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A two dimensional axis-symmetric fluid model is developed to present the streamer discharge process in transformer oil under positive and negative sub-microsecond impulse voltage, which is based on the hydrodynamic drift-diffusion approximation to simulate the dynamics of discharge inception and propagation. The streamers' spatial evolution with time are obtained, i.e., the distribution of electric field and space charge density under different conditions such as the amplitude of applied voltage and rise time. Simulation results verify the breakdown voltage has a distinct polar effect: negative streamer requires more than 2 times voltage threshold than that of positive one. It is revealed that the negative streamer, which usually has a bushy profile, is much more diffusive than the positive one. The magnitude and rise time of the applied voltage have evident influences on the shape and speed of the streamer in oil. The formation of positive or negative streamers are both characterized by a self-generated field enhancement at the head, and space charges enhance the front field of ionization region to assist the growing discharge channel to propagate longer. In terms of the generation of charged particles, it is considered that field-dependent molecular ionization predominates the charge generation mechanism of streamer discharge processes in transformer oil. [ABSTRACT FROM AUTHOR]

An electrothermal hydrodynamic model is presented to evaluate effects of the applied lightning impulse voltage parameters such as polarity, magnitude, and rise time on the initiation and propagation of the streamers formed in an IEC defined needle-sphere electrode geometry filled with transformer oil. Instantaneous velocity, column diameter, head curvature, maximum electric field, and the volume charge density have been investigated as the main characteristics of the streamer. Modeling results indicate that greater applied voltage peak amplitudes form streamers with higher velocity, greater head curvatures, and thicker columns. The bushy negative streamers usually initiate at almost twice the applied voltage magnitude and propagate slower than filamentary positive streamers. Results also show that in transformer oil at the same impulse voltage peak amplitude, shorter rise times create thicker positive and negative streamers. [ABSTRACT FROM PUBLISHER]

Subnanosecond breakdown of low-pressure gaseous spark gaps has been investigated by using a combined modeling of the volume discharge in gas and the electron emission at the electrodes. Simulation results predict minimum breakdown delays of about 400 ps for the geometries considered in this paper. These results are in good accordance with the measurements. [ABSTRACT FROM AUTHOR]