Secondary Voltage and Frequency Restoration Control of Droop-Controlled Inverter-Based Microgrids

Though microgrid technology has many advantages, stable operation with paralleled distributed generation (DG) must be ensured with proper controls. Droop control ensures distributed loading of DGs, but it has some limitations, namely, a large frequency deviation may occur if a large load demands reactive power sharing, the errors due to disproportional line impedance, etc. In order correct the voltage and frequency steady-state deviations of controlled inverter-based microgrids, the secondary control is needed. This paper proposes primary and secondary control for islanded microgrids (MGs) based on the P-V/Q-f droop control strategy. Based on the droop control method and the inverter's double-loop control, the primary control of hierarchical structure controls the distributed generations and loads, while secondary control allows restoration and improvement of the quality of frequency and voltage magnitude after each load change. The virtual impedance is proposed in order to improve the power sharing performance, by redesigning the converters' equivalent output impedance as pure resistance. An islanded MG test system consisting of four DGs is built in MATLAB and RT-EVENTS of OPAL-RT Technologies to illustrate the design approach. The effectiveness of the proposed control methodology is verified by offline simulation.