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Energy Procedia 74 (2015) 844 - 853

International Conference on Technologies and Materials for Renewable Energy, Environment

and Sustainability, TMREES15

Cascaded H-bridge Asymmetrical Seven-level Inverter Using THIPWM for High Power Induction Motor

R. Taleba*, D. Benyoucef, M. Helaimia, Z. Boudjemaaa, H. Saidib

aElectrical Engineering Department, Hassiba Benbouali University, Chief, Algeria Laboratoire Génie Electrique et Energies Renouvelables (LGEER) bElectrical Engineering Department, University of Science and Technology of Oran-Mohamed Boudiaf (USTO-MB), Oran, Algeria Laboratoire d'électronique de puissance d'énergie solaire et d'automatique (LEPESA)

Abstract

Multilevel inverters are well used in high power electronic applications because of their ability to generate a very good quality of waveforms, reducing switching frequency, and their low voltage stress across the power devices. This paper presents the Third Harmonic Injection Pulse Width Modulation (THIPWM) strategy of a Seven-level Uniform Step Cascaded H-bridge Asymmetrical Inverter (7-level USCHBAI). The THIPWM approach is compared to the well-known Sinusoidal PWM (SPWM) strategy. Simulation results demonstrate the better performances and technical advantages of the THIPWM controller in feeding a High Power Induction Motor (HPIM).

© 2015TheAuthors. Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD)

Keywords: Seven-level Uniform Step Cascaded H-bridge Asymmetrical Inverter; Third Harmonic Injection PWM; Sinusoidal PWM; High Power Induction Motor.

1. Introduction

Inverters are widely used in modern power grids; a great focus is therefore made in different research fields in order to develop their performance. Three-level inverters are now conventional apparatus but other topologies have been

* Corresponding author. Tel.: +213 (0)5 54 90 15 21; fax: +213 0(27) 72 17 94. E-mail address: r.taleb@univ-chlef.dz

1876-6102 © 2015 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/4 .0/).

Peer-review under responsibility of the Euro-Mediterranean Institute for Sustainable Development (EUMISD) doi:10.1016/j.egypro.2015.07.820

attempted this last decade for different kinds of applications. Among them, Neutral Point Clamped (NPC) inverters, flying capacitors inverters also called imbricated cells, and series connected cells inverters called cascaded H-bridge inverters [1-3].

This paper is a study about a three-phase multilevel converter based on series connected single phase inverters (partial cells) in each phase. A multilevel converter with k partial inverters connected in serial is presented by Fig. 1. In this configuration, each cell of rank j — 1.. .k is supplied by a dc-voltage source udj. It has been shown that feeding partial cells with unequal dc-voltages (asymmetric feeding) increases the number of levels of the generated output voltage without any supplemental complexity to the existing topology [4, 5]. These inverters are referred to as "Cascaded H-bridge Asymmetrical Multilevel Inverters" or CHBAMI.

Fig. 1. Three-phase structure of a multilevel converter with k H-bridge inverters series connected per phase

Some applications such as active power filtering need inverters with high performances [6]. These performances are obtained if there are still any harmonics at the output voltages and currents. Different Pulse-Width Modulation (PWM) control-techniques have been proposed in order to reduce the residual harmonics at the output and to increase the performances of the inverters [7, 8]. The most popular one is probably the Sinusoidal PWM technique (SPWM) which shifts the harmonics to high frequencies by using high-frequency carriers [9, 10].

To minimize the Total Harmonic Distortion (THD), and to increase the maximum amplitude fundamental of the output voltage of the CHBAMI, we have applied the Third Harmonic Injection PWM strategy (THIPWM) [11-13]. In this study we compare the SPWM strategy and THIPWM strategy applied to the control of a Seven-level Uniform Step Cascaded H-bridge Asymmetrical Inverter (7-level USCHBAI). As well we compare the performances related to the association 7-level USCHBAI-HPIM for both strategies. Simulation results demonstrate the better performances and technical advantages of the THIPWM controller in feeding a high power induction motor.

