Download High-Frequency Link: A Solution for Using Only One DC Source in

3884
IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 9, SEPTEMBER 2011
High-Frequency Link: A Solution for Using Only
One DC Source in Asymmetric Cascaded
Multilevel Inverters
Javier Pereda, Student Member, IEEE, and Juan Dixon, Senior Member, IEEE
Abstract—Multilevel inverters are in state-of-the-art power conversion systems due to their improved voltage and current waveforms. Cascaded H-bridge (CHB) multilevel inverters have been
considered as an alternative in the medium-voltage converter market and experimental electric vehicles. Their variant, the asymmetrical CHB (ACHB) inverter, optimizes the number of voltage levels
by using dc supplies with different voltages. However, the CHB
and ACHB inverters require a large number of bidirectional and
isolated dc supplies that must be balanced, and as any multilevel
inverter, they reduce the power quality with the voltage amplitude.
This paper presents a solution to improve the already mentioned
drawbacks of ACHB inverters by using a high-frequency link
using only one dc power source. This single power source can be selected according to the application (regenerative, nonregenerative,
and with variable or permanent voltage amplitude). This paper
shows the experimental results of a 27-level ACHB inverter with
a variable and single dc source, but the strategy can be applied
to any ACHB inverter with any single dc source. As a result,
the reduction of active semiconductors, transformers, and total
harmonic distortion was achieved using only one dc power source.
Index Terms—Asymmetrical multilevel inverters, cascaded
H-bridges (CHBs), multilevel converters, power conversion.
I. I NTRODUCTION
M
ULTILEVEL inverters have become more popular every
day [1], and the cascaded H-bridge (CHB) inverter [2]
has been gaining importance in the market because it can
achieve a high range of voltages and power [3]–[6] and has important advantages such as high power quality that allows high
motor performance, low total harmonic distortion (THD) that
eliminates output filters [7], reduced common-mode and derivative voltages (dv/dt) that decrease motor insulation damage
and torque jerk, low switching frequency that reduces switching
losses, and high modularity that reduces cost and increment
reliability [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; date
of current version August 12, 2011. This work was supported in part by the
Comisión Nacional de Investigación Científica y Tecnológica through Project
Fondecyt 1100175, by ABB Chile, and by Iniciativa Científica Milenio through
NEIM Project P-07-087-F.
The authors are with the Department of Electrical Engineering, Pontificia
Universidad Católica de Chile, Santiago 7820436, Chile (e-mail: jepereda@
ing.puc.cl; jdixon@ing.puc.cl).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TIE.2010.2103532
as the large number of active semiconductors, transformers, and
isolated dc supplies that must be balanced. Moreover, as any
multilevel inverter, the CHB inverter reduces the power quality
(number of levels) with the voltage amplitude [9].
Asymmetrical CHB (ACHB) inverter uses dc supplies
with different voltages [10], increasing the power quality
(number of levels), and it can maximize the number of levels
if the dc supply voltages are scaled in power of three [11].
Even more, ACHB improves the efficiency because the more
powerful (main) H-bridges manage the 80% of the power and
operate at the fundamental frequency, reducing the switching
losses. Moreover, the THD is highly reduced; therefore, output
filters can be eliminated, and the cable length to the motor is
less restricted. However, ACHB introduces a partial lack of
modularity because some H-bridges work at different voltages,
and the smaller or auxiliary (aux) H-bridges return power at
some operation ranges (voltage amplitudes), even when the
machine is motoring; therefore, auxiliary H-bridges must be
bidirectional [12], [13].
Some solutions to overcome CHB and ACHB drawbacks
have been proposed, such as the use of floating capacitors
as supplies, unidirectional supplies with special modulation
techniques [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 to
isolate the load [19], but they are only useful for constantfrequency applications, such as active filters [20]–[22].
This paper proposes a novel solution to eliminate the main
drawbacks and to keep the advantages of ACHB inverters. The
advantages of the proposed solution are as follows: 1) only one
dc power source instead of one isolated source per H-bridge;
2) automatic voltage balance among H-bridges because only
one dc source is used; 3) very low and constant THD at all
operation ranges; and 4) simpler regenerative operation.
