Distributed Secondary Control for Accurate Reactive and Harmonic Power Sharing among VSIs with Integrated Grid-Forming and Grid-Following Capabilities

The article proposes a novel hybrid control (HC) scheme that integrates grid-forming (GFM) and grid-following (GFL) functionalities within a single voltage source inverter (VSI). This HC system serves as the primary control layer, enabling voltage and frequency regulation, providing essential grid support services, and ensuring precise tracking of reference powers without relying on a phase-locked loop. However, achieving precise load sharing among HC-equipped inverter-based resources (IBRs) with mismatched feeder impedances is challenging due to the increasing proliferation of linear and nonlinear loads within islanded AC power distribution systems. To overcome this challenge, a new distributed secondary control scheme is also proposed to ensure accurate reactive and harmonic power sharing among HC-equipped IBRs. This scheme actively compensates for corresponding circulating voltage drops without prior knowledge of mismatched feeder impedances. The incorporation of an enhanced voltage control scheme featuring decoupled fundamental and harmonic voltage control branches enables the simultaneous and autonomous sharing of reactive and harmonic power among HC-based IBRs. Through extensive simulation studies conducted on a standalone distribution system employing three parallel VSIs, the feasibility and effectiveness of the proposed hierarchical control framework are validated under various operating conditions.