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Friday 27 March 2015

Single-Stage DC-AC Converter for Photovoltaic System

Single-Stage DC-AC Converter for Photovoltaic Systems



ABSTRACT:


This paper presents a DC-AC converter that merges a DC-DC converter and an inverter in a single-stage topology to be used as an interface converter between photovoltaic systems and the electrical AC grid. This topology is based on a full bridge converter with three levels output voltage, where two diodes and one inductor have been added in order to create a Boost converter. The control system of the proposed converter is based on two hysteretic controllers: one for the grid injected current and the other for controlling the panel current. A prototype of the proposed converter including power and control circuits was developed. The MPPT algorithm is not yet implemented and, therefore, to obtain experimental results an additional power supply is used to emulate the PV panel. Theoretical analysis and design criteria are presented together with simulated results to validate the proposed concepts. Experimental results are obtained in a lab prototype to evidence the feasibility and performance of the converter.


SOFTWARE: MATLAB/SIMULINK

 BLOCK DIAGRAM:


Fig.1. Power Grid connection of a PV array by means of a two stages converter.



Fig.2. Proposed converter: single-stage DC-AC converter for PV systems.

EXPECTED SIMULATION RESULTS:



 Fig.3. Experimental results: a) iLR (blue trace) with a gain of 1A/div, iLP (green trace) with a gain of 1A/div; b) grid voltage (magenta trace) with a gain of 50 V/div; c) vCF voltage (red trace) with a gain of 100V/div and iLR (blue trace) with a gain of 1A/div.

      
                                     
      
 Fig.4. Simulation results: a) iLR (blue trace) with a gain of 1A/div, iLP (green trace) with a gain of 1A/div; b) Obsetvation of iLP perturbations; c) verification of the maximum iLR switching frequency value, fSRmax.

CONCLUSION:

The new contribution of this paper consisted in the proposal of a new DC-AC converter for PV systems that includes two Boost converters and a full-bridge inverter in a single-stage topology. The unique restriction imposed by the converter is the minimum VCF voltage which must be greater than the sum of the maximum values of the panel and the grid voltage. Due to the high voltage gain given by the two input boosts, the topology is suitable to operate with low panel voltages. The theoretical concepts introduced in the paper were proved by the preliminary results obtained in the experimental tests of the converter prototype that is still in development. A converter efficiency of 90.5% was achieved. The prototype used to obtain the preliminary experimental results presented in the paper is not yet optimized in terms of layout and power density. It is expected that, in what concerns circuit layout, the reduction of the leakage inductances will result in a significant reduction of the dissipated power which will end up in efficiency increase.

REFERENCES:

[1] Johan H. R. Enslin, Mario S. Wolf, Daniel B. Snyman, and Wernher Swiegers, “Integrated Photovoltaic Maximum Power Point Tracking Converter”, in IEEE Transactions on Industrial Electronics, Vol. 44, no. 6, December 1997.
[2] Soeren B. Kjaer, John K. Pedersen and Frede Blaabjerg, ‘‘A Review of Single-Phase Grid-Connected Inverters for Photovoltaic Modules’’, in IEEE Transactions on Industry Applications, Vol. 41, No. 5, 2005.
[3] Fritz Schimpf and Lars E. Norum, “Grid connected Converters for Photovoltaic, State of the Art, Ideas for Improvement of Transformerless Inverters”, Nordic Workshop on Power and Industrial Electronics, 2008.
[4] Mario Fortunato, Alessandro Giustiniani, Giovanni Petrone, Giovanni Spagnuolo, and Massimo Vitelli, “Maximum Power Point Tracking in a One-Cycle-Controlled Single-Stage Photovoltaic Inverter”, in IEEE Transactions on Industrial Electronics, vol. 55, no. 7, pp. 2684-2693, July 2008.
[5] Yeong-Chau Kuo, Tsorng-Juu Liang, and Jiann-Fuh Chen, “Novel Maximum-Power-Point-Tracking Controller for Photovoltaic Energy Conversion System”, in IEEE Transactions on Power Electronics, vol. 48, no. 3, pp. 594-601, June 2001