Non-Uniform Field Breakdown of SF6 Insulation Under Combined AC and Switching Impulse Voltages

A number of rod-plane gaps in SF6 prestressed by 60 Hz ac voltage are tested under positive switching impulse voltages using a circuit which employs a 60 Hz test transformer as the only power supply. In addition to testing under switching impulse voltages, tests are also performed under superposed ac and impulse voltages with an impulse-to-ac ratio varying from 0 to 1.5. The effects of the compound stress on the breakdown probability distribution and the time-to-breakdown are investigated. The compound stress is found to have no effect on breakdown at low probabilities if the ac voltage component is below corona onset. Unsystematic variations in breakdown data are detected at the pressures where corona stabilization is most evident. Prestressing SF6 insulation with ac voltage shortens the time-to-breakdown under switching impulse voltages, particularly at those gas pressures where corona stabilization is present.