Prevention of Transformer Tank Explosion: Part 1 — Experimental Tests on Large Transformers

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.