IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER 2004 1323

Flexible Control of Small Wind Turbines WithGrid Failure Detection Operating in

Stand-Alone and Grid-Connected ModeRemus Teodorescu, Senior Member, IEEE, and Frede Blaabjerg, Fellow, IEEE

Abstract—This paper presents the development and testof a flexible control strategy for an 11-kW wind turbine witha back-to-back power converter capable of working in bothstand-alone and grid-connection mode. The stand-alone controlis featured with a complex output voltage controller capable ofhandling nonlinear load and excess or deficit of generated power.Grid-connection mode with current control is also enabled forthe case of isolated local grid involving other dispersed powergenerators such as other wind turbines or diesel generators.

A novel automatic mode switch method based on a phase-lockedloop controller is developed in order to detect the grid failure or re-covery and switch the operation mode accordingly. A flexible dig-ital signal processor (DSP) system that allows user-friendly codedevelopment and online tuning is used to implement and test thedifferent control strategies.

The back-to-back power conversion configuration is chosenwhere the generator converter uses a built-in standard flux vectorcontrol to control the speed of the turbine shaft while the grid-sideconverter uses a standard pulse-width modulation active rectifiercontrol strategy implemented in a DSP controller. The design ofthe longitudinal conversion loss filter and of the involved PI-con-trollers are described in detail. Test results show the proposedmethods works properly.

Index Terms—Digital signal processor (DSP) control, gridfailure, grid monitoring, phase-locked loop (PLL), renewableenergy, wind turbine.

I. INTRODUCTION

THE USE OF squirrel-cage induction generators (SCIG)for direct grid-connection wind energy conversion systems

(WEC) is well established. With the last advances in powerelectronics, the use of variable-speed SCIG with a double stageac–dc–ac power conversion has become quite attractive [1], inboth low power and very high power levels [1]. Increased energyproduction at low wind, elimination of the excitation capacitorbank, and especially the possibility for stand-alone operationmode can thus be achieved at the expense of a back-to-back con-verter with a flexible control [2].

Small efficient wind turbines (WT) in the range of 11 kW asdepicted in Fig. 1 can be used to supply private houses or farmsin remote locations in developing countries where low-cost ac-cess to the electrical power grid is impractical. The trend is that

Manuscript received June 1, 2004; revised July 21, 2004. This work was sup-ported by the Danish Research Agency, Danish Technical Research Councilthrough the Project “Reliable Grid Condition Detection” 2058–03–0003. Rec-ommended by Associate Editor F. Z. Peng.

The authors are with the Institute of Energy Technology, Power ElectronicsSystems Section, Aalborg University, Aalborg 9220, Denmark (e-mail:ret@iet.aau.dk; fbl@iet.aau.dk).

Digital Object Identifier 10.1109/TPEL.2004.833452

they should be able to work in stand-alone mode but also con-nected to isolated local loads/grid in parallel with other gen-erators such as other wind turbines, photovoltaic generators,or diesel generators in the so called hybrid generator systems(HGS). Thus some new challenges on the control side of theseWT occurred like reasonable voltage regulation in stand-alonemode with nonlinear load, grid-connection mode enabled, andautomatic detection of grid failure. But in order to achieve thesegoals, relative complex control strategies need to be developed.Also grid failure detection and an automatic mode switching arerequired.

A seamless transfer method from grid-connected to stand-alone and vice versa for critical loads is described in [3] wherean extra static switch is required. The algorithm matches themagnitude and phase of the inverter voltage and the grid voltageat the time of disconnecting or reconnecting to the grid to min-imize any sudden voltage change across the load.

A phase-locked loop (PLL) technique is commonly used ingrid-connected converters mainly for grid-synchronization [5],[6]. In this paper, a novel PLL-based method for grid failuredetection and automatic mode switching is proposed where thephase difference between the grid and the inverter is used todetermine grid failure and restoring.

