XLPE Insulation Research Articles

Переходные процессы и перенапряжения в однофазных кабельных линиях высокого напряжения

High-voltage lines with overhead and cable sections are getting high popularity in nowadays energetics. These transmission lines are normally constructed using a single-phase cables with XLPE insulation. On the other hand an application of this type of cables leads to certain unsolved problems, for instance- a numerous amount of breakdowns at the cable joints. This paper takes as an example the power line of 330kV in the city of Saint-Petersburg, which connects substations Severnaya and Vasileostrovskaya. There were considered three accidents, which led to a number of breakdowns of cable joints along the entire line. Basing on the simulation results it was proposed, that the accidents were caused by a significant overvoltage at the joints during the short-circuits. Firstly, it concerns high-frequency overvoltage, which cannot be bounded by a surge arrestor. Secondly, excessive current flows going through the arrestors.

A time-resolved current method and TSC under vacuum conditions of SEM: Trapping and detrapping processes in thermal aged XLPE insulation cables

Thermal aging of cross-linked polyethylene (XLPE) can cause serious concerns in the safety operation in high voltage system. To get a more detailed picture on the effect of thermal aging on the trapping and detrapping process of XLPE in the melting temperature range, Thermal Stimulated Current (TSC) have been implemented in a Scanning Electron Microscope (SEM) with a specific arrangement. The XLPE specimens are molded and aged at two temperatures (120°C and 140°C) situated close to the melting temperature of the material. The use of SEM allows us to measure both leakage and displacement currents induced in samples under electron irradiation. The first represents the conduction process of XLPE and the second gives information on the trapping of charges in the bulk of the material. TSC associated to the SEM leads to show spectra of XLPE discharge under thermal stimulation using both currents measured after electron irradiation. It was found that leakage current in the charging process may be related to the physical defects resulting in crystallinity variation under thermal aging. However the trapped charge can be affected by the carbonyl groups resulting from the thermo-oxidation degradation and the disorder in the material. It is evidenced from the TSC spectra of unaged XLPE that there is no detrapping charge under heat stimulation. Whereas the presence of peaks in the TSC spectra of thermally aged samples indicates that there is some amount of trapped charge released by heating. The detrapping behavior of aged XLPE is supported by the supposition of the existence of two trap levels: shallow traps and deep traps. Overall, physico-chemical reactions under thermal aging at high temperatures leads to the enhancement of shallow traps density and changes in range of traps depth. These changes induce degradation of electrical properties of XLPE.

Efficiency of using cables on 6-110 kV with sticked polyethylene isolation

The expediency of using more expensive 6-10 kV cables with XLPE insulation was proved by us on the basis of the method of multi-purpose optimization, which took into account both the resulted costs for the construction and operation of cable lines, and the reliability of its operation, as well as the convenience of installation. Without compensation of capacitive currents, permissible difference in the height of the cable laying, the effect of cable lines on the environment.

Investigating Thermal Effect on a Cross Linked Polyethylene Power Cable

<p>One of the agents responsible of the degradation of power cables in electrical distribution network is the temperature. In this paper, numerical modelling of temperature effect on the cross linked polyethylene (XLPE) insulation of a medium voltage cable containing internal defects, which are air void cavity and water tree cavities, is developed by using the finite element method and simulated by COMSOL Multiphysics Software. The experimental investigation is conducted through studying parial discharge inception voltages in XLPE insulation before and after 23 heating cycles of a 7 meters sample of a medium voltage cable at 100°C and 120°C temperatures. Partial discharge inception voltages detection were performed using the IEC60270 test method. The simulation results and experimental measurements assessed the thermal effect on the degradation of XLPE insulation.</p>

Online Loss Parameters Diagnosis Device Development of Power Cable Insulation

In southern China, power cable lines often running in water, and the state of XLPE insulation is difficult to detection effectively. This paper studied loss parameters online diagnosis technology of power cable insulation. An online loss parameters diagnosis device was development, which consists of current comparator, voltage controlled current source (VCCS), standard capacitor, GPRS wireless transmission system, control system and the data processing unit. 108 cables were detected using this device, and it was found out that one cable appeared to middle aging and eight cables appeared to slight aging. The butterfly water trees were observed in middle aging cable, which prove the effectiveness of online testing in the loss current measurement.

