Simulation and analysis of multilevel inverter with reduced number of switches

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SIMULATION AND ANALYSIS OF MULTILEVEL
INVERTER WITH REDUCED NUMBER OF SWITCHES
NIMITHA MURALEEDHARAN1
, REMANI T2
1
EEE, Govt. Engineering College, Thrissur,
2
Professor, EEE, Govt. Engineering College, Thrissur,
ABSTRACT
As compared to conventional two level inverters, multilevel inverters have been widely accepted for high-power
and high-voltage applications due to their added advantages of low switching stress and lower total harmonic distortion
(THD), hence reducing the size and bulk of the passive filters. In this paper a new five level inverter topology is
presented with reduced number of switches as compared to conventional cascaded H-bridge multilevel inverter, and can
be extended to any number of levels. The performance of the new topology and its controller are validated with
simulation results using the MATLAB/SIMULINK software.
Keywords: Multilevel inverter, SPWM, THD
1. INTRODUCTION
Among all the modern power converters, the voltage source inverter (VSI) is the simplest and most widely used
device with power ratings ranging from fractions of kilowatt to megawatt level. It converts fixed DC voltage to AC
voltage with controllable frequency and magnitude. To improve the efficiency, performance and reliability of the system,
most of the loads are connected to the AC power line through power converters. In recent years, the multilevel voltage
inverter has received wide attention in high-power applications such as large induction motor drives, UPS systems and
flexible AC transmission systems [1]-[8]. Multilevel inverter synthesizes a desired stepped output voltage from several
input DC voltage sources. With an increasing number of input DC sources, the inverter output voltage waveform
approaches nearly sinusoidal waveform. As compared to traditional two-level inverters, the multilevel inverters have
more advantages, which include lower semiconductor voltage stress, better harmonic performance, low electromagnetic
interference and lower switching losses. A multilevel inverter also enables the use of renewable energy sources.
Renewable energy sources such as photovoltaic, wind and fuel cells can be easily interfaced to a multilevel inverter
system for high power applications.
The three common topologies for multilevel inverter are: (i) Diode clamped [9] (ii) Flying capacitor and [10]
(iii) Cascaded H-bridge inverter. Among them, a cascaded H-bridge inverter is useful because it requires reduced number
of components to achieve the same number of output voltage levels among the conventional multilevel inverters [11]-
[15]. One of the disadvantages of multilevel inverter is the large number of power semiconductor switches required.
Every switch requires a gate driver circuit, therefore increasing the complexity and size of the overall circuit.
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This paper presents a new topology of cascaded multilevel inverter that produces the same output as the
conventional cascaded multilevel inverter for a given input but has fewer semiconductor switches and gate driver
circuits. Its performance is analysed using different sinusoidal PWM techniques [16]-[17]. The remaining paper is
organized as: Section 2 gives a brief explanation about the conventional cascaded multilevel inverter. Section 3 describes
the working and control algorithm of the new multilevel inverter topology. Section 4 presents the simulation results of
the conventional and the new cascaded multilevel inverter topology. Section 5 concludes the paper.
2. CASCADED MULTILEVEL INVERTER
This multilevel inverter uses set of series connected cascaded inverter with separate dc sources to synthesize a
desired voltage from several independent dc sources, which may be obtained from batteries, fuel cells, or solar cells. This
inverter can avoid extra clamping diodes or voltage balancing capacitors as in diode clamped multilevel inverters and
flying capacitor type multilevel inverters respectively. AC output of each of the inverter is connected in series such that
the synthesized voltage waveform is the sum of the inverter outputs. In this topology, the number of output phase voltage
levels is defined by m = 2s+1, where s is the number of dc sources.
Fig.1 shows a three level cascaded multilevel inverter. When switches S1 and S2 alone are conducting the output
of the inverter is Vdc, Fig.1.a. When switches S3 and S4 alone are conducting the output of the inverter is -Vdc, Fig.1.b.
When switches S1 and S3, are conducting, the output of the inverter is zero, Fig.1.c.
Fig.1. 3 level cascaded multilevel inverter
Fig.1.a. Switching combination required to generate output Fig.1.b. Switching combination required to generate output
voltage level +Vdc voltage level –Vdc

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Fig.1.c. Switching combination required to generate output voltage level zero
3. NEW MULTILEVEL TOPOLOGY
3.1. Working
In order to reduce the overall number of switching devices in conventional cascaded multilevel inverter
topologies, a new topology has been presented. The circuit configuration of the new five level inverter is shown in Fig.2.
It has four main switches in H-bridge configuration S3, S4, S5 and S6 and two auxiliary switches S1 and S2. The number of
dc sources (two) is kept unchanged as in similar five level conventional cascaded H-bridge multilevel inverter. Like other
conventional multilevel inverter topologies, this topology can be extended to any required number of levels.
The inverter can operate in four different modes according to the polarity of the load voltage and current.
• When switch S1, S3, S6 is conducting, the output voltage is +Vdc.
• When switch S2, S3, S6 is conducting, the output voltage is +2Vdc.
• When switch S1, S5, S4 is conducting, the output voltage is –Vdc.
• When switch S2, S5, S4 is conducting the output voltage is -2Vdc.
Fig.2. 5 level inverter with reduced number of switches
Fig.2.a. Switching combination required to generate Fig.2.b. Switching combination required to generate
output voltage level +2Vdc output voltage level +Vdc

