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With the increase of operation voltage to 800 kV-1100 kV, the sphere-plane gaps with radius of 0.5-1 meter are widely used in the converter station of UHVDC power transmission system. In order to ensure the safety of UHVDC system and reduce the investment, it is necessary to investigate the breakdown characteristic of large radius sphere-plane gap. This paper implemented experiments on three different gap configurations: rod-plane, sphere (R=0.55 m)-plane, and sphere (R=0.75 m)-plane. The 50% breakdown voltages and the current injected into the high voltage electrode were measured. Besides, the discharge process was observed by a high speed CCD camera. Three types of current waveforms related to different streamer progression characteristics were found. With the increase of sphere radius, it can be found that the streamer gradually becomes dominant in the discharge process, which leads to …