Test & measurement
HOW TO MAKE SURE SOLAR PV SYSTEMS RUN AT MAXIMUM EFFICIENCY AND OPTIMUM SAFETY LEVELS
t can be all too easy to get confused by the standards that govern the installation, testing and maintenance of photovoltaic (PV) systems but the key point to remember is that while efficiency is vital, nothing is more important than safety when it comes to setting up and operating solar power installations. This means that every party involved in installing and maintaining PV systems should be driven by the goal of ensuring maximum safety while, as a secondary consideration, seeking to achieve optimum efficiency. This rule applies whether it is a residential or commercial PV installation (typically producing between 100kW and 1MW of power) or it is an extensive decentralised array of PV panels in a field or ‘solar farm’. Typically, a utility solar installation of this kind will be capable of generating over 1MW of power from a single location. Whatever the set-up may be, the efficient operation of DC/AC inverters is essential to ensuring solar systems deliver their full potential. To check the efficiency of the inverters DC input power and AC output power is measured and their ratio is calculated and compared to the manufacturer’s specification.
I INSPECTION AND TESTING
Before a PV installation is connected to the grid and put into service, it is first checked according to the requirements of the standard IEC/EN 62446 part 1. Roughly speaking, the tests fall into two categories – safety and functional. The most important safety tests are the continuity tests of the earthing conductors, because each and every metal part of the PV installation must be electrically connected to these conductors which reduces the risk of electric shocks. Also, the insulation resistance of the wiring must be checked, especially because the PV modules are usually connected in strings and can generate high voltages of 1,000V to 1,500V. Other safety tests include visual inspection of DC circuits, polarity and combiner box tests and insulation resistance of DC circuits. On the other hand, functionality tests include checking open circuit voltage (VOC – the maximum voltage that can be delivered by a module) and short-circuit current (ISC) or operational current. It is always important to know the VOC and ISC because these are key indicators of the efficient operation of a PV system. To save time, sometimes it is possible to identify problems just by looking at those readings rather than the whole current-voltage
64 By Mark Bakker, Fluke
characteristic curve (I-V curve), checking of which can be time-consuming in large-scale installations. Both these parameters are provided in the manufacturer’s data sheet and relate not to safety but solely to checking whether the installation is performing as expected. Solar PV modules are usually mounted in a
string, and the output of this string is current (I) and voltage (V). The relation between the current and voltage is not linear, but follows a curve (as shown in the figure below).
One of the most crucial elements to consider within category 2 of the IEC/EN 62446-1 standard is I-V curve testing. As well as VOC and ISC, the operator needs to check voltage, the power characteristic curve, the fill factor (FF) and current and power at the maximum power point ( MPP in the figure) The MPP is the point at which a solar module is capable of generating the most power. It can also be found in the knee of the I-V curve and is calculated by multiplying the current at the maximum point by the voltage at the maximum point. It is important to have solar modules installed according to the MPP in order to maximise output.
The MPP is also a specification given by the manufacturer and is affected by irradiance levels (the amount of sunshine), so the inverter will continuously compensate for this in order to keep the solar panel module output to the MPP. This ‘MPP tracking’ is an established behaviour of a solar inverter to keep a PV module at its maximum performance.
Also, visual inspection is important. This test, it is not measuring – it simply means checking by sight if cables are installed correctly or are sufficiently thick to deal with the energy output required, if the panels are mounted correctly and if the inverter is dimensioned correctly. This category 1 activity is an element that is often
October 2023 Instrumentation Monthly
overlooked but it is absolutely essential that the inspector looks at an entire system to check if it is working as it should. Seeing how something is installed is a vital part of any inspection. I-V curve testing also involves troubleshooting at modules/PV arrays and the comparison of the PV string with datasheet values. While each individual solar module is allowed to deviate within set parameters, a solar multi-function tester (such as Fluke’s SMFT-1000) is capable of identifying whether the module is well-mounted and performing as expected within the manufacturer’s specifications. This represents a challenge because there are more than 100,000 different types of solar modules on the market today and each type is covered by its own unique specifications that have to be met, which is why I-V curve testing is so important. If the measured curve deviates from this expected form, then this is an indication something is wrong with the PV string. If, for example, steps or notches appear in the curve (see figure, dashed line) some modules of the string may be damaged or obscured.
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