Loop shaping based robust control for floating offshore wind turbines

Abstract

In this work, a thorough and complete methodology for the widely used SISO controller is described for floating offshore wind turbines (FOWTs). The motivation is to develop clear, easy implementable and automated design criteria of blade pitch control design, which takes both stability and performance into account for FOWTs without adding new sensors. The primary design criteria is to achieve a similar dynamic step response behaviour, i.e. overshooting, rise time and settling time across the operating points above rated wind speeds. The proposed design procedure can be performed by lower order numerical models with only two degrees of freedom, which can be derived analytically. The minimal required system information eases an early stage controller design, as well as the system engineering and integrated substructure design. The proposed design procedure is evaluated on three state of the art floating wind turbines. The resulting gain scheduling is quite different from the one for onshore turbines. The overall response is satisfying and comparable with an existing stability-oriented robust SISO controller at operation points where stability is critical. An improved performance is found for higher wind speeds.