A Survey of Multilevel Voltage Source Inverter Topologies, Controls, and Applications

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 9, No. 3, September 2018, pp. 1186~1201
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v9.i3.pp1186-1201  1186
Journal homepage: http://iaescore.com/journals/index.php/IJPEDS
A Survey of Multilevel Voltage Source Inverter Topologies,
Controls and Applications
Ibrahim Haruna Shanono1
, Nor Rul Hasma Abdullah2
, Aisha Muhammad3
1,2
Faculty of Electrical and Electronics Engineering, Universiti Malaysia Pahang, Malaysia
1
Department of Electrical, Faculty of the Engineering, Bayero University Kano, Nigeria
3
Department of Mechatronics, Faculty of the Engineering, Bayero University Kano, Nigeria
Article Info ABSTRACT
Article history:
Received Feb 9, 2018
Revised Jul 31, 2018
Accepted Aug 6, 2018
Multilevel converters tremendous positive contribution in the field of power
electronics and renewable energy has been the reason behind the surge in it
research interest. The converter has put hope in the minds of power
electronic engineers that a time will come when it will break a record by
providing an efficient means of utilising the abundant renewable energy
resources. In an effort to report the recent advances in renewable energy
conversion technology, this paper presents a review of multilevel voltage
source converters that are widely being used in engineering applications. It
reports the technological advancements in converter topologies of Flying
Capacitor (FC), Neutral Point (NPC) /Diode Clamped, and Cascaded H-
Bridge (CHB) with their respective advantages and disadvantages. Recent
customized/hybrid topologies of the three-phase multilevel inverter with
reduced component count and switching combination are reported. The paper
also reviewed different modulation techniques such as the multilevel
converter carrier base PWM, Space Vector Modulation techniques (SVM),
and Selective Harmonic Elimination method (SHE-PWM). Finally, various
multilevel converters areas of application were highlighted. This review will
expose the reader to the latest developments made in the multi-level
converters topology, modulation techniques, and applications.
Keyword:
Multilevel topology
Voltage source converters
Modulation technique
Reduced device count
Copyright © 2018 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
Ibrahim Haruna Shanono,
Faculty of Electrical and Electronics Engineering,
Universiti Malaysia Pahang, 26600 Pekan, Pahang, Malaysia
Email: snnibrahim01@gmail.com
1. INTRODUCTION
Over dependency on fossil fuels as the major source of energy poses a serious threat to the
continued existence of life on our beloved planet “Earth”. The number of greenhouse gases emitted into the
atmosphere on daily bases as the result of energy generation and industrial utilisation is quite alarming.
Fossils exploration and processing results in severe environmental pollution (both land and water), which
entails affects the natural structures and habitat living in them. As the result of the problem above,
researchers have focused their attention and resources towards finding an alternative and sustainable means
of energy generation that has zero or minimum environmental de-gradational effects compared to the
conventional fossil fuels [1]. To this effect, many power electronic converters were designed and had already
been placed at various distributed generation (DG) and grid-connected networks [2].
Multi-level converters have become an exciting research area for many power electronic engineers
and scientist; this could be mainly due to its high power handling capability making it suitable for both
medium and high power industrial applications. Figure 1 below depicts a graphical representation of the
significant areas of application. A number of research articles were published on the industrial applications of
multi-level converters especially the ones found in high power ac drives systems like electric trains, power

