Power electronic handbook
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448 S. Y. Hui and H. S. H. Chung
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17
Multilevel Power Converters
Surin Khomfoi and
Leon M. Tolbert, Ph.D.
The University of Tennessee,
Department of Electrical and
Computer Engineering,
Knoxville, Tennessee, USA
17.1 Introduction .......................................................................................... 451
17.2 Multilevel Power Converter Structures........................................................ 452
17.2.1 Cascaded H-bridges • 17.2.2 Diode-clamped Multilevel Inverter •
17.2.3 Flying-capacitor Multilevel Inverter • 17.2.4 Other Multilevel Inverter Structures
17.3 Multilevel Converter PWM Modulation Strategies ........................................ 459
17.3.1 Multilevel Carrier-based PWM • 17.3.2 Multilevel Space Vector PWM •
17.3.3 Selective Harmonic Elimination
17.4 Multilevel Converter Design Example......................................................... 470
17.4.1 Interface with Electrical System • 17.4.2 Number of Levels and Voltage Rating of
Active Devices • 17.4.3 Number and Voltage Rating of Clamping Diodes • 17.4.4 Current
Rating of Active Devices • 17.4.5 Current Rating of Clamping Diodes • 17.4.6 DC Link
Capacitor Specifications
17.5 Fault Diagnosis in Multilevel Converters ..................................................... 478
17.6 Renewable Energy Interface ...................................................................... 478
17.7 Conclusion ............................................................................................ 480
References ............................................................................................. 480
17.1 Introduction
Numerous industrial applications have begun to require higher
power apparatus in recent years. Some medium voltage motor
drives and utility applications require medium voltage and
megawatt power level. For a medium voltage grid, it is trou-
blesome to connect only one power semiconductor switch
directly. As a result, a multilevel power converter structure has
been introduced as an alternative in high power and medium
voltage situations. A multilevel converter not only achieves
high power ratings, but also enables the use of renewable
energy sources. Renewable energy sources such as photo-
voltaic, wind, and fuel cells can be easily interfaced to a
multilevel converter system for a high power application [1–3].
The concept of multilevel converters has been introduced
since 1975 [4]. The term multilevel began with the three-
level converter [5]. Subsequently, several multilevel converter
topologies have been developed [6–13]. However, the ele-
mentary concept of a multilevel converter to achieve higher
power is to use a series of power semiconductor switches with
several lower voltage dc sources to perform the power conver-
sion by synthesizing a staircase voltage waveform. Capacitors,
batteries, and renewable energy voltage sources can be used
as the multiple dc voltage sources. The commutation of the
power switches aggregate these multiple dc sources in order to
achieve high voltage at the output, however, the rated voltage
of the power semiconductor switches depends only upon the
rating of the dc voltage sources to which they are connected.
A multilevel converter has several advantages over a conven-
tional two-level converter that uses high switching frequency
pulse width modulation (PWM). The attractive features of a
multilevel converter can be briefly summarized as follows.
• Staircase waveform quality: Multilevel converters not
only can generate the output voltages with very low dis-
tortion, but also can reduce the dv/dt stresses, therefore
electromagnetic compatibility (EMC) problems can be
reduced.
•
Common-mode (CM) voltage: Multilevel converters pro-
duce smaller CM voltage, therefore, the stress in the
bearings of a motor connected to a multilevel motor drive
can be reduced. Furthermore, CM voltage can be elim-
inated by using advanced modulation strategies such as
that proposed in [14].
• Input current: Multilevel converters can draw input
current with low distortion.
• Switching frequency: Multilevel converters can oper-
ate at both fundamental switching frequency and high
switching frequency PWM. It should be noted that lower
Copyright © 2007, 2001, Elsevier Inc.
All rights reserved.
451
452 S. Khomfoi and L. M. Tolbert
switching frequency usually means lower switching loss
and higher efficiency.
Unfortunately, multilevel converters do have some disad-
vantages. One particular disadvantage is the greater number of
power semiconductor switches needed. Although lower voltage
rated switches can be utilized in a multilevel converter, each
switch requires a related gate drive circuit. This may cause the
overall system to be more expensive and complex.
