A Transformerless Grid Connected Photovoltaic Inverter with Switched Capacitors

ABSTRACT:

In the transformerless photovoltaic (PV) system, the common mode ground leakage current may appear due to the galvanic connection between the PV array and the ground, which causes the safety issues and reduces the efficiency. To solve this problem, a novel inverter topology with switched capacitors is proposed in this paper. By connecting one pole of the PV cell directly to the neutral line of the grid, the common mode current is eliminated. Meanwhile, the switched capacitor technology is applied to increase the DC voltage utilization rate. Furthermore, a modified unipolar sinusoidal pulse width modulation (SPWM) strategy is proposed to reduce the pulsating current caused by the charging and discharging operations of the switched capacitors. Also, several optimization principles are put forward to further reduce the pulsating current to improve the efficiency and reliability. Finally, the proposed topology and modulation strategy are verified with simulation and a 250W experimental prototype.

SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

Fig 1. Proposed topology.

EXPECTED SIMULATION RESULTS:

                                            Fig 2. Simulated Waveforms Of Output Current And Current Stress On S3

With Different Ratio Of C1/(C1+C2).

 Fig 3. Experimental Waveforms Of Igrid And Vgrid.

Fig 4. Current And Voltage Stress On S3.

(a)     C1/(C1+C2)=0.33 (C1 = 470μf, C2 = 940μf), Vbus = 400V

                                         (b)     C1/(C1+C2) =0.67 (C1 = 940μf, C2 = 470μf), Vbus = 400V

        

(C) C1/(C1+C2) =0.33 (C1 = 470μf, C2 = 940μf), Vbus = 380V

Fig 5. Current Stress On S3 Under Different Conditions.

CONCLUSION:

A novel transformerless inverter topology with the switched capacitors is proposed for the grid connected PV power generation system. Only five power switches are required in the proposed PV inverter topology. The common mode current is eliminated perfectly. The DC input voltage required is the same as the full bridge inverter. A modified unipolar SPWM strategy is proposed for the topology, enabling it to output three voltage levels. It is also guaranteed that C2 is charged every switching cycle under this SPWM strategy, so that the current pulse on the power devices caused by the switched capacitors is reduced. Furthermore, based on the quantitative analysis of the devices’ current stress, the principles for optimizing the capacitances of the switched capacitors C1 and C2 are given. The simulation and experimental results are provided to verify the theoretical analysis.

REFERENCES:

[1] Gonzalez R, Gubia E, Lopez J, Marroyo L, “Transformerless Single- Phase Multilevel-Based Photovoltaic Inverter,” IEEE Transactions on HIndustrial Electronics, Hvol. 55, Hno. 7, Hpp. 2694-2702, 2008.

[2] Lopez O, Freijedo F.D, Yepes A.G, Fernandez-Comesaa P, Malvar J, Teodorescu R, Doval-Gandoy J, “Eliminating Ground Current in a Transformerless Photovoltaic Application,” IEEE Transactions on Energy conversion, vol. 25, no. 1, pp. 140-147, 2010.

[3] Araujo S.V, Zacharias P, Sahan B, “Novel Grid-Connected Non- Isolated Converters for Photovoltaic Systems with Grounded Generator,” in HPower Electronics Specialists Conference, H2008, pp. 58-65.

[4] Lopez O, Teodorescu R, Doval-Gandoy J, “HMultilevel transformerless topologies for single-phase grid-connected converters,” in IEEE Industrial Electronics, IECON 2006-32nd Annual Conference on Digital Object Identifier, 2006, pp. 5191-5196.

Single-Phase Seven-Level Grid-Connected Inverter for Photovoltaic System

ABSTRACT:

This paper proposes a single-phase seven-level inverter for grid-connected photovoltaic systems, with a novel pulse width-modulated (PWM) control scheme. Three reference signals that are identical to each other with an offset that is equivalent to the amplitude of the triangular carrier signal were used to generate the PWM signals. The inverter is capable of producing seven levels of output-voltage levels (V_dc, 2V_dc/3, V_dc/3, 0,−V_dc,−2V_dc/3,−V_dc/3) from the dc supply voltage. A digital proportional–integral current-control algorithm was implemented in a TMS320F2812 DSP to keep the current injected into the grid sinusoidal. The proposed system was verified through simulation and implemented in a prototype.

KEYWORDS:

1.      Grid connected

2.      Modulation index

3.       Multilevel inverter

4.       Photovoltaic (PV) system

5.       Pulse width-modulated (PWM)

6.      Total harmonic distortion (THD)

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Proposed single-phase seven-level grid-connected inverter for photovoltaic

systems.

EXPECTED SIMULATION RESULTS:

            Fig. 2. PWM signals for S₁ and S₃.

        

Fig. 3. PWM signals for S₂ and S₄.

Fig. 4. PWM signals for S₅ and S₆.

