Scheduling of H∞ controllers in horizontal axis wind turbines

Safe pitch angle-based pole-zero-gain scheduling, and controller blending of H-infinity, controllers for a variable-speed variable-pitch wind turbine in the full load region are introduced in this paper. The design methodology ensures proper model reduction and modification, canonical controller realization, and cancellation of the hidden coupling terms that emerge from these scheduling procedures to maintain the stability of the closed-loop dynamics during wind turbine operation. In contrast to previous multi-model designs that either involve scheduling of complex transfer functions or depend on less-reliable wind speed estimations, in the presented framework only a portion of the controller is scheduled against the pitch angle. This approach markedly facilitates the implementation of the technique for typical H-infinity, synthesis controllers which are of high order. To provide further insight into the stability properties of the proposed control system, sufficient conditions for the stability of the closed-loop system are derived using the frozen-input theory. Moreover, in order to obtain a reliable uncertainty estimation, a time-marching simulation is utilized which requires only some basic characteristics of the wind turbine. Finally, explicit requirements for the pole-zero-gain scheduling and controller blending are derived and their performance is investigated using an HIL setup that connects validated FAST simulation codes to the actual control PLC of a 2MW industrial wind turbine.