Effect of moisture on breakdown strength of oil-paper insulation under different voltage types

Converter transformer is the important part of high voltage direct current transmission system (HVDC), whose safety reliability is related to the safety of HVDC. Compared to the traditional AC transformer, there is a complex AC-DC electric field inside the converter transformer, which is more likely to cause a decline in insulation performance, after the internal oil-paper insulation is dampened. In this paper, a test platform for insulation breakdown strength of oil-paper insulation was set up according to the standard in IEC60243. This paper analyzed the change regulation of moisture content of oil-paper insulation. The difference of breakdown strength of oil-paper insulation, which contain different percentage of moisture, were studied under AC, DC, different ratio of AC/DC voltage and polarity reversal voltage, and combined with double-layer dielectric theory, the effect of moisture content and voltage type on breakdown characteristics of oil-paper insulation was analyzed. The analysis showed that most of the moisture was concentrated in the insulating paper, with the increase of moisture content, the increasing amplitude of electrical conductivity of insulating paper is larger than that of insulating oil. As a result, the electric field strength on the insulation paper decreases and the electric field strength on the insulation oil increases, which made the insulation gap easier to breakdown.