Fig. 5. Inverter output voltage (V_inv).

Fig. 6. Grid voltage (V_grid) and grid current (I_grid).

CONCLUSION:

Multilevel inverters offer improved output waveforms and lower THD. This paper has presented a novel PWM switching scheme for the proposed multilevel inverter. It utilizes three reference signals and a triangular carrier signal to generate PWM switching signals. The behavior of the proposed multilevel inverter was analyzed in detail. By controlling the modulation index, the desired number of levels of the inverter’s output voltage can be achieved. A TMS320F2812 DSP optimized the performance of the inverter. The less THD in the seven-level inverter compared with that in the five- and three-level inverters is an attractive solution for grid-connected PV inverters.

REFERENCES:

[1] M. Calais and V. G. Agelidis, “Multilevel converters for single-phase grid connected photovoltaic systems—An overview,” in Proc. IEEE Int. Symp. Ind. Electron., 1998, vol. 1, pp. 224–229.

[2] S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, “A review of single-phase grid connected inverters for photovoltaic modules,” IEEE Trans. Ind. Appl., vol. 41, no. 5, pp. 1292–1306, Sep./Oct. 2005.

[3] P. K. Hinga, T. Ohnishi, and T. Suzuki, “A new PWM inverter for photovoltaic power generation system,” in Conf. Rec. IEEE Power Electron. Spec. Conf., 1994, pp. 391–395.

[4] Y. Cheng, C. Qian, M. L. Crow, S. Pekarek, and S. Atcitty, “A comparison of diode-clamped and cascaded multilevel converters for a STATCOM with energy storage,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1512– 1521, Oct. 2006.

[5] M. Saeedifard, R. Iravani, and J. Pou, “A space vector modulation strategy for a back-to-back five-level HVDC converter system,” IEEE Trans. Ind. Electron., vol. 56, no. 2, pp. 452–466, Feb. 2009.

Stability Enhancement of Wind Power System by using Energy Capacitor System

ABSTRACT:

This paper presents Permanent Magnet Synchronous generator (PMSG) based a variable speed wind turbine systems including energy capacitor system (ECS). The ECS is the combination of electric double layer capacitor (EDLC) known as super capacitor and power electronic devices for wind power application with its detailed modeling and control strategy which can supply smooth electrical power to the power grid and makes the system better stable and reliable. As generated power from wind fluctuates randomly, the objective of this control system is to select a line power reference level and to follow the reference level by absorbing or providing active power to or from ECS to smooth output power fluctuation penetrated to the grid and to keep the wind farm terminal voltage at a desired level by supplying necessary reactive power. The performance of the proposed system is investigated by simulation analysis using PSCAD/EMTDC software.

KEYWORDS:

1.      Variable speed wind generator

2.       Permanent Magnet Synchronous generator (PMSG)

3.       Energy Capacitor System (ECS)

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAM:

Fig. 1. Model System

 EXPECTED SIMULATION RESULTS:

Fig. 2. Response of real wind speed data [case-I]

Fig. 3. Response of PMSG generated Active Power [case-I]

 

Fig. 4. Grid terminal voltage without & with ECS [case-I]

Fig. 5. Grid Power with/without EDLC and EDLC power [case-I]

Fig. 6. Grid Active Power without and with EDLC [case-I]

Fig. 7. EDLC active Power [case-I]

Fig. 8. EDLC energy [case-I]

Fig. 9. Comparison with SMA and ECS

Fig. 10. Response of Wind speed [case-II]

Fig. 11. PMSG generated Active Power [MW] [case-II]

Fig. 12. Grid Active Power without and with EDLC [case-II]

Fig. 13. EDLC active power [case-II]

Fig. 14. EDLC energy [case-II]

Fig. 15. Grid terminal voltage without & with ECS [case-II].

Fig. 16. Frequency deviation of SMA, ECS & without ECS [case-I].

             

CONCLUSION:

The simulation results show that the quality of the terminal voltage and output power penetrated to the grid is not good but continuously varying without ECS system. Besides, when we used ECS system, the terminal voltage and grid power is almost constant and quality of voltage and power is excellent. So, using ECS system smoothed power can be supplied to the grid by charging and discharging of EDLC. By using low pass filter to calculate line power reference instead of SMA, EMA makes the system very simple, compact and cost effective. Therefore, it can be concluded that this proposed system can be applied effectively in power systems to generate high quality electrical power from the natural fluctuating wind.

REFERENCES:

[1] G. annual report, 2014; world wind energy association.

[2] Niu Jiangang, Baotou, “Investigation on the properties of fly ash concrete attacked by a Pseudo-capacitance Faradaic electrochemical storage with electron charge-transfer, achieved by redox reactions, intercalation or electrosorption. Rain,” IEEE, Conference, ICETCE, Lushan, DOI. 10, pp. 2335 – 2339, 22-24 April 2011.

