To the Special Issue (Section) “Fault Diagnosis and Fault‐Tolerant Control of Wind Turbine Systems” (2018)

The aim of this special section is to illustrate the potential of fault diagnoses and fault-tolerant control (FTC) methods to improve wind power technologies, with a special consideration on reliability, availability, and safety. These topics have begun to stimulate research and development in the wide control community, particularly for these installations that need a high degree of tolerance with respect to possible fault. Note that this topic represents a key point mainly for offshore wind turbines since they are characterized by expensive and safety critical maintenance works. In this case, a clear conflict exists between ensuring a high degree of availability and reducing maintenance times, which affect the final energy cost. On the other hand, wind turbines have highly nonlinear dynamics with a stochastic and uncontrollable driving force as input in the form of wind speed and self-induced loads, thus representing an interesting challenge also from the modeling point of view. Fault-tolerant control methods can provide a sustainable optimization of the energy conversion efficiency over wider conditions than normally expected ones. This special issue is devoted to any kind of fault detection, isolation, and FTC methods applied to wind turbine systems. The goal of this issue is to provide a state-of-the-art picture of model-based as well as model-free methods that take into account realistic conditions such as, in the presence of unknown uncontrollable inputs, structured and unstructured model uncertainties. Two contributions in the special issue are concerned with an active FTC on the basis of estimation methods. The paper "Fault estimation of wind turbines using combined adaptive and parameter estimation schemes" by M. Witczak, D. Rotondo, V. Puig, F. Nejjari, and M. Pazera addresses the problem of fault estimation in wind turbines using a joint fault and state estimation scheme. The scheme assumes a set of possible faults affecting the dynamics of the wind turbine. Here, a combined adaptive and parameter estimation scheme is developed taking into consideration that process disturbances and sensor noises are unknown but bounded in an ellipsoid. The investigation of a proportional multi-integral observer compared with a sliding-mode observer scheme used in an FTC procedure is the aim of the paper "Fault estimation and fault-tolerant control of the FAST NREL 5-MW reference wind turbine using a proportional multi-integral observer" by P. Kühne, F. Pöschke, and H. Schulte. To achieve FTC in the presence of actuator and sensor faults, the control inputs and measurement signals are modified with the estimated fault signals. In both papers, the effectiveness of the solution is shown by simulations using a wind turbine benchmark that represents relevant fault scenarios. In the paper "Fault diagnosis and condition monitoring of wind turbines" by H. Niemann, N. K. Poulsen, M. Mirzaei, and L. C. Henriksen, a multiblade coordinate transformation is proposed to detect asymmetries in the rotor caused by changes or faults in the turbine. It is shown that changes or faults in the rotor system will result in unique signatures in the set of modulation signals, and based on these, it is possible to detect and isolate which blade is faulty or has been changed. The paper "Health-aware model predictive control of wind turbines using fatigue prognosis" by H. E. Sanchez, T. Escobet, V. Puig, and P. F. Odgaard introduces an active fatigue load minimization at the blade root by model predictive control methods. The authors propose a control-oriented model of the fatigue on the basis of the rainflow-counting algorithm to obtain online information of the blades' accumulated damage that can be integrated with a model predictive control. The scheme is implemented and validated in simulation by a high-order 24-degree-of-freedom model with the aeroelastic code FAST by the NREL. Finally, a study in the field of wind farm control is described in the manuscript "Application of FMRAC to fault-tolerant cooperative control of a wind farm with decreased power generation due to blade erosion/debris buildup" by H. Badihi, Y. Zhang, and P. Pillay. A fault-tolerant cooperative control scheme in a wind farm is proposed on the basis of a fuzzy model reference adaptive control approach. The effectiveness and performance of the scheme is demonstrated by a series of simulations on a large offshore wind farm benchmark model in the presence of wind turbulence, measurement noise, load variations, and realistic fault scenarios. In summary, the authors participating in this special section provide a great collection of papers, which highlighted the role and the potential of condition and health-monitoring, fault diagnosis, and FTC in wind energy. We are happy to provide the readers of the International Journal of Adaptive Control and Signal Processing with this special section.