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。     

变频电机用聚酰亚胺薄膜电老化特性研究

为研究变频电机用耐电晕型聚酰亚胺材料耐局部放电老化机理,采用电导电流法分别测量了普通聚酰亚胺薄膜(100HN)和耐电晕型聚酰亚胺薄膜(100CR)电晕老化前、经10kV/mm、20kV/mm电晕强度8h老化后的电老化阈值。结果表明,2种薄膜的电老化阈值随电老化强度的增加而减小,且耐电晕型薄膜的电老化阈值始终高于普通膜的阈值;在2种薄膜的空间电荷限制电流区内,耐电晕型薄膜的电流增长率较大,说明其中含有更多的浅陷阱,测试结果与其170°C下退极化电流测试结果一致。这些浅陷阱有效调节了聚酰亚胺薄膜中电场的分布,改善其耐电晕性能。     

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。     

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     

Investigating dc polyethylene threshold by space charge. Currentand electroluminescence measurements

It has been proposed recently that the lowest level of dc field, above which electrical degradation may occur, is associated with the formation of space charges in insulation. Therefore, inference of field levels above which space charge accumulates in a given material may provide fundamental information for material characterization and system design. Threshold field values were estimated through evidence of space charge accumulation derived from conduction current, space charge amplitude and electroluminescence (EL) counts vs, electrical field characteristics. Press-molded plates of polyethylene-based materials were used for this investigation. In order to be able to detect even by EL experiments the presence of the space charge accumulated at low fields, measurements were carried out during both polarization and short-circuit of specimens. It is shown that the threshold corresponding to the onset of EL upon grounding the specimen is close to the space charge accumulation threshold derived from space charge and conduction current measurements. This earlier value is considerably lower than the one previously detected from constant dc-voltage EL measurements, thus confirming that even at low fields and in the presence of voltage reversals or transients, space charges can trigger luminescence phenomena. The methodology developed in this paper enlarges the applicability of the EL technique to accommodate a wider electrical field range in view of insulation characterization     

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.     

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.     

Evaluation of medium voltage enameled wire exposed to fast repetitive voltage pulses

The paper describes the investigation of the failure mechanisms of medium voltage inter-turn insulation, as a consequence of pulse aging. To study the performance and failure behaviour of enameled wires, models of inter-turn insulation are prepared with enameled wires having polyimide and nano-filled coatings. The aging of such bar samples is attained by sinusoidal (60Hz), fast repetitive unipolar voltage pulses, and high frequency AC waveforms. To establish the residual strength of the bar samples, DC breakdown voltages are determined before and after the aging. In addition, the thermally stimulated depolarization current (TSDC) is used to provide information about the relaxation processes, such as the disorientation of the dipoles and the release of charges from the trapping sites in enameled wires. The various trapping levels, which have discrete energy differences with distinct peaks in the thermograms, are discussed in terms of space charge polarization     

A space-charge based method for the estimation of apparent mobility and trap depth as markers for insulation degradation-theoretical basis and experimental validation

Assuming that aging of polymeric insulation consists of irreversible chemical-physical and microstructural modifications, effective ageing markers could be achieved from measurements of properties, which should vary with material microstructure. Among these properties this paper focuses on the estimation of apparent trap-controlled mobility and trap distribution, based on the results of space charge measurements. The depolarization characteristic obtained recording space charge magnitude when, after poling, electric field is removed and electrodes are short-circuited and grounded, is the experimental reference for the application of the expressions for apparent trap-controlled mobility and trap depth calculation derived in this paper. A simple relationship is proposed for apparent mobility, which is affected by significant approximations, but can be applied successfully for material characterization and aging investigation. It is applied to experimental results coming from space charge measurements carried out by the pulsed electroacoustic (PEA) technique on cross-linked polyethylene (XLPE) specimens, unaged and aged, cut from high-voltage cables. The trap depth distribution is calculated for the same material, and the results are cross-correlated with mobility estimates, revealing good agreement. Both apparent trap-controlled mobility and trap depth distribution reveal noticeable modifications with aging. The proposed methodology permits finding information on charge trapping/detrapping processes in polymeric materials, using a simple experimental procedure, as well as to extract quantities that seem very attractive as aging markers.     

Polarization and dielectric behavior of ac-aged polyethylene

High field polarization and dielectric relaxation behavior have been used to study the aging mechanisms of crosslinked polyethylene (XLPE) and low density polyethylene (LDPE) aged at constant alternating electrical field stress in a humid environment at room temperature. For this study, the dielectric spectroscopy data in the frequency range of 10^-5 to 10⁶ Hz and their comparative analysis, have been used to provide electrical analog models of the aging. The dielectric data in the frequency range of 10^-5 to 5×10 ^-2 Hz were calculated from the desorption current, whereas bridge measurements provide the data in the frequency range of 10 Hz to 10⁶ Hz. In addition, thermal transient current and thermally stimulated discharge current measurements have been carried out in order to determine the polarization and space charge accumulation in ac aged polyethylene. A correlation has been observed in the behavior of polarization and dielectric relaxation in the present work. The dielectric behavior of the XLPE cable samples and of the LDPE flat samples show a presence of both the inter-cluster (low frequency) charge exchange and the intra-cluster (high frequency) charge motion, the former mechanism becoming more dominant as the aging progresses. For the XLPE cable samples ac aged in a humid environment at room temperature, relaxation peaks obeying the fractional power law have been observed in addition to a quasi-dc process     

