PID (Potential Induced Degradation) in PV installations — cause and prevention

The [guide on insulation resistance measurement on the DC side of PV strings](/guides/nen-3140/isolatieweerstandsmeting-pv-strings-dc-zijde) covers how a periodic Riso measurement detects an insulation fault in the wiring or the panels themselves. PID is a different, slower-acting phenomenon that does not manifest as an acute insulation fault, but as a gradual, cumulative yield degradation — and for that reason is often mistaken for ordinary cell ageing until the yield drop becomes significant.

The mechanism: leakage current driven by cell voltage relative to ground

Within a PV string of series-connected panels, the individual solar cells, depending on their position in the string, have a varying voltage relative to the grounded frame edge and the grounded mounting structure. In a string with a substantial total string voltage, the cells at the "negative" end of the string can be hundreds of volts negative relative to ground. This voltage drives a small, but over the lifetime of the installation cumulatively damaging, leakage current through the glass layer and the encapsulation film of the panel, between the solar cell and the grounded frame. This leakage current causes ion migration (notably sodium ions from the cover glass) to the cell surface, which degrades the cell's electrical properties and gradually reduces the power of the affected panels — in severe cases up to several tens of percent yield loss for the most negatively polarized panels in the string.

Why the inverter's grounding configuration is decisive

The extent to which PID occurs depends strongly on how the PV inverter positions the DC side relative to ground:

With a transformerless inverter (the common design for most residential and many commercial installations), the DC side of the installation sits electrically relatively close to the grid's ground potential, which in practice often causes one end of each string to become strongly negative relative to ground — exactly the situation that promotes PID.
With an inverter with an isolation transformer (galvanically separated from the grid), the DC side can "float" more freely relative to ground, providing more room to center — or even actively shift — the string voltage relative to ground, one of the techniques covered below.

Two countermeasures: PID-resistant modules and active recovery

PID-resistant modules: panel manufacturers offer modules that have been specifically tested and qualified according to IEC 62804 (accelerated PID test procedure, typically at elevated temperature and humidity with an applied voltage for a standardized test duration) and whose cell technology and encapsulation materials are optimized to limit ion migration. This is the simplest, structural countermeasure, but requires a deliberate choice during panel selection — not every panel on the market is equally PID-resistant, even though all panels meet the common power and safety certifications.
Active PID recovery ("PID box"): an additional device that, outside daylight hours (when the string is not producing power), applies a reverse voltage across the string to partially or largely reverse the previously occurred ion migration. This is a corrective measure for an existing installation where PID has been identified, rather than a preventive design choice for a new installation.

Note: PID damage is, with timely intervention (especially via active recovery with a PID box), partially reversible because ion migration is a physico-chemical, not a destructive mechanism — unlike, for example, hotspot damage from a faulty bypass diode, which is permanent. However, the longer a string suffers untreated PID, the less complete the recovery turns out to be.

Practical relevance

For a PV installation that gradually delivers less yield than expected, without visible panel damage and without a regular Riso measurement showing an acute insulation fault, PID is one of the diagnoses that should be considered — especially for larger, transformerless installations with long strings and a high string voltage. A string-by-string I-V curve comparison (see the guide on I-V curve tracers) can distinguish PID from other degradation mechanisms because PID typically affects the panels at one end of the string more strongly than the panels in the middle.

Common mistakes

Automatically attributing a gradual yield decline to normal ageing, without considering PID as a possible cause, especially with a transformerless inverter with long, high-voltage strings.
Not selecting PID-resistant modules for a design with foreseeably high string voltages, when this choice is the simplest and most structural prevention at the new-build stage.
Confusing PID with a regular insulation fault — a Riso measurement can remain within norm while significant PID-related yield degradation is still occurring, because it involves a slow, cumulative leakage current rather than an acute breakdown.
Installing a PID box without first understanding the underlying string-voltage-relative-to-ground configuration, causing the applied reverse voltage not to effectively match the actual PID mechanism of that specific installation.