THE CURRENT ENERGY ARENAis changing. The feeling ofdependence on fossil fuelsand the progressive increaseof its cost is leading to theinvestment of huge amountsof resources, economical and
human, to develop new cheaper and clean-er energy resources not related to fossilfuels. In fact, for decades, renewable energyresources have been the focus forresearchers, and different families of powerconverters have been designed to make theintegration of these types of systems intothe distribution grid a current reality.Besides, in the transmission lines, high-power electronic systems are needed toassure the power distribution and the ener-gy quality. Therefore, power electronic con-verters have the responsibility to carry outthese tasks with high efficiency.
The increase of the world energy demandhas entailed the appearance of new powerconverter topologies and new semiconductor
LEOPOLDO G. FRANQUELO, JOSE RODRÍGUEZ, JOSE I. LEON,SAMIR KOURO, RAMON PORTILLO,and MARIA A.M. PRATS
A Review of a TechnologyThat HasPotential inCurrent andFuture PowerApplications
technology capable to drive all neededpower. A continuous race to develophigher-voltage and higher-currentpower semiconductors to drive high-power systems still goes on. In this way,the last-generation devices are suitableto support high voltages and currents(around 6.5 kV and 2.5 kA). However,currently there is tough competitionbetween the use of classic power con-verter topologies using high-voltagesemiconductors and new convertertopologies using medium-voltagedevices. This idea is shown in Figure 1,where multilevel converters builtusing mature medium-power semicon-ductors are fighting in a developmentrace with classic power convertersusing high-power semiconductorsthat are under continuous develop-ment and are not mature. Nowadays,multilevel converters are a good solu-tion for power applications due to thefact that they can achieve high powerusing mature medium-power semicon-ductor technology [1], [2].
Multilevel converters present greatadvantages compared with conven-
tional and very well-known two-levelconverters [1], [3]. These advantagesare fundamentally focused on im-provements in the output signal quali-ty and a nominal power increase in theconverter. In order to show theimproved quality of the output volt-ages of a multilevel converter, the out-put voltage of a single-phase two-levelconverter is compared to three- andnine-level voltage multilevel wave-forms in Figure 2. The power convert-er output voltage improves its qualityas the number of levels increasesreducing the total harmonic distortion(THD) of the output waveforms.
These properties make multilevelconverters very attractive to the indus-try and, nowadays, researchers all overthe world are spending great effortstrying to improve multilevel converterperformances such as the control sim-plification [4], [5] and the performanceof different optimization algorithms inorder to enhance the THD of the outputsignals [6], [7], the balancing of the dccapacitor voltage [8], [9], and the rip-ple of the currents [10], [11]. For
instance, nowadays researchers arefocused on the harmonic eliminationusing precalculated switching functions[12], harmonic mitigation to fulfill spe-cific grid codes [13], the developmentof new multilevel converter topologies(hybrid or new ones) [14], and newcontrol strategies [15], [16].
The most common multilevel con-verter topologies are the neutral-point-clamped converter (NPC)[17], flyingcapacitor converter (FC) [18], and cas-caded H-bridge converter (CHB). Theseconverters can be classified among the
FIGURE 1 — Classic two-level power converters versus most common multilevel power converters. Development race between two different solutionsin high-power applications.
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power converters for high-power appli-cations according to Figure 3. Severalsurveys on multilevel converters havebeen published to introduce thesetopologies [1], [2]. In the 1980s, powerelectronics concerns were focused onthe converter power increase (increas-ing voltage or current). In fact, currentsource inverters were the main focusfor researchers in order to increase thecurrent. However, other authors beganto work on the idea of increasing thevoltage instead of the current. In orderto achieve this objective, authors weredeveloping new converter topologies,and, in 1981, A. Nabae, I. Takahashi, andH. Akagi presented the first NPC pulsewidth modulation (PWM) converter,also named the diode-clamped convert-er [17]. This converter was based on amodification of the classic two-levelconverter topology adding two newpower semiconductors per phase (seeFigure 1). Using this new topology, eachpower device has to stand, at the most,half voltage compared with the two-level case with the same dc-link voltage.So, if these power semiconductors have
the same characteristics as the two-level case, the voltage can be doubled.The NPC converter was generalized in[21], [22] in order to increase the num-ber of output levels and was referred toas a multipoint clamped converter(MPC), although it has not reached themedium-voltage market yet.
