XLPE Insulation Materials Research Articles

Anomalous attenuation phenomenon of terahertz wave through the micron air gap defects in XLPE insulation material

During the fabrication process of the factory joint in submarine cables, manual cutting and polishing are required, which can easily result in scratches, pits, and other defects. Meanwhile, due to the limitations of insulation recovery and injection molding technology, air gap defects are prone to occur inside the joints, posing significant safety hazards. In order to preemptively detect such defects before the cable is put into operation, this study mimics the insulation recovery process of the cable joint to create micron air gaps inside the insulation and uses terahertz imaging technology to visualize them. The results indicate that 153 μm and 76 μm arc-shaped air gaps can be clearly recorded by terahertz imaging systems. Meanwhile, terahertz time-domain signals clearly show that the electromagnetic waves experience a significant attenuation of up to 45 % after passing through the arc-shaped air gap, and their energy exhibits an uneven distribution. To explain the propagation phenomena of terahertz waves in micron air gaps, this study proposes a propagation model based on absorption effects, reflection effects, partial interference effects, and off-axis Gaussian scattering effects. This paper proved the potential of terahertz waves in detecting air gap defects in cables joint. The associated theoretical models can be utilized to develop targeted imaging algorithms.

Effects of Overload on Thermal Decomposition Kinetics of Cross-Linked Polyethylene Copper Wires.

During an overload fault in an energized wire, the hot metal core modifies the structure of the insulation material. Therefore, understanding the thermal decomposition kinetics of the insulation materials of the overloaded wire is essential for fire prevention and control. This study investigates the thermal decomposition process of new and overloaded cross-linked polyethylene (XLPE) copper wires using thermogravimetry-Fourier-transform infrared spectroscopy and cone calorimetry. The thermal decomposition onset temperature and activation energy of the overloaded XLPE insulation materials were reduced by approximately 15 K and 20 kJ mol-1, respectively, and its reaction mechanism function changed from D-ZLT3 to A2 (0 < α < 0.5). The FTIR shows that the major spectral components produced during the pyrolysis of the XLPE insulation material are C-H stretching, H2O, CO2, C-H scissor vibrations, and C=O and C=C stretching. Additionally, the four functional groups in the PE chains produced the spectral components in the following decreasing order of wavenumber: C-H stretching > CO2 > C-H scissor vibration > C=O and C=C stretching.

Structural design of a new XLPE Insulation material without cross-linking by-products and theoretical study on the reaction mechanism: An alternative to peroxide crosslinking

The cross-linked polyethylene (XLPE) is used in most advanced power cable insulation. The cross-linking byproducts based on typically dicumyl peroxide (DCP) cross-linking process increase the electrical conductivity of the insulation material. In order to remove the hazardous byproducts, an essential time-consuming and energy-consuming procedure is needed. Hence, a new XLPE production process without cross-linking by-products receives considerable attention. Here, the structural design of a new XLPE based insulation materials have been established. The cross-linking of epoxy and reactive functional groups between two polyethylene copolymers in situ through reactive compounding is an alternative to DCP crosslinking without cross-linker and byproducts. Theoretical investigation on the reaction process of covalent bonds forming and reaction mechanism is accomplished by density functional theory. Two reaction mechanisms, synergistic reaction and step-by-step reaction, are proposed. The reaction potential energy information of the twelve reaction channels at B3LYP/6–311 +G(d,p) level are obtained. The calculation results show that the epoxy and reactive functional groups between two poly-ethylene copolymers can react in situ and crosslink through the formation of covalent bond to realize the networking of XLPE. The reaction of maleic anhydride with H2O (or CH3OH) is more kinetically and thermodynamically favorable than that of maleimide. The reactivity of carboxylic acid functional group is stronger among three cross-linking reaction systems considered. The reaction Gibbs energy barrier heights of synergistic reaction are lower than that of step-by-step reaction by about 0.6 eV. The theoretical study on the reaction mechanism of epoxy and reactive functional groups between two polyethylene copolymers would be beneficial to avoid the forming of hazardous cross-linking byproducts, which is a promising method to design thermoplastic insulation materials for future power cables.

