Wind Turbines with Doubly-Fed Induction Generator

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Wind Turbines with Doubly-Fed Induction Generator Systems with Improved Performance due to Grid Requirements D. Ehlert and H. Wrede, Member, IEEE

Abstract—Within more than 20 years of intensive research and development the wind industry has recently superseded the 3 MW size of machines successfully that have been in operation in the early 80th already. Interestingly the industry has realized that the basic electrical concept of these early day machines was very reasonable. Based now on a very solid background of operational experience with wind turbine generators (WTG) of various sizes and the development of high performing power electronics these latest WTG offer now the required and improved performances that will be necessary for a safe integration into the power systems. While WTG have manly been connected to distribution networks in the past, nowadays project and WTG sizes are calling for a direct connection to the transmission system level via a project specific sub-station. Due to this development an integrated approach is required to organize an optimized sharing of control responsibilities between sub-station control and WTG control. The use of doubly-fed induction generator (DFIG) systems offers a variety of options to implement sophisticated solutions. Different control strategies of the DFIG system are optimized for active and reactive power control (torque and speed control respectively) of WTG especially in steady state conditions. The dynamic control of the magnitude as well as the phase angel of the back-EMF voltage of the DFIG yields to a superior system performance to conventional power generators with grid parallel synchronous machines. Additional hard- and software solutions improve the fault ride through (FRT) capability of DFIG systems in case of transient voltage deviations as a consequence of grid faults. But also asymmetric voltage conditions can be counteracted by an independent control of positive and negative system of the wind turbine grid currents. An adequate control solution for a DFIG has not only to consider the transmission network needs, but also to secure the safe operation of the mechanical system of the WTG. Peak loads, especially at the moment of voltage recovery after transient network faults, have to be carefully analysed and respected in the design phase of a WTG.

Index Terms—Wind Power, Wind Turbine Generator, Doubly-Fed Induction Generator, Converter, Control System, Fault Ride Trough, Modelling, D. Ehlert is with REpower Systems AG, 22335 Hamburg, Germany (e-mail: [email protected]). H. Wrede is with SEG GmbH & Co. KG, 47906 Kempen, Germany (e-mail: [email protected]).

I. INTRODUCTION The use of doubly-fed induction generators (DFIG) in multi-megawatt wind turbine generators (WTG) has a long history of more than 20 years already. As a result of the oilprice crisis of the 70th significant efforts were spent in Germany and the USA to develop multi-MW WTG for the integration into electrical transmission and distribution systems. One of the most prominent WTG was the 3 MW GROWIAN (see Fig. 1), developed by a consortium of three German utilities, which was installed in 1983 at Marne, Germany [1, 2]. Fig. 1:

3 MW DFIG WTG GROWIAN (1983)

The Wind Energy Project Office of the NASA Lewis

Research Centre started to develop a 3.5 MVA WTG with a DFIG concept in the early 80th. The so called MOD-5B (see Fig. 2) was installed at Kahuku Village, Oahu, and started operation in July 1987 [3].

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Both machines were using thyristor based cyclo-converters, representing state of the art of power electronics technology at that time [4]. Although both machines haven’t been commercially successful, the experiences won with their operation were building up the required technical know-how for future developments. Nowadays the use of IGBT Fig. 2: MOD-5B (1987) technology and the solid know-how in multi-MW WTG design as a result of an unique and constant growth in number and size of the international WTG market make it possible to develop commercially successful 5 MW WTG. The integration of large scale (multi-hundredMW) wind farms equipped with WTG of the 5 MW class need an improved performance to secure network stability. The REpower 5M is following exactly that idea and is up to now being tested in various locations to proof its performance. With no doubts the machine installed near the BEATRICE-ALPHA oil platform in the Scottish North-Sea at a water depth of 44 m (Fig. 3) is facing the most challenging conditions.

turbine with a rotor diameter of 126 m that can be installed with hub heights of up to 120 m and therefore representing the biggest WTG ever been built worldwide.

