Diagnostic dielectric spectroscopy methods applied to water-treedcable

Considerable effort has gone into developing polymer formulations and cable designs to minimize failures through water tree growth. However, diagnostic techniques still are required to enable the estimation of the level of damage present within a service cable. This paper reports on progress regarding the application of dielectric spectroscopy to cable diagnostics. A 40 kV, crosslinked polyethylene (XLPE) insulated coaxial cable was used as a model power cable. Sample lengths were immersed in a potassium chloride solution and some of these were subjected to AC electrical stress. After an 8 week duration, a high density of tress was found in the electrically stressed cable. Dielectric spectra have been measured for both sample types in the frequency range of 10^-5 to 10⁵ Hz. Insertion loss measurements were also carried out in the frequency range of 3×10 ⁵ to 3×10⁹ Hz. From both types of measurement, it was possible to distinguish between the cables containing water trees and those that were free from water tree structures. These approaches could therefore be developed in order to provide diagnostics for the detection of water tree damage in electrical power cables     

Modeling of dielectric relaxation spectra of polymers in thecondensed phase

Discusses models and their possible implication in the interpretation of the relaxation spectra of polymers, particularly involving the concept of order/disorder systems. Furthermore, some examples of the application of the Dissado-Hill model have been provided in this work to explain quasi-dc (QDC) conduction processes, which have been observed in crosslinked polyethylene (XLPE) and low density polyethylene (LDPE), showing an ageing behavior with continuing use with an ac field in a humid environment. It is also shown in this paper how a measurement of dielectric relaxation properties on a wide frequency range may be used as a tool to investigate the ageing of polymeric power distribution cables with an ac field in a humid environment. Equivalent electrical analog circuits of dielectric relaxation spectra are also shown     

Transient current measurement for the detection of water treegrowth in polymeric power cables

In this paper, the relationship between the frequency and time domain windows is discussed. From this study an appropriate time window for transient current measurements may be chosen so that reliable low frequency dielectric data can be obtained. The suitability of applying either Fourier transform or the Hamon approximation for the derivation of low frequency responses is discussed for transient currents with different shapes. With water-treed LDPE samples aged in the laboratory, measurements of transient currents, with subsequent transformation into the frequency domain, show a loss peak at ~10^-4 to 10^-3 Hz when moisture is retained in the samples. Even with free water removed, a higher dielectric loss is observed in water treed samples compared to virgin samples. The relaxation behavior may serve as an indicator of water treeing in polymeric cable insulation. The results of both transient measurements and TSDC results can be affected significantly by the treatment conditions of the samples. The application of vacuum to a water treed sample can alter the activation energy of the conduction process and the TSDC spectra     

Dielectric response of ionically conducting liquids in thelow-frequency region

The selected dielectric studies are presented of ionically conducting materials (liquid crystal, epoxide resin) in the range of time, 10^-4 to 10⁴ s, or frequency, 10^-4 to 10⁴ Hz. The properties of the space charge at the blocking electrodes were taking into account. Investigations were focused on the time domain dielectric response of the model liquid crystalline pentylcyanobiphenyl. Similar studies were performed also on the epoxide resin Epidian 3. An influence of external parameters like temperature or concentration of impurities were included to interpret the obtained results. This interpretation was established on the principles of the two theoretical models for the space charge polarization     

Dielectric properties and spatial distribution of polarization inpolyethylene aged under ac voltage in a humid atmosphere

This paper reports the results of an investigation of the dielectric properties, growth of water tree density and spatial distribution of polarization in samples of non-stabilized LDPE. Samples were initially sand-blasted to accelerate initiation of water trees, and subsequently aged in salt solution for approximately three weeks under two regimes of temperature (room temperature and 65°C). Aging both with and without the presence of electric field stress was studied. The capacitance and dielectric loss of each sample were measured using a HV capacitance bridge, and compared to theoretical models over the frequency range 10^-5 to 10⁴ Hz. The results indicate a real movement of charges/ions in the polymer bulk which causes a deterioration of the electrical insulation properties even in the absence of AC fields. The presence of a field further degrades the insulator. Samples were studied using the LIMM (laser-intensity-modulation method) to determine the distribution of polarization, assuming mean polarization is zero and space charge is absent. In the absence of the external field there is a presence of ionic charges of opposite polarity, which increases with increasing aging time and is highest at room temperature. The length of the twenty longest vented water trees was measured after various aging times, and it was found that trees grown at room temperature are longer than those grown at 65°C. Also, the tree density was higher in specimens tested at room temperature     

Dielectric behavior of aged polyethylene under UV radiation

The present paper reports the results of a study of dielectric aging of low-density polyethylene (LDPE) by UV radiation. The extruded polymer contained carbon black particles, approximately 3% in weight. The aging studies were made for polymer without and with carbon black. LDPE flat specimens were aged under UV radiation for up to 300 h, resorting to dielectric spectroscopy measurements in the frequency range from 10/sup -5/ Hz to 10/sup 5/ Hz. The dielectric susceptibility behaviors indicate the presence of relaxation peaks obeying the asymptotic forms of Dissado and Hill susceptibility functions. As the aging progresses, quasi-DC and flat loss processes may be observed in the low frequency range, for the LDPE specimens without and with carbon black, respectively. In addition, dielectric spectroscopy results have been compared with other results obtained with additional techniques such as Fourier transform infrared spectroscopy and scanning electron microscopy.     

