Vegetable oils due to their unique properties of availability and biodegradability can be considered as a reliable substitute for petroleum-based oils in transformer cooling system. Therefore, the performance of waste cooking vegetable oil after transesterification process for application in electrical transformers has been evaluated. The important properties of this oil such as density, heat capacity, viscosity, thermal conductivity, breakdown voltage and flash point have been measured and compared with those of mineral oil. Furthermore, the thermal behavior of the cooling system of an actual transformer was numerically simulated using the measured properties. Based on these measured data, the breakdown voltage and thermal conductivity of the vegetable oil as the two main indexes of transformer oil are, respectively, 48% and 33%, higher than those of the mineral oil, while its viscosity is considerably lower. Furthermore, the transformer hotspot temperature with vegetable oil is 3 °C lower than the mineral oil and the transformer experiences considerably lower temperature in the thermally critical region. Present study proposed the waste cooking vegetable oil not only as an alternative cooling medium for liquid-filled transformers but also as a sustainable way of using the waste resources, decreasing the environmental pollution and promoting environmental and economic benefits.

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