Recent Advances and Industrial Applications of Multilevel Converters

TL;DR: This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry.

Abstract: Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.

Figures

Fig. 14. a) Transrapid Maglev train long linear synchronous motor with back-to-back 3L-NPC drive system [203], [204], and b) CHB transformerless front-end for power interface of a locomotive traction drive [207].

Fig. 13. Three-port UNIFLEX-PM system based on CHB multilevel converters, for power management of integrated power systems in distributed generation [117].

Fig. 12. a) 13-level CHB based 6.6kV 1MVA transformer-less STATCOM [232], b) 3L-NPC based active filter [239], and c) 7-level Flying Capacitor H-bridge active filter with tapped reactor connection for Marine power system [244].

Fig. 19. HVDC-plus transmission system, based on Modular Multilevel Converter (MMC) [55].

Fig. 17. Multilevel converter based photovoltaic grid connected systems: a) CHB-based, b) NPC-based.

Fig. 18. Back-to-back ANPC doubly-fed induction generator/motor for hydro-pumped energy storage application [11], [224].

Fig. 3. Three-phase five-level H-bridge NPC (5L-HNPC) [42]–[44].

Fig. 21. Regenerative downhill conveyor system based on two back-to-back 3L-NPCs, a 12-pulse transformer and SHE [219], [220].

Fig. 20. Nine-level asymmetric fed CHB class-D digital audio amplifier [222].

Fig. 16. Back-to-back NPC fully rated power converter for permanent magnet synchronous generator based variable speed wind turbine.

Fig. 4. Active NPCs (only phase a shown): a) 3L-ANPC featuring IGCTs [46], [47], b) 5L-ANPC featuring IGBTs (hybrid between 3L-ANPC and a 3L-FC) [48]–[50], c) Common cross converter stage plus 5L-ANPC hybrid 9-level converter [51].

Fig. 15. Tanker ship power system, featuring redundant 3L-NPC back-toback propulsion drive [200].

