It is essential to understand the complicated safety and measurement standards and categories when setting up and maintaining large photovoltaic installations, says Mark Bakker, Field Application Engineer at Fluke.
The most important standard that engineers working on high-voltage DC environments such as grid connected photovoltaic (PV) systems need to be aware of is IEC 62446-1, which specifies the tests that need to be completed before a PV system is put into service.
Although installers generally know about IEC 62446-1, it is prone to being misunderstood and/or misinterpreted. There is also a significant degree of confusion within the sector around measurement safety categories, especially when it comes to how they relate to overvoltage categories.
Divided into two categories
One key element that has the potential to confuse engineers is the fact that the testing of PV systems under IEC 62446-1 is divided into two categories, depending on the size of an installation. Category 1 relates to all types of PV generation systems whereas Category 2 is specifically to do with more vast and complex solar-based power plants (these are not to be confused with CAT I and CAT II measurement categories which are explained later). Under Category 1, a continuity test must be carried out to check if there are any DC earthing issues. The polarity of cables must also be confirmed before they are hooked up to the system.
Finally, a functional test is required to confirm that such control devices as circuit breakers and relay switches have been fitted correctly and are working effectively.
Other tests include checking insulation resistance and testing the combiner box to ensure all PV module ‘strings’ are connected properly. To check the voltage of the connected modules, a strings open circuit voltage test is required while a string circuit current test (short circuit or operational) confirms if there are any major areas of concern.
I-V curve tracing
Once all the Category 1 checks have been completed, Category 2 inspection can go ahead. Essentially, this involves using an I-V curve tracer to carry out a current-voltage test and inspection via a thermal camera to check heat generation of PV modules. The output of a string of solar PV modules is measured in current (I) and voltage (V) and the relation between the two follows a curve which provides a graphical representation of the I-V status.
Whether an operator is working on a small-scale solar system or a vast array in a field, I-V curve tracing is integral to the evaluation of PV module performance and the diagnosis of degradation in power output. As well as providing analysis based on I-V curves, multi-function devices like Fluke’s SMFT-1000 PV Tester, Performance Analyzer and I-V Curve Tracer can measure grounding continuity, polarity, open circuit voltage (VOC) and short circuit current (ISC), insulation resistance, DC and AC power, inverter efficiency and diodes.
The SMFT-1000 is designed for VOC measurement at the PV module/string up to 1000V DC and ISC measurement up to 20A DC. The device shows on-screen comparisons of on-location I-V curve results with manufacturers’ I-V curve data (ie the expected results curve), enabling operators to identify instantly if there are faults that could be affecting performance based on any deviations against the benchmark. The power vs voltage curve is also used to outline the peak or maximum power output benchmark, with the I-V curve evaluated to optimise performance so that it intersects with the maximum power peak (MPP) for maximum power generation.
If the measured curve deviates from the expected form, this is a clear indication that there is a problem in the PV string. For example, steps or notches in the curve would indicate that some modules of the string may be damaged or compromised in some way. Equally, if the current starts at a lower value, the modules may be degraded, the module is operating outside the set parameters, there are faulty connections or there is a manufacturing flaw. With the I-V curve data to hand in real-time, the operator is able to correct for those issues and re-do the measurement.
In addition to Category 1 and 2 tests, other tests that can be carried out include wet insulation tests, shade evaluation, blocking diode tests and resistivity measurements using a loop impedance meter.
A key factor in the installation and maintenance of any PV system is the documentation that is required to prove that the work has been carried out according to the relevant standards. It’s not just that a solar installation will not be allowed to operate without the correct reports; engineers will also not be paid until everything has been logged and recorded as required. Data management and reporting software such as Fluke’s TruTest is designed to work seamlessly with the SMFT-1000 and can be customised to meet a user’s specific documentation needs. Software like TruTest enables all data to be saved automatically and reports to be uploaded to a PC via USB. Reports can then be made available to all relevant parties – including, crucially, insurance companies.
Measurement categories under IEC 61010
Measurement categories – referred to as CAT II, CAT III and CAT IV – relate to the amount of energy that could be present at any point in a circuit (CAT I has been renamed as ‘Measuring circuits without a measurement category rating‘). This includes transient voltages, also known as surges or spikes, that propagate through the network from the point of impact that can cause normal voltage levels to be exceeded through, for example, an indirect lightning strike on a power line. CAT III focuses on electrical installations in buildings, such as industrial equipment, switches and circuit breakers, and solar installations are CAT III environments.
Within a measurement category, a higher working voltage rating also denotes a higher transient withstand rating. For example, a CAT III 1500V meter is resistant to much higher energy transients (and therefore offers better protection) than a meter rated CAT III 1000V.
Worst-case scenario
When choosing measurement tools for PV panels, it’s worth considering the worst-case scenario. First, choose a meter rated for the highest category you could be working in. Then, look for a voltage rating that matches your needs. Choosing the right CAT rated tool for your environment helps you and your team make reliable measurements while reducing risk.
The 393 FC Solar Clamp Meter meets the safety requirements for test equipment (under IEC 61010-2-032) corresponding to the overvoltage category level of the PV array electrical installation (covered by the IEC 61730-1 standard mentioned earlier). The clamp meter delivers safe and accurate voltage measurement up to 1500V in solar and battery applications, with several features designed to make the job easier. These include IP54 protection for dusty and rainy outdoor conditions and Fluke Connect software which gathers and stores data in the cloud for access anywhere.
Looking into the IEC 61010 standard further, IEC 61010-031 is specifically to do with test and measurement accessories while IEC 61010-2-030 3rd Edition relates to requirements for test equipment with measuring circuits. The standard clearly states that measurement categories are based on locations on the mains where measurements can be made. In addition, the IEC 61010-2-032 standard relates to current clamps that must be checked to measuring category CAT III.
When measurement equipment is connected to a PV system it is in turn connected to the external electric grid via an inverter. This configuration requires measurement equipment that satisfies measuring category CAT III. However, measurements that are carried out according to the IEC 62446-1 standard are done so on a PV panel string that has been disconnected from the inverter. Clearly, when there is no connection, there can be no electric events transmitted from outside and so the measurement category CAT requirements no longer apply. Exceptions are large-scale PV systems, where overvoltages can be introduced via cables. That’s why a measuring device with measurement category CAT III should always be used in these areas.
Conclusion
With all these various standards, categories and sub-categories, the obvious questions are why do we need them all, what is their purpose? The short answer is safety and functionality. The safety of operators working on a PV installation is paramount. At the same time, the reliability of that installation is essential to ensure not just efficient operation but also electrical accidents are eliminated that could have potentially catastrophic results.
It is much easier and safer to work with CAT III equipment in PV systems than try to determine whether a CAT II rated device will be sufficient for the job. All-in-one equipment such as Fluke’s SMFT-1000 mentioned earlier is certified to measurement CAT III and can be used safely for up to 1000V. It can never be over-emphasised that large-scale solar PV systems will always require measurement equipment that complies with category CAT III rating.
Finally, although safety and measurement categories in standards that govern the setting up and maintenance of large photovoltaic installations may be confusing, they needn’t be daunting. The number of PV installations around the world is increasing rapidly and engineers tasked with setting them up need to look to make their lives much easier by making sure they have the correct tools for the job and a thorough understanding of all the various standards and categories. If the wrong equipment is used, it can either become seriously damaged or, worse still, there could be fatal consequences. Engineers who work with a reliable supplier of top quality test equipment with decades of experience, such as Fluke, will have the peace of mind that comes with knowing they will always be adhering to the relevant standards that make PV testing safe.
