Numerical modeling of positive streamer propagation in alternative natural ester and traditional naphthenic oil under sub-microsecond pulsed voltages

In this paper, a two-dimensional axisymmetric numerical model of streamer discharge in alternative natural ester liquid and conventional naphthenic oil under a positive sub-microsecond impulse voltage is proposed. The model is based on the charge drift-diffusion approximation in a divergent electric field. Simulation shows that streamer propagation in naphthenic oil is divided into two stages: stage I with a small streamer velocity of 0-0.91 km s(-1) and stage II with a rapidly rising streamer velocity up to 213 km s(-1). In natural ester, the streamer velocity gradually slows down from 4.84 km s(-1) to 2.11 km s(-1). The electric field at the head of the streamer in natural ester ranges from 2.4 x 10(8) V m(-1) to 2.5 x 10(8) V m(-1) during propagation, which is visibly lower but more consistent than that in naphthenic oil (3.5 x 10(8) V m(-1) to 8.4 x 10(8) V m(-1)). It is revealed that the different streamer features within the two types of oil are caused by the different competition between the Laplacian electric field and space charge effect at the streamer head. For streamer propagation in naphthenic oil, stage I is dominated by the Laplacian electric field whereas stage II is dominated by the space charge effect. In natural ester, the whole streamer propagation is dominated by the Laplacian electric field. Further, it is indicated that the streamer head consists of two specific discharging regions, i.e. the front region of the streamer head which guides new space charge accumulation and the back region of streamer head which guides density reduction of accumulated space charge. The expansion of the streamer tunnel is caused by the alternating space charges in these two regions. The work in this paper provides a theoretical reference for the practical application of alternative transformer liquids.