高压直流电场与谐波耦合作用下金属化膜电容器失效研究

Mechanism Study on the Failure of Metalized Film Capacitor Under HVDC Field Superimposed Harmonics

直流支撑电容器作为柔性直流输电系统的核心设备,其特有的超高直流电场耦合多次复杂谐波的工况,导致直流支撑电容器寿命远低于传统电容器,并且目前几乎全部依赖进口。因此,该文基于直流支撑电容器的实际运行条件,设计并开展金属化膜电容器在高压直流叠加不同含量交流谐波电压环境的老化实验。电容器的测试结果显示,谐波含量升高会加速容量衰减,导致电容器寿命大幅度缩短。为探究失效机理,进一步对老化后的电容器进行拆解获得金属化膜和双向拉伸聚丙烯基膜(bi-axial oriented polypropylene,BOPP)。金属化膜的测试结果显示,金属化膜表面在高谐波含量作用下会出现电化学腐蚀,这也是导致电容量在老化初期快速衰减的主要原因。对BOPP基膜的研究发现,谐波含量升高会使聚丙烯部分化学键断裂、薄膜结晶度下降、并且无定形区分子链的运动活化能降低,这些结构劣化最终导致薄膜击穿场强下降。因此,金属化膜电容器在高压直流电场与谐波耦合作用下的加速失效,是金属化膜有效面积腐蚀损失和BOPP基膜绝缘性能下降的共同作用结果。

The DC-link capacitor is one of the key equipment in voltage source converter based high voltage direct current(VSC-HVDC)system,which still relys on importation nowadays.It is the extreme working condition of HVDC field superimposed harmonics that causes the lifetime of DC-link capacitor is much less than the normal capacitors.Consequently,in the present study,systematic researches are carried out based on the practical operation environment of DC-link capacitor.Results show that the high ratio of harmonics could lead to accelerated capacitance loss at the beginning of the ageing.Furthermore,to investigate the failure mechanism of metalized film capacitor,the aged capacitors are teared down and the extracted metalized films and bi-axial oriented polypropylene(BOPP)films are examined.The observation of metalized film proves that the accelerated capacitance loss is resulted from the electro-chemical corrosion brought by harmonics.The test of base BOPP films indicates that the harmonics result in the degradation of break-down strength,which is originated from the scission of molecules,the decrease of crystallinity,as well as the reduction of activation energy of molecular segment motions.As a result,the accelerated failure of metalized film capacitor under the condition of HVDC field superimposed harmonics could be concluded as the deterioration of breakdown strength of BOPP base film accompanied with the corrosion of metalized film.