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Performance enhancement of wind turbine systems with vibrationcontrol: A review

Mahmudur Rahman n,1, Ong Zhi Chao n,2, Chong Wen Tong, Sabariah Julai, Khoo Shin YeeQ1

Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 3 July 2014Received in revised form10 May 2015Accepted 26 May 2015

Keywords:Control policiesDamping devicesRenewable energyPerformance enhancementVibration mitigationWind turbines technology

a b s t r a c t

Renewable energy becomes an asset to the world's energy resource for its eco-friendly and low costenergy production feature. As an important renewable energy source, wind turbine technology hasbecome a significant contributor to the world energy production because of its feasible production cost,reliability and efficiency. Researchers are very active to optimize the effectiveness of wind turbineswhich may lead to increase the productivity of this source of energy. Vibration in the wind turbinesystem affects the productivity and thus reduces efficiency. Vibration of a system cannot be destroyedbut can be reduced or converted to energy using appropriate strategies. Vibration control systemimproves structural response of wind turbines and reliability which has impact on lifetime of thecomponents. Lowering the vibration amplitude of a system will provide a lesser amount of noise, assureuser and operating comport, maintain the high performance and production efficiency. These will assistthe system to prolong the lifetime of an industrial structure or machinery. Also vibration controlenhances the performance of wind turbines providing suitable work environment without externaldisturbance. This paper presents an ample review on performance enhancement of the wind turbines byvibration mitigation. The aim of this review is to provide a concise point for researchers to assess thecurrent trend to control vibration of wind turbines technology. This paper will focus on main vibrationcontrol techniques of wind turbine structures. It provides the applications of passive, active and semi-active and vibration control strategies for structures, especially for wind turbines. Besides, this paperreviews on damping devices needed for vibration mitigation of structures. These damping devices havebeen implemented extensively in wind turbines for increasing their efficiency by mitigating vibration.This paper also reviews and assesses the performance of different control policies to control the systeminput and power input of damping devices.

& 2015 Published by Elsevier Ltd.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Vibration control strategies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1. Passive control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2. Active control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3. Semi-active control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. Vibration control dampers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.1. Tuned mass damper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2. Tuned liquid damper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.3. Controllable fluid damper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.4. Other vibration control dampers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4. System controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Acknowledgement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

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Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/rser

Renewable and Sustainable Energy Reviews

http://dx.doi.org/10.1016/j.rser.2015.05.0781364-0321/& 2015 Published by Elsevier Ltd.

n Corresponding authors.E-mail addresses: [email protected] (M. Rahman), [email protected] (O.Z. Chao).1 Tel.: þ60 163443236.2 Tel.: þ60 379676815.

Please cite this article as: Rahman M, et al. Performance enhancement of wind turbine systems with vibration control: A review.Renewable and Sustainable Energy Reviews (2015), http://dx.doi.org/10.1016/j.rser.2015.05.078i

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www.elsevier.com/locate/rser

http://dx.doi.org/10.1016/j.rser.2015.05.078



mailto:[email protected]

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1. Introduction

Power generation using wind turbines is an effective andpotential source to meet the electricity requirement all over theworld nowadays. Wind energy is utilized for different purposessuch as rural electrification, at street lamp post, to charge electricvehicles and even to operate wind turbines to generate electricity.Wind energy is widely used in rural electrification mainly in lessdeveloped country and low-income households. Several studiespresented utilizations of wind energy as renewable energy sourcefor rural electrification [1–4]. Wind energy is also suggested to beused to charge electric vehicles in Netherlands in 2020 [5]. Windenergy is also used for outdoor/street lighting system [6]. Thereare many different types of wind turbines which were proposedand developed in the past depending on the orientations of theblade and configurations. But they can be grouped into twonamely: horizontal axis wind turbines (HAWTs) and vertical axiswind turbines (VAWTs) based on the orientation of their axis ofblade rotation. HAWTs have been the most implemented windturbines technology since few decades; however, VAWTs have alsobecome very prominent in this technology in the recent decades.The initial development of HAWTs and VAWTs is studied in [7].The blades of the HAWTs rotate horizontally and perpendicular tothe wind. Researchers proposed different types of HAWTs basedon different methods and configurations to enhance their effi-ciency [8–14]. HAWTs are typically tall which assists the system toface much higher and stronger winds to generate power effec-tively. However, installation and maintenance cost of HAWTs isrelatively high because the installation kits are placed on the top ofthe tower inside the nacelle. So it becomes difficult and costlywhen it needs to be repaired. On the other hand, VAWTs consist ofblades with the vertical axis of rotation and they do not have to bearranged to any specific direction to face the wind. Anotheradvantage of the VAWTs is that the installation box which includesgenerator, gearbox, etc. can be placed at the basement of thetower. It makes the maintenance and repair work easy and simple.The advantages, different designs and optimizations of VAWT arepresented in several research papers [15–18]. Different designsand configurations of VAWTs are also reviewed in [19,20]. Darrieustype wind turbine has the highest value of efficiency althoughproblems of low starting torque and poor building integration arethe major drawbacks of the system. This type of wind turbine iscalled as ‘lift type’ wind turbine where lift forces on the bladesresult the rotor to rotate and produce electricity. Savonius rotortype wind turbine is another type of VAWT which has less

