Accurate control of virtual oscillator-controlled islanded AC microgrids

Virtual oscillator control (VOC) is an emerging control strategy for grid-forming inverters. In contrast with the droop and virtual synchronous generator methods, VOC is a nonlinear and time-domain strategy that requires only the inverter output current measurement to control the inverter output. Hence, it is characterized by its good dynamic response and stable operation. To explore the VOC performance in islanded AC microgrids, this paper initially demonstrates for the first time the negative impacts of impedance mismatching of the interfacing feeders on power sharing among VOC-based inverters. Then, new control schemes to enhance the operation of VOC-based islanded AC microgrids are proposed. First, a fast and robust secondary control loop to restore voltage and frequency in the microgrid based on the adaptive tuning of the VOC voltage-scaling factor and the VOC inductance parameters is developed. Second, an optimal tuning approach of virdual complex impedance combines virtual inductance and resistance for each inverter is proposed to fully mitigate the impacts of impedance mismatch of the interfacing feeders. Consequently, accurate active and reactive power sharing among VOC-based inverters is achieved. Third, an online and non-invasive estimation technique for feeder impedance is embedded in the control loop of each inverter. Hence, prior knowledge of feeders’ impedances used to tune the virtual impedances of inverters is not required. Simulation and experimental results are presented to validate the efficacy of the proposed control scheme.