Transition to electrical treeing in polyethylene under a.c. voltage

The transition between the discharge-free electroluminescent state to micro-partial discharge state (early electrical tree propagation phase) is studied with a sensitivity reaching 0.01 pC. At the moment of tree initiation, electrical discharges from 0.04 to 0.1 pC occur in the positive half-cycle of the excitation voltage. They correspond to a motion of electrons from the dielectric towards the metallic needle. A very small electrical tree of 5 μm ion in length was observed at that time. We proposed an interpretation based on the similarity with grounding tree experiments in which the initiation of a local breakdown is mainly due to a strong Poisson field radiated by a space charge region in the vicinity of the needle tip