Int J Pow Elec & Dri Syst ISSN: 2088-8694 
A Survey of Multilevel Voltage Source Inverter Topologies,Controls …( Ibrahim Haruna Shanono)
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generation stations, crane conveyors and refineries [3]. In addition to its high power handling capability, it
also produces a better output voltage waveform closer to sinusoidal, with low output harmonic distortion and
reduced switching stress dv/dt on the power electronic switching devices [4]. These features have remarkably
put it on top of its conventional counterpart.
Figure 1. Multi-level inverter converter applications
Multi-level converters can be categorised into three main topologies namely Flying Capacitors (FC),
Neutral Point (NPC) /Diode Clamped and Cascaded H-bridge (CHB) [5]. They are invented to increase the
inverter output power and at the same time provide solutions to the inability of the ordinary conventional
inverters to withstand high switching frequency, high voltage and current stress which cumulatively lead to
reduced efficiency and induces substantial electromagnetic- interference (EMI) in the system [6].
The topology composed of some number of low voltage devices, arranged in such a way to share the
voltage/current stress across them, which is achieved by employing proper switching sequence on the
switches. An essential advantage of multi-level configuration is that without increasing the switching
frequency or output power, the harmonics in the converter output are reduced. The word multilevel converter
starts from three-level upward and as the level increases the total harmonic distortion (THD) decreases.
Voltage problems, clamping requirements, cost and other constraints limit the number of achievable output
voltage steps. Switching strategy plays a significant role in the performance of an inverter because it has a
direct link to the harmonic content of the inverter output voltage. That is why power electronic scientist has
suggested innovative techniques to minimise harmonic distortions in the converter output. Numerous
modulation methods such as Sinusoidal Pulse Width Modulation Techniques (SPWM), Space Vector
Modulation Technique (SVM) and Optimized Harmonic Stepped Waveform Technique (OHSW) are
embraced [7].
The SPWM and SVM techniques were designed to operate at a high switching frequency, which has
the merit of reduced output filter size and de-merit of increased device switching stress and heat loss. OHSW
technique operates the power devices at a low frequency, exerting less switching stress on the switches;
therefore dissipate less amount of heat. The reduced switching frequency results in increased output
harmonic distortion, which warrants the need for a large filter size. To realise a smaller size filter using this
technique, the dominant low-frequency harmonics needs to be shifted further to a high frequency using step
modulation and multiple level inverters, these results in increased inverter price and circuit complexity. To
address this problem, a different approach was introduced, were by notches are superimposed on the output
waveform at a predetermined angle; this pushed the low order dominant harmonics to a higher frequency so
that they can be easily eliminated using a small sized filter [8]. This new approach is called Selective
Harmonic Elimination Method or pre-calculated modulation technique. Patel et al. [9] were the first to
propose it. This is a non-carrier based technique because it only requires some pre-calculated angles that are
stored in a lookup table (memory) [10]. These angles are found by solving the transcendental non-linear
equation found through Fourier series expansion of the output voltage equation.
2. MULTILEVEL VOLTAGE SOURCE CONVERTER

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Int J Pow Elec & Dri Syst, Vol. 9, No. 3, September 2018 : 1186 – 1201
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As previously stated, quite a number of multi-level converter topologies have been mentioned in
several kinds of literature [11]. In this paper, the three most promising topologies that serve as the base for
hybrid configurations are reviewed in detail. The high voltage capability, fewer harmonics in the output
waveform and higher efficiency are some of the advantages that made multilevel voltage source noteworthy
[12]. A common drawback to the multilevel inverters is its complexity and cost due to the need for more
number of power switches [13]. Figure 2 summarizes the various classifications of high power converters.
Figure 2. Converter classifications
2.1. Converter terminologies
Here, some commonly used terminologies and assessment parameters in the field of multi-level
power converters are highlighted.
a. The reduced device counts multi-level inverter (RDC-MLI): Refers to the topology in which for a given
number of phase level the number of controlled switching devices are reduced.
b. Total voltage blocking capability: Refers to the maximum amount of reverse voltage the converter
switches are capable of blocking [14].
c. Symmetric and asymmetric source configuration: The term symmetric in an MLI refers to when the
voltages of the input dc level are equal; otherwise, it is termed asymmetric [15],[16].
d. Even power distribution: Also known as charge balance control or equal load sharing [17]. When each
input source of a multilevel conversion provides equal power to the load, such power distribution within
the sources is said to be ‘even’. In some literature, it is called charge balance control or equal load
sharing.
e. Level- Generation and Polarity-Generation: MLI generates stepped output waveform by controlling the
dc input source in such a way that it adds up and subtracts to produce a level waveform. The generated
output composed of both positive and negative polarities, allowing the realisation of an alternating signal
with positive and negative half cycle. The MLI converter switches in the polarity generation circuit have
to be able to withstand the operating voltage.
f. Fundamental frequency Switching: It is a frequently mentioned converter terminology. It is known that
switching frequency, current and blocking voltage are proportional to the switching losses in a converter
[18]. To minimise the losses, the MLI is operated at low frequency, which is called fundamental
frequency while reserving the output quality.
2.1.1. Converter assessment parameters
The performance measurement of any multilevel inverter topology is based on how well it performs
on the application in which it is meant for. The performance assessment of reduced device count multi-level
converters with regards to other topologies can be based on the following parameters:
a) Blocking voltage of the converter

