Coupling Mechanism and Stability Analysis of Parallel Grid-Forming Inverters in High-Frequency Band

With the increasing penetration of power electronics inverter (PEI)-interfaced renewable energy sources(RESs), the inertia of the power system is reduced, consequently resulting in the future electricity grid experiencing stability issues, due to the PEI-interfaced RESs working as a constant current/power source. To increase the system inertia, the grid-forming (GFM) operation of PEIs, which can emulate the conventional synchronous generator behavior in terms of inertia and damping support, is used instead of the constant current/power mode. However, the parallel operation of GFM inverters results in interactive oscillation issues in low and high-frequency bands, degrading the grid performances. The low-frequency interaction is caused by the power control loop, while the high-frequency interaction is caused by the voltage and current control loop. This paper models and analyzes the coupling mechanism in the high frequency of multiple-parallel GFM inverters based on their equivalent impedance models, and studies the stability based on the impedance stability criterion. The correctness and accuracy of theoretical analyses are clearly verified by the simulation results carried out in MATLAB/Simulink.