The flexible control strategy featuring stand-alone modeand grid-connected control as well as the automatic modeswitching strategy has been developed, implemented and testedfor a 11-kW WT. The control strategy has been implementedon a development platform based on a dSPACE digital signalprocessor (DSP) controller [4] that allows flexible code devel-opment in Simulink and online tuning and monitoring. Thus arelative short development time is achieved.

Both operation modes as well as the involved current andvoltage controllers design are described in detail. Also, the de-sign of the longitudinal conversion loss (LCL) filter togetherwith experimental results obtained with a drive-train emulatingthe 11-kW wind turbine are presented.

II. CONCEPTS FOR STAND-ALONE WIND TURBINES

The most common configurations of power converters forstand-alone SCIG WT are depicted in Fig. 2.

The diode rectifier topology in Fig. 2(a) can only be used inone quadrant, it is simple and it is not possible to control it.This configuration is especially suitable for fixed-speed windturbines and requires a capacitor bank between the generatorand the diode rectifier in order to magnetize the induction gen-erator.

The back-to-back voltage source converter is a bidirectionalpower converter consisting of two conventional voltage source

0885-8993/04$20.00 © 2004 IEEE

1324 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER 2004

Fig. 1. Small 11-kW GAIA (Denmark) wind turbine for households andremote location.

Fig. 2. Stand-alone wind turbine configurations with squirrel-cage inductiongenerators: (a) diode bridge rectifier and voltage source inverter, (b)back-to-back voltage source converter, and (c) matrix converter.

inverters as shown in Fig. 2(b). To achieve full control of theoutput, the dc-link voltage must be boosted to a level higher thanthe amplitude of the isolated grid voltage. The power flow ofthe grid side converter is controlled in order to keep the dc-linkvoltage constant, while the control of the generator side is setto suit the magnetization demand and the reference speed ortorque. A technical advantage of this topology is the capacitordecoupling between the grid converter and the generator con-verter. Besides affording some protection, this decoupling offersseparate control of the two converters, allowing compensationof asymmetry both on the generator side and on the grid side,independently. Though, the dc-link capacitor is bulky and ex-hibits relative reduced lifetime.

The matrix converter solution as shown in Fig. 2(c) shouldbe an all silicon solution with no passive components in thepower circuit [7], [8]. The basic idea of this converter is thatthe desired input current, output voltage and output frequency

can be obtained by properly connecting the output terminals ofthe converter to the input terminals. Some advantages of the ma-trix converters are less thermal stress of the semiconductors fora low output frequency compared with the back-to-back solu-tion and the absence of the dc-link capacitor that may increasethe efficiency and the lifetime. As drawbacks can be mentioned:the intrinsic limitation of the output voltage, the unavailabilityof a true bi-directional switch and there is no decoupling be-tween the input and the output of the converter as for the caseof back-to-back converter and this may lead to some instabilityissues.

A comparative study regarding different power convertertechnologies that can be used in wind turbine applications [9]concluded that the back-to-back converter is the least trou-blesome configuration mainly because it can use commercialconverters at the generator side.

III. SYSTEM DESCRIPTION

The power configuration of the developed system is depictedin Fig. 3. It consists in a three-phase squirrel-cage induction gen-erator, a generator converter to control the speed of the shaft, adc-link with breaking chopper, a full-bridge grid converter withLCL filter, in-rush resistors, and the three-phase 400-V/50-Hzgrid.

A flexible test and development platform has been built usinga variable-speed drive to emulate the wind turbine mechanicalcharacteristics.

The control diagram of this platform is shown in Fig. 4 wherethe control of the wind turbine emulator is also shown.

Three commercial frequency inverters rated for18.5 kVA/400 V are used. The first one of the type Dan-foss VLT5022 is feeding a 15-kW induction motor in orderto emulate the wind turbine shaft and uses a standard scalaropen-loop torque control.

The second converter of type Danfoss VLT5022 Flux is usedas generator converter and uses the built-in standard vector fluxcontrol to regulate the speed of the generator and provide themagnetizing flux for the induction generator.

