ENHANCEMENT OF FUNDAMENTAL RMS OUTPUT VOLTAGE OF...

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International Journal of Electrical Engineering & Technology (IJEET)

Volume 7, Issue 1, Jan-Feb, 2016, pp.17-29, Article ID: IJEET_07_01_002

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ENHANCEMENT OF FUNDAMENTAL RMS

OUTPUT VOLTAGE OF 5-LEVEL

CASCADED H-BRIDGE MULTILEVEL

INVERTER USING MODIFIED MULTI-

CARRIER PWM TECHNIQUE

CHINMAYI

Asst. Prof., Department of Electrical and Electronics Engineering,

East West Institute of Technology, Bengaluru, Research Scholar,

RRC-ECE, JSSATEB, VTU, Belagavi, India

Dr. B.G. SHIVALEELAVATHI

Professor, Department of Electronics and Communication Engineering,

JSSATE, VTU, Bengaluru, India

ABSTRACT

Cascaded H-bridge Multilevel Inverter (CHBMLI) is the most suitable

topology for the PV power converters. In this paper an effort has been made to

increase the performance of CHBMLI by improving the fundamental Root

Mean Square (RMS) value of the output voltage. This work proposes a

Modified Multi Carrier PWM (MMCPWM) technique where, reference sine

wave has been replaced by ellipse wave, resulting in enhanced performances

on the fundamental rms output voltage and lower Total Harmonic Distortion

(THD). Analysis of single phase 5-level CHBMLI with and without load are

carried for the different Multi Carrier PWM (MCPWM) techniques. Results

were compared for both MCPWM and MMCPWM at different modulation

indices. The proposed MMCPWM technique emerged as a very promising

technique in enhancing the fundamental output voltage and at the same time

mitigating the problem of THD. 5-level CHBMLI with the proposed control

strategy is simulated in MATLAB/SIMULINK. The results were compared with

the existing literature for validation of the proposed control strategy.

Key words: Cascaded H-Bridge Multilevel Inverter (CHBMLI), Modified

Multi Carrier PWM (MMCPWM), Multi Carrier PWM (MCPWM), Phase

Disposition (PDPWM), Phase Shifted PWM (PSPWM), Ellipse wave.

Chinmayi and Dr. B.G. Shivaleelavathi


Cite this Article: Chinmayi and Dr. B.G. Shivaleelavathi, Enhancement of

Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge Multilevel

Inverter Using Modified Multi-Carrier PWM Technique. International

Journal of Electrical Engineering & Technology, 7(1), 2016, pp. 17-29.

http://www.iaeme.com/IJEET/issues.asp?JType=IJEET&VType=7&IType=1

1. INTRODUCTION

The standard of living of a given country can be directly related to per capita energy

consumption. The per capita energy consumption is a measure of the prosperity of the

nation. Solar energy has the greatest potential of all the resources of renewable energy

and it is the most important supplies of energy especially when the other sources in

the country have depleted [1].

Therefore, the research need to be carried out to increase the efficiency of PV

power generation and to minimize the system cost. In this regard the inverters used to

convert power from DC to AC in solar power generation is having a very important

role to mitigate the problem of non sinusoidal output, high THD, high switching stress

and more number of switches. Multilevel inverters are the most promising in

overcoming the above problems. The advantages of MLI are:

Multilevel converters not only can generate the output voltages with very low

distortions, but also can reduce the dv/dt stresses.

Multilevel converters can draw input current with low distortion.

They can operate at both fundamental switching frequency and high switching

frequency PWM.

MLI topologies are classified into 3 types: Diode clamped inverters, Flying

capacitor inverters and Cascaded inverters [2].

In first two types for high levels, more number of diodes and capacitors are

required respectively so hence, the circuit will be bulky. And these two topologies

suits for single DC source input.

For a solar PV application CHBMLI are best suitable as it requires separate DC

sources for the real power conversion. From [2] [3] [4] and [5] CHBMLI is the best

suitable for solar PV application. The advantages of CHBMLI are:

CHBMLI uses less number of components compare to first two topology of MLI

No need of extra diodes and capacitors.

No voltage unbalancing problem compared to other two topology.

It has modular structure.

The transformers can be eliminated and this helps in enhancing the efficiency and

cost effectiveness.

Additional features such as its battery management capability, redundant

switching states in inverter operation and scalability make the CHBMLI the MLI of

choice [3]-[5].

