一种模块化多电平换流器交流侧预充电可控充电策略

为解决模块化多电平换流器在充电阶段过渡时易出现的电流过冲问题以及因不当投切引起的模块过充问题，深入分析了换流器内子模块电容的预充电过程，并根据子模块电容的充电特点提出了一种适用于可控充电阶段的有源交流侧均衡控制充电策略。通过母线电压定斜率控制固定上、下桥臂的子模块投切数目，避免数目突变。同时在充电过程中为使各臂子模块电容电压可准确跟踪其参考值，考虑了环流压降对桥臂子模块充电的影响，引入环流稳压控制环节对臂间子模块投切比例进行动态微调，从而保证子模块充电效果，提高相间充电一致性。最后，在PSCAD/EMTDC中搭建了双端有源换流器预充电仿真模型验证了上述充电策略的有效性，该策略控制下电容电压及直流母线电压平稳抬升，系统实现由不可控充电阶段到可控充电阶段的安全过渡。

Controllable Charging Strategy for AC-side Pre-charging of Modular Multilevel Converter

In order to solve the problem of impulse current and module overcharge caused by incorrect switching in the Modular Multilevel Converter during the transition of charging phase in the pre-charging process, this article deeply analyzed the charging process of the sub-module capacitor. According to the charging characteristics of the sub-module capacitance, this paper proposed an active AC side balancing control charging strategy suitable for the controllable charging phase. The number of switching sub-modules was controlled by bus voltage constant slope to avoid the sudden change of switching number of bridge arm sub-modules. And this strategy took into account the influence of circulation voltage drop on the charging of bridge arm sub-module, dynamically adjusted the switching ratio of arm sub-module through the circulation voltage stabilizing control link, so that the capacitance voltage of each arm sub-module could accurately track its reference value, so as to ensure the charging effect of the sub-module and improve the charging consistency. Finally, a simulation model of the dual-terminal active converter was built in PSCAD/EMTDC to verify the effectiveness of the above charging strategy. Under the control of this strategy, the capacitor voltage and the DC bus voltage are raised steadily and the safe transition in the charging phase is realized.