Sharing sequence components of reactive power in a three-phase four-wire islanded microgrid

Abstract

Under any load disturbances, the microgrid must maintain its voltage and frequency within the standard norms. In an isolated microgrid, under-loading and overloading conditions may emerge if the sequence components of powers are not appropriately shared by the power electronics interfaced distributed generation systems (DGSs). Hence, this paper tends to offer a droop-based control strategy for a three-level neutral point clamped (NPC) inverters-based islanded microgrid that enables supply and maintains power-sharing effectively. The proposed method constitutes enhanced voltage and current controllers, sequence components-based virtual impedance (VI) loop, and frequency restoration loop. With the proposed droop-based control, an equal and proportional power-sharing is achieved. Furthermore, the presented VI loop allows the connected DGs with uneven line impedances to share the positive and negative sequence components of the reactive power under unbalanced loads. Besides, the upgraded voltage controllers in the stationary reference frame ensure that the DG's terminal voltages remain balanced, even when the loads at point of common coupling (PCC) demand uneven currents, while the frequency restoration loop maintains the system frequency very close to the nominal value (i.e., 50 Hz). Various scenarios such as equal and proportional power sharing and multiple numbers of DGs are considered in the study to validate the efficacy of the proposed control scheme. The simulation results are carried out in MATLAB/Simulink environment to demonstrate the efficiency of the suggested control scheme.