218 Inernaional Conference on Physs, Compuing and Mahemaal Modeling (PCMM 218) ISBN: 978-6595-549- Bipolar-ype Dynam Volage Condiioner Using P-Leg and N-Leg Srucured AC-AC Converer Yue-yue LI 1, Dong-bo GUO 2, Yi-bo WANG 2,*, Yu MA 1 and Han-wen ZHANG 2 1 Sae Grid Shenyang Elecr Power Supply Company, China 2 School of Elecral Engineering, Norheas Elecr Power Universiy, China *Corresponding auhor Keywords: Residenial phoovola, Volage flucuaion, B-DVC, P-Leg and N-Leg, Commuaion problem. Absrac. The volage flucuaion caused by he high proporion of residenial phoovola (PV) grid-conneced no only resrs he capaciy of nework o absorb renewable energy, bu also seriously affecs he normal operaion of elecral equipmen. Volage problems in he low volage disribuion nework wih high proporion of residenial PV are described and a novel bipolar-ype dynam volage condiioner (B-DVC) is inroduced o solve he volage problem in his paper. The opology of he B-DVC adops he posiive curren leg (P-Leg) and he negaive curren leg (N-Leg). By combining wo separaion inducors ogeher, he volage disorion caused by he radiional full-bridge wo-way swh converer is overcome. Due o he use of he P-Leg and N-Leg srucure, he B-DVC can be shor- and open-circuied wihou damaging he swhing deves. Neiher lossy RC snubber nor a dedaed sof commuaion sraegy is required in he proposed B-DVC. Furhermore, B-DVC can no only oupu he volage wih he same polariy as he grid volage, bu also oupu he volage opposie he polariy, whh can ensure he sabiliy of he power grid volage and improve he capaciy of grid o absorb renewable energy. A 1-kW prooype of single-phase B-DVC is developed, and is performance is experimenally verified. Inroducion Wih he aggravaion of global energy shorages and environmen polluion problems, renewable energy, represened by phoovola (PV) and wind power generaion, has been more and more researched and uilized. However, as he number of residenial phoovola sysems inerconneced o he low volage disribuion nework (LVDN), many problems have arisen in he curren LVDN such as volage flucuaions, hree-phase imbalance [1] and increased harmon conen [2]. Among he above problems, volage flucuaions is also one of he imporan issues due o he volage flucuaion no only affecs he normal use of he user s power equipmen, bu also seriously resrs he capabiliy of Grid o absorb renewable energy[3-5]. Consequenly, i s of grea signifance o sudy he mehod of addressing volage flucuaion problems in LVDN wih a high percenage of renewable energy access. Unidirecional flow of power flow is a signifan feaure of LVDN, bu he curren high proporion of residenial phoovola access has caused he reverse of he power flow, whh signifanly increases he volage flucuaion risk. The connecion of LVDN is mosly residen load, bu he ypal residen s load and PV oupu peak imes are ofen differen. During he day, he excess of PV power leads o volage swell. While a nigh he PV can produce power bu i s he heavy load period ha resuls in he serious sag of he Grid volage. In addiion, he random access or exi of residenial PV, as well as he inermiency and volailiy of he residenial PV, resul in a serious problem of volage sag or swell in he curren LVDN. These problems, on he one hand, conribue o he power grid s capabiliy o absorb renewable energy, and on he oher hand affec he normal use of elecral equipmen, especially for daa ceners, hospials, semonducors manufacuring and precision machining have a greaer impac on users wih higher qualiy requiremens. 74
In order o address he volage flucuaion problems of LVDN, many expers and scholars a home and abroad have done exensive research on he opology and conrol sraegy of dynam volage regulaor[6]. In numerous researches, AC-AC converers have been exensively sudied due o he advanages of simple opology, single-sage power conversion, no DC links, high power facor and so on, so he dynam volage regulaor based on AC-AC conversion has also go grea developmen [7]. However, he dynam volage regulaor based on he convenional AC-AC conversion has he disadvanages of unreliable commuaion process, shor circui of bridge, dead ime and unipolar oupu volage [8-9]. The commuaion problem of convenional dynam volage regulaor based on BUCK converer is shown in Figure 1. As shown