Measuring instruments according to VDE-AR-N 4110 / <200 ms response time / P(f) function (active power reduction in case of frequency deviation)
Power generation controllers ensure that wind and solar farms feed into the grid at all times in a way that is compatible with the grid – and may soon also provide system services. However, this requires highly precise and dynamic measurement data in accordance with VDE-AR-N 4110/4120. There appears to be a need for further improvement here.
Photo: 50,2 Magazin
Frequency and voltage fluctuations
Jens Schübel, who measures the power quality in his home with a UMD 705E from PQ Plus, can confirm that this is no longer a theoretical problem: "I live in a rural area with many PV systems and regularly see short voltage dips of below 150 V during the summer months," reports Schübel, who is not only a trained electrical engineer but also an authorized signatory and technical director at the energy measurement technology provider PQ Plus. Such fluctuations are unproblematic for private households, as the power-connected devices used there generally have a high tolerance to voltage fluctuations. The situation is different for commercial customers who operate very sensitive electronic systems or devices. This can lead to production downtime or other expensive damage, which the responsible grid operator must always address in some form as part of the supplier's product liability for the electricity supplied. This is annoying enough for everyone affected, and in the long term, the risks may not be limited to isolated cases: "Given the rapid increase in renewable energy feed-ins – at all voltage levels – frequency fluctuations will occur more frequently and more pronounced," Schübel suspects. This could also have negative consequences for entire grid areas in the future.
The UMD 705 measuring device in an energielenker power generation controller. (Photo: Energielenker Management GmbH & Co.KG)
Standardized Power Generation Controllers and Measurement Technology
To avoid these problems, power generation controllers are used, which dynamically adjust the feed-in to requirements – whether the voltage level, the frequency, or the reactive power for voltage stabilization. The Technical Connection Rules for Medium Voltage (VDE-AR-N 4110) have been in effect for these systems since fall 2018 and have since regulated the grid connection of generating plants at lower voltage levels. The VDE-AR-N 4120 standard for high voltage has been in effect since 2015, but is currently gaining relevance as wind and solar farms are increasingly being connected to high voltage systems. Power generation controllers must be certified according to these standards, and clear specifications also apply to the measurement technology that provides the controller with the basis for control. These include, firstly, the measurement speed of the frequency measurement, which must not exceed 200 ms for rapid frequency changes. Secondly, the accuracy is defined: within a period of 200 ms, this is ±50 mHz. Those are the specifications. But do the measuring devices, some of which have been installed in wind and solar farms for many years, even consistently meet these requirements? This has hardly been questioned so far – for understandable reasons: Project developers and operators are naturally not experts in energy measurement technology and rely on service providers, manufacturers, certificates, and products that have been in use for years without any discernible problems. There has probably been little actual testing or even re-measurement.
Focus on Measurement Quality
However, project developers, plant operators, and technical service providers appear to be becoming more critical, as the technical director of PQ Plus observes. "For the first time, we have received a significant number of inquiries from customers asking whether our devices actually meet the requirements of the standard," reports Jens Schübel – and expressly welcomes this development: "Especially for volatile feeders such as PV and wind turbines, rapid adjustments are crucial for grid stability and system performance – and the controllers can only respond adequately to frequency and voltage fluctuations if they receive correspondingly high-resolution measurement values." PQ Plus sees itself as well-positioned in this area: "Our UMD measuring devices measure with an accuracy of ±10 and can even update and provide measurement values over 40 ms and 80 ms," reports Jens Schübel. Other measurement technology manufacturers have recently evaluated their own solutions, and PQ Plus has also conducted comparative measurements with competitor devices. The results are striking: “Many commonly used universal measuring devices are not optimized for this application and do not always meet the required measuring speed for precise and standard-compliant control,” summarizes Jens Schübel.
Comparative measurements between the PQ Plus UMD 705 (above) and a commonly used device from another manufacturer (below) show significant differences in the detection of frequency drops. (Photo: PQ Plus GmbH, Plexlog GmbH (measurements)
Review the technology used
This has clear consequences for operators of renewable energy plants and grid operators, says Jens Schübel: "The measurement technology used must comply with regulations – and all those involved are well advised to closely monitor that the applicable requirements are being implemented on-site – if necessary through random sampling." Of course, using more precise devices isn't making a mistake, he adds with a wink. Grid operators should also seize the opportunity to make low-voltage more transparent through suitable digital measurement technology – in order to reliably monitor frequency and voltage quality there as well. (pq)
