Wind Technologies Ltd, the result of a collaboration between Dr Richard McMahon at Cambridge University and Prof Peter Tavner at Durham University has developed the Brushless Doubly-Fed Generator (BDFG), a new maintenance-free generator which doubles the lifetime of generators in wind installations and significantly reduce maintenance costs and increase reliability.
According to this article, the BDFG system employs two 3-phase stator windings of different pole in a single frame in combination with a special form of ‘brushless’ rotor, which along with eliminating the need for brush replacement, also doubles the life time of the generator from 90,000 hours to 180,000 hours before failure, according to the managing director of Wind Technologies, Dr Ehsan Abdi.
Typically one stator winding is connected to the mains or grid, and hence has a fixed frequency, and the other is supplied with variable voltage at variable frequency from a converter.
In the majority (more than 90%) of newly-installed wind turbines in the world, generation is from a doubly-fed slip-ring induction generator (DFIG). There are drawbacks to the use of slip-ring generators, particularly the additional cost and bulk of a machine which incorporates slip-rings and the need to maintain brush-gears including replacement of the brushes on a regular basis. Studies have shown that problems with brush-gear are a significant issue in wind turbine operation and reliability, and that the problem will be more severe in machines deployed offshore where there are stronger winds and accessibility is impaired.
The University of Cambridge announced that the world's first commercial BDFG is to be installed on a 20kW turbine at or close to the Department's Electrical Engineering Division Building on the West Cambridge site by early 2008.
On a larger scale, a 600kW generator is to be tested on a DeWind D4 turbine in Germany, starting next spring. Its planned one-year test should demonstrate the improved performance of the BDFG technology to the key players in this industry.
"This will put Wind Technologies in a position of strength in pursuant discussions on technology trade sale, licensing or partnering with large generator manufacturers, which is the strategy of choice for tackling this concentrated marketplace", said Dr Ehsan Abdi.
“For a 3MW system, we expect the cost to replace the brush gear to be around US$3,000(£1,500) per change, which is done once or twice a year. So when you multiply that by the 20-year lifespan, it comes out at around US$60,000 (£30k) savings per unit. With the brushless system, this cost is completely removed.”
Furthermore, the production cost of the generator system incorporating a BDFG is expected to be 10-15% less than that of the DFIG.
This could seem to many a small amount of money to be saved, but wind power technology is becoming mature and improvements come in small increments. The reliability and lifetime are probably of more importance, especially on offshore turbines.