The last one is using only the power stage of a Danfoss VLT5022 inverter and is controlled by the dSPACE controller. It isused as grid converter and is connected to the grid through anLCL filter and employs a -axis control strategy for control-ling the active and reactive power independently. In-rush resis-tors are used to limit the dc-link capacitors charging current atstartup (see Fig. 3).

Both grid-connected and stand-alone mode for the grid con-verter have been implemented using the dSPACE controller. Amode switch can switch between the two operation modes auto-matically depending on the presence of the grid, as it is shownin VII.

A. Wind Turbine Emulator

The wind speed is calculated as an average value of thefixed-point wind speed over the whole rotor, and it takes thetower shadow and the rotational turbulences into account.The wind turbine mechanical shaft is emulated using a stan-dard six-pole 15-kW/400-V squirrel-cage induction motorcontrolled by a commercial frequency inverter operated inopen-loop torque control mode. The aerodynamic model of thewind turbine rotor is based the curve as shown in Fig. 5.

TEODORESCU AND BLAABJERG: FLEXIBLE CONTROL OF WIND TURBINES WITH GRID FAILURE DETECTION 1325

Fig. 3. Power configuration of the 11-kW variable-speed wind generator.

Fig. 4. Block diagram of the flexible development platform for the windturbine with 11-kW generator.

The tip speed ratio is calculated as

(1)

where is the shaft speed, is the blade radius, and is thewind speed.

The curve is very useful in modeling the torque pro-duction of the wind turbine at different wind speeds. Choosingthe optimal for the maximum torque coefficient , max-imum power can be extracted by setting the reference speed andtorque for the turbine emulator [1]

(2)

where is the optimal for the maximum torque coefficientas depicted in Fig. 5, and is the the air density.

Fig. 5. C � � performance curve for the wind turbine. C is the torquecoefficient.

The wind turbine emulator uses a scalar open-loop torquecontrolled induction motor where the torque reference is chosento be

(3)

in order to extract the maximum power from the wind turbine ata given wind speed.

B. Generator Control

The generator control is working in closed-loop speed controlmode where the speed reference converter is chosen to be equalwith the optimal speed in order to extract the maximum powerfrom the wind turbine at a certain wind speed

(4)

C. Grid Converter Control

The wind turbine grid converter can operate in both grid-con-nected control mode and stand-alone control mode.

Grid-Connected Control Mode: In grid-connected controlmode, basically all the available power that can be extractedfrom the wind turbine is transferred to the grid. Additionally,reactive power compensation is possible if required. Thecontrol structure for grid-connected control mode [10] is shownin Fig. 6. Standard PI-controllers are used to regulate the gridcurrents in the -synchronous frame in the inner control

1326 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER 2004

loops and the dc-voltage in the outer loop. A decoupling ofthe cross-coupling is implemented in order to compensate thecouplings due to the output filter [10], [11]. The referencecurrent in the -axis of the current loop is typically set to zero inorder to achieve zero phase angle between voltage and currentand so unity power factor can be achieved.

The output of the current controllers sets the voltage referencefor a standard space vector modulation (SVM) that controls theswitches of the grid converter via a fiber optic link. Synchronismwith grid is achieved by using a phase-locked loop (PLL) asdescribed in [5] and [6]. The principle of the PLL is depicted inFig. 7. The input signals are

(5)

where is the grid phase angle, and are the grid voltagecomponents in stationary -reference frame. The philosophyof the PLL is that the sinus of the difference between grid phaseangle and the inverter phase angle can be reduced to zerousing a PI-controller, and thus locking the grid inverter phase tothe grid, knowing that for small arguments

(6)

The output of the PI controller is the inverter output frequencythat is integrated to obtain the inverter phase . In order to im-prove the dynamic response at startup, the nominal frequency ofthe grid is feed-forwarded to the output of the PI-controller.

In Section VII, it will be described how the can be used fordetecting the presence or disappearance of the grid in order toautomatically switch between grid-connected and stand-alonecontrol mode.

