A High Efficient Micro-controlled Buck Converter with Maximum Power Point Tracking for Photovoltaic Systems

Transcription

1 Eurpean Assciatin fr the Develpment f Renewable Energies, Envirnment and Pwer Quality Internatinal Cnference n Renewable Energies and Pwer Quality (ICREPQ 09) Valencia (Spain), 15th t 17th April, 2009 A High Efficient Micr-cntrlled Buck Cnverter with Maximum Pwer Pint Tracking fr Phtvltaic Systems P. C. M. Bernard 1, Z. M. A. Peixt 1 and L.V. B. Machad Net 2 1 Graduate Prgram in Electrical Engineering 2 Grup f Studies in Energy GREEN Slar Pntifical Cathlic University f Minas Gerais PUC Minas Rua Dm Jsé Gaspar n. 500, Prédi 03, Sala Craçã Eucarístic Bel Hriznte - Minas Gerais - Brazil, pcmb@pucminas.br, assiszmp@pucminas.br, laur.vilhena@green.pucminas.br Abstract. In recent years, the use f slar energy has becme an alternative surce f energy f great imprtance. Several researches and effrts have been cncentrated n the imprvement f the efficiency f tvltaic systems and in the accessibility t this technlgy. In this sense, this wrk presents a buck cnverter cntrlled by P&O technique fr the maximum pwer pint tracking f the tvltaic panel. The system is simulated and implemented experimentally using a micrcntrller device. The analysis by simulatins and practical results shw a gd perfrmance f the cntrl technique and the imprvement f the efficiency f the system. Key wrds DC/DC Cnverter, Maximum Pwer Tracking, Phtvltaic Systems. 1. Intrductin In recent decades, researches n the use f slar energy as an alternative surce f energy have becme a rle f prminence in the field f electrical engineering. In parallel, new materials fr the manufacture f tvltaic panels and new methds f cntrl are being develped t reduce csts and increase the efficiency f pwer cnverters. Amng these techniques, a great effrt has been spent with the algrithms fr the maximum pwer tracking (MPPT) cnsidering the variatins f parameters such as temperature, slar irradiatin r the lad f the system. Several techniques fr maximum pwer pint tracking are based n the cmparisn between the measurements current and previus f the pwer delivered by tvltaic panel [1]. Frm these, ther algrithms f cntrl have been develped as the shrt circuit current r pen circuit vltage techniques and perturbatins methds [2-5]. In general, the first class f papers assumes slw variatins f parameters while the secnd wrks with rapidly changing envirnmental cnditins. Mst cmmnly, the climatic cnditins d nt change with great severity, becming apprpriate t use the methds based n the cmparisn f utput pwer. Usually, these methds are easier t the digital implementatin and they ffer a gd stability f peratin. This paper presents a buck cnverter cntrlled by Perturb and Observer (P&O) technique fr the maximum pwer pint tracking f the tvltaic panel. Firstly, a cmplete analysis f the tvltaic devices and cnverter is develped in the MatLab envirnment. Secndly, the MPPT Buck cnverter was implemented experimentally using micrcntrller Freescale HC08- QT4. The results shw the gd perfrmance f the cntrl technique and the imprving f the efficiency f the system. 2. Mdeling f the cell and slar panels A. Principle f Operatin and Equivalent Circuit The peratin f slar cells may be described frm a PN junctin where there are diffusin currents and drift currents fr the direct and reverse plarizatin, respectively. Usually, the cells perate in reverse directin s that the current drift is desirable. When the PN junctin is expsed t light, tns with energy greater than the gap f energy are absrbed, causing the emergence f pairs electrn-hle. These carriers are separated under the influence f electric fields within the junctin, creating a current that is prprtinal t the incidence f slar irradiatin [6]. Fig. 1 shws the equivalent circuit f the slar cell, frmed by a current surce I in anti-parallel with

