A three port resonant solid state transformer with minimized circulating reactive currents in the high frequency link

Multi-port dc-ac and dc-dc converters are of great interest for applications where electricity is generated through a variety of energy sources and where energy storage systems are required. In this paper, a three-port, dc-dc-ac Solid State Transformer (SST) is presented that can control the magnitude and direction of power transfer between a battery, a dc bus and the ac grid. The proposed SST features a low magnetic component count, has high power density, high efficiency and provides galvanic isolation between all three ports. The presented topology is operated with a fixed switching frequency and is phase shift modulated. In this paper, a mathematical analysis of the proposed SST is presented along with its sensitivity to parameter variation. The requirements for soft switching is derived and it is demonstrated that the converter can soft switch over large input voltage variation. The modulation parameters are optimized so that the high frequency link currents are minimized while soft switching is maintained. Additionally, in the simulation results, the ac grid current is modulated so that no Power Factor Correction (PFC) circuitry is required.