Defect Identification Method of Oil-impregnated Paper Bushing Based on Polarization/depolarization Current under High Voltage

Polarization/depolarization current method can sensitively reflect the insulation state of medium, and has been applied to the insulation state evaluation of oil paper insulation bushing. However, in the existing results, the polarization voltage applied to the oil-impregnated paper bushing is too low, and the test current is easily affected by environmental noise, so the test results can not accurately characterize the insulation state of the bushing. In order to study the polarization/depolarization characteristics of bushing under high voltage, the polarization/depolarization current method (PDC) was used to test the bushings under three different degrees of moisture. The results show that when the oil paper insulation bushing is affected by moisture, the strong polarity of moisture will aggravate the polarization process under the action of electric field, resulting in the increase of conductivity of oil paper insulation system, which is reflected in the increase of polarization/depolarization current. In addition, the polarization/depolarization current increases with the increase of the test voltage, but the trend of the curve is hardly affected by the voltage. Then the PDC test data of the bushing is analyzed by the charge difference method (CMD), and the charge difference curve obtained is a straight line with a constant slope. As the moisture content of the bushing increases, the difference between the polarization current and the depolarization current also increases, which leads to the increase of slope of charge difference curve. This characteristic is also more obvious under high voltage excitation. Therefore, the test and analysis of polarization/depolarization current of oil-impregnated paper bushing under high voltage can provide ideas for effectively identifying the moisture condition of bushing.