For obtaining accurately the internal design dimensions of the delivery valve hall of the ±800 kV UHV DC project at altitude above 2 500 m

the discharge characteristic test of the phase-to-phase and phase to ground gap is performed by the lifting and lowering method at Xining high-altitude test base at altitude of 2 500 m

the switching impulse discharge voltage

U

50

of the typical gap is obtained and the fitting formula of the discharge voltage and the gap distance is given. The test results show that the type of air gap determines the magnitude of the growth rate of the discharge characteristic curve

and the increase of the radius of curvature of the grading sphere electrode on the electrode can significantly improve the switching withstanding ability of the gap. However

as for the same kind of air gap

the electrode position applied by the positive switching impulse voltage will not directly lead to the difference of the discharge characteristic curve.