Impact of very fast transient overvoltage on the transformer bushing in open circuit condition or when using online monitoring

Abstract

Transformer bushings play a critical role in the insulation system of power transformers, and their reliability is essential for secure power system operation. One of the factors contributing to bushing failures is the occurrence of fast transient overvoltages (FTO) resulting from fast transient phenomena in power systems. In particular, the bushing Test tap, which is designed with a relatively low insulation level, is highly sensitive to such overvoltage impulses under open-circuit operating conditions or when connected to online monitoring systems. This paper investigates the impact of FTO on the reliability of transformer bushings through numerical simulations based on the finite element method (FEM). The simulation results show that the voltage on the Test tap can reach very high magnitudes during FTO events. Such overvoltages may lead to insulation breakdown in the Test tap chamber and potentially cause severe bushing failures when the Test tap is left open-circuited or connected to online monitoring equipment. The findings provide important insights into the associated risks and support the development of mitigation measures to suppress overvoltages on the tap conductor when online monitoring systems are applied to transformer bushings.