An improved model for the calculation of the electrical onset in gas insulated medium inhomogeneous gaps

Most hardware for use in HV systems requires a thorough geometric design concerning the inception of breakdown (partial discharges, corona). Work shows an interest in such corona onset design optimization criteria. Corona onset optimization is based on numerical E-field calculations and different methods for determination of onset/no onset. Practical HV equipment conductor surfaces will normally form nonuniform E-fields possessing a breakdown behaviour determined by both the surface electric field and the rate of decay of the field. Besides that, among other issues, one has to take into account the type of gas, the temperature and the pressure. Therefore it seems important to apply an accurate onset criteria in corona onset design. This work presents the first step of a validated onset criteria useful for practical electrode geometries. This is accomplished for atmospheric air on the basis of the well-known Townsend approach for nonuniform fields with modifications concerning the gas parameters and validation against IEC 52 sphere-sphere gap breakdown voltages. It was found that the onset/breakdown voltage for medium nonuniform fields can be calculated by means of the same criteria for varying electrode curvatures and degrees of field utilization with an accuracy better than 4%.