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b) Controllability of the topology
c) Total number of the power switch
d) Ability to employ asymmetric voltage ratios
2.2. Conventional Topologies
This section discusses the three conventional topologies within which most of the current hybrids
are drawn from.
2.2.1. Diode Clamped Topology
Diode clamp multi-level converter topology is an advancement of a three-level Neutral point
inverter invented in the early 80’s [19]. Later in the 1990s, the number of steps was increased up to six levels
[20]. Clamping diodes are used in this type of converter to reduce the device voltage stress. An md level
diode clamped converter needs (2md − 2) switching device, needs (md − 1) input voltage source and
(md − 1) (md − 2) number of diodes with a voltage Vdc across the diodes and switch [21]. Figure 3 shows
single-phase 5-level diode clamp converter. It comprises of four series connected capacitors in parallel with
the Dc bus. The Dc bus input voltage Vdc is shared equally across the four capacitors, each having
Vdc
4
across
it. The number of series connected capacitors determines the output voltage step. For M steps diode clamp
inverter, there are M-1 series connected DC bus capacitors. The clamping diodes have different voltage
ratings depending on its position in the circuit. Diode D3 in the figure should at least be able to block
3Vdc
4
voltages were D1 will only block
Vdc
4
[4]. Table 1 below shows the possible output voltage steps between
point Vx and Vo, with their allowable switching combination. Logic state1 and 0 corresponds to the switch
On and Off states.
Figure 3. 5-level diode clamp converter
Table 1. Diode-clamped Voltages with their corresponding switching combination
Voltage Switching States
𝐕𝐨𝐱 S1 S2 S3 S4 S5 S6 S7 S8
𝐕𝟏=0 0 0 0 0 1 1 1 `1
𝐕𝟐 =
𝐕𝐝𝐜
𝟒
0 0 0 1 1 1 1 0
𝐕𝟑 =
𝐕𝐝𝐜
𝟐
0 0 1 1 1 1 0 0
𝐕𝟒 =
𝟑𝐕𝐝𝐜
𝟒
0 1 1 1 1 0 0 0
𝐕𝟓 =𝐕𝐝𝐜 1 1 1 1 0 0 0 0
From the table, it can be seen that switch (S1, S5), (S2, S6), (S3, S7) and (S4, S8) are operated
complementary and in a sequential switching pattern. Each of the switching devices should at least be able to
block
𝑉𝑑𝑐
4
voltage level. The clamping diodes blocks unequal reverse voltage. For instance, when the
converter output voltage is at 𝑉1 = 0, all the lower arm switches are said to be at On state, in this case D3 has
to block
3𝑉𝑑𝑐
4
, D2 blocks
𝑉𝑑𝑐
2
, and D1 will block
𝑉𝑑𝑐
4
. In a situation where a converter is to be designed with the

