Frequency domain spectroscopy (FDS) has been widely used to evaluate the moisture content of oil-paper insulating materials in recent years. However, due to the differences of actual transformer insulation structure and application scope of characteristic fingerprint library to insulating materials, it can not be effectively applied to the evaluation of damp state of actual transformer on site. To overcome this drawback, we develop a new characteristic parameter called nonlinear coefficient α of U–I characteristic curve to indicate the moisture content in transformer insulation. The results show that under the excitation of 0.01 Hz, α is sensitive to water content, and the fitting relationship is given. By testing the composite insulation system models with different structures, it is found that the FDS curve is greatly affected by the insulation structure, and α It can accurately reflect the moisture content of the insulating paperboard without being affected by the structure, so it can be used as a characteristic parameter to evaluate the moisture content of the main insulation of the transformer.

oil-paper insulation,dielectric response,low frequency,nonlinear,moisture content