Collective Pitch Control Research Articles

Control of MW-Scale Wind Turbines for Fatigue Load Reduction and Performance Improvement

As the size of wind turbines are continuously increasing, fatigue loads must be reduced to obtain a more reliable condition for main components. Independent pitch control (IPC) strategy, based on the collective pitch control (CPC) strategy, provides an effective solution to load reduction on WT’s components caused by asymmetry loads. Meanwhile, IPC can also improve the performance of MW-scale wind turbine. This paper presents a dynamic model for 5 MW wind turbines of CSIC (Chongqing) Haizhuang Windpower Equipment Co., Ltd. Based on the dynamic model, an independent variable pitch controller has been investigated. Non-linear dynamic analysis has been conducted on wind turbine structures. During the turbine linearization, the blade flap-wise and tower F–A bending modes are considered. The tower, main shaft and blade are regarded as rigid bodies. Based on the dynamic model, the IPC controller is designed through d–q conversion. The results suggest that the IPC technique not only works well in stabilizing the output power, but may also effectively reduce the fatigue loads on the blades, main shaft, tower and hub. Under the identical conditions, the IPC presented in this paper has more advantages than a collective pitch controller, and can further reduce loads on key components. The IPC is of greater significance in control of wind turbines, and can lead to lower maintenance costs. However, IPC will raise the pitch-regulating speed, and further increase the fatigue load on the pitch mechanism. This problem requires further studies for a solution.

The influence of different mooring line models on the stochastic dynamic responses of floating wind turbines

Mooring line modelling plays an important role in predicting the dynamic response of the floating offshore wind turbines (FOWTs), especially under extreme load conditions. This paper investigates the influence of different mooring line models on the stochastic dynamic responses of a spar-type FOWT. A 16-degree-of-freedom (16-DOF) aero-hydro-servo-elastic model for the spar-type FOWT is first established using Euler-Lagrangian approach, taking into consideration the full coupling of the blade-drivetrain-tower-spar vibrations, a collective pitch controller and a generator controller. Three different mooring line models have been established and incorporated into the 16-DOF model, namely the linear spring model, the quasi-static model and the lumped-mass model, the last of which include the hydrodynamic loads, inertial force and damping force of the mooring cable. Stochastic dynamic analysis of the coupled FOWT-mooring line system is carried out using 3 different turbulent wind conditions and 4 different sea states and a total of 2160 10-min simulations. The mean value, the standard deviation and the extrapolated extreme value of the structural responses (blades, tower and mooring cable) as well as the fatigue equivalent loads are compared for the three different mooring line models.

Experimental investigation of dynamic inflow effects with a scaled wind turbine in a controlled wind tunnel environment

Dynamic inflow is a phenomenon that leads to a load overshooting, for example by fast pitch angle changes. Established engineering models for this effect are known to still have deficits. Advanced control mechanisms for turbines like collective pitch control in order to alleviate tower loads or individual pitch control increase the need for accurate dynamic inflow models. We performed fast pitching step experiments with our MoWiTO 1.8 (Model Wind Turbine Oldenburg 1.8 m) in the WindLab wind tunnel of the University of Oldenburg and investigated the load overshooting of blade loads, torque and thrust as well as velocity transients in the near wake. Dynamic inflow phenomena were clearly visible and their non-dimensional time constants matched existing full scale and lab size experiments. The transient process of the different measured loads and flow in the near wake is disussed in the paper to gain a deeper understanding of the underlying physical phenomena.

Vibration control of floating offshore wind turbines using liquid column dampers

This paper investigates the use of two different liquid column dampers for vibration control of spar-type floating offshore wind turbines (FOWTs). A 16-degree-of-freedom (16-DOF) aero-hydro-servo-elastic model for the FOWT is first established using multi-body based formulation and the Euler-Lagrangian equation, taking into consideration the full coupling of the blade-drivetrain-tower-spar vibrations, a collective pitch controller and a generator controller. It is found from the simulation results that due to the coupling to the spar rigid-body motion, the eigenfrequency of the tower vibration is significantly changed, which needs to be accounted for when tuning the liquid dampers. Tuned liquid column damper (TLCD) is investigated for controlling the lightly damped (due to low aerodynamic damping) tower side-side vibration and blade edgewise vibrations. Further, a newly proposed liquid column damper, the circular liquid column damper (CLCD) is also investigated for blade edgewise vibration control. The large centrifugal acceleration from the rotating blade makes it possible to use liquid column dampers with rather small masses for effectively suppressing edgewise vibrations. By properly tuning the dampers, both types of liquid column dampers are effective in mitigating tower and blade vibrations. Performances of the dampers are compared in terms of the control efficiency and practical considerations.

