high-frequency link: a solution for using only one...

3884 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 9, SEPTEMBER 2011

High-Frequency Link: A Solution for Using OnlyOne DC Source in Asymmetric Cascaded

Multilevel InvertersJavier Pereda, Student Member, IEEE, and Juan Dixon, Senior Member, IEEE

Abstract—Multilevel inverters are in state-of-the-art power con-version systems due to their improved voltage and current wave-forms. Cascaded H-bridge (CHB) multilevel inverters have beenconsidered as an alternative in the medium-voltage converter mar-ket and experimental electric vehicles. Their variant, the asymmet-rical CHB (ACHB) inverter, optimizes the number of voltage levelsby using dc supplies with different voltages. However, the CHBand ACHB inverters require a large number of bidirectional andisolated dc supplies that must be balanced, and as any multilevelinverter, they reduce the power quality with the voltage amplitude.This paper presents a solution to improve the already mentioneddrawbacks of ACHB inverters by using a high-frequency linkusing only one dc power source. This single power source can be se-lected according to the application (regenerative, nonregenerative,and with variable or permanent voltage amplitude). This papershows the experimental results of a 27-level ACHB inverter witha variable and single dc source, but the strategy can be appliedto any ACHB inverter with any single dc source. As a result,the reduction of active semiconductors, transformers, and totalharmonic distortion was achieved using only one dc power source.

Index Terms—Asymmetrical multilevel inverters, cascadedH-bridges (CHBs), multilevel converters, power conversion.

I. INTRODUCTION

MULTILEVEL inverters have become more popular everyday [1], and the cascaded H-bridge (CHB) inverter [2]

has been gaining importance in the market because it canachieve a high range of voltages and power [3]–[6] and has im-portant advantages such as high power quality that allows highmotor performance, low total harmonic distortion (THD) thateliminates output filters [7], reduced common-mode and deriv-ative voltages (dv/dt) that decrease motor insulation damageand torque jerk, low switching frequency that reduces switchinglosses, and high modularity that reduces cost and incrementreliability [8]. However, the CHB inverter has drawbacks such

Manuscript received January 20, 2010; revised May 16, 2010 and August 27,2010; accepted December 8, 2010. Date of publication January 6, 2011; dateof current version August 12, 2011. This work was supported in part by theComisión Nacional de Investigación Científica y Tecnológica through ProjectFondecyt 1100175, by ABB Chile, and by Iniciativa Científica Milenio throughNEIM Project P-07-087-F.

The authors are with the Department of Electrical Engineering, PontificiaUniversidad Católica de Chile, Santiago 7820436, Chile (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2010.2103532

as the large number of active semiconductors, transformers, andisolated dc supplies that must be balanced. Moreover, as anymultilevel inverter, the CHB inverter reduces the power quality(number of levels) with the voltage amplitude [9].

Asymmetrical CHB (ACHB) inverter uses dc supplieswith different voltages [10], increasing the power quality(number of levels), and it can maximize the number of levelsif the dc supply voltages are scaled in power of three [11].Even more, ACHB improves the efficiency because the morepowerful (main) H-bridges manage the 80% of the power andoperate at the fundamental frequency, reducing the switchinglosses. Moreover, the THD is highly reduced; therefore, outputfilters can be eliminated, and the cable length to the motor isless restricted. However, ACHB introduces a partial lack ofmodularity because some H-bridges work at different voltages,and the smaller or auxiliary (aux) H-bridges return power atsome operation ranges (voltage amplitudes), even when themachine is motoring; therefore, auxiliary H-bridges must bebidirectional [12], [13].

Some solutions to overcome CHB and ACHB drawbackshave been proposed, such as the use of floating capacitorsas supplies, unidirectional supplies with special modulationtechniques [14]–[17], and switched series/parallel dc sources[18], but these solutions are partials and reduce the quality.Other solutions use one dc source and output transformers toisolate the load [19], but they are only useful for constant-frequency applications, such as active filters [20]–[22].

