Conductivity Estimation for Reliable Assessment of Power Transformer Insulation at Reduced Time

In this paper, a method to estimate the conductivity of oil paper insulation is proposed for fast and effective diagnosis of power transformer insulation. Instead of conducting conventional frequency sweep from 1mHz to 1kHz, frequency domain spectroscopy (FDS) measurement was conducted from 50mHz to 1Hz and the imaginary part of complex permittivity (epsilon ''(omega)) of the oil paper insulation was obtained, based on which an equivalent model of the insulation system was developed. Two important parameters namely conductivity (sigma) and imaginary part of susceptibility (chi ''(omega)) were computed from the equivalent model. Finally these two aforesaid parameters were optimized to obtain the imaginary part of complex permittivity (epsilon ''(omega)) from 1mHz to 1Hz. Experimental verification on several laboratory prepared samples confirmed that the practicability of the proposed method. It was observed that the overall measurement time was reduced up to 78.5% enabling insulation diagnosis to be conducted quicker in comparison with conventional method.