Electrostatic and Hydrodynamic Effects in the Electrical Breakdown of Liquid Dielectrics

Optical studies of pre-breakdown events in insulating liquids enable one to distinguish three stages in the breakdown of negative point-plane gaps: (1) The creation of a rapidly expanding vapor cavity adjacent to the point electrode. (2) The instability of the surface of the cavity, characterized by the growth of wave-like disturbances, and (3) The runaway growth of the instability leading to vapor streamers that bridge the gap and cause the actual breakdown. This paper contains an analysis of the electrostatic and hydrodynamic forces acting on the cavity and on the resulting streamers. It is assumed that the vapor in the cavity is ionized due to the high Field, so that the potential at the bubble surface is close to that of the point electrode. This assunption enables one to calculate the electrostatic force expanding the cavity. The electrostatic force on the cavity wall also leads to an instability of the interface, and the runaway growth of the instability leads to the generation and propagation of streamers; by combining simple electrostatic and hydrodynamic concepts, we derive an equation for streamer velocity that is in approximate agreement with measured values.