cascade multilevel inverter with three phase transformers

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Published in IET Power Electronics

Received on 27th May 2011

Revised on 8th July 2012

doi: 10.1049/iet-pel.2011.0270

ISSN 1755-4535

Performance of cascaded multilevel inverter byemploying single and three-phase transformersA.K. Panda Y. Suresh

Department of Electrical Engineering, NIT Rourkela, India

E-mail: [email protected]

Abstract:Among the mature multilevel converter topologies, cascaded multilevel H-bridge inverter is promising one which is an

alternative for grid-connected photovoltaic/wind-power generator, flexible alternating current systems and motor driveapplications. In this study, a cascaded multilevel inverter with low-frequency three-phase transformers and a single dc power

source is proposed. This new topology aims to reduce the number of components and so reduce the complexity of the circuit.As three-phase transformer is employed harmonic components of the output voltage and switching losses can be diminishedconsiderably. To evaluate its relative performance it is compared with the conventional converters. Computer aidedsimulations are performed through Matlab/Simulink and simulation results are presented to verify the performance ofproposed cascaded multilevel inverter. Further it is validated with prototype experiments.

1 Introduction

In recent years, multilevel inverters have presented an

important development to reach high power with increasingvoltage levels. The research on multilevel inverter has beenreceiving wide attentions, and becoming hot point in theresearch on electrical and electronics. Multilevel converterpresent great advantages compared with conventional twolevel converters like; high-power quality of waveforms, lowswitching losses, high-voltage capability, lowelectromagnetic compatibility and so on [15]. Multilevelconverter can be divided into four remarkable topologies:diode-clamped multilevel converter (DCMC), flyingcapacitors multilevel converter (FCMC), P2 multilevelconverter (P2MC) and cascaded multilevel converter (CMC)with separate dc source [68]. These entire converters arecompared in terms of feasibility of their utilisation and itsapplications [9]. According to the MIL-HDBK-217Fstandard, the reliability of a system is indirectly proportionalto the number of its components [10, 11], so less thecomponents more the reliability. Compared with m-levelDCMC, FCMC and P2MC, which use m2 1 capacitors onthe dc bus, the CMC uses only (m2 1)/2 capacitors forsame m-level. Clamping diodes are not required for FCMC,P2MC and CMC. In overall P2MC undeniably requires toomany components as compared with other multilevelconverters; so it is not suitable for higher voltage levels.However, on comparing CMC with DCMC, FCMC andP2MC, it requires least number of components and itsdominant advantage is circuit layout with flexibility.

According to recent survey CHBMLI are extensively usedin compressors (82%), synchronous motors (92%), converters(98%) and power generation plants (47%), in addition it isbest suited for the power quality devices, such as static

synchronous compensators (STATCOMs) and universalpower quality conditioners [12, 13]. Although this invertertopology is more preferable still there are some aspects that

require further development and research. The primary issuethat strikes about conventional CHBMLI is that, it usesseparate dc source for each H-bridge or it utilises single dcsource by employing single-phase transformers, this notonly yield significant cost but also drastically effectsefficiency and reliability[2, 3, 14]of a converter. This issuebecomes the key motivation for this paper. In the presentpaper a CMC has been proposed, which employ single dcsource with isolated three-phase low-frequencytransformers. The proposed configuration employs reducenumber of transformers as compared with traditional three-phase multilevel inverters. In fact, topology has interestingfeatures like, less component count, twice output levels,improved output voltage waveforms, less switching losses,elimination of third and its multiple harmonic componentsand finally extremely flexible. Indeed one of the importantfindings of proposed archetype is converter able to produce13 level output voltages by using just nine H-bridge cells.Thus, converter not only enhances the output voltagequality but also improves the reliability of the system.Further, to verify the output voltage quality, necessaryharmonic verifications are presented for differentmodulation indexes and in depth assessment of topology isexplained in subsequent sections.

Rest of the paper is organised as follows: Section 2provides the details of the switching techniques. Section 3presents the details of conventional cascaded archetype with

single phase transformers and its working principle. Section4 demonstrates the proposed circuit topology with three-phase transformers and its working principles. Section 5provides the details of simulation and experimental

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verification of conventional and proposed approaches andtheir comparisons. Finally, conclusions are given in Section 6.