Full text

1
Recent Advances and Industrial Applications of
Multilevel Converters
Samir Kouro, Member, IEEE, Mariusz Malinowski, Senior Member, IEEE, K. Gopakumar, Senior Member, IEEE,
Josep Pou, Member, IEEE, Leopoldo G. Franquelo, Fellow, IEEE, Bin Wu, Fellow, IEEE, José Rodríguez, Senior
Member, IEEE Marcelo A. Pérez, Member, IEEE and Jose I. Leon, Member, IEEE
Abstract—Multilevel converters have been under research and
development for more than 3 decades, and have found successful
industrial application. However, this is still a technology under
development, and many new contributions and new commercial
topologies have been reported in the last few years. The aim of
this paper is to group and review these recent contributions, in
order to establish the current state of the art and trends of the
technology, to provide readers a comprehensive and insightful
review of where multilevel converter technology stands and is
heading. The paper first presents a brief overview of the well
established multilevel converters, strongly oriented to their cur-
rent state in industrial applications, to then center the discussion
on the new converters that have made their way to industry.
Also new promising topologies are discussed. Recent advances
made in modulation and control of multilevel converters are also
addressed. A great part of the paper is devoted to show non-
traditional applications powered by multilevel converters, and
how multilevel converters are becoming an enabling technology
in many industrial sectors. Finally, some future trends and
challenges in the further development of this technology are
discussed, to motivate future contributions that address open
problems and explore new possibilities.
Index Terms—Multilevel converters, modulation, control, high-
power applications, wind energy conversion, train traction,
marine propulsion, photovoltaic systems, FACTS, active filters,
HVDC transmission.
Manuscript received January 26, 2010. Accepted for publication April
3, 2010. This work was supported in part by the Chilean National Fund
of Scientific and Technological Development (FONDECYT) under Grant
1080582, in part by the Centro Cientifico-Tecnologico De Valparaiso (CCT-
Val) N
◦
FB0821, in part by Ryerson University, in part by the European Union
in the framework of European Social Fund through Center for Advanced
Studies Warsaw University of Technology, and in part by the Ministerio de
Ciencia y Tecnología of Spain under project ENE2007-67033-C03-00.
Copyright
c
° 2009 IEEE. Personal use of this material is permitted.
However, permission to use this material for any other purposes must be
obtained from the IEEE by sending a request to pubs-permissions@ieee.org.
S. Kouro and B. Wu are with the Department of Electrical and Computer
Engineering Ryerson University, M5B 2K3 Toronto, ON, Canada (e-mail:
samir.kouro@ieee.org; bwu@ee.ryerson.ca).
M. Malinowski is with the Institute of Control and Industrial Electronics,
Warsaw University of Technology, 00-662 Warsaw, Poland (e-mail: ma-
lin@isep.pw.edu.pl).
K. Gopakumar is with the Centre for Electronics Design and Tech-
nology, Indian Institute of Science, 560012 Bangalore, India (e-mail:
kgopa@cedt.iisc.ernet.in).
J. Pou is with the Electronic Engineering Department, Technical University
of Catalonia,08222 Terrassa, Catalonia, Spain (e-mail: pou@eel.upc.edu).
L. G. Franquelo and J. I. León are with the Department of Elec-
tronics Engineering, University of Seville, 41092 Seville, Spain (e-mail:
leopoldo@gte.esi.us.es; jileon@zipi.us.es).
J. Rodríguez and M. Pérez, are with the Electronics Engineering Depart-
ment, Universidad Técnica Federico Santa María, 2390123 Valparaíso, Chile
(e-mail: jrp@usm.cl; marcelo.perez@usm.cl).
I. INTRODUCTION
M
ULTILEVEL converters are finding increased attention
in industry and academia as one of the preferred choices
of electronic power conversion for high power applications
[1]–[10]. They have made their way successfully into industry
and therefore can be considered a mature and proven tech-
nology. Currently, they are commercialized in standard and
customized products that power a wide range of applications,
such as: compressors, extruders, pumps, fans, grinding mills,
rolling mills, conveyors, crushers, blast furnace blowers, gas
turbine starters, mixers, mine hoists, reactive power compen-
sation, marine propulsion, HVDC transmission, hydro pumped
storage, wind energy conversion, and railway traction to name
a few [1]–[10]. Converters for these applications are commer-
cially offered by a growing group of companies in the field
[11]–[26].
Although it is an enabling and already proven technol-
ogy, multilevel converters present a great deal of challenges,
and even more importantly, they offer such a wide range
of possibilities, that their research and development is still
growing in depth and width. Researchers all over the world are
contributing to further improve energy efficiency, reliability,
power density, simplicity and cost of multilevel converters, and
broaden their application field as they become more attractive
and competitive than classic topologies.
Recently, many publications have addressed multilevel con-
verter technology and stressed the growing importance of mul-
tilevel converters for high power applications [4]–[9]. These
works have a survey and tutorial nature and cover in depth the
traditional and well established multilevel converter topologies
like the Neutral Point Clamped (NPC), Cascaded H-bridge
(CHB) and the Flying Capacitor (FC), as well as the most used
modulation methods. Instead, this paper presents a technology
review, focused mainly on the most recent advances made
in this field in the past few years, covering new promising
topologies, modulations, controls and operational issues. In
addition, one of the most interesting topics in multilevel
converter technology is the rapidly increasing and diverse
application field, which is addressed in this work as well. Also
emerging trends, challenges and possible future directions
of the development in multilevel converter technology are
outlined to motivate further work in this field.
This paper is organized as follows: first, a brief overview
of classic multilevel topologies is presented in section II to
introduce basic concepts needed throughout the paper. This is

