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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 5, MAY 2011 1501

Fault-Ride-Through Capability ofOscillating-Water-Column-Based

Wave-Power-Generation Plants Equipped WithDoubly Fed Induction Generator and Airflow Control

Mikel Alberdi, Modesto Amundarain, Aitor J. Garrido,Member, IEEE,Izaskun Garrido,Member, IEEE, and Francisco Javier Maseda

AbstractThe increasing use of distributed power-generationsystems, as with the case of wave-power-generation plants, re-quires a reliable fault-ride-through capability. The effects of gridfault include uncontrolled turbogenerator acceleration, dangerousrotor peak currents, and high reactive-power consumption so thatthe plant may contribute to the voltage dip. A simple solutionis automatic disconnection from the grid, but this policy couldlead to a massive power-network failure. This is why new GridCodes oblige these systems to remain connected to the grid. In thispaper, an oscillating-water-column-based wave-power-generationplant equipped with a doubly fed induction generator is modeledand controlled to overcome balanced grid faults. The improvementrelies on the implementation of a control scheme that suitablycoordinates the airflow control, the active crowbar, and the rotor-and grid-side converters to allow the plant to remain in serviceduring grid fault, contributing to its attenuation by supplyingreactive power to the network and complying with new GridCode requirements. The simulated results show that it obtains agreat reduction of the rotor currents, improving the transients andavoiding rotor acceleration. Similar results are obtained from the

experimental implementation.Index TermsCrowbar, distributed power generation, doubly

fed induction generator (DFIG), fault-tolerant control, flow con-trol, low-voltage ride through, oscillating water column (OWC),voltage-source converters, wave energy, wells turbine.

NOMENCLATURE

Pwf,Pin Incident wave power, power available to turbine.Ed,g Wave-energy density, gravitational constant.vx,dp Airflow speed, pressure-drop across rotor.,w Air density, seawater density.a,r Area of turbine duct, mean radius.

Manuscript received January 25, 2010; revised May 26, 2010 and August 18,2010; accepted September 20, 2010. Date of publication November 9, 2010;date of current version April 13, 2011. This work was supported in part by theScience and Innovation Council MICINN throughResearch Projects ENE2009-07200 and ENE2010-18345, by the UE FP7 EFDA under Task WP09-DIA-02-01 WP III-2-c, and by the Basque Government through Research ProjectsS-PE08UN15 and S-PE09UN14.

The authors are with the Department of Automatic Control and SystemsEngineering, University of the Basque Country, 48012 Bilbao, Spain (e-mail:[email protected]; [email protected]; [email protected];[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2010.2090831

b,l Blade height, blade chord length.n,K Number of blades, turbine constant.q, Flow rate, flow coefficient.,Tw Wavelength, wave period.h,A Water depth, wave height.Cg ,J Wave group velocity, inertia of the system.,T Angular velocity, torque.Ca,Ct Power and torque coefficients.R,L, Resistance, inductance, flux.V,I,F Voltage, current, frequency.P,Q Active and reactive power.

Subscriptsd,q Direct and quadrature components.t,g,m Turbine, generator, magnetizing.gr,s,r Grid, stator, and rotor side.

I. INTRODUCTION

THE use of distributed energy resources is increasinglybeing pursued as a supplement and an alternative to large

conventional central power stations, often remotely controlled(see, e.g., [1]). The penetration of medium- and high-capacitypower-production plants, like wind farms and similar facilities,has reached such a level in diverse countries as Denmark,Germany, and Spain that represent a major impact on the char-acteristics of the power network [2]. In the particular case ofSpain, the Ministry of Industry, Tourism, and Trade publishedon October 4, 2006, Operating Procedure 12.3 regarding therequirement response to voltage dips of wind farms in which

the requirements to comply to the power-generation systems inthe special regime of medium and high capacity are stated, inorder to ensure the continuity of supply [3].

In the last couple of years, there has been a worldwideresurgent interest for wave energy, particularly in Europe. Thedevelopments in this sector are comparable with those in windenergy a few decades ago with similar economic potentials.Worldwide, the estimated technical and economical energy-production potential for ocean energy is estimated at about100 000 TWh/year. Today, there exist several ways to obtainenergy from the sea. In particular, the interest of this workfocuses on extracting energy from sea waves by using oscil-lating water columns (OWC) to transform the wave movement

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1502 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 5, MAY 2011

into pneumatic energy [4], [5]. This can be converted intomechanical energy with the use of a turbine [6][8], which, inturn, is used to move an induction generator [9][13]. Although,at present, there is no specific normative requirement on waveenergy, the arising issues with regard to power grid faults aresimilar and must also be solved by means of adequate fault-

tolerant control schemes.Many renewable power-generation plants incorporate a dou-bly fed induction generator (DFIG) to allow variable rotor-speed operation. The DFIG is directly connected to the networkthrough the stator, while the rotor is connected to the gridthrough a variable-frequency converter (VFC), which is onlyrequired to handle a fraction (25%30%) of the nominal powerto achieve total control of the generator. The VFC is composedof two voltage-source converters: one on the side of the rotor[rotor-side converter (RSC)] and the other one on the side ofthe grid [grid-side converter (GSC)], connected back to backthrough a capacitor [14][17].

In this paper, an OWC-based wave-power-generation plant iscontrolled by means of two complementary control strategies: arotational-speed control and an airflow control. The rotational-speed control by means of RSC provides a fast way to react toabrupt and short changes in the turbine speed, ensuring that theaverage power of the generator is adequately adjusted accordingto the incident-wave power level. In addition, a throttle valve isused to control the flow through the turbine so as to increasethe amount of energy produced, particularly at higher incident-wave power levels [32].