in Figure 1, in an ideal case, he swhes S 2 and S 3 are boh urned ON and OFF simulaneously, and hey are complemenary o he swhes S 1 and S 4 [1]. However, due o he inheren characeriss of he swhes iself, here always exiss a shor dead-ime or an overlap-ime beween swhes. During he dead-ime, he curren flowing in he oupu filer inducors (L ) has no circulaion loop, whh may cause he swhes o burs. Similarly, he swhes may be damaged by excessive curren when here is an overlap-ime beween swhes, because he inpu volage will be shor-circuied. In order o solve he commuaion problem, he usual mehod is o use lossy of resisor-capacior (RC) snubbers circui or o adop he sof swh commuaion sraegy. However, hese measures can reliably achieve commuaion when he volage crosses zero poin or he inpu volage is disored [11]. Therefore, radiional dynam volage regulaors for LVDN conaining high proporion of residenial PV can safely and reliably implemen volage regulaion. In his paper, he bipolar-ype dynam volage condiioner (B-DVC) based on a novel ype of direc AC-AC converer wihou he commuaion problem is proposed. The proposed converers adop he posiive curren leg (P-Leg) and he negaive curren leg (N-Leg) [12]. Because P-Leg and N-Leg srucure is adoped, he B-DVC proposed in his paper doesn need o se dead-ime and can avoid shoo-hrough problem. In addiion, i can operae reliably and robusly even when he volage or curren a he poin of common coupling are highly disored because safe commuaion is always guaraneed by he P-Leg and N-Leg srucure. The proposed B-DVC can change he polariy of he oupu volage by changing he posiive and negaive duy cycle (D), whh can effecively solve he problem of volage sag or swell. Therefore, i s possible o solve he curren LVDN s volage problem wih high proporion of renewable energy access. Based on he deailed heoreal analysis, a simulaion sysem is buil and an experimenal prooype is designed for he proposed opology. Finally, he feasibiliy and effeciveness of he proposed scheme are verified by simulaion and experimens. L L + + Vin C C R Vin C C R - - Figure 1. Commuaion of radiional dynam volage regulaor based on BUCK converer. Topology Srucure of B-DVC based on P-Leg and N-Leg P-Leg and N-Leg In order o address he commuaion problem in radiional converers, he proposed opology in his paper adops he P-Leg and N-Leg srucure, as shown in Figure 2. Each leg consiss of wo conrollable swhes (IGBT or MOSFET, ec.) and wo diodes. The cener erminal of he upper and lower pars of each leg is conneced o he curren sources or inducor. The curren ouflow direcion is 75
indaed by ( ) in Figure 2, he curren in every P-Leg always flows ou from he cener erminal, and he curren in N-Leg always flows ino he cener erminal. The wo upper and lower swhes in he radiional bridge-leg periodally work alernaely. In order o avoid shor circui caused by he wo swhes shor-hrough, he dead-ime mus be se during he swhing period. However, he separaion inducor is placed a he common por of he P-Leg and N-Leg bridge arms, Even if he swhing ime beween P-Leg and N-Leg swhes is overlapped, inducor and link sray inducor can limi he rae of curren change, so he dead-ime is no needed, so he D can be maximized and he conversion effiency of converer can be improved. A any momen, when he curren is in he posiive half period, i only flows hrough he P-Leg; when he curren is in he negaive half period, i flows hrough he N-Leg only. D3 D1 iacp L1 L2 iac D2 iacn vac P-Leg N-Leg D4 Figure 2. Evoluion of opology based on P-Leg and N-Leg. Topology srucure of B-DVC Figure 3 shows he proposed B-DVC opology. Compared wih he convenional dynam volage regulaors, he proposed B-DVC adops P-Leg and N-Leg srucure shown in Figure 2. v s L in L f v p v in Power Grid C in C f load AC S 1 S 5 Inpu S 3 S 4 S 6 S 8 Oupu S 2 Figure 3. B-DVC of opology based on P-Leg and N-Leg. As can be seen from Figure 3, he opology includes an LC inpu filer, a novel direc AC-AC converer, an LC oupu filer and a power frequency ransformer. The AC-AC converer is used o generae he compensaion volage V c, and he power frequency ransformer is used for elecral isolaion. Among hem, he novel direc AC-AC converer consiss of wo modules. Each module adops he P-Leg and N-Leg srucures described above and wo separaion inducors. Each of he P-Leg and N-Leg are respecively composed of wo swhes wih forward series diodes. Since he B-DVC is used in disribuion neworks, is power level and volage are relaively high, so he swhes are preferred o use insulaed-gae bipolar ransisors (IGBT) wih ani-parallel diodes. Due o he use of P-Leg and N-Leg in he opology, Compared wih he radiional half-bridge / full-bridge srucure, he proposed opology can effecively avoid he shoo-hrough and increase he reliabiliy of he sysem. In addiion, he proposed B-DVC can use MOSFETs in a cerain power level conversion. 76
