Grid-Forming Control Technologies for Cascaded H-Bridge Power Conversion Systems

The increasing penetration of renewable energy is driving power systems toward low-inertia and weak-grid operation, increasing the need for power conversion systems (PCSs) with active voltage and frequency support. Grid-forming (GFM) control enables converters to establish voltage and frequency references while providing synthetic inertia and damping. Transformerless cascaded H-bridge (CHB) energy-storage PCSs are particularly attractive for medium-voltage applications because their modular structure enables direct grid connection, multilevel voltage synthesis, and module-level state-of-charge (SOC) regulation. However, their cascaded power cells and independent battery strings tightly couple GFM control with power allocation, SOC balancing, modulation constraints, and system stability. This review summarizes the outer- and inner-loop structures and stability-enhancement methods of major GFM strategies for CHB-PCSs, including virtual synchronous generator, droop, and virtual oscillator control. It further examines the coordination of voltage formation with interphase and intraphase SOC balancing under weak-grid and fault conditions. Finally, key research directions are identified in multi-timescale coordination, fault current limiting, protection, and the integrated design of GFM control, energy balancing, and energy management.