A Comparative Study of Three Phase 2-Level VSI with 3-Level and 5

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459,ISO 9001:2008Certified Journal, Volume 4, Issue 4, April 2014)

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A Comparative Study of Three Phase 2-Level VSI with 3-Level

and 5-Level Diode Clamped Multilevel Inverter Darshan Prajapati

1, Vineetha Ravindran

2, Jil Sutaria

3, Pratik Patel

4

1,4Student

2,3Assistant Professor

Abstract—Multilevel inverters offer several advantages

compared to the conventional 3-phase bridge inverter in

terms of lower dv/dt stresses, lower electromagnetic

compatibility, smaller rating and better output features. This

paper deals with study of Three Phase Two Level, Three Level

and Five Level Voltage Source Inverter (VSI). The Three

Level and Five Level are realized by Neutral point clamped

(NPC) or Diode clamped (DC) inverter using Sinusoidal pulse

width modulation (SPWM) technique. Similarly three phase

VSI (two level) is also simulated using SPWM technique.

SPWM technique is one of the most popular PWM

techniques. Now-a-days harmonic reduction using SPWM is

most popularly used in industries for a Variable Frequency

Drives (VFDs). All the circuits are implemented using

MATLAB software and their outputs are shown in figures

step by step. Finally, this paper compares all the three

inverters i.e., Two Level, Three Level and Five Level based on

the total harmonic distortion of the output voltage waveform.

Keywords—Diode clamped (DC) inverter; multilevel

inverter (MLI); Sinusoidal pulse width modulation (SPWM);

Total harmonic distortion (THD).

I. INTRODUCTION

Nowadays multilevel inverters are most popularly used

in high power and high voltage applications like

transportation system and industrial motor drives.

Multilevel inverters are the better choice for the high power

end. The base behind the multilevel inverter is the fact that

the sine wave can be approximated to a stepped waveform

having large number of steps [1]. As the number of levels

increases, the steps in the output waveforms increases thus

it approaches nearer to the desired sine wave [2]. In a

single phase inverter there is a creation of problems due to

generated harmonics; advancements in development of

such techniques are required which can reduce the

generated harmonics. By using multilevel inverters such

harmonics can be reduced to a large extent. Different DC

levels are supported by connecting batteries or capacitors in

series.

The structure of multilevel inverter is such that lower

voltage rating devices can be used. The different topologies

of multilevel inverters can be categorized into Cascaded

multilevel inverter, Flying capacitor or Capacitor clamped

multilevel inverter and Diode Clamped or Neutral Point

Clamped Multilevel inverter. Number of single phase

inverter modules connected in series with separate DC

sources forms cascaded multilevel inverter. The drawback

of this topology is that isolated DC sources are required for

active power conversion. [1]Through manipulation of the

cascade inverter, with diodes blocking the sources, the

diode-clamped multilevel inverter was then derived [3] [4].

The structural design of the capacitor clamped MLI is

similar to that of the diode clamped multilevel inverter.

Main difference between these two is the capacitors are

used in place ofdiodes for clamping purpose.[7]

II. DIODE CLAMPED INVERTER

Fig. I. Three level Diode Clamped Inverter



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The power circuit diagram of three level diode clamped

multilevel inverter is as shown in the figure. The Mid-level

voltage is defined as a neutral point. So it is also called as

Neutral Point Clamped Multilevel Inverter. The basic

operation of three level Diode clamped inverter is

explained by understanding the inverter shown in fig. 1. In

this circuit, the DC bus voltage is split into three levelsby

two series connected capacitor, C1 and C2.The mid-point of

two capacitors is called the neutral point n. The switches

S11 and S14 are called as main switches. Switches S12 and

S13 are called auxiliary switches which helps to clamp the

output voltage to the neutral point with the help of

clamping diodes D10 and D10’.It is called the three level

multilevel inverter because it has three output voltages 0,

+Vdc/2, -Vdc/2.