Investigation of Water Tree Degradation in Dry‐Cured and Extruded Three‐Layer (E‐E Type) 6.6‐kV Removed XLPE Cables

SUMMARYDry‐cured and extruded three‐layer (E‐E type) 6.6‐kV cross‐linked polyethylene (XLPE) cables were introduced into electric power systems more than 30 years ago, but they do not experience failures because of water tree degradation. Also, the degradation index of water treeing for these cables has not been established. Therefore, investigating results of residual breakdown voltage and water tree degradation of these cables will help us plan for cable replacement and determine water tree degradation diagnosis scheduling, and will be fundamental data for cable lifetime evaluation. In this study, the authors measured the ac breakdown voltages of dry‐cured and E‐E type 6.6‐kV XLPE cables removed after 18 to 25 years of operation and observed the water trees in their XLPE insulation. As a result, it was observed that breakdown voltages were larger than the maximum operating voltage (6.9 kV) and the ac voltage for the dielectric withstanding test (10.3 kV). Water trees were mainly bow‐tie water trees and their maximum length was approximately 1 mm. Although the number of measured cables was limited, the lifetime of this type of cable was estimated to be approximately 40 years, even experiencing water immersion.

Effect of Degassing Treatment on the Crystalline Morphology of XLPE Insulation Used for 110kV XLPE Cable

Cross-linked polyethylene (XLPE) is a main kind of insulation materials, used in medium and high voltage power cable. High voltage XLPE cable production mainly adopts peroxide crosslinking method, and the performance of XLPE cable will be affected by decomposing by-products of crosslinking agent (DCP, dicumyl peroxide).In this paper, the XLPE cable was degassed with different times. The XLPE samples were analyzed by infrared spectrum analysis, DSC analysis and electron microscopy, and the influences of degassing on XLPE crystalline morphology were analyzed. The results showed that the crystal structure of XLPE in the cable was affected by degassing treatment. With the degassing treatment time went on, the by-products kept releasing. The content of by-products in 110kV XLPE cable decreased obviously in 2-7 days’ degassing treatment, and changed slowly after 7 days’. The growth and perfection of the original crystal in XLPE material were obtained, and the new small grains were formed in the material.

Accelerated inhomogeneous degradation of XLPE insulation caused by copper-rich impurities at elevated temperature

In this paper, the existence of copper-rich impurities in XLPE cable insulation is found by means of thermal-oxidative ageing. The emergence of copper-rich impurities, shown as dark islands in XLPE, is proposed to be a critical point to dramatic degradation of dielectric performance. It was deduced by XRD and DSC tests that thermal-oxidative ageing at elevated temperature leads to enhanced interplanar spacing and lamellar spacing, as well as decreased crystallinity for XLPE insulation around copper-rich impurities. Meanwhile, the existence of cracks in branch-type around impurities, which can be regarded as the generation of increased free volume in XLPE, was found by SEM. These results might be induced by chains scission and crystalline disruption due to copper-rich impurities. In addition, the Oxidative Induction Time (OIT) of XLPE insulation was measured and it was observed that antioxidant in XLPE insulation plays a key role in the generation of dark islands. The dark islands can be created only if the antioxidant is nearly exhausted, which shows that the generation of dark islands is a result of thermal-oxidative ageing of XLPE cable insulation. Copper-rich impurities play the role of potential catalysts in the oxidation reaction process. The existence of such impurities could lead to accelerated inhomogeneous deterioration of XLPE insulation. A model to illustrate the failure process of XLPE insulation with copper-rich impurities in thermal-oxidative ageing will be proposed.