Compared wih he IGBT, he MOSFET has lower swhing loss, resisance characeriss and high frequency. These advanages can ensure he oupu volage/curren waveform qualiy and can effecively reduce he size of he filer. The B-DVC can work in volage compensaion mode or bypass mode. When he grid volage is normal, he B-DVC operaes in he bypass mode adjusing he D of he PWM conrol signal o zero and he oupu compensaion volage o zero; when he grid volage sag or swell, he AC-AC converer generaes high-frequency volage conrolled by he D of he PWM signal. Afer filering, he sinusoidal AC volage V p in he same phase as he grid is obained. Finally, he V p is coupled o he disribuion nework hrough he power frequency ransformer, hereby ensuring he sabiliy of he volage a he load erminal. When he B-DVC solves he high volage problem, he polariy of he oupu volage is obained by changing he conrol sraegy, and here is no need o change he volage polariy hrough a mechanal swh or a solid sae relay. Therefore, compared wih he radiional volage regulaor, he B-DVC has he advanages of fas dynam response, high compensaion accuracy, safe and reliable operaion. Operaion of he Proposed B-DVC Swh Signal Modulaion The B-DVC can compensae boh volage sag and swell. A modulaion sraegy adaped o he opology of Figure 3 is designed according o he differen working modes of B-DVC. In his paper, a dual modulaion PWM modulaion sraegy is adoped. In his modulaion sraegy, he swhes S 1 S 2 S 3 and S 5, whh need o perform high frequency operaion, are conrolled by modulaion signal, while swhes S 4 S 6 S 7 and S 8 are conrolled by modulaing signal -. All high-frequency swhes ake he same riangular carrier signal. According o he differen modes of B-DVC, and he posiive or negaive half-wave of inpu volage, he modulaion sraegy can be divided ino Figure 4 and 5. The specif PWM conrol signal of he corresponding swhes is shown in Figure 6. Sg3 + 1 Sg1 + 1 Sg6 + 1 Sg5 Sg3 + 1 Sg2 Sg3 Sg8 Sg3 Sg4 Sg7 Sg3 (a) (b) (a) (b) Figure 4. PWM modulaion sraegy for volage sag. Figure 5. PWM modulaion sraegy for volage swell. 1 S g2 S g7 S g4 S g5 1 S g3 S g6 S g1 S g8 S g3 S g4 S g5 S g6 S g1 S g2 S g7 S g8 S g1 S g2 S g7 S g8 S g3 S g4 S g5 S g6 1 d -d 1 d -d S g1 S g6 S g5 S g2 S g8 S g3 S g4 S g7 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 (a) (b) Figure 6. The specif PWM conrol signal of B-DVC. 77
As shown in Figure 4, he PWM modulaion sraegy for volage sag, where (a) is he modulaion sraegy for he posiive half period of he inpu volage and (b) is he modulaion sraegy for he negaive half period of he inpu volage. Figure 5 shows he PWM modulaion sraegy when he volage swell, where (a) and (b) is he modulaion sraegy diagram a he ime of he posiive and negaive half period of he inpu volage respecively. In addiion, he gae drive signal of S g1 ~S g8 is S 1 ~S 8 in Figure 6, whh conrols he corresponding swhes. Commuaion Sae Analysis of B-DVC Commuaion Sae of Sep-down Mode. When he acual volage of he Grid is higher han he raed volage, B-DVC works in his mode. Taking he volage in he posiive half-period as an example o analyze he commuaion sae of he B-DVC. As shown in Figure 7(a), swhes S 2 and S 7 are always in he ON sae, swhes S 3, S 4, S 5, and S 6 are always in he OFF sae, and swhes S 1 and S 8 do high frequency acions o achieve a posiive half-period energy conrol of he volage. The commuaion sae is divided ino four saes, as shown in Figure 7 (a) (b) (c) (d). A ime ~ 1, as shown in Figure 7(a), he B-DVC operaes in an acive sae. A his poin, curren flows hrough S 1, inducors, capaciors, S 8, and finally back o he power sources. During his ime, he power source charges he capacior. A ime 1 ~ 