电磁继电器绝缘下降的失效分析与控制

文章通过对某电磁继电器不同引脚以及引脚与管壳之间绝缘下降进行失效分析,采用显微镜外部目检、扫描电镜检查和能谱分析等分析方法与手段,揭示了银离子迁移是导致该继电器绝缘下降的失效原因。阐述了银离子迁移的发生机理,从生产和使用两个方面提出了避免银离子迁移在继电器中发生的控制措施。     

基于Hilbert-Huang变换的1000kV输电线路雷电绕击与反击识别方法

在考虑雷电先导发展的基础上,提出了一种基于希尔伯特-黄变换(HHT)的1 000k V特高压输电线路雷电绕击与反击的识别方法。首先基于ATP-EMTP软件建立了考虑雷电先导发展过程的1 000k V输电线路雷电绕击与反击模型,研究了先导发展对输电线路雷击过电压的影响;然后对雷击输电线路后绕击和反击过电压的暂态过程进行了分析,基于信号的局部特征时间尺度,实现了线路雷电绕、反击信号的经验模式分解(EMD),对包含大部分高频信号的前四阶固有模式函数(IMF)进行Hilbert变换,将变换后的IMF瞬时幅值作为特征量,并计算其相应的方差贡献率的大小,进而完成输电线路雷电绕击和反击的判别。仿真结果和现场实测数据验证表明,该方法能够正确有效地识别输电线路雷电绕击与反击,不受线路接地电阻和雷电流幅值的影响,且克服了目前雷电流参数实测技术中获取数据工作量大和易误判的缺点,能为输电线路的防雷分析提供理论参考。     

大规模风电集中接入对电力系统暂态功角稳定性的影响（一）：理论基础

该文作为2篇系列文章的第1篇,旨在从理论上分析双馈风机组成的大规模风电场集中接入对电力系统暂态功角稳定性的影响。首先,根据双馈风机的暂态特性,提出了一种适用于电力系统暂态功角稳定分析的双馈风机简化模型,并讨论了利用直流潮流计算方法研究风电接入前后系统暂态功角稳定变化趋势的合理性。其次,在双馈风机简化模型和直流潮流计算方法的基础上,利用扩展等面积定则分析了双馈风机接入两机系统后系统暂态功角稳定性的变化,提出一种判断风电接入对两机系统暂态功角稳定影响的判据。最后,在双机互联系统中进行仿真,结果表明,提出的判据具有较高的准确率。     

电容换相换流器串联电容补偿度优化研究

为了探究电容换相换流器中串联电容的取值问题,建立了电容换相换流器(capacitor commutated converter,CCC)的稳态模型,并利用Matlab软件进行数学分析,探讨了串联电容补偿度与串联电容电压、阀电压、阀峰值电压、直流电流、换相重叠角、触发角、视在熄弧角和实际熄弧角之间的关系,最终给出合适的串联电容补偿度的取值范围并进行仿真验证。研究结果表明:补偿度的取值上、下限分别由阀峰值电压和实际熄弧角决定,其中阀峰值电压为主要限定因素;补偿度的可选范围涵盖欠补偿、全补偿和过补偿3种状态,且全补偿状态下CCC系统不会发生串联谐振;得出强受端系统下补偿度的可选范围为[0.92,1.4],而弱受端系统补偿度的可选范围为[0,1.08]。     

A non‐local continuum damage approach to model dynamic crack branching

Dynamic crack‐branching instabilities in a brittle material are studied numerically by using a non‐local damage model. PMMA is taken as our model brittle material. The simulated crack patterns, crack velocities, and dissipated energies compare favorably with experimental data gathered from the literature, as long as the critical strain for damage initiation as well as the parameters for a rate‐dependent damage law are carefully selected. Nonetheless, the transition from a straight crack propagation to the emergence of crack branches is very sensitive to the damage initiation threshold. The transition regime is thus a particularly interesting challenge for numerical approaches. We advocate using the present numerical study as a benchmark to test the robustness of alternative non‐local numerical approaches. Copyright © 2014 John Wiley & Sons, Ltd.     

