HF Transformer Based Grid-Connected Inverter Topology for Photovoltaic Systems

Transcription

1 1 HF Transformer Based Grid-Conneced Inverer Topology for Phoovolaic Sysems Abhiji Kulkarni and Vinod John Deparmen of Elecrical Engineering, IISc Bangalore, India. Absrac Inverers are required o conver he DC power from phoovolaic (PV) panels o AC. In grid conneced inverers, he AC oupu is direcly fed ino he elecric grid. In his paper, a high-frequency ransformer based inverer opology is presened. This leads o a very compac inverer. The modulaion of his inverer wih a lossless snubber circui is proposed. Proposed modulaion eliminaes he over volage spikes due o ransformer leakage inducance. Commuaion of he leakage inducance energy wih his modulaion is also discussed. The performance of he inverer wih he proposed modulaion is validaed using simulaion and experimenal resuls. Index Terms Phoovolaic sysems, inverers, ransformers, snubbers I. INTRODUCTION Phoovolaic panels are used in many grid-conneced inverers as he energy sources [1]. The PV panels generae DC power of hundreds of was normally a a low volage of less han 50V. In order o achieve a sandard single-phase ac volage of 230V rms and o inerface he PV inverers o he grid, a volage sepping up operaion is required which is normally achieved by ransformers. The ransformers also provide elecrical isolaion which is imporan from safey perspecive. There are differen inverer opologies possible. A simple opology is shown in Fig. 1. This opology uses a linefrequency (50Hz) ransformer o sep up he inverer volage and inerface o he grid. Noe ha he oupu of he ransformer can be conneced o loads insead of grid in sand-alone applicaions. The boos-sage used in he circui is opional. The inverer secion can be direcly fed in by he PV panels. Maximum power poin racking (MPPT) is required o obain he maximum available power from he panels. If he boos converer is used, as in Fig. 1, hen MPPT is performed on he boos converer swich. The inverer performs he dc bus volage regulaion and feeds curren ino he grid a uniy power facor. This opology is suiable for low power raings of few hundreds of was. As he power level increases, he size of he inerface ransformer increases. This resuls in bulky power circui and losses in magneics can also be significan. Anoher issue wih his opology is he inroducion of lower order harmonics in he grid curren [2]. There are many inverer opologies ha use a high-frequency (HF) link ransformer [3] [7]. This is because he size of he ransformer core reduces wih he increase in he operaing PV Array Fig. 1. L boos C in S boos D boos S 1 C dc V dc S 2 Boos-sage for MPPT 1-φ Inverer Transformer S 3 S 4 PV inverer wih line-frequency ransformer L fil i pri i sec V g ~ frequency. The circui opology discussed in his paper is shown in Fig. 2. Ideally, his opology does no require any snubber. However, due o he ransformer leakage inducance, a snubber circui is required. Oherwise he semiconducor swiches will be subjeced o high volage spikes. In his paper, a lossless snubber circui is considered which is shown in Fig. 2. A modulaion mehod is proposed which ensures proper operaion of he circui opology. The hardware for he inverer is buil in he laboraory. The operaion of he inverer wih he proposed modulaion has been validaed experimenally. II. OPERATION OF THE HF TRANSFORMER BASED A. Ideal Operaion INVERTER As shown in Fig. 2, he HF inverer consiss of he swiches S 1, S 2, S 3 and S 4. The HF recifier consiss of swiches S 5 hrough S 8. Oupu inverer is he convenional H-bridge wih an inducive filer conneced o grid. I can also be a sand-alone inverer feeding local loads. All he swiches used are assumed o be ideal. The HF ransformer is a sep up ransformer and i is assumed ha he ransformer does no have any leakage inducances and he magneizing inducance is assumed infinie. The complee circui is assumed o be lossless. The HF inverer and recifier are swiched in square wave mode. Tha is, for he HF inverer, S 1, S 4 are swiched ogeher o apply posiive volage across he ransformer primary. Similarly S 2, S 3 are swiched on o apply negaive volage o he ransformer. The swich pairs have a duy raio of 0.5. The HF recifier is also swiched in square wave mode. S 5, S 8 are swiched ogeher wih S 1, S 4. Similarly S 6, S 7 are swiched ogeher wih S 2, S 3. This ype of modulaion ideally resuls in a dc volage across he dc link. If inpu dc volage is V in

