Simulation study on oil pressure problems caused by internal faults in oil-immersed transformers

When internal faults occur in an oil-immersed transformer, the transformer oil inside the cabinet is subjected to thermal decomposition at high temperatures or arcing and produces a mixture of hydrocarbon gases with a risk of vapor explosion. Three-dimensional models of AC 220 kV, AC 500 kV, and DC ± 800 kV oil-immersed transformers were developed for numerical simulations. For these three transformers, the pressure distribution clouds inside the tank and the pressure variation curves at different positions discharge after the occurrence of a short-circuit arc fault between winding turns and at the connection between the tank body and the bushing. It was found that within 50 ms after the fault, the transformer oil vapor expanded rapidly and created a pressure difference at the oil-gas interface. As the oil vapor pressure continued to increase, the pressure inside the tank rose suddenly and transient pressure peaks of varying degrees (up to 0.65 MPa) appeared successively at different locations inside the tank. When the transformer tank pressure limit was exceeded, the tank cracked or exploded easily under uncontrolled conditions.