International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 11 - Apr 2014
Simulink Implementation of PWM Strategies
for Diode Clamped MLI
V.Harish1,B.Rajeev Reddy2 ,B.Nishanth3 ,E.SaiPrateekReddy4, B.Akhilesh Reddy5
12345
Dept. of EEE, KLUniversity, Vaddeswaram, Guntur, A.P. India
Abstract: Multilevel inverters has got more concern
over conventional two level inverters because of their
features like yielding output with low harmonics,
output tending to sinusoidal, the voltage stress across
each switching device is low. The diode clamped
inverter is widely multilevel inverter configuration
due to ease of operation when compared to other
topologies. The variation in output voltage magnitude
and frequency is achieved using various modulation
strategies. In this paper various PWM strategies are
implemented for three level and five level diode
clamped MLI and results are compared.
Keywords: multilevel inverter, diode clamped, pwm,
low harmonics
I.
INTRODUCTION
Multi-level inverters have been attracting
increasing interest recently, particularly because of
their increased power rating, improved harmonic
performance and reduced EMI emission. The most
common multi-level topologies are known as
Diode-clamped, Cascade and flying capacitor. The
typical three level diode clamped inverter is shown
below fig1. Each leg represents one phase. In a
single leg three level of voltages are possible
+vdc,0,-vdc.
Fig.2 Five level three phase diode clamped inverter
II.
Fig.1 Three level three phase diode clamped
inverter
In similar fashion five level inverter shown in
fig2.gives +vdc,+vdc/2,0,-vdc/2,-vdc.
ISSN: 2231-5381
CARRIER BASED PWM METHODS
The natural sampling techniques [2] for a multilevel
inverter are categorized into two and they are:
a)Single-Carrier SPWM (SCSPWM)
b)Sub-Harmonic PWM (SHPWM)
Sub-Harmonic PWM is an exclusive control
strategy for multilevel inverters and has further
classifications. They are:
1. Phase Shifted Carrier PWM method (PSPWM)
2. Carrier Disposition PWM methods
http://www.ijettjournal.org
Page 516
International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 11 - Apr 2014


Phase Disposition (PD)
Alternative Phase Opposition Disposition
(APOD)
 Phase Opposition Disposition (POD)
An m-level multilevel inverter using level-shifted
Multicarrier modulation scheme requires (m – 1)
triangular carriers, all having the same frequency
and amplitude. The (m – 1) triangular carriers are
vertically disposed such that the bands they occupy
are contiguous. The frequency modulation index is
given by mf = fcr/fm, which remains the same as the
phase-shifted modulation scheme whereas the
amplitude modulation index is defined as Where Vm
is the peak amplitude of the modulating wave vm
and ˆVcr is the peak amplitude of each carrier wave,
ma=Vm/(m-1)*Vcr
Fig.4 Pulses for upper switches in single leg of
three level
A) PHASE SHIFTED MODULATION
All the triangular carriers have the same frequency
and the same peak to peak amplitude. There is a
phase shift between any two adjacent carrier waves,
given by
Øcr = 360°/(m – 1)
For m=3, Øcr = 360°/2
Øcr = 180°
For m=5 Øcr = 360°/4
Øcr = 90°
Comparision of carrier and modulating waves for
three level and corresponding output voltages are
shown in fig.3, fig.5
Fig.5 Three phase line voltages of three level
DCMLI
Fig.3.Comparision of reference wave and phase
shifted carrier waves for three level
Fig.6 Comparision of reference wave and phase
shifted carrier waves for five level
ISSN: 2231-5381
http://www.ijettjournal.org
Page 517
International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 11 - Apr 2014
Fig.9 Three phase line voltages of three level
DCMLI
Fig.7 Three phase line voltages of five level
DCMLI
III.
CARRIER DISPOSITION PWM
METHODS
The widely used multi-carrier PWM methods are
known as Phase Shifted (PS), Phase Disposition
(PD), Phase Opposition Disposition (POD), and
Alternative Phase
Opposite Disposition (APOD).
