Frequency Stability of Synchronous Machines and Grid-Forming Power Converters

• Published in: IEEE journal of emerging and selected topics in power electronics Vol. 8; no. 2; pp. 1004 - 1018
• Main Authors: Tayyebi, Ali, Gross, Dominic, Anta, Adolfo, Kupzog, Friederich, Dorfler, Florian
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 01-06-2020; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Alternating current; Control stability; Dynamic stability; Electric converters; Forming techniques; Frequency conversion; Frequency stability; grid-forming converters (GFCs); Inertia; Load modeling; low-inertia power system; Power converters; Power system dynamics; Power system stability; Qualitative analysis; Stability analysis; Synchronous machines; synchronous machines (SMs); Systems stability; Voltage control; Windings
• ISSN: 2168-6777; 2168-6785
• DOI: 10.1109/JESTPE.2020.2966524

An inevitable consequence of the global power system transition toward nearly 100% renewable-based generation is the loss of conventional bulk generation by synchronous machines (SMs), their inertia, and accompanying frequency- and voltage-control mechanisms. This gradual transformation of the power system to a low-inertia system leads to critical challenges in maintaining system stability. Novel control techniques for converters, so-called grid-forming strategies, are expected to address these challenges and replicate functionalities that, so far, have been provided by SMs. This article presents a low-inertia case study that includes SMs and converters controlled under various grid-forming techniques. In this article, the positive impact of the grid-forming converters (GFCs) on the frequency stability of SMs is highlighted, a qualitative analysis that provides insights into the frequency stability of the system is presented, we explore the behavior of the grid-forming controls when imposing the converter dc and ac current limitations, the importance of the dc dynamics in grid-forming control design as well as the critical need for an effective ac current limitation scheme are reported, and finally, we analyze how and when the interaction between the fast GFC and the slow SM dynamics can contribute to the system instability.