Volume 71, Issue 12 - Special Issue: Dynamic Security Assessment of Electric Power Systems; Guest Editors: Martin Wolter and Fábio Ecke Bisogno

Zusammenfassung Ein stabiler Stromnetzbetrieb ist heute ohne die Synchrongeneratoren der konventionellen Kraftwerke nicht möglich, weil die derzeit dominierende Art der Wechselrichterregelung erneuerbarer Energien auf eine extern gebildete Netzspannung angewiesen ist. Ein grundlegender Paradigmenwechsel in der Wechselrichterregelung ist daher nötig, um in Zukunft rein wechselrichterbasierte und klimaneutrale Stromnetze zu ermöglichen. Dieser Beitrag zeigt die zukünftige Bedeutung dieser neuen Art der Regelung von Wechselrichtern für die Systemstabilität und damit verbundene Einschränkungen auf.

The present paper introduces a hybrid method based on both the Lyapunov’s direct approach using the Potential Energy Boundary Surface method and parallel time-domain simulation for fast power system dynamic security assessment. The main contribution in the presented method is an approach that combines the directional derivative of the potential energy function and the maximum potential energy criterion in order to reduce the conservativeness in the estimation of the stability region at the direct method stage. For detailed investigation of the critical contingencies, a parallel approach is applied at the time-domain simulation stage. From the simulation results, the proposed method achieves a high level of accuracy in classifying network contingencies and shows great computational performance potential in light of the requirement for fast dynamic security assessment.

The phase-out of nuclear and coal-based electricity production and their replacement by power-electronics based renewable energy sources, especially in Germany, leads to more and faster power system dynamics. Anyway, not all European countries plan to abandon bulk power generation, so there will still be a mix of inverter-based sources and synchronous machines in the future, justifying the necessity of rotor angle stability analysis. This task grows with the number of assets. It requires to numerically solve a large set of ordinary differential equations which gets more and more challenging to do in real-time during contingency analysis in power system operation. Therefore, this paper introduces a new approach to quickly estimate the transient stability of rotor angles without the need of solving differential equations. It is based on a linearized system description, some assumptions on the rotor angle displacement and uses the fault location and duration as parameters. It is proven, that this approach can be used to reduce the number of contingencies which need to be analyzed in detail during system operation and therefore helps transmission system operators to keep the power system stable and secure.

As renewable energy sources are increasingly integrated and distributed in the power system, the number of converters in the grid is growing. This leads to more and more dynamics in the distribution networks and may result in more frequent disturbances such as voltage fluctuations, power quality problems, etc. To prevent possible threats to the security and stability of the network due to the above issues, Dynamic Security Assessment (DSA) is necessary for a distribution network to ensure its safe and reliable operation. In this work, an innovative approach to DSA using Digital Twin (DT) technology, with a primary focus on small signal stability (SSS) analysis using an eigenvalue method is proposed. The DT of the distribution network is first constructed by incorporating machine learning algorithms and updated in real-time using data from sensors and SCADA systems to accurately represent the behaviour of the physical system. The DT will be used to continuously monitor and assess the stability of the distribution network characterised by the presence of power electronic converters. The DSA then uses SSS to analyse network behaviour under different operating conditions and identify potential risks or disturbances. This can include assessing the impact of sudden changes in load or renewable energy output, detecting potential faults or equipment failures, and identifying potential instabilities in the power system. Numerical case studies are used to verify the viability of the proposed approach. As a result, by integrating a DT into the DSA process for converter-based distribution networks, operators can improve the accuracy and efficiency of their security measures, reduce the risk of outages, and increase the overall reliability of the system.

Modern power systems require special features from the distributed energy resources especially to meet the power quality and dynamic security. The distribution static synchronous compensator (DSTATCOM) has a safe DC voltage operating range that is often unexplored. Self-excited induction generators (SEIGs) with a shunt DSTATCOM is a well-known solution for low-power, low-cost generation. These systems may have poor robustness against surges and contingencies. This paper proposes an ancillary controller as a supplement in the shunt compensation of an off-grid distributed generation system powered by SEIGs. The controller consists of a variable structure that uses an adequate DC bus capacitor voltage range to provide fast frequency response (FFR) with soft recovery. The control performance for fault rejection is evaluated as well as for load disturbances and generation contingency. Simulation results are presented to demonstrate system operation and compensator performance.

Zusammenfassung Die Grundlage eines resilienten Energieversorgungssystems bildet u. a. eine zuverlässige Netzsicherheitsbewertung. Der Abstand des aktuellen Netzbetriebspunktes zu den Grenzen des ihn umgebenden sicheren Betriebsbereiches kann hierbei durch die Optimierung von Blindleistungsarbeitspunkten verbessert werden. Bei der Wahl einer spannungsabhängigen Blindleistungsbereitstellung, der sogenannten Q ( U )-Regelung, müssen Umrichterinteraktionen von dezentralen Energieanlagen berücksichtigt werden. Dieser Beitrag widmet sich der Bewertung von Q ( U )-Charakteristiken in Hinblick auf veränderliche Netztopologien (N-1-Fälle) und Betriebsfälle. Im ersten Teil wird ein Stabilitätskriterium aus Vorarbeiten aufgegriffen, das auf einem zeitdiskreten Modell der Q ( U )-Regelung beruht, um zu beurteilen, wie Anstiege und Filterzeitkonstanten der einzelnen Energieanlagen gewählt werden können. Das Verfahren wird um die Berücksichtigung relevanter Netzzustände erweitert, die sich aus der Kombination von Topologiestörungen und Betriebsfällen ergeben. Der zweite Teil des Beitrages widmet sich der Applikation des Kriteriums unter Berücksichtigung dieser vielseitigen Netzzustände. Dabei werden Strategien zur Auswahl einer hinreichenden Anzahl an Netzzuständen hinsichtlich ihrer Kritikalität diskutiert. Als Bewertungsgröße wird dann die Filterkonstante einer Anlagenregelung eingeführt und die Kritikalität der Netzzustände anhand dieser verglichen. Abschließend erfolgt die Anwendung auf Benchmarknetze der Mittel- und Hochspannungsebene.