Heat treatment of cross-linked polyethylene and its effect on morphology and space charge evolution

It is believed that space charge build-up in cross-linked polyethylene (XLPE) insulation is the main cause of premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is the process of subjecting the XLPE to a moderately high temperature, thereby removing volatile residual by-products and additives that are commonly employed in cable manufacturing to improve insulation performance. While there have been reports comparing space charge activities (as-received) un-degassed and degassed samples, little attention has been placed on degassing conditions effect on space charge. In this paper, investigations of morphology and space charge distribution of planar XLPE samples was carried out as parameters such as degassing temperature, degassing duration and sample thickness were adjusted. Changes in the morphological structure of the material were probed with the differential scanning calorimeter and the transmission electron microscopy, whereas space charge distribution of the samples subjected to DC electric stress was monitored for 24 h using the laser induced pressure pulsed technique. It was revealed that degassing temperature, duration and the thickness of the degassed sample has a direct effect on the morphological structure and space charge distributions of the samples. It is believed that the presence and amount of by-products takes precedence over the morphology of the material as far as space charge evolution is concerned.     

Some factors for the space charge formation in polyethylene

Information on space-charge behavior in thick insulated samples aids in understanding the dc characteristics of polymer-insulated dc cables. The pulsed electroacoustic method is used to investigate several space charge formation factors in 2 mm-thick polyethylene (PE). The following results were obtained. For measurement factors: (1) A polymeric semiconducting electrode provides a more accurate measurement than does a metal electrode as a result of better matching of acoustic impedance with PE. (2) Within a dc electrical stress range of several tens kV/mm, the space charge distributions under and after dc voltage application are almost the same; this is due to a comparatively long time of space-charge decay. (3) The space-charge distribution of a plate sample agrees with that of a cable sample having the same insulation thickness. For insulating material factors: (1) The amount of space charge in crosslinked polyethylene (XLPE) is much larger than that in low-density PE (base of XLPE). The space charge of XLPE continues to increase even after dc voltage application (24 h); that of LDPE reaches equilibrium with a few hours. (2) The aforementioned space charge difference between XLPE and LDPE is assumed to be caused by ionic impurities in XLPE, not by the additives themselves (acetophenon and cumylalcohol as byproducts of cross linking and antioxidant).     

半导电材料对纳米MgO/XLPE复合介质空间电荷影响的研究

聚合物纳米复合介质中空间电荷的注入与半导电电极材料密切相关,文中采用电声脉冲(PEA)法测量了预压-60 kV/mm电场1 h后,对比研究了六种不同半导电电极材料下交联聚乙烯(XLPE)和MgO/XLPE复合介质中的空间电荷分布;并对不同半导电电极材料下MgO/XLPE复合介质中的平均电荷密度进行了计算。对比实验表明:配方不同的半导电电极材料确实对试样中空间电荷的分布以及空间电荷量影响很大;以乙烯醋酸乙烯共聚物(EVA)为基础材料、添加30wt%炭黑的第二种半导电材料对MgO/XLPE复合介质中空间电荷的抑制效果最好。     

Temperature effect on space charge dynamics in XLPE insulation

This paper reports on space charge evolution in crosslinked polyethylene (XLPE) planar samples approximately 1.20 mm thick subjected to electric stress level of 30 kV_dc/mm at four temperatures 25, 50, 70 and 90 degC for 24 h. Space charge profiles in both as-received and degassed samples were measured using the laser induced pressure pulse (LIPP) technique. The DC threshold stresses at which space charge initiates are greatly affected by testing temperatures. The results suggest that testing temperature has numerous effects on space charge dynamics such as enhancement of ionic dissociation of polar crosslinked by-products, charge injection, charge mobility and electrical conductivity. Space charge distributions of very different nature were seen at lower temperatures when comparing the results of as-received samples with degassed samples. However at higher temperature, the space charge distribution took the same form, although of lower concentration in degassed samples. Space charge distributions are dominated by positive charge when tested at high temperatures regardless of sample treatment and positive charge propagation enhances as testing temperature increases. This can be a major cause of concern as positive charge propagation has been reported to be related to insulation breakdown     

Electrical properties of silane crosslinked polyethylene incomparison with DCP crosslinked polyethylene

