MMC子模块故障下桥臂不对称运行特性分析与故障容错控制

Operation Characteristics of the MMC With Asymmetrical Arms Under Sub-module Faults and the Fault-tolerant Control

针对基于模块化多电平换流器的柔性直流输电系统(modular multilevel converter based high voltage direct current,MMC-HVDC),研究子模块故障发生后,桥臂子模块实际运行个数不对称情况下的运行特性,揭示其造成上、下桥臂电容电压基值不对称、各次不对称环流、直流电流波动、交直流侧电压偏置、各桥臂电流直流分量不对称等故障机理。基于平衡上、下桥臂基频电压分量的思想,提出一种具有子模块故障容错能力的环流抑制控制器,其通过在传统二倍频比例谐振环流控制的基础上引入基频谐振控制器,以解决不对称桥臂引起的一系列不平衡问题。基于PSCAD/EMTDC搭建双端201电平MMC-HVDC系统,仿真结果验证了所提控制策略的有效性。

The operation characteristic of the modular multilevel converter （MMC） with asymmetrical sub-module （SM） numbers of the arms caused by sub-module fault was analyzed. The fault characteristics, including the asymmetrical capacitor voltage of the upper and lower arms, the asymmetrical harmonic loop currents of all orders, the DC current fluctuation, the voltage offset at the DC or AC side, and the asymmetrical DC components of the arm current of the arms were revealed. A loop current suppressing controller with SM fault-tolerant ability was proposed based on the concept of rebalancing the fundamental frequency component of the upper and lower arm voltage. The controller was designed by adding a fundamental frequency resonant controller to the traditional second-order frequency proportional resonant （PR） loop current controller, hence it can solve the series of problems caused by asymmetrical arms. A 2-terminal 201-level MMC-HVDC system was established in PSCAD/EMTDC and the simulation results validated the efficiency of the proposed control strategy.