Collective Blade Pitch Research Articles

Robust blade pitch control of semi-submersible floating offshore wind turbines based on the modified super-twisting sliding-mode algorithm

In this paper, a novel robust collective blade pitch controller (CBPC) is proposed for the semi-submersible floating offshore wind turbine (FOWT) above the rated wind speed. The proposed CBPC is based on the modified super-twisting sliding-mode (MSTSM) algorithm. Firstly, based on a control-oriented model of the semi-submersible FOWT, the dynamics of the rotor speed and the platform pitch rate considering the lumped disturbances, which consist of external disturbances, parametric uncertainties and unmodeled dynamics, are derived. Afterward, the MSTSM algorithm-based CBPC (MSTSM-CBPC) is designed for regulating the generator power to its rated value and reducing the platform pitching motion. Comparative co-simulation tests among the gain-scheduling proportional-integral CBPC, the standard STSM algorithm-based CBPC and the proposed MSTSM-CBPC are performed. Simulation results validate the effectiveness and the superiority of the proposed MSTSM-CBPC.

Reduced-order modelling of floating offshore wind turbine: Aero-hydro-elastic stability analysis

A reduced-order aero-hydro-elastic analysis tool for performing the stability analysis on floating offshore wind turbines is presented. In this work, a high-order non-linear aero-elastic model initially developed for onshore wind turbines is extended by utilizing a super-element body, which models the floating platform, the mooring system and the hydrodynamic contributions. The turbine structure is modelled by a Finite beam Element Method, and the aerodynamic loads are modelled by the Blade Element Momentum method coupled with a Beddoes-Leishman type dynamic stall model in a state-space formulation. The linearization is performed around steady state equilibrium at any given mean wind speeds, rotor speeds and collective blade pitch angles utilizing Coleman transformation to remove the periodic terms. The order reduction is based on two projections to reduce the number of structural states and aerodynamic states separately using the structural modal project matrix and the aerodynamic shape functions. Concurrently, we investigate the effect of unsteady aerodynamic states on the stability analysis of a floating wind turbine. The results show the low-order aero-hydro-elastic model expands the scope of numerical tools available for conducting structural modal analysis and aero-hydro-elastic stability analysis on floating wind turbines.

Comparison of 25 MW downwind and upwind turbine designs with individual pitch control

As conventional upwind wind turbines grow larger, the increased mass and flexibility of the longer blades present challenges concerning costs, structural loads, and safety constraints such as tower clearance. At extreme scales, wind turbines in a downwind configuration may provide a feasible alternative to address these challenges by allowing lightweight, flexible blades that can reduce capital costs and blade loads while maintaining safety margins. Downwind turbine blades suffer from increased fatigue loading due to the tower shadow effect. In this study, novel downwind, three-bladed wind turbine designs at 25 MW rating with lightweight, flexible blades are evaluated and compared in terms of power production and structural loading. To obtain a baseline performance, a standard collective blade pitch wind turbine controller is implemented for the two downwind and one upwind turbine designs. Individual pitch control is then added for the downwind turbines to reduce structural fatigue on the turbine blades. In summary, the two downwind turbine designs that differ in rotor pre-coning and shaft tilt angles using collective and individual pitch control are compared against a conventional upwind turbine with collective pitch control at the same scale under turbulent wind conditions.

Observer based pitch control for load mitigation and power regulation of floating offshore wind turbines

Abstract Most commercial wind turbines use proportional-integral (PI) collective blade-pitch control to regulate rotor speed in the above-rated wind speed regime. A significant drawback of this type of controller is that it assumes that the blades have identical structural properties and are subject to similar aerodynamic loads, which is seldom the case. Also, these controllers are designed to regulate the rotor speed and are not designed for structural vibration/load reduction. However, it is well known that blade pitch control can reduce structural loads on wind turbines. This opens up the possibility of designing controllers that use existing actuators and sensors like the blade pitch actuators to reduce structural loads/vibrations while maintaining the required rotor speed. Recent studies have investigated individual blade pitch control (IPC) to address these shortcomings. However, the vast majority of studies published in the literature depend on the availability of state measurement. Although sensors are commonly placed on all wind turbines, and some information is readily available, the measurement required by the typical state-feedback controllers is usually not available. Displacements and velocities of the blade, the tower and the floating platform are difficult to measure. This paper develops an observer-based individual blade pitch controller for load mitigation and power regulation of floating offshore wind turbines. We propose to use a Kalman filter to estimate the state from the accelerometer and strain gauge measurement for use in the state-feedback controller. The state-feedback controller was proposed previously by the authors that showed excellent performance. This paper extends the capability of the state-feedback controller by designing an observer (Kalman filter) to estimate the state from limited measurements. The proposed observer based controller is compared against a baseline proportional integral collective blade pitch controller and full state-feedback controllers to evaluate its performance. Numerical results show that the proposed output feedback controller offers performance improvements over the baseline controller, similar to the full state-feedback controller.

