基于金属化薄膜电容器状态在线监测的剩余寿命评估方法

Remaining Lifetime Assessment Method Based on On-Line Condition Monitoring of Metallized Film Capacitors

模块化多电平变流器(MMC)是柔性直流输电系统的核心装备,其中子模块(SM)金属化薄膜电容器是MMC中的薄弱环节之一,故研究金属化薄膜电容器的状态监测和寿命评估对于提高MMC运行可靠性和制定系统维护计划具有重要意义。目前,现有金属化薄膜电容器的寿命评估方法未考虑电容器老化进程对电容值和等效串联电阻(ESR)的影响,难以准确实现不同服役状态下的电容器剩余寿命评估问题。因此,该文提出考虑当前电容值状态在线监测的金属化薄膜电容器的剩余寿命评估方法。首先,在考虑电容器老化速率与温度耦合关系的基础上建立了电容器ESR和电容值的老化模型。其次,在进行寿命计算时考虑到电容器的老化效应,根据提出的容值在线监测方法得到当前的电容值,推断出电容器已使用年限及电容器目前的ESR值,并代入建立的场路协同仿真电热耦合模型中计算得到电容器温度。最后,根据提出的剩余寿命模型对电容器当前剩余寿命进行预测。该文研究成果将为设备选型、系统设计和MMC系统子模块电容器的可靠性评估提供参考。

Modular multilevel converter(MMC)is the core equipment in flexible direct current transmission systems,where the sub-module(SM)metalized film capacitors represent one of the weak links within MMCs.Thus,state monitoring and lifetime assessment of metalized film capacitors are of significant importance for enhancing the operational reliability of MMCs and formulating system maintenance plans.Current lifetime assessment methods for metalized film capacitors do not consider the impact of capacitor aging on capacitance value and equivalent series resistance(ESR).It is difficult to accurately assess the remaining lifetime of capacitors under different service conditions.Therefore,this paper proposes a method to assess the remaining lifetime of metalized film capacitors,considering the current capacitance state for online monitoring.The main research contents include the following aspects.(1)A method based on capacitance value and hotspot temperature is proposed.An analytical lifetime model is established according to the failure mechanism of metalized film capacitors.Subsequently,considering the impact of aging on ESR and capacitance value,a time-varying degradation model for the capacitance and ESR is developed.The nonlinear cumulative damage and degradation process are obtained under actual operational conditions.Based on the coupling relationship between the state of the capacitor,ESR,capacitance value,aging rate,and temperature,the change in failure rate is analyzed at different aging levels.(2)A non-invasive method for monitoring capacitance value based on sliding window cumulative sum detection is proposed.Firstly,the coupling relationship between changes in capacitor voltage and switching signals is established based on the basic working principle of MMC.Then,the principle of the bilateral cumulative sum algorithm is described,and a sliding window is introduced to process the capacitor voltage,allowing for accurate determination of charging and discharging times within the sliding window.Thus,capacitance values can be calculated online.The effectiveness of the proposed method is validated through simulation and experiment,and the maximum error under different power levels is less than 0.6%.This method uses the existing capacitor voltage and arm current without affecting the original operating state of the MMC system,achieving non-invasive monitoring.(3)An electrothermal co-simulation model based on field-circuit coupling is proposed to precisely simulate capacitors’electrical and thermal characteristics under actual MMC conditions.The circuit model of the sub-module capacitor is constructed according to the circuit structure of the MMC system and the equivalent circuit of the capacitor.The temperature field model of the capacitor group is established on a finite element simulation platform of the actual physical structure of the capacitor components.Subsequently,a metalized film capacitor group's electrothermal coupling model is established through the mutual transfer of power loss and temperature parameters in Matlab.Finally,the proposed modeling method is analyzed and validated.