The mineral insulating oil and pressboard are important insulation dielectrics in AC-DC converter transformers. Thus, it is very significant to understand the dielectric properties of the oil and oil-impregnated pressboard. In this paper, the influences of the temperature, field strength and aging on the resistivity of oil and oil-impregnated pressboard have been investigated. Samples used in the experiments are new oil, aged oil, pressboard immersed in new oil and aged oil respectively. The measuring device of resistivity was full of oil and isolated from outside. All samples were measured at temperature range of 25-80 °C under the electric field strength range of 2-10 kV/mm for oil and 3-30 kV/mm for oil-impregnated pressboard. The results showed that the resistivity of oil-impregnated pressboard decreased along with the increase of field strength and temperature. For all oil specimens and pressboard immersed in aged oil, the resistivity decreasing rate ( lg / E ) along with electric field strength showed a downtrend with the increase of temperature; while, for pressboard immersed in new oil, the decreasing rate showed an uptrend with the increase of temperature. For oil, the resistivity reduction rate with aging (r) decreased along with the increase of field strength and temperature. For pressboard immersed in oil, the reduction rate with aging (r) showed an uptrend with the increase of temperature.
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 oilpressboard. 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 o 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 oilpressboard 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.
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