Line-Frequency-Isolation Flexible AC-Link Converter Based on Direct AC-AC Choppers

Line-frequency-isolation flexible ac-link converter based on direct ac-ac choppers (FACL) is a novel ac-ac power conversion device expected to realize voltage compensation and power flow control. Compared with the traditional voltage source inverter (VSI) based devices, FACL has no dc-link capacitors that lead to high equipment failure rate. First, this article systematically introduces FACL. The power conversion principle of FACL is analyzed, and the output comparison between the unipolar and bipolar FACL is presented. Then, a high-performance bipolar FACL is proposed. It eliminates the shortcomings of the existing FACL topologies, such as bidirectional switches, undesirable phase shift, and restricted output range. Additionally, for facilitating the control of FACLs, a variant of the conventional dq reference frame called DQ-ABC synchronous rotating reference frame (DSRRF) is adopted. DSRRF provides a platform for the flexible operation in the form of dc between two three-phase voltages, which is beneficial to FACL control. On the basis of DSRRF, the closed-loop control strategy of FACLs is designed so that the decoupling control and closed-loop control of the output voltage amplitude and phase can be achieved conveniently. Finally, the open-loop and closed-loop experimental tests are carried out based on a 1-kW experimental prototype to verify the theoretical analysis.