One of the main features of Microgrids is the ability to operate in both grid-connected mode and islanding mode. In each mode of operation, distributed energy resources (DERs) can be operated under grid-forming or grid-following control strategies. In grid-connected mode, DERs usually work under grid-following control strategy, while at least one of the DERs must operate in grid-forming strategy in islanding mode. A microgrid may experience remarkable fluctuations in voltage and current due to an unintentional islanding event. To achieve a smooth transition to islanding mode and mitigate disturbance effect, this paper proposes a control strategy includes a) a linear voltage controller with capacitor current feedback as an input to the voltage controller and output current feedforward as an input to current controller, and b) modified droop control to emulate the inertia response of a synchronous generator. The proposed controller can suppress voltage, current and frequency fluctuations and also guarantee a smooth transition. A small signal analysis of the proposed control strategy is developed to design its coefficients as well as the destabilizing effect of constant power load (CPL). Experimental results are provided to verify the effectiveness of the proposed control strategy.

Dual mode operation capability of distributed energy resources in microgrids is an attractive feature that makes these systems a promising solution for improving reliability and economy of the power system. However, the transition between microgrid operation modes (grid-connected and islanding) may lead to a significant deviation in voltage and current because of inconsistency in phase, frequency, and voltage amplitude. To minimize the fluctuations and provide a smooth transition, this paper presents an optimal control framework based on linear quadratic regulator. The framework includes two regulators that separately designed for each transition mode: 1) grid-connected to islanding smooth regulator, and 2) islanding to grid-connected smooth regulator. Optimality based on optimizing solution, smooth transition, ease of implementation due to its simple state feedback form and compatibility with familiar cascade loops are the main advantages of the proposed approach. Experimental results are presented to validate the efficacy of the proposed approach.