An isolated three-phase AC-AC converter with bipolar output voltage

This paper proposes a new topology of three-phase high-frequency transformer isolated bipolar ac-ac (HFTI-BP-AC) converter. It has a simple circuit topology consisting of nine active switches, three high-frequency transformers with primary and secondary series capacitors, and output LC filters. It can generate both non-inverting and inverting outputs with a symmetric bipolar voltage gain of ±𝒏𝒅, where n is the turns ratio of transformer and d is 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 high-frequency transformer (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 three-phase dynamic voltage restorer (DVR). The inverting and non-inverting operations can inject the anti-phase 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. An in-depth circuit analysis and operation is provided with proposed switch modulation strategies together with component design guidelines. Finally, the hardware measurements are taken on a laboratory build circuit to confirm the operation of the proposed converter.