DC-DC Boost Converter with Constant Output Voltage for Grid Connected Photovoltaic Application System

Transcription

1 DC-DC Boos Converer wih Consan Oupu Volage for Grid Conneced Phoovolaic Applicaion Sysem Pui-Weng Chan, Syafrudin Masri Universii Sains Malaysia Absrac The main purpose of his paper is o inroduce an approach o design a DC-DC boos converer wih consan oupu volage for grid conneced phoovolaic applicaion sysem. The boos converer is designed o sep up a flucuaing solar panel volage o a higher consan DC volage. I uses volage feedback o keep he oupu volage consan. To do so, a microconroller is used as he hear of he conrol sysem which i racks and provides pulse-widh-modulaion signal o conrol power elecronic device in boos converer. The boos converer will be able o direc couple wih grid-ied inverer for grid conneced phoovolaic sysem. Simulaions were performed o describe he proposed design. Experimenal works were carried ou wih he designed boos converer which has a power raing of 100 W and 24 V oupu volage operaed in coninuous conducion mode a 20 khz swiching frequency. The es resuls show ha he proposed design exhibis a good performance. 1. Inroducion Nowadays, power generaion using solar power had increased dramaically because i is polluion free as compare o power generaion using fossil fuel. Besides, i needs low mainenance and no noise and wear due o he absence of moving pars which make solar power aracive o he people. Solar power uses solar panel o conver sun irradiaion ino elecric energy using phoovolaic (PV) effec. The oupu volage of a solar panel is varying depending on sun irradiaion and emperaure [1]. As he sun irradiaion and emperaure changes, oupu volage changing as well. Since he volage produced is flucuaing, a lo of elecronic equipmens are unable o be direcly conneced. Therefore, a DC-DC boos converer wih consan oupu volage is needed. The boos converer will sep up he solar panel volage o he suiable volage required by elecronic equipmens. For AC elecrical equipmens, he sysem requires an addiional AC-DC inverer which convers he consan DC volage o AC volage. This sysem is called dual power processing sage sysem. Figure 1 shows a grid conneced PV applicaion sysem using dual power processing sysem. From he block diagram, he sysem does no use any baeries o sore energy produced by solar panel. Any power produced by solar panel is direcly deliver o he grid. Baeries are excluded from he sysem because baery banks need high mainenance which had o be handled carefully in order o have a long lifeime and safe environmen. Besides, baeries are he second major cos conribuor for he sysem [2]. Therefore, he exclusion of baeries as he energy sorage is economically advanageous [3]. DC Boos Converer DC Inverer Solar Panel Uiliy Grid Figure 1. Block diagram of a grid conneced PV applicaion sysem. 2. Basic Operaion The boos converer is a medium of power ransmission o perform energy absorpion and injecion from solar panel o grid-ied inverer. The process of energy absorpion and injecion in boos converer is performed by a combinaion of four componens which are inducor, elecronic swich, diode and oupu capacior. The connecion of a boos converer is shown in Figure 2 [4]. The process of energy absorpion and injecion will consiue a swiching cycle [5]. In oher word, he average oupu volage is conrolled by he swiching on and off ime duraion. A consan swiching frequency, adjusing he on and off duraion of he swich is called pulse-widh-modulaion (PWM) swiching. The swiching duy cycle, k is defined as he raio of he on duraion o he swiching ime period. The energy absorpion and injecion wih he relaive lengh of swiching period will operae he converer in wo differen modes known as coninuous conducion mode (CCM) and disconinuous conducion mode (DCM) [4][6]. AC Figure 2. Schemaic of boos converer.

