String to inverter: BoS component failures


Editor’s note: This article is the second in a series which will discuss specific system reliability issues seen in North American systems (the first article can be read here). Each article will focus on a specific failure mode, giving an overview and examples of defect presentation. A longer summary article diving into a broader industry context and defect prevalence will follow this series. This second article is a collaboration between Heliolytics and Strata Solar Services.

by Dr. Rob Andrews and Kristine Sinclair (Heliolytics) and Mike Loeser (Strata Solar)

Electrical Balance Of System (BOS) components can be key failure points in PV installations, and will be the subject of this article. All components on the DC side from module interconnections up to and including the inverter are essential for delivering the power produced by the modules to the grid. The further up the chain the component, the more influential the failure to power production and therefore site revenue. Figure 1 gives an example aerial Infrared image where a string and combiner outages can be observed. If there is a fault in the Electrical BOS chain, the electricity produced by the panels cannot be properly extracted, and is instead burnt off locally as heat in the panels.

Figure 1 — Example aerial Infrared image of a string and a combiner outage in a ground-mount system (yellow hot).


On a 1.6 GW subset of unique sites that Heliolytics has inspected (more than 5 GW scanned to date), it was found that the category of Electrical BOS issues is the single biggest contributor to DC unavailability. Depending on site size, anywhere from 40% up to close to 80% of defects observed during aerial Infrared surveys can be attributed to this category (see Figure 2).

Figure 2 — Summary of defects observed during aerial Infrared inspections by defect category, grouped by site size range, for a 1.6 GW subset of North American sites.


Outages at the string, combiner, and inverter level can have a wide variety of root causes arising from the power grid, nature, components aging, or installation workmanship.  Some specific examples of root causes are below:

  • String
    • Cracked module (see Figure 3)
    • Internal module issues
    • Damage to circuit wiring
    • Fuse failure
    • DC connector failure
    • Lightning strikes
  • Combiner
    • Combiner box flooding
    • Pest intrusion
    • Failed internal components
  • Inverter
    • Ground fault
    • Grid voltage issues
    • Pest intrusion
    • Transformer/medium voltage fault
Figure 3 – Example of a crack in the backsheet of a module that led to a blown string fuse.

Strata Solar


No matter the root cause of any outage or fault, active remote monitoring and a robust preventive maintenance program are the best defenses against nasty surprises, and are key to reducing energy losses due to BOS faults.  In-depth performance analysis can alert operators immediately to significant outages, while detailed remote monitoring systems can target more granular sources of underperformance.  The best chance of catching degrading components before a costly outage is with diligent inspection of components on site combined with infrared imagery.  This allows for fixes at the earlier stages of mechanical or electrical failure.

Want to learn more about quality issues with PV plants? Be sure to attend pv magazine’s Quality Roundtable at the Solar Power International trade show on September 12 in Las Vegas. Register here.

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