TJPRC: International Journal of Signal
Processing Systems (TJPRC: IJSPS)
Vol. 2, Issue 1, Jun 2016, 23-30
© TJPRC Pvt. Ltd.
COMPARITIVE ANALYSIS OF CASCADED H-BRIDGE MULTILEVEL INVERTER
WITH DIFFERENT PWM TECHNIQUES
K.S.S.PRASAD RAJU1 & K.VAISAKH2
1
Electrical and Electronics department, S.R. K.R. Engineering College Chinnamiram, Bhimavaram, India
2
Professor, Electrical Department, Andhra University College of Engineering (A), Visakhapatnam, India
ABSTRACT
The Multilevel inverters can synthesize output AC waveform with better harmonic spectrum and faithful output.
In this paper a comparative study is carried out for cascaded H-bridge multilevel inverters with different line shifted
modulation techniques such as Phase Disposition (PD), Phase Opposition Disposition (POD) and Alternative Phase
Opposition Disposition (APOD) and the Total harmonic distortion (THD) of the phase output voltages are observed for
various modulation index in MATLAB/SIMULINK background. The results obtained with different modulation
techniques are compared in terms of Total harmonic distortion (THD) at different modulation indices.
KEYWORDS: Cascaded Multilevel Inverter, PWM Techniques, Total Harmonic Distortion (THD)
I. INTRODUCTION
Multilevel inverters are widely used in controlling motor drive as static var compensators [7] . The
advantages of multilevel inverters are good power quality and high voltage capability. Multilevel inverters are most
Original Article
Received: Mar 28, 2016; Accepted: Apr 08, 2016; Published: Apr 18, 2016; Paper Id.: TJPRC: IJSPSJUN20163
advanced & latest type of power electronic converters ,with multilevel inverters by taking sufficient number of DC
sources at input side nearly sinusoidal voltage can be synthesized at the output side. The unique structure of the
multilevel voltage source inverters allows them to reach high voltage with low harmonics without the use of
transformers . In Multilevel inverters semiconductor switches must be turned ON & OFF to synthesize output AC
waveform from a DC input waveform such that desired fundamental is obtained with minimum harmonic distortion.
Comparing with two level inverter topologies at the same power ratings, multilevel inverters have the advantages
that the harmonic components of line-to-line voltages fed to load are reduced owing to its switching frequencies.
Therefore, the multilevel inverters also have lower dv/dt ratios to prevent induction or discharge failures on the
loads. The basic multilevel inverter topologies are: Diode Clamped Multilevel Inverters, Flying Capacitor multilevel
Inverters, and Cascaded multilevel Inverters.
There are different types of approaches for the selection of switching techniques for the multilevel inverters.
The main objective of the present paper is to review some of the SPWM methods used in multilevel inverters.
The paper is organized as follows cascaded H bridge multilevel inverter configure uration and its principle
of operation are given in Section II. Different SPWM techniques to cascaded H bridge multilevel inverter are
discussed in Section III. Simulation results are presented in Section IV. Conclusions are given in Section V.
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K.S.S.Prasad Raju & K.Vaisakh
II. CASCADED H-BRIDGE INVERTER CONFIGURE URATION
•
Basic Principle of Operation of Multilevel Invereter
The one full H-bridge cell of the cascaded H-bridge multilevel inverter is shown in Figure 1. The full H-bridge
inverter module includes four power switches and four clamping diodes to form an H-bridge [2][4].
Figure 1: Three Level Full Bridge Inverter
The switching inputs to the H-bridge in figure 1 generate the following output voltages.
The ratio of DC voltage source naturally affects the output levels of a cascade multilevel inverter.
•
Cascaded H-Bridge Multilevel Inverter
The H-bridge cells are connected serially to get AC outputs as shown in Figure 2 to get an increased phase
voltage levels such H-bridge cells are connected in series. Such that the total output level is the synthesize of cells’ output.
Cascaded H- bridge inverters are symmetrical and asymmetrical in symmetrical configure uration all the D.C sources are
having equal value in asymmetrical configure uration all the D.C sources having different values. Considering symmetrical
configure uration by assuming the number of inverter’s DC sources as “n”, the output levels of each phase and line voltage
will be
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Comparitive Analysis of Cascaded H-Bridge Multilevel Inverter with different Pwm Techniques
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Figure 2: Three Phase Cascaded Multilevel Inverter
as shown in (1) and (2) respectively.
m=2s+1
(1)
l=2m-1
(2)
The resulting phase voltages are synthesized by the addition of the voltages generated by the individual cells as in
(3).
VAN=VH1+VH2+VH3+---------+V Hn
(3)
Where n=1,2,3,………depends on number H-Bridges.
Three such legs as shown in figure 2 are connected to form a three phase inverter.
III PWM TECHNIQUES
Gating signals to the cascaded H-bridges can be generated by different methods. . The intersection of a
modulating sine wave with a carrier triangular wave generate firing pulses to the cascaded H-bridges . The carrier based
modulation schemes for multilevel inverters can be classified into two categories: phased shifted and level shifted
modulation The level shifted modulation techniques used in this work are given below [3].
•
Phase Disposition(PD) PWM strategy.
•
Phase Opposition Disposition (POD)PWM strategy.
•
Alternate Phase Opposition Disposition (APOD)PWM strategy.
