Abrupt Changes in Streamer Propagation Velocity Driven by Electron Velocity Saturation and Microscopic Inhomogeneities

Causes of streamer acceleration caused by electron velocity saturation at intense electric fields are investigated in liquid dielectrics. Findings of this paper explain regimes of higher positive streamer velocity using a nonlinear electric-field-dependent electron velocity model that describes the streamer velocity more accurately when compared with the experimental results in the literature. Streamer branching in inhomogeneous transformer oil is also studied as an alternative cause of sudden changes in streamer velocity magnitude and direction, because after branching, the velocities of the born branches are significantly higher than the main streamer column velocity. In this paper, a 3-D model is established to incorporate nonsymmetrical variations in the streamer shape. The modeling results show that the streamer branching has both stochastic and deterministic origins. The stochastic parameters are determined by the content of inhomogeneity in the liquid and the deterministic origins of branching are defined by the structure of the volume charge layer at the streamer head. The results indicate that the sudden changes in streamer velocities are due to the electric-field-dependent electron mobility, electric-field-dependent ionization potential of dielectric molecules, and the branching capability of streamers even in extremely homogeneous liquid bulk.