Photovoltaic modules and the PID effect: how to protect yourself?

The PID effect (Potential Induced Degradation) reduces the output power of the photovoltaic modules already after 2-4 years of operation and is a problem to consider when choosing a photovoltaic system. Designers and installers of photovoltaic modules they should take into account suitable solutions to manage this phenomenon.

Given that the causes of PID in photovoltaic modules are not yet clear, it seems that the acceleration factors include high humidity and temperatures, associated with the application of negative voltage.

The 'potential induced degradation', the PID effect, is a problem for i photovoltaic modules with crystalline and thin-film cells. The effect is triggered by the exposure of the system to an external potential and produces a degradation that can be reversible (polarization) or irreversible (then we speak of electrocorrosion).

When i photovoltaic modules have a negative potential to earth, the negative voltage between the module cells and the frame, usually connected to earth for safety reasons, is very high. The electrical voltage attracts the electrons of the materials used in the photovoltaic modules and discharges through the grounded frame, resulting in a small amount of leakage current. This migration of charge factors results in a disturbance due to the photovoltaic effect.

In 2011, the results of tests conducted by the Fraunhofer Center for Silicon were released, according to which the PID effect is responsible for a reduction in power of almost 70 percent. Some manufacturers of photovoltaic modules they replied that the problem exists, but their products are exempt thanks to the use of special materials such as insulating resins for the back of the modules and sealing resins.

However, research confirms that the main solution to PID is to avoid the application of a potential external to photovoltaic modules. Otherwise, the risk of power loss persists and becomes an economic problem in consideration of the fact that photovoltaic systems are generally financed with a life cycle of 25 years.

The PID effect can be addressed at the system architecture level, avoiding the presence of any external potential in the photovoltaic modules. In fact, the grounding of the negative pole of the photovoltaic string can definitively solve the problem of potential induced degradation.

You can easily ground the negative pole of the photovoltaic module when the inverter is equipped with galvanic separation, but a much more convenient and practical solution would be a solution where a transformerless inverter is provided.

The point is that, normally, a transformerless inverter is not a solution to the PID problem at all photovoltaic modules, since it applies negative voltage to the photovoltaic array. However, transformerless inverters have recently been designed and manufactured with special internal circuits capable of keeping the negative pole at the same potential as the neutral, as if it were virtually connected to earth. An advanced solution of this type is Omron's Kp 100L photovoltaic inverter, which uses the internal Zigzag-connected Chopper Circuit (ZCC) to do without the internal or external transformer and any other additional hardware.

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