfter a quarter of phase of a Acycle and (5 phase ms), the C arecurrents always of phase equal, A but and in the oppositephase direction, C both reach after thezero current at the ofmoment phase Bof decreasest5. Thus, the to zero phenomenon at the moment of the ofpowert4, which frequency is also the timeextinguishing of the contacts arcing of phase is observed. A and Statistical phase C results start to according separate. to The our field breaking tests show arc duration that the breaking of phase B is arc duration of the first‐pole‐to‐clear (phase B in this test) is almost 1.0–4.5 ms. ∆t1 = 1.7 ms (∆t1 = t4 ´ t3). After a quarter of a cycle (5 ms), the currents of phase A and phase C both reach zero at the moment of t5. Thus, the phenomenon of the power frequency extinguishing arcing is observed. Statistical results according to our field tests show that the breaking arc duration of the first-pole-to-clear (phase B in this test) is almost 1.0–4.5 ms. Energies 2016, 9, 88 10 of 14

Energies 2016, 9, 88 Energies 2016, 9, 88 Figure 11b illustrates the corresponding opening voltage waveforms in Case 4. The maximum Figure 11b illustrates the corresponding opening voltage waveforms in Case 4. The maximum of the opening voltage in this case occurs in the first-pole-to-clear (phase B), and its value is 18.45 kV of theFigure opening 11b voltage illustrates in this the casecorresponding occurs in the opening first‐pole voltage‐to‐clear waveforms (phase B), in and Case its 4. value The ismaximum 18.45 kV (2.26of(2.26 p.u.). the p.u.). opening Residual Residual voltage charge charge in this of of the case the capacitors capacitorsoccurs in the remainsremains first‐pole more more‐to ‐thanclear than 30 (phase 30 power power B), frequency and frequency its value cycles cyclesis 18.45after after thekV the capacitors(2.26capacitors p.u.). are disconnected.areResidual disconnected. charge Thus, of Thus, the the capacitors the switching switching remains off operation more than is is 3010 10 powermin min later. later.frequency cycles after the capacitors are disconnected. Thus, the switching off operation is 10 min later. 0.4 20 0.4 Phase A 20 Phase A PhasePhase AB PhasePhase AB 0.2 PhasePhase BC 10 PhasePhase BC U 0.2 Phase C 10 Phaseab C U ab 0.0 0

0.0 0

Current I/kA Current Voltage U/kV Current I/kA Current -0.2 Voltage U/kV -10 -0.2 -10

-0.4 -20 0.05 0.10 0.15 0.20 0.05 0.10 0.15 0.20 -0.4 -20 0.05 0.10Time/s 0.15 0.20 0.05 0.10Time/s 0.15 0.20 0.4 Time/s 20 Time/s 0.4 20

0.2 10 0.2 10

0.0 0

0.0 0

Current I/kA Current Voltage U/kV Voltage Current I/kA Current

-0.2 U/kV Voltage -10 -0.2 -10

-0.4 -20 0.06 0.07 0.08 0.09 0.06 0.07 0.08 0.09 -0.4 -20 0.06 0.07Time/s 0.08 0.09 0.06 0.07Time/s 0.08 0.09 Time/s Time/s

Figure 10. Waveforms of the closing current and capacitor voltage in Case 3. (a) Current waveform; Figure 10. Waveforms of the closing current and capacitor voltage in Case 3. (a) Current waveform; Figureand (b )10. voltage Waveforms waveform. of the closing current and capacitor voltage in Case 3. (a) Current waveform; and (andb) voltage (b) voltage waveform. waveform. 1.2 20 0.3 1.2 Phase A 20 Phase A 0.30.0 PhasePhase AB PhasePhase AB

0.8 Phase C 10 Phase C -0.30.0 Phase B Phase B

Current/kA -0.1742 -0.1740 0.8 Phase C 10 Phase C -0.3 Current/kA -0.1742Time/s -0.1740 0.4 Time/s 0

