Generally, an inverter is required to convert DC power generated by PV cell into AC power. A multilevel inverter (MLI) can synthesizes a staircase waveform. In this paper, a comparative study is made on performance of 5-level Z Source H Bridge inverter by considering advanced pulse width Modulation (PWM) strategies. PWM strategies of proposed ZSI’s is quite similar to the traditional carried-based PWM control method, the only difference is that to turn null states into shoot through states and keep the active switching states unchanged, so that the reliability of the inverter is greatly improved because miss-gating can no longer destroy the circuit. Keywords: Z source Inverter; PWM Schemes; Shoot Through; Photo voltai

Kumar, D. V. Ashok

Kumar, M. Vijay

Manjunath, B M

English

International Institute for Science, Technology and Education (IISTE): E-Journals

Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  45 Improved DC utilization using advanced modulation techniques with Z source Inverter B M Manjunath Research scholar, Department of Electrical & Electronics Engineering RGMCET, Nandyal, India, E-Mail: manjumtech003@gmail.com  Dr. D. V. Ashok Kumar Principal SDITW, Nandyal, India E.Mail: principal.sdit@gmail.com  Dr. M. Vijay Kumar Professor, Department of Electrical and Electronics Engineering. JNT University, Anantapur, India, E-Mail:mvk_2004@gmail.com Abstract Generally, an inverter is required to convert DC power generated by PV cell into AC power. A multilevel inverter (MLI) can synthesizes a staircase waveform. In this paper, a comparative study is made on performance of 5-level Z Source H Bridge inverter by considering advanced pulse width Modulation (PWM) strategies. PWM strategies of proposed ZSI’s is quite similar to the traditional carried-based PWM control method, the only difference is that to turn null states into shoot through states and keep the active switching states unchanged, so that the reliability of the inverter is greatly improved because miss-gating can no longer destroy the circuit.  Keywords: Z source Inverter; PWM Schemes; Shoot Through; Photo voltaic   I. Introduction Photovoltaic cells are static energy conversion devices, which converts the solar energy directly into DC electrical energy. Conventional Voltage Source Inverters and Current Source Inverters are used for AC machine drives applications. The AC drive system requires to operate the machine over a wide speed range. It is much preferable that the power converter should be capable of doing both buck-boost operations. The conventional inverter will suffers from following problems · Limited output voltage  · Either Buck or Boost operation only · Shoot-through or open circuit problem-reliability · Not inter changeable main circuit · Vulnerable to EMI noise · Difficult to use IGBT module and IPM for I-source · Start-up difficulty All these drawbacks can be overcome by using ZSI [1], [2]  II. Z-SOURCE INVERTER  Fig. 1 presents the of basic Z source inverter (ZSI). In attribute from VSI and CSI inverters, on DC side of ZSI a diode D and impedance network in “X” shape is placed, composed of two capacitors C1 and C2 and two inductive chokes L1 and L2. It employs a unique impedance network (or circuit) to couple the converter main circuit to the power source & load. The diode D prevents prohibited reversed current flow. For this reason application of ZSI is possible only where there is no necessity of regeneration, further it is even prohibited in case of fuel cell or photo-voltaic cell. The ZSI provides unique features i.e., buck & boost, that cannot be Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  46 observed in the traditional Voltage and current source converters where a capacitor and inductor are used, respectively.   Fig.1: General Structure of ZSI with PV cell  The firing control of the Z-source inverter includes the shoot through states.  Features of Z-source inverter:  · Able to produce any desired output AC voltage, even greater than the line voltage regardless of the input voltage. · Shoot through is allowable · Able to provide ride-through during voltage sags without any additional energy storage elements. · Immunity to EMI noise greatly increases reliability. · Improve power factor and reduces harmonic current and common-mode voltage. The ZSI will operate in three states: active state, zero-state and shoot-through state. In active mode (six states) and zero-state (null state) mode, the ZSI operates as traditional inverter.  