Two-stage filtration for decentralized frequency regulation and stability improvement in economically dispatched virtual synchronous generators within islanded microgrid

Incorporating the incremental cost (IC) function into the local control loop of dispatchable virtual synchronous generators (VSGs) facilitates decentralized economic dispatch (ED) in islanded microgrids (MGs). This approach optimizes power production expenses while simultaneously delivering virtual inertia support to improve frequency stability. Nevertheless, the scheme's decentralized structure introduces persistent frequency deviations, necessitating secondary frequency regulation. Current decentralized secondary control strategies address these deviations but compromise the ED objective. This paper presents a practical decentralized control framework for grid-forming VSGs, utilizing a feedforward integral controller and a high-pass filter to overcome this challenge. The framework restores the islanded MG frequency while maintaining decentralized ED. Although the structure ensures ED, a minor, manageable frequency error is introduced to sustain the decentralized nature. Additionally, while embedding IC into the VSG reduces the rate of change of frequency (RoCoF) by compensating for inertia deficits, increasing inertia can shift the system's eigenvalues towards the real axis, leading to power oscillations. Due to the IC's nonlinearity, these oscillations become more pronounced as load demand rises, ultimately degrading MG stability. The proposed framework employs a two-stage filtration mechanism to reduce the RoCoF without increasing inertia gain, thereby enhancing control over VSG dynamics. Additionally, it decouples the IC nonlinearity from the MG's frequency dynamics, thereby improving MG stability. Comprehensive small-signal stability analyses validate the stability improvements. Time-domain results and real-time control-in-the-loop experiments, using OPAL-RT on an IEEE 38-bus MG system, confirm the controller's effectiveness. The analysis includes testing the proposed model against diverse disturbances, such as load fluctuations, line disconnections, and generator outages. Moreover, extensive simulations on a larger system, reinforced by theoretical insights, showcase the controller's scalability. Additionally, the study dives deep into how tuning individual parameters influences key performance measures like frequency deviation and RoCoF, offering practical guidance for real-world applications.