Integrated Pitch Control System Design of a Wind Turbine by Using Multiobjective Optimization

Abstract The integrated pitch control system of a three-bladed, variable-speed, horizontal-axis large-sized wind turbines consists of three coupled control loops. The first one is known as collective pitch control (CPC). The second one is the active tower damping control (ATDC), which is used to damp tower oscillation due to the pitching activity. Finally, the third one is the individual pitch control (IPC), which is used to mitigate loads on the blades. In the present work, the coupled controllers are jointly tuned by using multiobjective optimization. Simulation results show that the proposed tuning approach yields satisfactory performance for all control loops.

Collective Pitch Control with Active Tower Damping of a Wind Turbine by Using a Nonlinear PID Approach

The present work describes the results obtained from a nonlinear PID control (NPID) approach for the pitch control with top tower active damping of a large wind turbine. The control algorithm is implemented by using a well-known approach based on hyperbolic secant functions. A nonlinear PID controller and a nonlinear P controller are used for the collective pitch control and the active tower damping, respectively. As simulation platform, the 5-MW reference turbine developed by NREL and the software FAST are used. Results show that the NPID approach can provide significant improvements in the control performance in comparison to the classic control approach, in particular when the wind speed goes far from its rated value.

Fault-Tolerant Individual Pitch Control of a Wind Turbine with Actuator Faults

The sizes of wind turbines have been steadily increasing over the past decade, especially on the offshore sites. The greater structural flexibility of such machines necessitates considerations of reliable load mitigation techniques in order to alleviate the effects of asymmetric wind loads and fatigue. This paper proposes a reliable load mitigation scheme, referred to as ‘fault-tolerant individual pitch control’, which involves adjustments in pitch angles of the wind turbine blades individually in the presence of blade pitch actuator faults. The proposed scheme consists of a collective pitch control augmented with an individual pitch control, and a fault detection and diagnosis system. Simulation results obtained for a large-scale offshore wind turbine with a stochastic wind field illustrate effectiveness as well as fault-tolerance of the proposed scheme compared to the scheme based on control of pitch angles collectively. Moreover, the proposed scheme not only ensures more uniform output power with reduced loads on the turbine, but also tolerates the effects of possible faults in pith actuators of the blades.

Reliability analysis of active tendon-controlled wind turbines by a computationally efficient wavelet-based probability density evolution method

Summary We propose a computationally efficient stochastic framework and an associated mathematical formulation to assess the improvement in the vibration response of wind turbines with active structural controllers. Uncertainties in the turbulent wind speed as well as in the structural parameters are considered in the formulation for analyzing the 2 important (rated and cut-out) wind speed conditions. A multimodal wind turbine model based on a NREL 5-MW wind turbine model is adopted for analysis, and an active tendon controller is used to develop the stochastic framework. Dynamic interaction between the rotor blades and the supporting tower, as well as the coupling induced by the pretwist of the blades, and a collective pitch controller are considered. A time-evolving phase spectrum method together with a phase delay spectrum model is used to simulate the stochastic wind field, which is computationally less expensive. The probability density contours and the extreme value distributions of the blade tip displacements with and without the active controller are obtained by the wavelet-based probability density evolution method, followed by the computation of the corresponding failure probabilities. It is observed that the improvement in reliability and performance of the rotor blades with the controller becomes statistically more effective as the wind speed increases (e.g., significant beneficial effects are noticed from the probability distributions in terms of the spread at the cut-out wind speed).

Blade-pitch system for tidal current turbines with reduced variation pitch control strategy based on tidal current velocity preview

A collective pitch control system is designed using a rack and pinion gear set and hydraulic drive to provide an available blade pitch angle from 0° to 180°. This response accounts for the tidal current bi-directionality caused by flood and ebb under the moon's gravitational force. A front-installed hydraulic cylinder drives the rack-and-pinion system to synchronously turn the blades. Mechanisms are designed to overcome the difficulties of an outer hydraulic oil being deposited into the rotary hub and the pitch angle detection in a rotary hub. This simple and compact structure in a narrow hub reduces current blocking. Experimental results demonstrate that the pitch system has excellent dynamic characteristics. A tidal current velocity preview (CVP) method is proposed to overcome the difficulty of current velocity detection at the rotary impeller. Sealing is important for these underwater conditions. Hence, a reduced variation pitch control strategy is proposed based on CVP to reduce pitch action, which decreases mechanical wear of the sealing structure while extending the tidal current turbine's working life. Comparative results are obtained by using both an approximate sinusoidal current velocity data set simulating periods of real flow and a sea trial measured data set containing highly-turbulent and velocity-sheared flow in a semi-physical test. This test validated the significant reduction of pitch action and relatively high efficiency for energy generation.

