Breakdown Characteristics of Rod-Plate Electrode under Composite AC and DC Voltage

With enhancement of voltage level, the parts such as fittings, shields and insulator of converter valve which is an important equipment in the high voltage direct current (HVDC) transmission systems suffer from greater volume of composite AC and DC voltage and harmonic voltage. At present, the breakdown characteristics of air gap under composite voltage have been short of research. In this paper, based on the test platform with parallel circuit which means that AC and DC high voltage are applied to one electrode at the same time and the other electrode is grounded, the breakdown characteristics of the rod-plate electrode under the composite voltage are studied. The effects of different electrode distances and different DC components on breakdown voltage are compared and the following conclusions are obtained: when the electrode distance is 0.5 cm or 3 cm, the breakdown voltage basically has no change with DC component; when the electrode distance is from 1 cm to 2.5 cm, with the increase of DC component, the breakdown voltage is first maintained at the value of the AC breakdown voltage, then increases quickly to the DC breakdown voltage and finally basically stay at this value. By analyzing the voltage and current waveforms during breakdown, it can be concluded that the positive corona discharge pulses before breakdown is the key factor affecting the breakdown voltage. The research results of this paper can lay the foundation for the subsequent research on the breakdown mechanism under composite voltage and the prediction of composite breakdown voltage.