2 2 i source i link V pv PV Array C dc S 1 S 3 S 2 S 4 S 5 S 7 L lp 1:n L ls i sec D s Vlink HF Transformer S 6 S 8 C s S S 9 S 11 C sray S 10 S 12 L i grid V g ~ Grid HF Inverer HF Recifier Lossless Snubber Oupu Inverer Fig. 2. PV inverer wih high-frequency link ransformer and he ransformer urns raio is 1 : n, he link volage V link will be a pure dc equalling nv in. The various waveforms for he ideal operaion are shown in Fig. 3. Noe ha he sawooh carrier and riangle carrier V s S 1,S 3 S 5,S 7 S 2,S 4 S 6,S 8 V pri V in -V in V sec nv in -nv in V link nv in V ri i link i L T s T s (c) (d) (e) (f) (g) (h) Fig. 3. Waveforms during he ideal operaion. Sawooh carrier, and (c) Gaing pulses for swiches S 1 o S 8, (d) Primary volage of he ransformer, (e) Secondary volage, (f) Link volage, (g) Triangle carrier and (h) Link curren. are used for he HF inverer-recifier and oupu inverer respecively. These carriers mus be synchronized. B. Operaion of he Pracical Circui Due o he swiching of he inverer devices, he dc link curren i link shown in Fig. 3(h) is disconinuous. If S 9 and V ref -V ref S 11 are he swiching saes of he swiches S 9 and S 11 respecively, hen he link curren can be expressed as: i link = ( S 9 S 11 )i L (1) Where i L is he oupu curren which is sinusoidal and regulaed. S 9 and S 11 ake values of zero and one. Thus depending on he value of he swiching saes, he link curren is eiher zero or +i L or i L. In sine-riangle PWM, wih uniy power facor (upf) operaion, i can be verified ha i link is eiher zero or he absolue value of i L. Thus here are wo ransiions: one wih very high and posiive di link /d and oher is very high and negaive di link /d. Pracically he ransformer has a leakage inducance. The effecive leakage inducance seen from he secondary side is given by L leakage = n 2 L lp + L ls (2) In (2), L lp and L ls are he ransformer primary and secondary leakage inducances respecively and ransformer has a urns raio of 1 : n. Due o he leakage inducance, he link volage and he device volages will see large overvolages because of L leakage di link /d. There will be a very large posiive overvolage during he posiive ransiion and during he negaive ransiion he volage goes o zero as i canno go negaive due o he back diodes of he swiches. Thus, he link volage will have large spikes and he sress on he semiconducor devices will be very large which is highly undesirable. III. CLOSED-LOOP CONTROL AND MODULATION A. Conrol of he Injeced Grid Curren The curren injeced ino he grid depends on he available inpu power from he PV panels for a given grid volage ampliude. The closed-loop conrol of he complee opology involves he conrol of he oupu inverer o injec a curren wih minimal harmonic disorion ino he grid. Curren is normally conrolled a uniy power facor. However, when he energy from he panels is no available, he inverer can be operaed in STATCOM mode [8]. MPPT algorihm needs o be used o compue he maximum power available from he panels. From his, he required curren reference is generaed. The conrol block diagram is shown in Fig. 4. The curren conroller block shown in he

