Reconfigurable Power Electronics Topologies

Reconfi gurabl e Power Elect ronics Topol ogies Haochen Li, Jo nathan Shek In stitute for Energy Systems , School o f Engin eering, The Univer sit y of Edinb urgh, The King ’s Buil dings , Mayf iel d R oad, Ed inb urgh , EH9 3 DW , U nited K ingdom Abstract This paper presents tw o novel to pologies for automatically transforming power converter topology from three - phase 3 - level cascaded H - bridge to three - phase 2 - level conve rter design. These technique s are imple mented b y flicking specific switches to rearrange circuit con nections. The switch es can be controlled by signal s in order to re alize autom ation. 1. Introduction In t his paper , topologies are designed so that they can be transformed fro m thre e - phase 3 - leve l converter s to 2 - level according to demand s. The processes and transfo rmations are simulate d with MATLAB/ SIMUL INK. The top olog ies have 6 separate DC sources as input s and produce three - phase 3 - level or 2 - level AC outp ut s , with signal generat ors includi ng step signal source, pulse generat or and 2 - level PW M gen erator. R ec onfigurable pow er converter topologies have potential to be used in HVDC transmission . A s the HVDC is commo nly use d to t rans mi t power nowadays and pow er converter are essenti al to transfer power from DC sources into thre e - phase AC voltage in order to be us ed for grid. The designed topologie s also have a wide app licati on potenti al to build powe r converter s for renewabl e energy. For example, one of the most popular renewable energy nowadays – wi nd power need s thi s technique basically. Cascad ed inverte rs a re essential to compose the joint between AC g rid and separate DC sources which is usually used as renewab le energ y sources such as wind turbines , photo volta ics or fuel cells [1] . Also reco nfigu rable power convert er provid es mobili ty and flexi bil ity in practice. The circuits build by it ca n be trans formed betw een 3 - level AC ou tput and 2 - level AC ou tput e asily by c ontrolling signal. It inspires a metho d to bu ild recon figurable topo logies comp rised by d ifferent kinds of multilevel co nverter and 2 - level c o nverter. And the output AC voltage will change after transformation, which is a potential application for differ ent requirement of voltage. MATLAB/ SIMUL INK sof twa re is gener al ly use d to si mula te dynam ic systems with a block diagram environment. It is i ntegrated in the MATLAB and assists desi gn, simulation, t est, analysis and verificati on of em bedded systems [2] . This software is used in this work to des ign, test and a nalyse the reconfigurable po wer converter topo logies. 2. Background Three - phase volt ag e source inverter s are essential in m edium and h igh level power applications to supply a three - phase voltage source to AC grid. Its a mplitude, phase and frequency can be c hanged as required [3] . The concept s of m ultilevel pow er converters appeared in 1975 [4] and have developed a lot since then. The multilevel converte r synthesizes a sinusoidal voltage with several voltage levels that obtained from capacitor voltage sources [5] . Multil eve l conver te r has sev era l advant age s compar ed to a tradi ti ona l 2 - le vel converter includi ng better staircase waveform quality , s mall er co mmon - mode ( CM) vol tage , drawing input current with less distortion and being able to operate at various frequencies. Ho wever, a great number of power semiconductor s switches are needed an d causes the overall system to be expensive as well as complicated [1] . There are three kinds of multilev el conv erter structure majorl y used in indus try: cascaded H - bridge converter with separate DC sources, diode clamped (neutral clamped) and flying c apacitors (capacit or clamped) [1] . In this paper, 3- leve l cascad ed H - bridge power converter will be used to design three - phase reconfigurable po wer converter topo logies, which provide s foun dation and inspiration for further reconfigurable top ologies us ing other kinds of multilevel power converter structures or power converter with high er l evel. 3. De sign of Reconfigurable Topologies 3.1 Explanation To build a reconfigura ble power converter topology, it is necessary to co mbine the two topolo gies – t hree - phase 3 - level ca scaded H - bridge converter and three - phase 2 - level bridge in verter. At first the tw o topologies need to b e simula ted and tested to ensure that they ar e function al . T hen an analysis will be performed to find the w ay combin ing them , and t he le ast amount of sw itches shall be used whil e all functions work na tural ly . 