2011 IEEE. Reprinted, with permission, from David Dorrell, Design and comparison of 11 kv multilevel voltage source converters for local grid based

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1 2 IEEE. Reprinted, with permission, from David Dorrell, Design and omparison of k multilevel voltage soure onverters for loal grid based renewable energy systems. IECON 2-37th Annual Conferene on IEEE Industrial Eletronis Soiety, November 2. This material is posted here with permission of the IEEE. Suh permission of the IEEE does not in any way imply IEEE endorsement of any of the University of Tehnology, Sydney's produts or servies. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for reating new olletive works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions@ieee.org. By hoosing to view this doument, you agree to all provisions of the opyright laws proteting it

2 Design and Comparison of k Multilevel SCs for Loal Grid Based Renewable Energy Systems Md. Rabiul Islam, Youguang Guo, Jian Guo Zhu, and David Dorrell Shool of Eletrial, Mehanial and Mehatroni Systems, University of Tehnology Sydney, Sydney, Australia md.islam@uts.edu.au, rabiulbd@hotmail.om Abstrat- Beause the availability of renewable energy is highly variable and the power demand by the onsumers ould have a very different harateristi, it is very desirable to operate a renewable generation system with grid interfaing. In this respet, the k multilevel oltage Soure Converter (SC) is the transformer less, ost effetive solution to interfae the renewable generation system to the loal grid diretly. This paper presents the design and omparison of a Five-Level Neutral Point Clamped (5L-NPC), a Five-Level Flying Capaitor (5L-FC), a Five-Level Series Conneted H-Bridge (5L-SCHB), an Eleven-Level Neutral Point Clamped (L- NPC), an Eleven-Level Flying Capaitor (L-FC), and an Eleven-Level Series Conneted H-Bridge (5L-SCHB) SC for an k loal grid based onverter. The ost of power semiondutors and apaitors, modulation shemes and harmoni spetra of the onverters are the bases for omparison. Keywords Renewable energy, k multilevel onverter, transformer less diret onnetion, MATLAB simulation. I. INTRODUCTION The availability of renewable energy soures has strong daily and seasonal patterns and the power demand by the onsumers ould have a very different harateristi. Therefore, it is diffiult to operate a power system installed with only one type of renewable energy resoure. The loal grid based renewable generation is the only solution to overome this problem but due to the variable nature of renewable energy soures, output voltage and frequeny adjustments are the hallenging issues to onnet these systems to power grids. Different power eletroni onverters have been developed using onventional topologies to fulfill the requirements of renewable generations. In order to step up the onverter output voltage to grid level, an additional power transformer is required to interonnet the renewable generation with the grid []. The additional bulky and weighty transformer again inreases the system size, weight, ost and power loss. An isolation transformer may represent 3%-5% of the total system size and 5%-7% of the system weight [2]. An -MA transformer generates a large amount of heat energy (up to 68 Btu/h) and requires a signifiant amount of air onditioning. Thus, the transformer less solution would result in large amount of energy saving. In order to stabilize the system operation, the harmoni ontrol is also important. To mitigate this harmoni effet it is essential to use a filter oil, whih also inreases the system omplexity and ost. Output voltage waveforms of the onverter ould be improved by inreasing the level of the onverter, whih may redue the size of the input and output filter requirements. Output voltage Frequeny Spetrum (FS) of two level and eleven level onverter is shown in Fig.. (a) (b) Fig.. Frequeny spetrum of (a) two level (b) eleven level SCs On the other hand, a ontinuous rae to develop higher voltage and higher urrent power semiondutors to drive high power systems still goes on. In this way, last generation devies are suitable to support medium voltages but high voltage semiondutor is still under development [3]. The prie of power semiondutor devies inreases rapidly with their power ratings as shown in Fig. 2 [4]. The series-parallel onnetion of lower rated semiondutors ould be the ost effetive solution for high voltage appliations. So, it is required and not diffiult to develop an k multilevel voltage soure onverter, whih an be able to onnet the renewable generation system to the loal grid without introduing a power transformer. Fig. 3 shows the basi blok diagram of the proposed onverters. This paper ompares a 5L-NPC onverter, a 5L-FC onverter a 5L- SCHB onverter, an L-NPC onverter, an L-FC onverter and an L-SCHB onverter for an k loal grid based renewable generation systems. The ost of power

