Phase-Oriented Control of a Modular 3-Phase 3-Level 4-Leg Inverter AC Power Source Supplying Floating or Grounded Loads

Transcription

1 13 IEEE Proceedngs of the IEEE Energy Converson Congress and Exposton (ECCE USA 13), Denver, Colorado, USA, Septeber 1519, 13 PhaseOrented Control of a Modular 3Phase 3Level 4Leg Inverter AC Power Source Supplyng Floatng or Grounded Loads P. Cortés, D. O. Bollat, J. W. Kolar Ths ateral s publshed n order to provde access to research results of the Power Electronc Systes Laboratory / DITET / ETH Zurch. Internal or personal use of ths ateral s pertted. However, persson to reprnt/republsh ths ateral for advertsng or prootonal purposes or for creatng new collectve works for resale or redstrbuton ust be obtaned fro the copyrght holder. By choosng to vew ths docuent, you agree to all provsons of the copyrght laws protectng t.

2 PhaseOrented Control of a Modular 3Phase 3Level 4Leg Inverter AC Power Source Supplyng Floatng or Grounded Loads Patrco Cortes, Davd O. Bollat, Johann W. Kolar Power Electronc Systes Laboratory, ETH Zurch, Swtzerland cortes@le.ee.ethz.ch Abstract A control schee for a hghperforance threephase AC power source s presented. The fourleg nverter output stage uses three brdge legs to generate the phase output voltages wth reference to the neutral pont potental, whch s defned by the fourth brdge leg. The nverter s controlled usng a phaseorented control n order to acheve precse control of each output phase wth unbalanced voltages and any knd of load. The neutral potental s controlled for axu odulaton range n case of floatng load and controlled to zero for grounded load. The control schee for the nput PWM rectfer stage consders the control of the zero sequence voltage n order to prevent the appearance of crculatng currents through the ground loop when the load star pont s connected to ground. The control of the crculatng currents also allows the safe parallel connecton of two or ore AC sources. Sulaton results under balanced and unbalanced condtons deonstrate the perforance of the AC source n ters of output voltage control and preventon of crculatng ground currents. I. INTRODUCTION Hghperforance AC power sources are a very portant tool for testng new power electroncs converters suppled fro snglephase or threephase AC ans. They allow to verfy the behavor of the new equpent under dfferent operatng condtons such as the presence of haroncs n the supply voltage, and grd falures lke loss of a phase of a threephase supply [1] [3]. Accordngly, an portant requreent for the AC source s the generaton of balanced or unbalanced voltages, even DC, as well as operaton wth unbalanced or nonlnear loads, ncludng snglephase loads. In case of loads wth a ground connecton, the AC source ust ensure that no current s crculatng through the ground loop occurrng through the ground connecton of the ans star pont. It s also hghly desrable to allow the parallel connecton of two or ore AC sources, usng the as odules n order to ncrease the power ratng of the syste. Any currents crculatng between the odules ust be avoded. In order to handle unbalanced or nonlnear loads, a fourth leg s ncluded n the output stage provdng a neutral pont connecton. In addton, the use of a fourth leg allows to ncrease the output voltage range and to reduce the DClnk capactors, n coparson to the drect connecton of the neutral pont of the load to the dpont of the DClnk [4]. Modelng and control of 4leg systes have been studed n the lterature and several odulaton and control schees have been proposed. Most of the control schees consder a coordnates transforaton usng a statc or rotatng reference frae and an equvalent crcut, representng the 4leg syste as three snglephase systes [4] [7]. The use of carrerbased pulse wdth odulaton (PWM) [8], as well as space vector odulaton (SVM) usng a threedensonal representaton of the voltage vectors have been proposed n the lterature [4], [9], [1]. All these approaches consder threephase snusodal voltages and are not sutable or easly applcable f the reference voltages can have any prograed wavefor, ncludng DC. Consderng the AC source as a odule, three types of crculatng currents can be dentfed: zero sequence currents, odule ground currents and, for parallel connected odules, nter odule crculatng currents. Zero sequence currents crculate nsde the power converter and can be controlled n order to provde soe addtonal advantage n the operaton of the converter, e.g axze the output odulaton range of the nverter. Module ground currents can appear when supplyng voltage to loads wth connecton to ground, as the star pont of the ans supplyng the AC source s ted to ground. Ths effect s ore lkely to appear wth unbalanced loads, nonlnear loads and actve loads. Accordngly, the desgn of the AC source crcut and control schee ust consder the tgaton of the odule ground currents, provdng safe operaton wth ground connected loads. The ncluson of an solaton stage could prevent the appearance of crculatng currents but t requres a hgher pleentaton effort. Inter odule currents crculate between parallel connected AC source odules, and can generate uneven power dstrbuton between the odules and dstorton of the nput currents. Ths proble has been dscussed n the lterature consderng the parallel connecton of voltage source rectfers [11] [13], current source rectfers [14] and voltage source nverters [15], [16]. The control schee of each AC source odule ust consder the regulaton of the zerosequence currents at the nput sde n order to have a balanced operaton of the odules wthout nter odule currents. At the output sde, a asterslave confguraton of the odules can be pleented n order to guarantee equal sharng of the load current. In ths paper, the fourleg nverter uses a phaseorented control schee, allowng ndependent operaton of each phase of the power source wth any type of voltage references /13/$ IEEE 1357

