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IEEE TRANSACTIONS ON ENERGY CONVERSION 1

Variable Speed Wind Turbines Based onElectromechanical Differential Systems

Markus Waldner and István Erlich, Senior Member, IEEE

Abstract—This paper introduces a new electromechanical en-ergy conversion concept for use in wind-based power generationsystems. Modern wind turbines use frequency converters to meetthe speed control requirement. But voltage source-based frequencyconverters typically have limited overload capability. In addition,they fully or partially decouple the rotating masses of wind en-ergy converter systems, thereby removing altogether or reducingthe inertia as a medium of energy storage and modulation fromthe perspective of the grid during transient conditions. In the pro-posed concept, a drive system based on electromechanical differ-ential gear including a servo motor is used for generator torquecontrol, while using a synchronous machine as the main genera-tor. The system thus provides the speed variability required foroptimal utilization of wind energy combined with the advantagesof a directly grid-connected synchronous generator. It as a resultpermits the supply of high currents during faults to the grid toprovide more effective voltage support as there are no electroniccircuits in the main power path, which would limit the overload ca-pability. Furthermore, the inertia of the system comes fully to bear,thus contributing to the overall grid inertia. The performance ofthe system has been studied experimentally on a prototype systemand the results of the conceptual analysis have been verified. Usingthe mathematical model of the system, a grid fault simulation hasbeen performed, and the response during fault and the dampingbehavior has been studied. The results show the excellent damp-ing behavior due to the active damping control by the differentialdrive.

Index Terms—Differential drive, speed control, variable speed,wind energy converter (WEC).

I. INTRODUCTION

THE optimal utilization of wind energy presupposes vari-able rotor speed [1]. To ensure this basic requirement,all wind energy converter (WEC) systems, which have estab-lished themselves in the marked, are using frequency convert-ers. In WEC based on doubly fed induction generators (DFIG),the three phase rotor circuits are connected through a voltagesource converter to the grid. On the rotor side, the frequency,respectively, the phase angle, and also the voltage amplitudeare controlled by the converter. The rotor speed, as the differ-ence of the two rotating magnetic fields—one from the stator

Manuscript received February 10, 2013; revised June 12, 2013 and September19, 2013; accepted October 6, 2013. Paper no. TEC-00080-2013.

M. Waldner is with the SET Sustainable Energy Technology GmbH, 9020Klagenfurt, Austria (e-mail: [email protected]).

I. Erlich is with the Department of Electrical Power Systems, UniversityDuisburg-Essen, 47057 Duisburg, Germany (e-mail: [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/TEC.2013.2287260

and another from the rotor side-can vary in the range of ±30%around the synchronous speed. In the full converter-based WECconcept, the whole generated power is supplied through thevoltage source converter. Therefore, on the generator side, thefrequency can be adapted to the wind speed regardless of thegrid frequency. Both concept are very popular and represent thestate-of-the-art technology in modern wind turbines.

However, due to the power electronic components, the over-load capacity of these WEC is very limited. It is just slightlyabove the nominal current. As a consequence, the wind turbinesdo not contribute considerably to the short-circuit currents in theevent of grid fault. Also, the voltage support capability duringgrid faults is limited due to the current limitation. Besides, therotating inertia as a medium of energy storage is decoupled fromthe grid by the fast acting converter control. Discharging energyfrom the rotating masses, as is the case in conventional syn-chronous generators, without any control action is not possiblein modern WEC.

The aforementioned drawbacks of the current WEC systemshave prompted the quest for some new WEC concepts. The ideais to keep conventional synchronous machines directly con-nected to the grid and make the speed variable on the gearboxside. One approach resulting in a commercial product based onhydraulic gearbox was developed by the company VOITH [2].

In this paper, the authors are introducing a new concept usingan electromechanical differential system (DS) from the com-pany SET [3]. It is called DSgen-set R©. The system design willbe described in detail, and measurements as well as simulationresults intended to demonstrate the performance of the systemare presented toward the end of the paper. Finally, a comparisonis provided with the established DFIG system with respect tothe response following grid faults.