In the shoot-through mode, all the switches in one or two or three legs are turned on & the shoot-through state is forbidden in the traditional inverter. In shoot through mode the dc capacitor voltage can be boosted to the desired value. Assume the inductors (L1&L2) and capacitors (C1 & C2) have the same inductance and capacitance values. [2], [3], [4]  The equivalent circuit of ZSI’s operating in null, active & shoot through state shown in Fig. 2, Fig. 3 & Fig. 4 respectively.         Fig.2: ZSI Equivalent circuit in null sates The inverter is in one of the 2 traditional zero states shown in Fig.2 and the inductor current decreases to zero, thus a new operation mode appears. In this Mode, the diode stops conducting and the inverter is an open circuit i.e., Ii=0. The inductor current becomes zero and maintains zero until the next switching action. Therefore in this mode, the Z-source circuit is isolated from both the dc source and the load.          Fig.3: Approximate Equivalent circuit in active states Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  47 In active state the inverter will operate as normal inverter but input voltage to the inverter is boosted shown in Fig.3.       Fig.4: Equivalent circuit in shoot through state  The circuit is in a shoot-through zero state, the sum of the two capacitors voltage is greater than the dc source voltage (VC1+VC2 > V0), the diode is reverse biased, and the capacitors charge the inductors. The voltages across the inductors are: VL1 = VC1 & VL2 =VC2. The inductor current increases linearly assuming the capacitor voltage is constant during this period. Because of the symmetry (L1=L2=L and C1=C2=C) of the circuit, one has VL1 = VL2 = VL, IL1 = IL2 = IL, & VC1 = VC2 = VC  III. Operating Principle & Control  When the dc voltage is high enough to generate the desired ac voltage, the traditional PWM can be used. While the dc voltage is not sufficient to directly generate a desired output voltage, a modified PWM controller is used so as to meet the requirement. A uniqueness of PWM control in ZSI is the ability of the shoot-through state to boost to a desired output voltage (greater than available dc-link voltage) and to buck the voltage under single stage process. PWM pulses can be generated by  1) Simple Boost PWM 2) Maximum Boost PWM  3) Maximum Constant PWM.  In this work the Simple boost control method is used to generate the shoot through states, Simple boost control utilizes two straight lines as shown in Fig. 5. Illustrate about the single phase configuration of H-Bridge inverter & simulated for three phase configuration. When the triangular carrier waveform is greater than the upper constant value or lower than the bottom constant value, the circuit turns into shoot-through state [6], [7], [8], [10] Otherwise it operates just as traditional carrier-based PWM. Fig. 5 shows the pulse generation for one phase switch. This method is very straightforward. In order to produce an output voltage that requires a high voltage gain, a small modulation index (ma) has to be used.  Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  48                                  Fig.5: a. Pulses generation for ZSI b. ZSI H-Bridge Inverter  Shoot-through zero states are evenly allocated into each phase. That is, the active states are unchanged. However, the equivalent dc-link voltage to the inverter is boosted because of the shoot-through states. VdcD1 L1L2C1 C2S1 S2S3S4G1G2G3G4D2 L3L4C2 C3S5 S6S7S8G5G6G7G8VdcJournal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  49         Fig.6: Pulse generation using alternate phase disposition          Fig.7: Pulse generation using phase disposition        Fig.8: Pulse generation using phase opposition disposition  Fig. 6 to 8 shows the carrier & reference waveform for generating PWM pulses.  ü Alternate phase disposition (APOD): Carrier waveform is in out of phase with its neighbor carrier by 180° shown in Fig.6. ü Phase disposition (PD) - All carrier waveforms are in phase shown in Fig.7.  