Three-Phase Flexible Transformer Based on Bipolar Direct AC/AC Chopper and its Control Strategy

Dynamic states (e.g., voltage sags and swells) in the power grid can cause faults and defects in sensitive loads and in consequence financial losses. This paper deals with a flexible transformer based on bipolar direct AC/AC chopper (BD-AC). It is a combination of BD-AC and traditional power frequency transformer, and it can realize the flexible regulation of voltage amplitude and phase angle respectively according to voltage requirements. Firstly, the topology and modulation strategy of the selected BD-AC are analyzed. On this basis, the flexible transformer system topology based on BD-AC and transformer is proposed. Then, based on the structure of flexible transformer system, its working principle is analyzed in detail, and the regulation range of voltage amplitude and phase angle are quantitatively studied. Furthermore, considering the principle of voltage regulation, a triangle angle theory based control strategy is proposed. Finally, the correctness and effectiveness of the theoretical analysis of the proposed flexible transformer system are carried out by detailed computer simulation.