Conduction mechanisms and high-field effects in syntheticinsulating polymers

The absorption and conduction currents in synthetic insulating polymers, viz., polyethylene, polypropylene, polyethylene-terephthalate and polyvinylidenefluoride, with dc fields may be electronic or ionic both and it may be difficult to determine their origins unambiguously. The charges which are present in the bulk of the polymer may be responsible for both the absorption and the conduction current. A limited displacement of charges together with the rotation of side chains may provide a decaying transient current in the presence of an externally impressed field whereas the conduction current may originate from a hopping of charge carriers in which the charges transfer between localized states. The complex dielectric response of these polymers with ac field may also be explained by intra- and inter cluster charge movement which would be consistent with the unified model, stated above for the case of dc fields     

A generalized dielectric polarization evolution equation

In this paper a non-equilibrium statistical-mechanical theory of dielectric relaxation is developed. This approach differs from previous work in that a generalized nonlocal evolution equation for the polarization is constructed. General equations of motion are presented for the polarization, internal energy, and entropy which include effects of memory. These equations can be expressed in terms of reduced-correlation functions, and are valid for non-equilibrium and arbitrary field strengths. Expressions for an effective local field also are developed. The Fourier transform of the evolution equation yields a general compact expression for the Fourier transform of the memory function and a specific form for the susceptibility. The kernel, Fourier transform of the memory function are developed, and relaxation-time functions for special cases. In the limit of a single relaxation time, a Debye response is obtained. In the subsequent special cases exponential and Gaussian forms for the memory functions are assumed. The final special case relates a power-law circuit transfer function to the theory of Dissado and Hill. In this case the memory kernel and relaxation times are derived from the Dissado-Hill response function     

Influence of bulk dielectric polarization upon partial dischargetransients effect of heterogeneous dielectric geometry

A physically valid theory of partial discharge (PD) transients is based upon the concept of the charge induced upon the detecting electrode by the PD. This induced charge consists of two components. One is associated with the actual space charge in the void, while the other is related to changes in the polarization of the bulk dielectric. These changes are brought about by the field produced by the space charge. The magnitude of the induced charge and its components are examined for several heterogeneous dielectric systems. It is demonstrated that, in relation to a homogeneous dielectric system, the magnitude of the induced charge either increases or decreases depending on the ratio of the dielectric permittivities and within which dielectric the void is located. It is shown that this behavior is directly related to the magnitude and polarity of the polarization component of the induced charge. Furthermore, we demonstrate that the geometry of the dielectric system and the physical dimensions of the different dielectrics influence in a similar manner the magnitude of the induced charge, although to a lesser degree     

Influence of bulk dielectric polarization upon partial discharge transients: effect of void geometry and orientation

The induced charge arising from a partial discharge consists of 2 components. One is associated with the actual space charge in the void. The other is related to changes in the polarization of the bulk dielectric. These changes are a direct consequence of the field produced by the space charge. The influence of the void geometry upon the polarization component of the induced charge is examined for a heterogeneous bulk dielectric system. It is demonstrated that, depending on the ratio of the dielectric permittivities and within which dielectric the void is located, the relative magnitude of this component may increase or decrease. The magnitude of this effect is also strongly dependent upon the prolateness/oblateness of the void geometry, and on the orientation of the void with respect to the detecting electrode.     

Advances in Electrohydrodynamic Conduction Pumping

A detailed review of progress-to-date in the field of electrohydrodynamic conduction pumping is presented. Analytical, numerical, and experimental research studies that have been published in this field are summarized and a detailed scientific overview of the conduction pumping model is provided. Conduction pumping has already been investigated successfully to resolve several commonplace issues relating to pumping, flow-distribution, and heat transfer and these applications are also presented.     

Molecular Conduction: Paradigms and Possibilities

We discuss the factors that determine the overall shape and magnitude of the current-voltage (I-V) characteristics of a variety of molecular conductors sandwiched between two metallic contacts. We analyze the individual influences of the contact geometry, the molecular chemistry, the electrostatics of the environment, and charging on molecular conduction. Current conduction depends sensitively on the experimental geometry, as well as on the theoretical model for the molecule and the contacts. Computing molecular I-V characteristics will thus require theoretical understanding on several fronts, in particular, in the scheme for calculating the molecular energy levels, as well as on the position of the contact Fermi energy relative to those levels.     

Unlocking the value of flexibility of behind-the-meter prosumers: An overview of mechanisms to esteemed trends

Behind-the-meter (BTM) resources is being recognized as a viable solution to offer grid services including flexibility procurement which is required for volatile renewable power systems. This paper brings an overview of current and esteemed frameworks and respective challenges revolving around BTM flexibility notion and mechanisms. To begin with, we review grid architectures, e.g., microgrids and virtual power plants, capable of accommodating BTM flexibility and desirable flexibility market designs, including peer-to-peer trading. The role of machine learning initiatives, including reinforcement learning and probabilistic forecasting, in designing reliable energy management systems is extensively deliberated. Last but not least, supplementary discussions in making this concept a reality, which can be regarded as future research, are given.     

Analysis of Trap-controlled Conduction in Modified PTFE

The reasonable conduction property is beneficial to release the deposited charges by conduction current in di- electric space charging environment, thus the level of the deep charging and the pulse dis...     

