 Open Access
 Total Downloads : 16
 Authors : M. Vijaya Krishna, D. Lakshman Kumar, Kamal Kishore. Y
 Paper ID : IJERTCONV4IS07008
 Volume & Issue : ETE – 2016 (Volume 4 – Issue 07)
 Published (First Online): 24042018
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Comparison of Various Switching Techniques for 7Level Cascaded Multilevel Inverter: A Review
1M. Vijaya Krishna Assistant Professor EEE Department
Ballari Institute of Technology and Management, Ballari, Karnataka, India.
2D. Lakshman Kumar Assistant Professor EEE Department
Shri Vishnu Engineering College for Women, Bhimavaram, Andhra Pradesh, India.
3Kamal Kishore. Y Assistant Professor EEE Department
Ballari Institute of Technology and Management, Ballari , Karnataka, India.
Abstract Utilization of power semiconductor apparatus to enhance power quality may typically use activepower devices optimally operating with very high switching frequencies. This paper deals with comparison of various switching techniques like Stepped wave, Inphase Disposition carrier and Hybrid Level shifted carrier Pulse width modulation techniques for the control of a 7level cascaded multi level inverter. Comparison is done in terms of design of the pulses and their operation.
Keywords Cascaded Multilevel inverter (CMLI), Hybrid modulation, sequential switching pulse, Multilevel Sinusoidal PWM, Hybrid Inphase Disposition (HIPD).

INTRODUCTION
MLI is improved. It also has the advantage of equal power dissipation among the power devices in a cell.
Comparison of the 7level cascaded multilevel inverter with stepped pulses, Inphase Disposition and Hybrid IPD in terms of the voltage levels and the harmonics content is considered.
v
v
+ S1 S3
Vdc –
S4 S2
The multilevel converters achieve highvoltage switching by the use of series of voltage steps, each of the individual power devices are within the ratings. Among the
+
Vdc –
S5 S7 L O
A
multilevel inverters, the cascaded Hbridge topology is attractive in highvoltage applications, because it requires the least number of components to obtain the same number of
S8 S6 D
voltage level.
Highvoltage capability with voltage limited devices;
+
Vdc –
S9 S 11
low harmonic distortion and increased efficiency are some of the special features of multilevel inverter. The cascaded multilevel inverter appears to be superior to other at high
S 12
S10
power rating because of its modular nature of modulation, control and protection requirements of each full bridge inverter [26]. Many new modulations have been developed to meet the growing number of MLI topologies. They are aimed to generate a stepped switched waveform that approximates an arbitrary reference signal with adjustable amplitude, frequency and phase fundamental component.
Most of the modulation methods developed for multilevel inverters are based on multiplecarrier arrangements with PWM. The carriers can be arranged with vertical shifts or with horizontal displacements. In this paper, vertical displacements is considered i.e., level shifted carrier. With the use of hybrid modulation the performance of the
Fig.1. Schematic diagram of the inverter topology used to verify the proposed hybrid modulations.

PWM TECHNIQUE FOR CHB INVERTER

Stepped Pulses
The stepped pulses, this is the conventional topology for triggering of the cascaded multilevel inverter. Output voltage is a staircase wave, there will be rise of level with the pulse given to the corresponding Hbridge.

In Phase Disposition PWM (IPDPWM)
Fig.2 shows the inphased disposition multicarrier modulation scheme. A multilevel inverter with M no. of voltage levels it may requires (M1) triangular carriers. In the
level shifted multicarrier modulation scheme, all the carriers have equalized magnitude & frequency[9]. The modulation index is
frequency and staying switches are operated with multilevel sinusoidal switching frequency.
In this topology all the four switches in the HBridge are
Frequency modulation
=
()
(1)
operated at two various frequencies; three switches of 7level HBridge are operated at Fundamental Frequency and remaining three switches are operated at the multilevel
= (2)
sinusoidal PWM frequency.
Comp –
Where Ar is the amplitude of reference sinusoidal waveform from peak to peak; Ac is the amplitude of carrier waveform; Fc is the frequency of the carrier waveform; Fr is the frequency of reference waveform.
Fig.2. Level Shifted 5Level IPD Carrier PWM

