The DC breakdown of mineral insulating oil and oil-pressboard

AC-DC converter transformer is one of the most important equipments in the high-voltage D.C. transmission system. And the mineral insulating oil and pressboard immersing in oil are important insulation dielectrics used in AC-DC converter transformers. Thus, it is very significant to understand the dielectric properties of the mineral insulating oil and oil-pressboard. In this paper, the DC breakdown of two new mineral insulating oils and a pressboard immersing in these oils under different temperature has been systematically investigated. The oil and oil-pressboard was degassed and dehydrated before the test. In the testing progress, the test device of breakdown strength was protected by nitrogen and isolated from outside. The breakdown strength of two kinds of mineral insulating oil were tested at five temperatures with the range of 25-80°C, and the breakdown strength of pressboard immersed in both oil were also tested at three temperature with the same range. From the results, it can be obtained that the breakdown strength of oil increased with the increase of temperature, whereas the oil-pressboard showed opposite tendency. This paper enhanced the understanding of the breakdown strength for mineral insulating oil and pressboard immersed in oil, provided the theoretical and test basis to expand the engineering application of this mineral insulating oil and pressboard.