纳米SiO2对交联聚乙烯交/直流击穿强度和耐电树枝性能影响

为系统地研究纳米SiO₂对交联聚乙烯（XLPE）交/直流击穿强度和交/直流耐电树枝特性的影响,使用平行双螺杆分别制备了含0.5wt%和1wt%纳米SiO₂的纳米SiO₂/XLPE复合材料,以商用直流电缆料和普通XLPE作为参照,测试了掺杂纳米SiO₂对XLPE交流电树枝和直流接地电树枝的引发和生长特性及交/直流击穿强度的影响。实验结果表明,商用直流电缆料的直流击穿强度与普通XLPE相近,但其直流接地电树枝的引发更困难,树枝生长也更缓慢;随着纳米SiO₂添加量增大,纳米SiO₂/XLPE复合材料交/直流击穿强度的作用增强,对交/直流电树枝引发的抑制作用也增强,1wt%纳米SiO₂/XLPE复合材料具有显著抑制直流接地电树枝生长的效果,其直流接地电树枝引发和生长特性均优于商用电缆料;1wt%纳米SiO₂/XLPE复合材料的交流击穿强度和交流电树枝起始电压均高于普通XLPE,但其对交流电树枝的生长抑制作用仅局限在电树枝生长初期,电树枝生长达到一定阶段后,1wt%纳米SiO₂/XLPE复合材料中的电树枝生长速度超过普通XLPE。      

Fractal analysis of side channels for breakdown structures in XLPE cable insulation

The role of frequency in the range 20 through 300 Hz on the breakdown voltage and the breakdown path is studied in cross-linked polyethylene (XLPE) cable insulation using embedded needle. A maximum breakdown voltage of 25 kV is found at 240 Hz, and side channels are observed on the flank of the main channel of the electrical breakdown path. Fractal analysis of the side channel is carried out and it is induced that the frequency dependence of the fractal dimension D of the side channel are similar to that of the electrical trees before breakdown. It is suggested that the space charge can be injected from the needle tip. This leads to partial discharge causing progress of the electrical tree and the breakdown path. Space charge will also result in field-moderating cloud around the needle tip and turn to sidewall charges in the side channels. The frequency dependence of the breakdown voltage of the XLPE with the embedded needle can be clarified based on the fractal analysis of the side channel and the electrical tree of the XLPE insulation.     

Electrical characteristics of high-Tc superconducting mini-model cable under mechanical stresses in liquid nitrogen

To develop 22.9 kV class high-T_c superconducting (HTS) cable in Korea, we have been studying electrical insulation properties of dielectric paper, such as breakdown voltage, partial discharge, which is one of the HTS cable structure elements. However, the research on the mechanical stress of dielectric paper compared to breakdown properties of dielectric paper is insufficient. A cracking and variation of the electrical insulation due to mechanical stresses during cooling and bending of HTS cables in cryogenic temperature is a serious problem. Thus, we investigated tensile stress and breakdown stress of dielectric paper under mechanical stress. Moreover, we manufactured mini-model cables investigated breakdown stress under bending stress to design a cable drum for conveyance. In the AC, impulse and partial discharge properties, all test results showed a similar tendency, and the suitable bending radius ratio R/r was decided to be more than 25.     

Study on nonlinear conductivity and breakdown characteristics of zinc oxide–hexagonal boron nitride/EPDM composites

The ethylene–propylene–diene monomer (EPDM) has been widely used in HVDC cables accessories. The nonlinear conductivity of EPDM-based composites plays an important role on relieving the distortion of electric field. In this study, the zinc oxide (ZnO) particles are selected as fillers for improving the nonlinear conductivity of EPDM. The result shows that nonlinear conductivity characteristics of ZnO/EPDM becomes more and more pronounced with the increase of ZnO doping content, however, the breakdown strength of ZnO/EPDM composites has been seriously deteriorated with the increase of ZnO doping content. The excellent breakdown strength of composites is very important for ensuring the safe operation of cable accessories, so the hexagonal boron nitride (h-BN) with good electrical insulation has been employed for improving the breakdown strength of ZnO/EPDM. The results indicate that both of non-linear conductivity and good breakdown strength have been obtained in ZnO–h composite-BN/EPDM composites. This work provides a novel way for constructing the composites with excellent electrical performances which are used for cable accessories.     