Stand-Alone Control Mode: In stand-alone control mode, nogrid exists so the output voltages need to be controlled in termsof amplitude and frequency and thus, the reactive and, respec-tively, active power flow is controlled [1]. In the case of unbal-ance between the generated and the load-required power, adjust-ment of the speed of the generator can regulate the producedpower in a limited range. The potential excess of power willbe quickly dissipated in the damp resistor by starting thechopper control (see Fig. 4).

The control structure for stand-alone control mode is depictedin Fig. 8 and it consists of output voltage controller, dc-linkvoltage controller, damping chopper control, and current lim-iter.

The output voltage controller is aiming to control the outputvoltage with a minimal influence from the shape of the nonlinearload currents or load transients. A standard PI controller oper-ating in the synchronously rotating coordinate system whereis kept to zero is used. The dc-voltage PI controller maintainsthe dc voltage to the reference.

The dc-link voltage controller is acting only when the dc-linkis below the reference and it lowers the voltage reference of the

Fig. 6. Control structure for grid-connected control mode of operation.

Fig. 7. PLL structure used to synchronize the inverter voltage with the grid.

Fig. 8. Control structure for stand-alone mode of operation.

main voltage controller in order to avoid inverter saturation. Forfast response there is a direct forward connection to the voltagecontroller output.

When the dc-link is higher than the reference, the dampingchopper control is activated and discharges the energy in excessstored in the dc-link and thus reduces the dc-link voltage. Thecontrol is linear and increases the duty-cycle as a function ofthe overvoltage size. In the case of an over-production, the levelof produced power can be reduced also by changing the shaftspeed in the allowed speed range.

When the load current exceeds the rated current, the currentlimiter will decrease the output voltage reference in the allowedrange, and for fast response there is a direct forward connectionto the voltage controller output. This situation can occur if at acertain load, the generated power decreases due to lower windor during overloading.

TEODORESCU AND BLAABJERG: FLEXIBLE CONTROL OF WIND TURBINES WITH GRID FAILURE DETECTION 1327

D. Control Panel

The whole control strategy of the wind turbine emulator andwind turbine control is implemented in Simulink [12] using aDS1103 dSPACE controller. A virtual control panel developedin ControlDesk [12] that controls the whole system is shown inFig. 9. As it can be observed, the control layout is divided intotwo parts generator control and grid converter control. In thegenerator control panel, the reference for the generator speedand the mechanical power can be set. The speed and torque areshown in both graphs and digital displays.

In the grid-converter control panel the control mode willbe automatically switched between stand-alone and grid-con-nected according to the presence of the grid (see Section VII).In grid connection control mode, the references for the activeand reactive power to be transferred to the grid are given. Thedc-voltage reference can also be changed in the allowablerange.

In stand-alone control mode, the output voltage reference isset in terms of RMS value. The dc voltage reference can also bechanged in the allowable range. Grid currents and voltages aredisplayed in scope-like graphs. Digital displays show the powerbalance, including the dissipated power in the dc chopper, aswell as the output voltages and currents.

A test control panel for online tuning of the controllers hasalso been done. It proved to be a very convenient method tofine-tune the PI-controllers in the whole system.

In the following more details on the design of the LCL-filter,controllers in the two control modes and the Automatic ModeSwitching will be explained.

IV. LCL FILTER DESIGN

The LCL filter gives a better attenuation of the switchingripple compared to a classic L filter as it is a third-order filterand therefore it can be designed using smaller components.

The transfer function in the -plane of a single-phase LCL-filter (see Fig. 3) with passive damping is shown in (7) at thebottom of the page, where is the inverter voltage, is theinverter current, is the inverter-side inductance, is thegrid-side inductance, is the filter capacitance, and arethe damping series resistors.

