Partial discharge pulses at artificial protrusions in SF₆ under negative IEC60060-3 standard impulses

Protrusion defects on conductor in gas insulated switchgear (GIS) formed by manufacture, transportation, installation or operation can seriously degrade the withstand voltage strength. Partial discharge (PD) test under ac voltage is sometimes ineffective for this type of defect in practice. Thus, we carried out PD measurements for protrusion defects under four types of standard impulse voltages and explored some basic behaviors and characteristics of PDs, which could provide some guidance on impulse PD test which may be used as a more stringent measure in the future field test. In this paper, PD pulses at protrusions with tip radii of 25, 100 and 500 'm in SF₆ in gas pressure from 0.05 MPa to 0.55 MPa, were detected under four negative IEC60060-3 impulses by using a specially designed measuring system. The voltage-time lag characteristics of the first discharge under the impulses are investigated and described by the proposed theoretical 'voltage-time lag (V-t) curve of the first discharge', and the simulated results were compared with the experimentally measured V-t points. Seven different discharge current and light pulse sequences were distinguished and analyzed statistically. And then, the occurrences and mechanisms underlying these different sequences were discussed in detail.