A new method to evaluate the effectiveness of impulse voltage for detecting insulation defects in GIS equipment

Lightning impulse (LI) test is an effective means to detect insulation defects in gas-insulated metal-enclosed switchgear (GIS) equipment. However, the LI test wavefront time T _f will exceed the standard value of 1.2 μs ± 30% for large test load capacitance. Whether extending the T _f will influence defect detection effectiveness should be a concern. In this study, a generating system of impulses with different wavefront parameters was established. The insulation characteristics of a coaxial cylinder structure with an SF ₆ gas gap with conductive protrusion in a bus under impulses with different wavefront parameters were studied. Experimental results show that the voltage–time curve of the gap shows a bathtub trend that has a flat part in the range of 1 μs to 5 μs. With a T _f increase, the 50% breakdown voltage increases. Whether extending the T _f will influence defect detection effectiveness is analyzed from the standpoint of the different meaning of the breakdown voltage in voltage-time curve and 50% breakdown voltage. The essence of detection effectiveness under impulse voltages with different wavefront parameters is a comparison of the probability of detecting the same insulation defect. Based on the discharge probability distribution, a new quantitative method to evaluate the insulation defect detection effectiveness of impulse voltages for GIS equipment is proposed. The method, based on the 90% discharge voltage value of an insulation defect under a standard LI, normalizes the discharge probability under different wavefront parameters.