Individual Pitch Control Research Articles

Estimation and Control of Wind Turbine Tower Vibrations Based on Individual Blade-Pitch Strategies

In this brief, we present a method to estimate the tower fore-aft velocity based upon measurements from blade load sensors. In addition, a tower dampening control strategy is proposed based upon an individual blade pitch control architecture that employs this estimate. The observer design presented in this brief exploits the Coleman transformations that convert a time-varying turbine model into one that is linear and time-invariant, greatly simplifying the observability analysis and subsequent observer design. The proposed individual pitch-based tower controller is decoupled from the rotor speed regulation loop and hence does not interfere with the nominal turbine power regulation. Closed-loop results, obtained from high fidelity turbine simulations, show close agreement between the tower estimates and the actual tower velocity. Furthermore, the individual-pitch-based tower controller achieves a similar performance compared with the collective-pitch-based approach but with negligible impact upon the nominal turbine power output.

Multivariable robust blade pitch control design to reject periodic loads on wind turbines

The demand on sustainable operations of large-scale wind turbines necessitates the concurrent advancement of power regulation and load mitigation through blade pitch control. Traditional collective pitch control (CPC) mechanisms can only deal with symmetric disturbances. The advent of individual pitch control (IPC) provides new opportunities to mitigate asymmetric or periodic loads on blades. Nevertheless, difficulties in control synthesis remain. In order for IPC to be truly effective, the complicated dynamic coupling between turbine components has to be accounted for. Moreover, wind turbine dynamics is highly nonlinear, and significant modeling uncertainties exist. In this research, a multivariable robust IPC framework is developed, aiming at rejecting periodic loads. The inter-blade coupling is explicitly modeled to provide response characteristics in the frequency domain. Subsequently, the structured singular values (μ)-synthesis strategy is adopted, as it shows distinct capability of dealing with periodic loads. In particular, weighting functions can be tailored to suppress response peaks at periodic frequencies with guaranteed robustness. Systematic case investigations indicate that, with the proposed IPC strategy, one can achieve significant periodic load mitigation as well as fatigue alleviation in speed-varying wind fields.

Development of Hardware-in-the-Loop-Simulation Testbed for Pitch Control System Performance Test

This paper deals with the development of a wind turbine pitch control system and the construction of a Hardware-in-the-Loop-Simulation (HILS) testbed for the performance test of the pitch control system. When the wind speed exceeds the rated wind speed, the wind turbine pitch controller adjusts the blade pitch angles collectively to ensure that the rotor speed maintains the rated rotor speed. The pitch controller with the individual pitch control function can add individual pitch angles into the collective pitch angles to reduce the mechanical load applied to the blade periodically due to wind shear. Large wind turbines often experience mechanical loads caused by wind shear phenomena. To verify the performance of the pitch control system before applying it to an actual wind turbine, the pitch control system is tested on the HILS testbed, which acts like an actual wind turbine system. The testbed for evaluating the developed pitch control system consists of the pitch control system, a real-time unit for simulating the wind and the operations of the wind turbine, an operational computer with a human–machine interface, a load system for simulating the actual wind load applied to each blade, and a real pitch bearing. Through the several tests based on HILS test bed, how well the pitch controller performed the given roles for each area in the entire wind speed area from cut-in to cut-out wind speed can be shown.

Vibration and Performance Analyses Using Individual Blade Pitch Controls for Lift-Offset Rotors

This work attempts to reduce the hub vibratory loads of a lift-offset rotor using IBC (individual blade pitch control) in high-speed forward flight. As a lift-offset rotor for the present study, the rigid coaxial rotor of a XH-59A compound helicopter is considered and CAMRAD II is used to predict the hub vibration and rotor performance. Using the IBC with a single harmonic input at 200 knots, the vibration index of the XH-59A rotor is minimized by about 62% when the 3/rev actuation frequency is applied with the IBC amplitude of 1° and control phase angle of 270° (3P/1°/270°); however, the rotor effective lift-to-drag ratio decreases by 3.43%. When the 2/rev actuation frequency with the amplitude of 2° and control phase angle of 270° (2P/2°/270°) and the 3/rev actuation frequency using the magnitude of 1° and control phase angle of 210° (3P/1°/210°) are used in combination for the IBC with multiple harmonic inputs, the vibration index is reduced by about 62%, while the rotor effective lift-to-drag ratio increases by 0.37% at a flight speed of 200 knots. This study shows that the hub vibration of the lift-offset rotor in high-speed flight can be reduced significantly but the rotor performance increases slightly, using the IBC with multiple harmonic inputs.