Space charge development and breakdown in XLPE under DC field

The correlation between space charge accumulation and insulation failure has been investigated in four, polyethylene-based materials subjected to a DC field of 50 kV/mm. Two of the materials contained tree-retardant additives and all four materials were aged for extended periods up to 19,000 h. The specimens were removed periodically from the external field and subjected to space charge density measurements under no-voltage conditions using the thermal step (TS) technique. Not a single parameter that would consistently correlate with times to insulation failure could be derived from the space charge measurements. However, the space charge measurements provided valuable insight into a possible link between the dynamics of space charge development and the time of DC aging. In particular, a consistent surge in the amount of accumulated space charge shortly before insulation breakdown was observed in practically all samples. Also, a consistent relationship between time to breakdown and the position of the internal space charge density peaks in the sample was observed. Samples that had space charge density peaks further apart consistently survived longer than specimens for which the peaks were closer together. These phenomena, if confirmed, could be used to screen new materials for DC voltage applications.     

交联聚乙烯电缆空间电荷与理化性能的关系

针对高压交联XLPE电缆,通过对未老化、加速老化1年和实际运行30年的电缆绝缘的空间电荷特性、力学性能及理化性能进行研究,分析了老化过程中电缆绝缘空间电荷分布与理化性能之间的关系。结果表明:沿电缆径向由内向外,未老化电缆电荷积累量增加,加速老化1年的电缆电荷积累量呈下降趋势,实际运行30年的电缆电荷积累量上升。结合力学性能及理化分析认为,加速老化电缆绝缘老化起始于绝缘内侧,并且影响到绝缘中间部位;而实际运行30年的电缆绝缘老化起始于绝缘外侧。     

基于介质损耗分析研究变频电机绝缘老化特性

变频牵引调速电机匝间绝缘承受来自逆变器的连续高压脉冲方波的作用,其老化机理与工频正弦电压作用下具有很大差异。笔者通过采用两种电压源对模拟变频电机匝间绝缘和对地绝缘的绞线对试样进行老化试验,测量了tanδ随老化时间的变化趋势,分析了电压形式对匝间绝缘和对地绝缘介质损耗产生和发展的作用机理。试验结果表明:脉冲电压下,匝间绝缘内部有放电发生时,放电与空间电荷协同作用使绝缘缺陷迅速增多;无放电时空间电荷反复注入和抽出造成绝缘损伤。交流电压下绝缘介质发热主要由局部放电造成,而脉冲电压下主要是由空间电荷所导致。对地绝缘由于其绝缘结构,呈现槽放电现象,空间电荷的引入大大加速了绝缘的老化。     

不同热老化程度油纸绝缘介质在直流电场中的空间电荷特性

为理解油纸绝缘介质的热老化程度与其体内空间电荷特性间的关系,对由#25矿物变压器油和普通植物纤维素绝缘纸组合而成的油浸绝缘纸试样进行加速热老化试验,应用电声脉冲（PEA）法测量了不同热老化程度下试样在直流电场（20 kV/mm）作用时和作用后的空间电荷密度分布图像,讨论了热老化程度对试样内部空间电荷分布及电荷总量的影响。结果表明：随着油浸绝缘纸试样热老化程度的加深,外加直流电场时试样中部空间电荷注入量呈增大趋势,电荷密度到达极值所需时间延长;去压后试样内部积聚电荷消散所需时间明显延长,将致使材料内部场强畸变时间增加。研究得到去压后试样内空间电荷密度极值及电荷总量与消散时间呈现指数衰减关系,可用衰减时间常数τ来表征不同老化状态油纸绝缘的电荷消散能力。该研究成果为进一步评价油纸绝缘介质在不同热老化程度下的空间电荷特性提供了参考依据。     

基于界面电荷极化特性的变压器油纸绝缘寿命预测方法研究

准确预测变压器油纸绝缘寿命对于保障电力系统安全稳定运行具有重要意义。根据微观介电响应机制剖析界面极化与油纸绝缘老化的内在机理联系,并基于混合极化模型——一种在扩展德拜模型基础上引入界面极化支路的等效电路拓扑,探究界面电荷极化特性在油纸绝缘寿命预测中的应用。首先,分析温升过程中频谱特性变化背后蕴含的普适弛豫极化规律,基于混合极化模型重构回复电压极化谱。结合温度对主时间常数和模型参数的影响规律,提出一种单一温度测试下的油纸绝缘活化能计算方法,通过界面极化电容构建出频温平移因子。其次,剖析介质损耗因数随老化加深的变化规律,探究损耗峰偏移现象与界面极化弛豫时间的内在联系,凝练全新表征油纸绝缘聚合度含量的频域特征参量——界面极化支路极点Ph2。最后,联合界面极化支路极点Ph2、频温平移因子和损失累积动力学方程构建油纸绝缘寿命预测模型,并将不同运行年限变压器现场实测数据代入模型验证实效性,为油纸绝缘寿命预测提供了一种新思路。     

