XXVth Int. Symp. on Discharges and Electrical Insulation in Vacuum - Tomsk – 2012

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Concept of Series Connected Vacuum Interrupters

Sandeep Kulkarni1, Hemachander M1, Arun Kumar S1, Lalichan Andrews1, Viren Acharya1,

Maheswaran C1, Srinivas Rayudu2

1Global R&D Center, Crompton Greaves Limited, Mumbai 2Crompton Greaves limited, Vacuum Interrupters and Instrment Transformer Division, Aurangabad

Abstract- Vacuum Interrupters could be connected in series to increase the voltage withstand capacity. This series connected vacuum interrupters would especially be useful for application of vacuum circuit breakers at transmission voltages (145 kV, 245 kV). An important aspect of series connected vacuum interrupters is sharing of voltage across the two gaps. This paper presents results of power frequency and impulse experiments on series connected vacuum interrupters. Keywords- grading capacitor- voltage distribution- stray capacitances- high voltage-uniform gap-non uniform gap.

I. INTRODUCTION For High Voltage circuit breakers, SF6 is widely used

as an insulating and arc quenching medium. Vacuum is predominant as arc quenching medium for switchgear rated up to 40.5 kV. SF6 has been identified as gas with highest green- house warming potential and hence efforts are being made to find an alternative to high voltage SF6 breakers. Vacuum is being widely explored as this alternative. Single vacuum interrupters could be used up to 72.5 kV. For breakers rated 145 kV and above, multiple vacuum interrupters in series would be an economically more viable option. This is because the contact gap of a single interrupter would more than the sum of the contact gaps of two interrupters connected in series for the same voltage. This is explained in detail in the next section. Series connection of interrupter brings along with it the problems like possible unequal voltage sharing during the tests and operation. This paper presents the results of investigation of series connected interrupters for power frequency voltage and impulse voltage. The usefulness of grading capacitors for equalising the voltage sharing is also presented.

II. 1. SINGLE AND MULTIPLE VACUUM GAPS

The dependency of breakdown voltage (BDV) on electrode gap for various insulating materials is shown in figure 1. It can be seen that for SF6 gas and air the relationship between the BDV and electrode gap is more or less linear. The BDV of a vacuum gap is more than for an SF6 gap at 5 bar pressure up to an electrode gap of about 5 mm. At larger gaps the curve for vacuum is asymptotic and hence at higher voltages a higher gap is

required in vacuum as compared to that in SF6. Hence for higher voltages, a much larger gap would be required for a vacuum interrupter. A larger gap would result in higher stress on the bellow, higher energy mechanism and a much bigger interrupter.

Figure 1. Breakdown voltage of different insulation media as a

function of contact separation [1]

Another effect of the larger gap is the reduction of the axial magnetic field. Hence a single interrupter would be feasible economically up to possible 72.5 kV voltage class. Beyond 72.5 kV, two interrupters connected in series could possibly be a more practical alternative.

By using a series arrangement of two or more vacuum interrupters it is basically possible to double or multiply the dielectric strength. In the arrangement of two series connected interrupters the total contact gap is divided into two smaller gaps. The relationship between breakdown voltage and contact gap is as follows [2], [3]

Ub = k (d) α (1)

where, Ub is the breakdown voltage, k a constant, d is the gap distance between the electrodes, α is a scaling factor varying from 0.4 -0.7.

If the single gap d is divided into two equal gaps d/2 and the two gaps are connected in series, the equation changes to:

978-1-4673-1266-0/12/$31.00 ©2012 IEEE

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Ubseries = k (d/2) α + k (d/2) α (2)

Ubseries = 2k (d/2) α (3)

K = Ubseries / Ub = 2(1-α) (4)

Thus the breakdown voltage for two vacuum interrupters in series each having a gap (d/2) is 2(1-α) times of a single interrupter with a gap d. For α = 0.7, K = 1.23. Thus the series connected system would have a breakdown voltage which is 1.23 times higher than a single interrupter. The above hypothesis stands true under ideal conditions in which the two interrupters are identical, the gaps are exactly equal and there is no effect of stray capacitance. In practical conditions the capacitance of the two interrupters may not be exactly equal. It is possible that the contact gaps are not equal. Also stray capacitance would always be present. These three factors would result in unequal voltage distribution across the interrupters resulting in lowering the breakdown voltage for the series connected system. This paper investigates the effect of unequal gaps and the effect of stray capacitance on the voltage distribution across the series connected interrupters. The study has been performed for power frequency voltage and impulse voltage.

