I-V (current-voltage) curve measurement is one of the well-established methods used for defect (or fault) diagnosis and performance analysis of Solar-PV installations. Assuming you have your IV-Curve established, here are the set of characteristics to look for to diagnose module level or string level defects.
First, we look at the region close to the Isc (Short Circuit Current) where shunt resistance is a factor. A lower shunt resistance causes power losses by providing an alternate path for current to flow through and consequently lowers the voltage. The result is a steep slope in the Isc region as compared to the baseline I-V curve measured. Studies have correlated the above phenomenon to several physical defects: loss in transparency in the glass, glass corrosion, delamination and Isc mismatch among the solar cells on the solar module. On a string level, to decouple these effects and achieve module level diagnostics it is imperative to measure the I-V curve of each module in the string.
Second, analyze the Voc region. Is the slope lower than expected? The Voc (Open Circuit Voltage) region can be affected by the series resistance in the PV. Series resistance can be correlated to several sources – the movement of current across the base of the solar cell, the contact resistance between the metal and silicon, and lastly the resistance of the top and rear metal contacts. A high series resistance, as manifested in the Voc region, is caused aggregately by soldering connection issues. Soldering connection issues could be solder corrosion, soldering discolorations, and broken interconnect ribbons.
These two scenarios as seen in the Isc and Voc regions of the I-V curve can greatly affect fill factor and thus the maximum power output of the PV module or string. For situations wherein there is very high series resistance the effect will be a reduction in short circuit current as well. One of the most striking behavior as manifested in the I-V curve is when at the Pmax (Maximum Power) region there are inflection points or notches. These notches are caused by bypass diode failures, broken cells, cell mismatch, and shading on a module or a series of modules in a string.
When the actual Voc value is reduced significantly compared to baseline I-V curve possible correlation to root cause could be failed cell interconnects, short circuits from cell to cell, and bypass diode failures. Also, a lower Voc for crystalline silicon technologies can be attributed to either light induced degradation or potential induced degradation. In case of light induced degradation, where power loss is not significant than nameplate – it’s not considered a defect. In contrast, potential induced degradation is a well-known phenomenon where a module’s potential and current leakage drives ions to move across the module. This is exacerbated by environmental factors such as humidity, temperature, and voltage potential coming together.
I-V curve measurement is one method of determining the degradation models discussed above and can be an important factor for predicting and managing performance to achieve expected ROI.