Flashover characteristics of barrier insulation under uni-directional and bi-directional oscillatory impulse voltages

The authors describe a series of unidirectional and bidirectional oscillatory impulse voltage tests carried out on an experimental insulation system representative of a typical barrier insulation assembly used in present-day equipment. The influence that the type of oscillatory impulse voltage, the oscillatory frequency, and the voltage levels applied have on the ultimate flashover levels are investigated. Results of tests on polymethylmethacrylate, polycarbonate, and synthetic resin bonded paper used as the insulation barrier are presented. It is shown that a wide dispersion of flashover voltage exists with some insulation materials but not others. This characteristic, as well as that of flashover on the second and third peaks of the oscillatory wave, is explained in terms of stored surface charge