Wind tunnel setup for experimental validation of wind turbine control concepts under tailor-made reproducible wind conditions

To speed up validation and prototyping of novel control and design concepts in wind energy, wind tunnel experiments should be used in synergy with numerical simulations and field experiments. The paper presents a wind tunnel setup consisting of a scaled wind turbine model MoWiTO and an active grid for generation of tailor-made and reproducible inflow conditions. To demonstrate that the presented experimental setup is suitable for fast validation of advanced control concepts, several control algorithms have been implemented and validated in various inflow conditions, such as turbulent and sheared inflows, and wind gusts. The tested control algorithms include the baseline collective pitch and torque control, individual pitch control for load reduction, and a control algorithms for compensation of static aerodynamic imbalances. All control algorithms resulted in an expected behaviour of the model wind turbine, thus emphasizing that the presented experimental setup can be used for fast and inexpensive testing of control concepts.

Rejection of varying-frequency periodic load disturbances in wind-turbines through active disturbance rejection-based control

Wind turbines are complex mechatronic systems subject to different kinds of disturbances. Among all disturbance sources, those from periodic nature are the most relevant and important for maintaining the structural health of the entire wind turbine. However, periodic disturbances in wind turbines, mainly given by wind shear, tower shadow and rotational sampling of turbulence, have a variable fundamental frequency that changes as function of the turbine angular speed. In consequence, conventional controllers have issues to mitigate such varying-frequency periodic loads. This paper proposes two individual pitch control schemes to attenuate the main periodic load components of blade flap-wise and hub yaw/tilt-wise bending moments in horizontal-axis wind turbines under varying-frequency conditions. Both proposals are founded on the paradigm of Active Disturbance Rejection Control (ADRC): the former is formulated in the time-domain using a Linear-Time-Invariant (LTI) standard controller, while the latter is designed in the angular position domain using a Linear-Position-Invariant (LPI) approach. Several realistic simulations with different wind turbulence intensities and average wind speeds are performed using a 5 MW three-bladed horizontal-axis wind turbine implemented in the FAST code by NREL. Simulation results of the proposed control schemes show improved load reductions compared with other control techniques.

Effectiveness of individual pitch control on a 5 MW downwind turbine

A novel experimental study of individual pitch control on downwind turbines is conducted. The experiment is accomplished using a sub-scale model of a 5 MW downwind turbine. It is shown that individual pitch control based on sinusoidal/cosinusoidal pitching schemes that are locked to the phase of the rotor rotation further improves the power output, yaw stability and mitigates the unsteady loads of a downwind turbine through the intelligent control system. The optimum pitching amplitude is observed to be insensitive to yaw angle. Furthermore, a negative sinusoidal pitching amplitude improves power and reduces overall unsteadiness at zero yaw angle. In the case of positive yaw; a positive cosinusoidal pitching amplitude improves power, reduces overall unsteadiness and improves yaw stability. In the case of negative yaw; when a positive sinusoidal pitching amplitude alleviates overall unsteadiness, improves yaw stability but reduces power while a positive cosinusoidal pitching amplitude increases power coefficient, alleviates overall unsteadiness but reduces yaw stability. Moreover, positive rotor yaw angle which is more stable than negative can be used to alleviate loads on downwind turbines during operation because overall unsteadiness is observed larger when the nacelle has a negative yaw angle, compared to when nacelle has a positive yaw angle.