2 dide driven by a current I d. The dide determines the current-vltage utput characteristic (IxV) and includes the vltage and current dependence f the cell in relatin t the temperature. V+ R I s AVT I = I I [( e 1)] (3) Replacing the term V KT T = (thermal vltage), the q equatin f the utput vltage f the cell will be given by: Fig. 1: Circuit equivalent f cmplete slar cell, cnsidered the internal lsses. V AKT I I + I = IRs + ln( ) (4) q I Based n this cmplete equivalent circuit, the utput current f the slar cell can be calculated as, I = I I I (1) d Rp where I is the current generated by irradiatin als called shrt-circuit current, I d and I Rp represent the diffusin current f the internal dide and the lss f current in R. p Cnsidering the characteristic IxV the PN junctin, the utput current f the cell can be rewritten f the way: I = I I ( e q( V + IRs ) V AKT O + IRs 1 ) (2) R p where I and V O are, respectively, the utput current and vltage f the slar cell. I is the reverse saturatin current f the dide, q is the charge f the electrn, A is the dide ideality factr, K is the Bltzmann cnstant, T is the temperature in ºK, R s and R p represent the internal series and parallel resistances, respectively. Accrding [7], the parallel resistance has greater influence in the regin f lw vltages where the current thrugh f the equivalent circuit is very small. This resistance is mainly due t the leakage f current in the cell surface, acting significantly nly when the cells are in lw brightness. The series resistance has rigin in the metallic cntacts and it ffers significant influence n the perfrmance f the slar cell. Other apprximate mdels can be built in accrdance with the type f applicatin. A simplified mdel is shwn in Fig. 2 and its equatins are given by (3) and (4). The utput pwer f the slar cell is calculated as: P = IV (5) B. Dependency with temperature and slar irradiatin. Lking up the equatins (1) t (4), it can be bserved a high dependence f the utput current f the cell in relatin t the temperature and slar irradiatin. In functin f this, the utput pwer f the cell is strngly dependent f the climate cnditins in which it is placed. Experimental bservatins shw that between the dawn and the dusk r n cludy days and/r rainy, the yield f the tvltaic generatr sets appreciable variatins in terms f generated pwer [8] [9]. The slar cells can be cnnected s as t frm blcks with values pre-defined f utput vltage, named mdule r slar panel. The mst cmmn way t cnnect the cell is series cnnectin where each unit cntributes with a vltage f arund 0.5 V [9]. 3. The mdel using MatLab Simulink Fig. 3 shws the mdel f the tvltaic panel implemented in Simulink-MatLab envirnment. This mdel allws the chice f the ysical parameters f the cell and t verify the behavir f panel in relatin t the external variables (temperature and irradiatin). The variables cnsidered in the mdel include the current, vltage and pwer generated by slar cell ( I cel, V cel and P cel, respectively). Multiplying these variables by the number f cells ( n cel ), it can be calculated the variables relative t the slar panel. The diagram includes a slar cell blck that was implemented by equatin (4), a surce cntrlled by vltage cnnected t the utput f the cell and a cntrlled resistive lad, respnsible by the changes f the peratin pint under fixed values f temperature and irradiatin. Fig. 4 and 5 shw the utput current versus vltage and utput pwer versus vltage characteristic curves f the Fig. 2: Simplified equivalent circuit f the slar cell.

3 slar panel, respectively. In bth, were used 5 levels f 2 irradiatin within f a range 200 t 1000W m. efficiency f the tvltaic system, it is fundamental that the cntrl f the cnverter guarantees its peratin near t the maximum pwer pint, independent f the cnditins f slar irradiatin and/r temperature. In this sectin, it will be presented the design and simulatin f a buck cnverter cntrlled by the maximum pwer pint tracking algrithm. The algrithm used is the Perturb and Observer (P&O) that ffers gd cnditins t its digital implementatin [10]. The basic structure f a DC-DC cnverter, called step-dwn r Buck, is presented in Fig. 6. Fig. 3: Diagram fr the simulatin f the slar panel. Fig. 6: Simplified diagram f a cnverter buck. The cnditins used in the design f the buck cnverter are input vltage equal (18 VDC ± 10%), utput vltage (12 VDC), switching frequency ( f = 40kHz ), inductr current ripple ( i L = 75mA ), capacitr vltage ripple ( V C = 500µ V ) and an utput current equal t I = 1. 2A fr a resistive lad f 10Ω. Fig. 4: IxV characteristic f slar panel with 36 cells. A. The maximum pwer pint tracking algrithm Due t the lw efficiency f slar panels, ne f the majr challenges in relatin t the design f tvltaic generatrs refers t the maximum utilizatin f energy generated. In general, the pulse width mdulatin (PWM) techniques are used t prvide the cntrl f the pwer cnverter, respnsible fr the transfer f energy frm the slar panel t the lad. Every variatin f temperature r slar irradiatin causes changes n the electric characteristics f the slar panel and it is needed a new pint f peratin. T guarantee an peratin under better delivery pwer cnditins, a cmmn type f cntrl used is the Maximum Pwer Pint Tracking Algrithm (MPPT). Fig. 5: PxV curves f slar panel with 36 cells. 4. The Buck Cnverter and the MPPT Algrithm Due t the variatins f the temperature and slar irradiatin, the vltage generated by the slar panel must be applied t pwer cnverters that prvide the cnditining required by the type f lad. T increase the Fig. 7: Flw chart f the P&O cntrl algrithm.

