Crackle and Hum

Roger Brownlie looks at how ABB has designed a new ecologically friendly dry transformer that brings the power closer to you

Porcelain, paper and oil; electrical materials of an older era? Not yet. Visit any substation and the crackle and hum of century-old transformer technology might shock you. A failure of this old technology and the stray voltage will do more than make your hair stand on end. When stakes are this high, it is safe design that defines efficient or elegant design. But technology that simplifies the old transformer and reduces the risks is becoming available.

ABB’s common claim that its new Dryformer technology will `transform the world’ may have some truth to it. Transformers with a combined rating of more than 700000MVA have been shipped from ABB factories in Ludvika, Sweden, leaving ABB as the largest transformer producer in the world.

Keeping ahead of the competition, ABB’s innovation is a simple but radical departure from convention. Mats Leijon of the ABB research centre says, ‘the idea is to use dry polymer cables instead of paper insulated cables submerged in oil, a technology that is used in our high voltage generator, Powerformer.’

Research on the ‘dry transformer’ was started in 1996 by Albert Jaksts of ABB, a specialist in electrical insulation. The evolution of Dryformer has been a form of reverse engineering. It uses the principle materials as Powerformer but reversed to reduce voltage instead.

The cable is the key component. High voltage cables insulated with chemically cross-linked polyethylene (XLPE) are used to construct the windings. Conventional cables have an oil-cellulose insulation, bringing the surface close to ground potential.

Soren Petersson, a design engineer at ABB Transformers told Design Engineering: ‘We have no outer grounded shield of oil or paper. Usually the cable has a grounded shield on the outer circumference but that has been taken away in both Powerformer and Dryformer.’ Using the XLPE cable, the electric field is contained within the cable while its surface remains at ground potential. This leaves the magnetic field used for induction free from any interference.

Another difference is with the Dryformer’s core. The core conductor is cylindrical giving an even electric field distribution as derived from Maxwell’s equations.

‘The copper core is not homogenous copper.’ says Petersson. ‘It is composed of several strands, of about 2mm diameter, and isolated from each other with varnish. This brings down losses caused by a circulating current when several cables are wound and increases the magnetic flux. We can provide a very low reactive impedance in a transformer.’

This new found freedom from the electric field opens up possibilities for the internal design of transformers and the co-ordination of electric and magnetic fields in general.

‘We can mix turns from the low voltage, turns from the high voltage and turns from the regulating winds, and so on.’ says Petersson. ‘Winds can easily be mixed with one another and we still have no problems with dielectric field strength. In conventional windings we have to separate low voltage windings from high voltage windings by a rather big oil duct. But here we can mix them freely.’

The origin of all Dryformer advantages lies with the simplification of the electric field’s containment. The benefits are manifold.

An oil-free power transformer has clear advantages for the environment. There can be no spills and design parameters associated with the risk of leakage, such as fire regulations, are reduced.

There is less inflammable material in a Dryformer compared to a conventional transformer, so fire-fighting equipment can be reduced and personnel safety increased. It is even possible for a Dryformer to be installed indoors or in highly populated urban areas.

If an internal arc develops within the winding arrangement, a huge amount of energy is released into the fault. The pressure wave that occurs during a short circuit can rupture a tank in an oil-filled transformer. But a pressure wave is less severe through air. In Dryformer, the energy is absorbed both by the XLPE cable, which stretches, and the air surrounding the unit.

The overload capacity of conventional transformers depends on the finite lifetime of the oil impregnated paper surrounding the conductors and of the oil itself. The Dryformer lifetime is only limited by the temperature of the XLPE cable. As long as the windings are kept below 80 degrees C, the ageing of XLPE is negligible. A cooling fan enclosure forces air across the windings to ensure sub 80 degrees C conditions.

And so the only requirements for safe operation are thermometers in the windings and control of the cooling equipment. The traditional transformer’s oil pit, oil level gauge, gas alarm and oil thermometer are now unnecessary.

Vibrations in the core reverberate inside a conventional transformer, emitting that familiar but frightening buzz. But the Dryformer is surrounded by air which is acoustically less conductive than oil. The result – peace and quiet.

Keeping in mind the purpose of transformers – using high voltages to cut losses in electricity transmission – reducing risk at a substation allows a more flexible choice of its location.

‘As with all transformers, energy losses are minimised by optimising the transformers. One way to minimise losses is to site the transformers as close as possible to the point of energy consumption – to use the benefits of high voltage transmission for as long as possible,’ explains Lena Melzer, development engineer at ABB Transformers.

In the UK, high energy users in industry, such as paper mills and aluminium producers, can already negotiate their own electricity prices. If industries also own their own substation in situ, they could then tap the grid directly for even greater savings. As more private electricity companies appear on the electricity market this could be a real possibility.

Currently the most expensive part of the Dryformer is the XLPE cable supplied by ABB High Voltage Cables in Karlskrona, Sweden. Petersson is hoping that production improvements in Karlskrona and high volume orders for Dryformer and XLPE will lower cable costs.

Petersson warns: ‘I can tell you that this Dryformer will not be cheap. I heard our boss here in Ludvika, our deputy md Sten Jacobsson, saying to potential customers that you will have to pay for it. It has its advantages but yes, you will have to pay.’

Perhaps there are British cable engineers out there who won’t make us pay quite so much?

ABB Tel: +46 240 782 000