THE ROLE OF ELECTROSTATIC AND HYDRODYNAMIC-FORCES IN THE NEGATIVE-POINT BREAKDOWN OF LIQUID DIELECTRICS

Optical studies of prebreakdown processes in insulating liquids enable one to identify several stages in the breakdown of negative point/plane gaps. In volatile liquids the process starts with the creation of a rapidly expanding vapor cavity adjacent to the point electrode. As it expands, the surface of this cavity becomes unstable and one observes the growth of an electrohydrodynamic instability. In some liquids the runaway growth of this instability leads to streamers that bridge the gap and cause the actual breakdown; in other cases, instabilities lead to bush-like growths. By combining simple electrostatic and hydrodynamic concepts we can derive equations for cavity growth, the instability growth rate, and streamer velocity; these results are in reasonable agreement with experiments at voltages near breakdown, in short gaps. In high-viscosity fluids other hydrodynamic growth processes are observed; thus in 100 and 1000 cSt DC-200 silicone fluids a vortex of hot liquid forms near the point cathode and this vortex develops into a vortex ring which grows and decays; at high fields the vortex undergoes an unstable growth process, leading to multiple vortices.