Nevertheless, renewable-power-generation plants based onDFIG are very sensitive to voltage dips. When a grid fault oc-curs on the transmission system, the speed of the turbogenerator

group increases due to the imbalance between the mechanicaltorque imposed by the turbine and the electromagnetic torque ofthe induction generator. Also, during the fault period and faultrecovery, the induction generator injects large peak currents,with the risk of damaging the rotor converter and increasing theconsumption of reactive power so that if the plant would notbe tripped from the grid, it would contribute to the voltage dip[18][22]. However, the politics of disconnecting the systemfrom the grid may initiate collapse of the power network.

As indicated previously, new Grid Codes oblige distributedpower-generation systems to remain connected to the powernetwork during the fault to avoid massive chain disconnections

so that the implementation of an adequate fault-ride-through(FRT) capability is indispensable in this kind of systems toensure its stability and uninterrupted operation [23][27].

A solution sometimes employed is the use of a crowbar,which guarantees connection to the grid. The crowbar shortcircuits the rotor windings, protecting the RSC from overvolt-ages and overcurrents that arise when grid fault takes place[30]. Nevertheless, under these conditions, the DFIG becomes asquirrel-cage motor and begins to absorb an amount of reactivepower that the RSC cannot regulate since it remains blocked.At the same time, the Wells turbine continues rotating by theaction of the incident waves so that an undesired excessiveacceleration of the turbogenerator group can be produced be-

cause the generator is out of control while the crowbar remainsconnected.

Fig. 1. Representative spectrum of the wave climate.

This paper proposes a novel control scheme that allows theplant to manage grid faults by adequately coordinating theairflow control, the active crowbar, and the VFC so as to remainin service during the grid fault, composing a brand-new topicas in the case of FRT capability in wave-power-generationplants. The flow is reduced by regulating the throttle air valvein order to control the acceleration of the group. Meanwhile,the crowbar is activated during a fraction of the fault, i.e., justthe necessary time to avoid dangerous current values. When thecrowbar is switched off, the RSC regains control to reduce theactive- and reactive-power references. At the fault clearance,

the crowbar may be launched again if necessary. Finally, after ashort recovery period, the RSC is resumed, and the DFIG startsto provide reactive power.

The rest of this paper is organized as follows: Section IIprovides the necessary background on OWC-based wave-powergeneration. In Section III, the proposed control scheme ispresented, coordinating the airflow control, crowbar control,RSC, and GSC. In Section IV, some demonstrative simulationexamples are given in order to test the performance of thecontroller. Section V is devoted to the corresponding experi-mental results, and finally, concluding remarks end this paperin Section VI.

II. BACKGROUND ONOWC-BASEDWAVE-P OWERGENERATION

A. Wave Model

Wave motion and wave-energy absorption are composed oftime-varying oscillatory phenomena. For the study of regularwaves, it is necessary to take into account the spectrum ofthe wave climate that indicates the amount of wave energy atdifferent wave frequencies, as it may be observed in Fig. 1(see also [9]). This representative spectrum of the wave climateis obtained by an offshore wave rider buoy in deep water,

measuring a wide range of oceanographic parameters (waveheight, period, and direction).


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ALBERDIet al.: FRT CAPAB IL IT Y O F OWC-BA SE D WAVE-POWE R-GE NE RAT ION PL AN TS EQU IP PE D WI TH DFIG 1503

Fig. 2. Capture chamber and ocean wave.

Then, the regular wave is modeled, taking its height andfrequency to be equal to the peak one in Fig. 1, and can bewritten as follows:

Pwf = CgEd=wg A

2

16Tw

1 +

4 h/

sinh(4 h/)

(1)

=g T2w

2 tanh(2 h/). (2)

B. OWC System

As shown schematically in Fig. 2, OWC is basically adevice that converts the hydraulic energy of the waves intoan oscillating airflow. The principal component of an OWC isthe capture chamber, which is composed of a fixed structurewith its bottom open to the sea. The wave motion alternatelycompresses and decompresses the air above the water levelinside the chamber. A conical duct is erected at the top ofthe chamber with the power-takeoff system, consisting of theturbine and the generator, located within this duct [10], [11].

The OWC energy equations are similar to those used for windturbines. In this way, the power available from the airflow inthe OWC chamber may be expressed as (3), where it can benoted that the airflow kinetic-energy term v3xa/2is common

to wind-turbine analysis, whereas the air-pressure term dpvxais a representative term to this application. From (3), it can beobserved that the size of the duct and the airflow through theduct play a significant role in the OWC system design. For acomplete description, see [11]

Pin =

dp + v2x/2

vxa. (3)

C. Wells-Turbine Description

The Wells turbine is a specially designed axial-flow turbinethat converts an oscillating flow into a unidirectional rotary

motion for driving an electrical generator. That is, it alwaysrotates in the same direction both for inbound and outbound

Fig. 3. Power coefficient versus flow coefficient.

airflow. The Wells turbine used in this work consists of a rotorwith eight blades and their chord lines lying in the plane ofrotation [6]. The equations used for the modeling of the turbineare given by (see [7])

dp= Ca K(1/a)

v2x+ (r r)2

(4)

Tt = Ct K r

v2x+ (r r)2

(5)

Tt = dp Ct r a/Ca (6)

= vx/(r r) (7)

q= vxa (8)

K= b l n/2. (9)

For a Wells turbine and a given rotational speed, a linearrelationship can be established between the pressure drop andthe flow rate. This fact is employed to achieve matching be-tween the turbine and the OWC, which also presents a similarcharacteristic.

