A variable frequency model of a transformer bushing with localised moisture content

Moisture ingress within a high voltage power transformer bushing can be detrimental to the insulation and causes the majority of bushing failures. In practice, wet bushings are known to have moisture accumulate within the lower region of the condenser. However, traditional power frequency measurements are often too insensitive to adequately evaluate these moisture conditions. Variable frequency analysis can provide a more sensitive estimation of moisture content, but bushing frequency responses are not well documented. In this paper, a simplified bushing model is used to evaluate moisture content within the insulation, for both uniformly distributed and localised wet regions. Numerical software is used to model the dissipation factor and capacitance, as a function of frequency and temperature. It was found that a localised wet region which had a 1.0% moisture content, with an impregnation of between 5% and 20%, was not distinguishable from a uniformly distributed 0.5% moisture content. However, when the localised region moisture percentage increased to 4.0%, a distinction could be made by a steeper response at frequencies of less than 0.1Hz.