Circuit Breaker Transient Recovery Voltage in Presence of Source Side Shunt Capacitor Bank

Circuit Breaker Transient Recovery Voltage in Presence of Source Side Shunt Capacitor Bank Murali Kandakatla, B Kondala Rao, Member, IEEE, and Gopal Gajjar, Member, IEEE

Abstract— Reactive power compensation through connection of shunt capacitor banks is a common practice in medium voltage systems. But the presence of capacitor bank leads to some interesting phenomena in switching transients. Clearing of a fault on line side of breaker in the presence of a source side capacitor bank is one such phenomenon. While clearing the fault if there is any re-ignition, presence of capacitor bank will develop high frequency discharge current that is superimposed on the power frequency fault current. Interruption of this high frequency current would cause considerable increase in the TRV magnitude. This paper describes the above phenomenon with PSCAD simulation results of a practical system. It also explains the effect of various parameters on the breaker TRV magnitude. Index Terms—Arcing time, High frequency discharge current, Initial Transient Recovery Voltage (ITRV), Rate of Rise of Recovery Voltage (RRRV), Re-ignition, Restrike, Shunt capacitor bank, Transient Recovery Voltage (TRV).

I. INTRODUCTION

A

SIMPLIFIED single line diagram of a system is shown in Fig. 1. It shows a 33 kV medium voltage system supplied by a 110 kV line through a three winding transformer. A 2.5 MVAR capacitor bank is connected to the 10 kV tertiary winding bus. There are four outgoing overhead radial feeders that supply utility loads. During one incident of LL fault near breaker terminal of feeder 3, the breaker CB3 and Bus breaker failed to clear the fault. The fault was ultimately cleared by the 110 kV side breaker. On further investigation it was found that failure mode was identical in both CB3 and Bus Breaker. The current had continued through the two poles of breakers connected to the faulty phases, while there was successful interruption in the poles connected to healthy phase. From the event recorders it was established that the breakers had operated in sequential manner, initially CB3 operated and when the fault current continued the Bus Breaker operated after a delay of 300 ms. Unfortunately no disturbance recorders were installed in the system. In past these breakers had cleared several LG faults Murali Kandakatla is with ABB Ltd, Vadodara, India (e-mail: [email protected]). B Kondala Rao is with ABB Ltd, Vadodara, India (e-mail: [email protected]). Gopal Gajjar is with ABB Ltd, Vadodara, India (e-mail: [email protected]).

978-1-4244-1762-9/08/$25.00 ©2008 IEEE

successfully. This interesting incident was studied in detail. Many possibilities to explain this kind of failures were evaluated. The possibilities of phenomena like Short Line Fault, fault current exceeding the rated breaking capacity were eliminated through detail calculation and simulation. Then the focus shifted to the role played by the shunt capacitor bank in the event. 110 kV HV Supply 110 kV B reaker Cap. Bank 10 kV bus 33 kV bus CB 1

CB 2

Bus B reaker CB 3

C B4

Fault

Fig. 1. Single line diagram of practical system.

The rest of the paper analyses the possibility of abnormal TRV on breakers due to presence of the shunt capacitor bank on source side. II. EFFECT OF SHUNT CAPACITOR BANK Clearing of faults near shunt capacitor banks is a concern for the breaker as explained in [1]. If a re-ignition occurs in the breaker, presence of capacitor bank would cause high frequency discharge current superimposed on the fault current. Clearing of this high frequency current at next current zero causes increase in TRV magnitude. A. Theory Behind the Phenomenon A system can be represented as shown in Fig. 2 to explain the theoretical aspects of the phenomenon. In this system the capacitor bank is connected to the feeder bus. Whenever a short circuit occurs near the capacitor bank, the energized bank discharges to the fault on nearby feeder. The current at

the instant of fault is combination of source and capacitive discharge current. HV Supply

CB Fault Cap. Bank Fig. 2. Simplified single line diagram.