II. 2. EXPERIMENTAL SETUP

The experimental set-up is shown in figure 2. The fixed ends of the two vacuum interrupters are connected together. An arrangement is made to create the desired contact gap in both the vacuum interrupters. The created gap is measured with the help of a pointer and scale arrangement. The pointer is attached to the moving electrode.

Figure 2. Experimental setup for measuring voltage distribution

Each interrupter is rated at 12 kV. High voltage lead is connected to the moving electrode of one VI; the other moving electrode is grounded. The Vacuum Interrupter to which HV is connected is designated at VI1. The other vacuum interrupter is designated at VI2.

III. EXPERIMENTAL RESULTS

3.1 Power Frequency Test

3.1.1 Voltage distribution at low equal gaps

A very small gap of 1mm was created in both the interrupters. Voltage was applied across the series connection and voltage across both the interrupters was

measured. The variation is depicted in figure 3. As seen from the graph, the voltage sharing across the two VI is unequal though the gaps are same. VI1, which is the line side VI, shares a greater voltage. This unequal sharing is the result of the stray capacitance.

Voltage distribution 1mm- 1mm gap

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Figure 3. Voltage distribution across VI1, VI2 at a gap distance of 1mm-1mm

3.1.2 Voltage distribution at low un- equal gaps The gap distance is adjusted as VI1: VI2 = 2mm-0.5mm as shown in figure 4. Thus capacitance of VI1 is lower than VI2 by about 4 times. In other words the capacitive reactance of VI 1 is higher than VI2 by 4 times. Hence VI1 should share 4 times the voltage as that of VI 2. However in practice it is observed that the ratio of voltage across VI1 to VI2 varies between 10 and 20. The higher voltage drop resulting from un-equal voltage sharing causes a flashover across VI1. At the instant of the flashover, the entire applied voltage appears across VI2 resulting an internal breakdown. This example illustrates the effect of unequal gaps which can occur if the vacuum interrupters do not open simultaneously. In such cases the TRV can get un-equally distributed resulting in breakdown of the series connected VI system.

Voltage distribution 2mm - 0.5mm gap

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Figure 4. Voltage distribution across VI1, VI2 at a gap distance of 2-0.5mm

The other case is also investigated where-in the VI1 (1.5 mm) has a lower gap than VI 2 (2mm). Figure 5 depicts the voltage sharing for such a case.

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In this case the capacitance of VI1 is higher and hence it should share lesser voltage. However as seen from the graph VI1 is sharing the higher voltage as compared to VI1. This could be the effect of stray capacitance. The voltage across VI1 linearly rises up to 35 kV and then starts decreasing. At the same voltage, the voltage across VI2 starts suddenly increasing.

Voltage distribution 1.5mm-2mm gap

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Figure 5. Voltage distributions across VI1, VI2 at a gap distance of 1.5-2mm

This could be the effect of the pre-breakdown phenomena which would have been initiated in VI1 owing to the lower gap and higher voltage drop. As the pre-breakdown currents starts to flow, the potential of the fixed electrode of VI1 (and of VI2) starts increasing resulting in the rise of potential across VI2.

3.2 Lightning Impulse voltage test

Lightning impulse voltage test were carried out using Recurrent surge Generator (RSG) with an amplitude of 50V and wave shape of 1.2/50μs [4]. Again, as in the previous case, HV is connected to moving end of VI1 and moving end of VI2 is grounded. Voltage across each interrupter is measured.

3.2.1 Voltage Distribution across Series Connected Interrupter Without Grading Capacitor.

In this study the gap in VI2 is kept constant. Gap in VI1 is varied from 1 mm to 8 mm to understand the impulse voltage distribution across the VI’s at different unequal gaps. Figure 7 shows the variation of voltage across VI2 for a gap of 1 mm and 2 mm in VI1. The voltage across VI2 decreases as the gap in VI1 increases from 1 mm to 8 mm. At a gap of 2 mm in VI1, with 2 mm gap in VI 2, the voltage across VI2 is about 7 volt. The total applied voltage is 50V. Thus only 14% voltage appears across VI2. This is the result of the stray capacitance.