Plentiful multilevel converter topologies have been pro-
posed during the last two decades. Contemporary research
has engaged novel converter topologies and unique modu-
lation schemes. Moreover, three different major multilevel
converter structures have been reported in the literature:
cascaded H-bridges converter with separate dc sources, diode
clamped (neutral clamped), and flying capacitors (capaci-
tor clamped). Moreover, abundant modulation techniques
and control paradigms have been developed for multilevel con-
verters such as sinusoidal pulse width modulation (SPWM),
selective harmonic elimination (SHE-PWM), space vector
modulation (SVM), and others. In addition, many multilevel
converter applications focus on industrial medium-voltage
motor drives [11, 15], utility interface for renewable energy
systems [16], flexible ac transmission system (FACTS) [17],
and traction drive systems [18].
This chapter reviews state of the art of multilevel power
converter technology. Fundamental multilevel converter struc-
tures and modulation paradigms are discussed including
the pros and cons of each technique. Particular concentra-
tion is addressed in modern and more practical industrial
applications of multilevel converters. A procedure for calcu-
lating the required ratings for the active switches, clamping
diodes, and dc link capacitors including a design example
are described. Finally, the possible future developments of
multilevel converter technology are noted.
17.2 Multilevel Power Converter
Structures
As previously mentioned, three different major multilevel con-
verter structures have been applied in industrial applications:
cascaded H-bridges converter with separate dc sources, diode
clamped, and flying capacitors. Before continuing discussion
in this topic, it should be noted that the term multilevel con-
verter is utilized to refer to a power electronic circuit that could
operate in an inverter or rectifier mode. The multilevel inverter
structures are the focus of this chapter, however, the illustrated
structures can be implemented for rectifying operation as well.
17.2.1 Cascaded H-bridges
A single-phase structure of an m-level cascaded inverter is
illustrated in Fig. 17.1. Each separate dc source (SDCS) is
connected to a single-phase full-bridge or H-bridge inverter.
v
a
v
a[(m-1)/2]
v
a[(m-1)/2-1]
v
a2
v
a1
n
S
1
S
3
S
2
V
dc
V
dc
V
dc
V
dc
S
4
+
+
+
+
−
−
−
−
SDCS
SDCS
SDCS
SDCS
S
1
S
2
S
3
S
4
S
1
S
1
S
2
S
4
S
3
S
3
S
2
S
4
FIGURE 17.1 Single-phase structure of a multilevel cascaded H-bridges
inverter.
Each inverter level can generate three different voltage out-
puts, +V
dc
,0,and−V
dc
by connecting the dc source to the
ac output by different combinations of the four switches, S
1
,
S
2
,S
3
, and S
4
. To obtain +V
dc
, switches S
1
and S
4
are turned
on, whereas −V
dc
can be obtained by turning on switches S
2
and S
3
. By turning on S
1
and S
2
or S
3
and S
4
, the output volt-
age is Zero. The ac outputs of each of the different full-bridge
inverter levels are connected in series such that the synthe-
sized voltage waveform is the sum of the inverter outputs.
The number of output phase voltage levels m in a cascade
inverter is defined by m = 2s + 1, where s is the number
of separate dc sources. An example phase voltage waveform
for an 11-level cascaded H-bridge inverter with five SDCSs
and five full bridges is shown in Fig. 17.2. The phase voltage
v
an
= v
a1
+v
a2
+v
a3
+v
a4
+v
a5
.