2
IGCT
a
N
+
_
V
dc
V
dc
/2
V
dc
/2
MV-IGBT
a
N
+
_
V
dc
V
dc
/2
V
dc
/2
V
dc
/2
a
LV-IGBT
V
dc
V
dc
N
(a) (b) (c)
Fig. 1. Classic multilevel converter topologies (only one phase shown): a)
Three-level Neutral Point Clamped (NPC) featuring IGCTs, b) Three-level
Flying Capacitor (FC) featuring MV-IGBTs and c) Five-level Cascaded H-
Bridge (CHB) featuring LV-IGBTs.
followed by a review of recent advances in multilevel converter
topologies in section III, where those already found in practice
and those currently under development are addressed. Section
IV covers the latest developments in multilevel modulation
methods. Latest contributions on multilevel converter control
and different operational issues, such as capacitor voltage
balance and fault tolerant operation are reviewed in section V.
New and future promising applications of multilevel converters
are described in section VI. Finally, in Section VII future
trends and challenges of multilevel converter technology are
discussed, which is followed by concluding remarks in section
VIII.
II. CLASSIC MULTILEVEL TOPOLOGIES OVERVIEW
For completeness and better understanding of the advances
in multilevel technology, it is necessary to cover classic
multilevel converter topologies. However, in order to focus
the content of this paper on the most recent advances and
ongoing research lines, the well established topologies will be
only briefly introduced and referred to existing literature. In
the following, classic topologies will be referred to those that
have been extensively analyzed and documented and have been
commercialized and used in practical applications for more
than a decade.
Multilevel converter technology started with the introduc-
tion of the multilevel stepped waveform concept with a Se-
ries Connected H-Bridge, also known as Cascaded H-Bridge
(CHB) converter in the late 60s [27]. This was followed
closely by a low power development of a flying capacitor (FC)
topology the same year [28]. Finally, in the late 70s, the diode
clamped converter (DCC) [29] was first introduced. The DCC
concept evolved into the three-level Neutral Point Clamped
Converter (3L-NPC) we know today as it was proposed in
[30]–[32], and can be considered as the first real multilevel
power converter for medium voltage applications. Later, the
CHB would be reintroduced in the late 80s [33], although it
would reach more industrial relevance in the mid 90s [34]. In
the same way, the early concept of the FC circuit introduced
for low power in the 60s developed into the medium voltage
multilevel converter topology we know today in the early 90s
[35]. Through the years the FC has also been reported as the
imbricated-cell and multi-cell converter (the latter is also a
name used for the CHB, since both are modular and made by
interconnection of power cells).
These three multilevel converter topologies could be consid-
ered now as the classic or traditional multilevel topologies that
first made it into real industrial products during the last two
decades. The power circuits of a single-phase leg of these three
topologies are shown in Fig. 1, featuring the corresponding
commonly used semiconductor device. These converters are
commercialized by several manufacturers in the field [11]–
[26], offering different power ratings, front end configurations,
cooling systems, semiconductor devices and control schemes,
among other technical specifications. The most relevant pa-
rameters and ratings for each od these classic topologies are
listed in Table I. The parameters for each category are given for
the different manufacturers, whose corresponding reference is
given at the bottom of the table. As can be observed from
the table, the 3L-NPC and the CHB are the most popular
multilevel topologies used in industry. It is not straight forward
or fair to compare the commercially available 3L-NPC with
the 7L- to 17L-CHB listed in Table I, since the first will have
worse power quality and the second a more complex circuit
structure. However, some evident differences between them
can be concluded from Table I:
• The NPC features medium/high voltage devices (IGCT
and MV/HV-IGBTs), while the CHB uses exclusively low
voltage IGBTs.
• The CHB reaches higher voltage and higher power levels.
• The NPC is definitely more suitable for back-to-back
regenerative applications. The CHB needs substantially
higher number of devices to achieve a regenerative option
(a 3-phase 2-level VSI per cell).
• The CHB needs a phase shifting transformer, usually
to conform a 36 pulse rectifier system. This is more
expensive but improves input power quality.
• The NPC has a simpler circuit structure, leading to a
smaller footprint.
• Although both topologies generate same amount of lev-
els when using same number of power switches, the
commercially available CHBs have more output voltage
levels (up to 17 compared to 3 of the NPC). Hence,
lower average device switching frequencies are possible
for same output voltage waveform quality. Therefore, air
cooling and higher fundamental output frequency can be
achieved without derating and without use of output filter.
These multilevel voltage source converter topologies belong
to the medium-voltage-high-power converter family, whose
classification is shown in Fig. 2. Note that generally speaking
the medium-voltage range is considered in the power converter
industry from 2.3 to 6.6kV, and high power from 1MW
to 50MW. The classification also includes the direct ac-
ac converters and current source converters, which currently
are the main competitors of multilevel technology: mainly
the cycloconverter and load commutated inverters for very
high power, high torque and low speed applications, and the