Fig. 4. Doubly-fed induction generator wind turbine generator system

The functional principle of the variable speed generator is based upon the DFIG in combination with a 4-quadrant ac-toac frequency converter equipped with IGBT technology. The system assures efficient power production thanks to variable rotor speed, which adjusts itself automatically in accordance with prevailing wind speeds. Speed variability is made possible by the directionally dependent transfer of slip power via the frequency converter. o In the sub-synchronous operating mode (partial load range) the stator of the DFIG feeds all generated electrical power to the grid, and additionally makes slip power available which is fed from the frequency converter to the rotor via the generator’s slip rings. o However, in the super-synchronous operating mode (nominal load range) total power consists of the components fed by the stator of the DFIG plus slip power, which is fed from the rotor to the grid via the frequency converter. At full load active power, which is fed to the grid via the converter, amounts to roughly 25% of total power. Advantages of the system include, amongst others, low losses, which assures a high overall efficiency, and an outstanding availability due to the compact design with a minimal number of components. The stator windings of the DFIG system as shown in Fig. 4 are gently switched directly to the low-voltage side of the grid. Technical data of the 5M machine are: • •

Fig. 3: REpower 5M (2006)

In this paper the REpower 5M is presented as an example of a DFIG system based WTG with improved performances available today. II. WIND TURBINE SYSTEM The REpower 5M is a variable speed WTG using pitch control and a DFIG system (Fig. 4). It is a three-bladed

Nominal power Speed range

Pel = 5300 kW n = 670 to 1170 RPM

A specific maximum power for the average value is associated with each rotational speed by a characteristic curve of the blades which must not be exceeded due to design reasons. III. CONVERTER SYSTEM AND DFIG CONTROL The adjustable speed generator system (Fig. 4) offers improved system efficiency because inverter rating is typically 25…30% of the total system power, while the speed

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range of the DFIG is +/- 30% around the synchronous speed. Cost of the inverter and filters are reduced compared to a fullsized converter system. The converter enables decoupled control of active and reactive power of the generator [5-12], moreover reactive power control can be implemented at lower cost because the DFIG system (4-quadrant converter and induction machine) basically operates similar to a synchronous generator. The converter has to provide only excitation energy.

dΨS + j ⋅ ωS ⋅ Ψ S dt d Ψ′R + j ⋅ (ω S − ω ) ⋅ Ψ ′ R v R = RR′ ⋅ i ′ R + dt Ψ S = LS ⋅ i S + Lm ⋅ i R

v S = RS ⋅ i S +

(1)

Ψ R = LR ⋅ i R + Lm ⋅ i S

{

3 * T = − ⋅ p ⋅ Im Ψ S ⋅ i S 2

}

In Fig. 6 a vector controller block diagram for the DFIG is shown. Regard that the synchronous reference frame is linked to the stator voltage space vector vS and not to the stator or rotor flux vector as is common in field oriented controllers. Fig. 6. Vector controller block diagram for the DFIG

Fig. 5. Single line diagram of the 5M converter system

A. System Overview Fig. 5 shows the single line diagram of the 5M converter system. Each of the four back-to-back voltage source converters consist of two converters based on water-cooled 1700V-IGBT modules, a low-inductive DC-bus, a dv/dt-filter on the machine side and a grid choke on the line side. They are paralleled with their DC-links as well as on the machine and line side in the way, that in case of a converter failure two converters can be switched out of the system. With this redundancy the wind turbine can be maintained in operation with half of the system power in such failure mode, which improves the system reliability and gives a significant benefit especially for offshore wind turbines. B. Control of the DFIG To guarantee stable operation and to enable independent control of active and reactive power of the DFIG, a model based feed-forward controller is implemented using the dynamic model equations of the DFIG.

All measured quantities, i.e. stator and rotor current iS and iR are transformed into the synchronous reference frame. A decoupling circuit calculates from the demanded active and reactive power reference signals the rotor voltage command vRd and vRq. A reverse vector rotation computes magnitude and phase of the rotor voltage command in a stationary reference frame. The measured rotor current signals are used for rotor current regulation to

minimize the effects of parameter detuning and inverter gain errors. Note that system performance depends on speed due to the coupling between d and q variables. The dynamic control of active and reactive power by a frequency converter with sampling and switching frequencies above 2kHz is equivalent to a high dynamic control of the magnitude as well as the phase angel of the back-EMF voltage of the DFIG and yields to a superior system performance to


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component in the transmission system are shown. The appearance of positive and negative sequence components in grid voltages (uN1, uN2) and line currents (iN1, iN2) of the wind turbine leads to an oscillation of the instantaneous system power with twice the grid frequency of 50 Hz and result therefore in an oscillation of the electrical torque of the DFIG. Implementing an active compensation of the negative sequence current component in a negative sequence current controller yields to symmetrical line currents of the WTG even under asymmetrical voltage conditions and also leads to a reduction of the electrical torque oscillation of the DFIG. The line current space vector curve of one WTG of the simulated wind farm including an active negative sequence current compensation is shown in Fig 9. c), note that the negative sequence current component iN2 is fully compensated. This leads in case of the 10% negative sequence voltage component in the transmission system to a strong reduction of the electrical torque oscillation of the DFIG system which can be seen in d).

