A Comprehensive Review of Optimal Control Approaches for Inverter Dynamic Voltage Support in Weak and Faulted Grids
The high rate of replacement of synchronous machines by inverter-based resources (IBRs) has increased the role of Dynamic Voltage Support (DVS) during grid disturbances, especially on weak and failed grid conditions. Traditional reactive-current-based grid-support rules can have a poor performance with large R/X ratios, deep voltage sags, and hard inverter current or power constraints. To this end, much research has been done on the development of the best control measures that can help in maximizing positive- sequence voltage, improving low-voltage ride-through capability, and reducing synchronization instability. The review gives a cohesive and thorough evaluation of the best DVS strategies, including analytical global-optimality models, model-driven optimization, sensitivity-based methods, active/reactive power allocation models, and newly developed model-free and real-time optimum-seeking controllers. The review identifies the importance of current limits, active power availability, and synchronization stability limitations in determining inverter behavior, and contrasts the outputs of various approaches to these problems in the presence of varying grid strengths. Aspects in practical implementation of photovoltaic, wind and storage inverters are discussed and also robustness in the face of parameter uncertainties. Future directions in research (such as adaptive optimization, controllers based on learning, multi-inverter coordination, and grid-forming DVS) are determined. This review summarizes the latest knowledge and defines the directions of resilient, optimal, and grid-code-conforming voltage support of next- generation power electronic inverters.