2. Uniform Step Cascaded H-bridge Asymmetrical Multilevel Inverter (USCHBAMI)

Multilevel inverters generate at the ac-terminal several voltage levels as close as possible to the input signal. Fig. 2 for example illustrates the N voltage levels us1, us2...usN composing a typical sinusoidal output voltage waveform. The output voltage step is defined by the difference between two consecutive voltages. A multilevel converter has a uniform or regular voltage step, if the steps Au between all voltage levels are equal. In this case the step is equal to the smallest dc-voltage, ud1 [14]. This can be expressed by

Udl = Au = Us2 - Usl = Us3 - Us2 = ... = UsN - Us(N-1)

If this is not the case, the converter is called a non uniform step CHBAMI or irregular CHBAMI. An USCHBAMI is based on dc-voltage sources to supply the partial cells (inverters) composing its topology which respects to the following conditions:

Ud1 < Ud2 < ... < Udk j-1

Udj < 1 + 2^ Udl

where k represents the number of partial cells per phase and j — 1... k. The number of levels of the output voltage can be deduced from

N = 1 + 2^ Ud

This relationship fundamentally modifies the number of levels generated by the multilevel topology. Indeed, the value of N depends on the number of cells per phase and the corresponding supplying dc-voltages.

UsN Us(N-1)

Us2 Us1

L : ,.r Y 1 Us 3n/2

. 01 02 ..... dp _ n x/2 _ Y ^J 2n " /

Fig. 2. Typical output voltage waveform of a multilevel inverter

Equation (3) accepts different solutions. With k — 3 for example, there are two possible combinations of supply voltages for the partial inverters in order to generate a 13-level global output, i.e., (ud1, ud2, ud3) e {(1, 1, 4), (1, 2, 3)}, and there are three possible combinations to generate a 15-level global output, i.e., (ud1, ud2, ud3) e {(1, 1, 5), (1, 2, 4), (1, 3, 3)}. Fig. 3 shows the possible output voltages of the two partial cells of the 7-level USCHBAI with k — 2. The dc-voltages of the two cells are ud1 — 1p.u. and ud2 — 2p.u. The output voltages of each partial inverter are noted up1 and up2 and can take three different values: up1 e {-1, 0, 1} and up2 e {-2, 0, 2}. The result is a generated output voltage with 7-levels: us e {-3, -2, -1, 0, 1, 2, 3}. Some levels of the output voltage can be generated by different commutation sequences. For example, there are two possible commutation sequences resulting in us — 1p.u.: (up1, up2) e {(-1, 2), (1, 0)}. The dashed lines in Fig. 3 show the commutation sequence (up1, up2) — (1, 0). These redundant combinations can be selected in order to optimize the switching process of the inverter [15].

These different possibilities offered by the output voltage of the partial inverters, and the redundancies among them to deliver a same output voltage level, can be considered as degrees of freedom which can be exploited in order to optimize the use of a USCHBAMI.

parcial cells

Fig. 3. Possible output voltages of each partial inverter to generate N - 7 levels with k - 2 cells per phase (ud\ - Ip.u. and ud2 - 2p.u)

3. Multilevel Inverters Control Strategies

Among several modulation strategies, the multi-carrier sub-harmonic PWM technique has been receiving an increasing attention for symmetrical multilevel converters [16]. This modulation method can also be used to control asymmetrical multilevel power converters. Other kinds of modulation techniques can also be used in the case of CHBAMI. This section presents the both strategies SPWM and THIPWM, these control strategies will be compared by computer simulations.

