In order to offer a good power quality for end users in islanded microgrids, the power sharing between distributed generators (DGs) is the first challenge that must be ensured. In this paper, a droop control method based primary control for two parallel single-phase VSIs forming an islanded microgrid is presented. The proposed control consists of two DGs local controllers, where each controller includes voltage and current inner controllers, virtual output-impedance loop and droop control method. DG primary controller acts as synchronous generator that reduces the VSI voltage frequency when the active power increases. This concept can be applied in parallel-connected VSIs without any communication links between them, using the P/Q droop strategy. The main aim of the designed control scheme is to achieve proper active and reactive power sharing among DG units at the load bus using droop control. A Simulation study is presented in order to show the effectiveness of the proposed primary control when DG units plug-in/-out and under load variation. The obtained results have demonstrated the effectiveness and robustness of the proposed control scheme for shared active and reactive power under different operating conditions.

In grid-connected systems, a control that can ensure power flows with better performances and assure the injection of proper power quality into the utility grid is of prime interest. In this paper, an optimum design of a current controller based on Second-Order Generalized Integrator Frequency-Locked Loop scheme (SOGI-FLL) for single-phase LCL-filtered grid-connected photovoltaic VSI is presented. The main aim of this work is to ensure the grid current control with good transient response, injection of proper power quality, and optimization of the computation time. To this end, the SOGIFLL scheme which is characterized by its simplicity, good transient response, and better filtering performances is introduced into the system control. The SOGI-FLL is utilized to estimate proper filtered orthogonal components and frequency of the grid voltage and current to be used by the proposed controller which can enhance its performances. In addition, an improvement in the inner control loops, based on the manipulation of sinusoidal signals instead of dq components through PI controllers is also proposed. This improvement can optimize the computation time by eliminating some PI controllers and dq transformation blocks. Simulation results are provided for a single-phase grid-connected PV, showing the effectiveness of the proposed controller.