Now we have enough information to calculate what our PV array should be producing by multiplying the STC watts by the three main loss factors:
Expected inverter output (W) = STC watts × Module temperature factor × Irradiance factor × Other losses factor = 3,000 W × 0.86 × 0.875 × 0.82 = 1,851 W
Recall that the output was 925 W as shown on the inverter display. According to calculations, this PV array should be producing about twice what the inverter was showing. This is not even close to the expected output. We need to find out what is wrong, but where do we start?
With troubleshooting, anything is possible. Although these procedures are a good place to start, don’t limit yourself to them. Common sense, experience, and keen observation can help find solutions.
Error codes on your inverter or remote monitor.This could be an LED light, a specific code flashing on the display, or a ground-fault indicator. Consult your installer or the inverter’s manual, or call technical support to determine what the code means. If there is indication of a ground fault, immediately contact your PV installer or other qualified person, as there could be danger of electric shock on parts that are not normally energized (such as metal conduit, module frames, ground wires, metal enclosures, etc.). Do not continue investigating the system by yourself if there is a ground fault!
Inspect your PV array. Is any of the glass cracked? Do you notice any missing parts? Make sure the modules are aligned and no corners are sticking up or down—heating and cooling cause expansion and contraction, which can lead to mechanical failure within the rack and modules. Look for loose bolts, bent frames, and cracked glass. Snow load can also damage an array.
Look for yellow or brown burn marks on both the front and the back of the modules. If you can’t easily access the array, you can do this inspection from the ground with binoculars. Diodes and solder connections within the module and its junction box can fail, causing areas to heat up and burn or melt.
Check underneath the array. Are there any loose, dangling wires? Expansion and contraction can also cause taut wires to pull loose from their connectors, or cause the connectors to partially open. Poor connections will create heat from increased resistance and can melt the connectors, junction boxes, or leave burn marks on the white back-sheet.
Is any of the wire insulation missing or damaged? Small animals will often build nests underneath arrays and seem to have an appetite for wire insulation. Has debris accumulated? Leaf litter buildup can create a place for moisture to collect, and loose connections contacting moisture can then cause intermittent faults (especially when the insulation is missing) and, eventually, corrosion.
Try to find problem strings or modules. If you have module-level monitoring, such as with microinverters or DC optimizers, it should be easy to see if one or more modules are underperforming. If your PV system doesn’t use this technology, each series string’s open-circuit voltage and operating voltage should be tested and recorded, as well as its operating current (using a clamp-on meter). You can then compare string or module measurements with each other or against calculated expectations to find any poor performers. Note that any work or measurements on exposed energized parts should be done only by trained individuals. These are often high-voltage systems, which can kill or injure.
Using an infrared camera can also help spot problems within modules, but only if you can get your hands on one and know how to use it! (For more information, see the “Tools of the Trade” sidebar and “Potential PV Problems & New Tools for Troubleshooting” in HP143.)
Check production history. Examining past electric bills, and inverter and system monitoring data can help you pin down when the array started underperforming. Is the problem intermittent? This might be due to loose connections or wet debris facilitating a short. Was there a sudden drop in production, or did it seem to happen gradually? Has it always been this way? Perhaps a wire was pinched during the installation and the array has never been producing full power. Finding out the time of year or coordinating the loss of production with other events (storms, grid outages, roof repairs, etc.) can also provide clues about the underlying issue.
Take the problem area apart. If the array’s voltage and current meet specifications at the array output, but not at the inverter input, then the problem lies between these points. Check junction and combiner boxes for loose connections, compromised wiring, or blown fuses. Thermal cycling can cause wire connections to loosen; even properly installed wires can come loose over time. Check the inverter and disconnects for the same. If the inverter input meets specs, but its output does not, then it’s likely there is an issue within the inverter itself, and the manufacturer will need to be consulted.
If you’ve narrowed down the problem to the array, sometimes it’s necessary to inspect the array wiring and rack up close, as well as isolate and test modules. Check all of the module frames, junction boxes, connectors, glass, and backsheets. An IV curve tracer can be helpful in finding underperforming series strings (see “Tools of the Trade” sidebar) so the entire array does not need to be disassembled. Note: Only qualified individuals should work on the array, move modules, or expose or take apart any of the wiring.
Lena Wilensky is co-owner of Nunatak Alternative Energy Solutions, a small Colorado RE company. She is a Solar Energy International instructor and a NABCEP-certified PV installer, and is certified by ISPQ as a PV Affiliated Master Trainer. She is also the proud mother of a future solar sister.
“Photovoltaic Degradation Rates” • bit.ly/PVdegrade
“Pump up the Power” by Jeremy Taylor in HP127
“PV System Commissioning” by Blake Gleason in SolarPro 2.6
“Seeking Peak Performance” by Brian Mehalic in HP133