The high frequency capacitive discharge current dies down within 2 to 3 power frequency cycles due to losses in the network. Normally in medium voltage system the breaker will operate in about 5 cycles (i.e., 100 ms) from the fault inception, so the fault current interrupted by the breaker is a power frequency current. When the circuit breaker clears fault current, immediately after the arc extinction, the power network reacts with a TRV that stresses the gap. The presence of a large source side capacitor bank influences the TRV across the breaker contacts. In this case RRRV is reduced, because the source side capacitor bank provides the time delay prior to the initial rate of rise of the source side TRV. Hence the breaker will be able to interrupt the arc at first current zero after contacts separation. If the arcing time is less than minimum arcing time of breaker, the small arcing contact gap is unable to withstand the developed TRV, causing re-ignition in the breaker. During the TRV build up period the shunt capacitor bank gets charged to the source side TRV magnitude. When the reignition occurs, the shunt capacitor bank discharges into the fault through the circuit breaker. High frequency discharge current of the capacitor bank with very high amplitude will flow through the breaker. The amplitude, frequency and damping factor of these high frequency currents depends on the instantaneous voltage of the capacitor bank, the inductance between the capacitor bank and the fault location and dynamic resistance between the capacitor bank and the fault location. This high frequency discharge current is superimposed on the power frequency fault current and summation of these currents flow through the circuit breaker arcing gap. Because of this high frequency current, there may be additional current zeros other than the power frequency current zero. The breaker tries to interrupt the arc at current zeros of this high frequency current. This interruption will be successful if the di/dt of this high frequency current at current zero is less than the rated quenching capability of the breaker [2]. In most cases the di/dt

condition satisfies and interruption will be successful at first current zero of high frequency current. But this may lead to higher overvoltages and may even result in voltage escalation. When the breaker interrupts at first current zero of this high frequency current, the current flowing through the source inductance is not zero and the magnetic energy is stored in this inductance. After interruption, the current through the inductance charges the already charged capacitor bank to higher voltage which will appear as TRV across the breaker contacts [1]. If the recovery voltage exceeds the breaker dielectric withstand capability, restrike occurs due to the dielectric break down of the contact gap. The current through the breaker is established again. The breaker may interrupt this current at next current zero and also withstand the resulting TRV, leading to successful interruption. Other possibility is that breaker may not interrupt at next current zero due to prolongation of arcing time or if it interrupts it may not withstand the resulting high TRV leading to failure of interruption. As the breaker is not designed for this type of phenomena complete failure of the breaker may occur. B. Practical Aspects Common system configuration in medium voltage substation is to have reactive power compensation through shunt capacitor bank connected to the same bus as outgoing feeders. Similar failure of breakers is not very wide spread problem in practical system. The phenomenon described in section-A. may lead to two severe problems. One is the TRV magnitude may exceed the dielectric strength of breaker causing restrikes and can lead to failure of the breaker. The second one is di/dt of high frequency current at current zero may reach greater than the current quenching capability of breaker, causing prolongation of arcing time and breaker fails to interrupt the fault current. This phenomenon is not common and it is very much dependent on system parameters like source side inductance and capacitance value. The magnitude of TRV after re-ignition depends on re-ignition voltage and capacitive current magnitude at the high frequency current zero. The above two parameters are not controllable as re-ignition can occur at any instant on the TRV waveform. The standards body recognizes this aspect. The problem of high TRV is mentioned but at the same time no mandatory test is recommended to prove the breaker performance under such conditions. For number of reasons the breakers have been successful in operating under such conditions [3]. The major reason in this case is that practically ITRV is not negligible and the circuit breaker will not interrupt fault current at very short arcing time and the sequence described above will not occur. But in the worst-case, this phenomenon may lead to the failure of the breakers. III. CASE SIMULATION The incident introduced in Section-I is examined in detail to check whether there is any role of the shunt capacitor bank

