Influence of space charge buildup on the transition to electrical treeing in PE under ac voltage

Our investigation is focused on the understanding of electrical aging in polymeric materials under 50 Hz ac voltage. In this report, a needle electrode molded into the insulant to simulate defects producing local field enhancement is used. Special emphasis is given to low density polyethylene. The transition between the discharge-free electroluminescent state to micropartial discharge (PD) state (early electrical tree propagation phase) is studied with a sensitivity reaching 0.01 pC. Optical diagnosis is carried out simultaneously. At the moment of tree initiation, electrical discharges from 0.04 to 0.1 pC occur in the positive half cycle. A very small electrical tree of 5 μm was observed. Using the phase angle of the first measurable PD with respect to the applied voltage offers additional information helping to understand tree initiation. Then a correlation between the magnitude of electrical discharges and the characteristic traces of local breakdown (electrical tree) is reported. 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