容 性 部 分 能 量 转 移 型 多 端 口 特 高 压 大 容 量 直 流 变 压 器

The ultra-high voltage (UHV) large-capacity direct current (DC) transformer is key equipment for realizing DC collection and transmission of large-scale renewable energy bases in desert Gobi and other arid areas. This paper proposes a multi-port UHV large-capacity DC transformer based on the principle of capacitive partial energy transfer. Each arm of the DC transformer is connected between the collection side and the transmission side, only requiring bearing the voltage difference between the collection and transmission ports, and can directly transfer the DC power from the collection side to the transmission side. Meanwhile, the arm alternately connects in parallel on the collection side and in series between the collection side and the transmission side to realize current switching, and the inner sub-module capacitors of the arm achieve efficient capacitive energy balance by absorbing and releasing power. In addition, the DC transformer adopts thyristor and diode valve groups, and can reduce equipment costs and improve operational efficiency. The DC transformer has a multi-port structure, and can isolate faults at the collection port without affecting the normal operation of other collection ports. This paper introduces the details of topological structure, operating principle, parameter design method, control strategy, fault blocking and post-fault switching operation strategy of the proposed DC transformer. The effectiveness of the proposed DC transformer and its operating principle design is verified through simulation cases, and quantitative comparison results with modular multilevel converters in terms of the number of power devices, energy storage, and power losses are presented.