Control of Grid Connected Three-Phase Inverter for Hybrid Renewable Systems using Sliding Mode Controller

Transcription

1 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, ol. 9, No., 08 Control of Grd Connected Three-Phase Inverter for Hybrd Renewable Systems usng Sng Mode Controller Sam Youns Techncal College at Dammam KSA Techncal and ocatonal Tranng Corporaton Nejb Hamroun aboratory of Analyss and Treatment of Energetc and Electrc Systems (ATEES) Scence Faculty of Tuns-Unversty of Tuns El Manar Abstract Ths paper presents a power control approach of a grd connected 3-phase nverter for hybrd renewable energy systems that conssts of wnd generator, flywheel energy storage system and desel generator. A sng controller s developed around the grd connected nverter to control the njected currents whch leads to control the actve and reactve powers requested by grd and/or solated loads. In seres wth the controller, a Space ector Pulse wdth Modulaton method s used to drve the sx nverter swtches to generate 3-phase voltages and currents for transferrng the desred powers requested by the alternatve sde. Smulatons under Matlab- Smulnk software of the hybrd renewable energy systems are made to show performances gven by the developed sng mode controller. Keywos Grd connected systems; sng controller; hybrd renewable systems; SPWM I. INTRODUCTION The nternatonal renewable energy program s a strategc ntatve for the wor that ams to substantally ncrease the share of renewable energy n the total energy mx. In 07, worwde, renewable power capacty had a 3% share of total power capacty, edgng out coal []. In terms of total power output, renewable energy s predcted to make 8% of global electrcty output by 0[]. Ths strategy s followed after the reducton of fossl fuel sources such as ol and gas. There are many renewable sources of energy such us wnd energy, solar energy, and bomass. They appear clearly to complete the used energy. Ther producton fluctuates and t s not predctable [,3,4]. These problems affect drectly the qualty of the actve and reactve power transferred to the grd whch are very fluctuatng and whch pose unbalances consumptonproducton. Ths lmts ther rate of penetraton nto the grd [5,6]. These problems of the grd connecton and the qualty of the energy transmtted can be solved by the good control of the actve and reactve powers transferred from the renewable sources. Ths control s possble by the acton on the nverter whch s the prncple element of connecton between renewable sources and the grd. There are many researches who worked on the control of actve and reactve powers generated by renewable wnd, solar and hybrd energy systems and they have used varous methods [7,8]. In ths paper we present the actve and reactve powers control for a 3-phase grd connected nverter of hybrd renewable energy system (HRES) that consst of wnd system, flywheel energy storage system and desel generator. A sng mode controller s developed from the mathematc lne model to control the njected currents whch leads to control the actve and reactve powers requested by grd and/or solated loads. Ths method has been used n many works wth wnd and solar renewable energy systems generally to extract the maxmum power (MPPT)[9], to control the grd connected photovoltac system [0] and to control the electrcal machnes n wnd energy systems[,3,]. In seres wth the sng controller a Space ector Pulse Wdth Modulaton (SPWM) method s used to drve the sx nverter swtches whch generate consequently the necessary 3-phases voltages and currents for transferrng the powers requested by the grd. Ths method s developed n ths paper because t s easy to mplement on the calculator and t can elmnate the low oer harmoncs generated by the nverter. The experment results of the SPWM show the hgh relablty of ths method. Smulatons under Matlab-Smulnk software are made to show the results gven by the developed sng mode controller. These results show the advantages of ths controller that offers a compromse between good speed of system response and relablty, especally snce the system s very fluctuatng n very short tme dependng on the wnd speed. Ths paper s organzed as follows. Secton II descrbes the