2 Switching techniques

Several modulation techniques have been proposed forcascaded multilevel inverters. A high number of powerelectronic devices and switching redundancies bring a

higher level of complexity compared with a two-levelinverter counterpart. However, this complexity could beused to add additional capabilities to the modulationtechnique, namely, reducing the switching frequency,minimising the common-mode voltage, or balancing the dcvoltages [15, 16]. Recurrently numerous pulse widthmodulation (PWM) techniques are evolved and applied tocascaded multilevel inverters and some of the prominenttechniques are: (i) sinusoidal PWM (SPWM), (ii) selectiveharmonic elimination, (iii) hybrid modulation, (iv) nearestlevel control and (v) space vector PWM (SVM) [23]. To bespecific, sinusoidal carrier-based PWM approaches are quitegood to handle and Peng et al. [17] had reported aboutmulticarrier-based PWM techniques. According to literaturesurvey in multicarrier approaches two major carrier-basedPWM techniques are prominently used. Namely, phase-shifted PWM and level-shifted PWM techniques. However,an in-depth assessment between PWM methods can befound in [18, 19]. In brief, rather than level-shifted PWM,phase-shifted PWM technique had finite merits such as, norotation in switching, less switching losses and easy toimplement. Indeed, in the present paper all productivetopologies are implemented with sinusoidal PWMapproach. Next sections provide the details of conventionaltopologies and their operating principles and finally figureout the merits and demerits.

3 Conventional cascaded H-bridge multileveltopologies

3.1 Cascaded multilevel inverter with separatedc source

Architecture shown in Fig. 1 is a series H-bridge inverterappeared in 1975[20], but several recent patents have beenobtained for this topology as well. Since this topologyconsist of series power conversion cells, the voltage andpower level may be easily scaled. Numerous advantageshave been figured out using this topology, which areextensively used in medium- and high-power applications.Fig. 1b provides output characteristics of seven-levelH-bridge cell. Examining Fig. 1b, the output phase voltagecan be expressed as V v1+ v2+ v3, this is because all theinverters are connected in series. Each single-phase full-bridge inverter can generate three-level outputs; Vdc, 0 andVdcand this is made possible by connecting the dc sourcessequentially to the ac side via the four switching devices.Minimum harmonic distortion can be obtained bycontrolling the conducting angles at different inverter levels.

However, this class of converter avoid extra clampingdiodes or voltage balancing capacitors. Each separate dcsource is associated with single balancing capacitors andwith single phase H-bridge converter. All such threeinverters together forms cascaded multilevel inverter.

Fig. 1b shows seven-level cascaded H-bridge multilevelphase voltage waveform. Phase output voltage (V0) is thesum of all three inverter outputs [21]. The primarydrawback of the H-bridge inverter is that, there is a

possibility of short circuit of the input dc voltage sources.On the other hand, Fig. 2 demonstrates the cascadedmultilevel inverter with single-phase transformers by usingsingle dc source. At present scenario, these archetypes arehighly preferred because they can be easily coupled tohigh-power transmission systems. Further, because of thetransformers on the secondary high-quality waveforms areobtained. Although, this class of converters are highly

preferred, still there are some aspects to be considered, thatis, it uses single-phase transformers on the secondary sideand as level increases transformer count drastically increase,which further effects the reliability of the system. On theother hand, we suggest a topology with three-phasetransformers. Adopting such prospects in the system resultsin improving output voltage quality and consistency. Indepth assessment of topology is presented in the next section.

4 Proposed cascaded H-bridge multilevelinverter

The proposed configuration is demonstrated in Fig. 3. It uses

single dc source and three-phase low-frequency transformersand the main attributes of proposed approach is; it can beeasily connected in series to handle medium voltage levels(6.6 kV) in distributed generation, based on renewableenergy and fuel cells [22]. Coming to structure point ofview, each primary terminal of the transformer is connectedto an H-bridge module so as to synthesise output voltagesof +Vdc, Zero, 2Vdc. Every secondary of transformer isconnected in series to pile of output level up. Further, eachphase terminal is delta connected to restrain the thirdharmonic component. Fig. 3 illustrates, primary of eachphase is three-phase and secondary is single phase terminal.All three terminals are series connected to generate phasevoltage.

