Quantitative diagnosis of moisture content in oil-paper condenser bushing insulation based on frequency domain spectroscopy and polarisation and depolarisation current

Dielectric response techniques have been proven useful for evaluating the moisture content (K-m.c) of oil-paper insulation. However, an accurate model for oil-paper condenser bushing has not been presented. This study designed a laboratory oil-impregnated paper (OIP) bushing model to simulate moistened defects in oil-paper bushings. Moreover, a 10 kV OIP bushing model was simulated using COMSOL multiphysics AC/DC to confirm that the distribution of potential meets the design requirements. Furthermore, this work investigates the effect of moisture content on the dielectric frequency response and polarisation and depolarisation current (PDC) of models of oil-paper bushings. A group of characteristic parameters was extracted from the dielectric dissipation factor curves and the DC conductivities of different moisture contents were computed by charge difference method from PDC curves. The obtained results suggest that the dissipation factor and the capacitance increase with the increase of moisture content in the whole frequency range, and the quantitative relationships among the characteristic parameters, DC conductivities, and the moisture content were established and follow an exponential law.