The following design considerations are taken into account[10]:

1) the capacitor value is limited by the tolerable decrease ofthe power factor at rated power (less than 5%). The totalvalue of the inductance should be lower than 10% to limitthe dc-link voltage;

2) the voltage drop across the filter during operation in orderto limit the dc-link voltage;

3) the resonance frequency should be included in a rangebetween ten times the line frequency and one half ofthe switching frequency in order not to create resonanceproblems in the lower and higher parts of the harmonic

Fig. 9. ControlDesk panel for testing the wind turbine control strategy bothstand alone and grid-connected control modes.

spectrum. The passive resistors should be chosen as acompromise between the necessary damping and thelosses in the system.

The following values for the LCL-filter have been calculated:mH, mH, F, and .

V. GRID CONNECTION CONTROL MODE—CURRENT

CONTROLLER DESIGN

In order to design the PI current controllers in the syn-chronous -frame, the transfer function of the filter, con-sidered as the system plant, is discretized using a zero-orderhold method at a sampling frequency of 8 kHz and it therebybecomes

(8)

An additional delay of one switching period is inserted in themodel due to the PWM control of the inverter. Then the transferfunction of the PI-controller is

(9)

The block diagram of the current control loop is first sim-ulated in Simulink using the model from Fig. 10 where onesample delay due to the effect of PWM inverter is included.

A standard criterion to achieve an acceptable step response isused for tuning the PI controller [11].

Fig. 11(a) shows pole-zero plot of open loop system withoutthe controller. There are two complex conjugated poles and onereal pole. Because of complex conjugated poles, compensationtechniques cannot be used. Instead a pole-zero placement tech-nique is used. An integrator time constant ms is selectedas a compromise between good dynamics and still a good noise

(7)

1328 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER 2004

Fig. 10. Simulink model of current control loop in z-domain for the windturbine.

Fig. 11. Pole-zero placement of (a) open-loop system and (b) closed loopsystem for the current controllers in grid-connected control mode.

rejection. The gain is selected using root locus method sothat the dominant poles have a damping of 0.7. In Fig. 11(b),the pole-zero placement for optimized closed loop of the controlis shown. The dominant poles have now a damping of 0.7 at afrequency of 580 Hz. The gain of the controller is found to be

.

VI. STAND-ALONE CONTROL MODE—VOLTAGE

CONTROLLER DESIGN

The output voltage controller is aiming to control the outputvoltage with minimal influence of the shape of the nonlinearload currents or load transients. The transfer function of the gridwith a load impedance in terms of grid voltage and in-verter voltage is representing the plant and can be expressedin the -domain as (10) shown at the bottom of the page, where

is the grid impedance at nominal power.Synthesis of the voltage regulator is again made in the

-plane, using a Simulink equivalent model, where the gridtransfer function has been converted into the -plane usingzero-order-hold method [12]. The switching frequency ischosen to be 8 kHz. The pole-zero placement of the plantshown in Fig. 12(a) show, that there is a real pole that can becompensated by setting the integral time of PI-controller to

ms. Using the root locus method [12] depictedin Fig. 12(b), a gain of can be chosen to achievea damping of the loop of . From the compensatedBode-plot shown in Fig. 12(c), the bandwidth of the voltagecontroller can be determined as the minimum frequency for3-dB attenuation or 45 phase [12] as Hz.

Fig. 12. Design of the voltage controller in stand-alone control mode:(a) pole-zero map for the plant, (b) root-locus map for closed-loop system, and(c) Bode-plot of the closed loop system.

The controller is now tested at very light load conditions (1%of nominal load) and a low damping at the resonance frequencyof the LCL-filter was noticed as shown in Fig. 13(a). The re-sponse of the system depicted in Fig. 13(b) shows high oscilla-tions due to a high gain.

Redesigning the controller for light load using the same tech-nique (root locus) leads to a lower gain of . TheBode-plot shown in Fig. 14(a) shows much better attenuationat the switching and sampling frequency of 8 kHz and the stepresponse depicted in Fig. 14(b) was considered acceptable interms of compromise between the speed and damping of thehigher order harmonics at low load conditions.

From the compensated Bode-plot shown in Fig. 14(a), thebandwidth of the voltage controller can be obtained as

Hz. The final parameters for the main output voltage con-troller are , ms.

