Schräder, Johannes

To ensure a future-proof electrical energy supply, the expansion of renewable energy sources and grid infrastructure is essential. This challenging task largely falls to distribution grid operators, who must also ensure that inverter-connected systems in Germany are tested for resonance stability in the future. This white paper provides a practical introduction to the topic, guided by the impedance-based stability criterion and findings from grid and inverter impedance measurements. Results from measurement campaigns to determine grid and inverter impedances are interpreted within the context of the stability criterion, identifying how resonance stability should be ensured in the future grid.

Zur Messung der zeit- und frequenzabhängigen Netzimpedanz werden an der Professur für elektrische Energiesysteme der Helmut-Schmidt-Universität/Universität der Bundeswehr Hamburg mobile Messanlagen für die Mittel- und Hochspannung entwickelt. Diese Arbeit beschreibt den Lösungsansatz zum Erlangen der Spannungsfestigkeit der 110-kV-Messanlage unter dem Einsatz minimaler Mittel und der Verwendung des bestehenden Designs.

Das Bundesministerium für Wirtschaft und Klimaschutz fordert bis 2030 ein Verfahren zur Prüfung der Resonanzstabilität von stromrichtergekoppelten Anlagen am Netz. Grundlage eines solchen Verfahrens ist das impedanzbasierte Stabilitätskriterium, welches die Kenntnis der Netz- und Anlagenimpedanzen erfordert. In diesem Beitrag werden die Impedanzen von Wechselrichtern, als wichtige Komponente von PV-Anlagen, unter verschiedenen Betriebsbedingungen gemessen und im Kontext des Arbeitspunkts und der gewählten Anregungsmethode bewertet. Die Messergebnisse zeigen, dass die Impedanzcharakteristik von Wechselrichtern vom Arbeitspunkt abhängig und zur Ermittlung der Mitsystemimpedanz eine symmetrische, dreiphasige Anregung des Wechselrichters notwendig ist.

Climate change is influencing traditionally stable factors such as meteorological characteristics and soil conditions, impacting the planning process of electrical energy grids, especially energy cables. Supported by real-life data from the metropolitan region of Hamburg, this study examines the sensitivity of electric energy cables to seasonal and climate related changes, aiming to address inevitable future climate impacts. Using the thermal impedance model by the International Electrotechnical Commission, combined with 32 years of local soil and weather data, permissible current levels were calculated for a specific cable configuration. Comparisons with static boundaries reveal that shifts in environmental conditions can undermine the planning process, affecting maximum current limits and casting doubt on the current method’s validity. Analysis shows that seasonal transitions significantly alter soil parameters within each annual cycle, causing up to a 10 % variation in energy transfer potential, depending on soil, cable, and regional specifics. Static standards also overestimate ampacity by up to 12 % for the studied region and timeframe. Climate change leads to shifting soil and weather conditions, causing unused energy transfer capacities, overestimations, and potential structural damage. As climate effects intensify, both seasonal and historical shifts are expected to have greater impacts, highlighting the limitations of the current static planning model without additional monitoring systems. As limited transmission capacities increasingly demand costly congestion management and equipment redundancies diminish, the need to optimize current resources and plan for a changing future becomes even more critical.

Mit dem Ausbau der erneuerbaren Energien und der Netzinfrastruktur soll die elektrische Energieversorgung zukunftssicher gestaltet werden. Diese anspruchsvolle Aufgabe liegt zu großen Teilen bei den Verteilnetzbetreibern, die unter anderem dafür sorgen müssen, dass stromrichter-gekoppelte Anlagen zukünftig auch auf Resonanzstabilität geprüft werden. Dieses Whitepaper bietet einen praxisorientierten Einstieg in das Thema, orientiert am impedanzbasierten Stabilitätskriterium und Erkenntnissen der Netz- und Anlagenimpedanzmessung. Die Ergebnisse aus Messkampagnen zur Bestimmung der Netz- und Wechselrichterimpedanzen werden im Kontext des Stabilitätskriteriums interpretiert. Daraus werden grundlegende Thesen zur Resonanzstabilität abgeleitet, die eine Einschätzung der Prüfnotwendigkeit beim Netzanschluss ermöglichen. Der Beitrag schließt mit einem Vorschlag für ein Prüfschema, das auf den gewonnenen Erkenntnissen basiert.

