Electricity markets are very competitive and in order to limit costs, companies often reduce their investments by using aging equipment and by overloading their transformers. For these reasons, oil-filled transformer explosions are becoming more and more frequent. They are caused by electrical arcs occurring in transformer tanks. Within milliseconds, arcs vaporize the surrounding oil and the generated gas is pressurized because the liquid inertia prevents its expansion. The pressure difference between the gas bubble and the surrounding liquid oil generates a dynamic pressure peak, which propagates and interacts with the tank. Then, the reflections generate pressure waves that build up the static pressure, leading to tank rupture since tanks are not designed to withstand such levels of static pressure. This results in dangerous explosions, expensive damages and possible environmental pollution. Despite all these risks, and contrarily to usual pressure vessels, no specific standard has been set to protect sealed transformer tanks subjected to large dynamic overpressures. To limit the consequences of an explosion, protective walls surrounding transformers can contain the explosion while sprinklers may extinguish the induced fire. In order to extend this chain of protections to the transformer itself, a strategy to avoid transformer tank rupture was developed and presented at the previous PVP08 Conference (PVP2008-61526 - Prevention of Transformer Tank Explosion: Part 1). The concept of this strategy is based on the direct mechanical response of a depressurization set to the inner dynamic pressure induced by electrical faults. In the same paper, the efficiency of this depressurization strategy was experimentally shown: if the oil evacuation through the depressurization set is activated within milliseconds by the first dynamic pressure peak before static pressure increases, the explosion can be prevented. The use of these protections eliminates the need to design transformer tanks as pressure vessels, which by application of the ASME standard would require a significant increase of the the shell thickness. Complementarily, a compressible two-phase flow numerical simulation tool based on a 3D finite volume method was developed to study transformer explosions and possible strategies for their prevention. Its theoretical bases were detailed in the PVP08 ASME Conference (PVP2008-61453 - Prevention of Transformer Tank Explosion: Part 2). The current paper shows the applications of this simulation software as a decision making tool, especially toward improving the design of real mechanical transformer protections. Some guidelines to optimize the efficiency of transformer protections are suggested thus contributing to a possible standard setting.

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