VII. AUTOMATIC MODE SWITCHING

Switching between stand-alone and grid-connection opera-tion modes is done using the difference between the measuredgrid voltage angle and the angle generated by the phase-locked loop (PLL) [5] as control variable (see Fig. 7). Whenthis phase error (see Fig. 15) is exceeding a certainlevel grid failure is considered and the operation mode is con-sequently switched.

(10)

TEODORESCU AND BLAABJERG: FLEXIBLE CONTROL OF WIND TURBINES WITH GRID FAILURE DETECTION 1329

Fig. 13. Voltage controller characteristics at 1% load in stand-alone control mode with K = 0:32: (a) Bode plot and (b) step response.

Fig. 14. Voltage controller characteristics at 1% load in stand-alone control mode with K = 0:1: (a) Bode-plot and (b) step response.

Fig. 15. Phase angle detection with PLL in xy-coordinate system.

In grid-connection mode the system is using the PLL-gener-ated phase angle that is less noise sensitive than the phase angledirectly calculated from the voltage -components using arctanfunction [11].

Fig. 7 depicts the PLL loop principle. A settling time of 20 mswas considered in order to get a satisfied high bandwidth of

the PLL controller. When the grid fails, the phase error isstarting to increase and when it exceeds a certain threshold thesystem switches to stand-alone mode. In stand-alone mode thePLL is disabled and the reference frequency is integrated togenerate the angle (see. Fig. 8).

The grid-connection to stand-alone transition is demon-strated experimentally in Fig. 16 at nominal load of 11 kW. Thethreshold for the phase error was set to 0.3 radians. Thusan efficient way to detect the grid failure is achieved, and as itcan be observed, in this case the system continues to operatein stand-alone mode.

In stand-alone operation the system uses its own generatedvoltage phase angle, which is normally different from gridphase voltage angle. When the grid is recovered, due to thefact that the two voltage systems are not synchronized, a highvoltage across the LCL filter would cause high currents to flowthrough the inverter. The high currents will trip the inverterand the presence of the grid is investigated by measurements.If the grid is recovered, the coasted inverter waits for the PLLto perform synchronization. The system is then considered

1330 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER 2004

Fig. 16. Measured grid-connection to stand-alone transition: (a) phase error�� (etheta) and synchronization signal (PLLe), (b) phase currents, and (c) phasevoltages;.

synchronized again when the phase error falls below thethreshold and now the inverter is started in grid-connectionmode, as it can be seen in Fig. 17 where the threshold wasset again to 0.3 radians.

A state-machine has been built and implemented in order tohandle the transition between the three states: idle (I), stand-alone (SA), and grid-connected (GC) as depicted in Fig. 18,where and represent the presence and, respectively,nonpresence of the grid voltage condition, PLLe represents thecondition , TRIP and /TRIP stands for tripsignal enabled, and, respectively, not-enabled and START and

Fig. 17. Measured stand-alone to grid-connection transition: (a) phase error�� (etheta) and synchronization signal (PLLe), (b) phase currents, and (c) phasevoltages.

STOP stands for the running mode, and, respectively, coastingmode (idling).

The advantage of this method is that it does not require extrahardware for mode switching as described in [3].

The shortcoming of this method is that the PLL controllerhas to be fast enough in order to limit the current rise below thepermitted limit during the detection period.

VIII. EXPERIMENTAL RESULTS

The hardware realization of the back-to-back converter isshown in Fig. 19. As it can be observed, the system has beenhoused in a mobile cabinet in order to ease the field-testing.

TEODORESCU AND BLAABJERG: FLEXIBLE CONTROL OF WIND TURBINES WITH GRID FAILURE DETECTION 1331

I-GC = V and /PLLe and /TRIP and STARTI-SA = /V and /TRIP and STARTGC-I = TRIP or STOPSA-I = TRIP or STOP or PLLeGC-SA = PLLe

Fig. 18. State-machine for controlling the automatic mode switch.

