Surface charge characteristics on insulators in vacuum under nanosecond pulse excitation

Various partial discharge of an organic and a polyethylene insulation materials in vacuum under impulse voltage (rise time of 50 ns, halfwidth of 350 ns and 3 μs) excitations were observed, and the characteristics of surface charge were investigated by an electrostatic probe, a current monitor system and a luminescence signal measurement system. The experimental results show that, a partial discharge current may be observed near the peak, the tail of pulse, or even close to zero voltage at pulse end. The partial discharge is the basic reason for the surface charge under nanosecond pulse excitations. Positive surface charge was measured after the partial discharge happen, and the charge density near cathode is alittle more than that near anode for the electron emission began from the cathode.The partial discharge and surface charge on organic glass are easier been observed than those on polyethylene, and the organic glass insulator flashover voltage is lower. The partial discharge induces the surface charge, whose physical mechanism is the trap centers of insulator and the collisional ionization during the process of the secondary electron emission avalanche.