This paper provides a novel hierarchical control for VSI-based microgrids. The advantage of the provided control scheme is to maintain the frequency and voltage stability and load sharing against large-signal disturbances. A hierarchical control, consisting of three levels, is described. A new control loop based on PI controller, is presented. The new control loop has a great impact on increasing the stability margins, by moving the poles. In next steps, secondary and tertiary control levels are described. Then, the voltage droop equation is improved by a fuzzy controller. This controller generates a floating reactive power reference value, by a fuzzy logic. The role of floating reactive power reference value is to compensate the drastic changes in the voltage amplitude by changing the reactive power reference value. To verify the performance of the provided control scheme, a microgrid including four VSI-based DGs is simulated in islanded and grid-connected modes, by MATLAB/ SIMULINK. The simulation results show that the micogrid can maintain the frequency and voltage stability, in term of large-signal disturbances such as 3-phase and 1-phase short circuits. With this method microgrid load sharing is not altered after disturbances.

Keywords: Droop Control, Fuzzy Control, Hierarchical Control, Large-signal Disturbance, Microgrid, Power Sharing, Stability, Voltage Source Inverter (VSI).

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