Load reduction on a clipper liberty wind turbine with linear parameter‐varying individual blade pitch control

Abstract

The increasing size of modern wind turbines also increases the structural loads caused by effects such as turbulence or asymmetries in the inflowing wind field. Consequently, the use of advanced control algorithms for active load reduction has become a relevant part of current wind turbine control systems. In this paper, an individual blade pitch control law is designed using multivariable linear parameter-varying control techniques. It reduces the structural loads both on the rotating and non-rotating parts of the turbine. Classical individual blade pitch control strategies rely on single-control loops with low bandwidth. The proposed approach makes it possible to use a higher bandwidth since it accounts for coupling at higher frequencies. A controller is designed for the utility-scale 2.5 MW Liberty research turbine operated by the University of Minnesota. Stability and performance are verified using the high-fidelity nonlinear simulation and baseline controllers that were directly obtained from the manufacturer. pitch control algorithm should not change the thrust and the torque of the turbine in order to maintain the power output at its desired level.Sophisticated, model-based state space control approaches also became possible by transforming the whole periodic system into the non-rotating frame. 7 Multivariable control designs were investigated in Bossanyi 2 on the basis of a linear quadratic regulator and in Geyler and Caselitz 8 and Vali et al. 9 on the basis of H 1 -norm optimal control. Adaptive control techniques for individual blade pitch control are presented in Magar et al. 10 Periodic control for load reduction using individual blade-pitch control was developed and tested in Stol et al. 11 These controllers aimed at the once-per-revolution (1P) blade loads and were shown to yield similar results as the classical two-SISO-loops control strategy. A direct extension of this control strategy to target loads at higher frequencies is addressed in van Engelen 12 and Petrović et al. 13 additionally using complex, higher-order transformations of the sensor data. Successful field test results of such controllers for two-bladed and three-bladed turbines is presented in Bossanyi et al. 14 However, in this case, a linear analysis of the closed-loop model to verify robustness and performance is not straightforward anymore. Further, it also requires several design steps instead of a single one. This is also true for the feedforward filters added in each of the two SISO loops to cope with the 3P loads on the nacelle proposed in Bossanyi. 15 The feedforward filters are designed using a constrained numerical optimizations technique. Because of these reasons, the classical two-SISO-loops control strategy serves for comparison in this paper. The reduction of higher frequency loads was also achieved in Barlas et al. 16 and Unguran and Kühn 17 by employing additional trailing edge flaps and in Larsen et al. 18 by using additional flow measurement sensors. The disturbance accommodation techniques that are presented in W...

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