The electrical degradation threshold of polyethylene investigatedby space charge and conduction current measurements

Measurement of dc charging current and space-charge observations, performed on specimens made by low-density polyethylene (PE) and crosslinked PE, are considered for dc electrical threshold investigation. The threshold is inferred from charging-current measurements through steady-state voltage-current characteristics, as well as from the total trapped charge, obtained by space-charge measurements carried out at 0 V and under short-circuit after poling for a pre-selected time. It is shown that both techniques provide similar information regarding the electrical threshold values, which vary for the two tested materials. Being the threshold associated with mechanisms of charge storage, its evaluation can help in insulation design and material characterization when electric field, especially dc, is a significant stress in service. Other significant information for design purposes comes from the rate of charge accumulation as function of the applied field, which is estimated through the charge-field characteristic derived from space-charge measurements     

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     

Electroluminescence excitation mechanisms in an epoxy resin underdivergent and uniform field

Electroluminescence (EL) excitation mechanisms have been investigated in epoxy resin under divergent and uniform field situations. Metallic wires embedded in the resin were used to produce field divergence whereas film samples were metallised to obtain a uniform field. EL under divergent field was stimulated by a pulse voltage. Light was emitted on the positive and negative fronts of the pulses when the field exceeded 20 kV/mm at the wire surface, with equal intensity and without polarity dependence. There was evidence of space charge accumulation around the wires in multiple-pulse experiments. Charge injection and extraction occurring at both fronts of the pulse provide the condition for EL excitation. Further excitation of the EL during the plateau of the voltage pulse is prevented by the opposite field of the trapped charge. Field computation with and without space charge supports the proposed interpretation. A dc voltage was used for the uniform field experiments. A continuous level of EL is found at 175 kV/mm. Charging/discharging current measurements and space charge profile analyses using the pulsed electroacoustic (PEA) technique were performed at different fields up to the EL level. Dipolar orientation generates a long lasting transient current that prevents the conduction level being reached within the experimental protocol (one hour poling time). The continuous EL emission is nevertheless associated with a regime where the conduction becomes dominant over the orientational polarization. Polarization and space charge contribute to the PEA charge profiles. Homocharge injection at anode and cathode is seen at 20 kV/mm and a penetration of positive space charge in the bulk is detected above 100 kV/mm, suggesting an excitation of the continuous EL by bipolar charge recombination     

Polymeric HVDC Cable Design and Space Charge Accumulation. Part 1: Insulation/Semicon Interface

From theory and experiments, it can be deduced that materials for DC applications should not accumulate a large amount of space charge if accelerated degradation of the insulation system is to be avoided. Therefore, the characterization of DC insulation must take into account the evaluation of space charge accumulation. This cannot be done exhaustively without taking a system approach considering both the semiconductive material and the insulation, in particular, the properties of the semicon/insulation interface. The latter interface, in fact, plays a major role in space charge injection/accumulation in the insulation bulk. Having analyzed different semiconductive and insulating materials candidate for HVDC cable applications, the best solution to be exploited for HVDC cable design would be the combination showing a high threshold for space charge accumulation, a small rate of charge accumulation as a function of electric field and a small activation energy, i.e., a space charge amount less dependent on temperature. Therefore, space charge measurements will provide important information to cable material manufacturers with the aim of tailoring insulation and semicon specifically for HVDC application and, thus, improving the reliability of polymeric cables.     

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。     

Electrical Breakdown Properties and Space Charge Formation in High Temperature Region in Ultraviolet Ray Irradiated PVC

Polyvinyl chloride (PVC) is the most popular insulating material for electric wiring instruments. However, an exothermic reaction above 150 °C may cause deterioration of the insulating properties of PVC. Therefore, it is important to clarify the heat degradation in PVC, not only to investigate the ignition of electrical wiring products but also to use electrical products safely. It is known that ultraviolet (UV) irradiation causes chemical deterioration of PVC and an increase in its conductivity. Generally, it has been thought that the electrical breakdown properties, electrical conduction, and insulating performance are affected by space charge accumulation in an insulating material. A high temperature pulsed electroacoustic (PEA) system usable up to 250 °C has been developed, and the PEA system can measure the space charge distribution and conduction current in the high temperature range simultaneously. In this investigation, the space charge distribution and conduction current were measured up to electrical breakdown in a non‐UV irradiated sample (normal PVC) and in 353 nm and 253 nm UV‐irradiated PVC samples in the range from room temperature to 200 °C in a DC electric field. In the short wavelength UV irradiated PVC sample (253 nm, 300 h), a deterioration of breakdown strength at 90 °C to 150 °C and negative packet‐like charges were observed at 60 °C and 100 °C, a positive charge accumulated in front of both the anode and cathode above 90 °C, and a higher electric field near the cathode side because the positive charge of the cathode side was greater.     

