Loss Analysis of a 100kW PV Inverter

Transcription

1 Inernaional Power, Elecronics and Maerials Engineering Conference (IPEMEC 215) Loss Analysis of a 1kW PV Inverer Ruiming YUAN 1, a, Hengchun DING 2, b, Jihong QIAN 3, c, Ying CHEN 4, d, Xiaoyu XI 5, e and Xiaobo YANG 6, f 1 Elecric Power Research Insiue, Sae Grid JIBEI Elecric Power Co., Ld, Beijing, 153, China 2 Sae Grid JIBEI Elecric Power Co., Ld, Beijing, 153, China 3 Wasion Group, Changsha, 4125, China 4 School of Elecrical Engineering, Xi'an Jiaoong Universiy, Xi'an, 7149,China 5 Elecric Power Research Insiue, Sae Grid JIBEI Elecric Power Co., Ld, Beijing, 153, China 6 Elecric Power Research Insiue, Sae Grid JIBEI Elecric Power Co., Ld, Beijing, 153, China a yuan.ruiming@ nc.sgcc.com.cn, b ding.hengchun@nc.sgcc.com.cn, c @qq.com, d ying.zc.g@gmail.com, e xi.xiaoyu@nc.sgcc.com.cn, f @163.com Keywords: Phoovolaic (PV) Generaion Sysem; PV inverer; Power Loss Absrac. This paper deals wih he power loss of a phoovolaic inverer sysem. The research aims a revealing he loss mechanism of differen pars and developing corresponding mahemaical calculaion mehods of he loss. In he firs par he srucure of a commonly used hree-phase grid-conneced PV inverer is inroduced. The possible losses in his sysem come from he IGBTs, diodes, capacior on he DC side, LCL filer, conrol circui, cooling sysem, ec. In he following pars, he losses of each par are analyzed and calculaed. Finally, he validiy of he proposed calculaion mehodology is assessed by comparing he heoreical and experimenal resuls. Inroducion In recen years, solar energy and oher renewable energy sources have araced much aenion and have been developing fas all over he world. In 21, he oal global insallaions of phoovolaic power plan reached 18.2 GW [1]. And China sees a rapid increase of marke share of global solar cell producion from 1% in 21 o over 59% worldwide in 214 [2]. The percenage of elecriciy produced by PV generaion sysems is growing and he power loss inside he sysems is worh consideraion. The loss in a grid-conneced PV sysem mainly consiss of phoovolaic array loss, maximum power poin racking (MPPT) loss, DC cable loss, inverer sysem loss, AC cable loss, ec. In large-scale PV sysems hese losses can be criical. This paper deals wih he inverer sysem loss. Inverer sysem loss mainly consiss of loss in he IGBTs and diodes, loss in he DC-side capacior, loss in LCL filer and oher losses. Currenly for he firs 3 kinds of losses here are various ways o calculae hem precisely, bu wih raher complicaed formula and parameers difficul o ge. Consequenly hese calculaion mehods are no easily implemened in pracice. In his paper, he losses of PV inverer sysem are firs analyzed. A compromise had been made beween calculaion precision and complexiy and, hus, pracical mehods are seleced o calculae he losses while preserving high accuracy. The validiy and uiliy of he proposed loss calculaion mehod are assessed by comparing he heoreical and experimenal resuls for a 1kW PV inverer. Srucure of a Three-phase PV Inverer In PV generaion sysems, he role of PV inverers is o conver he DC power generaed by PV cells ino AC power. Currenly, he inverer sysem of large-scale PV generaion saions mosly ake he opology as shown in Fig The auhors - Published by Alanis Press 746