Therefore each phase can be expressed independently. As aresult each phase multilevel inverter can be depicted as anisolated H-bridge cascaded multilevel inverter. We canobtain the relation between input and output voltages ofthree-phase transformer as

[VAk;VBk; VCk] =N

3

2 1 1

1 2 1

1 1 2

VakVbk

Vck

(1)

Vak, Vbk and Vck are primary terminal voltages of phase a,phase b and phase c, respectively, and herein, VAk, VBkandVCk define summation of three primary voltages. T is the

transformation ratio and defined as

T=

2/3 1/3 1

1/3 2/3 1/3

1/3 1/3 2/3

(2)

where Nis the transformation ratio (n2/n1) between primaryand secondary. If there is a balanced input, then sum of theeach phase voltage would become zero.

5 Switching patterns and outputcharacteristics

5.1 Switching and phase shifting

For multicell switching, phase shifting is important criterion.Present cascaded multilevel inverter utilises nine H-bridge

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cells and for each phase, it uses three cells. Thus six carriersare chosen with appropriate phase shift. In general, a multicellconverter requires (m2 1) triangle carriers. However, in thepresent case (m2 1)/2 carriers are required, which signifies

that carrier count is drastically reduced. For example forthirteen level output waveform (13-1) carriers are needed inconventional structures, where as it is half in present case,that is, (m2 1)/2. On the other hand triangle carriers have

same frequency and same peak to peak amplitude but thenphase difference between two adjacent carriers are given by

wcr= 360/(m 1)/2 (3)

By using above equation appropriate phase shift can beincluded.

Fig. 2 Details of cascaded multilevel inverter with single dc source by employing single-phase transformers

Fig. 1 Series H-bridge inverter

a Conventional cascaded H-bridge multilevel inverter (seven-level)b Operational waveforms for seven-level inverter



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Fig. 3 Details of cascaded-inverters with single dc source by employing three-phase transformers

Fig. 4 Details of single H-bridge operation and unipolar switching criteria

a Details of single H-bridge cellb Unipolar modulation of one arm of H-bridge of VSC

Fig. 5 Switching pattern of phase each output voltage of three-phase transformer

a Case 1: 0 ak p/6, p/6b Case 2: p/6 ak p/3c Case 3: p/3 ak p/2



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5.2 Unipolar switching

Unipolar switching scheme is considered for generatingpulses. Figs. 4a and b provides details of single H-bridgeoperation and unipolar switching criteria. Herein, the 13-level voltage source modulation is accomplished bycomparing the duty cycles with a set of carrier waveforms.The switching function Vsin is compared with triangular

carrierVtri of frequency fs and with definite amplitude.Switching functionVsinis modulated with carrier followingthe principle of unipolar PWM, that is,

Condition: 1.1 Vsin . Vtri. Then SW1 is on and resultantvoltageVA0 +1/2Vdc.

Condition: 1.2 Vsin , Vtri. Then SW4 is on and resultantvoltageVA0 21/2Vdc.

In similar fashion for other phase leg of the H-bridge,

Condition: 2.1 Vsin , Vtri. Then SW3 is on and resultantvoltageVBO +1/2Vdc.Condition: 2.2 Vsin . Vtri. Then SW2 is on and resultant

voltageVBO 21/2Vdc.

Therefore net voltage levels obtained for one H-bridge isVdc, 0, 2Vdc. The process is repeated for other H-bridgeswith a carrier phase shifted by the corresponding angles.Sum of all these three voltages results in producingresultant output waveform.

Fig. 6 Simulation results and verification

a Output voltage waveform for conventional cascaded multilevel inverter with nine H-bridge cells (at modulation index: 1.0, 0.85, 0.5 and 0.1 from top to bottom)b Output voltage waveform for proposed cascaded multilevel inverter with nine H-bridge cells (at modulation index: 1.0, 0.85, 0.5 and 0.1 from top to bottom)



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5.3 Output voltage characteristics

Output voltage of the three-phase transformer will bedetermined by combination of A, B and C voltages andthere are three possibilities in output voltage of transformer.Switching pattern of each phase and output voltage of eachtransformer is shown in Fig. 5. Output voltage of proposedinverter is sum of secondary terminal voltages oftransformer which are connected in series and all these areindependent of switching range from 0 , ak, p/2.

