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.

Partial discharge (PD) in a void reflects degradation of insulating material and, therefore, is utilized for insulation diagnosis. Although it is well known that PD pattern changes with voltage application, it is not clear how the pattern change is related to material degradation or how it is related to final breakdown. We simultaneously observed PD characteristics and light emission from partial discharge and the results were compared with those of surface observation. Both in an open void where atmospheric condition does not change and in a closed void, PD activity become concentrated in a small spot after certain time of voltage application. This seems to be caused by field concentration due to deposit of degradation products. In an open void, the intensity of each PD pulse is strong but the time to final breakdown is long. This is explained by the fact that the PD spot does not remain in the same place but constantly moves around, leading to smaller local energy input. On the other hand in a closed void, though the intensity of each PD pulse is small, the time before final breakdown is relatively short. This is explained by the experimental result that the PD spot in this case remains in the same place and the local energy input seems large. Finally, by terminating voltage application just before the final breakdown it is confirmed that the breakdown starts at the spot of PD concentration where the deposit exists. As mentioned above, the PD characteristics, void surface degradation and final breakdown are related to each other and this relation is expected to be useful for better insulation diagnosis.

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We have developed a partial discharge measurement and analysis system using a personal computer, which allows one to obtain reproduced partial discharge pulse data with the phase information in an applied alternating current voltage and the number of cycles. For the CIGRE method II electrode system using 0.1 mm thick polyethylene film, temporal changes of partial discharge characteristics were measured up to breakdown of the sample. The average inception phase theta inc was proposed as a new statistical partial discharge parameter, which was obtained by making use of the capability of the time-sequential partial discharge data acquisition of our system; theta inc is defined as the average of theta inc of the number of cycles in a measurement; theta inc is a phase at which the first partial discharge pulse occurs in the half cycle after the polarity reversal of the previous partial discharge pulses. The experimental results revealed that the partial discharge pulse occurrence phase distribution ( phi -q-n distribution) exhibited a good correlation with ageing time. It was suggested that theta inc is a candidate signal for diagnosis of insulation performance of the system from the initial to middle degradation stage. We also discuss the physics of the temporal change of the phi -q-n distribution and theta inc as the ageing process proceeded.