An innovative combined hydraulic and gear-train power transmissions system for Mega-Watt scale wind turbines is proposed herein. The proposed concept targets large-scale wind turbines for an efficient and reliable conversion of the mechanical power of the rotating blades to electrical power. The novel hybrid system presented in this approach takes advantage of the benefits of both hydraulic and conventional gearbox systems, without introducing their potential inherent undesirable attributes at large scale. The proposed design first converts the mechanical power of the turbine blades to hydraulic power at a relatively high-pressure (about 2,500 psi) under a relatively low-speed (about 4 in/sec). The hydraulic fluid exiting the discharge port of the low-speed hydraulic pump is branched out into plurality of hydraulic lines for the purpose of dividing the total mechanical power of the wind turbine into multitude of lower hydraulic power lines. Each hydraulic line then delivers its hydraulic power into the corresponding intake port of a hydraulic motor having a low-speed-high-torque output shaft. The output shaft of each of the hydraulic motors then drives the input shaft of a mechanically matched gearbox to increase the rotary speed. Finally, the high-speed output shaft of each gearbox (about 1800 RPM) drives a corresponding matched electric generator. A preliminary design for a variable displacement vane pump has been proposed in this paper. This work includes a theoretical analysis of the overall efficiency of the system. The combined volumetric, mechanical, and overall efficiency of a typical proposed system was shown to be about 98%.

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