HRES confguraton. Secton III presents the control approach wthn the sng controller. Secton I presents the smulaton results and dscussons. II. SYSTEM COMPONENTS AND DESCRIPTION The smplest structure of the hybrd renewable energy system that studed s shown on fgure. It conssts of power electronc converters whch ncrease the system effcency and smplfy the control methods. Ths confguraton based on permanent magnetc synchronous machne () offers a good effcency for low power [,3,4]. The converters are used to connect hybrd system wth the DC-lnk. It s a bdrectonal converter, to allow the energy transfer between the DC-lnk and the flywheel n the two drectons. The DC-lnk and the AC grd are connected by capactor and 3 phase P a g e

2 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, ol. 9, No., 08 nverter. An R flter can be also added to smooth the current. Salent parameters of the system are presented n table. e en Fg.. The Grd Connected Hybrd System Confguraton. III. CONTRO STRATEGY In our prevous works [,3] we have developed local methods to control subsystems of the overall HRES and a global supervsor able to generate the erence powers requested by the grd. The am of ths paper s to control, by sng controller, the 3-phase nverter whch connects the DC-lnk bus to the grd. The control of actve and reactve powers transferred to the grd s carred out by the control of grd njected currents. The control vector U T od I D _ I dc c DC-nk that represents the output erence voltages of the nverter n Park-dq axs s calculated by sng mode method for current control. The erence values of currents are calculated by usng the actve and reactve powers erence values P req_ and Q req_ that requested by grd and/or solated loads. These erence powers are calculated by usng the powers generated by hybrd renewable energy system and by consderng the power n the capactor for regulaton of DC lnk voltage U c and losses n the electronc converters. The PWM sgnals are determned by usng SM method to drve the sx nverter IGBT`s of nverter. They generate the real nverter output voltages from ther erence values calculated by the sng mode controller and transfer the necessary actve and reactve powers to the alternatve sde. Fgure shows the control strategy of a three-phase-nverter for HRES. A. DC- nk oltage Control The man am of the voltage control loop s to regulate the DC-lnk voltage at a specfed value and to provde the erence current. The DC-lnk voltage s set to 55 whle the grd lne voltage ( rms ) s set to 0. The closed loop control of the DC-bus voltage s needed because the output power of the renewable sources s varable wth clmatc condtons. The nverter nput erence current e- s descrbed by the followng relaton: e DC/AC U c AC Grd R 3 Isolated loads S e dc PI ( Uc Uc ) () For the control of the DC-lnk voltage, a proportonal ntegral corrector (PI) has been used. It s parameterzed accong to the capactor and the dynamc of the regulaton loop. By neglectng losses n the nverter swtcher, the power balancng yes: P. U req e c The erence actve power njected nto the grd s gven by: P. U req _ e c B. Sng Mode Control of 3-Phase Inverter The sng controller requres the knowng of the sng surface, the condton of convergence and the control law. The general form of the sng surface s gven n[]: r S(x) ( x ) e(x) t (4) Wth: e(x) x x, s the error of the varable to be regulated x s a postve constant whch nterprets the band-wdth of the desred control. r: s a number of tmes that t s necessary to derve the output varables to reveal the command. S(X)=0: A lnear dfferental equaton whose sngle soluton s e(x) 0. DC-lnk 3-ph Inventer l U c C O r PWM, PWM3,4 PWM5,6 U c U c- Control crcut and optocopler Spacevector PWM computng Powers and DC lnk Controller od_ P req_ Fg.. Control Strategy Dagram. The general form of the sng controller s U=U eq +U nl. U eq and U nl are respectvely the equvalent controller that s the soluton of the equaton S ( X ) 0 and the nonlnear part R v PARK 3/ v 3ph-Grd Sng mode controller Q req_ () (3) P a g e

