Pressure Characteristics in the Nitrogen-Sealed Power Transformers under Internal Faults

The explosion-proof performance is an important index for oil-immersed transformers. The nitrogen-sealed transformer is a new type of transformer with nitrogen gas in the upper space, which can buffer against internal stress increase caused by arc faults. However, the pressure changes in the transformer under internal faults are unclear. The authors of this study propose a method based on finite element simulation to analyze the pressure changes and the stress on the tank. First, the calculation process of arc energy and the pressure of the bubbles caused by the arc are derived. Second, the dynamic pressure wave propagation model and acoustic-solid coupling model are established. Last, the finite element simulation model is built to analyze the pressure characteristics. Taking the winding turn-to-turn and phase-to-phase short circuit faults as the analysis situations, the pressure changes in the 110 kV/20 MVA nitrogen-sealed transformer are simulated. Due to the pressure wave refraction and reflection, the pressure changes show oscillatory characteristics with time after the occurrence of an internal short circuit fault. The pressure wave travels from the arc fault position to the periphery. Compared to the conventional transformer, the pressure changes with slower variations under an internal short circuit fault and the tank suffer less stress, which indicates that the nitrogen-sealed transformer is more effective in the explosion-proof performance.