3
TABLE I
CLASSIC MULTILEVEL TOPOLOGIES COMMERCIAL RATINGS AND SPECIFICATIONS .
Parameter
Multilevel topology
3L-NPC CHB 4L-FC
Max. power 27MW
(1)
, 31.5MVA
(2)
, 40MVA
(3)
, 44MW
(4)
, 120MW
(2)
, 15MW
(3)
, 5.6MW
(7)
, 2.24MW
(15)
33.6MW
(5)
, 3.7MW
(6,9)
, 27MVA
(8)
, 10MW
(14)
10MVA
(10)
, 11.1MVA
(11)
, 6MVA
(12)
, 6250kVA
(13)
Output voltage [kV] 2.3/3.3/4.0/4.16
(1,2)
, 2.3/3.3/4.16
(4,6,8,9,14)
, 2.3–13.8
(2)
, 3.3/6.6
(3,12)
, 2.3/4.16/6/11
(7)
, 2.3/3.3/4.16
(15)
3.3/6.6
(5)
3/6/10
(10)
, 3/4/6/10
(11)
, 3/3.3/4.16/6/6.6/10
(13)
Max. output freq. [Hz] 82.5
(1)
, 250
(2)
, 90
(3)
, 140
(4,14)
, 300
(5)
, 120
(6)
, 100
(8,9)
330
(2)
, 120
(3,7,11−13)
, 50
(10)
120
(15)
Diode front end [# pulses] 12/24
(1−5,8)
, 24
(6)
, 12/18
(9)
, 12/24/36
(14)
18/36
(2,3,12)
, 30
(7)
, 36
(11)
, 24/30/42/48
(13)
18/24/36 (diode+SCR)
(15)
Active front end option 3L-NPC in back-to-back
(1−5,8,14)
3-phase VSI per cell
(10)
4L-FC in back-to-back
(15)
Power semiconductor IGCT
(1,2,4,8)
, MV/HV-IGBT
(2,5,6,8,9,14)
, IEGT
(3,8)
LV-IGBT
(2,3,7,10−13)
MV-IGBT
(15)
Cooling system air/water
(1,2,4,8,14)
, water
(3,5)
, air
(9)
air/water
(2,13)
, air
(3,7,11,12)
air
(15)
Modulation method PWM
(2−6,14)
, SHE
(3,9)
, SVM
(8,9)
PS-PWM
(2,3,7,10−13)
PS-PWM
(15)
Control methods DTC
(1)
, v/f and FOC
(2−4,14)
, FOC
(5,6,8)
, v/f
(9)
, v/f and FOC
(2,3,7,11,12)
, FOC
(10,13)
v/f and FOC
(15)
DPC
(1)
, VOC
(2−5,8,14)
# voltage levels 3 9/13
(2)
, 7/13
(3,12)
, 11
(7)
, 7/11/13/19)
(10)
, 4
(15)
13
(11)
, 9/11/15/17
(13)
# power cells — 4/6
(2)
, 3/6
(3,12)
, 5
(7)
, 3/5/6/9
(10)
, 3
(15)
6
(11)
, 4/5/7/8
(13)
References
(1)
[11],
(2)
[12],
(3)
[13],
(4)
[14],
(5)
[15],
(6)
[16],
(7)
[17],
(8)
[18],
(9)
[19],
(10)
[20],
(11)
[21],
(12)
[22],
(13)
[23],
(14)
[24],
(15)
[25]
Note: Information provided in the table is to the authors best knowledge valid to the submission date of this paper, hence some differences or unintentional omissions could be
possible.
5L-ANPC
Multilevel Matrix
converters
Stacked Flying
capacitor
NPC +
Cascaded H-bridge
Flying capacitor +
Cascaded H-bridge
Other
MMC
(Cascaded Half-Bridge)
Flying Capacitor
High Power
2-Level VSI
Indirect conversion
(DC-Lnk) ac-dc-ac
Direct conversion
(ac-ac)
Matrix Converter
PWM Current
Source Inverter
Load Commutated
Inverter
Equal DC
sources
IGBT-based
IGCT-based
NPC Cascaded Topologies
Unequal DC
sources
Thyristor-based
3L-ANPC
H-NPC
Cascaded NPCs
(open winding loads)
Hybrid topologies
IGBT (bidirectional switch)
Voltage Source
CHB
(Cascaded H-Bridge)
Current Source
High Power Converters
Multilevel
Converters
CCC + 5L-ANPC
Cycloconverter
Transistor clamped
TCC (or NPP)
Fig. 2. Multilevel converter classification.
