Study on assessing the ageing condition of oilpaper insulation by Polarization/Depolarization current

The power transformer is one of the important equipment for ensuring the safe and stable operation in the power system. Accurately assessing the ageing condition of oil-paper insulation in transformer is essential and important. Polarization and Depolarization Current (PDC) technique can sensitive to reflect the insulation condition of oil-paper insulation system. The PDC behavior of mineral oil-paper insulation has been used in the transformer oil paper insulation moisture assessment, but there is no report about the analysis of mineral oil-paper insulation ageing condition by PDC. In this paper, the current of oil-paper insulation under different aging time has been measured at 110°C thermal aging test. A new characteristic parameter (the depolarization charge quantity) for assessing the ageing condition of mineral oil-paper insulation by analyzing the change of PDC current characteristic was proposed. Results show that the depolarization charge quantity of mineral oil-paper insulation sample is increasing with its ageing condition, but the tendency is weak. And it is also very sensitive to its ageing condition, the two characteristic parameters collected form the curve and the degree of polymerization are satisfied by a kinetic model equation. So the depolarization charge quantity could be used to predict the ageing condition of mineral oil-paper insulation.