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same rated diodes, then for a Dn-1 positioned diode, n-1 series connected diodes will be required to block
(𝑛 − 1)
𝑉𝑑𝑐
𝑛
reverse voltage [4]. Below is the list of advantages and disadvantages of diode clamped converter
topology [22].
Advantages:
1.It requires single isolated Dc supply; this warrants
the possibility of having a back-to-back
connection.
2.The Dc bank capacitors can be charged
simultaneously.
3.It provides a better efficiency when operated at the
fundamental switching frequency.
Disadvantages:
1.The number of clamping diodes increases with
increase in voltage level. This brought about the
additional cost and circuit complexity, especially
in higher steps converters.
2.Unequal switching stress across the switching
device, i.e. inner switches conduct for a short time
compared with the inner switches.
3.Not suitable for redundancy.
2.2.2. Flying Capacitor Topology
Flying capacitor (FC) is another converter topology that was invented in the early 90’s by Maynard
et al. [23]. It circuit connection is similar to that of a diode clamp, only that capacitors were placed instead of
diodes. An m level diode clamped converter needs (2𝑚 − 2) switching device and needs (𝑚 − 1) number of
capacitors with a voltage Vdc across the `capacitors and switch [21]. Figure 4 shows its 3-phase circuit
connection. The topology comprises of a ladder-like arranged dc link capacitors, each having a different
voltage rating. The voltage difference between two adjacent capacitors determines the amplitude value of
each step. To generate M-level phase voltage steps at Vxo, M-1 capacitors need to be connected serially at
the dc-bus. The line voltage Vxy, in this case, will be (2M -1) levels. One of the benefits of flying capacitor
over diode clamp is its redundancy property. It can generate a particular output voltage using multiple
switching sequences. For an M-level flying capacitor converter designed with equal rated capacitors, the
number of phase capacitors can be found using the expression ((M-1)(M-2))/2 [19].
Figure 4. Five-Level Flying capacitor Multilevel converter
The merits and demerits of the multi-level flying capacitor are stated as follows [19].

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Advantages:
1. High-value capacitors can serve as a backup
during a power failure and voltage sagging.
2. The topology allows the possibility of
switching redundancy, which results in
voltage stress balancing across the
capacitors.
3. Allows the control flow of real and reactive
power.
4. It requires single isolated dc supply voltage
source.
Disadvantages:
1. The number of capacitors is determined by the
inverter output level. This makes the circuit looks
bulky and difficult to package.
2. A complex control mechanism that requires
feedback is involved. This is to maintain voltage
across the capacitors.
3. The converter has poor efficiency due to high
switching losses.
2.2.3. Cascaded H-bridge Topology
A cascaded H-bridge is a promising converter topology made up of series a connected H-bridge
inverter module. Figure 5 shows a single phase 5-level circuit configuration of the inverter. The circuit
comprises of four full bridge modules with each having its independent dc supply. The nine-level voltage
steps are generated by adding up the individual output voltages generated by each module. By employing a
systematic switching function on the switches, each of the H-bridge cells can synthesise three different
voltage levels, positive (+E), zero (0) and negative (-E) [3]. The sequence of phase voltage steps is expressed
in a unique and distinctive pattern from the earlier mentioned topologies. In this case, it is express as N=
2Pdc+1, where Pdc stands for the number of independent Dc source.
Figure 5. Single phase 5-level Cascaded H-bridge Converter
From the Figure 5, switch Sa1 and Sa2 are called the converter leg and are always operated
complimentary to avoid short-circuiting the supply. The five-level switching sequences and their
corresponding output voltage is given in Table 2.
Table 2. The five-level Cascaded H-bridge Converter switching sequences
Voltage Switching States
Vab S1 S2 S3 S4 S5 S6 S7 S8
V1=-2E 0 1 1 0 0 1 1 `0
V2 =−E 0 1 1 0 0 1 0 1
V3 =0 0 0 0 0 0 0 0 0
V4 =+E 1 0 0 1 1 0 1 0
V5 =+2E 1 0 0 1 0 0 0 1

