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Windtech: Hydraulic transmission - Potential variable speed breakthrough

High-pressure oil hydraulic transmissions have long been considered for wind turbine drive trains, but the part-load efficiencies of the available hydraulic pumps and motors have been too poor for any of the systems tested over the years to have moved into commercial production.

That could be about to change. Artemis Intelligent Power, based in Edinburgh, Scotland, believes it has solved the conundrum with its high-efficiency "digital displacement" technology. The new transmission will be lighter than systems to date, the company says, and offers high levels of controllability and electrical power quality. It will be suitable for retrofitting to existing machines and for use in the next generations of much larger turbines. Artemis says it is working with a number of turbine manufacturers and hopes to develop the concept so that it can be tested on a prototype machine.

A main advantage of hydraulic rather than mechanical transmissions for wind power is that, while the rotor operates at variable speed, the generator can run at constant speed. Once a generator operates at constant speed, a high-speed synchronous generator becomes the cheaper option compared with the asynchronous generators used in most wind turbines today. Synchronous generators are widely used in power generation, are inexpensive compared with asynchronous generators and can deliver good-quality power that meets the requirements of grid codes. Variable-speed operation maximises energy capture, as the tip-speed/wind-speed ratio can be kept near its optimum value for the prevailing wind strength. This ensures that the peak aerodynamic efficiency of the rotor is maintained, but requires a continuously variable transmission ratio if the electrical generator is to operate at constant speed. It is this facility that the Artemis transmission can provide, says the company.

In variable-speed wind turbines available today, the generator speed changes to match that of the rotor, favouring use of asynchronous generators. Investment in power conditioning equipment is subsequently necessary to produce grid quality electricity. Drive trains in most large modern wind turbines use mechanical gearboxes to couple the slow rotation of the blades to the high speed of the generator. Gearboxes, however, are heavy, making them a costly element, and prone to failure. In situ replacement of a failed gearbox is expensive. To avoid use of a gearbox, alternative direct-drive wind turbines use low-speed electrical generators, not as readily available as high speed generators as yet, that are directly coupled to the shaft of the wind rotor. While these machines eliminate the risk of gearbox failure, they are heavy and need full-power electronic frequency and voltage converters to condition their power for supply to the electrical network. This weight and complexity tends to push up their capital cost.

A continuously variable transmission ratio, however, allows the rotor to be operated at the best speed for optimal power capture and use of a synchronous generator driven at a much higher constant speed. A hydraulic transmission shares its load across many pumping modules and so avoids the few highly stressed line contacts that are inherent in gearboxes, says Artemis. The ability of hydraulics to limit pressure to a safe working value avoids potential over-stressing of the driveline, the company adds. Short-term storage in accumulators can smooth out wind turbulence, it says.

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