Operational Analysis and Control of a Multi-Port Solid-State Transformer with Integrated Batteries

The modular multilevel type multi-port solid-state transformer (SST), is a promising topology to connect and control the bidirectional power flow among the MVAC grid, the MVDC grid, and LVDC bus simultaneously and with full degree of flexibility. Identical monolithic isolated DC-DC converters containing medium-frequency transformers are thereto embedded in the submodules to provide voltage conversion and galvanic isolation between medium-voltage and low-voltage sides. This work introduces a medium-voltage battery energy storage variant with battery packs directly integrated into each cell of the system. The basic operating principle and system control is discussed based on an analytical average model. And performance of the approach is demonstrated by simulations. Special focus is put on energy balancing control between the MMC cells, where leg circulating currents of the MMC are used. Finally, the converter operation, proposed energy balancing methods and control approaches are validated by simulation of the average model of a 3 MVA, 4kV MVAC, 8kV MVDC and 1500V LVDC system.