The ACHB inverter is supplied only by one dc source by
using a high-frequency link (HFL) that generates all the isolated dc supplies of the auxiliary H-bridges with an automatic
balance. The main H-bridges are supplied in parallel by the
single dc source, so an isolated winding motor connection must
be used (Fig. 2). The single dc source has variable voltage
control, so the amplitude voltage of the motor is controlled by
the single dc source using all the levels all the time. A very
fast dynamic operation can be achieved by using a conventional
pulsewidth modulation (PWM) strategy among the H-bridges
until 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
Fig. 1.
Conventional ACHB inverter with 27 levels.
II. ACHB M ULTILEVEL I NVERTER
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Fig. 2. Proposed ACHB inverter with 27 levels.
TABLE I
P OSSIBLE DC S OURCES FOR THE P ROPOSED ACHB I NVERTER
A. Conventional ACHB Topology
Fig. 1 shows a conventional ACHB multilevel inverter with
27 levels (3N +1 levels), where N is the number of auxiliary
H-bridges per phase. Each H-bridge needs an isolated dc supply, which is scaled in power of three to maximize the number
of levels, so the voltage ratio is Vdc : Vdc /3 : Vdc /9.
As will be demonstrated in Section II-D, most of the power
delivered to the machine comes from the most powerful (main)
H-bridges. At nominal operation, more than 80% of the real
power is delivered by the main H-bridges, and less than 20% is
delivered by the auxiliary H-bridges [12], [13].
In a conventional ACHB, a huge and complicated multiwinding transformer or a large number of transformers are necessary
to create the isolated dc supplies. Moreover, all the auxiliary
rectifiers must be bidirectional because the voltage amplitude
is controlled by changing the combination of the H-bridges, so
the topology produces regeneration in the auxiliary H-bridges
at some voltage amplitude, even when the machine is motoring.
Also, the voltage supply of each H-bridge must be carefully
controlled (balanced) to avoid voltage distortion.
B. Proposed ACHB Topology
Fig. 2 shows the proposed ACHB multilevel inverter using an
HFL to supply the auxiliary H-bridges. The HFL manages less
than 20% of the total power system, regardless of the number
of H-bridges used in the ACHB inverter. The HFL generates
a square-wave voltage of 10–20 kHz by using a fast H-bridge
with MOSFETs or insulated-gate bipolar transistors (IGBTs), a
small toroidal ferrite transformer, and simple diode rectifiers.
The HFL replaces all the auxiliary transformers and PWM
rectifiers, and it will be analyzed in Section III. The motor
windings must be isolated to connect the main H-bridges at
the same dc source. This motor connection represents a small
disadvantage compared with the converter of Fig. 1, which can
generate more than 27 levels in each motor winding due to the
floating neutral that the star connection produces. However, the
difference 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 selected
according to the application (Table I). The single dc source must
be bidirectional for regenerative applications, and it may have
variable voltage control for high dynamic drives. A very high
dynamic operation can be achieved by using a classic PWM
among H-bridges until the dc source reaches the reference
voltage.
The proposed ACHB inverter can be implemented in a large
number of applications as is shown in Table II. Most of the
applications in industry are nonregenerative, so ACHB can be
supplied by a diode rectifier or by a thyristor (SCR) rectifier
to control the voltage amplitude. In electric vehicles, like cars,
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 9, SEPTEMBER 2011
TABLE II
F IELD A PPLICATION OF THE P ROPOSED ACHB I NVERTER
Fig. 4.
used.
Experimental THD of the voltage as a function of the number of levels
proportionally reduced, increasing the THD of the voltage as
shown in Fig. 4.
D. Power Distribution in ACHB Inverters
Fig. 3. Output voltage of the ACHB inverter using NLC modulation.
buses, trolleybuses, subways, and trucks, the dc source should
be a dc/dc converter (e.g., chopper buck–boost) because dc
energy sources are intrinsically used (batteries, fuel cells, or dc
catenaries). For mining trucks, locomotives, and naval ships fed
by diesel or turbines, thyristor rectifiers should be used.
