Space charge and insulation properties of nano-Al 2 O 3 -modified oil-impregnated paper used for HVDC convertor transformer

Oil-paper (cellulose) insulation is the main insulation of converter transformer, and the space charge accumulation inside the oil-impregnated paper is a serious problem. To decrease the space charge accumulation and improve the breakdown field strength, the oil-impregnated paper (OIP) with different Al 2 O 3 nanoparticles content was prepared. The test results show that the space charge accumulation in the non-modified paper and 3.75 wt% Al 2 O 3 paper is serious under DC electric field, and the decay amplitude and decay speed of space charge are larger and faster in the short circuit test. Suppression effect of space charge in the 2.5 wt% Al 2 O 3 paper is the best, and the decay amplitude and decay speed of space charge are the lest and slowest. Moreover, the surface potential of non-modified paper decay faster, and that of 2.5 wt% Al 2 O 3 paper decay slowest. The breakdown field strength and resistivity of Al 2 O 3 -modified OIP are larger than those of the non-modified paper. The breakdown field strength of 2.5 wt% Al 2 O 3 paper increases most, about 16%. Suitable Nano-Al 2 O 3 -modified increases the traps level and density of deep traps in oil-impregnated paper, resulting in the space charge suppression and breakdown field strength improvement.