Partial discharges at a triple junction metal/solid insulator/gas and simulation of inception voltage

In gas-insulated systems, the presence of solid insulators used to hold conductors at high voltage induces the presence of triple interfaces where the metal, solid insulator and gas are in contact. These regions, usually named “triple junction”, constitute weak points for the insulation, mainly due to the mismatch between permittivities of solid and gas, which induces an enhancement of the electric field. If the design of these regions is not carried out with great care, partial discharges (PDs) may appear and accelerate the ageing of the insulator. This paper deals with an experimental study of PDs in a “triple junction” electrode configuration, carried out with sensitive electrical and optical PD measurements. Experiments are done in two pure gases, nitrogen and SF6. Two discharge regimes can be observed: very small discharges (<1 pC) occurring at low voltage, probably initiated by defects, and more intense and homogeneous discharges spreading around the triple junction. A model based on the calculation of the electric field close to the triple interface and on the application of the Townsend breakdown criterion is then presented and compared to experiments.