Investigation of the gas-solid insulating system in DC GIS exposed to AC-DC mixed voltage for offshore bipole applications at ±525 kV DC

Abstract:
In order to develop offshore converter stations in the most space-saving way possible, high-voltage components should be gas-insulated, where feasible. This not only holds for the DC and the AC switchyard, but also the space between the main transformers and the converters. At this point in bipolar HVDC systems, the equipment is subjected to a mixed voltage stress consisting of AC and DC components. In this paper, the electric field stresses of typical AC-DC mixed voltages to the gas-solid insulation system of a DC GIS are determined by calculations using the finite element method. They are compared with the stresses in the case of pure direct voltage and different test voltages. It is found that the field stresses occurring in the gas-solid insulation system under AC-DC mixed voltage are always lower than the stresses under pure DC voltage, when considering the same maximal voltage value. Finally, the dielectric test requirements of the standards for converter transformers and bushings under AC-DC mixed voltage are compared with the recommendations for dielectric testing of gas-insulated DC systems under DC voltage.