CONTROL DESIGN OF CONVERTERS FOR WIND ENERGY CONVERSION SYSTEMS APPLIED TO BATTERY CHARGING Herminio M. de Oliveira Filho, René P. T. Bascopé, Luiz H. S. C. Barreto, Fernado L. M. Antunes and Demercil S. Oliveira Jr. Federal University of Ceará – UFC, Group of Energy Processing and Control - GPEC PO Box 6001, P. Code 60455-760, Tel.: +55(85) 33669586 email@example.com, firstname.lastname@example.org Abstract – This paper presents a control design for a (400 W – 5 kW) associated with battery chargers are verysmall size wind generation system for battery charging useful .applications. A boost converter is used to regulate the The directed connection of a three-phase rectifier tobattery bank voltage, in order to implement a MPPT and batteries is a common practice adopted by somelimit the maximum current through the battery. The manufacturers . Although there is simplicity andsystem overview and modeling are presented including robustness, several problems associated with this solutioncharacteristics of wind turbine, generator, power result , such as the reduction of batteries useful life andconverter, control system, and supervisory system. increase of power losses. A dc/dc converter can be insertedSimulations of the system are performed using between the rectifier output and the battery to change theMATLAB/SIMULINK software for the case of battery apparent DC bus impedance seen by the generator  andbank voltage regulation. improved efficiency can be obtained with the implementation of maximum power point tracking algorithms (MPPT). Keywords – Battery chargers, boost converters, control Within this context, this paper presents a system whichmethods, wind generation, maximum power point employs a boost converter in cascade with the rectifier totracking. regulate the battery bank voltage, in order to implement a MPPT algorithm and reduce the machine rotation in front of I. INTRODUCTION high current levels through the battery. The system model, control method, and supervisory system strategy are Wind energy is a renewable energy source that does not investigated. Simulations results regarding the regulation ofpollute and is present everywhere. This kind of energy has the battery bank voltage are also presented.become popular in electric power generation since 1970’s.Within this period the world faced the Oil Crisis, whichcaused an emphatic investment in wind systems to try toreduce the employ of petroleum, which is used in powerstations. The installed wind capacity in the world has increasedmore than 30% per year over the last decade . The currentsurge in wind energy development is driven by multipleforces in favor of wind power including its tremendousenvironmental, social, and economic benefits, technologicalmaturity, the deregulation of electricity markets throughoutthe world, public support, and government incentives . A wind generation system can be used basically in three Fig. 1. Proposed topology.distinct applications: isolated systems, hybrid systems andgrid connected systems. Basic characteristics of the systems II. PROPOSED TOPOLOGYinclude power and energy system storage capability.Generally, small size isolated systems demand energy The proposed system (Figure 1) is composed of a windstorage by the use of batteries or in the form of gravitational turbine directly connected to a permanent magnetpotential energy in order to store the water pumped in synchronous generator (PMSG) rated at 40 V/1000 W inreservoir raised for posterior use. More specifically, for cascade with a battery charger which supplies four batterieselectric generation in isolated systems, small wind turbines rated at 12 V/50 Ah.