For output voltage Vdc/2, switches S11 and S12 needs to

be turned on; for –Vdc/2, switches S13 and S14 needs to be

turned on and for 0, switches S12 and S13 needs to be turned

on. The clamping diodes D10 and D10’ in this topology

distinguishes it from conventional two level inverter. These

two diodes clamp the switch voltage to half the level of

DC-bus voltage. When switch S11 and S12 turned on, the

voltage across a and n is Vdc/2, i.e, Van=Vdc/2.In this

case, the capacitor D10’ balances out the voltage sharing

between S13 and S14.S13 blocks the voltage across capacitor

C1 and S14 blocks the voltage across C2.When switches S12

and S13 turned on, Van=0; when switches S13 and S14

turned on, Van=-Vdc/2.

III. SINUSOIDAL PULSE WIDTH MODULATION

Fig.II. Comparison of Sine and triangular wave for Three phase VSI

SPWM

Fig. III. Comparison of Sine and triangular wave for Three Level

SPWM

Fig. IV.Comparison of Sine and triangular wave for Five Level

SPWM

Fig.V. Sinusoidal Pulse Width Modulation(SPWM) output

Sinusoidal Pulse Width Modulation technique is most

used effective technique to reduce the harmonic contents as

compared to any other PWM technique. In SPWM

technique the desired frequency sine wave is compared

with the high frequency triangle wave. Intersection of these

two waves determines the commutation of the modulated

pulse and switching instants of it. Comparison of

fundamental SINE wave and high frequency triangular

wave is as shown in figure. The compared triangular wave

is called as carrier waves and the desired sine wave is

termed as reference wave. Both these waves are compared

in a comparator and the output of the comparator is given

to the switching devices like TCRs, MOSFETs, IGBTs or

any other switching devices. The comparator output is high

when the magnitude of the sine wave is higher than the

magnitude of the triangular wave and vice-versa.



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The modulation index (MI) is:

MI=

Where =Magnitude of the reference sine wave

=Magnitude of the carrier triangular wave

The MI can never be more than unity i.e., 1.So the

outputvoltage can be controlled by varying Modulation

Index.

As the number of levels increases in multilevel

inverterthe switching devices required is increases. For

example,for a five level diode clamped multilevel inverter

of oneleg the switching devices used are 8, 4 for positive

halfcycle and other 4 for negative half cycle. So one

sinewave is compared with a four triangular wave and each

one’scompared output is given to the each switching

devices. For the next leg these waves are shifted 120° and

same for the third leg.

IV. SWITCHING STRATEGY

The switching strategies of each switch for three level

output voltage is shown in table I.

Fig. VI.Desired pole voltage for three level DCMLI

Fig.VII. One Leg of Three Level Diode Clamped Inverter

TABLE I

SWITCHING STRATEGY FOR ONE LEG OF THREE LEVEL DCMLI

Period S1 S2 S3 S4 Vo(pole

voltage)

0°-

60°

0 1 1 0 0

60°-

120°

1 1 0 0

120°-

180°

0 1 1 0 0

180°-

240°

0 1 1 0 0

240°-

300°

0 0 1 1

300°-

360°

0 1 1 0 0

The switching strategies of each switch for five level

output Voltage is shown in table II.

Fig.VIII. One Leg of Five Level Diode Clamped Inverter



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Fig. IX. Desired pole voltage for five level DCMLI

TABLE II

SWITCHING STRATEGY FOR ONE LEG OF FIVE LEVEL DCMLI

Period S1 S

2

S

3

S

4

S

5

S

6

S

7

S

8

Vo

(Pole

Voltag)

0°-

30°

0 0 1 1 1 1 0 0 0

30°-

60°

0 1 1 1 1 0 0 0

60°-

90°

1 1 1 1 0 0 0 0

90°-

120°

1 1 1 1 0 0 0 0

120°-

150°

0 1 1 1 1 0 0 0

150°-

180°

0 0 1 1 1 1 0 0 0

180°-

210°

0 0 1 1 1 1 0 0 0

210°-

240°

0 0 0 1 1 1 1 0

240°-

270°

0 0 0 0 1 1 1 1

270°-

300°

0 0 0 0 1 1 1 1

300°-

330°

0 0 0 1 1 1 1 0

330°-

360°

0 0 1 1 1 1 0 0 0

V. SIMULATION RESULTS

This simulation was done in the MATLAB R2010a

software with the DC input voltage of 200 volts.

