��European Wind Energy Conference & Exhibition. February-March 2006, Athens. ANALYSIS OF CONTROL STRATEGIES OF A FULL CONVERTER IN A DIRECT DRIVE WIND TURBINE E. ROBLES, U. AGUIRRE , J. L. VILLATE, I. GABIOLA, S. API�ANIZ ROBOTIKER , Parque Tecnol�gico Edif. 202 48170 Zamudio (Bizkaia), Spain. [e-mail] T�l : + 34 94 600 22 66, Fax : + 34 94 600 22 99 ABSTRACT: As wind energy evolves, it is tending towards a direct drive connection using synchronous generators without a gearbox. Variable speed wind turbines with synchronous machines require the conversion of the full power. One alternative is using full converters with a DC link. The objective of this paper is to study different control strategies of a back-to-back DC-link full converter for grid connected direct drive wind turbines. Traditionally, the generator side converter controls the electromagnetic torque, and thus, the generated power, while the grid side converter regulates the DC link voltage as well as the input power factor. In this paper we reverse the control function of each converter, so that, the generator side converter will regulate the DC link voltage, and the grid side converter will control the electromagnetic torque. Both alternatives are analysed and compared by means of simulations based on Matlab/Simulink models. The behaviour of both strategies is examined under abrupt wind speed variations and grid disturbances. Differences in rotor speed tracking, power generated, DC voltage, and grid currents are also analysed. Keywords: Variable Speed control, wind energy, PM synchronous generator, full converter. 1 Introduction an uncontrolled 3 phase diode rectifier [4],[5]. This way the DC link varies in an uncontrolled manner [6] The existing wind turbine designs have several and a DC/DC converter is inserted between the technical differences, and these are reflected in their rectifier and the inverter. For this purpose, it is interaction with the power system. In fixed speed common the use of a boost converter [7], [5], systems, the rotor is coupled to the system through a [8],[9],[10]. Since the diode rectifier increases the gearbox and an induction generator [1]. They require current amplitude and distortion of the PMSG [11], reactive power from the grid, nearly always the most extended topology uses controlled compensated by capacitors. They have the advantage converters in both sides. Here, the use of field of being simpler and cheaper. But these systems run orientation control allows the generator to operate at constant rotor speed, and wind speed fluctuations, near its optimal working point in order to minimize translate into drive train torque fluctuations, which the losses in the generator and power electronic could cause the undesirable  flicker effect [2]. circuit[12]. Variable speed wind turbines can produce more In all the consulted bibliography, the generator side energy for a given wind speed [3], by controlling the converter controls the electromagnetic torque, and tip speed ratio for maximum efficiency. In this case, therefore the extracted power, while the grid side power converters are necessary to decouple converter controls both the DC link voltage and the mechanical rotor frequency and electrical grid power factor. Moreover, when designing the control frequency. strategy, it seems that the generator-side converter must control the extracted power as it is located Nowadays the two variable speed designs use the closer to the incoming power. Hence, the grid-side doubly fed induction generator (DFIG), and the converter would control the DC voltage. In this paper permanent magnet synchronous generator (PMSG). we try to analyse the extent to which this is true. The first option is for the moment the most widespread, however, PMSG allows low speed operation and gearless direct drive connection which 2 Control Strategies results a very attracting choice. 2.1 System Configuration This paper is focused in a PMSG based direct drive turbine, connected to the grid by means of a full A complete model of the wind turbine has been power converter. There are several ways of designing developed, from the blades to the grid. It comprises a the conversion system, depending on power permanent magnet synchronous generator, a rectifier electronics, but it is necessary the conversion of the (generator-side converter) and an inverter (grid-side full power, using an AC/DC converter connected to converter) connected through a DC link. The model the generator (generator side converter) and a DC/AC is integrated into a simulation platform based on converter connected to the grid (grid side converter), Matlab/Simulink where both control strategies have with a DC-link between them. Some references use been modelled: 1 European Wind Energy Conference & Exhibition. February-March 2006, Athens. 1. Traditional strategy. The grid side converter the wind speed, as shown in Fig. 2. From this, it maintains the DC voltage, and the generator follows that for each wind speed, there is an optimal side converter controls the rotor speed, and rotor speed for extracting the maximum power. thus the power, by means of the generator current. 