High Voltage Breakdown Performance and Circuit Isolation Capability of Vacuum Interrupters

High voltage breakdown experiments determined the ability of vacuum interrupters to provide adequate circuit isolation in the open position. The vacuum interrupters were rated for a 38kV system voltage, and contained either Cu-W (10 wt.%) or Cu-Cr contacts. Sample interrupters were voltage conditioned with a peak AC voltage of 160kV. The statistical distribution of the BIL breakdown voltage for 3 and 5 mm contact gaps were determined using the up-down method for newly conditioned contacts. A three-parameter Weibull distribution fit the breakdown data, implying the existence of a threshold value below which no breakdowns occurs. These results are discussed in terms of the high voltage capabilities of Cu-W contacts and with respect of the peak open-circuit voltages for typical 38 kV applications. The vacuum breakdown behavior is also compared to the breakdown in SF6