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Energy Procedía 62 (2014) 543 - 554

6th International Conference on Sustainability in Energy and Buildings, SEB-14

Triphase symmetrical cascaded multilevel inverter (5L) for PV systems controlled by various multicarrier PWM strategies

Abdelaziz Fria **, Rachid El Bachtirib, Abdelaiz El Ghzizalc, Aziz Naamaned

abc Labo. LESSI, FSDM. Equipe. REEPER, EST; Univercity. USMBA Fez, Maroc dAix Marseille Université, CNRS, ENSAM, Université de Toulon, LSIS UMR 7296, 13397, Marseille, Fra

Abstract

This paper presents a comparative study in the Matlab/Simulink environment among six multicarrier pulse width modulation strategies (MCPWM), of triphase symmetrical cascaded H-bridge multilevel inverter (five level). We will consider the Phase Disposition PWM, the Phase Opposition Disposition PWM, the Alternate Phase Opposition Disposition PWM, the Carrier Overlapping PWM, the Variable frequency PWM and the Phase Shifted Carrier PWM. Each type of these MCSPWM controls the same inverter. The symmetric voltage sources, supplying inverters cells are simulated using a DC voltage representing photovoltaic arrays (PV). Comparison of these MCPWMs is based on the spectral quality and magnitude of the output compound and simple voltages. The different strategies are carried out in Matlab/Simulink software environment with simulation results.

© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selectionand peer-reviewunder responsibilityof KES International

Keywords:

1. Introduction

Recently, the development of high power devices and related control techniques, multilevel inverters (MLIs) has become highly attractive to researches and industrial companies [1]. Multilevel inverters have achieved an increasing contribution in high performance applications [2]-[5]. The advantages of MLIs have improved the quality of the output voltages, lowest output voltage total harmonic distortion (THD), reduced voltage stress on semiconductors switches and decreases EMI problems. The different multilevel inverter structures are cascaded H-

* Corresponding author. Tel.: +212663686830. E-mail address: Abdelaziz.fri@usmba.ac.ma.

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of KES International

doi: 10.1016/j.egypro.2014.12.416

bridge, diode clamped and flying capacitors multilevel. Several studies in the literature confirm that cascaded H-bridge multi-level inverter topology is the most suitable for photovoltaic systems [6]-[11]. The advantages of MLIs increase more with the best choice of the right control. Multicarrier sinusoidal Pulse width modulation MCPWMs strategies for power conversion have received much attention recently [12]-[14].

In this work, different types of MCPWMs are used for controlling the proposed inverter: MCSPWM, PDPWM, PODPWM, APODPWM, COPWM and VFPWM. They are all based on the multicarrier concept. The main objective is to compare reference and multicarrier waves using a single phase five level cascaded H-Bridge inverter.

2. five level Cascaded H-bridge inverter

2.1. Structure

Fig. 1 shows a schematic diagram of a DC/AC full H-bridge inverter, Voltage-sourced inverter (VSI), for producing an AC voltage and employing four power switches. Each power switch is realized by antiparallel connection of a fully controllable unidirectional switch (MOSFET transistors) and a diode. This VSI is the basic cell structure of cascaded H-bridge multilevel inverters. The continuous symmetrical voltage VDCi are provided by PV panels.

Fig.1. Full H-bridge structure inverter connected to a PV panel

Each of the power switches has the following parameters.

Table 1. Power switches parameters

Parameters Symbol Values

FET resistance Ron (Œ) 0.1

Internal diode inductance Lon (H) 0

Internal diode resistance Rd (Q) 0.01

Internal diode forward voltage Vf (V) 0

Snubber resistance Rs (fl) 1e5

Snubber capacitance Cs (F) inf

The number of the level of simple output voltage depends on the number of cells.

N=2.C+1

With : N: number of levels and C:number of cells Fig. 2 shows the triphase five level cascaded H-bridge inverter considered in this work.