For a stepped waveform such as the one depicted in
Fig. 17.2 with s steps, the Fourier Transform for this waveform
follows [14, 18]
V (ωt) =
4V
dc
π
n
[
cos
(
nθ
1
)
+cos
(
nθ
2
)
+···+cos
(
nθ
s
)
]
×
sin
(
nωt
)
n
, where n = 1, 3, 5, 7, ... (17.1)
From Eq. (17.1), the magnitudes of the Fourier coefficients
when normalized with respect to V
dc
are as follows
H
(
n
)
=
4
πn
[
cos
(
nθ
1
)
+cos
(
nθ
2
)
+···
+cos
(
nθ
s
)
]
, where n = 1, 3, 5, 7, ... (17.2)
17 Multilevel Power Converters 453
–5V
dc
0
π/2
3π/2
2π
π
v
a-n
v
a-n
*
v
a5
v
a4
v
a3
v
a2
v
a1
0
0
V
dc
–V
dc
5V
dc
P1
P2
P3
P4
P5
P1
P2
P3
P4
P5
θ
5
θ
4
θ
1
π−θ
1
π−θ
2
π−θ
3
π−θ
4
π−θ
5
θ
2
θ
3
FIGURE 17.2 Output phase voltage waveform of an 11-level cascade
inverter with five separate dc sources.
The conducting angles, θ
1
, θ
2
, ..., θ
s
, can be chosen such
that the voltage total harmonic distortion is minimum. Gen-
erally, these angles are chosen so that predominant lower
frequency harmonics, 5th, 7th, 11th, and 13th harmonics
are eliminated [19]. More details on harmonic elimination
techniques will be presented in the next section.
Multilevel cascaded inverters have been proposed for
such applications as static var generation, an interface with
renewable energy sources, and for battery-based applications.
H-
Bridge
INV.
Motor
To
Charger
Charge/Drive
Switch
AC
H-Bridge Inverter
48 V
Battery
DC
+
−
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
H-
Bridge
INV.
FIGURE 17.3 Three-phase wye-connection structure for electric vehicle motor drive and battery charging.
Three-phase cascaded inverters can be connected in wye, as
shown in Fig. 17.3, or in delta. Peng has demonstrated a pro-
totype multilevel cascaded static var generator connected in
parallel with the electrical system that could supply or draw
reactive current from an electrical system [20–23]. The inverter
could be controlled to either regulate the power factor of the
current drawn from the source or the bus voltage of the elec-
trical system where the inverter was connected. Peng [20]
and Joos [24] have also shown that a cascade inverter can be
directly connected in series with the electrical system for static
var compensation. Cascaded inverters are ideal for connecting
renewable energy sources with an ac grid, because of the need
for separate dc sources, which is the case in applications such
as photovoltaics or fuel cells.
Cascaded inverters have also been proposed for use as the
main traction drive in electric vehicles, where several batteries
or ultracapacitors are well suited to serve as SDCSs [18, 25].
The cascaded inverter could also serve as a rectifier/charger
for the batteries of an electric vehicle while the vehicle was
connected to an ac supply as shown in Fig. 17.3. Additionally,
the cascade inverter can act as a rectifier in a vehicle that uses
regenerative braking.
Manjrekar has proposed a cascade topology that uses mul-
tiple dc levels, which instead of being identical in value are
multiples of each other [26, 27]. He also uses a combination
of fundamental frequency switching for some of the levels and
PWM switching for part of the levels to achieve the output
voltage waveform. This approach enables a wider diversity of
output voltage magnitudes, however, it also results in unequal
voltage and current ratings for each of the levels and loses the
advantage of being able to use identical modular units for each
level.
The main advantages and disadvantages of multilevel cas-
caded H-bridge converters are as follows [28, 29].
454 S. Khomfoi and L. M. Tolbert
Advantages:
• The number of possible output voltage levels is more than
twice the number of dc sources (m = 2s + 1).
• The series of H-bridges makes for modularized lay-
out and packaging. This will enable the manufacturing
process to be done more quickly and cheaply.
Disadvantages:
• Separate dc sources are required for each of the
H-bridges. This will limit its application to products that
already have multiple SDCSs readily available.
Another kind of cascaded multilevel converter with trans-
formers using standard three-phase bi-level converters has
been proposed [14]. The circuit is shown in Fig. 17.4. The
converter uses output transformers to add different voltages.
In order for the converter output voltages to be added up,
the outputs of the three converters need to be synchronized
with a separation of 120
◦
between each phase. For exam-
ple, obtaining a three-level voltage between outputs a and b,
the output voltage can be synthesized by V
ab
= V
a1−b1
+
V
b1−a2
+ V
a2−b2
. An isolated transformer is used to provide
voltage boost. With three converters synchronized, the volt-
ages V
a1−b1
, V
b1−a2
, V
a2−b2
, are all in phase, thus, the output
level can be tripled [1].