Level-shifted multicarrier modulation scheme requires (m – 1) triangular carriers, where m is the number of phase
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K.S.S.Prasad Raju & K.Vaisakh
levels in the output voltage . The (m – 1) triangular carriers are vertically disposed such that the bands they occupy are
contiguous. The frequency modulation index is Mf=fcr/fm and the amplitude modulation is given by Ma=2Am/(m-1)Ac.
Where
fc r– Frequency of the carrier signal
fm – Frequency of the reference signal
Am –Amplitude of the reference signal
Ac – Amplitude of the carrier signal
m – number of levels.
Figure 3: Phase Disposition(PD) PWM
In phase disposition (PD) PWM techniques all the carriers are in the same phase and peak to peak values are also
same.
Figure 4: Phase Opposition and Disposition(POD)PWM
In phase opposition disposition technique carrier waveforms above zero are in phase and they are 180 degrees out
of phase with those below zero and peak to peak amplitude of all carriers are same .
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Comparitive Analysis of Cascaded H-Bridge Multilevel Inverter with different Pwm Techniques
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Figure 5: Alternative Phase Opposition and Disposition(APOD) PWM
Carriers are arranged in such a manner that each carrier is out of phase with its neighbour by 180 degrees.
When sinusoidal modulation signal is compared with upper carrier reference waveform switching signal is
generated to S11 in figure 2 when it is compared with second reference signal switching signal is generated to S12,when it
is compared with third reference signal switching is generated to S13,when it is compared with fourth reference signal
switching is generated to S33,when it is compared with fifth reference signal switching signal is generated to S32,when it
is compared with sixth reference signal is generated toS31.The gating signals to the lower switches in each bridge is the
complimentary to the upper switches (i.e S41 is the compliment of S11,S21 is the compliment of S31 and so on).
Table1: Switching Patterns to Get Seven Level in the Output Voltage
IV SIMULATION RESULTS
A three phase seven level cascaded H-bridge multilevel inverter is simulated with the MATLAB/SIMULINK
software with the design parameters [1].
DC input voltage Vdc=100V
Reference switching frequency fm=50Hz
Carrier switching frequency fc=3.5KHz
Load resistance R=50Ω
Load inductance L=50mH
The output phase voltage is having seven level as shown in Figure 6 and all the three phase voltages VAN,VBN,VCN
are phase shifted by1200 as shown in Figure 10,Figure 11 and figure 12.The harmonic spectrum for VAN phase voltage with
different switching techniques such as APOD,POD and PD is shown in Figure 7,Figure 8 and Figure 9 respectively.
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K.S.S.Prasad Raju & K.Vaisakh
Figure 6: Phase Voltage of Seven Level CH-MLI
Figure 7: Harmonic Spectrum With APOD Technique
Figure 8: Harmonic Spectrum with POD Technique
Figure 9: Harmonic Spectrum with PD Technique
Figure 10: VAN Phase Voltage of Seven Level CH-MLI
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Comparitive Analysis of Cascaded H-Bridge Multilevel Inverter with different Pwm Techniques
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Figure 11: VBN phase Voltage of Seven Level CH-MLI
Figure 12: VCN Phase Voltage of Seven Level CH-MLI
Table 2: Comparison Table of THD for Different Modulation Index
V CONCLUSIONS
In this paper a three phase seven level cascaded H-bridge Multilevel inverter is designed and implemented with
different sinusoidal pulse width modulation (SPWM) techniques. The comparison is done for the Phase voltage Total
harmonic distortion (THD) for different modulation indices. From the THD analysis it is observed that Phase opposition
disposition (POD) technique is having low Total harmonic distortion (THD) compared with other techniques for different
modulation techniques.
VI REFERENCES
1)
Vivek Kumar Gupta , R. Mahanty “Optimized switching scheme of cascaded H-bridge Multilevel inverter using PSO”
International journal of Electrical power and energy systems 64(2015) 699-707.
2)
Krishna Kumar Gupta ,Alekh Raja , Pallavee Bhatnagar, Lalit Kumar Sahu and Sailendra Kumar Jain “Multilevel inverter
topologies with reduced device count” IEEE Transactions on Power Electronics.
3)
Jose Rodriguez, Jih-Sheng Lai and Fang Zheng Peng “Multilevel Inverters: A Survey of Topologies, controls, and
applications” IEEE Transactions on Industrial Electronics, VOL. 49, NO. 4, August 2002.
4)
E.Sambath, S.P. Natrajan and C.R. Balamurugan “Performance Evaluation of Multi carrier based PWM techniques for single
phase five level H-bridge type FCMLI”IOSR journal of Engineering vol. 02, Issue. 7,PP 82-90.
5)
Giuseppe Carrara, Simone Gardella, Mario Marchesoni, Raffaele Salutari and Giuseppe Sciutto “A new level PWM method:
A theoretical analysis”, IEEE trans on power electronics, Vol. 7, no. 3 July 1992.
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K.S.S.Prasad Raju & K.Vaisakh
6)
Mariusz Malinowskki, K. Gopakumar, Jose Rodriguez and Marcelo A. Perez “ A survey on cascaded Multilevel Inverters”
IEEE trans on Industrial Electronics, Vol. 57,no. 7,july 2010.
7)
Bin Wu “High Power Converters and AC drives” IEEE press, a John Wiley & sons, Inc., Publication
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