3.1. Sinusoidal Pulse-Width Modulation (SPWM)

The SPWM is also known as the multi-carrier PWM because it relies on a comparison between a sinusoidal reference waveform and vertically shifted carrier waveforms. N -1 carriers are therefore required to generate N levels. The carriers are in continuous bands around the zero reference. They have the same amplitude Ac and the same frequency fc. The sine reference waveform has a frequency fr and an amplitude Ar. At each instant, the result of the comparison is 1 if the triangular carrier is greater than the reference signal and 0 otherwise. The output of the modulator is the sum of the different comparisons which represents the voltage level. The strategy is therefore characterized by the two following parameters, respectively called the modulation index and the modulation rate [10, 17]:

m - f (4)

r = — A (5)

N -1 Ac

The reference voltages are given as follows:

uri - urmax sin(2f - (j -1) , (i, j) ^ {(a, 1), (b, 2), (c, 3)} (6)

We propose to develop a seven-level uniform step cascaded H-bridge asymmetrical inverter composed of k - 2 partial inverters per phase with the following dc-voltage sources: ud1 - 1p.u. and ud2 - 2p.u.. The output voltages upa1 and upa2 of each partial inverter, the resulting voltage ua for the first phase and its frequency representation are presented by Fig. 4 for r - 0.85, with m - 15 and 16.

harmonic ranks harmonic ranks

Fig. 4. Output voltages of each partial inverter, total output voltage ua and its frequency representation of the 7-level USCHBAI

controlled by the SPWM for m - 15 (left) and m - 16 (right)

We note that:

• For odd m, we have symmetry to it, therefore only the odd harmonics exist, for even m, we have any symmetry, so there are both, the odd harmonics and the even harmonics;

• The voltage harmonics are grouped into centered families around multiple frequencies 6mfr. The first centered family around the mfr frequency is the most important amplitude;

• Increasing the modulation index m allows pushing the harmonics to high frequencies and therefore easily filtered by the inductance of the induction motor.

3.2. Third Harmonic Injection Pulse Width Modulation (THIPWM)

As harmonics with rank multiple three are null for the line-line voltages of three phase inverters, we can then inject these harmonics in the reference voltages in order to increase the maximum fundamental amplitude of the output voltage of the 7-level USCHBAI. If we only inject the third harmonic with an injection rate the new reference voltages will be:

u'ri = urf + <urmax sin(3<yrt) with i - (a, b, c) and rnr - 2ifr (7)

To find the optimal value for Ç allowing u'ra (i = a) to take its maximum value u'rmax, it must that its derivative be null:

du'ra/d&rt = urmax cos(®r0 + 3&rmax cos(3®r0 = 0

The solution of equation (8) yields:

cos(®rt ) = ±^19Ç-1/12Ç The reference voltage u'ra can also be set as follows:

u'ra = (1 + 3Ç)Urmax Sin(®r?) - 4Çurmax(Sin(®r?))3

Substituting (9) into (10), we get: ura max = 8Çurmax

The optimal value of Çrate is equal to the maximum value of u'ramal, such that: du 'ra max/ dÇ = umac ((6Ç - 1)/3^)V(3^ + 1)/12Ç = 0

The value Ç= 1/3 is excluded because it cancels u'ramax, this leads us to the optimal value of the injection rate Ç — 1/6.

The corresponding voltage u'ra — urmax(sin(art) + (1/6)sin(crt)) has a zero derivative and is maximum for art — л/3 or 2л/3. (by contrary, for a>rt — л/2, there is a minimum between two maxima). Therefore we get, for art — л/3:

(S/2)urmax + 0 = u'ra

Which gives Urmax = 1.154«'^^ instead of Urmax = u'ramax, an increase of 15.4%. Thus, the fundamental theoretical maximum amplitude of the output phase voltages passes Urmax with SPWM strategy to 1.154Urmax with THIPWM strategy. Figure 5 shows the signals required for generating a voltage of a three phase 7-level USCHBAI.

