Partial discharge characteristics and trap parameters analysis of oil-paper after thermal aging

The degradation of oil-paper caused by thermal aging seriously affects the lifetime of power equipment such as transformers. In order to investigate the relationship between partial discharge (PD) and degradation of oil-paper insulation, PD characteristics and trap parameters of the paper are measured. In this work, we fabricate oil-paper model with cavity discharge, and the adopted oil-immersed paper specimens with different degree of polymerization (DP) are achieved by thermally accelerated aging to simulate transformers with different operation time. For each sample, the phase resolved partial discharge (PRPD) spectrogram is recorded. The trap parameters such as density and energy of different aged paper specimen are obtained by thermally stimulated depolarization current (TSDC) method. Furthermore, the microstructure of each paper specimen is observed by scanning electron microscope (SEM). Experimental results show two levels of trap existing on the surface of oil-paper: shallow (0.10-0.59 eV) and relatively deep (0.60-0.90 eV) energy level. Further analysis illustrates that thermally aging makes the trap energy deeper. It is considered that the change of PD and trap characteristics is of great significance for insulation condition assessment, and the trap parameter is promising to be new characteristic for aged paper insulation.