The selection of appropriate PWM technique has a greater role in producing

quality output voltage and current for inverters. The main modulation techniques used

in MLI are: Multilevel Sinusoidal PWM (carrier based PWM), Space Vector PWM

and Multilevel Selective Harmonic Elimination (MSHE).For controlling the output

voltage of CHBMLI several types of modulation techniques have been proposed in

the literature, namely multistep or stair case modulation techniques, Multi Carrier

PWM (MCPWM) techniques and space vector modulation technique [4] and [6].

Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge

Multilevel Inverter Using Modified Multi-Carrier PWM Technique


The general principle of a MCPWM technique is the comparison of a sinusoidal

waveform with a carrier waveform, this typically being a triangular waveform. The

Carrier frequency depends on the switching frequency of the converter and the

elimination of high order harmonic components of the output voltage. The multi

carrier techniques are divided into the following categories [7]:

Level shifted PWM (LSPWM): Depending on the phase relation between the

individual carriers, there are three variants in LSPWM: Alternative Phase Opposition

Disposition (APOD), Phase Opposition Disposition (POD) and Phase Disposition

(PD) [8].

The unequal device conduction periods of the LSPWM technique has resulted as

main disadvantage particularly, in photovoltaic power generation. To overcome this

problem many techniques are discussed in literature [8]-[12].

Phase Shifted PWM (PSPWM): PSPWM technique is generally used modulation

technique in CHBMLI, as it offers even power distribution among the modules and

results in uniform utilization of inverter switches within a module [12].

In this paper, a MMCPWM technique is proposed, where in, the conventional sine

wave reference is been replaced by an ellipse wave. The simulation was carried out

for MCPWM and MMCPWM techniques on single phase 5-level CHBMLI. It is

found that, for the same CHBMLI, the overall performance of MMCPWM techniques

is superior compared to conventional MCPWM techniques, in terms of fundamental

output voltage and total harmonic distortions.

2. CASCADED H-BRIDGE INVERTER TOPOLOGY

2.1. 5 level - CHBMLI

The single phase cascaded five level inverter topology is as shown in Fig.1a and the

corresponding output voltage waveform in Fig. 1b. The circuit consists of eight main

switches in two series connected H-bridge configuration S1 to S4, and S5 to S8. The

number of DC sources are two, so the output voltage of the CHBMLI is given by (1):

Vo= V1+V2 (1)

Table I shows the switching sequence of devices and the respective output

voltages.

(a) (b)

Figure 1 a) 5-Level CHBMLI b) Output Voltage Waveform



Table I Switching Sequence of 5-level CHBMLI.

Switches turn on Output Voltage level

S1, S2 +Vdc

S1,S2,S5, S6 +2Vdc

S4,S2,S8,S6 0

S3,S4 -Vdc

S3,S4,S7,S8 -2Vdc

2.2. Multi Carrier Modulation Techniques

The principle of the MCPWM technique is based on a comparison of a sinusoidal

reference waveform with triangular carrier waveforms. For n-level inverter, (n-1)

carriers are required to compare. The carriers are continuous bands around the

reference zero. They have the same amplitude, Ac and the same frequency, fc. The sine

reference waveform has a frequency fr and amplitude of Af. Comparison at each time

will generate a high if carrier signal is greater than sine else a low [7]. The amplitude

modulation index ma and modulation frequency mf can be given by:

(2) (for PD, POD, APOD) and

(3) (for PSPWM)

(4)

The different types of MCPWM techniques are presented in [7] - [11]. Comparing

them [7]-[12] for suitability with CHBMLI, the following are considered for the

analysis:

PSPWM: The carriers are phase shifted by 360/(n-1), where n is the output levels in

the MLI. For a 5 level-CHBMLI, the four carriers are phase shifted by 90° each and

compared with reference wave having a frequency of fundamental output voltage.

PDPWM: The (n-1) carriers of equal amplitude and frequency are in phase but

shifted vertically for n-level MLI. For 5-level- CHBMLI, four carriers are vertically

shifted and they will be in phase.

APODPWM: Similar to PDPWM, but the (n-1) carriers are phase displaced from

another by 180° alternatively.

2.3. Proposed Modulation Technique

In conventional method of MCPWM technique, the sine wave with the fundamental

frequency is applied as reference wave. The expression for the sine wave is given by,

Y= amp * sin (2*pi*f*t) (5)

Where in the (5), amp is the amplitude and f is the frequency of the sine wave.

In the proposed MMCPWM modulation technique, the ellipse wave is compared

with carrier wave. The expressions used to generate the ellipse wave are given by:

X= x(i) + a * cos(t) (6)

Y= y(i) + amp * sin(t) (7)




Where in the (6) and (7), a is the radius of the ellipse which gives the frequency

term and amp is the amplitude of the ellipse wave.