An investigation of the potential of polypropylene and its blends for use in recyclable high voltage cable insulation systems

Most modern extruded high voltage cables employ cross-linked polyethylene (XLPE) as the insulation material. XLPE has excellent thermo-mechanical properties, is relatively cheap and has a low dielectric loss, which make it an ideal material for this application. Unfortunately, XLPE is not easily recycled at the end of its lifetime leading to questions concerning its long-term sustainability. A previous investigation in this series considered the potential of a range of ethylene-based systems to provide suitable recyclable alternatives to XLPE. Whilst blending could allow systems having similar thermo-mechanical and electrical properties to XLPE to be designed, it was not possible to obtain better performance than XLPE using these systems. Polypropylene offers, potentially, a route to improved insulation systems by virtue of its higher melting point and excellent dielectric properties. However, traditional isotactic polypropylenes have always had the problem of being too brittle for inclusion into practical cable designs. Recently a broad range of propylene co-polymers having improved ductility have become available, which may prove more suitable. The current study compares traditional isotactic and syndiotactic polypropylenes to a range of commercially available propylene co-polymers and focuses on their morphology, thermal, thermo-mechanical and electrical properties. These parameters were then taken together to identify the most suitable candidate materials for future cable applications. The use of blending as a means to further optimise the various material properties was also explored.     

钛酸铜钙纳米纤维/液体硅橡胶复合介质非线性电导性能

为解决直流电缆附件内因温度梯度和材料电导率差异而引起的局部电场畸变的难题,本文通过静电纺丝方法制备了钛酸铜钙（CaCu₃Ti₄O₁₂）纳米纤维,并将其分散在液体硅橡胶中合成了具有非线性电导特性的CaCu₃Ti₄O₁₂纳米纤维/液体硅橡胶复合介质。采用XRD和SEM对CaCu₃Ti₄O₁₂纳米纤维和CaCu₃Ti₄O₁₂纳米纤维/硅橡胶复合介质进行微观结构表征,并对CaCu₃Ti₄O₁₂纳米纤维/硅橡胶复合介质的介电特性、空间电荷特性及在30℃、50℃、70℃条件下电导率随电场强度变化规律和击穿强度进行测试,最后建立电缆附件模型,并对附件应力锥根部电场进行仿真。结果发现:CaCu₃Ti₄O₁₂纳米纤维/硅橡胶复合材料的介电常数和电导率都随着CaCu₃Ti₄O₁₂纳米纤维含量的增加而增大,当纳米纤维达到3vol%时复合介质的相对介电常数增加到3.27,非线性电导率也变化了近4个数量级,经过空间电荷测试发现,空间电荷的消散量与CaCu₃Ti₄O₁₂纳米纤维含量也正相关,复合材料的直流击穿强度随纳米纤维含量的增加而降低,通过对附件进行稳态电压作用下的电场分布仿真分析发现,当CaCu₃Ti₄O₁₂纳米纤维的含量为2vol%时,应力锥根部最大电场强度已经从增强绝缘中转移到电缆主绝缘中,在正、反极性雷电冲击电压作用下,3vol%含量的CaCu₃Ti₄O₁₂纳米纤维/硅橡胶复合介质作为增强绝缘材料时最大电场强度均远远低于其击穿强度。以上实验结果表明,CaCu₃Ti₄O₁₂纳米纤维作为填充相在较低的掺杂浓度实现了对液体硅橡胶的改性,满足了复合介质应用于电缆附件的电气绝缘性能需求。      

The method of testing of XLPE cable insulation resistance to partial discharges and electrical treeing

The paper gives a method of model XLPE medium voltage cable insulation testing for partial discharges and electrical treeing with a point to plane test geometry. Based on this method, a comparative estimate of insulation resistance to treeing and partial discharges have been made. XLPE crosslinked by different methods (steam medium and hot nitrogen medium) and with varying contents of inorganic filler kaolin were tested. The characteristics of partial discharges were measured by means of a pulse height analyser characterized with a microscope. The data have been analysed statistically.     