FromFig. 5, output voltage of a phase (VA) is symmetricalin nature. Fourier expression can be written as

VA1 =1n=1

bnksin(nu) (4)

wherebnkis a constant.

Output characteristics of phase A voltage can beobtained between three switching ranges, that is, 0 akp/6, p/6 ak p/3, p/3 ak p/2. These are showninFig. 5.

For case-1, that is, 0 ak p/6

bnk=4Vdcp

p/3akak

sin(nu) du+3

3

p/3+akp/3+ak

sin(nu) du

+

4

3p/2p/2+ak sin(nu) du

that is, bnk=

4Vdcp

cos(na) (5)

Fig. 6 Continued



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For case-2, that is, p/6 ak p/3, p/3;

bnk=4Vdcp

akp/3ak

sin(nu) du+3

3

2p/3akak

sin(nu) du

+2

3 p/2

2p/3ak

sin(nu) du

that is,bnk=

4Vdcp

cos(na)

(6)

For case-3, that is, p/3 ak p/2

bnk=4Vdcp

2p/3akakp/3

sin(nu) du

+

2

3

p/2ak

sin(nu) du

bnk=4Vdcp

cos(na)

(7)

From (5), (6) and (7), Fourier progression is same. In a similar

fashion, Fourier transform for primary voltages of transformerVak, VbkandVckare given as

Vak=1n=1

bnksin(nu)

Vbk=1n=1

bnksin nu2np

3

Vck=1n=1

bnksin nu+2np

3

which are 1208

apart from each phase and coefficients ofbnkandVakare half-wave symmetries; hence forth odd functioncan be written as

bnk=4Vdcp

cos(na) (8)

Using (1), output phase voltage ofVAKcan be expressed as

VAK=1n=1

bnksin(nu) 1/3

1

n=1

bnk sin(nu) + 2 sin(nu) cos 2np

3 (9)

In above equation if n 3, 9, . . . , 3 (p2 2), 3p, thenequation becomes zero.

that is, VAk= 0 (10)

This represents, all triplen harmonic component does notappear in the three-phase output voltage.

Ifn 1, 5, 7, 11, . . . , p, then equation becomes

VAK=1

n=1

bnsin(nu)

Thus, least harmonic in output waveform starts from 5, 7,11, . . ., p 2 2, p.

On the other hand in conventional converter, similarcalculation follows. However, from (1), we can concludethat transformation ratio is quite different and multiplyingsuch transformation in (9) will result as

VAK=

1

n=1

bnksin(nu)

1n=1

bnk sin(nu) + 2 sin(nu)cos 2np

3

(11)

From the above equation if n 1, 2, 3 and 4, . . . , n, theequation becomes

VAK=1n=1

bnsin(nu) (12)

Thus, it signifies presence of all harmonic components inphase output voltage.

6 Simulation and experimental verification

6.1 Simulation verification

In order to verify the proposed structure we performedextensive simulation by using nine H-bridge cells, withdifferent modulation indexes varying from 0.1 to 1 andsimulations were performed using Matlab/Simulink.However, for effective comparison conventional architectureperformances are also incorporated. Fig. 6a providessimulation results for the conventional topology at themodulation indexes 1, 0.8, 0.4 and 0.1, using single phasetransformers. Certainly because of three bridges and single-

phase transformers seven-level performance is observed atthe modulation index 1 and its total harmonic distortion(THD) is about 12.58%. Further, as modulation index isreduced THD keep on increasing and at 0.1 modulationindex, THD is about 50%. In fact, THD spectrumsignificantly specifies presence of lower order harmonicsand it is notable that for all modulation indexes third and itsmultiple harmonics are present in the output voltage.Fig. 6b provides the detail simulation verifications for theproposed architecture. As of before, simulation were carriedout for modulation indexes 1, 0.8, 0.4 and 0.1. Atmodulation index 1 output voltage waveform constitute 13levels and this defines the inherent potential of the proposedconverter. With the same number of H-bridge cells theconverter is able to produce twice output voltage levels,which is a very important finding. Further, THD of outputvoltage is about 6 and 33.18% at modulation indexes 1 and0.1, respectively, which is a huge marginal gain in THD.Moreover, it is evident that for all modulation indexes thirdand its multiple harmonics are completely eliminatedbecause of delta connected transformers on the secondaryside. Thus, power quality has been improved predominantlywhen compared with conventional one.