高频脉冲下牵引电机绝缘的局部放电特性

变频牵引电机定子绝缘承受来自变流器的连续高压方波脉冲,可能导致绝缘过早失效,其老化机理与工频交流电压作用下差异很大。局部放电是导致变频调速牵引电机绝缘失效的主要原因之一。研究高频方波脉冲下的局部放电特性为牵引电机绝缘材料的改进和优化提供理论基础,试验分析了普通和聚酰亚胺纳米复合薄膜制成的电磁线寿命,结果发现纳米试样的寿命得到很大提高。针对普通和聚酰亚胺纳米复合薄膜研究了不同脉冲频率、上升时间和温度对放电起始电压、平均放电量和放电次数的影响。结果表明,频率的增加、上升时间的缩短和温度的增加加剧了局部放电活动,普通薄膜试样的局部放电活动更强,其原因是纳米粒子的添加导致纳米复合薄膜存在大量界面区和电导率增加,电荷的迁移速率增大,空间电荷累积效应减弱。     

Electrical aging and life models: the role of space charge

This paper has the aim of providing a view of a lively debated topic which has broad impact on the design of electrical apparatus and new insulating materials, that is, the interaction between space charge and aging processes of polymeric insulation. Aging models developed in recent decades that consider explicitly or implicitly the contribution of space charge to insulation degradation, under both DC and AC voltage, are dealt with, with the intention to point out their range of validity. Some conventional phenomenological models that have been used for much more than two decades without referring to space charges can be exploited to account for electrical field and activation energy modification due to space charge. These, together with models conceived considering space charges as the driving force for aging, are especially examined. In addition, recent models that disregard the action of space charge as an ageing factor, but consider space charge as the consequence of degradation processes are also discussed.     

Luminescence and space charge in polymeric dielectrics - [whitehead memorial lecture (2008)]

Polymers are extensively used as insulating material in high voltage devices, such as underground power cables, power capacitors and transformers. During normal operation the polymeric insulation of a power device is not only subjected to electrical stresses, but could also be subjected to other stresses that can cause the degradation and ultimately lead to insulation failure. It has been well established that electroluminescence and charge injection, which gives rise to space charge in the polymeric insulation, occur at ac, dc and impulse field above a certain threshold value. Space charge can cause dissipative energetic processes such as photon and phonon emission, increase the local electric field and reduce the withstand voltage of the insulation. This paper describes the characteristics of electroluminescence in polymeric insulation subjected to ac voltage and shows its relevance to space charge injection in the material. It is shown that insulation subjected to high voltage could emit various types of light but electroluminescence emission which is related to space charge injection can be clearly distinguished from these other types of light emission. The electroluminescence technique is a valuable tool to evaluate the dielectric properties of novel insulating materials, such as nanodielectrics.     

Electrical measurements for the characterization of electret properties of cellular polymers

Cellular polymeric materials may show interesting quasi-piezoelectric properties, upon charging under specific conditions of electrical field, temperature, pressure and environment. Electrical charging is the trigger of electret properties. It has been proved that the occurrence of electrical avalanches in the cavities which are present in a material is associated with the appearance of electret behavior, but the precise nature of the phenomena involved in this transformation is still under investigation. This paper provides a contribution to the study of these materials focusing on the relation between the appearance of electret properties and the presence of charge injection and partial discharge phenomena. For this purpose, space charge, partial discharges and low-frequency polarization measurements were carried out on polypropylene (PP) cellular materials, at different fields and temperatures. The existence of a threshold field for the start up of electret behavior that is larger than the threshold for space charge injection and partial discharge inception was observed. Steady charge is accumulated in the material, thus generating electret behavior, only if partial discharge repetition rate becomes sufficiently high and space charge is injected from electrodes. Space charge measurements were performed through the pulsed electroacoustic technique, in spite of significant attenuation of acoustic waves due to the cellular nature of the materials. Partial discharge measurements were carried out by an advanced system recording each discharge pulse, thus allowing observation of single and multiple pulses, besides achieving statistics of discharge pulse amplitude and phase. By such approach, space charge and partial discharge measurements proved to constitute a valuable tool to investigate the phenomena leading to piezoelectric behavior and, also, to infer electret time stability and optimize charging methodologies.     

交流电压下基于空间电荷效应的XLPE电缆绝缘老化研究现状

交联聚乙烯(XLPE)电缆以其优良的机械和电气性能广泛应用于现代电力系统。研究表明,在直流电压作用下绝缘中容易形成空间电荷,导致电场畸变,加速绝缘老化。国内外很少关于交流电压下空间电荷对XLPE电缆绝缘影响的研究。本文综述了交流电压下空间电荷对XLPE电缆绝缘老化的影响及其作用机理,并介绍了交流电压下测量空间电荷分布的改进的电声脉冲法。结果表明,交流电压下,空间电荷分布特性影响XLPE电缆绝缘老化。