This paper proposes a novel solution to eliminate the maindrawbacks and to keep the advantages of ACHB inverters. Theadvantages of the proposed solution are as follows: 1) only onedc power source instead of one isolated source per H-bridge;2) automatic voltage balance among H-bridges because onlyone dc source is used; 3) very low and constant THD at alloperation ranges; and 4) simpler regenerative operation.

The ACHB inverter is supplied only by one dc source byusing a high-frequency link (HFL) that generates all the iso-lated dc supplies of the auxiliary H-bridges with an automaticbalance. The main H-bridges are supplied in parallel by thesingle dc source, so an isolated winding motor connection mustbe used (Fig. 2). The single dc source has variable voltagecontrol, so the amplitude voltage of the motor is controlled bythe single dc source using all the levels all the time. A veryfast dynamic operation can be achieved by using a conventionalpulsewidth modulation (PWM) strategy among the H-bridgesuntil the single dc source reaches the proper voltage.

0278-0046/$26.00 © 2011 IEEE

PEREDA AND DIXON: HFL: SOLUTION FOR USING ONLY ONE DC SOURCE IN CASCADED MULTILEVEL INVERTERS 3885

Fig. 1. Conventional ACHB inverter with 27 levels.

II. ACHB MULTILEVEL INVERTER

A. Conventional ACHB Topology

Fig. 1 shows a conventional ACHB multilevel inverter with27 levels (3N+1 levels), where N is the number of auxiliaryH-bridges per phase. Each H-bridge needs an isolated dc sup-ply, which is scaled in power of three to maximize the numberof levels, so the voltage ratio is Vdc : Vdc/3 : Vdc/9.

As will be demonstrated in Section II-D, most of the powerdelivered to the machine comes from the most powerful (main)H-bridges. At nominal operation, more than 80% of the realpower is delivered by the main H-bridges, and less than 20% isdelivered by the auxiliary H-bridges [12], [13].

In a conventional ACHB, a huge and complicated multiwind-ing transformer or a large number of transformers are necessaryto create the isolated dc supplies. Moreover, all the auxiliaryrectifiers must be bidirectional because the voltage amplitudeis controlled by changing the combination of the H-bridges, sothe topology produces regeneration in the auxiliary H-bridgesat some voltage amplitude, even when the machine is motoring.Also, the voltage supply of each H-bridge must be carefullycontrolled (balanced) to avoid voltage distortion.

B. Proposed ACHB Topology

Fig. 2 shows the proposed ACHB multilevel inverter using anHFL to supply the auxiliary H-bridges. The HFL manages lessthan 20% of the total power system, regardless of the numberof H-bridges used in the ACHB inverter. The HFL generatesa square-wave voltage of 10–20 kHz by using a fast H-bridgewith MOSFETs or insulated-gate bipolar transistors (IGBTs), asmall toroidal ferrite transformer, and simple diode rectifiers.

Fig. 2. Proposed ACHB inverter with 27 levels.

TABLE IPOSSIBLE DC SOURCES FOR THE PROPOSED ACHB INVERTER

The HFL replaces all the auxiliary transformers and PWMrectifiers, and it will be analyzed in Section III. The motorwindings must be isolated to connect the main H-bridges atthe same dc source. This motor connection represents a smalldisadvantage compared with the converter of Fig. 1, which cangenerate more than 27 levels in each motor winding due to thefloating neutral that the star connection produces. However, thedifference of the THD currents is negligible between both.

Anyway, the entire system is powered by only one source(the ac/dc converter shown in Fig. 2), which should be selectedaccording to the application (Table I). The single dc source mustbe bidirectional for regenerative applications, and it may havevariable voltage control for high dynamic drives. A very highdynamic operation can be achieved by using a classic PWMamong H-bridges until the dc source reaches the referencevoltage.

