Interpreting I-V Curve Deviations
Inside this Article
When a measured I-V curve differs substantially from the predicted curve, commissioning agents or service technicians can use the nature of the deviation to screen for potential performance problems.
As PV arrays age, there are many potential causes of system underperformance. Some may be expected, such as soiling losses or long-term array degradation. Some may be unexpected, such as bypass diode failure, cracked modules and so forth.
Because I-V curve tracers capture all of the current and voltage operating points of a PV source, they are uniquely capable of identifying symptoms of underperformance in PV systems. As I describe in “Field Applications for I-V Curve Tracers” (SolarPro, August/September 2011), every module datasheet provides a model I-V curve that represents all the current and voltage combinations at which you can operate or load the module under Standard Test Conditions (STC). When a measured I-V curve differs significantly in height, width or shape from the predicted I-V curve—which is based on the model I-V curve, but adjusted for actual irradiance and temperature conditions—the nature of the deviation provides clues about potential performance problems.
Here I provide an overview of the process used to gather I-V curves and identify normal traces associated with healthy modules and source circuits. I then explain how to interpret differences between measured and predicted I-V curves. I discuss basic types of I-V curve deviations, all of which indicate that PV power is reduced, and consider possible causes. The discussion of I-V curve deviations is organized according to a troubleshooting flowchart process that is designed for optimal workflow efficiency (see Inside this Article). I present strategies for identifying PV modules with performance problems. I also cover best practices for taking irradiance and temperature measurements, which can improve the accuracy of measured and predicted I-V curves.
Safety is the first consideration when performing any type of electrical work. Before beginning to troubleshoot a PV system, make sure that you know how the system is constructed and how it operates. Verify that the test equipment is properly rated for the current and voltage you will expose it to. Use the necessary tools, procedures and personal protective equipment detailed in NFPA 70E, known as the Standard for Electrical Safety in the Workplace.
While PV systems present unique electrical hazards, using I-V curve tracers can improve safety relative to other testing methods. PV circuits do not need to be under inverter load for you to use an I-V tracer to look for a bad source circuit. Wade Webb, the vice president of quality assurance at Martifer Solar, explains: “To use a current clamp to test for bad strings, the technician has to work in a combiner box that is connected to an operating inverter, perhaps via a downstream recombiner. This is the main reason we prefer to look for bad strings using an I-V curve tracer. Besides the fact that an I-V curve tracer provides more detailed information than you can get using a clamp meter, it may also provide an additional level of safety by reducing the arc-flash hazard that the technician is exposed to.”
Basic test procedure. In commercial and utility-scale PV systems, I-V curve traces are generally measured in combiner boxes that are electrically isolated from the rest of the PV system. For example, perhaps zone-level monitoring in the inverter indicates that a particular combiner box is underperforming. Unless immediate action is required, asset managers will likely flag that combiner box for inspection during a regularly scheduled site maintenance visit. Once on-site, you can electrically isolate the combiner box by locking out and tagging out an equipment disconnecting means. If a visual inspection of the PV modules does not point to a likely cause, you can use an I-V curve tracer to identify underperforming source circuits.
For calibrated performance measurements, install an irradiance sensor in the plane of the array and stick a temperature sensor to the backside of a thermally representative module. After ensuring that the PV source circuits are not under load, open each touch-safe fuseholder in the combiner box. Using an alligator clip or similar connector, connect one test lead to the positive busbar and another to the negative busbar. You can now test each of the PV source circuits one at a time by closing the appropriate fuseholder and initiating an I-V curve trace. The test process can take as little as 10 to 15 seconds per source circuit, and the data are saved electronically. (While the process described here represents a scenario commonly encountered in the field, test procedures and measurement times may vary somewhat in practice depending on the specifics of the PV system, the BOS equipment, the I-V curve tracer or the test goals.)