Electric Field Distribution in DC Bushings under Anisotropic Bulk Conductivity: Effect of Non-Uniform Temperature Field Considering Dielectric Harmonic Loss

This study investigates a ±800 kV converter transformer valve-side bushing. By experimentally characterizing the anisotropic material parameters and performing coupled electro-thermal simulations, the work systematically examines (i) how dielectric harmonic losses contribute to bushing temperature rise, and (ii) how resulting temperature gradients modulate the electric field distribution at the gas–solid interface under anisotropic bulk conductivity. The findings reveal that both excessively steep and shallow radial temperature gradients impair insulation performance in distinct spatial regions. Specifically, enhanced radial temperature gradients driven by high-frequency harmonic losses exacerbate radial electric field nonuniformity. As a result, the peak radial electric field increases by 3.5 kV/mm, and the bulk dielectric strength between the conductor and flange is degraded. Conversely, weaken radial temperature gradients, even when nearly reaching thermal homogenization, redirect electric field concentration toward the inner core layer. Under anisotropic condition, this phenomenon intensifies the electric field within these regions and promoting significant negative charge accumulation at the gas–solid interface. Under this condition, the maximum negative surface charge density reaches −33.26 pC/mm², and the interfacial electric field peaks at 3.48 kV/mm, which indicates a potential compromise in surface insulation reliability at the gas–solid interface.