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An Augmented Modular Multilevel Converter for Clearing Fault in DC Grid

1.State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources (North China Electric Power University), Beijing 102206, China;2.University of Electronic Science and Technology of China, Zhongshan 528400, China

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This work is supported by National Key R&D Program of China (No. 2018YFB0904600) and National Natural Science Foundation of China (No. 51777072).

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    With the increase of voltage and capacity of high voltage direct current (HVDC) system, the requirement for rapid clearance and isolation of DC faults is getting higher and higher. As the most effective solution, hybrid DC circuit breaker is not mature enough, which restricts its engineering application. Accordingly, this paper proposes a low-cost DC fault clearance scheme suitable for clearing fault in DC grid. In this scheme, the traditional half-bridge modular multilevel converter (MMC) is partially innovated to have the ability of auxiliary breaking operation. With the effect of the MMC auxiliary breaking operation at both ends of the fault line, the DC fault current can be quickly interrupted and isolated by low-cost circuit breaker unit installed in the DC transmission line. The equivalent circuit in the fault isolation process is modelled and analyzed, and the fault isolation sequence is designed. Furthermore, the comparative analysis of devices usage among typical schemes is performed. Finally, a simulation model with the proposed scheme is built, and an MMC prototype is developed. The simulation and experimental results validate the effectiveness of the proposed scheme.

    表 1 Table 1
    图1 增强型MMC方案结构Fig.1 Structure of augmented MMC scheme
    图2 故障清除过程Fig.2 Process of fault clearance
    图3 故障隔离过程Fig.3 Fault isolation process
    图4 所提方案与混合式DCCB方案对比Fig.4 Comparison between the proposed scheme and hybrid DCCB scheme
    图5 正常运行阶段的等效电路Fig.5 Equivalent circuit in the normal operation
    图6 故障发生瞬间的等效电路Fig.6 Equivalent circuit at the moment of fault occurrence
    图7 故障清除阶段的等效电路Fig.7 Equivalent circuit in the fault clearance stage
    图8 能量耗散阶段的等效电路Fig.8 Equivalent circuit in the energy dissipation stage
    图 四端双极直流电网仿真模型Fig. Simulation model of four-terminal bipolar DC grid
    图 正常运行时MMC1_1动态响应波形图:(a)有功功率;(b)直流电压Fig. Dynamic response diagram of MMC1_1 under normal state: (a) active power; (b) DC voltage
    图 直流故障穿越过程:(a)故障线路UFD电流波形图;(b)故障清除过程中换流器两端电压;(c) 故障清除过程中的各桥臂电流波形图;(d)换流器子模块电容电压波形图Fig. DC fault isolation process: (a) fault line current; (b) DC voltage; (c) arm current of MMC; (d) SMs’ capacitor voltage
    图 硬件实验接线图Fig. Wiring diagram of the prototype
    表 2 Table 2
    图 硬件平台实物图Fig. Practicality picture of the prototype
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ZHAO Chengyong,LI Shuai,ZHANG Jiyuan,et al.An Augmented Modular Multilevel Converter for Clearing Fault in DC Grid[J].Automation of Electric Power Systems,2020,44(5):60-67.DOI:10.7500/AEPS20190128005

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  • Received:January 28,2019
  • Revised:June 30,2019
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  • Online: March 08,2020
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