The phenomenon of Potential Induced Degradation (PID) negatively affects the performance of many PV installations around the world.
PID leads to the progressive degradation of modules and strings, representing a serious threat to the performance of the whole installation. What can be done about it?
It’s a complex issue, but services are available to identify modules affected by PID, assess the long-term impact, and launch appropriate remediation action.
The phenomenon of Potential Induced Degradation (PID) negatively affects the performance of many PV installations around the world. PID leads to the progressive degradation of modules and strings, representing a serious threat to the performance of the whole installation. What can be done about it? It’s a complex issue, but services are available to identify modules affected by PID, assess the long-term impact, and launch appropriate remediation action.
Potential Induced Degradation (PID) is the phenomenon of ion diffusion occurring in the material layers between PV cells and the module’s grounded frame, leading to power loss in these cells. The underlying cause is the high negative potential relative to earth (up to 1000 or 1500 V) which in combination with an inadequate design or use of inferior materials can lead to a leakage current. In such cases, the ion diffusion is a continuous process, first degrading the cells closest to the frame and progressively affecting further cells and modules.
While most PV plant operators are still unfamiliar with PID, tests have revealed that about 80% of commonly used modules are susceptible to it. The process typically occurs a few years after commissioning, beginning at the periphery but gradually affecting a larger number of cells and modules. Over time, affected modules may lose up to 80% of their power generation capacity. Consequently, the issue can pose a serious threat to PV plant profitability if it is not addressed appropriately and in good time.
So, how do PV plant owners or operators know if their installation is at risk or is already suffering from power loss? And what can they do about it? PID is a tricky phenomenon requiring meticulous investigation of PV plant circumstances and careful evaluation of the possibilities for remedial action. It is therefore essential to approach the issue systematically.
The first step is to carry out a preliminary assessment of the plant’s susceptibility to PID. This involves analysis of the PV plant’s technical specifications, including module design, materials used, and the grounding configuration. If the technical set-up shows low vulnerability to PID, it is safe to assume that the plant’s business case is not at risk because of the phenomenon and no further action is needed.
Where there is significant risk, a more comprehensive assessment should be conducted. The plant should be investigated to identify the presence of PID. Where needed, this might include drone-assisted aerial inspection of the entire plant and, following the initial global diagnosis, carrying out IV-curve measurements on carefully selected modules and strings. Any power loss resulting from PID can then be quantified.
However, it is equally important to also assess long-term impact, given the progressive nature of PID. This requires in-depth analysis of the measurement data and the installation’s technical specifications, including comparison with reference data from similar PV plants. An assessment of the future evolution of plant performance can then be made, including the impact on financial profitability. This will allow asset managers to make informed and calculated decisions to resolve the issues.
Depending on the measured power loss and plant specifications, different remediation solutions can be developed.
One strategy could involve installing a grounding kit to prevent further PID. Grounding the appropriate pole of the affected PV array, depending on the configuration, will remove the voltage bias that causes PID. However, installing a grounding kit can be challenging.
For example, a transformer must be present to ensure galvanic isolation, otherwise the DC residual current will flow on the AC side. Functional grounding is therefore not an option for the transformerless string inverters used in most PV plants.
In addition, there are many complications involved in installing a grounding kit. For safety reasons, grounding must be reserved for nighttime hours; fire hazard needs to be under control, isolation measurements need to be carried out and compatibility with inverter warranty must be assured. This is certainly a task requiring a high level of expertise.
While grounding might be a viable option under some circumstances, an easier way to address PID is to install PID boxes. These charge equalizers raise the PV array to a high positive potential of perhaps +1000 V, creating a reversed polarization effect. As a result, the ionization effect in the materials is reversed and the affected PV cells are regenerated.
Positive results have been obtained with different types of PID boxes. A utility-scale test program carried out in 2018 showed remarkable results, confirmed by both IV-curve measurements and drone-assisted inspection. However, a few things need to be taken care of when installing PID boxes. For example, it is essential to assure compatibility with the inverter warranty and the plant’s insulation resistance (RISO) must be monitored.
Susceptibility to PID and the resulting impact on plant performance varies depending on the type of plant. It is therefore essential that the service provider have comprehensive experience analyzing different types of solar plants, ranging from first-generation PV plants to leading-edge bifacial installations. They should also be familiar with all the available PID mitigation solutions, keep a close watch on the ever-evolving PV market, and remain independent from equipment suppliers.
It is also important to bear in mind that any PID investigation project could be used as an opportunity to concurrently examine other aspects that might negatively impact plant quality and performance. During inspection, experts might also identify other phenomena such as backsheet issues, soiling or unexpected inverter downtimes arising from reduced insulation resistance. This will allow targeted remedial actions.
This article is by ENGIE Laborelec’s expert team specializing in solar power generation. The team provides expert advice, auditing services, performance improvement solutions and field assistance for power plant operators and industry.
If this article prompts questions about the performance of your PV plant, please feel free to contact us at firstname.lastname@example.org.