Experimental Study On Corona Onset Characteristics Of High Voltage Electrode Under Composite AC-DC Voltage

High voltage direct current transmission is developing rapidly and has become one of indispensable elements of the strong smart grid. With enhancement of voltage level, as the key equipment in the high voltage direct system, the volume of DC converter valve tower turn to be greater and internal circuit wiring are more complex. Except for those, valve internals inevitably withstand AC and DC composite voltages and harmonic interference, so that the internal electric field distribution is more complex. If the surface electric field on high potential areas has not been controlled strictly, corona discharge phenomenon will occur. In order to improve the converter valve insulation level, it is necessary to carry out experimental studies on the corona characteristics for electrodes in the valve under composite voltages.In this paper, a rod-plane electrode with extremely non-uniform electric field and the scale model of typical valve electrode with slightly non-uniform electric field are selected. The experimental platforms for the measurement of the corona characteristics of two kinds of electrodes are set up in laboratory. The characteristics of the series and parallel loop for AC-DC composite voltage are compared by experiment and simulation, and the circuit in series is finally due to its little influence between the DC and AC sources.In terms of the rod-plate electrode, the ultraviolet imaging detection technology is applied to measure the ultraviolet photons characteristics of the rod-plane electrode under different voltage ratios between DC and AC voltages. According to curves of the photon count with voltage under different types of corona discharge, the criterion for corona inception is proposed.Based on the experimental platform and corona inception voltage criterion, the corona inception characteristics of the rod-plate electrode were studied. The influences of the mode of applied voltages, the voltage ratios and the size of the electrode on the corona onset voltage are investigated in our work. Besides, the electric field distribution characteristics of electrode are obtained by electric field simulation. The presented results show that when the electrode is at the corona inception point, the AC component decreases linearly with the increase of DC component, and the linear superposition value of the two components increases with the increase of DC.In the end, the corona onset characteristics for the scale electrode of typical valve electrode typed as slightly non-uniform electric field are investigated. The influences of the ratio of AC-DC components and the geometrical size of the electrodes on corona onset voltage are studied. The electric field distributions of electrodes under different AC-DC composite voltages are also obtained by electric field simulation. The experimental results show that the variation of AC-DC component of the typical electrode at the corona point is the same as that of the rod-plate electrode. The surface electric field intensity at the corona onset voltage decreases with the increase of the electrode gap distance and the top fillet value. Increasing the distance of the electrode gap and suitable fillet value can effectively decrease the surface electric field strength of electrodes, which can improve the electric distribution and insulation level of the valve electrode.