SELECT PROFILE CABLE IN DISTRIBUTION NETWORK VOLTAGE 10(6) кV ECONOMY

The article analyzes the choice of cables on economic current density in the current economic conditions. The values of economic current density for paper insulated cables with XLPE insulation. It is shown that the application of the criteria Wald, Hurwitz, Laplace, Savage is closely linked with the choice of cables on economic current density at its various values in the range of uncertainty.

Neural networks and fuzzy logic approaches to predict mechanical properties of XLPE insulation cables under thermal aging

The widespread use of cross-linked polyethylene (XLPE) as insulation in the manufacturing of medium- and high-voltage cables may be attributed to its outstanding mechanical and electrical properties. However, it is well known that degradation under service conditions is the major problem in the use of XLPE as cable insulation. Laboratory investigations of the insulations aging are time-consuming and cost-effective. To avoid such costs, we have developed two models which are based on artificial neural networks (ANNs) and fuzzy logic (FL) to predict the insulation properties under thermal aging. The proposed ANN is a supervised one based on radial basis function Gaussian and trained by random optimization method algorithm. The FL model is based on the use of fuzzy inference system. Both models are used to predict the mechanical properties of thermally aged XLPE. The obtained results are evaluated and compared to the experimental data in depth by using many statistical parameters. It is concluded that both models give practically the same prediction quality. In particular, they have ability to reproduce the nonlinear behavior of the insulation properties under thermal aging within acceptable error. Furthermore, our ANN and FL models can be used in the generalization phase where the prediction of the future state (not reached experimentally) of the insulation is made possible. Additionally, costs and time could be reduced.

Influence of Fuzzy Parameters on the Modeling Quality of XLPE Insulation Properties under Thermal Aging

In this work, we have used the fuzzy logic approach to predict mechanical properties (hot set test) of cross-linked polyethylene (XLPE) used as insulation in high voltage cables. The studied property presents non linear variations according to the aging time under high temperatures. So it is very difficult to find a theoretical or experimental model of the properties evolution under thermal aging. For that reason, several factors have been considered such as aging time and applied temperature. The obtained results are very encouraging and pointed out that the fuzzy logic is a powerful tool to predict the insulation proprieties. In other words, the obtained results are in good accordance with the experimental results with an acceptable error margin.

乾式架橋・三層同時押出（E-E方式）の6.6kV撤去CVケーブルの水トリー劣化調査

Dry-cured and extruded three-layer (E-E type) 6.6kV cross-linked polyethylene (XLPE) cables were introduced into electric power systems more than 30 years ago, but they do not experience failures because of water tree degradation. Also, the degradation index of water treeing for these cables has not been established. Therefore, investigating results of residual breakdown voltage and water tree degradation of these cables will help us plan for cable replacement and determine water tree degradation diagnosis scheduling, and will be fundamental data for cable lifetime evaluation. In this study, the authors measured the AC breakdown voltages of dry-cured and E-E type 6.6kV XLPE cables removed after 18-25 years of operation and observed the water trees in their XLPE insulation. As a result, it was observed that breakdown voltages were larger than the maximum operating voltage (6.9kV) and the AC voltage for the dielectric withstanding test (10.3kV). Water trees were mainly bow-tie water trees and their maximum length was approximately 1mm. Although the number of measured cables was limited, the lifetime of this type of cable was estimated to be approximately 40 years, even experiencing water immersion.