in breaker failure. A. System Description The practical system is fed by 110 kV, 50 Hz source through a 3-ph 3 winding transformer. The transformer rating is 110/35/10kV, 31.5 MVA, YNyn0d11. The leakage impedance of transformer, HV-MV is 10.7%, HV-LV is 18.2% and MV-LV is 6.4%, on 31.5 MVA base. Tertiary winding is connected to a 2.5 MVAR ungrounded star connected capacitor bank consisting of reactor and capacitor in series. The value of capacitance and series reactor in each phase is 82.3 µF and 7.7 µH respectively. The neutrals at 110 kV and 35 kV are solidly grounded. The 35 kV winding is connected to a 33 kV bus bar to which four feeders are connected. B. PSCAD Model The breaker TRV is mostly a localized phenomenon, so proper representation of stray capacitances and inductances of station equipment is very important in the simulation modeling. The complete system PSCAD model is shown in Fig. 3. RRL IS

50 [kohm]

0.0024 [ohm]

0.0077[mH]

#1

0.000855 [uF]

82.3 [uF]

0.001236 [uF]

0.000444 [uF]

50.0 [kohm]

#2

0.0024 [ohm]

0.0077[mH]

20[kohm]

0.002196 [uF]

C. Checking the Possibility of Re-ignitions and Restrikes The fault current waveform through the breaker for a LLL fault on the feeder 3 is shown in Fig. 4. The fault instance is at 125 ms, as shown by mark 1. The magnitude of symmetrical fault current is around 6.5 kA peak. From Fig. 4, it is observed that at fault instance the high frequency capacitive current is flowing through the breaker, but it dies down within 4 cycles. The first arc interruption occurs at 233 ms, 5 cycles after fault inception as shown by mark 2, at this instance there is no high frequency current present.

ME2 BBRK ME1

CBRK

82.3 [uF]

#3

Vm

0.0024 [ohm]

0.001122 [uF]

0.0077[mH]

IC

82.3 [uF]

Three phase 3 winding transformer is modeled using classical model and this model is extended to high frequencies by adding winding capacitances externally, the resistive damping is considered to approximate the practical situation. 33 kV feeder lines are modeled using frequency dependent distributed parameter model, the most accurate model for transient calculations. All 33 kV feeder bus bars are represented by lumped parameter model using coupled-π sections. CTs and PTs are modeled as capacitance to ground. Load is modeled as a series and parallel resistor and inductor branches [4]. Capacitor bank was modeled as a star ungrounded bank with capacitance of 82.3 µF and series inductance of 7.7 µH. The bus breaker and all the 33 kV feeder breakers were rated at 36 kV and their rated breaking current is 31.5 kA. Normal PSCAD breaker is not capable of self re-ignitions and restrikes. A breaker model capable of self re-ignition and restrikes is developed and used. The modeling details and working is explained in the [5].

MEB

8

IM

4

PI SECTION

IP2

COUPLED

IP1

PI SECTION

Current (kA)

270 [pF]

COUPLED

PI SECTION

COUPLED

PI SECTION

COUPLED

COUPLED

COUPLED

PI SECTION

PI SECTION PI SECTION

COUPLED

PI SECTION

COUPLED

COUPLED

PI SECTION

PI SECTION

COUPLED

PI SECTION

COUPLED

B315

B310 BRK

B312

E1

B311

160.0 [pF]

E2 #1 #2

T TLine2

T TLine5

IF

T TLine3

Timed Fault Logic

1.0 [uF]

200 [ohm]

500 [ohm]

3.0 [H]

4 [ohm]

200 [ohm]

200 [ohm]

Fig. 3. System PSCAD model.

1.27 [H]

4 [ohm] 0.127 [H]

4 [ohm] 0.127 [H]

4 [ohm] 0.127 [H]

ABC

1.27 [H]

140

Mark2 160

180

200 Time (ms)

220

240

260

Fig. 4. Fault current waveform in one of the phase with high frequency component.

