Prevention of Transformer Tank Explosion: Part 2 — Development and Application of a Numerical Simulation Tool

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.