One to depend on

Designers of a new type of wind turbine generator hope to find applications with offshore wind farms where low maintenance and reliability is key.

Cambridge University spin-out Wind Technologies plans to install the first commercial brushless doubly-fed generator (BDFG) on a 20kW turbine on the university's West site by early next year. It will convert wind energy to electrical energy and help the university meet legislation that requires all new UK buildings to obtain 10 per cent of its electricity from renewable sources.

'The concept behind this generator is not new, but nobody has been able to design a commercial one before,' said Ehsan Abdi, Wind Technologies managing director. 'Understanding the performance of this generator is pretty tough.'

Abdi and a team of PhD researchers from Cambridge have spent the last eight years designing the generator. It works with the same efficiency as wind turbine generators, which are doubly fed induction generators that use the properties of electromagnetic induction to produce electrical voltage — a difference in electrical charge.

As a turbine turns, an exciter sends an electrical current to the generator's rotor. The rotor is a series of large electromagnets that spins inside a tightly-wound coil of copper wire called the stator. The magnetic field between the coil and the magnets creates an electric current (typically alternating current) driven out through power lines for distribution.

These generators rely on slip rings in their construction. One connects the rotor shaft to a frequency converter, while the other connects the stator to the power grid.

Abdi's generator is unique as it does not use slip rings. 'Our technology has an extra winding on the stator coil that enables the rotor to be brushless,' he said. 'Therefore, one winding is connected directly to the grid, and instead of having the rotor connected to the frequency converter by a slip ring, we connect the second stator winding to the converter to control the frequency.'

The lack of slip rings is the key to the low maintenance. 'Slip rings need to be maintained and decrease the reliability of the turbine,' said Abdi.

His research has shown BDFG performs correctly for twice as long as a slip-ring generator before failing, lowering the maintenance burden and keeping the turbine active for longer periods. The system is also cheaper to install, saving 10 to 15 per cent on the price of a conventional generator.

'The West Cambridge mid-size turbine should successfully demonstrate the reliability of the new generator and we hope it will encourage the developers of other new construction projects to consider local wind-powered electricity generation to meet their obligations,' said Abdi.

His team plans to test its generator on a larger 660kW system in Germany next year. The one-year test should demonstrate the applicability of WTL's BDFG technology to large-scale commercial turbines of the kind used on wind farms throughout the world.

'At the moment we have a 10kW laboratory prototype,' said Abdi. 'We haven't tried any commercial applications with offshore wind farms yet because nobody will allow us to put a 3MW or higher generator in their developments until we convince them it can handle large applications.

'This 600kW generator in Germany will be an intermediate step. Once we've done this we should be able to convince somebody to put it in an offshore or onshore in a large turbine 3MW or more.'