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Oil-filled transformer explosions are caused by low impedance faults that result in arcing in transformer tanks once the oil loses its dielectric properties. Within milliseconds, oil is then vaporized and the generated gas is pressurized because the liquid inertia prevents its expansion. The pressure difference between the gas bubbles and the surrounding liquid oil generates pressure waves, which propagate and interact with the tank. Then, the pressure peak reflections are building up the static pressure, which rises and leads to the tank explosion since tanks are not designed to withstand the resulting static pressure. This typical transformer incident is common. Indeed, conventional transformer protections are unable to react fast enough to prevent tank explosion that usually result in very expensive blackouts and fire damages for electricity facilities. This paper describes a transformer explosion prevention technology based on the direct mechanical response of a Depressurization Set to the tank inner dynamic pressure induced by electrical faults. Since transformers always rupture at their weakest point because of the static pressure increase, the Depressurization Set is designed to be this weakest point in term of inertia to break before the tank explodes. To evaluate its efficiency, experiments and computer simulations have been performed. Two experimental test campaigns were carried out, first by Electricite´ de France in 2002 and second, by CEPEL, Brazil, in 2004 on large scale transformers equipped with that prevention technology. These tests consisted in creating low impedance faults in oil filled transformer tanks. The 62 tests confirmed that the arc first creates a huge volume of gas that is quickly pressurized, generating one high pressure peak that propagates in the oil and activates the transformer protection within milliseconds before static pressure increases, thus preventing the tank explosion. Beside the experiments, a compressible two-phase flow numerical simulation tool was developed. The theoretical bases of this tool are presented in a parallel paper [1] and it is used here to study the pressure increase in an unprotected tank when subjected to an internal arcing which properties are similar to those used during the experiments. The fast tank depressurization induced by the transformer protection and its protective effects are thus highlighted.

Power transformers rank highly among the most dangerous electrical equipments because of the large quantity of oil they contain in direct contact with high voltage elements. Low impedance faults resulting in arcing can appear in transformer tanks if the oil loses its dielectric properties. Vaporization of the oil generates pressurized gas because the liquid inertia prevents expansion. The pressure difference between the gas bubbles and the surrounding liquid oil generates dynamic pressure waves which propagate and interact with the sealed tank structure. Simultaneously, the static pressure inside of the tank climbs and causes the tank to explode, resulting in fires and very expensive damages for electricity facilities. Despite all these risks, and contrary to usual pressure vessels, no specific standard exists as of yet to protect sealed transformer tanks subjected to large dynamic overpressures. This paper describes a complete numerical model for transformer explosions, which helps to understand all processes involved in such dramatic events, and helps design and optimize an efficient explosion prevention technology. Such a model includes various physical phenomena from the electrical arc description to the evaluation of the stress loads the transformer tank must withstand. The simulation tool kernel is based on a reduced 5 equation model introduced by Kapila et al. [8] to describe the hydrodynamic behavior of compressible 2-phase flows. It consists of a set of conservation laws for each phase partial-mass, the mixture momentum and energy, and volume fraction evolution equation. The closure is isobaric, and both phases have the same velocity at a given point. Each phase is described by its own equation of state. Physical effects such as electromagnetic forces, viscosity, thermal and gravity effects are also taken into account. These equations are solved on complex 3D transformer geometries using a finite volume strategy with unstructured meshes. Computer simulations are then used to study a fast-direct-tank-depressurization-based method to prevent the transformer explosions. Numerical results compare well with experimental results collected during arcing tests in oil filled transformers.

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