Study of charge injection in insulators submitted to divergingfields

Various theoretical models have been proposed to explain the behavior of insulator-electrode interfaces in the case of strongly divergent electric fields. However until recently a direct measurement of the charge transferred at these interfaces could not be performed and very indirect ways had to be used. In this paper we propose a structure which allows for direct measurements of this charge using the PWP (pressure wave propagation) method. We present evidence of charge transferred around small diameter wires, embedded in an insulator, submitted to an applied voltage     

Effect of additives on morphology and space charge accumulation in low density polyethylene

The effects of an antioxidant additive on polymer morphology and space charge formation have been investigated, as a function of temperature, in melt-quenched films of low-density polyethylene. On quenching, the additive-free polymer crystallizes to give extensive spherulitic structures; similar objects are not seen in the system containing the antioxidant. The pulsed electro-acoustic method was then used dynamically to follow both the formation of space charge at high voltage and its subsequent decay under short circuit conditions in both material systems. Data were obtained at room temperature, 50 and 70/spl deg/C. Comparing the additive free material with identical polymer containing a standard anti-oxidant package indicates that the addition of the antioxidant markedly changes both space charge formation and decay processes. At room temperature, both materials were found to behave similarly, suggesting that the changes in gross morphology described above are of secondary importance. At higher temperatures, the addition of the antioxidant enhances negative charge accumulation in the material, implying its association with relatively deep traps for the negative charges within the bulk.     

Demonstrating a threshold for trapped space charge accumulation in solid dielectrics under DC field

This paper presents a discussion of the concept of threshold for trapped space charge accumulation in solid dielectrics submitted to a DC field. The starting point is the fact that it is often possible to define a critical field separating an ohmic type of conduction from a nonlinear regime in the current-voltage characteristic of solid dielectrics. In the space charge limited conduction theory, this critical field corresponds to the onset of space charge accumulation. However, other conduction processes, such as hopping conduction for example, can also explain nonlinearity in the current-voltage characteristic, which does not involve space charge. It is proposed to check for the existence of a critical field for space charge accumulation using complementary techniques, i.e., space charge detection and electroluminescence techniques. Polyethylene, polyester and polycarbonate were investigated as being representative of three different families of polymers. It is shown that similar values of thresholds are found for a given material using the three above-mentioned techniques, lending support to the physical explanation of a threshold for trapped space charge accumulation.     

共混改善聚乙烯空间电荷分布的研究

在高场强下用电声脉冲法（PEA）测量了低密度聚乙烯和线性低密度聚乙烯共混物中的空间电荷分布,用差示扫描量热法（DSC）研究了共混物的形态。实验结果表明：与纯低密度聚乙烯相比,共混可以降低空间电荷。文章最后分析了共混后对空间电荷影响的机理。     

空间氢镍电池充电技术研究

精确、可靠的充电技术是氢镍电池在空间电源系统中成功应用的关键。针对国内卫星平台技术的特点和卫星自动控制技术的进展,对用于地球同步轨道卫星的氢镍电池多种充电技术,进行了分析和评述。     

加聚酰亚胺薄膜阻挡层的聚乙烯中空间电荷分布特性的研究

选用聚酰亚胺(PI)薄膜作为电极注入的阻挡层,用电声脉冲法装置测量茂金属聚乙烯(MPE)试样中空间电荷的分布。试验结果表明:在高场强下,放置在上电极和MPE之间的阻挡层,既能有效地抑制负电极的电子的注入,也能阻止正电极的空穴注入到MPE。用聚合物的能带和界面理论解释了实验现象。     

Relation between space charge accumulation and partial discharge activity in enameled wires under PWM-like voltage waveforms

It has been observed that voltage waveforms generated by power electronic converters may affect significantly the reliability of electric motor insulation. Since partial discharges are considered to be the main cause of the reliability loss, new enamel insulations for magnet wires are being developed in order to withstand better stress amplification. The electrical characterization of these insulating materials is often carried out through aging tests which may provide estimation of life under different stress levels and conditions. However, deeper investigation of aging phenomena due to supply voltage waveforms is needed, especially regarding the relation between aging factors and stress conditions. This paper deals with this topic, showing experimental evidences of relation between partial discharge quantities (e.g., inception voltage, repetition frequency, amplitude) and electrical properties, associated with charge accumulation, which can be directly evaluated through space charge measurements. Characterization of insulating materials and comparison of materials candidate for application in power electronic waveform environment can be carried out resorting to the methodology proposed here. This approach can provide, therefore, a useful feedback to wire manufacturers regarding, e.g., the choice of additive nature and enamel components for magnet wires in power-electronic controlled motors.     

