A novel communication-less frequency restoration technique in droop controlled inverter-based islanded AC Microgrid

Active and Reactive power is shared between the inverters in an islanded AC Microgrid (MG) using droop control technique in a decentralized manner. Deviations in the frequency and voltage magnitudes, from their nominal values, are inevitable in the droop based power-sharing. To maintain the grid code, it is important to restore these parameters to their rated values, which is basically done by a secondary control layer in the hierarchical control system of an islanded AC Microgrid. This paper proposes a communication-less frequency restoration technique in the primary layer by adopting the property of Recursive Discrete Fourier Transform (RDFT) and Inverse Recursive Discrete Fourier Transform (IRDFT) in the droop control equation. This technique replicates the frequency response of a Band Pass Filter (BPF) and thus extracts the nominal frequency if there is a drift in frequency due to the active power-sharing. The proposed technique is validated with the help of simulation studies for the parallel operation of two three-phase inverters for various test cases.