A Study on the Capability of Backward Swept Blades to Mitigate Loads of Wind Turbines in Shear Flow

The vertical wind shear is one of the major sources of fatigue loads on the blades of a horizontal axis wind turbine. Traditionally, the active individual pitch control system is used to alleviate the cyclic load fluctuations, which requires sensors and actuators. In this paper, a passive load control technique by using backward swept blades is explored of its potential capability of mitigating load variations in shear wind. An advanced aeroelastic model based on free wake vortex lattice model (FWVLM) and geometrically exact beam theory (GEBT) is developed for the study. Both the aerodynamic and structural solvers are able to account for the three-dimensional (3D) shape effects of the undeformed and deformed swept blades. The NREL 5-MW reference wind turbine is analyzed by adding backward sweep to the baseline blade. Comparisons are made between the baseline and swept blades for the time-varying root moments and rotor moments under sheared inflow. Different amounts of tip sweep are discussed. An effective reduction in the amplitude of the flapwise root moment variations is obtained, while the edgewise root moment is less influenced. Reduced mean values of the rotor yaw and tilt moments are also achieved, at the cost of increased blade root torsion moment.

Active load alleviation potential of adaptive wind turbine blades using shape memory alloy actuators

AbstractUpscaling of wind turbine blades calls for implementation of innovative active load control concepts that will facilitate the flawless operation of the machine and reduce the fatigue and ultimate loads that hinder its service life. Based on aeroelastic simulations that prove the enhanced capabilities of combined individual pitch and individual flap control at global wind turbine scale level, a shape adaptive concept that encompasses an articulated mechanism consisting of two subparts is presented. Shape memory alloy (SMA) actuators are investigated and assessed as means to control the shape adaptive mechanism at airfoil section level in order to alleviate the developed structural loads. The concept is embedded in the trailing edge region of the blade of a 10‐MW horizontal axis wind turbine and acts as a flap mechanism. Numerical simulations are performed considering various wind velocities and morphing target shapes and trajectories for both normal and extreme turbulence conditions. The results prove the potential of the concept, since the SMA controlled actuators can accurately follow the target trajectories. Power requirements are estimated at 0.22% of the AEP of the machine, while fatigue and ultimate load reduction of the flap‐wise bending moment at the blade root is 27.6% and 7.4%, respectively.

Analysis and optimal individual pitch control decoupling by inclusion of an azimuth offset in the multiblade coordinate transformation

AbstractWith the trend of increasing wind turbine rotor diameters, the mitigation of blade fatigue loadings is of special interest to extend the turbine lifetime. Fatigue load reductions can be partly accomplished using individual pitch control (IPC) facilitated by the so‐called multiblade coordinate (MBC) transformation. This operation transforms and decouples the blade load signals in a yaw‐axis and tilt‐axis. However, in practical scenarios, the resulting transformed system still shows coupling between the axes, posing a need for more advanced multiple input multiple output (MIMO) control architectures. This paper presents a novel analysis and design framework for decoupling of the nonrotating axes by the inclusion of an azimuth offset in the reverse MBC transformation, enabling the application of simple single‐input single‐output (SISO) controllers. A thorough analysis is given by including the azimuth offset in a frequency‐domain representation. The result is evaluated on simplified blade models, as well as linearizations obtained from the NREL 5–MW reference wind turbine. A sensitivity and decoupling assessment justify the application of decentralized SISO control loops for IPC. Furthermore, closed‐loop high‐fidelity simulations show beneficial effects on pitch actuation and blade fatigue load reductions.

Wavelet-based individual blade pitch control for vibration control of wind turbine blades

Wavelet-based individual blade pitch control for vibration control of wind turbine blades

Combined Pitch and Trailing Edge Flap Control for Load Mitigation of Wind Turbines

Using active control methods for load mitigation in wind turbines could greatly reduce the cost of per kilowatt hour of wind power. In this work, the combined pitch and trailing edge flap control (CPFC) for load mitigation of wind turbines is investigated. The CPFC includes an individual pitch control (IPC) loop and a trailing edge flap control (TEFC) loop, which are combined by a load frequency division control algorithm. The IPC loop is mainly used to mitigate the low frequency loads, and the TEFC loop is mainly used to mitigate the high frequency loads. The CPFC adopts both an azimuth angle feed-forward and a loads feedback control strategy. The azimuth angle feed-forward control strategy should mitigate the asymmetrical loads caused by observable disturbances. and the loads feedback control strategy should decrease asymmetrical loads by closed loop control. A simulation is carried out on the joint platform of FAST and MATLAB. The simulation results show that the damage equivalent load (DEL) of blade root out-of-plane bending moment is reduced by 53.7% while using CPFC, compared to collective pitch control (CPC); and the standard deviation of blade tip out-of-plane deflection is reduced by 50.2% while using CPFC, compared to CPC. The results demonstrate that the CPFC can mitigate the fatigue loads of wind turbines as anticipated.

