V-t Characteristics of Gas Insulating Spacer Model for Sustained AC Voltage

In order to discuss the reliability of a gas insulated switchgear (GIS) insulation under the operating conditions, the deterioration characteristics (V-t characteristics) of GIS insulating spacer model was studied by AC voltage application. The deterioration factor n which indicates the decrease of breakdown voltage under the application up to 1000 hours was found to change at the two grade with the lapse. And, the relation between partial discharge (PD) inception voltage and breakdown voltage was studied after 1000 hours applications. As the result, it was found that both voltage (V50) at a 50% cumulative probability were equal to the initial values. But, dispersion of PD inception and breakdown voltage have a tendency to be larger than initial values. Moreover, in oder to make the deterioration phenomenon clear, the relation between the ratio of applied voltage (Va) and the initial PD inception voltage (Vp) and lifetime was obtained. The inclination of curve indicated at the minimum value in VaJVp-t characteristics has similar tendency with V-t characteristics and the minimum value of Va/Vp is 0.4. This value corresponds to deterioration of 3.4 σ (σ is standard deviation) from V50 of PD inception. From the above characteristics, the threshold value of dielectric strength for V-t characteristics on GIS insulating spacer model is assumed to be a 4 σ from V50 of PD inception.