Study on the voltage endurance characteristics of mineral insulation oil impregnated pressboard

As the growing emphasis on high voltage direct current (HVDC) transmission in power system, the reliability of converter transformer that is the vital equipment in DC transmission system becomes more important. The insulation system of converter transformer is mainly composed of insulating oil and pressboard, therefore the studies on DC voltage endurance (V-t) characteristics of the oil-impregnated pressboard are of great guiding significance for the insulation system design. In this paper, V-t characteristics of unused mineral insulation oil-impregnated pressboard have been studied in a completely seal V-t characteristics test system at 50°C. Firstly, the oil needs undergo a treating process to diminish the impurities, water and oxygen content in oil, and satisfy the requirements of water content <;10ppm and oxygen content <;500ppm. The transfer and store of the treated oils are all operated in the completely seal. The insulation board is uniform in thickness of 1mm, and dried 48h at 105°C before oil-impregnated. Then, the continuous boost condition method is adopted in the experiment to obtain the breakdown field strength of samples, and the voltage endurance tests of samples are carried out at eight different voltage levels respectively. The results show that the voltage endurance curve of oil-impregnated pressboard is approximately fitted exponential relationship with time in two time domains during 25 days. The life index n can be calculated by means of the inverse-power model of electrical aging. Based on the results, the electric life of oil-impregnated pressboard can be estimated under different voltage level. This study may have important implications for the insulation system design under DC field strength.