Robust Model Predictive Control for Collective Pitch in Wind Energy Conversion Systems

When wind speeds exceed the rated values, wind turbines operate in region 3. In region 3, collective pitch control (CPC) is the main tool to regulate the turbine’s speed and generated power. The main challenges that face a CPC design are the modeling uncertainties, constraints on the control actions, and immeasurable system states. A tube-based model-predictive output-feedback controller is proposed here to design a CPC. The proposed controller is an optimal controller that respects constraints and accommodates uncertainties without a need to measure all states. Applications to a typical 5-MW offshore wind turbine show through simulations the superiority of the proposed controller.

Iterative feedback tuning of wind turbine controllers

Abstract. Traditionally, wind turbine controllers are designed using first principles or linearized or identified models. The aim of this paper is to show that with an automated, online, and model-free tuning strategy, wind turbine control performance can be significantly increased. For this purpose, iterative feedback tuning (IFT) is applied to two different turbine controllers: drivetrain damping and collective pitch control. The results, obtained by high-fidelity simulations using the NREL 5MW wind turbine, indicate significant performance improvements over baseline controllers, which were designed using classical loop-shaping techniques. It is concluded that iterative feedback tuning of turbine controllers has the potential to become a valuable tool to improve wind turbine performance.

Disturbance Accommodating Control Design for Wind Turbines Using Solvability Conditions

In this paper, solvability conditions for disturbance accommodating control (DAC) have been discussed and applied on wind turbine controller design in above-rated wind speed to regulate rotor speed and to mitigate turbine structural loads. An asymptotically stabilizing DAC controller with disturbance impact on the wind turbine being totally canceled out can be found if certain conditions are fulfilled. Designing a rotor speed regulation controller without steady-state error is important for applying linear control methodology such as DAC on wind turbines. Therefore, solvability conditions of DAC without steady-state error are attractive and can be taken as examples when designing a multitask turbine controller. DAC controllers solved via Moore–Penrose Pseudoinverse and the Kronecker product are discussed, and solvability conditions of using them are given. Additionally, a new solvability condition based on inverting the feed-through D term is proposed for the sake of reducing computational burden in the Kronecker product. Applications of designing collective pitch and independent pitch controllers based on DAC are presented. Recommendations of designing a DAC-based wind turbine controller are given. A DAC controller motivated by the proposed solvability condition that utilizes the inverse of feed-through D term is developed to mitigate the blade flapwise once-per-revolution bending moment together with a standard proportional integral controller in the control loop to assist rotor speed regulation. Simulation studies verify the discussed solvability conditions of DAC and show the effectiveness of the proposed DAC control design methodology.

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.

Turbulence upstream of wind turbines: A large-eddy simulation approach to investigate the use of wind lidars

Despite the evolution of wind turbines, the way in which in-situ meteorological information is obtained has not evolved much. Wind vane and cup anemometers, installed at the turbines nacelle, right behind the blades, are still used. This near-blade monitoring does not provide any time to readjust the profile of the wind turbine, and subjects the blades and structure to wind gusts and extreme incoming wind conditions. A solution is to install wind lidar devices on the turbine's nacelle. This technique is currently under development as an alternative to traditional in-situ wind anemometry because it can measure the wind vector at substantial distances upwind. However, most used wind lidar systems are optimized for measuring within a fixed upwind range, but at what upwind distance should they interrogate the atmosphere? This work uses Large Eddy Simulations to create a realistic atmospheric flow to evaluate optimal scanning distances to learn about the incoming turbulence as a function of wind farm configuration and atmospheric stratification. A correlation model, based on a modified truncated normal distribution, has also been developed, which could be implemented within the feed-forward collective pitch control of the turbine, allowing for improved wind turbine readjustments.

