Partial discharges triggered by metal-particle on insulator surface under standard oscillating impulses in SF₆ gas

The characteristics of partial discharge (PD) triggered by a metal particle on an insulator's surface were studied experimentally under standard oscillating lightning impulse (OLI) and standard oscillating switching impulse (OSI) conditions. Light emission detection was carried out to assist in PD measurement. Experiments showed that the largest number of successive PDs occurred during the rise time of each oscillation period of applied impulses and that a small number of reversed-polarity PDs occurred during the fall time. The composition of the PD sequence was determined by the position of the particle on the insulator's surface. Specifically, when a needle particle detached from the plate electrodes, the PD sequences consisted of both negative point PDs and positive point PDs. Two types of negative discharge, three types of positive discharge, and reversed-polarity discharge were differentiated by their current and light pulse waveforms. In these experiments, detectable partial discharge inception voltages (PDIVs) of the three particle positions were investigated at gas pressures from 0.05 to 0.35 MPa. The light emission inception voltages (LEIVs) were lower than PDIVs under OLI, OSI, and AC voltages. These inception voltages of different particle positions could be sorted as follows: LEIV < PDIV₊ (d = 30 mm) ≤ PDIV_- (d = 0 mm) < PDIV_+/- (d = 15 mm). We demonstrated that the OLI voltages could excite PDs from defects at lower equivalent voltage levels than for OSI and a 50 Hz AC voltage, and the PDs under OLI were more active in terms of occurrence frequency than those under OSI and AC voltages. A simple model was proposed to explain PD inception and propagation under oscillating impulses. The rapid change of background electric field and the formation and accumulation of charges on the insulator's surface were considered the main causes of the greater efficiencies of OLI and OSI at exciting PDs compared with the AC voltage.