空间电荷测量表征聚乙烯中电荷输运特性

通过电声脉冲(PEA)法测量了低密度聚乙烯(LDPE)在不同场强作用下的去极化特性。根据空间电荷限制电流(SCLC)理论,通过公式推导求解了聚乙烯的平均电荷体密度、视在迁移率、陷阱深度分布及阈值场强。结果表明聚乙烯中的陷阱或缺陷,即其微观形态与宏观上空间电荷的分布密切相关。     

茂金属聚乙烯改性低密度聚乙烯中电荷入陷与传输特性

聚合物的电性能与其物理、化学、微观结构密切相关.通过电声脉冲(PEA)法测量少量茂金属聚乙烯(MPE)与低密度聚乙烯(LDPE)共混物在不同场强作用下的去极化特性.根据空间电荷限制电流(SCLC)理论,通过公式推导求解共混物平均电荷体密度、视在迁移率、陷阱深度分布及阈值场强.结果表明,1%MPE与LDPE共混,能有效减少深陷阱密度,增加浅陷阱密度,提高电荷迁移率和阈值场强.     

Quantities extracted from space-charge measurements as markers for insulation aging

The relation of space charge-derived quantities with aging stresses and time is discussed in this paper. Polyethylene-based materials, aged for different times under electrical and/or thermal stress, are subjected to space charge measurements. Quantities are derived, that is, the threshold for space charge accumulation and the apparent, trap-controlled mobility, obtained by the threshold and the depolarisation characteristics, respectively, which show time variations depending on the level of applied stress and aging time, i.e. on the extent of degradation. This indicates that such quantities can be considered as aging markers useful for the diagnosis of bulk degradation in insulation systems aged under electrical field, dc or ac, and temperature.     

热老化对纤维素绝缘纸的空间电荷及陷阱特性的影响

为理解变压器用纤维素绝缘纸在其热老化过程中的空间电荷特性演变规律及其影响机制,采用电声脉冲法研究不同老化程度绝缘纸在不同电压等级(30、15和5kV/mm)下的加压和去压后的电荷密度分布规律,分析了绝缘纸热老化程度与其空间电荷特性演变规律间的联系。结果表明:纤维素绝缘纸体内电荷注入量及注入速率与外施电场等级成正比关系。热老化能够降低纤维素绝缘纸空间电荷注入的起始阈值,并减缓绝缘纸体内电荷注入和迁移的速率,从而削弱了体内电荷的复合现象。随着热老化程度的增加,纤维素绝缘纸体内陷阱的分布密度和陷阱能级的深度都将增大,致使材料体内残留电荷不仅总量逐渐增大,而且衰减速率逐渐降低。研究成果为理解热老化对纤维素绝缘纸空间电荷积聚和消散的影响机制提供了理论支撑。     

形态对聚乙烯中空间电荷包运动特性的影响

空间电荷相关研究是直流电介质材料特性研究的重要领域,聚乙烯则是主要的内绝缘材料之一。为了对高压直流条件下聚乙烯中的空间电荷现象进行深入研究,应用电声脉冲法空间电荷测量系统对高场强下低密度聚乙烯中的空间电荷现象进行测量,观察到了空间电荷包现象。研究结合不同热处理方法得到了不同微观形态的聚乙烯试品,并在不同场强下结合微观形态的变化对低密度聚乙烯的空间电荷包运动速率进行了分析。结果表明,聚乙烯的微观形态对空间电荷包特性有显著的影响,也对空间电荷包运动速率有明显的影响,结晶度越高,空间电荷包运动速率越小。     

Deep trapping centers in crosslinked polyethylene investigated bymolecular modeling and luminescence techniques

