Analysis of frequency domain spectroscopy measurements on power transformers by the use of a Finite Element based model

Dielectric spectroscopy (FDS) is widely used to assess the condition of transformer oil-paper insulation systems. Typically the frequency dependent complex capacitance C_HL between the high- and low-voltage windings is measured. This capacitance can be modeled analytically using simple RC-equivalent circuits, due to the homogenous electrical field distribution in the main insulation duct (XY or Pancake-model) [2]. A curve fitting of this model to the measured data enables extraction of dielectric parameters like oil conductivity and moisture content in pressboard. Apart from C_HL it is also possible to measure the complex capacitance between HV windings and tank (C_H) where we also have an oil-paper insulation system. The corresponding measurement data can´t however be fitted to the conventional Pancake-model, because of the inhomogeneous field distribution between HV windings and tank. Hence a Finite Elements Analysis (FEA) based model which describes the dielectric behavior of the transformers isolation system between HV windings and tank was developed in this paper. This numerical model was combined with an optimization algorithm to determine the unknown dielectric parameters from dielectric spectroscopy data for C_H. FDS measurements of C_H and C_HL from three power transformers were analyzed by the use of the conventional Pancake-model for C_HL and the new FEA-model for C_H. The results showed good agreement in dielectric parameters of oil and pressboard what leads to a strong verification of dielectric diagnosis results for a transformer by two independent models and measurements.