A Small-AC-Signal Injection Based Decentralized Secondary Voltage Control Method for Multi-Bus AC Microgrids

In AC microgrids, grid-forming inverters with droop control provide voltage and frequency support but suffer from steady-state voltage deviations, necessitating secondary voltage restoration. To eliminate the need of extra communication networks, this paper proposes a decentralized secondary voltage regulation strategy utilizing small-AC-signal injection for multi-bus AC microgrids. In the proposed secondary voltage control scheme, a droop characteristic is established between the small-AC-signal frequency and the estimated global voltage magnitude, while another droop characteristic links the small-AC-signal active power to the estimated voltage. By means of these droop relationships, the estimated global voltage magnitudes converge to the arithmetic mean of the node voltages where inverters equipped with small-AC-signal injection are connected. Subsequently, a PI controller is employed to restored this estimated global voltage magnitude to its rated value. The proposed method is validated through a case study involving a three-inverter four-bus microgrid. Therefore, the proposed method offers a communication-free and model-independent solution for voltage secondary restoration in multi-bus AC microgrids.