MMC子模块母排直流故障失效机理分析

Failure Mechanism Analysis of Busbar of MMC Submodule Under DC Fault

模块化多电平变流器(modular multilevel converter,MMC)发生直流故障后,子模块内会出现显著的故障过电流,将在子模块母排上产生极大的电磁力与温升,可能造成母排永久性失效。为探究MMC子模块母排在直流故障下的失效机理,采用MMC直流故障工况计算与母排多物理场仿真相结合的方法,对故障电流冲击下母排的电磁–温度–应力耦合场进行了分析。首先,基于桥臂等值模型,对换流器桥臂故障电流进行了精确计算,探究了故障时刻对桥臂故障电流的影响,对子模块母排上可能出现的最严重故障电流工况进行了求解。其次,建立了母排在故障电流冲击下的电磁–温度–应力耦合场模型,对MMC子模块各部分母排的电磁力与焦耳热进行了建模,求解了子模块各部分母排在直流故障下的温升与应力分布。计算结果显示,在直流故障下MMC子模块母排温升很小,但机械应力较大、变形较为严重,母排所连旁路晶闸管端面螺栓承受的轴向力最大,可能出现螺栓松动与脱离失效。该结论可为MMC子模块母排直流故障工况可靠性评估与优化设计提供一定的参考。

Overcurrent induced by DC fault may cause great electromagnetic force and temperature rise on the busbar of modular multilevel converter(MMC)submodule,which may result in permanent failure of the busbar.In order to explore the failure mechanism of the busbar under DC fault,we analyzed the electromagnetic-temperature-mechanical coupling field of the busbar based on DC fault calculation and multiphysical field simulation.First of all,based on the arm equivalent model,we accurately calculated the arm fault current,analyzed the influence of fault time on the arm fault current,and obtained the most serious fault current magnitude that may occur on the submodule busbar.Secondly,the electromagnetic force and Joule heat of each part submodule busbar were modeled,and the temperature rise and stress distribution of each part of busbar were solved.The calculation results show that the temperature rise of the busbar is negligible,but the mechanical stress is large,the deformation is serious,and the axial force on the bypass thyristor bolts is the largest of all,which may cause bolt detachment failure.This paper provides useful information for MMC submodule busbar failure analysis and design.