The torque and power developed by the turbine can be com-puted based on the power coefficient and the torque coefficientagainst the flow coefficient. These relationships compose thecharacteristic curves of the turbine under study, and their shapemay be seen in Figs. 3 and 4. The performance of the Wellsturbine is limited by the onset of the stalling phenomenon on theturbine blades, which is clearly observable in Fig. 4, when theflow coefficient approaches the 0.3 value. As shown, the turbineefficiency drops drastically when the airflow rate exceeds thiscritical value, depending on the rotational speed. The undesiredstalling behavior can be avoided or delayed if the turbineaccelerates fast enough in response to the incoming airflow. Theequation for the turbogenerator may be written as [8]

J(r/t) =Tt Tg. (10)

This expression shows that the values of the turbine torqueand the electromagnetic torque, averaged over any sufficiently


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Fig. 4. Torque coefficient versus flow coefficient.

long period of time (several wave periods), present approxi-mately the same magnitude.

D. Technical Connection Requirements to Power Grids

The main requirements to be met by power-generation plantsof medium and high capacity, as in the case of wind farmsor wave-energy converters, to be able to operate within theelectricity market on equal conditions with conventional powerplants are established as follows: controllability of the activepower, controllability of the reactive power, and FRT capabilityduring voltage drops in the transmission system (see [18]).

1) Active-Power Control: The mismatch between the gen-erated and consumed active powers in the network may causedangerous changes in the frequency of the grid. In particular, ifpower-production plants were disconnected when a grid faultoccurs on the transmission system, it may contribute to thevoltage dip. Therefore, this actuation is not allowed.

2) Reactive-Power Control: The voltage regulation of thegrid is essential in electricity supply since a mismatch betweengenerated and consumed reactive power would cause dangerousvariations in the grid voltage. If the supply of reactive power isless than the demanded one, the line voltage decreases, favoringa grid fault.

3) FRT Capability: The stator flux of DFIG is calculatedusing the following (see, e.g., [14]):

s = s0+

t0

(Vs RsIs)dt. (11)

When a grid fault occurs on the transmission system causinga voltage dip, the stator flux cannot follow the stator-voltagevariation, which provokes an increase of the current in the statorwindings. Because of the magnetic coupling between the statorand the rotor, this current will also flow in the rotor. Moreover,the rotor keeps rotating, and the high-slip generated introduces

overvoltages and overcurrents in the rotor that can damage theRSC and the rotor (see [15] and [19]). This situation must

be avoided by implementing an adequate fault-tolerant controlscheme in the plant.

III. CONTROLSTATEMENT

The control scheme is shown in Fig. 5, and its operation

is described hereafter. As shown in this figure, the DFIG isattached to the Wells turbine by means of a gear box. The flowthrough the turbine is regulated by equipping the device withair valves. In this work, the use of a throttle valve mounted inseries with the turbine has been considered, located in the ductconnecting the chamber and the atmosphere.

During the normal working regime of the system, the con-troller regulates the active power generated by the stator toobtain the maximum allowed active power without entering intothe Wells-turbine stalling behavior [13], [32]. The power refer-ence is then obtained by applying the equation Psref=K 3r ,where the angular speed has been established to maximize theenergy production by avoiding the turbine stalling behavior andwhere the constant Kdepends on the characteristic values ofthe turbine. In turn, the proportionality to 3r(law derived fromturbomachinery dimensional analysis) ensures that the averagepower of the generator adjusts itself to the incident-wave-powerlevel.

However, when a grid fault is detected, the primary objectiveof the implemented control is the uninterrupted-operation fea-ture of the wave-energy plant during the fault. For this purpose,the rotor is short circuited by a crowbar, and the RSC is blockedto protect it from the rotor high currents, causing loss of controlof active power (Ps) and reactive power (Qs) of the DFIG.At the same time, as the Wells turbine continues rotating by

the action of the incident waves and in order to control theacceleration of the turbogenerator group, the flow is reducedaccordingly with the modified power reference, regulating thethrottle air valve.

When the rotor current and the dc-link voltage are lowenough, the crowbar is turned off, and the RSC is restarted. Thestator active- and reactive-power references are also reduced inorder to limit the currents. On the other hand, the GSC keepsthe dc-link capacitor voltage constant.

After voltage recovery, a second crowbar-circuit activationmay happen if the dc-link voltage or the rotor currents exceedtheir maximum allowed values. Meanwhile, the reactive-power

controllability of the GSC(Qgr)is useful during the process ofvoltage reestablishment. Finally, after a short recovery period,the RSC is resumed to modify the stator references in order toprovide active and reactive power to the grid.

When the voltage and frequency of the network return tosteady-state values, the references are modified again, restoringthe normal functioning of the system.

Therefore, when a grid fault takes place, the power referencesmust be modified. To do so, numerous simulations have beencarried out in order to study the variation of the currents fordifferent pressure and voltage drops. The results of this studyare shown in Table I, where the reference change for thedifferent pressure and voltage variations may be observed. The

operation of the different parts of the control scheme is detailedin what follows.


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Fig. 5. Configuration scheme.

TABLE IPRESSUREDROP VERSUSVOLTAGEDROPAND

CONTROLLERREFERENCES

A. Airflow Control

In order to understand the dynamics of wave-power-generation plants, we must take into account that the efficiencyof OWC-based plants equipped with Wells turbines is particu-larly affected by the intrinsically unsteady flow of air displacedby the wave motion inside the chamber and because increasingthe airflow rate above a limit depending on the rotational speed

of the turbine is known to give rise to a rapid drop in the poweroutput of the turbine. The use of a throttle valve to controlthe flow through the turbine and, in this way, to prevent orreduce the stalling losses at the turbine is expected to increasethe amount of energy produced by the plant, particularly athigher incident-wave power levels. This increase is expected tobe quite relevant for turbines whose performance is drasticallyaffected by rotor stalling, as in the case of the turbine used inthis work and most Wells turbines of fixed-pitch type.