Fig.X, Fig.XI, and Fig.XIIshows the Pole Voltage

waveforms of the three phase VSI, three level inverter and

the five level inverter respectively. And Fig. XIII, Fig.

XIV, Fig.XVshows the Line to Line Voltage waveforms of

the three phase VSI, three level inverter and the five level

inverter respectively. As the output is not connected to any

load, all the outputs obtained are on No-Load.

A. Pole voltage waveforms

Fig.X. Pole Voltage of Three Phase Voltage Source Inverter

Fig.XI. Pole Voltage of Three Level Diode Clamped Inverter

Fig. XII. Pole Voltage of Five Level Diode Clamped Inverter



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B. Line voltage waveforms

Fig. XIII. Line to Line Voltage for three phase VSI

Fig. XIV. Line to Line Voltage for three level DCMLI

Fig. XV. Line to Line Voltage for fivelevel DCMLI

C. Comparison of total harmonic distortion

The total harmonic distortion (THD) any wave form is

defined as the closeness to the fundamental wave shape.

FFT analysis is done to determine the THD of the output

waveforms. The FFT analysis of the voltage waveforms

shown in Fig.XIII, Fig.XIV, and Fig.XV is done using the

power GUI block in the MATLAB software. The results

are shown in Fig.IX, Fig.X, and Fig.XI.

Fig.XVI. FFT Analysis of Three Phase Voltage Source Inverter

Fig.XVII. FFT Analysis of Three Phase Three Level DCMLI

Fig.XVIII. FFT Analysis of Three Phase Five Level DCMLI



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TABLE III

COMPARISON OF TOTAL HARMONIC DISTORTION

Inverter Total Harmonic

Distortion(Line to Line)

3-Phase VSI(2 level) 50.67%

3-level DCMLI 34.88%

5-level DCMLI 16.55%

As seen from the. Fig.X, Fig.XI, and Fig.XII the output

of five level inverter is more nearer to sine wave than the

output of three level inverter. Similarly the output of three

level inverter is more nearer to sine wave than the output of

two level inverter.

And as seen from the Fig.XVI, Fig.XVII, Fig.XVIII and

Table III, the total harmonic distortion (THD) reduces as

the number of levels increases.

VI. CONCLUSION

The simulation and analysis of three phase VSI, three

phase three level Diode Clamped inverter and five level

Diode clamped inverter has been done using MATLAB

software. The PWM technique used is SPWM which is

easier to implement in hardware. The output pole voltage

has been taken on no load. It has been observed from the

output voltage waveform that it appears more like sine

wave as the number of level increases.

This has been in fact proved by doing FFT analysis on

the voltage waveforms and it was found that the total

harmonic distortion decreases as the number of level

increases. This leads to decrease in the requirement of

filter. The number of level of the multilevel inverter to be

used for a particular application can thus be decided upon

the allowed THD of the system.

REFERENCES

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Using Sinusoidal SPWM”,IJETT,Vol.6(2),pp.97-103,Dec-

2013(Article) [3] C.R.Balamurugan,S.P.Natarajan,R.Revathy,”Analysis Of Control

Strategies For Diode Clamped Multilevel

Inverter”,IJIAS,Vol.3(1),pp.19-34,May-2013(Article) [4] RituChaturvede,”A Single Diode Clamped Multilevel Inverter And

Its Switching Function”,Journal Of Innovation Trends In Science,Pharmacy and Technology,Vol.1(1),pp.63-66,2014(Article)

[5] Nazmul Islam Raju, Md. ShahimurIslam, AhmedAhsanUddin,”Sinusoidal PWM Signal Generation Technique

For Three Phase Voltage Source Inverter With Analog Circuit &

Simulation Of PWM Inverter For Standalone Load & Micro-Grid System”,International Journal Of Renewable Energy

Research,Vol.3(3),pp.647-658(Article) [6] Manasa S.,Balaji Ramakrishna S.,MadhuraS.,Mohan H M,”Design

And Simulation Of Three Phase Five Level And Seven Level

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Documents

A Comparative Study of Three Phase 2-Level VSI with 3-Level and 5