Fig. 2 shows the comparison between the sine and ellipse wave with same

amplitude and frequency. The shape of the ellipse towards the peaks is wider than

sine, resulting in cutting more carrier waves. This increases the width of switching

pulses in the upper and lower peak of the output voltages, in turn reducing THD and

enhancing the rms of the fundamental output voltage. The improvement of output

voltage is achieved without entering the over modulation range.

Figure 2 Comparison of Sine and Ellipse wave

3. PERFORMANCE COMPARISON OF PWM TECHNIQUES

To analyze the different MCPWM techniques, single phase 5 - level CHBMLI with

R-L load is simulated in MATLAB/SIMULINK software platform. Both PSPWM

and LSPWM techniques were simulated.

In PDPWM and APODPWM techniques as the carriers are vertically shifted, the

width of the switching pulses is less at the peak of the fundamental output voltage and

also the switching frequency is almost equal in all intervals as shown in Fig. 3a. This

result in reducing the fundamental rms output voltage and increasing THD compare to

PSPWM technique. In PSPWM technique as the carriers are phase shifted, there will

be more overlapping of carrier waves leading to more switching width at peak of both

the half cycle and less in between of the fundamental output voltage as shown in Fig.

3b. The Switching pulses of PDPWM technique using sine and ellipse wave are

shown in Fig. 4a and Fig. 4b respectively, which clearly shows more switching pulse

width at peak of the reference wave in ellipse case.

(a)



(b)

Figure 3 Comparison of Sine and Ellipse wave for 5- level CHBMLI a) for PDPWM

technique b) for PSPWM technique

(a)

(b)

Figure 4 PDPWM Switching pulses for 5-level CHBMLI with a) Sine wave as

reference b) Ellipse wave as reference

4. PERFORMANCE COMPARISON OF OUTPUT VOLTAGE

The different modulation techniques discussed in the previous section were applied to

5- level CHBMLI and simulation was carried out using MATLAB/SIMULINK

platform. Fig. 5 shows a single phase 5 level CHBMLI simulation circuit to which an

RL-load is connected. The circuit parameters details are given in Table. II. First , all




the PWM techniques were simulated with Sine as reference wave having a frequency

of 50Hz. Carrier frequency was set for 1350Hz, keeping Mf = 27. PSPWM gives

better performance compared to PDPWM and APODPWM technique. The analysis

for %THD is noted by varying the modulation indices from 0.6 to 1.

Figure 5 Simulation circuit of 5-level CHBMLI

TABLE II Circuit Parameters of 5-level CHBMLI

Simulation Parameter Values

DC Voltage, Vdc 115V

Reference frequency ( fundamental output frequency) 50Hz

Carrier frequency 1350Hz

RL-Load 100Ω and 5mH

Filter inductance 50mH

Filter capacitance 40µF

The proposed MMCPWM technique was verified by replacing the reference sine

wave to ellipse wave. In all the cases, the proposed MMCPWM result in enhanced

output voltage as it produces more switching pulse width at both the peak of output

voltage. Even the total harmonic distortion has reduced comparatively. The output

voltages were compared without load and with RL-load for PDPWM technique for

both sine and ellipse as reference on a single window as shown in Fig. 6 and Fig. 7. It

shows that the MMCPWM technique produces more peak output voltage compare to

MCPWM technique. Henceforth the RMS output voltage is comparatively more in

proposed technique.



(a) (b)

Figure 6 a) No-Load output Voltage of 5-level- CHBMLI with MCPWM and

MMCPWM in a single scope b) Comparison of output voltages for a single cycle

showing the more output width of the MMCPWM

Figure 7 Output voltage of PDPWM fed 5-level CHBMLI with RL-Load.

(Comparison of Sine and Ellipse wave performance)

Fig. 8a and Fig. 8b shows the simulated output voltage for the Modified PDPWM

and Modified PSPWM techniques without load. Fig.9 shows the output voltage and

current waveform for 5-level CHBMLI with Modified PDPWM technique. The

corresponding FFT analysis of the output voltages for PDPWM and PSPWM

technique are shown in Fig. 10 and Fig. 11 respectively. The FFT analysis of the

output voltage for Modified PDPWM and Modified PSPWM are shown in Fig.12 and

Fig.13 respectively. The %THD has reduced in the proposed MMCPWM technique

for PDPWM. It is very clear from the figures, that the 3rd

harmonic component is

contributing more to THD. Fig. 14 shows the FFT list for fundamental output voltage,

peak output voltage with the corresponding %THD for PSPWM and Modified

PSPWM technique. The RMS output voltage and %THD is much better from

modulation indices of 0.8 onwards in Modified PSPWM technique.