2, as shown in Figure 7(b), he B-DVC operaes in a freewheeling sae, curren flows hrough S 2, inducors, capaciors, and S 8, and he capacior is discharged. Because S 2 is always in he ON sae, when S 1 is urning on o off, he curren loop is always exis, and herefore swhes are no impaced. A ime 2 ~ 3, as shown in Figure 7(c), he operaing sae of he B-DVC is he same as he ime ~ 1, and will no be described again. A ime 3 ~ 4, as shown in Figure 7 (d), he B-DVC is in a freewheeling sae. A his ime, he curren flows hrough S 1, inducors, capaciors and S 7. When he volage a a negaive half-period, he working sae is i is similar o he posiive half-period, so i will no be repeaed. The specif swhes on and off follow he PWM signal shown in Figure 6 (a). iou iou vou vou (a ) (c ) iou iou vou vou (b ) Figure 7. Working process diagram of B-DVC wih volage sag and posiive half-period. Commuaion Sae of Sep-up Mode. When he acual volage of he Grid is higher han he raed volage, B-DVC works in his mode. Taking he volage in he posiive half-period as an example o analyze he commuaion sae of he B-DVC. I can be from Figure 6 (b) ha swhes S 3 and S 6 are always in he ON sae, he swhes S 1, S 2, S 7 and S 8 are always in he OFF sae, and swhes S 4 and S 6 do high frequency acions o achieve a posiive half-period energy conrol of he volage a his ime. The commuaion sae is divided ino four saes, as shown in Figure 8 (a) (b) (c) (d). A ime ~ 1, as shown in Figure 8(a), he B-DVC operaes in an acive sae. A his poin, curren flows hrough S 5, inducors, capaciors, S 4, and finally back o he power sources. During his ime, he power source (d ) 78
charges he capaciors. A ime 1 ~ 2, as shown in Figure 8(b), he B-DVC operaes in a freewheeling sae, he curren flows hrough S 4, inducors, capaciors and S 6, and he capacior is discharged. Because he swh S 4 is always in he ON sae, when he swh S 1 is urning on o off, he curren loop is always exis and herefore he swhes are no impaced. A ime 2 ~ 3, as shown in Figure 8(c), he operaing sae of B-DVC is he same as he ime ~ 1, and will no be described again. A ime 3 ~ 4, as shown in Figure 8 (d), he B-DVC is in a freewheeling sae. A his ime, he curren flows hrough S 3, inducors, capaciors and S 5. When he volage a a negaive half-period, he working sae is similar o he posiive half-period, so i will no be repeaed. The specif swhes on and off follow he PWM signal shown in Figure 6 (b). iou iou vou vou (a ) (c ) iou iou vou vou (b ) (d ) Figure 8. Working process diagram of B-DVC wih volage swell and posiive half-period. Simulaion and Experimenal Sudy Simulaion Resuls To prove he validiy and advanages of he B-DVC proposed, deailed simulaion resuls are presened in his secion. Suppose he grid volage peak is 311V and he load is 1kW pure resisive. The simulaion waveform when he volage sag and swell is shown in Figure 9 and Figure 1, respecively. Volage /V Compensaion volage Vp Load volage Vou volage/v 4 Inpu volage Vin Compensaion volage Vp Load volage Vou 2 2-2 -2-4. 8. 1. 12. 14. 16. 18. 2 时间 Time/S. 8. 1. 12. 14. 16. 18. 2 时间 Time/S Figure 9. Simulaion waveform diagram of volage sag. Figure 1. Simulaion waveform diagram of volage swell. When he volage sag of grid occurs, he oupu of B-DVC is in he same phase and polariy as he volage of grid. The red, green and blue waveform represen he grid volage, he load volage and he compensaion volage of he B-DVC oupu, respecively. As can be seen from Figure 9 ha he grid volage has fallen before.1s, and hen he grid volage coninues o drop. A his ime, he oupu volage of B-DVC increases simulaneously. We can see ha B-DVC compensaion speed is fas, i 79
