Space charge characteristics of insulating materials under AC voltage and DC voltage are equally important. There is a huge amount of AC transformers in the power transmission system. However, there is no report about the space charge characteristics of the oil-paper insulation materials under the AC voltage. In this paper, firstly, the space charge characteristics of the oil immersed insulating paper under the positive/negative DC field were investigated using the pulsed electro-acoustic (PEA) method. Secondly, the space charge characteristics of the oil immersed insulating paper under power frequency voltage were studied using the improved PEA system. Results show that the electric field polarity has a decisive impact on the charge distribution. When the DC electrical field is positive polarity, the polarity of the charge accumulated at the paper-paper interface near the aluminum electrode is positive, while it is negative when the DC field strength is negative. The charge polarity of the charge accumulated at the aluminum electrode is negative in the positive half cycle under the power frequency voltage. It is positive in the negative half cycle of the power frequency voltage. The charge polarity of the charges accumulated at the internal interface of the oil-immersed insulation paper are opposite for 90° and 270° phase in the voltage applied process. The charges accumulated within the oil-immersed insulating paper under the power frequency voltage show a significant reduction compared with the results under DC voltage.
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