Analysis of Different Modulation Techniques for
Multilevel Inverters
Jani Rushiraj G.
Prof. P.N. Kapil
Department of Electrical Engineering,
Institute Of Technology, Nirma University
Ahmedabad,
14meep10@nirmauni.ac.in
Department of Electrical Engineering,
Institute Of Technology, Nirma University
Ahmedabad,
pnkapil@nirmauni.ac.in
Abstract—Analysis of different modulation techniques for
multilevel inverter is discussed. To control the multilevel
inverters, different modulation techniques have been used.
Amongst them multi carrier Sine Pulse Width Modulation
(SPWM) techniques are widely used for different multilevel
inverter topologies. These modulation techniques are derived
from the conventional two level inverter SPWM technique.
Different modulation techniques are compared on the basis of the
output THD, utilization of DC link voltages and common mode
voltage. The detailed analysis of modulation techniques are
shown on the basis of the output THD offered by the five level
Neutral Point Clamped or Diode Clamped (NPC) inverter.
separate DC source can be easily obtained. The simulation
results are shown for five level NPC type topology [3].
The conventional five level NPC topology is shown in fig.1. Here four different, equal valued DC source of VDC/4 are
used.
Keywords— Multi carrier pulse width modulation; multilevel
inverter; Total Harmonic Distortion (THD)
I. INTRODUCTION
The Pulse Width Modulation (PWM) techniques for
conventional two level inverter can be used in Adjustable
Speed Drive (ASD) to provide easy control on output voltage
and frequency. But in high power, medium voltage
applications, conventional two level inverter has severe
limitations. Advantages of multilevel inverter over the
conventional two level inverter is discussed so far [1]. Due to
these advantages, different multilevel inverter topologies
becomes more popular in recent years, mostly in induction
motor drive systems. Total Harmonic Distortion (THD) at
output voltage improved as the number of pole voltage level
increased, but the controlling and bus structure of inverter
becomes complex as the number of level increased above five
[2].
There are mainly three topologies used in multilevel
inverters. There are Neutral Point Clamped (NPC), Flying
Capacitor (FC), and Cascaded H-Bridge (CHB) [1]. Selection
among these topologies depends upon application. NPC
multilevel inverter required separate DC source and complex
control algorithm when real power need to be delivered at load
side. Flying Capacitor type topology is used for both active and
reactive power supply, but the capacitor voltage balancing
becomes problem in this topology, when inverter is used for
reactive power compensation. Cascaded H-Bridge (CHB)
inverter requires separate DC sources, hence this topology is
mainly suitable for non-conventional energy sources, where
Fig. 1 Five level Neutral Point Clamped (NPC) inverter
topology
As shown in the fig.-1, in this configuration, in one leg of
inverter switches S11 and S15, S12 and S16, S13 and S17 and S14
and S18 operates in complementary nature, i.e. if one switch is
in ON state the other must be in OFF state [4]. This
configuration of the Diode clamped inverter is referred as
asymmetrical neutral point clamped five level inverter, because
the blocking voltage rating of diodes are not same. Switching
table of the inverter configuration corresponding to the pole
voltage is shown in Table-1.
Table-1 Switching Table of NPC for one phase
Switching State
with the reference sine wave and in this manner a modulating
signal is to be obtained.
III. LEVEL SHIFTED MULTI CARRIER SPWM TECHNIQUES
S11
S12
S13
S14
S15
S16
S17
S18
Output
Pole
Voltage
1
1
1
1
0
0
0
0
VDC/2
0
1
1
1
1
0
0
0
VDC/4
0
0
1
1
1
1
0
0
0
0
0
0
1
1
1
1
0
-VDC/4
The basic SPWM techniques are listed in previous section.
In level shifted SPWM total (n-1) carrier signals are needed to
obtain n voltage levels at the pole of inverter, and these carrier
signals are vertically displaced with different phase
displacement [6]. Orientation of carrier wave with respect to
the reference wave decides the THD in output of the inverter.
Here, in simple SPWM reference signal is sine wave only.
