PID stands for Potential Induced Degradation which occurs when the modules are exposed to high negative voltage with respect to the ground. PID happens in ungrounded PV systems and is more prominent near modules towards the negative end of the string, exposed to negative potential with reference to ground.
The main causes of PID are:
Migration of positive ions from the glass plate through the EVA and ARC to the cell. The presence of these ions causes an effective shunt path across the cell and reduces the output.Buildup of the negative potential