pulse width modulated current source inverter for high power
variable speed drives. Other multilevel converter topologies
also appear in this classification, some of them have recently
found practical application, and will be discussed later in this
paper.
The operating principles, multilevel waveform generation,
special characteristics, modulation schemes and other infor-
mation related to the NPC, FC, and CHB can be found with
plenty of details and useful references to previous works in
[2]–[9], and therefore will not covered in this paper devoted
to present research topics.
A number of papers have been published recently comparing
the three topologies for specific applications in terms of the
losses and the output voltage quality [36]–[38]. A few conclu-
sions from these papers are worth mentioning. The 3L-NPC
has become quite popular because of a simple transformer
rectifier power circuit structure, with a lower device count
when considering both the inverter and rectifier, and less
number of capacitors. Although the NPC structure can be
extended to higher number of levels, these are less attractive,
because of higher losses and uneven distribution of losses
in the outer and inner devices [5]. Specially the clamping
diodes, which have to be connected in series to block the
higher voltages, introduce more conduction losses and produce
reverse recovery currents during commutation that affects
switching losses of the other devices even more. Furthermore,
dc-link capacitors voltage balance becomes unattainable in
higher-level topologies with a passive front end when using
conventional modulation strategies [39]–[41]. In this case the
classic multilevel stepped waveform cannot be retained and
higher dv/dts (more than one-level transitions) are necessary
to balance the capacitors for certain modulation indexes.
On the other hand, the CHB is well suited for high power
applications because of the modular structure that enables
higher voltage operation with classic low voltage semicon-
ductors. The phase-shifting of the carrier signals moves the
frequency harmonics to the higher frequency side, and this to-
gether with the high number of levels enables the reduction of
the average device switching frequency (≤500 Hz), allowing
air cooling and lower losses. However, it requires large number
of isolated dc sources, which have to be fed from phase-
shifting isolation transformers, which are more expensive and
bulky compared the standard transformer used for the NPC.
Nevertheless, this has been effectively used to improve the
input power factor of this converter, reducing input current
harmonics.
Although the flying capacitor is modular in structure, like