0
0
0
0
1
1
1
1
-VDC/2
A. In Phase Disposition PWM (IPDPWM)
II. DIFFERENT PWM TECHNIQUES
The PWM techniques for multilevel inverters are broadly
classified as:
•
Sine Pulse Width Modulation (SPWM)
•
Space Vector Pulse Width Modulation (SVPWM)
•
Discontinuous Pulse Width Modulation (DPWM)
In this modulation scheme all the carrier signals are in
phase but they are vertically displaced as shown in fig.-2. But
for each phase of inverter, the carrier signal should be equally
displaced in phase as the reference signal is displaced. Here all
the four carriers have 1 kHz frequency and are in phase with
reference sine wave which has 50 Hz frequency.
Here SPWM and SVPWM are the types of Continuous
PWM (CPWM) technique. In these PWM techniques, the
reference signal is kept within the limits of carrier band, so
intersection of carrier signals and reference signal occurs
continuously, hence switching occurs continuously. But in
discontinuous PWM (DPWM) the reference signal is clamped
to either positive or negative maximum of carrier band, hence
during this time interval no switching takes place [5].
Depending upon the orientation and frequency of different
carrier waves with respect to the reference wave in Sine Pulse
Width Modulation (SPWM) categorized into following
manner:
1.
In Phase Disposition PWM (IPDPWM)
2.
Phase Opposition Disposition PWM (PODPWM)
3.
Alternate
(APOD)
4.
Phase Opposition Disposition
Frequency PWM (PODfPWM)
Phase
Opposition
Disposition
with
PWM
Variable
Fig. 2 In Phase Disposition PWM (IPDPWM)
B. Phase Opposition Disposition PWM (PODPWM)
In the Phase Opposition Disposition PWM technique, all
the carrier above and below zero reference line are in phase
with each other, but the carriers above and below zero
reference are in phase opposition [6] as shown in the fig.-3.
Among these PWM techniques, first three techniques offers
same DC link utilization but different harmonic spectrum. The
fourth PWM technique is used to reduce the switching losses as
two different carrier band with different switching frequencies
are being used in this technique. SVPWM offers more
advantages over SPWM, but implementation of SVPWM
becomes more complex as number of pole voltage levels
increased. Hence, SVPWM techniques is more suitable up to
three level inverter. DPWM (Depenbrock’s Discontinuous
Pulse Width Modulation) are modified PWM techniques.
In Carrier based Space Vector Pulse Width Modulation
(SVPWM) and Discontinuous Pulse Width Modulation
(DPWM) a zero sequence signal has to be obtained and add
Fig. 3 Phase Opposition Disposition (PODPWM)
C. Alternate Phase Opposition Disposition PWM
(APODPWM)
In Alternate Phase Opposition Disposition (APOD) all
carriers are in phase opposition with each other [6] as shown
in fig.-4.
Fig. 6 Reference Signal of Carrier Based SVPWM
B. Depenbrock’s Discontinuous PWM -1 (DPWM1)
Fig. 4 Alternate Phase Opposition Disposition (APOD)
It is discontinuous PWM technique. In this PWM
technique a zero sequence signal which is injected in to the
sine wave is different than a triangular wave. Here the
reference signal is clamped to maximum positive and negative
values of carrier signals [7]. The modulating signal generated
in this PWM technique is shown in fig.-7.
D. Phase Opposition Disposition with Variable Frequency
PWM (PODfPWM)
In this PWM technique, two band of carriers having
different frequencies are being used. The outer band of
carriers having less frequency compared to inner band of
carriers [6] as shown in fig.-5. Here the outer band of carrier
have frequency of 550 Hz and inner band of carrier have 1
kHz.
Fig. 7 Depenbrock’s Discontinuous PWM-1 (DPWM1)
C. Depenbrock’s Discontinuous PWM -3 (DPWM3)
It is another type of discontinuous PWM technique. Here
injecting zero sequence signal and the generated reference
signal is shown in the fig.-8.
Fig. 5 Phase Opposition Disposition with Variable Frequency
(PODfPWM)
IV. ZERO SEQUENCE SIGNAL INJECTION
A. Carrie Based Space Vector Pulse Width Modulation
In this PWM technique a zero sequence triangular wave is
added to the sine wave and the resultant modulating signal
shown in the fig.-6 is compared to the triangular carrier signals
[5].