conventional power generators with grid parallel synchronous machines. IV. OPTIONAL SOLUTIONS FOR IMPROVED SYSTEM PERFORMANCE

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A negative sequence component in the grid voltage due to asymmetric faults or system conditions yields typically to a negative sequence current component of any electrical device especially passive components but also electric machines as synchronous generators. As can be seen in Fig. 8 a) the voltages on the high voltage side of a simulated 20 x 2MW wind farm contain a 25% negative sequence component due to a 2-phase fault in the transmission system. By calculating of instantaneous active and reactive power based on the instantaneous power theory [14] and getting the actual system power by filtering of this signals (Qist), the reactive power support of one WTG with state of the art power controllers like mentioned above is shown in Fig. 8 b) and c) respectively. It can be seen that the reactive power support in the positive sequence system Q1 lags behind the demanded reference value Qsoll. Fig. 8 d) shows the reactive power if a decoupled control of the positive and negative current components of the DFIG system is used [15] an the reactive power in the positive sequence system Q1 is controlled due to the reference value Qsoll.

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Fig. 9. Simulation results (pu.) in case of a 10% negative sequence voltage component in the transmission system, line current space vector curve a) and electrical torque of the DFIG b) with conventional power control and with active compensation of the negative sequence current component c) resp. d) 0

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Fig. 8. Simulation results (pu.) in case of a 2phase fault in the transmission system, voltages on the high voltage side of the wind farm a), reactive power response of a conventional power controller calculated due to instantaneous power theory b) and in positive and negative sequence systems c) as well as with decoupled control d)

Furthermore mechanical stress on the wind turbine can be reduced with an improved decoupled control in the positive and negative system. In Fig. 9 the line current space vector curve iNab a) of a WTG with a conventional power controller in the simulated wind farm and the electrical torque of the DFIG b) as a result of a 10% negative sequence voltage

The capability of dynamic decoupled power respectively current control in the positive and negative sequence systems yields additionally to a superior system performance to conventional power generators with grid parallel synchronous machines and can further be used to improve system operation. V. WTG MODELLING AND DESIGN To be able to design a modern 5 MW WTG that can be commercially successful, a depth knowledge of the transient electro-mechanical behaviour of such machines is essential. This basic prerequisite might have been one of the major


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S ta to rflu s s [V s ]

R o to rflu s s [V s ]

problems, wherefore the machines designed in the early 80th could not meet expectations. The powerful performance of nowadays computers and the increasing amount of algorithms and computing tools in the wind industry over the last two decades allow a detailed testing of WTG design prior to the prototype installation. The most important step forward for the design of the REpower 5M was a detailed validation with such algorithms and tools. Especially the advanced requirements with respect to power system integration to cope with high transient grid faults such as deep voltage dips create a potential risk of mechanical overloading that needs to be carefully analysed. In addition to the traditional fatigue and extreme loading scenarios defined as per IEC 61400-1, transient network events can lead to significant peak loads that need a more detailed consideration [16]. The high transient character of these events have to be respected by the right degree of detailed modelling. While network stability analysis software often reduces the degree of detail to a 3rd order model to limit the simulation efforts with respect to the huge size of the networks and the relevant elements, the mechanical design impact needs to be analysed with a higher order. The mechanical time constants seem to be in contradiction to this necessity, but a mechanical peak load has to be seen as a result of a high transient electrical impact. A typical example is the crowbar activation that is electrically needed to protect the DC-link circuit from overvoltages. Fig. 10. illustrates the different degree of detail for the flux calculations in stator and rotor and the resulting torque based on 3rd and 5th order simulation models. It can easily been seen that the lower order model is neglecting some relevant peaks. 2 0 -2 2 0 -2

M [k N m ]

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Fig. 10. Comparison of accuracy between 3rd (blue) and 5th (red) order simulation models