Years later, other multilevel convert-er topologies such as the FC [18] orCHB [19], [20] appeared. These multi-level converters present different char-acteristics compared with NPC, such asthe number of components, modulari-ty, control complexity, efficiency, andfault tolerance. Depending on the appli-cation, the multilevel converter topolo-gy can be chosen taking into accountthese factors as shown in Table 1.
Nowadays, there are several com-mercial multilevel converter topolo-gies that are sold as industrialproducts for high-power applications[23]–[25]. However, although theadvantages of using multilevel convert-ers have been demonstrated, there hasnot been an industrial boom in theapplication of these power systems in
the electrical grid in spite of theirdemonstrated good features to beused as medium-voltage drives. Maybetechnological problems such as relia-bility, efficiency, the increase of thecontrol complexity, and the design ofsimple and fast modulation methodshave been the barrier that has sloweddown the application of multilevel con-verters all over the world. Finally, theeffort of researchers has overcome thistechnical barrier and it can be affirmedthat multilevel converters are pre-pared to be applied as a mature powersystem in the electric energy arena.
This work is devoted to review andanalyze the most relevant characteris-tics of multilevel converters, to moti-vate possible solutions, and to showthat we are in a decisive instant inwhich energy companies have to beton these converters as a good solutioncompared with classic two-level con-verters. This article presents a briefoverview of the actual applications ofmultilevel converters and provides anintroduction of the modeling tech-niques and the most common modula-tion strategies. It also addresses theoperational and technological issues.
Multilevel Converter-Driven ApplicationsMultilevel converters are consideredtoday as a very attractive solution formedium-voltage high-power applica-tions. In fact, several major manufactur-ers commercialize NPC, FC, or CHBtopologies with a wide variety of controlmethods, each one strongly dependingon the application. Particularly, the NPChas found an important market in moreconventional high-power ac motor driveapplications like conveyors, pumps,fans, and mills, among others, whichoffer solutions for industries includingoil and gas, metals, power, mining,water, marine, and chemistry [26], [27].
The back-to-back configuration forregenerative applications has alsobeen a major plus of this topology,used, for example, in regenerative con-veyors for the mining industry [28] orgrid interfacing of renewable energysources like wind power [29], [30]. Onthe other hand, FC converters havefound particular applications for high
FIGURE 2 — Comparison of output phase voltage waveforms: (a) two-level inverter, (b) three-levelinverter, and (c) nine-level inverter.
1
−1
0
Vol
tage
[pu]
0 0.005 0.01(a)
(b)
(c)Time [s]
0.015 0.02 0.025 0.03
1
−1
0
Vol
tage
[pu]
0 0.005 0.01 0.015 0.02 0.025 0.03
1
−1
0
Vol
tage
[pu]
0 0.005 0.01 0.015 0.02 0.025 0.03
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bandwidth–high switching frequencyapplications such as medium-voltagetraction drives [31]. Finally the cas-caded H-bridge has been successfullycommercialized for very high-powerand power-quality demanding applica-tions up to a range of 31 MVA, due toits series expansion capability. Thistopology has also been reported foractive filter and reactive power com-pensation applications [32], electricand hybrid vehicles [33], [34], photo-voltaic power conversion [35]–[37],uninterruptible power supplies [38],and magnetic resonance imaging [39].As an example of a commercial multi-level power converter, a 34-kV–15-MWthree-phase, six-cell CHB converterfrom Siemens for regenerative drivesis shown in Figure 4. A summary ofmultilevel converter-driven applica-tions is illustrated in Figure 5.
Models: A Tool to EnhanceMultilevel Converter PossibilitiesThe simulation and the determinationof “input to output (I/O)” relations are afundamental task in the study anddesign process of the multilevel con-verters. These I/O relations becomeessential for the development of suit-
able models, which allows one to obtainall the necessary information about theconverter prior to the implementationstage. The modeling of multilevel con-verters is not a trivial task since theyare made up of linear and nonlinearcomponents. Historically, modelingtechniques applied to dc power elec-tronics converters have been adaptedto be used in the study of ac devices,giving place to different approximationsthat achieve, according to their objec-tives, snubber circuits design, controlschemes, and controllers development;steady-state study; dynamic and tran-sient response study; stability analysis,etc. The operation of the multilevel con-verter is a periodic sequencing of its
possible states corresponding to dis-crete states of the switches. Figure 6shows a single-phase three-level NPCphase has and the two possible model-ing techniques. Taking these remarksinto account, two types of models canbe developed: equivalent circuit simula-tion or state-space averaged.