3 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, ol. 9, No., 08 that can be nterpreted as the overage value of the controller whch permts to mantan the state of the system on the sng surface [3,4]. The nonlnear command (U nl ) s gven to guarantee the attractvely of the varable to be controlled towas the sng surface and to satsfy the condton of convergence. The smplest functon s n the form of relay. It s gven by U nl =K.sgn (S(X)) wth K s a postve constant [3,4]. C. Applcaton to the nverter The control vector s carred out by the model of the Rlne flter. Ths model s gven, n Park dq- axs, by: Od Oq R R d d s s Wth od and oq are the nverter output voltages n dqaxs ; and are the grd voltages n dq-axs ; R and are respectvely the resstance and the nductance of the flter; s s the frequency of njected currents. The control vector T U od _ s gven by the drect nversng of Eq. 5. It s expressed as follows: od _ s s The output s the currents vector T Il. S and S consttute respectvely the sng surfaces of the ext varables and. They represent the errors of the drect and quadratc currents. They are expressed as follows: S S I I By usng the equatons of system (6), the frst dervaton of Eq. 7 gves: S I _ R s { R s od S I _ oq The relatve degree of the system s equal to because the command appears n the frst dervaton of the varables to be controlled. Equaton 8 can be replaced by: S B A { S B A Wth : (5) (6) (7) (8) (9) B I _ A B I _ A R R s s (0) The soluton of equaton S 0 leads to the equvalent controller (U eq ). Its expresson s gven by: _ eq _ eq B A B A The nonlnear part of the sng controller s gven by: _ nl _ nl Wth: K sgns d K sgns q K d = K q = K= ǀ max ǀ. The global command U T od _ () () s the sum of the equvalent and the nonlnear parts gven by equatons and. It s expressed as follows: od _ I I R R w w s s K sgns K sgns q d (3) The erence currents, gven n 3, are expressed as functons of the actve and reactve erence powers requested by the grd and / or solated loads. They are gven by: I I P P req _ req _.. Q Q req _ req _.. (4) D. Three-Phase Inverter SPWM The SPWM (Fg.3) s a technque used to drve the nverter swtches to generate equlbrate three-phase voltages wth desred ampltude and frequency; oa, ob and oc, from the erence voltages oa_, ob_ and oc_. These voltages are calculated by the Park transformaton (dq-abc) of the nverter voltages od_, provded by the sng controller bloc [5,6]. As ndcated by fgure 4, the algorthm of SPWM s based on the knowledge of the erence vector whch rotates n sx sectors n the swtchng hexagon. Its coonates are the components α and β, and the change values for each angle θ s whch allowng the calculaton of the swtchng tmes P a g e

4 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, ol. 9, No., 08 Fg. 3. SPWM Sgnals Generaton. Te s the perod corresponds to the count number of the (TCNT) of DSP (TMS30F40). It s used to compare the correspondng values to the nverter swtchng tmes. F h s the clock frequency of the DSP fxed at 0 MHz. n s the clock frequency of the DSP dvder. The perod of the nverter output varables s gven by: (7) The frequency of the nverter three-phase output voltages s 50Hz (T=0.0s). To set ths frequency value the followng parameters are selected: k = 360. T e = 38. n = 4. Fgure5 shows the IGBT drvng sgnals PWM for ={, 6}. Fg. 4. Reference ector araton n αβ-axs. Ampltude araton of Output oltages The components and are expressed as follows: I. SIMUATION STUDY Smulatons by Matlab-Smulnk software were performed to test and verfy the 3-phase nverter control presented and developed n the prevous secton and to study the output nverter voltages, the njected currents, and the actve and reactve powers transferred from the hybrd renewable energy system to the grd. PMW { (5) The maxmum radus of the crcle wthn the swtchng polygon s The 3-phase nverter controlled by SPWM can generate three phase- voltages wth maxmum value equal to. Theore, by selectng the varable ρ, we can set the ampltude of the nverter output sgnals. Frequency araton of Output oltages The dscretzaton of the erence vector requres a good choce of the number of samples per revoluton, and theore the samplng perod on whch to bu t. The maxmum number of samples per perod s lmted by the speed of calculaton of the DSP TMS30F40.In general, for a number faster than 7 (5 for a sample) the erence vector s contnuously varable over tme. To fx the perod of the erence vector and the perod of the nverter output voltages, t s necessary to fx the modulaton perod gven by [7]: T m Wth n. Te. F h. (6) PMW PMW3 PMW4 PMW5 PMW6 Fg. 5. Drvng Sgnals. These smulatons have been obtaned under varous values of power requested by grd and/or loads. They are selected to demonstrate the most sgnfcant performances of the control approach. The erence and measure are respectvely represented by dotted and contnuous lne. The nteracton between the renewable energy sources s not P a g e

