| Summary: | In the hierarchically connected ultra-high voltage (UHV) DC projects, the minimum air clearance on the 1000 kV AC side of converter transformer directly affects the layout and area of the valve hall. The minimum air clearance is determined by the overvoltage and the flashover characteristics of different electrodes. Since the UHV DC projects are hierarchically connected to the AC power grid, the overvoltage and the electrode flashover characteristics are different from that of general UHV AC projects, necessitating the relevant overvoltage calculation and test studies. The electromagnetic transients software PSCAD is used for modelling of the ±1100 kV Wannan convertor station and the related AC system. The power frequency overvoltage, switching overvoltage, time-to-crest value, and coefficient α of phase-to-phase overvoltage on the AC side of the 1000 kV converter transformer are calculated. Tests are carried out on air clearances between bundle conductors, tubular buses, and equipment grading rings under switching impulse with 1000 μs long wave-front. The minimum phase-to-earth and phase-to-phase air clearances on the 1000 kV AC side of the converter transformer are given.
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