3.2 C onstruction of three - phase 3 - level cascaded H - bridge converter The three - phase 3 - level cascaded H - bridge converter is designed and test ed first, whose block diagram is shown in Fig.5, and the output w aveform in Fig.6. Fig.5 Bl ock diagram of the thre e - phase 3 - level cascad ed H - bridge converter Fig.6 Outpu t waveform of the three - phase 3 - level c ascaded H - bridge converter 3.3 C onstruction of three - phase 2 - level bri dge invert er Next, the th ree - phase 2 - leve l bridge in verter is de signed an d test ed , whose block diagram is shown in Fig.7, and the output waveform in Fig.8. Note that the state of the two ideal swit ch es are 0 , represe nting that they are open. Fig.7 Bl oc k diagram of the three - phase 2 - level brid ge inverter Fig.8 Outpu t waveform of the three - phase 2 - level bridge inverter 3.4 Combination of the two to polog ies into a reconfigurab le topology As both topolo gies (Fig.5, Fig.7) run well, they can be c om bine d to gether and proceed to the posterior process . The blo ck diagram of the overall reconfigur able topology is shown in Fig.9, which has output wavefo rm s as Fig.10 an d Fig.11. Here we use value 1e12 as the resistan ce of the ideal sw itches , and they are driven by 2 step signals with step time of 3s, in other word s, the top ology is tr ansforme d with a time interval of 3s . A lso each DC voltage sources is set to output 48V. Fig.9 Block diagram of reconfigurable topology 1 Fig.10 Out put waveform of three - phase 3 - level cascaded H - bridge inverter among reconfigurabl e topology 1 Fig.11 Out put waveform of three - phase 2 - level bridge inve rter amon g reconfig urable top ology 1 3.5 Extension of three - phase 2 - level bridge inverter Also th ere is an extensi on topolog y fro m th ree - phase 2 - level bridge inverter (Fig.7), which use H - bridge IGBTs in stead of separa ted p ulse generators . This to pology is de signed and tested, the blo ck diagram of which is shown in Fig.12 , and the ou tput wavefo rm in Fig.13. As shown in Fig.12, it is neater than befo re (Fig.9). Fig.12 Bl ock diagra m of the e xtension t hree - phase 2 - level bridg e inverter Fig.13 Out put waveform of the ext ension thr ee - phase 2 - level brid ge inverter 3.6 Another re configurable topology Anothe r proposed reconfigurable topology is co mposed by three - phase 3 - level H - bridge converter (Fig.5) and the ex tens e three - phase 2 - level bridg e inverter (F ig.12). The block diag ram of ov erall reconfigurable top ology is shown in F ig.14 and has output w aveform s as Fig.15 and Fig.16. Fig.14 Bl ock diagra m of reconfigurable topology 2 Fig.15 Outpu t wavef orm of three - phase 3 - level ca scaded H - bridge invert er among reconfi gurable topology 2 Fig.16 Outpu t wavef orm of extension three - phase 2 - level bridg e inverter am ong rec on figurable topolo gy 2 4. Features and A nalysis 4.1 The amount of used switches The tot al number of di fferent switch es are counted and shown in Tab .1. It is apparent that the total amounts of switches are about the same, while the second topology us es slight fewer switches. Reconfi gurabl e topology 1 Reconfi gurabl e topology 2 IGBT 24 24 3- way swit ch 6 12 Ideal switch 34 26 Total 64 62 Tab.1 Statis tics of s witches 4.2 Phase voltage difference range T he three - phase 3 - level casca ded H - br idge co nverters act ideally in both rec onfigurable to pologies as all the phase voltage are the same and equal to 96V which equals to the total voltage of DC voltage sources in se ries . However, as we can see in Fig.17 and Fig.18, the phase voltage of both 2 - level br idge inverter are dif ferent in amp litude, w hile that of reconfigu rable top ology 1 fluc tuate m ore acu tely as the maximu m differe nce rang e of reconfi gurab le topol ogy 1 is 3V compare d to 1V of reconf igura ble topology 2. Thus, rec onfigurable topology 2 has more st able output vol tage and works better . Fig.17 I llustrati on on phase voltage difference of 2 - level bridge inverter in r econfigur able topolo gy 1 Fig.18 I llustrati on on phase voltage difference of 2 - level bridge inverter in r econfigur able top ology 1 