During recent years, an increasing demand of compact solutions for HVDC transmission systems can be stated. In this context, gas-insulated systems (GIS) for HVDC operation enable space-saving, low- loss and reliable realizations. For developing a reliable design, the behaviour under extreme temperature conditions and simultaneous voltage stress is essential, since the strong nonlinear dependence of the electric properties on temperature or gas humidity brings particular challenges compared to AC systems. For that reason, investigations with real-sized insulators for DC gas- insulated systems were performed at ambient temperatures between -30°C and +50°C with simultaneous dielectric stresses. Particularly DC voltages up to ±825 kV and lightning impulse voltages up to ±1550 kV were applied to the gas-solid insulating system.
Resulting from the investigations, it was found that a significant lower conductivity of the solid material at -30 °C and further potential influencing factors, such as electron emission from electrodes under presence of low-conducting epoxy material, did not have a considerable influence on the dielectric performance of real-sized DC GIS insulators under dry conditions.
Considering a higher gas moisture content, especially during temperature changes from -30 °C to +50 °C, a strong reduction of the withstand voltages was observed. Hence, the moisture content of the gas has to be taken into account for gas-insulated HVDC systems. Anyhow, the application of drying agents is a suitable measure to overcome this constraint.