Publication
This paper presents a robust design of a megawattscale medium-voltage (MV) direct current transformer (DCT) that features a bidirectional LLC resonant converter topology. This topology harnesses the inherent ability to control power flow seamlessly between two MVDC buses without relying on closedloop control, maximizing efficiency and stability. Integrated gatecommutated thyristors (IGCTs) were chosen as the power devices due to their superb on-state properties, enhancing the performance of this resonant DCT. The power stages incorporate a three-level neutral point clamped (3L-NPC) topology, paired with a split-capacitor DC link, enabling three-level modulation during soft-start and two-level modulation during normal operation. Taking advantage of the resonant operation, the record-high switching frequency in the range of 5kHz is achieved. The custom-designed medium frequency transformer (MFT) uses oilinsulated copper pipes for the windings and nanocrystalline core while integrating the inductive elements of the resonant tank. Power-in-the-loop experiments are presented that demonstrate the system efficiency approaching 99%.