efficiency value than Darrieus type and not used for high powerapplications. However, self-starting capability is the importantadvantage of savonius type wind turbine compared to lift typewind turbines. Four aerodynamic models for VAWT have beenanalysed in Ref. [20] to highlight the performances as well asadvantages and disadvantages. The blade element momentum andcascade models have good power predictions capability and fastcomputational times but may fail to predict instantaneous bladeforces. However, vortex and panel models can simulate the wakeof the VAWT and have capability to model a rotor which consistsof multiple rotating bodies. The rotation axis for horizontal windturbine is horizontal or parallel to the ground. For big wind areas,HAWTs are very popular and produce more energy. However,HAWTs need big space and are generally heavier which are notefficient enough in turbulent winds. HAWTs are generally installedin high wind areas, especially in sea areas. The rotation axis ofvertical wind turbine is perpendicular to the ground and mostlyused in residential applications. VAWTs work well under turbulentwind conditions, thus VAWTs are efficient where wind is notconsistent. VAWTs are capable of generating power from low windand they are generally positioned in the low wind urban environ-ment. Unlike HAWT, VAWT does not have a tower and is situatedat the ground where the wind is low and turbulent. Also thedirection of blade rotation encourages the structure to be at lowwind environment for better performance. There are several otherfactors that affect the efficiency of HAWTs and VAWTs. Few factorsand possible solutions for them are presented in [21]. Severalchallenges have been highlighted for performance degradation ofwind turbine technology which are maintaining performanceefficiency, intermittent nature of wind supply, global industrializa-tion, fossil fuel energy market, social acceptability of on-shorewind power, cost, technical and climate change of off-shore windpower, competition from other clean energy competitors, policyinstability, etc. The possible solutions are suggested in the study toenhance the performance of wind turbines and some of theimportant solutions are proper maintenance of different machin-ing parts such as blades, gearbox, bearing, etc. to ensure goodperformance, ensuring adequate nature wind supply, reducingnoise of heavy weight wind turbines, reducing cost and solvetechnical issues such as place of installation, designs, etc. Further-more, the HAWTs need higher wind speeds for generating asmaller amount of electricity and thus they become less importantin the urban environment. HAWTs and VAWTs have differentefficiency levels based on their direction of rotation, configurationsand they are compared in [22–24]. Table 1 shows the comparison

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Nomenclature

Abrreviations

HAWT horizontal axis wind turbineVAWT vertical axis wind turbineTMD tuned mass damperSTMD semi-active tuned mass damperTLCD tuned liquid column damperMTMD multiple tuned mass damperTLD tuned liquid damperER electrorheological

MR magnetorheologicalBVA ball vibration absorberLQR linear quadratic regulatorLQG linear quadratic GaussianMPC model predictive control

Symbols

U actuating forcek spring co-efficientsc damping co-efficients

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between HAWT and VAWT to highlight their advantages anddisadvantages based on configurations and efficiency.

From the comparison it can be concluded that HAWTs aregenerally installed in high wind areas and VAWTs in urban andlow wind environments. HAWTs have larger structures and moreproduction efficiency. VAWT installation and maintenance cost is lowcompared to HAWT because of small structure and operatingcomponents placed in the ground. The actual examples of thesetypes of wind turbines with different performance analyses arepresented in Refs. [22–25]. When wind turbine technology becomesone of the most used technologies for power generation, researchershave indicated few limitations of wind turbines technology based onthe dynamics, structure, and control systems. Therefore, researchfocus on wind turbine technology has been increased to improve theproductivity and effectiveness. One of the major limitations of thistechnology is the structural complexity. One of the main reasons ofthis complexity is exciting force of the wind turbines which involvesdifferent levels of vibration in the system. Based on the aboveresearch papers and study, the dynamic behaviour of HAWTs andVAWTs is analysed. Because of the tall towers and high wind speed,high vibration occurs in the HAWTs towers and blades. HAWTs alsoexperience vibration problem because of the heavy loads on the topof the tower which includes generator, gearbox and other installationkits. VAWTs are also affected by vibration because the ground airflow creates turbulent flow. Vibration in the wind turbines systemreduces efficiency and thus requires implementing vibration controlsystem. The level of vibration is then controlled with designoptimization using different vibration control methods. Wind

