Hydrostatic Transmission for Wind Turbines: An Old Concept, New Dynamics Available to Purchase

Hydrostatic drives are commonly used in mobile machinery. A new application for this technology is the renewable energy sector, especially wind power. Despite using the same basic components the dynamics of these new drive systems are somewhat different compared to those used in mobile applications. In order to design an appropriate control system for a wind turbine it is necessary to understand these differences and how they affect the system. In this paper, the system behavior of a hydrostatic transmission for wind turbines is compared to commonly used hydrostatic drives in mobile machinery. The analysis begins by explaining that the characteristics of the loading acting on a turbine are fundamentally different to the load torque present in a standard application. Using mathematical models of both systems these differences are highlighted and discussed with special reference to how changes in system parameters can affect stability and lead to non-minimum phase behavior. These theoretical results are validated using measurements of a 1 MW hydrostatic transmission installed on a test bench.