Modeling of Streamers in Natural Ester and Naphthenic Oil: Basic Physical Properties and Propagation Modes

This article presents a 2-D axisymmetric numerical model of streamer propagation in transformer oils, constructed based on the drift-diffusion control equation of charged carriers. This model is energized by submicrosecond positive pulsed voltage to simulate streamer behaviors in various voltage levels, gaps, and oils. The simulation results indicate that a smaller gap and a higher voltage level result in faster streamer propagation. The average velocity of streamers varies in 2-400 km/s, but in most cases, this value is concentrated in 11-95 km/s. The field strengths for streamers in naphthenic oil and natural ester range in 3.5-6.2 x 10(8) and 1.7-4.2 x 10(8) V/m, respectively, whereas the space charge densities are 500-2200 and 200-900 C/m(3), respectively. Streamers in both oils rapidly accelerate around the needle electrodes. However, the streamer in natural ester experiences deceleration as it moves away from the needle electrodes at lower voltages, whereas at higher voltages, its velocity is either comparable to or exceeds that of the naphthenic oil. In the process of streamer propagation, the field strength and space charge density at the streamer head are mainly related to the liquid nature. The increase of the applied voltage can improve the average propagation speed of streamer, but the change of the real-time propagation speed is mainly determined by the propagation stage where the streamer is. Furthermore, by summarizing the simulation results in different cases, a theoretical model of streamer propagation, i.e., competition between the Laplacian electric field and the space charge effect, is proposed from the perspective of the behavioral mechanism of space charge. With the propagation of streamer, the extent to which the space charge effect can offset the diminishing Laplacian electric field, becomes crucial in determining the streamer's acceleration potential. In regions far from the needle electrode, the enhanced space charge effect emerges as the overriding factor, propelling the streamer's acceleration.