Thermal and electrical assessment of flexible

The electrical and thermal is described of a flexible liquid nitrogen cooled, cryoresistive cable with plastic tape insulation. The tape materials considered are low-loss polypropylene paper laminate (LLPP/P), polyethylene, melinex, and polypropylene. The effects of the variation of conductor size, circuit length, liquid nitrogen pressure and temperature, thermal resistivity of the insulation and coolant-flow arrangements, on the cable ratings are investigated. For lengths of 10 km between pumping and refrigeration stations at 138 kV the capability for all the tape materials is seen to be 1 GVA and at 500 kV in the order of 3 GVA. At 1200 kV, LLPP/paper appears to be the only material suitable.     

纳米TiO2/液体硅橡胶直流电缆附件绝缘复合材料的介电性能

高压直流电缆附件在电力系统运行中,由于复合绝缘电导率不匹配极易导致电场畸变引发绝缘故障。针对这一问题,采用直接共混法制备了不同掺杂浓度的纳米TiO₂/液体硅橡胶（LSR）复合材料,并对其微观形貌和介电性能进行了测试研究。结果表明:纳米TiO₂粒子在LSR基体中分散较均匀,随着TiO₂掺杂含量的增加,纳米TiO₂/LSR复合材料试样的相对介电常数和介质损耗因数增大。当纳米TiO₂粒子添加量为4wt%时,纳米TiO₂/LSR复合材料的电导率与电缆主绝缘交联聚乙烯（XLPE）的电导率近似相等,且随着电场强度的增大,两者的电导率变化趋势也基本一致。电声脉冲法（PEA）测量结果表明,添加4wt% TiO₂的纳米TiO₂/LSR复合材料内积聚的空间电荷最少。纳米TiO₂粒子的掺杂,提高了TiO₂/LSR复合材料电缆附件绝缘电导率对电场强度的响应依赖特性,使其能与XLPE绝缘电导率较好地匹配,同时一定程度地抑制了空间电荷的积累,有助于直流电缆附件内复合绝缘电场的均匀分布。      

核电站电缆料的研究进展

依据核电站电缆的种类,介绍了核电站电缆料的种类、性能、生产工艺和国内外的研究现状。研究发现,制备核电站电缆绝缘料主要用交联聚乙烯(XLPE)和硅橡胶(或硅橡胶复合材料),此两种原料具有优良的耐热性、电绝缘性、耐低温性、耐化学性和良好的耐辐射性;核电站电缆料应具有低烟无卤、耐阻燃性和耐环境性能;其制备方法主要是挤出法,此方法生产工艺简单、成本低。最后展望了核电站电缆料的发展趋势。     

An investigation of the potential of ethylene vinyl acetate/polyethylene blends for use in recyclable high voltage cable insulation systems

Ethylene vinyl acetate (EVA) co-polymers can potentially provide novel materials for inclusion into extruded high voltage cable systems, providing a degree of electrical conductivity whilst avoiding the dispersion problems associated with conventional particulate fillers or conducting polymers. Although a degree of conductivity can decrease the electrical breakdown performance, it can help to suppress the development of space charge and increase the tree initiation voltage leading to enhanced dielectric properties. In addition, novel two phase morphologies can be formulated leading to the ability to control key thermal and mechanical properties and the ability to tailor these to suit the application. In addition, one of the problems with conventional cross-linked polyethylene (XLPE) is that it cannot easily be recycled; therefore, in this time of increasing environmental awareness, it is prudent to begin investigations into alternative recyclable materials to replace XLPE in extruded cables for the medium to long term. The current article focuses on the crystallisation behaviour, morphology, mechanical and dielectric properties of a range of polymeric insulation systems based on an EVA co-polymer together with a high density polyethylene (HDPE) component. The morphology was controlled by choosing co-polymers containing different vinyl acetate contents together with appropriate crystallisation routes. The relationships between the morphology and the mechanical and dielectric properties were explored. Blends containing a low vinyl acetate content co-polymer combined with HDPE have significant potential to replace XLPE in cable systems and have the advantage of being easily recycled at the end of their service life.     

Cryopower transmission studies in Europe

European studies on cryopower transmission are focused on semiflexible and totally flexible superconducting cables. Intense investigations of cable components are being performed. Nb/Cu and Nb/Al composite conductors have been developed, which exhibit 50 Hz surfaces losses well below the design limit of ac cables at 4–6 K and surface peak flux densities up to 0.1 T. Measurements of breakdown voltages and dissipation factors at 4 K indicate that wrapped tape electrical insulation, impregnated with helium, may be suitable for superconducting cables, the achievable dielectrical strength being about 10 kV mm^?1 (rms). But further investigations are necessary within this topic. Different types of dc and ac cable joint and terminal have been designed or already constructed. A number of larger laboratory model tests with cable sections up to 50 m length have been performed or are planned for the near future with the aim of studying size effects, the interaction of different components and to demonstrate the feasibility and performance of superconducting cables.     