3 Input Phase a Input Phase b Input Phase c DClnk Balancer p Phase A Phase B Phase C Phase N a b L c, L d, Input Flter L d,1 L c,1 L boost a u DC,p C DC,p L DM,1 A L DM, R D L D A,out A B PE c b L u,pe,out C c,out N C d, y C d,1 u DC,n C DC,n,out gnd u y C y Input Stage n Stage C DM,1 C DM, Fg. 1: Input and output stages of the AC source. Input Stage Stage L c, gnd L d,1 /3 L eq zs N C eq, u y C eq,1 u Fg. : Modellng of the ground current equvalent crcut for a grounded load. and any type of load. The neutral potental s controlled for axu odulaton range n case of floatng load and controlled to zero for grounded load. The control schee of the nput PWM rectfer consders the control of the zero sequence voltages n order to prevent the appearance odule ground current and nter odule currents, allowng the operaton wth ground connected loads and the parallel connecton of several AC power source odules. II. PROPOSED CONTROL SCHEME A. Control Schee for the AC Source (One odule) The power crcut of the AC source consdered n ths work s shown n Fg. 1. The power converter s based on the threelevel neutral pont claped (NPC) topology. At the nput sde, a threephase voltage source rectfer provdes controlled DClnk voltages and snusodal nput currents. An nput flter s ncluded and t s assued that the neutral pont of the grd s connected to ground. The star pont of the flter capactors y s connected to the dpont of the DClnk through another capactor C y, allowng the control of the dpont potental wth respect to ground. In order to ensure the proper balance of the DClnk capactor voltages under any operatng condtons, a balancer crcut s ncluded n the nput stage of the converter. The output sde has four legs wth twostage LC flters provdng controlled output voltages at the phase ternals A, B, C and at the neutral ternal N. The output voltages f phases A, B and C are defned wth respect to the neutral ternal N. A fully syetrcal output flter s selected for the output stage, wth dentcal twostage LC flters for the four output phases. Ths flter structure allows decoupled control of each phase voltage. The reference value for the neutral phase voltage can be selected n order to axze the output voltage range of the three output phases or to be zero n case of loads connected to ground. The load can be balanced or unbalanced, lnear or nonlnear, and wth or wthout connecton to ground. In order to desgn a controller for the ground current, the odel shown n Fg. s consdered. The nput sde of the AC source s odeled n ters of the zero sequence coponents of the converter voltages and currents, and the coon ode equvalent crcut of the nput flter. The output stage consders only the eleents that affect the behavor of the ground current,.e. the voltage at the neutral pont N, wth respect to the dpont of the dclnk, and the ground connecton of the load. When a grounded load s connected at the output ternals, the neutral pont output voltage defnes the potental of the dpont wth respect to ground. For ths reason the reference output voltage u N ust be set to zero for operaton wth grounded load. As t can be observed n Fg., the ground current can be controlled by eans of the voltage u y (the output voltage can be consdered a dsturbance n the control loop), whch can be odfed by the zero sequence current coponent zs. Ths dea leads to a cascaded control structure as shown n Fg. 3. A PI controller regulates the ground current gnd wth a reference value equal to zero. The output of ths controller and a feedforward loop of the output neutral voltage generate the reference voltage u y. The second PI controller regulates the capactor voltage u y and ts output s the reference value for the zero sequence current controller. Ths thrd controller provdes a fast control of the zero sequence current zs and generates the reference for the 1358

