solar energy
Proving Performance Analyzing populations of I-V curves by Willard MacDonald
DATA IS VITAL TO UNDERSTANDING THE HEALTH OF ANY
PV installation. Capturing I-V curves during the commissioning or O&M of a large-scale PV plant can result in a significant amount of data. This article details ways of analyzing this data and effectively verifying operation and performance. I-V curves are plots of current (I) versus voltage (V), and are the most effective way to test the health of a PV string. I-V curves measured during commissioning demonstrate functionality and performance prior to sign-off by the owner, as well as establish a baseline for tracking the health of the arrays during the life of the system. During O&M, they provide a way to monitor on-going performance, troubleshoot problems, and substantiate warranty claims. While open-circuit voltage and clamp-meter measurements are commonly employed during PV measurements, they only provide a limited view. A typical I-V curve contains 100 points or more, whereas Voc and Isc (or Imp) are just two points on the curve; the shape of the curve between these two points can highlight a variety of problems, including bad bypass diodes, cracked cells, shading, high resistance connections, and cell degradation. During commissioning, I-V curves are typically captured for every string within a combiner box. Outliers are identified and fixed, and then the next combiner box is measured, until all strings in the system have been measured. During O&M, a particular string may be measured repeatedly over the course of years. Accumulating this information can highlight cell degradation, and potentially Figure 1: Typical I-V curve
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of the data. For example, it can plot the I-V curves for the family of strings in each combiner box, as shown in Figure 2. I-V curves scale vertically with irradiance or uniform module soiling; they scale horizontally with temperature. This is normal behavior for a healthy string, but variations in irradiance, temperature, and soiling across a population of strings can make is harder to detect anomalies in string performance. However, by normalizing the I-V curves we can remove these effects from the data, highlighting outliers and certain classes of problems, as shown in Figure 3. Here, the curves have been normalized to an arbitrary Isc of 10A and module Voc of 45V. This pins the Isc and Voc points of each curve together. Notice how the shapes of some curves stand out - If you were investigating a low performing combiner box, these stand outs would be the first to look at.
Figure 2: Population of string I-V curves
OUTPERFORMS DURING THE WINDIEST OF STORMS
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be used for a warranty claim if the degradation is greater than expected. I-V curve tracing is also used to evaluate PV modules or strings that show up with problems in infrared imaging. It is convenient to break up the measuring and analysis of large PV plants into smaller blocks. For example, a block might be the 250 strings associated with a single 3MW inverter. Testing this block will result in 250 I-V data sets. Datasets from one or more blocks can be imported into a data analysis tool (supplied by the curve tracer manufacturer). The data analysis tool provides a variety of views
Figure 3: The same curves normalized to the same Isc and Voc The I-V curves can alternatively be translated to the Standard Test Conditions (STC) of 1000W/m2 and 25oC. This facilitates comparing the I-V curves to the data sheet for the modules, particularly the Isc and Voc. For instance, you can easily see if a string has uniform soiling or has degraded by comparing the STC-translated Isc to the module data sheet Isc. Or you can see if there
