Design and Implementation of Silicon Carbide based ...

European Journal of Scientific Research ISSN 1450-216X Vol.49 No.2 (2011), pp.225-233 © EuroJournals Publishing, Inc. 2011 http://www.eurojournals.com/ejsr.htm

Design and Implementation of Silicon Carbide based Hybrid Cascaded Multilevel Inverter using THIPDPWM Technique R. Seyezhai Department of EEE, SSN College of Engineering, Chennai, Tamilnadu, India E-mail: [email protected] B. L.Mathur Department of EEE, SSN College of Engineering, Chennai, Tamilnadu, India E-mail: [email protected] A. Jaibunisha Department of EEE, Rajalakshmi Engineering College, Tamilnadu, Chennai E-mail: [email protected] Abstract Multilevel inverter has emerged as an effective and practical solution for increasing power and reducing harmonics of ac waveforms. This paper presents the performance of SiC (Silicon Carbide) based Hybrid Cascaded Multilevel Inverter (HCMLI) based on a Third Harmonic Injected Phase Disposition Pulse Width Modulation (THIPDPWM) technique. A comparison study is made between THIPDPWM and Conventional PDPWM technique. The performance parameter chosen in this work includes Total Harmonic Distortion (THD), fundamental voltage value and switching loss. Both the HCMLI circuit topology and its control scheme are described in detail and their performance is verified based on simulation and experimental results.

Keywords: HCMLI, THIPDPWM, PDPWM, SiC

1. Introduction Today the world is in need of high efficiency, high power density and high temperature operation of power electronics devices so a change of technology from Si to SiC will certainly revolutionize the field of electronics. SiC device has advantages such as wider bandgap, higher thermal conductivity, and higher critical breakdown field strength. SiC devices are capable of operating at high voltages, high frequencies, and at high junction temperatures. Significant reduction in weight and size of power converters with an increase in efficiency is an added advantage of SiC. Multilevel Inverter has emerged recently as a very important alternative in the areas of high power medium voltage energy control. By synthesizing the ac output terminal voltage from several levels of dc voltages, staircase waveform can be produced. Multilevel Inverter allows for higher output voltage and lowers the switches voltage stress. Multilevel Inverters have become an effective and practical solution for reducing switching losses. As the number of voltage level increases in the dc side, the synthesized output adds more steps, producing an output which approaches the sinusoidal wave with minimum harmonic distortion. The various topologies of multilevel inverter are diode-clamped multilevel inverter, flying capacitor multilevel inverter and cascaded H-bridge multilevel inverter. Among the topologies , cascaded H-

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bridge multilevel inverter has more advantages [1] because of the following reasons: it requires least number of components, no extra clamping diodes and voltage balancing capacitors and switching redundancy for an inner voltage level. The main drawback of this topology is the isolated dc power supply requirement for each of its stages which is overcome by HCMLI. From the various modulation strategies reported in the literature , THIPDPWM technique has been employed. Advantage of THIPDPWM is to improve the fundamental voltage without entering the over modulation range. So any carrier employed for this reference will enhance the fundamental by 15% without increasing the harmonics. In this paper a comparative evaluation is made between Conventional PDPWM and THIPDPWM of hybrid cascaded multilevel inverter. The Total harmonic Distortion (THD) of THIPDPWM is less when compared to conventional PDPWM. Also a comparative study is made between all Si based HCMLI and Si MOSFET-SiC Schottky diode based HCMLI.

2. Hybrid Cascade Multilevel Inverter Conventionally, each phase of a cascaded multilevel converter requires ‘n’ dc sources for 2n + 1 level [2, 3]. For the cascade H-bridge multilevel converter, a scheme is proposed in this paper that allows the use of two unequal dc sources (such as battery or fuel cell) in the cascaded H-bridges multilevel converter. A Seven - level hybrid cascaded H-bridge multilevel converter has two H-bridges for each phase. One H-bridge is connected to a dc source of value Vdc and another is connected to a dc source of value Vdc/2, as shown in Fig.1. The dc source for the first H-bridge (H1) could be a battery or fuel cell with an output voltage of Vdc, and the dc source for the second H-bridge (H2) could also be a battery or fuel cell with an output voltage of Vdc/2. The output voltage of the first H-bridge is denoted by V1, and the output of the second H-bridge is denoted by V2 so that the output voltage of the cascaded multilevel converter is V (t) = V1 (t) +V2 (t). By opening and closing the switches of H1 appropriately, the output voltage V1 can be made equal to -Vdc, 0, or +Vdc while the output voltage of H2 can be made equal to – Vdc/2, 0, or Vdc/2. Therefore, the output voltage of the converter is a combination of Vdc and Vdc/2 as shown in Fig.2. Figure 1: Hybrid Cascaded Multilevel Inverter

V (t) =V1 (t) + V2 (t) Figure 2: Output Voltage Waveform of Hybrid Cascaded Seven Level Inverter.