excitation, earthquake excitation and waves (for offshore turbines)are the major sources of structural vibration of wind turbine bladesand tower. Significant interest has been shown to research anddevelopment to overcome the vibration problem in recent years inwind turbine technology. As a consequence, the main vibrationcontrol devices such as passive, active and semi-active devices havebeen implemented with huge interest. Definitions and applicationsof these three major classes of vibration control devices in structureswere reviewed in [26]. The passive control system is capable ofenhancing the structural damping, stiffness and strength. Active andsemi-active control systems employ external controllable forcedevices with integrated sensors, controllers and signal processing[27]. The goal of these control methods is to maximize the perfor-mance of wind turbine energy generation by minimizing the vibra-tions of the wind turbine drive train and tower. For energydissipation, damping devices are generally used in the system toprovide necessary damping forces. There are different types ofdamping devices that have been implemented for structural vibra-tion control such as tuned mass dampers, tuned liquid mass damper,controllable fluid dampers, etc. The use and effectiveness of thesedampers in wind turbine structures are reviewed in this study.System controllers play an important role to maximize the perfor-mance of the system. Therefore, different control policies applied formitigating vibration in wind turbines are also reviewed. The contentstructure of this review is shown in Fig. 1.

To prepare this study, an extensive review of the previousresearch works on wind turbine technology and its vibrationcontrol method is conducted. Suitable research papers are

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Table 1Comparison between HAWT and VAWT [20,23,25].

HAWT VAWT

Rotating axis of the wind turbine remains horizontal, or parallel with the ground Rotating axis remains vertical, or perpendicular to the groundIt is able to produce more electricity from a given amount of wind It produces up to 50% more electricity on an annual basis versus conventional

turbines with the same swept areaIt is suitable for big wind application It is suitable for small wind projects and residential applicationsComparatively heavier and not suitable for turbulent winds Lighter and produce well in tumultuous wind conditionsHAWTs only are powered with the wind of specific direction Vertical axis turbines are powered by wind coming from all 3601, and even

turbines are powered when the wind blows from top to bottomNot suitable to generate electricity from the wind speed below 6 m/s and generally cutout speed 25 m/s

Generates electricity in winds as low as 2 m/s and continues to generatepower in wind speeds up to 65 m/s based on the model

They cannot withstand extreme weather conditions due to frost, freezing rain or heavysnow plus heavy winds in excess of 50 m/s

Withstands extreme weather such as frost, ice, sand, salt, humidity, and veryhigh wind conditions in excess of 60 m/s

Birds are injured or killed by the propellers since they are not solid objects so the birdsfly into the blades

Does not harm wildlife as birds can detect a solid object and can be seen onaircraft radar

Most are self-starting Low starting torque and may require energy to start turningDifficult to transport and install Lower construction and transportation costsControl system, generator and gearbox are positioned at the top of the tower at around100 m above the ground which require larger and stronger structures.

Transmission and generation systems are positioned at the bottom of thestructures which requires small support

Environment is quite noisy because of larger structures VAWT has less noise while in operation compared to HAWT

Fig. 1. Content structure of the review.

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collected from different online databases. A short summary of thereferences with the database list is provided in Table 2.

2. Vibration control strategies

There are several areas that can be improved to enhance thewind turbine technology, an important area being structuralcontrol of the turbine blades, tower and other supporting compo-nents. Several vibration control approaches have been developedand implemented in the past on different structures such as tallbuildings, long bridges and wind turbines to reduce vibration fordifferent external excitations. The necessity and applications ofstructural control approaches were presented in Ref. [28]. Threeimportant vibration control techniques which are being usedfrequently in the structures are mentioned below:

� Passive control.� Active control.� Semi-active control.