2.3 Power Control 2. New strategy. The generator side converter In this work, we base the power control on the rotor maintains the DC voltage, while the grid side speed control. The speed control loop in Fig.3, converter controls the rotor speed, and thus assures that the wind turbine achieves the optimal the power, by means of the grid current. speed reference. This control consists of a PID controller that, depending on the rotor speed error, generates the reference of the generator current (1. strategy) or grid current (2. strategy) that is to be achieved in the generator side (1) or grid side (2) converter. C To grid PMSG Fig.1: Configuration of the electrical power generation system. Fig.3: Rotor speed control loop. The system configuration is the same in both control For each wind speed, electrical power and rotor speed strategies. The turbine supplies the mechanical torque are linked as shown in Fig. 2. The optimum speed is to the generator depending on the wind speed and the the one that makes the turbine work in the peak of the rotor speed. The generator is decoupled from the grid curve. When the turbine is in the ascending part of by means of a DC-link. the curve, we demand more speed to the control, the current reference falls and so does the output power Both strategies work with the same controllers, but (electrical) with regard to the input power they are exchanged between generator side and grid (mechanical). Hence, the turbine speeds up until side converter. We will therefore, explain the control input and output power are equal. When the turbine is and after, analyse how it is applied to each strategy. in the descending part, rotor speed reference falls, increasing current reference and consequently output power. In this case, the turbine decelerates. The 2.2 Aerodynamics difference between mechanical and electrical power causes oscillations in the rotor speed. The mechanical input power at the generator shaft can be obtained as: Synchronous generator model and internal switching 1 3 control of each converter is realised in dq coordinates Pm = �AV Cp(�) (1) [13], [8]. A rotating reference system fixed to the 2 rotor has been used [16]. According to this reference For a given wind turbine, the maximum power system, and the adopted sign criterion in the Park depends on the wind speed and the power coefficient, transformation, the output current reference of the which is function of the tip speed ratio � and the rotor speed control loop, will be the id reference. Iq aerodynamic design. reference will be set to 0 in this paper because we do not do reactive control. Fig.4: Generator side converter (1)/ Grid side converter (2) control in dq coordinates. Fig.2: Electrical power vs. rotor speed for several wind speeds. With this control, we obtain the rotor speed reference For a given Cp(�) [7], power curves can be plotted as from the measured wind speed. It is necessary to a function of the rotor speed for different values of filter this speed since its high frequency components 2 European Wind Energy Conference & Exhibition. February-March 2006, Athens. do not supply energy but are an undesirable noise they rapidly extract the maximum power for each source for the controller. The wind speed wind speed Fig.7(b). measurement is not usually reliable, thus the additional use of a maximum power point tracking algorithm [14] is convenient. In case of wind speed measurement error, this algorithm can take the rotor to the real optimal speed which extracts the maximum power. 2.4 DC-Link Control In the traditional control strategy, the grid side converter obtains the desired power factor, and maintains the DC voltage to a previously fixed value. The control is shown in Fig.5. This is the first strategy of this work. The second strategy applies the Fig.6: Simulated average wind speed. same control of Fig.5 to the generator side converter, which will maintain the DC voltage to the desired The only difference under this disturbance, is the value. maintenance of the DC-link voltage. In the traditional strategy (1), a little oscillation can hardly be seen. However, as shown in Fig. 8, in the new strategy (2), where the generator side converter maintains the DC- link voltage, abrupt wind speed variations provoke big oscillations in the DC voltage. Fig.5: DC_Link Control Loop. Grid side converter (1)/ Generator side converter (2) control in dq coordinates. The DC voltage error is the input of a PI controller, that obtains the id reference for the Grid (1) / Generator (2) currents. In this control, iq reference is also set to 0, but it can be used for controlling the (a) reactive power. 3 Simulation results The aim of this work is to compare both control strategies under external disturbances. For this purpose, two kind of simulations have been carried out: under wind speed variations, and under a voltage dip. We analyse the behaviour of the rotor speed, generated power, DC voltage, and grid currents. The disturbances are introduced from the steady state. Both models work in the same conditions for a plausible comparison. The parameters of the (b) synchronous generator are set for 5 MW and 6,3kV. The turbine is connected to a 10kV grid, through a Fig.7: (a) Rotor speed, (b) input/output power bus of 16kV, using a modulation index of 0,9. behaviour under wind speed variation. 