5 Powers (W) oltage () Current (A) Wnd speed (m/s) Powers (AR) (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, ol. 9, No., 08 dscussed n ths paper. Only the nverter control s consdered n ths paper. value 0.0H 0.9μF 0.05Ω 34/s 3 55 TABE I. symbols C R w s K d=k q U c PARAMETERS OF THE GRID CONNECTED SYSTEM Parameters Inductance of the flter Capactance of the DC-lnk Resstance of the flter Pulsaton of the grd voltage Constant of the sng control DC-lnk voltage erence A varable wnd sgnal, shown on fgure 6, s appled to the turbnes. It vared between 7 and m/s regang the maxmal power pont trackng zone tme (s) Fg. 6. Wnd Speed araton. As ndcated by fgure 7, the erence actve power requested by the grd and/or loads (P req_ ) s consdered varable wthn the tme whle the erence reactve power shou be zero n oer to obtan the lne current n phase wth the grd voltage. On fgure 7a, t s shown that the actve power generated by the hybrd system satsfes the requested power however the reactve power transferred to the grd oscllates around ts erence fxed at zero (Fg. 7b). Accong to those fgures, the nverter output reactve power remans equal to zero whle the actve power vared wth the demand of the load n the alternatve sde. 0.3 (b) tme (s) Fg. 7. Inverter output powers (measure and erence): : actve powers, (b): reactve powers. Fgure 8a presents the DC-current provded by the renewable energy system to satsfy the power requested by the load. The current vared proportonally wth the power values njected nto the alternatve sde whle the DC-lnk voltage s constant and follows ts erence (U c_ ) fxed at 55. The DC-lnk voltage presents some nstantaneously overshoots n each change of requested power load (Fg. 8b) Qreq- Qgen tme (s) (b) Uc- Uc 00 Preq- Pgen tme (s) tme(s) Fg. 8. Input characterstcs of the nverter; : DC-current provded by the renewable energy system, (b): DC-lnk voltage (measure and erence) P a g e

6 Current (A) oltage () oltage () (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, ol. 9, No., od- od tme (s) 00 oq- oq tme (s) Fg. 9. Inverter output voltage (measures and erences); : drect components; (b): quadratc components. Fgure 9 llustrates the dq-components of the nverter output voltage under varable load powers. As t can be seen by ths fgure, the sng mode controller presents good performances n terms of stablty and response tme. The nverter output voltages reach nstantaneously ther erence values provded by the controller wthn the changes of the requested load power. The drect and the quadratc components of the nverter output current q are represented respectvely by fgures 0a and 0b. As t can be seen by those fgures, the nverter output currents reach nstantaneously ther erence values provded by the sng controller (b) I- I tme (s) Fg. 0. Inverter output current (measures and erences); : drect components; (b): quadratc components. Fnally, the control unt composed by the sng controller, the DC-lnk voltage regulator and the SPWM block, acheves the control of the 3-phase grd connected nverter of hybrd renewable energy system. It permts to regulate nstantaneously, the DC-lnk voltage, the nverter output powers (actve and reactve powers) and the currents njected nto the grd. The prevously fgures show the relablty and the short tme response of the global system wth the developed controller, even wth the wnd system that s very fluctuatng and the desel generator whch has a very slow dynamcs snce t s a mechancal system.. CONCUSION In ths paper, a control strategy based sng mode controller, has been proposed. The command approach ensures the control of the 3-phase nverter output currents whch