Space charge in crosslinked polyethylene (XLPE) has been detected under both ac and dc fields. Its role in electrical aging and breakdown is recognized, but not deeply understood. It is thought that identification of the trapping centers in this material would help improve the modeling of conduction and electrical aging as well as making possible the design of crosslinked materials with improved properties. We have developed theoretical and experimental approaches to this problem which emphasize the role of chemical traps acting as deep trapping centers. Molecular modeling is used to estimate the trap depth for negative and positive charge carriers associated with the main by-products of crosslinking reactions (using dicumyl peroxide as a crosslinking agent) since their aromatic structure makes them candidates for deep traps. Calculations on acetophenone, n-methyl styrene and cumyl alcohol show that they indeed can act as deep traps. Because such deep traps can act as recombination centers, their involvement in charge trapping can be checked in specially designed luminescence experiments. In our experiments, charges of both polarities are generated at the surface of the material under study by using a non-reactive cold plasma in helium. The analysis of the decay kinetics and emission spectrum of the subsequent luminescence allows us to define unambiguously the time range in which charge recombination is the dominant excitation process of the luminescence. The emission spectra obtained within this time range provide the optical fingerprint of chromophores acting as deep traps in the material. The low-density polyethylene (LDPE) doped with crosslinking byproducts and XLPE (film and cable peeling) have been investigated. Their role in charge trapping is apparent in the luminescence experiments and in space charge distribution analysis. In thermally treated XLPE, it is shown that other species strongly bonded to the polymer chain are also able to trap electrical charges     

聚合物绝缘材料表面陷阱与电致发光现象研究

聚合物材料老化前会出现电致发光现象，其紫外分量可能导致材料的劣化。因此电致发光的研究可为分析材料老化和击穿机理提供依据。该文首先基于等温电流衰减理论，设计了材料表面陷阱参数测量装置，实验中采用多针电极对低密度聚乙烯(polyethylene，LDPE)、聚丙烯(polypropylene，PP)和聚四氟乙烯(polytetrafluoroetylene，PTFE)3种材料进行表面充电，记录表面电位衰减并按照等温电流衰减理论计算得到材料表面陷阱参数。结果显示，LDPE的陷阱密度较PTFE和PP大，LDPE和PTFE的电子空穴陷阱分布成对称结构，PP的电子陷阱密度明显大于空穴陷阱密度，这与材料的分子结构的对称性有密切关系。同时基于单光子计数模块设计了微弱发光数据采集系统，对上述材料在真空中交流电压下的电致发光特性进行了测量和分析。材料的陷阱密度对其电致发光特性具有重要的影响，材料的陷阱密度越高，发光越弱，反之则越强。     

Space charge phenomenology in polymeric insulating materials

Topics related to space charge phenomenology, particularly the origin of space charge and its interaction with the host insulating polymer, have been widely discussed in recent years. Due to the development of reliable techniques for direct observation of space charge it is now possible to obtain, besides obvious indications on space charge and electric field magnitude and location, quantities that provide additional information on space charge accumulation and depletion rate, apparent mobility and trap depth distribution. Therefore, commonly debated topics such as the nature of charge carriers, the conduction mechanism, the charge dynamics at interfaces and the relation with nanostructure can be re-examined on the basis of space charge observation and this is, indeed, the purpose of this paper.     

Elemental strain and trapped space charge in thermoelectrical agingof insulating materials: life modeling

On the basis of the new concept of elemental strain, used to describe a possible aging process occurring in polymeric materials, the expression for the life model, already proposed in a previous paper, must be modified, taking into account the possibility that the local strain is an elemental property of the polymer morphology, as suggested by molecular quantum calculations. By doing so, a connection can be established between the life model and space charge densities as revealed by experiments. The derivation of this new model and the correlation with analytical measurements is the subject of this paper. Applying this connection to electrical aging and space charge data for the same crosslinked polyethylene (XLPE) material has resulted in a set of material factors that we have demonstrated are both consistent with experiments and also produce physically feasible space charge fields that are capable of driving the proposed aging mechanism to the extent required     

Potential buildup in samples charged by successive low-energypulses

We present a calculation for the charging of a dielectric sample submitted in open circuit to a series of consecutive, identical, electron pulses of low energy. The calculation is based on deep trap kinematics and takes full account of space charge effects. Electric field profiles and the potential buildup after n⩽10 pulses are presented, allowing one to infer the important parameter μτ of the material (μ the mobility and τ the trapping time). A comparison with the results of a simple calculation which neglects space charge effects is also given     

