中文摘要：

模块化多电平换流器（modular multilevel converter, MMC）多用于基于高压直流输电（high voltage direct current transmission, HVDC）的多端输电系统。针对该系统中直流电缆或架空线路的意外短路或断路故障，影响输电线路和换流站安全的问题，提出一种基于三端电压裕度控制下的旁路晶闸管和直流侧IGBT断路器（IGBT-Circuit Breaker）的混合保护方案。分析了子模块闭锁前直流电流超调的数学模型及故障机理，根据直流短路电流的峰值及上升时间动态特性，选取了MMC的参数。利用仿真软件PSCAD／EMTDC对控制方法及保护策略的效果进行验证，结果表明剩余系统主站切换及时，能够持续隔离发生永久故障的线路，使得其余线路继续运行，实现了系统直流故障时及时保护，提高了系统稳定性。

英文摘要：

Modular multilevel converters （ MMC ） are suitable for the high voltage direct current （ HVDC ） transmission in multi-terminal DC systems. However, the accidental dc faults in the cable or overhead lines in multi-terminal DC system, could give rise to safety concerns on transmission lines and converter stations. In order to solve the problem, a hybrid protection scheme was proposed based on the three termi- nal voltage margin which controls the bypass thyristor and IGBT circuit breaker at DC side. The Mathe- matical model and fault mechanism of DC current overshoot before sub module locking was studied, and the parameters used for MMC were selected by analysing the dynamic characteristics in the rise time and the peak of DC short circuit current. In addition, the effectiveness of the control method and the protection strategy is verified by using the simulation software PSCAD/EMTDC, the results showing that the main station of the residual system is switched in time, and the permanent fault line can be isolated, so that the remaining lines continue to operate, and the system can be protected timely, so the stability of the system is improved.