The valve is governed by an actuator, which is designed toallow modulation, in order to reduce or increase the flow ratethrough the turbine accordingly with wave-activity variations.The actuator drives the throttle valve into the demanded posi-

tion against a counterbalance weight. Once in position, it is heldsteady by an electromagnetic brake. In the event of a control

failure or if an emergency closure is demanded, the brakesupply is interrupted, and the valve closes under the influenceof the weight. For a detailed description, see [12].

In this paper, we use a modified antiwindup proportional-integral differential (PID)-based controller to govern the outputof the generator so that the variable controlled is the powergenerated, and the manipulated variable is the flow through theturbine.

PID control schemes are composed of simple and well-known techniques, and they are currently the most commonly

used controllers in industrial control real applications. In thisrespect, these may be recalled in the last work of strm andMiller Feedback Systems (see [28]), where it is stated thatmore than 95% of all industrial control problems are solvedby PID control. In addition, since the final objective of theproposed control system is to be implemented over a real plant,one of the requisites was to maintain the air-valve controlas simple as possible while meeting the system performancerequirements. In this sense, other control schemes were initiallyconsidered, as artificial-neural-network-based controllers (see[13]) or robust sliding-mode controllers ([29]), but they werefinally discarded for the sake of implementation reliability.

One of the issues present when dealing with PID-basedcontrollers is related to the need of an appropriate tuning thatmust be usually readjusted over the real system, due to thedifferences that always exist between the model and the realsystem. Nevertheless, results showed that the tuning procedurewas easily performed and provided an adequate response of thesystem within the operating range of the system.

As usual, in this kind of controllers, the reference and thefeedback (averaged mean value of the generated power) gen-erate the error signal e(t), which will serve as input to thecontroller. Then, the control signal u(t) drives the valve intothe demanded position. In this way, the modulation of the valveaims to adjust the pressure drop across the Wells-turbine rotor.

The general expression for a traditional PID, including aproportional action modulated by an integral action to eliminate


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Fig. 6. Modified antiwindup PID-based airflow control.

the steady-state error and derivative action to stabilize thesystem is given by

u(t) =K1e(t) + K2

e(t)dt + K3

de(t)

dt (12)

whose Laplace transform can be expressed as

U(s) =K1

1 +

1

Tis+ Tds

E(s). (13)

Nevertheless, since the control actuator is a throttle valvesubject to saturation, it was necessary to consider an integralwindup effect. It must be considered that the control valvehas physical limits so that once it has saturated, increasing themagnitude of the control signal further has no effect. In thiscase, there exists a difference between desired and measured

output-power values, and the resulting error will cause a steadyincrement in the integral term. When the error term changesits sign, the integral term starts to unwind, and this may causelong time delays and possible instability. When this happens,the feedback loop is broken, and the system runs in open loop,independent of the output power, as long as the valve remainssaturated.

In order to avoid this phenomenon, the following modifiedantiwindup controller, shown in Fig. 6, has been considered,where an extra feedback path has been included that is gen-erated by measuring the actual valve output u(t) subject tosaturation and defining the saturation error signal esat(t) asthe difference between the output of the controller upresat(t)and the valve output. This corrective error signal is fed to theinput of the integrator through the gain Kcso that when there isno saturation, its value is zero, having no effect on the controlsignal, but when the throttle valve saturates, the signal is fedback to the integrator in such a way that the integral actionui(t) is decreased accordingly with the saturation error. Thisimplies that controller output upresat(t) is kept close to thevalve saturation limit, and integral windup is avoided.

B. VFC-Controlled DFIG Description

In order to understand the operation of the control scheme,

it is recommendable to previously describe the functioningof the DFIG. This device composes a well-known induction-

machine configuration (see [16]) with a huge potential for thedevelopment of distributed renewable-energy sources and, inparticular, for wave energy [17], [20][22]. The functioningof the VFC-controlled DFIG can be described as follows.As shown in Fig. 5, the DFIG stator windings are connected

directly to the grid, while the rotor windings are connected tothe VFC. The power flow between the rotor and the grid mustbe controlled both in magnitude and phase to produce electricalpower, maintaining constant voltage and frequency values fora wide range of speed sets. This function is performed by theVFC, which is composed of a GSC connected to the grid, andan RSC connected to the wound-rotor windings. They bothconsist of insulated-gate bipolar transistor (IGBT) pulsewidthmodulation (PWM) converters, connected back-to-back by adc-link capacitor (see [23]). One of its main advantages isthe fact that this well-known topology represents a low-costcommercial solution compared with other schemes. Further,

the generator and the grid are decoupled, within certain limits,owing to the dc-link phase. This allows independent control ofboth converters.

The control scheme of the RSC is shown in Fig. 7. This con-verter controls the active power (Ps)and reactive power(Qs)of the DFIG. In order to achieve independent control of them,the instantaneous three-phase rotor currents irabcare sampledand transformed intodqcomponentsiqr and idrin the stator-flux-oriented reference frame. In this context, Psand Qscan berepresented as functions of the individual current components.Then, the reference active power is compared with the powerlosses and compensated by the stator-flux estimator to formthe reference q-current i

qr

, which is then passed through astandard proportionalintegral (PI) controller. Its output vqr1isin turn compensated by vqr2 to generate the q-voltage signalvqr . The RSC reactive-power control is also used to maintaina constant stator voltage within the desired range when theDFIG is connected to weak power networks without reactive-power compensation. When connected to strong power grids,this control may be set to zero.

The reference reactive power is compared with its actualmeasurement to generate the error signal, which is passedthrough a PI controller to provide the reference signal (idr).Then, it is compared with its actual signal (idr) to generatean error, which is then used to provide the required q-voltage

signal(vdr1)by means of a PI controller. In turn, this voltage iscompensated byvdr2 to generate thed-voltage signal vdr and


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Fig. 7. Control scheme of the RSC.

