Investigation of the Partial Discharge Characteristics on PTFE and PMMA Surfaces Under Positive Ramp High Voltages with Variable Rate of Rise

摘要

Polymeric insulators when exposed to electric stress their surface dielectric behaviour greatly determines system insulation performance. Depending on insulating surface material and state as well as type and polarity of the applied voltage, partial discharges may develop adhering to the insulating surface or mainly in air away from the surface. In withstand or flashover tests on HVDC equipment, the rate of rise of the applied voltage is expected to affect partial discharge activity, thus also the estimated values of withstand or flash-over voltages. In this study, the characteristics of partial discharges along PTFE and PMMA insulating surfaces bridging a non-uniform electric field electrode arrangement (short sphere-plane gap), under positive ramp high voltages. Discharge regimes are identified based on current measurements conducted at applied voltages just sufficient for discharge inception up to voltages causing flashover. From the obtained I-V characteristics the discharge inception and flashover voltages could be determined. Surface discharge initiates in the form of small current pulses, at applied voltages lower for the PMMA than PTFE insulator and lower than those corresponding to the inception of the burst corona in air. However, the glow discharge, established in air around the sphere electrode, initiates at higher applied voltages for the insulating surfaces than in air alone, and also higher for the PMMA than PTFE specimen. Consequently, flashover occurs through a spark developing in air away from the insulating surface, at voltages closely related to the glow inception voltage. When the applied voltage is normalized with respect to the glow inception voltage, the effects of the rate of rise of the applied voltage on discharge currents are well accounted for; an empirical expression has been introduced describing well the I-V characteristics for all investigated cases (air alone and insulating surfaces).