Bidirectional Power Flow in Direct AC/AC SST with Selective Harmonic Elimination and Pulse Density Modulation

Solid state transformers (SSTs) are integral in interfacing conventional grid to microgrid, any AC/DC source to the conventional grid, any kind of load to conventional grid and microgrid to microgrid. Direct AC/AC SST is a single stage SST with no electrolytic capacitors. It presents the advantages of high efficiency, power density and reliability over multi-stage SSTs. Bidirectional power flow is an essential functionality of any SST topology. With four-quadrant switches, the direct AC/AC SST is capable of bidirectional power flow. Bidirectional power flow in direct AC/AC SST is available in the literature using the space vector based synchronous PWM strategies. However, these synchronous strategies for direct AC/AC SST lead to non-sinusoidal current in the high frequency transformer (HFT) windings. This limits the switching frequency of the SST due to the HFT leakage inductance. The asynchronous PWM technique with selective harmonic elimination (SHE) and pulse density modulation (PDM) removes this limitation by making the current in HFT windings sinusoidal while also removing LC filter associated resonance in HFT voltage and current. bidirectional power flow using the asynchronous PWM strategy with SHE and PDM has not been discussed in the literature. This paper proposes a bidirectional power flow strategy for direct AC/AC SST between two grids of same or different frequencies using the asynchronous PWM strategy consisting of SHE and PDM to bridge this gap. The proposed strategy is verified through simulations in MATLAB Simulink.