Simulation tool for mismatch conditions at cell level


Researchers from Portugal's Instituto Superior Técnico have developed a framework to simulate photovoltaic arrays under mismatch conditions. They claim the framework considers different mismatch factors and non-uniform conditions, in addition to variations in irradiance and temperature.

The proposed tool was validated under standard test conditions, with cell temperatures of 25 C and irradiance of 1,000 W/m2 on a 72-cell PV module made by China's Suntech. The scientists looked at 15 different mismatch scenarios, including partial shading and, most importantly, short circuit failures on the PV cells.

The academics claim their tool marks the first attempt to compare similar experimental curves with simulations based on the single-diode equivalent circuit model, using predefined database parameters. These parameters, which relate to hundreds of commercially available solar modules, are available on the PV Performance Modeling Collaborative website run by the Sandia National Laboratories and PVSyst software.

“In addition to consulting the databases, a set of reference parameters for each model was estimated based on the I–V curves measured at uniform conditions,” they added.

The scientists said they obtained experimental I–V curves under controlled mismatch conditions.

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“For each curve, the error of the maximum power point (MPP) position and the divergence in curve shape were observed,” they explained. “To have smooth I–V curves describing all the relevant nuances, we traced them with a hundred equal voltage steps between 0 and open-circuit voltage. For arrays with larger open-circuit voltage, the number of steps should be increased to preserve the voltage step below 2 V.”

The tests showed that the simulation tool is particularly good at simulating small-sized PV systems with few module strings, which is the type of array usually linked to string inverters or combined boxes. “For large arrays, a lower resolution should be used to maintain a reasonable processing time,” they added.

The scientists claimed that the tool is fully implementable, replicable, and useful in understanding failure impacts on PV strings. They said that it could be also used to develop increasingly efficient maximum power point tracking (MPPT) algorithms. They describe their work in “A complete framework for the simulation of photovoltaic arrays under mismatch conditions,” which was recently published in Solar Energy.

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