2.1. Passive control

The passive control method is simple and reliable. This controlmethod does not require external force [29]. The passive controlmethod is easy to implement to reduce the structural vibrationand it is widely used in wind turbine technology for enhancement.A conventional vibration control technique, consisting of springsand dampers only, is referred as passive control device. Theworking principle of the springs and dampers in a passive controldevice is to either absorb vibration energy or loading the trans-mission path of the disturbing vibration. Good vibration control ofthe structure is the impact of this device. Basic principles of the

passive control method are presented in [27]. The passive controlsystem may include sensors to measure the excitation amplitude.Since there are no external control forces, the vibration amplitudeis minimized by controlling internal forces provided by the motionof the points of attachment [30]. System controllers are alsodesigned to control the input forces. Several researchers adoptedthe passive control system in various applications for vibrationcontrol for its simplicity and efficiency [31–33]. Fig. 2 shows thebasic structure of passive vibration control approach where noexternal forces are required. The parameters k and c representspring and damping co-efficient respectively for a single-degree-of-freedom structural model.

Passive control approach is simple and effective; it has beenextensively used in structural loading and vibration control [34–36]. Passive control devices have also been developed and imple-mented in wind turbines with a large number for performanceenhancements. A passive control method was presented byMurtagh et al. [37] to mitigate the vibration of the blades andtower. A tuned mass damper (TMD) was used for energy dissipa-tion and Fourier transform approach was implemented to find thedisplacement at the top of the tower. To improve the structuralresponse of wind turbines, passive control approach was pre-sented in [38]. Also this study presented calculation of optimalparameters using parametric study for the passive control method.In another study, passive structural responses at offshore windturbines are simulated [39]. The wind turbines are modelled asmultiple degree of freedom structure and simulated subjected toboth strong and moderate wind and wave loadings. Passivestructural control was also presented in [40]. In this study, theeffect of passive damping on a 40 m high and 500 kW pitchregulated three-bladed horizontal wind turbine was investigated.

2.2. Active control

The active control system provides enhanced structural beha-viour of a system and it consists of force delivery devices, real-timedata processors and sensors. Active control devices employ one ormore actuators which apply torques or forces to the structureaccording to a control law. External actuators provide requiredforces to mitigate the structural vibration. Real-time data proces-sors process measured information and calculate the necessaryforces to counter the measured vibration amplitude. Sensors areused to measure excitation amplitude, structural response ampli-tudes, etc. [41]. The input control forces are provided based on theacquired information from sensors that measure the excitationinput and the response of the system structure. The control forcesare generated by electro-hydraulic actuators which require largepower sources. Active control systems enhance the performance inresponse control with the capacity of control using actuators.Researchers applied active control system to various applications

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Table 2Summary of references of the review paper.

Section Reference numbers

Introduction 1–27Vibration control strategies � Passive: 27–40

� Active: 27, 41–55� Semi-active: 27,41, 49–69

Vibration control dampers � Tuned mass damper: 37–38, 70–78� Tuned liquid damper: 39, 79–92� Controllable fluid damper: 69, 93–100� Other vibration control dampers: 101–109

System controllers 29,67, 68, 110–113

Fig. 2. (a) Structure with Passive Energy Dissipation (PED) [27] and (b) typical construction of passive control.






to mitigate vibration of the system [42–45]. Fig. 3 shows the basicstructure of active vibration control approach where externalcontrol actuating force (U) is applied. The parameters k and crepresent spring and damping co-efficient respectively for asingle-degree-of-freedom structural model. Under excitation, thestructural behaviour is measured through sensors and the pro-cessors provide the required force to control actuator. Then thecontrol actuator inputs required force to minimize unwantedamplitude of the structure.

The implementation of active control approach has receivedsignificant attention in recent years to deal with structural vibra-tion due to different excitations. The design, installation andperformance of the active control techniques on a full-scalestructure were presented in [46]. Some other investigations onactive control approach for different structural vibration controlswere presented in [47–51]. Passive vibration control can becombined with active vibration control and an integrated pas-sive–active vibration control system is evaluated to reduce vibra-tory power transmission for a rigid body connected to a platestructure in [52].

Active control approach is another useful and widely usedoptimization method in wind turbines to control structural vibra-tion. Staino and Basu proposed a modelling and control of windturbine vibrations due to the change of rotational speed of theblades [53]. An active controller is presented by Krenk et al. [54] toreduce wind induced edgewise vibrations and the simulations ofthis method show the improvement of the blade response in termsof vibration. The active vibration control method is also applied tothree bladed wind turbine rotors with 42 m blades where actua-tor–sensor pairs are implemented to each blade. The actuators aretuned to supply resonant damping of the collective and whirlingmodes by using separate resonance characteristics of both themodes. In another study, an aerodynamic device, synthetic jet

actuator for active vibration control approach was investigatedexperimentally to control the vibration of wind turbine blades[55].