2 As can be seen from his opology, he possible losses caused by imperfecion of he power devices may come from he IGBTs, diodes, DC-side capacior, LCL filer, wire, circui breaker, conacor, ec. The loss of each componen will be analyzed and calculaed in he following pars. PV Combiner Box DC Breaker Fuse A B C Three-phase IGBT Bridge LCL Filer Fig. 1 Diagram of a hree-phase PV inverer wih LCL filer Loss Analysis of a Three-phase PV Inverer Losses of grid-conneced PV inverers mainly come from he semiconducor devices (IGBTs and diodes), DC-side capacior, LCL filer (inducors and capaciors), and oher pars (cooling sysem, conrol sysem, ec.). The model of losses in each par is buil and analyzed as follows. Power elecronic device losses. Due o he imperfecion of he IGBTs and he diodes, losses exis in real applicaions of hese devices. The losses in hese wo devices can be divided ino 2 caegories: swiching loss and conducion loss. 1) Swiching loss For IGBTs, swiching loss includes he urn-on and urn-off loss. For diodes, i includes he urn-on loss and he reverse-recovery loss. The urn-on loss of he diode, which is much smaller han he reverse-recovery loss, is ofen negleced in pracice. S x v, i T T off on off V d on I o T = 1/ f s s off V d c( on) d ( off ) c ( off ) d ( on) d P T VI o P sw ( on ) Pon sae Fig. 2 Losses of an IGBT Losses of a single IGBT is shown in Fig. 2. When he sae of he swich changes, here will be a ime delay afer he command signal unil he compleion of he change. Swiching losses are generaed during hese delay periods. For a hree-phase PWM inverer wih swiching frequency f s, he swiching loss of an IGBT can be calculaed by [3] 1+ cosθ Vd I Psw, IGBT = fs ( Eon + Eoff ) (1) 2π V I CEN CN P sw ( off ) 747

3 where, V CEN is he nominal collecor-emier volage; I CN is he nominal collecor curren; E on and E off are he urn-on and urn-off energy of a single IGBT wih raed volage and curren, respecively; θ is he phase difference beween oupu volage and curren. For he diode, only he reverse-recovery loss is considered, consequenly he swiching loss for a single diode can be calculaed by [3] P f 1+ cosθ Vd I sw, Diode = s off 2 E (2) π V I N N where, V N and I N are he nominal volage and curren, respecively; E off is he urn-off energy. 2) Conducion loss For a hree-phase PWM inverer, he conducion loss of a single IGBT, caused by non-zero volage drop during conducion sae, is calculaed by [4] 1 M 2 1 M Pi = + cosθ ri T CM + + cosθ VF ICM (3) 8 3π 2π 8 where, V F and r T are he forward hreshold volage and resisance of he IGBT, respecively; I CM is he peak curren of he inverer oupu; M is he PWM modulaion raio. Similarly, he conducion loss for a single diode is 1 M 2 1 M Pd = cosθ ri D CM + cosθ VDICM (4) 8 3π 2π 8 where V D and r D are he forward hreshold volage and resisance of he diode, respecively; DC-side capacior loss. By aking ino accoun he loss behavior, he real capacior can be modeled as an ideal capacior in series wih an equivalen resiser (see Fig. 3) [5]. In Fig. 3, Z refers o he oal impedance, C is he equivalen series capacior, R s refers o he equivalen series resisor (ESR), δ is he dielecric loss angle, θ is he phase angle beween R s and Z (supplemenary angle of δ), D is he dissipaion facor (anδ) of capacior. Rs θ δ R s C jx c j Z Fig. 3 Equivalen circui model of a real capacior The relaionship beween hese coefficiens is 1 T Z = RS jx C; X C = = ; ωc 2πC. (5) RS D T anδ D= an δ = = RS ωc; RS = D XC = = XC ωc 2πC The loss of he capacior is calculaed by T /2 2 E RS I() d =. (6) For he capacior on he DC side, he loss is mainly caused by he curren ripple. This curren ripple, using SVPWM modulaion, is calculaed by [6] I = Im 2m + cos ϕ m 4π π 16 (7) where I m is he peak curren on he AC side of he inverer, m refers o he modulaion raio of SVPWM, φ is he phase difference of he oupu volage and curren. 748