The proposed ACHB inverter can be implemented in a largenumber of applications as is shown in Table II. Most of theapplications in industry are nonregenerative, so ACHB can besupplied by a diode rectifier or by a thyristor (SCR) rectifierto control the voltage amplitude. In electric vehicles, like cars,


TABLE IIFIELD APPLICATION OF THE PROPOSED ACHB INVERTER

Fig. 3. Output voltage of the ACHB inverter using NLC modulation.

buses, trolleybuses, subways, and trucks, the dc source shouldbe a dc/dc converter (e.g., chopper buck–boost) because dcenergy sources are intrinsically used (batteries, fuel cells, or dccatenaries). For mining trucks, locomotives, and naval ships fedby diesel or turbines, thyristor rectifiers should be used.

In regenerative operation, the proposed ACHB inverter usesonly the main H-bridges; therefore, the auxiliary H-bridges arenot used, and the inverter works as a three-level inverter.

C. Output Voltage and Modulation of the ACHB Inverter

CHB and ACHB multilevel inverters can be modulated withnearest level control (NLC) [13], which chooses the voltagelevel nearest to the reference voltage. The NLC gives an excel-lent output voltage quality, and it produces an inverse relation-ship between frequency and delivered power by each H-bridge,improving the overall efficiency (the switching frequency ofthe main H-bridges is the fundamental load frequency). Fig. 3shows the output voltage of the proposed ACHB inverter usingNLC. Then, if the dc source voltage (Vdc) is modified, theoutput voltage (Vload) varies proportionally, keeping the fullnumber of levels and the lowest THD at any output voltagemagnitudes (Fig. 4). On the other hand, when Vdc is constant,the output voltage must be controlled by using PWM directlyon the H-bridges, as in conventional ACHB. This conventionalPWM reduces the waveform quality of the output voltagewhen its amplitude decreases, because the number of levels is

Fig. 4. Experimental THD of the voltage as a function of the number of levelsused.

proportionally reduced, increasing the THD of the voltage asshown in Fig. 4.

D. Power Distribution in ACHB Inverters

As was already mentioned, in ACHB inverters with dc sup-plies scaled in power of three, the main H-bridges managemore than 80% of the power system, which makes possible thesolution proposed: an HFL that manages less than 20% of thepower to supply the auxiliary H-bridges. This feature will bedemonstrated in the following lines.

The full power per phase delivered for an ACHB inverterwith N auxiliary H-bridges (3N+1 levels) is

Pload =N∑

j=0

(V 1

rms · I1rms · cos ϕ

)j, j =

⎧⎪⎪⎨⎪⎪⎩

0 = main1 = aux-1...N = aux-N

(1)

where V 1rms, I1

rms, and cos ϕ are the fundamentals of the volt-ages, currents, and displacement factor, respectively. As theH-bridges are in series, all the currents are the same

(I1rms

)load

=(I1rms

)main

=(I1rms

)aux-1 = · · · =

(I1rms

)aux-N .

(2)

Moreover, (V 1rms)main, (V 1

rms)aux-1, and (V 1rms)aux-N are in

phase as shown in Fig. 5, so the power factor is the samefor all H-bridges. For these reasons, the percentage of powerdistribution is the same as the rms voltage distribution.

From (1) and (2)

(V 1

rms

)load

=(V 1

rms

)main

+(V 1

rms

)aux-1 + · · ·

+(V 1

rms

)aux-N (3)(

V 1max

)load

=(V 1

max

)main

+(V 1

max

)aux-1 + · · ·

+(V 1

max

)aux-N . (4)

As the H-bridges are scaled in power of three, the size of eachlevel is VL = Vdc/3N as is shown in Fig. 6.

Using Fourier series decomposition, (V 1max)load can be eval-

uated with the integration of each rectangle of Fig. 6 step


Fig. 5. Voltage waveforms and fundamental voltages of an ACHB inverterwith 27 levels (N = 2) and using the NLC modulation.