0.4 0

Voltage U/kV Voltage Current I/kA Current 0.0 Voltage U/kV Voltage

Current I/kA Current -10 0.0 -10 -0.4 -20 -0.4-0.19 -0.18 -0.17 -0.16 -0.15 -0.19 -0.18 -0.17 -0.16 -0.15 -20 -0.19 -0.18Time/s -0.17 -0.16 -0.15 -0.19 -0.18Time/s -0.17 -0.16 -0.15 0.6 Time/s 20 Time/s 0.6 20

0.3 10 0.3 10

0.0 0

0.0 0

Current I/kA Voltage U/kV Voltage Current I/kA

-0.3 U/kV Voltage -10 -0.3 -10

-0.6 -20 -0.1726 -0.16800 -0.16792 -0.172 -0.170 -0.168 -0.166 -0.164 -0.6 -20 -0.1726 -0.16800Time/s -0.16792 -0.172 -0.170Time/s -0.168 -0.166 -0.164 Time/s Time/s

Figure 11. Waveforms of the opening current and capacitor voltage in Case 4. (a) Current waveform; Figure 11. Waveforms of the opening current and capacitor voltage in Case 4. (a) Current waveform; Figureand 11. (bWaveforms) voltage waveform. of the opening current and capacitor voltage in Case 4. (a) Current waveform; and (b) voltage waveform. and (b) voltage waveform.

Energies 2016, 9, 88 11 of 14 Energies 2016, 9, 88

4.5.4.5. Case Case 5 5 BeforeBefore thethe switching-offswitching‐off operations operations in in Case Case 5, #65301,5, #65301, #65302, #65302, and and #65303 #65303 are all are closed. all closed. The circuit The circuitbreaker breaker of this of switching this switching operation operation is still #653.is still Figure #653. Figure 12a shows 12a shows the opening the opening shunt capacitorsshunt capacitors banks’ banks’current current waveform waveform in Case in 5 Case without 5 without phase selectionphase selection but with but three with groups three groups of shunt of capacitorshunt capacitor banks. banks.The breaker The breaker contacts contacts of phase of phase B start B start to separate to separate at tat6, t and6, and phase phase B B is is alsoalso the the first first-pole-to-clear.‐pole‐to‐clear. The breaking arc durationduration ofof phase phase B B is is∆ Δt2t2= = 4.14.1 msms ((∆Δtt22 == t7 − ´t6t).6). The The currents currents of of phases phases A A and and C C are are equalequal but opposite inin directiondirection from fromt 7t7to tot 8t8,, during during which which the the current current of of phase phase B B reaches reaches zero. zero. After Aftert8, tthe8, the currents currents of of the the three three phases phases remain remain zero. zero. FigureFigure 12b12b shows the opening voltage waveforms in Case 5. The The maximum maximum value value of of the the opening opening voltage is − ´18.2018.20 kV kV (2.23 (2.23 p.u.), p.u.), and and it it still still occurs occurs in in the the first first-pole-to-clear‐pole‐to‐clear (phase (phase B) B) in in this this case. case.

2.7 20 0.8 Phase A Phase A 0.0 Phase B Phase B

1.8 Phase C 10 Phase C

Current/kA -0.8 -0.047 -0.046 Time/s 0.9 0

Current I/kA 0.0 U/kV Voltage -10

-0.9 -20 -0.06 -0.05 -0.04 -0.03 -0.02 -0.06 -0.05 -0.04 -0.03 -0.02 Time/s Time/s 1.8 8

0.9 0

0.0

-8 Current I/kA Voltage U/kV Voltage -0.9 -16

-1.8 -24 -0.0428 -0.038 -0.044 -0.042 -0.040 -0.038 -0.036 Time/s Time/s

FigureFigure 12. WaveformsWaveforms of the opening current current and capacitor capacitor voltage voltage in in Case Case 5. 5. ( (aa)) Current Current waveform; waveform; andand ( (b)) voltage waveform.