Data-driven repetitive control: Wind tunnel experiments under turbulent conditions

A commonly applied method to reduce the cost of wind energy, is alleviating the periodic loads on turbine blades using Individual Pitch Control (IPC). In this paper, a data-driven IPC methodology called Subspace Predictive Repetitive Control (SPRC) is employed. The effectiveness of SPRC will be demonstrated on a scaled 2-bladed wind turbine. An open-jet wind tunnel with an innovative active grid is employed to generate reproducible turbulent wind conditions. A significant load reduction with limited actuator duty is achieved even under these high turbulent conditions. Furthermore, it will be demonstrated that SPRC is able to adapt to changing operating conditions.

Optimization and control of offshore wind systems with energy storage

Wind energy is widely exploited as a promising renewable energy source worldwide. In this article, an optimization method for the control and operation of the offshore wind farm as an integrated system considering its operational, economic and environmental impacts is proposed. The state of the offshore wind farm development, and the challenges faced by the offshore wind farms, including the challenges of economic costs, operational reliability, and environmental impacts are reviewed and summarized. A systematic methodology is proposed to optimize the costs of an offshore wind farm system based on a framework that extends from the supervisory level dispatch strategies to individual pitch control for load reduction and fatigue mitigation. This holistic approach is able to improve the efficiency and economic performance of a wind farm through overall system optimization, while explicitly operating each wind turbine using a formally designed control framework leading to an extension of their service lifespan. This work provides thorough introduction and innovative solutions for many economic and environmental issues in offshore wind farm development and has strong application potential.

Design optimization of hull size for spar-based floating offshore wind turbines

A newly evolving concept in utility-scale floating wind turbines is investigated through design optimization. The ideal hull length is selected from seven new designs, based on optimizing the trade-off between minimizing structural weight and maximizing electricity harvest. The seven new spar-buoy designs have drafts ranging from 62 to 120 m. Each floater hull has been developed in conformance with industry-standard structural guide API Bulletin 2U. Performance is assessed with the Loose software using time-domain analysis and irregular winds and waves. The simulation method is based on multi-body theory in Euler space, which does not rely on small-angle assumptions. Nonlinear hydrostatic restoring forces and moments are derived for large rotational angles in the pitch-roll-yaw Euler angle sequence. Environmental forcing by wind is computed using aero-elastic theory. A new individual pitch control (IPC) method based on DISCON control routine (Jonkman) is investigated as a possible means to improve generating efficiency. The pitch angle of each blade is individually controlled relative to a mean angle computed by DISCON to maintain the ideal angle of attack by removing the angle change caused by inclining tower. A comparison of the generating efficiency between IPC and traditional collective pitch control shows that IPC is not an effective way to increase output power.

Assessment of load reduction capabilities using passive and active control methods on a 10MW-scale wind turbine

In the paper, the potential to alleviate wind turbine loads through combined implementation of different passive and active control methods is assessed. Passive control of loads is accomplished through blade designs with build in material and/or geometric bend-twist coupling (BTC). The first is materialized by introducing an offset angle on the plies of the uni-directional material over the spar caps of the blade, while the latter by sweeping the blade elastic axis with respect to the pitch axis. Active control of loads is considered through individual pitch control (IPC) or concurrent use of individual pitch and flap control (IPC+IFC). Different combinations of the abovementioned techniques are tested in the paper with the aim to obtain maximum possible load reduction levels but also confine key design parameters of the various methods within reasonable limits that by no means exceed manufacturing constraints. The performance of the different control options is assessed through aeroelastic simulations for the 10MW DTU Reference Wind Turbine (RWT). A subset of representative fatigue and ultimate design load cases (DLCs) of the IEC is simulated and load reduction levels are assessed with respect to the baseline RWT configuration with no aeroelastic control of loads.