2.3. Semi-active control

To achieve the full advantages of passive and active controlapproaches in structural control applications, a new controltechnique has been introduced in recent years. Semi-active controlapproach extracts some best features from passive and activecontrol approaches to provide best possible structural vibrationcontrol. Compared to the passive control method where thecontrol forces are developed from the motion of the structureitself, appropriate adjustable mechanical devices are used toprovide control forces for semi-active method. Semi-activeapproach has become attractive for structural vibration controlapplications due to controllable damping and low power require-ment for operating damping devices. Several studies presentedsemi-active vibration control method for different applications[56–58]. Fig. 4 shows the basic structure of semi-active vibrationcontrol approach where controllable damping device is imple-mented. The structure of a semi-active control system is quitesimilar to active control system except the external control force.Semi-active control system consists of external devices capable ofproviding adjustable control forces. Therefore, semi-active devicesare often called as controllable passive devices [41].

A lot of research proposed semi-active vibration control toautomotive applications [59–61]. However, for the first time in thefield of structural engineering, semi-active structural controlapproach was proposed subjected to environmental loads byHrovat et al. [62] few decades ago. From then, this approachbecame very attractive for structural vibration control wherevibration occurs due to earthquakes and heavy wind loads.

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Fig. 3. (a) Structure with active control [27] and (b) typical construction of active control.

Fig. 4. (a) Structure with semi-active control [27] and (b) typical construction of semi-active control.






Researchers found semi-active vibration control as a low cost andeffective control strategy with controllable damping devices [63–67].

Wind turbines technology is effective to generate power butsometimes vibration occurring in the system affects its effective-ness. The effectiveness of wind turbines decreases when powerrequirement is high to control the vibration. Semi-active controlapproach is an effective solution to control the vibration level withlow power consumption which makes the wind turbine moreeffective. Semi-active vibration control approach was investigatedto mitigate wind turbines' vibration by many researchers. Karimiet al. investigated semi-active control device to mitigate thevibration of offshore wind turbine tower [68]. Tuned liquid columndamper was used with a controllable valve as an external dampingdevice to deal with wind turbine vibrations due to wind load andearthquake excitations. Kirkegaard et al. also applied semi-activesystem to wind turbine tower to reduce the vibration numericallyand experimentally [69]. The wind turbine response was mea-sured using a shaking table experimental model with a control-lable fluid damper attached as a damping device. Semi-activecontrol approach appears to be most attractive nowadays for itsnature of structure which offers reliability of passive and adapt-ability of active devices.

3. Vibration control dampers

Vibration control dampers are very important to dissipateenergy from a structure. A wide range of vibration controldamping device has been developed to control unwanted vibra-tion in the structure. Among them, few devices such as tuned massdamper, tuned liquid damper, and controllable fluid dampers havebecome very popular for their capability of improving structuralresponse. These dampers are implemented in the structure aspassive, active and semi-active damping devices. The applicationsand effectiveness of these damping devices in structural vibrationcontrol are discussed in this section.

3.1. Tuned mass damper

The concept of tuned mass-damper (TMD) was first presentedin the 1940s by Den Hartog [70]. TMD consists of a secondary massplaced top of the primary structure with spring and dampingelements. It provides a frequency-dependent hysteresis character-istic that increases damping in the main structure. The efficiencyof TMD in reducing structural vibrations is now well establishedand is widely used in tall buildings, bridges, and towers forvibration control purpose. It is also a well known strategy foroptimizing wind turbine power generation efficiency by mitiga-tion vibration. TMD can be used as a simple passive device;however, the effectiveness of the passive TMD can also beimproved using external actuator force (called as active TMD) orcontrollable force (called as semi-active TMD). A TMD was con-structed in the wind turbine nacelle in [71] to mitigate thevibration of the tower. Generally a TMD, representing a springdamper system, is placed on top of the main structure to counter-act and reduce extraneous vibration. The construction of TMD forvibration control of wind turbine tower is shown in Fig. 5.

Recently, wind turbine tower vibration control using passiveand active TMD has been investigated in [38] and [72] respectively.In the first paper, Lackner and Rotea performed a parametric studyto achieve optimal parameters of passive TMD and then simulatedwind turbine models with equipped TMD. The results showed theimprovement in the response of the offshore wind turbinetechnology and demonstrated the potential of the active TMD inwind turbines. Rotea et al. designed and constructed active TMD

for wind turbine structural control in another study. Simulationstudy showed a clear improvement of response using active TMDcompared to passive control. The responses of wind turbine towerusing TMD for passive and active control approaches are comparedin Fig. 6.

