基于混合型MMC和直流开关的柔性直流电网直流故障保护研究

提出了采用混合型模块化多电平换流器(hybrid modular multilevel converter,hybrid MMC)和直流开关构建柔性直流电网进行架空线远距离电能传输的方案。针对由全桥型子模块和半桥型子模块组成的混合型MMC,分析了其拓扑结构、基本运行原理和直流电压运行区间,提出了混合型MMC的三自由度控制架构,并详细分析了直流故障穿越控制策略,进而设计了混合型MMC构成的柔性直流电网的故障清除策略和多次重启动时序。故障期间,混合型MMC无须闭锁IGBT,可控制故障电流至0,从而保持不间断运行、持续向交流系统提供无功支撑。3次重启动失败后,架空柔性直流电网配置的直流开关在零故障电流下开断以隔离故障电流通道,直流电网重启,线路潮流发生转移。最后在PSCAD/EM TDC仿真平台验证了所提出的故障清除策略及重启动时序的可行性。

DC Fault Protection of VSC-HVDC Grid Based on Hybrid MMC and DC Switch

To transmit the bulk renewable power using overhead lines,this paper proposes a VSC-HVDC grid transmission system based on hybrid modular multilevel converter (hybrid MMC) and DC switch.According to the hybrid MMC composed of half bridge sub-modules and full bridge sub-modules,this paper analyzes its topology,basic operating principle and DC voltage operating region,proposes a control framework embedding three-channel control freedom for hybrid MMC,analyzes the DC fault ride through control strategy in detail,and then designs the fault clearing method and multiple restart sequence of the VSC-HVDC DC grid based on hybrid MMC.During the DC fault,hybrid MMC can control the fault current to be zero without tripping the IGBTs.Therefore,the DC grid remains continuous operation to provide reactive power to the AC system.During post-fault,if the system fails to restart after three restart attempts,the DC switches installed at the overhead lines will be tripped to isolate the fault path at zero DC current.After fault clearance,as the change of network topology,the power flow will be re-distributed.At last,the feasibility of the fault clearing strategy and restart sequence is validated in PSCAD/EMTDC.