Hybrid Inphase Disposition
Fig.3 depicts the hybrid inphase disposition 7 – level carrier waveforms for 7level carrier PWM topology with three carrier waveforms for 7levels correspondingly a sinusoidal reference. The mi of this system is comparatively
Reference Signal
Carrier Signal
+2Ac
+Ac
Ac
2Ac
3AC
3
Comp – 2
Comp – 1
Comp – 4
Comp – 5
Comp – 6
S9
S5
S1
S3
S7
S11
however the plentifulness to be viewed as Ar is the sufficiency of the reference sinusoidal waveform from least to most extreme amplitudes. The modulation index is
Fig.4 Block Diagram of 7 Level IPDPWM
Fig.4. shows the modulation of IPDPWM it has one reference wave which is compared with the six carrier waves
=
(3)
and pulses were generated. This is the level shifted carrier
()
PWM method. In addition to this 7level HIPDPWM is developed. Table.1 gives the requirement of number of carrier waves for the IPD and Hybrid IPDPWM.
Fundamental PWM
Generator
Sequential PWM
Generator A
B
Uni polar Comparator 1 C
Fig. 3 Hybrid IPD 7 Level Carrier PWM


SEQUENTIAL SWITCHING HYBRID MODULATION SCHEME

Basic Principle of Modulation:
In the above topology a sinusoidal fundamental frequency is compared with the multiple carrier waves which are in vertical disposition to other as in Fig.2.
Refernce Signal
Carrier signal
+Ac
+Ac
Comparator 2 D
Comparator 3 E
In this HIPD topology all the concern switches in H bridge are operated at two diverse frequencies; three switches of 7level Hbridge are worked at fundamental switched
Fig.5. Sequential Switching of Hybrid Modulation.

Hybrid Modulation Scheme
A successive type sequential switching pulse type (SSP) will be square wave with 50% of duty ratios and it is a large portion of basic fundamental frequency. This signal makes each power switch to operate at MSPWM as well as FPWM arrangement to equalize the power loss among the power devices.
TABLE. I. REQUIRED NUMBER OF CARRIERS
Type of Method
Reference Wave
Carrier Wave
IPD
1
6
HIPD
1
3
FPWM (Fundamental Frequency Scheme) is additionally a square wave signal, synchronized with the modulated waveform. SSP (Sequential Switching Pulse) and FPWM pulses are same for all inverter cells. The general structure of the SSHM (Sequential Switching Hybrid Modulation) is demonstrated in Fig.5 [1].

Base PWM Circulator
A basic base modulation scheme is to get resultant hybrid PWM flow among the power modules. Where as if there should be arise an occurrence of 7levels HIPDPWM the clock frequency is diminished to f0/6 with three 3:1 multiplexers, and choose one from the three WMs focused around the time determination clock signal.

Hybrid Modulation Controller
This controller combines SSP, FPWM & MSPWM that achieves SSHM pulses. It is designed by using a simple combinational logic for a 7level HIPDPWM is expressed (3) as [10]
S1=B +B S7 = +
S2= + S8 = +
S3= + S9 = + B
S4= + S10 = +
S5=B +B S11 = +
S6= + S12 = + (3)
Fig.6. Sequential Switching Hybrid Modulation for Seven Levels
Fig.7. Low and High Frequency IPDPWM Pulses At mi=1 And Fc=1050.
The switching pulses which are obtained from various modulation schemes are as follows in (a) and (b).
Fig. 8(a), (b) indicates the stepped pulses for 7 level cascaded multilevel inverter. Fig.8 (a) indicates the pulses fed for left half of the HBridge and Fig.8 (b) represents the pulses fed for the right half of the HBridges in Fig.1.
Fig 9(a) represents the 7level IPDPWM pulses fed to the switches serving the positive half of the output voltage. Fig. 9(b) indicates the 7level IPDPWM pulses fed to the switches serving the negative half of the output voltage.
(a)
(b)
Fig. 8 (a), (b) Stepped pulses for 7level CMLI
(a)
(b)
Fig.9 (a),(b). 7level Pulses for CMLI using IPD PWM
(a)
(b)
Fig.10 (a), (b). Seven Level Sequential Switching IPDPWM Pulses.
Fig.7 depicts the representation the low and high frequency IPDPWM, in that A is the F0, B is the F0/2, C, D & E are the pulses from the comparators 1, 2 & 3. The C, D & E are the fusion of C, D & E, such that for the first cycle the particular pulse will be the yield for the second cycle an alternate pulse will be the yield pulse. This is acquired by utilizing the base PWM flow strategy. Fig.10 is the seven levels IPDPWM pulses produced from the circuit demonstrated in Fig.7. All the three techniques got have encouraged to the MLI with the comparing pulse names given to the separate switches of MLI [8].