Fig.2. Triphase cascaded H-bridge five level inverter and PV arrays

Each cell can generate three different output voltages, +VXDCi, 0, or -VXDCi (X=A,B, C and i=1,2) by connecting the DC source to the AC output by different combinations of the four switches. The AC inverter outputs is the sum of the voltages of the different cells connected in series.

2.2. Control

The 5 Level inverter will be controlled by the strategies of MCPWMs in order to compare their advantages and disavantages . This study will be developed in the following sections.

Each leg of the cells is controlled by comparing a reference signal (sinusoidal wave) and its own triangular carrier. For this work it is necessary to use four carriers.

Fig.3. Principle of PWM control

2.3. Quality of inverter output voltage

The quality of an inverter output is evaluated in terms of its harmonic factor p, distortion factor ^ and total harmonic distortion THD. For voltage-sourced inverters, the factors are redefined in terms of the output voltage harmonics as follows.

= "fJ„

The distortion factor for an individual harmonic is

V- _ P-

The total harmonic distortion is

3. Multicarrier PWM Strategies

Multicarrier PWMs strategies are the most widely used for MLI. It is similar to that of the PWM strategy except the fact that several carriers are used. MCPWM is one in which several triangular carrier signals are compared with a sinusoidal modulating signal. The number of carriers required to produce N-level output is N-1. All carriers have the same peak to peak magnitude Ac and same frequency fc except for VFPWM. The reference waveform has peak to peak magnitude Am and frequency fm. The reference is continuously compared with each carrier signals and whenever the reference is greater than the carrier signal, pulses are generated. There are many carrier arrangements to implement the PWM strategies. In this work the following strategies are carried out.

• Phase disposition PWM strategy;

• Phase opposition disposition PWM strategy;

• Alternate phase opposition disposition PWM strategy;

• Carrier overlapping PWM strategy;

• Variable frequency PWM strategy and

• Phase shifted carrier PWM strategy.

3.1. Phase disposition PWM strategy (PDPWM)

Fig.4 shows the sinusoidal pulse width modulation of a five level inverter; four carriers with the same frequency fc and same magnitude Ac are positioned such that the bands they occupy are contiguous. If the reference wave is more than a carrier signal, then the active devices corresponding to that carrier are switched ON. Otherwise, the devices are switched OFF.

2Am Am

with N = 5

(N— l)Ac Ac

Fig.4. Carrier arrangement for PDPWM strategy (ma=1 and mf=20)

3.2. Phase opposition disposition PWM strategy (PODPWM)

In PODPWM strategy the carrier waveforms above the zero reference are in phase. The carrier waveforms below are also in phase, but are 180 degrees phase shifted from those above zero as shown in Fig.5.

3.3. Alternate phase opposition disposition PWM Strategy (APODPWM)

In APODPWM strategy the carriers of same magnitude are phase displaced from each other by 180 degrees alternately. The carrier arrangement is shown in Fig.6.

3.4. Carrier overlapping PWM strategy (COPWM)

In COPWM strategy, carriers with the same frequency fc and same peak-to-peak magnitude Ac are chosen such that the bands they occupy overlap each other; the overlapping vertical distance between each carrier is Ac/2.

0.01 0.015 0.02 0.025 0.03

Fig.7. Carrier arrangement for COPWM strategy (ma=1 and mf=20)

3.5. Variable frequency PWM strategy (VFPWM)

The number of switching for upper and lower devices of chosen MLI is much more than those of intermediate switches in other PWM using constant frequency carriers. In order to equalize the number of switching for all the switches, variable frequency PWM strategy is used.

ma —

2 Am (N-l)Ac

Fig.8. Carrier arrangement for VFPWM strategy (ma=1 and mf=20)

3.6. Phase shifted carrier PWM strategy (PSCPWM)

Fig.9 shows the PSCPWM strategy, this strategy technique uses four carrier signals of the same magnitude and frequency which are shifted by 180°/(N-1) degrees to one another to generate the five level inverter output voltage.

Fig.9. Carrier arrangement for PSCPWM strategy (ma=1 and mf=20)

4. Simulation and results

4.1. Simulation

Fig.10 shows the simulation scheme in Matlab/Simulink environment of the studied triphase cascaded H-bridge inverter (5L). DC voltage sources represent the PV arrays and the following parameter values used for simulation. VXDC1=VXDC2=120V, (X=A,B,C) fc=1000 Hz and fm=50Hz.