(a) (b)

Figure 8 CHBMLI No-Load Output Voltage waveform with Ellipse reference for

a) PDPWM technique b) PSPWM technique

Figure 9 Output Voltage and Current for PDPWM CHBMLI with Ellipse reference

Figure 10 FFT Spectrum of output Voltage Figure 11 FFT Spectrum of Output

Voltage for PDPWM with Sine wave for PSPWM with Sine wave



Figure 12 FFT Spectrum of Output Voltage Figure 13 FFT Spectrum of Output

Voltage for PDPWM with Ellipse wave for PSPWM with Ellipse wave

(a) (b)

Figure 14 a) FFT List for Ellipse wave and b) FFT List for Sine wave for PSPWM

with ma = 0.8

The simulated fundamental RMS output voltage and % THD for different

modulation indices has been tabulated in Table III. The proposed MMCPWM

technique have improved performance in all the types of PWM techniques compare to

MCPWM technique. For PSPWM of MCPWM technique at ma = 1, %THD is low. In

MMCPWM technique the %THD is not increasing much with reduction in

modulation indices, which shows the superior performance of the proposed technique.

MMCPWM –APOD technique have much reduced harmonics compare to MCPWM –

APOD. The fundamental RMS output voltage is enhanced in all types of MMCPWM

technique. Fig.15-18 shows the graph plot for fundamental output voltage against

modulation indices in PS and PDPWM technique, where, it shows the excellent

performance of the proposed MMCPWM technique.




TABLE III Performance Comparison of MCPWM AND MMCPWM TECHNIQUE

for 5-level CHBMLI with RL-Load

Figure 15 Comparison of RMS output Figure 16 Comparison of % THD Vs

voltage VS Modulation indices Modulation indices

Figure 17 Comparison of RMS output Figure 18 Comparison of THD Vs

Voltage Vs Modulation indices Modulation indices

0

50

100

150

200

0.4 0.6 0.8 1

RM

S P

has

e V

olt

age

Ma

RMS Output Voltage Vs Modulation

Index for PDPWM

Sine

Ellipse

0

5

10

15

20

0.4 0.6 0.8 1

%TH

D

Ma

%THD Vs Modulation Index

for PDPWM

Sine 5- level

Ellipse 5 level

0

50

100

150

200

0.4 0.6 0.8 1 RM

S P

has

e V

olt

age

Ma

RMS Output Voltage Vs

Modulation Index

PSPWM

Sine

Ellipse 0

5

10

15

20

0.4 0.6 0.8 1

%TH

D

Ma

%THD Vs Modulation Index

PSPWM

Sine 5- level

Ellipse 5 level

SINE WAVE

(MCPWM)

ELLIPSE WAVE

(MMCPWM)

FIVE Ma Output % Output %

LEVEL

Voltage THD Voltage THD

1 147.4 13.08 163.3 10.65

PD 0.8 102.8 14.68 114.2 11.39

PWM 0.6 56.81 17.5 64.21 11.65

0.4 13.18 12.01 16.32 5.66

1 144.8 8.55 163.2 10.39

PS 0.8 101.4 11.36 113.8 9.93

PWM 0.6 56.85 15.04 64.03 9.87

0.4 20.74 18.87 23.95 11.76

1 146.6 15.43 163.4 11.01

APOD 0.8 102.2 17.02 114.3 11.15

PWM 0.6 56.52 19.53 63.97 11.16

0.4 13.15 14.6 15.72 5.64



4. CONCLUSION

This paper investigates the performance analysis of three MCPWM and MMCPWM

techniques namely PSPWM, APODPWM and PDPWM for single phase 5-level

CHBMLI. The performance evaluation is carried out in terms of RMS output voltage

and %THD. Control strategy was first verified for its functioning on

MATLAB/SIMULINK software. It is found that for the same circuit parameters, the

overall performance of MMCPWM is superior to that MCPWM technique as it is

producing more fundamental RMS output voltage and hence increasing the DC bus

utilization and reducing %THD. Further, a suitable technique may be developed to

improve the performance with respect to %THD by eliminating third harmonic

component. Hence the proposed MMCPWM technique is a most promising technique

to improve the performance of a CHBMLI for a PV Power generation in renewable

energy source.

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