子模块混合型MMC-HVDC直流故障穿越控制策略

A DC Fault Ride-through Strategy for Cell-hybrid Modular Multilevel Converter Based HVDC Transmission Systems

半桥和全桥子模块混合型模块化多电平换流器在具备直流故障穿越能力的同时降低了开关器件的数量。介绍其拓扑结构以及子模块数量的确定方法。阐述半桥和全桥子模块阀段自身平机理和调制电压基本分配原则,并结合最近电平逼近调制提出一种半桥和全桥阀段间平衡的控制策略。分析直流故障期间换流器的等效电路,为了减少暂态期间直流故障电流对子模块电容电压平衡的影响,提出一种基于虚拟电阻的优化控制策略。整个故障穿越期间无需闭锁换流器,且还能持续保证交流系统对无功功率的需求。基于PSCAD/EMTDC,搭建两端子模块混合型模块化多电平换流器HVDC仿真模型,针对双极直流短路工况进行仿真分析,验证了所提出的控制策略的有效性。

The half- and full-bridge based cell-hybrid modular multilevel converter（MMC） also has the dc faults ride-through capability, while needs less switching devices compared with the full-bridge based MMC. The topology and the method for calculating the half- and full-bridge sub-module numbers of cell-hybrid MMC was introduced. Then, the balancing mechanism between half- and full-cells and the basic principle of arm-voltage reference value setting method for different valves were illustrated. Combining with the nearest level modulation for MMC, a voltage balancing strategy for half- and full-cells was proposed. During the dc-line fault period, the equivalent circuit of the converter was analyzed. On this basis, a virtual resistance based optimal control strategy during the transient period was proposed to reduce the influence of the oscillating current in the dc line to the balance of sub-module capacitor voltage. It does not need blocking action during the fault ride-through period, while still meets the requirement of reactive power of the ac system. Finally, based on PSCAD/ EMTDC software environment, a two ends cell-hybrid MMC-HVDC model was set up, and the simulation results under a DC line pole-to-pole short fault were analyzed, which prove the effectiveness of the proposed control strategy.