Soft-Switching Solid State Transformer (S4T)
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Solid state transformer is a device that can convert single- or multi-phase AC to another single or multi-phase AC or DC with high-frequency isolation between the input and the output. It provides features of bidirectional power flow control, reactive power compensation, harmonics suppression, voltage regulation, sag correction, and fault isolation. Conventional topologies to implement the SST have issues of complex multi-stage structure, high device count, hard-switching with severe parasitic impact, low efficiency, and poor reliability. This research proposed a novel soft-switching solid state transformer (S4T) topology, which has the features of: 1) minimal power conversion stage with low device count, 2) soft-switching feature for all devices over the full load range, 3) fully bidirectional active and reactive power flow control with voltage buck and boost capability, 4) arbitrary power factors and frequencies for input/output, 5) eliminating large DC capacitors to avoid inrush and to improve reliability, 6) fast dynamic response to transients, 7) ease of control to achieve soft-switching operation, 8) flexible configurations for two- or multi-terminal DC, single- and multi-phase AC systems such that it acts as a universal power converter, and 9) modular structure to scale to medium voltage and high power though stacking the converter modules.