Voltage escalation in vacuum switching operations

The ability of vacuum switching devices to interrupt high-frequency currents can lead to repeated prestrikes and clearings when such switches are being closed and repeated reignitions and clearings when the switches are opening. Attention is focused on the interruption of a motor inrush current (aborted starts), where the successive trapping of magnetic stored energy in the motor winding and its subsequent release to the motor and cable capacitance can result in a sequence of escalating voltage surges. Of particular concern is the sudden collapse of voltage when sequential reignitions occur and the impact of this on the interturn insulation of the motor winding. The authors report on simulated tests with circuit breakers of different manufacturers and attempt to analyze cause and effect with respect to the magnitudes of the voltages developed and the characteristics of the switch. Particular attention is given to the rate of dielectric recovery of the contact gap and the dI/dt interrupting capability of the switch