Harnessed PV arrays are solar installations where multiple strings of photovoltaic (PV) modules are connected in parallel upstream of the inverter, reducing the number of combiner boxes and associated wiring. This configuration offers cost savings, but it also poses challenges for performance testing. In this guide, you’ll learn how to set up, calibrate, and analyze I-V curves using advanced tools like the Fluke PVA-1500, and discover best practices for optimizing performance in harnessed PV arrays.
Why Use Harnessed PV Arrays in a Solar Installation?
Harnessing is a strategy used to reduce the number of combiner boxes and associated feeder runs by paralleling strings upstream of the inverter. The harness is typically pre-fabricated with in-line fuses, allowing more than two strings to be paralleled into a single pair of home run conductors. These conductors are then protected by another fuse in the combiner box or at the inverter. Harnessing is particularly beneficial for thin-film (TF) arrays, which usually feature higher voltages and lower module currents.
While harnessing reduces upfront system costs, it can complicate performance testing. Isolating harnessed strings for I-V curve measurements or other electrical tests is challenging, because the parallel connections are made in the array wiring harness, not at a combiner box, which removes the convenient testing point at the combiner. This makes it harder to identify issues in individual strings; the larger the number of paralleled strings, the lower the sensitivity to localized performance issues. The other issue with harnessed PV arrays is that each time a parallel connection is made, the current of the circuit is increased. These parallel connections often exceed the maximum current rating of the test device.
The Role of I-V Curve Testing
I-V curve testing is crucial for commissioning, operations, maintenance, and troubleshooting of PV arrays. It provides a comprehensive performance measurement that helps detect outliers and establish a baseline for future inquiries. The process involves tracing the I-V curve of a PV string or module using a variable load, which helps assess the health and performance of solar modules and arrays effectively.
How to Measure I-V Curves in Harnessed PV Arrays
- Setup and Calibration
- Consistent setup and calibration processes help maintain accuracy across multiple measurements.
- Begin by ensuring all array connections are secure and properly configured. The harness should be installed correctly, with in-line fuses in place.
- Using a reference senso paired with the I-V curve test tool, like the SolSensor in the Fluke PVA-1500 PV Analyzer, helps you achieve accurate measurements. The sensor should be installed to orient the irradiance sensor to the plane-of-array, and the temperature sensor should be in direct contact with the backside of a module.
- Measurement Process
- Identify what section of the array to test.
- Connect your measurement device to the string. Select the appropriate number of I-V curve points (typically 100 or 500) to balance detail and speed.
- Make sure you are aware of the number of strings paralleled. The expected power and current will be higher than a single string, so it is important to ensure the string current doesn’t exceed the maximum current rating of the I-V test tool.
- The device will sweep the voltage from zero to the open-circuit voltage (Voc) and measure the resulting current. This data is used to plot the I-V curve, which shows the relationship between current and voltage under specific conditions.
- Additionally, the device can generate the P-V (power vs. voltage) curve, Isc (short-circuit current), Voc (open-circuit voltage), Imp (current at maximum power), Vmp (voltage at maximum power), Pmax (maximum power), fill factor, and performance factor.
- Advanced PV analyzers like the PVA-1500HE2 can handle high-efficiency modules and operate efficiently in high-temperature environments.
- Data Analysis and Reporting
- Use software tools to save and analyze the measurements. The software should allow you to save your measurements by selecting branches in a customized array tree, automatically calculating the expected I-V curve and displaying the performance factor.
- Effective analysis requires compiling and comparing data over time. Using software tools that provide immediate feedback on performance metrics, like the performance factor, can help you quickly identify and address potential issues.
- Advanced software tools, like the Fluke Solmetric PVA Application and Data Analysis Tool, can validate field data and analyze performance metrics. They compile key PV parameters, flag non-conforming strings, and provide a statistical overview of the entire array.
- The Data Analysis Tool (DAT) also generates detailed reports that visualize data trends and flags inconsistencies. It can combine string I-V curves at the combiner or inverter level and generate histograms for PV parameters, which can be added to customizable reports and exported as PDFs.
Advanced Testing Capabilities
To measure high-efficiency PV modules accurately, make sure that your testing equipment is capable of handling modules with high efficiency (>19%). This is crucial — high-efficiency modules can pose a challenge for some I-V curve tracers due to their higher capacitance, which can lead to current overloads during measurements. The Fluke PVA-1500HE2 PV Analyzer, for example, is designed to handle high-efficiency modules up to 30A, delivering accurate performance assessments even under challenging conditions.
Efficient Data Analysis and Reporting
Software tools designed for comprehensive data analysis and reporting are crucial to accurate analysis and reporting. The Fluke PVA-1500’s software allows for efficient data capture in the field, and its DAT validates and presents the results in multiple formats. This tool compiles key PV parameters, flags non-conforming strings, and provides a statistical overview of the entire array. Its visualization features combine string I-V curves at the combiner box level, offering clear depictions of consistency and identifying atypical strings. It can generate histograms and customizable reports to support very detailed performance analysis and documentation.
Conclusion
Measuring I-V curves in harnessed PV arrays is a critical task that requires careful setup, accurate measurement, and detailed analysis. By following best practices and utilizing advanced tools like the Fluke PVA-1500, you can ensure the optimal performance of your PV systems, maximize energy production, and extend the lifespan of your solar installations.