Fig. 19. Back-to-back converter mobile cabinet build for the 11-kW windturbine test setup with dSPACE DS1103controller.

First the system is tested in grid-connection mode at a 10-Astep in the -current reference. The system response depictedin Fig. 20 and demonstrates good performance of the currentcontroller. The presence of the low-order harmonics in the cur-rent can be observed as a consequence of the grid voltage back-ground harmonic distortion.

The system is then tested in stand-alone mode generating thenominal power of 11 kW having a linear balanced resistive load.The dc-voltage was set to 620 V, and the output phase voltagewas set to 230 V.

The experimental results are shown in Fig. 21 and reveallower harmonic distortion. The phase shift of 30 between themeasured voltage and current is due to the fact that the measuredvoltage is line-to-line voltage.

The wind turbine is tested in stand-alone mode generatingthe nominal power of 11 kW having a nonlinear load createdwith a three-phase diode rectifier with resistive load (Fig. 22).

Fig. 20. Experimental test of grid-connection control mode. Current responseat 10-A step in d-axis reference. Grid phase voltage V [250 V/div], dc-voltageV [250 V/div] and grid current I [5 A/div].

Fig. 21. Experimental test with linear load in stand-alone mode. Grid line-linevoltage V [250 V/div], dc-voltage V [250 V/div], and grid current I[10 A/div].

Fig. 22. Experimental test with an 11-kW nonlinear load: (a) grid phasevoltage V and (b) grid current I .

The dc-voltage was set to 700 V, and the output phase voltagewas set to 230 V. The measured voltage THD was 9.8% at acurrent THD of 34.7% indicating a reasonable voltage regula-tion. The regulation can be improved by using other controllerstructures like resonant controllers which are more suitable forsinus tracking [11]. The switching ripple in the waveforms fromFig. 22 is removed as they are sampled once in the middle of

1332 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER 2004

the switching period using the dSPACE controller. The THD ismeasured with a Voltech PM3000 power analyzer directly.

IX. CONCLUSION

This paper presents the development and test of a flexiblecontrol system for an 11-kW wind turbine system with adouble back-to-back power converter capable of working instand-alone mode but also in grid-connection mode on a localgrid. A novel strategy for automatic switching between thetwo operating modes based on PLL controller is presented andsuccessfully tested.

The most relevant feature of the system is the flexibility,ability to develop the control strategy entirely in Simulink andtest it in a short time. The test results show high-performancefeatures which can be used in future wind turbine systems.

REFERENCES

[1] F. Blaabjerg, Z. Chen, and S. B. Kjaer, “Power electronics as efficientinterface in dispersed power generation systems,” IEEE Trans. PowerElectron., vol. 19, pp. ??????–??????, Sept. 2004.

[2] R. Pena, R. Cardenas, R. Blasco, G. Asher, and J. Clare, “A cage induc-tion generator using back-to-back PWM converters for variable speedgrid connected wind energy system,” in Proc. IECON’01 Conf., vol. 2,2001, pp. 1376–1381.

[3] R. Tirumala and N. Mohan, “Seamless transfer of a grid-connectedPWM inverters between utility-interactive and stand-alone modes,” inProc. APEC’02 Conf., vol. 2, 2002, pp. 1081–1086.

[4] R. Otterbach, T. Pohlmann, A. Rukgauer, and J. Vater, “DS1103PPC controller board—rapid prototyping with combined RISC andDSP power for motion control,” in Proc. PCIM’98 Conf., Nürnberg,Germany, 1998.

[5] G.-C. Hsich and J. Hung, “Phase-locked loop techniques—A survey,”IEEE Trans. Ind. Electron., vol. 43, pp. 609–615, Dec. 1996.

[6] S.-K. Chung, “Phase-locked loop for grid-connected three-phasepower conversion systems,” Proc. Inst. Elect. Eng., vol. 147, no. 3, pp.213–219, May 2000.