The electrical properties such as water tree length, electrical conduction, ac breakdown strength and space charge of silane crosslinked polyethylene (SXLPE) were investigated, with a purpose to compare this material with ordinary dicumyl peroxide (DCP) crosslinked polyethylene (XLPE). Experimental results show that SXLPE has a smaller water tree length, much lower conduction current density, a little bit higher ac breakdown strength than XLPE, and on SXLPE only a very small homocharge is seen. The better behaviour of SXLPE under electric stress is attributed to the absence of residual curing byproducts present in XLPE and the change of chemical structure by grafting and curing reactions     

Space charge accumulation in power cable XLPE insulation

The accumulation of space charge in the crosslinked polyethylene (XLPE) insulation of a power cable sample under dc stress was investigated. The sample was held at 82°C and atmospheric pressure, and at 82°C and 0.1 Pa, for periods up to several days. Such conditioning removed pre-existing space charge, When an external dc field was applied after conditioning at atmospheric pressure and 82°C, space charge accumulated at a rate considerably faster than that observed before conditioning. However the rate of accumulation following conditioning at 82°C under reduced pressure was much slower than that observed before conditioning. The faster rate could be recovered by reconditioning the sample at atmospheric pressure after conditioning under reduced pressure. Inversion of the equilibrium space charge profile, following reversal of the applied field, was observed. This observation is consistent with a model incorporating a spatially-inhomogeneous polarization in the amorphous volume of the XLPE, and electron injection/extraction at the electrodes. The electron transfer between electrode and XLPE in either direction involves the same narrow window of combined donor and acceptor states in the insulator, centered on the Fermi level. The spatially-inhomogeneous polarization in the XLPE originates in a spatially-inhomogeneous distribution of dipole complexes. It is suggested that the dipoles are formed by a process equivalent to nonpermanent oxidation of the XLPE, requiring water as a catalyst     

Electrodes and charge injection in low-density polyethylene usingthe pulsed electroacoustic technique

The effects of electrode materials on space charge formation in low-density polyethylene (LDPE) have been investigated experimentally using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either the laboratory or power cable industry were selected, i.e. aluminum, gold and carbon loaded crosslinked polyethylene (XLPE), and space charge accumulation after the application of high electric stress was monitored. Experimental results demonstrated that charge injection processes take place in all cases once the applied stress has exceeded a threshold. However the amount of charge, and the polarity of the dominant injected charges showed a significant dependence on the electrode materials under the same applied electric stress     

Study of space charge accumulation in polyolefins submitted to acstress

The aim of the present study was to follow space charge formation under ac field (50 Hz) in polyolefins. The thermal step method was used, and charge accumulation was observed in crosslinked polyethylene (XLPE), very low density polyethylene (VLDPE), isotactic polypropylene (PP) and a PE /PP copolymer, the temperature varying from 45 to 90°C. It was found that the amount of charge accumulated under ac conditions may reach a maximum value at a given temperature, while the highest space charge was found in the polyolefin with the lowest crystallinity ratio. The conduction current characteristics of XLPE specimens show the existence of traps in the polymer at a level deeper than the potential barrier electrode/dielectric, explaining the accumulation of injected charge in the material. The injection and conduction mechanisms observed were interpreted by Schottky and Poole-Frenkel processes and used to explain the maximum accumulation of charge occurring at certain temperatures     

Space charge and anomalous discharge currents in crosslinkedpolyethylene

Transient currents in cross-linked polyethylene (XLPE) have been investigated by using different parameters: temperature T, electrical field E_p and hydrostatic pressure P. Anomalous discharging currents flowing in the same direction as charging currents were observed from T=30 to 70°C, P=0.1 to 30 MPa and E_p=13 to 33 kV/mm. The space charge was measured using the pressure wave propagation method (PWP). Theoretical calculation revealed that this anomalous behavior can result from charge migration under a partial blocking condition at the electrodes. We discuss theoretical results which are in reasonable agreement with our discharge current measurements     

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.     

