We have investigated the partial discharge (PD) due to electrical treeing degradation in low-density polyethylene (LDPE), ethylene - vinyl acetate copolymer (EVA) and ethylene - acrylic acid copolymer (EAA) by a computer-aided partial discharge measurement system which allowed us to obtain phase-resolved PD pulse data. The experimental results revealed that the PD magnitude was strongly affected by the instantaneous applied voltage and that the occurrence of a PD was determined by the time derivative of the applied voltage (). The PD pulse-sequence analysis revealed the following: (i) a PD occurs in a discharge path which consists of a tree trunk and branches extending from the trunk; (ii) in each discharge path at most one PD occurs per half cycle. Based on these facts, a model of PDs due to electrical treeing was proposed. The influences of applied voltage and frequency were investigated by applying a triangular voltage. The number and average magnitude of PDs increased linearly with applied voltage whereas the PD charge per cycle increased quadratically. These results are in good agreement with the model.

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In order to investigate partial discharge(PD) degradation in composite insulating systems such as a prefabricated cable joint(PJ), we proposed an electrode system to simulate PD degradation at a solid-solid interface. It was observed that the tree at the EPR-epoxy resin interface tended to propagate into the epoxy resin bulk, even though the breakdown strength of epoxy resin was higher than that of EPR. To clarify the mechanisrri of this unsy inmetric degladarion at the solid-solid interface, we investigated effects of permittivity and mechanical stress on the tree propagation. When high-permittivity material1 was put behind the electrode, the tree tended to propagate to the high-permittivity-material side under voltages higher than the tree-inception voltage. This suggests that unsymmetry of the electric field or polarization charge has an effect on the tree propagation. At the interface of EPR and LDPE with about the same permittivities, the tree along the interface tended to propagate into the LDPE bulk as high voltage is applied. This suggests that factors other than permittivity also influence the tree propagation. When a high pressure was applied to the EPR-epoxy resin interface, the tree tended to propagate into the epoxy resin bulk. Mechanical properties and conditions also contribute to the unsymmetric tree propagation.