[7] P. Nielsen, F. Blaabjerg, and J. K. Pedersen, “New protection issues ofa matrix converter design considerations for adjustable speed drives,”IEEE Trans. Ind. Applicat., vol. 35, pp. 1150–1161, Sept./Oct. 1998.

[8] J. Chang, T. Sun, and A. Wang, “Highly compact AC-AC converterachieving a high voltage transfer rate,” IEEE Trans. Ind. Electron., vol.49, pp. 345–352, Apr. 2002.

[9] L. H. Hansen, L. Helle, F. Blaabjerg, E. Ritchie, S. Munk-Nielsen, H.Bindner, P. Soerensen, and B. Bak-Jensen, “Conceptual survey of gen-erators and power electronics for wind turbines,” Tech. Rep. Risø-R-1205(EN), 2001.

[10] M. Liserre, A. Dell’Aquila, and F. Blaabjerg, “Design and control ofa three-phase rectifier under nonideal operating conditions,” in Proc.IAS’02 Conf., vol. 2, 2002, pp. 1181–1188.

[11] M. P. Kazmierkowski, R. Krishnan, and F. Blaabjerg, Control in PowerElectronics. Selected Problems. New York: Academic, 2002.

[12] A. Tewari, Modern Control Design With Matlab and Simulink. NewYork: Wiley, 2002.

Remus Teodorescu (S’94–M’99–SM’02) receivedthe Dipl.Ing. degree in electrical engineering fromthe Polytechnical University of Bucharest, Romania,in 1989 and the Ph.D. degree in power electronicsfrom Galati University, Romania, in 1994.

From 1989 to 1990, he was with the Iron and SteelPlant, Galati, and then was with Galati University,where he was an Assistant with the Electrical En-gineering Department, and since 1994, an AssistantProfessor. In 1996, he was appointed head of thePower Electronics Research Group (PERG), Galati

University. In 1998, he joined the Institute of Energy Technology, PowerElectronics and Drives Department, Aalborg University, Aalborg, Denmark,where he is currently an Associate Professor. He has more then 60 paperspublished, one book, and two patents. His areas of interests include powerconverters for renewable energy systems, multi-level inverters, digital controland computer simulations of advanced electrical drives.

Dr. Teodorescu received the Technical Committee Prize Paper Awards, andthe IEEE-IAS’98 and OPTIM-ABB Prize Paper Awards at OPTIM’02.

Frede Blaabjerg (S’86–M’88–SM’97–F’03)received the M.Sc.EE. and Ph.D. degrees fromAalborg University, Aalborg, Denmark, in 1987 and1995, respectively.

He was with ABB-Scandia, Randers, Denmark,from 1987 to 1988. He became an Assistant Pro-fessor at Aalborg University in 1992, an AssociateProfessor in 1996, and a Full Professor in powerelectronics and drives in 1998. He is the authoror coauthor of more than 300 publications inhis research fields including Control in Power

Electronics (New York: Academic, 2002). He is an Associate Editor for theJournal of Power Electronics and the Danish journal Elteknik. His researchareas are in power electronics, static power converters, ac drives, switchedreluctance drives, modeling, characterization of power semiconductor devicesand simulation, wind turbines, and green power inverters.

Dr. Blaabjerg received the 1995 Angelos Award for his contribution inmodulation technique and control of electric drives, the Annual Teacher Prizeat Aalborg University in 1995, the Outstanding Young Power ElectronicsEngineer Award from the IEEE Power Electronics Society in 1998, four IEEEPrize paper awards during the last six years, the C. Y. O’Connor Fellowshipfrom Perth, Australia in 2002, the Statoil-Prize in 2003 for his contributionsin power electronics, and the Grundfos-prize in 2004 for his contributionsin power electronics and drives. He is an Associate Editor of the IEEETRANSACTIONS ON INDUSTRY APPLICATIONS and the IEEE TRANSACTIONS ON

POWER ELECTRONICS. He has held a number of chairman positions in researchpolicy and research funding bodies in Denmark.