500 [pF]

160.0 [pF]

1.27 [H]

Mark1

EB

160.0 [pF]

T TLine1

-4

-8 120

A V

160.0 [pF]

BN VBUS

0

A re-ignition occurs after opening of breaker and the fault current re-establishes. Now this current has large high frequency component, as can be seen in portion after mark 2 of Fig. 4. When the breaker clears fault current, the capacitor bank influences TRV appearing across the breaker. Due to capacitor bank the RRRV is low, but amplitude of TRV is higher than without capacitor bank. This low RRRV can force the breaker to interrupt the arc at first current zero of fault current after opening. This early interruption can cause a re-ignition as dielectric withstand capability is less due to small contact gap. The rated TRV magnitude for 36 kV systems is considered

as 66 kV, with first pole to clear factor as 1.5 and amplitude factor as 1.5. The dielectric strength of this breaker is considered to be increasing with a constant rate of 25 V/µs. If the TRV developed is higher than the dielectric strength of breaker, arc may re-establish and current continues to flow. If this happens within one-quarter cycle after current zero it is considered as re-ignition. After re-ignition the total fault current has components of fault contribution from source and capacitive discharge current as shown in Fig. 4. The fault current after re-ignition has high frequency oscillations, the frequency of these oscillations is around 650 Hz, and peak magnitude of fault current is around 7 kA. At current zero the di/dt of rated breaking current (31.5 kA) is 13.9 A/µsec, for high frequency current it is 5.34 A/µsec. After re-ignition breaker interrupts the arc at first current zero of this high frequency fault current as it has less di/dt than rated. As discussed in the above sections interruption of this high frequency current can lead to higher over voltages that can cause restrike due to dielectric breakdown of contact gap. Following sections will discuss effect of high frequency interruption on breaker TRV. IV. BREAKER TRV UNDER DIFFERENT FAULT CONDITIONS A. LG Fault An LG fault is created at the breaker terminals of feeder 3. Effect of re-ignition on the breaker TRV is studied considering two cases one is with capacitor bank and another one is without capacitor bank. The results of simulation for the LG fault are shown in Table I. The results show that the effect of re-ignition on the TRV rise, without capacitor bank is negligible. In the presence of capacitor bank if the re-ignition occurs in the faulted phase the high frequency capacitive current is established as shown in Fig. 4. The % Rise is calculated based on TRV value with no re-ignition. TABLE I BREAKER TRV UNDER LG FAULT Cap Bank No Yes

RRRV (kV/µs) 0.316 0.036

TRV without Re-ignition (kV) 37.94 39.78

TRV with Re-ignition (kV) 39.40 51.32

% Rise 3.84 29.00

B. LL Fault An LL terminal fault is created between R & Y phases of feeder 3. Effect of re-ignition on the breaker TRV with and without capacitor bank is shown in Table II. TABLE II BREAKER TRV UNDER LL FAULT Cap Bank No Yes

RRRV (kV/µs) 0.309 0.034

TRV without Re-ignition (kV) 38.00 43.00

TRV with Re-ignition (kV) 39.30 59.00

% Rise 3.42 37.20

The effect of re-ignition on the TRV rise is more in the presence of capacitor bank. If the re-ignition occurs in Rphase, it will force the re-ignition in Y-phase also. The high

frequency capacitive current is established in both R & Y phases. C. LLL Fault An LLL fault is considered on the line side breaker terminals of the feeder 3 and the effect of capacitor bank on the breaker TRV is studied. The TRV developed in the successfully cleared first pole with and without capacitor bank is shown in Table III. If the re-ignition occurs in the first cleared pole, the high frequency current established in that phase gets divided into two equal parts and flow through the remaining two phases. TABLE III BREAKER TRV OF FIRST CLEARED POLE UNDER LLL FAULT Cap Bank No Yes