3 semiondutors and passive omponents is alulated and ompared for all available multilevel onverter topologies. Level Shifted (LS) Sine-Pulse Width Modulation (SPWM) and Phase Shifted (PS) SPWM are used to ompare the performanes. Renewable Energy Conversions (a) (b) Fig. 2. Prie of IGBT (AU$) when (a) rated urrent is.4 A (b) rated voltage is.7 K Renewable Energy Conversions Booster/ Retifier MPPT Booster / Retifier MPPT DC link DC link Inf. 5L/L NPC/FC Controller (a) 5L/L SCHB Controller Filter Grid Informations LC filter Grid Informations k Loal Grid (b) Fig. 3. Basi blok diagram of loal grid based diretly onneted (a) 5L/L-NPC/FC SC (b) 5L/L-SCHB SC k Loal Grid II. CONERTER DESIGN Design onditions identifiation is the vital part of design proess. The onditions and basi onverter data are shown in Table I. The minimum DC-link voltage to ahieve an output line-to-line voltage of k an be alulated by d(min) = 2 ll ( rms) = 2 k = 5, () To determine the nominal DC-link voltage of the onverter, a voltage reserve of 4% is assumed, i.e. d( nom) =. 4 d,min = = 6,78. 7 (2) The apparent onverter output power an be alulated by S = I 3 ll( rms) p( rms) = 3 k 25A = 4. 76MA (3) TABLE I BASIC CONERTER DATA Tehnial Data Abbreviations alue Converter line-to-line voltage ll(rms) k Minimum DC-link voltage d(min) 5,556.4 Nominal DC-link voltage d(nom) 6,78.7 Phase urrent I p(rms) 25 A Apparent onverter output power S 4.76 MA Converter arrier frequeny f -2 khz Output frequeny f o 5 Hz Maximum juntion temperature (IGBT, diode) Heat sink temperature T 25 o C j(max) h 8 C Converter effiieny η 98% The semiondutor utilization is a very important part to evaluate high-voltage topologies due to the high share of semiondutor osts. Considering the nominal DC-link voltage and osmi ray effet the IGBTs and diodes voltage rating is hosen. Aording to the output apaity and voltage ratings of the onverters the availability of IGBT and diode modules in the market is also onsidered in design proess. Two 4.5 k series onneted IGBTs are onsidered in plae of single swith for all 5L onverter topologies. Table II summarizes the design of the power semiondutors for the onverter speifiation of Table I, with a arrier frequeny of khz. To enable a onverter output phase urrent of 25 A, the 4 A urrent rating is hosen for the power semiondutors. To ompute the apaity of flying apaitor, a maximum apaitor voltage ripple ( ) is assumed to be 5% of the DC-link voltage and the DC output urrent ( I ) is d T

4 approximated to the maximum amplitude of the phase urrent I ) [5], i.e. ( p(rms) C Iˆ = (4) ( n f p( rms) where n is the number of series-onneted flying apaitor ells. A large number of different modulation shemes have been adapted or developed depending on the appliation and the onverter topology, and eah has its unique advantages and disadvantages. The most ommon modulation method in ) industry is the arrier-based sine-triangle modulation. The LS-SPWM method is espeially useful for Neutral Point Clamped (NPC) onverters, sine eah arrier an be easily assoiated to two power swithes of the onverter and the PS- SPWM method is espeially useful for Flying Capaitor (FC) and Series Casaded H Bridge (SCHB) onverters. In this paper an LS-SPWM sheme is used for NPC topologies and a PS-SPWM sheme is used for FC and SCHB topologies to ompare the onverter performanes. TABLE II POWER SEMICONDUCTOR RATING 5L-NPC 5L-FC 5L-HB L-NPC L-FC L-HB d(nom) 6,78.7 6,78.7 6,78.7 6,78.7 6,78.7 6,78.7 Rated devie voltage (IGBT) 2x4.5k 2x4.5k 2x4.5k 3.3k 3.3k 3.3k Commutation voltage of respetive ommutation ells, om 2, , , The devie ommutation voltage for a devie reliability of 2x225 FIT due to osmi radiation, FIT 2x225 2x Devie voltage utilization fator, om / FIT III. SIMULATION RESULTS The L NPC and SCHB onverters output voltage and output urrent (without and with the filter) are shown in Figs. 4- respetively. In order to measure the harmoni ontent of the output urrent and the harmoni losses in the load the harmoni spetrum of the line voltage is evaluated. Amplitude (olts) Current (Amp) 2 x Fig. 4. Simulated line voltage (without filter) of L-NPC SC Fig. 5. Simulated line urrent (without filter) of L-NPC SC The harmoni ontent of the output voltage (without filter) of 5L and L NPC, FC and SCHB onverters are shown in Figs. 2-7 respetively. Due to page limitation only a few of results are presented in this paper. Amplitude (olts) Amplitude (Amp) 2 x Fig. 6. Simulated line voltage (with filter) of L-NPC SC Fig. 7. Simulated line urrent (with filter) of L-NPC SC

