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                                      What causes Potentially Induced Degradation (PID)?

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                                      Potentially Induced Degradation (PID) can significantly impact PV system output and be a real headache for system owners. But what is PID, what causes it, and what can be done to prevent it?

                                      Potentially Induced Degradation (PID) is a defect experienced by some PV modules when installed such that the PV cells are operated at a negative voltage with respect to ground (module frames are electrically grounded). The greater the negative potential difference relative to ground, the more severe the PID effect. However, not all module designs are inclined to PID and not all system designs trigger the failure mechanism (see Figure 1).

                                       

                                      Schematic illustrating different grounding scenarios and associated PID risk. DNV-GL

                                      Figure 1 – Schematic illustrating different grounding scenarios and associated PID risk. DNV-GL

                                       

                                      PID risk can be mitigated by designing the PV system to avoid exposing the modules to negative bias relative to ground (standard in North America), designing the system to reverse the voltage on the modules periodically to reverse the effects of PID, or designing the modules to not be susceptible to exposure. Unfortunately, each of these approaches have drawbacks:

                                      • Advanced system designs targeting more efficient or safer operation often require at least part of the array exposed to negative voltage.
                                      • System designs that periodically reverse the effects of PID spend time operating with some level of active PID, and an imperfect degree of reversibility may lead to progressive impacts over time.
                                      • Modules designed for minimal susceptibility to PID are still relatively recently deployed in the field and are under active research. In addition, low conductivity encapsulants to protect susceptible cells lack redundancy and are susceptible to quality control deviations and component degradation over time. DNV GL therefore recommends testing for PID (for instance to the existing standard IEC 62804) on a sample of the modules sent to the project site.

                                      Although PID is rare in North America due electrical system design regulations avoiding the conditions that allow for PID to occur, it has been observed by Helioytics. 

                                       

                                      Aerial thermal image of PID

                                      Figure 2 - A thermal image of PID. Heliolytics

                                       

                                      The site highlighted in this article is from the US. Although PID is less frequent in the US than in the rest of the world, it needs to be considered by system operators and owners. A combination of lab and field tests supported by engineering understanding is an efficient approach to identify and mitigate PID.

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                                      This blog post was originally featured on PV Magazine USA.

                                      This article is the one in a series which discusses faults in solar power plants and specific system reliability issues seen in North American systems (click here for the next). Each article will focus on a specific failure mode, giving an overview and examples of defect presentation, followed by a longer summary article diving into a broader industry context and defect prevalence will follow this series. This first article focusing on PID is a collaboration between Heliolytics and DNV GL.