A satisfactory testing technique is developed for investigating the recovery characteristics of various power system insulations.
Duration of the fault current, for a vertical test gap, has little effect on the dielectric recovery. Five cycle fault current duration decreases the initial rate of recovery, for a horizontal test gap, below that for a half cycle, but for longer time delays thermal convection increases the rate of recovery above that for a half cycle duration.
The 11 inch test gap has a lower percentage recovery than the 6inch test gap. The lower the boiling point and the lower the ionization potential of the electrode material, the lower the rate of recovery.
The increase in the rate of recovery produced by winds up to 1000 feet per minute is mainly due to displacement of the ionized gases from the test electrodes. For recovery based on this displacement of the ionized gases from the electrodes, the axial wind recovery should be half of the perpendicular. However, the axial wind decreases the cross section of the arc, heat transfer away from the ionized gases is increased, and the axial recovery is greater than half of the perpendicular recovery. Wind velocities less than 100 feet per minute have a negligible effect on the dielectric recovery characteristics.
A 3000 frame per second camera is used to make a high speed photographic study of the arc. By using a system of mirrors, two mutually perpendicular views of the arc are obtained simultaneously, on each frame of the film. Breakdown occurs in the weakest path through the ionized gases, even though this path is 2 to 3 times as long as the electrode separation.
The extent of ionization of the luminous gases is obtained with the aid of a microphotometer by taking density readings directly from the image of the arc on the 16 mm. negative film. The average density variation with time follows the general shape of the curve based ondiffusion as the only important deionizing agent, for a short time after current zero.
The local coefficient of heat transfer obtained from the analogy of the arc to a hot cylindrical solid body increases with the wind velocity and the density of ionization of the luminous gases decreases with increasing wind velocity.
The diameter and density of the arc are obtained from the photographic study and the recovery voltage calculated, at various time delays, from Slepian's theory for the critical breakdown gradient of the arc column. The results are in general qualitative agreement with the experimental results.
The minimum reclosing time for high-speed automatic-reclosing circuit breakers on a transmission system can be reduced by a factor of five for a wind of 1000 feet per minute (15 miles per hour) blowing across the ionized fault path.
