Control of Three-Phase Solid-State Transformer with Phase-Separated Configuration for Minimized Energy Storage Capacitors
Journal article, Peer reviewed
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- SINTEF Energi 
This paper presents the control structure of a solidstate 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 an 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.