Robust digital control of a wind turbine for rated-speed and variable-power operation regime

The design of a double-loop robust digital control algorithm for a wind turbine (WT) containing a doubly-fed induction machine is presented. The operating zone considered is that obtained when the rotational speed is limited to its rated value and the generated electricity has not reached its corresponding rated value. It is not particularly easy to find descriptions of controllers for this working zone in the existing literature. The rotational sampling effect may produce drive-train torque oscillations when a WT operates at constant speed. Consequently, its control system, which consists of two levels of control loops, has to be able to address these oscillations and be able to minimise flicker emission and dynamical loads in the drive train. These specifications, along with the robustness of the synthesised controller, lead to a reduced cost for the generated electricity and to a higher power quality, compared to those obtained using more classical WT control strategies that do not account for the peculiarities of the considered operation zone. The inner control loops govern the machine electromagnetic torque. The outer control-loop is synthesised using the discrete pole-placement with sensitivity function shaping method, to keep the rotational speed at its rated value and to reduce electromagnetic torque oscillations. The designed and synthesised control algorithms are tested on a simulation model validated using field data. Simulation results show that the proposed regulator reduces both flicker emission and dynamical drive-train loads, when compared to a classical PI regulator tuned to show an identical cut-off frequency and integral action.