A power system with high voltage transmission is necessary to transmit a large amount of electrical energy from generating stations to consumers. 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 effects of temperature on the breakdown voltage and partial discharge patterns of natural oil. The oil used was BIOTEMP. The range temperature used was from room temperature to 60o. The breakdown voltage was measured using Liquid Dielectric Cell Test, LD 60 from Phenix. The bi-spherical electrodes with the separation of 2.5 mm was used in accordance with IEC 156 standard. The AC voltage was applied to the electrode filled with BIOTEMP at the rate of 2 kV/s. 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 in the experimental temperature range the breakdown voltage increased significantly with temperature. The relative water content may play an important role. Partial discharge measurement results indicated that negative PD appear early than those of positive PD. The inception of negative PD slightly lower than those of positive PD. The inception voltage decreased with temperature. 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.

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