Resonance and Design Constraints in Nacelle-to-Ground Hydrostatic Transmission Concept for Wind Turbines

Abstract

This paper presents design principles for the nacelle-to-ground hydrostatic transmission concept for wind turbines including the analysis of resonance originating from the turbine tower. The design has the pump at the top inside the nacelle and the hydraulic motor at ground level driving the generator that is directly connected to the grid. It has no frequency converter. Resonance can originate from the excitation of the natural frequency of the hydrostatic transmission due to the rotor tower shadow effect. The low natural frequency is a consequence of the large rotor mass moment of inertia and high volume of the pressure lines. Through a non-linear model validated by a prototype, it is shown that the nacelle-to-ground hydrostatic transmission concept can supply electricity to the grid with a frequency error that adheres to regulatory limits. 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 delivery to the grid for a meaningful range of mid-size wind turbines. In the case of turbines with a capacity of over 600 kW, the resonance may be present during the power delivery to the grid, which might impose a limit maximum power to this concept. However, other design constraints, such as excessive load loss, pressure surges and availability of hydraulic components, can reduce this maximum power limit. A prototype was built with only off-the-shelf components and served, as shown in this paper, as a proof of concept.