4 Several algrithms have been prpsed, amng these, the algrithm Perturb and Observer (P&O) is used in this wrk [1]. Fig 7 and 8 shw the flw chart and the blck diagram with the necessary steps fr the implementatin f the P & O algrithm. Fig. 8: Blck diagram f the P&O algrithm in the Matlab - Simulink. Samples f current and vltage f the system are acquired and used in the calculatin f the current and previus pwer. A delay time was intrduced after the sampling t calculate the previus pwer and t prvide the cmparisn between the pwer signals accrding the strategy f the algrithm P&O. Fig. 10: Output signals f the slar panel with resistive lad variable. Fig. 11 shws the wavefrms f the input and utput vltage and current lad f the pwer cnverter. The actin f the P&O cntrl algrithm can be bserved by the presence f scillatins in the input vltage. 5. The high-efficient cnverter tvltaic The cmplete diagram f the cnverter with cntrl MPPT is shwn in Fig. 9, including all elements described previusly. Fig. 11: Output wavefrms btained by simulatin f the chpper with resistive lad. Fig. 12 and 13 shw the simulatin f the tvltaic panel perating clse t the maximum pwer pint. Fig. 9: Cmplete diagram f the tvltaic cnverter with MPPT algrithm. Fig. 10 shws the wavefrms f vltage, current and average pwer f the slar panel, btained at utput f the system. The lad used is variable and the irradiatin and temperature are cnsidered cnstant. The average utput pwer f the panel is ver 40 W, indicating values in the regin f maximum pwer, accrding t the curves shwn in Fig. 5. Fig. 12: IxV characteristic curve f the slar panel.

5 The MPPT algrithm has been prgrammed int micrcntrller HC08QT4 - Freescale, which is respnsible fr sampling f the utput vltage and current f the slar panel. In fllwing, the micr-cntrller runs the P&O algrithm and it prvides the signals t the cntrl f switching devices f the Buck cnverter. Fig. 13: P xv characteristic curve f the slar panel Fig. 14 shws, in detail, the PxV characteristic f the slar panel. It can be nted that the utput pwer f the slar panel is arund the 43 W which, accrding t Fig. 5, indicates prximity f the pint f maximum pwer. Fig. 14: Details f the peratin f the system at maximum pwer pint. The switching device f the cnverter is cmpsed by tw transistrs, ne f high gain f current f the type Darlingtn (TIP 122) and anther f fast switching and medium pwer (TIP 36). The cnverter als has an inductr f 2 mh, an electrlytic capacitr f 470µ F / 100V and a fast dide pwer with the functins f filtering and limitatin f induced vltage, respectively. The cmpsitin f the experimental set can be described, basically, as: - A KYOCERA Slar Panel, mdel KC-50, cmprising 36 rectangular cells f plycrystalline silicn with maximum pwer f 50 W; pen circuit vltage f 21.5 V, shrt-circuit current f 3.1 A; maximum vltage f pwer 16.7 V and current f maximum pwer f 3 A; - A Buck cnverter as previusly described; - An acquisitin and measurement system f slar irradiatin cnsisting f a Data Acquisitin System Natinal Field Pint 200 and a piranmeter Eppley 8-48, respectively; - An acquisitin system and measuring fr the input vltage and current f the cnverter Buck, f type Scpe Meter Fluke mdel 199; - Resistive lad f 5 Ω r battery 12VDC. Fig. 16 shws the results f measurements f slar irradiatin, current and vltage available at the utput f tvltaic panel, during an expsitin f apprximately 04:50hrs, in the range f 10:42hrs t 15:32hrs. The range f sampling was set t 10s. During apprximately the first 200 samples was used as a resistive lad f 5 Ω, and then was dne cnnecting the battery t 12VDC utput f the cnverter. 6. Experimental results Essentially, the prttype develped is cmpsed f ne tvltaic panel assciated with a Buck cnverter cntrlled by P&O algrithm. Initially, the utput f the system was cnnected t a resistive lad equal t 5 Ω and, after this, t a 12 VDC battery. Fig. 15 shws the electrical diagram f the experimental set. Fig. 16: Measurements f slar irradiatin, current and vltage f input pwer cnverter. Fig. 15: Electric diagram f prttype t the tvltaic generatr. Nte the crrelatin between slar irradiatin and utput current f the panel, already discunted the lss in relatin t the internal tcurrent generated. The vltage utput f the slar panel remains virtually cnstant.

