Influence of Sample Surface Oil on the Accuracy of Space Charge Measurement in Oil-Paper

Space charge in oil-paper under dc electric field is a significant factor for the development of converter transformer. The space charge in oil-paper is usually measured by a pulsed electroacoustic (PEA) method. But, its accuracy is not good that the surface charge amount at electrode cannot match the bulk charge-induced charge. In this article, space charge distributions in oil-paper samples with and without surface oil are measured and analyzed. The amount of bulk charge in the sample without surface oil is larger than that in the sample with surface oil. In the sample with surface oil, the amount of surface charge at lower electrode has a distinct error that it is much higher than the bulk charge-induced charge. The space charge-induced charge at lower electrode is calculated and compared with the measured surface charge. It is found that surface oil on the sample can greatly influence the amount of surface charge at electrodes. In the sample without surface oil, the measured surface charge at lower electrode has similar amount with the bulk charge-induced charge. In the sample with surface oil, the quantity of the measured surface charge is much higher than that of the induced charge. Besides, the bulk charge amount is less in the sample with surface oil. It indicates that the space charge injected from upper electrode to surface oil interferes the development of surface charge at electrodes during voltage application. During voltage removal, the charges in the surface oil induce more charges at lower electrode. Therefore, it is suggested to remove surface oil on oil-paper samples for an accurate space charge measurement.