Gearless Transmissions for Large Wind Turbines: The history and Future of Hydraulic Drives.

Hydraulic transmissions are routinely used for large powers, converting high-speed rotation of electric and internalcombustion prime movers to low speed rotation for driving machines where large shocks must be handled. Examplesare rock crushers, sugar cane pulping mills and paper mills. If these drives were operated in the opposite sense, as isrequired by renewable power plant, which starts with a low-speed, irregular input and drives an electrical generator,they would provide a gearless solution for the wind turbine industry.The traditional ruggedness and reliability characteristics which make them attractive for large power conversion inother industries could be usefully transferred to wind. More challengingly the electrical characteristics around syn-chronous generation and the hydraulic buffering capability of a hydraulic transmission open new design possibilitiesfor the turbine industry. Conventional variable speed hydraulic drives already exhibit the ruggedness, weight and con-trollability required for large wind turbines, however hydraulic solutions generally have unacceptable efficiency at part-load; a result of the loss mechanisms internal to the pumps and motors. Artemis Intelligent Power Ltd has developedhydraulic machines, which use a different principle to off-load the unused capacity, in a manner which results in verylow parasitic loss. Because of the speed at which this can be done, these machines can be controlled with a highbandwidth to follow a demand signal with good linearity and low hysteresis. Large wind turbines, in the great majorityof cases, use mechanical speed-up gearboxes to couple the input rotor to a high-speed generator. Power electronicsare inserted between the generator and the line frequency in order that the drive can be operated at variable speed.Artemis will describe how applying their proprietary digital displacement technology to traditional hydraulics allows thedesign of an efficient, flexible, hydraulic transmission to replace the traditional gearbox, and much of the power elec-tronics requirement for large turbines.