电气设备传导干扰检测的研究

分别研究了电气设备的电流、电压和功率传导干扰的检测技术 ,阐述了其测量方法和经验。最后例举几种开关电源的电流、电压和功率的谐波含量的测量方法     

中点钳位型三电平逆变器通态损耗分析

研究了中点钳位型三电平逆变器通态损耗的一种简单计算方法.根据电流、电压关系确定器件导通规律,进而计算出器件导通占空比,由此推导出用于分析计算的导通损耗表达式.该表达式与器件静特性、功率因数、调制度及负载电流有关,利用其计算不同载波调制下通态损耗时只需改动导通占空比.另外,采用该损耗模型对正弦波调制和三次谐波注入法调制下的三电平逆变器通态损耗进行了分析比较,所得结论为三电平逆变器损耗研究奠定了基础.     

Prebreakdown phenomena in dielectric liquids

The aim of this paper is to make a synthesis of a number of characteristic features of prebreakdown phenomena in liquids, e.g. those events which precede arc development. The subject is restricted to short term processes in the bulk, more specifically of hydrocarbons. First, the basic physical properties involved in the prebreakdown mechanisms are considered, with special emphasis on the influence of pressure and temperature on the physical and electronic properties of liquids and their vapor. Following this, the advantages of using a point-plane electrode geometry to study separately cathodic and anodic prebreakdown stages are demonstrated. To begin with, there is injection of carriers which consequently produces avalanches in the liquid followed by bubble generation. These mechanisms are now well characterized. Eventually, discharge channels are developed, streamers or leaders. The initiation and the propagation of these channels depend on many experimental conditions which are discussed. Characterization and understanding of these phenomena are in progress. Finally, prebreakdown phenomena in uniform fields are considered, either at short (μm) gaps or at large gaps, in the presence of large protrusions or free moving particles     

高介电复合材料及其介电性能的研究

研究了金属／铁电体／聚合物三元复合材料的介电常数和介电损耗。发现金属粒子的加入除了可以提高铁电体／聚合物复合材料的介电常数外，还可以非常明显地降低其介质损耗。金属／铁电体／聚合物三元复合材料是一种很有应用前景的高介电材料。     

可控电抗器晶闸管导通角的单片机控制

介绍了可控电抗器晶闸管导通角的微机控制及应注意的问题 ,并给出了试验结果     

超临界水冷堆水棒导热性能研究

在热谱超临界水冷堆设计中,工质在堆芯内剧烈的密度变化使堆芯轴向方向慢化不均匀,需要引入水棒结构减小这种影响。为描述超临界水冷堆堆芯热工水力现象和中子慢化性能,更好地实现超临界水冷堆的设计,需要研究水棒导热性能的影响。针对特定的超临界水冷堆组件,选取不同水棒中间材料的热导率,编制程序对堆芯燃料、冷却剂、慢化剂进行联合求解,计算得出各种工况下的温度分布,并进行对比分析。结果表明,燃料通道和水棒内工质随着流动的方向其温度都是呈现不断升高的趋势;不合理的材料选取,可能出现慢化剂与冷却剂反向传热的现象;在水棒材料选取时,应尽量选择热导率比较小的材料。     

Space charge and dielectric response in polar fluids

The assumption of "low signal" response is important in relation to the interpretation of the interaction of the electrical field (dc and ac) with mobile charge carriers. The existence of such a response is assumed in linear samples under the condition of absence of space charge. Experimental evidence as well as theoretical arguments of formation of hetero-charge in liquid-like materials have been reported. The materials, of course, are of low electrical conductivity. It has been demonstrated that the existence of space charge field is not always followed by sample nonlinearity. It is the case, where space charge field acts as a macrodipole with particular time or frequency response function. In principle, dc and ac sample responses should be the same because of the lack of nonlinearities. However, there is experimental evidence that this statement is not always true. The important problem seems to be the detection of space charge effects, which is particularly difficult in the case of ac measurements. There is a proposal to use dc combined with ac or dc measurements     

泵技术进展与发展趋势

泵作为一种典型的水力机械,在国民经济中扮演着重要的角色,其技术进展一直受到多个行业的关注。在各种应用需求的驱动以及材料、力学、信息和计算机等相关学科的带动下,泵理论和技术研究已取得巨大进展。人们对泵内流机理和运行特性的认识更为深入,数字化设计和信息化运维已得到广泛应用,泵产品已逐步具有生态友好和智能化特征。本文以2000年以来泵领域所取得的进展为重点,围绕旋涡泵、离心泵、混流泵、轴流泵和贯流泵等叶片式泵,从产品应用需求出发,针对泵的水动力学、设计技术、振动噪声、节能与运维技术等四个部分对技术进展进行综述,并对泵的发展趋势进行总结和展望。     

Status and trends in biomedical engineering education

An overview of graduate and undergraduate programs is given, including a brief history of each. With respect to undergraduate studies, the issues of accreditation, department status versus option programs, employment, registration, and medical and professional school opportunities are examined. Enrollment trends are examined. A survey of biomedical engineering textbooks and a bibliography containing 102 references are given in an appendix.