In [17] an enhanced reduced order model (ROM/E) is proposed for power system stability studies which fills the simulation gap between a 5th resp. full order (FOM) and a 3rd resp. reduced order model (ROM) with minimised additional calculation effort compared to a 3rd order simulation. With the proposed approach the alternating components of the rotor currents are considered as can be seen in Fig. 11, so that the converter DC-link can be modeled realistically. Consequently, it is possible to consider the correct response of converter control and crowbar switching to voltage sags caused by grid faults. When simulating power system dynamic behavior with increased integration of WTG, it is essential to consider the true control sequence and crowbar switching of DFIG-based wind turbines which can with this model be implemented in reduced order power system simulations. Fig. 11. Rotor currents following a three-phase voltage sag from rated voltage down to 15 %

For a secure WTG design a sophisticated 5th order model of the whole electro-mechanical system is required. SEG and REpower put a lot of efforts in developing such sophisticated models. Building up the model is only half the way of course. It will not be able to build up the required amount of confidence to the simulation results without a careful validation. Starting with a half-load laboratory test of a 1.5 MW system at a SEG test stand, REpower carried out full scale field tests on a 2 MW WTG analysing various fault scenarios. The field test confirmed the capability of the REpower WTG equipped with SEG inverters to stay connected during voltage dips resulting from symmetrical and asymmetrical system faults at rated and partial power. By using the electrical and mechanical measurement results of both tests, REpower and SEG were able to improve the accuracy of the electrical as well as the mechanical model to an extraordinary level. The following Fig. 12 a) shows the

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simulation result of a 3-phase fault resulting in a voltage dip down to 22% nominal voltage. Fig. 12 b) below shows the comparable measurement result of the same fault scenario.

another proof of the preciseness of the developed models. Fig. 13. Simulated (top) and measured (below) generator torque during forced decoupling from the grid using a crowbar activation

With the described modelling of DFIG-based WTG a project specific analysis of the fault scenarios and the required turbine behaviour in terms of its active and reactive power supply during the fault and after fault clearance as required to meet relevant grid codes can be carried out.

VI. CONCLUSION

Fig. 12. Simulated a) and measured b) active (green, blue = ref. value) and reactive power (turq., red = ref. value) flow during a 22% 3-phase fault

With comparisons like the above it could be shown that the developed model is precisely simulating the high transient effects inside the electrical system of the WTG in terms of the principle behaviour as well as the quantitative results. Only saturation effects within the transformers led to deviations. Having a validated 5th order model of the WTG and inverter system in place, it is possible now to analyse the electrical behaviour of the WTG with respect to the impact of the power system stability as well as the consequences for the mechanical system of the WTG. A comparison of simulated and measured values could also be carried out on the mechanical loading scenarios. One of the essential effects that could happen during a fault is the activation of the crowbar. Such a comparison is illustrated in Fig. 13. The comparison is 15

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The use of DFIG systems in WTG has tremendously matured over the last 20 years. At least with the appearance of the 5 MW class that is planned to be installed in huge multihundred-megawatt off-shore wind farms simulation, analysis and solutions for improved performance in steady-state operation and during transient fault conditions of the transmission network they are connected to are required. As a prominent example of this next generation of WTG the REpower 5M is exactly designed to that purpose. The dynamic control of active and reactive power by a frequency converter is equivalent to a high dynamic control of the magnitude as well as the phase angel of the back-EMF voltage of the DFIG and yields to a superior system performance to conventional power generators with grid parallel synchronous machines. The decoupled control of the WTG currents in the positive and negative sequence systems gives further improvement of the superior system performance. As a result of such an improved electrical performance the mechanical system of the WTG is facing loads that were not considered in the classical turbine design. To secure the WTG capability to face the highest possible loads during the worst case scenario out of various grid codes and project specific circumstances a reliable model of the whole electromechanical system is required. By having a validated 5th order model of the WTG and inverter system in place, it is possible to analyse the electrical behaviour of the WTG with respect on the impact of the power system stability as well as the consequences for the mechanical system of the WTG. This allows a project specific analysis of the fault scenarios and the required turbine behaviour in terms of its active and reactive power supply during the fault and after fault clearance as per the relevant grid codes.