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Out of the three mentioned topologies in this paper, cascaded H-bridge topology uses the least
number of power electronic components. The advantages and disadvantages of this topology are summarized
as follows [3].
Advantages:
1. There is an automatic voltage sharing across
the switches in a module due to the usage of the
independent voltage source. Therefore, reduces
restriction in the switching sequence.
2. The converter produces more output voltage
levels, providing a smooth and steady voltage
change across the load, resulting in less THD
for a particular operating frequency.
3. Its modular nature makes its production,
maintenance and redundancy integration much
easier.
4. Higher voltage level can be achieved by series-
connecting more H-bridges.
Disadvantages:
1. The need for separate independent dc
supply by each of the H-bridge modules. Increases
the device cost and size.
3. HYBRID CONVERTER TOPOLOGIES WITH REDUCED DEVICE COUNT
This section reports three phase-modified topologies with reduced device count. Some of this
topologies are hybrids or a slightly modified existing configuration.
3.1 Three-phase Multi-level DC link inverter (MLDCL)
This is an inverter circuit proposed by Rao et al. [24], it associated gate drive circuitry was reduced
as it utilizes fewer switches in contrast to the 36 switches in the conventional three-phase cascaded inverter
topology. For m number of voltage level, MLDCL requires m+3 active switches per phase, which is half the
number of switches and clamping diode required in diode-clamped topology and also half the voltage
capacitor and clamping capacitors needed in flying capacitor topology. Figure 6 below shows the circuitry of
the proposed hybrid MLDCL converter.
Figure 6. Three phase Multilevel DC link inverter topology

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Sub-harmonic and modified space vector pulse width modulation are the control techniques used in
the proposed topology. Table 3 shows the proposed topology single phase switching states and the respective
generated voltage.
Table 3. Switching States for a single phase seven level MLDCL inverter
3.2 Three-phase Reduced Switch Multi-level Inverter (RSMLI) Topology
This topology produces a seven-level output voltage that is almost sinusoidal, thereby reducing the
converter lower order harmonics. The converter uses three H-bridge modules with one module per phase and
3 additional connected switches that allows the possibility of achieving the desired seven level waveform.
There are a total of 21 switches and 9 dc sources with three each per phase. In order to determine the
switching angles for generating a fundamental output voltage, it uses fundamental switching scheme. Below
is the circuit of a 3-phase 21 switch MLI [25].
Figure 7. Three phase seven level Multilevel Inverter
The proposed topology has three operating modes, namely; powering mode, freewheeling mode and
regenerating mode. For the powering mode, the load current and voltage polarities are the same. In the
freewheeling operating mode, one of the main switches is off and passage of the load current is due to load
inductance, whereas in regenerating mode the stored energy in the load inductance is feedback to the source,
the load current has to be positive during the negative half cycle and negative for positive half cycle [25].
Table 4 shows the per phase switching states of the power electronic switches and the corresponding
generated output voltage. The same switching combination applie to the remaining two phase.
Table 4. Switching states per phase for seven level Inverter
V0 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
0 1 0 0 1 0 1 0 1 0 1
E 1 0 0 1 0 1 0 1 1 0
2E 1 0 0 1 0 1 1 0 1 0
3E 1 0 0 1 1 0 1 0 1 0
-E 0 1 1 0 0 1 0 1 1 0
-2E 0 1 1 0 0 1 1 0 1 0
-3E 0 1 1 0 1 0 1 0 1 0

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Switches in ON
states
Output voltage [Vao(t)]
Q1, Q4, Q5 E
Q1, Q4, Q6 2E
Q1, Q4, Q7 3E
Q1, Q2, 0
Q2, Q3,Q5 -E
Q2, Q3,Q6 -2E
Q2, Q3,Q7 -3E
Q3,Q4 0
3.3. Reverse Voltage Multilevel Inverter Topology (RVMLIT)
Najafi and Yatin [26] proposed a new MLIT that has dual operational functionalities. The first part
utilizes high-frequency switches for the generation of level states, and the second part employs low frequency
for the generation of the polarity of the output voltage. Therefore, these two high and low frequencies
produce the required output voltage. To generate a seven-level output voltage, two different topologies are
integrated per phase, which requires 10 switches in a phase. The upper six switches generate the required
non-polarized output waveform levels while the lower 4 switches in the full bridge circuits do generates the
polarity. Duplicating the middle stage increases the output voltage level. As seen from the circuitry, to
generate a particularly desired polarity, the output voltage is the input to the full bridge converter with the
converter output fed into the primary of a transformer whose secondary is arranged in the delta and to the
three-phase system. The circuit diagram is shown in Figure 8.
Figure 8. Reverse voltage multilevel inverter topology
3.4 Three-phase Asymmetrical Cascaded Multilevel Inverter Topology (ACMLIT)
The converter is called asymmetrical cascaded MLIT due to its unequal magnitude of the input DC
sources. This MLI topology can be integrated with renewable energy sources as it requires less number of
DC sources [27]. Figure 9 shows the circuit diagram of the asymmetrical cascaded topology.