0 0.005 0.01 0.015 0.02

Fig. 5. Different signals THIPWM strategy of the 7-level USCHBAI

The output voltages upa1 and upa2 of each partial inverter, the resulting voltage ua for the first phase and its frequency representation are presented by Fig. 6 for r - 0.85, with m - 15 and 16.

time (s)

harmonic ranks

3 0 -1

fe 0.6

time (s)

harmonic ranks

Fig. 6. Output voltages of each partial inverter, total output voltage ua and its frequency representation of the 7-level USCHBAI

controlled by the THIPWM for m - 15 (left) and m - 16 (right)

We note that:

• For even m, we have symmetry to i, therefore only the odd harmonics exist, for odd m, we have any symmetry, so there are both, the odd harmonics and the even harmonics;

• The voltage harmonics are grouped into centered families around multiple frequencies 6mf.. The first centered family around the mfr frequency is the most important amplitude;

• Increasing the modulation index m allows pushing the harmonics to high frequencies and therefore easily filtered by the inductance of the induction motor.

4. Application of High Power Induction Motor

In order to evaluate the performance of the proposed approach, a 7-level USCHBAI is used to supply a High Power Induction Motor (HPIM). This approach is compared to the SPWM strategy in controlling the 7-level USCHBAI. The objective is to use the proposed strategy in order to minimize the harmonics absorbed by the induction motor.

Figures 7 and 8 show the results of a high power induction motor with the following data: rated power Pn -20MW, rated voltage 5.5kV, stator resistance Rs - 0.397Q, rotor resistance Rr - 0.081Q, stator inductance Ls -0.0089H, rotor inductance Lr - 0.0085H, mutual inductance Lm - 0.0082H, number of pole pairs P - 2, rotor inertia J - 1400kg.m2, viscous friction coefficient Kf - 0.009Nm.s.rad_1.

Performances obtained with both methods are summarized in Table 1. The analysis of figures and Table 1, show that:

For THIPWM strategy: decrease 38.24% of THD and increase 15.4% of the fundamental output voltage compared to the SPWM strategy, the stator current closer to the sinusoid;

For SPWM strategy: the electromagnetic torque continuously oscillates at a frequency f (50Hz), because of the harmonics of rank tow which is present in the stator current. The torque oscillates at 2f (100Hz) with the THIPWM approach.

■■■II11111II

1 1.5 2

time (s)

2.5 3 3.5 4

Y H ■

l1 1

0 0.5 1 1.5 2 2.5 3 3.5 4

time (s)

harmonic ranks

time (s)

t-i -0.5

Fig. 7. Performance of the HPIM fed by a 7-level USCHBAI controlled by the SPWM

0 0.5 1 1.5 2 2.5 3 3.5 4

time (s)

II ml**0*" y

II INil ■ill

! in

0.5 1 1.5 2 2.5 3 3.5 4 time (s) x 104

harmonic ranks

time (s)

Fig. 8. Performance of the HPIM fed by a 7-level USCHBAI controlled by the THIPWM Table 1. Performances of the control methods.

Control Ua Amplitude fTe A Te

method THD (%) of fund. THD (%) (Hz) (kNm)

SPWM 20.03 U1 11.21 f 50.23

THIPWM 12.37 1.154«! 6.50 2f 32.78

5. Conclusion

The improved performance of a drive system of an induction motor passes through the choice of a best strategy of the control inverter. In this work, we have shown by simulation that the THIPWM strategy presents better performances than the SPWM strategy. Indeed, it ensures a highest quality torque, to minimize the current harmonics and increase the maximum amplitude of the fundamental output voltage of the 7-level USCHBAI. Therefore the choice of this strategy in controlling a uniform step cascaded H-bridge asymmetrical multilevel inverter feeding a high power induction motor.

References

[1] J. Rodriguez, S. Bemet, P. K. Steimer, I. E. Lizama, A Survey on Neutral-Point-Clamped Inverters, IEEE Transactions on Industrial Electronics, vol. 57, no. 7, pp. 2219-2230, July 2010.

[2] J. Huang, K. A. Corzine, Extended operation of flying capacitor multilevel inverters, IEEE Transactions on Power Electronics, vol. 21, no. 1, pp. 140-147, January 2006.