Effect of blade aspect ratio on the performance tradeoff between figure of merit and bending-torsion dynamics of wind turbines with synthetic jets

In this experimental study, two three-bladed rotors with a blade aspect ratio of 7.79 and 9.74 representing a 25% increase were tested. Each blade contains an S809 airfoil with zero pre-built twist and 20 high-performance synthetic jet actuators distributed along the span. The rotor was tested at four rotor speeds, Ω of 250, 500, 750, and 1,000 revolutions per minute (RPM) and three collective blade pitch angles, θc of 2, 5, and 8 degrees. Rotor thrust was measured using a high-capacity load cell, and blade bending and torsion was measured at four equally spaced radial locations on the blade defined by r/R= 0.24, 0.48, 0.72, and 0.96 using four onboard inertial measurement units. The spanwise flow over the suction surface of the blade was measured using laser Doppler velocimetry (LDV) techniques. The local Reynolds number produced with the test rotor speeds at the measurement locations ranges from 8.31 × 104≤Re∞≤ 9.49 × 105. It was found that rotor aerodynamic efficiency as quantified by the figure of merit is directly linked to blade aspect ratio and the magnitude of spanwise flow over the blade. The rotor with a higher blade aspect ratio produces less overall spanwise flow and is more aerodynamically efficient, however the blade also generates larger mean and fluctuating bending and torsion displacements, which are undesirable for aeroelastic stability. The use of synthetic jets on the blade mitigates the root mean square of the bending and pitch displacements above Ω= 250 RPM and θc= 2°. It was found that at Ω= 1,000 RPM, the blade structures undergo a second mode bending response indicating a condition of structural instability due to higher aerodynamic loading. At this rotor speed, flow control on the outboard of the blade reduces unsteady bending as quantified by arms up to a maximum of 36.5%, and unsteady torsion, θrms up to 22.9% compared to the baseline for the rotor with an aspect ratio of 9.74.

Real-Time Repositioning of Floating Wind Turbines Using Model Predictive Control for Position and Power Regulation

As offshore wind capacity could grow substantially in the coming years, floating offshore wind turbines (FOWTs) are particularly expected to make a significant contribution to the anticipated global installed capacity. However, FOWTs are prone to several issues due partly to environmental perturbations and their system configuration which affect their performances and jeopardize their structural integrity. Therefore, advanced control mechanisms are required to ensure good performance and operation of FOWTs. In this study, a model predictive control (MPC) is proposed to regulate FOWTs’ power, reposition their platforms to reach predefined target positions and ensure their structural stability. An efficient nonlinear state space model is used as the internal MPC predictive model. The control strategy is based on the direct manipulation of the thrust force using three control inputs, namely the yaw angle, the collective blade pitch angle, and the generator torque without the necessity of additional actuators. The proposed controller accounts for the environmental perturbations and satisfies the system constraints to ensure good performance and operation of the FOWTs. A realistic scenario for a 5-MW reference wind turbine, modeled using OpenFAST and Simulink, has been provided to demonstrate the robustness of the proposed MPC controller. Furthermore, the comparison of the MPC model and a proportional-integral-derivative (PID) model to satisfy the three predefined objectives indicates the superior performances of the MPC controller.

LQG control for hydrodynamic compensation on large floating wind turbines

This work proposes a novel Linear Quadratic Gaussian (LQG)-based blade pitch control method for floating offshore wind turbines, in which a state-space model of the turbine and water hydrodynamics is included in the LQG design. The actuation considered is collective blade pitch control with the objective of generator power stabilisation and platform motion reduction. A linear Kalman filter is used to estimate un-measurable states relating to wave excitation and radiation through measurements of generator speed, platform pitch, and wind disturbance. Controller design models were validated with the full order nonlinear model under various testing conditions. The new controller design is tested on a nonlinear high-fidelity simulation model of the 15 Mega-Watt (MW) floating semi-submersible wind turbine. In simulations with realistic stochastic wind and wave disturbances, the new controller achieves 32% lower generator speed Root Mean Square Error (RMSE) and 16% lower platform pitch RMSE compared to a standard LQG controller that does not include hydrodynamic states, for equivalent levels of pitch actuation and with a 2°/sec rate limit on pitch. The inclusion of hydrodynamics in the controller design not only reduced platform pitching fluctuation, but also had a strong effect of hub-height factors such as the generator speed.

