Comparative Analysis of Electric Field Simulation Under Isotropic and Anisotropic Bulk Conductivity in DC Bushing

The UHVDC converter transformer valve-side bushing is a critical component in DC transmission systems. However, the anisotropic properties of bushing insulating materials have been overlooked in previous studies. This paper investigated on the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm \mathbf{8 0 0 ~ k V}$</tex> converter transformer valve-side bushing by measuring the bulk conductivity of epoxyimpregnated paper flake specimens cut from an actual bushing both parallel and perpendicular to the insulation paper surface. The study revealed that the bulk conductivity in the perpendicular direction was significantly higher than that in the parallel direction. A steady-state coupled electrical-thermalfluid simulation model for the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 800 \text{kV}$</tex> converter transformer valve-side bushing was further established. Simulations were conducted under both isotropic and anisotropic bulk conductivity conditions, and the results were comparatively analyzed. The study revealed that on the bushing epoxy core airgap surface near the shield, isotropic bulk conductivity resulted in minor negative charge accumulation, whereas anisotropic bulk conductivity induced significant negative charge accumulation. Compared to isotropic bulk conductivity, anisotropic conditions reduced the air-gap terminal potential by 14 kV and increased the normal/tangential components of the electric field on the SF6 side of the air-gap surface by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.9 \text{kV} / \text{mm}$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$0.51 \text{kV} / \text{mm}$</tex>, respectively, degrading the insulation performance of the air-gap region.