C. S. Kalpage

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Transformer mineral oil has been replaced by alternative oils such as synthetic oil and natural esters due to their biodegradability and environmentally friendly nature.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.

Laboratory investigation of treated coconut oil as transformer liquid insulation

This paper presents laboratory investigation of treated coconut oil as alternative to liquid insulation of power transformers. Thirteen different virgin and treated coconut oil samples consisting three different brands were used for the investigation. Frequency Dielectric Spectroscopy measurements were conducted on the samples at room temperature and at high temperature of 65°C. The conductivity, loss tangent, permittivity and the activation energy were calculated and used for the analysis. It was found that the conductivity improved slightly with heat treatment and neutralization process. However, during one week ageing, the conductivity improved significantly. It suggested that with proper treatment, coconut oil can be used as alternative transformer liquid insulation.

Coconut oil as transformer liquid insulation — Ageing and simulated thermal and electrical faults

This paper presents performance comparison of coconut oil as an alternative liquid insulation for transformer mineral oil during ageing and under simulated electrical and thermal fault conditions. In the ageing study, 6 wet coconut oil, 6 wet and 6 dry transformer oil samples were aged (each half as sealed and unsealed with inclusions of Cu, Al, Fe and Zn metal substances) inside an oven at 120°C. Six sample sets were taken out at three different ageing intervals i.e. 2, 5 and 7 weeks and were tested by visual inspection (color), chemical tests (interfacial tension, acidity, DGA) and electrical tests (breakdown voltage, FDS). In the electrical fault study, 2 coconut oil and 2 transformer oil samples were exposed to corona for four hours and for 20 electrical breakdowns with a point-plane gap of 15mm. In the thermal fault study, 2 coconut oil and 2 transformer oil samples were aged at 160°C for 12 hours and at 200°C for 1 hour. The DGA and FDS measurements were taken on the aged samples. FDS Results were analyzed by using Davidson-Cole Model. It was found that the coconut oil samples gave comparable results with those for transformer oil.