Fig. 2. Control Design. The transfer functions of controlled plants were obtained I L (s) Voapplying the ac model for small signals of the PWM switch GI L _ d ( s) = = (8). d (s) s ⋅ L The block diagram of the implemented control system is The Bode diagram for open-loop transfer function withoutshown in Figure 2. The internal controller CiL(s) regulates the compensation OLNi (s) and the compensator Ci(s) is shown inaverage inductor current IL. This is compared with a current Figure 3. The compensator was designed to obtain an open-reference provided by an intermediate controller, which closed transfer function with compensation OLCi(s) withregulates the input voltage or dc bus voltage Vi. The input crossover frequency of 5 kHz, slope of -20 dB/decade acrossvoltage reference is provided by the external controller that zero magnitude, and phase margin equal to 55º. Figure 4regulates the battery bank voltage. This signal is compared represents OLCi(s), which is given by the product betweenwith a voltage reference one provided by the supervisory OLNi(s) and Ci (s).system. 1) Average Inductor Current Control - The transferfunction of the plant in (8) is obtained considering only theduty cycle d(s) and inductor current perturbations IL(s). Theremaining magnitudes are considered time invariant. Theinductor resistance is neglected to simplify the analysis. Fig. 4. Bode diagram for OLCi(s). 2) Input Voltage Control - The transfer function of the plant in (9) is obtained considering only inductor current IL(s) and input voltage Vi (s) perturbations. The other magnitudes are considered time invariant. The input capacitance resistance RCi is considered in this plant because it affects the dc bus voltage ripple. Fig. 3. Bode diagram for OLNi(s) and Ci(s). Vi (s ) s + 1 RCi ⋅ Ci GVi _ I L ( s ) = =-RCi ⋅ (9) The plant for the aforementioned control system is formed I L ( s) sby (8) and PWM modulator, whose transfer function is Fm(s).
dB/decade across zero magnitude, and phase margin equal to 84º. The input voltage controller is slower than the average current one to minimize the inductor current ripple. Figure 6 shows OLCVe(s), which is obtained from the product between OLNVe(s) and CVe(s). 3) Output Voltage Control - The transfer function of the plant in (10) is obtained considering only input voltage Vi(s) and output voltage Vo(s) perturbations. The other magnitudes are considered time invariant. The battery bank model is formed by capacitance C2 in series with resistance Rb. It is very important because it causes the battery bank voltage ripple. Inductor, output capacitance and switch resistances are ignored to simplify the analysis. The battery bank capacitance model is obtained in (11). The closed-loop transfer function with compensation CLTFVe(s) obtained for (8) has 18 poles. To simplify the analysis of the control system, it was approximated for a first order transfer function observing the unit step response for CLTFVe(s). The obtained result is given in (12). Therefore, the plant for this control system is constituted by (10) and (12). Rb DL s + 1 RbC2 GVo _Ve ( s )= ⋅ 2 2 (10) LC2 Rb DL DL Fig. 5. Bode diagram for OLNve(s) and CVe(s). s +s + 3 2 s+ LC1 LC1C2 Where: DL - complementary duty cycle. 3600 ⋅ Cap C2 = (11) N ⋅Vb Cap - battery capacity; N - number of batteries. 200 CLTFVi ( s ) = (12) s + 200 Fig. 6. Bode diagram for OLCve(s). The plant analyzed for this control project is formed by(9) and closed-loop transfer function with compensationCLTFi(s) obtained for (8). The Bode diagram for open-loop transfer function withoutcompensation OLNVe(s) and compensator CVe(s) designed ispresented in Figure 5. The compensator was designed toprovide an open-closed transfer function with compensationOLCi(s) with crossover frequency of 400 Hz, slope of -20 Fig. 7. Bode diagram for OLNvo(s).