Fig. 8. Control scheme of the GSC.

used by the PWM module to generate the IGBT gate-controlsignals necessary to drive the RSC jointly with the q-componentsignalvqralready obtained.

Analogously, Fig. 8 shows the control scheme of the GSC.This converter controls the dc voltage and the reactive power

Qgrexchanged with the grid, with its objective being to keep

the dc-link capacitor voltage constant regardless of the mag-nitude and phase of the rotor power. In addition, the reactive-power Qgrcontrol plays a crucial role during the fault recovery.

Its operation is somehow similar to the RSC: The actualq-current signal (iqg) is compared with its reference signal(iqref)to generate the error signal, which is used to provide therequiredq-voltage signal(vqg1)by means of a PI controller. Inturn, it is compensated by vqs and vqg2and used by the PWMmodule to generate the IGBT gate control signals necessary todrive the GSC converter. Similarly, the actual d-current signal(idg)is compared with its reference signal(idg)to generate theerror signal, which is used to provide the required PI-controlled

q-voltage signal (vdg1). In turn, it is compensated by vdsandvdg2and used by the PWM module to generate the IGBT gate-

control signals necessary to drive the GSC converter jointlywith theq-component signals.

For a more detailed study about DFIG control devices bymeans of RSC and GSC, see [20] and [21].

C. Crowbar Control

The use of crowbar circuits is a temporary measure usuallyemployed to protect the power-electronic components in theDFIG controller when a grid fault takes place on the transmis-sion system.

As shown in Figs. 9 and 10, it is basically composed ofa circuit mounted in parallel with the RSC, which activateswhen the voltage at the dc-link capacitor reaches its maximumvalue or when the rotor-current limit is exceeded (see [24]).In response to these situations, the rotor windings are shortcircuited by the crowbar circuit in order to protect the RSC,causing loss of vector control. In this way, during the crowbar

activation time, the stator is not disconnected from the grid so asto retrieve the generator control as soon as possible, as required


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Fig. 9. Diode-bridge crowbar.

Fig. 10. Antiparallel thyristor crowbar.

by the Grid Code. Even when the fault has been cleared, high-current transients may also produce a new activation of thecrowbar circuit.

The crowbar circuits may be implemented with different con-figurations. Some of the most popular ones are the antiparallelthyristor crowbar, shown in Fig. 10, in which each rotor phaseis connected to external resistances and the three-phase diode-bridge crowbar, shown in Fig. 9, where the resistive load iscoupled to a thyristor. For our case study, diverse crowbars withdifferent resistance values were modeled in order to identifytheir optimum value. It must be taken into account that lowresistance values keep currents within safe limits but take a longtime to deenergize the rotor, while a high-resistance crowbardeenergizes the machine rapidly but produce undesired current

peaks. On the other hand, it must be observed that the reactive-power consumption is significant while the crowbar is activated,

regardless of the resistance value since, under these conditions,the DFIG acts as a conventional induction generator.

In order to regain control of the DFIG as soon as possible,the short circuit performed by the crowbar has to be removedwhenever needed, and this can only be securely achieved byreplacing the thyristor by a gate turnoff (GTO) thyristor or anIGBT. This modified circuit is called active crowbar since theyare fully controllable [30]. Although the diode-bridge config-uration is a simpler device containing just a GTO thyristor,in this paper, the antiparallel GTO thyristor circuit shown inFig. 10 has been chosen. In this way, the control law onlyactivates the crowbar in response to exceeding rotor currentor dc-link voltage signals and just for the necessary time toprevent damages in the rotor converter, reconnecting it as soon

as possible to avoid the negative effects of a prolonged RSCblocking.


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TABLE IITURBINE ANDINDUCTION-GENERATOR PARAMETERS

TABLE IIISYSTEMPARAMETERS

IV. SIMULATIONRESULTS

In this section, the model of the Wells turbine and the OWCmodule, jointly with the VFC-controlled DFIG, the crowbar,

and the airflow control presented in Sections II and III, re-spectively, have been implemented following the scheme shownin Fig. 5. The turbine-generator parameters used are shown inTable II, and the system parameters are shown in Table III.

The final objective of this work is to be applied to a realexperimental plant called NEREIDA MOWC. This project isintended to demonstrate the successful incorporation of theOWC technology with Wells-turbine power takeoff into a newlyconstructed rubble-mound breakwater in the Basque location ofMutriku, in the northern coast of Spain. The aim is to provethe viability of this technology for future commercial plants.For this reason, the simulation data and wave model have been

chosen, taking into account the spectrum of the wave climateof Mutriku (see [9]). In this way, the input pressure drop maybe experimentally modeled, as shown in Fig. 11. On the otherhand, the balanced grid fault has been implemented as an85% reduction of the grid voltage applied at 5.0 s and clearedat 5.5 s.

For comparison purposes, an uncontrolled case study hasbeen initially studied without considering protective actions,either airflow control or crowbar control. Under these condi-tions, the effect of the fault over the stator and rotor phasecurrents may be observed in Figs. 12 and 13, presenting asudden increase with large peaks when the voltage dip startsand recovers. In particular, it can be seen in Fig. 13 that the rotor

circuit suffers current peaks of about four times the prefaultlevels within a few milliseconds that, as indicated before, could

Fig. 11. Input pressure drop (in pascal).