Fig. 6. Size of the voltage levels for an ACHB inverter scaled in power ofthree.

by step

(V 1

max

)load

=8

ωT· Vdc

3N

·

⎛⎜⎜⎝

cos−1( 13N+1 )∫

0

cos(ωt)dωt

+

cos−1( 33N+1 )∫

0

cos(ωt)dωt

+

cos−1( 53N+1 )∫

0

cos(ωt)dωt + · · ·

⎞⎟⎟⎠

(V 1

max

)load

=4π

Vdc

3N·

3N+1−12∑

j=0

⎛⎜⎜⎝

cos−1( 2j+13N+1 )∫

0

cos(ωt)dωt

⎞⎟⎟⎠ . (5)

Equation (5) allows getting the values of (V 1max)load for any

number of auxiliary H-bridges, from a three-level inverter (zero

TABLE IIIPOWER DISTRIBUTION

auxiliary H-bridges or N = 0) to a theoretically infinite-levelinverter

(V 1

max

)load

=

⎧⎪⎪⎪⎪⎨⎪⎪⎪⎪⎩

1.20 · Vdc, if N = 0 (3 levels)1.44 · Vdc, if N = 1 (9 levels)1.49 · Vdc, if N = 2 (27 levels)...1.50 · Vdc, if N = ∞ (∞ levels).

(6)

The values of each (V 1max)aux-N can be obtained in the

following way:(V 1

max

)aux-N =

(V 1

max

)load

|N −(V 1

max

)load

|N−1 (7)

(V 1

max

)aux-N =

⎧⎪⎪⎪⎪⎨⎪⎪⎪⎪⎩

0.24 · Vdc, if N = 10.05 · Vdc, if N = 20.01 · Vdc, if N = 3...0.00 · Vdc, if N = ∞.

(8)

Then, the voltage relation for N = 2 in terms of (V 1max)load

by using (6) is(V 1

max

)j

(V 1max)load

∣∣∣∣∣N=2

=

{ 0.81 · Vdc, if j = 0 (main)0.16 · Vdc, if j = 1 (aux-1)0.03 · Vdc, if j = 2 (aux-2).

(9)

Then, for an ACHB inverter with three H-bridges per phase(N = 2), the 81% of the total power comes from the main H-bridges. It is important to realize that the minimum amount ofpower from the main H-bridges to the load is produced whenN → ∞(

V 1max

)main

(V 1max)load

∣∣∣∣∣N=∞

=

(V 1

max

)load

∣∣N=0

(V 1max)load|

N=∞ =1.20 · Vdc

1.50 · Vdc= 0.8.

(10)

Then, no matter what the amount of auxiliary H-bridges inthe chain, the total power delivered from them to the load willnever go larger than 20% because the main H-bridges take atleast 80% of the total power. Table III shows the percentage ofpower that each H-bridge manages as a function of the numberof auxiliary H-bridges (N) or the number of levels (3N+1

levels).When a constant dc source is used, the output voltage

Vload must be controlled directly by a PWM among theH-bridges, reducing the number of levels used (classic ACHBcontrol). Fig. 7 shows the power distribution among eachH-bridge at different reference voltage amplitudes in an ACHB


Fig. 7. Average power of each H-bridge for different reference voltages.

inverter with 27 levels (N = 2). At full reference voltage,81% of the nominal converter power is managed by the mainH-bridge, with 16% by the aux-1 H-bridge and 3% by the aux-2H-bridge.

ACHB inverters have a big drawback when constant dc sup-plies are used because, in some reference voltage amplitudes,the average power of auxiliary H-bridges is negative, producingregeneration. To solve this problem, bidirectional rectifiers ordissipative resistors must be used [12], and the result is a morecomplicated and less efficient converter. These operation rangescan be avoided to inhibit regeneration by jumping those levelsthrough a special PWM strategy [14]. However, the proposedACHB inverter operates with all the number of levels per-manently, maintaining the best power distribution and voltagequality and avoiding the regeneration in motor mode.