II. SYSTEM DESIGN

Figs. 1 and 2 show the principle of an electromechanicalsystem consisting of a differential gear (1–3) and a differentialdrive (4, 5). The WEC rotor drives the main gearbox, which isusually a three-stage gear with two planetary stages and a spurgear. Between the main gearbox and the generator (6), thereis the differential stage (1–3), which represents the core of theentire system. The generator (6), preferably a separately excitedhigh-voltage synchronous generator, is connected to the ringgear (2) of the differential gear and driven by it. The pinion(3) on the differential gear (sun gear) is connected to the servomotor (5) by means of a spur gear (7). The differential driveconsists of the servo motor (5), typically a low-voltage three-phase generator, and the frequency converter (4). The frequencyconverter (4) is connected to the high-voltage grid (8) via the

0885-8969 © 2013 IEEE


2 IEEE TRANSACTIONS ON ENERGY CONVERSION

Fig. 1. Principal design of a WEC based on electromechanical differential.

Fig. 2. Sectional view of the differential gear.

transformer (9), which also has a 400-V outlet for supplyingthe WEC auxiliaries. The speed of the differential drive (4, 5)is controlled, on one hand, to ensure that the variable speed ofthe WEC rotor is converted to constant speed at the generator(6 and 2), and, on the other, to control the torque throughout theentire WEC drive train [3].

A major advantage of the schema shown in Figs. 1 and 2 is thesimplicity of the differential gear and, as a result, the high effi-ciency of the differential stage. In addition, the differential gearcan also be manufactured as a separate assembly and, therefore,installed and maintained independently of the main gearbox.

The speed equation for the DS is

ωServoISpur

+ I0ds · ωGen −1 + I0dsIMainGB

· ωWECRotor = 0 (1)

The parameter I0DS is the transmission ratio of the differen-tial gear and stands for the gear ratio between the ring gear andthe sun gear. The ring gear is directly connected to the gener-ator shaft, and the sun gear is connected to the servo shaft via

the spur gear with the transmission ratio ISpur . IMainGB is thetransmission ratio of the main gearbox. ωServo and ωGen are theservo and WEC rotor speeds, respectively.

The torque at the high-speed shaft (HSS) THSS is determinedby the available wind and the aerodynamic efficiency of therotor. The ratio between the torque at the rotor shaft and that atthe differential drive is constant, thus enabling the torque in thedrive train to be controlled by the differential drive:

TServo = −THSS

(1 + I0DS) ISpur(2)

Finally, the transport of the generated wind rotor powerthrough the differential gear can be described as follows:

PWECRotor = ωWECRotor · TWECRotor . (3)

Without losses in the main gear stage, one obtains for theinput shaft:

PWECRotor = PHSS (4)

and hence

PHSS = PServo + PGen (5)

Considering (1) and (2), we get

PServo = ωServo · TServo

=(

1 + I0DSIMainGB

· ωWECRotor − I0DS · ωGen)· THSS1 + I0DS

(6)

PGen = ωGen · TGen = ωGen ·I0DS

1 + I0DS· THSS (7)

ωGen is constant and given by the grid frequency. So, the inputpower is split up in two paths through the differential gear, theservo path, and the generator path. Only a fraction of the totalpower (


WALDNER AND ERLICH: VARIABLE SPEED WIND TURBINES BASED ON ELECTROMECHANICAL DIFFERENTIAL SYSTEMS 3

Fig. 3. Prototype of a 3-MW/10-kV WEC generator including the electrome-chanical DS and corresponding converter on the test bench.

TABLE ITECHNICAL DATA

Fig. 4. Speed and power ratios for a differential stage.

Fig. 5. Possible wind farm configuration using WECs with differencialsystem.

Fig. 6. Efficiency of the DSgen-set R© compared with permanent magnet gen-erator full-scale converter (PMG-FSC) concept.

it is operated as a motor (−) in the range less than the basicspeed and as a generator (+) in the range greater than the basicspeed. This means that power is fed to the differential stage inthe motor range and withdrawn from the differential stage in thegenerator range. If the differential drive is electric, the poweris preferably withdrawn from and fed into the grid. The sum ofgenerator power and differential drive power is equal to the totalpower delivered to the grid for an electric differential drive.

