Power Flow Control of a Single-Stage AC-AC Solid-State Transformer for AC Distribution System

Recently, there has been a growing demand for bidirectional ac-ac solid-state transformers (SST) that can meet both active and reactive power demands of the low- and medium-voltage ac distribution grids. Single-stage SSTs can be of significant interest in these grids because they employ fewer passive components besides offering a simpler power flow control. In this paper, a closed-loop feed-forward PQ controller is investigated for a single-stage bidirectional ac-ac SST that consists of two H-bridge cells and a high-frequency transformer in a dual active bridge (DAB) configuration. Single phase-shift PWM is used to meet the active and reactive powers demands of the single-phase ac grids. A second-order generalized integrator (SOGI) in conjunction with a phase-locked loop (PLL) ensures proper grid synchronization. The operation of the controller is validated through simulations performed on a 3.6 kVA, 120/120 V SST. With the designed controller, the SST can meet 20% of its rated kVA as reactive power without exceeding the maximum allowable total harmonic distortion (THD) injection to the grid as per the IEEE 519-2014 standard.