The electric field distribution in oil-paper insulation under combined AC-DC voltage

The valve winding in the converter transformer withstands several types of voltage, including the AC, DC, and the combined AC-DC voltage. It is important to consider the effects of voltage type to the electric field distribution. In this paper, a combined AC-DC source was constructed. The electric field distribution of oil paper insulation model was measured by Kerr electro-optic effect, under combined AC-DC voltage with different composite ratios of DC/AC. The results indicate that: 1) DC electric field distribution is asymmetric, which is caused by the difference of the charge accumulation status in different area. The electric field intensity in the left area accumulated negative charge is lower than that in the right area accumulated positive charge. 2) AC electric field distribution depends on the geometric structure and is symmetric. 3) The AC electric field intensity rises with the increase of AC component linearly. However, the relationship of the DC electric field and the DC component is complicated and non-linear in most area.