Electric Field Analysis of a DC Bushing: Effects of the Anisotropic Bulk Conductivity of Epoxy Resin-Impregnated Paper

The valve-side bushing of the converter transformer used in Ultra-High Voltage Direct Current (UHVDC) systems is an important component of DC transmission networks, where the operational reliability of the bushing fundamentally determines system-level safety performance. However, research on bushings rarely considers the anisotropic bulk conductivity of epoxy resin-impregnated paper (RIP). Accordingly, this study systematically characterizes the anisotropic bulk conductivity and temperature-dependent behavior of the RIP composites. A transient multi-physics simulation of electric-thermal-fluid coupling is carried out, with anisotropic bulk conductivity parameters established for the valve-side bushing of a ±800 kV converter transformer. The numerical results indicate that anisotropic bulk conductivity induces bipolar charge segregation: The epoxy core’s outer surface exposed to SF₆ gas maintains positive charge accumulation, while the air-gap and arc-corner surfaces develop negative charge densities of up to –14.82 pC/mm². Under isotropic bulk conductivity, the air-gap termination region maintains a uniform electric field below 0.1 kV/mm. In contrast, anisotropic bulk conductivity induces field distortions characterized by tangential/normal components reaching 0.5/1.8 kV/mm. These distortions elevate the risk of surface flashover on the bushing because of air-gap discharge.