适用于大规模风、光直流送出系统的改进型MMC子模块拓扑

An improved MMC submodule topology for large-scale wind and optical DC delivery systems

高压直流输电系统中多采用模块化多点平换流器,但传统半桥型子模块不具备直流故障自清除能力;逆阻型半桥子模块在换流器闭锁时,下管IGBT两端会出现过电压现象;部分新型直流故障自清除能力的子模块拓扑在闭锁后,故障阻断能力不强。文章将改进型子模块与半桥型子模块相结合,提出一种新型MMC桥臂设计方案,分析了其子模块的拓扑结构、各种运行工况和故障阻断机理,从耐压性与经济性两个方面,与其他经典的子模块作比较分析。最后,在PSCAD/EMTDC中搭建双端21电平MMC直流系统模型进行验证,仿真结果表明,文章所提改进子模块拓扑能够快速有效阻断直流侧故障电流,且子模块下管IGBT两端不会出现过电压,兼具故障自清除与低损耗的特点。

Modular multi-level converter(MMC)has become a new development trend to realize HVDC transmission in large-scale wind power and photovoltaic centralized delivery systems.But the traditional half-bridge submodule(HBSM)does not have the ability of DC fault self-cleaning.The reverse resistance type half bridge submodule(RB-HBSM)will have a large overvoltage at both ends of IGBT when the converter is locked,which may burn the device.In some new submodule topologies with DC fault self-cleaning ability,only part of the capacitance in the submodule is involved in blocking the fault current after locking,and the fault blocking ability is not very strong.To solve these problems,an improved MMC submodule topology is proposed,and the topology structure,various operating conditions and fault blocking mechanism of the submodule are analyzed.On this basis,in order to have the characteristics of fault self-cleaning and low loss,a hybrid MMC bridge arm design with sub-module and half-bridge sub-module is proposed.Then,from two aspects of pressure resistance and economy,it is compared with other classical submodules.Finally,a two-terminal 21-level MMC DC system model was built in PSCAD/EMTDC to verify the blocking current characteristics of the improved submodule.The simulation results show that the proposed improved submodule topology can quickly and effectively block the fault current on the DC side and there is no overvoltage at both ends of the submodule's lower tube IGBT.