Surface discharge initiated by immobilized metallic particles attached to gas insulated substation insulators: process and features

In order to study the developing process of surface discharges initiated by immobilized metallic particles attached to Gas Insulated Substation (GIS) insulators under gradually-raised applying voltages, long-term laboratory tests were conducted on a well-established 220 kV test assembly to observe the entire evaluation process of surface discharges from its inception to gradual expansion and the eventual occurrence of surface flashover. Real-time detection was conducted to record conventional impulse current (CIC) signals, UHF signals, ultrasonic signals and light emission signals from PD. Features of surface discharges at different developing stages were captured from the discharge trend curves, scatter diagrams, two-dimensional histograms, and grey-scale maps of PD. The test findings indicate that i) surface discharge initiated by immobilized metallic particles attached to GIS insulators undergoes three developing stages from corona discharge domination in the incipient stage, to co-existence of corona discharge and surface discharge in the expansion stage, and finally the surface discharge domination in the final stage; ii) the shape and phase distribution shown in scatter diagram, histogram and grey-scale map could be used as effective criteria to infer different evolutionary stages of surface discharges produced by immobilized metallic particles attached to GIS insulators.