Structural Changes and Very-Low-Frequency Nonlinear Dielectric Response of XLPE Cable Insulation under Thermal Aging.

The structural changes and very-low-frequency (VLF) nonlinear dielectric responses are measured to evaluate the aging state of cross-linked polyethylene (XLPE) in power cables under various thermal aging conditions. For this purpose, the accelerated thermal aging experiments were performed on XLPE insulation materials at 90 °C, 120 °C and 150 °C with different durations of 240 h, 480 h and 720 h, respectively. The Fourier transform infrared spectrum (FTIR) characterization and differential scanning calorimeter (DSC) were tested to analyze the influence of different aging on physicochemical properties of XLPE insulation. Besides, the VLF dielectric spectra show that the permittivity and dielectric loss change significantly in the VLF range from 1 mHz to 0.2 Hz. A voltage-current (U-I) hysteresis curve referring to a standard sinusoidal voltage and the response current were introduced to characterize the nonlinear dielectric properties of XLPE insulation caused by thermal aging.

Comparative Analysis of XLPE and Thermoplastic Insulation-Based HVDC Power Cables

The application of cross-linked polyethylene (XLPE) cables to voltage sourced converter (VSC)-based high voltage direct current (HVDC) systems has already been technically verified and has become common, and thermoplastic (TP) is attracting attention as an insulation material for next-generation cables due to the recent development of material-related technologies. However, studies related to TP cables are mainly focused on improving material properties, and studies related to cable systems are insufficient. In this paper, XLPE and TP cables were designed for application to VSC-based HVDC systems, and major characteristics such as electric field distribution and thermal stability were compared and analyzed through overvoltage simulation. The insulation design method of HVDC cable was presented, and the design was performed using XLPE and TP insulation materials. The temperature and electric field profiles of the cables were also analyzed through a finite element method simulation. To analyze the performance of the designed cable, it was simulated with the PSCAD/EMTDC program. Based on the simulation results, the major characteristics of XLPE and TP cables were compared and analyzed. Results showed that in the case of TP cables, insulation properties were excellent, but thermal conductivity was relatively low; therefore, countermeasures are needed.

Research on Defects of High Voltage Cable Fusion Joint Using XLPE Insulation Material

High-voltage cable fusion joints are made on site. Affected by environmental conditions, fusion joints are prone to small defects that are not easy to find, resulting in frequent accidents after the joints are put into operation. There is currently no effective method to detect small defects inside the joint. Therefore, this paper proposes to use digital radiography nondestructive test technology to detect the main insulation and conductor recovery process of high-voltage cable fusion joints. The results show that the digital radiography nondestructive test method can effectively find defects such as microporous bubbles, conductor bending, and eccentricity in the main insulation of XLPE fusion joints. The on-site anatomy demonstrates the accuracy of the test results, and the digital radiography nondestructive test can effectively detect the defects of high-voltage cable fusion joints. Experiments have verified that the decrease of cross-linked pipe temperature and nitrogen pressure can lead to the generation of microporous bubbles in the XLPE main insulation. For conductor bending and eccentricity defects, the limit value of the welding conductor bending gap is 11mm through simulation calculation.

Ameliorated Dielectric Performances of UV-Initiated Auxiliary Crosslinking Polyethylene

In photon-initiated crosslinking reactions of polyethylene molecules, the auxiliary crosslinkers in form of either monomer or homopolymer will cause bridging connections between polymeric molecules by transforming the irradiated photon energy to chemical energy under the assistance of photon-initiators, which can improve photon-initiation quantum efficiency and crosslinking uniformity. In the present study, the auxiliary crosslinkers of TAIC, TAC and TMPTA combining the macromolecular photon-initiator of BPL are employed into the ultraviolet (UV)-initiation technology to develop high-level crosslinked polyethylene (XLPE) insulation materials, whilst elucidating the structural and electrical mechanisms of the dielectric amelioration deriving from auxiliary crosslinking schemes. The specified photosensitive auxiliary crosslinkers can chemically bridge polyethylene molecules in the UV-initiated polyethylene crosslinking process, which can effectively promote polyethylene crosslinking degree but will slightly abate polyethylene crystallinity. Whereas, the orientation polarization and relaxation of molecular electric-dipoles on auxiliary crosslinkers cause additional dielectric permittivity and loss respectively, which will probably reduce insulation performances of XLPE. By contrast to XLPE benchmark, especially for grafting auxiliary crosslinker TAIC with the multiple-coupling carbonyl groups in a ring-conjugation, the preferable deep charge-traps can be introduced into polyethylene matrix to effectively improve electrical conductance and AC dielectric breakdown strength. This study provides experimental basis for developing the photon-initiated XLPE insulation materials with advanced dielectric performances required for manufacturing high-voltage grade cables.

