Independent Effects of Aged Oil and Aged Paper on Moisture Evaluation of Power Transformers

Abstract

Estimating the moisture content of insulation paper is important in transformer condition evaluation, the results of which are affected by thermal aging. This study investigates the independent effects of aged oil and aged paper on moisture equilibrium charts to determine which of the two primarily causes the inaccuracy in moisture estimation of aged transformers. A thermal aging experiment was conducted. At five aging degrees, four kinds of oil–paper insulation systems were formed: new oil–new paper, aged oil–new paper, new oil–aged paper, and aged oil–aged paper. Moisture equilibrium curves were drawn for each insulation system. The relative moisture content of oil and that of paper were calculated. Results showed that the moisture content of insulation paper decreased with aging, which led to the migration of the moisture equilibrium curves and further influenced the moisture estimation results. A comparative analysis showed that the migration of the moisture equilibrium curves was mainly caused by aged paper at the early and middle aging stages and by the synergistic effects of aged paper and aged oil at the late aging stage. The effects of aging on moisture distribution were verified using a retired transformer.