Electrical performances of palm oil and rice bran oil under alternating current and impulse stresses for transformer application

Transformer oil does not only serve as an insulating liquid but also in removing heat from the windings and cores. Mineral oil (MO) has been widely used in transformers for more than 150 years. Recently, researchers have attempted to search for alternative insulating oils due to the possibility that MO will run out in future and the concern on fire safety and environmental pollution. Among the potential oils are palm oil (PO) and rice bran oil (RBO). These oils offer attractive characteristics such as non-toxic, highly biodegrade, and higher flash and fire points than mineral oil. To use these oils in transformers, the oil physicochemical properties and dielectric properties must be thoroughly investigated. This work presents the studies of the physicochemical properties such as the viscosity, flash point, pour point and moisture content of these vegetable oils. Besides that, this research also investigates the dielectric properties of the oils (permittivity, resistivity, tan delta, AC breakdown voltage and lightning impulse behaviour) under various electric fields and gap distances. The results revealed that the viscosity, flash and fire points of PO and RBO fulfilled the minimum requirement of the IEC standards. In terms of dielectric properties such as the resistivity of the oil samples, RBO has the highest resistivity with 6.372 Tfkm followed by PO with 5.341 T.Qcm and MO with 3.042 T.Qcm. Based on this resistivity study, RBO and PO have fulfilled one of the important criteria to be transformer insulation oil. However, the dielectric dissipation factor and relative permittivity ofRBO and PO were 93% higher than MO. Further assessment was carried out on the 50% AC breakdown voltage which shows that RBO has the highest breakdown voltage for all gap distances. This is followed by PO and MO. The electrical performances of lightning impulse tests show that RBO and PO have lower lightning impulse breakdown voltage than MO under both uniform and non-uniform electric fields . However, the difference in LI breakdown voltages between RBO, PO and MO is slightly small which is less than 20%. In addition, there is no significant effect in the various testing methods under both uniform field and non-uniform field where the percentages of difference are less than 12% and 8% respectively. The data of AC and lightning impulse breakdown voltage were statistically analysed using Weibull distribution to predict the withstand voltage (1 %) of the oil samples. From the results, the Weibull distributions of MO, PO and RBO have fitted well with the experiment data at higher probability. The withstand voltages of the oil samples obtained in this study were higher than the Weidmann curve (normally used in the transformer industry for transformer insulation design). Finally, the relationship between lightning impulse voltages under a non-uniform field with various parameters of PO and RBO was obtained and proposed. From this work, it can be concluded that PO and RBO show promising results to be considered as an alternative to MO in transformer applications.