2 3. Boos Converer Analysis 3.1. Coninuous Conducion Mode Under CCM, i is divided ino wo modes. Mode 1 begins when he swich SW is urned on a = 0 as shown in Figure 3. The inpu curren which rises flows hrough inducor L and swich SW. During his mode, energy is sored in he inducor and load is supplied by capacior curren. Mode 2 begins when he swich is urned off a = kt. The curren ha was flowing hrough he swich would now flow hrough inducor L, diode D, oupu capacior C, and load R as shown in Figure 4. The inducor curren falls unil he swich is urned on again in he nex cycle. During his ime, energy sored in he inducor is ransferred o he load ogeher wih he inpu volage. Therefore, he oupu volage is greaer han he inpu volage and is expressed as V 1 ou V (1) 1 in k where V ou is he oupu volage, k is duy cycle, and V in is inpu volage [4]. PWM - Vou ILmax ILmin 0 kt T Figure 5. Boos converer waveforms a CCM Disconinuous Conducion Mode Under DCM, inducor curren I L does no flow coninuously. There is an inerval of ime which he curren is zero before he nex urn on of swich SW. The swiching waveforms are shown in Figure 6. Taking inegral of inducor volage over one ime period o zero, k T V in Vou 1 T 0 k Vou V (5) in where T is swiching period, 1 is ime period for negaive inducor volage and 2 is ime period for zero inducor volage [4]. vl il Figure 3. Circui diagram of boos converer during Mode 1. PWM vl - Vou Figure 4. Circui diagram of boos converer during Mode 2. In order o operae he converer in CCM, he inducance is calculaed such ha he inducor curren I L flows coninuously and never falls o zero as shown in Figure 5. Thus, L is given by 2 k k R Lmin (2) 2 f where L min is he minimum inducance, R is oupu resisance, and f is he swiching frequency of swich SW [4]. The oupu capaciance o give he desired oupu volage ripple is given by k C (3) min R f V r where C min is he minimum capaciance and V r is oupu volage ripple facor [4]. V r can be expressed as Vou Vr (4) V ou ILmax 0 kt Δ1T T Δ2T Figure 6. Boos converer waveforms a DCM. As he value of L min calculaed previously is he minimum inducance o operae in CCM, herefore any values of L min below han he minimum inducance will resul in he boos converer o operae in DCM. The calculaion for peak-o-peak ripple in he oupu volage for DCM is he same as (3). 4. Proposed Sysem In his paper, a boos converer operaed in CCM is designed o sep up a flucuaing solar panel volage o a higher consan oupu volage of 24 V. Referring o [7], he range of he duy cycle is beween 0 o 75% due o he insabiliy cause by he parasiic componens. The specificaion of he proposed design is shown in Table I. il

3 TABLE I SPECIFICATION OF PROPOSED BOOST CONVERTER Mode Power Raing (P) Oupu Volage (V ou ) Oupu Curren (I ou ) Swiching Frequency (f) Inpu Volage (V in ) CCM 100 W 24 V 4.2 A 20 khz 6 23 V In order o produce a consan oupu volage, volage feedback conrol sysem is used. In his conrol sysem, oupu volage will be measured and compared wih a reference volage and he differenial value is used o produce a PWM signal. Any changes in he oupu volage will lead o he changes of duy cycle in PWM signal. To produce a se of PWM signal, a microconroller is used. PIC16F877 microconroller is seleced as i is having a successive approximaion analog-o-digial converer, comparaor and PWM generaor. PWM signal wih frequency 20 khz can be generaed when PIC16F877 is driven by a 20 MHz clock cycle. Conrol sraegy for volage feedback conrol flow char as shown in Figure 7 is wrien and load ino PIC16F877 microconroller Selecion of inducor Equaion (2) is he minimum inducance for boos converer o operae in CCM, herefore he selecion of he inducor should be higher han he calculaed value. Inducors wih a ferrie core or equivalen are recommended Selecion of diode Diode reverse volage raing is he main consideraion for selecing he diode. Oher imporan consideraion is is abiliy o block he required off sae volage sress and have sufficien peak and average curren handling capabiliy, fas swiching characerisics, low reverse recovery, and low forward volage drop Selecion of capacior Equaion (3) is he calculaion for oupu volage ripple using capaciance. The selecion of capacior should be higher han he calculaed value o make sure ha he converer s oupu volage ripple says wihin he specific range. Anoher imporan consideraion is is equivalen series resisance (ESR). Since he capacior s ESR affecs efficiency, low-esr capaciors will be used for bes performance. ESR can be reduced by connecing few capaciors in parallel. Yes Sar Oupu pulse (PWM) Read feedback value, V ou Feedback = desired value? Table II shows he componens used in he proposed design based on he equaions and consideraion menion previously. TABLE II BOOST CONVERTER CALCULATED PARAMETERS Componens Value / Type Elecronic swich Inducor Diode IRF540N 2.5 mh MBR1060 Oupu capacior 940 µf Load 5.76 Ω Yes Reduce duy cycle Figure 7. Conrol flow char Selecion of elecronic swich The elecronic swich SW in Figure 2 has been chosen based on is volage and curren raing which have o be higher han he maximum inpu volage and curren. From he proposed sysem, he raing of he converer is 100 W wih an inpu volage ranging from 6 V o 23 V. Therefore, elecronic swich such as power MOSFET, IGBT, BJT and hyrisor handling capabiliy should mee he specificaion of he proposed design. No Feedback > desired value? No Increase duy cycle 5. Simulaion and Experimenal Resuls 5.1. Simulaion resuls Based on he proposed design, compuer sofware PSIM is used o simulae he designed boos converer s performance. The simulaion resuls of he boos converer wih differen inpu volages and duy cycles are shown in Figure 8 and Figure 9. I L V ou I ou Figure 8. Simulaion waveforms for oupu volage, oupu curren and inducor curren a 25% duy cycle.