Neural network-based supertwisting control for floating wind turbine in region III

In this paper, a hybrid control strategy based on the super-twisting sliding mode approach and artificial neural network method has been proposed for collective blade pitch (CBP) control of floating wind turbines (FWT) above the rated wind speed. Besides the presence of uncertainties and external disturbances due to the complexity of the model of wind turbines, this paper uses the radial basis function (RBF) neural network to approximate model uncertainties and unmodeled dynamics, reducing the controller dependency on the exact model of the system. The implemented neural network adaptive law has been achieved based on the Lyapunov stability, and the convergence of the closed-loop system is guaranteed by adjusting the learning rate. As the floating wind turbine is a highly nonlinear system, the main objectives are limitation of platform pitch motion and related fatigues, blade fatigue load reduction, and essentially power regulation. Here, using the FAST simulator, the proposed controller has been tested by achieving the required dynamic and static performance. The simulation results illustrate the efficiency of the investigated strategy by comparing it with and without RBF neural network on the FWT.

A novel resonant controller for sea-induced rotor blade vibratory loads reduction on floating offshore wind turbines

A novel multi-layer control approach for offshore floating wind turbines is presented, suitable for alleviating the vibratory loads while keeping the generator power output stable. It consists of the simultaneous application of different control strategies, each optimized for a specific objective. The proposed multi-layer scheme’s main scope is to reduce the Levelized Cost of Energy. Two resonant controllers based on collective blade pitch actuation are applied for the rejection of the vibratory loads induced by sea waves. In contrast, a proportional–integral controller, fed by measured cyclic blade root flapping moments, provides the blade cyclic pitch to be actuated to reduce blade root loads at the rotor revolution frequency. The proposed control strategy is validated by simulations on: (i) the NREL (U.S. National Renewable Energy Laboratory) 5 MW wind turbine, supported by a spar buoy or a semi-submersible platform; (ii) the 15 MW IEA (International Energy Agency) wind turbine supported by a semi-submersible platform. These show that significant reductions of the vibratory blade root flapping moments and rotor nacelle assembly loads are achievable, thus demonstrating the good potential performance of the controller introduced.

Aeroelastic response of a propulsive rotor blade with synthetic jets

Understanding the aeroelastic response of a propulsive rotor blade is critical to implementing active flow control techniques to mitigate blade structural vibration. In this experimental study, the coupling between rotor blade aerodynamic loading and first flap mode bending and torsion is explored. The three-bladed rotor is 2.58 m in diameter, and contains a NACA 0012 airfoil with zero pre-built twist. Each blade contains 20 high-performance synthetic jet actuators distributed along the span and are activated with a constant mean jet velocity of 66.3 m/s. The rotor was tested at rotor speeds, Ω of 250, 500, 750, and 1000 revolutions per minute (RPM) and collective blade pitch angles, θc of 2, 5, and 8 degrees. Rotor thrust and torque were measured using a high-capacity load cell, and the flow near the blade tip was measured using laser Doppler velocimetry (LDV) techniques. Blade bending and torsion was measured at three equally spaced radial stations on the blade root, middle, tip regions defined by r/R= 0.32, 0.64, and 0.96 respectively using three onboard electronic accelerometers. Synthetic jets reduce the near-wall streamwise velocity deficit and turbulence intensity in the near wall flow, improving the sectional lift coefficient of the blade and delaying flow separation which results in marginally higher thrust and lower torque loading of up to 7.4% and 4.3 % respectively. At higher rotor speeds of 750 and 1000 RPM, there is a pronounced non-linear increase in bending and torsion along blade span. In the blade tip region, the streamwise flow undergoes a significant decrease in velocity and momentum, which contributes to instabilities in the boundary layer with early-stage unsteady flow separation at θc= 8°. This is correlated to a dramatic rise in the baseline power spectral density (PSD) at the blade’s natural frequency along with an increase in root mean square (rms) of bending acceleration and blade pitch angle, where the maximum values measured are arms= 0.74 g and θrms= 1.45°.

