Assessment of the Moisture Content of Oil–Paper Insulation Based on the Nonlinear Coefficient of Low-Frequency – Per-Unit Curve

Moisture is one of the critical factors that lead to the deterioration of the insulation of oil-immersed power transformers. This study develops a novel feature parameter called nonlinear coefficient $\alpha $ of ${U}$ – ${I}$ per-unit curve to assess the moisture content in transformer insulation. Results reveal that the nonlinear coefficient $\alpha $ of oil-impregnated paperboard increases with the increase in moisture content or the decrease in excitation frequency. A power function relationship exists between $\alpha $ and moisture content at a given excitation frequency and is minimally affected by insulation structure and degree of polymerization (DP). The validity of the new characteristic parameters proposed in this study is also verified using a model transformer in a laboratory and a real transformer in the field.