Oil filled transformer explosions and their prevention is a complex industrial issue. Experimental tests showed that when an electrical fault occurs in a transformer, it generates dynamic pressure waves that propagate in the oil. Reflections of these waves on the walls build up high static pressure which transformer tanks cannot withstand. The tank’s ability to withstand this pressure is thus one of the key parameters of transformer explosion prevention, and a numerical tool was developed to simulate the phenomena highlighted during the tests, especially the pressure wave propagation. The present paper’s aim is thus to complete this numerical tool so that the mechanical behavior of the tank can be accurately studied. The hydrodynamic numerical tool was subsequently coupled with a dynamic structure analysis package: the open source software Code_ASTER. A weak coupling strategy was first developed by applying the simulated pressures to the structure geometry in order to evaluate stresses and deformations. This strategy has evolved with the development of a strong coupling strategy which required establishing a moving mesh technique for the hydrodynamic code to accept displacement data from the structure code and complete the exchange between hydrodynamic and structure codes. First encouraging results are shown.
Electrical arcing can occur within the oil-filled tanks of transformers and On Load Tap Changers (OLTCs) when the oil dielectric properties are reduced. The electrical arc then, within milliseconds, vaporizes a portion of the oil, creating a highly pressurized gas bubble. The rapid pressure rise inside the generated bubble creates a dynamic pressure peak which then propagates and interacts with the tank structure. The wave reflections on the walls generate secondary pressure waves that raise the static pressure inside the tank. This static pressure increase leads to tank rupture since tanks are not designed as pressure vessels and therefore cannot withstand such levels of static pressure. This results in explosions, fire, expensive damages and possible environmental pollution. Despite all these risks, conventional protections are not effective to avoid explosion and contrarily to usual pressure vessels, no specific standard has been set to protect sealed tanks subjected to large dynamic overpressures. In order to study transformer and OLTC explosions and their prevention, experiments were performed on large scale transformers. Nevertheless, live tests require specific laboratory equipment and they are thus expensive and can be dangerous. In order to limit the costs, to reduce risks and to get a deep insight on the physical phenomena, numerical simulation tools are necessary. The development of such tools was presented at previous ASME Conferences. The present paper’s goal is to illustrate the use of this tool by an investigation of an arcing event in an OLTC. This arcing was simulated in the OLTC in both the unprotected case and also when the OLTC has been protected using a fast depressurization method. The stress and strain results are then used to determine how effectively the depressurization relieved pressures in the tank.
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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