Enhanced Field Calculation for HVDC GIS

The high reliability and compactness achieved by compressed gas insulation in HVAC systems, has led to the development of HVDC Gas Insulated Substations (GIS). As in case of HVAC GIS, the basic insulation components of HVDC GIS are SF6 gas and solid spacers. In this respect, it has been recognized that the breakdown strength of GIS is mainly influenced by spacers. For HVAC GIS, the problems related to spacers can be eliminated to a great extent by a careful electric stress control design1-3. However, for HVDC GIS stress control designs based on dielectric interfaces with no trapped charges are not valid, since charges get accumulated on spacer surfaces which distort the designed Laplacian field distribution. In this context, it is also kept in mind that SF6 insulation is very sensitive to local enhancement of electric field. According to this surface charge accumulation and its effects in HVDC GIS have been studied in various aspects by several researchers4-11. In order to improve the dielectric performance of epoxy spacers by properly shaping the gas-dielectric interfaces, studies on electric field optimization along the profile of the gas-dielectric interface in gas insulated systems have been reported in literature12,13. For HVDC GIS, investigations on the effects of spacer shapes on flashover characteristics have also been carried out14. Since, in contrary to HVAC GIS, the electric field distribution at steady-state in HVDC GIS is mainly controlled by the conductivity k, of the dielectric media, studies on the influence of volume and surface resistivities on the electric field distribution around GIS spacers have been done15.