Since the synchronous generator is connected directly to thegrid, the implemented excitation of the WEC presented hereallows controlling the terminal voltage and the supply of re-active power to the network as it is common in conventionalpower plants. Besides, the short-circuit contribution is consid-erably higher than that of converter-based WEC systems. As thegearbox itself is small, losses are only minimal.

As the generator can be designed directly for voltages≥10 kV,under circumstances, there is no need for additional transformersfor connecting to the wind farm. The generator terminal voltageis ≥10 kV. This allows a wind farm configuration as shown inFig. 5

The servo, which is actually required to control the system,is also used as a generator to produce electricity. This enablesan overall system efficiency (DSgen-set R© including the syn-chronous generator) of 97%. A comparison with permanentmagnet full rated converter WEC is shown in Fig. 6.



Fig. 7. Speed and power ratios for the locked mode and the DS mode.

As the generator is directly connected to the grid and thereare no electronic circuits in the main power path, power can befed directly into the grid at medium-voltage level, or individualWECs in a wind farm can be connected without high lossesoccurring.

A special feature of the DSgen-set R© is the possibility to lockthe differential gear. This allows an extension of the workingrange toward low power (low wind). In this mode, the syn-chronous generator is disconnected from the grid; however, theservo drive runs in generator mode like a full converter machine.The pure DSgen-set R© allows a minimum wind rotor speed ofapproximately 70% of the nominal speed. This is a disadvantagecompared to full converter systems. However, with the lockedsystem, the speed range can be extended to a cut-in speed of30% of the nominal speed (see Fig. 7). The maximum outputpower in this mode depends on the used servo drive and is about20% of the nominal power.

From a cost perspective, the relatively small servo driveplus the utilization of a conventional medium-voltage syn-chronous generator do substantially reduce the cost for powerelectr(on)ics, which includes generator, converter, cables, andtransformer. Typically, the achievable reduction of costs forpower electr(on)ics of a DSgen-set R© over a PMG-FSC can reachup to about 50%.

IV. MODELING

In the course of the system development, plenty of investiga-tions have been carried out. One objective of the study was thedrive-train analysis with respect to grid faults, in particular theimpact of the peak torque caused by the generator on the tower,nacelle, blades, etc. On the other hand, electricity utilities areinterested in the system performance in response to grid faults. Itis expected that modern wind turbines are able to ride-throughdeep voltage sags caused by faults like three-phase short cir-cuits and provide, at the same time, voltage support by injectingcapacitive reactive current. To quantify the performance of theDSgen-set R©, detailed simulations results will be compared inthis paper with the well-known DFIG wind turbine technology.

Fig. 8. Schematic diagram of the of the servo controller model.

From the simulation point of view, three subsystems have tobe considered: the mechanical, the electrical, and one whichrepresents the control system [10].

The model of the electrical subsystem consists of well-knowncomponents like the synchronous generator, the transformer,etc. The specific electrical parameters for these could be takenfrom the real components of the DSgen-set R© and were providedby the supplier of these components. The controller subsystem(see Fig. 8) is actually a feed-forward system to control the servotorque.

It consists of a lookup table which defines the relation betweenthe input speed and the expected HSS torque. This describesthe WEC characteristic and has to be adapted from case tocase. Furthermore, a low-pass filter is included with the gainrepresenting the gear ratio to calculate the corresponding servotorque. A limiter is required in order to avoid overloads. Theentire dynamics of the control part is dominated by the combinedservo and converter response, which is represented by a transferfunction [will be discussed later in conjunction with (11)].

The electrical and the control subsystems have a large impacton the behavior of the whole system, but their effect is straight-forward and easy to predict. The model of the mechanical part ismore complex and has been developed from the scratch. In casethat mechanical subsystem is not correctly modeled—even if theother two subsystems are perfect—the results of the completesimulation would not be accurate enough.