III. HFL

A. Concept of HFL

The HFL feeds the auxiliary H-bridges. As can be seen inFig. 2, the circuit is quite simple because it only needs thefollowing: 1) a square-wave generator (H-bridge) of high fre-quency, rated at 20% of full power; 2) one multiwinding ferritetransformer; and 3) some bridge rectifiers with simple fast-recovery diodes. The H-bridge only needs to generate a squarewaveform of voltage, and hence, no control is required for itsoperation. High-power HFL can be designed and implementedtoday for up to 400 kW [23]. Then, up to 6 MW, converters arepossible if three independent ferrite transformers of 320 kW areused for each aux-1 H-bridge and a fourth ferrite transformer of180 kW is used for the three aux-2 H-bridges.

This solution (HFL) reduces the number of floating powersources from N + 1 per phase to only one per phase and onlyone for the entire system if the motor is connected with isolatedwindings or if all the main H-bridges are replaced by a three-phase inverter, as shown in Fig. 10. This HFL means small size,weight, and cost when it is compared with other alternativesused in machine drives. On the other hand, this solution matchesperfectly with the requirement for other applications, like trac-tion drives, where one dc power source is mandatory.

Fig. 8. Flux and voltage in a square-wave transformer.

Fig. 9. Toroidal transformer for a 100-kW converter (used in the experiments).

B. Toroidal Transformer Design

As the transformer works with a square-wave voltage, itneeds a different design. To generate a square-wave voltage, theflux must be a triangular function, as shown in Fig. 8. The slopeof the triangular wave defines the amplitude of the voltage.

According to Fig. 8

ϕ(t) =

{ϕmaxT/4 ·

(t − T

4

), 0 ≤ t ≤ T

2

−ϕmaxT/4 ·

(t − 3T

4

), T

2 ≤ t ≤ T(11)

v(t) =N · dϕ

dt

{N · ϕmax

T/4 = Vmax, 0 < t < T2

−N · ϕmaxT/4 = −Vmax,

T2 < t < T

(12)

Vrms =

√√√√ 1T

·(∫ T/2

0

V 2maxdt −

∫ T

T/2

V 2maxdt

)

=Vmax = N · ϕmax

T/4= 4 · f · N · ϕmax

= 4 · f · N · A · Bmax (13)

where f is the frequency, T is the period, N is the number ofturns, A is the core area, and Bmax is the flux density. As theHFL works at a very high frequency, its size and weight becomevery small.

For example, in a 100-kW ACHB inverter, the HFL powerwill never be larger than 20 kW (20%). Assuming a 27-levelinverter (N = 3) with a single dc source of 300 V, the windingsof the HFL transformer have the following characteristics:

−−Primary of HFL toroid : 19.4 kW 300 V 64.7 A

−−Each secondary aux-1 : 5.4 kW 100 V 54.0 A

−−Each secondary aux-2 : 1.1 kW 33.3 V 33.0 A.


Fig. 10. Scheme of the proposed system and power flow in motor and braking generator mode using an ACHB or hybrid cascaded inverter.

Fig. 11. Output voltage and currents in a reverse speed operation (simulation).

If the HFL works at 20 kHz using a core transformer of9 cm2 (3 cm × 3 cm) and a flux density of 0.2 T, the number ofturns required by the primary of the toroidal transformer is

N =Vrms

4 · f · A · Bmax=

3004 · 20 × 103 · 9 × 10−4 · 0.2

= 21.

(14)

Therefore, the primary winding must have at least 21 turns,and the design should consider 27 turns to satisfy the volt-ages scaled in power of three. Then, only nine turns for eachaux-1 winding and just three turns for each aux-2 winding arerequired. For the primary current (64.7 A), a 20-mm2 copperwire is enough. For the current windings of aux-1 (54.0 A)and aux-2 (33.3 A), 18- and 10-mm2 copper wires are required,respectively.

