基于模块化多电平的柔性直流系统故障稳态特性分析

直流故障稳态特性对直流电网设计、保护配置以及一次设备选型等具有重要意义。针对模块化多电平的柔性直流系统，考虑了桥臂电抗对两极短路故障不控整流稳态运行期间换相重叠的影响，分析了可能出现的换相重叠角范围及各自对应的故障场景。针对不同的换相重叠角范围，分析了相应的桥臂导通工作情况，并据此推导出直流故障后稳态阶段直流电流、电压、桥臂电流、交流电流的精确计算公式。最后，在PSCAD/EMTDC仿真平台上搭建了基于模块化多电平的柔性直流输电系统模型，通过不同故障距离的仿真算例验证不同换相重叠角范围下理论计算的准确性。

Stable fault characteristic analysis of the DC system based on modular multilevel converter

The stable characteristics of the DC fault are significant for the design and programming of the DC system, the configuration of the protection system, as well as the selection of the primary equipments. As for flexible DC system based on modular multilevel converter (MMC), the influence of the arm reactors on the commutation overlap during the uncontrolled rectification stage of the DC fault is addressed. This paper analyses the possible commutation overlapping angle ranges and the corresponding fault conditions. For different commutation overlapping angle ranges, the arm conducting working conditions are discussed, and then the calculation methods of the DC current, DC voltage, arm current and AC current during the stable state of the DC fault are proposed. Finally, the MMC-HVDC system model is built on the PSCAD/EMTDC platform. Simulation tests with different fault distances are carried out to verify the accuracy of the theoretical calculation.