A very detailed mechanical subsystem model has been devel-oped by using the MBS software [5]. It provides the flexibilityand the unavoidable nonlinearities necessary for accurate mod-eling. In order to simulate complex mechanical behavior ofthe system, the MBS software utilizes a reduced finite-elementapproach based on the Craig-Bampton method and modal de-scription [11].

For analyzing the mechatronic models with respect to itsdynamic behavior, a couple of tools have been used [4], [5]. Formore details, the reader is referred to [6]–[8].



Fig. 9. Image of the MBS model. The measurement points give the positionof the results shown in Figs. 10–12.

Fig. 10. Oscillation in x-direction regarding the servo (see also Fig. 9).

For the verification of the 3-D (six degrees of freedom) mul-timass model of the DSgen-set R© also components of the test-bench were considered. Fig. 9 shows this model, which hasbeen used for validation of the DSgen-set R© results within thetest bench.

Figs. 10–12 show comparison between simulation and mea-surement in terms of servo housing and HSS.

Finally, when the results of this tuning procedure were foundto be satisfactory, the characteristics of each mechanical com-ponent were transformed to a 1-D (one degree of freedom:torsional) multimass model, which can be used for dynamic

Fig. 11. Oscillation in z-direction regarding the servo (see also Fig. 9).

Fig. 12. Oscillation regarding the input shaft (HSS) (see also Fig. 9).



Fig. 13. Speed comparison of each pinion shaft for a tested wind profile (blue: measured, red: simulated values).

simulations in combination with the synchronous generator,servo, and converter.

The central point of the 1-D multimass model is the differen-tial gear with the transmission ratios.

The pinions of the gear are connected to the generator shaftand the HSS, which are modeled as a typical two-mass–springsystem. The servo shaft is more complex and is split into a coupleof submodules of a mass–spring systems. In total, the modelconsists of 15 masses, 9 in the servo path, 2 in the generatorpath, and 3 in the HSS path.

The drive-train model also considers friction and dampingfactors. During the model verification tests, these factors weretuned according to the results provided by the real system onthe test bench (see Fig. 3). Figs. 13 and 14 show the resultsregarding speed and torque of a model verification test run onthe test bench.

V. CONTROL

The system has different operating modes and each modeneeds separate control algorithms. Basically, one can distinguishbetween the two different states of the system.

1) Generator is not synchronized to the grid.2) Generator is synchronized to the grid.This paper will focus on state 2, when the generator is already

synchronized to the grid. Due to the constant speed on the gen-erator side controlling, the power is equivalent to controlling thetorque. The characteristic of the differential gear regarding the

Fig. 14. HSS torque comparison for a testet wind profile (blue: measured, red:simulated values).

torque is

TRing = −I0DS · TSun (8)

and

TCarrier = (I0DS + 1) · TSun . (9)



The ring gear is connected to the generator, so that TRingrepresents the torque of the generator shaft. The carrier gear isconnected to the HSS, so that TCarrier represents the torque ofthe HSS, and TSun is the torque of the sun gear shaft. The latterhas to be provided by the servo drive. However, in the analyzeddesign within the path of the servo shaft, a spur gear with thegear ratio ISpur is included. This has to be considered for thecalculation of the servo torque. Given the above requirements,the generator torque results in the relation:

TGen = −I0DS · ISpur · TServo . (10)

The parameter I0DS stands for the gear ratio between thering gear and the sun gear. The servo torque is controlled by anunderlying torque control loop. Due to this feedback control,the dynamics of the servo drive can be described with a sim-ple transfer function. This transfer function was identified byanalyzing a step response and is given as follows:

GServo(s) =ToutTin

=11025

s2 + 126s + 11025(11)

where Tin is the leading function of the torque controller andTout is the desired servo torque Tservo . The leading torque Tinfor the servo can be calculated by using (2). But note that theservo has to provide a counter torque with respect to the HSS anda driving torque for the generator. Hence, the sign of the desiredTServo is opposite to that of THSS as indicated in (2). Finally,THSS is the torque on the input shaft (behind the main gear),which depends on the control characteristic of the WEC. Thefollowing equation is the known relationship for an optimizedcontrol regime with just one input variable, the wind rotor speedvW :

