This paper studies a single-stage bidirectional three-phase Buck-Boost high-frequency isolated AC-DC converter. The topology employs a Y-connected three-half-bridge AC side and a three-phase full-bridge DC side

achieving isolated AC-DC power conversion with only 12 power switches. For this topology

based on the power transfer model per switching cycle

a novel symmetric DC-side modulation strategy is proposed. By redistributing the space vector

it achieves central symmetry in the transformer’s DC-side voltage

simplifying analysis. Analytical derivations establish the functional relationships between space vector durations and phase shift angle

defining three operating modes and their boundaries. Three-degree-of-freedom coordinated control enables sinusoidal grid current regulation over the entire line cycle and extends the power factor (PF). Finally

a 6.6 kVA experimental platform (380V/50Hz AC

400V DC)is built. The experimental results show that the converter can achieve wide load range

and the maximum efficiency reaches 96.58%. Under full-load conditions

the total harmonic distortion (THD) of the grid current is as low as 2.11%

and the power factor is continuously adjustable.