Design and testing of an arc resistant power transformer with a reinforced turret

This paper presents the design and testing of a full-scale 510-kV, 415-MVA HVDC converter transformer prototype designed to withstand the resulting pressure of a 20 MJ low-impedance fault in oil occurring inside a bushing turret, which is one of the major causes of transformer fires. In addition, there is currently few studies and recommendation regarding this failure mode, which is the main motivation for initiating this study. In total, two full-scale units were designed and built: First consisting of a reinforced turret and a transformer tank designed for arc resistance using nonlinear static finite-element analysis with design pressures retrieved from explicit dynamic simulations, and a second reference unit, with a conventional turret design. Both units were then experimentally tested with a methodology based on injecting pressurized air into the water-filled units, with test equipment dimensioned and calibrated to achieve a linear injection of energy equivalent to a 23 MJ arc in oil in 100 ms. The reinforced turret and transformer tank was able to contain the arc energy and performed as predicted by the nonlinear static and explicit dynamic simulations. The energy levels used in tests are, to our knowledge, the highest reported for a low-impedance fault occurring inside a turret, demonstrating the feasibility of designing a transformer to safely withstand mentioned faults without rupturing. The suggested reinforced turret is a robust and maintenance free solution offering passive protection against internal arcing, without the need of additional relive devices. It will significantly reduce the risk of fires, improve overall safety of the substation, and has the potential to be optimized for different arc energy levels and voltage class for turrets, cable boxes and chimneys.