Effect of Dibenzyl Disulfide Corrosion on Accelerated Thermal Aging Crosslinked Polyethylene Cable Materials with Copper Core and Insulation Layer

Crosslinked polyethylene (XLPE) cable has been widely used and studied with its specific failure mechanism. Among them, although thermal aging of XLPE insulation materials has been widely studied, the effect of accelerated thermal aging on the sulfur corrosion of XLPE cable has not been studied much. For further understand the mechanism of sulfur corrosion, the accelerated thermal aging method was taken with 200 mg/kg DBDS based on the distinguish of temperature. The macro and micro analysis contrast test before and after corrosion were also taken to investigate the morphology difference, the element distribution, the influence of thermal aging and DBDS addition the corrosion of copper core, and the assessment of insulation layer was also carried out by Fourier transform infrared spectra and spectroscopy, scanning electron microscopy to distinguish the microstructure and composition. Under the accelerated thermal aging experiment with DBDS, both copper core and insulation layer of sample No. 1 exhibit the most serious corrosion and aging condition. On the surface of the copper core, the DBDS-Cu complexes decompose to produce Cu2S and the temperature has a direct effect on this reaction. The insulation layer of sample No. 1 showed the most severe aging situation, with characteristic peaks of carbonyl appearing at 1720 cm-1 in the Fourier transform infrared tests and more defects, holes and cracks found in spectroscopy, scanning electron microscopy images.

Assessment of thermally aged XLPE insulation material under extreme operating temperatures

The degradation of cross-linked polyethylene (XLPE) insulation material is critical for the safe operation of cables. While there are extensive studies on the thermo-oxidative ageing of XLPE at or below 130 °C, the maximum overload conductor operating temperature, the short circuit temperature can rise above 150 °C, and the ageing patterns at these situations have not been fully understood. Here, chemical, crystalline structure, cross-linking density as well as the mechanical properties were characterized. The results demonstrate the importance of antioxidants in the high temperature situation. Before antioxidants are completely consumed, most of the mechanical and physicochemical properties remain stable. While antioxidant is depleted, the thermo-oxidative degradation is detrimental to the structure and properties of the insulation. The activation energy is around 160 kJ/mol for the thermo-oxidative ageing, from which the lifetime allowed before serious destruction of the insulation at different shortage or overload temperatures can be calculated.

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.

Study on Inhibition of Water Tree in XLPE Insulation Material

The formation of water tree in XLPE insulation material was inhibited by adding EAA(Ethylene Acrylic Acid),DBS(dibenzylidene sorbitol) into cross-linked polyethylene-based materials. The mixture was crosslinked through mixing and tabletting the blend at 160 degrees. The effect of additive content on the length of the water tree was investigated. The water tree cultivation experiment showed that the length of water tree decreased with increasing of the content of EAA and DBS in the modified compound.Its full property experiments results showed that the property of the insulation material fully meet the related requirements for the use of anti-watertreeing XLPE cable.