伽马射线对低密度聚乙烯的空间电荷陷阱特征影响

为研究伽马射线辐照后的低密度聚乙烯材料在直流电场作用前后的空间电荷形成和衰减的陷阱特征,在60Co伽马射线的辐照源下得到最大辐照剂量为50 kGy的聚乙烯样品,利用电声脉冲法测量其空间电荷分布,进而提出了一种离散陷阱分布模型来解释射线辐照的空间电荷衰减特征。结果表明:在阴极附近出现异号的正电荷积累并随着辐照剂量增加而增加;分析认为射线辐照后样品出现的异号正电荷是由于材料发生了化学变化(氧化)而导致更深能级陷阱的产生;同时,利用所提出的模型计算了陷阱的能级深度和陷阱密度参数,并分析了它们与辐照氧化的微观机制。由陷阱能级和陷阱密度围成的曲线族变化可作为材料辐照老化的特征参量。     

Space charge characterization in aged LDPE amalgamated insulationregions from underwater telecommunication systems

The requirement of insertion of optical amplifier units and joints, into long-distance submarine telecommunication systems, results in regions of the polymeric insulation which have to be reinstated, using injection molding techniques. These techniques introduce internal amalgamated interfaces, which subsequently can be the sites of possible electrical insulation failure in the system. This paper reports on space charge measurements, (using the pulsed electroacoustic (PEA) technique), from microtomed thin film samples taken from the interface region of typical system joints which have undergone accelerated aging tests under a dc voltage at room temperature. The space charge distributions obtained for the samples are discussed with reference to the underlying mechanisms of both the bulk low-density polyethylene (LDPE) and the surface (LDPE/electrode interface). The effect of varying the electrode material (i.e. Au and Al) is discussed with reference to previously published data. Finally the effects of long-term aging on space charge accumulation within LDPE are considered briefly     

绝缘油老化对油纸绝缘介质空间电荷形成及迁移特性的影响

探索绝缘油老化或绝缘纸老化对油纸绝缘介质在老化过程中空间电荷形成及迁移特性的影响,能为有效抑制油纸介质空间电荷的形成提供科学依据。首先在130℃下将绝缘油进行22 d加速热老化,并对其老化状态进行表征;然后通过电声脉冲法测量由不同老化程度绝缘油浸渍的油纸试品,分析在加压、瞬时去压和去压情况下绝缘油老化对油纸试品直流空间电荷动态行为的影响;最后通过计算分析绝缘油老化对油纸试品的空间电荷总量、表面陷阱能级分布和电场畸变率的影响。结果表明:在负极性电源电压下,贴近阳极(铝板)侧绝缘纸层内注入的正电荷密度以及在靠近阳极侧绝缘纸–绝缘纸界面处积聚的负电荷密度均随绝缘油老化程度加深而增大;绝缘油老化越严重,相应绝缘油浸渍油纸试品的空间电荷总量、表面陷阱能级密度和电场畸变率也越大。     

Dielectric material properties investigated through space charge measurements

This paper investigates means of exploitation of space charge measurement results in order to achieve information about chemical, physical and microstructural properties of dielectric materials. The derivation of quantities as mean volume density of space charge, threshold for DC space charge accumulation, apparent-trap controlled mobility is discussed and various applications to dielectric materials are shown. In particular, the differences of trapping behavior of polyethylene-based materials, as low-density polyethylene, cross-linked polyethylene and high-density polyethylene, the effect of additives containing polar groups, the evaluation of cellular polymeric electrets are discussed on the basis of the results of space charge measurements and the relevant extracted quantities.     

