Single-stage high-frequency-isolated three-phase four-leg buck-boost inverter with unbalanced load

A three-phase four-leg inverter shows its preponderance on providing energy to unbalanced load and high DC-link utilisation. To increase the power density of the traditional three-phase four-leg inverter with power frequency isolation, this study proposes a single-stage isolated three-phase four-leg inverter. To ensure the symmetry of three-phase output voltages, the proposed converter provides the path for neutral current resulted by the unbalanced load. It achieves buck-boost DC/AC conversion by introducing the transformer whose turns ratio provides additional voltage gain. The working frequency of the transformer is in accord with the switching frequency, so high-frequency electrical isolation is realised with a single-stage structure as well. The specific mode analysis and the relationship between the switching states and the voltage vectors are presented. A modified three-dimensional space vector pulse width modulation algorithm with a simplified section partition method is proposed, which guarantees the voltage-second balance of the transformer within one switching cycle as well. In order to explore the feasibility of the topology and control strategy, the simulation is done by MATLAB/Simulink. Then a prototype was built in the lab and the experimental results verify that the inverter can produce balanced voltages under both balanced and unbalanced load.