A. Phase Disposition
If all carriers are selected with the same phase, the
method is known as Phase Disposition (PD)
method. It is generally accepted that this method
gives rise to the lowest harmonic distortion in
higher modulation indices when compared to other
disposition methods. This method is also well
applicable to diode clamped inverters. The
waveform of carriers of this method is illustrated in
Fig. 8.
Fig.8 Comparision of reference wave and phase
disposition carrier waves for three level
ISSN: 2231-5381
Fig.10 Comparision of reference wave and phase
disposition carrier waves for five level
Fig.11 Three phase line voltages of five level
DCMLI
http://www.ijettjournal.org
Page 518
International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 11 - Apr 2014
B. PHASE OPPOSITION DISPOSITION
The Phase Opposition Disposition (POD) method,
having the carriers above the zero line of reference
voltage out of phase with those of below this line
by 180 degrees as shown in Fig. 12 is one another
of the carriers’ disposition group. Compared to the
PD method, this method has better results from the
viewpoint of harmonic performances in lower
modulation indices. In POD method, there is no
harmonic at the carrier frequency and its multiples
and the dispersion of harmonics occurs around
them.
Fig.14 Comparison of reference wave and phase
opposition disposition carrier waves for five level
Fig.12 Comparison of reference wave and phase
opposition disposition carrier waves for three level
Fig.15 Three phase line voltages of five level
DCMLI
Fig.13 Three phase line voltages of five level
DCMLI
ISSN: 2231-5381
C. ALTERNATE PHASE OPPOSITION
DISPOSITION
The third member of the carriers’ disposition group
is known as Alternative Phase Opposition
Disposition (APOD) method. Each carrier of this
method is phase shifted by 180 degrees from its
adjacent one. It should be noted that POD and
APOD methods are exactly the same for a 3-level
Inverter. This method gives almost the same results
as the POD method. The major differences are the
larger amount of third order harmonics which is not
important because of their cancellation in line
voltages. Thus, this method results in a better THD
for line voltages when comparing to the POD
method. The carrier waveforms of this method are
illustrated in Fig 16.
http://www.ijettjournal.org
Page 519
International Journal of Engineering Trends and Technology (IJETT) – Volume 10 Number 11 - Apr 2014
Fig.16 Comparison of reference wave and alternate
phase opposition disposition carrier waves for five
level
Fig.17 Three phase line voltages of five level
DCMLI
The FFT analysis of three level and five level diode
clamped MLI for various modulation schemes.
REFERENCES
Modulation Scheme
Three
level
51.68
34.60
Phase Shifted
Phase disposition
Phase Opposition
Alternate
Phase 36.83
OppositionDisposition
Five
level
60.87
17.86
21.83
25.8
[1]
[2]
[3]
[4]
CONCLUSION
Three phase three level and five level diode
clamped multilevel inverter configuration are
simulated using SIMULINK. The different
modulation schemes like phase shifted, phase
disposition, phase opposition, alternate phase
opposition disposition are implemented and
corresponding voltages foe each configuration are
obtained and analysed.
ISSN: 2231-5381
[5]
[6]
Akira Nabae et al., “A New Neutral-Point-Clamped
PWM Inverter”, IEEE Transactions on Industry
Applications, Vol. IA-17, No. 5. September/October
1981
Muhammad H. Rashid, Power Electronics Circuits,
Devices, and Applications, third edition, 2003.
Ned Mohan, Power Electronics, Converters,
Applications and Design, third edition,
2006
J. Rodriguez, J.-S. Lai, and F. Z. Peng, “Multilevel
inverters: A survey of topologies, controls, and
applications,” IEEE Trans. Ind. Electron.,vol. 49,
no. 4, pp. 724–738, Aug. 2002.
J. S. Lai and F. Z. Peng, “Multilevel converter—A
new breed of power converter,” in Proc. IAS'95
Conf., 1995, pp. 2348–2356.
X. Yuan and I. Barbi, “Fundamentals of a newdiode
clamping multilevel inverter,” IEEE Trans. Power
Electron., vol. 15, pp. 711–718, July 2000.
http://www.ijettjournal.org
Page 520