Grid-Tied PV System Performance Factors: Page 4 of 4

Intermediate

Inside this Article

Cleaning solar modules
Grid-Tied PV System Performance Factors
A remote meter
A remote meter can help you keep track of how well your array is performing.
Utility meter
Taking readings from the PV system production meter can help you identify system problems, but only if you have previous records to compare to.
A temperature gun
A temperature gun checks infrared radiation to give quick readings of cell temperature.
Measuring cell temperature
Placed against the back of a module, a sensor (attached to a meter) can be used to measure cell temperature. Be sure to take readings on several spots and average the results.
Pyranometer sensor
To accurately determine the irradiance striking your PV array, be sure the pyranometer sensor is on the same plane as the modules.
PV module label
While this label does not show the PV module’s power tolerance, it does give the minimum figure, which computes to be 91% of the rated power (154.7 ÷ 170.0).
Dirty PV module
Dirty PV modules can significantly reduce output. Clean ’em!
Dusty PV module
Dirty PV modules can significantly reduce output. Clean ’em!
Partial shading of a PV module
Shading can negatively affect your PV system’s output. For testing purposes, be sure your array is in full sun.
Misshapen junction box
Warped, misshapen boxes or insulation indicate electromechanical problems with one or more PV modules.
Burn marks
Burn marks indicate electromechanical problems with one or more PV modules.
Monitoring display
Finding individual underperforming PV modules is possible with module-level tracking technologies, such as microinverters or DC optimizers.
Measuring the array’s operating voltage
Measuring the array’s operating voltage at the positive and negative terminals, with the array connected to the inverter.
A clamp-on meter
A clamp-on meter is used to measure the current while the array is connected to the inverter.
Cleaning solar modules
A remote meter
Utility meter
A temperature gun
Measuring cell temperature
Pyranometer sensor
PV module label
Dirty PV module
Dusty PV module
Partial shading of a PV module
Misshapen junction box
Burn marks
Monitoring display
Measuring the array’s operating voltage
A clamp-on meter

Putting it All Together

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?

Troubleshooting

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.

Access

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.

Resources:

“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

Comments (3)

Justine Sanchez's picture

Hi Gary,
Thanks for posting! Yes a production meter is an extremely valuable asset to any grid-tied PV system. Separate production meters (and faceplate inverter output meters) can give us a good idea if the PV system is meeting energy (kWh) production expectations overtime. But if your kWhs aren't stacking up to what is expected, this article provides the next step in troubleshooting, and describes a method of spot checking the wattage of the system, so that we can determine if there is actually a problem with the system itself, rather than something other such as a cloudier than usual spring or perhaps partial morning or afternoon array shading due to tree growth over time, etc..And yes the method described here does require some specialty tools (ex/irradiance meter and temp gun) which will be common to the installer, but not necessarily to the average homeowner.
Cheers,
Justine Sanchez
Home Power Magazine

gary beckwith's picture

this is an interesting article with some good information for the techies and tinkerers. but for practical purposes, the easiest way to check on your system is to install a separate meter for production if there isn't one already. personally I think it's a good idea to put a production meter on your system even if it isn't required, just for this purpose. then go to the PVWatts website and put in your location, tilt angle, temperature etc, and see what you should be getting every month and per year. the PVWatts calculator already factors in all the derate issues mentioned in this article. So if you're consistently below what PVWatts says, then you should call your installer and have them troubleshoot your system.

Robert Sczech_2's picture

A 3KW system will generate roughly 3 MWH of electricity per year - a market value of roughly $300. If one is really to perform all the maintenance tasks and checks on the system as described in the article and if one values ones own life time at only $10 per hour, then the value of ones human lifetime invested into these activities could easily eat up a significant part of the electricity created.

A personal experience: The state of NJ has mandated the installation of so called revenue grade meters for all solar installation in order to accurately measure the amount of electricity generated. In my case, the process of installing that meter started last July and is still not complete. During the past year many emails and phone calls needed to be made to get the contractor, the permits and the final inspections. The inspection process alone takes many months because of the paperwork requirements and repeated reinspections. If all the human labor in getting these meters installed is all added up, then the gain from renewable does not appear to be that significant. Lots of that gain is lost in the inefficiencies of the full installation process.

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