Effect of Short-Circuit Ratio and Current Limiting on the Stability of a Virtual Synchronous Machine Type Gridforming Converter

Abstract

This paper analyzes the impact of ac system strength on the stability of a virtual synchronous machine (VSM) type grid-forming converter. A grid-forming VSM without additional control loops, such as overcurrent limiting, is first considered and used as a reference for comparison with a more realistic VSM with current limiting control loops. A small-signal model of the system is developed for both cases. The models are validated by comparing their transient responses with a detailed electromagnetic transient (EMT) simulation. Subsequent analysis using the developed small-signal model shows that a VSM type grid-forming converter with current limiting capability exhibits poorer damping when the connected ac network has a high short-circuit ratio (SCR). This poor damping arises from the addition of the current control loop. The paper will show that when the current controller is absent, the SCR of the ac network has minimal impact on the stability behavior of the VSM.