The strings in a PV system comprises of various modules connected in series to deliver the requisite voltage as per the input voltage requirement of the inverter. However the output voltage of the string is dictated by the operating voltage of individual modules .We need to understand that the string open circuit voltages, the voltage which is delivered by a series combination of different PV modules .The open circuit voltage of each module may have variation ,due to voltage mismatch. If a module is of 35 V, the actual voltage (VOC) may vary between 33-36V. Which will lead to voltage mismatch in a series.The overall voltage of the string will be sum of voltages of individual modules connected in series as shown in the figure. Similarly individual modules also subjected to current mismatch and the overall output of a string is dictated by the output current of the weakest module .It should be noted that voltage mismatch between parallel string is not that a big problem as current mismatch in series of modules.
If there are 3 strings and each of them have a different VOC under the same condition (Radiance and Temperature) as shown in figure. The low voltage string will absorb some of the VOC from the high voltage string and the overall VOC combination is typically average of the VOC measured at individual string .It is important to use I-V tracer to study the IV curves of each string and access the characteristic of strings connected in parallel while conducting field tests .
Dr.Sanjay Vashishtha & Rishikesh Muthyal