Despite 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 longevity of equipment, safety, ROI, warranties.
Step 1: Photovoltaic system design and production
To find the expected production at your site, determine your solar resource and take into account shading. The solar resource is measured in peak sun hours, which are the number of hours you receive 1,000 watts per square meter per day. For instance, if you are in many parts of California the solar resource is great – 6,000 watts per square meter, or 6 peak sun hours. Use the Amprobe SOLAR-100 to determine the actual solar irradiance (watts/m2) and shading at the site to develop a baseline.
Let’s say you have a 10 kW PV array. You can calculate expected annual production by multiplying the 10-kW array x 6 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 for us per year, or 51 kWh per day.
Step 2: Measuring
Once your system is installed, make sure it is 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, it maximizes its power output by capturing the current and voltage - since power is voltage x current - at which the string is producing the most power. 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 and 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, you need to know the Voc and Isc, which are listed on the module datasheet. measure the Voc and Isc before and after installation.
Voc is measured by using the Fluke 376 FC True-RMS Clamp Meter or the Fluke 325 True RMS Clamp Meter to determine the voltage between the positive and negative terminals. 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). Also check the conductor polarity while testing Voc. If it is reversed it can mean that in the combiner box 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.
Use your system’s computer interface to read the actual power output of the array.
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 GEO Earth Ground Tester to measure earth ground resistance to ensure a resistance of less than 5 ohms.
Step 3: Comparing and diagnosing
Even when installed correctly, a PV system may not be producing the expected electrical production. It is 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 not output any power.
Scenario 1: Open circuit voltage or short circuit current is higher or lower than on the datasheet
In this case, your string has a module(s) whose characteristics are not to specification. Open circuit voltage out of range means your inverter may not output power. Short circuit current out of range indicates you may have module mismatch, which can severely degrade your 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 you see that your power output is lower than expected, you may have 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 fire. Thermal imagers can quickly identify hot spots.
Ground faults are another, but they are harder to diagnose and require testing the voltage and current of each conductor and the equipment grounding conductor (EGC), which carries stray current to ground. If there is voltage and current on the EGC, it is an indication of a ground fault. Ground faults 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 your location. Use a solar pathfinder to find any new sources of shading and remove, if possible. While it may not be feasible to change the tilt and compass direction of the array to point more directly at 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 1555 FC 10 kV Insulation Tester, which can test up to 10,000 volts.
For systems with batteries, compare the expected battery voltage and state of charge with the actual using the Fluke 500 Series Battery Analyzer.
About the expert
Michael Ginsberg is a solar energy expert, trainer for the U.S. Department of State, author and Doctor of Engineering Science candidate at Columbia University. He is also chief executive officer of Mastering Green, where he has trained nearly a thousand technicians worldwide in solar PV installation, maintenance, and operation.