特高压柔性直流阀组投入过程中混合型MMC启动充电策略

在基于两个阀组串联构成一极的特高压柔性直流输电系统主接线方案中，为了实现混合型模块化多电平换流器(MMC)阀组的在线投入并简化其操作流程，混合型MMC需具备在直流侧短接情况下完成启动充电的能力。首先分析了直流侧短接时混合型MMC在不控充电阶段的工作原理，发现该阶段所有半桥子模块始终处于旁路状态，其自取能电源无法启动。为解决这一问题，提出了一种基于负向电流支路部分子模块切出的可控充电控制策略。该方法不仅可保证半桥子模块电容能够串入回路中进行充电，而且可维持子模块间的电压均衡，并最终将其电容充电到额定电压附近。最后，在仿真模型以及实验样机上验证了所提策略的有效性以及性能分析的正确性。

Start-up Charging Strategy for Hybrid MMC in Valve Switching-on Process of VSC-UHVDC System

In a voltage source converter based ultra-high voltage direct current(VSC-UHVDC)system, where one pole is formed by connecting two hybrid modular multi-level converter(MMC)in series, the hybrid MMC needs to possess the start-up ability under the condition of DC-side short-circuit, so as to realize and simplify the valve online switching-on process. The uncontrolled charging stage of hybrid MMC is firstly analyzed when the DC-side is in short-circuit. All half-bridge sub-modules are bypassed, and their self-energized power supplies are failed to start. Then a novel controlled charging strategy is proposed based on the bypassing parts of sub-modules in negative current branches. This strategy can not only ensure half-bridge sub-modules to be charged and maintain the DC capacitor voltage balance, but also can raise the capacitor voltage to rated value. The effectiveness of the proposed start-up strategy and the accuracy of performance analysis are proved on the simulation model and experimental prototype.