Assuming a toroidal transformer with a hole five times largerthan the total area required for all the windings, the holeshould have an area of (27 × 20 + 27 × 18 + 9 × 10) × 5 =5600 mm2 (8-cm internal diameter).

Fig. 9 shows the design and experimental high-frequencytransformer for a 100-kW converter using 27 turns for the single

Fig. 12. (a) Experimental prototype. (b) Primary, secondary aux-1, andsecondary aux-2 voltages of the toroidal transformer (HFL).

Fig. 13. Output voltages using only NLC modulation and using NLC modu-lation with PWM among the levels.

primary winding, 9 turns per secondary aux-1 winding, and3 turns per secondary aux-2 winding (27×1, 9×3, and 3×3).

C. Efficiency

The toroidal transformer uses a ferrite core, which has lowcoercivity to reduce the hysteresis losses and high resistivity tominimize the Foucault currents. As the number of turns is smalland the core is also small, the wires are very short, reducing thecopper losses. Even more, flat laminated copper or a litz wire(cable made with many thin wires) can be used to reduce theskin effect losses.

Despite that the HFL works at a high frequency (10–20 kHz),this solution allows the main H-bridges to work at a very lowfrequency (fundamental frequency of the motor). This char-acteristic reduces the switching losses, improving the overallefficiency of the main inverters, which represent at least 80% ofthe total power.


Fig. 14. Output voltages using variable PWM and constant dc supply (conventional ACHB) and using constant PWM and variable dc supply (proposed ACHB).

D. Limitations

As a dc source with voltage amplitude control is used in theproposed system, the dynamic of the ACHB inverter is limitedby the dc-link capacitor, which limits the voltage speed rate ofthe amplitude modulation. However, this speed rate limitationis solved by using conventional PWM among the H-bridgesduring transient operation, only until the capacitor reaches thereference voltage. Therefore, very high dynamic operation isallowed.

IV. REGENERATIVE BRAKING

To allow regenerative braking, the single ac/dc or dc/dcconverter shown in Figs. 2 and 10 must be bidirectional, ora dissipative option must be used to consume the energy ina resistor. Despite this modification, the regenerative brakingwill only work at a three-level configuration because all theauxiliary H-bridges are diode rectifiers (unidirectional). Tooperate the proposed system as a three-level converter forbraking operation, all the auxiliary H-bridges are switched to0-V operation (the two upper or lower transistors of theH-bridges in “ON state”). As only the main H-bridges are used,the power regeneration is limited by the power capacity of themain H-bridges, which manage 80% of the full power atleast.

To have full-level operation for regenerative braking, the sys-tem becomes more complicated because each auxiliary rectifiermust be replaced by a PWM regenerative rectifier or by a dis-sipative device (resistor). These solutions increase complexityand cost, which is unnecessary because three-level operationis more than enough for regenerative braking, which generallyworks for short periods.

V. SIMULATIONS

To probe the high dynamic performance of the proposedACHB inverter, a reversible speed operation was simulatedusing direct torque control. For this fast operation, a conven-

tional PWM among the H-bridges was used to control themotor voltage amplitude through a reduction or increment inthe number of voltage levels. This strategy achieves a very highspeed rate of the voltage variation, as shown in Fig. 11.

VI. EXPERIMENTS

A 3-kW experimental prototype was assembled to test theproposed ACHB inverter. The HFL works with a square wave-form of voltage at 20 kHz (Fig. 12).

Fig. 13 shows a comparison between the voltages of asquirrel-cage induction motor using the following two modula-tion methods: 1) a single NLC modulation and 2) a combinationof NLC with PWM among each voltage level. The PWM amongeach voltage level is optional if high transient operation isrequired, but the single NLC has better efficiency than thesecond one because it has lower switching frequency.