Numerical investigation of azimuth dependent smart rotor control on a large-scale offshore wind turbine

The smart fatigue load control of a large-scale wind turbine blade was numerically investigated on our newly integrated aero-servo-elastic platform with emphasis on the effect of azimuth angles. It was found that the smart control effectively reversed the phases of the flapwise aerodynamic force or the acceleration through the controllable deformable trailing edge flap (DTEF) activation within most of rotor azimuth angle range, turning in-phased flow-blade interaction into an anti-phased one at primary 1P mode, significantly enhancing the damping of the fluid-structure system and subsequently contributing to the greatly attenuatedflapwise fatigue loads on the blade and turbine performances. This aero-elastic control physics was most drastic as the investigations were focused on the case beyond the rated wind velocity, leading to the maximum reduction percentages in the time-averaged and azimuth-averaged fatigue loads up to about 30.0%, in contrast to the collective pitch control method. In addition, the finding pointed to a crucial role that the suppression of the coupled flow-blade system dependent on azimuth angles played in the smart blade control, which might guarantee better effectiveness if it would be considered in the development of the DTEF controller.

Optimal tuning of multivariable disturbance‐observer‐based control for flicker mitigation using individual pitch control of wind turbine

Multivariable disturbance accommodated observer based control (DOBC) scheme is presented to mitigate loads generated due to wind shear and tower shadow using individual blade pitch for above-rated wind speed condition of wind turbine. Wind shear and tower shadow add flickers as 1p, 3p, 6p and so on, (p is the rotor rotational frequency) for three-bladed wind turbine. Novel DOBC with individual pitch control (IPC) to mitigate the flickers is presented and linear state-space model of wind turbine with tower dynamics is developed. The proposed controller is tuned using optimal control theory to reduce fatigue of drive-train, tower and to regulate output power. The authors have tested the controller on NREL's 5 MW wind turbine, FAST (fatigue, aerodynamics, structures and turbulence) code is used for load modelling and MATLAB/Simulink is used for the simulation. A comparison of power spectral density of generator speed, drive-train torsion and tower fore-aft moment shows better mitigation to the flickers by proposed controller as compared with proportional–integral (PI) and disturbance accommodation control (DAC) with collective pitch control. Furthermore, it shows less degradation in the performance as moving away from the operating point for above-rated wind speed condition of wind turbine. It is concluded that proposed multivariable controller shows better mitigation to turbulent and cyclic aerodynamic loads, provide better regulation to output power using IPC of wind turbine and increased the lifetime of drive-train torsion and tower as compared with PI and DAC.

Wind turbine blade optimisation with individual pitch and trailing edge flap control

Individual pitch control and trailing edge flaps have been shown to be capable of reducing flapwise fatigue loads on wind turbine blades, with research to date focusing on controller development and performance assessment. This work covers development of a blade optimisation process which integrates synthesis of individual pitch and trailing edge flap controllers to evaluate their impact on the levelised cost of energy. The optimisation process selects blade chord, twist and material distributions, along with the spar cap width, and integrates a turbine cost and mass model with existing simulation codes. Constraints based on ultimate stresses, fatigue damage, blade deflection, resonant frequency, and rotor thrust are considered. Using the NREL 5 MW reference turbine as an initial point, reductions in the levelised cost of energy of 1.05% are obtained with collective pitch control only, while the addition of individual pitch control increases this reduction to 1.17%. The use of trailing edge flaps on top of individual pitch control increases the reduction in the levelised cost of energy to 1.27%. Blade mass and material cost reductions from 13.6 to 16.4% and 18.1–21.5% respectively are also obtained. Optimised blade designs are driven by blade deflection, rotor thrust and ultimate stresses in the spar cap.