Circuit Simulation Modelingof Multilevel ConvertersA model of the converter can beobtained with the help of powerful sim-ulation tools such as SPICE-based sim-ulators. In this case, the modeling ofthe multilevel converters is reduced tothe generation of an adequate electriccircuit model that fully includes the
FIGURE 3 — High-power converters classification.
High Power Converters
Direct Conversion
Cycloconverter Current Source
PWM CurrentSource Inverter
Load CommutatedInverter
Single dc Source
NPC Flying Capacitor Cascaded H-Bridge
Equal dc Sources
Multicell Structures (Modular)
Unequal dcSourcesHigh Power Semiconductors
Medium Power Semiconductors
High Power 2-LevelVSI
Multiple Isolateddc Sources
Voltage Sources
MultilevelConverters
Indirect Conversion (dc-Lnk)
TABLE 1—COMPARISON OF MULTILEVEL CONVERTER TOPOLOGIESDEPENDING ON IMPLEMENTATION FACTORS.
NPC FC CHBSpecific requirements Clamping diodes Additional capacitors Isolated dc sources
Modularity Low High High
Design and implementation Low Medium (capacitors) High (input complexity transformer)
Control concerns Voltage balancing Voltage setup Power sharing
Fault tolerance Difficult Easy Easy
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nonlinearities of the switches allowingthe complete characterization of the sys-tem dynamics. Considering ideal switch-es, a linear description of the convertercan be obtained for every switchingstate of the power converter. Figure 6shows one phase of a three-level NPCwhere the switches have been replacedby an ideal switch, and it can be easilyseen that the phase acts like a voltagesource for every switch position, so a lin-ear equivalent circuit description of the
converter phase can be obtained foreach one. With this model, a linear piece-wise simulation can be carried out. If theintegration method for the model equa-tions is properly chosen [40], the simula-tion time and results accuracy are goodenough. However, this modelingapproach often leads to large simulationtimes and possible unreliable results dueto convergence problems. The maindrawbacks of this modeling techniqueare that the integration of advanced con-
trol techniques with the model is almostimpossible [40] and that the model isusually complex, with its use for controldesign often being troublesome [41],[42]. These models can be used in thetuning process of the control loops andto evaluate the high-order harmonicsdue to switching that can be seen oncurrents shown in Figure 6.
State-Space Averaged Modelingof Multilevel ConvertersState-space averaged models can beeasily obtained from the discrete mod-els when varying quantities areassumed as their averaged value over aswitching period. Since in ac convertersthese quantities are time varying even inthe steady state, it is necessary to makea change of coordinates to convert acsinusoidal quantities to dc quantitiesprior to the averaging process [43], [44].Time-invariant system controller designtechniques can be used with these mod-els when important components otherthan the fundamental harmonics are notpresent in the system. With the transfor-mation to this “rotating referenceframe,” dc quantities correspond to thefundamental harmonic of the signals,
FIGURE 4 — Multilevel cascaded H-bridge converter with six cells per phase, 13 levels, and 15 MWfor regenerative drives.
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but some multilevel converter topolo-gies are not completely characterizedby only the first harmonic, and it is nec-essary to draw on the “harmonic mod-els” where a greater number ofharmonics are taken into account,obtaining an adequate modeling of theconverter [41]. These harmonic modelsare complex and only some advancedcomplex control techniques are suitableto be applied to them [42].
Recently, a new state-space averag-ing modeling technique has been intro-duced based on approximations overthe exact averaged linear piecewisecharacteristics of the converter [30]. Inthe phase of the three-level diode-clamped converter shown in Figure 6,the ideal switch will be switchingbetween the three possible states so anaverage model can be deduced consid-
ering δa as the averaged value of theswitch position. Figure 6 shows thegraphic representation of the exactaveraged linear piecewise approxima-tion and the proposed quadraticapproximation [29]. This techniqueprovides simple enough models to beused in the controller design [45] andcarries out fast simulations withoutconvergence problems due to the con-tinuous nature of the obtained equa-tions. Therefore, the use of thesemodels overcomes one of the techno-logical handicaps in which the multi-level converters are involved, makingthe design stage of multilevel powersystems a more accessible task. Figure6 shows the currents obtained with thiskind of model, and when comparedwith those obtained with the equiva-lent circuit simulation, it can be seen
that the results are almost the sameexcept for the high-order harmonics.
