As the world of solar energy continues to grow, testing on the acceptance and maintenance sides will grow with it. In typical installations, we have been asked to test only the basic electrical apparatus. This typically consists of circuit breakers, transformers, instrument transformers, relays, cables, grounding, and functional testing. Inverters are often held for the manufacturer to set up and commission as they tend to have proprietary software for their systems. At the photovoltaic (PV) array, we have been limited to verifying circuits, grounding, and possibly circuit breakers if the system is so equipped.
USE THE I-V CURVE
In truth, the arrays or panels themselves should undergo testing utilizing the I-V (current-voltage) curve and basic test equipment. Plotting the I-V curve can help the engineers make changes to their model using real-world scenarios and capture maximum efficiency from the PV system.
There are many influences on a PV system that affect its output. These can be as simple as weather (clouds, smog, fog, snow, ice), environmental issues (smoke, dust, dirt, shade, reflection), and the age of the system.
Utilizing the correct test equipment, you can plot an I-V curve and examine the efficiency of the array. Ignoring other components of a PV system and focusing only on the PV array itself, we recommend the following test equipment:
- Irradiance sensor
- DC clamp-on ammeter
- Multimeter capable of measuring direct current voltage (DCV)
- Insulation resistance meter
- Infrared camera
Note that several companies produce PV array test equipment that can increase testing efficiency by pulling output data of the array into one system to plot the I-V curve.
The I-V curve measures the performance of the array under the current conditions (Figure 1). This takes into account the irradiance (light), temperature, and output (voltage and current) of the array. This helps determine how to align the array or, in instances where a tracking array follows the sun, how to best time the tracking. Several tracking systems can now adjust for the time of year, the position of the Earth relative to the Sun, and other inputs such as weather forecasts.
Where:
Isc = Short circuit current
Imp = Max power current
Vmp = Max power voltage
Voc = Open circuit voltage
The maximum power point is at the knee of a normal I-V curve (where Imp and Vmp intersect). This is the point at which the array generates maximum electrical power. If conditions remained constant in a perfect world, you would maintain this maximum power — but this is seldom the case. Many factors affect efficiency, even in the most complex arrays that are designed to track and maximize efficiency.
PROVE SYSTEM OPERATION AND EFFICIENCY
Along with the normal testing of the apparatus that we are all accustomed to, we can use the test equipment listed above to plot an I-V curve and prove the operation and efficiency of the system. Looking at our I-V curve, we can also evaluate the fill factor (FF) of the PV array. FF looks at the squareness of the curve.
FF = (Imp*Vmp)/(Isc*Voc)
FF is an important indicator of performance. Currently, PV systems cannot achieve the perfect rectangle represented by the FF because any outside interference will affect the FF. The FF will also be affected by design and module technology. The left side of the curve shows the shunt losses; the down-slope or right side of the curve shows the series losses. These losses will be combined with mismatch losses like shading or other outside influences.
I-V curves can give significant information by themselves but are greatly enhanced when utilized with a proper PV model. Think of this as performing sweep frequency response analysis (SFRA) on a transformer and comparing the results with factory results. Creating that overlay allows you to obtain the greatest amount of information.
For a PV model to have value, the following information is required:
- PV module characteristics
- Number of PV modules wired in series
- Number of modules or strings wired in parallel
- Length and gauge of wire between the modules or strings
- Irradiance in the plane of the array
- Cell temperature
ENSURE EFFICIENT OUTPUT
Many things can affect the efficiency and output of an array. Some of the more common issues include:
- Uniform or non-uniform soiling caused by dust, dirt, grime, or clouds and fog can affect the array (as discussed earlier) by blocking the irradiance.Degradationis typically a slow process based on the quality of materials used to build the panels. Over time, the sun damages the covering material on the panels, decreasing their ability to produce. We have all seen vehicles with clouded headlights due to environmental factors.
- Incorrect modules being selected for the model is rare, but it can also greatly affect the predicted model expectations compared to real-world output and efficiency. Verifying that the correct modules are in use is important.
CONDITIONS DURING TESTING
It should go without saying that looking at the conditions during testing is always important for electrical testing. Making sure that PV testing is performed close to the time the model shows optimum production will yield more accurate results. Rising or setting sun conditions can vastly affect PV production.
Other issues can be as simple as improper positioning of the irradiance sensor or not considering reflection. Reflections from a variety of sources, including a window across from the array at a certain time of day or even the window on the technician’s vehicle, can greatly affect PV output. Reflections from nearby water or other objects are sometimes overlooked as well.
Temperature is very important in correctly assessing the measured curve versus the predictive model. Higher temperatures will result in a lower Voc. This could be due to improper measurement techniques including, but not limited to, taking measurements on the face of the cells versus the back of the module or a poor connection to the measurement device.
Notched I-V curves can indicate shading or, worse, a damaged PV cell. If a cell is damaged, it can become electrically isolated and mirror the effects of shading. This shows the importance of visual/mechanical inspection of all components in the system.
CONCLUSION
As with all NETA testing, PV testing will advance as the technology advances. With the growing use of solar power, testing will have to grow with it. As systems being implemented today near the end of life, it will be important to determine the most cost-effective time to replace components rather than keeping them in service. As technology advances, we should see longer lifespans and greater harnessing of this endless energy source.
Jason Carlson is Vice President of CBS Field Services based out of the Pacific Northwest. He started his career in critical power, specializing in uninterruptible power systems and DC systems before joining a NETA testing firm. He has worked between the two industries for more than two decades. Carlson served in the United States Marine Corp. from 1993 to 1999.