Fig.10. Triphase cascaded H-bridge 5L scheme of simulation Fig. 11 gives the scheme of control circuit simulated by Matlab/Simulink.

-»] .......

Ljrorj-

......... .I Or —

pnhM Calls _

1 1 V 115 Wj.^5 50111(82

-»I Lwfcqi [-

Ir->] boolean |—Iior [—

Fig.11. Scheme of simulation of control circuit

-►j boofcaii |—i-

»] boo ban |-1»|hOT [■

->| boofean [■ ->] boofcan |—i»|HOT [-

4.2. Waveforms and spectral analysis of the output voltage ■ PDPWM. for ma = 1 and mf = 20

Fig.12. Compound output voltages generated by PDPWM

Fig.13. FFT plot for compound output voltage of PDPWM

Fig.14. Simple output voltages generated by PDPWM

PODPWM. for ma = 1 and mf = 20

Fig.15. FFT plot for simple output voltage of PDPWM

Fig.16. Compound output voltages generated by PODPWM

Fig. 17. FFT plot for compound output voltage of PODPWM Fundamental (5GHz) = 240,5 . THD= 27.25%

20 30 40 50

Harmonic order

Fig.18. Simple output voltages generated by PODPWM

Fig.19. FFT plot for simple output voltage of PODPWM

' APODPWM. for ma = 1 and mf = 20

Fig. 20. Compound output voltages generated by APODPWM

Fig. 21. FFT plot for compound output voltage of APODPWM Fundamental (50Hz) = 240 . T\-\D= 26.51%

0 0.01 0.02 0.03 0.04 0.05

Fig. 22. Simple output voltages generated by APODPWM

COPWM. for ma = 1 and mf = 20

20 30 40 0.06 Harmonic order

Fig. 23. FFT plot for simple output voltage of APODPWM

Fig. 24. Compound output voltages generated by COPWM

Fig. 25. FFT plot for compound output voltage of COPWM

Fig. 26. Simple output voltages generated by COPWM

Fig. 27. FFT plot for simple output voltage of COPWM

■ VFDPWM. for ma = 1 and mf = 20

Fig. 28. Compound output voltages generated by VFDPWM

Fig. 29. FFT plot for compound output voltage of VFDPWM

Fig. 30. Simple output voltages generated by VFDPWM

' PSCDPWM. for ma = 1 and mf = 20

Fig. 31. FFT plot for simple output voltage of VFDPWM

Fig. 32. Compound output voltages generated by PSCDPWM

Fig. 33. FFT plot for compound output voltage of PSCDPWM

Fundamental (5GHz) = 229.8 . THCt 33.38%

Fig. 34. Simple output voltages generated by PSCDPWM

5. Results

Harmonic order

Fig. 35. FFT plot for simple output voltage of PSCDPWM

The Table 2 below summarizes the simulation results of the modulation index (ma) between 0.1 and 1.1 for the various multicarrier PWM strategies applied for a triphase H-bridge 5L. They give the magnitude of the fundamental and the value of harmonic distortion output voltages.

Table 2. Fundamental compound output voltages and THD% for various MCPWM

PDPWM PODPWM APODPWM COPWM VFPWM PSCPWM

ma Compound Volt. output Compound Volt. output Compound Volt. output Compound Volt. output Compound Volt. output Compound Volt. output

Fund. THD % Fund. THD % Fund. THD % Fund. THD % Fund. THD % Fund. THD %

0.1 26.08 212.23 24.82 319.65 24.82 319.65 48.31 150.56 26.08 212.23 20.75 324.68

