Novel method to determine insulation temperature during dielectric measurements on power transformers

Frequency domain measurements of oil-paper insulation systems provide novel diagnostic methods for assessing the aging and moisture content in the insulation layers of power transformers. It has however been observed that the results of these measurements are highly influenced by the temperature. Due to long measuring times, temperature changes inside the power transformer lead to inaccurate moisture analysis. The oil temperature which is normally read and used as the insulation temperature is insufficient for this purpose. In this paper, a novel temperature determination method is presented. The winding resistance of the transformer is measured as a function of time during a FDS-measurement on a power transformer. The resistance values are then used to calculate the winding temperature. This method promises more accurate results for the temperature measurement and the subsequent moisture determination.