Diagnosis of the PID Phenomenon

What is PID

Potential Induced Degradation (PID) is the phenomenon of reducing output power of a photovoltaic module (PV module). This arises from the electrical current leakage from the cell of a photovoltaic module to the frame, which removes useful electrons from semiconductors, in other parts of the plant such as glass and frame. The polarization created changes the electrical characteristics of the module (PV module) causing gradual degradation in output power.

Basic Factors

The factors that trigger the phenomenon of the PID exist in any installed system.

For example:

  • Humidity and temperature at which the panels are exposed may cause PID. Indeed, the very high temperature and humidity may lead to rapid deterioration of a panel's efficiency
  • The quality and the technical characteristics of constructing materials (glass, encapsulation materials, cell) of panels play a crucial role in the emergence of the phenomenon. The use of a) glass with a low sodium content b) low permeability membrane encapsulation to moisture c) material anti-reflective coatings cells that do not significantly affect the electron mobility may limit the phenomenon.
  • The grounding and the total voltage of each array can contribute to the emergence of the phenomenon. Thus the topology of the inverter, grounding, dimensioning, radiation and temperature can potentially limit the phenomenon.

Indications

The sharp decline in the performance of a PV installation is the first sign of the appearance of PID. The decrease that is due to PID, leads to a reduction of Mpp (Maximum Power Point) and Voc (Open Circuit Voltage) of an array. To determine the problem, the Voc in the two poles of the array that identifies the problem is measured and compared.

Ideally, the positive and the negative pole of the array must be symmetrical to the potential (grounded) neutral conductor. In case of PID, we will observe reduced Voc to the negative pole of the array and that is why the phenomenon mainly affects the panels which are closest to the negative pole.

In case of PID, the extent of the problem and ways of treatment should be examined.

Workaround

In installations with inverters with a transformer, the PID can be prevented by grounding the negative pole of the generator, since it shifts the potential of the entire array to positive. In systems with inverters without a transformer, the PID can be prevented by reversing voltage potential.