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Design breakthrough cuts CAPEX and OPEX

09 July 2012

New entrant to the wind turbine market, Wind Technologies has recently completed testing on a radically different type of generator – one that could result in substantial cuts to the cost of operation and maintenance of wind turbines.

Operating and maintenance costs are serious issues for the wind turbine industry, and not least for operators of offshore wind farms. Going some way towards ameliorating the current situation, Wind Technologies new brushless, doubly-fed induction generator concept, which not only increases the reliability of wind turbines, but also reduces the size requirement for the associated converter to one-third of the generator rating, has just undergone serious field trials.

This generator technology was originally developed more than ten years ago by teams from Cambridge and Durham Universities, and a 20kW machine was first tested in a Gazelle wind turbine in 2009; this was to be the world’s first brushless doubly-fed induction generator (brushless DFIG) fitted into a wind turbine. While this important milestone completely proved the theoretical models, it was just the first step in a process that will ultimately see a multi-MW brushless DFIG machine installed and running.

The team opted for intensive testing of a 250kW unit, built to its specifications by ATB Laurence Scott of Norwich. Wind Technologies designed a test rig with double power feed – mains power and connection via the controlling inverter drive – with a Control Techniques Unidrive providing grid connection.

Wind Technologies’ brushless DFIG has two stator windings of different pole numbers in a single frame (chosen so that there is no direct coupling between them) in combination with a special form of rotor that can couple both fields. This configuration offers a number of benefits.

The brushless design ensures a significant cut in build cost (CAPEX), as well as reductions in the level and frequency of maintenance (OPEX). The generator no longer requires brush carbon extraction equipment, further reducing manufacturing costs, and the incidence of carbon brush dust spreading into the turbine nacelle due to brush wear is completely eliminated. This clearly removes a major maintenance routine as far as the generator is concerned, an advantage that becomes even more significant for offshore wind turbines.

Because the brushless DFIG has doubly-fed operation, it only requires a fractionally rated converter, again saving overall costs. It also shows greater compatibility with grid codes than the conventional DFIG, thereby providing further CAPEX benefits. 

Wind Technologies’ brushless DFIG is, by nature, a 'medium-speed' generator, so best operation in a wind turbine is achieved when it is coupled to a one or two stage gearbox. This offers an important reliability improvement by eliminating the high-speed stage(s) of the gearbox and hence simplifying the mechanical system. Independent studies have shown that Wind Technologies’ drive-train is 10 percent more reliable than a conventional drive-train comprising a DFIG and three stage gearbox. 

The brushless DFIG has recently completed trials at the ATB Laurence Scott factory in Norwich. The grid-side connection is via a 90kW Control Techniques Unidrive SP ac drive, operating in regeneration mode; the configuration provides ride-through in the event of a mains fault of duration up to two seconds. A control algorithm immediately reduces the power from the generator to zero, eliminating the need for a ‘crowbar’, used for dumping excess power. The test rig has sensors to measure rotor current, flux and rotor temperature, unusually making use of Bluetooth wireless technology to transmit the data.

A multi-MW drive-train is now being designed by Wind Technologies’ engineering team and this is expected to be incorporated into a wind turbine for field testing next year. The drive-train will include the brushless DFIG and its associated frequency converter and control system, coupled to a two-stage gearbox. 

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