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

Abstract

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.

Investigation of the gas bubble dynamics induced by an electric arc in insulation oil

“…Furthermore, the calculation results of different gas generation theories are directly compared in figure 10. The orange solid-line curve shows the result obtained from the previous linear theory [1,14], which suggested a linear relationship of 500 ml kJ −1 between the generated gas volume and the arc energy at normal pressure and 2000 K. For the yellow solid-line curve, the gas volume was estimated by the formula V=0.44ln(W arc +5474.3)-3.8 according to the logarithmic theory from [6,7]. It is apparent that these two steady-state theories produce significant differences compared with the experimental data.…”

“…Considering the severe consequences mentioned above, since the 1970s, significant effort has been devoted to the investigation of prevention and mitigation methods that alleviate damage [2][3][4][5][6][7]. Meanwhile, studies have also been conducted to investigate the dynamic pressure rises and mechanical responses of oil-immersed transformers or other power equipment under internal arcing faults [8][9][10][11][12].…”

“…According to experiments [8,14,15], a constant gas generation rate of 85 ml kJ −1 was chosen at standard pressure and temperature. In contrast, some scholars claimed that the gas volume is a logarithmic function of the arc energy [6,7]. Owing to the constraints of the measurement methods and experimental conditions, previous studies did not describe the gas dynamic growth in the arc duration but instead obtained relations between the total volume of the generated gas and the total energy.…”

Study on High Energy Discharge Characteristics Caused by Arc Faults in Transformer Turret

“…Additionally, experiments have been conducted to advance the understanding of this field further. [13][14][15][16][17].…”

“…While relatively simple, safe and reproducible, this method does not generate gas as rapidly as a real arc. Between 2002 and 2004, 62 experiments on transformer internal faults were carried out in France and Brazil [13]. Although the primary objective of these experiments was to verify the performance of French SERGI's oil discharge and nitrogen injection equipment on transformer protection, they served as valuable reference cases for conducting arc fault tests by using real transformers.…”