Robust Stability Analysis of the Simplified Virtual Synchronous Compensator for Grid Services and Grid Support

Grid-connected converters that are controlled according to the Virtual Synchronous Machine (VSM) concept can provide ancillary services (e.g., the inertial behavior and the grid support during faults). The VSM is typically called VSG when it is in charge of processing the full power generation of the converter. The VSM can also operate as a Virtual Synchronous Compensator (VSC) when the virtual part only provides ancillary services and the conventional control structure of the converter is in charge of the power generation. This way, the VSC always operates at a very small load angle, showing higher damping and better transient stability with respect to a VSG. Independently of the operation mode (VSG or VSC), plant uncertainties such as wrong grid impedance estimation, grid reconfigurations and the influence of neighboring converters might affect the stability of grid-connected converters. Recent papers applied the µ-analysis to the VSG to study its robust stability for a given set of plant uncertainties. Nevertheless, the differences in terms of robustness between a VSC and a VSG have never been investigated in the literature. Therefore, this paper proposes a robust stability analysis applied to the Simplified Virtual Synchronous Compensator (S-VSC), a solution available in the literature that can operate as a virtual compensator or as a generator. The theoretical analysis demonstrates that a VSM working as VSC is more robust than the VSG mode operation. Moreover, this paper provides a method to experimentally validate the µ-analysis outcomes that confirm the theoretical results.