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3. SiC Schottky Diode In its basic form, a Schottky diode is formed when a metal such ac aluminum is in direct contact with SiC. Due to difference in the absolute potential energy of the electrons present in the metal to that of the electrons present in the semiconductor, a depletion region is formed at the metal-semiconductor interface with the metal acquiring a negative charge and the semiconductor acquiring a positive charge. The performance of CSD100060 SiC Schottky diode was comparable to that of MUR1560 Si diode which was obtained by simulation using PSPICE. The SiC diode has less forward voltage, high reverse breakdown voltage and less reverse recovery current and hence less switching loss [4-6]. Advantages of SiC diode are • High power efficiency • Reduction in size & weight (33%-60%) • High switching speed • Low conduction & Low switching losses • High temperature operating capabilities The V-I characteristics of SiC diode is shown in Figure.3. the reverse recovery characteristics of SiC and Si diode is shown in Figure.4

4. Control Strategies of HCMLI The control strategies of HCMLI can be classified into fundamental switching frequency and high switching frequency [6, 7]. Further high switching frequency can be classified into space vector PWM and sinusoidal PWM as shown. This paper focuses on sinusoidal PWM technique. Sinusoidal PWM technique is classified based on modulating signal and carrier signal. A According to modulating signal sinusoidal PWM can be classified into 1) Pure Sinusoidal PWM (PSPWM) 2) Third Harmonic Injection PWM (THIPWM) Figure 3: V-I characteristics of SiC diode

For hybrid cascaded multilevel inverter several sinusoidal PWM techniques have been developed. Sinusoidal PWM technique by modulating reference sine waveform can be classified in to pure sinusoidal PWM technique, third harmonic injection PWM technique and dead band PWM technique. This paper focuses on third harmonic injection PDPWM technique and it is compared with conventional PDPWM technique

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Figure 4: Reverse Recovery Characteristics of SiC and Si diode

5. Third Harmonic Injection PDPWM Technique (THIPDPWM) In this method carriers are in same frequency, amplitude and phases, but they are in different DC offset. The modulating signal used is third harmonic injected sine wave form which is shown in Figure.5. A method to improve the gain of the pulse width modulator is to inject the third harmonic [810]. This method is derived from conventional sinusoidal PWM with the addition of a 17% third harmonic component to the sine reference waveform. This is an alternative to improve the fundamental voltage without entering the over-modulation range. So, any carriers employed for this reference will enhance the fundamental by 15% without increasing the harmonics. For third harmonic injection PWM, the reference waveform is defined as F(wot) = 1.15M sin(wot) +0.19Msin (wot), 0≤wot≤2П Where M is the modulation index ratio. Figure 5: Modulating, Carrier waveforms and gating pattern for the THI-PDPWM

6. Simulation Results The simulated output waveform of conventional and third harmonic injected PDPWM is shown in Figures.6, 7.

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R. Seyezhai, B. L.Mathur and A. Jaibunisha Figure 6: Line-line voltage of Conventional PDPWM HCMLI

Figure 7: Line-line voltage of THIPDPWM HCMLI

From the simulated line-line output voltage THIPDPWM HCMLI shows the higher value than conventional PDPWM for same value of amplitude modulation index (ma)=0.95 and fc=3 KHz. Figure 8: Comparison of the fundamental line-line output voltage for THIPDPWM and Conventional

PDPWM

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Figure 9: Comparison of THD values for THIPDPWM and Conventional PDPWM

The work presents the comparison between THIPDPWM technique and conventional PDPWM which is shown in Figures 8 and 9.The investigation is made in terms of total harmonic distortion (THD) and fundamental line-line output voltage. The above comparison chart shows THD values for different amplitude modulation index (ma) and fundamental line-line output voltage for different switching frequency for THIPDPWM and conventional PDPWM. For all amplitude modulation index values (ma) the THIPDPWM method shows lowest THD and the fundamental line-line voltage is higher in THIPDPWM than in conventional PDPWM. Using SiC diode in the HCMLI replacing Si diode has considerably reduced the switching loss and hence the efficiency of the inverter is increased. The switching loss for SiC and Si diode with increase in frequency for HCMLI is shown in Figure.12. The comparison of total loss of the inverter with SiC and Si diode is shown in Figure.13. the loss reduction in percentage using SiC diode is shown in Figure.14. Figure 10: Simulated output and FFT waveforms for THIPDPWM

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R. Seyezhai, B. L.Mathur and A. Jaibunisha Figure 11: Simulated output and FFT waveforms for conventional PDPWM

Figure 12: Comparison of switching loss of SiC and Si diode

Figure 13: Comparison of total loss of HCMLI with SiC and Si diode

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Figure 14: Loss reduction in percentage using SiC diode

7. Experimental Results A prototype of single –phase hybrid cascaded multilevel inverter using Power MOSFET and SiC diode has been built in order to verify the proposed THIPDPWM technique. Gating pattern has been generated using PIC18F4550.The seven-level output (V1 = 30V and V2 = 15V)is shown in Figure 15. Figure 15: Experimental output waveform of SiC based Hybrid Multilevel inverter

8. Conclusion From the simulation study conducted several distinct features of the seven levels hybrid multilevel inverter with THIPDPWM scheme from the aspect of line voltage can be identified. The line voltage yields better spectral performance for THIPDPWM compared to conventional PDPWM and this reduces the need for output filter. At high modulation index the proposed PWM technique introduces the lowest line voltage THD. The hybrid multilevel inverter using proposed THIPDPWM is suitable for high power fuel cell modules. The THIPDPWM yields of 7.23% for ma=0.95 and mf =21 is verified experimentally. HCMLI with SiC diode has considerable loss reduction of 11.24% for 3kHz when compared to HCMLI with Si diode.

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Acknowledgement We thank the AICTE, New Delhi, India for having sponsored the project titled, “Design of silicon carbide based hard-switched DC-AC power converter’ under RPS scheme which was completed in the year 2010.

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