Influence of thermal treatment and residues on space charge accumulation in XLPE for DC power cable application

The effects of cross-linking by-products (residues) quantity and thermal treatment on space charge accumulation and decay in manufacturer modified XLPEs for DC power cable application have been investigated using the pulsed electro-acoustic technique. The threshold stress for space charge generation among the modified and reference XLPEs during voltage-ramping was found to show considerable variation and to depend upon the material and the amount of residue present. However, the modified XLPE material was found to exhibit a higher threshold for space charge accumulation than the reference XLPE whatever the conditions. De-gassed samples were found to exhibit the highest threshold stress, with that of the modified de-gassed XLPE accumulating no space charge at all even after 24 h stressing at 70 kV. In general heterocharge regions were formed when the residues were present and homocharge or no charge was formed when the residues were removed by degassing. Differences were also found in the space charge decay following short-circuit (volts-off), with the decay of heterocharge being rapid, whereas that of homocharge being slow. The charge accumulations have been tentatively explained by the mechanisms of ion-pair separation when residues are present, and interfacial injection when residues are absent. Decay of the heterocharge is governed by ion-pair displacement just as the generation and will have a similar time scale. In contrast homocharge decay will be governed by charge de-trapping and extend to time scales well beyond that of injection for charge in the deepest traps     

表面电荷积聚对绝缘子沿面闪络影响的研究

通过对典型绝缘子表面电荷分布特点的分析，研究了表面电荷与放电起始间的关系。研究表明在表面电荷作用下绝缘子沿面闪络起始电压会发生变化。对110kV三相共箱式GIS绝缘子的闪络实验表明，表面电荷可使绝缘子沿面闪络电压下降23．4％。对位移电流作用下的绝缘子沿面闪络先导发展模型进行改进，补充了表面电荷对该模型的影响。指出表面电荷产生的附加电场会影响流注电晕内部正负电荷的分离速度及放电的进一步发展。     

Optical emission due to space charge effects in electricallystressed polymers

Electrical wear-out and dielectric breakdown of insulating polymers involve dissipation to the lattice of the energy of mobile or trapped space charges. Optical emissions are proof of such energy releases and can be used to define the onset of electrical aging and to investigate the dissipation mechanisms. They also could provide a basis for monitoring the degradation rate. The situation considered is that of a material subjected to a voltage stress in the absence of any gas discharge, i.e., the light does not originate from a gaseous plasma. Depending on the type of material and experimental configuration, light emission can be due to excited states of the solid itself and/or of dissolved or adsorbed gas molecules. The polymers of concern are practical materials used as electrical insulation in cables, capacitors, motors, transformers, etc. This review attempts to bring together the published data on electroluminescence of insulating polymers and to discuss the relationship between light emission, space charge, and polymer degradation     

Space charge measurement techniques and space charge inpolyethylene

Recently, several new techniques such as LIPP, PIPS, PEA and TP methods have been developed to measure directly the space charge distributions in insulating polymers. Many papers have been published on space charge in insulating materials. In this paper, the space charge measurement techniques and space charge in polyethylene are reviewed. The space charge distributions in polyethylene depend strongly upon additives (antioxidants, antistatic agents, etc.), oxidation products, byproducts from the crosslinking reaction (acetophenone, etc.). Some of them enhance electron (or hole) injection from the electrode and, as a result, homo space charge is formed. Ionic carriers are supplied from some impurities to form hetero space charge. Space charge distributions are also sensitive to the electrode material and the interface between different materials. From the space charge behavior observed, the space charge effects on the high-field conduction and breakdown phenomena have been discussed quantitatively and some of the high-field phenomena in polyethylene have been elucidated     

Electric field reduction due to charge accumulation in adielectric-covered electrode system

Outlines for increased insulation performance of an air gap through the use of dielectric coatings are given. Theoretically, it is shown that the homogeneous electric field in a plane-parallel electrode system can be reduced if the electrodes are covered with a thick dielectric coating. Free charges will be affected by the electric field between the electrodes and are deposited at the dielectric surfaces. As a consequence, a counteracting electric field component results, which accordingly causes a reduction of the total electric field in the air gap and an enhancement of the field in the dielectric layers, i.e. the electric field is forced into the dielectric coatings by the charges. This effect has important implications in HV engineering. Introductory experiments supporting the idea have been carried out with promising results. It was confirmed that the withstand voltage of a plane-parallel electrode geometry with an open air gap, for dc as well as unipolar impulse voltage, could be increased considerably if the electrodes were covered with thick polymeric layers. Charge formation at the electrode surfaces as well as in the air gap is believed to be responsible for this effect. In today's insulation systems, this is known to work only for time-independent electric fields, i.e. generally dc voltages. Further experimental work is required and will be performed in order to scrutinize thoroughly and clarify the phenomenon, its capabilities and limitations     