Multilevel Modulation MethodsMultilevel converter modulation andcontrol methods have attracted muchresearch and development attentionover the last decade [1], [2], [46], [47].Among the reasons are the challenge toextend traditional modulation methodsto the multilevel case, the inherent addi-tional complexity of having more powerelectronics devices to control, and thepossibility to take advantage of the extradegrees of freedom provided by theadditional switching states generated bythese topologies. As a consequence, alarge number of different modulationalgorithms have been developed, eachone with unique features and draw-backs, depending on the application.
FIGURE 6 — Equivalent circuit and state-space modeling of multilevel converters.
Averaged Modeling Using a asAveraged Voltage of the Power
Converter Phase Over a SwitchingPeriod
Averaged Modeling Using a asAveraged Voltage of the Power
Converter Phase Over a SwitchingPeriod
iαβ- Equivalent Circuit Simulation iαβ- State-Space Averaged Model
20
10
0
−10
30iαβ- Equivalent Circuit Simulation iαβ- State-Space Averaged Model
A classification of the modulationmethods for multilevel inverters is pre-sented in Figure 7. The modulation algo-rithms are divided into two main groupsdepending on the domain in which theyoperate: the state-space vector domain,in which the operating principle isbased on the voltage vector generation,and the time domain, in which themethod is based on the voltage levelgeneration over a time frame. In addi-tion, in Figure 7 the different methodsare labeled depending on the switchingfrequency they produce. In general, lowswitching frequency methods are pre-ferred for high-power applications dueto the reduction of switching losses,while the better output power qualityand higher bandwidth of high switchingfrequency algorithms are more suitablefor high dynamic range applications.
Multilevel Converters PWM StrategiesTraditional PWM techniques [48] havebeen successfully extended for multi-level converter topologies, by usingmultiple carriers to control each powerswitch of the converter. Therefore, theyare known as multicarrier PWM meth-ods as shown in Figure 7. For multicell
topologies, like FC and CHB, each carri-er can be associated to a particularpower cell to be modulated independ-ently using sinusoidal bipolar PWM andunipolar PWM, respectively, providingan even power distribution among thecells. For a converter with m cells, acarrier phase shift of 180◦/m for theCHB and of 360◦/m for the FC is intro-duced across the cells to generate thestepped multilevel output waveformwith low distortion [23]. Therefore, thismethod is known as phase shiftedPWM (PS-PWM). The differencebetween the phase shifts and the typeof PWM (unipolar or bipolar) isbecause one CHB cell generates three-level outputs, while one FC cell gener-ates two-level outputs. This methodnaturally balances the capacitor volt-ages for the FC and also mitigates inputcurrent harmonics for the CHB.
The carriers can also be arrangedwith shifts in amplitude relating eachcarrier with each possible output volt-age level generated by the inverter. Thisstrategy is known as level shifted PWM(LS-PWM), and depending on the dispo-sition of the carriers, they can be inphase disposition (PD-PWM), phase
opposition disposition (POD-PWM), andalternate phase opposition disposition(APOD-PWM) [49], all shown in Figure 7.
An in-depth assessment betweenthese PWM methods can be found in[50]. LS-PWM methods can be imple-mented for any multilevel topology;however, they are more suited for theNPC, since each carrier signal can beeasily related to each power semicon-ductor. Particularly, LS-PWM methodsare not very attractive for CHB invert-ers, since the vertical shifts relateeach carrier and output level to a par-ticular cell, producing an unevenpower distribution among the cells.This power unbalance disables theinput current harmonic mitigationthat can be achieved with the multi-pulse input isolation transformer,reducing the power quality.