TWECRotor =cp (λopt) · ρ · π3 · R5

2 · λ3opt · 302· v2W . (12)

This, on the other hand, can be used to calculate the THSSneeded in (11):

THSS =TWECRotorIMainGB

. (13)

In principle, to control the active power of the generator, one hasonly to measure the speed of the wind rotor. Via the torque/speedcharacteristic of the WEC, the desired HSS torque is determinedand so the desired leading torque for the servo as well. Theservo torque will be adjusted with the torque control loop. Thecharacteristic of the differential gear provides the appropriatetorque to the synchronous generator and so, in the end, thedesired active power (see also Fig. 8).

VI. RESPONSE TO GRID FAULTS

For testing the grid performance of the DSgen-set R©, a three-phase fault has been simulated in the test grid shown in Fig. 15.It represents a typical medium-voltage distribution grid.

For comparison, the same faults are simulated by using aDFIG wind turbine model [9] of a real system, which was usedin a previous study. The size of both WEC is the same. During thefault and the subsequent recovery period, the WEC shows elec-

Fig. 15. Test grid.

Fig. 16. Active power of DSgen-set R© following grid short circuit.

Fig. 17. Comparison of DSgen-set R© and DFIG active power behavior follow-ing grid short circuit.

tromechanical oscillations typical for directly grid-connectedsynchronous machines (see Figs. 16 and 17).

In the classical synchronous generators, it may lead to sometransient stability problems due to the fact that the drive torquecannot be reduced considerably during this time period. How-ever, the DSgen-set R© is controlled electronically and the dif-ferential drive reacts very fast. Therefore, the mechanical drivetorque is reduced nearly simultaneously as the terminal powerdrops, so the balance is always maintained and the synchronousgenerator remains in synchronism. The damping of the elec-tromechanical oscillations is excellent due to the active dampingcontrol by the differential drive. Comparing both WES systems,it is apparent that no electromechanical oscillations exist whenrunning the DFIG WEC. On the other hand, this fact indicates



Fig. 18. Comparison of DSgen-set R© and DFIG reactive power behavior fol-lowing grid short circuit.

Fig. 19. Comparison of DSgen-set R© and DFIG terminal voltage behaviorfollowing grid short circuit.

Fig. 20. Comparison of DSgen-set R© and DFIG reactive current supplied fol-lowing grid short circuit.

also that DFIG WEC does not contribute to the grid inertia,whereas the DSgen-set R© does.

Another important advantage of the DSgen-set R© is demon-strated in Figs. 18–20 concerning the grid voltage support capa-bility. In any DFIG WEC systems, the maximum current sup-plied to the grid is limited by the power converter connected tothe rotor side. The same is valid for full rated converter WECsystem, where the entire generated power is supplied throughthe converter. By contrast, the DSgen-set R© is able to supply

high currents to the grid and so helps to mitigate the voltagedrop during the low-voltage period. Following fault clearing,the voltage recovery is faster than that in the DFIG. However, itcan also result in voltage overshoot. The slower voltage recov-ery with the DSgen-set R© is well known from the conventionalsynchronous generators and does not represent a real concern.Finally, an important aspect is the electrical behavior in termsof the severity of the fault the machine can ride-through and themaximum level of a voltage drop resulting from a fault. Thesynchronous generator is able to withstand voltage drops of upto 100%. DFIGs, on the other hand, can support maximum volt-age drops of up to 80%, beyond which the control capability islost. This is a huge advantage for the operating company sincethe WEC can fulfill the most stringent grid code requirementswithout any compromise.

VII. CONCLUSION

The DSgen-set R© WEC introduced in this paper represents apromising alternative to the established WEC systems. It pro-vides the speed variability required for optimal utilization ofwind energy combined with the advantages of directly grid-connected synchronous generators. In particular, it allows thesupply of high currents during faults to the grid and thus pro-vides more effective voltage support. Furthermore, there is acontribution to the overall grid inertia which is expected todecrease with severe consequences for the entire grid whenonly converter-based WEC systems are used. Besides, it pro-vides higher efficiency and reliability than the established WECsystems.

