Control of Three-Phase Solid-State Transformer With Phase-Separated Configuration for Minimized Energy Storage Capacitors

This paper presents the control structure of a solid-state transformer for three-phase ac/ac, to reduce the required size of capacitors. The structure consists of an ac/dc converter based on cascaded H-bridge converters, isolated dc/dc converters, and a dc/ac inverter. The phase separated configuration requires a high capacitance for the smoothing capacitors owing to the instantaneous power oscillation at the double-line frequency. This paper reviews control methods of the dc/dc converter to achieve synchronous instantaneous power with the ac/dc converter, and thereby cancel the double-line frequency component applied to the capacitors between the ac/dc and dc/dc converters. This study also proposes incorporating a control method to cancel the oscillating instantaneous power caused by unbalanced load current. The overall control was demonstrated using a 6-kVA laboratory prototype. The required minimum capacitance, which is decided by the switching frequency, is discussed, and the demonstration of a design case study on a 300-kVA system is described. The likely increase in loss in the isolated dc/dc converter owing to the power oscillation controls was evaluated with a 10-kW loss evaluation-model of a dc/dc converter.