What is PID (Potential Induced Degradation)?
Potential induced degradation (also called PID) is a performance degradation in crystalline solar PV modules, induced by potential (voltage). This effect may cause power loss in the power rating of a solar panel by up to 40 percent.
The cause of the harmful leakage currents, besides the structure of the solar cell, is the voltage of the individual photovoltaic (PV) modules with reference to the ground. Nowadays most solar systems are of grid type. These systems use grid interactive type transformerless inverter. Hence it is not possible to ground the solar module or string in such systems. In most ungrounded PV systems, the PV modules with a positive or negative voltage to the ground are exposed to PID. PID occurs mostly at the negative voltage with respect to the ground potential and is accelerated by high system voltages, high temperatures, and high humidity. In fact, nowadays most on-grid systems have a rated voltage of 1000 Volts and some inverters even go up to 1500 Volts. In case of these high voltages, it is common for lower quality and cheap modules to exhibit PID.
PID in case of an individual module
The PID effect is stronger in cells that are located at the borders (closer to the aluminum frame). The PID effect is less strong in cells that are located towards the centre of the solar panel.
PID in case of a string
The PID effect is stronger in solar modules of a string that are closer to the negative side of the string. This is because the negative potential with respect to earth is the highest at that side.
What are common causes of PID
Usually one of the most common causes is cheap and low quality raw materials. In fact it is important to use the right quality of solar raw materials such as EVA, backsheet, Solar cells, etc which are PID resistant.
Further usage of solar modules in higher voltage strings and systems such as 1000vdc also drastically increases the chance of PID. Most on grid systems today are mostly having a 1000 volts system , hence it is not possible to change this.
Temperature and humidity further increase the speed of PID in solar modules.
Further PID effect also increases with time.
Ways to prevent PID
It is important that solar modules are made with PID resistant materials.
Also the solar cells used must be of A grade quality with no microcracks or other defects so that they do not start deteorating due to PID effect.
Further a timely checking of the solar string and system can help in early detection and avoidance of PID in solar modules.