4 out gnd a b c gnd * = DC L c, u DC a,n b,n c,n Flter y C y c b a L c,1 Lboost u,pe * zs zs u Flter L d,1 u y y Î * sn(ωt) u ay cos(ωt) PLL α * α u by u cy zs abc αβ b c u α u β u αβ abc u a u b u a u c Input Stage Stage u A u N N A * A L DM,1 L DM, A,out *,out,out,out A B C A,out,out,out N R D C DM,1 L D C DM, AN N n/ax N AN BN CN Fg. 3: Dagra of the control schee of the nput and output stages. zero sequence voltage u. In the nput stage, a PI controller regulates the total DClnk voltage u DC and generates the reference for the nput phase current apltude Î, ncludng a feedforward loop of the output current out, whch can be obtaned fro the output stage controller varables: out = u AN A,out u BN,out u CN,out u DC. (1) A ovng average flter s used for the dclnk voltage u DC and load current out n order to avod dstortons n the nput currents due to perodc varatons of the dclnk voltage generated by unbalanced operaton of the output stage, e.g. wth a sngle phase load. The nput currents are controlled n the αβ reference frae. The reference current for each coponent s calculated usng the nput voltage wavefors and the current agntude Î. A phaselocked loop (PLL) extract the nput voltage angle fro the capactor voltages easureents, whch s requred for the generaton of the α and β coponents of the current references. Then, the reference voltages u α, u β and u are transfored to the abc reference frae and a feedforward loop of the nput flter capactor voltages s ncluded. At the output stage, the voltage references can have any prograed wavefor, ncludng threephase voltages, snglephase wth dfferent frequences and DC voltages. Consderng these knd or references, a space vector representaton s not sutable, and the use of a rotatng reference frae or resonant controllers does not allow the requred flexblty requred to handle dfferent snglephase references. Consequently, a phaseorented control s proposed for the output stage. Each phase s controlled ndependently usng a cascaded structure wth an nner current control loop and an outer voltage control loop, ncludng feedforward loops of the load phase currents and reference voltages. The selecton of the control schee for hghbandwdth AC sources s dscussed n detal n [17]. By usng a hghbandwdth controller, the steady state error for snusodal references s neglgble. The reference for the neutral phase voltage s calculated n order to axze the voltage range of the other three output phases: u N = n(u AN, u BN, u CN ) ax(u AN, u BN, u CN ). () In case of ground connecton of phase N, the reference voltage s set to zero,.e. u N =. B. Control Schee for Parallel Connecton of AC Sources When two or ore AC sources are connected n parallel, the control schee of each AC source reans the sae at the nput sde, but a asterslave schee s consdered for the output stages. Ths eans, only one AC source perfors the control of the output voltages (aster) and provdes the current references for the other AC sources (slaves). A dagra of the connecton of two AC sources s shown n Fg. 4. Here, the output voltage and current references are transfered fro the aster odule to the slave odule. III. SIMULATION RESULTS Sulatons of the AC power source crcut shown n Fg. 1 wth the control schee shown n Fg. 3 are setup usng GeckoCIRCUITS [18]. The behavor of the output stage of the AC source s shown n Fg. 5 for balanced threephase output voltages 1359

5 PE a b c a1 b1 c1 AC Source 1 (Master) A,out1,out1,out1,out1 A B C N u * AN a b c u * BN u * CN u * N * A * B * C * N AC Source (Slave) A,out,out,out,out Fg. 4: Parallel connecton of two AC sources. and a floatng threephase load. Results for a balanced load are shown n Fg. 5(a). The output phase voltages (easured wth respect to the neutral phase N) are snusodal whle the output voltages wth respect to the DClnk dpont are optzed to axze the output voltage range. The neutral phase voltage reference value s calculated accordng to (). The current n the neutral phase s equal to zero. An unbalanced load, wth a % hgher resstance n phase C, has been used for the results shown n Fg. 5(b). The current n the neutral phase s dfferent than zero allowng the operaton of the AC source wth unbalanced threephase currents. The operaton of the AC source wth balanced threephase voltages and a nonlnear load s shown n Fg. 6(a). A threephase dode brdge rectfer s used as a load. The neutral ternal N s not connected. Three ndependent snglephase sources are consdered n the results shown n Fg. 6(b). Each phase has a dfferent reference, phase A wth a 5Hz snusodal reference, phase B wth a 1Hz snusodal reference and phase C as a DC source. The star pont of the threephase load s connected to N. The neutral phase voltage, as well as the phase voltages wth respect to the dpont of the DClnk are optzed for axzaton of the output voltage range. The operaton of the AC source wth a ground connected load s shown n Fg. 7 for an unbalanced load. Balanced threephase references are set for the output voltages and the neutral phase voltage u N s set to zero. In order to assess the effectveness of the proposed schee, results for an nput control schee wthout ground current control are shown n Fg. 7(a). In ths schee, the zero sequence current zs s controlled wth a reference zs =. It can be observed that due to the sall varatons n the output neutral pont voltage, a current wth a agntude of 1 A crculates n the ground loop. When the ground current s controlled usng the schee shown n Fg. 3, the ground current gnd can be reduced to less than 1 A, as shown n Fg. 7(b). The parallel connecton of two AC sources s sulated usng a asterslave confguraton for the output stage control (cf. Fg. 4). Results for a step change n the agntude of (a) Balanced load (b) Unbalanced load Fg. 5: Sulaton results for the AC source operatng wth balanced threephase voltages. The star pont of the threephase load s connected to N. The unbalanced load presents a % hgher resstance n phase C. the threephase reference voltages are shown n Fg. 8. An unbalanced and grounded threephase load s used for these results. It can be observed n Fg. 8(a) that the agntude of the nput currents change due to the step change n the output stage and equal charng of the nput currents s acheved. The ground current gnd s kept near zero, except durng the transent where ths current presents a peak value of 15 A durng a very shot perod of te. At the output stage, the output voltages present a fast transent wth neglgble overshoot. The neutral phase voltage presents a devaton of 3 V durng the transent. Results for a step change n the load resstance s shown n Fg. 9. The power of a balanced threephase load s changed fro 1 kw to kw. The nput currents are well dstrbuted between the two converters and a sall peak n the ground u A u B u C A u A u B u C A 136