Inflow-based flap control on a 10MW-scale wind turbine using a spinner anemometer

In the paper, an individual flap control (IFC) algorithm based on wind speed measurements obtained with a spinner anemometer is presented. The controller uses flap actuators with the aim to remove any deterministic source of load variation on blades, associated with asymmetries of the inflow. Such load variations are concentrated on multiples of the rotational frequency (p multiples) and they are mainly due to wind yaw, inclination and atmospheric boundary layer (ABL) shear. The aim of the controller is to assist operation of the conventional feed-back individual pitch controller (IPC) and thereby reduce its control duty cycle. The performance of the proposed controller is assessed through aero-elastic simulations for the 10MW DTU Reference Wind Turbine. A subset of normal operation fatigue and ultimate Design Load Cases (DLCs) of the IEC has been simulated and load reduction capabilities of the control algorithm have been compared against those of the standard individual pitch control (IPC) loop.

Validating subspace predictive repetitive control under turbulent wind conditions with wind tunnel experiment

To reduce the cost of wind energy, it is essential to reduce loads on turbine blades to increase lifetime and decrease maintenance cost. To achieve this, Individual Pitch Control (IPC) received an increasing amount of attention in recent years. In this paper, a data-driven IPC algorithm called Subspace Predictive Repetitive Control (SPRC) is used to alleviate periodic loads on a scaled 2-bladed wind turbine in turbulent wind conditions. These wind conditions are created in an open-jet wind tunnel with an active grid, enabling unique reproducible high turbulent wind conditions. Significant load reductions are achieved even under these high turbulent conditions.

Investigation on the potential of individual blade control for lifetime extension

In recent years the focus of wind energy industry is on reducing levelized cost of energy by rotor upscaling. Moreover, a current topic of interest to both industry and academia is the extension of lifetime to existing wind turbines approaching the end of initial design span. Thus, the need for load alleviation technologies integrated in the design process or for retrofit purposes is becoming more relevant. One of these is individual blade pitch control, a recurring topic in research, with known advantages and weaknesses namely the pitch actuator and bearing wear. The present work suggests such a system incorporating three independent controllers with input the root bending moments on the rotating frame. The linear system used for controller design is based on black box identification of non-linear simulations and filters are used both for the input and output. Different setups of the independent blade control scheme are applied on a 10 MW reference turbine, with a large and highly flexible rotor representative of the current industrial status, under wind conditions as defined by relevant certification standards. The investigation aims on evaluating the system’s performance based on the fatigue load alleviation potential for different components as well as identifying the tradeoff for each design choice. Finally, based on basic assumptions the reductions are translated to possible life time extension for each component based on a combined operation where the new controllers are applied for a percentage of the initial 20 year lifetime.

Experimental Determination of Thrust Loading of a 2-Bladed Vertical Axis Wind Turbine

Large floating offshore wind turbines are beginning to show promise as a technology with several pilot projects being completed in recent years with more on the near horizon. Due to the complexities of the floating configuration there are substantial costs associated with the platform and mooring systems for these types of deep water machines. The vertical axis wind turbine has been proposed as a potential solution for lowering the overall costs of turbine installations. This is achieved through a lower center of gravity and a greater tolerance to platform motions than an equivalent horizontal axis machine. The cost of the platform system is related to the overturn moment of the turbine in crucial operational states. The largest contribution to this moment is the rotor thrust. In this work, an experimental wind tunnel model has been made to study the loading of a 2-bladed H-type VAWT. The model is capable of individual active pitch control and is equipped with sensors to measure thrust and side loading with respect to the turbine. This paper introduces the experimental wind tunnel model referred to as PitchVAWT, discusses the method of determining rotor thrust and side loads, and presents measured results for a fixed pitch case with varying tip speed ratio. The data presented will be made available for further evaluation and potential validation of turbine numerical codes.