Observer-based decentralised control of a wind turbine with a hydrostatic transmission

In this paper, a decentralised control approach for a 5 MW wind turbine with a hydrostatic transmission is presented that covers the whole range from low to very high wind speeds. In addition to a linear control of the pitch angle, a multi-variable gain-scheduled PI state feedback control based on LQR techniques is proposed for the angular velocities of both the rotor and the generator. The sixth-order simulation model comprises the complete drive train dynamics, the generator torque dynamics as well as the rotor aerodynamics and is derived from first principles. To reduce the implementation effort, the multi-variable control structure is based on a simplified statespace model with three states and two inputs. Moreover, a reduced-order observer estimates the rotor torque as well as an unknown leakage volume flow for a disturbance compensation. The control performance is illustrated by simulation results, which show an excellent tracking behaviour for the controlled variables.