Grid impedance estimation based hybrid islanding detection method for AC microgrids

This paper focuses on a hybrid islanding detection algorithm for parallel-inverters-based microgrids. The proposed algorithm is implemented on the unit ensuring the control of the intelligent bypass switch connecting or disconnecting the microgrid from the utility. This method employs a grid impedance estimation technique that uses resonance excitation once grid fault occurs. It includes two stages: the first one is to detect grid impedance variations resulting from a grid fault; and the second one is to excite the resonance by using a virtual resistance in order to extract the grid impedance parameters. Grid impedance variations detection algorithm is based on grid current measurements temporal redundancies with fast current acquisition period. Once the grid impedance variation is detected, the excitation resonance is performed by injecting a resonance frequency in only one inverter control to avoid interactions with other units. The selected inverter will be the one closest to the controllable distributed generation system or to a healthy grid side in case of meshed microgrid with multiple-grid connections. The detection algorithm is applied to quickly detect the resonance phenomena, so that the resonance excitation is canceled and the resistive and inductive grid impedance parts are estimated. Simulation results are carried out to illustrate the effectiveness of the proposed method.