Resonance and Design Constraintsin Nacelle-to-Ground HydrostaticTransmission Conceptfor Wind Turbines

This paper presents design principles for the nacelle-to-ground hydrostatictransmission concept for wind turbines including the analysis of resonanceoriginating from the turbine tower. The design has the pump at the top insidethe nacelle and the hydraulic motor at ground level driving the generator thatis directly connected to the grid. It has no frequency converter. Resonancecan originate from the excitation of the natural frequency of the hydrostatictransmission due to the rotor tower shadow effect. The low natural frequencyis a consequence of the large rotor mass moment of inertia and high volumeof the pressure lines. Through a non-linear model validated by a prototype,it is shown that the nacelle-to-ground hydrostatic transmission concept cansupply electricity to the grid with a frequency error that adheres to regulatorylimits. Furthermore, using a simplified model, a procedure is proposed to determine the natural frequency and compare with excitation frequencies.It is shown that resonance does not occur in the region of power deliveryto the grid for a meaningful range of mid-size wind turbines. In the case ofturbines with a capacity of over 600 kW, the resonance may be present duringthe power delivery to the grid, which might impose a limit maximum powerto this concept. However, other design constraints, such as excessive loadloss, pressure surges and availability of hydraulic components, can reducethis maximum power limit. A prototype was built with only off-the-shelfcomponents and served, as shown in this paper, as a proof of concept.