ENHANCEMENT OF FUNDAMENTAL RMS OUTPUT VOLTAGE OF 5-LEVEL CASCADED H-BRIDGE MULTILEVEL INVERTER USING MODIFIED MULTI- CARRIER PWM TECHNIQUE

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1 International Journal of Electrical Engineering & Technology (IJEET) Volume 7, Issue 1, Jan-Feb, 2016, pp.17-29, Article ID: IJEET_07_01_002 Available online at ISSN Print: and ISSN Online: Journal Impact Factor (2016): (Calculated by GISI) IAEME Publication 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 This work proposes a Modified Multi Carrier PWM (MMCPWM) technique where, reference sine wave has been replaced by ellipse wave, resulting in enhanced performances on the fundamental rms output voltage and lower Total Harmonic Distortion (THD).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 editor@iaeme.com

2 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 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] editor@iaeme.com

3 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 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): V o = V 1 +V 2 (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 19 editor@iaeme.com

4 Chinmayi and Dr. B.G. Shivaleelavathi Table I Switching Sequence of 5-level CHBMLI. Switches turn on Output Voltage level S1, S2 +V dc S1,S2,S5, S6 +2V dc S4,S2,S8,S6 0 S3,S4 S3,S4,S7,S8 -V dc -2V dc 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, A c and the same frequency, f c. The sine reference waveform has a frequency f r and amplitude of A f. Comparison at each time will generate a high if carrier signal is greater than sine else a low [7]. The amplitude modulation index m a and modulation frequency m f 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 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) 20 editor@iaeme.com

5 Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge Multilevel Inverter Using Modified Multi-Carrier PWM Technique 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) 21 editor@iaeme.com

6 Chinmayi and Dr. B.G. Shivaleelavathi (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 22 editor@iaeme.com

7 Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge Multilevel Inverter Using Modified Multi-Carrier PWM Technique the PWM techniques were simulated with Sine as reference wave having a frequency of 50Hz. Carrier frequency was set for 1350Hz, keeping M f = 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. DC Voltage, V dc Figure 5 Simulation circuit of 5-level CHBMLI TABLE II Circuit Parameters of 5-level CHBMLI Simulation Parameter Reference frequency ( fundamental output frequency) Carrier frequency RL-Load Filter inductance Values 115V 50Hz 1350Hz 100Ω and 5mH 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 editor@iaeme.com

8 Chinmayi and Dr. B.G. Shivaleelavathi (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 3 rd 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 editor@iaeme.com

9 Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge Multilevel Inverter Using Modified Multi-Carrier PWM 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 Voltage for PDPWM with Sine wave Figure 11 FFT Spectrum of Output for PSPWM with Sine wave 25 editor@iaeme.com

10 Chinmayi and Dr. B.G. Shivaleelavathi 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 m a = 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 m a = 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 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 editor@iaeme.com

11 RMS Phase Voltage %THD RMS Phase Voltage %THD Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge Multilevel Inverter Using Modified Multi-Carrier PWM Technique TABLE III Performance Comparison of MCPWM AND MMCPWM TECHNIQUE for 5-level CHBMLI with RL-Load SINE WAVE (MCPWM) ELLIPSE WAVE (MMCPWM) FIVE Ma Output % Output % LEVEL Voltage THD Voltage THD PD PWM PS PWM APOD PWM RMS Output Voltage Vs Modulation Index for PDPWM Ma Sine Ellipse Figure 15 Comparison of RMS output voltage VS Modulation indices RMS Output Voltage Vs Modulation Index PSPWM Ma Sine Ellipse Figure 17 Comparison of RMS output Voltage Vs Modulation indices %THD Vs Modulation Index for PDPWM M a Sine 5- level Ellipse 5 level Figure 16 Comparison of % THD Vs Modulation indices %THD Vs Modulation Index PSPWM Ma Sine 5- level Ellipse 5 level Figure 18 Comparison of THD Vs Modulation indices 27 editor@iaeme.com

