Samuel Brodeur

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.

High-energy internal failures of transformers are catastrophic events which are hardly predictable. For this reason, a full-scale controlled experiment represents a valuable learning opportunity to gather accurate information about sequence of events during the very short time in which the failure occurs. Controlled parameters include tank design, material properties, experimental load and measurements. In this paper, we present a detailed investigation using nonlinear finite-element analysis of a 210-MVA transformer high-pressure experiment. We begin by evaluating the relationship between internal arcing pressure rise and tank expansion characteristics. Since this relationship is not linear due to geometric and material nonlinearities, an iterative process is proposed to ensure result accuracy. Stress–strain material properties are retrieved by tension experiments of specimens extracted from the tested tank to enable accurate comparison of numerical and experimental results. It is shown in this paper that nonlinear material parameters have a small influence on the tank pressure rise, but a significant one on large strain prediction and therefore the true stress-strain curve is recommended. In addition, the ductile rupture criterion based on the ultimate plastic strain of the material correlates with the experimental and explicit dynamic analysis results. This can ensure a certain design margin for tank rupture prevention.

This paper presents the design and testing of a full scale 330-kV, 210-MVA transformer tank to withstand the pressure effects of a 20 MJ, three-cycle low impedance fault in oil, and fail at its cover in a safe and controlled manner above that amount of energy. The transformer tank, equipped with a mockup active part, was designed for arc resistance using nonlinear static finite-element analysis and a specific rupture criterion. It was then tested using an experimental methodology based on inject-ing pressurized air into the water-filled tank. To adequately reproduce the effects of an arc in oil, the test parameters were determined using explicit dynamic simulations. The transformer tank performed as predicted by the finite-element and explicit dynamic simulations. The energy levels used in the tests here are, to our knowledge, the largest ever reported. This study demonstrates that it is feasible to design a power transformer to safely withstand high-energy low-impedance faults of short duration.