HVDC wall bushing is the core equipment in the converter station, which connects the valve hall and the DC field, and withstands long-term full DC voltage of both positive and negative polarity. As for HVDC wall bushing with resin impregnated paper capacitor core filled with SF6 gas, long-term DC voltage test is required to verify its insulation performance under long-term operating conditions. In IEC 65700-2014, test method is positive DC voltage for 2h, with the value of 1.5 times of the long-term maximum operating voltage. However, the different polarity of DC voltage is not considered. The experience of positive DC voltage comes from detecting the internal defects of oil-paper insulated DC bushing, which is contrary to SF6 filled equipment with weak insulating performance under negative DC voltage. It is urgent to study the influence of DC voltage polarity, and evaluate the reliability of insulation performance in the life cycle of bushing.

The resin impregnated paper capacitor core of the DC wall bushing model contains 3 layers of aluminum foil, hollow insulator is adopted for external insulation. The test platform of DC wall bushing model under long-term DC voltage is established, which includes ±300kV DC voltage generator, DC wall bushing model and test platform support.

The irrecoverable insulation damage occurs in the resin impregnated paper capacitor core when continuous discharge pulse happen. During 2h long-term DC voltage duration test, discharge pulses above 500pC appear relatively stable in 90~120min, it is more appropriate to record the number and value of discharge pulses in this interval for condition assessment. As for the resin impregnated paper capacitor core, the number of discharge pulses under negative DC voltage is more than that of positive polarity, but for pulses with high value, this relationship is the opposite. As for the hollow insulator, the number and value of discharge pulses under positive DC voltage is higher than that under negative DC voltage.

The number and value of discharge pulses are 2 important parameters for evaluating DC partial discharge, both positive and negative DC voltage shall be used to test the long term DC insulation performance of HVDC wall bushing.
 

RIP UHVDC wall bushing,Internal discharge,DC voltage