ü Phase opposition disposition (POD): All carrier waveforms above zero reference are in phase and are out of phase with those below zero reference by 180° shown in Fig.8. ü Fig. 9 shows the pulse generation for ZSI          Fig.9: Pulse generation for ZSI by Phase opposing disposition  IV. MODELLING OF PV MODULE  The most commonly used model for PV-cell is one – diode equivalent circuit as shown in Fig.10. The shunt resistance Rsh is large, it is neglected. This simplified circuit is used in this paper for modelling of a PV-cell. Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  50        Fig10: One-diode equivalent circuit model for a PV cell. (a) Five parameters model; (b) Simplified four parameters model.  The non-linear of Vpv-Ipv and P-V curves are shown in Fig.11.           Fig11. Vpv-Ipv & P-Vpv characteristics of a PV cell  From Fig 10.b the relation between the output Vpv and the output current Ipv can be expressed as:    Where  IL= Light current; Io=  Saturation current; Rs= Series Resistance; α = Thermal voltage timing   completion factor.     The above four parameters are need to determined the I-V characteristics of PV-module. Thus, this model can be termed as Four-parameter model. The equations for determining the four parameters are given below:  4.1.  Light Current (IL)      Where G=irradiance (W/m2); Gref= reference irradiance (200 W/m2  is used in this study); ILref= light current at the reference condition (200 W/m2  and 25 ˚C); Tc  = PV cell temperature (˚C); Tcref = reference temperature (25 ˚C is used in this study); µIsc = temperature coefficient of the short-circuit current (A/˚C).                  From the above equation for light current it can be observed that IL is a function of both temperature and irradiance. Both ILref and µIsc can be obtained from manufacturer data sheet.  4.2. Saturation Current (IO)   (3) Where Ioref= saturation current at the reference condition (A); egap  = band gap of the material 1.17 eV; Ns  = number of cells in series of a PV module; q = charge of an electron (1.60217733×10-19 C); αref = the value of α at  reference condition.  Ioref can be calculated as:      Where Vocref = the open circuit voltage of the PV module at reference condition (V).  4.3. Calculation of α Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  51        The value of αref can be calculated as:                    Where Vmpref = maximum power point voltage at the reference condition (V);    Impref  = maximum power point current at the reference condition (A);            Iscref = short circuit current at the reference condition (A).  4.4. Series Resistance(RS) Some manufacturers provide the value of Rs. If not provided, the following equation can be used to estimate its value:      RS is taken as a constant in the model of this study. 4.5. Thermal Model of PV From equations (1) to (7), it can be renowned that the temperature is acting an important role in the PV performance. Therefore, it is necessary to have a thermal model for a PV cell/module.  In this  study,  a  lumped  thermal  model  is developed for the PV module. The temperature of the PV module varies with surrounding temperature, irradiance, and its output current and voltage, and can be written as:    CPV  = the overall heat capacity per unit area of the PV cell /module[J/(˚C-m2)]; Kinpv= Transmittance-absorption product of PV cells; Kloss = overall heat loss coefficient[W/(˚C-m2)];Ta=ambient temperature (˚C); A = effective area of the PV cell/module (m2).  V. Simulation Results The Z source network values for L & C is 1mh & 1600 µF respectively. The inputs to PV cell: temperature 25 °C, constant irradiance of 200 W/m2 & produces a constant voltage of 79.12V. Input voltage to each H-Bridge inverter is output of PV i.e., 79.12V & switching frequency is 5 kHz. The complete system is simulated & results are presented.  The model Ipv-Vpv characteristic curves under different irradiances are shown in Fig.10 at 25 °C. It is noted, that the higher is the irradiance the larger are the short-circuit current (Isc) and the open-circuit voltage (Voc). Obviously, the larger will be the maximum power (P), shown in Fig.11.   