Liquid insulations are widely used in high voltage equipments such as transformers. Under high electric stress and the presence of impurities or defects streamer discharges may take place in the liquid insulation. This study reports the experimental results on the characteristics of Partial Discharges (PD) silicone oil under sinusoidal and triangular voltages and a proposal for equivalent circuit for explaining the characteristics. Electrode used in the experiment was steel needle-plane arrangement with needle tip radius of curvature of 3 μm. Discharges occurred in the sample were detected with an RC detector. The experimental results shown that the PD inception voltage in silicone oil increased with viscosity. Negative PD pulses appeared at applied voltage of slightly higher than the inception voltage. At higher voltage PD took place at both positive and negative half cycles where the negative PD pulse number was higher but with smaller magnitude. This shown that initiation of negative PD from needle tip was easier to happen compared to those of positive PD from the oil side. PD in silicone oil took place around the peak of the applied voltage under both sinusoidal and triangular applied voltage which indicated that PD occurrence in silicone oil was strongly dependent on the instantaneous applied voltage. The magnitude of the PD also reflected the waveform of the applied voltage. Based on the experimental results equivalent circuits for PD in silicone oil is proposed. In order to obtain more deeply understanding of the streamer discharges in insulating liquid, a computer simulation was done by utilizing the proposed equivalent circuit. The simulation results were compared to discharge patterns obtained from the experiment. The results indicated that simulated discharge patterns were strongly similar to those obtained from experiment. This shown the validity of the proposed equivalent circuit.
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