Partial Discharges and Breakdown of Protrusion in GIS under Oscillating Lightning Impulses

Gas insulated switchgear (GIS) is important equipment in power system. To ensure its safety, alternating-current (AC) voltage withstand test is usually carried out in field. However, for GIS of high voltage level and big scale, it becomes non-effective to expose all defects for the low sensitivity of partial discharge (PD) detection. Then field impulse voltage withstand test, as an important supplement, is becoming increasingly important. Oscillating lightning impulse (OLI) is suitable for field use because of its easy generation. Sharp protrusions on HV conductor are typical defects in GIS. They are more critical under lightning impulses. This paper therefore studied its discharge characteristics under OLI. Four OLIs are generated whose wavefront was 1.2 μs, 3 μs, 7 μs and 13 μs respectively. The protrusion was 6 mm-long and was set on the inner conductor in a GIS chamber of 330 kV. An internal ultra high frequency (UHF) sensor was used to measure partial discharge and a capacitive voltage divider to measure applied voltage. Then the breakdown characteristics were experimentally studied. The results show that breakdowns always occur in the vicinity of each wave peak. 50% probability breakdown volage (BDV
₅₀
) is lower for positive polarity. Wavefront has strong influence on the discharge characteristics. BDV50 increase with the increase of wavefront. BDV50 of 13 μs wavefront is more than 10 % higher than that of 1.2 μs wavefront. Short wavefront indicates that first streamer will directly lead to breakdown. When wavefront is longer, first streamer becomes pre-breakdown discharge and forms corona stabilization, which makes it harder to breakdown. The results of the study are expected to be beneficial for the field impulse voltage withstand test for GIS.