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TRANSCRIPT
Comparison of Multi Carrier PWM
Techniques applied to Five Level
CHB Inverter
P S V Kishore1, P Suresh Kumar2,
K Ramesh3
1,2,3Vignan’s IIT, AP, India
April 14-15, 2017
Abstract
The use of Multi-level inverters has been increased due
to their high power applications and the ability for
getting nearly a sinusoidal output voltage compared to
normal two level inverters. The various multilevel
inverters in existence are Diode Clamped, Capacitor
Clamped, and Cascaded H-bridge inverters. In order to
get fewer harmonic various techniques are adopted to
control the switches in a multilevel inverter. In this
paper, multi carrier PWM techniques are used to
generate the pulses to the switches in the 5-Level
Cascaded H-bridge (CHB) inverter.
MATLAB/Simulation is used to simulate the circuit and
the results are tabulated.
Keywords: Multi carrier PWM, CHB Inverter, Multi-level
inverter, MATLAB/Simulation, five-level
1 Introduction
Due to the advantage of high voltage operation, multilevel inverters
have look forward for a wide range of research work. A part from
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high voltage operation they provide fewer harmonics which lead to
obtain a desired sine wave [1]. Due to this reason they were used in
power converter topology for high power & voltage applications
[2][3]. There are mainly three types of multilevel inverters.
1. Diode Clamped Multi-level Inverter (DCMI)
2. Capacitor Clamped Multi-level Inverter (CCMI) and
3. Cascaded H-Bridge Inverter (CHBI)
The advantages of CHB inverters over other multi-level inverters
are
1. It doesn’t require diodes or capacitors for clamping.
2. It doesn’t need any filter since the output waveform is nearly a
sine wave.
There are so many techniques available namely Sine PWM (SPWM),
Space Vector PWM (SVPWM) [4] to generate pulses to the switches
in these multilevel inverters so that the output voltage is nearly
sinusoidal and contains less number of harmonics. The SPWM
technique can be extended to multilevel inverters by using multiple
carrier signals so it is called as Multi Carrier PWM technique.
Different types of multicarrier PWM techniques have been applied
to Z-source inverter [5-8] and in [9], multi carrier techniques are
applied to diode clamped multi-level inverter. In this paper, three
types of multicarrier techniques have been applied to five-level CHB
inverter.
2 CHB Inverter
Fig1: Five-level CHB inverter Fig2: Output of five-level CHB inverter
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The major differences of DCMLI (Diode Clamped Multi-level
inverter), CCMLI (Capacitor Clamped Multi-level Inverter) from
CHBI is the methodology of obtaining the output voltage waveforms.
A separate DC source is being used along with each CHB Inverter to
create a stepped waveform. A simple single phase leg of a 5-level
CHB inverter is shown in figure 1. The single H-bridge itself acts as
a 3-level inverter. Here two H-bridge modules are used for getting 5
different voltage levels as shown in figure 2. Table 1 shows the
switching pattern and the different output voltage levels for the five-
level CHB inverter.
Table 1: Switching pattern and output voltage levels of five-
level CHB
Switches in H-bridge 1 Switches in H-bridge 2 Voltage
level S1 S2 S3 S4 S5 S6 S7 S8
1 1 0 0 1 1 0 0 2V
1 1 0 0 0 1 0 1
V 1 1 0 0 1 0 1 0
0 1 0 1 1 1 0 0
1 0 1 0 1 1 0 0
0 1 0 1 0 1 0 1
0
0 1 0 1 1 0 1 0
1 0 1 0 0 1 0 1
1 0 1 0 1 0 1 0
1 1 0 0 0 0 1 1
0 0 1 1 1 1 0 0
0 0 1 1 1 0 1 0
-V 0 0 1 1 0 1 0 1
0 1 0 1 0 0 1 1
1 0 1 0 0 0 1 1
0 0 1 1 0 0 1 1 -2V
3 Multi Carrier PWM techniques
In a Sinusoidal PWM technique, a single sine wave is compared
with a triangular (carrier) wave in order to generate pulses for the
switching operations of an inverter. A sine PWM technique has been
extended to multi-level inverter modules by taking more number of
carriers. Hence it is simply called as multi carrier pulse width
modulation technique. It requires (n-1) carrier waves for n level
inverter. It is again simplified as Level Shifted PWM (carrier signals
are arranged vertically) and Phase Shifted PWM (carrier signals are
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arranged horizontally). LSPWM is again divided into three types.