4 Consequenly, by using he equivalen resisance and curren ripple, he loss of he capacior on he DC side can be calculaed. LCL filer loss. For high-power inverers, he ofen used filer is he 3 rd order LCL filer. According o he opology, losses come from he inducors and he capaciors. Generally he losses caused by he inducors are relaively larger han ha caused by he capaciors. 1) Capacior loss According o he capacior model shown in Fig. 3 and loss calculaion mehod (Eq. 6), he loss of capaciors on AC-side can be deduced. In his case, he AC curren as well as he harmonics should be aken ino consideraion insead of he curren ripple compared o he DC-side capacior. The loss can be expressed as [5] h= hmax C _ oal shωh h h= 1 P = C R U (8) where h refers o he order of he harmonic, R sh =hr s1, ω h =2πf h, and U h is he rms value of he h-h harmonic volage. Defining he loss facor anδ as anδ = RSωC, (9) Eq. 8 can be wrien as h= hmax 2 C _ oal (an δh) ωh h h= 1 P = C U (1) 2) Inducor loss The inducor loss can be divided ino wo pars: iron loss (core loss) and copper loss (winding loss). The copper loss is generaed by he resisance of he winding and is calculaed as [7] 2 Pcu = RacIrms (11) where R ac is he ac resisance, I rms is he rms value of he curren, and pracically R ac can be deduced by 4 ( ro / d ) Rac = Rdc (12) ( ro / d ) where r o is he radius of he round conducor and δ is he skin deph which is calculaed by 1 δ = (13) π f µσ wih f, μ, σ referring o he wave frequency, permeabiliy of he conducor and conduciviy of he conducor, respecively. R dc is he dc resisance and is pracically calculaed by Rdc = N( MLT )( ρ2)[1 + a2(tmax 2)] (14) where N is he number of winding urns, MLT is he mean lengh of a urn, ρ 2 is he dc resisance per cenimeer of he maerial, T max is he maximum emperaure of he device (emperaure rise ΔT plus he ambien emperaure T a ). The values of all he parameers can be found in he daashee of he inducor. The iron loss, caused by he change of magneic field in he core, is divided ino hyseresis loss, eddy-curren loss and anomalous loss. In his case he firs loss is much larger han he oher wo losses, so only he hyseresis loss is considered. I is ofen calculaed by he Seinmez equaion as Pfe = Kc f a B β max (15) where f is he frequency, B max is he maximum magneic inensiy; K c, α, β are Seinmez parameers. However, in power elecronic applicaions, flux waveforms are no sinusoidal due o he swiching converers. In his case he Seinmez equaion is no longer precise especially when he swiching frequency is higher han 1 khz. The improved generalized Seinmez equaion is hus proposed [7,8] 749

5 α 1 T db() β α β α1 T db() Pv = k i B d ki B d T = d T d (16) α β α db() = ki B d where α, β, k i are Seinmez parameers, and Kc ki =. (17) 2π β 1 α 1 α 2 π cosθ dθ Since he formula is quie complicaed, in he considered frequency range sill he Seinmez equaion is applied wih he precision requiremen saisfied, and DB ( V i V o ) DT Bmax = = (18) 2 2NAc where V i and V o are he inpu and oupu volage; DT is he urn-on ime of he swich; N is he number of urns and A c is he cross-secion area of he core. Oher losses. Oher pars of he losses include he losses of he conrol and cooling sysem, and hea generaed in devices such as DC breaker, conacor, and surge arreser when curren flows hrough because of heir inner resisance. The firs wo can be evaluaed from daashees or hrough experimens, bu he oher losses are difficul o calculae since he inner parasiic resisance parameers are difficul o evaluae. In pracice, hese losses are considered consan according o engineering experiences. α Simulaion and Experimenal Verificaion The losses in he PV inverer sysem are analyzed in deail in he above pars. In order o verify he proposed loss calculaion mehodology, measuremen resuls are compared wih he calculaed ones. Since he loss of each par is difficul o be separaed from he experimen resuls, simulaion mehods are uilized. In his aricle he loss of semiconducor devices is evaluaed by simulaion. Therefore, he verificaion procedure includes he following wo seps: 1) A well-elaboraed simulaion based on MATLAB/Simulink is performed in order o verify he correcness of he loss calculaion mehod of power elecronic devices (IGBTs and diodes). The simulaion model includes he real-ime measuremen of swiching and conducion losses of devices and is illusraed in deail in he following pars. 2) The verificaion of losses in oher pars (inducors, capaciors and oher pars) is realized by experimen. This is done by comparing he overall calculaed loss wih he measuremen resuls a differen percenages of oupu power. The power diagram of he sysem and he organizaion of he verificaion is shown in Fig. 4. A 1kW hree-phase PV inverer wih opology in Fig. 1 is uilized o validae he calculaion scheme, and he sysem parameers are given in Table 1. DC Cap. Experimenal validaion IGBT + Diode LCL Filer Ohers Inpu DC Power Simulaion validaion Oupu AC Power Fig. 4 Power diagram and loss validaion Simulaion verificaion for power elecronic devices. Losses of IGBTs and diodes and heir calculaion mehods are considered in his par. The chosen IGBT module is FS3R12KE3 and he inpu DC volage is 46V. 75