0.2 71.54 110.51 71.42 163.39 71.42 163.39 93.55 80.26 72.61 108.84 41.41 219.56

0.3 115 56.26 111.7 108.20 111.7 108.20 156.1 47.22 115.3 56.71 140.9 77.80

0.4 158.6 43.59 155.9 75.93 155.9 75.93 204.8 35.67 157.6 43.45 176 63.84

0.5 199.3 35.76 199.7 47.01 199.7 47.01 262 28.56 201.6 35.51 200.5 53.96

0.6 249.1 27.39 250.3 37.97 250.4 27.63 302.8 25.92 247.3 34.78 224.3 48.62

0.7 288.9 24.04 287.1 38.49 286.4 28 349.5 19.49 288.4 33.48 302. 33.42

0.8 330.8 22.55 331.5 35.50 331 31.09 384.1 20.88 332.3 28.46 338.3 32.05

0.9 373.4 17.62 371.9 30.38 373.2 29.08 413 21.49 374.2 22.65 368.9 30.83

1 415.2 17.58 413.8 22.03 414.7 25.74 444.7 21.02 414.7 20.76 398.6 28.18

1.1 441.8 16.64 441.4 16.63 439.9 20.87 463.3 20.05 439.9 19.15 436.1 21.69

Table 3 . Fundamental simple output voltages and THD% for various MCPWM

PDPWM PODPWM APODPWM COPWM VFPWM PSCPWM

ma Simple Volt. output Simple Volt. output Simple Volt. output Simple Volt. output Simple Volt. output Simple Volt. output

Fund. THD % Fund. THD % Fund. THD % Fund. THD % Fund. THD % Fund. THD %

0.1 15.29 336.34 15.34 335.21 15.34 335.21 28.25 390.39 15.29 317.51 12.02 341.82

0.2 41.33 169.58 42.54 167.89 42.54 167.89 54.47 208.27 42.51 167.29 23.92 232.24

0.3 65.57 117.39 65.11 117.38 65.11 117.38 88.88 122.04 67.59 114.74 79.97 88.36

0.4 92.97 82.61 92.61 84.41 92.61 84.41 117.7 96.85 90.59 83.67 102.4 68.23

0.5 115.4 57.39 115.6 58.08 115.6 58.08 150.8 74.34 116.4 58.07 115.5 60.63

0.6 144.4 45.25 146.7 42.69 147.3 42.99 175.3 64.57 142.1 46.58 130.2 55.50

0.7 165.7 43.41 165.5 44.50 164.3 43.95 201.8 51.09 166.6 43.13 174.1 44.14

0.8 190.8 39.47 191.1 38.82 191.6 39.3 222.8 44.19 192.2 38.23 194.4 39.95

0.9 216.5 33.82 215.3 33.85 216.3 34.03 239.4 39.62 215.6 33.37 214.5 36.41

1 240.2 27.49 240.5 27.25 240 26.51 255.5 35.02 239 27.57 229.8 33.38

1.1 254.6 23.65 256.5 23.02 254 21.96 267.9 30.92 255 22.98 252 26.44

One can observe from Table 2 and 3 that APODPWM method provides simple output voltages with relative low distortion for different values of ma. among the six strategies developed, the PDPWM is the best one showing that the THD % is considered for compound voltages.

6. Conclusion

In this paper, six multicarrier PWMs strategies, for triphase cascaded H-bridge inverter 5 levels have been presented, namely the PDPWM, PODPWM, APODPWM, COPWM, VFPWM and PSCPWM. Performance factors like THD% and fundamental of compound and simple output voltages have been measured, presented and simulated in Matlab/Simulink environment.

Considering the THD% (ma=1) for:

• Simple voltage, it's clear that APODPWM, PODPWM, PDPWM, and VFPWM give the best way to control the inverter with a little advantage for the first one.

• Compound voltages, we conclude that PDPWM is the better strategy among the six strategies developed.

In future, we can consider other criteria for the choice of the suitable strategy that is the best for eliminating harmonics, in this case PSCPWM generates harmonics at high frequency for the simple and compound voltages Figures 33, 35 (40th harmonic) which give it another advantage. Moreover, this type of strategies allows a supplement control parameter by introducing an adjustable shift among carriers.

On the other hand, the COPWM strategy gives the highest value for the fundamental of both simple and

compound voltages. References

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