Finally, the hybrid modulation is inpart a PWM-based method that is spe-cially conceived for the CHB withunequal dc sources [14], [51]–[53].The basic idea is to take advantage ofthe different power rates among thecells of the converters to reduceswitching losses and improve the con-verter efficiency. This is achieved by
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controlling the high-power cells at afundamental switching frequency byturning on and off each switch of eachcell only one time per cycle, while thelow-power cell is controlled usingunipolar PWM. Also, asymmetric orhybrid topologies have been proposedbased on the MPC structure [54].
Space Vector Modulation TechniquesSpace vector modulation (SVM) is atechnique where the reference voltageis represented as a reference vector tobe generated by the power converter.All the discrete possible switchingstates of the converter lead to discreteoutput voltages and they can be alsorepresented as the possible voltagevectors (usually named state vectors)that can be achieved. The SVM tech-nique generates the voltage referencevector as a linear combination of thestate vectors obtaining an averagedoutput voltage equal to the referenceover one switching period [55].
In recent years, several space vectoralgorithms extended to multilevel con-verters have been found in theresearch. Most of them are particularlydesigned for a specific number of levelsof the converter and the computationalcost and the algorithm complexity areincreased with the number of levels.Besides, these general modulation tech-niques for multilevel converters involvetrigonometric function calculations,look-up tables, or coordinated systemtransformations, which increase thecomputational load.
Recent SVM strategies have drasti-cally reduced the computational effortand the complexity of the algorithmscompared with other conventionalSVM and sinusoidal PWM modulationtechniques [56]–[62]. A survey ofrecent SVM algorithms for power volt-age source multilevel converters waspresented in [63]. These techniquesprovide the nearest state vectors to thereference vector forming the switchingsequence and calculating the corre-sponding duty cycles using extremelysimple calculations without involvingtrigonometric functions, look-up tables,or coordinate system transformations.Therefore, these methods drasticallyreduce the computational load main-
tained, permitting the online computa-tion of the switching sequence and theon-state durations of the respectiveswitching state vectors. In addition, thelow computational cost of the pro-posed methods is always the same andit is independent of the number of lev-els of the converter.
The three-dimensional SVM (3D-SVM) technique presented in [59] is ageneralization of the well known two-dimensional (2D)-SVM strategy [60]used when the power system is bal-anced (without triple harmonics) and,therefore, the state vectors are locatedin a plane (alpha-beta plane). However,it is necessary to generalize to a 3Dspace if the system is unbalanced or ifthere is zero sequence or triple har-monics, because in this case state vec-tors are not on a plane. The 3D-SVMtechnique for multilevel converters issuccessfully used for compensatingzero sequence in active power filterswith neutral single-phase distortingloads that generate large neutral cur-rents. In general, 3D-SVM is useful insystems with or without neutral, unbal-anced load, triple harmonics, and forgenerating any 3D control vector.Moreover, this technique also permitsbalancing the dc-link capacitor voltage.
The strategy proposed in [59] is thefirst 3D-SVM technique for multilevelconverters that permits the on-line cal-culation of the sequence of the nearestspace vector for generating the refer-ence voltage vector. The computation-al cost of the proposed method is verylow and it is independent of the num-ber of levels of the converter. Thistechnique can be used as a modulationalgorithm in all applications that pro-vide a 3D vector control.
Finally, four-leg multilevel convertersare finding relevance in active power fil-ters and fault-tolerant three-phase recti-fiers with the capability for loadbalancing and distortion mitigationthanks to their ability to meet theincreasing demand of power ratings andpower quality associated with reducedharmonic distortion and lower EMI [64],[65]. A four-leg multilevel converter per-mits a precise control of neutral currentdue to an extended range for the zerosequence voltages and currents.
A generalized and optimized 3D-SVM algorithm for four-leg multilevelconverters has been recently present-ed in [66]. The proposed techniquedirectly allows the optimization of theswitching sequence minimizing thenumber of switching in four-leg sys-tems. As in [56]–[61], the computation-al complexity has been reduced up tominimum. This technique can be usedas a modulation algorithm in all appli-cations needing a 3D control vectorsuch as four-leg active, where the con-ventional 2D-SVM cannot be used.
