To improve the frequency stability of islanded microgrid

we propose a hierarchical frequency control scheme based on the synchronous reference frame

in which the inner control level consists of voltage and current loops for tracking quickly and improving the quality of waveforms.The virtual synchronous generator with power-frequency droop mechanism is used for primary control in order to increase inertia and improve the stability of the microgrid.Then a frequency restoring strategy

i.e.

secondary control is developed based on the internal model control.The effectiveness of the proposed control method is verified using a microgrid test system

which includes inverters with different kinds of control strategies

in PSCAD/EMTDC.The results show good robustness of the proposed secondary frequency control strategy based on the internal model control even considering communication delays.