Electroluminescence measurements to detect accumulated charge atthe electrode-insulator interface

Electroluminescence (EL) spectrum analysis is proposed to detect charge accumulation in insulators. The objective is to find the nature of accumulated space charges at the electrode-insulator interface by using only EL measurements recorded under a moderate ac 50 Hz field. The differences between spectra obtained with virgin samples and treated samples are analyzed. Some experience obtained with 500 μm thick low-density polyethylene (LDPE) samples are described to illustrate the expected modification of the EL spectrum vs, interfacial space charges. Direct space charge measurements, using the laser induced pressure propagation (LIPP) method are used to verify our assumptions. Good agreement is found between EL analysis and experimental data of space charges     

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.     

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.     

Recent developments in the pressure wave propagation method - [Feature Article]

Performing measurements is an essential part of understanding physical phenomena. Hence, numerous measuring techniques have been developed to study space charge build-up in dielectrics. This macroscopic accumulation of charge modifies the electric field and can lead to aging and premature failure of electrical insulation. On the other hand, charge accumulation is essential to the operation of devices such as electret- or piezoelectric-based sensors. It is therefore essential to study charge build-up in order to design insulating materials in which excessive charge does not accumulate, or electrets in which a sufficiently large amount of charge accumulates to ensure adequate device operation, and the stability of that charge in time is sufficient to ensure an acceptable device life-time. This paper focuses on the measurement method using a pressure wave to perturb the charges. That method is commonly called the pressure-wave-propagation (PWP) method. A pressure wave enters the sample through one of its interfaces. As it travels through the material at the velocity of sound, the charges encountered are mechanically displaced.     

Time-resolved space charge and electroluminescence measurements in polyethylene under ac stress

Time-resolved space charge (SC) and electroluminescence (EL) measurements are carried out on polyethylene films stressed under ac voltage at industrial frequency to probe injection, trapping and recombination of charge. SC is measured by pulsed electro-acoustic (PEA) technique at each zero voltage cross-over point of the ac voltage, thereby getting rid of the capacitive charge. EL is detected using photon counting techniques with a time resolution down to 50 /spl mu/s. Both SC and EL exhibit a threshold response as a function of the applied voltage. Numerical values of these thresholds are in good correspondence for SC and EL measurements in every tested material. Observations are consistent with a model of bipolar injection with an unbalanced situation in positive and negative charge behavior. It is also shown that different polyethylene materials behave differently relative to the field above which the charge is detected, providing thereby a way to compare the space charge behavior of those materials under ac field.     

Understanding treeing phenomena and space charge effect in gamma-irradiated XLPE cable insulation

In the present work, electrical trees were experimentally generated in virgin and gamma-irradiated XLPE cable insulation, under AC voltages. Fibrillar type of electrical tree structure forms from the defect site in gamma-irradiated specimens. Failure times due to electrical trees were analysed by adopting Weibull distribution studies. The results of the study indicate that gamma-irradiated specimens, where the rate of tree propagation is high, causing early failure of insulation. Pulsed electro acoustic emission (PEAE) technique was adopted to understand the space charge formation in XLPE cable insulation. The results of the study indicate that homo and hetero charges form near to the electrodes especially in gamma-irradiated XLPE cable insulation. Variations in space charge density at different voltage magnitudes with time were analysed. It is observed that the electric field in the insulating material becomes highly non-uniform on occurrence of hetero charges. It is also realized that on removal of applied voltage, the charge decay is less with gamma-irradiated specimens. Based on the present study, it could be concluded that Space charge density in insulating material and life of insulating material due to electrical tree formation shows inverse relationship.     

Space charge behavior in XLPE cable insulation under 0.2-1.2 MV/cmdc fields

This report deals with space charge behavior in PE (polyethylene) under dc fields. Direct observation of time-dependent space charge profiles in 3-mm thick XLPE (crosslinked low-density polyethylene) cable insulation under dc electric fields was performed using the pulsed electroacoustic method. Stable hetero charges were formed when the field was as low as 0.2 MV/cm, and intermittent generation of packet shaped space charges and their propagation through the insulation were observed when the field was as high as 0.7 MV/cm. These phenomena were reproduced in sheet specimens of XLPE and LDPE (low-density polyethylene). It was found that hetero charges resulted from heat treatment of the XLPE specimen containing antioxidant and acetophenone, which is one of the crosslinking by-products, suggesting dissociation of the antioxidant through solvation at high temperature by acetophenone. The packet charges were easily detected when acetophenone was diffused into the LDPE specimen. However, uniformity of acetophenone distribution prevented the packet charge generation. It is suggested on the basis of several experimental results that local ionization of impurities in the insulation through solvation by acetophenone takes place assisted by high field and leads to the packet charge generation. A numerical simulation was carried out based on the above model     

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.     