전극형상 및 보이드에 따른 XLPE의 부분방전 특성

Transmission equipment is mainly used for the XLPE (cross-linked polyethylene) insulation cable for ultra high voltage power to minimize power loss. The experiment examined the partial discharge characteristics according to the insertion of the bar electrode and needle electrode into the XLPE specimen and the air voids. XLPE insulation cable manufactured by T. company and tungsten electrode material by K. company were used for specimens, by adhering conductive tape on the semi-conductive material of the lower electrode of XLPE specimen with the dimension of <TEX>$16{\times}40{\times}30$</TEX> [mm] was used as negative electrode. In order to investigate the PD with <TEX>${\phi}$</TEX>-q-n of XLPE specimen according to the electrode shape and the size of air voids. we examined the PD by varying the voltage after applying voltage of 3~20 kV on the electrode. Therefore, it was confirmed from the result of PD characteristics of specimen that the larger the air void than the gap between electrode (+) and electrode(-), the larger effect on the discharge when the bar electrode and needle electrode inserted into XLPE, and the closer the distance between the insulation and the needle electrode, the faster insulation breakdown.

基于核磁共振的XLPE电缆绝缘热老化试验研究

随着交联聚乙烯电缆在电力系统中的广泛应用，其老化行为成为电力系统长期安全运行的瓶颈，迫切需要我们研究其老化规律和机理。交联聚乙烯尽管具有较好的耐热性能和电性能。但在实际运行中长期处于高温下而发生化学变化，使得绝缘材料的物理性能和电气性能发生改变。本文按照IEC60287标准对XLPE绝缘材料进行热老化试验，利用核磁共振分析仪，选取共振峰积分面积和纵向弛豫时间为反应绝缘老化程度的特征量，对绝缘材料分子结构进行分析，得到更直观的老化程度结果；同时利用差示扫描量热分析技术对不同热老化程度的绝缘材料进行了热性能分析，提出了以玻璃转化温度、结晶度、结晶速率为指标的热性能分析技术。 XLPE cables are widely used in the power system; the aging behavior of XLPE cable insulation be-comes the bottleneck of long-term safe operation of the power system. The urgent need is to study the law and mechanism of aging. Although the XLPE has good heat resistance and electrical prop-erties, in actual long-term operations, cables are always under high temperature, physical and electrical properties of the insulating material are slowly changed. This article did the heat aging test of XLPE insulation material according to IEC 60287 Standard. With the help of nuclear magnetic resonance analyzer, the resonance peak integration area and longitudinal relaxation time are selected as the feature quantities to characterize the degree of aging of the insulation. The molecular structure of the insulation material is analyzed to obtain a more intuitive aging results while taking advantage of differential scanning calorimetry analysis of the different thermal aging of insulation material thermal performance analysis, determined the glass transition temperature, crystallinity and crystallization rate as index of thermal performance analysis techniques.

Accelerated aging study on 15 kV XLPE and EPR cables insulation caused by switching impulses

Cross-linked polyethylene (XLPE) and Ethylene Propylene Rubber (EPR) are the common polymeric materials used for power cable insulation in the USA. While in service, the main threats to the cable insulation include electrical stress, thermal stress, and mechanical stress. The electrical stress, from either operation voltages or abnormal over-voltages during transients, has negative impacts to the power cables insulation. The excessive electrical stress will trigger the aging of insulation materials, which will lead to the failures of cables, resulting in outages of power systems. The aging phenomena of polymeric power cables have been extensively studied during the past decades. Although the study of aging mechanisms was done on molded samples, little is known about the aging phenomena of the XLPE and EPR cables by switching impulses. In this experiment, the 15 kV XLPE and EPR cable samples were aged up to 10,000 switching impulses. The study helped to assess the reliability of the XLPE and EPR cables serving in electrical power systems.

(IJCESEN)

This paper studies the electrical constraints within an XLPE insulated cable containing micro-cavities. The purpose of this work is to determine by simulation the distortions caused by micro-cavities on the electric field distribution in the insulating layer of cable. The computation method used to determine the electric field is based on numerical resolution of Laplace equation using the finite element method. The threshold of partial discharges ignition in gaseous cavities contained in XLPE insulation of 30 kV cable is studied. The computation of the critical field by approached formula of Paschen permits us to elucidate the harmfulness caused by a cavity according to its size and its position.