They are
1. Phase Disposition-PWM (PD-PWM),
2. Phase Opposition Disposition-PWM (POD-PWM)
3. Alternate Phase Opposition Disposition-PWM(APOD-PWM) [5].
In PD-PWM, all the carrier waves are in phase (fig 3), in POD-
PWM, All the carrier waves above zero are in phase among them
and all the carrier waves below the zero are in phase among them
but in opposition to the earlier carrier waves (fig 4) and in APOD-
PWM, all the carrier waves are in opposition alternatively from top
to bottom. (fig 5)
Here the peak to peak voltage of 3V is taken for each carrier wave
and the sine reference wave is shown in figures 3-5 for the
modulation index of 0.8 (that is, peak value of sine
wave=6*0.8=4.8V). Here the figures 3-5 are shown for one cycle of
sine reference wave.
Fig3: PD-PWM
Fig4: POD-PWM
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Fig5: APOD-PWM
4 Simulation Results
All the three multi carrier techniques are applied to five-level CHB
inverter. Simulation is carried out in MATLAB/Simulink by taking
the parameters as follows.
1. DC bus voltage=100V,
2. Modulation index (ratio of peak value of the reference wave to the
peak value of the upper carrier wave), m=0.8, 1 and
3. Carrier frequency, fc =2500Hz, therefore frequency modulation
ratio (ratio of carrier frequency to reference frequency), mf = 50
If p is the number of voltage levels in phase voltage of a multilevel
inverter then the number of levels that the line voltage contain is
(2p-1). Therefore for 5-level inverter, the number of levels in the line
voltage is nine. When the reference waveform is more than the
carrier wave, a pulse is produced. The pulses produced by the
comparison of reference sine wave with upper and lower carriers,
are given to the H-bridge 1 and the pulses produced by comparison
of reference sine wave with the middle two carrier waves, are given
to H-bridge 2. One phase of the simulation circuit is shown in fig 6
and the circuit for the three-phase is constructed by taking the
reference sine wave with a phase shift of 120 and 240 degrees for
the other two phases.
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Fig 6: Simulation diagram for one leg for PD-PWM technique.
The output voltages (both phase and line) and corresponding
harmonic patterns are taken. Figure 7 and 8 shows the phase
voltage, line voltage respectively and the corresponding harmonic
pattern also shown when the PDPWM applied to 5-level CHB when
the modulation index is one. Similarly the waveforms obtained when
the POD PWM and APOD PWM applied to 5 level CHB when m=1,
are shown in the figures 9-12. In all the figures, the harmonics in
the line voltage decreases significantly since it contains more levels
than the phase voltage. All these output voltages and harmonic
content values are sown in table 2 for the modulation indexes of 0.8
and 1. When the modulation index increases from 0.8 to 1, the
voltage peak voltage has increased and the harmonics decreased.
Fig7: Phase voltage and harmonics for PD-PWM technique applied to 5-level CHB
when m=1
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Fig8: Line voltage and harmonics for PD-PWM technique applied to 5-level CHB
when m=1
Fig9: Phase voltage and harmonics for POD-PWM technique applied to 5-level CHB
when m=1
Fig10: Line voltage and harmonics for POD-PWM technique applied to 5-level CHB
when m=1
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Fig 11: Phase voltage and harmonics for APOD_PWM technique applied to 5-level
CHB,m=1
Fig12: Phase voltage and harmonics for APOD-PWM technique applied to 5-level
CHB, m=1
Table 2: Output voltages and percentage of harmonics for different modulation
indexes.
m=0.8 m=1
PWM
techni
que
Peak of
the
Fundam
ental
(Vph)(V)
Harmo
nics
(%)
Peak of
the
fundam
ental
(Vline)(V)
Harmo
nics
(%)
Peak of
the
fundam
ental
(Vph)(V)
Harmo
nics
(%)
Peak of
the
fundam
ental
(Vline)(V)
Harmo
nics
(%)
PD 157.1 38.24 271.9 21.34 196.2 26.68 339.8 16.89
POD 157 37.52 272 35.35 196.1 26.59 339.9 21.34
APOD 157 37.62 271.9 29.50 196.2 26.63 339.8 25.34
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5 Conclusion
The three types of multicarrier PWM techniques (PD-PWM,
POD-PWM and APOD-PWM) have been applied to the five-level
CHB inverter for different modulation indexes. The phase and line
voltage waveforms and the corresponding harmonic content is
shown for the modulation index of unity and the results are
tabulated for both modulation indexes. All the three techniques gave
nearly the same fundamental voltage but Phase Disposition Pulse
Width Modulation (PD-PWM) technique gives the less harmonic
content.
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