中文摘要：

下一代高压直流电网技术成为未来电网发展的重要方向,高压直流变压器是实现不同电压等级的直流电网线路之间互联的关键设备,也是制约直流电网推广的技术瓶颈之一。然而有关模块化多电平换流器型高压直流变压器（modular multilevel converter based HVDC transformer,M-HVDCT）拓扑的故障响应特性和故障穿越能力的研究尚未见诸报道。在搭建PSCAD/EMTDC仿真模型的基础上,首先分析了M-HVDCT的直流侧线路双极短路、单极接地短路故障机理,分析了系统的故障输出特性。结果表明,双极短路时的子模块电容放电是造成瞬态电流冲击的主因,而变压器耦合的阀侧馈能则是稳态故障电流的原因;M-HVDCT对双极短路故障具有故障隔离能力,而单极接地短路故障则会因为变压器的耦合作用而传到非故障侧,从而给系统带来负面影响。仿真实验结果验证了理论分析的正确性,研究结果将为直流电网技术的工程化研究提供重要的理论和工程应用基础。

英文摘要：

Next generation HVDC grid has become an important direction for future utility grid development. HVDC transformer, one of technical bottlenecks restricting promotion of future HVDC grid, is a key equipment for interconnection between dc power lines with different voltage levels. However, there are few reports published on fault characteristics and fault ride-through capability of modular multilevel converter based HVDC transformer（M-HVDCT）, quite different from conventional MMC-HVDC system. Based on simulation model established in PSCAD/EMTDC, DC fault mechanism of pole-to-pole, pole-to-ground and corresponding output performance, are studied in detail. Results indicate that sub-module capacitor energy and transformer feeding ac current are main causes generating transient and steady fault current respectively. M-HVDCT has fault isolation capability for DC pole-to-pole fault, while it transmits DC pole-to-ground fault from fault-side to non-fault-side, negatively affecting system. Feasibility of theoretical analysis is verified with simulation results. This research provides essential theoretical and practical basis for industrial application in future HVDC grids.