6 A (a) Nonlnear load u A u B u C Input Ground current [A] gnd Mdpont voltage [V]. 1 u,pe (a) Wthout odule ground current control a b c A (b) Unbalanced voltages Fg. 6: Sulaton results for the AC source operatng wth nonlnear load and wth unbalanced voltages. (a) A dode brdge rectfer s used as a nonlnear load. (b) Each phase works as an ndependent source: phase A wth a 5Hz snusodal reference, phase B wth a 1Hz snusodal reference and phase C as a DC source. The star pont of the threephase load s connected to N. current os present durng the transent, as shown n Fg. 9(a). The good regulaton of the output voltages s observed and a 1 V devaton n the neutral phase voltage s present durng the transent, as shown n Fg. 9(b). Equal sharng of the output currents s acheved wth the asterslave schee. IV. CONCLUSIONS A control schee for a hghperforance AC source s presented. The addtonal degree of freedo ntroduced by the suppleentary brdgeleg for the neutral phase can be utlzed to axze the 3phase output voltages for floatng loads; or t can be used to control the neutral pont potental to zero for grounded loads. A phaseorented control schee u A u B u C Input Ground current [A] gnd Mdpont voltage [V] 1 u,pe (b) Wth odule ground current control Fg. 7: Sulaton results for the AC source operatng wth balanced threephase output voltages and unbalanced load. The unbalanced load presents a % hgher resstance n phase C. allows ndependent control of each output phase voltage and the operaton wth any knd of reference. A hghbandwdth controller s requred for neglgble steady state errors wth snusodal references. The control of the nput stage uses the zero sequence voltage for controllng the ground current between the ans star pont and the load neutral pont n case of grounded loads. The proposed cascaded control schee takes nto account the ultstage structure of the nput flter and the nfluence of the output stage to effectvely reduce the ground current. Ths control also prevents the crculaton of currents between the odules n case of parallel operaton of several AC sources. Sulaton results under dfferent operatng condtons verfy the proposed control schee. Further verfcaton wll be perfored wth easureents on a 1 kw laboratory prototype. a b c 1361

7 Input Input Input c c gnd, Mdpont voltages [V] 4 u,pe,1 u,pe, a1 b1 a b gnd,1 Input Input Input c c gnd, Mdpont voltages [V] 4 u,pe,1 u,pe, a1 b1 a b gnd,1 (a) Input currents. (a) Input currents. voltages [V] voltages [V] A,out1,out1,out1,out A,out1,out1,out1,out Neutral voltages [V] N,out A,out,out,out,1, Neutral voltages [V] N,out A,out,out,out,1, (b) voltages and currents. Fg. 8: Results for a step change n apltude of the reference voltages for parallel connecton of two AC sources feedng a grounded unbalanced load (phase C presents a % hgher resstance). A asterslave schee s used for the output stage control. (b) voltages and currents. Fg. 9: Results for a step change n the load resstance for parallel connecton of two AC sources feedng a grounded balanced load (the load power s ncreased fro 1 kw to kw). A asterslave schee s used for the output stage control. 136

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