Apart from string current display function, the purpose of string current sampling is used for I-V curve scanning and diagnosis for smart maintaining work. The PV plant has a large number of PV strings. Meanwhile, one PV string also consists of multiple PV modules (PV panels). The fact is that any PV module or electrical connection can have potential failures or risks that can lead to power generation loss. For example, shadow, dust shielding, and glass panel cracking can cause current mismatch in the string. Diode short circuit, cable disconnect, Potential-induced degradation (PID) and hotspot can cause low string open circuit voltage.

The challenge in the PV system is how to accurately and rapidly locate and process these failures or risks. The traditional method is offline manual inspection which has very low efficiency and high cost. The current popular way is online I-V curve scanning and diagnosis to improve the efficiency and accuracy of PV system fault identification.

Figure 2-2 shows an example of normal and abnormal I-V curves scanning and diagnosis. As abnormalities of the PV system can cause different changes in the I-V characteristic curve, the I-V curve monitoring results can be used to analyze the potential failures or risks during PV system operation. So, the accuracy of string current and voltage sampling is one of the key factors to determine the final failure diagnosis accuracy, and indirectly determine the power generation efficiency. This is very important to commercial-industrial PV plant and utility PV plant which care a lot about output.

Also notice that maximum power point tracking (MPPT) in string inverter is usually implemented in PV array level, while I-V curve scanning is implemented in single string level.

Figure 2-2 Example of Normal and Abnormal I-V Curves Scanning and Diagnosis