The advantage of the cascaded multilevel converters with
transformers using standard three-phase bi-level converters
is that the three converters are identical and thus control is
more simple. However, the three converters need separate DC
sources, and a transformer is needed to add up the output
voltages.
V4
V3
V2
V1
V0
C4
C3
C2
C1
V5
C5
Va
Vc
Vb
Sa5
Sa4
Sa3
Sa2
Sa1
Sa'5
Sa'4
Sa'3
Sa'2
Sa'1
Sb5
Sb4
Sb3
Sb2
Sb1
Sb'5
Sb'4
Sb'3
Sb'2
Sb'1
Sc5
Sc4
Sc3
Sc2
D1
D2
D3
Sc1
D4
Sc'5
Sc'4
Sc'3
Sc'2
D4
D3
D2
Sc'1
D1
Vdc
Vdc
Vdc
Vdc
Vdc
+
+
+
+
+
-
-
-
-
-D
1
D2
D3
D4
D1
D2
D3
D4
D4
D3
D2
D1
D4
D3
D2
D1
FIGURE 17.5 Three-phase six-level structure of a diode-clamped inverter.
a
1
a
2
a
3
b
1
b
2
b
3
c
1
c
2
c
3
M
a
b
c
Inverter 1
Inverter 2
Inverter 3
T1
T2T3
FIGURE 17.4 Cascaded multilevel converter with transformers using
standard three-phase bi-level converters.
17.2.2 Diode-clamped Multilevel Inverter
The neutral point converter proposed by Nabae, Takahashi,
and Akagi in 1981 was essentially a three-level diode-clamped
inverter [5]. In the 1990s, several researchers published arti-
cles that have reported experimental results for four-, five-,
and six-level diode-clamped converters for uses such as
static var compensation, variable speed motor drives, and
high-voltage system interconnections [17–30]. A three-phase
six-level diode-clamped inverter is shown in Fig. 17.5. Each
of the three phases of the inverter shares a common dc bus,
17 Multilevel Power Converters 455
TABLE 17.1 Diode-clamped six-level inverter voltage levels and
corresponding switch states
Voltage V
a0
Switch state
S
a5
S
a4
S
a3
S
a2
S
a1
S
a
5
S
a
4
S
a
3
S
a
2
S
a
1
V
5
= 5V
dc
1111100000
V
4
= 4V
dc
0111110000
V
3
= 3V
dc
0011111000
V
2
= 2V
dc
0001111100
V
1
= 1V
dc
0000111110
V
0
= 0 0000011111
which has been subdivided by five capacitors into six levels.
The voltage across each capacitor is V
dc
, and the voltage stress
across each switching device is limited to V
dc
through the
clamping diodes. Table 17.1 lists the output voltage levels pos-
sible for one phase of the inverter with the negative dc rail
voltage V
0
as a reference. State condition 1 means the switch is
on, and 0 means the switch is off. Each phase has five comple-
mentary switch pairs such that turning on one of the switches
of the pair requires that the other complementary switch be
turned off. The complementary switch pairs for phase leg a are
(S
a1
, S
a
1
), (S
a2
, S
a
2
), (S
a3
, S
a
3
), (S
a4
, S
a
4
), and (S
a5
, S
a
5
).
Table 17.1 also shows that in a diode-clamped inverter, the
switches that are on for a particular phase leg are always adja-
cent and in series. For a six-level inverter, a set of five switches
is on at any given time.
Figure 17.6 shows one of the three line–line voltage wave-
forms for a six-level inverter. The line voltage V
ab
consists of a
phase-leg a voltage and a phase-leg b voltage. The resulting line
voltage is an 11-level staircase waveform. This means that an
m-level diode-clamped inverter has an m-level output phase
voltage and a (2m − 1)-level output line voltage.
