Most PV modules are typically connected in series to form a string to match with the inverter voltage. The typical voltage at module level is about 30V and at Inverter level is about 300-700V and accordingly the number of panels in series and parallel are decided. Potential Induced Degradation (PID) is the degradation of modules due to the leakage current between the solar cells and panel frame surface. PID is more prone to the cells having the negative potential. In a typical string the cells are at a voltage level of -200V at the negative end o fthe string and +200V at the positive end of the string. The aluminium frame of the panel is grounded due to security reasons, and the high potential difference between the module frame and the negative cell voltage causes the leakage current flow from the cell surface to the module frame and subsequent to the ground, which results in the degradation of the cell as well as reduced generation.
To prevent PID, the solar cells must not have a negative voltage relative to their surroundings. Grounding the negative pole of the PV array resolves this problem. A PV Offset Box can be used for transformerless inverters that cannot be grounded.