Other MultilevelModulation AlgorithmsAlthough SVM and multicarrier PWMare widely accepted and have reacheda certain maturity for multilevel appli-cations, other algorithms have beendeveloped to satisfy particular needs ofdifferent applications. Selective har-monic elimination (SHE), for example,has been extended to the multilevelcase for high-power applications due tothe strong reduction in the switchinglosses [6], [12], [67]. However, SHEalgorithms are very limited to open-loop or low-bandwidth applications,since the switching angles are comput-ed offline and stored in tables, whichare then interpolated according to theoperating conditions. In addition, SHE-based methods become very complexto design and implement for converterswith a high number of levels (abovefive), due to the increase of switchingangles, hence equations, that need tobe solved. In this case, other lowswitching frequency methods are moresuitable. For example, multilevel spacevector control (SVC) takes advantageof the high number of voltage vectorsgenerated by a converter with a highnumber of levels by approximating thereference to the closest generable vec-tor [68]. This principle results in a natu-ral fundamental switching frequencywith reduced switching losses, like inSHE, that can be easily implemented inclosed-loop and high-bandwidth sys-tems. The time-domain version of SVCis the nearest level control (NLC),which in essence is the same principlebut considering the closest voltagelevel that can be generated by the
inverter instead of the closest vector[69]. Both methods are suitable forinverters with a high number of levels,since the operating principle is basedon an approximation and not a modula-tion with a time average of the refer-ence; also, due to the low and variableswitching frequency, they present high-er total harmonic distortion for invert-ers with a lower number of levels andalso for low modulation indexes.
As mentioned above, not all of themodulation schemes mentioned beforeand illustrated in Figure 7 are suitablefor each topology; moreover, somealgorithms are not applicable to someconverters. Figure 8 summarizes thecompatibility between the modulationmethods and the multilevel topologies.
Operational and Technological IssuesMultilevel converters offer very attrac-tive characteristics for high-power appli-cations; however, the power circuits ofthe multilevel topologies have morecomplex structures than classic con-verters and sometimes their operationis not straightforward and particularproblems need to be addressed. Inother occasions this extra complexitycan also be embraced as an opportunityto introduce enhanced operating char-acteristics like efficiency, power quality,and fault-tolerant operation, which arenot feasible in classic topologies.
One of the most analyzed and exten-sively addressed drawbacks of multi-level technology is the neutral point
control or capacitor voltage balancenecessary for NPC converters. The NPCexperiences a capacitor unbalance forcertain operating conditions, depend-ing on the modulation index, dynamicbehavior, and load conditions, amongothers, which produce a voltage differ-ence between both capacitors, shiftingthe neutral point and causing undesir-able distortion at the converter output.This drawback has been addressed inmany works for different modulationmethods, both in vector and timedomain [70]–[71], and is widely accept-ed as a solved problem. The neutralpoint control of NPC converters and thepower circuit structure becomes evenmore complex for nontraditional config-urations with more output levels (fiveand up), especially due to the amountof clamping diodes needed. Therefore,mainly three-level NPC converters arefound on the market.
FC converters, on the contrary, havea natural voltage balancing operation[31], but the capacitor voltages have tobe precharged at startup close to theirnominal values, also know as initializa-tion. This can be performed via an addi-tional and simple control logic of theswitches of the converter by connect-ing successively each of the capacitorsto the source and disconnecting themwhen the desired voltage is reached.Although the topology is modular instructure and can be increased in anarbitrary number of cells, the additionalflying capacitors and the involved costshas kept traditional configurations up
to about four levels. In addition, morecells do not necessarily signify anincrease of the power rating of the con-verter, since the output voltage ampli-tude does not vary—only the numberof levels, hence the power quality.
CHB converters have also no volt-age balancing problems due to theindependent and isolated dc sourcesprovided by the multipulse secondarywindings of the input transformer.Furthermore, they do not need specialinitialization, and their circuit struc-ture enables series connection toreach power levels for very high-powerapplications (maximum rates 13.8 KV,1,400 A and 31,000 KVA), where it hasfound industrial acceptance. However,the isolation transformer is nonstan-dard due to the amount of secondariesand to the angle shifts between wind-ings for input current harmonic mitiga-tion. This is an important drawbackthat has kept this topology with asmaller market penetration. Neverthe-less, transformer-less applications, likephotovoltaic power conversion, activefilters, and battery-powered electricvehicles, have been reported as suit-able applications [32]–[39]. The com-plicated transformer has also beenavoided using a standard transformerto power only one cell (per phase) ofthe converter and use the controlstrategy to control the circulatingpower to keep the other power cells’dc links charged at desired values [76].