Fig. 14 shows a comparative sequence of typical voltagewaveforms in two possible function modes: 1) when the dcsource has constant voltage and 2) when the dc source hasvariable voltage (both cases use NLC). In the first case, theamplitude of the output voltage has to be controlled directlyinto the main and auxiliary H-bridges, changing the switchingmodulation through the NLC strategy. As can be seen, thenumber of levels decreases when the output voltage is reduced,losing the quality and raising the THD as is shown in Fig. 4. Bycontrast, in the second case, the voltage remains its waveformat any operation range because the NLC modulation is constant,so the output voltage amplitude is controlled by modifying themagnitude of the single dc source. As can be seen, the numberof levels remains for all output voltage amplitudes, and hence,the THD remains at its minimum value (3%). This is anothergreat advantage of this new topology.

Figs. 15 and 16 show that the full 27 levels remain withoutdistortion, when the voltage of the single dc source changes, aswas verified in Fig. 14. As can be seen, the speed rate of thevoltage amplitude is limited by the time constant RC, given bythe dc-link capacitor.


Fig. 15. Output voltages (Vload) using full level (27 levels) permanently anddecreasing the amplitude voltage with the single dc supply (Vdc).

Fig. 16. Output voltages (Vload) using full level (27 levels) permanently andincreasing the amplitude voltage with the single dc supply (Vdc).

Fig. 17. Output voltage and current in one phase. (a) Transition from motor toregeneration mode. (b) Motor mode when the HFL fails (three-level operation).

Fig. 17(a) shows a transition from motoring operation toregenerative operation. The transition is very clean, and thecurrent changes from 27-level operation to 3-level operationwithout problems. The three-level operation also makes thesystem more reliable. Fig. 17(b) shows a motor mode operationwhen the HFL fails, so the inverter works only with the mainH-bridges, operating at 80% of the full power at least.

VII. CONCLUSION

An asymmetric cascaded multilevel inverter using only onepower source has been implemented and tested. To eliminatethe dc sources of the auxiliary converters, the system uses anHFL, based on a square-wave generator and a multiwindingtoroidal transformer. This paper has focused on a 27-levelACHB inverter, but the idea can be applied to converters withany number of levels. The topology also permits regenera-tive braking in three-level operation by using only the mainH-bridges. The solution proposed permits the following twofunction modes: 1) constant dc source using NLC with variable

sinusoidal reference (conventional operation) and 2) variabledc source using NLC with constant sinusoidal reference. Inthe second case, the number of levels remains constant for allvoltage amplitudes, keeping a very low THD voltage (3%).The proposed solution can be applied to converters of up to6 MW because it is limited for restriction design of the HFLtransformer. Nevertheless, this power permits satisfying a widerange of applications.

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Javier Pereda (S’09) was born in Santiago, Chile.He received the Electrical engineering degree (withhighest honors) from the Pontificia UniversidadCatólica de Chile, Santiago, in 2009, where he iscurrently working toward the Ph.D. degree.

He is a Research Assistant in power electronics,electrical machines, power generation, and electrictraction with the Department of Electrical Engineer-ing, Pontificia Universidad Católica de Chile, wherehe is also part of the Electric Vehicle Laboratory. Heis currently working on ac motor drives, direct torque

control, and new multilevel inverter topologies.Mr. Pereda is a member of Millennium Nucleus Power Electronics, Mecha-

tronics and Control Process (NEIM) and a Comisión Nacional de InvestigaciónCientífica y Tecnológica scholarship holder.

Juan Dixon (M’90–SM’95) was born in Santiago,Chile. He received the Ms.Eng. and Ph.D. degreesfrom McGill University, Montreal, QC, Canada, in1986 and 1988, respectively.

Since 1979, he has been with the Departmentof Electrical Engineering, Pontificia UniversidadCatólica de Chile, Santiago, where he is currentlya Professor. He has presented more than 70 worksin international conferences and has published morethan 40 papers related with power electronics inIEEE transactions and IEE proceedings. His main

areas of interest are in electric traction, pulsewidth modulation rectifiers, activefilters, power factor compensators, and multilevel converters. He has created theElectric Vehicle Laboratory, where state-of-the-art vehicles are investigated.

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