An Isolated Three-Phase AC-AC Converter With Bipolar Output Voltage

This article proposes a new topology of a three-phase high-frequency transformer (HFT) isolated bipolar ac-ac converter. It has a simple circuit topology consisting of nine active switches, three HFTs with primary and secondary series capacitors, and output LC filters. It can generate both noninverting and inverting outputs with a symmetric bipolar voltage gain of +/- nd, where n is the turns ratio of the transformer and d is the switch duty cycle. The proposed converter can provide step-down output by setting the turns ratio to unity; however, step-up output operation can also be achieved by increasing n beyond one when required by a particular application. The proposed converter integrates HFT-based isolation and, therefore, can eliminate the sizeable low-frequency transformer for applications requiring grid interface. Owing to the use of series-blocking capacitors, the HFT is devoid of any line-frequency component. A safe-commutation-based switch modulation strategy is developed to avoid switch current and voltage overshoots. The proposed converter is well suitable for application as a three-phase dynamic voltage restorer. The inverting and noninverting operations can inject the antiphase and same-phase series voltages to mitigate grid voltage swell and sag, respectively. Moreover, the additional line-frequency isolation and voltage injection transformers can be saved. In-depth circuit analysis and operation are provided with proposed switch modulation strategies together with component design guidelines. Finally, the hardware measurements are taken on a laboratory-built circuit to confirm the operation of the proposed converter.