Individual/collective blade pitch control of floating wind turbine based on adaptive second order sliding mode

A new control strategy based on adaptive second order sliding mode approach is applied to a floating wind turbine system in the above rated region. This adaptive controller is well adapted to a highly nonlinear system as floating wind turbine, and can be easily implemented with very reduced knowledge of modeling. The proposed controller partially based on multi-blade coordinates transformation combines collective and individual collective blade pitch control, for power regulation, platform pitch motion reduction and reduction of blades fatigue load. The proposed controller is implemented on FAST simulator and shows high level of performances.

Field testing of multi-variable individual pitch control on a utility-scale wind turbine

The ongoing increase in size of modern wind turbines creates the demand of better control algorithms to counteract the increased loads acting on them. Individual blade pitch control (IPC) algorithms to alleviate blade loads are becoming a crucial part of current wind turbine control systems. The state-of-the-art individual blade pitch load reduction controller mostly relies on multiple single-input single-output (SISO) designs. This approach, however, constrains the achievable bandwidth of the controller as relevant couplings in the dynamics are ignored. These couplings become more pronounced for bigger turbines. Model-based multiple-input multiple output (MIMO) control designs can be used to account for these couplings and hence reduce loads for future, larger turbines. In this article we present the results of an intensive field test campaign of an H∞-design based MIMO IPC. This controller is designed for and tested on the utility-scale 2.5 MW Clipper Liberty research turbine operated by the University of Minnesota. The article guides the reader through the turbine’s open loop dynamics, the IPC design and the relevant implementation steps on the turbine. Finally, the developed H∞-based IPC is compared using experimental field data against no IPC (collective blade pitch control) and a classical IPC based on decoupled SISO loops.

Adaptive super-twisting control of floating wind turbines with collective blade pitch control

Abstract This paper proposes an adaptive super-twisting controller for floating wind turbine based on collective blade pitch control. This adaptive second order sliding mode control scheme being efficient for systems with uncertainties and external perturbations, which is compared with the traditional gain scheduled PI (GSPI) controller. It is shown that this kind of controller requires reduced knowledge about the system (only the relative degree) and the gain tuning effort is also reduced given that the operating domain is large versus GSPI controller. Simulation results show high performances of the proposed controller for rotor speed regulation and reduction of platform oscillations in Region 3 of the floating wind turbine.

Linear Quadratic Optimal Control of a Spar-Type Floating Offshore Wind Turbine in the Presence of Turbulent Wind and Different Sea States

This paper presents the design of a linear quadratic (LQ) optimal controller for a spar-type floating offshore wind turbine (FOWT). The FOWT is exposed to different sea states and constant wind turbulence intensity above rated wind speed. A new LQ control objective is specified for the floater-turbine coupled control, in accordance with standard requirements, to reduce both rotor speed fluctuations and floater pitch motion in each relevant sea state compared with a baseline proportional-integral (PI) controller. The LQ weighting matrices are selected using time series of the wind/wave disturbances generated for the relevant sea states. A linearized state-space model is developed, including the floater surge/pitch motions, rotor speed, collective blade pitch actuation, and unmeasured environmental disturbances. The wind disturbance modeling is based on the Kaimal spectrum and aerodynamic thrust/torque coefficients. The wave disturbance modeling is based on the Pierson–Moskowitz spectrum and linearized Morison equation. A high-fidelity FOWT simulator is used to verify the control-oriented model. The simulation results for the OC3-Hywind FOWT subjected to turbulent wind show that a single LQ controller can yield both rotor speed fluctuation reduction of 32–72% and floater pitch motion reduction of 22–44% in moderate to very rough sea states compared with the baseline PI controller.

Modern methods for investigating the stability of a pitching floating platform wind turbine

Abstract. The QBlade implementation of the lifting-line free vortex wake (LLFVW) method was tested in conditions analogous to floating platform motion. Comparisons against two independent test cases using a variety of simulation methods show good agreement in thrust forces, rotor power, blade forces and rotor plane induction. Along with the many verifications already undertaken in the literature, it seems that the code performs solidly even in these challenging cases. Further to this, the key steps are presented from a new formulation of the instantaneous aerodynamic thrust damping of a wind turbine rotor. A test case with harmonic platform motion and collective blade pitch is used to demonstrate how combining such tools can lead to a better understanding of aeroelastic stability. A second case demonstrates a non-harmonic blade pitch manoeuvre showing the versatility of the instantaneous damping method.