Although each active switching device is required to block
only a voltage level of V
dc
, the clamping diodes require
different ratings for reverse voltage blocking. Using phase a
of Fig. 17.5 as an example, when all the lower switches S
a
1
−V
5
0
π/2
3π/2
2π
Fundamental
component of V
ab
V
ab
V
0
V
5
−V
4
−V
3
−V
2
−V
1
V
4
V
3
V
2
V
1
FIGURE 17.6 Line voltage waveform for a six-level diode-clamped
inverter.
through S
a
5
are turned on, D
4
must block four voltage levels,
or 4V
dc
. Similarly, D
3
must block 3V
dc
, D
2
must block 2V
dc
,
and D
1
must block V
dc
. If the inverter is designed such that
each blocking diode has the same voltage rating as the active
switches, D
n
will require n diodes in series; consequently, the
number of diodes required for each phase would be (m −1) ×
(m − 2). Thus, the number of blocking diodes is quadrat-
ically related to the number of levels in a diode-clamped
converter [28].
One application of the multilevel diode-clamped inverter
is an interface between a high-voltage dc transmission line
and an ac transmission line [29]. Another application would
be as a variable speed drive for high-power medium-voltage
(2.4–13.8 kV) motors as proposed in [3, 6, 19, 28–30].
Static var compensation is an additional function which
several authors have proposed for the diode-clamped con-
verter. The main advantages and disadvantages of multilevel
diode-clamped converters are as follows [1–3].
Advantages:
• All of the phases share a common dc bus, which mini-
mizes the capacitance requirements of the converter. For
this reason, a back-to-back topology is not only pos-
sible but also practical for uses such as a high-voltage
back-to-back inter-connection or an adjustable speed
drive.
• The capacitors can be pre-charged as a group.
• Efficiency is high for fundamental frequency switching.
Disadvantages:
• Real power flow is difficult for a single inverter because
the intermediate dc levels will tend to overcharge or
discharge without precise monitoring and control.
• The number of clamping diodes required is quadrat-
ically related to the number of levels, which can be
cumbersome for units with a high number of levels.
17.2.3 Flying-capacitor Multilevel Inverter
Meynard and Foch introduced a flying-capacitor-based
inverter in 1992 [31]. The structure of this inverter is similar to
that of the diode-clamped inverter except that instead of using
clamping diodes, the inverter uses capacitors in their place.
The circuit topology of the flying-capacitor multilevel inverter
is shown in Fig. 17.7. This topology has a ladder structure of
dc side capacitors, where the voltage on each capacitor differs
from that of the next capacitor. The voltage increment between
two adjacent capacitor legs gives the size of the voltage steps
in the output waveform.
One advantage of the flying-capacitor-based inverter is that
it has redundancies for inner voltage levels, in other words, two
or more valid switch combinations can synthesize an output
voltage. Table 17.2 shows a list of all the combinations of phase
voltage levels that are possible for the six-level circuit shown
456 S. Khomfoi and L. M. Tolbert
V
4
V3
V2
V1
V0
C2
C3
C4
C5
V5
C1
Va
Vc
Vb
Sc5
Sc4
Sc3
Sc2
Sc1
Sc'5
Sc'4
Sc'3
Sc'2
Sc'1
Vdc
Cc1
Cc2
Cc2
Cc3
Cc3
Cc3
Cc4
Cc4
Cc4
Cc4
Sb5
Sb4
Sb3
Sb2
Sb1
Sb'5
Sb'4
Sb'3
Sb'2
Sb'1
Cb1
Cb2
Cb2
Cb3
Cb3
Cb3
Cb4
Cb4
Cb4
Cb4
Sa5
Sa4
Sa3
Sa2
Sa1
Sa'5
Sa'4
Sa'3
Sa'2
Sa'1
Ca1
Ca2
Ca2
Ca3
Ca3
Ca3
Ca4
Ca4
Ca4
Ca4
FIGURE 17.7 Three-phase six-level structure of a flying-capacitor inverter.
in Fig. 17.7. Unlike the diode-clamped inverter, the flying-
capacitor inverter does not require all of the switches that
are on (conducting) be in a consecutive series. Moreover, the
flying-capacitor inverter has phase redundancies, whereas
the diode-clamped inverter has only line–line redundancies
[2, 3, 32]. These redundancies allow a choice of charging/
discharging specific capacitors and can be incorporated in the
control system for balancing the voltages across the various
levels.