4
the CHB, it has found less industrial penetration compared to
the NPC and CHB, mainly because higher switching frequen-
cies are necessary to keep the capacitors properly balanced,
whether a self balancing or a control assisted balancing
modulation method is used (e.g. greater than 1200 Hz) [5].
These switching frequencies are not feasible for high power
applications, where usually they are limited in a range from
500 to 700 Hz. This topology also requires initialization of
the flying capacitor voltages.
III. RECENT ADVANCES IN TOPOLOGIES
Since the introduction of the first multilevel topologies
almost four decades ago [27], perhaps dozens of variants and
new multilevel converters have been proposed in literature.
Most of them are variations to the three classic multilevel
topologies, discussed in previous section, or hybrids between
them. However, not so many have made their way to in-
dustry yet. Among the newer topologies that currently have
found practical application are: the five-level H-bridge NPC
(5L-HNPC), the three-level Active NPC (3L-ANP), the 5L-
ANPC, the Modular Multilevel Converter (MMC) and the
Cascaded Matrix Converter (CMC). Apart from these, several
other topologies have been proposed and are currently under
development, among them: the Transistor Clamped Converter
(TCC), the CHB fed with unequal dc-sources or asymmetric
CHB, the cascaded NPC feeding open-end loads, the hybrid
NPC-CHB and hybrid FC-CHB topologies, the stacked flying
capacitor or stacked multi-cell to name a few. All these
topologies are addressed in the following subsections, and can
be found in the medium voltage converter classification of Fig.
2.
A. Five-level H-bridge NPC (5L-HNPC)
This converter is composed by the H-bridge connection of
two classic 3L-NPC phase legs as shown in Fig. 3, form-
ing a five-level HNPC (5L-HNPC) converter, and was first
introduced in [45]. This topology has been commercialized
by two medium voltage drive manufacturers [11], [13], and
has received increased attention over the years [42]–[44].
The combination of the three levels of each leg of the
NPC (V
dc
/2, 0, −V
dc
/2) results in the five different output
levels (V
dc
, V
dc
/2, 0, −V
dc
/2, −V
dc
). As with the traditional
36-pulse rectifier system
a
3-phase, 5-level H-NPC
b
c
n
N
-20°
0°
+20°
-20°
0°
+20°
30°
0°
V
dc
2
V
dc
2
6-pulse
rectifier
6-pulse
rectifier
Fig. 3. Three-phase five-level H-bridge NPC (5L-HNPC) [42]–[44].
H-bridge, this topology requires an isolated dc-source for each
H-bridge to avoid short-circuit of the dc-links. Therefore a
transformer with three dedicated secondary three-phase wind-
ings is necessary to supply the H-bridge of each phase of the
converter. Moreover, since the semiconductors of the 3L-NPC
leg block half of the total dc-link voltage, higher voltage can
be reached with a series connection of two diode bridges. This
can lead to a 36-pulse rectifier system as can be seen in Fig.
3.
The disadvantage of a more complex transformer comes
along with an attractive feature which is the enhanced input
side power quality obtained with the phase-shifting trans-
former and multipulse rectifier configuration. Low order har-
monics are effectively canceled up to the 25th (for a 36 pulse
rectifier) improving greatly the input current THD, eliminating
the needs of filters just like with the CHB topology [34]. In
fact this topology features an identical transformer to the one
that would be used for a five-level two cell CHB, with the
same amount of diode rectifier bridges, number of capacitors
and switching semiconductors, but with the addition of 12
clamping diodes, and need to control the neutral point of each
H-bridge.
This converter can be found in practice with a 36-pulse
rectifier system, featuring IGCTs, and for a 2 to 7 MW
power range air cooled or 5 to 22 MW water cooled. Other
characteristics are: it is controlled with direct torque control,
reaches output frequency up to 250Hz and output voltages up
to 6.9kV [11]. Alternatively, several configurations of the 5L-
HNPC are available from [13]: with 24 or 36 pulse diode
rectifier front end, with medium voltage IGBTs, IEGTs or
GCTs, up to 7.8 kV maximum output voltage, up to 120Hz
output frequency, air or water cooled, vector controlled, with
a power range up to 120 MVA.
B. Three-level Active NPC (3L-ANPC)
One of the drawbacks of the 3L-NPC topology is the
unequal share of losses between the inner and outer switching
devices in each converter leg. Since the semiconductors are
cooled with separate heat sinks and cooling system it results
in an unsymmetrical semiconductor-junction temperature dis-
tribution which affects the cooling system design, limits the
maximum power rate, output current and switching frequency
of the converter for a specific semiconductor technology
(usually the IGCT) [46], [47]. This issue can be solved by
replacing the neutral clamping diodes with clamping switches
to provide a controllable path for the neutral current, and
hence control the loss distribution among the switches of
the converter. In other words, with clamping diodes like in
the 3L-NPC, the current freewheels through the upper or
lower clamping diode depending on the current polarity when
the zero voltage level is generated. Instead, with clamping
switches, the current can be forced to go through the upper or
lower clamping path. This can be used to control the power
loss distribution and overcome the limitations of the 3L-NPC,
enabling substantially higher power rates. These additional
devices are called the active neutral clamping switches and