5 oltage (olts) Current (A) - x 4 Fig. 8 Simulated line voltage (without filter) of L-SCHB SC Fig. 9 Simulated line urrent (without filter) of L-SCHB SC 2 x 4 Fig. 2 Harmoni spetrum of line voltage of 5L-NPC SC Fig. 3 Harmoni spetrum of line voltage of 5L-FC SC Amplitude (olts) Fig. Simulated line voltage (with filter) of L-SCHB SC Fig. 4 Harmoni spetrum of line voltage of 5L-SCHB SC Amplitude (Amp) Fig. Simulated line urrent (with filter) of L-SCHB SC Fig. 5 Harmoni spetrum of line voltage of L-NPC SC

6 Fig. 6 Harmoni spetrum of line voltage of L-FC SC Fig. 7 Harmoni spetrum of line voltage of L-SCHB SC I. CONERTER COMPARISON In NPC onverter topology eah ative swithing devie is only required to blok a voltage level of d ( nom ) /( m ) but the lamping diodes must have different voltage ratings for reverse voltage bloking. If it is assumed that eah bloking diode voltage rating is the same as the ative devie voltage rating, the number of diodes required for eah phase will be (m-) (m-2). When m is suffiiently high, the number of diodes required will make the system impratial to implement. A total of 9 numbers of diodes is required for eah phase of L onverter. This large number of diodes affets the reverse reovery of the lamping diodes whih is the major design hallenge in high-voltage high-power systems. A list of number of power omponents for eah onverter topologies is shown in Table III. Aording to the output apaity and voltage ratings of the onverters the availability of IGBT and diode modules in the market is also onsidered in design proess. For 5L NPC/FC inverter topologies eah IGBT swith is implemented by series onnetion of 6.5k and.7 k IGBTs so the number of IGBTs is To enable a onverter output phase urrent of 25 A, the simulation result is used to determine urrent rating of the power semiondutors. In booster setion eah IGBT is implemented by parallel onnetion of two series onneted 6.5k and.7 k IGBTs. Similar to NPC topology, the FC topology requires a large number of bulk apaitors to lamp the voltage. Provided that the voltage rating of eah apaitor used is the same as that of the main power swith, an m-level onverter will require a total of (m- ) (m-2)/2 lamping apaitors per phase. A total of 45 numbers of lamping apaitor is required for eah phase of L onverter. These large numbers of bulky and heavy apaitors inrease the onverter size and ost and redue the overall lifetime of the onverter. Moreover the apaitor voltage balaning problem beomes hallenging issue with high level numbers. There are no bloking diodes or lamping apaitors in SCHB topology. The omponent number of this topology sales linearly with the number of levels. So, overall number of total omponents is muh lower than that with other topologies. The individual modules are similar and totally modular in onstrution, whih makes it easy to implement for any levels. The higher number of levels attainability provides more sope for reduing harmonis. The high number of levels means that it is possible to onnet the onverter to the AC network diretly. To evaluate the harmoni spetrum of the line to line voltage the total harmoni distortion (THD) is onsidered. The THD an be omputed by THD 2 ll, n n= 2 (%) = ll, Table I shows the THD of different multilevel onverter topologies. Among these three onverter topologies the NPC onverter topology has the best harmoni performane. The harmoni performane of SCHB topology is not as good as that of NPC onverter topology. The harmoni ontent dereases rapidly with inreasing number of levels and as well as redues the size of the LC filter. This means that by inreasing the levels of the onverter it is possible to keep the output voltage total harmoni distortion of less than or equal to 5% (aording to the standard IEEE ). The SCHB onverter is more eonomial than the others. The L SCHB onverter is the low ost high performane onverter for k loal grid based diretly onneted renewable generation systems. The prie data of semiondutor devies and apaitors were olleted from the Galo Industrial Eletronis and Farnell atalogue [4, 6], where devies were hosen from the same family where possible to fit with requirements. But the filter indutor and booster indutor need to be ustomized. Therefore, their osts depend on the orresponding design. Due to large urrent and high frequeny, the amorphous metal is hosen for the magneti ore and the opper tube is used for the windings of the indutors. The material osts of the indutor an thus be estimated. The IGBTs where hosen with integral freewheel diodes and hene these diodes did not appear in onsiderations. The urrent rating of most of devies is seleted on the basis of simulation results. Table shows the estimated ost of different onverter topologies. The number of semiondutor inreases with the number of levels but the hange of ost is small beause prie of lower rated devie is omparatively lower. Due to lower voltage and urrent requirements the total semiondutor ost of L-SCHB onverter is lower than all other topologies. (5)

