An Improved Fixed-Frequency Control Method for Inverter-Dominated Islanded Microgrids Under Various Operating Conditions

Synchronized fixed-frequency current control (SFFC) enables inverter-dominated islanded microgrids to regulate power without relying on frequency deviations. However, conventional SFFC may exhibit power oscillations when battery storage systems (BSSs) transition from discharging to charging, while unbalanced and nonlinear loads can degrade voltage quality. To address these challenges, this paper proposes an improved SFFC scheme integrating power decoupling, multi-frequency voltage compensation, and current-capacity coordination. First, the mechanism of power oscillations during BSS discharging-to-charging transitions is analyzed, and a voltage-vector-oriented controller is developed to decouple active- and reactive-power regulation. Second, the effects of negative-sequence and harmonic currents on voltage distortion are analyzed while considering the inverter equivalent impedance. Accordingly, a compensation method combining notch and low-pass filters is developed to extract and compensate for the fundamental negative-sequence and selected harmonic components. Third, a residual-current-capacity-based allocation strategy is developed to coordinate reactive-power support, negative-sequence voltage compensation, and harmonic suppression within inverter current limits. Furthermore, a satellite-synchronized mode-transition strategy is incorporated to facilitate transitions between grid-connected and islanded operation. Simulation and experimental results demonstrate improved power regulation and voltage quality under different operating conditions. Specifically, the experimental line-voltage unbalance index decreases from 3.83% to 1.64%, while the line-voltage total harmonic distortion decreases from 4.89% to 1.98%. Stable BSS charging and discharging operation and grid-connected-to-islanded transitions are also experimentally demonstrated.