5
are shown in Fig. 4a, and give this converter its name 3L-
ANPC. A detailed analysis on the loss distribution and how
to control it through the new switching states provided by the
additional clamping switches is performed in [46].
The 3L-ANPC was developed during the past five years
[47], [52], and has been recently introduced with a back-to-
back regenerative configuration as a commercial product [11].
It covers a power range from 20 to 200 MVA and can be
connected with a transformer from a 6kV to a 220kV grid.
Recently a variation of the ANPC concept has been pro-
posed, namely a five-level hybrid multilevel converter that
combines a three-level ANPC leg with a three-level FC power
cell connected between the internal ANPC switching devices
as shown in Fig. 4b. Although it is a hybrid topology, it has
been called 5L-ANPC [48]–[50], [53]. It effectively increases
the number of levels of the converter with the levels introduced
by the FC cell. The flying capacitor is controlled to V
dc
/4
so that its series connection to the ANPC dc-link capacitors
at V
dc
/2 or to the neutral at zero volt, using an appropriate
switching state generates the additional intermediate voltage
levels completing a total of five levels (±V
dc
/2, ±V
dc
/4,
and 0). There are several redundancies that can be used to
control the flying capacitor voltage. This hybrid ANPC-FC
concept enables somehow the modularity factor that lacks the
classic NPC converter family by just adding FC cells to reach
easily higher level values [53], without the need to add series
connected diodes. Moreover, because only a 3L-NPC leg is
used, the problems of capacitor voltage balancing when using
passive front ends in higher number of level NPCs explained
further in Section V is avoided as well. These advantages
a
+
_
V
dc
V
dc
/2
V
dc
/2
V
dc
/4
a
N
+
_
V
dc
IGCT
a
N
+
_
V
dc
V
dc
/2
V
dc
/2
V
dc
/2
V
dc
/2
V
dc
/4
V
dc
/8
(a)
(b)
(c)
N
N
ccc
IGBT
Fig. 4. Active NPCs (only phase a shown): a) 3L-ANPC featuring IGCTs
[46], [47], b) 5L-ANPC featuring IGBTs (hybrid between 3L-ANPC and a
3L-FC) [48]–[50], c) Common cross converter stage plus 5L-ANPC hybrid
9-level converter [51].
come at expense of a more complex circuit structure, and
with the need to control the flying capacitor voltages and
their initialization, besides the NPC dc-link capacitors voltage
unbalance control. In contrast to the CHB topology, this
modularity does not increase the power rating of the converter
but only the number of levels and the power quality, since the
flying capacitor adds an intermediate voltage level and does
not provide active power, so the power rating is still limited by
the ANPC part. Note that instead of IGCTs, series connected
IGBTs are used in the NPC part of the converter, probably
to keep all semiconductors of the same type. This inherently
introduces more conduction and switching losses, and requires
a special gate driver to ensure simultaneous control of both
switches. Nevertheless, because of the extra levels and the
particular configuration of the power circuit, the outer switches
commutate at lower switching frequency which compensates
for the series losses. Other ratios for the flying capacitor
voltage can be used to increase the number of voltage levels at
the output. However, in that case higher switching frequencies
are necessary to control the capacitor voltage properly and
makes it less attractive.
A commercial version of this topology has been recently
introduced [11], [50], aimed at medium voltage but not high
power. Configurations are available from 0.4 to 1MVA, rated
at 6 to 6.9kV, air cooled, with maximum output frequency of
75 Hz, exclusively in back-to-back configuration.
In addition, a variation to the hybrid 5L-ANPC has been
proposed by adding a common cross converter (CCC) stage
introduced in [54] to the 5L-ANPC resulting in a 9-level
hybrid converter introduced in [51] and shown in Fig. 4c. This
converter stage can connect any nodes of its input to any node
of its output through a set of direct and diagonal connected
switches. When this stage is added between the 3L-ANPC
part and the FC part of the circuit, the CCC stage capacitor
can be clamped in any polarity between the 3L-ANPC dc-
link capacitors and the flying capacitor producing even more
levels. If V
dc
/8 is chosen for the CCC stage capacitor (i.e.
a ratio between the ANPC dc-link voltge, the FC voltage
and the CCC voltage equal to V
dc
/2 : V
dc
/4 : V
dc
/8 =
1 : 0.5 : 0.25), 9 different output levels can be generated
(±V
dc
/2, ±3V
dc
/8, ±V
dc
/4, ±V
dc
/8 and 0). More CCC stages
can be added to increase the number of levels. Although this
additional stage increases the number of levels which greatly
improves power quality, this comes at expense of a complex
circuit structure and the need to balance both, the CCC and FC
stage capacitor voltages. Moreover, the voltage ratio chosen
for the CCC stage capacitor can affect the overall maximum
modulation index achieved by this topology, which for the
ratio 1:0.5:0.25 is M=0.925 at full active power [51], limiting
its application field. As with the 5L-ANPC this converter is
also limited in power range to the ANPC stage, and the CCC
does not supply additional active power. On the attractive
side, since the CCC stage is common to the three phases this
converter reaches 9 levels with less components than the CHB
with equal dc-sources, and with a much simpler transformer-
rectifier system.