7 TABLE III NUMBER OF POWER COMPONENTS 5L-NPC 5L-FC 5L-HB L-NPC L-FC L-HB Number of IGBTs Inverter setion Number of diodes (NPC) Number of flying apaitors Sub total Number of indutors Number of IGBTs 2(+) 4 2(+) Booster setion Number of diodes Number of DC link apaitors Sub total Number of diodes Retifier setion Number of filter apaitors Sub total All setions Total omponent ount Many publiations have addressed the limitation of SCHB as the requirements of multiple isolated DC soures, and therefore, its appliation is not straightforward. Multiple independent generator stator windings and multistring photovoltai onfiguration ould be the possible solution to overome the above limitation. Cost of semiondutors Costs of passive omponents TABLE I THD OF THE LINE TO LINE OLTAGE Converter Topology % of THD 5L-NPC L-FC 7.8 5L-HB 8.3 L-NPC 7.7 L-FC 7.28 L-HB 8. Although the harmoni performane of NPC onverter is better than the others, the NPC and FC onverters have the disadvantages that the number of omponents sales quadratially with respet to the number of output levels. Moreover in FC onverters the apaitor voltage balaning problems beome hallenging issue with high level numbers. This means that NPC and FC topologies are not feasible for high level onverters. Aording to onverter ost, omplexity and performane the SCHB topology is feasible for high level onverters. The high number of levels means that it is possible to onnet the onverter to the loal AC network diretly. This diret onnetion means elimination of heavy, bulky, lossy and ostly transformer from the system. Transformer less operation will improve the performane of existing renewable generation system. The improved performane should lead to signifiant ost savings in the long run.. CONCLUSIONS TABLE ESTIMATED COST FOR POWER COMPONENTS 5L-NPC 5L-FC 5L-HB L-NPC L-FC L-HB Costs of IGBTs (inverter) 82,27 82,27 82,27 82,59 82,59 82,59 Costs of diodes (NPC) 8, , Costs of IGBTs (retifier) 35,38 35,38 4,4 27,386 27,386 2,75 Costs of Diodes (retifier) 2,99 2,99 4, ,83 Total ost of semiondutors 29,334 2,399 27,527 44,98,44 97,72 Material osts of LC filter 5,5 7, 2,5,4, 2,3 Costs of flying apaitors , , Costs of DC apaitors 6,72 6,72 9,288 4,8 4,8 2,48 Costs of boost indutors 3,3 3,3 25,2 2,4 2,4 8,9 Total osts of passive omponents 5,972 9,872 54,988 35,6 76,7 43,68 Total Total osts ($) 8,36 2,27 82,55 8,58 98,4 4,392 REFERENCES [] S. Kouro, M. Malinowski, K. Gopakumar and et al., Reent advanes and industrial appliations of multilevel onverters, IEEE Transations on Industrial Eletronis, vol. 57, no. 8, pp , 2. [2] H. Abu-Rub, J. Holtz, J. Rodriguez and G. Baoming, Medium voltage multilevel onverters-state of the art, hallenges, and requirements in industrial appliations, IEEE Transations on Industrial Eletronis, vol. 57, no. 8, pp , 2. [3] J. Rodriguez, L. G. Franquelo, J. I. Leon and et al, Multilevel onverters: an enabling tehnology for high-power appliations, Pro. IEEE, vol. 97, no., pp , 29. [4] [Online].Available at: [5] D. Krug, S. Bernet, S. S. Fazel, and et al., Comparison of 2.3 k medium voltage multilevel onverters for industrial medium voltage drives, IEEE Transations on Industrial Eletronis, vol. 54, no. 6, pp , 27. [6] [Online]. Available at: