The modelling of electric field, capacitance and dissipation factor of a high voltage bushing over varying frequency

Within a high voltage substation, power transformer bushings are key components and an assessment of their condition is important to ensure operational reliability. Bushings represent a large proportion of transformer failures, where most issues are as a result of moisture ingress. Moisture ingress affects the distribution of the electric field and changes the capacitance and dissipation factor (tanδ) of the insulation. In this paper, a numerical model is built for an oil impregnated paper (OIP) condenser bushing. The simulated dielectric frequency response (DFR) dependence of transformer oil and OIP with varying moisture content is given. Simulations are made of the AC and very low frequency (approximated to DC) potential and electric field distributions. Furthermore, the capacitance and dissipation factor over a frequency range of between 1mHz to 1kHz is presented. The results are found to be consistent with typical bushing manufacturers' data.