The modular multilevel DC transformer (MMDCT) exhibits underdamped characteristics because of the primary side series-connected submodule capacitors

arm inductors and parasitic resistors. This often leads to frequent and persistent underdamped oscillations in actual operation

posing challenges to the safe and reliable operation of the system. To address this issue

an equivalent circulating current model of the primary side of the MMDCT is first established

revealing the mechanism behind the occurrence of underdamped oscillations. Second

a small correction angle is introduced to adjust the circulating current suppression voltage every half switching cycle

which actively controls the circulating current’s variation trend and effectively enhances the internal damping of the system

thus suppressing the underdamped oscillations in the transient process. A power feedforward-capacitor voltage and circulating current dual-loop control strategy is adopted. With proper parameter design

the system ensures good dynamic response performance in various scenarios and operating conditions. Finally

the effectiveness of the proposed control strategy in mitigating the underdamped characteristics of the MMDCT is verified through simulations and experiments.