12 Chinmayi and Dr. B.G. Shivaleelavathi 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. REFERENCES [1] B. H. Khan, Non-Conventional Energy Resources, Second Edition, McGraw Hill Education India Pvt. Ltd., 2014, pp.1-2. [2] Venkatachalam Kumar Chinnaiyan, Jovitha Jerome and J. Karpagam, An Experimental Investigation on a Multilevel Inverter for solar energy applications, Electrical Power and Energy Systems 47, 2013, ELSEVIER, pp [3] Mr. Pharne I. D. and Prof. Bhosale Y.N, A Review on Multilevel Inverter Topology, Dept. Of Electrical Engineering, RIT, Islampur, India, International Conference on Power, Energy and Control(ICPEC)-2013, ISSN: , IEEE, pp [4] Nupur Mittal, Bindeshwar Singh, S.P.Singh, Rahul Dixit and Dasharath Kumar, Multilevel Inverters: A Literature Survey on Topologies and Control Strategies, 2 nd International Conference on Power, Control and Embedded Systems, 2012, ISSN: , IEEE. [5] Sergio Daher, jurgen Schmid and Fernado L.M. Antunes, Multilevel Inverter Topologies for Stand-Alone PV Systems, IEEE Transactions on Industrial Electronics, 55(7), July 2008, DOI: /TIE , PP [6] E. Sambath, S.P. Natarajan, and C.R. Balamurugan, Performance Evaluation of Multi Carrier Based PWM Techniques for Single Phase Five Level H-Bridge Type FCMLI, Dept. Of EEE, Annamalai University, Chidambaram, Tamilnadu, India, IOSR Journal of Engineering, ISSN: , 2(7), 2012, PP [7] V.Manimala, Mrs.N.Geetha.M.E, Dr.P.Renuga, Design and Simulation of Five Level Cascaded Inverter using Multilevel Sinusoidal Pulse Width Modulation Strategies, IEEE 2011, [8] Dharmavarapu Sreenivasarao, Pramod Agarwal and Biswarup Das, Performance evaluation of carrier rotation strategy in level-shifted pulse-width modulation technique, Electrical Engineering Dept., IITR, Roorkee, India, IET Power Electron., 2014, vol.7, Iss.3, DOI: /iet-pel , pp [9] Subhransu sekhar Dash, P. Palanivel and S. Premalatha, Performance Analysis of Multilevel Inverters Using Variable Switching Frequency Carrier Based PWM Technique, Dept. Of E&E, SRM University, Chennai-Tamilnadu, ICREPQ 12- Spain 28 th 30 th March 2012, EA4EPQ. [10] P. Sai Prasanna, V. Lavanya, Rajan, Comparative Analysis of Cascaded Half Bridge Inverter Using Different Modulation Techniques, Department of EEE, GMR Institute of Technology, Vol. 4,Iss.3, March 2015, Andhra Pradesh, India, International Journal of Science and Research (IJSR) ISSN (Online): editor@iaeme.com

13 Enhancement of Fundamental RMS Output Voltage of 5-Level Cascaded H-Bridge Multilevel Inverter Using Modified Multi-Carrier PWM Technique [11] Bharathi M. Sudhakaran, G. Muthuselvi M. Shanmugasundaram and S. Subathra, Comparative Analysis of PD and APOD for Cascaded H-Bridge Multilevel Inverter, Department of EEE, Ganadipathy Tulsi s Jain Engineering Collège, Kaniyambadi, Tamilnadu, India, IJIRST International Journal for Innovative Research in Science & Technology, Volume 2, Iss.05, October 2015 ISSN (online): [12] M. S. Rajan and R. Seyezhai, Comparative Study of Multicarrier PWM Techniques for a Modular Multilevel Inverter, Research Scholar-Department of EEE, SSN College of Engineering, Rajiv Gandhi Salai (OMR), Kalavakkam , Tamil nadu, INDIA, International Journal of Engineering and Technology (IJET),Vol.5,no. 6, Dec 2013-Jan , ISSN : [13] Roshny Mary B and Neethu S, A Comparative Study of Cascaded H Bridge and Reversing Voltage Multilevel Inverter Topologies. International Journal of Electrical Engineering & Technology, 5(12), 2014, pp [14] Shimi S.L, Dr. Thilak Thakur, Dr. Jagdish Kumar, Dr. S Chatterji and Dnyaneshwar Karanjkar, Anfis Controller For Solar Powered Cascade Multilevel Inverter. International Journal of Electrical Engineering & Technology, 3(3), 2012, pp [15] Indrajit Sarkar and B. G. Fernandes, Modified Hybrid Multi-Carrier PWM Technique for Cascaded H-Bridge Multilevel Inverter, Department of Electrical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai , India , 2014, Industrial Electronics Society, IECON th Annual Conference of the IEEE. [16] Vishal Rathore and Dr. Manisha Dubey, Speed Control of Asynchronous Motor Using Space Vector PWM Technique. International Journal of Electrical Engineering & Technology, 3(3), 2012, pp editor@iaeme.com