Time and Space Dependent Characteristics of HVDC Electric Field Stress in Oil-pressboard Composite Insulation Systems

For an enhancement of HVDC electrical insulation performance in power equipment, we need to investigate more details of DC-dependent charge activities and resultant DC-peculiar electric field distributions. In this paper, we focused our investigations on time and space dependent characteristics of charge behavior and DC electric field in oil-pressboard (PB) composite insulation systems. By using different structure models with different field non-uniformity factors, and based on the material properties of oil and PB, we calculated the time transition of accumulated charge density on PB surface and electric field distributions, from DC-on to DC steady-state. As a result, in quasi- and non-uniform fields, we found the electric field distributions and the charge accumulations on PB were to be built up nonlinearly, depending on the local electric field stress. Time constants of electric field in oil differs depending on locations and the final distributions were formed. We finally discussed the time dependent characteristics of the electric field stress as functions of electric field non-uniformity factors and the magnitude of local electric fields.