1195
Figure 9. Three phase Asymmetrical Cascaded Multilevel Inverter Topology
The zero and E-level output voltage level are generated using two switches and a single DC source.
In order not to short-circuit the DC source, the switches S1 and S2 are operated in a complementary manner.
The two DC sources and four switches of each phase of the proposed topology forms a module structure.
Three basic unit can be added based on the required output voltage levels. It generates the PWM signals
through comparison of the reference sine wave with the carrier wave thereby generating pulse signals A1, A2,
A3.
3.5. Three-phase Symmetrical Multilevel Inverter Topology (SMLIT)
This is another modular 3-phase symmetrical topology, which produces 3-phase output voltage level
using 12 switches [28]. Each phase comprises a battery and two switches in series and parallels respectively.
Both switches connected to the battery are operated in complementary to each other. Increasing the number
of phase elements (components) increases the number of the voltage step.
Figure 10. Three-phase Symmetrical Multilevel Inverter Topology
Equation 1 to 3 shows an expression of this topology that outlines the relationship between the
number of cells, output levels, power electronic switches.
Mlevel= 2Ncell +3 (1)
Nsw = 3(2Ncell +2) (2)
Nps = 3Ncell +1 (3)

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Where Mlevel = number of output levels, New =number of power switches, Nps=number of power electronic
switches, Ncell=number of cells
3.6. 5-level three-phase cascaded hybrid Multilevel inverter
Thongprasri [29] proposed a three-phase cascaded hybrid multilevel inverter comprising of a 3-
phase inverter in series with an H-bridge module. Each module has a separate independent dc source. He uses
multi-carrier based modulation sub-harmonic PWM (MC-SH PWM) on FPGA board to generate the control
signals. Figure 11a shows the 3-phase topology of the proposed multilevel inverter.
Figure 11a. 5-level three-phase cascaded hybrid multilevel inverter
The proposed topology was tested using 3 different types of the load; 18W fluorescent lamp- ballast,
RL, and 1HP 3-phase induction motor; without filtering. The per-phase circuit configuration is given in
Figure 11b.
Figure 11b. 5-level Single-phase cascaded hybrid multilevel inverter
4. MULTILEVEL CONVERTERS MODULATION TECHNIQUES (MT)
Due to the rapid advancements in the field of power electronics and the tremendous breakthrough
and confidence poised by multi-level converter technology, researchers are exploring every aspect of it, with
the aim of improving its overall performance. With the subsequent invention and modification of topologies,
the need for new and compatible modulation techniques to suit the new topologies can never be
overemphasized. This is the reason why modulation technique is trending in the field of power electronics

