Measurement, modeling and computer simulation of partial discharges in natural liquid insulation for high voltage transformer

High voltage transformers are widely used in electric power systems. Oil insulation is important component in the transformers. The oil serves as electrical insulation and coolant in transformers. The integrity of the oil insulation is important factor in keeping the normal operation of the transformer. In particular condition an excessive electric field may occur and partial discharges may take place in the oil insulation which may degrade the performance of the insulation. The occurrence of the partial discharge (PD) is also an indication of insulation abnormality. There are 3 kinds of oil insulations. They are mineral, synthetic and natural oils. Due to environmentally friendly, the last oil is being popular. This paper reports the measurement, modeling and simulation of partial discharges in natural oil. The oil used was BIOTEMP. The partial discharges were generated using needle-plane electrode system under sinusoidal voltage. Partial discharge measurement was conducted using phase-resolved measurement system which able to measure the PD magnitude (q), phase of PD occurrence (ϕ)as well as PD number (n). The analysis of the PD data was conducted by utilizing ϕ-q-n and ϕ-n pattern. The experimental results showed that PD took place at around the peak of the applied voltage. Phase-resolved analysis indicated that PD magnitude as well as PD occurrence were strongly dependent on the instantaneous applied voltage. Based on the experimental results, a PD model in natural oil was proposed based on the Whitehead PD equivalent circuit. The model was used to simulate PD in the oil. The simulation was able to generate ϕ-q-n and ϕ-n patterns similar to those obtained from the measurement.