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Fig.2.c. Switching combination required to generate
output voltage level -Vdc
Fig.2.d. Switching combination required to generate
output voltage level -2Vdc
Considering the load to be a RL load,
Powering Mode: This occurs when both the load current and voltage have the same polarity. In the positive
half cycle, when the output voltage is Vdc, the current path comprises; the lower supply, S1, S3, load, S6, and back to the
lower supply. When the output voltage is 2Vdc, current path is; the lower source, S2, the upper source, S3, load, S6, and
back to the lower source. In the negative half cycle, S3 and S6 are replaced by S4 and S5 respectively.
TABLE 1: Operating modes
Output Voltage Level Conducting Switches
+Vdc S1, S3, S6
+2Vdc S2, S3, S6
-Vdc S1, S5, S4
-2Vdc S2, S5, S4
Free-Wheeling Mode: Free-wheeling modes exist when one of the main switches is turned-off while the load current
needs to continue its path due to load inductance. This is achieved with the help of the anti-parallel diodes of the
switches, and the load circuit is disconnected from the source terminals. In this mode, the positive half cycle current path
comprises; S3, load, and D5 or load, S6 and D4, while in the negative half cycle the current path includes S5, load, and D3
or load, S4 and D6.
Regenerating Mode: In this mode, part of the energy stored in the load inductance is returned back to the source. This
happens during the intervals when the load current is negative during the positive half cycle and vice-versa. When the
output voltage is Vdc, the positive current path comprises; load, D5, D1, the lower source, and D4, while the negative
current path comprises; load, D3, D1, the lower source, and D6. When the output voltage is 2Vdc, the positive current path
comprises; load, D5, upper source, D2, lower source and D4, while the negative current path comprises; load, D3, upper
source, D2, the lower source, and D6.
3.2. Switching Techniques
A single sinusoidal reference is compared with each carrier signal to determine the output voltage for the
inverter. Three dispositions of the carrier signal are considered to generate the PWM signal.
3.2.1 Phase disposition (PD): In PDPWM Strategy for a m-level inverter, (m-1) carriers with the same frequency
and same amplitude are all in phase with each other. If the reference wave is more than a carrier signal, then the active
devices corresponding to that carrier are switched on. Otherwise, the devices switch off.
3.2.2 Alternative phase opposition disposition (APOD): In APOD strategy the carriers of same amplitude are phase
displaced from each other by 180 degrees alternatively.
3.2.3 Phase opposition disposition (POD): In POD strategy the carrier waveforms above the zero reference are in
phase. The carrier waveforms below zero reference are also in phase, but are 180 degrees phase shifted from those above.

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(a)
(b)
©
Fig.3. Carrier and reference signal arrangements for: (a) Phase disposition (PD). (b) Alternative phase opposition
disposition (APOD). (c) Phase opposition disposition (POD).
TABLE 2: Comparison Of Number Of Components
Number Of Switches
Inverter Type 5 LEVEL 7 LEVEL 9 LEVEL 11 LEVEL
Cascaded H-Bridge 8 10 12 14
New Topology 6 7 8 9

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Fig.4.a. Simulink model of conventional five level cascaded multilevel inverter
Fig.4.b. Simulink model of five level cascaded multilevel inverter with reduced number of switches
4. SIMULATION RESULTS
A conventional five level cascaded multilevel inverter is first simulated using the following parameters and the
simulink model is shown in Fig.4.a. Fig.4.c shows the output voltage of conventional five level cascaded multilevel
inverter.
i. Input voltage: 100V
ii. Switching frequency: 1KHz
iii. Modulation index: 0.8
iv. R load =100Ω
Simulation is done for the new five level multilevel inverter topology in MATLAB/SIMULINK using the
following parameters. The simulink model is shown in Fig.4.b. Fig.4.d and Fig.4.e shows the switching signals given to
the switches. Fig.4.f shows the output voltage.
i. Input voltage: 100V
ii. Switching frequency: 1KHz
iii. Modulation index: 0.8
iv. R load =100Ω

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Fig.4.c. Output voltage of conventional five level cascaded multilevel inverter
Fig.4.d. Switching signals to the switches S1, S2
Fig.4.e. Switching signals to the switches S3, S4, S5, S6
Fig.4.f. Output voltage of the new multilevel inverter topology

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Fig.4.g and Fig.4.h shows the output voltage and output current of the new multilevel inverter topology with
filter at the output. The filter parameters used are: C=22 µf and L= 30 mH. The FFT analysis of output voltage of the new
multilevel inverter topology with filter at the output is shown in Fig.4.i.
Fig.4.g. Output voltage of the new multilevel inverter with filter at the output
Fig.4.h. Output current of the new multilevel inverter with filter at the output
Fig.4.i. FFT analysis of output voltage with filter

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Fig.4.j. Voltage across auxiliary switch of the conventional 5 level inverter
Fig.4.k. Voltage across auxiliary switch S1 of the new 5 level inverter
Fig.4.l. Voltage across auxiliary switch S2 of the new 5 level inverter
Fig.4.j shows the voltage across each auxiliary switch of the conventional 5 level cascaded inverter. Fig.4.k and
Fig.4.l shows the voltage across the auxiliary switches S1 and S2 of the new 5 level inverter topology.

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TABLE 3: Comparison Of THD And Number Of Switches
5 Level Cascaded Multilevel Inverter Number Of Switches THD
Conventional Topology 8 26.52%
New Topology 6
24.58%(POD)
25.58%(APOD)
26.68%(PD)
5. CONCLUSION
In this paper, a new cascaded inverter topology has been presented which has superior features over
conventional cascaded multilevel inverter topologies in terms of the fewer power switches, control requirements, cost,
and reliability. Three different SPWM control methods are used to drive the inverter and their performance in terms of
THD is compared and its found that POD PWM strategy gives an output voltage with less THD as compared to other
PWM strategies. Simulation of the new five level inverter topology is done for an input of 100V in
MATLAB/SIMULINK and the results are shown.
.
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