Testing of a Covered Conductor’s Fault Detectors

The majority of medium-voltage overhead lines are currently operated with AlFe conductors which have a simple construction and which rank among the cheapest versions of overhead lines from the point of view of initial investments. These overhead lines can be susceptible, however, to faults primarily in mountain and wooded areas, with an ensuing decrease in safety and operations reliability, as well as with limitations to the electric power supply. This draws attention to outdoor overhead lines with covered conductors, which are relied upon for use for connecting “defined” systems (SMART GRIDS) with an external electric power system. These conductors, with mostly XLPE and PE basic insulation, are able to resist both mechanical and electric strain caused by falling trees or branches and consequently their total number of faults is significantly lower. Massive usage of overhead lines with covered conductors is currently contradicted by prevailing difficulties in detecting faults. This article describes the possibility of detecting covered conductor faults and verifying the methodology under both laboratory and real conditions.

Temperature dependent signal propagation velocity: possible indicator for mv cable dynamic rating

Insulation temperature is the critical parameter for dynamic rating of underground power cables. Power cables on medium voltage (MV) level are too numerous and widespread to economically justify solutions like optical fibers as temperature sensors, often employed for high voltage power cables. This paper proposes to utilize the propagation velocity of high frequency signals as indicator for MV cable dynamic rating. Laboratory scale tests are performed for both PILC and XLPE cables. Test results show that the high frequency signal propagation velocity for XLPE insulation increases with the temperature rise while PILC cable shows opposite behavior. The variation of propagation velocity with temperature is consistent with the variation of permittivity measured on test samples of both materials. The variation of propagation velocity of XLPE is confirmed by data of a power cable in service subjected to strong load cycling monitored over a week. Field data on propagation velocity also matches load variation observed for a PILC cable for load cycling recorded over one week. From laboratory tests and field measurements, it seems feasible to extract thermal information from MV cables for dynamic rating by means of high frequency propagation velocity.

Расчет температуры жилы однофазного высоковольтного кабеля с изоляцией из сшитого полиэтилена в режиме реального времени

Расчет температуры жилы однофазного высоковольтного кабеля с изоляцией из сшитого полиэтилена в режиме реального времени

Diagnosis of Water Tree Aging in XLPE Cable by the Loss Current Harmonic Component Under Variable Frequency Power

Water treeing is one of the main aging in XLPE cable insulation, which has the direct effect on the reliability of cable long-term operation. The diagnosis of water tree aging is a premise to ensure the safe operation of cables. The water treed XLPE insulation shows a nonlinear conductivity characteristic, which leads to a harmonic component in the corresponding lose current under the standard sinusoidal voltage. So the loss current harmonic component method can be an effective diagnosis of water tree aging. The loss current measurement system based on the high voltage current comparator bridge is used in this paper, which is designed by Harbin University of Science and Technology and Jiangsu Electric Power Company Research Institute. The feasibility of the application of variable frequency series resonant power source in the loss current measurement, which is aimed to diagnose the water treeing in XLPE insulation, is studied in the laboratory and field test. The results show that the output voltage waveform of the variable frequency series resonant power source is quite close to the standard sinusoidal voltage, although there are some high frequency sharp peaks. So the variable frequency series resonant power source can be used as the high voltage test power supply in the loss current harmonic component measurement system. The loss current harmonic component has power frequency interference during the field test, but this can be solved by differential frequency principle. The microscopic observation of water tree structures during the anatomy of the cable not only indicates the effectiveness of the results and diagnosis during the field test, but also proves the feasibility of the application of variable frequency series resonant power source in the loss current measurement. The existence and degree of the water tree can be estimated according to the test results, which can be a strong basis of the standardization of water tree aging diagnosis. Furthermore, the expensive standard sinusoidal wave generator and linear power generator is replaced by the variable frequency series resonant power source, which reduces the cost and weight of the testing system.