Another paper investigated the along wind forced vibrationcontrol system using passive TMD for a simplified wind turbinesystem [37]. Recently, TMD has been used in the floating windturbine model by national renewable energy laboratory to controldeflections of tower and blades [73]. Using this method, reductionrates of deflections at tower and blades are 50% and 40% respec-tively. There are two main types of vibration occurring in the windturbine blades. Edgewise vibration is the vibration which occurs inthe plane of rotation of the blades, whereas flapwise vibration isout of the rotation of the blades. Fitzgerald et al. investigated theuse of active TMD for mitigation of both edgewise [74] andflapwise [75] vibrations of wind turbine. A Euler–Lagrangian windturbine mathematical model has been implemented for thispurpose by considering the structural dynamic system and theinteraction between in-plane and out-of-plane vibrations.Recently, semi-active tuned mass damper (STMD) was proposedto control flapwise vibrations in wind turbine blades and simula-tion results show that STMD successfully reduced the response ofthe wind turbine due to wind induced excitation [76]. However,based on the effectiveness of TMD for structural vibration control,researchers proposed an optimization method of TMD which wasnamed as multiple tuned mass dampers (MTMD) to improve thevibration control system. Collins et al. investigated multiple tunedmass dampers (MTMD) to optimize wind turbine system where

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Fig. 5. Construction of TMD in wind turbine [71].

Fig. 6. Comparison of wind turbine tower response between passive control andactive TMD control system [72].






the multiple mass damping system was connected with theturbine towers [77]. The use of MTMD was also investigated formitigation of edgewise vibration of tower/nacelle and spar of spar-type floating wind turbine [78]. The results showed that thismethod with passive control approach can reduce the nacellesway displacement up to 50% compared to TMD which can reduceup to 40%. As of today, the use of TMD in wind turbine technologybecomes very popular because of its simplicity and effectiveness;thus it remains an important issue to study.

3.2. Tuned liquid damper

Another important and commonly used damping device forstructural vibration control is tuned liquid damper (TLD). A TLD isalso used to design passive, active and semi-active device forstructural vibration control. Among different types of TLD, TunedLiquid Column Damper (TLCD) is the most feasible and efficientdamper to solve vibration problem due to high excitation force. ATLCD is generally modelled as U-shaped tube which is partiallyfilled with a volume of liquid and it acts as a mass of the damper.When a TLCD is attached to a structure and the structure isexcited, liquid of TLCD oscillates through the column and re-establishes the system to equilibrium. A TLCD is typically placedon top of the structures. Two decades ago, Sakai et al. proposed aTLCD to suppress wind-induced horizontal motion of high risebuildings [79]. The performances of single-tuned liquid column

damper (STLCD) were investigated in [80], whereas multiple-tuned liquid column damper (MTLCD) was investigated for windapplications in [81,82]. A hybrid tuned liquid damper was alsoproposed for structural vibration control in [83]. Other than U-tube shape, rectangular and crossed tube-like containers were alsoproposed for tuned liquid damper (TLD) for structural vibrationcontrol in [84,85] respectively. Different types of TLDs and theirapplications in structural vibration have been studied in [86]. Theuse of TLD for building has been adopted to control vibration atwind turbine because of its success in structural engineering. Fig. 7shows the schematic diagram of the wind turbine vibrationcontrol method by using TLCD where two TLCDs are applied.

The effectiveness of TLD in reducing the structural response ofwind turbines under harmonic and random excitations wasinvestigated in [88]. Annular TLD was proposed in this study andsimulation was done based on single-degree of freedom hybridwind turbine model. This study found that the used TLD method iseffective for small amplitude of excitations. For large amplitude ofexcitations, TLCD damper was investigated in many studies forstructural vibration control. Colwell and Basu investigated TLCD onmultiple-degree-of-freedom offshore wind turbine system to sup-press the vibration subjected to both ‘moderate and strong’ windand wave loadings [39]. This study found that TLCD is effective forlarge amplitude of excitations and can reduce vibration up to 55%of peak response of wind turbines compared to the same systemwithout TLCD. TLCD has also been studied to control the structural

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100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132Fig. 7. (a) Schematic diagram of wind turbine with TLCDs and (b) TLCD model [87].






response of wind turbines in [89]. The study found that TLCDenables significant damping to the wind turbine system and thusoffers cost reduction to increase the efficiency of the system.Structural response minimizations and reliability improvement ofwind turbine technology using TLCD were presented in [87]. Thestudy simulated a wind turbine model with TLCD and the resultsshow significant reductions in tip displacement and bendingmoment which are shown in Fig. 8.

