To increase productivity of a wind power plant, the overall loss of its drivetrain should be minimized. For hydrostatic transmission wind turbine (HTSWT), aerodynamic efficiency of the rotor and the hydrostatic efficiency of pump and motor determine the overall loss. In this study, optimal control theory is utilized to develop a control law that minimizes the overall loss. A nonlinear model is considered for the drivetrain and a performance index (PI) is defined for the overall loss subject to system constraints which were then used to derive the optimal control law based on Pontryagin Minimum Principle (PMP),. Simulation results verified that the controller was able to maximize the drive-train efficiency. At very low wind speed, a trade-off is observed between aerodynamic and hydrostatic efficiency. For higher wind speed where the hydrostatic efficiency asymptotically reaches its maximum efficiency maximum output power coincided with maximum power point tracking of input wind power.

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