Influence of frequency, electrode material and superimposed dc onac electroluminescence in polymer films

Summarizes new results on the influence of some experimental parameters on ac electroluminescence (EL) of polymers used as insulating materials in electrical engineering. Electroluminescence under a uniform ac field is investigated in low density polyethylene (PE) and polyethylene naphthalate (PEN) films with semi-transparent electrodes on both sides. Particular emphasis is given to polyethylene. The dependence of EL on the voltage frequency is investigated in the range of 10^-2 Hz to 10⁺² Hz with the help of phase-resolved measurements. The influence of the electrode/polymer contact is investigated by changing the electrode material (gold, silver, and calcium). The effect of a superimposed dc bias on the ac voltage is also reported     

Polarization and space charge behavior of unaged and electricallyaged crosslinked polyethylene

The behavior of crosslinked polyethylene (XLPE) for HV cable insulation under electrical stress is discussed in the paper, referring to specimens unaged and electrically aged. Polarization and space charge features are considered. The behavior of the examined insulation is evaluated through the `universal' model for polarization and space-charge associated quantities, such as the dc threshold for space charge build up and the total charge accumulated into insulation. It is shown that aging affects the magnitude and the frequency of low and intermediate frequency polarization peaks, as well as the magnitude of accumulated space charge and threshold. Charge injected or formed in the bulk, as well as microstructural modifications, could explain changes of polarization and space charge magnitude with aging     

Carrier mobility in LDPE at high temperature and pressure

Current peaks due to transient SCLC (space charge limited current) were observed in LDPE (low-density polyethylene) at 70°C. From the time at which the current peak occurs, carrier mobilities ranging from 5×10^-15 to 7×10^-14 m²/Vs were obtained. The mobility values obtained for LDPE depend on both applied field and hydrostatic pressure and verify hopping mechanism. The presence of space charge has been confirmed by analyzing discharge currents measurements     

Mechanism for impulse conversion of water trees to electrical treesin XLPE

A lightning impulse causes substantial capacitive current in a water tree channel which, as a result of its small cross section, has relatively low conductance. Transient, nonlinear finite element computations with coupled thermal and electric fields for the geometry of a 15 kV XLPE dielectric cable indicate that an 80 kV lightning impulse can cause the water within a water tree channel to boil over a range of four orders of magnitude in water conductivity, from 10^-2 S/m to 10² S/m. Boiling of the water in a water tree channel reduces substantially the yield stress of the XLPE, raises the pressure within the water tree channel, and is likely to leave a cavity which can support partial discharge resulting in electrical tree initiation     

Influence of aging on electrical properties of polyethylenenaphthalate-2,6 dicarboxylate films

Poly(ethylene naphthalate 2,6 dicarboxylate), thanks to its heat resistance, may be used as insulating material in motors and laminated chip capacitors. In these specific applications the occurrence of partial or corona discharges leads to insulation aging and possible breakdown. An experimental investigation has been carried out to determine the changes in dielectric properties as function of the aging conditions, e.g. the temperature of irradiation. Dielectric loss values have been determined in the 10² to 10⁶ Hz frequency range from 150 to 200°C. Three relaxation peaks were found α,β*,β, in the order of decreasing temperature     

Glass transition of an epoxy resin. A wideband dielectricinvestigation

The dynamics of an epoxy resin, poly(phenyl glycidyl ether-co-formaldehyde), has been investigated in the supercooled and glassy phases by wideband dielectric spectroscopy (10^-2 to 3×10⁹ Hz) and compared with that of the previously investigated epoxy resin, diglycidyl ether of bisphenol-A. The temperature evolution of the dynamics of the system is monitored through the characteristic parameters of the relaxations, namely the relaxation times, the relaxation strengths and the shape parameters. Two transition regions are revealed: the glass transition and the split between structural and fast secondary relaxation, where the onset of the structural relaxation is located also     

