Streamer-to-Leader Transition Characteristics of Long Air Gap Between Sphere and Plane with Burr Defects at High Altitudes

In the valve hall of the converter station of a UHV transmission project at high altitudes, the shielding sphere and the wall/floor form a large-size sphere–plane long air gap. Burr defects on the surface of the shielding sphere can affect its discharge characteristics. The streamer-to-leader transition is a key process in the discharge of the long air gap. The existing research is limited to the discharge characteristics of small-size electrodes at low altitudes and cannot be directly extended to those of large-size electrodes at high altitudes. Therefore, this paper constructs a discharge test platform with optical–electrical synchronous detection at an altitude of 2200 m. The instantaneous optical power, electric field intensity, high potential current, and other physical parameters during the discharge in the long air gap of a 1.3 m diameter sphere–plane system were collected for both a sphere electrode with burrs and one without burrs. The injection current of the initial streamer was used as the input variable and substituted into Gallimberti’s model to analyse the transformation process of the streamer stem’s vibrational energy into translational energy. A modified model that is more suitable for high altitudes was developed by taking into account convective diffusion and the thermal expansion of the streamer, and the influence of burr defects on the characteristics of the transition from streamer to leader was analysed and compared with the experimental results. Overall, burr defects reduced the duration of the streamer-to-leader transition and facilitated discharge. The analysis results generally agree with the experimental results. The research results are of great significance for the design of the valve hall insulation in converter stations at high altitudes.