构 网 型 与 跟 网 型 逆 变 器 并 联 系 统 精 确 频 域 建 模 及 简 化 稳 定 判 据

The parallel grid-forming and grid-following inverter system is widely used in the islanded AC microgrid with distributed integration of renewable energy. The power-frequency droop control loop of grid-forming inverter and the phase locked loop (PLL) of grid-following inverter both generate fundamental frequency small-signal dynamics. Consequently, there exists the strong fundamental frequency interaction between the parallel power loops and PLLs, which has a considerable effect on the stability of the system. Regarding the above mentioned issue, first, based on terminal impedances and frequency-domain frequency-current terminal characteristics, the accurate small-signal frequency-domain models are constructed for the grid forming inverter, the grid-following inverter and the parallel inverter system, which include the main circuit dynamics, the voltage and current inner-loop dynamics, the power loop and PLL frequency dynamics, and the interaction dynamics. Furthermore, based on the frequency-domain model, the characteristics and effects of frequency dynamics are analyzed, and a simplified stability criterion based on the principal diagonal entries of the return-ratio matrix for the parallel system is derived using the Gerschgorin circle theorem under the typical operation conditions. Finally, simulation and experimental results validate the accuracy and effectiveness of the proposed frequency-domain model and simplified stability criterion.