Coordinated Control of Grid-Forming and Grid-Following Inverters With Enhanced Dynamic Performance

While most grid-tied inverters are grid-forming (GFL), it is foreseeable from the current development of grid-following (GFM) techniques that the modern grid will face a mix of both GFL and GFM inverters. This work addresses coordinated secondary control of an islanded microgrid that contain both GFM and GFL inverters; such a system possesses complex dynamics due to its different operating characteristics. A unified control framework is developed that coordinates both GFM and GFL, and a set of distributed secondary control methods is proposed to provide secondary restoration of the system's operating frequency and voltage and to enforce proportional sharing of active and reactive power across all GFM and GFL. Compared with the conventional control that focuses on the steady state, the proposed method explicitly bounds over the dynamics of power fluctuations induced by large load disturbance, which resulted in enhanced system dynamic performance. The scheme does not require precise system parameters, and could be applied to a wide range of grid topologies without additional communication needs. The Lyapunov stability of the controller has been proved, and its performance has been extensively analysed in both steady and dynamic states. A controller-hardware-in-the-loop testbed for an islanded microgrid with two GFL and two GFM inverters have been developed and the performance of the proposed coordinated control has been validated under various scenarios.