3.1 Wind Speed Variations Simulation starts with an average wind speed of 10 m/s. Then, there is a step variation until 8 m/s. Once it is stable, wind changes abruptly until 12 m/s. Fig.6. None of the strategies has any problem in following the optimum rotor speed control Fig.7(a). Therefore, 3 European Wind Energy Conference & Exhibition. February-March 2006, Athens. (b) Fig.8: DC-link voltage under wind speed variations. 3.2 Voltage dip It is important to analyse the machine behaviour under voltage dips. New grid codes do not allow grid disconnection, thus, wind turbines must support some particular types of voltage dips, depending on the country. Nowadays, there are different techniques to reduce the impact of voltage dips, such as: Dynamic Voltage Restorer (DVR) and crowbar, but until their reaction, voltage dips affect the wind turbine. (c) Fig.9: Simulated voltage dip. To compare both strategies, a single phase voltage dip (with the same shape as defined on E.on grid code) is introduced at second 121. Fig.9. In both (d) cases rotor speed suffers a light variation but it stabilizes rapidly. Fig. 10 (a). Fig.10: (a) Rotor speed, (b) DC-link voltage, (c) Grid current in traditional strategy, (d) Grid current in new strategy. As shown in Fig.10 (b), under a voltage dip DC-link voltage suffers a great oscillation in case of the traditional strategy (1). The new strategy (2), where the generator converter maintains the DC-link, hardly notices the variation in the bus voltage. Grid currents have a similar behaviour. However, in the new strategy (2) they seem to have a slightly smaller peak and a faster recovery. (a) 4 Conclusions Different simulations have been carried out for both control strategies. Results show that in normal conditions both have a similar performance, and that the behaviour of each control strategy depends on the 4 European Wind Energy Conference & Exhibition. February-March 2006, Athens. kind of disturbance. We have proved that the [7] Rodr�guez Amenedo J.L, Burgos D�az J.C, converter closest to the disturbance has real problems Arnalte G�mez S,  Sistemas e�licos de in maintaining a constant DC voltage. When there is producci�n de energ�a el�ctrica, Ed. Rueda, a wind speed variation, the best strategy is to control 2003. the bus through the grid-side converter. Under [8] M. Malinowski, S. Bernet,  Simple Control voltage dips, it is better to make the control of the DC Scheme of 3level PWM Converter connecting voltage with the generator-side converter. wind turbine with grid, Nordic Wind Power Conference (Chalmers University of In conclusion, both strategies have advantages and Technology), 1-2 March, 2004. disadvantages. Under wind speed variations, there are other buffer components (blades, pitch control, [9] D.C. Aliprantis, S.A. Papathanassiou, M.P. generator) between the wind and the power converter. Papadopoulos, A.G.Kaladas,  Modeling and Whereas under grid voltage dips, there is nothing to control of a variable-speed wind turbine equipped smooth over the disturbance and it is transmitted with permanent magnet synchronous generator, directly to the converter. Therefore, it could be better Proc. Of ICEM/2000, Vol.3, pp.558-562. to control the bus with the generator-side converter. [10] A. Haniotis, S. Papathanassiou, A. Kladas, The present work is mainly based on simulation. M. Papadopoulus,  Control issues of a Permanent Future objective is the experimental validation of Magnet Generator variable-speed Wind Turbine, these results using a small-scale test bench with a 15 Journal on Wind Engineering, Vol. 26, no 6, pp. kW permanent magnet generator. 371-381, 2002. [11] Hao, S. Hunter, G. Ramsden, V. Patterson, D., 5 Acknoledgements  Control system design for a 20 kW wind turbine generator with a boost converter and battery bank This work has been developed with the support of the load, Power Electronics Specialists Conference, Education and Science Ministry of Spain under the 2001. PESC. 2001 IEEE 32nd Annual , Volume: programme  Torres Quevedo for young researchers. 4 , 2001,pp: 2203  2206. [12] Schiemenz, I.; Stiebler, M.,  Control of a 6 References permanent magnet synchronous generator used in a variable speed wind energy system, Electric [1] H. Sharma, T. Pryor, S. Islam,  Effect of pitch Machines and Drives Conference, 2000. IEMDC control and power conditioning on power quality 2001. IEEE International, 2001,pp 872  877. of variable speed wind turbine generators, AUPEC 2001, 23-26 September 2001, Perth, Australia, pp 95-100. [13] B. Kwon, J. Youm,  A Line-Voltage-Sensorless Synchronous Rectifier, IEEE Transactions on [2] H. Slootweg, E. de Vries,  Wind Turbines: Fixed Power Electronics, vol. 14, n�. 