AC partial discharges propagating on an unpenetrated crack wall

The purpose of this study is to make clear the influences of partial discharges (PD) propagating in the walls of an unpenetrated crack. The high densities of charges due to PD are expected to accumulate in the unpenetrated crack. ne unpenetrated crack in this study was formed with walls of opposed two insulator blocks (PMMA) on an insulating board (PMMA). Two blocks on the board were set in the needle-plate electrode system. AC high voltage (30 kV, 50 Hz) was applied to a needle electrode. The crack width studied ranged from 10 μm to 1000 μm. The length of the crack mainly studied was 40 mm. The results indicate that the PD propagation into the crack is strictly changed at a critical width of the crack. When the crack width is from 10 μm to 200 μm, strong positive pulses with Q>2×10^-8 C appears. When the crack width s>500 μm, the strong positive pulses disappears. The critical width exists between 200 μm and 500 μm. Negative pulses of relatively high magnitude are observed just before the phase where strong positive pulses appear. The strong positive PD will be generated when the negative pulses neutralize the accumulated positive charge on the wall of the crack