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[3]. The ever-rising complication of power control systems as the result of additional auxiliaries, the need to
reduce switching losses and stress on individual power switches, have facilitated significantly in the
advancement so far recorded
in the field of modulation techniques [30]. Several techniques have been developed with each having its
merits, de-merits and preferential area of application. Figure 12 shows the various classifications of multi-
level converter modulation techniques. From the figure, it can be seen that the techniques are categorized into
two broader groups based on their domain of operation. These are the state space vector techniques and the
voltage level based approach [31]. The former operates based on the voltage space vectors while the latter
operates on voltage level over a period of time.
Multi-level
Modulation
Spaced
Vector Based
Algorithms
Voltage level
Based
Algorithms
Spaced Vector
Modulation (SVM)
Spaced
Vector
Control (SVC)
Hybrid
Modulation
Selective
Harmonic
Elimination
(SHE)
Multicarrier
PWM
Nearest Level
Control
2-D
Algorithms
3-D
Algorithms
3-Leg
Inverters
4-Leg
Inverters
Phase Shifted
PWM
Level Shifted
PWM
Opposition
Disposition
(OD)
Alternate
Phase
Opposition
Disposition
PWM (APOD)
Phase
Disposition
(PD)
Figure 12. List of Multilevel converter Modulation techniques
4.1. Carrier Based Pulse Width Modulation (CB-PWM)
The early PWM techniques were transformed and upgraded to be applied to the current multi-level
topologies. The new technique unlike the conventional counterparts produces the control signal using
multiple carrier signals, hence called multi-level PWM [32]. FC and CHB topologies can both be configured
in modular form, hence giving the leverage for each module to be modulated independently using bipolar and
unipolar sinusoidal carrier PWM, this amount to equal power distribution in the module cells. An N cell of
FC and CHB will require (180°)/N and (360°)/N carrier phase shift respectively to generates a low harmonic
distorted multi-level voltage waveform [33]. This technique is denoted as Multi-level Phase Shift Sinusoidal
Pulse Width Modulation (MPSSPWM). Its major advantage is its ability to suppress the harmonics at the
inputs of CHB’s converters and also it can provide a balanced dc voltage in FC converter topologies. A new
different approach called Level Shift Pulse width modulation (LSPWM) was introduced, it requires
superimposing two carrier signals on each other. This technique is further sub-classified into 3 categories
namely Alternate phase opposition (APOD), Phase opposition disposition (POD-PWM) and Phase
disposition (PD) on the bases of the spatial orientation of the carrier signal [32].
4.2. Space Vector Modulation (SVM)
This technique involves comparison of converter gating signal with the vector representation of a
sinusoidal reference (Vref), whereby a replica of the reference voltage is reproduced at the converter output
[33]. SVM technique is currently applied on various converter topologies, resulting in the development of
new algorithms that suits the existing topologies, this can be seen in the various reported literature. The
reported techniques are application specifics that are designed to work on a fixed converter output steps. The
higher the output steps, the more complex the control algorithms and the mathematical computations. The
drawback poised by the complex algorithms and rigorous computations was later addressed in the subsequent
literature [34].
Franquelo et al. [35] in his article “Space vector modulation technique for multi-level converters”,
proposed a new algorithm that uses simple computational procedures to produce a reference value close to
the state vector, hence eliminating the need for lookup tables. In addition to reducing tedious computations, it
also disputes the notion that says the more the output steps, the more the computational complexity. In
another literature, a new 2 dimensional modified SVM (MSVM) technique was presented in [31]. The
technique is suitable for application in 3-phase balanced system where all triple harmonics have cancelled. A

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more sophisticated algorithm that warrants an online vector calculation was presented in [36], it is called 3
Dimensional algorithms because it is an upgraded version of the earlier discussed 2D, it has further
minimised mathematical computations and eliminates the links between circuit complexity and converter
output step. This technique applies to both balanced and unbalanced systems [36].
4.3. Selective Harmonic Elimination Method (SHE-PWM)
Another name given to this modulation technique is offline or pre-calculated modulation technique.
It was first proposed by Patel et al. [9]. This is a non-carrier based technique because it only requires some
pre-calculated angles that are stored in a lookup table (memory) [37]. These angles are generated by first
finding the Fourier expansion of the voltage waveform, which happens to be a non-linear trace-dental
equation as shown in equation (4).
Hn = {
4*E
nπ
{(-1)
N
(1+2 ∑ (-1)
K
cos(nαK)
N
K=1 )} for odd n
0 for even n
(4)
Based on the odd quarter wave symmetry theory, the dc components and even harmonics will cancel
out will be left with the fundamental and odd harmonic components [38]. The fundamental component is
equated to a constant value which is determined by the selected modulation index, and the odd harmonics are
equated to zero. The generated equations have no direct solution. Hence, numerical techniques which involve
iterative procedures as in Newton Raphson method [39]. Mathematical resultant method, which uses the
concept of polynomials, optimization techniques or Hybrid genetic algorithm, are capable of providing an
approximate value of the angles [10]. SHE method has drastically reduced switching loss, which is the reason
why it has been extended to high power multi-level converters [37].
Its drawback is its restrictions to open loop systems and the complexity in finding the switching
angles for a higher number of steps. These limitations resulted in the invention of other low switching loss
techniques with feedbacks that are suitable for higher level applications [31].
4.4. Sinusoidal PWM (SPWM)
This technique works based on the comparison of a referenced sinusoidal wave and a triangular
carrier wave. The output voltage waveform is formed as a result of the gating signal been fed into the inverter
switches. These gating pulses are formed when the reference wave is larger than the carrier wave as depicted
in the figure below [40].
Figure 13. Pulse generation using reference and carrier signal
4.5. Phase Disposition (PD) PWM
This technique enhances output voltage of a multi-level converter by optimally suppressing the
output harmonics. Hence, minimising the distortion level in the voltage waveform. It employs several
optimisation techniques, the likes of gradient optimisation algorithms [41].
4.6. Staircase Modulation
The staircase modulation method is most suitable for elimination of device stresses and switching
losses in higher voltage and power energy conversion. It does so by determining the optimal primary values

