中高压直流变压器是直流输配电系统中进行新能源发电直流汇集与电压变换的关键设备之一。模块化多电平型直流变压器（modular multilevel DC transformer，MMDCT）模块化程度高、电压扩展能力强，是高压大功率直流电能变换的主流技术方案，呈现出迅猛的发展趋势。首先梳理了中高压直流变压器的典型应用场景，然后归纳整理了MMDCT的典型拓扑结构。在此基础上，分别对MMDCT的调制策略、电容电压均衡控制方法、环流抑制策略、功率控制方案、实验样机和应用等关键技术问题进行归纳分析与总结。最后，展望了MMDCT今后的发展趋势，总结了在未来直流电网中MMDCT所承担的多功能角色以及多方向的技术挑战。

Medium and high voltage direct current (DC) transformer is one of the key equipments for DC collection of new energy generation and voltage conversion in DC transmission and distribution system. Modular multilevel DC transformer (MMDCT) has high modularity and strong voltage expansion ability, which is the mainstream technical solution of high voltage and high power DC energy conversion, showing a rapid development trend. The typical application scenarios of medium and high voltage DC transformers are summarized, and then the typical topology of MMDCT is summarized. On this basis, the key technical problems of MMDCT such as modulation strategy, capacitor voltage balancing control method, circulating current suppression strategy, power control scheme, prototype and application, are analyzed and summarized. Finally, the development trends of MMDCT in the future are prospected, and the multi-functional roles and the multi-direction technical challenges of MMDCT in the future DC power grid are summarized.