Interactions of Grid-Forming Power Converters and Synchronous Machines -- A Comparative Study

An inevitable consequence of the global power system transition towards nearly 100% renewable-based generation is the loss of conventional bulk generation by synchronous machines, 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 synchronous machines. This article reviews existing grid-forming converter control techniques and presents a high-fidelity case study that includes grid-forming converters as well as synchronous machines. We provide a performance comparison that accounts for the interactions between synchronous machines and converters and analyze the response of various grid-forming control approaches to contingencies, i.e., large changes in load and the loss of a synchronous machine. Our case study highlights the (possibly surprising) positive impact of the grid-forming converters on frequency stability and analyzes the potential limitations of each control technique when interacting with synchronous machines. Our studies also demonstrate how and when the interaction between the fast grid-forming converter, the DC source current limitations, and the slow synchronous machine dynamics contributes to system instability.

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