Electric field in polymeric cable due to space charge accumulation under DC and temperature gradient

When power cables are loaded under high voltage direct current (HVDC), an accumulation of space charge and a radial distribution of temperature gradient are developed across the insulation material. Such existence and accumulation of space charge within the insulating material poses a threat to the reliability of the operation of dc power cables. The electric field of a practical dc power cable is affected by the conductivity of the material, which is a function of both temperature and electric field. This causes difficulties in identifying the electric field distribution. In this paper, a method of determining the electric field distribution in dc power cables was proposed by considering the influence of space charge on the conductivity of the insulating material under different temperatures. Commercial 11 kV ac cross-linked polyethylene (XLPE) power cables were used and the space charge in these cables under dc conditions was measured using a modified pulsed electroacoustic (PEA) system with an attached current transformer. Therefore, a replica of a power cable under load conditions is obtained, which allows an investigation of the formation, migration and accumulation of space charge in a power cable with and without temperature gradients across the insulating material. COMSOL Multiphysics software package was used to accurately determine the electric field distribution in the dc power cable with consideration of the influence of electric field on the conductivity of the insulating material. The numerical modelling is based on the hopping conduction mechanism and its parameters were obtained from experiments carried out on the XLPE insulation material.

A Review of Possible Methods for Defining Tree Retardant Crosslinked Polyethylene (TRXLPE)

This article documents the activities and discussions of the ICC discussion group A4D comparing the performance of TRXLPE and XLPE insulation material used in medium voltage underground power cables. It was not possible at the time to use water tree growth tests, carried out according to ASTM 6097-97, to provide a definition of a water tree retardant material. This was due to the relatively small number of tests carried out, the large scatter in the data of repeat tests on the same material, and the overlap in the data observed between XLPE and TRXLPE insulations.

A Field-circuit Coupled Method to Accurately Determine Intrinsic Complex Permittivity of XLPE Insulation Material

To identify the location of partial discharge occurring in a power cable, it is essential to obtain the intrinsic values of permittivity of cross-linked polyethylene (XLPE) material. This paper reveals that when the conventional two-parallel-electrode method is used, and due to the fringing effect, the measured permittivity of XLPE material pronouncedly differs from its intrinsic value, consequently significant measurement error could occur. An effective method based on the field-circuit coupled theory is presented in this paper to investigate the difference between the measured and intrinsic permittivity of XLPE insulation material. A capacitor constructed from a concentrically cylindrical XLPE sample is used. By assuming a certain value of the intrinsic permittivity for XLPE material, the electric field distribution in the XLPE material and the ambient is calculated. Then the total energy is determined, as well as the measurable permittivity. By assuming different values of intrinsic permittivity, corresponding measurable values of permittivity are determined, and a curve of measurable permittivity against its intrinsic value is then obtained, from which the intrinsic value can be found once its measurable value is obtained from experiment. The influence factors of the correction curve are analyzed, i.e. the sizes of sample and electrode. A measurement was carried out to verify this method.

Electron spin resonance spectral study of PVC and XLPE insulation materials and their life time analysis

Electron spin resonance (ESR) study is carried out to characterize thermal endurance of insulating materials used in power cable industry. The presented work provides ESR investigation and evaluation of widely used cable insulation materials, namely polyvinyl chloride (PVC) and cross-linked polyethylene (XLPE). The results confirm the fact that PVC is rapidly degrades than XLPE. The study also indicates that colorants and cable's manufacturing processes enhance the thermal resistance of the PVC. It also verifies the powerfulness and the importance of the ESR-testing of insulation materials compared to other tests assumed by International Electrotechnical Commission (IEC) Standard 216-procedure, e.g. weight loss (WL), electric strength (ES) or tensile strength (TS). The estimated thermal endurance parameters by ESR-method show that the other standard methods overestimate these parameters and produce less accurate thermal life time curves of cable insulation materials.

Development of Heat-resistant XLPE Cable and Accessories

We have developed heat-resistant XLPE cable and accessories that can be operated at 105°C as the maximum permissible conductor temperature in normal operation. Through this cable system, greater transmission capacity can be achieved using existing cable ducts and without increasing the conductor size of the cable. We have developed heat-resistant XLPE insulation material which has a higher melting point than that of conventional XLPE. The breakdown strength of heat-resistant XLPE cable at 105°C is almost the same as that of conventional XLPE cable at 90°C. The heat deformation of the new cable at 105°C is almost the same as that of conventional XLPE cable at 90°C. Conventional self-pressurized rubber joints can be applied to heat-resistant cable lines with the new waterproof joint compound with low heat resistivity.