Fig.10:  IPV-VPV characteristic curves of the PV model under different irradiances  Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  52              Fig. 11:  P-Vpv characteristic curves of the PV model under different irradiances            Fig.12. Output Line to Line voltage of Conventional H-Bridge Inverter           Fig.13: Spectrum analysis of line to line voltage of Conventional H-Bridge Inverter          Fig.14: Line to Line voltage of ZSI H-Bridge with PD    Fig.15: Spectrum analysis of line to line voltage of ZSI H-Bridge Inverter with PD Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  53    Fig.16: Line to Line voltage of ZSI H-Bridge with APOD    Fig.17: Spectrum analysis of line to line voltage of ZSI H-Bridge Inverter with APOD           Fig.18: Line to Line voltage of ZSI H-Bridge with POD   Fig.19: Spectrum analysis of line to line voltage of ZSI H-Bridge Inverter with POD  Table 1 shows the comparison results of the ZSI system by considering the Line to Line voltage. With constant PV output voltage of 79.12V with Boost factor (K) of 0.9. if K =1 ZSI operates as a normal inverter. Table 1: Comparison of Output voltage (RMS) of the ZSI & Conventional Inverter with three PWM Technique  PWM Technique Fundamental Voltage  RMS output voltage THD(%) ZSI PD 285.2 209 17.32 POD 285 208.6 24.60 APOD 292.4 215.8 32.77 Conventional PD 273 195 16.84 POD 273 197 21.48 APOD 258.3 195 29.93  Journal of Energy Technologies and Policy                                                                                                                                      www.iiste.org ISSN 2224-3232 (Paper)   ISSN 2225-0573 (Online) Vol.4, No.6, 2014  54  VI. Conclusion From the results tabulated, the work carried out in this paper has the following conclusions ü In view of conventional & ZSI outputs, it is concluded that ZSI has good fundamental voltages at output. ü ZSI has good DC utilization using advanced modulation technique such as PD, POD & APOD ü APOD based ZSI has better DC utilization among other modulating techniques ü The better design values of Z-source i.e., L & C values will help to improve the system performance.  REFERENCES  [1] F.Z.Peng, “Z-Source inverter”, IEEE Trans. Ind. Applicat.Mag.,vol.39,no.2,pp.504-510, Mar/Apr.2003 [2] Yu Tang, Member, Shaojun Xie and Chaohua Zhang “An Improved - Source Inverter” IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 26, NO. 12, DECEMBER 2011 [3] Saravanan. V, Rampradesh. T, Ramanujam. R & Arumugam. M “Comprehensive analysis of Z Source Inverter topologies for photovoltaic systems”, IEEE transactions on Power electronics. Publication Year: 2011, Page(s): 97 – 103 [4] Dos Santos, E.C.; Pimentel Filho, E.P.X.; Oliveira, A.C.; da Silva, E.R.C. “Hybrid pulse width modulation for Z-source inverters”, IEEE transaction on Circuits and Systems. Publication Year: 2010, Page(s): 2888 – 2892 [5] M. von Zimmer, M. Lechler, B. Piepenbreier, “Z-Source Drive Inverter using modified SVPWM for low Output Voltage and regenerating Operation”, IEEE Transaction on Industry Applications, 2009 [6] Poh Chiang Loh,  Feng Gao and Frede Blaabjerg “Topological and Modulation Design of three level Z-Source inverter” IEEE Transactions on power electronics, VOL. 23, NO. 5, SEPTEMBER 2008 [7] D. Floricau, E. Floricau, and G. Gateau, “Three-level active NPC converter: PWM strategies and loss distribution,” in Proc. IEEE IECON, Nov. 2008, pp. 3333–3338. [8] P. C. Loh, F. Blaabjerg, S. Y. Feng, and K. N. Soon, “Pulse-width  modulated Z-source neutral-point-clamped inverter,” IEEE Transactions on Industrial Applications. vol. 43, no. 5, pp. 1295–1308, Oct. 2007 [9] F. Z. Peng, M. Shen and K. Holland, “Application of Z-Source Inverter for Traction Drive of Fuel Cell Battery Hybrid Electric Vehicles,” IEEE Transactions on Power Electronics, vol. 22, no. 3, pp. 1054-1061, 2007. [10] P. C. Loh, D. M. Vilathgamuwa, Y. S. Lai, G. T. Chua, and Y. Li, "Pulse width modulation of Z-source inverters," IEEE Transactions on Power Electron. vol. 20, pp. 1346-1355, 2005.  The IISTE is a pioneer in the Open-Access hosting service and academic event management.  The aim of the firm is Accelerating Global Knowledge Sharing.  More information about the firm can be found on the homepage:  http://www.iiste.org  CALL FOR JOURNAL PAPERS There are more than 30 peer-reviewed academic journals hosted under the hosting platform.   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Improved DC utilization using advanced modulation techniques with Z source Inverter

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Improved DC utilization using advanced modulation techniques with Z source Inverter

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