The vegetable oil transformer has excellent fire resistance, environmentally friendly performances and electrical properties, which is the development direction of oil-filled transformer in the future. At present, the fault diagnosis methods are limited to pure electrical fault or thermal fault, also the diagnosis of combined electro-thermal fault on RAPO vegetable oil needs to be developed. In this paper, an electrothermal fault experiment platform was established. The gas generation

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... stics of RAPO vegetable oil under pure electrical faults, pure thermal faults and combined electric-thermal faults were obtained. Results show that the characteristic gases under pure electrical fault are methane and acetylene, while under pure thermal fault are ethane. Ethane is the characteristic gas of electro-thermal faults except combined high energy breakdown and low temperature thermal fault, of which the characteristic gas is acetylene. Carbon dioxide quickly reached the saturation solubility at the corresponding temperature in the early stage of the above faults, and its content remained stable until the end of the experiment. Therefore, carbon dioxide could not be used as the characteristic gas to diagnose faults. Under different electro-thermal fault conditions, the gas production trend and gas production quantity of hydrogen and carbon monoxide are generally similar, but still different to some extent. Then, the fault diagnosis method of RAPO vegetable oil was established based on the characteristic gas and the ratio criterion. The above study can provide reference for establishing the fault diagnosis method of RAPO vegetable oil and promoting the practical application of vegetable oil transformer. INDEX TERMS Characteristic gas, electro-thermal fault, fault diagnosis, ratio criterion, vegetable oil transformer.