Performance of coconut oil as an alternative transformer liquid insulation

Transformer mineral oil has been replaced by alternative oils such as synthetic oil and natural esters due to their biodegradability and environmentally friendly nature. This paper presents performance of coconut oil as such an alternative. Generally, as the other alternative oils, coconut oil has high conductivity due to the presence of free fatty acids. In this study, three different types of coconut oil samples consisting of virgin, copra and RBD (refined, bleached and deodorized) were initially tested by frequency dielectric spectroscopy (FDS) measurements to see how the conductivity was improved by dehydration and neutralization. The FDS results were fitted by inverse power dependence and Cole-Cole models to estimate the conductivity and response functions. Afterwards, a set of new coconut oil samples extracted from copra were thermally aged at 120°C under sealed conditions and compared with that of mineral oil. The performances of oil samples were evaluated using the test results of breakdown voltage, acidity, interfacial tension and FDS measurements under different aging periods such as 2, 5 and 7 weeks. Another 4 sets of new coconut and mineral oil samples were subjected to simulated thermal faults and electrical faults which include aging for 12 hours at 160°C, one hour at 200°C, exposing to partial discharges for four hours and subjecting to 20 low energy breakdowns. The performance comparisons were done by FDS measurements and dissolved gas analysis. In parallel, field-aged coconut oil samples collected from a sealed distribution transformer with 11 years of service were also tested by FDS measurements. In general, coconut oil shows its suitability as an alternative to the mineral oils for transformers, despite limitations found in some of their physical properties. It was found that the FDS results were in good agreement with chemical test results and with the estimated conductivity values.