Design and simulation of a continuously variable hydraulic power-split drivetrain for wind turbines

Abstract. This paper presents the development and analysis of a novel drivetrain concept for large-scale wind turbines. The proposed concept builds on a predominantly mechanical transmission, with a small fraction of power transferred by a hydraulic system. The hydraulic power path, an adjustable hydrostatic transmission with a pump and a motor, enables full rotational speed variability while transmitting only a limited portion of the total power. As a result, a higher overall efficiency can be achieved compared to fully hydrostatic drivetrains. Furthermore, it is possible to couple the generator directly to the grid, thus omitting the frequency converter which contributes to system complexity, losses, and failure susceptibility. A simulation model incorporating component-level loss representations is developed to evaluate different transmission layouts and design variants. It is investigated whether a hydromechanical power-split drivetrain can achieve efficiency and energy yield levels comparable to conventional geared drivetrains while referring to the characteristics of currently available hydraulic components. The results show that the drivetrain efficiency strongly depends on the transmission layout and the site-specific wind conditions. While lower efficiencies are observed for sites with low mean annual wind speeds, specific design configurations achieve high efficiencies and energy yields comparable to those of a conventional geared reference drivetrain at sites with higher mean annual wind speeds. Overall, this paper extends previous research by minimizing the rated power of the hydraulic power path, estimating efficiencies for different drivetrain configurations, and looking into a mechanical design.