Preventing transformer explosions

"Transformer failures have cost human lives when things have gone seriously wrong," says Håkon Nordhagen, a materials specialist at SINTEF in Trondheim, Norway, the largest independent research organisation in Scandinavia.

Large oil filled transformers are found in all power and switching stations, as well as in many large buildings. If an internal short-circuit occurs, an electrical arc, gas formation and pressure increase will be the result.

According to Nordhagen, the likelihood that Norway could experience major explosions caused by short-circuits of this sort is low. But precisely because lives can be lost and property damaged if they do occur, he and his colleagues now want the electricity supply sector to adopt safety measures used by vehicle manufacturers.

"Today, transformers are installed in stiff steel tanks. The risk of explosions can be reduced if the industry changes to "soft" housings that absorb energy in the same way as modern car bodies," he says.

Pilot project underway

The idea of using automotive industry technology in transformer housings has led to a pilot project in which SINTEF's materials and electricity experts are collaborating.

Almost four years ago, the initiators of the project began to seek solutions to the problem. Now they want to acquire more fundamental knowledge within this area, and in the pilot project, they have been joined by four Norwegian electricity generators.

Oil mist expands

All large transformers use oil for insulation and cooling, as this greatly reduces energy losses and improves reliability compared to dry type transformers. The increasing focus on energy efficiency actions might also enforce a substitution of many smaller dry isolated transformers with oil filled transformers.

Statistics suggest that internal failure will occur on average in about 15 of Norway's 3000 transformers every year. The arc which occurs in such situations makes the oil evaporate and form combustible gasses. In the wake of a large internal fault, the gas expands violently, causing a powerful and extremely rapid pressure build-up in the transformer tank.

If the tank ruptures, combustible gases will leak out. If these are ignited, an explosion will follow, and any combustible materials in the vicinity of the transformer can catch fire. The fire may also be fed with oil from the tank. If so, it will keep burning.

Stiffness has side-effects

Two different measures are usually adopted to prevent fire and explosion incidents of this sort:

• Transformers are equipped with relay protection that disconnect them from the grid within a few power cycles when internal short-circuits occur, thus rapidly extinguishing the arc.

• Additionally, transformer tanks are reinforced by means of welded steel beams, in order to make them withstand pressure rises, and allowing for vacuum filling of oil.

High internal pressure

"However, the stiffness that is usually incorporated has an unfortunate side-effect," Nordhagen points out.

On one hand, the transformer tanks are able to withstand more pressure than they could have done without the reinforcement, on the other, the extra stiffness means that they cannot expand when the electric arc is burning, causing the pressure in the oil and gas to rise dramatically.

High internal pressure is undesirable for many reasons, says the SINTEF scientist:

"Combustible gases and oil can leak out of gaskets and penetrators, which may ignite fires or explosions. Moreover, stresses occur within the material of the tank, increasing the probability that it will rupture. And if this happens, bolts, hatch-covers and similar components may be slung out with great force, depending on how high the internal pressure is."

This is why Nordhagen and his colleagues are so keen on developing an alternative "bodywork" solution.

Energy-absorbing tanks

"Car bodywork is designed with soft zones that absorb energy from collisions by crumpling. Our idea is to design transformer tanks in such a way that they expand when their internal pressure increases, without risking that major rises in tension and weak points in their material occur as a result of the expansion. If such an expansion is allowed, there will be more time available for the transformer to be disconnected from the grid before rupture occurs. It is perfectly possible to achieve this, while at the same time ensuring that the housing is stiff enough to withstand transport stresses and the vacuum treatment.

The aim of the recently launched transformer safety pilot project is to pave the way for a large-scale competence-building main project.

"With better knowledge and new mathematical models, we will be able to observe the relationships between the physics of electric arcs, the transformer tank and the electrical protection system. This will offer us new prospects of modelling safe and reliable solutions," says Håkon Nordhagen.