Comparison of Multi Carrier PWM

Techniques applied to Five Level

CHB Inverter

P S V Kishore1, P Suresh Kumar2,

K Ramesh3

1,2,3Vignan’s IIT, AP, India

pulavarthi.kishore@gmail.com

suresh0260@gmail.com

rameshkoyyana@gmail.com

April 14-15, 2017

Abstract

The use of Multi-level inverters has been increased due

to their high power applications and the ability for

getting nearly a sinusoidal output voltage compared to

normal two level inverters. The various multilevel

inverters in existence are Diode Clamped, Capacitor

Clamped, and Cascaded H-bridge inverters. In order to

get fewer harmonic various techniques are adopted to

control the switches in a multilevel inverter. In this

paper, multi carrier PWM techniques are used to

generate the pulses to the switches in the 5-Level

Cascaded H-bridge (CHB) inverter.

MATLAB/Simulation is used to simulate the circuit and

the results are tabulated.

Keywords: Multi carrier PWM, CHB Inverter, Multi-level

inverter, MATLAB/Simulation, five-level

1 Introduction

Due to the advantage of high voltage operation, multilevel inverters

have look forward for a wide range of research work. A part from

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high voltage operation they provide fewer harmonics which lead to

obtain a desired sine wave [1]. Due to this reason they were used in

power converter topology for high power & voltage applications

[2][3]. There are mainly three types of multilevel inverters.

1. Diode Clamped Multi-level Inverter (DCMI)

2. Capacitor Clamped Multi-level Inverter (CCMI) and

3. Cascaded H-Bridge Inverter (CHBI)

The advantages of CHB inverters over other multi-level inverters

are

1. It doesn’t require diodes or capacitors for clamping.

2. It doesn’t need any filter since the output waveform is nearly a

sine wave.

There are so many techniques available namely Sine PWM (SPWM),

Space Vector PWM (SVPWM) [4] to generate pulses to the switches

in these multilevel inverters so that the output voltage is nearly

sinusoidal and contains less number of harmonics. The SPWM

technique can be extended to multilevel inverters by using multiple

carrier signals so it is called as Multi Carrier PWM technique.

Different types of multicarrier PWM techniques have been applied

to Z-source inverter [5-8] and in [9], multi carrier techniques are

applied to diode clamped multi-level inverter. In this paper, three

types of multicarrier techniques have been applied to five-level CHB

inverter.

2 CHB Inverter

Fig1: Five-level CHB inverter Fig2: Output of five-level CHB inverter

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The major differences of DCMLI (Diode Clamped Multi-level

inverter), CCMLI (Capacitor Clamped Multi-level Inverter) from

CHBI is the methodology of obtaining the output voltage waveforms.

A separate DC source is being used along with each CHB Inverter to

create a stepped waveform. A simple single phase leg of a 5-level

CHB inverter is shown in figure 1. The single H-bridge itself acts as

a 3-level inverter. Here two H-bridge modules are used for getting 5

different voltage levels as shown in figure 2. Table 1 shows the

switching pattern and the different output voltage levels for the five-

level CHB inverter.

Table 1: Switching pattern and output voltage levels of five-

level CHB

Switches in H-bridge 1 Switches in H-bridge 2 Voltage

level S1 S2 S3 S4 S5 S6 S7 S8

1 1 0 0 1 1 0 0 2V

1 1 0 0 0 1 0 1

V 1 1 0 0 1 0 1 0

0 1 0 1 1 1 0 0

1 0 1 0 1 1 0 0

0 1 0 1 0 1 0 1

0

0 1 0 1 1 0 1 0

1 0 1 0 0 1 0 1

1 0 1 0 1 0 1 0

1 1 0 0 0 0 1 1

0 0 1 1 1 1 0 0

0 0 1 1 1 0 1 0

-V 0 0 1 1 0 1 0 1

0 1 0 1 0 0 1 1

1 0 1 0 0 0 1 1

0 0 1 1 0 0 1 1 -2V

3 Multi Carrier PWM techniques

In a Sinusoidal PWM technique, a single sine wave is compared

with a triangular (carrier) wave in order to generate pulses for the

switching operations of an inverter. A sine PWM technique has been

extended to multi-level inverter modules by taking more number of

carriers. Hence it is simply called as multi carrier pulse width

modulation technique. It requires (n-1) carrier waves for n level

inverter. It is again simplified as Level Shifted PWM (carrier signals

are arranged vertically) and Phase Shifted PWM (carrier signals are

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arranged horizontally). LSPWM is again divided into three types.