Frequently asked questions

Q1. How many thyristors need to be connected in series?

Since the transmission voltage and total power can reach up to 800kV and 7GW [12], many thyristors need to be connected in series. 

Q2. What contributions have the authors mentioned in the paper "Recent advances and industrial applications of multilevel converters" ?

However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers a comprehensive and insightful review of where multilevel converter technology stands and is heading. The paper first presents a brief overview of the well established multilevel converters, strongly oriented to their current state in industrial applications, to then center the discussion on the new converters that have made their way to industry. Also new promising topologies are discussed. A great part of the paper is devoted to show nontraditional applications powered by multilevel converters, and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed, to motivate future contributions that address open problems and explore new possibilities. 

Q3. What future works have the authors mentioned in the paper "Recent advances and industrial applications of multilevel converters" ?

The main reason is to improve efficiency, to extend the device limits, and to have a practically feasible cooling system. However, an assessment comparing the classic and newer multilevel converter topologies in relation to witching and conduction losses is something still pending and is a challenge for further research that can provide valuable insight on the newer topologies. Reliability is also a key ingredient in the future development of multilevel converters. Nonetheless, the possibility to actually use this strength relies on the ability ( accuracy and speed ) to detect and diagnose a fault, so the fault tolerant reconfiguration of the converter can be performed before damage generated by the fault takes place. 

Q4. What is the main advantage of the multilevel current source?

Instead of avoiding series connection of devices to reach higher voltage operation while improving voltage waveform quality with a voltage source multilevel converters, the multilevel current source avoids parallel connection of devices and reaches higher output currents while improving current waveform quality. 

Q5. What are the main issues that are important to multilevel converters?

Apart from the development of modulation methods and the extension of control methods for multilevel converters, some operation specific issues like capacitor voltage control, common-mode voltage reduction/elimination and fault detection, diagnose and tolerant operation of multilevel converters are equally important. 

Q6. What is the main drawback of the phase-shifting transformer?

Although the phase-shifting transformer is needed to enable the series connection of matrix converter cells to reach medium voltage and high power operation, and also improves the input/output power quality, it adds volume and weight affecting negatively one of the features of classic matrix converters. 

Q7. What is the future of multilevel converters?

In the future, high voltage SiC power semiconductors could allow extending the applicability of multilevel converters to higher voltage applications, especially for those related to electrical utilities. 

Q8. What is the way to avoid the gearbox?

If multipole generators are considered, the gearbox can be avoided by achieving electromechanically the speed conversion between the low speed rotor shaft (around 15 rpm) to the grid frequency (usually 50 or 60 Hz). 

Q9. What is the main purpose of multilevel converters?

Multilevel converters can be used to interconnect the photovoltaic strings in a more intelligent way to reach higher voltages closer or even of same value of the point of common coupling. 

Q10. How many dcs are needed to produce the same air-gap flux?

In order to produce the same air-gap flux in the machine, the voltage amplitude required for the motor is divided in the two total dc voltages of the converters at both sides of the stator, in two equal or unequal parts (depending on the dc ratio used). 

Q11. Why do diode clamped converters have not found industrial acceptance?

they have not found industrial acceptance because the series connection of diodes are necessary to block the increased voltage produced by the series connection of the capacitors above and below the node where the output phase has been clamped. 

Q12. Why is the power quality of multiphase machines very welcome?

The power quality in particular is very welcome because of EMI/EMC issues, which imposestrong requirements in the automotive industry. 

Q13. What is the important parameter that is strongly related to the modulation stage?

As stressed throughout the paper, one of the most important parameters that is strongly related to the modulation stage is the average device switching frequency. 

Q14. What is the key challenge for the development of multilevel converter technology?

Since multilevel converters have better performance and power quality than these two topologies, it is the availability, reliability, efficiency, size and costs the key challenges for the development of multilevel converter technology to make it more competitive against these topologies for these applications. 

Q15. What is the main advantage of the use of predictive control in the field of multilevel converters?

In effect, the use of predictive control avoids the need of modulators and linear controllers to generate, for example, controlled currents to the load. 

Q16. How many converters are needed to handle the full power of state of the art turbines?

To reach the power levels of state of the art turbines, several converters in parallel are needed to handle the full power, since the current rating is generally high considering the output voltage of the conversion systems is usually around 690 V. 

Q17. How much of variable speed can improve efficiency at different load and operating conditions?

it has been shown that even a small % of variable speed operation above and below synchronous speed can improve efficiency at different load and operating conditions. 

Q18. What is the main reason why the newer topologies discussed in this paper are able?

This is why many of the newer topologies discussed in this paper are able to generate more voltage levels (5L-HNPC, modular multilevel converter, cascaded matric converter, etc.). 

Q19. How can the converter be used for three-phase drives?

It can also be used for three-phase drives with open end stator windings, by connecting two of the converter phases to each winding of the motor. 

Q20. What is the need to include between the propulsion converter and the ship switchboard?

As these systems become more complex there is need to include passive or active filters or other FACTS, between the propulsion converter and the ship switchboard to compensate and support the power system. 

Q21. What is the main reason why some works propose additional control mechanisms?

Therefore some works propose additional control mechanisms also based on switching state redundancies, to improve the dynamic performance of the voltage balance.