RRRV (kV/µs) 0.145 0.05

TRV without Re-ignition (kV) 56.80 62.00

TRV with Re-ignition (kV) 58.00 70.92

% Rise 2.11 14.38

D. LLLG Fault An LLLG fault is considered on the line side breaker terminals of the feeder 3. In case of LLLG fault the first pole to clear factor is 1.0. Moreover, if there is re-ignition during clearance of first pole the high frequency current in that phase does not get distributed into other two phases. No effect of high frequency current interruption is observed in case of LLLG fault. After re-ignition in the first cleared pole the TRV magnitude is severe in the last cleared pole. Hence the TRV developed in the last cleared pole is shown in Table IV. TABLE IV BREAKER TRV OF LAST CLEARED POLE UNDER LLLG FAULT Cap Bank No Yes

RRRV (kV/µs) 0.092 0.038

TRV without Re-ignition (kV) 39.57 40.49

TRV with Re-ignition (kV) 39.23 38.76

% Rise -0.86 -4.27

E. Summary From the above analysis the effect of capacitor bank on the breaker TRV is present in the case of LG, LL and LLL fault. The % Rise in TRV is high (37.20) for the LL fault case. Compared to % Rise in LG fault (29.00), the rise in the case of LLL fault (14.38) is low, but the TRV magnitude in the case of LLL fault is higher because of first pole to clear factor. The effect of capacitor bank on breaker TRV is severe in the case of LL and LLL fault. Further analysis is done in LL and LLL fault cases in the next sections. V. FURTHER ANALYSIS IN THE CASE OF LL AND LLL FAULTS A. Analysis of breaker TRV under LL Fault An LL terminal fault is created between R & Y phases of feeder 3 and the breaker connected to feeder 3 clears the fault. The normal TRV developed with capacitor bank is 43 kV as shown in Fig. 5. This TRV is under normal condition without any re-ignition. 43 kV TRV magnitude will be considered as a base value of TRV for LL faults to calculate relative increase in TRV due to interruption of high frequency current after initial re-ignition.

40

Voltage (kV)

20

0

-20

-40

-60 252

254 C

256

Time (ms)

258

260

®

262

O

O

®

Fig. 5. TRV developed in the 3-phases of breaker with capacitor bank.

If re-ignition occurs because of low arcing time, the TRV waveform in all the 3-phases is as shown in Fig. 6. Here the peak TRV value is 59 kV, which shows that due to re-ignition increase in TRV is 16 kV. 60

40

Voltage (kV)

20

0

-20

-40

-60 252

254

256

258

Time (ms)

260

262

264

266

Fig. 6. TRV developed in the 3-phases of breaker with re-ignition and with capacitor bank.

The TRV magnitude after re-ignition depends on two parameters one is re-ignition voltage and second one is capacitive current magnitude at the instant of high frequency current zero after re-ignition. The effect of above parameters on TRV after re-ignition is shown in the Table V. TABLE V EFFECT OF RE-IGNITION VOLTAGE AND CAPACITIVE DISCHARGE CURRENT ON THE FEEDER BREAKER TRV Series reactor value of 7.7 µH Case No RV (kV) ICapPeak (kA) ICapInst (kA) TRV (kV) 1 40.63 4.76 2.23 54.13 2 34.20 3.67 3.10 59.45 Series reactor value of 2.7 µH 3 35.65 4.05 3.32 61.17 4 30.60 3.22 2.00 52.50 5 25.30 2.05 0.56 41.00 Where, RV IcapPeak IcapInst