1199
of the switching angles. Particle Swarm optimising algorithm can be utilised in solving the transcendental
equations because it makes the switching angle solution converges at the global minimal [42].
4.7. Stepped Modulation
Here, the modulating signal is a stepped waveform, divided into intervals with each interval
controlled separately to minimized specific harmonics. Converters controlled using this technique has less
distortion compared to the conventional PWM modulation method [40].
4.8 Comparison of Modified/Hybrid Three Phase Multilevel Inverter Topologies
With the aim of keeping the THD within the standard limit and lesser device count, researchers have
proposed different control strategies. Table 5 shows the comparison of some of the hybrid/ modified three-
phase topologies based on modulation techniques, number of diodes and switches, Dc source and percentage
of THD in the output.
Table 5. Three phase converter topology parameter comparison
S/N Author Topology Ns Nd Nc Ndc Nt MT THD
1 [43] BDSI 6 0 2 1 0 PWM 25
2 [44] CCHB 36 0 0 9 0 Sub-H
PWM
10.78
MSVM 8.79
3 [30] HMLIT 18 0 2 4 0 MCSVM 15.6
4 [26] RVMLIT 30 0 0 3 3 PD
PWM
3.85
5 [24] MLDCL
T
30 0 0 9 0 Sub-H
PWM
9.02
MCSVM 6.84
6 [26] RSMLIT 21 0 0 9 0 SHE
PWM
7.8
7 [28] ACMLIT 12 0 0 6 0 SPWM 25.609
8 [29] SMLVI 12 0 0 4 0 LFM 16.88
SingleCSP
WM
34.83
Two
CSPWM
35.35
Note: Some of the notations used in the table are mentioned in the text, the rest stands for the
following: Ns stands for the number of switches, Nd number of diodes, Nc number of balanced capacitors, Ndc
stands for the number of DC inputs, Nt number of transformers, BDSI stands for Bi=derictional switched
inverter, CCHB stands for conventional cascaded H-bridge, HMLIT stands for Hybrid Multilevel inverter
topology, Sub-H PWM stands for Sub-harmonic Pulse width modulation, LFM stands for Low-frequency
modulation, CSPWM stands for Carrier Sinusoidal PWM.
5. CONCLUSION
This paper presented a comprehensive survey on multilevel converters. With emphasis given to the
conventional topologies, that serves as the basis of the modern hybrid topologies. Converters operational
characteristic, advantages, disadvantages and major areas of applications were outlined. The paper also
reviewed some recently proposed three-phase hybrid multilevel inverter topologies with reduced device
count and modified switching function. The hybrid topologies appeared to have low total harmonic
distortion, less number of power electronic components and an additional number of output steps compared

 ISSN: 2088-8694
1200
to the conventional topologies. The paper reported the promising and widely used modulation techniques the
likes of multilevel carrier base PWM, space vector modulation techniques (SVM), selective harmonic
elimination method (SHE-PWM) and phase disposition (PD) modulation techniques. It is hoped that this
review will serve as a source of literature to the vast amount of new researchers in the field of multilevel
voltage source converters.
6. ACKNOWLEDGEMENTS
This work is supported by the Faculty of Electrical and Electronic Engineering, Universiti Malaysia
Pahang (UMP).
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