The DSgen-set R© is fully developed and tested by using aprototype of a 3-MW/10-kV system.

REFERENCES

[1] T. Ackermann, Ed., Wind Power in Power Systems. 2 nd ed., New York,NY, USA: Wiley, Apr. 2012. (1st ed., Jan. 2005).

[2] A Unique Solution to Generating Electricity from the Wind. WinDriveTechnology. (2011). [Online]. Available: http://voith.com/en/products-services/power-transmission-383.html?page=2&limit=10&searchString=&productCategory=variable_speed_drives&language=&publicationCategory=#literature

[3] SET Sustainable Energy Technologies GmbH. (2013). [Online]. Avail-able: http://www.set-solutions.net/technology.html

[4] SimPowerSystems and SimDriveline Toolbox, Mathworks, Natick, MA,USA, 2011.

[5] G. Offner and H. H. Priebsch, “Flexible multi-body dynamicssimulation—A powerful method for prediction of structure borne noiseof internal combustion engines,” in Proc. Int. Conf. Noise Vibration Eng.,2006, p. 14.

[6] D. Bestle, Analyse und Optimierung von Mehrkörpersystemen. Berlin,Germany: Springer-Verlag, 1994.

[7] O. Wallrapp and S. Wiedemann, “Comparison of results in flexible multi-body dynamics using various approaches,” in Nonlinear Dynamics. vol.34, Norwell, MA, USA: Kluwer, 2003, pp. 189–206.

[8] O. A. Bauchau, Flexible Multibody Dynamics. New York, NY, USA:Springer, 2011.

[9] I. Erlich, J. Kretschmann, J. Fortmann, S. Engelhardt, and H. Wrede,“Modeling of wind turbines based on doubly-fed induction generators forpower system stability studies,” IEEE Trans. Power Syst., vol. 22, no. 3,pp. 909–919, Aug. 2007.

[10] UpWind—The largest EU- funded wind project ever. (2011). [Online].Available: http://www.upwind.eu/publications/

[11] AVL EXCITE Power Unit (MBS software). (2012). [Online]. Available:https://www.avl.com/web/ast/excite



Markus Waldner was born in 1968. He receivedthe Dipl.Ing degree in electrical engineering from theGraz University of Technology, Graz, Austria.

From 1997 to 1998, he was a Control Engineerand Developer with Philips. From 1998 to 2002, hewas a Digital Designer with Infineon Technologiesand from 2002 to 2008, he was a Systems Engineerwith Miconas. During this time, he has been involvedin the design and testing of digital signal processingsystems and video processing. From 2008 to 2010with Grace Semiconductor, he was responsible for

the technical marketing in Europe and Asia. Since 2008, he has a teachingassignment for “Applied automatic control engineering” at the FH Pinkafeld,Austria. In 2010, he started his career at SET Sustainable Energy TechnologyGmbH. Since 2012, he has been the Director of the department “Analyticalcalculation,” where he is leading a team of experienced engineers for modelingand simulating power train systems in terms of mechanical and electrical aspects.

István Erlich (SM’08) was born in 1953. He re-ceived the Dipl.-Ing. degree in electrical engineeringand the Ph.D. degree from the University of Dresden,Dresden, Germany, in 1976 and 1983, respectively.

From 1979 to 1991, he was with the Departmentof Electrical Power Systems, University of Dresden.From 1991 to 1998, he worked with the consultingcompany EAB, Berlin, and the Fraunhofer InstituteIITB Dresden. During this time, he also had a teach-ing assignment at the University of Dresden. Since1998, he has been a Professor and Head of the In-

stitute of Electrical Power Systems, University of Duisburg-Essen, Duisburg,Germany. His major scientific interests include power system stability and con-trol, modeling, and simulation of power system dynamics, including intelligentsystem applications.

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