Investigation of an Internal Arcing Event in an On Load Tap Changer Using Fluid Structure Interaction

Abstract

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.