3 3 Fig. 4. i grid,ref (From MPPT) + - i feedback Curren Conroller Closed-loop conrol block diagram. + + Feed Forward V ref PWM S9 - S 12 figure is a PR conroller wih he following ransfer funcion. k p, and k r are designed using he sandard design procedure available [9]. The grid fundamenal frequency is ω 0 rad/s. G P R (s) = k p + k rs s 2 + ω0 2 (3) The feed-forward erm indicaed in Fig. 4 can be any harmonic compensaing signal due o dead-ime effec ec. In sand-alone applicaion, he oupu volage will be conrolled. Similar o he curren conroller, a PR conroller can be used o regulae he oupu volage. B. Modulaion of he Semiconducor Swiches The HF inverer and recifier swiches are modulaed in square-wave mode as explained in Secion II-A and indicaed in Fig. 3. The oupu inverer swiches are o be swiched using a pulse widh modulaion (PWM) echnique o conrol he oupu curren in grid conneced operaion or oupu volage in sand-alone operaion. The PWM mehod can be convenional sine-riangle PWM, space vecor PWM or some advanced PWMs [10], [11]. In his paper, sine-riangle PWM is used. The volage reference for his PWM is generaed by he closedloop conroller. The modulaion for he snubber swich S is proposed as follows. This swich is urned on during every acive sae of he oupu inverer. Tha is, whenever he diagonal swiches of oupu inverer are urned on, he snubber swich is also urned on. I is urned off when he zero sae sars. If S 9 and S 11 are he swiching saes of he swiches S 9 and S 11 respecively, hen he swiching sae for he snubber swich S is given by S = S 9 S 11 (4) One imporan poin relaed o he modulaion mehod proposed in his paper is he synchronizaion beween he riangle and he sawooh carriers. They mus be synchronized in such a way ha he polariy reversal of he primary/secondary volage occurs during he zero sae of he oupu inverer. This ype of synchronizaion can be achieved by coinciding he zero of sawooh waveform wih he negaive peak of he riangle shown in Fig. 3. If his is no followed hen he snubber acion will no be effecive. IV. COMMUTATION OF THE LEAKAGE INDUCTANCE ENERGY In his secion, he commuaion of energy in leakage inducance is explained qualiaively in he following poins. 1) Consider he insan when he oupu inverer is in zero sae i.e., i link = 0. The oupu curren is assumed o circulae beween S 9 and he diode of S 11. Snubber swich S is off as per (4). The ransformer primary/secondary -- volages are assumed o be posiive and he snubber capacior C s is being charged via diode D s. 2) A he end of he zero sae, he acive sae of he oupu inverer begins which means i link = i grid. Bu now S is urned on and hence C s discharges hrough S o supply he necessary i link and hence he ransformer curren does no have any large di/d. This means no volage spike on he semiconducor devices. 3) A he end of he acive sae, anoher zero sae begins and C s will again sar geing charged via diode D s. 4) During his zero sae, he ransformer applied volage changes polariy (see Fig. 3(e) and Fig. 3(h) a = T s /2). Then he secondary curren will sar flowing hrough S 6, sray capaciance C sray and S 7. I discharges C sray so ha he link volage goes o zero. Then i will decay o zero wih a slope of nv in /L leakage. The same seps will repea in he negaive half cycle of he ransformer applied volage. Thus, i is concluded ha here are no over-volage spikes. However, he link volage is discharged o zero whenever he polariy of he ransformer applied volage reveres. V. SELECTION OF SNUBBER CAPACITOR The snubber capaciance value mus be such ha he resonance frequency beween C s and L leakage is lower han he swiching frequency of he inverer. Oherwise, i can be shown ha he over-volage spikes canno be eliminaed effecively. The resonance frequency is given by f res = 1 2π L leakage C s (5) The value of he snubber capaciance has a direc effec on he ransformer rms and peak currens. The variaion of secondary peak and rms currens versus snubber capaciance value are shown in Fig. 5. I can be observed ha beyond Fig. 5. Peak and rms curren of he ransformer secondary versus snubber capaciance for uniy power facor. 10µF, he rms curren variaion is very small. However, he peak curren variaion becomes small only for a value of beyond 1000µF. If here is a sric requiremen on peak curren reducion, hen higher capaciance has o be seleced. However, higher capaciance would mean he choice of he capacior will be elecrolyic. For he smaller capaciance designs, polypropylene capaciors can be used which have higher lifeime and increase he sysem reliabiliy.

4 4 VI. SIMULATION AND EXPERIMENTAL RESULTS In his Secion, he performance of he inverer is verified in ime-domain simulaions and experimens. The sysem raings are specified in Table I. The experimens are performed a a power level of 800W. TABLE I SYSTEM PARAMETERS. Parameer Value PV panel inpu volage (V pv) 35 40V Transformer urns raio (1 : n) 1:10 Maximum power raing 3kW Nominal grid volage (V g) 230V rms Filer inducance (L) 8.8mH Swiching frequency (f sw) 20kHz Ne leakage inducance w.r. secondary (L leakage ) 34µH Fig. 6 shows he simulaed link volage of he opology wihou any snubber. I can be seen ha he volage has large spikes of upo 2.5kV. This is highly undesirable. The Fig. 7. Experimenal Link volage (V link ) waveform wihou any snubber wih he lossless snubber. 400 Snubber Capacior Volage(V) V ime(ms) Fig. 6. Simulaed Link volage (V link ) waveform wihou any snubber wih he lossless snubber 50V/div link volage wih he lossless snubber circui is shown in Fig. 6. I can be observed ha he volage does no have any overvolage spikes. I is also no a pure dc quaniy. This is because of he ransformer curren rese ha happens during he ransformer volage polariy reversal. The resuls are validaed experimenally as shown in Fig. 7. The inpu volage used for his experimen is only 10V. This is because, he peak volage ha can be seen in he link volage wihou any snubber, in Fig. 7, is 170V. A raed inpu volage of abou 40V would resul in a repeiive peak of 680V which can damage he 600V IGBTs used in he inverer. The resul wih lossless snubber wih proposed modulaion is shown in Fig. 7. I can be clearly observed ha he overvolage spikes are compleely eliminaed wih he proposed modulaion. The snubber capacior volage for he circui is shown in Fig. 8. The simulaion resul is shown in Fig. 8 and he 0V 4ms/div Fig. 8. Volage waveform across snubber capacior C s. Simulaion resul and Experimenal resul. experimenal resul is shown in Fig. 8. I can be seen ha he capacior volage has a 100Hz ripple due o he single phase operaion. The average capacior volage in simulaion is 376V and in experimens i is observed o be 364V. The small deviaion is mainly due o he circui non-idealiies such as device volage drops, winding and rack resisances. The simulaion and experimenal resuls for he oupu curren are shown in Fig. 9 and Fig. 9 respecively. The secondary volage and he primary currens are shown