4.3 E ffect of ideal switches resistance Durin g experiments it is found that the re sistance of ideal switches inf luences the per formance of reconfigurable topo logies significantly. If the resistance of all ideal switches in reconf igurable to pology 1 are changed from 1e 12 to 1e9, we can get output waveform shown in Fig.19 and Fig.20 (comp ared to Fig.10 and Fig.11) . And reco nfigurab le to pology 2 resu lts in outputs as Fig.21 and Fig.22 (comp ared to Fig.15 an d Fig.16) . Fig.19 Out put waveform of three - phase 3 - level cascaded H - bridge converter among reconfigurabl e topology 1 with ideal swit ches with resistance 1e9 Fig.20 Out put waveform of three - phase 2 - level bridge inve rter amon g reconfig urable top ology 1 wit h ideal switches with resistance 1e9 Fig. 21 Output wavefor m of three - phase 3 - level cascaded H - bridge convert er among reconfigur able topology 2 with ideal swit ches with resistance 1e9 Fig.22 Out put waveform of three - phase 2 - level bridge inve rter amon g reconfig urable top ology 2 wit h ideal switches with resistance 1e9 As the ou tputs change ob vious ly duri ng the “Off” st ates a nd cannot act as ideal as befor e. It is apparent that the resistances of switch es have remarka ble influence on the output of th e overall circuit and cannot be neglec ted. As for the practical simulation time of 10s, reconfigurable t opology 1 takes about 42s while i t is around 33s for the second topology (simulated by a computer with 8 GB memory and Intel Core i5 processor ). 4.4 Fault on DC volt age sources B esides, if one of the upper D C voltage s ources is sho rted or opened, t he 2 - level bridge in verter topology can still work normally . For example, for the reconfigurabl e topology 1, if the upper DC voltage source of the secon d parallel line is op ened, w e can get voltage output waveform as shown in F ig.23 and Fig.24. If the DC voltage source is shorted, sim ilar vo ltage o utput waveform will be generated. And the same situation h appens to reconfigurable top ology 2. Fig.23 Out put waveform of three - phase 3 - le vel cascaded H - bridge converter among reconfigurable topology 1 with an opened DC voltage s ource Fig.24 Out put waveform of three - phase 2 - level bridge inve rter amon g reconfig urable top ology 1 wit h an opened DC voltage source Fig.23 ind icates that the phase voltage of the shorted/opened DC volt age source red uces to half of the other two normal phases voltage. And in Fig.24 the waveform is almost the same with all components work normally. T hus, if any DC voltage source breakdown , the reco nfigurab le powe r conv erter c an be transform ed to the sec ond topolo gy to minim ize nega tive effect on grid. 5. Conclus ion s and Future Work In this p aper, two kinds of reconfig urable power conv erter top ologies are designed, imp lemented and simulated with MATL AB/SIMU LINK . I t turned out that the y are functional to be utilized in reality. They can be ap plied to transform between three - phase 3 - leve l cascaded H - bridge converter and three - phase 2 - level b ridge in verter . This paper provides inspirat ion and innovat ion on the design of reconfigurable power electronic s topologies. Accord ing to t he result of simulation and testing , rec onfigura ble to pology 2 has a more stabl e and id eal appearance than re configurab le topolog y 1. As it is able to work normall y when fault s occur, such as a voltage source is shorte d or opene d . In the future work , the principal a nd na ture ab out the fault conditions a nd the furthe r meth ods to prevent and deal with them are to be researched. Also other further re configurable power electronics topologies can be d eveloped based on the ones implemented in this paper. Refere nce [1] S. Khomfoi and L. M. Tolbert, “Mult ilevel Power Convert ers,” in Power Elect ronics Handbook, M. H. Rashid, Ed. 2, Els evier Inc., 2007, Ch. 17, pp. 451 - 454. [2] Math Works In c., Si muli nk U ser’ s G uide , 2015. [3] S. Khomfoi and L. M. Tolbert, “Mult ilevel Power Convert ers,” in Power Elect ronics Handbook, M. H. Rashid, Ed. 2, Els evier Inc., 2007, Ch. 15, pp. 363 - 364 [4] R. H. Baker a nd L. H. Bannis ter, “Elect ric P ower Conve rter, ” U.S. Patent 3 867 643, Feb. 197 5. [5] J. S. Lai and F. Z. Peng, “Multilevel Converters – A New Bree d of Powe r Con verte rs” IEEE Transact ions on Industry Applicat ions. v ol. 32, no. 3, May/Jun. 1996, pp. 509 - 513