Moisture determination and degradation of solid insulation system of power transformers

The reliability and operational risk of power transformers is strongly dependant on the condition of the electrical insulation system. Different kind of stress can be distinguished, normally a combination of electrical and thermal stress is present as well as the influence of humidity in the insulation system has to be considered. The so called multi-stress load results in an accelerated ageing process and the degradation of the insulation system. The condition of the mineral oil can be determined by sampling and analyzing the oil by the means of DGA and chemical-physical analysis. The solid insulation system mainly consists of transformerboard. The condition determination of the cellulose can be done by sampling, with is quite expensive and difficult. New dielectric methods which operate on the effect of response function have established in the last years. In this paper the focus should be directed on the mechanism of cellulose ageing and dielectric measurement.