In addition to the (m − 1) dc link capacitors, the m-level
flying-capacitor multilevel inverter will require (m − 1) ×
(m − 2)/2 auxiliary capacitors per phase if the voltage rating
of the capacitors is identical to that of the main switches. One
application proposed in the literature for the multilevel flying
capacitor is static var generation [2, 3]. The main advantages
and disadvantages of multilevel flying-capacitor converters are
as follows [2, 3].
Advantages:
• Phase redundancies are available for balancing the voltage
levels of the capacitors.
• Real and reactive power flow can be controlled.
• The large number of capacitors enables the inverter to
ride through short duration outages and deep voltage
sags.
Disadvantages:
•
Control is complicated to track the voltage levels for all
of the capacitors. Also, precharging all of the capacitors
to the same voltage level and startup are complex.
•
Switching utilization and efficiency are poor for real
power transmission.
• The large number of capacitors are both more expensive
and bulky than clamping diodes in multilevel diode-
clamped converters. Packaging is also more difficult in
inverters with a high number of levels.
17.2.4 Other Multilevel Inverter Structures
Besides the three basic multilevel inverter topologies previ-
ously discussed, other multilevel converter topologies have
been proposed, however, most of these are “hybrid” circuits
that are combinations of two of the basic multilevel topologies
or slight variations to them. Additionally, the combination of
multilevel power converters can be designed to match with a
specific application based on the basic topologies. In the inter-
est of completeness, some of these will be identified and briefly
described.
A. Generalized Multilevel Topology
Existing multilevel converters such as diode-clamped and
capacitor-clamped multilevel converters can be derived from
the generalized converter topology called P2 topology pro-
posed by Peng [33] as illustrated in Fig. 17.8. The generalized
multilevel converter topology can balance each voltage level
by itself regardless of load characteristics, active or reactive
power conversion, and without any assistance from other cir-
cuits at any number of levels automatically. Thus, the topology
17 Multilevel Power Converters 457
TABLE 17.2 Flying-capacitor six-level inverter redundant voltage levels and corresponding switch states
Voltage V
a0
Switch state
S
a5
S
a4
S
a3
S
a2
S
a1
S
a
5
S
a
4
S
a
3
S
a
2
S
a
1
V
a0
= 5V
dc
(no redundancies)
5V
dc
1111100000
V
a0
= 4V
dc
(4 redundancies)
5V
dc
−V
dc
1111000001
4V
dc
0111110000
5V
dc
−4V
dc
+3V
dc
1011101000
5V
dc
−3V
dc
+2V
dc
1101100100
5V
dc
−2V
dc
+V
dc
1110100010
V
a0
= 3V
dc
(5 redundancies)
5V
dc
−2V
dc
1110000011
4V
dc
−V
dc
0111010001
3V
dc
0011111000
5V
dc
−4V
dc
+3V
dc
−V
dc
1011001001
5V
dc
−3V
dc
+V
dc
1100100110
4V
dc
−2V
dc
+V
dc
0110110010
V
a0
= 2V
dc
(6 redundancies)
5V
dc
−3V
dc
1100000111
5V
dc
−4V
dc
+V
dc
1000101110
4V
dc
−2V
dc
0110010011
4V
dc
−3V
dc
+V
dc
0100110110
3V
dc
−V
dc
0011011001
3V
dc
−2V
dc
+V
dc
0010111010
2V
dc
0001111100
V
a0
= V
dc
(4 redundancies)
5V
dc
−4V
dc
1000001111
4V
dc
−3V
dc
0100010111
3V
dc
−2V
dc
0010011011
2V
dc
−V
dc
0001011101
V
dc
0000111110
V
a0
= 0 (no redundancies)
0 0000011111
provides a complete multilevel topology that embraces the
existing multilevel converters in principle.
