A Four-Port Single-stage AC-AC Converter for SST Applications

Abstract

The solid state transformer (SST) standard concept utilizes a three-stage solution. The DC-DC stage provides the medium voltage (MV) and low voltage (LV) sides with isolation requirements in medium frequency (MF). These DC ports allows a hybrid distribution system and can integrate renewable energy sources into the grid. A single-stage solution is possible with the Matrix converter, however, DC ports are not available. This paper proposes a new single-phase single-stage bidirectional ACAC converter topology with MV and LV DC ports available, employing reduced switch count compared with the single-phase three-stage solution. The converter utilizes the interleaved and dual-active bridge (DAB) concept. The MV side employs the neutral-point clamped (NPC) converter and for the LV side are presented two alternatives, half-bridge (HB) and full-bridge (FB) configuration. The FB configuration is analysed. At AC ports there are five-level voltages, and the current frequency of the low-pass filter inductors is twice the switching frequency. A carrier-based pulse-width-modulation (CB-PWM) is proposed as well as control strategy. The converter power flow is analysed. Simulation results in closed loop operation demonstrate the feasibility of the proposed converter. The simulations are performed in multiple scenarios with the prototype parameters.