In regenerative operation, the proposed ACHB inverter uses
only the main H-bridges; therefore, the auxiliary H-bridges are
not 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 with
nearest level control (NLC) [13], which chooses the voltage
level nearest to the reference voltage. The NLC gives an excellent output voltage quality, and it produces an inverse relationship between frequency and delivered power by each H-bridge,
improving the overall efficiency (the switching frequency of
the main H-bridges is the fundamental load frequency). Fig. 3
shows the output voltage of the proposed ACHB inverter using
NLC. Then, if the dc source voltage (Vdc ) is modified, the
output voltage (Vload ) varies proportionally, keeping the full
number of levels and the lowest THD at any output voltage
magnitudes (Fig. 4). On the other hand, when Vdc is constant,
the output voltage must be controlled by using PWM directly
on the H-bridges, as in conventional ACHB. This conventional
PWM reduces the waveform quality of the output voltage
when its amplitude decreases, because the number of levels is
As was already mentioned, in ACHB inverters with dc supplies scaled in power of three, the main H-bridges manage
more than 80% of the power system, which makes possible the
solution proposed: an HFL that manages less than 20% of the
power to supply the auxiliary H-bridges. This feature will be
demonstrated in the following lines.
The full power per phase delivered for an ACHB inverter
with N auxiliary H-bridges (3N +1 levels) is
⎧
0 = main
⎪
⎪
N
⎨ 1 = aux-1
1
1
Vrms · Irms
Pload =
· cos ϕ j ,
j = ..
⎪
⎪
j=0
⎩.
N = aux-N
(1)
1
1
where Vrms
, Irms
, and cos ϕ are the fundamentals of the voltages, currents, and displacement factor, respectively. As the
H-bridges are in series, all the currents are the same
1
Irms
load
1 1 1 = Irms
= Irms
= · · · = Irms
.
main
aux-1
aux-N
(2)
1
1
1
Moreover, (Vrms
)main , (Vrms
)aux-1 , and (Vrms
)aux-N are in
phase as shown in Fig. 5, so the power factor is the same
for all H-bridges. For these reasons, the percentage of power
distribution is the same as the rms voltage distribution.
From (1) and (2)
1
Vrms
load
1
Vmax
load
1 1 = Vrms
+ Vrms
+ ···
main
aux-1
1 + Vrms aux-N
1 1 = Vmax
+ Vmax
+ ···
main
aux-1
1 + Vmax aux-N .
(3)
(4)
As the H-bridges are scaled in power of three, the size of each
level is VL = Vdc /3N as is shown in Fig. 6.
1
)load can be evalUsing Fourier series decomposition, (Vmax
uated with the integration of each rectangle of Fig. 6 step
PEREDA AND DIXON: HFL: SOLUTION FOR USING ONLY ONE DC SOURCE IN CASCADED MULTILEVEL INVERTERS
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TABLE III
P OWER D ISTRIBUTION
Fig. 5. Voltage waveforms and fundamental voltages of an ACHB inverter
with 27 levels (N = 2) and using the NLC modulation.
auxiliary H-bridges or N = 0) to a theoretically infinite-level
inverter
⎧
1.20 · Vdc ,
if N = 0 (3 levels)
⎪
⎪
⎪
if N = 1 (9 levels)
⎪ 1.44 · Vdc ,
⎨
1 if N = 2 (27 levels)
Vmax load = 1.49 · Vdc ,
⎪
.
⎪.
⎪
⎪
⎩.
1.50 · Vdc ,
if N = ∞ (∞ levels).
(6)
1
The values of each (Vmax
)aux-N can be obtained in the
following way:
1 1 1 Vmax aux-N = Vmax
|N − Vmax
|N −1 (7)
load
load
⎧
0.24 · Vdc ,
if N = 1
⎪
⎪
⎪
,
if N = 2
0.05
·
V
⎪
dc
⎨
1 ,
if N = 3
0.01
·
V
dc
Vmax aux-N =
(8)
⎪
..
⎪
⎪
⎪
⎩.
0.00 · Vdc ,
if N = ∞.
Fig. 6. Size of the voltage levels for an ACHB inverter scaled in power of
three.
by step
1
Vmax
load
=
8 Vdc
·
ωT 3N
⎛
cos−1 ( N1+1 )
3
⎜
·⎜
cos(ωt)dωt
⎝
0
cos−1 (
3N +1 )
cos(ωt)dωt
+
3
0
⎞
5
( 3N +1 )
⎟
cos(ωt)dωt + · · ·⎟
⎠
−1
cos
+
0
3N +1 −1
2
⎛
⎜
1 4 Vdc
⎜
·
Vmax load =
⎝
π 3N
j=0
⎞
2j+1
3N +1 )
⎟
cos(ωt)dωt⎟
⎠ . (5)
cos−1 (
0
1
Equation (5) allows getting the values of (Vmax
)load for any
number of auxiliary H-bridges, from a three-level inverter (zero
1
Then, the voltage relation for N = 2 in terms of (Vmax
)load
by using (6) is
1 N =2 if j = 0 (main)
0.81 · Vdc ,
Vmax j ,
if j = 1 (aux-1)
0.16
·
V
=
dc
1 )
(Vmax
load if j = 2 (aux-2).
