Introduction To Potential Induced Degradation

Potential-induced degradation (PID) has received considerable attention in recent years due to its detrimental impact on photovoltaic (PV) module performance under field conditions. Both crystalline silicon (c-Si) and thin-film PV modules are susceptible to PID and PID drastically impact the reliability of the c-Si and thin film modules.

PV Diagnostics will release a series of articles to talk about PID mechanism in both, c-Si and thin-film PV modules, modelling of leakage current paths in the module package, dependence of temperature, humidity and voltage on the spreading PID and preventive measures for PID particularly at system level.

In this article, we have discussed the basic cause of PID, detection of PID in modules installed in the plant and overview of PID recovery.

PID in photovoltaic modules which occurs when the modules are exposed to high voltage with respect to the ground, mainly by stray currents. PID happens only in ungrounded PV systems, more prominent at negative potential modules with reference to earth. Our previous studies across the country show that number of PV modules suffering from PID ranges from 5-25% across different locations.
The chart below illustrates different factors affecting PID:

The standard reliability qualification PV Module tests [IEC 62804-1] required module manufacturers to test the modules for PID and have PID free test certifications, but the danger of PID looms over due to several reason such as improper grounding, environmental factor such as humidity etc.

What causes PID?

The high relative voltage forces the sodium ions to diffuse from glass through encapsulate and eventually accumulate on the cell surface. This diffusion increases the surface recombination, causes increased local shunting and thereby reduce the fill factor. Modules with negative potential with respect to ground are mostly affected. In PID affected modules, there is high potential difference between cell and the module frame, which results in leakage currents which flows from cell to encapsulate to glass and frame. Degradation due to PID increases over a period of time.

PID can lead to catastrophic failures of PV modules in the field under outdoor conditions. In problem is likely to become more severe in the future as the plants move towards increasing maximum system voltage to 1500 V.

Detection of PID on field

Detection of PID can be done by analyzing  IV-curves, IV data and EL images. With IV data we can observe the reduction in shunt resistance, Voc and declining fill factor.

EL images below correspond to the 4 different stages of PID in modules of same make, followed by the IV curves for these modules. As you can see, with the increased shunt resistance, the darker areas in the EL images are increasing. PID is stronger in modules closer to the negative side of the string.

 

 

IV curves for the 4 modules affected by PID (shown above). We can see that with the increasing
PID degradation the fill factor and Voc of the modules decreases.

What can we do about PID?

The effect of PID and its impact on PV systems is not immediately visible. Further, it is difficult to get insurance cover for modules degraded due to PID. Therefore, it is important that we take a preventive and precautionary approach and conduct regular audits to check for initial PID states.

PID can be prevented by ensuring that the inverter is negatively grounded. In polycrystalline modules, PID causes surface polarization which can be reversed to recover considerable power from the module.  In thin-film modules, PID causes electro-corrosion of conducting layer, which is not reversible.

PID can be reversed through 2 methodologies

  1. Heat recovery – storing PID affected modules at 100 deg. C for ~10 hours. In the long term, it could impact module stability due to exposure to high temperatures
  2. Voltage bias – applying positive bias to the modules. It is a much slower process, but doesn’t affect the health of modules in the long term

 

With the rapid growth in the solar industry, awareness about the causes and consequences of PID is extremely important to ensure better quality installations and longevity of PV systems. More and more newer installations are now coming up with properly functional negative grounding to prevent PID. Also, the higher degradation in some of the older installations is not much of a concern, as most of the power loss can be recovered through right engineering.

Despite improvements in negative grounding systems, PID as a challenge remains a  potential reality. With due care, losses due to PID can be prevented and mitigated.