ELIMINATION OF SURFACE LEADER AND INCREASE IN FLASHOVER STRENGTH BY USE OF HOLLOW INSULATORS

For an insulator with an inner electrode, negative surface leaders appear under ac voltage in air. When a hollow type insulator is used, the negative surface leader is almost eliminated at an optimum size of the well. With the optimum design, the number of negative leaders is approximately 1/100 of that for regular electrodes at 40 kVp. Dust figures of surface charge distribution reveal that negative charge, due to partial discharges in ac voltage, is deposited on the surface of the insulator within the well. It is believed that the elimination of the negative leaders results from field stabilization due to the deposited negative charge. An increase in ac flashover voltage is observed when the hollow insulator is used. The increase in flashover voltage with the hollow insulator under an impulse voltage is less than that under ac voltage.