Breakdown voltage, partial discharge and electric field of palm oil-based transformer liquids under AC stress

Mineral oil has been widely used in the liquid insulation of power transformers more than 150 years. However, it is poorly biodegradable and could cause serious contamination to the environment if a spill occurs. Recently, researchers have attempted to search for alternative insulating liquids due to the possibility that mineral oils will run out in future and the concern on the fire safety and environmental consideration. One of the potential alternative liquids is palm oil. This oil offers attractive factors such as non-toxicity, highly biodegradable, high flash and fire point which are better than mineral oil. To adopt these oils in a large power transformer, the oil's dielectric strength and electrical performances such as AC breakdown voltage and Partial Discharge (PD) must be thoroughly investigated. This work presents the studies on the AC breakdown voltage under contamination condition and water contents. The PD was also considered by testing the effects of extraneous factors to determine the extent which the oil can withstand. A simulation using the ANSYS software on the electric field distributions of the electrode systems was also performed due to time consuming and cost on doing testing. The simulation approach is done in this study to simulate the relationships between PDIV and PDIF. By testing the effect of particles and water contents, the result indicates that the AC breakdown voltage of palm oils is more than 30 kV, which is in good agreement with the IEC standard of the minimum breakdown voltage. Minor increase in the mean breakdown value observed upon the addition of 1.2 mL of distilled water to the palm oils indicates that these oils have the capability to absorb water 8-11 % from the oil. The result on PD test shows that palm oils have better insulating performances to mineral oil in terms of higher PDIV value up to 20 kV and lower number of PD repetition rate. Further assessment was carried out on the effect of thermally ageing oil with Kraft paper for palm oils brought better changes on the PDIV value with 4% improvement after 30 days of ageing. Finally, when comparing PDIV and PDIF from simulation, the PDIF of mineral oil at IO mm gap distance is 38% lower than palm oil. This indicates that mineral oil shows higher electric field stress compared to palm oil. From this works, it can be concluded that palm oil shows promising results to be considered as alternative to mineral oil in transformer application.