Abstract: Considering the importance of power transformers condition monitoring concerning the improvement of power quality, the paper presents information regarding the degradation processes of the transformer insulation system (paper-oil) and the methods for their lifetime estimation (standard and activation energy method). Further, the experiments carried out in order to determine the lifetime curves (through both methods) are presented. The insulation lifetime calculation corresponding to constant operating temperature and the loss and remaining lifetimes in case of variable temperatures of the oil and paper are presented in this paper, as well. In the end, a new equation to calculate the remaining lifetime of an in-service transformer based on monitoring of its oil resistivity is proposed. In this equation, the temperature changes due to daily load variation is no longer involved.
Keywords: power transformers monitoring, mineral oil, transformer paper, insulation systems, ageing, lifetime