leads the control of the actve and reactve powers transferred nto the grd and/or the solated loads. In seres wth the sng controller a SPWM was developed to drve the sx nverter swtches to generate 3-phase voltages necessary to transfer the desred powers. Ths command approach (sng controller wth SPWM) has gven very acceptable results n terms of response tme and accuracy of the controlled varables, especally snce ths system s very varable and has dsturbances accong to the wnd speed. ACKNOWEDGMENT Ths work s part of the project Projets Jeunes chercheurs funded by the Tunsan Mnstry of Hgh Educaton and Scentfc Research. The support of the mnstry s kndly acknowledged. REFERENCES [] E. Gosden, "Global renewable power capacty overtakes coal as 500,000 solar panels nstalled every day, The Telegraph, ondon,06. [] S. Youns, M. Jrad, N. Hamroun and A.Cherf, "Modellng and control of hybrd renewable energy system connected to AC grd", Internatonal journal of computer scences and engneerng (IJCSE), ol. 3 No., 0. [3] S. Youns, M. Jrad, N. Hamroun and A. Cherf, "Artfcal neural network control of hybrd renewable energy system connected to AC grd", Internatonal journal of computatonal ntellgence technque (IJCIT) ol., no., pp. 44-5, 0. [4] S. Youns, M. Jrad and A. Cherf, "Modellng and control of wnd farm assocated wth a flywheel energy storage system and a desel generator connected to the network", Journées Tunsennes d Electrotechnque et d Automatque (JTEA 00), 6-8 Mars, Tunsa, P a g e

7 (IJACSA) Internatonal Journal of Advanced Computer Scence and Applcatons, ol. 9, No., 08 [5] S. El AIMANI, "Modélsaton de dfférentes technologes d éolennes ntégrées dans un réseau de moyen tenson", Thèse de doctorat de l école centrale de lle, 003. [6] Gabrel-Octavan CIMUCA, "Système de stockage d énerge assocé à des générateurs éolennes", Thèse de Doctorat de l ENSAM, 004. [7] H. Ozbak and M.Kesler, "Actve and reactve power control of grd-ted three phase nverter for P system, Internatonal journal of hydrogen energy, August 06. [8] M. Bnh and M. T. Dat, "Actve and reactve power controller of grdconnected sngle phase photovoltac system", Internatonal journal of hydrogen energy. [9] D. Rekoua and A. Y. Achour, "Trackng Power Photovoltac system wth sng mode control strategy", Energy Proceda, ol. 36, pp. 9-30, 03. [0] F. Pres,"Dual-nverter for grd connected photovoltac system: modelng and sng control", solar energy, vol. 86, no. 7, pp. 06-5, 0. [] H. Ammeur and D. Aouzelleg, "Sng Mode Control of a dual-stator nducton generator for wnd energy converson systems", Internatonal Journal of Electrcal Power & Energy Systems.vol. 4 no., pp , 0. [].I. Utkn, " Sng mode n control optmzaton", sprnger-verlag, Berln 99. [3] H. Buhler, "Réglage par mode de glssement", presse polytechnque romande, 986. [4].I. Utkn, "Sng mode control desgn prncples and applcaton to electrc drves", IEEE Trans on Elect, vol. 40 pp. 3-36, 993. [5] H. djaghloud and H. Benallah, "Space ector Pulse Wdth Modulaton Appled to Three-level oltage Inverter", 5th Internatonal Conference on Technology and Automaton ICTA 05, Thessalonk, Grece Oct. 00. [6] J. H. Seo, C. H. Cho and D. S. Hyun " A new Smplfed Space ector PWM Method for Three-level Inverter " IEEE Transactons on Power Electroncs, vol. 6, no. 4, pp , 00. [7] K.B. Prya, J. B.. Subrahmanyam, C. Skanth and M. Ayoub, Space vector PWM Technque for 3phase voltage source nverter usng Artfcal Neural Network", Internatonal Journal of Engneerng and Innovatve Technology, ol., 0. AUTHOR'S PROFIE Nejb Hamroun receved hs engneerng degree from the Natonal Engneerng School of Sfax, n 000 and the PHD from the Natonal Engneerng School of Tuns, n 009, both n electrcal engneerng. He s an Assstant professor at Natonal Engneerng School of Gabés from 00 to 05. Snce September 05 he s an assstant professor at ISSAT of Mateur. He has partcpated n several research and cooperaton projects, and s the author of more than 0 nternatonal communcatons and publcatons. Sam Youns obtaned hs engneerng degree from the Natonal Engneerng School of Sfax and hs PHD n electrcal engneerng n 03 from the Scence Faculty of Tuns. He s an Assstant professor at the Insttute of Technologes of Tuns P a g e