Development of the new polymer insulating materials for HVDC cable

The properties of modified HDPE, selected from various polymer materials by evaluations of DC characteristics, were examined in both sheet and cable samples. It was found that the modification, which introduced a small amount of polar group into HDPE, considerably enhanced DC breakdown strength to as high as 1.5-2.0 times those of XLPE (cross-linked polyethylene). Under a poling voltage of up to 30 kV/mm, the modified HDPE exhibits particular behavior, with its space charge decreasing as stress is increasing. In addition, the additives in HDPE have a large influence on space-charge characteristics, thereby affecting the DC breakdown strength. The evaluation of cable insulated with optimum modified HDPE was conducted to determine the breakdown strength under various voltage applications. Compared with the XLPE cable, modified HDPE cable exhibited excellent characteristics under all kinds of voltage applications, particularly DC dielectric breakdown strength, which was almost twice that of XLPE     

Study of space-charge characteristics in polyethylene for power cable insulation by laser-induced pressure-pulse technique

By utilizing the laser induced pressure-pulse (LIPP) technique, the behavior of space charge in low-density polyethylene (LDPE) and crosslinked polyethylene (XLPE) films in contact with metal or carbon-loaded semiconducting layers was studied quantitatively to clarify the space-charge characteristics in power cables. Negative heterospace charge near the anode and positive space charge in the bulk were observed in unoxidized LDPE under the fields above 120 kV/mm. The amount of negative space charge increased with applied field, while positive space charge in the bulk disappeared with increasing applied field. This indicates that electron injection and ionization are enhanced by applied field. Prominent negative homospace charge was formed near the cathode in oxidized LDPE, which indicates that oxidation enhanced electron injection. The depth of charge centroid from the cathode became larger with increasing temperature. This indicates that the effective electron mobility increases with temperature. Negative space charge also was formed in the bulk in XLPE films with metal electrodes, which indicates that crosslinking enhanced electron injection. XLPE films with a carbon-loaded semiconducting layer showed both negative and positive homospace charges near the semiconducting layers, which indicates that both electrons and holes were injected from the semiconducting layer.     

The influence of electrodes and conditioning on space chargeaccumulation in XLPE

The accumulation of space charge in planar crosslinked polyethylene (XLPE) samples under dc electric fields at room temperature was investigated using the pulsed electroacoustic (PEA) method. Three different organic semiconductor (semicon) materials containing carbon black at concentrations ~30 %wt, and vacuum-evaporated gold, were used as electrodes. Three different conditioning procedures were investigated. Unconditioned samples developed heterocharge with density increasing from zero at the electrodes, to a maximum at roughly one-third of the thickness, and then falling to zero around the center. They also developed homocharge close to the electrodes. The choice of electrode material had little effect on the heterocharge profile in unconditioned samples. Conditioning by holding at 80°C for four days, at rotary pump pressure or at atmospheric pressure, suppressed the accumulation of heterocharge. Homocharge accumulation close to the electrodes in samples with semicon electrodes was affected little by this conditioning, but was reduced considerably in samples with gold electrodes. Conditioning by holding at room temperature for seven days at rotary pump pressure had little effect. The heterocharge originates in inhomogeneous polarization due to a spatially inhomogeneous distribution of permanent dipole molecules, probably acetophenone. The dipole concentration decreases in going from the electrodes towards the center of the samples, as a result of diffusion of the molecules in the opposite direction. The homocharge originates in electronic charge injection/extraction across the electrode/XLPE interfaces     

Factors of hetero space charge generation in XLPE under dc electric field of 20 kV/mm

This report deals with the mechanism of space charge accumulation in cross‐linked polyethylene (XLPE) under dc electric field. Space charge was measured by the pulsed‐electroacoustic method with applying dc stress of 20 kV/mm. A large amount of hetero space charge accumulated in fresh XLPE samples. Factors influencing the space charge accumulation were analyzed in regard to cross‐linking by‐products and antioxidant. No space charge was seen when the fresh sample was degassed to remove cross‐linking by‐products. Introducing acetophenone, one of the cross‐linking by‐products, in a degassed sample produces no space charge, suggesting that acetophenone itself could not be the direct factor of space charge formation. However, heating this sample up to 150 °C results in formation of hetero space charges as in virgin samples. Hence, it is concluded that hetero space charges may be formed when impurities, such as an antioxidant, dissociate thermally with the help of acetophenone and that the dissociated products are attracted toward both electrodes under a dc field to form the hetero space charges. © 1999 Scripta Technica, Electr Eng Jpn, 129(2): 13–21, 1999     

