A Hybrid Synchronization Control Method with Active Voltage-Frequency Support Ability for Parallel Inverter System

The grid-forming (GFM) inverters are suitable for working in low short-circuit-ratio (SCR) power grids, but the output power oscillations and even instability phenomena occur under strong grids. The performance of grid-following (GFL) inverters on different SCRs is the opposite. The inverters controlled by the two separate schemes cannot meet the stability requirements of a wide varying range of SCR. A unified synchronous control is proposed for islanded and grid-connected parallel inverter systems with various SCRs. The active support capability and robust stability of the system are improved by reconstructing the external characteristics of the grid-connected inverter under GFM and GFL control combined, and breaking the limitation of existing hybrid synchronous control methods only adapt to the grid-connected inverter system. Finally, the stability of the system is analyzed based on impedance modeling and the Nyquist stability criterion. The potency and workability of the put forward method are confirmed by simulation and physical experiments.