PID (potential induced degradation)
Potential induced degradation — leakage current driven by high system voltage degrades cells, showing up as a patchwork of warm cells that is worst near one end of each string. It is progressive and, caught early, partly reversible.
Thermal signature
A distinctive patchwork or checkerboard of individually warmer cells scattered across a module, affecting many modules at once. The intensity follows string position: modules closest to the negative end of the string (on common p-type systems) are visibly worse, fading toward the other end.
Typical ΔT
3–10 °C per affected cell against unaffected cells on the same module (typical industry interpretation aligned with IEC TS 62446-3). The pattern and its string-position gradient matter more than any single cell's ΔT.
What causes it
- High negative bias between cells and the grounded frame, driving ion migration through the glass and encapsulant
- Hot, humid conditions accelerating leakage currents
- System grounding topology that leaves strings floating at large negative potential
- Module types with encapsulants or glass more susceptible to PID
Power impact
PID losses commonly run from a few percent to 30% or more per affected module and keep growing while the driving conditions persist. Because it affects many modules in a position-dependent pattern, the site-level loss can dwarf any individual fault.
Recommended action
Fix: within 30 days
Confirm and plan mitigation within 30 days: verify with I-V curves and electroluminescence on a sample of worst-position modules, review the inverter grounding topology, and evaluate anti-PID measures (functional grounding, offset boxes) or module replacement for far-gone units. PID caught early is substantially recoverable.
Frequently asked questions
Is PID reversible?
Often, partially. If the driving voltage stress is removed or reversed — via grounding changes or anti-PID hardware that applies a recovery bias overnight — affected cells can regain much of their performance. Severely degraded modules may not fully recover, which is why early detection matters.
Why is PID worse at one end of the string?
The degradation is driven by the voltage between the cells and the grounded frame. Modules near the negative end of a string sit at the largest negative potential relative to ground, so they experience the strongest leakage current and degrade first — producing the tell-tale gradient along the string.
How is PID distinguished from soiling in a thermal image?
By pattern. Soiling follows physical deposition — bands along the lower edge, random droppings — and shows on the RGB pair. PID follows electrical position: a cell-level checkerboard that intensifies toward one end of each string, repeated across many strings, with clean glass.
Related anomalies
Suspect pid (potential induced degradation) on your site?
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ΔT figures and abnormality classes on this page describe the typical industry interpretation aligned with IEC TS 62446-3 — indicative engineering guidance, not normative text from the specification. Temperature differentials are only meaningful when captured at ≥600 W/m² irradiance (per the TS) and verified on site before repair work.