Multivariable Representation and Analysis of Controllers Used for Individual Pitch Control of Large Wind Energy Converters

Large wind energy systems use pitch control not only to regulate the power in the case of overrated wind speed but also for load alleviation. The reduction of blade loads is based on the so-called individual pitch control (IPC), which is a complicated configuration that includes either a Coleman or a Clarke transformation. As a result, the IPC provides good performance for its design objective, namely load reduction, but its dynamic properties are poorly known and difficult to study. The consequence is that sometimes the control system presents unexpected behaviors and difficulties in getting the controller tuned. The present contribution reformulates the IPC approaches as transfer matrices such that they are better understood from a systemic point of view, several unknown properties are derived, and some theoretic limitations are established. In particular, proportional-resonant controllers, which are normally hidden, play an important role in the dynamics. These outcomes can help to simplify the controller design as well as the parameter setting.