Fig. 12. Stator phase currents (in p.u.) for the uncontrolled case.

damage the RSC. As a result, the wave-power-generation plantmight be tripped out. Since this behavior is not allowed by thenew Grid Code, this is one of the problems that the proposedcontroller must deal with. Analogously, the stator presentsovercurrents during the start and clearance of the fault, reachingvalues of 1.5 per unit (p.u.) that quickly decrease, as shownin Fig. 12. However, this behavior of the stator current has

a relative relevance since no vulnerable power electronics isinvolved in the stator connection.On the other hand, Fig. 14 shows the active and reactive

power generated at the stator terminals. It can be observed thatthe active power drops rapidly down to zero when the fault isintroduced, while the reactive power consumption is slightlyincreased. After the clearance of the fault, the active power risesrapidly to its reference-power value after a transitory periodwith strong oscillations. Analogously, the reactive power dropsdown and, after a transitory period, recovers its reference value.It can be seen that during the fault and, particularly, duringthis transitory period, the generator absorbs a large amount ofreactive power from the grid contributing to the network fault.

As has been indicated before, this is another of the issues thatmust be solved by the control scheme.


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Fig. 13. Rotor phase currents (in p.u.) for the uncontrolled case.

Fig. 14. Active and reactive power for the uncontrolled case.

In order to observe more clearly the effects of the faultover the dc-link voltage and the rotor speed, a longer voltagedip starting at 20 s and cleared at 40 s has been considered.The results are shown in Figs. 1517. It may be seen that thedc-link voltage presents an oscillating dynamics with valuesranging from 765 to 845 V. The reason is the RSCs inability

to deal with the high currents coming from the rotor. Moreover,the rotor speed increases due to the imbalance between themechanical torque and the electromagnetic torque, as shownin Fig. 16, causing clearly the uncontrolled acceleration of theturbogenerator. This behavior can be dangerous and must bealso avoided by the controller. It can be seen in Fig. 17 thatthe turbine torque value decreases during the voltage dip. Itmay also be observed from (5) that Ttdepends onCtand r.Similarly, it may be seen in Fig. 16 that thervalue increasesdue to the voltage dip, as expected. This increment affects thevalue ofsince, from (7), decreases when r increases sothat, since the turbine is operating within the first zone of Fig. 4(stalling avoidance), the torque coefficient Ctis decreasing, and

as a consequence,Ttdecreases also despite the r increment.Nevertheless, this torque decrement is not sufficient to avoid

Fig. 15. VDC-link voltage (in volts) for the uncontrolled case.

Fig. 16. Rotor speed (in p.u.) for the uncontrolled case.

Fig. 17. Turbine torque (in newton meters) for the uncontrolled case.

the uncontrolled acceleration of the turbine-generator group, asshown in Fig. 16.

For comparison purposes, the same examples have beenperformed for the controlled case study, by considering the


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Fig. 18. Control action u(t)for the controlled case.

Fig. 19. Stator phase currents (in p.u.) for the controlled case.

proposed airflow control and crowbar control. In the simula-tions conducted, a double activation of the crowbar is observed,one at the beginning of the fault and the second one at thefault recovery. This is due to the aforementioned high rotorcurrents produced by the fault (see Fig. 13) so that whenthe fault is detected (some milliseconds), since the generatorcannot be disconnected from the grid, it is necessary to takeprotective actions to block the RSC by activating the crowbar

circuit. At the same time, the airflow control acts, closingthe air valve and diminishing the turbine throttle. When thevoltage at the dc-link capacitor and the rotor phase currentsare low enough, approximately one hundred milliseconds afterapplying the short circuit, the crowbar is turned off, and theRSC starts switching, regaining the control to reduce both theactive- and reactive-power references. Once the grid voltagehas been recovered, similar disturbances affect the generator,as shown earlier, and therefore, the crowbar is launched again.Finally, after a short recovery period, the plant reference steady-state values are retrieved.

Fig. 18 shows the control action generated to regulate the fluxthrough the turbine. It may be observed that the valve is almost

closed during the voltage dip. In this way, it can be observedthat the stator and rotor phase currents shown in Figs. 19

Fig. 20. Rotor phase currents (in p.u.) for the controlled case.

Fig. 21. Active and reactive power for the controlled case.

and 20, respectively, are significantly lower than those inFigs. 12 and 13, mainly due to controller activity and, inparticular, to the crowbar activation that allows partial energydissipation through suitably selected external resistances.

Analogously, the active and the reactive power generatedby the DFIG, shown in Fig. 21, may be compared with the

results of the uncontrolled case shown in Fig. 14: The active-power overshoot at voltage recovery is eliminated, reducing itsoscillatory dynamics, while giving reactive power to the gridso that it contributes to the attenuation of the voltage dip, incontrast with the uncontrolled case. In this context, Fig. 21shows that the plant generates active and reactive power duringthe fault period and, particularly, during the fault recovery, andthat only during 100 ms after the fault recovery, the generatorabsorbs a little amount of power (less than 60% of the nominalpower).

In the same way, it may be observed in Fig. 22 that theGSC controller improves the performance of the dc-link voltageperformance compared with the uncontrolled case (Fig. 15),

significantly reducing the voltage overshoots during the faultperiod and fault recovery.


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Fig. 22. VDC-link voltage (in volts) for the controlled case.

Fig. 23. Rotor speed (in p.u.) for the controlled case.

Similarly, Fig. 23 shows the rotor speed for the controlledcase where it can be clearly observed that the uncontrolledacceleration shown in Fig. 16 has been avoided by the controllerby adequately regulating the airflow valve. In this sense, it canbe also observed that the loss of the vector control at faultinitiation and clearance while the RSC is blocked has no effecton the rotor speed. Fig. 24 shows the turbine-torque reductionas a consequence of the partial air valve closing during thevoltage dip. This reduction, stronger than that of Fig. 17, allowssuccessfully controlling the turbine speed.

These results indicate that the proposed control scheme iseffective for the dynamic voltage control, even when the plantremains connected to the power network during a grid fault.In this way, the controller significantly enhances the capabilityof the wave-power-generation plant to ride through the griddisturbances, improving its performance.