6 Table 1 Main parameers Parameer Value Parameer Value P ac,nom 1[kW] L in.15[mh] V ac,nom 22[V] L ou.35[mh] f ac 5±.5[Hz] C LCL 4[uF] C dc 1.72[mF] U dc 33-6[V] The simulaion model is shown in Fig. 1. In he calculaion block of he simulaion model (see Fig. 5), he swich loss for a single IGBT module is calculaed by evaluaing he real-ime swiching and conducion loss and summing hese wo losses. The look-up ables sore he daa of he devices from he daashee, including he V CE -I C curve, he V F -I F curve, E(E on, E off, E rec )-I C curve, ec. i IGBT IGBT swiching loss Look-up Table v IGBT i IGBT IGBT conducion loss P IGBT i Diode Reverse-recovery loss Look-up Table v Diode i Diode Diode conducion loss P Diode Fig.5 Semiconducor swich loss calculaion block of he simulaion (for one IGBT module) The simulaions of he IGBT swiching loss and he diode reverse-recovery loss are shown in Fig. 6. The logic par discerns he urn-on and urn-on acions of he devices. When a urn-on or urn-off acion is recognized, he sysem checks he look-up able for a correc ΔE o add o he oal loss. Sar Yes i(k-1)=? Yes Turn-on E =E on(i(k)) Iniialize E= i(k)>? i(k-1)>? End P sw = E/ T Yes Turn-off (a) IGBT swiching loss E =E off(i(k-1)) Yes Sar Iniialize E= i(k)>? i(k-1)>? End P sw = E/ T Fig. 6 Calculaion of swiching loss in simulaion Yes Turn-off E =E off(i(k-1)) (b) Diode swiching loss Fig. 7 Power loss of he IGBTs and diodes 751

7 Calculaion of he losses of power elecronic devices and he real-ime simulaion of hese losses are applied a he same ime for comparison. The calculaed and simulaed power loss of he IGBTs and diodes wih differen oupu power are illusraed in Fig. 7. From Fig. 7, i is apparen ha he calculaed resuls have a good maching wih he simulaed resuls, a differen oupu power. In his sense, he proposed model of IGBT and diode loss calculaion, wih is simpliciy in he form, can be uilized o calculae he swich loss precisely. Experimen verificaion. The opology diagram of inverer sysem is shown in Fig. 1 and he parameers are given in Table 1. The inpu and oupu power of he sysem a differen percenages of raed oupu power are measured a 4 C, and he comparison resuls are shown in Fig. 8 and Fig. 9. Fig. 8 Overall power loss analysis Fig. 9 Efficiency a differen oupu power percenage From Figs. 8 and 9, i is obvious ha he calculaed resuls are precise especially wih low oupu power. When he oupu power increases o 1%, he error is he larges (1.7%). In all he oher cases, he error is smaller han 9%. In his sense, he proposed calculaion model can be used o calculae and analyze he sysem loss in a relaively precise way. Using he calculaion resuls in Fig. 8, he loss of each par can be evaluaed and, hence, specific improvemens can be harnessed in order o decrease he major losses. This resul helps o beer design he inverer sysem wih lower loss. Conclusion In his paper, he losses of PV inverer sysem are analyzed, and pracical mehods of loss calculaion are proposed o evaluae he loss of each componen. Simulaion and experimenal resuls for a hree-phase 1kW PV inverer validaed he proposed calculaion mehodology. The resul is beneficial for a beer undersanding of loss mechanisms and for he design of low-loss inverer sysems. 752

8 References [1] Informaion on hp:// [2] Morgan, R. ; SEMI, San Jose, CA, USA ; Weiss, B. ; Berwind, J. ; Raihel, S. The impac of a rapidly changing global PV marke on he PV manufacuring supply chain[c], Phoovolaic Specialiss Conference (PVSC), h IEEE. [3] LIU Yao. Loss Analysis and Opimal Design of PV Grid-conneced Converer [D]. Beijing Jiaoong Universiy, 211. [4] Kolar, J.W.; Zach, F.C. Losses in PWM inverers using IGBTs [J], Elecric Power Applicaions, IEE Proceedings -, Jul 1995, [5] Huang, Chaofeng ; Cymer, Inc., San Diego, CA ; Melcher, P. ; Ferguson, G. ; Ness, R. Loss Esimaion of Capacior in High Rep-Rae Pulsed Power Sysem. [C] Pulsed Power Conference, 25 IEEE. [6] Kolar J W, Wolbank T M, Schrodl M. Analyical calculaion of he RMS curren sress on he DC link capacior of volage DC link PWM converer sysems [J] [7] W.G. Hurley, W.H. Wölfle. Transformers and Inducors for Power Elecronics: Theory, Design and Applicaions. ISBN: , 37 pages, April 213. [8] Shimizu T, Kakazu K, Masumori H, e al. Iron loss eveluaion of filer inducor used in PWM inverers[c]. Energy Conversion Congress and Exposiion (ECCE), 211 IEEE