4.6. Case 6 4.6. Case 6 Before the switching‐off operations in Case 6, #653 is closed, and #65302 and #65303 are opened. Before the switching-off operations in Case 6, #653 is closed, and #65302 and #65303 are opened. The circuit breaker of this switching operation is #65301. Figure 13a shows the opening current The circuit breaker of this switching operation is #65301. Figure 13a shows the opening current waveforms in Case 6 with phase selection. The strategy for opening shunt capacitor banks with phase waveforms in Case 6 with phase selection. The strategy for opening shunt capacitor banks with phase selection is also addressed in Section 2. The breaker contacts of phase B start to separate 2.67 ms selection is also addressed in Section2. The breaker contacts of phase B start to separate 2.67 ms (Δt3 = 2.67ms) later than its last current zero‐crossing point at t9, and phase B is the first‐pole‐to‐clear. (∆t3 = 2.67ms) later than its last current zero-crossing point at t9, and phase B is the first-pole-to-clear. The transient currents of phase A and phase C are always equal, but in the opposite direction, after The transient currents of phase A and phase C are always equal, but in the opposite direction, after the current of phase B decreases to zero at the moment of t10, which is also the time that the contacts the current of phase B decreases to zero at the moment of t10, which is also the time that the contacts of phase A and phase C start to separate. The breaking arc duration of phase B is Δt4 = 7.4 ms of phase A and phase C start to separate. The breaking arc duration of phase B is ∆t4 = 7.4 ms (Δt4 = t10 − t9). After a quarter of a cycle (5 ms), the currents of phase A and phase C both reach zero (∆t4 = t10 ´ t9). After a quarter of a cycle (5 ms), the currents of phase A and phase C both reach at the moment of t11. Therefore, the power frequency extinguishing arcing still occurs in this case. zero at the moment of t11. Therefore, the power frequency extinguishing arcing still occurs in this Statistical results according to our field tests indicate that the breaking arc duration of the case. Statistical results according to our field tests indicate that the breaking arc duration of the first‐pole‐to‐clear (phase B in this test) is about 7.5 ms, and the time error of the phase‐controlled first-pole-to-clear (phase B in this test) is about 7.5 ms, and the time error of the phase-controlled circuit circuit breaker for opening shunt capacitor banks is below ±0.3 ms. breaker for opening shunt capacitor banks is below ˘0.3 ms. Figure 13b illustrates the opening voltage waveform in Case 6. The maximum of the opening voltage in this case occurs in the first‐pole‐to‐clear (phase B), and its value is − 12.83 kV (1.57 p.u.).

Energies 2016, 9, 88 12 of 14

Figure 13b illustrates the opening voltage waveform in Case 6. The maximum of the opening Energies 2016, 9, 88 voltage in this case occurs in the first-pole-to-clear (phase B), and its value is ´12.83 kV (1.57 p.u.). The residualThe residual charge charge of capacitorsof capacitors remains remains more more thanthan 30 power power frequency frequency cycles cycles after after the thecapacitors capacitors are disconnected.are disconnected.