Review of space charge measurements in dielectrics

In the last two decades, a significant effort has been directed to understand the internal charge distributions in dielectrics. This has resulted in the development of a number of methods which give detailed information about space charge distributions. This review presents the evolution of several experimental approaches for the determination of spatial charge and field distributions in dielectrics, including the nondestructive methods of direct probing such as the thermal-pulse methods and the methods using pressure waves that propagate through the charged sample. Detailed information is also provided on the principles used in each technique, typical resolution, limitations and advantages associated with each method. The applications of the methods to several insulating materials charged with various mechanisms are described. The paper concludes with a discussion on a number of quite interesting techniques which seem most promising     

Simulation of space charge in unbounded geometries

In corona ion devices the electric field distribution is highly dependent on the form of the electrostatic space-charge distribution. In many instances these devices also have essentially unbounded geometries. A numerical technique is presented based on the combined method of charge simulation and the method of characteristics to evaluate fields in such geometries. The numerical procedure utilizes a nonrigid grid structure which, after convergence to a self-consistent solution, is a map of the field lines and equipotential contours. The numerical procedure is applied to a rod-to-plane geometry, and the simulation results show good agreement with experimental data     

空间氢镍蓄电池充电控制方法研究

氢镍蓄电池在轨可靠、稳定的运行,除了电池自身的制造水平、地面贮存环境等因素外,合理的在轨充电控制方法也是最关键的因素.对我国空间氢镍蓄电池用的多种充电控制方法设置原理、适用性等进行了研究分析、评述.     

Mechanisms of surface charge accumulation in SF6

Contents Special geometries of electrodes were designed to determine the dominant mechanism of surface charge accumulation. It was found that the charge transport in the gas is dominant, and consequently, the normal field at spacer surfaces as well as the ionization behaviour at electrode surfaces play important roles in charge accumulation in SF₆.

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Akkumulationsmechanismus von Oberflächenladungen in SF₆

Übersicht Zur Bestimmung des Akkumulationsmechanismus von Oberflächenladungen wurden spezielle Elektrodenkonfigurationen entwickelt. Der Ladungstransport im Gas wurde als dominierend erkannt. Folglich spielen die Größe der Normalkomponente des elektrischen Feldes auf der Stützeroberfläche und das Ioinisationsverhalten an der Elektrodenoberfläche eine wichtige Rolle hinsichtlich der Oberflächenladungsakkumulation in SF₆.

     

A rational consideration of space charge

Space charge formation and its increase with field under DC conditions is not a mystical phenomenon. The increase in space charge with applied field is an obvious and inevitable result of the interaction of field-dependent current density with spatially inhomogeneous resistivity. For common polymeric dielectrics, the current density makes a transition from a linear function of field to an exponential function of field at around 10 kV/mm. This causes a similar transition in the sample space charge in the same field region. However, this transition has no obvious connection with aging, and if it does prove to have a connection, the driving force is the rapidly increasing current density and not the space charge, which is a result thereof. As to the issue of the accuracy of space charge measurements, it may be more important to focus on providing a physical mechanism for 1-eV deep traps at an average separation of 3 nm within a polymeric dielectric. This would bring about the ability to engineer important dielectric properties as well as improved understanding of the physical basis of aging and other important phenomena in dielectrics.     

Feature article - Polymeric HVDC cable design and space charge accumulation. Part 2: insulation interfaces

The electric field distribution of cable insulation systems under HVDC can be affected significantly at interfaces due to space charge build-up. In this article, the second part of a three-article series, face and space charge accumulation are analyzed first in terms of macroscopic physics, then through approximate mathematical models that will be used to fit experimental data obtained for model cables having two insulation layers and constituting cylindrical interfaces.     

Measurement of space charge distribution in epoxy resin

Internal space charge behavior of insulating materials has recently attracted attention of many researchers, and a large number of experimental studies were carried out by using the materials for dc cables, such as XLPE, LDPE, and HDPE. Epoxy resins are used for insulation under strong electric fields in power apparatus and in electronic devices, and we investigated the behavior of internal space charge using the pulsed electroacoustic method. Two types of epoxy resins were studied: insulation-grade and craft-grade. When dc electric fields were applied to the craft resins treated by immersing them into room-temperature water for 8 and 24 h, positive and negative charges accumulated near the anode and the cathode, respectively, and the charge distribution changed with the immersion time. On the other hand, no charge was observed in the insulation-grade epoxy resin. Next, we treated the samples by immersing them into 100 °C water for 8 h. When the sample was treated for 8 h, hetero charge distribution, which means positive charges near the cathode and negative charges near the anode, was observed. This result is consistent with a previous paper reporting that by chemical analysis, secondary decomposition had occurred. These results show that water and temperature influence the internal space charge behavior of epoxy resins. © 1999 Scripta Technica, Electr Eng Jpn, 129(3): 9–16, 1999