PV plants are not built to last an eternity, but they still have to serve their purpose fault-free for a very long time. Regular plant inspections are therefore a basic requirement for economical operation. The previous articles dealt with visual inspection systems at the module level and their importance. We now shift the focus to more thorough measurement-based inspections.
But first, a few words on the current situation and price developments in the solar panel market. For standard mainstream modules and high-efficiency products, prices have flattened out due to a certain degree of oversupply. For a number of more-or-less well-known brands, PV modules are beginning to pile up in Rotterdam. This is due in part to the long delays in customs processing and even more so to the fact that ordered modules are not being collected, or only with very long delays, because other components are missing. The result is that large quantities of modules ordered for projects are coming onto the market at moderate prices. All-black modules and solar shingles are still very difficult to get. Rather than waiting and thumb-twiddling until promised deliveries arrive, it makes sense to dedicate resources to servicing installed systems, routinely checking existing installations and taking a closer look at whether yields are falling.
Photovoltaic installations are electrical systems in the sense of DIN VDE 0100. This includes all of the electrical components of the photovoltaic system, which is equipment covered in DIN VDE 0100-712 and 0100-600. And let’s not forget the requirements outlined in VDE 0105-100 and VDE 0126-23 (DIN EN 62446) which prescribes a periodic inspection, in which generally accepted best practices should be observed in the version in the standard valid at the time of the construction of the electrical system. The electrician in charge of the installation bears responsibility for evaluating and implementing the details of the applicable standards.
These obligatory system tests create a factual basis designed mainly to enable safe operation of the system, but they also ensure economical operation of the plant. This testing ensures that equipment malfunctions such as faulty solar modules, inverters, increased contact resistance due to loose terminals or connectors, and visually undetectable degradation, can be detected in good time. This helps to keep repair costs down by eliminating damaged components as soon as possible.
Unfortunately, this prescribed and continuous system inspection is not always carried out. When faulty system components are detected, the potential faults should be narrowed down in advance with the electrician in charge, and the exact approach to dealing with the problem should be discussed. This applies not only to large rooftop and ground-mounted systems, but also to small and medium-sized rooftop systems. The inspection always starts with visual inspection of the plant components, followed by testing and measurement. All AC and DC circuits have to be tested in line with the requirements contained in the standards.
One test that is very often underestimated and neglected is the continuity test. This applies not only to grounding and equipotential bonding connections in the electrical system, but also includes switches, fuses, electrical connections, conductors and other components. Evaluating resistance gives an indication of the voltage drop that can be expected. Proper grounding of photovoltaic systems reduces the risk of electric shock and the effects of direct or indirect power surges.
The insulation resistance of electrical equipment is also essential for personnel and system protection in photovoltaic installations. Insulation resistance also provides valuable insights about the electrical condition of all the system components. However, insulation resistance is always subject to natural aging as well as moisture, dust, installation errors, rodent damage, etc., which can have a significant impact on the insulation strength. Discrepancies in the applied safety thresholds (0.25 MΩ, 1 MΩ, 2 MΩ) are often present because these values merely describe the state. In practice, higher values should be achieved. In outdoor applications, for instance, insulation values must have adequate insulation even under varying weather conditions, such as changing heat and humidity levels. The experience of the electrician is often required here to make a final assessment and decide whether the system can continue operation.
Another measure for quality tests of installed solar modules and for determining output (related to STC values for checking compliance with manufacturer performance guarantees) is a field I-V curve measurement. In addition to the basic values of current, voltage and power, other parameters of the solar panels can be measured or calculated. For example, when measuring at the string level, the series resistance provides information about cabling and connectors errors. The parallel resistance allows conclusions to be drawn about fill factor, i.e. panel quality. The measurement of the internal series resistance at the module level checks the cell contacts, connectors and cables. This can only serve as an indicator, however, since in the vast majority of warranty or guarantee cases performance measurements are required at the module level and these must be carried out under laboratory conditions or ideally by an accredited laboratory. A number of conditions must be met in the field in order to achieve the accuracy required by the manufacturer for a warranty claim. With the right measuring equipment, it is usually possible to perform a combination of measurements that allow the open-circuit voltage, polarity and short-circuit current to be verified.
In order to get a quick overall impression of the array, drones equipped with thermographic cameras are often used to overfly the plant. With hot spots thus made visible, conspicuous parts of the plant can be evaluated in advance and later investigated in a more targeted manner. This thermographic inspection includes checking electrical wiring, distribution boards, fuses, switches, inverters, transformer stations, connectors and electrical connections. This safe inspection of electrical system components can reveal faults fairly quickly and identify possible causes such as loose contacts or damage caused by surges. This inspection thus contributes significantly to fire protection in the electrical system, while ensuring power generation availability and checking the quality of the solar panels. Thermographic imaging is a measuring technique that provides a lot of benefit with comparatively little effort and can be employed during system operation.
Electroluminescence (EL) testing of solar modules is also increasingly becoming a quality standard for commissioning and inspection of existing systems in the field. Electroluminescent radiation, which is invisible to the naked eye, is emitted by reverse current flow in modules and can be detected by highly sensitive cameras. If a connection to the cell area is degraded or completely broken, low-level current flows, which shows up as a dark spot on the EL image. Mechanically damaged cells in which so-called microcracks are present are one of the most common defects in existing plants and can be caused by various factors. The EL method cannot be used to directly infer performance, however.
These camera-based inspection methods are a viable approach to quick and cost-effective identification of manufacturing defects or faults caused by storms, hail, transportation and assembly in solar panels that would not have been detected by ordinary STC measurement or thermographic inspection.
These examples show that photovoltaic systems are complex technical installations and have a wide range of different electrical equipment. Standards-compliant installation and periodic testing of the system should be the norm. In our experience, this is unfortunately often not the case and these essential inspections are often skipped for cost or other reasons. A detailed plant audit and metrological inspection with the above-mentioned methods often offers great potential for plant optimization. In the course of the inspection, any causes for warranty claims should also be checked and verified. Furthermore, there may be additional repowering potential to expand the existing plant.
The co-author of this article, Falko Krause, is co-founder and CTO of GME clean power AG. As a TÜV-certified appraiser, he is nationally and internationally active in technical advice, system planning and quality assessment. GME clean power AG focuses on optimizing existing systems (revamping & repowering) and developing ecologically and economically sensible photovoltaic projects.
Overview of price points broken down by technology in August 2022 including changes over the previous month (as of 15 August 2022):