Figure 17.8 shows the P2 multilevel converter structure per
phase leg. Each switching device, diode, or capacitor’s voltage
is 1 V
dc
, for instance, 1/(m − 1) of the dc-link voltage. Any
converter with any number of levels, including the conven-
tional bi-level converter can be obtained using this generalized
topology [1, 33].
B. Mixed-level Hybrid Multilevel Converter
To reduce the number of separate dc sources for high-
voltage, high-power applications with multilevel converters,
diode-clamped or capacitor-clamped converters could be used
to replace the full-bridge cell in a cascaded converter [34].
An example is shown in Fig. 17.9. The nine-level cascade con-
verter incorporates a three-level diode-clamped converter as
the cell. The original cascaded H-bridge multilevel converter
requires four separate dc sources for one phase leg and twelve
for a three-phase converter. If a five-level converter replaces
the full-bridge cell, the voltage level is effectively doubled for
each cell. Thus, to achieve the same nine voltage levels for
each phase, only two separate dc sources are needed for one
phase leg and six for a three-phase converter. The configura-
tion has mixed-level hybrid multilevel units because it embeds
multilevel cells as the building block of the cascade converter.
The advantage of the topology is it needs less separate dc
sources. The disadvantage for the topology is its control will
be complicated due to its hybrid structure.
C. Soft-switched Multilevel Converter
Some soft-switching methods can be implemented for differ-
ent multilevel converters to reduce the switching loss and
458 S. Khomfoi and L. M. Tolbert
Basic P2 cell
V
dc
V
dc
V
dc
V
dc
V
dc
V
dc
V
dc
V
dc
V
dc
V
dc
V
d
c
V
d
c
V
d
c
V
d
c
V
o
2-level
3-level
4-level
5-level
n-level
V
dc
FIGURE 17.8 Generalized P2 multilevel converter topology for one
phase leg.
n
0.5V
dc
0.5V
dc
a
V
dc
0.5V
dc
0.5V
dc
V
dc
FIGURE 17.9 Mixed-level hybrid unit configuration using the three-
level diode-clamped converter as the cascaded converter cell to increase
the voltage levels.
C
r
S
B−
S
A−
S
B+
C
2
C
2
S
A+
S
1
S
2
S
3
S
4
a
n
C
1
L
r1
L
r2
C
r
C
r
C
r
FIGURE 17.10 Zero-voltage-switching capacitor-clamped inverter
circuit.
to increase efficiency. For the cascaded converter, because
each converter cell is a bi-level circuit, the implementa-
tion of soft switching is not at all different from that of
conventional bi-level converters. For capacitor-clamped or
diode-clamped converters, soft-switching circuits have been
proposed with different circuit combinations. One of the
soft-switching circuits is a zero-voltage-switching type which
includes auxiliary resonant commutated pole (ARCP), cou-
pled inductor with zero-voltage transition (ZVT), and their
combinations [1, 35] as shown in Fig. 17.10.
D. Back-to-back Diode-clamped Converter
Two multilevel converters can be connected in a back-to-back
arrangement and then the combination can be connected to
the electrical system in a series–parallel arrangement as shown
in Fig. 17.11. Both the current demanded from the utility
and the voltage delivered to the load can be controlled at the
same time. This series–parallel active power filter has been
referred to as a universal power conditioner [36–42] when
used on electrical distribution systems and as a universal power
flow controller [43–47] when applied at the transmission
level. Previously, Lai and Peng [29] proposed the back-to-
back diode-clamped topology shown in Fig. 17.12 for use as
a high-voltage dc inter-connection between two asynchronous
ac systems or as a rectifier/inverter for an adjustable speed
drive for high-voltage motors. The diode-clamped inverter has
been chosen over the other two basic multilevel circuit topolo-
gies for use in a universal power conditioner for the following
reasons:
• All six phases (three on each inverter) can share a com-
mon dc link. Conversely, the cascade inverter requires
that each dc level be separate, and this is not conducive
to a back-to-back arrangement.
• The multilevel flying-capacitor converter also shares a
common dc link however, each phase leg requires sev-
eral additional auxiliary capacitors. These extra capacitors
would add substantially to the cost and the size of the
conditioner.