can mee he requiremens of volage flucuaion conrol in LVDN. Similarly, when he volage swell of grid occurs, he compensaion volage of B-DVC oupu is in he same phase and opposie polariy o he grid volage. I can be seen from Figure 1 ha he waveform is slighly disored when he polariy of he oupu volage is reversed, bu he duraion is very shor and doesn affec he accuracy of compensaion. Experimenal Resuls On he basis of he previous analysis, a 1-kW prooype B-DVC is developed and esed, as shown in Figure 13. Table I liss he elecral specifaions of he proposed B-DVC. Due o he limiaion of laboraory condiions, i s assumed ha he raed volage ampliude and frequency of LVDN are 155.5V and 5Hz, respecively. When he grid volage ampliude falls o 1V, ha is, when he grid volage falls 35.7%, he waveform is shown in Figure 14 (a). In addiion, when he volage sag from 2% o a sudden rise of 4%, he waveform is show in Figure 14 (b). From Figure 14 (a) and (b) shows ha he B-DVC proposed in his paper can accuraely realize he compensaion of volage sags and swells, especially when crossing zero poin, here is no commuaion problems and volage isn disored. Inpu filer DSP Oupu filer IGBT Cooling panel DSP Simulaor Driver Figure 13. B-DVC experimenal prooype. Grid volage Vin Load volage Vou Compensaion volage Vp Compensaion volage Vp Load volage Vou Parameer Inpu filer inducor L in /uh (a) (b) Figure 14. Waveform diagram of compensaing volage sag and swell. Inpu filer capacio r C in /uh Table 1. Parameers of he B-DVC proposed. IGBT S 1 ~S 8 Swhing frequency /Hz Oupu filer inducor L f /uh Oupu filer capaciance C f /uf Transformer capaciy S/MVA Transformer raio Figure 5 5 4A/1.2kV 2k 1 2 1 1:1 8
Conclusion In order o address he problem of volage sag or swell in LVDN wih high proporion of residenial PV access, a Bipolar-ype Dynam Volage Condiioner is proposed. The B-DVC adops a direc AC-AC converer based on P-Leg and N-Leg srucure, whh overcomes he volage disorion caused by radiional full-bridge bi-direcional swhing converer. A he same ime, he B-DVC does no need o se dead-ime, so he D is maximized, and he effiency of he B-DVC is improved. In addiion, he B-DVC can operae reliably and robusly due o safe commuaion is always guaraneed by he P-Leg and N-Leg srucure even when he volage or curren a he poin of common coupling are highly disored. Moreover, i can oupu bipolar volage wihou changing he polariy of oupu volage by mechanal swhes. On he basis of heoreal analysis, he simulaion and experimenal resuls show ha he B-DVC proposed can qukly adjus he volage and ensure he sabiliy of he load erminal volage, whh can effecively solve he problem of volage flucuaion in he LVDN and improve he abiliy of he LVDN o absorb renewable energy. Acknowledgemen This research was financially suppored by he science and echnology projec of Sae Grid Liaoning Elecr Power Supply Co. Ld. (5222SY16M). References [1] El-Naggar A, Erlh I. Conrol approach of hree-phase grid conneced PV inverers for volage unbalance miigaion in low-volage disribuion grids[j]. IET Renewable Power Generaion, 217, 1(1):1577586. [2] Fekee K, Kla Z, Majdandz L. Expansion of he residenial phoovola sysems and is harmon impac on he disribuion grid[j]. Renewable Energy, 212, 43:1448. [3] Weckx S, Driesen J. Opimal Local Reacive Power Conrol by PV Inverers[J]. IEEE Transacions on Susainable Energy, 216, 7(4):1624633. [4] Procopiou A T, Ochoa L F. Volage conrol in pv-rh LV neworks wihou remoe monioring[c]// IEEE Power & Energy Sociey General Meeing. IEEE, 217:1. [5] Long C, Ochoa L. Volage conrol of PV-Rh LV neworks: OLTC-fied ransformer and capacior banks[c]// Power and Energy Sociey General Meeing. IEEE, 216:1. [6] Babaei E, Kangarlu M F, Sabahi M. Dynam volage resorer based on mulilevel inverer wih adjusable dc-link volage[j]. IET Power Elecrons, 214, 7(3):576-59. [7] Barros J D, Silva J F. Mulilevel Opimal Predive Dynam Volage Resorer[J]. IEEE Transacions on Indusrial Elecrons, 21, 57(8):2747-276. [8] Johibasu S, Mishra M K. A Conrol Scheme for Sorageless DVR Based on Characerizaion of Volage Sags[J]. IEEE Transacions on Power Delivery, 214, 29(5):2261-2269. [9] Liu H, Wang J. Analysis and conrol of a single phase AC chopper in series connecion wih an auo-ransformer[c]// Inernaional Conference on Auomaion and Compuing. IEEE, 212:1-6. [1] Shin H H, Cha H, Kim H G, e al. Novel Single-Phase PWM AC AC Converers Solg Commuaion Problem Using Swhing Cell Srucure and Coupled Inducor[J]. Power Elecrons IEEE Transacions on, 215, 3(4):2137-2147. [11] Kaniewski J, Szczesniak P, Jarnu M, e al. Hybrid Volage Sag/Swell Compensaors: A Review of Hybrid AC/AC Converers[J]. IEEE Indusrial Elecrons Magazine, 215, 9(4):37-48. [12] Li S, Tolber L M, Wang F, e al. Sray Inducance Reducion of Commuaion Loop in he P-cell and N-cell-Based IGBT Phase Leg Module[J]. IEEE Transacions on Power Elecrons, 214, 29(7):3616-3624. 81