What happens when… bypass diodes fail?

This hypothetical case is realistic and shows how O&M service provider Enovos would respond.

The plant: a 5 MW ground-mounted plant, commissioned in 2011. The modules are oriented toward the south at a 30 degree angle. The system uses a central inverter and has string-level monitoring. The performance ratio of the system is calculated via a solar radiation sensor.

The monitoring system reports: In many of the strings there are negative deviations to current compared with other strings that have a similar number of connected modules, angle of inclination, and alignment. The performance ratio of the plant is lower than in the same month of the previous year on days with comparable solar radiation. The monitoring software reports a fault. A closer look at the yield curves on days with high levels of solar irradiance reveals occasional kinks in the curves; that is, sudden drops of string current that do not rise again.

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The hypothesis: The spontaneous drops in the string current indicate the failure of one or more bypass diodes in different modules. The failure of a bypass diode in a module usually results in the switching-off of one of three cell strings. The result is a sudden drop in output by one third. A decline in yield of this magnitude and in several modules can be detected in the yield curve of a string with good solar radiation conditions. In low sunlight, this effect can be considerably less visible. After all, the failure of a single bypass diode in a measuring channel with 48 modules results in a deviation of less than 1% of the string current.

Since these effects occur in the I-V curves of several strings, it is probable that this is not a single local problem, but that the problems are scattered throughout the system. Nevertheless, it is not possible to be 100% sure that problem is down to failed bypass diodes. The problem could be dirty panels, for example, but this would not occur suddenly. In the case of a lightning strike, on the other hand, the problems generally only occur in a certain part of the system, and not in the entire plant.

Identifying the fault and the affected modules: The total yield of the plant has already fallen by more than 4%. The 2011 system receives a FIT of €0.2111/kWh. If the plant were functioning smoothly, it would generate around five million kilowatt hours per year, which it would feed into the public grid for a price of €1,055,500 per year. A 4% reduction in yield equates to an annual revenue shortfall of more than €42,000. At this price, looking into the problem is definitely worthwhile, as the cost of troubleshooting is likely to be lower than the potential benefit.

Since the fault appears to be distributed over the entire system, the first step is to conduct spot checks (with a thermographic camera and other equipment) in order to obtain a picture of the defects. Measurements of I-V curves or similar tests on each string or module are too costly at this stage. After the discovery of failed diodes on a considerable number of modules, it is time to determine the exact extent of the problem and the location of the failed diodes. For this reason, the operations manager decides to conduct a thermographic examination of the entire plant using a drone. A rough estimate of the price per megawatt is roughly €1,000. A drone pilot can take the photos in less than one day, provided that it is a day with consistently high solar radiation.

Other defects can be seen in the thermographic image, such as inactive or overheated cells. If the problem is not with the bypass diodes, the images will still help. Since a high level of solar radiation is a prerequisite for usable images in which even minor abnormalities are visible, it is necessary to plan accordingly. In Germany, it can take a few days or weeks until weather conditions are good enough. So time is of the essence.

Enovos Renewables O & M GmbH, or Erom for short, is a classic manufacturer-independent service provider for technical operations management. Head of Operations & Maintenance is Richard Rath. From its headquarters in Berlin, photovoltaic systems in Berlin and in the metropolitan area of Brandenburg are fully managed by their own staff. Facilities in other locations and abroad are supplied by local partners. The control room works with all standard monitoring systems. Erom is the O & M department of the former Q-Cells. In the meantime, O & M contracts of Solon also have been taken over. The portfolio currently comprises 460 megawatts.

Evaluating the survey: The thermographic image shows that around 10% of the modules have one or more failed cell strings. This confirms the assumption that the reduction in yield is caused by failed bypass diodes.

Results of the investigation: The product warranty for these modules is not valid after six years. However, since the output of a module decreases by at least 30% in the event of a permanently failed bypass diode, the operator can invoke the performance guarantee granted by the manufacturer. Such warranties generally guarantee an output of 80% of the rated capacity even after 20 years of operation. Although the costs for dismantling the defective modules and replacing them with new ones have to be borne by the operator, the high yield losses make it worthwhile to invoke the warranty and replace the defective modules.

If the module manufacturer is no longer a going concern and its original module warranty has become worthless, it pays to take a closer look at the junction box and the diodes. Plug-in diodes that can be replaced in a few easy steps were often used during this time, and with material costs in the range of just a few cents, a fully functional module can be obtained. If the diodes are soldered in, the effort increases. But if the junction boxes are sealed, the diodes can no longer be replaced.

Effort of troublesooting in this case

Estimate of the cost of troubleshooting upon detection of decreased performance in monitoring:
around €5,000 for the thermographic survey + one workday.

Estimated waiting time until the faulty modules were located:
a few weeks



pv magazine troubleshooting series:

If you are interested in participating in similar cost estimates or have suggestions for a more efficient handling of the faults, we look forward to receiving an e-mail from you: Please contact michael.fuhs@pv-magazine.com (subject line: “troubleshooting”).