Transient field distribution in gas-solid insulation systems under DC voltages

The development of electric fields in gas-solid insulation systems under dc voltage stress goes hand in hand with the accumulation and decay of charges on dielectric interfaces. The charge transport through the solid insulator and the gas is determined by the electric properties of the involved insulating materials. Therefore, the nonlinearly field-dependent volume conductivity of a gas, resulting from a constant charge generation rate by natural ionization has to be considered. The transient field distribution on insulators under dc voltage is investigated theoretically, using a gas model, which takes into account the physical characteristics of insulating gases like air and SF₆ as well as the effects resulting from electron emission from the electrodes. The influence of the ion pair generation rate in the gas, the electric field strength, and the volume and surface conductivity of the solid insulating material on the development of the electric field is investigated on cylindrical and conical insulators which are typical for gas insulated systems. Results of calculations using the gas model are compared to simulations assuming constant gas conductivity. The simulation results are verified by measurements of the surface potential on cylindrical epoxy resin insulators in air.