Thermal Ageing Condition Assessment for Transformer Using Natural Ester-Paper Insulation Based on Polarization and Depolarization Current

Accurate diagnosis of transformer aging state is very important for ensuring safe operation of transformer. Polarization and Depolarization Current (PDC) behaviors of mineral oil-paper insulation have been widely investigated. With the popularization and application of natural ester, it is very important to diagnose the aging state of natural ester transformer. In this paper, thermal ageing condition assessment for natural ester-paper insulation based on PDC was investigated. Results show that the degradation of polymeric insulation systems increases the space charge density and the concentration of electrical dipoles. The longer the ageing time, the higher amplitude the depolarization current values of natural ester-paper insulation sample (NEP). There is no consistent change for the absorption ratio and the polarization index of the natural ester-paper insulation in the aging process. The polarization index changing behavior for NEP is the same to mineral oil-paper insulation sample (MOP). While the absorption ratio changing behavior for NEP is different to MOP. The MOP has higher polarization index than the NEP when the DP falling down to 500. The NEP has higher absorption ratio than the MOP when the DP falling down to 800. According to the ratio of depolarized total charge quantity growth, the aging stage can be clearly divided into four stages, which is DP>800, 800>DP>600, 600>DP>400, DP<400. It is found that the depolarized total charge quantity growth can be used to predict the ageing condition of oil-paper insulation.