Bidirectional Hybrid Isolated DC Transformer for All-DC Collection and Transmission System

Bidirectional DC transformer (DCT) is the key energy conversion equipment for all-DC collection and transmission system. Cost, efficiency, and DC fault blocking capability are important considerations that are currently difficult to address simultaneously for the existing modular multilevel converter (MMC) based DCT. In this paper, a bidirectional hybrid isolated DC transformer (BHIT) as an alternative to the existing MMC based DCT is proposed. By replacing half of the submodules (SMs) by anti-paralleled thyristors, the proposed BHIT can reduce 36.7% device cost, 32.5% device loss, and 64.8% total SM capacitance compared with the existing MMC based DCTs. By tailoring the trapezoidal waveforms of the thyristors, continuous DC currents are ensured without any filtering effort. In addition, an active fault current limiting control (AFCC) is further proposed to reduce the fault current during short-circuit faults. The detailed operational analysis, control schemes, simulation results, and scale-down prototype are provided to validate the effectiveness of proposed topology.