Analasis Of Photovoltaic Cell An Application Of A Level Shifted Cascaded Multilevel Inverter

Analasis Of Photovoltaic Cell An Application Of A Level Shifted Cascaded Multilevel Inverter

Analasis Of Photovoltaic Cell An Application Of A Level Shifted Cascaded Multilevel Inverter

*Prof in dept of EEE, Narayana Engg College, Nellore gaddam.

**Professor, S.V.Univesity, Tirupati.

***M.Tech Student Scholar Narayana and Engg. , Nellore A.P, India

Abstract- The past few years have seen many milestones in the development of streamlined, standardized requirements for utility interconnection of small-scale renewable generating facilities, particularly solar photovoltaic (PV) systems. The PV array generates DC power with a variable voltage and current. It is possible to supply DC loads directly from the PV array. Direct coupling of PV array and load is typically applied for applications exhibit intrinsic storage capabilities like water pumping or cooling systems. If AC loads are to be supplied, an inverter is required, transforming the DC power from the PV array to AC power at prescribed voltage and frequency. Battery is needed as a storage device that supplies to the load incase of indirect supply of power from the PV array. This paper focuses on the grid connected PV system. These systems are usually connected to the low voltage distribution grid. Today, grid connected PV systems, they always supply the maximum available power while voltage and frequency are determined by the mains. They are cheaper than the standalone systems and are easy to operate, as they require no storage equipment.

Keywords-Amplitude modulation (AM), dc-ac power conversion, insulated-gate bipolar transistor (IGBT), power electronics, pulse-width modulation, voltage-source converter (VSC)

1. Introduction

   The energy sources we have become accustomed tothe fossil fuels and nuclear fission have ceased to be the easy answers to our ever-growing need for

   electric power. Burning oil, coal and natural gas pumps nitrogen oxide, sulfur dioxide, and mercury and

   other toxic metals into our atmosphere, directly cause environmental pollution. Nuclear fission produces radioactive waste, material that will remain deadly for thousands of years, for which we have yet to discover a safe method of storage. As we learn more about the inter-connectedness of all the ecosystems that permit and sustain life on our planet, the poisonous

   results of the various pollutants created by the use of these fuels are becoming increasingly harder to justify.

   In a grid-connected PV system, PV modules, wired together to form a PV array, pass DC electricity through an inverter to convert it into AC power. If the PV system AC power is greater than the owner's needs, the inverter sends the surplus to the utility grid for use by others. The utility provides AC power to the owner at night and during times when the owner's requirements exceed the capability of the PV system.Using grid-connected PV power can have economic as well as environmental advantages. Where utility power is available, consumers can use a grid- connected PV system to supply some of the power they need and use utility-generated power at night and on very cloudy days. When the PV system supplies power to the grid as well as to a specific building or piece of equipment, the utility becomes a kind of storage device or battery for PV-generated power.

   Distributed power generation systems (DPGS) are widely exploited according to the development of renewable energy systems [1,2]. DPGSs cover wide power ranges from 1kW class residential applications to several hundred MW class generation parks. Medium and small scale DPGSs are normally connected to grid systems through utility interactive inverters that inject grid current by current control mode operation

   II MODEL FOR PV CELL

   The building block of the PV array is the solar cell, which is

   basically a pn semiconductor junction that directly converts light energy into electricity. The equivalent circuit is shown in Fig. 1

   Fig. 1 Equivalent circuit for a PV cell.

   To simulate a PV array, a PV simulation model which was used based on the following equation:

   Where IPV is the PV array output current (A); VPV is the PV array output voltage (V); ns is the number of cells connected in series; np is the number of strings connected in parallel; q is the charge of an electron; k is Boltzmanns constant; A is the p-n junction ideality factor; T is the cell temperature (K); and Irs is the cell reverse saturation current. The factor A in Eq. (1) determines the cell deviation from the ideal pn junction characteristics. The ideal value ranges between 1 and 5 and in our case, A equals 2.15. The cell reverse saturation current Irs varies with temperature and the photocurrent Iph depends on the solar radiation and the cell temperature as shown in the following equation:

   III . Cascaded H-Bridge Multilevel Inverter Full H-Bridge

   Fig.2 shows the Full H-Bridge Configuration.

   By using single H-Bridge we can get 3 voltage levels. The number output voltage levels of cascaded Full H- Bridge are given by 2n+1 and voltage step of each level is given by Vdc/n. Where n is number of H- bridges connected in cascaded. The switching table is given in Table 1 and 2.