The 3 KVA prototype model of the proposed andconventional converter is developed and verified in thelaboratory. The experiment was carried out for a starconnected load. The details of prototype are given in

Table 1and for the experimentation field programming gatearray (FPGA)-based module was utilised. An analogueexpansion daughter board interfaced between the FPGAmodule and insulated gate bipolar transistor inverter.



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Proposed circuit topology and control strategy was realisedwith code composer. Fig. 7 indicates the construction ofcascaded multilevel inverter using three-phase transformers.The measured quantities are the load current, load voltages,which are measured with Hall-effect current and voltagetransducers. The transformer ratios are scaled in power of1. Input dc voltage is taken as 60 V. The load parametersare R 400 V, L 1000 mH per phase. Fig. 8 highlightsthe performance of conventional cascaded multilevelinverter and provides experimental output voltages and FFTresult of the phase A, at modulation indexes 1, 0.85, 0.5and 0.1, respectively. At modulation index 1 correspondingFFT reports presence of lower-order harmonics and leastharmonic component found to be 13th, rest all theharmonics are suppressed and as modulation index reduces,all odd harmonic components are noted. However, for allmodulation indexes third and its multiples harmonics arenoted. Further THD and DF for various modulation indexesare demonstrated in Fig. 10a. Fig. 9 highlights theperformance of the proposed cascaded multilevel inverterand provides experimental output voltages and FFT resultof the phase a, at modulation indexes 1, 0.85, 0.5 and 0.1,respectively. At modulation index 1 corresponding FFT

reports complete elimination of lower-order harmonics and

least harmonic component is 25th, rest all the harmonics aresuppressed and as modulation index reduces, all oddharmonic components are noted. Interesting features ofproposed converter is that, it produces 13 level outputwaveform with nine bridge cells. This defines the inherentpotential of a converter. In fact, converter produces almostsinusoidal waveforms with improved THD. In Fig. 10bTHD and DF variations of output voltages based on

different modulation indexes are shown. It is observed thatTHD of voltage harmonics gradually increases withdecrease in modulation index and at modulation index 1 itsTHD is 6% and at 0.1 it is about 33%. However, for all themodulation indexes it is evident that third harmoniccomponent is completely absent. The next section presentsthe important features of the proposed converter.

Investigating, the proposed architecture uses (m2 1)/4transformers for same output levels. Where as inconventional it is about (m 2 1)/2 3. Further it is foundthat the conventional cascaded multilevel inverter needs(m2 1) 2 3 number of switches, but on other side theproposed configuration requires (m2 1) 3 switchingdevices to generate the same output voltage level. Thecomponent comparison is presented in Table 2. To makethe matter lucid, for generating 13 output voltage levelsconventional arrangement required 18 single phasetransformers. However, in the proposed configurations ituses just three three-phase transformers. Further when wecompare components, convetnional cascaded multilevelinverter requires 72 switches. However, proposedconfiguration requires only 36 switches, signifies switchcount is considerably reduced, thus size and cost ofequipment comes down. Thus conventional convertersrequire too many components to generate same level ofoutput voltage. Besides, the conventional cascadedmultilevel inverters use a circulating switch pattern in order

to maintain the same ratio in switch utilisation. Therefore

Fig. 7 Details of prototype set-up for proposed CMC with three-phase transformers