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。     

外施电压频率对XLPE电缆绝缘中电树枝生长特性的影响

本文研究了半结晶交联聚乙烯(XLPE)电缆绝缘试样在50～2000Hz正弦电压下电树枝的结构特征与生长特性,发现由于XLPE的半结晶聚集态物理结构,在小于250Hz施压频率下会生成枝状、枝状与丛林混合状及纯丛林状三类电树枝,在500Hz以上高频下则只能生成稠密枝状电树枝,分别对应于不同的生长机理.低频下电树枝生长特性和电树枝结构与材料的聚集态密切相关,而高频下的电树枝生长特性与材料的聚集态关系不大.高频电树枝与环氧树脂、有机玻璃等高聚物材料中的生长规律相同.半结晶高聚物在低频下的电树枝生长特性主要取决于晶界与无定形界面的微孔、杂质集中情况以及针尖电极与晶块或无定形区所处的相对位置,而在高频下电极向介质中注入与抽出电荷的过程较低频下猛烈,会形成较均匀的介质弱区,因此高频电树枝引发与生长规律较为单一.     

Space change behavior in full scale ±250 kV dc XLPE cables

Space charge in full size ±250 kV dc cables was measured using a pulse-electroacoustic method. Two newly developed types of dc XLPE cables with 20 mm thick insulation were subjected to measurements at a dc voltage of 500 kV, keeping the conductor temperature either at room temperature or at 85°C. Qualitative analyses of the space charge distribution and the quantitative analysis of the electric fields in the vicinity of semicon interfaces were made. It was shown that the field in the vicinity of the inner semiconductor tends to increase by 10 to 40% when the polarity of the applied voltage was switched, in the case when the conductor was kept at 85°C. However, the distortion of the electric field was significantly smaller than that expected with conventional XLPE cables. As a result, dc cables are considered to have stable dc characteristics from the viewpoint of space charge behavior. © 1998 Scripta Technica, Electr Eng Jpn, 124(2): 16–28, 1998     

退役高压XLPE电缆绝缘空间电荷行为研究

文中对两回110 k V退役高压交联电缆进行180 a预鉴定试验,目的在于研究不同运行年限的退役高压交联电缆老化前后的空间电荷行为差异,并评估电缆重新投入实际运行的可靠性。通过电声脉冲(PEA)法测量试验前后绝缘层交联聚乙烯(XLPE)试样的空间电荷分布,结合傅里叶红外光谱(FTIR)实验、X射线衍射(XRD)实验揭示试样的微观结构、聚集态结构状态变化与空间电荷的迁移、积聚和消散过程的潜在关系。实验结果表明:实际运行16 a的电缆绝缘由于存在大量杂质使得空间电荷的积聚严重,老化试验后,杂质的减少和淬火反应使得空间电荷的积聚降低且消散过程加快;实际运行32 a的电缆绝缘空间电荷的积聚和消散过程缓和,老化试验后,由于绝缘降解作用和晶态结构的破坏使得空间电荷的积聚加剧且消散过程变得缓慢。     

Electrical properties of biodegradable polylactic acid film

We measured the basic electrical insulation characteristic of biodegradable polylactic acid (PLA), and the following results were obtained. The volume resistivity, dielectric constant, and dielectric loss tangent measured at room temperature were almost the same as those of crosslinked polyethylene (XLPE) currently used as insulating material for cables and electric wires. The mean impulse breakdown strength of PLA was about 1.3 times that of XLPE. Also, space charge accumulation in PLA and XLPE was measured. The amount of space charge accumulation in PLA was one-half that in XLPE when a DC voltage was applied for a short time.     

Computational and Experimental Study of Ionic Space Charge Generated by Combined Corona–Electrostatic Electrode Systems

Numerical computation of the electric field intensity and space charge density in electrode systems consisting of ionizing and nonionizing elements, connected at the same direct current (dc) high-voltage supply and facing a grounded plate, is a difficult problem, which is of interest to several electrostatic processes applications. The aim of the present paper is to demonstrate the effectiveness of an original method of field computation in the analysis of the factors that influence the distribution of the ionic space charge in such combined corona–electrostatic electrode systems. The computations and the experiments were carried out for an ionizing wire of diameter 0.3 mm, located at different distances$h$(10–30 mm) from a tubular support of diameter 25 mm. Several interelectrode distances (20–45 mm) were simulated. The extension of the zone at the surface of the grounded electrode, which is affected by the space charge, diminishes when reducing the intervals between these elements of the electrode system, and, at similar applied voltage, the density of the corresponding corona current increases. The experimental data were in good agreement with the computed results, validating the accuracy of the numerical method of space-charge calculation in this special electrode configuration.