This paper presents the architecture of a high voltage DC transformer that uses a single phase modular multilevel converter (MMC) to reduce significant energy losses in high voltage AC transmission and address the issue of wide frequency oscillations in flexible DC transmission.This design effectively supports the integration and distribution of renewable energy technologies.Both the high voltage and medium voltage sides use a single phase MMC configuration

with a 400 Hz medium frequency transformer for electrical isolation.A vector control technique using virtual dq components is employed to enable decoupling control between the high voltage and medium voltage sides in the view of single current voltage characteristic of the single phase MMC.A suppression method using a resonant controller is deve loped to reduce double frequency circulating currents in the single phase MMC.Finally

simulations of steady state operation and circulating current suppression are conducted

and the results confirm the effectiveness of the proposed topology and control strategy.