5 5 Oupu curren(a) 0 5A/div ime(ms) Fig. 11. Picure of he inverer buil in laboraory showing (1) HF Inverer, (2) HF Recifier and oupu inverer, (3) HF ransformer, (4) Oupu inducor, (5) Gae-drive circui, (6) FPGA conroller board, (7) CPLD based proecion and delay card, and (8) Volage and curren sensor board. Fig. 9. Oupu curren Simulaion resul and Experimenal resul. larger han he one shown in Fig. 11. The core volume of he HF ransformer is cm 3. However, he core volume is cm 3 for an equally raed line frequency ransformer which is abou 28 imes larger. Thus he HF ransformer based inverer is considerably compac and hence ligh-weigh compared o he line-frequency ransformer in he opology shown in Fig. 1. in he simulaion and experimenal resuls respecively in Fig. 10 and Fig. 10. The resuls are in agreemen. The sysem efficiency is observed o be around 87%. I can be improved furher by designing he HF ransformer wih inerleaved windings which reduces he copper losses. VII. CONCLUSION In his paper, a high-frequency ransformer based inverer is presened for phoovolaic energy conversion sysem. The requiremen of such a opology in comparison o a line frequency inerface ransformer is explained and i is shown o be very compac. The operaion of he circui and pracical difficulies are discussed. A lossless snubber is effecive in reducing he peak volage sress in he power semiconducor swiches. The modulaion and closed-loop conrol for he inverer opology is presened. The effeciveness of he proposed modulaion scheme is verified using simulaions and experimenal resuls. REFERENCES Fig. 10. Secondary volage and primary curren of he HF ransformer Simulaion resul and Experimenal resul. The picure of he hardware prooype buil in he laboraory is shown in Fig. 11. All he differen circui boards have been labelled in he figure. If he opology shown in Fig. 1 were used, he size of he ransformer would have been much [1] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, A review of single-phase grid-conneced inverers for phoovolaic modules, IEEE Trans. Ind. Appl., vol. 41, no. 5, pp , Sep./Oc [2] A. Kulkarni and V. John, Miigaion of lower order harmonics in a grid-conneced single-phase PV inverer, IEEE Trans. Power Elecron., vol. 28, no. 11, pp , Nov [3] K. Basu, N. Mohan, A high-frequency link single-sage PWM inverer wih common-mode volage suppression and source-based commuaion of leakage energy, IEEE Trans. Power Elecron., vol. 29, no. 4, pp , Aug [4] D. De and V. Ramanarayanan, A proporional + muliresonan conroller for hree-phase four-wire high-frequency link inverer, IEEE Trans. Power Elecron., vol. 25, no. 4, pp , Apr [5] T. Shimizu, K. Wada, and N. Nakamura, Flyback-ype single-phase uiliy ineracive inverer wih power pulsaion decoupling on he DC inpu for an AC phoovolaic module sysem, IEEE Trans. Power Elecron., vol. 21, no. 5, pp , Sep [6] X. Li and A. K. S. Bha, A uiliy-inerfaced phase-modulaed highfrequency isolaed dual LCL DC/AC converer, IEEE Trans. Ind. Elecron., vol. 59, no. 2, pp , Feb [7] H. Qin and J. W. Kimball, Closed-loop conrol of DC DC dual-acivebridge converers driving single-phase inverers, IEEE Trans. Power Elecron., vol. 29, no. 2, pp , Feb [8] X. Zhenglong, S. Liping and Y. Xiaodong, Conrol sraegy of cascade STATCOM under unbalanced grid condiions, IETE Tech. Rev., vol. 31, no. 2, pp , Mar-Apr

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