Experimental Investigation of Partial Discharge Inception Characteristics on Polymeric Insulating Surfaces under Positive Ramp High Voltages

Partial discharge inception in a short sphere-ring gap bridged by polymeric insulating surfaces is investigated under positive ramp high voltages with variable rate of rise. Partial discharge inception probability distributions were obtained through successive inception tests, based on simultaneous applied voltage and discharge current measurements. The partial discharge inception voltage is lower for smaller sphere radius and higher material relative permittivity. These effects could be accounted for theoretically, based on electrostatic field simulations and the equivalent avalanche concept, by assuming a poor contact between the sphere and insulating specimen. Using the ratio of sphere radius to material relative permittivity, a modified Peek's formula yields satisfactory estimates of the partial discharge inception field.