Electricity and Control May 2023

PLANT MAINTENANCE, TEST + MEASUREMENT

Commissioning, monitoring and maintaining PV systems High precision handheld devices used to measure the solar irradiance on photovoltaic installation sites and the performance of the system are increasingly in demand. Fluke, locally represented by Comtest, provides a range of tools that support initial design and the safe and effective installation, commissioning and maintenance of PV systems.

I mportantly, when commissioning a new installation, the baseline performance should be established – for customer acceptance as well as for ongoing maintenance. This also serves to optimise the longevity of equipment and ensure safety, ROI, and warranties. System design and the solar resource The first step in considering a new installation is to determine expected solar energy output at the site by measuring the solar resource and taking into account any shading of the panels that may occur. The solar resource is measured in peak sun hours, which is the number of hours the installation achieves 1 000 watts per square metre per day. If the solar resource is good, this may be 6 000 watts per square metre, or six peak sun hours. The Fluke IRR-1 Solar Irradiance Meter can be used to determine the solar irradiance (watts/m 2 ) and shading at the site to establish a baseline. For example, in a 10 kW PV array, the expected annual production can be calculated by multiplying the 10‑kW array x 6 peak sun hours x 365 days per year x 0.85 (allowing 15% derating due to power losses in wiring and the inverter). This array should produce 18 615 kWh of energy for use per year, or 51 kWh per day.

Commissioning a photovoltaic system for maximum performance.

and the power output of the array should be measured to make sure it is operating as designed. The performance of a PV array is based on its current voltage (IV) curve. The inverter used in PV systems to convert dc to ac also maximises 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 in this state 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). The values of the Voc and Isc will be listed on the module datasheet and, to check that an array is working as designed, the Voc and Isc should be measured before and after installation, to ensure they align with the specifications. Voc can be measured 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 1 500 V / CAT IV 600 V rated, which makes it safe and reliable for making measurements in CAT III environments like solar installations. The Fluke 64 MAX IR Thermometer can be used to determine the temperature of the modules to account for the effect of temperature on Voc (the lower the temperature, the higher the voltage and vice versa).

Measuring performance Once the system is installed, its electrical characteristics

The current-voltage (IV) curve of a PV module.

22 Electricity + Control MAY 2023