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Potential-Induced Degradation

Why PID Testing Matters

A 1.2 MW solar site in Spain was commissioned in 2019 and thermal imaging at that time did not show any anomalies. However, a repeat of thermal imaging one year later revealed four strings that were approximately 20°C hotter than the others on the site.

Researchers at York University, UK, performed further analysis via in–field EL imaging and determined that a PID issue had affected the four strings. The PID was instigated during periods of inverter ground faults when the module strings were stuck at open–circuit voltage.

A performance ratio (PR) analysis was conducted across the four affected strings (string 1 to 4) compared to a healthy string (string 5). The results indicate that in some cases PID can lead to rapid reduction in power: over one year the PID–affected strings had a maximum 40% decrease in
PR, while the healthy string remained stable. The researchers also observed that PID can affect strings at different rates: some strings degraded quicker than others.

Anti–PID devices were utilized on all four strings and a similar analysis was done a year later. The module surface temperature of the four strings had decreased, but they were still hotter than others in the array. An analysis of PR did not show an improvement in module power output. This indicates that in
this case the PID issue could not be reversed.

Following Kiwa’s Module Procurement Best Practices including PVEL PQP testing, BOM specification in procurement contracts and batch testing can greatly reduce the likelihood of procuring PID susceptible modules. This is important for both transformerless inverter sites and central inverter sites when open–circuited strings can instigate PID, as shown in this case study.

Credit: Badran, G., Dhimish, M. Field study on the severity of photovoltaic potential induced degradation. Sci Rep 12, 22094 (2022). https://doi.org/10.1038/s41598-022-26310-y

EL images of PID-affected modules; Bottom: EL images from a non-PID affected string.

Performance ratio of the tested strings. A baseline of 90% was selected to indicate low performance; the red box plots indicate the initiation of PID for each string.

Glass, Glass Coating
Front Encapsulant
Back Encapsulant
Cells

Materials Assessed

Leakage current can flow from the cells through the encapsulant and glass to the module frame which can result in static charge buildup and/or sodium ions penetrating the cell surface, reducing energy generation. These materials can therefore impact the module’s PID susceptibility:

  • Cells
  • Glass
  • Encapsulant
  • Glass Coating
Explore PVEL’s Test Methodology
Key Takeaways
Scroll through the key takeaways.

Median degradation similar to last Scorecard, but negative outliers exist.

79% of BOMs tested were PID Top Performers, but 5% of BOMs had over 5% power loss. Recovery can typically occur when using the IEC 61215 post-PID UV exposure, but industry research is still ongoing to see how field-representative this is.

Closely aligned results across laminate types and crystalline cell technologies.

The average and median degradation rates for glass//glass and glass//backsheet BOMs were indistinguishable. PVEL also saw very strong PID results for CdTe, while PERC, HJT and TOPCon median degradation all ranged from 1.1 to 1.6%.

Encapsulant choice affects PERC PID results.

The choice of encapsulant had a meaningful impact on PID susceptibility for PERC modules. Modules using EVA for rear-side encapsulant had an average median degradation four times higher than those using POE or EPE.

Four manufacturers experienced higher than expected PID power loss.

In one case, two almost identical BOMs were submitted and only a change in encapsulant supplier was enough to increase PID power loss from less than 1% to almost 4.5%.

Test Procedure

Modules are placed in an environmental chamber at 85°C and 85% relative humidity for 192 hours while a voltage bias equal to the maximum system voltage rating of the module (typically + and/or − 1500V) is applied. These conditions cause PID to occur in susceptible modules. The test duration is twice that of IEC 61215:2021’s PID test.

Power Degradation of Each Model Type

pid degradation results

All 2013/2014 and 2015 results are from 600 hours of PID testing. This was decreased to 192 hours for 2016 results and beyond.

View Box Plot Interpretation Guide

See Top
Performers

Click here to see the 126 model types listed as Potential-Induced Degradation Top Performers

PID Test Result Spotlight

Over more than a decade PVEL’s PQP testing has shown that materials impact module reliability and performance. This was evident again in the case shown here, with two almost identical glass//glass BOMs from the same manufacturer. The cells, glass and almost all other materials were identical; the only differences were using encapsulants from two different suppliers (although both were front side EVA + rear side POE), and different frame and junction box sealants. What some may consider as minor BOM changes resulted in one BOM having four times higher PID susceptibility.
PID192(-) - BOM-1

Post-PID192(-) on one BOM with 1.1% power degradation.

PID192(-) - BOM-2

Post-PID192(-) on an almost identical BOM with 4.5% power degradation.

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