Fig. 8 Depenbrock’s Discontinuous PWM-3 (DPWM3)
V. SIMULATION RESULTS
For simulation of different PWM technique for multilevel
inverter, five level NPC topology is used. The targeted load
for the simulation purpose is 5.4 Hp, 400 V, 1430 rpm
induction motor is used. DC link voltage for the inverter is
400 V.
To obtain proper results, i.e. symmetry in pole voltages for
all three legs of the inverter, all the carrier signals must be
phase displaced as the modulating signals. As the carrier
signals are displaced, instantaneous comparison between
carriers and modulating signals remains same for all the
phases.
Fig.-9 (a), (b), (c), and (d) shows the pole voltage of
inverter for one leg in IPDPWM, PODPWM, APODPWM,
and PODfPWM respectively. Here in this simulation except
PODfPWM all the carrier signals have switching frequency of
1 kHz and reference wave is sine wave with 50 Hz frequency.
So, the frequency modulation ratio (mf) for these scheme is 20
and amplitude modulation ratio (ma) is 0.8 which is kept fixed
for analysis.
Similarly fig.-10 (a), (b), (c), and (d) shows FFT analysis of
the line voltage of the inverter for IDPWM, PODPWM,
APODPWM and PODfPWM respectively.
(d)
Fig. 9 Pole Voltage for Level Shifted SPWM using (a)
IPDPWM (b) PODPWM (c) APODPWM (d) PODfPWM
(a)
(a)
(b)
(b)
(c)
(c)
(d)
Fig. 10 FFT Analysis of line voltage for (a) IPDPWM (b)
PODPWM (c) APODPWM (d) PODfPWM
(a)
(a)
(b)
Fig. 12 (a) Pole voltage of inverter for DPWM1 (b) FFT
analysis of line voltage for DPWM1
Similarly, pole voltage of inverter for DPWM3 and FFT
analysis of line voltage of the inverter is shown in Fig.-13(a)
and (b) respectively.
(b)
Fig. 11 (a) Pole Voltage SVPWM (b) FFT Analysis of Line
Voltage
As shown in fig.-11 (a) the pole voltage of one leg of
inverter obtained by comparing reference signal shown in fig.6 and carrier signals shown in fig.-4. The FFT analysis of the
line to line voltage of inverter is shown in fig.-11 (b). The
output voltage can be obtained by using every level shifted
carrier signals.
Similarly, fig.-12 (a) shows the pole voltage of inverter
using DPWM1 PWM technique. It is the discontinuous PWM
technique. So as shown in the fig.-11 (a) the comparison is not
taken place for some portion of the reference signal. Fig.-11
(b) shows the FFT analysis of line to line voltage of the
inverter.
(a)
(b)
Fig. 13 (a) Pole voltage of inverter for DPWM3 (b) FFT
analysis of line voltage for DPWM3
REFERENCES
VI. CONCLUSION
From all discussed modulating signals with discussed for
level shifted carrier arrangement, different PWM techniques
can be obtained. Different PWM techniques offers different
harmonic distortion in output line voltage waveforms. In the
level shifted SPWM technique, the significant harmonic
component is shifted to the frequency modulation ratio (mf).
By giving proper phase shift in carrier signals better
performance of the inverter is obtained. In the alternate phase
opposition disposition PWM, the significant harmonics are
appeared as the sidebands around carrier frequency, but this
method with any modulating wave offers proper half wave and
quarter wave symmetry. In variable frequency PODfPWM,
lower order harmonics are appeared compared to the other
methods, but this method offers lower switching losses as the
outer carrier band has lower frequency, as switching for
switching devices reduced due to decrease in comparison in
outer carrier band. Carrier based SVPWM offers more DC
link utilization and lower harmonic distortion compared to the
SPWM. So, carrier based SVPWM with POD carrier
arrangement offers lowest harmonic distortion with maximum
DC link utilization.
In discontinuous PWM technique, the reference wave is
modified, hence harmonic spectrum of line to line voltage is
different than the SPWM. Among the all PWM combination
DPWM3 with POD carrier arrangement offers lower THD
than DPWM1.
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