The effect of needle curvature and aluminum foil on the transition of discharge mechanism under needle-plate configuration

The measurement system detecting ns discharge current pulse during partial discharge (PD) was constructed to study the effect of the variation of needle curvature and aluminum foil diameter on the discharge parameters. Five types of needle electrode (with the curvature radius of 0.19 mm, 0.40 mm, 0.59 mm, 0.78 mm, 0.98 mm, 1.45 mm) and three types of round aluminum (with the diameter of 5 mm, 8 mm, 12 mm) were employed in our paper. As the area of aluminum foil increased, the peak value and rise part of current pulse increased and the falling part and pulse width decreased. However, the abnormal phenomenon for negative discharge that minimum rise part appeared with relatively small diameter of aluminum foil (5 mm). When no aluminum foil covered the dielectric surface, the peak value of positive discharge current was larger than that of negative one, its slew and falling part were smaller. These could be explained well by the streamer theory. When the aluminum foil covered the dielectric surface, the similar differences between positive discharge and negative one were obtained when the curvature radius was smaller (0.19 mm). However, as curvature radius increased (≥0.98 mm), the distinct result that pulse width of negative discharge was smaller and its current peak value was larger compared with positive discharge. This is explained in terms of the different speed of avalanche development of positive and negative pulses.