0.03 · Vdc ,
(9)
Then, for an ACHB inverter with three H-bridges per phase
(N = 2), the 81% of the total power comes from the main Hbridges. It is important to realize that the minimum amount of
power from the main H-bridges to the load is produced when
N →∞
N =∞
N =0
1 1 Vmax load Vmax main 1.20 · Vdc
=
=
= 0.8.
N
=∞
1
1
(Vmax )load 1.50 · Vdc
(Vmax )load |
(10)
Then, no matter what the amount of auxiliary H-bridges in
the chain, the total power delivered from them to the load will
never go larger than 20% because the main H-bridges take at
least 80% of the total power. Table III shows the percentage of
power that each H-bridge manages as a function of the number
of 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 the
H-bridges, reducing the number of levels used (classic ACHB
control). Fig. 7 shows the power distribution among each
H-bridge at different reference voltage amplitudes in an ACHB
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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 58, NO. 9, SEPTEMBER 2011
Fig. 8.
Flux and voltage in a square-wave transformer.
Fig. 9.
Toroidal transformer for a 100-kW converter (used in the experiments).
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 main
H-bridge, with 16% by the aux-1 H-bridge and 3% by the aux-2
H-bridge.
ACHB inverters have a big drawback when constant dc supplies are used because, in some reference voltage amplitudes,
the average power of auxiliary H-bridges is negative, producing
regeneration. To solve this problem, bidirectional rectifiers or
dissipative resistors must be used [12], and the result is a more
complicated and less efficient converter. These operation ranges
can be avoided to inhibit regeneration by jumping those levels
through a special PWM strategy [14]. However, the proposed
ACHB inverter operates with all the number of levels permanently, maintaining the best power distribution and voltage
quality and avoiding the regeneration in motor mode.
III. HFL
A. Concept of HFL
The HFL feeds the auxiliary H-bridges. As can be seen in
Fig. 2, the circuit is quite simple because it only needs the
following: 1) a square-wave generator (H-bridge) of high frequency, rated at 20% of full power; 2) one multiwinding ferrite
transformer; and 3) some bridge rectifiers with simple fastrecovery diodes. The H-bridge only needs to generate a square
waveform of voltage, and hence, no control is required for its
operation. High-power HFL can be designed and implemented
today for up to 400 kW [23]. Then, up to 6 MW, converters are
possible if three independent ferrite transformers of 320 kW are
used for each aux-1 H-bridge and a fourth ferrite transformer of
180 kW is used for the three aux-2 H-bridges.
This solution (HFL) reduces the number of floating power
sources from N + 1 per phase to only one per phase and only
one for the entire system if the motor is connected with isolated
windings or if all the main H-bridges are replaced by a threephase inverter, as shown in Fig. 10. This HFL means small size,
weight, and cost when it is compared with other alternatives
used in machine drives. On the other hand, this solution matches
perfectly with the requirement for other applications, like traction drives, where one dc power source is mandatory.
B. Toroidal Transformer Design
As the transformer works with a square-wave voltage, it
needs a different design. To generate a square-wave voltage, the
flux must be a triangular function, as shown in Fig. 8. The slope
of the triangular wave defines the amplitude of the voltage.
According to Fig. 8
ϕ
T
max
0 ≤ t ≤ T2
T /4 · t− 4 , (11)
ϕ(t) =
3T
T
− ϕTmax
/4 · t − 4 ,
2 ≤t≤T
ϕ
0 < t < T2
dϕ N · Tmax
/4 = Vmax ,
v(t) = N ·
(12)
ϕmax
dt −N · T /4 = −Vmax , T2 < t < T
T
T /2
1
2 dt −
2 dt
Vrms = ·
Vmax
Vmax
T
0
T /2
ϕmax
= 4 · f · N · ϕmax
T /4
= 4 · f · N · A · Bmax
= Vmax = N ·
(13)
where f is the frequency, T is the period, N is the number of
turns, A is the core area, and Bmax is the flux density. As the
HFL works at a very high frequency, its size and weight become
very small.
