Even with great engineering, no system is failproof. That’s where commissioning comes in, establishing a baseline of performance for customer acceptance and follow-on maintenance. Commissioning is important not only for photovoltaic (PV) system performance, but also for the longevity of equipment, safety, ROI, and warranties. Fluke is experiencing increasing demand for high-precision handheld devices which can measure photovoltaic (PV) systems. This is the procedure.
Step 1: Photovoltaic system design and production
To find the expected production at the site, determine the solar resource and take into account any shading that may occur on the panels. The solar resource is measured in peak sun hours, which is the number of hours the installation achieves 1000 W/m2 per day. If the solar resource is good this will be 6000 W/m2 or six peak sun hours.
The Fluke IRR-1 solar irradiance meter can be used to determine the actual solar irradiance (W/m2) and shading at the site to develop a baseline. Let’s say it’s a 10 kW PV array. Calculate the expected annual production by multiplying the 10 kW array x six peak sun hours x 365 days per year x 0,85 (15% derating due to power losses in wiring and inverter). This array should produce 18 615 kWh of energy per year, or 51 kWh per day.
Step 2: Measuring PV performance
Once the system is installed, make sure it’s operating as designed by measuring its electrical characteristics and the actual power output of the array. The performance of a PV array is based on its current-voltage (IV) curve. Not only does an inverter convert DC to AC, but it also maximises its power output by capturing the current and voltage at which the string is producing the most power, since power is voltage x current. The short circuit current (Isc) is the maximum current from a cell, and no power will be produced because there is no voltage difference; the positive and negative wires are touching. The open circuit voltage (Voc) is the maximum voltage from a cell. No power will be produced because the circuit is open. The point at which the module produces the most power is called the maximum power point (mpp).
Current-voltage (IV) curve of a PV module.
To know if an array is working as designed, know the values of the Voc and Isc, listed on the module datasheet. Measure the Voc and Isc before and after installation.
Voc is measured by using the Fluke 393 FC CAT III solar clamp to determine the voltage between the positive and negative terminals. The 393 FC is CAT III 1500 V/CAT IV 600V rated, making it safe and reliable for making measurements in CAT III environments like solar installations. Use the Fluke 64 MAX IR thermometer to determine the temperature of the module to account for the effect of temperature on Voc (the lower the temperature, the higher the voltage and vice versa). The 393 FC provides audio polarity warning while testing Voc. If it’s reversed, the combiner box or other circuits may be unintentionally connected in series, resulting in voltages over the maximum inverter input voltage.
To test Isc, disconnect all parallel circuits, and safely short the circuit. Measure the current between the positive and negative terminals through a multimeter. Set the dial to a current greater than expected. Record the values of Isc and Voc on the Fluke Connect app and save them for trending and reporting
Check the insulation resistance of your conductors, the connections between modules and between modules and racking, and your resistance to ground. Use the Fluke 1625-2 FC earth ground tester to measure earth ground resistance to ensure a resistance of less than 25 ohms.
Step 3: Diagnosing variances
Even when installed correctly, a PV system may not meet the expected electrical production. It’s very important for a module to have the electrical characteristics specified, because an inverter has a minimum and maximum input current, below and above which it will have no power output.
Scenario 1: Open circuit voltage or short circuit current is higher or lower than on the datasheet
In this case, the string has one or more modules whose characteristics don’t meet specification. Open circuit voltage out of range means the inverter may not output power. Short circuit current out of range indicates there may have a module mismatch, which can severely degrade the array’s performance because the current of a string is limited by the module with the lowest current. Identify and replace the modules.
Scenario 2: Power output is low
If the power output is lower than expected, there may be a problem. While some fluctuation in output is expected, consistently less than predicted output could be a sign of a faulty string, a ground fault, or shading. One reason could be hot spots − the accumulation of current and heat on a short-circuited cell, leading to reduced performance and possible fire. Thermal imagers like the Fluke Ti480 PRO infrared camera or the Fluke TiS75+ thermal camera can quickly identify hot spots.
Ground faults are another reason, but they’re harder to diagnose and require testing the voltage and current of each conductor and the equipment grounding conductor (EGC), which carries stray current to the ground. Voltage and current on the EGC indicate a ground fault. Ground faults can occur due to damaged conductor insulation, improper installation, pinched wires and water, which can create an electrical connection between a conductor and the EGC. Find the source of the problem and replace the damaged wires or improve the conditions.
Other reasons for low power output could be shading, and poor tilt and compass direction (azimuth angle) for the location. Use a solar pathfinder to find any new sources of shading and remove them if possible. While it may not be feasible to change the tilt and compass direction of the array to point the panels more directly toward the sun, you should know the tilt and azimuth angles to establish a baseline for future reference.
In large-scale PV systems, the power from a solar system goes through transformers after being inverted to step up the voltage, then to switchgear and medium voltage cables, where decreased insulation resistance is a common issue. For medium and high voltage cables, use the Fluke 1555C FC 10 kV insulation tester, which can test up to 10 000 V. For systems with batteries, compare the expected battery voltage and state of charge with the actual using the Fluke 500 Series battery analyser.
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