V. EXPERIMENTALRESULTS

In order to prove the feasibility and goodness of the pro-posed schemes, in this Section, we are going to implement the

Fig. 24. Turbine torque (in newton meters) for the controlled case.

proposed control design over an experimental system where adigital signal processor (DSP)-based induction motor has beenused for turbine emulation purposes. The corresponding exper-imental control-system scheme may be observed in Fig. 25,while Fig. 26 shows the physical system used for the realimplementation.

The emulation of the Wells turbine is performed by meansof a motor under speed control using the Micro MasterSiemens frequency converter. The dynamic behavior of the tur-bine has been implemented using MATLAB/Simulink environ-ment (PC1) and the commercial real-time hardware dSPACEDS1104 DSP-based digital-to-analog-converter system. In or-der to model the waves, it is necessary to consider the spectrum

of the wave climate. From these data and taking into accountthe characteristics of the real chamber, it is possible to estimatean average wave power for the standard input pressure drop sothat the turbine input may be experimentally modeled, as maybe seen in Fig. 11. The throttle valve controls the flow throughthe turbine, and taking into account the linear relationshipbetween pressure drop and flow rate, the antiwindup PID-based controller modulates the input pressure drop so that from(4)(9), the corresponding speed is obtained, which serves asreference to the VFC for turbine emulation purposes.

The DFIG system includes a wound-rotor ac generator andthe VFC. The VFC control structure and the PWM scheme,

as shown in Fig. 25, have been also implemented over a DSPcontrolled by a second computer serving as Master (PC2),concretely on a hybrid DSP56F807, the most powerful DSP ofthe 56000 family, with a velocity of up to 40 million instructionsper second at 80-MHz core frequency. The main features ofthis DSP are the following: twin periphery with two 6-channelPWM modulator, four 4-channel 12-b analog-to-digital con-verters, and two quadrature decoders. Further, this DSP presentsthe advantage of a wide connectivity (CAN, SCI, SPI, andJTAG ports for debugging). The software is developed usingthe CodeWarrior C-compiler of Metrowerk. The programmingof the different peripherals and algorithms of estimation andcontrol are developed based on this software.

The active crowbar is implemented by connecting an externalresistor across each phase of the rotor circuit. The value of


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Fig. 25. Experimental control-system scheme.

Fig. 26. Real-system implementation.

the external resistance has been chosen as Rext= 20Rr. Thecrowbar controller has been programmed to enable and disablethe rotor converter IGBTs and the crowbar GTO thyristors as a

function of measured rotor currents.In order to produce the grid voltage dip, the stator of thegenerator has been connected to a voltage-dip generator, asshown in Fig. 25. This device is based on an inductive dividercomposed of a series and a parallel branch (see [31]). Thecontroller has been performed by means of a purpose-builtanalog-circuit hardware board. In this manner, the grid faultstarts when the circuit breaker S is closed and ends when thebreaker opens and clears the fault current.

Referring to Figs. 2730, it can be seen in Fig. 28 and Fig. 30that the stator and rotor phase currents of the controlled caseare lower during the fault and recovery periods compared withthe results of the uncontrolled case of Figs. 27 and 29, due to

the action of the airflow controller and, mainly, to the doubleactivation of the crowbar circuit at the beginning and clearance

of the fault. When the fault is detected and the RSC is blockedby the crowbar, the control over the active- and reactive-poweroutputs is temporarily lost (see Figs. 3134). However, thecontrol of the active and reactive power is regained as soon aspossible, as observed in the aforementioned figures. On faultclearance, the RSC is disabled again, and a peak reactive powerof 26% of the rated power occurs. Finally, after a short recoveryperiod, the RSC is reengaged to provide active and reactivepower to the grid. The experiments presented in this Sectionprovide similar results to those of the numerical simulations

of Section IV, verifying the FRT capability of the proposedcontrol.

VI. CONCLUSION

The increasing penetration of renewable distributed power-generation systems within the sometimes deregulated elec-tricity markets has given rise to new technical requirements.One of the most demanded requirements is an FRT capabilityduring voltage drops in the transmission system. In this sense,although, at present, a specific normative requirement on waveenergy does not exist, the arising issues regarding power grid

faults are similar to those of more extended renewable energiesas in the case of wind energy and must also be solved by meansof adequate fault-tolerant control schemes. When a grid faultoccurs on the transmission system, the speed of the turbogen-erator group increases, the induction generator injects largepeak currents, and the plant tends to increment the reactive-power consumption so that they might intensify the voltage dipand contribute to the collapse of the power network. A simplesolution would be the automatic disconnection of the plant fromthe grid in response to the power fault, but this policy could leadto a series of chain disconnections that would produce a massivepower-network failure.

This paper has investigated the application of a new control

scheme to achieve the uninterrupted operation of OWC-basedwave-power-generation plants equipped with DFIGs during


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Fig. 27. Experimental results. Stator A and B phase currents (in p.u.) for the uncontrolled case.

Fig. 28. Experimental results. Stator A and B phase currents (in p.u.) for the controlled case.

Fig. 29. Experimental results. Rotor A and B phase currents (in p.u.) for the uncontrolled case.

Fig. 30. Experimental results. Rotor A and B phase currents (in p.u.) for the controlled case.

balanced grid faults. For this purpose, an airflow valve control

and crowbar control has been proposed and coupled to theVFC-controlled DFIG.

The controller operation can be detailed as follows: When

the fault is detected, the flow through the Wells turbine isreduced according to the modified power reference, regulating


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Fig. 31. Experimental results. Active power (in p.u.) for the uncontrolled case.

Fig. 32. Experimental results. Active power (in p.u.) for the controlled case.