Rotor aeromechanics study using two different blade property data sets

Purpose This paper aims to compare the comprehensive rotorcraft analyses using the two different blade section property data sets for the blade natural frequencies, airloads, elastic deformations, the trimmed rotor pitch control angles and the blade structural loads of a small-scale model rotor in a blade vortex interaction (BVI) phenomenon. Design/methodology/approach The two different blade section property data sets for the first Higher-harmonic control Aeroacoustic Rotor Test (HART-I) are considered for the present rotor aeromechanics analyses. One is the blade property data set using the predicted values which is one of the estimated data sets used for the previous validation works. The other data set uses the measured values for an uninstrumented blade. A comprehensive rotorcraft analysis code, CAMRAD II (comprehensive analytical model of rotorcraft aerodynamics and dynamics II), is used to predict the rotor aeromechanics such as the blade natural frequencies, airloads, elastic deformations, the trimmed rotor pitch control angles and the blade structural loads for the three test cases with and without higher-harmonic control pitch inputs. In CAMRAD II modelling with the two different blade property data sets, the blade is represented as a geometrically nonlinear elastic beam, and the multiple-trailer wake with consolidation model is used to consider more elaborately the BVI effect in low-speed descending flight. The aeromechanics analysis result sets using the two different blade section property data sets are compared with each other as well as are correlated with the wind-tunnel test data. Findings The predicted blade natural frequencies using the two different blade section property data sets at non-rotating condition are quite similar to each other except for the natural frequency in the fourth flap mode. However, the natural frequencies using the predicted blade properties at nominal rotating condition are lower than those with the measured blade properties except for the second lead-lag frequency. The trimmed collective pitch control angle with the predicted blade properties is higher than both the wind-tunnel test data and the result using the measured blade properties in all the three test cases. The two different blade property data sets both give reasonable predictions on the blade section normal forces with BVI in the three test cases, and the two analysis results are reasonably similar to each other. The blade elastic deformations at the tip using the measured blade properties are correlated more closely with the wind-tunnel test data than those using the predicted blade properties in most correlation examples. In addition, the predictions of blade structural loads can be slightly or moderately improved by using the measured blade properties particularly for the oscillatory flap bending moments. Finally, the movement of the sectional centre of gravity location of the uninstrumented blade has a moderate influence on the blade elastic twist at the tip in the baseline case and the oscillatory flap bending moment in the minimum noise case. Practical implications The present comparison study on rotor aeromechanics analyses using the two different blade property data sets will show the influence of blade section properties on rotor aeromechanics analysis. Originality/value This paper is the first attempt to compare the aeromechanics analysis results using the two different blade section property data sets for all three test cases (baseline, minimum noise and minimum vibration) of HART-I in low-speed descending flight.

Control design for a two-bladed downwind teeterless damped free-yaw wind turbine

In this paper, a control architecture for a two-bladed downwind teeterless damped free-yaw wind turbine is developed. The wind turbine features a physical yaw damper which provides damping to the yawing motion of the rotor-nacelle assembly. Individual Pitch Control (IPC)11List of abbreviations: Individual Pitch Control (IPC); Collective Pitch Control (CPC); Linear Individual Pitch Control (LIPC); Multi-Blade Coordinate (MBC); Multi-Blade Coordinate (MBC); Out-of-Plane (OoP); In-Plane (IP); Variance Accounted For (VAF); Damage Equivalent Load (DEL); Extreme Direction Change (EDC); Proportional Integral (PI).List of symbols: Ttrq: Demanded generator torque for speed regulation; Tdtd: Demanded generator torque for drivetrain damping; Tgen: Demanded generator torque; Ωrated: Rated generator speed setpoint; Ωgen: Generator speed control setpoint; θcol: Collective blade pitch angle; θcol: Collective blade pitch angle; θ1: Individual blade 1 pitch angle; θ2: Individual blade 2 pitch angle; Θ1: Blade 1 pitch angle; Θ2: Blade 2 pitch angle; θtilt: Non-rotating blade pitch setpoint for tilt coordinate; θyaw: Non-rotating blade pitch setpoint for yaw coordinate; My, 1: Blade 1 Out-of-Plane root bending moment; My, 2: Blade 2 Out-of-Plane root bending moment; Mx, 1: Blade 1 In-Plane root bending moment; Mx, 2: Blade 2 In-Plane root bending moment; Mtilt: Rotor tilt moment; Myaw: Rotor yaw moment; ψ: Rotor azimuth angle; ψoff: Azimuth angle offset; ϕref: Yaw setpoint angle; ϕ˙yb: Yaw bearing angular velocity; n: n’th harmonic of load; ϕyaw: Yaw misalignment (between turbine yaw position and wind direction); ϕ: Yaw error (controller input); Lθ: Low-pass filter for individual pitch angles; Lϕ: Low-pass filter for yaw misalignment signal; HIPC: High-pass filter for individual blade pitch signals; Lyb: Low-pass filter for yaw bearing angular velocity signal; fIPC: Cut-off frequency of high-pass filter for individual pitch control κ: Gain of yaw model; τ: Time constant of yaw model; Td: Time delay in yaw model. is employed to obtain yaw control so as to actively track the wind direction and to reduce the turbine loads. The objectives of both load and yaw control by IPC are conflicting and therefore two decoupling strategies are presented and compared in terms of controller design, stability, and turbine loads. The design of the different controllers and the physical yaw damping are coupled and have a large impact on the turbine loads. It is shown that the tuning of the controllers and the choice of the yaw damping value involve a tradeoff between blade and tower loads. All results have been obtained by high-fidelity simulations of the state-of-the-art 2-B Energy 2B6 wind turbine.