They are

1. Phase Disposition-PWM (PD-PWM),

2. Phase Opposition Disposition-PWM (POD-PWM)

3. Alternate Phase Opposition Disposition-PWM(APOD-PWM) [5].

In PD-PWM, all the carrier waves are in phase (fig 3), in POD-

PWM, All the carrier waves above zero are in phase among them

and all the carrier waves below the zero are in phase among them

but in opposition to the earlier carrier waves (fig 4) and in APOD-

PWM, all the carrier waves are in opposition alternatively from top

to bottom. (fig 5)

Here the peak to peak voltage of 3V is taken for each carrier wave

and the sine reference wave is shown in figures 3-5 for the

modulation index of 0.8 (that is, peak value of sine

wave=6*0.8=4.8V). Here the figures 3-5 are shown for one cycle of

sine reference wave.

Fig3: PD-PWM

Fig4: POD-PWM

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Fig5: APOD-PWM

4 Simulation Results

All the three multi carrier techniques are applied to five-level CHB

inverter. Simulation is carried out in MATLAB/Simulink by taking

the parameters as follows.

1. DC bus voltage=100V,

2. Modulation index (ratio of peak value of the reference wave to the

peak value of the upper carrier wave), m=0.8, 1 and

3. Carrier frequency, fc =2500Hz, therefore frequency modulation

ratio (ratio of carrier frequency to reference frequency), mf = 50

If p is the number of voltage levels in phase voltage of a multilevel

inverter then the number of levels that the line voltage contain is

(2p-1). Therefore for 5-level inverter, the number of levels in the line

voltage is nine. When the reference waveform is more than the

carrier wave, a pulse is produced. The pulses produced by the

comparison of reference sine wave with upper and lower carriers,

are given to the H-bridge 1 and the pulses produced by comparison

of reference sine wave with the middle two carrier waves, are given

to H-bridge 2. One phase of the simulation circuit is shown in fig 6

and the circuit for the three-phase is constructed by taking the

reference sine wave with a phase shift of 120 and 240 degrees for

the other two phases.

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Fig 6: Simulation diagram for one leg for PD-PWM technique.

The output voltages (both phase and line) and corresponding

harmonic patterns are taken. Figure 7 and 8 shows the phase

voltage, line voltage respectively and the corresponding harmonic

pattern also shown when the PDPWM applied to 5-level CHB when

the modulation index is one. Similarly the waveforms obtained when

the POD PWM and APOD PWM applied to 5 level CHB when m=1,

are shown in the figures 9-12. In all the figures, the harmonics in

the line voltage decreases significantly since it contains more levels

than the phase voltage. All these output voltages and harmonic

content values are sown in table 2 for the modulation indexes of 0.8

and 1. When the modulation index increases from 0.8 to 1, the

voltage peak voltage has increased and the harmonics decreased.

Fig7: Phase voltage and harmonics for PD-PWM technique applied to 5-level CHB

when m=1

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Fig8: Line voltage and harmonics for PD-PWM technique applied to 5-level CHB

when m=1

Fig9: Phase voltage and harmonics for POD-PWM technique applied to 5-level CHB

when m=1

Fig10: Line voltage and harmonics for POD-PWM technique applied to 5-level CHB

when m=1

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Fig 11: Phase voltage and harmonics for APOD_PWM technique applied to 5-level

CHB,m=1

Fig12: Phase voltage and harmonics for APOD-PWM technique applied to 5-level

CHB, m=1

Table 2: Output voltages and percentage of harmonics for different modulation

indexes.

m=0.8 m=1

PWM

techni

que

Peak of

the

Fundam

ental

(Vph)(V)

Harmo

nics

(%)

Peak of

the

fundam

ental

(Vline)(V)

Harmo

nics

(%)

Peak of

the

fundam

ental

(Vph)(V)

Harmo

nics

(%)

Peak of

the

fundam

ental

(Vline)(V)

Harmo

nics

(%)

PD 157.1 38.24 271.9 21.34 196.2 26.68 339.8 16.89

POD 157 37.52 272 35.35 196.1 26.59 339.9 21.34

APOD 157 37.62 271.9 29.50 196.2 26.63 339.8 25.34

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5 Conclusion

The three types of multicarrier PWM techniques (PD-PWM,

POD-PWM and APOD-PWM) have been applied to the five-level

CHB inverter for different modulation indexes. The phase and line

voltage waveforms and the corresponding harmonic content is

shown for the modulation index of unity and the results are

tabulated for both modulation indexes. All the three techniques gave

nearly the same fundamental voltage but Phase Disposition Pulse

Width Modulation (PD-PWM) technique gives the less harmonic

content.

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