Grid and system impedances are crucial inputs for the integration of renewables, evaluations of system stability and novel monitoring methods. They can be determined through various setups and excitation methods. However, the increasing use of grid-connected converters creates subperiodic effects, challenging the requirement of time-invariance in most an-alytical methodologies. This paper presents a novel indicator, the Subperiodic Impedance Variance (SIV), to quantify the impact of the time-variance of a given system. Laboratory measurements and simulations with different impedance meas-urement methods of systems with varying degrees of time-variance have been conducted to estimate provisional threshold values for the application of broadband excitation impedance measurements. Extensive field measurements were analyzed to determine real-world SIV values across various grid nodes.

The harmonic grid impedance is a crucial system property utilized in various applications. However, traditional simulation-based methods are often limited by the lack of access to essential information, such as equipment characteristics, load behaviors, and grid topologies. In response, measurement-based black-box techniques have been developed, with initial commercial solutions available for low-voltage applications. Unfortunately, for the medium-voltage level, only a limited number of prototypes exist. Unlike other techniques, which require large, immobile setups, the measurement system described in measurement system is the first of its kind to be successfully applied in several research projects at various grid connection points. This paper presents, for the first time, real-world grid impedance measurements collected from multiple sites over extended time periods on the medium-voltage level. In addition to the measurement data, this paper provides comprehensive metadata for each measurement site, which is presented in measurement sites and metadata and the appendix, significantly enhancing the usability of these results for further research. The measurement results have been thoroughly analyzed, providing valuable insights into medium-voltage grid behavior. The evaluation of the measurement data in presentation and evaluation of the measurement data includes general observations comparing grid impedances at various connection points, as well as graphical and statistical evaluations of the time-dependent behavior of grid impedance. Special attention is given to resonance points, and the influence of renewable energy installations on grid impedance. Finally, the results are summarized and discussed in conclusion of the measurement results.

Impedance-based analysis methods enable a more specific and earlier foundation for assessing harmonic stability in decentralized converter-based power plants compared to the conventional compliance testing in the grid connection process. Essentially, they can be implemented as black-box model approaches without the necessity to disclose internal control models. Initially, only knowledge of the input impedances of the planned grid connection point and the planned PV system is required for application. For this purpose, the method of impedance spectroscopy for inverters has already been developed as a means to determine the effective impedance profile and the internal harmonic sources of inverters, allowing for the description of the frequency-dependent behavior of individual units. The time- and frequency-dependent grid impedance at the grid connection point (GCP) has also been successfully measured in several campaigns on medium and low-voltage grids. Through the coordinated application of both measurement methods, a predictive harmonic assessment is intended in the future, ensuring high planning reliability and grid quality even in grids with a high penetration of power electronics-coupled systems. This paper provides an overview of the current state of research on impedance-based stability criteria and presents measurement methods for practical implementation. Furthermore, it outlines remaining open questions until application in the field.

Die Übertragungskapazitäten von Freileitungen und Energiekabeln werden durch thermische Modelle bestimmt, die konservative und statische Grenzwerte mit substantiellen Sicherheitsfaktoren festlegen. Ungenutzte Potenziale verstärken die Kapazitätsengpässe der Netzinfrastruktur. Um die Überlastung von Betriebsmitteln zu verhindern, werden die teuren und nicht-ökologischen Maßnahmen des Netzengpassmanagements eingesetzt (Redispatch, Einspeisemanagement und Netzreservekraftwerke). Dieser Beitrag untersucht, inwiefern sich Erhöhungen der Übertragungskapazitäten auf den Einsatz des Netzengpassmanagements auswirken. In der 50Hertz-Zone wurde festgestellt, dass eine statische Erhöhung der Übertragungskapazität um durchschnittlich 7,86 % bzw. 3,83 % für 220 kV- bzw. 380 kV-Systeme ausreicht, um mindestens 50 % der Einspeisungsreduktion zu verhindern. Um 90 % der Reduktion zu verhindern, sind durchschnittlich 23,70 % bzw. 11,90 % für 220 kV- bzw. 380 kV-Systeme notwendig. Bereits geringe Erhöhungen der Energieübertragungskapazität führen folglich zu signifikanten Reduktionen des Netzengpassmanagements.