Analysis, Simulation, and Experiments of Dynamics and Control of a Hydrostatic Wind Turbine

Abstract

Kω^2 control, also called torque control, is a popular tool for maximizing wind turbine power in region 2. For hydrostatic wind turbines, the Kω^2 law relates pressure and rotor speed because pressure is proportional to torque. The Kω2 control law becomes pressure control with pc=K'ω2. A new control law, Inverse Kω2 control, is proposed for rotor speed control with ωc=(p/K')1/2. Both pressure- and rotor speed-regulation methods are investigated using P-, PI- and PID-control. This work analyzes the nonlinear dynamic interaction between HST wind turbines and the two Kω2 control methods.Dimensionless, linearized models of these two approaches are used to investigate dynamics and control. Analysis shows that the mechanical rotor dynamics are much slower than the hydraulic transmission dynamics and that frictional and leakage losses have a negligible effect on system dynamics. Root locus analysis shows how systems responses change with variation of PID controller gains. Both control approaches require derivative controller action to sufficiently dampen their responses; both are also fundamentally limited in their speed of response by a slow stable pole regardless of their controller loop gains. Nonlinear system simulation shows that both control approaches track the maximum power point with nearly identical transient behavior and have nearly identical power losses when using suboptimal values of the control law gain K. Experiments using the power regenerative hydrostatic test stand at the University of Minnesota – Twin Cities show that the control approaches have different transient responses but capture comparable power within 2% under steady, turbulent and nonideal conditions.