Higher TRV due to higher instantaneous capacitive current is as per expectation. At high frequency current zero the value of inductive and capacitive current through the breaker is equal. Higher value of capacitive current implies equally high value of the power frequency current flowing through the source side inductive circuit. Sudden interruption of the breaker current diverts this inductive current in the capacitor bank, giving rise to higher TRV. The initial peak value of the capacitive current depends on the value of re-ignition voltage. But the instantaneous value of capacitive current at the high frequency current zero depends on its frequency and phase relationship with the power frequency fault current, the rate of damping of the capacitive current and also the magnitude of the fault current. Test was done for the same LL fault with the transformer leakage impedance of MV-LV changed from 6.4% to 5.4% and resulting TRV has shown in Table VI. In this case the initial magnitude of the high frequency capacitive current magnitude is higher. The fault current magnitude remains almost same as it is governed by leakage impedance between HV-MV winding of transformer. The TRV generated after interruption at high frequency current zero is greater than 66 kV. It is possible that the breaker may fail to clear the fault because of restrikes.

% Rise 25.88 38.25 42.25 22.09 -4.60

: Re-ignition Voltage : Initial peak value of the capacitive current after re-ignition measured at 10 kV bus. : Instantaneous value of capacitive current at high frequency current zero.

It can be seen that TRV generated after interruption at high frequency current zero has a wide range of variations. But, it can be deduced that the peak value of TRV depends on the instantaneous value of capacitive current at the high frequency current zero. The initial peak value of capacitive current after re-ignition is directly proportional to the re-ignition voltage.

TABLE VI EFFECT OF TRANSFORMER LEAKAGE IMPEDANCE OF MV-LV ON THE FEEDER BREAKER TRV Case MV-LV If peak No %imp (kA) 1 6.40 5.60 2 5.40 5.60

ICapPeak (kA) 3.67 5.27

Freq (Hz) 685 751

ICapInst (kA) 3.10 4.80

TRV (kV) 59.45 72.64

% Rise 38.25 68.90

The variation of transformer leakage impedance between HV and MV is considered and the effect on the breaker TRV is shown in Table VII. The results show that variation in HVMV impedance is not much influencing breaker TRV. TABLE VII EFFECT OF TRANSFORMER LEAKAGE IMPEDANCE OF HV-MV ON THE FEEDER BREAKER TRV Case HV-MV If peak ICapPeak No %Imp (kA) (kA) 1 18.20 5.60 3.67 2 19.00 5.53 4.17 3 17.00 5.63 3.43

Freq (Hz) 685 689 685

ICapInst (kA) 3.10 3.28 2.66

TRV (kV) 59.45 60.17 56.74

% Rise 38.25 39.93 31.95

The fault resistance also one of the deciding factor in this phenomenon. The effect of fault resistance on the breaker TRV is shown in Table VIII. TABLE VIII EFFECT OF FAULT RESISTANCE ON THE FEEDER BREAKER TRV Case No 1 2 3

Rf (ohm) 0.005 0.50 5.00

If peak (kA) 5.60 5.39 4.60

ICapPeak (kA) 3.65 3.09 2.43

ICapInst (kA) 3.15 2.81 0.55

TRV (kV) 59.52 52.76 47.54

% Rise 38.42 22.70 10.56

As the fault resistance increases the severity of the TRV magnitude after re-ignition decreases. The capacitive current after re-ignition, which is the major cause for the high TRV, is quickly damped with the increase in fault resistance. B. Analysis of Breaker TRV in LLL Fault The case of LLL fault is complicated to analyze through the simulation circuit used in this paper. An LLL fault is considered on the line side breaker terminals of the feeder 3 and the effect of capacitor bank on the breaker TRV has been studied. The normal TRV is calculated without any re-ignition. The peak TRV value is 56.8 kV due to the first pole to clear factor of 1.5. This is taken as the base value of TRV for LLL fault case for calculation of rise in TRV due to effect of high frequency capacitive current. If re-ignition occurs on the phase cleared first then the fault current along with the addition of the high frequency capacitive current continues to flow through that phase. As the fault is LLL, the high frequency current flowing through the re-ignited phase gets coupled to other phase. The high frequency current gets divided into two equal parts and flow through the remaining two phases. This causes high frequency current zero in the next phase reaching to power frequency current zero. When the other phase interrupts at its current zero the TRV developed in it will have component of first pole to clear factor and the rise in TRV due to interruption at high frequency current zero. If re-ignition occurs in the first cleared phase the TRV developed in all the three phases is as shown in Fig. 7. The TRV developed after re-ignition in one phase is reaching to 70.92 kV. 50

