Complex Behavior and Instability Mechanisms of Grid-Forming and Grid-Following Converter Systems

Description

The penetration of renewable energy sources (RES), driven by global efforts to mitigate global warming, has been increasing in recent power system developments. The power generated from RES is converted and delivered to the grids through grid-connected converters. However, these converters have a significant impact on the grids' dynamics and stability, leading to power interruptions, oscillations, and instability in connected subsystems.Grid-connected converters can generally be classified as grid-following and gridforming converters (GFLCs and GFMCs). To facilitate the integration of distributed RES, these converters can cluster together to form an islanded microgrid. The behavior of grid-connected converters and multi-converter islanded microgrids can have a significant impact on system stability. To better understand the complexity of GFLCs and GFMCs, several critical challenges need to be addressed. Firstly, the impact of dclink controls and non-smooth components on the transient stability of GFLCs and GFMCs has not been thoroughly studied, and the similarities and differences in their nonlinear behaviors remain unexplored. Secondly, the small-signal and transient stability of an islanded microgrid with multiple GFMCs and GFLCs is not fully understood. Thirdly, the use of distributed secondary control in multi-converter islanded microgrids and coordination of multiple microgrids as a large islanded system using a two-layer distributed control have not been extensively studied in terms of their impact on the system's nonlinear dynamics. This limits the development of more effective methods to enhance system stability. Lastly, future power grids will be dominated by GFLCs and GFMCs, and the complex behavior of these converters is the root cause of all instability events. However, the current classical framework does not adequately consider this complex behavior. Therefore, it is crucial to develop a new framework for studying the complex behavior and stability of converter-based systems.Based on these considerations, our proposal has four objectives. The first objective begins with a study of the complex behavior of a GFLC/GFMC connected to an infinite bus. The second objective is to investigate the complex behavior of a multi-converter islanded microgrid using only primary control. The third objective is to analyze the complex behavior of multi-converter islanded microgrids and multi-microgrid islanded systems using distributed controls. Finally, the fourth objective is to propose a new framework for comprehensive analysis of the complex behavior of GFMCs and GFLCs.