The Bode diagram for open-loop transfer function without V. SUPERVISORY SYSTEMcompensation OLNVo(s) and compensator CVo(s) is shown inFigure 7. The compensator was designed to obtain an open- The supervisory system flow chart is shown in Figure 9. Itclosed transfer function with compensation OLCVo(s) with gives the set point for the control system and is composed ofcrossover frequency of 0.1 Hz, slope of -20 dB/decade across three functions: battery current regulation, output voltagezero magnitude, and phase margin equal to 90º. The voltage regulation and MPPT algorithm.controller is the slowest one. Slow response is necessary to The first and second functions have priority because theyreduce inductor current and input voltage ripple. Moreover, it are necessary to avoid reduction on battery useful life due toreduces noise. Figure 8 shows the Bode plot for OLCvo(s). overvoltage and overcurrent, respectively. They regulate the converter through the operation of the wind turbine out of its maximum power point. The limit for battery current is 20% of battery capacity and the limit for output voltage is voltage recommended by the battery manufactor. The third function consists in regulating the converter to operate the wind generator at maximum power point using a MMPT algorithm. VI. SIMULATED SYSTEM The simulations have the objective to evaluate and validate the proposed system model. This paper will only be concerned with battery voltage regulation, as simulations are performed using MATLAB/SIMULINK. The simulated circuit is shown in Figure 10. The average current model for boost converter was used to simplify the analysis and reduce simulation time. The output voltage reference was chosen to regulate battery voltage bank around the floating point, defined as 53.7 V. Wind speed was varied from 6 to 11 m/s in 100 seconds. The simulated waveforms are given in Figure 11. It is observed that power coefficient practically assumes its maximum value, but mechanical speed is low. This fact occurred because wind turbine is out of maximum power point due to fixed reference signal of the controller. The wind turbine speed is decreased and large part of generated Fig. 8. Bode diagram for OLCvo(s) power is waste in the generator internal resistance. The battery voltage regulation can be seen in Figure 12. Voltage ripple assumes values between 53.7 and 54.4 V, corresponding to a variation of 0.7 V. Figure 12 also shows that input voltage ripple is very small, but inductor current one is greater due to the response of the input voltage controller, as one can see in their respective reference signals. VII. CONCLUSIONS This paper has presented an alternative for control design of boost converter for small size wind generator systems applied to battery charging. The system was designed and studied via simulated results. One has verified that control system performance is satisfactory for battery bank voltage regulation. It was possible to break the turbine without the use of external resistances or converters. Although simulations have demonstrated good results, it will be harder to implement the complete system due to high complexity of the plant. Future perspectives for additional studies include simulating the complete supervisory system and the Fig. 9. Flow chart for supervisory system. development of an experimental prototype.
Fig. 10. Schematic of the simulated circuit. Velocity (m/s) 10 ACKNOWLEDGEMENT Wind 5 The authors acknowledge the members of Group of 0 10 20 30 40 50 60 70 80 90 100 Energy Processing and Control (GPEC) for the friendship Speed (rad/sec) 100 Mechanical and overall support. 50 0 0 10 20 30 40 50 60 70 80 90 100 REFERENCES Coefficient 0.4  P. Gipe, “Soaring to new heights: the word energy market”, Power 0.2 Renewable Energy World, vol. 5, no. 4, pp. 33-47, 0 10 20 30 40 50 60 70 80 90 100 September/October 2003.  Q. Wang, L. Chang, “An Intelligent Maximum PowerPower (W)Generated 2000 Extraction Algorithm for Inverter-Based Variable Speed Wind 1000 Turbine Systems”, IEEE Transactions on Power Electronics, 0 0 10 20 30 40 50 60 70 80 90 100 vol. 19, no. 5, September 2004.  I. R. Machado, H. M. Oliveira F., L. H. S. C. Barreto, D. S. Power (W) 500 DC Bus Oliveira Jr., “Wind Generation System for Charging Batteries”, in Proc. of Brazilian Power Electronics 0 0 10 20 30 40 50 60 70 80 90 100 Conference, COBEP, pp. 371-376, October 2007. Time (sec)  www.enersud.com.br, access in 12/04/2008, 11:20 AM.  T. Tafticht, K. Agbossou, A. Chériti, M. L. Doumbia, “Output Fig. 11. Simulation results. Maximization of a Permanent Magnet Synchronous Generator Based Stand-Alone Wind Turbine, IEEE ISIE, pp. 2412-2416, 50 July 2006. Current (A)  S. Heier, Grid Integration of Wind Energy Conversion Inductor Systems, John Wiley Sons, 1st Edition, West Sussex, 0 England, 1998. 0 20 40 60 80 100 15  Z. Guo, L. Chang, “FEM Study of Permanent Magnet Voltage (V) Synchronous Generators for Small Wind Turbines” Canadian Input 10 Conference on Electrical and Computer Engineering, pp. 641- 5 0 20 40 60 80 100 644, May 2005. 54.5  R. W. Erickson, D. Maksimovic, Fundamentals of Power Voltage (V) Electronics, Kluwer Academic Publishers, 2nd Edition, New Battery 54 York, USA, 2004. 53.5 0 20 40 60 80 100  V. Vorpérian, “Simplified Analysis of PWM Converters Using 1.195 the Model of the PWM Switch Part I: Continuous ConductionReference Voltage Mode”, IEEE Transactions on Aerospace and Electronics Input 1.19 Systems, vol. 26, no. 3, pp. 490-496, May 2003. 1.185 0 20 40 60 80 100 10 BIOGRAPHIES Reference Inductor Current 5 Herminio Miguel de Oliveira Filho was born in Taguatinga, Distrito 0 Federal, Brazil, on 1983. He received the B.Sc. degree in Electrical 0 20 40 60 80 100 Time (sec) Engineering from the Federal University of Ceará, Brazil, in 2007. Currently, he is an engineer and M.Sc. student with the Group of Energy Fig. 12. Simulation results.