small value of fault resistance, this smaller damping time does not offset the effect of distribution of high frequency current between two phases. For LLL fault even though the contribution from high frequency current interruption is lower, higher magnitude of TRV is observed because of the first pole to clear factor. TABLE IX EFFECT OF CAPACITIVE DISCHARGE CURRENT ON THE BREAKER TRV Case No RV (kV) ICapPeak (kA) ICapInst (kA) 1 59.60 4.00 3.16 2 52.80 3.49 2.75 3 49.40 3.42 2.68 4 41.50 2.68 2.08

Voltage (kV)

% Rise 14.38 10.80 9.67 5.64

VI. CONCLUSIONS The effect of source side shunt capacitor bank on the breaker TRV of a medium voltage substation is studied through PSCAD simulation. It can be concluded that in case of LG, LL and LLL faults there is increase of TRV observed by the breaker during fault clearance. The rise in TRV is higher in LL fault than other type of faults. The TRV magnitude is higher in case of LLL fault because of first pole to clear factor. Hence the possibility of restrike is more in LLL fault. This rise in TRV magnitude depends on various parameters, which are uncontrollable like re-ignition voltage, capacitive current magnitude at high frequency current zero and fault resistance. It exhibits a wide variation due to reasonable small changes in these parameters. The probability of breaker failure due to such phenomena is quite remote and practically it may be possible to continue to operate the system without any special care. But it is suggested not to overlook this phenomenon, in the worst case it can lead to failure of the breaker.

25

VII. REFERENCES

0

[1]

-25

[2]

-50

[3]

-75 460

TRV (kV) 70.92 68.70 68.00 65.50

462

464

466

Time (ms)

468

470

472

474

Fig. 7. TRV developed in the 3-phases of breaker with re-ignition in the first cleared phase.

It can be noted that although the peak value of TRV is generally higher, the percentage rise due to high frequency current interruption is less than that in case of LL fault. This can be explained in terms that for same level of re-ignition voltage the high frequency current interruption is half the magnitude because of the current distribution. The effect of capacitive discharge current on the breaker TRV after reignition has shown in Table IX. The effect of damping in the capacitive current is less in case of LLL fault as the next interruption is after 1/6th of power cycle instead of half cycle as in case of LL fault. For

[4] [5]

Lou Vander Sluis and A L J Janssen, “Clearing Faults near shunt capacitor banks”, IEEE Transactions on Power Delivery, July 1990 “Current Interruption in HV Networks”, edited by Klaus Ragaller, Plenum press, NewYork, 1978 Guide For Application Of IEC 62271-100 And IEC 62271-1 Part 2 Making And Breaking Tests, Cigre 305, 2006 Tutorial on “ Modeling and Analysis of System Transients using Digital Programs” by IEEE working group 15.08.09, IEEE, Power Engineering Society. B. Kondala Rao and Gopal Gajjar, “ Development and Application of Vacuum Circuit Breaker model in Electro Magnetic Transient Simulation“ Power India Conference, IEEE April 2006.

VIII. BIOGRAPHIES Murali Kandakatla received M.Tech in Power System Engineering from NIT Warangal, India in 2007. His interest includes power system transients and power system protection. B Kondala Rao received M.Tech in Power System Engineering from NIT Warangal, India in 2005. He is actively involved in the area of power system transients and power system protection. Gopal R Gajjar received M.Tech in Power Electronics and Power System from IIT Bombay, Mumbai, in 2001. His research interest includes power system transients, power system protection and electromagnetic simulation.