DC–DC Autotransformer With Bidirectional DC Fault Isolating Capability

This paper studies the control and dc fault isolation of a dc-dc autotransformer topology (DC AUTO). The operating principle and power flow analysis of a DC AUTO are studied. Internal dynamic study shows that dynamics of the common bus ac voltage is a purely algebraic equation. A control strategy of using VSC2 to control common ac bus voltage, and VSC1 (3) to control the transferred dc power is then proposed. System responses to dc fault are analyzed. Corresponding design methods that enable bidirectional dc fault isolating are then proposed and their impacts on component cost are analyzed. A family of possible DC AUTO topologies is also proposed. Extensive simulations on power step change, dc and ac faults confirmed the theoretical studies of a DC AUTO employing the modular multilevel converter topology. Taking a ±320-kV/±500-kV DC AUTO transferring 1000-MW dc power as an example, conventional dc-ac-dc technology requires 2000-MW total converter rating with power loss ratio of 1.8%, while the DC AUTO technology only requires 1020-MW total converter rating with power loss ratio of about 0.8%. The DC AUTO is able to achieve exactly the same functions as a dc-ac-dc with significantly reduced investment and operating cost under low- and medium-dc voltage stepping ratio.