Coordinated Control of an Energy-Storage-Integrated Modular Multi-Level AC–AC Converter for Equal-Frequency Flexible Interconnection in Distribution Networks

To address equal-frequency AC–AC flexible interconnection and cross-regional power-flow regulation in medium-voltage distribution networks with a high penetration of distributed generation and flexible loads, this paper proposes a coordinated control strategy for an energy-storage-integrated modular multi-level AC–AC converter. The converter adopts a back-to-back MMC topology with distributed energy storage and enables controllable power exchange among multiple feeders. Feeder states, net-load conditions, loading limits, and SOC-dependent storage boundaries are mapped into four operating zones. Under normal conditions, the strategy coordinates port power and energy-storage buffering to balance feeder loading. When the storage reaches its SOC limits, photovoltaic curtailment or non-critical load shedding maintains the active-power balance. Under feeder faults, the hierarchical support and master–slave reconfiguration restore islanded loads and rebuild the DC-voltage reference. Electromagnetic-transient simulations show that the proposed control completes load balancing or reconfiguration within 37.0–62.0 ms, limits the maximum DC-bus voltage deviation to 3.323%, and restores 99.96–100% of the off-grid demand. Compared with a conventional SOP benchmark, it reduces the Zone 1 loading-excess integral by 98.23% and avoids 3.80–10.75 MW of unsupported demand under feeder-fault conditions.