The PID or Potential Induced Degradation is a phenomenon that has appeared in PV modules in the past few years. The reason for this appearance is the designing of PV installations with long strings that develop high voltage. This voltage in many cases nears 1000 V and serves the reduced energy losses in the pipelines.

However, the high voltage creates a large potential difference between the PV modules and the mounting system (mounts), which is grounded. As a result, there is leakage of current from the PV cells to the grounded frame and mounting system especially in PV modules located near the negative pole. This phenomenon is called PID.

As a result of this leakage, PV modules develop less power, and as a result, have significantly lower output. Therefore, it is necessary to detect PID in PV installations early on in order to avoid significant panel degradation that will call into question any chances of reversing the phenomenon. 

Early diagnosis and documented confirmation of PID can only be performed with electroluminescence testing and low radiation power measurement. This is because:

• At the initial stage (power reduction up to ~6-7%), it is impossible to detect PID with a thermal camera or from the developing electrical characteristics of the PV module. In fact, the electroluminescence test requires an experienced operator, using the appropriate settings in order to identify and record the problem.
  
• At the intermediate stage (power reduction ~10-20%), PID cannot be detected with a thermal camera as the temperature differences are imperceptible and the difference of the commonly measured Voc and Isc values from the nominal values is minimally affected at the level of instrument measurement error. At this stage, the existence of PID can only be indicated by measuring each PV module under load.
  String measurements, at this stage, cannot confirm or disprove the existence of PID, as measurement under load is usually incorrectly done with the string connected to the inverter. It is known that the inverter regulates/determines the developing voltage and intensity of the string (of all the PV modules) in order to achieve maximum power under specific weather conditions (radiation, temperature, etc.), but not per PV module.
  Therefore, even if an attempt is made to compare the measurements of a non-problematic string with a string suspected of PID, the validity of the comparison is in no way ensured, as there is no way to confirm the absence of PV modules with PID in the non-problematic string.
  
  
• The advanced stage (power reduction of more than 25%) (μείωση ισχύος άνω το 25%)usually occurs 2-3 years after the phenomenon first appears. At this stage, power reduction of more than 55% has been measured in PV modules. In these cases, it is ‘easier’ to speculate the existence of PID with a thermal camera, but it is not certain. Special knowledge of thermography and testing under specific weather conditions are required. As a matter of fact, there can be no proof of PID with the use of thermography as ‘temperature disparities’ in PV modules do not necessarily indicate PID. Additionally, the electrical characteristics must be measured in a simulator under load, in order to be accurate and usable.

The electroluminescence test can draw completely accurate conclusions about the existence or not of PID, at any stage of the phenomenon.

On the one hand, the early diagnosis of PID protects the investor against loss of production and revenue, and on the other hand, enables the reversal of the phenomenon to a significant extent.

Making use of the innovative Mobile PV Testcenter Kmetrics is able to diagnose PID at an early stage and propose the appropriate solution, which varies in each case (replacement under warranty, use of reversal devices, etc.)