Flashover tests on air gap of ??800kV DC transmission line under composite DC and switching impulse voltage

This paper deals with the air gap insulation of ±800 kV DC transmission systems under composite DC and switching impulse voltage. The gaps studied were representative of ±800 kV DC transmission line, as rod-to-plane and conductor-to-tower gaps. The flashover voltages of the gaps at an altitude of 2100 m were determined for a pure switching impulse and composite voltages of DC and switching impulse. It is found that there are clear increase in the composite flashover voltage of both rod-to-plane and conductor-to-tower gaps, compared with those in the gap that only impulse voltage was applied. From the test results, the flashover voltage increases by 17-23%in the rod-to-plane gap and 5-10% in the conductor-to-tower gap, when the DC voltage of 800 kV is pre-stressed. The standard deviations have little obvious variation under the pre-stressed DC voltage and its average value is 4-4.5% in the tests. Additionally, a 1,000,000 fps high speed CCD camera is used to observe the breakdown process for investigation on the influence of pre-stressed DC voltage on the conductor. The leader inception and breakdown process properties were studied. These data and explanation are useful for ±800kV DC insulation design and testing.