空间电荷与直流电导联合测试技术用于纳米MgO抑制XLPE中空间电荷的研究

粒径70nm的MgO以不同浓度与交联聚乙烯(XLPE)共混制成聚合物纳米复合介质。采用自主研发的四电极系统同时测量复合介质的高场电导特性和空间电荷分布。通过强场电导实验发现,在室温下,XLPE及纳米MgO/XLPE复合介质的电导机理不是单纯的空间电荷限制电流(SCLC)。此外,添加纳米MgO可以明显地提高空间电荷的注入阈值,并且在低于空间电荷注入阈值的电场下,复合介质的电导电流密度随纳米MgO浓度的增加,先减小后增大。最后从空间电荷的实验数据验证了纳米MgO能有效抑制XLPE中空间电荷,并进一步定性地认为纳米Mgo的添加提高了电子注入的电场强度阈值。     

Electrical Breakdown and Space Charge Formation at High‐Temperature Region in Long‐Time Heating Treatment PVC

Polyvinyl chloride (PVC) is widely used as an insulating material in various electrical products. It is reported that an exothermic reaction reaching temperatures above 150 °C can be caused by overload currents or inferior electrical wire connections before the ignition of electrical products. The exothermic phenomenon may cause deterioration of insulating properties in PVC due to its chemical decomposition. It is necessary to clarify the degradation of insulating properties in PVC under thermal stress exceeding 150 °C for the safe use of electric products. In this investigation the space charge distribution and conduction current in the heat‐treated PVC sheet were measured in the range from room temperature to 200 °C in the presence of a dc electric field, using a high‐temperature PEA system. Positive charge injection and increasing conduction currents were observed before breakdown above 100 °C in 100 °C 300‐h heat‐treated samples and in non–heat‐treated samples. The results indicate the thermal breakdown process from the analysis of conduction currents and electric fields. In samples exposed to higher temperatures (150 °C 100 h), the breakdown strength deteriorated strongly in the range from room temperature to 90 °C. Increases in conduction current were observed in the entire temperature range before breakdown of the 150 °C 100‐h heat‐treated PVC. This indicates that heat treatment above 150 °C degrades the breakdown properties in the range from room temperature to 90 °C due to thermal decomposition accompanied by dehydrochlorination in PVC. The electric field is intensified near the cathode due to positive charge accumulation, and the breakdown strength begins to deteriorate only above 90 °C. This shows that thermal stress exceeding 150 °C causes deterioration of insulating properties and that the breakdown process is affected by space charge formation in PVC.     

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.     

直流XLPE电缆绝缘中空间电荷的抑制方法综述

交联聚乙烯(XLPE)因其优异的介电、理化性能而被广泛应用于电缆绝缘领域。在电缆的服役过程中,电缆绝缘内部会积聚空间电荷,严重时可引发电场畸变,导致电缆击穿事故发生。对于直流XLPE电缆,空间电荷的积聚及影响更加不容忽视。针对直流XLPE电缆绝缘中产生的空间电荷积聚效应,目前学界主要采用共混改性、聚合物链段接枝极性基团、纳米掺杂改性及制备高纯净绝缘料等方法来进行控制,改性后的直流XLPE电缆绝缘对空间电荷产生的抑制效果均有所提升。文中首先对上述直流XLPE电缆绝缘中空间电荷的抑制方法进行综述,介绍其抑制原理以及相应的抑制效果,然后对比总结不同抑制空间电荷方法的优缺点,最后对未来直流XLPE电缆绝缘中空间电荷抑制方法的研究发展作出展望。     

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     

油纸绝缘介质的空间电荷积聚与消散特性

油纸材料的绝缘问题在换流变压器、直流套管、直流电缆等高压大型直流设备大量应用的情况下显得十分突出。为探讨高压直流设备绝缘的最主要问题—空间电荷效应,应用电声脉冲法(PEA)对油纸绝缘材料的空间电荷特性进行了研究。有关外加场强对油纸材料中空间电荷积聚情况的影响和在较高场强下油纸材料的击穿破坏与空间电荷关系的研究结果表明:①低场强下油纸材料中空间电荷以电离产生为主;而在较高场强下,先后在阴极和阳极产生了同极性载流子注入。②相对于聚乙烯而言,由于油纸材料的电导率较大,材料内的空间电荷在外加电场撤去后很快消散。③空间电荷的注入和运动会导致油纸材料的劣化和破坏。油纸材料中的空间电荷快速消散现象有利于直流设备在极性反转条件下的运行,为阐释油纸绝缘良好的长期性能提供了有重要意义的试验依据。