A dielectric frequency response model to evaluate the moisture content within an oil impregnated paper condenser bushing

Increased moisture content within an oil-paper insulation system can significantly reduce its life expectancy, and for oil impregnated paper condenser type bushings, it is the most common cause of failure. Distinction between moisture contents using traditional power frequency tests of dissipation factor and capacitance can be difficult, particularly at ambient temperatures. Dielectric frequency response is becoming an established technique to measure the dissipation factor and capacitance of a bushing, but to date an accurate model to evaluate the condenser moisture content has not been presented using this technique. In this study, a dielectric frequency response bushing model is proposed and finite element method software is used to simulate the variation in dissipation factor and capacitance of a bushing with varying moisture content, as a function of frequency and temperature. The modelled results are compared with measurements reported in the literature and from the authors' own field measurements, where a good agreement is demonstrated. It is shown that the distinction in dissipation factor between moisture contents at frequencies <0.1 Hz is in excess of ten times greater than at 50 Hz. The proposed model can be used by practitioners to evaluate moisture content within the condenser insulation and distinguish between moisture contents ranging from 0.2 to 4.0%.