5, pp. 966-972, vs. Variable speed, Renewable Energy World, Sep. 1999. Feb. 2003. [14] R. J. Spiegel,  Assessment of a wind turbine [3] D.S. Zinger, E. Muljadi,  Annualized Wind intelligent controller for enhanced energy Energy Improvement Using Variable Speeds, production and pollution reduction, Wind IEEE Trans. on Industry Applications, Vol. 33, Engineering Vol.25, No.1, pp. 23-32, 2001. n�6, Nov/Dec 1997, pp. 1444-1447. [15] Newman MJ, Holmes DG, Nielsen JG, [4] T. Zouaghi,  Variable Speed Drive modelling of Blaabjerg F,  A dynamic voltage restorer (DVR) Wind Turbine Permanent Magnet Synchronous with selective harmonic compensation at medium Generator, ICREP 04 International Conference voltage level, IEEE Trans. on Industry on Renewable Energy and Power Quality, Applications, Vol. 41, n�6, Nov/Dec 2005, pp. Barcelona, Spain, 2004. 1744-1753. [5] J. Marques, H. Pinheiro, H. A. Gr�ndling, J. R. [16] CHEE-MUN ONG "Dynamic Simulation of Pinheiro and H. L. Hey,  A survey on variable Electric Machines Using MATLAB/Simulink" speed wind turbine system, Congresso Brasileiro Editorial "Prentice Hall", 1998. de Eletr�nica de Pot�ncia (COBEP), Fortaleza, CE. [6] Z. Chen, E. Spooner,  Grid interface options for variable speed, permanent-magnet generators, IEEE Proc.-Electro. Power Appl., Vol. 145, N� 4, July 1998. 5 ANALYSIS OF CONT OL ST ATEGIES OF A FULL ANALYSIS OF CONT OL ST ATEGIES OF A FULL ANALYSIS OF CONT OL ST ATEGIES OF A FULL ANALYSIS OF CONT OL ST ATEGIES OF A FULL CONVE TE IN A DI ECT D IVE WIND TU BINE CONVE TE IN A DI ECT D IVE WIND TU BINE CONVE TE IN A DI ECT D IVE WIND TU BINE CONVE TE IN A DI ECT D IVE WIND TU BINE E. obles, U. Aguirre, J.L.Villate, I.Gabiola, S. Api�aniz E. obles, U. Aguirre, J.L.Villate, I.Gabiola, S. Api�aniz E. obles, U. Aguirre, J.L.Villate, I.Gabiola, S. Api�aniz E. obles, U. Aguirre, J.L.Villate, I.Gabiola, S. Api�aniz OBOTIKE ENE GY BUSINESS UNIT OBOTIKE ENE GY BUSINESS UNIT OBOTIKE ENE GY BUSINESS UNIT OBOTIKE ENE GY BUSINESS UNIT [e-mail] [e-mail] [e-mail] [e-mail] ABST ACT: ABST ACT ABST ACT ABST ACT As wind energy evolves, it is tending towards a direct drive connection using synchronous generators without a gearbox. Variable speed wind turbines with synchronous machines require the conversion of the full power. One alternative is using full converters with a DC link. The objective of this paper is to study different control strategies of a back-to-back DC-link full converter for grid connected direct drive wind turbines. Traditionally, the generator side converter controls the electromagnetic torque, and thus, the generated power, while the grid side converter regulates the DC link voltage as well as the power factor. In this paper we reverse the control function of each converter, so that, generator side converter will regulate the DC link voltage, and the grid side converter will control the electromagnetic torque. Both alternatives are analysed and compared by means of simulations based on Matlab/Simulink models. The behaviour of both strategies is examined under abrupt wind speed variations and grid disturbances. Differences in rotor speed tracking, power generated, DC voltage, and grid currents are also analysed. TRADITIONAL STRATEGY NEW STRATEGY The grid side converter maintains the DC The generator side converter voltage, and the generator side converter maintains the DC voltage, while CONTROL controls the rotor speed, and thus the the grid side converter controls the power, by means of the generator rotor speed, and thus the power, STRATEGIES current. by means of the grid current. VOLTAGE WIND C To grid PMSG DIP VARIATION Direct Driven Wind Turbine with PM Synchronous Direct Driven Wind Turbine with PM Synchronous Direct Driven Wind Turbine with PM Synchronous Direct Driven Wind Turbine with PM Synchronous Generator and a Full Power Converter Generator and a Full Power Converter Generator and a Full Power Converter Generator and a Full Power Converter Similar behaviour in wind speed Similar behaviour in wind speed Similar behaviour in wind speed Similar behaviour in wind speed tracking tracking tracking tracking Differences in the maintenance of Differences in the maintenance of Differences in the maintenance of Differences in the maintenance of the DC-Link Voltage. the DC-Link Voltage. the DC-Link Voltage. the DC-Link Voltage. The converter closest to the The converter closest to the The converter closest to the The converter closest to the disturbance has real problems in disturbance has real problems in disturbance has real problems in disturbance has real problems in maintaining a constant DC voltage maintaining a constant DC voltage maintaining a constant DC voltage maintaining a constant DC voltage CONCLUSIONS: FUTURE WORK: " In normal conditions both have a similar performance. " Experimental validation of the results using a small- " The behaviour of each control strategy depends on the kind of scale test bench with a 15 disturbance. kW permanent magnet " The converter closest to the disturbance has real problems in generator. maintaining a constant DC voltage. When there is a wind speed " otor outer PM low speed variation, the best strategy is to control the bus through the grid- innovative generator. side converter. Under voltage dips, it is better to make the control of the DC voltage with the generator-side converter.