[3] Harden F, Bleijis JAM, Jones R, Bromely P, Ruddell AJ, “Application of power-controlled flywheel drive for wind power conditioning in a wind /diesel power system,” Ninth international conference on Electrical Machines and Drives, Canterbury, paper no. 468, pp. 65-70.

[4] Senjyu T., Sakamoto R., Urasaki N., Funabashi T. Fujita H., SekineH.,“Output power leveling of wind turbine Generator for all operating regions by pitch angle control,” Energy Conversion, IEEE Transactions, Vol. 21, pp. 467 - 475, 2006.

[5] Ali MH, Murata T, Tamura J, “Minimization of fluctuations of line power and terminal voltage of wind generator by fuzzy logiccontrolled SMES,” international review of Electrical engineering, vol. 1, pp. 559-566, 2006.

Modeling and Simulation of a Stand-alone Photovoltaic System

ABSTRACT:

In the future solar energy will be very important energy source. More than 45% of necessary energy in the world will be generated by photovoltaic module. Therefore it is necessary to concentrate our forces in order to reduce the application costs and to increment their performances. In order to reach this last aspect, it is important to note that the output characteristic of a photovoltaic module is nonlinear and changes with solar radiation and temperature. Therefore a maximum power point tracking (MPPT) technique is needed to track the peak power in order to make full utilization of PV array output power under varying conditions. This paper presents two widely-adopted MPPT algorithms, perturbation & observation (P&O) and incremental conductance (IC). These algorithms are widely used in PV systems as a result of their easy implementation as well as their low cost. These techniques were analyzed and their performance was evaluated by using the Matlab tool Simulink.

KEYWORDS:

1.      Photovoltaic system

2.      MPPT

3.      Perturbation and Observation

4.      Incremental conductance

SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:

Fig. 1. Block diagram of the stand-alone PV system.

CIRCUIT DIAGRAM

Fig. 2. Model of the photovoltaic module

Fig. 3. Schematic diagram of a DC Buck-Boost converter.

EXPECTED SIMULATION RESULTS:

Fig. 4. Output current of PV module

Fig. 5. Output voltage of PV module

Fig. 6 Output power of PV module

Fig. 7. Output current of MPPT+DC-DC converter

Fig. 8. Output voltage of MPPT+DC-DC converter

Fig. 9. Output power of MPPT+DC-DC converter

Fig 10 : PV-Output power with and without MPPT+DC-DC converter

Fig. 11. Output current of MPPT+DC-DC converter

Fig. 12. Output voltage of MPPT+DC-DC converter

Fig. 13. Output power of MPPT+DC-DC converter

Fig. 14. PV-Output power with and without MPPT+DC-DC converter

             

CONCLUSION:

In this work, we presented a modeling and simulation of a stand-alone PV system. One-diode model for simulation of PV module was selected; Buck-Boost converter is studied and applied to test the system efficiency. Two Maximum Power Point Tracking techniques, P&O and IC, are presented and analyzed. The proposed system was simulated using the mathematical equations of each component in Matlab/Simulink. The simulation analysis shows that P&O method is simple, but has considerable power loss because PV module can only run in oscillation way around the maximum power point. IC method has more precise control and faster response, but has correspondingly higher hardware requirement. In practice, in order to achieve maximum efficiency of photovoltaic power generation, a reasonable and economical control method should be chosen. The following of this work is based on optimizing the performance of PV modules and stand-alone systems using more efficient algorithms to minimize the influence of the meteorological parameters on the PV energy production.

REFERENCES:

[1] A.KH. Mozaffari Niapour, S. Danyali, M.B.B. Sharifian, M.R. Feyzi, “Brushless DC motor drives supplied by PV power system based on Zsource inverter and FL-IC MPPT controller”, Energy Conversion and Management 52, pp. 3043–3059, 2011.

[2] Reza Noroozian, Gevorg B. Gharehpetian, “An investigation on combined operation of active power filter with photovoltaic arrays”, International Journal of Electrical Power & Energy Systems, Vol. 46, Pages 392-399, March 2013.

[3] N. Femia, D. Granozio, G. Petrone, G. Spaguuolo, and M. Vitelli, “Optimized one-cycle control in photovoltaic grid connected applications”, IEEE Trans. Aerosp. Electron. Syst., Vol. 42, pp. 954- 972, 2006.

[4] T. L. Kottas, Y. S. Boutalis, and A. D. Karlis, “New maximum power point tracker for PV arrays using fuzzy controller in close cooperation with fuzzy cognitive net-work”, IEEE Trans. Energy Conv., Vol. 21, pp. 793–803, 2006.

[5] Mohamed A. Eltawil, Zhengming Zhao, “MPPT techniques for photovoltaic applications”, Renewable and Sustainable Energy Reviews, Vol. 25, P. 793-813, 2013.