Application of UHF PD Online Monitoring to Oil-Paper Insulation Life Assessment for Transformers

Pattern of number of PD events ("NE" for short) variation detected by UHF online monitoring was found through experiments, demonstrating that in the first half life of oil-paper insulation the NE value declined at the beginning and then kept stable, while it increased sharply in the later half. Least square linear regression was applied to calculate the changing rate ("CR" for short) of the latest 168 measurements of NE, which turned out to be an appropriate indicator of life stage for its monotonicity over time from the initiation of PD till final breakdown. The Grubbs method was also applied to exclude erroneous data and to determine the credible variation of CRs.