Breakdown characteristics of protrusion on HV conductor in GIS under oscillating and standard lightning impulse voltages

Gas insulated switchgear (GIS) is widely used in power system. To improve its reliability, field impulse voltage withstand test is an important step. Standard lightning impulse (SLI) is normally adopted in factory withstand test for GIS equipment. However, oscillating lightning impulse (OLI) is often used in field. Differences exist between OLI and OSI concerning their capability of insulation assessment and defect exposition. Sharp protrusions or particles on HV conductor are typical defects in GIS and are critical at lightning impulse voltages. This paper therefore aims at breakdown characteristics study of protrusion on HV conductor under SLI and OLI. Firstly, the impulse voltage generator was established to generate SLI and OLI. They had the same front time and time to half value. Secondly, a 25mm-long protrusion defect on HV conductor was set up in a 330kV GIS chamber. And then the 50% probability breakdown voltage (BDV₅₀) and Voltage-time characteristics (V-t characteristics) were experimentally studied. The results show that for positive and negative polarity, BDV₅₀ under OLI is 0.4% and 10.9% higher than that under SLI respectively. V-t plots under SLI show a steeper downward slope than that under OLI. Breakdowns under positive OLI concentrate in the vicinity of the first wave peak, while under negative OLI, breakdowns concentrate in the first three peaks and minimal breakdown voltage (V_min) occurs in the third peak.