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