For the case of CHB with unequal dcsources, the same drawback of theequally fed case applies with the differ-ence that the input transformer haseven power rate differences betweenwindings, and, in addition, no inputcurrent harmonic compensation isachieved. Another drawback is the lossof modularity since the asymmetricpower distribution between cells forcesdifferent ratings of the components(mainly the voltage rate of the capaci-tors and semiconductors). Neverthe-less, these topologies offer very highpower quality waveforms with lesspower semiconductors (reduction insize and cost, while an increase in relia-bility), and lower switching losses,since the high-power cells only commu-tate at a fundamental switchingFIGURE 8 — Applicability of modulation methods to multilevel topologies.
Topologies
SVM
LS-PWM
PS-PWMHybrid
ModulationSHE
SVC
NLC
Mod
ulat
ion
Met
hods
Applicable/Recommended Not Applicable Applicable/Not Recommended
NPC FC CHB
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frequency. Moreover, the complicatedtransformer can be avoided by similarcontrol strategies applied to the sym-metric case, or in transformer-lessapplications (especially active filters).Another issue with the asymmetricCHB is that the low-power cells regen-erate power during some operatingconditions (they vary depending on theasymmetry, the modulation index, andthe load), even if the power converteris in motoring mode [77]. If this poweris not handled appropriately by usingan active front-end rectifier or by resis-tive dissipation, the lower-power cells’dc link voltages will drift and becomeunbalanced, generating output voltagedistortion. This problem can be mini-mized using appropriate voltage asym-metries between the cells [14].
Although common-mode voltagesand bearing currents are stronglyreduced when using multilevel con-verters, due to the reduced voltagederivatives and more sinusoidal out-puts, this is still a subject underresearch, and several contributionshave been reported [78]–[81].
Since CHB and FC have a modularstructure, they can be more directlyadapted to operate under internal faultconditions. This is a very attractivecapability for industry applications,especially considering those down-times (and the associated costs) canbe avoided, or greatly reduced, while amore organized and scheduled repara-tion is prepared. Fault operation isonly possible if the malfunction isproperly and timely detected, makingthe fault diagnostic an important issue.Several contributions have beenreported, from simply bypassing faultycells to more complex reference prec-ompensation methods for enhancedoperation [82]–[85]. Different faultdetection mechanisms have also beenreported, for example, based on thespectral analysis of the carrier andsidebands harmonics of the outputvoltage [86], [87].
The three main topologies analyzedin the article present unique featuresand drawbacks, making each one spe-cial for a particular application. Theyhave been compared in terms of struc-ture, cost, and efficiency in [88].
ConclusionsMultilevel converters have maturedfrom being an emerging technology toa well-established and attractive solu-tion for medium-voltage high-powerdrives. As presented in this article,these converters have overcome thetechnical barriers that had been thecurb for their deep use as an opti-mized solution in the power market.Modeling, control strategies design,and modulation methods developmenthave been introduced in recent yearsto carry out this technical revolution.Nowadays, multilevel convertertopologies such as NPC, FC, and CHBown very interesting features in termsof power quality, power range, modu-larity, and other characteristics achiev-ing high-quality output signals beingspecially designed for medium- andhigh-power applications. Therefore, it’sthe time for betting on this technologyfor actual and future power applica-tions just now when the market is mov-ing forward with more powerful anddistributed energy sources. The cur-rent trends and challenges faced byenergy applications, such as renewablepower conversion and distributed gen-eration systems, together with therecent developments in multilevel con-verter technology, are opening a newvast area of applications where thistechnology has a lot to offer. It is just aquestion of time before multilevel con-verters will reach an important marketshare in these applications. You couldsay it is time for multilevel converters.
BiographiesLeopoldo G. Franquelo received theM.Sc. and Ph.D. in electrical engineer-ing from the University of Seville,Spain, in 1977 and 1980, respectively.In 1978, he joined the University ofSeville and has been a professor since1986. From 1998 to 2005, he was thedirector of the Department of Elec-tronic Engineering. He was the vice-president of the IEEE IndustrialElectronics Society (IES) SpanishChapter (2002–2003) and member atlarge of IES AdCom (2002–2003). Hehas been the vice-president for confer-ences of the IES (2004–2007), in whichhe has also been a distinguished lec-
turer since 2006. He has been an asso-ciate editor for the IEEE Transactionson Industrial Electronics since 2007 andcurrently is IES president elect. Hiscurrent research interest lies in modu-lation techniques for multilevel invert-ers and their application to powerelectronic systems for renewable ener-gy systems. He leads a large researchand teaching team in Spain. In the lastfive years, he has been an author of40 publications in international jour-nals and 165 in international confer-ences. He is the holder of ten patentsand he is an advisor for ten Ph.D. dis-sertations and 96 R&D projects.
