Optimising inverter and array design


The performance and reliability of PV system is greatly affected through mismatch of arrays. Mismatch occurs due to the interconnection of cells with different performance characteristics. This condition results in two detrimental effects: power loss and reliability degradation.

When the characteristics of cells within a PV array differ, individual cells may operate sub optimally, causing a reduction in array output power. This is especially the case for cells connected in series with variations in their current outputs. Such a situation may also arise when two different types of PV modules/ arrays are connected in tandem with a single inverter.

Below Figure shows a typical example of mismatch where two different modules are selected in a PV system array and connected in series for a normal power output.  The resultant current-voltage ( I – V ) relationship shows that the current output of the two 145 W modules of two different brands differs significantly and limits the current and hence power output of the array. In extreme cases, almost the entire array output can be lost due to such array mismatch. While some of the recent installation has adopted two different classes of modules in their array systems, the care has been taken in terms of selecting the right module/array/ inverter combination. Some of the developers such as Mahindra Solar have recently tried the use of a combination of thin film as well as crystalline technologies along with the single axis tracker systems for their installation in Baap substation. The system has been successful as the design takes care of the right selection of inverter/array combination and voltage optimisation approaches.

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