Thermal gradient effects on surface charge of HVDC spacer in gas insulated system

HVDC transmission might increasingly be realized in the form of GIS and GIL. And surface charge accumulation on spacers is one of the main factors that lead to the reduction of insulation level in dc gas insulated system. Due to dc, the electric fields are not capacitively but resistively determined. Further, the electrical conductivity of polymeric materials strongly depends on the temperature. Since actual dc operation is connected with ohmic heating of the conductor, inhomogeneous temperature distribution is inside the spacer. Therefore, the thermal gradient effects on surface charge must be considered. In this paper, surface charge distributions on a model GIL spacer in 0.1 MPa SF6 under DC voltage were obtained by the electrostatic probe method. With the help of a simulation model, the observed results with and without thermal gradient are compared and discussed. It shows that bulk conduction dominates the surface charge accumulation and thermal gradient facilitates the charge injection from the bulk. Surface charge density is higher under thermal gradient and follows an exponential distribution, which coincides with the thermal-dependent conductivity distribution.