MODEL-BASED CONTROL OF UTILITY-SCALE WIND TURBINES

As wind turbine size increases, small increases in efficiency can lead to significant monetary and lifespan improvements. As utility scale wind turbines approach structural efficiency limits, control systems have become the primary means of improving power efficiency and reducing fatigue. In variable speed turbines, collective or individual blade pitch control is one of the most common control schemes for above-rated speed operation. This paper presents the design and implementation of a model-based controller for jointly controlling the wind turbines power and blade loading reduction. First, a high-fidelity simulation model of a 3MW wind turbine is devel-oped followed by the control development using a computationally efficient version of model predictive control which benefits from short-term wind speed forecasts. The proposed controller is com-prehensively tested under multiple practical scenarios and compared with conventional wind turbine control methods.

外乱抑制FRIT法を用いたPI制御ゲインチューニングの浮体式洋上風車への応用

Floating offshore wind turbines (FOWTs) have attracted interest in recent years. FOWTs operate under irregular wind and waves, and these disturbances cause generator output fluctuation and platform motions. In this paper, a PI controller in collective blade pitch control is used to mitigate these fluctuation and motions. The PI controller gain parameters are tuned by the FRIT (Fictitious Reference Iterative Tuning) method, and we design a gain-scheduled PI controller based on tuned gains at multiple operating points. We show that the gain-scheduled PI controller improves the disturbance suppression performance.

Novel parameter settings for gain-scheduled feedback control of rotational speed in a floating offshore wind turbine–generator system

Novel parameter settings for gain-scheduled feedback control of rotational speed using collective blade pitch manipulation in a spar-buoy floating offshore wind turbine–generator system were developed in order to reduce both power-output fluctuation and platform motion. The development was conducted through numerical simulation using the aeroelastic simulation model (FAST), measured high wind speed data, and simulated irregular sea waves. In this gain-scheduled feedback control of rotational speed, a proportional-plus-integral action was employed, and the proportional gain was varied to the blade pitch angle. First, the sensitivity analysis of the control parameter settings, that is, the proportional gain, integral time, and the generator torque manipulation strategy, to the system performances was carried out. Then, a new guideline of parameter settings for the gain-scheduled feedback control was theoretically revealed by investigating the damped oscillation characteristics of the feedback control loop. Novel parameter settings yielded a natural frequency of the feedback control loop higher than that of the platform pitching motion and a damping coefficient larger than 1.0. Under these parameter settings, the platform pitching motion was similar to that under settings previously developed for this floating offshore system, while the power-output fluctuation was drastically reduced. High power–generation performance and a significant reduction in the damage equivalent fatigue loads at the main parts of the system were also obtained.

風力発電システムの独立翼ピッチ操作による翼荷重ゲインスケジューリング制御

A gain-scheduled control approach of blade loads using individual blade pitch manipulation is developed to reduce variations in aerodynamic loads of wind turbines during rotation. The motivation of the study is that the dynamic characteristics of the blade loads depend on the wind speed and blade pitch. The developed approach consists of the gain-scheduled feedback control (PI action) of the rotor speed using collective blade pitch and generator torque manipulations and the gain-scheduled feedback control (P action) of the d- and q-axis blade loads using individual blade pitch manipulation. The gain-scheduling strategy for the blade load control is developed through a numerical analysis of the NREL 5MW-onshore wind turbine-generator system using the aeroelastic simulation model (FAST) and the observed high wind speed data. The sensitive analysis of the proportional gain for the blade load control reveals that the stability limit gain decreases with the increase in the wind speed. The gain-scheduling strategy based on the stability limit gain reduces the variation in the blade loads during rotation as compared with the individual blade pitch manipulation using the fixed gain as well as the collective blade pitch manipulation without the blade load control.

Design of Control Algorithm for Floating Offshore Wind Turbines

We present a collective blade pitch control algorithm for floating offshore wind turbines. Control objectives are to regulate the rated power of a generator and to maintain the stability of platforms in region 3. To achieve these, the optimal control method with integral action is basically used with a Kalman filter. The Kalman filter is designed to estimate the wind speed as well as wind turbine and platform states. The performances of the designed controllers are tested and verified by comparing the standard deviations of the assigned parameters of our controllers with those of the standard controllers, according to the IEC standard conditions.