电声脉冲法研究热老化对160kV直流电缆绝缘材料陷阱特性的影响

交联聚乙烯(XLPE)老化会引起内部陷阱特性的变化,进而造成空间电荷分布的变化,威胁直流电缆的安全运行。对160k V直流电缆切片进行了90℃和135℃不同程度的热老化试验,并利用电声脉冲法(PEA)探究了热老化对直流电缆空间电荷分布的影响。通过分析空间电荷的衰减规律并结合等温松弛理论,提出了利用PEA法分析绝缘材料陷阱特性的方法,并对老化前后试样内的陷阱特性进行了分析研究。研究结果表明,90℃热老化前期由于后交联反应,试样部分深陷阱转为浅陷阱,直流电缆内部空间电荷消散特性变好;但随着老化时间的进一步增加,试样内的陷阱深度及密度均有一定程度的增长,空间电荷积累量也随之增加。试样在135℃高温热老化情况下,内部发生强烈的热氧反应,结晶形态遭到严重破坏,陷阱能级及密度均大大增加。     

From LDPE to XLPE: investigating the change of electrical properties. Part I. space charge, conduction and lifetime

This paper discusses the results of space charge, conduction current and electrical lifetime measurements performed on widely-used materials for electrical insulation, that is, low density polyethylene (LDPE) and cross-linked polyethylene (XLPE). Space charge accumulation profiles were compared in LDPE, low density polyethylene plus antioxidant (LDPE+AO) and XLPE, with consideration of thermal treatment effects in LDPE and XLPE. Significant variation (decrease) of accumulated space charge and apparent mobility, as well as slight decrease of conduction current, can be seen going from LDPE to LDPE+AO and XLPE, which may be associated with formation of deeper trap levels (or an increase of their density). On the contrary, electrical life under AC voltage does not show significant differences from LDPE to XLPE. This latter result underlines that life under high AC electrical stress is mostly determined by defects (weak points) rather than material characteristics associated with charge injection and transport.     

聚合物电介质中空间电荷陷阱深度的模拟计算

利用分子动力学(MD)与密度泛函理论(DFT),建立聚合物电介质中空间电荷陷阱深度的计算模型,得出了聚乙烯分子的化学结构缺陷和过氧化二异丙苯(DCP)、α-甲基苯乙烯等杂质分子的空间电荷陷阱深度,并结合聚乙烯空间电荷陷阱能级的变化,分析了聚合物分子缺陷处空间电荷陷阱特性。模拟结果表明聚合物中苯环、羰基、醛基和共轭双键等化学键及键和链的作用是空间电荷陷阱存在的重要原因。其结果同现有的文献符合得比较好。     

Influence of oil aging on the space charge dynamics of oil‐immersed paper insulation under a DC electric field

The formation of space charge in oil/paper insulation system can lead to material degradation in the region of high electric field. Therefore, it is important to understand the factors that affect space charge formation in oil/paper insulation system. In this paper, the effect of oil aging on space charge dynamics in oil/paper insulation system is investigated using the pulsed electroacoustic (PEA) technique under a DC electric field at room temperature. The ultraviolet/visible (UV/vis) spectrum of the oil shifts to longer wavelengths, and the oil acidity increases as the aging time increases. It has been found that the oil property has a significant effect on the space charge distribution in the oil/paper insulation sample. The more the deterioration of the oil, the larger amount of negative and positive charges accumulated in the bulk of the oil/paper insulation sample. The total amount of slow moving charges, the surface trap energy density, and the electric field distortion of the oil/paper insulation sample increased with the degree of deterioration due to oil aging. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

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.     

Evaluation of dc insulation performance based on space-chargemeasurements and accelerated life tests

Space charge observation is becoming the most widely used technique to evaluate polymeric materials for dc insulation applications, particularly HV cables. However, quantities are still lacking that can help to summarize and interpret the huge amount of data resulting from space charge measurements, and that also are associated with the electrical performance of the insulation. The purpose of this paper is to propose test methods and parameters, based on the pulsed electroacoustic technique, which can be used for the evaluation of the electrical performance of polymeric insulation under dc fields. These parameters are related to the space charge magnitude and mobility as well as to the internal electric field amplification, the rate of charge accumulation, and the threshold field above which charge is stored in the insulation. In order to investigate the meaning of the parameters proposed for the assessment of insulation performance, the correlation between these parameters associated with space charge and insulation life is discussed. Examples are reported with reference to five different candidate materials for dc cable insulation