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Parallel operation of a synchronous generator and a virtual synchronous generator under unbalanced loading condition in microgrids

Parallel operation of a synchronous generator (SG) and an inverter-interfaced distributed generator (DG) is required in some islanded microgrids. However, dynamic performance of a small SG is usually poor, i.e. the rotor speed usually deviates largely during a loading transition, due to small inertia and slow governor response. Moreover, unbalanced SG current should be prevented to protect the SG, which makes this issue more challenging. In this paper, a control method for the inverter-interfaced DG based on virtual synchronous generator (VSG) concept is proposed, to improve the overall performance of parallel operation of SG and DG. Double decoupled synchronous reference frame (DDSRF) decomposition, SG negative-sequence current compensation, modified calculation methods for power and voltage, and transient virtual stator impedance are integrated into the proposed VSG control scheme, and tuning methods of main control parameters are discussed. It is demonstrated by simulation results that the proposed DG control method alleviates the SG rotor speed deviation and DG overcurrent during a loading transition, and eliminates the negative-sequence components of the SG output current.