Insulation characteristics of GIS bus with conductive protrusion attached to the high-voltage electrode under lightning impulses with different wavefront times

The wavefront time T
_f
of lightning impulse plays an important role in the detection of insulation defects in metal enclosed gas insulated switchgear (GIS). Usually, a rod-plane electrode system with different geometric parameters is adopted to simulate local field enhancements in GIS. But, for GIS, the defects may occur on the high-voltage electrode which can not be properly simulated by a rod-plane electrode. To research the influence of waveform parameters on actual defects in GIS, cylinder-plane, coaxial cylinder, and basin-type insulator with a conductive protrusion, were used in this study. A generating system of double-exponential impulses with wavefront times in the range of 0.08 to 15 μs and wave tail times of approximately 50 μs was used to simulate different impulse test waveforms based on a fully enclosed oil-immersed Marx generator. The results show that for the cylinder-plane gap with a protrusion, when the gap distance of the needle tip to the plane is constant, increasing the length of the needle decreases the 50% breakdown voltage. Comparing with the insulation characteristics of rod-plane gap, the U
₅₀
%-T
_f
curves of the cylinder-plane electrode with needle defects thereafter no longer show a U shape as the rod-plane electrode, but increase with an increase in the wavefront time. The difference can be explained by the shielding effect of the cylinder electrode on the defect, which changes the electric field distribution of the gap. The U
₅₀
%-T
_f
curves of GIS bus with different voltage levels and basin-type insulator with surface conductive defect also show the similar increase trend. For V-t characteristics of GIS bus with different voltage levels, we find that the higher the voltage level, the more obvious the trend of the V-t characteristic bathtub curve.