Introduction to Aerial Inspections: Page 2 of 3
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Benefits of Aerial Inspections
The value propositions associated with aerial inspections include more-comprehensive site coverage, enhanced visibility into plant performance issues and improved site safety.
Comprehensive coverage. Operators can use high-quality aerial inspections in lieu of labor-intensive preventative maintenance activities, including manual I-V curve traces, voltage and current measurements, handheld IR thermography, module electrical connection tests and visual inspections. In comparison to these manual tests, aerial inspections not only identify dc performance issues with a higher degree of accuracy (and less labor), but also allow operators to characterize an entire plant under consistent operating conditions.
Where operators rely exclusively on manual preventative maintenance tests, technicians generally characterize only a representative subset of source circuits at a site each year. Because it takes a lot of time to conduct manual tests, especially on multi-megawatt sites that cover hundreds if not thousands of acres, the test conditions are inherently more variable, which complicates the process of comparing and analyzing the results. This piecemeal approach can result in undetected losses.
Enhanced visibility. Heliolytics has inspected more than 2.5 GW of PV projects internationally. We performed a comparative analysis on 1.6 GW of this portfolio across 280 sites, ranging from 60 kW to 250 MW, which is representative of systems from across North America, and filtered that selection to exclude systems with failure rates over 10%. Analyzing these representative data in the aggregate, we find that phantom dc capacity losses are as high as 1.25% of installed capacity across all sites, based on the expected performance impacts of observed faults. The average capacity losses for projects under 10 MW are 1.29% versus 1% for projects over 10 MW. String-level failures account for 84% of the capacity losses, with module-level faults making up the balance of the phantom losses.
Most importantly, all these data come from sites with active O&M and data analysis programs in place. In most cases, technicians had conducted I-V curve traces and handheld IR inspections for 10%–25% of the modules at each site. Therefore, these failure rates represent losses associated with faults that are slipping through the cracks because traditional data analytics and manual inspections are incapable of or ill-suited to identifying all the phantom losses that sap PV system performance and revenue.
By contrast, annual aerial thermal inspection results provide technicians with data that are both granular and highly actionable. Aerial IR imagery tells technicians exactly where to locate and remedy dc performance problems within an array. Because string-level failures are relatively consistent throughout the life of a system and account for the majority of the expected capacity losses, technicians can quickly repair these problems and increase system production.
Since aerial surveys can identify 100% of the faults at a given site, operators can use these data to classify all the fault mechanisms at a site and potentially identify systemic or serial issues. Figure 3, for example, is a map for a 20 MW solar farm where each color corresponds with a specific manufacturing batch and the letter X identifies locations of diode failures. Whereas the overall diode failure rate was only 0.2%, we observed that the majority of these failed diodes were associated with a specific manufacturing batch (dark orange). Identifying this systemic issue allowed the owner to prosecute for warranty remediation proactively, before the data acquisition system even had visibility into the progress of this fault mode.
Site safety. When operators use aerial inspections in lieu of manual dc inspections, technicians spend less time accessing combiner boxes and inverters. This effectively reduces worker exposure to electrical hazards. Technicians are exposed to electrical shock hazards whenever they open a combiner box or inverter; in large-scale systems deployed with central inverters, technicians are also potentially exposed to dc arc-flash hazards. (See “Calculating DC Arc-Flash Hazards in PV Systems,” SolarPro, February/March 2014.)
While it is possible to control these risks with personal protective equipment, there remains opportunity for human error or equipment failure. In the long term, the more effective and sustainable safety practice is to simply eliminate unnecessary manual inspection activities wherever possible. Viewed from this perspective, aerial inspections provide operators and organizations with an opportunity to implement a higher level of hazard control in accordance with OSHA’s hierarchy of controls methodology. Though workers may still need to open combiners, disconnects or inverters to conduct periodic visual and IR inspections, the hazards associated with these visual inspection activities are less severe than the hazards associated with physically accessing busbars and fuseholders to perform electrical characterization tests.