Field Applications for I-V Curve Tracers
Inside this Article
While indispensible to PV cell and module manufacturing processes, I-V curve tracers have historically held a limited role in the field. Now that the technology is more widely accessible, the role of curve tracers is expanding beyond the laboratory.
Silicon PV modules are highly reliable, but performance problems do arise, and the industry needs fast and accurate ways to detect them. The stakeholders in newly built systems want to verify that all the PV modules are of a consistent quality, that they were not damaged during shipment or assembly, and that the array is producing at the contracted capacity. These stakeholders would also like a permanent record of the as-built system performance, a benchmark for comparison as arrays age and degrade—particularly in cases where warranty negotiations are required. Later in the system’s life cycle, operations and management (O&M) or asset management companies want to evaluate the health of older arrays and have the ability to efficiently locate an ailing module.
These are all potential applications for I-V curve tracers, which can provide both a qualitative visual representation and a quantitative measure of PV performance. Curve tracing equipment was developed for testing transistors and diodes in the semiconductor industry. Now it is a workhorse in PV R&D and manufacturing, for use with both individual cells and modules. It also has a long history of use in field testing of PV arrays, a use that is likely to increase in frequency as more affordable and user-friendly products become available.
In an effort to demystify I-V curve tracers, here I explain how these devices work and how they can be used to commission and troubleshoot PV arrays. The basic characteristics of a healthy I-V curve are described, as well as characteristics that indicate the most common classes of PV array performance impairments. I present rules of thumb for the successful use of I-V curve tracers in the field, which is inherently more challenging than taking measurements in controlled settings like a factory or laboratory. I also provide tips on how to avoid common measurement and data analysis mistakes. When properly attained and analyzed, I-V curve traces provide the most comprehensive measurement possible of PV module or array performance.
I-V Curve Measurements
I-V curves or traces are measured by sweeping the load on a PV source over a range of currents and voltages. Curve tracers accomplish this by loading a PV module or string at different points across its operating range between 0 V and Voc. At each point, the output current and voltage are measured simultaneously. The load presented by the curve tracer may be resistive, reactive (typically capacitive) or electronic. Field test gear uses resistive or capacitive loading, whereas reference I-V test systems at research facilities tend to use electronic loads. The I-V curve may be swept in either direction.
In field test equipment, the actual I-V measurement sweep typically requires less than a second. However, there is a sweep speed limit for certain cell types. High-efficiency cell technologies from Sanyo, SunPower and other manufacturers cannot be swept arbitrarily fast. Because these cells store considerably more charge, more time is required for the cells to reach steady-state operating conditions at each point in the curve. A rough guideline is that the sweep rate for high-efficiency cells should not exceed 10 V per second per cell.
I-V CURVE REFRESHER
I-V curves, which appear on every PV module datasheet, represent all of the combinations of current and voltage at which the module can be operated or loaded. Normally simple in shape, these curves actually provide the most complete measure of the health and capacity of a PV module or array, providing much more information than traditional electrical test methods.
A normal-shaped I-V curve is shown in Figure 1 (above). The maximum power point (Pmp) of the I-V curve—the product of the maximum power current (Imp) and the maximum power voltage (Vmp)—is located at the knee of the curve. At lower voltages, between the knee and the shortcircuit current (Isc), the current is less dependent on voltage. At higher voltages, between the knee and open-circuit voltage (Voc), the current drops steeply with increasing voltage. The output current of a typical crystalline silicon PV module drops 65% in the upper 10% of its output voltage range. It is not uncommon for an I-V curve to be displayed with its associated power-voltage (P-V) curve, which is also shown in Figure 1. The value of power at each voltage point is calculated using the corresponding current from the I-V curve. The peak of the P-V curve (Pmax), of course, occurs at Vmp.