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VII. REFERENCES [1]

[2]

[3] [4]

[5] [6]

[7]

[8]

[9] [10] [11]

[12]

[13]

[14]

[15]

[16] [17]

Warneke, O.;”Use of a double-fed induction machine in the large GROWIAN wind energy converter.”; Siemens Energietech (Germany, Federal Republic of) Vol. 5:6. Coden: SIEED; pp. 364-367. 1983 Braun, D., Kloeppel, V., Marsch, G., Meggle, R., Mehlhose, R. Schoebe, B. Wennekers, R.; Wind energy converter GROWIAN II; Original Title: Windenergieanlage GROWIAN II; Corporate Source: Bundesministerium für Forschung und Technologie, Bonn (Germany, F.R.); Publication Date: April 1984, p. 148. NASA; “MOD-5B Wind Turbine System Final Report, Volume II, Detailed Report.” NASA CR-180897, March 1988. Carlin, P.W. ; Laxson , A.S. ; Muljadi, E.B. “The History and State of the Art of Variable-Speed Wind Turbine Technology”; National Renewable Energy Laboratory; February 2001 T. Burton, D. Sharpe, N. Jenkins, E. Bossanvi, Wind Energy Handbook, John Wiley & Sons, Ltd, 2001 Müller, S.; Deicke, M.; De Doncker, R. W.:Adjustable speed generators for wind turbines based on doubly-fed induction machines and 4-quadrant IGBT converters linked to the rotor. Records of the IEEE IAS Conferernce, Rome, CD, 2000 Datta R., Ranganathan V. T.: Decoupled control of active and reactive power for a grid-connected doublyfed wound rotor induction machine without position sensors, In Conference Record of the 1999 IEEE Industry Applications Conference. Thirty-Fourth IAS Annual Meeting (Cat. No.99CH36370), pp. 2623-2628 Geniusz A., Krzeminski Z.: Control system based on the modified multiscalar model for the Double Fed Machine, Records of the PCIM Conference, Nürnberg, 2005 Krzeminski Z.: Sensorless Multiscalar Control of Double Fed Machine for Wind Power Generators, Osaka 2002 Leonhard W.: Control of Electrical Drives. Springer-Verlag, 2nd Edition, 1996 Peresada, S., Tilli A., Tonielli A.: Power control of a doubly fed induction machine via output feedback, Control Engineering Practice, 12, pp. 41-57, 2004 Petersson A.: Analysis, Modelling and Control of Doubly-Fed Induction Generators for Wind Turbines, Thesis for the degree of licentiate of engineering, Department of Electric Power Engineering, Chalmers University of Technology Goteborg, Sweden 2003 A. Geniusz, S. Engelhardt: . Riding through Grid Faults with Modified Multiscalar Control of Doubly Fed Asynchronous Generators for Wind Power Systems Records of the PCIM Conference, Nürnberg, 2006 H. Akagi, Y. Kanazawa, A. Nabae: Generalized Theory of the Instantaneous Reactive Power in Three-Phase Circuits. International Conference on Power Electronics, Tokyo, 1983, p 1375-1386 S. Engelhardt, H. Wrede, J. Kretschmann: Power Control of Wind Power Stations with Doubly Fed Asynchronous Machine under Asymmetrical Grid Conditions (in german). Records of the VDI/VDA-Conference, 27./28. Sept. 2006, Böblingen, Germany, 2006 IEC 61400-1, ed. 2: Wind Turbine Generator Systems - Part 1: Safety Requirements, 1999 Kretschmann, H.Wrede, S. Mueller-Engelhardt, I. Erlich „Enhanced Reduced Order Model of Wind Turbines with DFIG for Power System Stability Studies”, PECon Kuala Lumpur, 28-29 November 2006

VIII. BIOGRAPHIES Dirk Ehlert was born in Berlin, Germany, on March 4 in 1963. He received his Dipl.-Ing. degree in electrical engineering from the Technical University Berlin, Germany, in 1991. In 1993 he has installed his first WTG in a project that he has developed by himself. Since 1994 he is working in the wind industry and was with several WTG manufactures during this time. He was involved in various international wind farm projects and was therefore in close touch with network operators all over the globe to discuss the grid code compliance issues. Since 2002 he is with REpower Systems AG, Hamburg/Germany, presently head of the Department Sales Support and therefore responsible for grid connection related customer support and the product portfolio development process with respect to grid codes. Holger Wrede (M´2003) was born in Freiburg, Germany, on November 11 in 1971. He received his Dipl.-Ing. degree in electrical engineering from the Technical University Braunschweig, Germany, in 1998. From 1998 to 2004 he joined the Institute for Electrical Power Engineering and Power Electronics of the RuhrUniversity Bochum, Germany, where he worked on FACTS devices and power quality in transmission and distribution systems as well as on power theory and compensation strategies for STATCOMs and received his doctor´s degree in electrical engineering and power electronics in 2004. Since 2004 he is with SEG GmbH & Co. KG, Kempen/Germany, presently manager of the group Innovation / Converter Technology and responsible for power electronics and converter designs, system simulations and control strategies as well as patents.