Table 1 Hardware specifications

Items Specifications and features

switching devices FGH20N60UFD 600 V, 20 A fair child

semi-conductors

transformers El lamination (3 EA) 1:1 ratio

input voltage 1-single DC, 60 V

output voltage 13-level, 220 V

FPGA Xilinx Spartan3 device generatePWM signals



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they employ switches which are identical in the voltage andcurrent ratings. Assuming that the magnitude of the outputvoltages and output power are equivalent, the voltageratings of each switch are determined by the number of

series-connected switches. Consequently, we can say thatthe proposed method is more advantageous in switch costand system size compared with the conventional approachesbecause the proposed method can reduce the number of

switches. In addition, the proposed multilevel inverteremploy three-phase low-frequency transformers at theoutput terminal, which has a finite merit, considering thatthe output voltage is synthesised by an accumulation of

each transformer output, it does not require an additionaltransformer for galvanic isolation. However, with thesuggest approach power quality has been improvedpredominantly when compared with conventional

Fig. 8 Performance of cascaded multilevel inverter using single-phase transformers at

a andb Modulation index of 1c anddModulation index of 0.8e andf Modulation index of 0.4g andh Modulation index of 0.1



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Fig. 9 Performance of cascaded multilevel inverter using single-phase transformers at

a andb Modulation index of 1c anddModulation index of 0.8e andf Modulation index of 0.5g andh Modulation index of 0.3i andj Modulation index of 0.1



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converters. In fact, all these features of the proposed cascadedmultilevel inverter with PWM approach can be easilyimplemented for utility applications such as; STATCOM,static series compensators, unified power qualityconditioners and so on.

However, since the proposed scheme needs three three-phase transformers, the cost and size will be slightlyincreased because the capacity of the transformer is one-thirdof the transformer, which is applied to the conventionalmethod. Further since low-frequency transformers are usedon the ac side, the converter is not suitable for variablefrequency application like motor drives. However, merits ofthe proposed converter outweigh its demerits.

7 Discussion and conclusion

This paper proposed a cascaded multilevel inverter, whichemployed a single dc input source and low-frequency three-phase transformers with PWM approach. The effectivenessand validity of proposed approach is demonstrated withprototype experiments. Significance of proposed converteris well demonstrated by comparing with conventionalsingle-phase cascaded multilevel inverter. Since lessnumber of components are utilised, the proposed structureis reliable, efficient, cost-effective and compact. Theattractive features of the proposed converters are; lowswitching frequency and reduce electromagneticinterference problems, increase utilisation rate because ofsingle dc source, removal of third harmonic component asthree-phase transformer are employed on secondary side.Consequently, this characteristic allows one to achievehigh-quality output voltages and input currents. It also has

outstanding availability because of their intrinsic componentredundancy. Owing to these features, proposed architectureis superior over conventional structures. The remarkableattributes of the proposed converter is well suitable for

grid-connected photovoltaic/wind-power generator andflexible alternating current systems.

8 References

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3 Sirisukprasert, S., Xu, Z., Zhang, B., Lai, J.S., Huang, A.Q.: A high-frequency 1.5 MVA H-bridge building block for cascaded multilevelconverters using emitter turn-offthyrister. Proc. IEEE-APEC, 2002,pp. 27 32

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Fig. 10 THD and DF of output voltage on the variation of modulation index for

a Cascaded multilevel inverter with single-phase transformersb Cascaded multilevel inverter with three-phase transformers

Table 2 Components comparison with conventional cascaded multilevel inverter

Components CMI using single phase transformers CMI using three phase transformers

main switching devices (m2 1) 2 3 (m2 1) 3

main diodes (m2 1) 2 3 (m2 l) 3clamping diodes 0 0

dc-bus capacitors 1 1

balancing capacitors 0 0

input dc sources 1 1

output transformers (m2 1)/23 (m2 1)/4 (valid form 4n2 3 wherenis 1,2,3,4, . . . n)



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14 Song, S.G., Kang, F.S.: Cascaded multilevel inverter employing three-phase transformers and single DC input, IEEE Trans. Ind. Electron.,2009, 56, (6), pp. 20032014

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22 Baker, R.H., Bannister, L.H.: Electric power converter. US Patent 3867 643, February, 975

23 Peng, F.Z., Lai, J.S.: Multilevel converters a new breed of powerconverters, IEEE Trans. Ind. Appl., 1996, 49, (3), pp. 509517



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