Platform position control of floating wind turbines using aerodynamic force
Introduction
Wind is a renewable, clean and readily available energy source which has become increasingly popular in the recent years. In the wind energy industry, a fast growing trend lately is to position the wind turbine systems offshore, where the wind condition is stronger and steadier while vast open space is available for wind farm construction [1]. The average offshore wind speed is estimated to be 90% greater than the average onshore [2], giving offshore wind farms the theoretical advantage for more power output. Furthermore, offshore wind fields have less spatial variations in speed due to the flat ocean surface, which reduces excitation of structural fatigue loading as the turbine blades travel through the more uniform wind field in circle.
To take advantage of the favorable offshore wind resources, wind turbines have to operate far away from the coastline at deep-water locations. While offshore wind turbines in shallow-water can be attached to the seabed underneath like the onshore ones, deep-water sites require floating platforms to host the wind turbines since fixed bottom structures are neither feasible nor economical for water depth larger than 50 m [3].
The floating platforms add extra degrees of freedom (DOFs) to the wind turbines by allowing the platform to translate and rotate under the excitation of wind and wave, which brings both challenges and opportunities in control. Control challenges include increased complexity in modelling [4], platform vibration suppression and resonance avoidance [5], as well as wave disturbance rejection [6]. On the other hand, the position DOFs of a floating platform provide the opportunity for real-time position control of the wind turbine, which is impossible for onshore or fixed-foundation offshore wind turbines. The benefit of real-time wind turbine position control is clear when a wind farm is considered.
When designing the layout of a wind farm, one of the most important considerations, besides the cost of infrastructure and transmission, is to reduce the wake effect. The wake effect is the undesirable aerodynamic interference where the upwind wind turbines in a wind farm block the available wind energy to the downwind ones and also pass on turbulence [1]. To mitigate the wake effect, real-time wind farm control methods have been developed, such as strategies based on the axial induction factor [[7], [8], [9]], wake redirection [[10], [11], [12]], and specifically for offshore cases, real-time layout optimization [[13], [14], [15]].
As for the wind farm layout optimization, the optimized layout is a function of, among other variables, wind speed and direction which may vary during the wind farm operation. Since fixed foundation wind turbines cannot change their positions after installation, they will suffer from reduced power and increase structure vibration unless wind turbines are installed sufficiently far apart each other. In contrast, movability of floating offshore wind turbines (FOWTs) allows the wind farm to track optimal layout with the least amount of wake influence in response to real-time wind condition changes [16]. Such advantage of FOWTs motivates this study on position control problem of a single FOWT in the context of real-time floating offshore wind farm layout optimization.
Since the FOWT is typically not designed with position transfer in mind, to generate the necessary actuation force for platform position transfer, either additional actuators should be installed or existing actuators need to be effectively utilized. Our preliminary work [17] explored the novel concept of FOWT position control through only passive utilization of the aerodynamic thrust force from wind. On the other hand, a group of researchers studied offshore wind farm layout optimization by assuming some finite range of movability for each FOWT based on conceptual floating platform design using a winch mechanism [18]. However, any prototyping progress based on this concept has not been reported to date. Another actuator concept is artificial muscle based active mooring line proposed in Ref. [19]. It was demonstrated to be effective for stabilization but may not have sufficient range of motion for position control. In comparison, since our proposed aerodynamically-actuated mechanism manipulates the thrust force from the wind by adjusting only commonly-available wind turbine control inputs, it is readily applicable to any FOWT without hardware modification. Furthermore, we proposed in Ref. [20] a computational method to determine the movable range of an FOWT under a given wind condition, within which system constraints (such as rotor speed and generator torque constraints) are met and position transfer is feasible at steady state. We also proposed in Ref. [20] open-loop constant control inputs to realize the desired platform position transfer, but such open-loop control has disadvantages in response speed.
In this paper, a feedback controller for the FOWT is proposed, which is capable of tracking the platform’s optimal position in a wind farm by the utilization of the aerodynamic thrust force. For obtaining sufficiently large thrust force to move an FOWT, a pitch-to-stall blade control strategy is adopted instead of the standard pitch-to-feather strategy, even though the pitch-to-stall blade control will increase the loads to the blades. The proposed controller has a structure with three sub-controllers, i.e., a power regulator, an inner-loop stabilizing controller, and a position controller. The power regulator based on the standard constant-power strategy guarantees that the FOWT generates the specified power during the platform position transfer and regulation, while the inner-loop stabilizing controller is required for subsequent position controller design. The desired position transfer and regulation are achieved by incorporating a Linear-Quadratic-Integral (LQI) controller in the overall control structure. To the best of our knowledge, there is only very few publications on FOWT platform position controller design.
This paper is organized as follows. In Section 2, a platform position control problem of an FOWT is formulated for a semi-submersible wind turbine, and the importance of the position control capability is explained from the viewpoint of wind farm’s total efficiency. The mechanism to adjust the aerodynamic thrust force by using only the standard FOWT control inputs is also briefly discussed in this section. To solve the formulated problem, a feedback control structure which involves a power regulator, an inner-loop stabilizing controller and an LQI position controller is proposed in Section 3. Section 4 examines the performance of the designed controller to track the position command. The performance is quantitatively compared with that of a Proportional-Integral-Derivative (PID) controller, an open-loop controller, and a gain-scheduling PI controller.
Section snippets
Position control problem of an FOWT
In this section, an FOWT to be controlled and its associated signals are introduced in Section 2.1. Then, for this FOWT, a position control problem is formulated in Section 2.2. The importance of this formulated FOWT position control problem in the context of real-time wind farm layout optimization and reconfiguration is discussed in Section 2.3. Finally, in Section 2.4, the mechanism to adjust the aerodynamic force for repositioning the FOWT platform is explained.
Feedback control structure
The proposed structure of the turbine level multi-objective controller is shown in Fig. 3. The controller receives target command in power () and position () from the wind farm level controller, as well as feedback signals () from the sensors, and then sends control input commands () to the FOWT. As can be seen, the proposed feedback controller consists of three sub-controllers, that is, Power Regulator, Stabilizing Controller, and Position Controller.
The Power Regulator adopts
Simulation results
This section examines the performance of the designed controllers on a specified position transfer task in simulations using FAST (Version 7.02) and Matlab/Simulink, with quasi-static mooring line model and first-order wave loading model realized in FAST v7. All the designed controller parameters are provided in Appendix A. Since the controller proposed in Section 3 is responsible for not only position transfer but also power and position regulation against disturbances, controller performances
Conclusion
This paper proposed a feedback control structure and a linear-quadratic-integrator controller for real-time platform position control of floating offshore wind turbine systems. The platform position of the wind turbine system was transferred to a target position and regulated there by adjusting the aerodynamic thrust force direction and magnitude with the control inputs which are commonly available in modern utility-scale wind turbines. It was demonstrated by simulations that, in the absence of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This research was financially supported by the Discovery Grant Program of the Natural Sciences and Engineering Research Council in Canada (NSERC) (RGPIN-2017-03753) and the Canada Research Chair Program.
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