Chen and Georgakis proposed a spherical TLD for vibrationcontrol in wind turbines [90]. The spherical TLD consists of twolayers of hemispherical shape containers which are partially filledwith water. The radius of spherical containers is determined by itsfrequency and the mass of the sloshing water. The dynamicresponses were reduced significantly using shake table test fordifferent excitation loads. The dynamic behaviour and vibrationcontrol response of wind turbine blades were investigated usingpassive TLCD in [91]. The effectiveness of using TLCD on windturbine blades in stationary horizontal position was presented toachieve desired vibration control response. Recently, Zhang et al.investigated the edgewise vibration control of wind turbine bladesusing TLCD [92] and achieved improved results.

The study found that TMD and TLCD are mostly implementedin HAWTs, but they can also be implemented in VAWTs. Aschematic diagram of vertical axis wind turbine is shown inFig. 9 which presents an idea for the implementation of TMD orTLCD. The wind flow direction for VAWT is not uniform, therefore,the direction of the external force either needs to be fixed or theposition of the dampers needs to be changed for the implementa-tion of TMD or TLCD in VAWTs.

3.3. Controllable fluid damper

Controllable fluid damper is a class of semi-active deviceswhich uses controllable fluids inside the damper. Electrorheologi-cal (ER) and Magnetorheological (MR) fluid dampers are the twomost commonly implemented controllable semi-active devices forstructural vibration control. These types of dampers are veryreliable because they contain no moving parts except piston.Controllable fluids of ER/MR damper have the ability to changefrom free-flowing state to semi-solid state when it comes to anelectric (ER) or magnetic (MR) field. The input current and forcecharacteristics of MR damper are evaluated using different dampermodels by Spencer et al. [93]. The schematic diagram of control-lable damper filled with ER/MR fluid is shown in Fig. 10.

The contribution of ER/MR damper in structural vibrationcontrol is significant [95–98]. The usefulness of these dampingdevices has also influenced the researchers to implement them inwind turbine technology for optimization purpose. Kirkegaardet al. [69] investigated semi-active vibration control of windturbine tower using MR damper. In this study, wind turbine towerwas modelled as two-degree-of-freedom system and MR damperwas used as external damping device. Shaking table scaled modelhas been used as wind turbine for experimental verification andMR damper is placed between the shaking table and top of theframe structure. The results showed improved response in termsof vibration reduction using MR damper compared to passivedamping. MR fluid was also implemented to design semi-activeTLCD in [99] to overcome the shortcoming of passive system andto enhance the reliability of the system. MR fluid was used in thisstudy to design controllable valve for semi-active TLCD. Recently,magnetorheological vibration absorber was implemented todesign the wind turbine tower-nacelle model for the purpose ofvibration suppression [100]. At present, the controllable fluiddampers such as ER/MR damper become much desired dampingdevice in structural control applications because of its reliability,versatility and adaptability.

3.4. Other vibration control dampers

Some other important dampers were proposed in the past instructural vibration control. These types of dampers also playimportant roles to deal with structural vibration due to wind,wave and earthquake excitations. Pendulum damper is one kind ofpassive TMD which has been utilized in many structural vibrationcontrol applications [101–103]. Pendulum damper is modelledwith a large mass that is hung in an oil bath. When base isexcited, the mass is released to swing at the opposite motion ofthe structure to provide counter force. The use of pendulumdamper is not limited to tall buildings but also effective for windturbines. A pendulum damper was modelled for vibration controlof wind turbine towers in [104] and it is simulated under whitenoise wind effect. In another study, the reduction of fatigueloading in wind turbine was investigated with a pendulum TMDin [105]. A simple two-mass model was used in this study. Anotherimportant damping device is ball vibration absorber (BVA) whichwas first investigated by Pirner [106] on two television towers.Recently Zhang et al. investigated the ball vibration absorberthrough a series of shake table tests for wind turbines vibration

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100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132Fig. 8. Reductions in (a) tip displacement and (b) bending moment of wind turbine tower using TLCD [87].






suppression theoretically and experimentally [107]. The BVA con-sists of a steel ball, an arc path and two steel plates which preventthe ball slides aside. When the base structure is excited, the ballrolls along the arc path and thus counters the excitation force byabsorbing energy. Experimental result found that vibrationresponse has been reduced to 39% of the displacement with theuse of BVA [108]. The structure of the BVA and the displacementresponse of the wind turbine tower with and without BVA areshown in Fig. 11.