Voltage distribution across VI2 ( VI1 gap- 1mm-8mm, VI2 - 1mm&2mm gap)

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Figure 6. Voltage distribution across VI2 for various gap distance of VI1

3.2.2 Voltage Distribution across Series Connected Interrupter With Grading Capacitor.

The results explained above indicate that the voltage is not shared equally possibly because of stray capacitance. One method to negate the effect of stray capacitance is to connect grading capacitors. The following section studies the affect of grading capacitors [5], [6], [7] on voltage distribution. The grading capacitors increase the overall capacitance of the vacuum interrupters. As the grading capacitors are connected in parallel, the voltage distribution is now governed by the capacitance of the grading capacitor rather than the capacitance of the vacuum interrupter. Thus even if the capacitance of the vacuum interrupters is not equal owing to different gaps or any other reason, the voltage is more or les equally distributed across the two capacitors.

Grading capacitor C1 is connected across VI1 and grading capacitor C2 is connected across VI2. A low voltage impulse of 50 V is applied across the series connection. Voltage across VI2 is measured. Three values of C2: 20 pF, 100 pF and 500 pF are selected. For each of these values of C2, C1 is varied between 0 and 1000 pF to find the correct combination of C1 and C2 for equal voltage distribution. The variation is shown in figure 7.

Figure 7. Voltage distribution across a VI2 with increasing C1

For C1 = C2 ~ 100 pF the voltage drop across VI2 is around 25 V indicating that the voltage is approximately equally distributed across the two VI’s.

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IV. CONCLUSIONS Series connection of vacuum interrupters is beneficial

for higher voltages like 72.5 kV and above. The solution results in lower individual gaps and hence lower travel for the mechanism. Also the series connection is advantageous from the point of view of axial magnetic field. However, the voltage distribution across the series connected interrupters gets affected by stray capacitance. The experiments have shown that even for uniform gaps the voltage is not equally distributed. The condition is even worse for non uniform gaps which could be a result of non-simultaneous opening. Both power frequency and impulse voltage distribution is affected because of stray capacitance. Because of the non-uniform voltage distribution, higher voltage would be dropped across one of the interrupters resulting in either an internal breakdown or flashover of the interrupter which would further lead to breakdown of the other interrupter. The problem could be mitigated by designing interrupter with higher withstand capacity. Other method of mitigation of the problem is to connect grading capacitors across the interrupters. As has been shown in one of the experiments, correct value of grading capacitors result in equal sharing of voltage. However, grading capacitors also come with problems like reliability and requirement of extra connections. Hence a judicious choice needs to be made between over-designing of VI and employment of grading capacitors. Further work needs to be done to understand

the effect of stray capacitance at higher voltages: 72.5 kV and 145 kV. Also the current interruption performance of the series connected vacuum interrupters would be investigated.

REFERENCES

[1] Schneider Electric, “Cahier Technique no.193” [2] Liu Don-hui et al., “Research on 750 kV Vacuum circuit

breakers composed of several Vacuum interrupter in series”. IEEE, PP-315-318, 2004

[3] “The Vacuum Interrupter : Theory, Design and Applications”, Paul G Slade, CRC press, Page 96

[4] T.Fugel and D.Konig, “Influence of grading capacitors on the breaking performance of a 24kV vacuum circuit breaker series design”, IEEE transactions on Dielectrics and Electrical insulation, vol.10, No.4, pp.569-575, August 2003.

[5] Thomas betz, Dieter Konig, “Influence of grading capacitor on the breaking capacity of two vacuum interrupter in series”, IEEE Transactions on Dielectrics and electrical insulation. PP: 405 -409. 1999.

[6] Naotaka Ide, Osamu Tanaka, Satoru Yanabu, Shuhei kaneko, Shigemitsu Okabe, Yoshihiko Matsui, “Interruption characteristics of double-break vacuum circuit breakers”, IEEE transactions on Dielectrics and Electrical insulation. PP: 1065-1078, 2008.

[7] S. Giere, H. C.Karner, H. Knobloch, “Dielectric strength of double and single- break vacuum interrupters Experiments with real HV demonstration bottles”, IEEE transactions on Dielectrics and Electrical insulation. PP: 43-47, 2001.

E-mail of the author(s): sandeep.kulkarni@cgglobal.com