   Switches Turn ON	Voltage Level
   S1,S2	Vdc
   S3,S4	-Vdc
   S4,D2	0

   Table 1. Switching table for H-Bridge

   Switches Turn On	Voltage Level
   S1, S2	Vdc
   S1,S2,S5,S6	2Vdc
   S4,D2,S8,D6	0
   S3,S4	-Vdc
   S3,S4,S7,S8	-2Vdc

   where Iscr is the cell short-circuit current at reference

   temperature and radiation, ki is the short-circuit current temperature coefficient, and s is the solar radiation in mW/cm2.

   Table 2. Switching table for Cascaded H-Bridge

   which remains the same as that for the phase-shifted modulation scheme. For PID modulation, the multilevel converter with multilevel requires (m-1) triangular carriers with same amplitude and frequency. The frequency modulation index mf which can be expressed as:

   mf = fcr/ fm

   where fm is modulating frequency and fcr are carrier waves frequency. The amplitude modulation

   index ma is defined by

   ma = Vm / Vcr (m-1) for 0 ma 1

   Figure. 2 Full H-Bridge

   1. LEVEL SHIFTED PWM METHOD

      Fig.3 Level shifted carrier and reference

      The Level shifted carrier pulse width modulation. An m-level Cascaded H-bridge inverter using level shifted modulation requires (m1) triangular carriers, all having the same frequency and amplitude. The frequency modulation index is given by

      mf = fcr/ fm,

      Where Vm is the peak value of the modulating wave and Vcr is the peak value of the each carrier wave [1].

      The amplitude modulation index, ma is 1 and the frequency modulation index, mf is 6. The triggering circuit is designed based on the three phase sinusoidal modulation waves, Va, Vb, and Vc Three of the sine wave sources have been obtained with same amplitude and frequency but displaced 120° out of the phase with each others. For carriers wave sources block parameters, the time values of each carrier waves are set to [0 1/600 1/300] while the outputs values are set according to the disposition of carrier waves. After comparing, the output sinals of comparator are transmitted to the IGBTs.

   2. SIMULATION RESULTS

5.1 Modeling of Cascaded H-Bridge Multilevel Converter

Fig.7 shows the Matlab/Simulink Model of five level Cascaded H-Bridge multilevel converter. Each H-bridge DC voltage is 50 V. In order to generate three phase output such legs are connected in star/delta. Each leg gating pulses are displaced by 120 degrees.

Figure. 4 Matlab/SImulink Model of CHB with grid connected

Fig.3 Shows the simulink model of the cascaded multilevel inverter which is connected to the grid. This is a closed system with PI controller.

Figure. 5Carrier Signals of Phase Shifted Carrier PWM

Fig.5 shows the Phase shifted Carrier PWM wave form. Here four carriers each are phase shifted by 90 degrees are compared with sine wave.

Fig.6 Source voltage

Fig.6shows the output voltage of the PV module

Fig.7DC-DC output voltage

Fig7shows the DC-Dc output voltage waveform which is maintained constant as the input to the CMI is a constant DC

Fig.8Five level phase voltage

Fig.8shows the phase voltage of level shifted carrier PWM CHB inverter. Fig.12 shows the line voltage of level shifted carrier PWM CHB inverter. Here phase voltage has five voltage levels where as line voltage has nine voltage levels.

Figure. 9 Nine Level Line Voltage

This paper presents a cascaded multilevel inverter with photovoltaic module connected to the system. The multilevel inverter is triggered by using the level shifted PWM method, with closed loop system to maintain the synchronization with the grid. Finally a five level single H-bridge is proposed. A SIMULINK based model is developed and Simulation results are presented.

1. J. McDonald, Leader or follower [The business scene], IEEE Power Energy Mag., vol. 6, no. 6, pp. 1890, Nov. 2008.

2. N. Flourentzou, V. G. Agelidis, and G. D. Demetriades, VSC-based HVDC power transmission systems: An overview, IEEE Trans. Power Electron., vol. 24, no. 3, pp. 592602, Mar. 2009.

3. A. A. Edris, S. Zelingher, L. Gyugyi, and L. J. Kovalsky, Squeezing more power from the grid, IEEE Power Eng. Rev., vol. 22, no. 6, pp. 46, Jun. 2002.

4. B. K. Perkins and M. R. Iravani, Dynamic modeling of high power static switching circuits in the dq-frame, IEEE Trans. Power Syst., vol. 14, no. 2, pp. 678684, May 1999.

5. P. Steimer, O. Apeldoorn, E. Carroll, and A. Nagel,

   IGCT technology baseline and future opportunities, in Proc. IEEE Transmi. Distrib. Conf. Expo., Oct. 2001, vol. 2, pp. 11821187.