For example, in a 100-kW ACHB inverter, the HFL power
will never be larger than 20 kW (20%). Assuming a 27-level
inverter (N = 3) with a single dc source of 300 V, the windings
of 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
−−Each secondary aux-2 : 1.1 kW
33.3 V 33.0 A.
54.0 A
PEREDA AND DIXON: HFL: SOLUTION FOR USING ONLY ONE DC SOURCE IN CASCADED MULTILEVEL INVERTERS
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Fig. 10. Scheme of the proposed system and power flow in motor and braking generator mode using an ACHB or hybrid cascaded inverter.
Fig. 12. (a) Experimental prototype. (b) Primary, secondary aux-1, and
secondary aux-2 voltages of the toroidal transformer (HFL).
Fig. 11. Output voltage and currents in a reverse speed operation (simulation).
If the HFL works at 20 kHz using a core transformer of
9 cm2 (3 cm × 3 cm) and a flux density of 0.2 T, the number of
turns required by the primary of the toroidal transformer is
N=
Vrms
300
= 21.
=
3
4 · f · A · Bmax
4 · 20 × 10 · 9 × 10−4 · 0.2
(14)
Therefore, the primary winding must have at least 21 turns,
and the design should consider 27 turns to satisfy the voltages scaled in power of three. Then, only nine turns for each
aux-1 winding and just three turns for each aux-2 winding are
required. For the primary current (64.7 A), a 20-mm2 copper
wire 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 larger
than the total area required for all the windings, the hole
should 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-frequency
transformer for a 100-kW converter using 27 turns for the single
Fig. 13. Output voltages using only NLC modulation and using NLC modulation with PWM among the levels.
primary winding, 9 turns per secondary aux-1 winding, and
3 turns per secondary aux-2 winding (27×1, 9×3, and 3×3).
C. Efficiency
The toroidal transformer uses a ferrite core, which has low
coercivity to reduce the hysteresis losses and high resistivity to
minimize the Foucault currents. As the number of turns is small
and the core is also small, the wires are very short, reducing the
copper losses. Even more, flat laminated copper or a litz wire
(cable made with many thin wires) can be used to reduce the
skin 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 low
frequency (fundamental frequency of the motor). This characteristic reduces the switching losses, improving the overall
efficiency of the main inverters, which represent at least 80% of
the total power.
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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 the
proposed system, the dynamic of the ACHB inverter is limited
by the dc-link capacitor, which limits the voltage speed rate of
the amplitude modulation. However, this speed rate limitation
is solved by using conventional PWM among the H-bridges
during transient operation, only until the capacitor reaches the
reference voltage. Therefore, very high dynamic operation is
allowed.
IV. R EGENERATIVE B RAKING
To allow regenerative braking, the single ac/dc or dc/dc
converter shown in Figs. 2 and 10 must be bidirectional, or
a dissipative option must be used to consume the energy in
a resistor. Despite this modification, the regenerative braking
will only work at a three-level configuration because all the
auxiliary H-bridges are diode rectifiers (unidirectional). To
operate the proposed system as a three-level converter for
braking operation, all the auxiliary H-bridges are switched to
0-V operation (the two upper or lower transistors of the
H-bridges in “ON state”). As only the main H-bridges are used,
the power regeneration is limited by the power capacity of the
main H-bridges, which manage 80% of the full power at
least.
To have full-level operation for regenerative braking, the system becomes more complicated because each auxiliary rectifier
must be replaced by a PWM regenerative rectifier or by a dissipative device (resistor). These solutions increase complexity
and cost, which is unnecessary because three-level operation
is more than enough for regenerative braking, which generally
works for short periods.
V. S IMULATIONS
To probe the high dynamic performance of the proposed
ACHB inverter, a reversible speed operation was simulated
using direct torque control. For this fast operation, a conven-
tional PWM among the H-bridges was used to control the
motor voltage amplitude through a reduction or increment in
the number of voltage levels. This strategy achieves a very high
speed rate of the voltage variation, as shown in Fig. 11.
VI. E XPERIMENTS
A 3-kW experimental prototype was assembled to test the
proposed ACHB inverter. The HFL works with a square waveform of voltage at 20 kHz (Fig. 12).
