Fault Diagnosis Methods for Vegetable Oil Transformers

Abstract

The vegetable oil, which has the advantages of high burning point, favorable biodegradability and low cost, is an ideal liquid insulating medium substitute for mineral oil. Due to the lack of operational data and production experience, the fault diagnosis methods of vegetable-oil-filled transformer are not perfect enough for large-scale applications and their use have been limited to distribution transformers. Firstly the classic Duval triangle 1, the Duval pentagon 1, Roger’s ratio, Dornenburg and IEC 60599 methods are compared. The direct application of mineral oil-based diagnose methods to vegetable oils would cause diagnostic error and therefore modifications are needed. Then, fault diagnosis methods for vegetable oil transformers are summarized. These methods include the modified method based on the mineral oil diagnosis method, such as the Duval triangle 3, Duval triangle 6 and Duval triangle 7. There are also new methods such as the Duval pentagon 2 and Duval pentagon 3. Finally, the electro-thermal joint faults experiment platform was built in this article. Simulation experiments of partial discharge combined with thermal faults in RAPO were carried out. Thermal faults were simulated under conditions of an overall temperature of $120 ^{\circ}\mathrm{C}$ and local hot stank temperatures of $200 ^{\circ}\mathrm{C}$, $500 ^{\circ}\mathrm{C}$, and $800 ^{\circ}\mathrm{C}$. Pre-experiments of partial discharge were carried out at $80 ^{\circ}\mathrm{C}$, and the voltage at which the stable partial discharge occurred was selected as the experimental voltage. The simulation conditions of electro-thermal fault in this paper are closer to the actual operation conditions of transformers than that of single fault simulations. The results show that $\mathbf{C}_{\mathbf{2}} \mathbf{H} _{\mathbf{2}}$ was generated in large quantities when local fault temperature was about $200 ^{\circ}\mathrm{C}$, which would not happen when the temperature was higher. This is in contrast to the individual thermal or electrical fault conditions. When the temperature of local thermal fault reached $800 ^{\circ}\mathrm{C}$, spark discharge occurs under partial discharge voltage. It indicates that if the electrical and thermal fault occur at the same time, and the starting conditions for severe faults will be reduced. The diagnosis methods of single fault are not suitable for the diagnosis of electro-thermal joint fault. More experiments of electro-thermal joint fault are supposed to be carried out, and new diagnose methods should be proposed.