[3] E. Babaei, A cascade Multilevel Converter Topology With Reduced Number of Switches, IEEE Transactions on Power Electronics, vol. 23, no. 6, pp. 2657-2664, November 2008.

[4] S. Mariethoz, Etude formelle pour la synthèse de convertisseurs multiniveaux asymétriques: topologies, modulation et commande (in french), Ph.D. thesis, EPF-Lausanne, Switzerland, 2005.

[5] J. Song-Manguelle, Convertisseurs multiniveaux asymétriques alimentés par transformateurs multi-secondaires basse-fréquence: réactions au réseau d'alimentation (in french), Ph.D. thesis, EPF-Lausanne, Switzerland, 2004.

[6] M. A. Ghani, A simple active power filter with 5-level NPC inverter, 4th International Conference on Engineering Technology and Technopreneuship, ICE2T'14, pp. 330-334, Kuala Lumpur, Malaysia, 27-29 August 2014.

[7] S. Bonan, W. Zhang, Q. Gao, D. Xu, Research on the Control Strategy of Modular Multilevel Converter, Third International Conference on Instrumentation, Measurement, Computer, Communication and Control, IMCCC'13, pp. 1678-1683, Shenyang, 21-23 September 2013.

[8] S. P. Singh, R. K. Tripathi, Performance comparison of SPWM and SVPWM technique in NPC bidirectional converter, Students Conference on Engineering and Systems, SCES'13, pp. 1-6, Allahabad, 12-14 April 2013.

[9] R. Taleb, A. Meroufel, A. Massoum, Control of a Uniform Step Asymmetrical 13-Level Inverter Using Particle Swarm Optimization, Automatika - Journal for Control, Measurement, Electronics, Computing and Communications, vol. 55, no. 1, pp. 79-89, 2014.

[10] R. Taleb, A. Meroufel, Control of asymmetrical multilevel inverter using artificial neural network, Elektronika ir Elektrotechnika, vol. 96, no. 8, pp. 93-98, 2009.

[11] D. Baimel, R. Rabinovici, S. Tapuchi, Phase shifted PWM with third harmonic injection for over-modulation range operation, pp. 753-757, Ischia, 18-20 June 2014.

[12] C. Aghion, O. Ursaru, M. Lucanu, C. Pavaluta, Motor control strategy based on ISCPWM and THIPWM, 10th International Symposium on Signals, Circuits and Systems, ISSCS'11, pp. 1-4, Lasi, 30 June-1 July 2011.

[13] J. Jose, G. N. Goyal, M. V. Aware, Improved inverter utilisation using third harmonic injection, Joint International Conference on Power Electronics, Drives and Energy Systems (PEDES) & 2010 Power India, pp. 1-6, New Delhi, 20-23 December 2010.

[14] R. Taleb, A. Derrouazin, USAMI Control with a Higher Order Harmonics Elimination Strategy based on the Resultant Theory, International Conference on Technologies and Materials for Renewable Energy, Environment and Sustainability, TMREES'14, Beirut, Lebanon, 10-13 April 2014.

[15] R. Taleb, A. Bouhani Ben Ziane, T. Bessaad, A. Derrouazin, Commande par la stratégie d'élimination d'harmoniques d'un onduleur asymétrique à onze niveaux (in french), 3rd International Seminar on New and Renewable Energies, SIENR'14, Ghardaïa, Algeria, 13-14 October 2014.

[16] F. Bouchafaa, Etude et commande de différentes cascades à onduler à neuf niveaux à structure NPC : Application à la conduite d'une MSAP (in french), Ph.D. thesis, ENP, Algiers, Algeria, 2008.

[17] R. Taleb, A. Meroufel, P. Wira, Control of a uniform step asymmetrical 9-level inverter based on artificial neural network strategy, Acta Polytechnica Hungarica, vol. 6, no. 4, pp. 137-156, 2009.