6 Fig. 17 and 18 shw the curves IxV f the panel used fr the levels f irradiatin equal t 1000, 800, 600, 400 and 200 W/m 2. In all cases, the peratin pints f the slar panel were lcated t the right f graics, always near t the pint f maximum pwer. With the battery as lad, shwed by Fig. 18, the system ffered better perfrmance, even fr lwer levels f slar irradiatin. maximum pwer pint tracking f the slar panel. Althugh the P&O algrithm presents sme technical limitatins facing sudden changes in weather cnditins, the simplicity and facility f its implementatin using prgrammable digital devices make it a great ptential fr applicatins f lw cst and high efficiency f telectric cnversin. References [1] Hsia Y.T. e C.H. Chen C.H., Maximum Pwer Tracking fr Phtvltaic Pwer System in Prceedings f IEEE Industry Applicatins Cnference - 37th IAS Annual Meeting, [2] Sera D., Kerekes T., Tedrescu R., Blaabjerg F., Imprved MPPT Algrithms fr Raplidly Changing Envirnmental Cnditins in Prceedings f Pwer Electrnics and Mtin Cntrl Cnference, EPE- PEMC 06, Fig. 17: Operatin pints f the slar panel with MPPT cntrl, cnsidering a resistive lad equal t 5 Ω. [3] T. Nguchi, S. Tgashi, R. Nakamt, Shrt- Current Pulse Based Maximum Pwer Pint Tracking Methd fr Multiple Phtvltaic and Cnverter Mdule System, IEEE Trans. Industrial Electrnics, [4] Ortiz R. E. I., A MPPT Methd based n the Apprximatin f a PVM Mdel using Fractinal Plynmials in Prceedings f IEEE Pwer Electrnics Specialists Cnference, PESC [5] D. Casadei; G. Grande e C. Rssi.: Single-Phase Single-Stage Phtvltaic Generatin System Based n a Ripple Crrelatin Cntrl Maximum Pwer Pint Tracking, IEEE Trans. Energy Cnversin, [6] Markvart T.: Slar Electricity, Jhn Wiley & Sns, Fig. 18: Operatin pints f the slar panel with MPPT cntrl, cnsidering as a 12VDC battery charge. 7. Cnclusin The lcatin f the peratin pints near the regin f maximum pwer pint indicates the gd perfrmance f the prttype develped. The wrk allwed t detailed analysis f a tvltaic generatr cmpsed by the assciatin f a slar panel, Buck cnverter with MPPT cntrl and lads type resistive r battery by means f resurces f simulatin and experimentally. Frm mathematical mdeling f the slar panel, it was develped a flexible structure t the simulatin accunting the effects f changes in slar irradiatin, temperature and lad. A Buck cnverter was designed fr the desired perating cnditins. The system was implemented experimentally with a micr-cntrlled Buck cnverter. The experimental results cnfirm the gd perfrmance f the P&O technique fr the [7] Lrenz E., Arauj G.L., Cuevas A., Egidi M.A., Miñan J.C., Ziles R.: Eletricidad Slar Ingenieria De Ls Sistemas Ftvltaics ; PROGENSA, 1ª edición, [8] CRESESBE/CEPEL: Energia slar Princípis e Aplicações. Centr de Referência para Energia Slar e Eólica Sergi de Salv Brit: Sistema Eletrbrás, Brasil, [9] ALTENER/GREENPRO: Energia Ftvltaica: Manual sbre tecnlgias prjet e instalaçã ; Uniã Eurpéia, [10] C. Cabal; C. Alns; A. Cid-Pastr; B. Estibals; L. Seguier, R. Leyva, G. Schweitz, J. Alzieu.: Adaptive digital MPPT cntrl fr tvltaic applicatins in Prceedings f IEEE Internatinal Sympsium n Industrial Electrnic, 2007.