When bypass diodes overheat

pv magazine’s third Quality Roundtable hosted during Intersolar Europe in June revealed a case in which junction boxes overheated and caused brown discoloration of the backsheets of PV panels. The case, discussed during the Roundtable event, reveals how the chain or responsibility can be extremely difficult to unpick, leaving investors scratching their heads and potentially footing the bill.

Had the module or junction box manufacturer been present during the discussion of this case at the third Roundtable they would have done well to slip out of the room. The final verdict of most of the participants was to find them guilty of supplying inferior quality components, earning them the “black sheep” tag. Issues and individual cases are discussed during the Roundtable without company or brand names.

With regard to the EPC, there was a more balanced view, tending more toward "gray" – neither black nor white. But the voting was just the entertaining part of the workshop. On a more serious note, this case demonstrated the lines of conflict that are drawn when this type of fault occurs. In fact, in this case – one of three that were discussed at the roundtable – any one of the parties involved might have been responsible.

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The discussion showed what one would actually have to find out in order to develop an opinion. “The investigations in this case are ongoing," said Andreas Kleefisch, an attorney at the Baumeister law firm, which initiated discussion of the case. However, there are a couple of preliminary results.

2,000 of 4,000 modules discolored

A routine inspection was conducted of a PV system with more than 1 MW of capacity built by an installation company in 2010, prior to the expiration of the five year warranty. The inspection revealed that around half of the more than 4,000 PV panels had brown discolorations on the backsheets, which were also visible on the front of the modules. The discolorations coincided exactly with the places where the junction boxes were attached to the back of the modules. Random I-V curve measurements indicated that the open-circuit voltage of several modules was too low. The evaluators suspected that this was due to one or more activated or failed bypass diodes. Some modules even had melted junction boxes.

The wholesaler that sold the PV panels many years earlier to the installer has since gone bankrupt. The installer, therefore, took the problem directly to the module manufacturer. The manufacturer then explained that the discoloration was nothing more than a purely visual defect which had no influence whatsoever on the longevity and performance of the modules. The producer therefore ruled out any compensation or replacement modules. It also ruled out engaging an arbitration expert, according to the parties involved.

Manufacturer sees no connection

At that point, the operator assumed the cost of investigating the affected modules at a testing laboratory and opened the junction box of a third module himself. In the junction box the operator opened, he found severely melted bypass diodes, according to the lawyer Kleefisch. In a preliminary report from a testing laboratory the junction boxes and their components were found to be conforming to certification standards, however that in one module two of the bypass diodes were broken and activated, and in the other module all three diodes were broken in the active mode.

Once confronted with the melted components in some of the junction boxes, the manufacturer offered to exchange the affected modules. Nevertheless, the producer still refuses to see a connection between the brown discoloration of the backsheets and overheating of the junction boxes. It is also refused to exchange modules that were merely discolored.

"If a connection can be established between the brown discoloration and the overheating of the junction boxes, it is likely that the rest of the modules will fail," says Kleefisch. After all, renegotiating damage compensation month after month is not a satisfactory solution, he said. "That would mean very high costs for the installer."

At the Quality Roundtable event in Munich, the case triggered heated discussions. There was little disagreement that the cause of the brown discoloration of the modules was very likely due to overheating in the junction boxes. However the question of why the junction boxes overheated and who was responsible for the fault is not yet clear at this stage of the analysis.

To find out what was behind the bypass diode activation and subsequent failures, pick up a copy of the November edition of pv magazine global. Alternatively, attend the fourth Quality Roundtable event next week, hosted alongside the Solar Power International trade show in Las Vegas. Register here. Mirco Sieg, Michael Fuhs

Questions for a discussion:

• Is backsheet browning itself a warranty case?

• Is browning a sign of material deterioration that will lead to premature module degradation or other faults in the future?

• If browning is not covered by warranties, but a sign of faster degradation, how should investors proceed?

• Who is liable for the bypass-diode overheating or: Which stress bypass diodes have to withstand?

• Join the Roundtable to take part of the discussion!

This article was translated by Tim Hanes and edited by Jonathan Gifford