Fig. 13 shows a comparison between the voltages of a
squirrel-cage induction motor using the following two modulation methods: 1) a single NLC modulation and 2) a combination
of NLC with PWM among each voltage level. The PWM among
each voltage level is optional if high transient operation is
required, but the single NLC has better efficiency than the
second one because it has lower switching frequency.
Fig. 14 shows a comparative sequence of typical voltage
waveforms in two possible function modes: 1) when the dc
source has constant voltage and 2) when the dc source has
variable voltage (both cases use NLC). In the first case, the
amplitude of the output voltage has to be controlled directly
into the main and auxiliary H-bridges, changing the switching
modulation through the NLC strategy. As can be seen, the
number of levels decreases when the output voltage is reduced,
losing the quality and raising the THD as is shown in Fig. 4. By
contrast, in the second case, the voltage remains its waveform
at any operation range because the NLC modulation is constant,
so the output voltage amplitude is controlled by modifying the
magnitude of the single dc source. As can be seen, the number
of levels remains for all output voltage amplitudes, and hence,
the THD remains at its minimum value (3%). This is another
great advantage of this new topology.
Figs. 15 and 16 show that the full 27 levels remain without
distortion, when the voltage of the single dc source changes, as
was verified in Fig. 14. As can be seen, the speed rate of the
voltage amplitude is limited by the time constant RC, given by
the dc-link capacitor.
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sinusoidal reference (conventional operation) and 2) variable
dc source using NLC with constant sinusoidal reference. In
the second case, the number of levels remains constant for all
voltage amplitudes, keeping a very low THD voltage (3%).
The proposed solution can be applied to converters of up to
6 MW because it is limited for restriction design of the HFL
transformer. Nevertheless, this power permits satisfying a wide
range of applications.
Fig. 15. Output voltages (Vload ) using full level (27 levels) permanently and
decreasing the amplitude voltage with the single dc supply (Vdc ).
Fig. 16. Output voltages (Vload ) using full level (27 levels) permanently and
increasing the amplitude voltage with the single dc supply (Vdc ).
Fig. 17. Output voltage and current in one phase. (a) Transition from motor to
regeneration mode. (b) Motor mode when the HFL fails (three-level operation).
Fig. 17(a) shows a transition from motoring operation to
regenerative operation. The transition is very clean, and the
current changes from 27-level operation to 3-level operation
without problems. The three-level operation also makes the
system more reliable. Fig. 17(b) shows a motor mode operation
when the HFL fails, so the inverter works only with the main
H-bridges, operating at 80% of the full power at least.
VII. C ONCLUSION
An asymmetric cascaded multilevel inverter using only one
power source has been implemented and tested. To eliminate
the dc sources of the auxiliary converters, the system uses an
HFL, based on a square-wave generator and a multiwinding
toroidal transformer. This paper has focused on a 27-level
ACHB inverter, but the idea can be applied to converters with
any number of levels. The topology also permits regenerative braking in three-level operation by using only the main
H-bridges. The solution proposed permits the following two
function modes: 1) constant dc source using NLC with variable
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Javier Pereda (S’09) was born in Santiago, Chile.
He received the Electrical engineering degree (with
highest honors) from the Pontificia Universidad
Católica de Chile, Santiago, in 2009, where he is
currently working toward the Ph.D. degree.
He is a Research Assistant in power electronics,
electrical machines, power generation, and electric
traction with the Department of Electrical Engineering, Pontificia Universidad Católica de Chile, where
he is also part of the Electric Vehicle Laboratory. He
is currently working on ac motor drives, direct torque
control, and new multilevel inverter topologies.
Mr. Pereda is a member of Millennium Nucleus Power Electronics, Mechatronics and Control Process (NEIM) and a Comisión Nacional de Investigación
Científica y Tecnológica scholarship holder.
Juan Dixon (M’90–SM’95) was born in Santiago,
Chile. He received the Ms.Eng. and Ph.D. degrees
from McGill University, Montreal, QC, Canada, in
1986 and 1988, respectively.
Since 1979, he has been with the Department
of Electrical Engineering, Pontificia Universidad
Católica de Chile, Santiago, where he is currently
a Professor. He has presented more than 70 works
in international conferences and has published more
than 40 papers related with power electronics in
IEEE transactions and IEE proceedings. His main
areas of interest are in electric traction, pulsewidth modulation rectifiers, active
filters, power factor compensators, and multilevel converters. He has created the
Electric Vehicle Laboratory, where state-of-the-art vehicles are investigated.