4 Efficiency (%) V ou I L I ou Figure 9. Simulaion waveforms for oupu volage, oupu curren and inducor curren a 75% duy cycle. From he simulaion resuls, he proposed converer is able o give a consan 24 V oupu volage a 100 W loads Experimenal resuls Experimenal measuremens are being carried ou in order o verify he performance of he boos converer proposed. A power supply is conneced o V in and a load resisance is conneced o V ou as shown in Figure 2. Figure 10 shows he experimenal resuls for differen inpu volages. Inpu volage (V) TABLE III BOOST CONVERTER EXPERIMENTAL RESULTS Inpu Curren (A) Duy cycle Oupu Volage (V) Eff. (%) The efficiency of he boos converer is calculaed by he raio of oupu power o inpu power. Efficiency can be increase by reducing he oal losses of he converer. The losses in a boos converer consiss of swiching loss, conducion loss, inducor s eddy curren and hyseresis loss, ESR and elecromagneic inerference. Sof swiching such as zero-volage and zero-curren swiching is able o reduce swiching losses in a boos converer Efficiency vs Oupu Power 0.0 Oupu Power (W) Figure 11. Efficiency versus oupu power for boos converer. (a) (b) Figure 10. Oupu volage waveform and PWM signal from PIC16F877 (a) wih 19 V inpu volage and 20% duy cycle (b) wih 7 V inpu volage and 70% duy cycle. From he experimenal resuls, i shows ha he proposed design is able o produce a consan 24 V oupu volage wih a duy cycle of 70% and 20%. Table III shows he experimenal resuls for he proposed boos converer whereas Figure 11 shows he efficiency of he boos converer for differen loads. 6. Conclusion From he proposed design, he boos converer is able o produce a consan oupu volage of 24 V from a variable volage of solar panel. The boos converer is able o deliver power wih he highes efficiency of 95%. Componens had been chosen based on he consideraion made. PIC16F877 microconroller is able o perform he volage feedback conrol echnique. Acknowledgemen The auhors wish o hank School of Elecrical and Elecronic, Universiy Science Malaysia for providing laboraory and equipmens o suppor his work. This work is funded in par by he USM Fellowship Incenive Gran/RCMO wih gran number 1001.PELECT References [1] E. Kouroulis, K. Kalaizakis and N. C. Voulgaris, Developmen of a microconroller based phoovolaic maximum power poin racking sysem, IEEE Trans. Power Elecronics, vol. 16, no. 1, pp , 2001.

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