Jose Rodríguez received the Engi-neer’s degree in electrical engineeringfrom the Universidad Técnica FedericoSanta Maria (UTFSM), Valparaíso, Chile,in 1977, and the Dr.Ing. degree in electri-cal engineering from the University ofErlangen, Germany, in 1985. Since 1977,he has been a professor with theUTFSM, where from 2001 to 2004 he wasappointed as director of the ElectronicsEngineering Department, from 2004 to2005 he was the vice rector of academ-ic affairs, and since 2005 has been therector. During his sabbatical leave in1996, he was responsible for the MiningDivision, Siemens Corporation, Santia-go, Chile. Prof. Rodriguez has been anactive associate editor with the IEEEPower Electronics and Industrial Elec-tronics Societies since 2002. He hasserved as guest editor of IEEE Transac-tions on Industrial Electronics four times.He has consulting experience in themining industry, particularly in theapplication of large drives such ascycloconverter-fed synchronousmotors for SAG mills, high-power con-veyors, controlled ac drives for shovels,and power-quality issues. His mainresearch interests include multilevelinverters, new converter topologies,and adjustable-speed drives. He hasdirected over 40 R&D projects in thefield of industrial electronics, he hascoauthored over 50 journal and 130conference papers, and he has con-tributed one book chapter. His researchgroup has been recognized as one ofthe two centers of excellence in engi-neering in Chile from 2005–2008. He is aSenior Member of the IEEE.
Jose I. Leon received the B.S., M.S.,and Ph.D. in telecommunications engi-neering from the University of Seville,Seville, Spain, in 1999, 2001, and 2006,respectively. In 2002, he joined thePower Electronics Group, University ofSeville, working on R&D projects. He iscurrently an associate professor withthe Department of Electronic Engineer-ing, University of Seville. His researchinterests include electronic power sys-tems; modeling, modulation, and con-trol of power-electronic convertersand industrial drives; and power quali-ty in renewable generation plants.
Samir Kouro received the M.Sc.and Ph.D. degrees in electronics engi-neering from the Universidad TécnicaFederico Santa María (UTFSM),Valparaíso, Chile, in 2004 and 2008,respectively. In 2004, he joined theElectronics Engineering Department atUTFSM, where he is currently an asso-ciate researcher. In 2004, he was distin-guished as the youngest researcher ofChile granted with a governmental-funded research project (FONDECYT)as principal researcher. His researchinterests include power convertersand adjustable speed drives.
Ramon Portillo received the B.S.and M.S. degrees in industrial engineer-ing from the University of Seville in2002, where he is currently workingtoward the Ph.D. in electrical engineer-ing with the Power Electronics Group.In 2001, he joined the Power Electron-ics Group, working on R&D projects.Since 2002, he has been an associateprofessor with the Department of Elec-tronic Engineering, University ofSeville. His research interests includeelectronic power systems applied toenergy conditioning and generation,power quality in renewable generationplants, applications of fuzzy systems inindustry and wind farms, and modelingand control of power-electronic con-verters and industrial drives.
Maria A.M. Prats received theLicenciado and Doctor degrees inphysics from the University of Seville,Spain, in 1996 and 2003, respectively.In 1996, she joined the SpanishAerospatial Technical National Insti-tute (INTA), where she worked in theRenewable Energy Department. In
1998, she joined the Department ofElectrical Engineering, University ofHuelva, Spain. Since 2000, she hasbeen an assistant professor with theDepartment of Electronics Engineer-ing, University of Seville. Since 2006she has been the IEEE WIE Spanishsection president. Her research inter-ests focus on multilevel convertersand fuel-cell power-conditioner sys-tems. She is involved in industrialapplications for the design and devel-opment of power converters appliedto renewable-energy technologies.
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