Rotational Control Method with Transformer Loss Savings in Solid-State Transformer based Converter System

This paper presents a novel control strategy for high-frequency transformer loss reduction in modular solid-state transformer (SST) based converter system. The modular SST design is based on an ac-dc power-electronic building block (PEBB) comprised of an H-bridge at the AC input and a bi-directional soft-switching isolated dc-dc converter with a high frequency transformer. The AC terminals of input H-bridges are connected in series and directly coupled to the AC line, while the output DC terminals are all connected in parallel at low voltage. The key innovation described in the paper is the control scheme that reduces core and winding losses associated with the dc-dc resonant stage by turning off the selected dc-dc converters in synchronism with the line voltage. The prototype for test described in this paper is based on a modular SST where each module is rated at 50 kVA, with a 200 kHz transformer using 1.7 kV SiC MOSFETs, but the described control concepts are more broadly applicable to similar topologies.