Electrical relaxation in iron-containing glasses

Glasses with composition 70 mol%(SiO₂, B₂O ₃, P₂O₅, TeO₂)-15 mol% Fe ₂O₃-15 mol%(BaO, CaO) were prepared by the conventional melt quenching technique. The electrical relaxation of these glasses has been studied in the frequency range 20 to 10⁵ Hz. The small polaron hopping between iron ions in a different valence state Fe²⁺ to Fe³⁺ is found to be the principal conduction mechanism. The ratio of Fe²⁺ ions to the total iron content, C=Fe²⁺/Fe_tot, is one of the factors determining the electrical conductivity. The glass former has a minor influence on dc conductivity, except of TeO₂ glass where conductivity is three order of magnitude higher than those of other glasses. The ac conductivity as a function frequency is divided into two domains, a dc plateau, followed by a power law in frequency. These two regions are well distinguished in the complex plot of electric modulus where all data points for different temperatures reside on the single plot. The results are discussed in the frame of the Hunt theory of dielectric relaxation in glasses containing mobile charge carriers     

Dielectric and electrical properties of polycrystalline rocks atvarious hydration levels

The dielectric properties of low porosity natural polycrystalline rock material (marble) were investigated using broadband 10^-2 to 10⁶ Hz) dielectric relaxation spectroscopy at several hydration levels, from dry to fully hydrated samples, with the water content varying from 0 to 0.08% g water/g dry material. The measured dielectric response is characterized by large low frequency dispersions, which depend strongly on the water content. Upon varying the sample thickness, dielectric and water sorption measurements reveal that bulk rather than electrode/sample interface effects are responsible for the main part of the high values obtained for the complex permittivity ε*. Comparative analysis of the experimental data within the complex permittivity ε*, impedance Z* and modulus M*, allows a detailed investigation of the AC conductivity mechanisms existing in the rocks and of water content dependency. Concerning the rock-water interaction, a critical water content h_c, (h_c~0.04%, the water content for ambient humidity conditions) has been found     

Measurement of space charge in XLPE insulation under 50 Hz acelectric stresses using the LIPP method

Space charge is believed to play an important role in ac electrical ageing of polymeric insulation, particularly where electrical treeing is the dominant cause of premature failure. The majority of work to date has been on the space charge characteristics under dc electric stresses, whereas work on the dynamics and the role of space charge on electrical breakdown under 50 Hz ac conditions have only received limited attention. Consequently, a full understanding of space charge trapping and dynamics under ac operating conditions is required if ever more reliable polymer insulated cable systems are to be developed. The present paper reports on aspects of an on-going investigation into the measurement of space charge in XLPE insulation under 50 Hz ac stress using the laser induced pressure propagation (LIPP) technique. Some important features of the method are described such as, point on wave control of the laser and use of correction factors to offset variations in the laser power and ablation of the target material. Emphasis has also been placed on establishing a simple method for analysing data, termed "X-plots". Some preliminary results of the space charge and electric stress distribution of un-degassed and degassed crosslinked polyethylene (XLPE) plaques aged electrically under ac stress (30 kV_peak/mm) for 24 h are also presented     

Comparative laboratory evaluation of TR-XLPE and XLPE cables with Super-smooth conductor shields

This paper provides data on four commercial tree retardant crosslinked polyethylene (TR-XLPE) and one cross-linked polyethylene (XLPE) insulated 15 kV cables supplied by three manufacturers. The cables have "super-smooth" conductor shields and "extra-clean" insulation and insulation shields. AC and impulse voltage breakdown and selected other characterization data are presented for cables that were aged immersed in room temperature water (15-30/spl deg/C) up to 24 months of a planned 48 months aging program. The five cables have high ac voltage breakdown strength, three of the TR-XLPE cables, actually increased in breakdown strength during aging. The one TR-XLPE cable that had the lowest ac voltage breakdown had vented trees at the insulation shield and high dissipation factor, which the other cables did not have. The impulse voltage breakdown strength of all cables decreased during aging; the cable with the lowest ac voltage breakdown also has the lowest impulse voltage breakdown. The dissimilar performance of the TR-XLPE cables and the excellent performance of the XLPE cable indicates evaluations at longer times are required to differentiate between modern TR-XLPE and XLPE insulated cables.     

Electrical studies of leaves over wide frequency range

Dielectric properties of fresh leaves samples from different plant species were studied over the frequency range 10^-3 to 10⁴ Hz. The dynamic behaviour of the conductance and capacitance in leaf samples has been observed to follow fractional power law. The observed power-law dispersions have been modelled using frequency dependent resistive and capacitive elements of a conventional equivalent circuit     

66 kV抗水树XLPE绝缘轻型海底电缆研制

介绍一种额定电压66 kV抗水树XLPE绝缘轻型海底电缆的研制方法。通过对模型电缆进行工频电压和雷电冲击电压击穿性能测试,按照CIGRE TB 722:2018规范对研制样品进行500 Hz/3000 h、50 Hz/8750 h、50 Hz/17500 h 3种湿式绝缘质量鉴定试验,按照CIGRE TB 490:2012和CIGRE TB 623:2015规范对研制产品进行型式试验,试验结果完全满足设计规范要求。在未来的深远海、大功率海上风机互联阵列海缆选型中,66 kV抗水树XLPE绝缘轻型海底电缆可以完美替代35 kV XLPE绝缘海底电缆。     

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 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