Space charge effect on tree propagation in epoxy block subjected to polarity-reversed voltage pulses

This paper presents an experimental study on electrical tree propagation from a point electrode in an epoxy block subjected to polarity-reversed repetitive pulses after dc prestress. When the epoxy block at one end of a long coaxial cable is charged by a dc prestress and then directly grounded by closing a switch at the other end of the cable without any resistance, repetitive voltage pulses are applied to the epoxy block with a change in its polarity. In test for polarity-reversed tree with -dc prestress, tree length tends to be shorter than that for an positive impulse tree without prestress. However, the shape of the polarity-reversed tree becomes more busy. In test for polarity reversed tree with +dc prestress, tree length at room temperature is very short for both the pulses with and without the prestress because of the poor propagation capability in negative streamer. However, polarity-reversed tree at 120degC is very large because the negative streamer propagation is followed by the successive positive streamer caused by the second polarity reversal.