Processing and Control in the Federal University of Ceará. His interest areas the IEEE Power Electronics Society and Industrial Application Society andinclude control applications, switching power supplies, and renewable the Brazilian Power Electronics Society (SOBRAEP).energy applications. Fernando Luiz Marcelo Antunes, Ph.D. B.Sc. degree in ElectricalRené Pastor Torrico Bascopé, Dr. was born in Cochabamba, Bolivia, on Engineering from Federal University of Ceará, Brazil, in 1978, B.Sc. degree1967. He received the BSc from San Simón University of Cochabamba, in Business and Administration from Estate University of Ceará, Brazil,Bolivia, in 1992, and its MSc and Dr. degrees from the Federal University of MSc. degree from the University of São Paulo, Brazil, in 1980 and PhDSanta Catarina, Brazil, in 1994 and 2000, respectively. During 1995 and degree Loughborough University of Technology, United Kingdom (UK) in1996, he was Research Engineer in the Power Electronics Institute of 1991. He is a lecturer at Federal University of Ceará, Brazil, teaching PowerFederal University of Santa Catarina. From March 2001 to May 2003, he has Electronics and Electric Machines at graduate and undergraduate levels. Hebeen an Assistant Professor at the Univali – University of Itajaí, Brazil. coordinates the Energy Processing and Control Group of the UFC whereCurrently, he is Professor of the Federal University of Ceará. His main research projects are carried out with financial support of researchresearch interests include power supplies, power factor correction development agencies and public and private companies. He is member oftechniques, UPS systems, and renewable energy applications. the SOBRAEP, SBA and IEEE.Prof. Luiz Henrique Silva Colado Barreto, Dr. He received the B.Sc. Prof. Demercil de Souza Oliveira Júnior, Dr. has B.Sc. (1999) and M.Sc.degree in Electrical Engineering from Universidade Federal de Mato Grosso, degrees (2001) in Electrical Engineering from Federal University ofBrazil, in 1997 and the M.Sc. and Ph.D. degrees from the Universidade Uberlândia, Brazil, and Dr. degree (2004) from Federal University of SantaFederal de Uberlândia, Brazil, in 1999 and 2003 respectively. Since June Catarina, Brazil. His fields of interest include power electronics, soft-2003, he has been with the Electrical Engineering Department, Universidade switching, wind energy systems, DC/DC converters, etc. At the moment heFederal do Ceará, Brazil, where he is a Professor of Electrical Engineering. is Professor and Researcher in Federal University of Ceará, Brazil, and isHis research interest areas include high-frequency power conversion, reviewer from IEEE Transactions on Industrial Electronics and IEEEmodeling and control of converters, power factor correction circuits, new Transactions on Power Electronics.converters topologies, UPS system and fuel cell. Dr. Barreto is member of