Characterising Double Cavity Partial Discharges

This paper presents an investigation into characteristics of double cavity partial discharges. High Density Polyethylene (HDPE) is used as the insulation in the test specimen. Firstly, an electric field simulation using FEMM was used to observe the influence of the gap distance between the two cavities. This was to observe the electric field magnitude in the cavities for the same applied voltage across the parallel plate electrodes. It was observed that the electric field within the cavities is inversely proportional to the gap size between the cavities. For identically sized 1 mm diameter cavities, the electric field magnitudes within cavities influenced each other for gap length less than 3.5 mm. Beyond 3.5 mm, the crossinterference of the electric fields in the two cavities became negligible. Based on the simulation results, a gap length of 1 mm was used in the physical experiments. The test samples were prepared by placing a defected HDPE disc between two healthy discs. The resultant sample is compressed together using plane brass electrodes. An IEC60270 standard PD measurement circuit was used for the measurement of PD. The results show that the double cavity PD magnitude is greater than the single cavity by an average of 15%. There are more discharges per cycle for double cavities compared to single cavity. The results are corroborated through simulations using the ABC PD model in MATLAB Simulink™. These findings can be useful in informing condition monitoring and factory acceptance tests for high voltage insulation.