Voltage conditioning of vacuum interrupters with application of out of phase voltage to the two contacts

Conditioning or seasoning is an important process step for a vacuum interrupter (VI) manufacturing. Conditioning process suppresses/burns the micro-protrusions and makes the VI suitable for withstanding the impulse voltage and capacitor switching. The conditioning process is divided basically in two types: Voltage conditioning and Current conditioning. Voltage conditioning can be further divided in three types: power frequency voltage conditioning, impulse voltage conditioning, and high frequency pulse voltage conditioning. In all these methods the applied voltage creates a stress at the micro-protrusions. When the stress is higher than the work function, field emission is initiated at the cathode and emission current flows from the anode to cathode. This current heats and eventually evaporates the protrusions. In power frequency voltage conditioning, voltage at 50/60 Hz is applied across the electrodes. The value of voltage depends on the gap but, as a general thumb rule, the peak of the voltage is approximately equal to the impulse withstand voltage. As the rated voltage of VI increases so does the impulse withstand voltage. And hence the conditioning voltage also increases proportionately. In today's scenario, vacuum interrupters are being increasingly used at voltages classes of 40.5, 52 and 72.5 kV. For these rated voltages the conditioning voltages would be of the order of 200 kV, 250kV and 325 kV. This would increase the size of the transformer. Also, as the rated voltage of transformer increases the clearances of the test lab would increase. The alternatives to mitigate this issue is to use either impulse conditioning or high frequency pulse conditioning. Another approach is to increase the stress across the electrodes by applying voltages of opposite polarity. With such an approach it is estimated that lower conditioning voltages can be employed to achieve the same robustness of process. This paper aims to study the voltage patterns, field profiles and stress values to assess the effectiveness of using voltages of opposite polarity. The paper compares the situation with conventional method. The paper would also enlist the challenges of this method.