Brittle and ductile failure of rocks: Embedded discontinuity approach for representing mode I and mode II failure mechanisms

In this work, we present a discrete beam lattice model with embedded discontinuities capable of simulating rock failure as a result of propagating cracks through rock mass. The developed model is a two‐dimensional (plane strain) microscale representation of rocks as a two‐phase heterogeneous material. Phase I is chosen for intact rock part, while phase II stands for pre‐existing microcracks and other defects. The proposed model relies on Timoshenko beam elements enhanced with additional kinematics to describe localized failure mechanisms. The model can properly take into account the fracture process zone with pre‐existing microcracks coalescence, along with localized failure modes, mode I of tensile opening and mode II of shear sliding. Furthermore, we give the very detailed presentation for two different approaches to capturing the evolution of modes I and II, and their interaction and combination. The first approach is to deal with modes I and II separately, where mode II can be activated but compression force may still be transferred through rock mass which is not yet completely damaged. The second approach is to represent both modes I and II being activated simultaneously at a point where complete failure is reached. A novel numerical procedure for dealing with two modes failure within framework of method of incompatible modes is presented in detail and validated by a set of numerical examples. Copyright © 2015 John Wiley & Sons, Ltd.     

Statistical Inference for Power-Law Process With Competing Risks

The focus of this article is on failure history of a repairable system for which the relevant data comprise successive event times for a recurrent phenomenon along with an event-count indicator. We undertake an investigation for analyzing failures from repairable systems that are subject to multiple failure modes. Failure data representing a cluster of recurrent events from a single system are studied under the parametric framework of a power-law process, a model that has found considerable attention in industrial applications. Some interesting and nonstandard asymptotic results ensue in this context that are discussed in detail. Extensive simulation has been carried out that supplements the theoretical findings. An extension to the case where the specific cause of failure may be missing is investigated in detail. The methodology has been implemented on recurrent failure data obtained from a warranty claim database for a fleet of automobiles. Supplementary material for this article is available online.     

复合材料宏观强度准则预测能力分析

对复合材料宏观强度准则进行了总结和评述,对最具有代表性的5种强度准则的预测能力进行了综合评估。首先,由各强度准则得到了不同平面应力状态下AS4/3501-6材料的理论失效包线,以此阐明了各强度准则的物理本质。然后,考虑到强度的分散性,采用Monte-Carlo方法并基于各强度准则建立了4种材料的概率失效包线和强度分散带。最后,结合多组试验数据,客观评估了各强度准则的预测能力。评估结果表明:各强度准则的预测结果都不可能完好地吻合所有的试验结果;相比于Max-Stress准则、Hashin准则和Tsai-Wu准则,Puck准则和LaRC03准则的预测能力相对较好,且对复合材料的损伤机理有更为合理的解释。     

基于渐进损伤分析的复合材料螺栓连接强度包线法研究

针对中国民机采用T800级复合材料这一新材料体系而基础数据匮乏的现状,采用渐进损伤分析(PDA)替代试验以显著降低研究周期和成本。综合渐进损伤方法和工程算法各自的优点,提出以渐进损伤分析替代应力集中减缓因子(SCRFs)测定试验,进而建立强度包线,并进行多钉连接强度预测的数值策略。为验证该数值策略的可行性,针对典型铺层应力集中减缓因子,测定试样,并开展渐进损伤分析,获得了试验件强度预测值来计算应力集中减缓因子,采用旁路载荷修正的强度包线法,绘制了典型铺层复合材料多钉连接旁路载荷修正强度包线,预测多钉连接的失效载荷,并与试验结果进行对比。结果表明:采用该数值策略预测的强度包线、多钉连接的失效载荷和失效模式均与试验结果吻合良好,证明了该数值策略的可行性。     

复合材料波纹梁冲击试验与数值模拟

为了探究复合材料波纹梁的吸能性能,针对铺层形式分别为[(±45)3/(0,90)/(±45)3]、[(±45)8]和[(±45)7]的3种复合材料波纹梁元件,进行了动态冲击试验,得到了吸能载荷-位移曲线,并对其损伤破坏形貌进行了分析。以连续损伤力学为基础,结合改进的Hashin损伤判定准则以及损伤演化规律,提出了针对波纹梁耐撞性损伤分析的刚度退化模型,并基于有限元软件平台开发了适用于波纹梁渐进损伤分析的子程序。对3种不同结构形式的波纹梁进行了渐进失效数值分析,模拟得到了能量评估参数比吸能(SEA)和平均载荷值,并将模拟结果与试验结果进行了对比分析。比较分析了不同薄弱环节复合材料波纹梁的吸能能力。结果表明:波纹梁在冲击载荷作用下发生了渐进压溃失效;平均压溃载荷的相对误差不超过12%,能够满足工程应用要求;薄弱环节的设置需综合考虑复合材料性能和铺层方式等因素。