6. V. G. Agelidis and G. Joos, On applying graph theory toward a unified analysis of three-phase PWM inverter topologies, in Proc. IEEE Power Electronics Specialists Conf., Seattle, WA, Jun. 1993, pp. 408 415.

7. J. Arrillaga, Y. H. Liu, and N. RWatson, Flexible Power Transmission:

   The HVDC options. Hoboken, NJ: Wiley, 2007.

8. G. Asplund, Application of HVDC light to power system enhancement,in Proc. IEEE Power Eng. Soc. Winter Meeting, Singapore, Jan. 2000, vol. 4, pp. 24982503.

9. P. N. Enjeti, P. D. Ziogas, and M. Ehsani,

   UnbalancedPWMconverter analysis and corrective measures, in Proc. IEEE Industry Applications Soc. Annu. Meet., San Diego, CA, Oct. 1989, pp. 861870.

10. P. N. Enjeti and W. Shireen, A new technique to reject dc-link voltage ripple for inverters operating on programmedPWMFigwurea.v1e3fNoirnme Lse,vel LIEinEe EVolTtargae ns. Power Electron., vol. 7, no. 1, pp. 171180, Jan. 1992.

11. J. Y. Lee and Y. Y. Sun, Adaptive harmonic control in PWM inverters with fluctuating input voltage, IEEE Trans. Ind. Electron., vol. IE-33, no. 1, pp. 9298, Feb. 1986.

12. S. Funabiki and Y. Sawada, Computative decision of pulse width in three-phase PWM inverter, in Proc. IEEE Industry Applications Soc. Annu. Meet., Pittsburgh, PA, Oct. 1988, pp. 694699.

13. T. Kato, Precise PWM waveform analysis of inverter for selected harmonic elimination, in Proc. IEEE Industry Appl. Soc. Annu. Meeting, Piscataway, NJ, Sep. 1986, vol. 1, pp. 611616.

14. B. P. McGrath and D. G. Holmes, A general analytical method for calculating inverter dc-link current harmonics, in Proc. IEEE Ind. Appl. Soc. Annu. Meeting, Edmonton, AB, Canada, Oct. 2008, pp. 18.

15. A. M. Cross, P. D. Evans, and A. J. Forsyth, DC link current in PWM

    inverters with unbalanced and nonlinear loads, Proc. Inst. Elect. Eng., Elect. Power Appl., vol. 146, no. 6, pp. 620626, Nov. 1999.

16. M. H. Bierhoff and F. W. Fuchs, DC-link harmonics of three-phase voltage-source converters influenced by the pulsewidth-modulation strategy An analysis, IEEE Trans. Ind. Electron., vol. 55, no. 5, pp. 20852092, May 2008.

17. M. N. Anwar and M. Teimor, An analytical method for selecting dc-link-capacitor of a voltage stiff inverter, in Proc. 37th IAS Annu.Meeting IEEE

    Industry Applications Conf., Dearborn, MI, Oct. 2002, vol. 2, pp. 803810.

18. F. D. Kieferndorf, M. Forster, and T. A. Lipo,

    Reduction of dc-bus capacitor ripple current with PAM/PWM converter, IEEE Trans. Ind. Appl., vol. 40, no. 2, pp. 607614, Mar. 2004.

19. P. N. Enjeti, P. D. Ziogas, and J. F. Lindsay,

    Programmed PWM techniques to eliminate harmonics: A critical evaluation, IEEE Trans. Ind.

    Appl., vol. 26, no. 2, pp. 302316, Mar. 1990.

20. V. G. Agelidis, A. Balouktsis, I. Balouktsis, and

    C. Cossar, Multiple sets of solutions for harmonic elimination PWM bipolar waveforms: Analysis and experimental verification, IEEE Trans. Power Electron., vol. 21, no. 2, pp. 415421, Mar. 2006.

21. L. Ran, L. Holdsworth, and G. A. Putrus,

    Dynamic selective harmonic elimination of a three- level inverter used for static VAr compensation, Proc. Inst. Elect. Eng., Gen., Transm. Distrib., vol. 149, no. 1, pp. 8389, Jan. 2002.

22. S. Filizadeh and A. M. Gole, Harmonic performance analysis of an OPWM-controlled STATCOM in network applications, IEEE Trans. Power Del., vol. 20, no. 2, pt. 1, pp. 10011008, Apr. 2005.

23. C. Hochgraf and R. H. Lasseter, Statcom controls for operation with unbalanced voltages, IEEE Trans. Power Del., vol. 13, no. 2, pp. 538544, Apr. 1998.

24. B. Blazic and I. Papic, Improved D-STATCOM control for operation

with unbalanced currents and voltages, IEEE Trans. Power Del., vol. 21, no. 1, pp. 225233, Jan. 2006.