Another contributing damping device is tuned rolling-balldamper which is quite similar to TLCD in shape. Tuned rollingdamper consists of one or multiple steel balls in place of liquidcompared to TLCD. Chen and Georgakis [109] studied tuned rollingball damper modelled with one-layer (spherical) and two-layer(hemispherical) containers for wind turbine shake table scaledmodel. This study presented vibration control response of

developed wind turbine test model for different excitation loadsand number of balls in the container.

4. System controllers

System controllers are designed to improve the response of thesystem based on desired output. For vibration problem of astructure, controllers are designed to provide stability of a struc-ture so that unwanted vibration is minimized or mitigated. Sinceexcitation forces are high in civil structures and wind turbines,appropriate controller design is important to avoid structuredamage. The importance of controller design for wind turbineshas been highlighted in [110]. Numerous controllers weredesigned in the past to provide structural stability with structuralvibration control. The applications and effectiveness of few wide-spread controllers for structural vibration control were presentedin [29]. H-infinity, linear quadratic regulator/Gaussian (LQR/LQG),fuzzy logic, neural network are the most applied control techni-ques for wind turbine to control vibration. LQR/LQG controllers aredescribed as operating dynamic systems with providing minimumcost using quadratic function and it controls multiple input multi-ple output (MIMO) problems. These controllers are suitable forboth linear and non-linear problems. Eide and Karimi [111]investigated the performance of Disturbance AccommodatingControl (DAC), Linear Quadratic Control (LQR) and Linear Quad-ratic Gaussian (LQG) control for vibration mitigation on windturbine systems. LQG controller has also been utilized for MRdamper based semi-active vibration control of an offshore plat-form [67]. In another research, Karimi et al. showed how H-infinity

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Fig. 10. Schematic of controllable fluid damper [94].

Fig. 9. Schematic diagram of VAWT with TMD/TLCD.






control law can be designed and implemented on wind turbinesystems for vibration control [68]. Result shows a significantimprovement in vibration reduction using the controller comparedto uncontrolled system. Robust control improves the performancesand provides stability in the presence of uncertainties and theuncertainties are described probabilistically [29]. The performanceof a robust model predictive control (MPC) was investigated in[112] to mitigate the vibration of wind turbine blades. Theeffectiveness of H-infinity and robust MPC control policies areshown in Fig. 12(a) and (b) respectively.

To optimize the performance of wind turbines, fuzzy logiccontrollers were designed and implemented in [113]. To provideimproved structural response with vibration control, suitablecontroller must be designed. Researchers are giving significantfocus to design system controllers because of its usefulness tooptimize wind turbine power generation with respect to vibrationdue to different excitation loadings.

5. Conclusion

Wind turbine system is the state of the art technology toprovide high amount of energy source. This technology is growingfast all over the world because of its capability in supplyingrequired energy. High vibration in wind turbines often reducesthe efficiency of energy generation, thus, implementation of

vibration control in wind turbines becomes very important. Thisstudy focuses on vibration control strategies with suitable damp-ing devices to optimize the efficiency of wind turbine technology.The commonly used control algorithms for vibration controlapplications have been highlighted in this review. This reviewpresents past and present trends to control vibration of windturbines and it may lead to find new trends to apply in windturbine system. A possible future research can be the developmentof new approaches of vibration control with designing suitablecontroller which can be a combination of different control algo-rithms for providing better vibration control. A probable hybridcontroller can be integrated of classical PID controller and modernfuzzy controller which develops fuzzy-PID controller to achieveimproved performance compared to conventional methods. Thiscontroller can overcome the limitations of PID controller by usingfuzzy rules to change PID gains according to the excitationfrequency.

Acknowledgement

The authors would like to acknowledge the financial supportand advice of University of Malaya Research Grant (RP022D-2013AET), High Impact Research Grant under UM.C/HIR/MOHE/ENG/15 (D000015-16001). The authors also wish to thank allmembers from Advanced Shock and Vibration Research (ASVR)

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Fig. 12. Wind turbine vibration control response using (a) H-infinity [68] and (b) robust MPC controller [112].

Fig. 11. (a) The BVA and (b) top displacement of wind tower with and without BVA [108].






Group of University of Malaya and other project collaborators fortheir continuous support, guidance, help and constructive views.

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