Fig. 33. Experimental results. Reactive power (in p.u.) for the uncontrolled case.

the throttle air valve during all the voltage-dip periods and thefollowing recovery period. Simultaneously, the RSC is blockedby the crowbar in order to protect it from the overcurrentsin the rotor circuit. When the dc-link capacitor voltages andthe rotor phase currents decrease to the operating values, thecrowbar is switched off, and the RSC regains generator controlto reduce the active- and reactive-power references. Throughthe whole process, the GSC remains connected, contributingto the voltage reestablishment by regulating the reactive-powergeneration to the grid. At the grid-voltage recovery, a secondcrowbar-circuit activation may be required since high currentpeaks and dc-link voltage instability may take place again.During this voltage recovery period, the stator active- andreactive-power references are adequately adjusted in order to

provide active and reactive power to the grid, and the RSC isreconnected as soon as possible by the crowbar circuit. Finally,

when the voltage and frequency of the network return to steady-state values, the references are modified again, restoring normalfunctioning of the system.

In particular, the simulations performed and the experimentalresults obtained show that the aforementioned key problems,rotor overcurrents, reactive-power increase, and uncontrolledrotor acceleration, have been avoided or significantly improved.On the one hand, the valve control, based on a modifiedantiwindup PID control law, successfully governs the air valvethat modulates the pressure drop across the turbine in orderto control the acceleration of the turbine. On the other hand,the crowbar provides great reductions of the stator and rotorcurrents. In addition, the RSC successfully controls the activepower and reactive power of the DFIG while the crowbar is not

active, and the GSC controls the dc voltage and the reactivepower exchanged with the grid during all the process.


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Fig. 34. Experimental results. Reactive power (in p.u.) for the controlled case.

It has been shown that the proposed control design allowsthe uninterrupted operation of the wave-power-generation plantduring grid faults, contributing to the fault clearance by in-creasing the reactive-power production during the voltage dip.

Moreover, a significant improvement in machine control duringthe fault and recovery periods is demonstrated, compared withthe uncontrolled case. In particular, the uninterrupted opera-tion achieved complies with the Spanish Operating Procedure12.3 regarding response requirements to voltage dips in windfarms, which states that 150 ms after the onset of the fault and150 ms after its recovery, the reactive-power consumption mustbe under 60% of the nominal power.

ACKNOWLEDGMENT

The authors would like to thank the anonymous reviewerswho have helped to improve the initial version of this paper.

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[32] M. Amundarain, M. Alberdi, A. J. Garrido, and I. Garrido, Modelingand simulation of wave energy generation plants: Output powercontrol, IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 105117,Jan. 2011.

Mikel Alberdiwas born in Bilbao, Basque Country,Spain in 1965. He received the B.S.I.E. degree inelectrical engineering and the M.Sc. degree in elec-tronic engineering from the University of the BasqueCountry, Bilbao, where he is currently working

toward the Ph.D. degree in communications, elec-tronics, and control.

He is currently an Associate Professor of systemsand control engineering with the University of theBasque Country. His present main research inter-est area is the applied control of dynamic systems,

particularly induction machines and power converters, and its application torenewable energy systems, in particular, to wave-power generation plants.

Modesto Amundarainwas born in Plentzia, Basque

Country, Spain in 1964. He received the B.S.I.E.degree in electrical engineering and the M.Sc. degreein electronic engineering from the University of theBasque Country, Bilbao, Spain, where he is currentlyworkingtoward thePh.D. degree in communications,electronics, and control.

He is currently an Associate Professor of systemsand control engineering with the University of theBasque Country. His present main research inter-est area is the applied control of dynamic systems,

particularly induction machines and power converters, and its application torenewable energy systems, in particular to wave-power generation plants.

Aitor J. Garrido(M07) was born in Bilbao, Spainin 1972. He received the M.Sc. degree in appliedphysics, the M.Sc. degree in electronic engineering,and the Ph.D. degree in control systems and au-tomation from the University of the Basque Country,Bilbao, in 1999, 2001, and 2003, respectively.

Since 2000, he has held several research andteaching positions with the Automatic Control and

Systems Engineering Department, University of theBasque Country, where he is currently an AssociateProfessor of systems and control engineering. He has

more than 100 papers published in main international conferences of the area,book chapters, and JCR(ISI)-indexed journals, has served as reviewer in severalinternational indexed journals and conferences, and has supervised severalPh.D. theses. His present main research interest area is the applied control ofdynamic systems, particularly induction machines, and its application to wavepower, as well as nuclear-fusion processes.

Izaskun Garrido(M07) was born in Bilbao, Spainin 1967. She received the M.Sc. degree in appliedmathematics from the University of the BasqueCountry (UPV-EHU), Bilbao, and the M.Sc. de-gree in numerical analysis and programming andthe Ph.D. degree in finite elements from DundeeUniversity, Dundee, U.K., in 1999.

She has held several positions with PIK(Germany), ZIB (Germany) and UiB (Norway).She has been an invited Researcher in institutionssuch as Stanford University or LLNL (U.S.). Since

2004, she has been an Associate Professor of systems and control engineeringwith the Automatic Control and Systems Engineering Department, UPV-EHU,where she is also Vice Dean of Research and International Relations. Shehas over 100 publications and has served as reviewer in international journalsand conferences. She has supervised several Ph.D. theses, and her presentmain research interest area is numerical simulation and control applied towave-power generation plants, as well as fusion.

Francisco Javier Maseda was born in Bilbao,Spain, in 1959. He received the B.Eng. degree,

the M.Sc. degree in electronic engineering, and thePh.D. degree in control systems and automation fromthe University of the Basque Country, Bilbao, in1983, 2002, and 2006, respectively.

Since 1986, he has been an Assistant Professorwith the Department of Control Systems and Au-tomation, University of the Basque Country and iscurrently a Full Professor of systems engineeringand automatic control. He has worked in industrial

companies in the fields of electrical and electronic engineering. He has severalpublications in the fields of automation and advanced control of dynamicsystems and its application in induction-machine drives.

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