SIMULATION
Fig.11. Simulation circuit of MLI.
The simulation is done by using MATLAB/SIMULINK software. The output voltage waveforms are observed for the three PWM methods. Fig.11 shows the MATLAB/Simulink model of cascaded multilevel inverter of 7Level, in this three HBridges are connected in series connection.
Fig.12 depicts the voltage waveform of the 7level Line to neutral voltage of the CMLI controlled by the stepped pulses.
Fig.13 Harmonic spectrum of stepped pulsed CMLI.
Fig.13 shows the Harmonic spectrum of the output voltage which is obtained with the stepped pulses controller with the THD of 16.8% and voltage magnitude of 276.4V.

RESULTS
For justification of comparison, a performance index namely total harmonic distortion (THD) was chosen to evaluate the three techniques and plotted. The THD is calculated using the equation (4). Up to 20th order of harmonics is taken in to consideration [8].
Fig.14 7level Line to neutral Voltage waveform with IPDPWM
Fig.14 depicts the Line to neutral voltage waveform of the 7level CMLI controlled by the Inphase disposition PWM.
Fig.15 shows the Harmonic spectrum of the 7level CMLI output voltage waveform with THD of 18.23% and magnitude of 300V.
50 2
THD=
2
1 (4)
Fig.15 Harmonic spectrum of the 7level CMLI with IPDPWM
Fig. 12 7level Line to neutral stepped inverter output voltage waveform
Fig.16 Line to neutral output voltage waveform for a 7level inverter at mi=1 and fc=1050 Hz of new HIPDPWM
Fig.17. Harmonic spectrum of the line to neutral output voltage of HIPDPWM
Fig.16 shows the phase voltage waveform of the 7level inverter with the Hybrid level shifted IPDPWM method under RLoad and Fig.17 shows the Spectrum analysis of the corresponding voltage waveform with the THD value of 18.20%.
Table II gives the list of harmonic content in terms of percentages for corresponding harmonic order from fundamental to 19th order with respective triggering methods of 7level CMLI with the expected output voltage of 300V and fundamental frequency of 50Hz.
TABLE. II HARMONIC CONTENTS

CONCLUSION
An optimized hybrid level shifted PWM control scheme for cascaded multilevel inverter is discussed in this paper. A hybrid IPDPWM controller is designed to generate gate pulses for the inverter switches. When compared with the conventional Stepped pulses and the IPDPWM techniques with the hybrid PWM it acts as a moderate in between the other two types of methods. From the obtained data it can be conclude that though the THD value of the HIPDPWM value is slight greater than the conventional triggering method, but while considering the individual harmonic order in HIPDPWM the harmonic percentage is considerably decreased when compared to stepped and IPDPWM.
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Harmonic Order 
Harmonic percentage 

Stepped 
IPD 
HIPD 

1 
100 
100 
100 
3 
9.76 
0.23 
0 
5 
2.23 
0.30 
0.01 
7 
0.02 
0.33 
0.01 
9 
1.28 
0.16 
0 
11 
2.49 
0.07 
0 
13 
7.82 
0.01 
0 
15 
6.55 
0.29 
0.01 
17 
1.81 
0.30 
0 
19 
0.67 
0.26 
0 