14 0.5 2.0 Phase A 0.0 Phase B

Phase C 7

Current/kA -0.5 1.0 0.0512 0.0514 Time/s Phase A Phase B 0

0.0 Phase C Current I/kA Voltage U/kV

-1.0 -7

-2.0 -14 0.04 0.05 0.06 0.07 0.08 0.04 0.05 0.06 0.07 0.08 Time/s Time/s 1.8 14

0.9 7

0.0 0

Current I/kA Voltage U/kV -0.9 -7

-1.8 -14 0.0586 0.0631 0.0632 0.0586 0.0588 0.0630 0.0632 0.0634 Time/s Time/s

Figure 13. Waveforms of the opening current and capacitor voltage in Case 6. (a) Current waveform; Figure 13. Waveforms of the opening current and capacitor voltage in Case 6. (a) Current waveform; and (b) voltage waveform. and (b) voltage waveform. 5. Conclusions 5. Conclusions Based on our previous work in this field [15,16], this study presents a complete comparison Basedbetween on ordinary our previous VCBs and work phase in‐ thiscontrolled field [VCBs15,16 ],in this the studyapplication presents of switching a complete 10 kV comparison shunt betweencapacitor ordinary banks VCBs through and field phase-controlled tests. The following VCBs conclusions in the are application drawn: of switching 10 kV shunt capacitor banksThe throughovercurrent field of tests. closing The 10 following kV shunt conclusionscapacitor banks are was drawn: about 2.12 p.u. with phase selection, and it was far below than those of ordinary VCBs (4.04 p.u. for Case 1 and 4.49 p.u. " The overcurrentfor Case 2). of Moreover, closing 10 high kV shunt‐frequency capacitor voltage banks oscillation was about did not 2.12 occur p.u. withfor switching phase selection, on and it wasshunt far capacitor below thanbanks those when of phase ordinary‐controlled VCBs VCBs (4.04 were p.u. used. for Case 1 and 4.49 p.u. for Case 2). Moreover, The high-frequencyovervoltage of opening voltage 10 oscillationkV shunt capacitor did not banks occur was for switchingabout 1.57 p.u. on shuntwith phase capacitor banks whenselection, phase-controlled and it was below VCBs those of were ordinary used. VCBs (2.26 p.u. for Case 4 and 2.23 p.u. for Case 5). " Theovervoltage The arc duration of opening of closing 10 kV shunt shunt capacitor capacitor banks banks without was phase about selection 1.57 p.u. was withabout phase 1.0–4.5 selection, ms. and it wasHowever, below for those the ofphase ordinary‐controlled VCBs VCBs, (2.26 the p.u. circuit for Case breaker 4 and of 2.23the first p.u.‐pole for‐ Caseto‐clear 5). was opened 2–3 ms later than its last current zero‐crossing point, which result in an average of " The arc duration of closing shunt capacitor banks without phase selection was about 1.0–4.5 ms. 7.5 ms arc duration of first‐pole‐to‐clear. However, for the phase-controlled VCBs, the circuit breaker of the first-pole-to-clear was opened  The time error of phase‐controlled VCBs for opening and closing shunt capacitor banks was 2–3 msbelow later than± 0.3 ms. its lastFurthermore, current zero-crossing the higher arc duration point, which increased result the in fracture an average gap distance of 7.5 of ms arc durationphase of first-pole-to-clear.‐controlled VCB contacts, and the probability of prestrike and re‐ignition was reduced. " The timeThis error can contribute of phase-controlled to keeping the VCBs power for system opening safe andand achieving closing shunt steady capacitor operation. banks was below ˘0.3 ms. Furthermore, the higher arc duration increased the fracture gap distance of Acknowledgments:phase-controlled This VCB work contacts, was supported and theby the probability National Natural of prestrike Science Foundation and re-ignition of China (51177182), was reduced. the National Natural Science Foundation of China (51477018), the National Natural Science Foundation of China This can contribute to keeping the power system safe and achieving steady operation. (51507019), and the Funds for Innovative Research Groups of China (51321063).

Author Contributions: Wenxia Sima contributed to the research idea and theoretical analysis of switching shunt Acknowledgments: This work was supported by the National Natural Science Foundation of China (51177182), capacitor banks. Mi Zou designed the experiments, and drafted the manuscript. Qing Yang, Ming Yang and the National Natural Science Foundation of China (51477018), the National Natural Science Foundation of China (51507019), and the Funds for Innovative Research Groups of China (51321063). Energies 2016, 9, 88 13 of 14

Author Contributions: Wenxia Sima contributed to the research idea and theoretical analysis of switching shunt capacitor banks. Mi Zou designed the experiments, and drafted the manuscript. Qing Yang, Ming Yang and Licheng Li worked on the data analysis and revision of the manuscript. All authors have read and approved the final manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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