In this paper, a 2-D axisymmetric numerical model of streamer discharging in alternative natural ester liquid and conventional naphthenic oil under positive sub-microsecond impulse voltage is proposed, which is based on the charge drift-diffusion approximation in divergent electric field. Simulation shows that streamer propagation in naphthenic oil is divided into two stages, i.e., stage I with a small streamer velocity of 0 to 0.91 km/s, stage II with a rapidly rising streamer velocity up to 213 km/s. In natural ester, the streamer velocity gradually slow down from 4.84 km/s to 2.11 km/s. The electric field at the head of streamer in natural ester ranges from 2.4×108 V/m to 2.5×108 V/m during propagation, which is visibly lower but more consistent than that in naphthenic oil, 3.5×108 V/m to 8.4×108 V/m. It is revealed that the different streamer features within the two types of oils are caused by the different competitions between Laplacian electric field and space-charge effect at the streamer head. For streamer propagation in naphthenic oil, stage I is dominated by Laplacian electric field whereas stage II is dominated by space-charge effect. In natural ester, the whole streamer propagation is dominated by Laplacian electric field. Further, it is indicated that the streamer head consists of two specific discharging regions, i.e., the front region of streamer head which guides for newly space charge accumulation, and the back region of streamer head which guides for density reduction of accumulated space charge at present. The expansion of streamer tunnel is caused by the alternatively changed space charges in these two regions. The content in this paper provides a theoretical reference for the practical application of alternative transformer liquids.
In this paper, the CEL (Coupled Eulerian-Lagrangian) fluid-solid coupling simulation method is adopted to simulate the mechanical behavior and bubble pulsation evolution process of the transformer oil filled in the converter transformer when a high-energy discharge failure occurs in the on-load tap-changer of the converter transformer. The mechanical response of the tap changer and its solid structure is analyzed. Based on this, the compensation performance and strength of the pressure compensated expansion joint installed on the tap changer are evaluated.
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