ABS/石墨复合材料的交直流导电特性及流变性能

研究了石墨填充丙烯腈-丁二烯-苯乙烯共聚物(ABS)复合材料的直流(DC)和交流(AC)导电特性和线性粘弹行为。电性能测试结果表明,石墨体积分数为13.21%~16.36%时,ABS/石墨复合材料的DC电阻率突降6个数量级,说明发生电学逾渗;同时,AC电阻率在低频区不随频率而变化,且AC阻抗复平面图中阻抗实部与阻抗虚部呈现半圆弧,进一步证明导电网络的形成。流变性能测试结果表明石墨体积分数为10.24%~13.21%时复合体系的储能模量和复数黏度出现跳跃,损耗因子(tanδ)的峰值减小且逐渐向高频移动,说明复合体系从"类液态"转变为"类固态",发生流变逾渗现象。流变逾渗阈值小于导电逾渗阈值是因为传递电子时石墨之间的距离比阻碍聚合物分子链运动时石墨之间的距离小。     

Aging characteristics of cryogenic insulator for development of HTS transformer

In the response to the demand for electrical energy, much effort aimed to develop and commercialize high temperature superconducting (HTS) power equipments has been made around the world. Especially, HTS transformer is one of the most promising devices. For the development of HTS transformer, the cryogenic insulation technology should be established. In this paper V−t characteristics of polyimide (Kapton) tape and GFRP used as turn-to-turn and structural insulations, respectively were studied. Moreover, breakdown hole site of GFRP after breakdown was also discussed. The experimental results show that the time to breakdown is conditioned on applied electric stress and the lifetime indices n of Kapton tape decrease slightly as the number of tape increases while the lifetime indices n of GFRP decrease strongly with increasing thickness. Furthermore, the breakdown holes of GFRP were not at the contact point, at which the electric field is maximum value, between sphere electrode and GFRP sample and its location depends on applied voltage as well as sphere diameter.     

Effect of MWCNTs/ZnO inorganic fillers on the electrical,mechanical and thermal properties of SiR-based composites

Rubber insulation materials were widely used in the fields of electrical and electronic engineering, especially, which have excellent nonlinear electrical conductivity and can be employed to homogenize the electric field distribution of cable accessories. To enable the rubber materials, such as silicon rubber (SiR), to possess excellent nonlinear electrical conductivity has been a hot issue. In this paper, MWCNTs/ZnO inorganic fillers were prepared by mixing a small amount of multi-wall carbon nanotubes (MWCNTs) with zinc oxide (ZnO) nanosheets, and MWCNTs/ZnO/SiR composites were prepared. The macroscopical properties results show that the nonlinear electrical conductivity characteristics can be induced by filling appropriate content of MWCNTs/ZnO fillers, and the threshold field strength corresponding to the nonlinear conductivity gradually decreases with the increase of MWCNTs filling content, which further decreases with the increase of measured temperature. The COMSOL simulation results also verify that MWCNTs/ZnO/SiR composite with nonlinear conductivity can effectively reduce the electric field strength at the stress cone of cable accessories. In addition, the thermal conductivity and tensile strength for MWCNTs/ZnO/SiR composite are also improved comparing to pristine SiR. This work demonstrates MWCNTs/ZnO/SiR composites possess outstanding overall properties and have good potential to be used in the cable accessory.

     

Correlation of electrical breakdown of supercritical helium in short gaps with partial discharge in cable samples

As part of a project to develop a superconducting power cable at Brookhaven National Laboratory (BNL), many experiments have been conducted on short coaxial cable insulation samples lapped with various polymeric tapes. These experiments have shown the partial discharge inception voltage to be a function of a sample construction and the density of the helium impregnant. It was concluded that the partial discharge inception voltage in the butt gaps of lapped tape insulation is determined by the product of helium density and gap depth, which corresponds to the tape thickness. This means that the inception voltage obeys the similarity law. This conclusion was based on additional experiments performed to determine the breakdown voltage of helium in a short gap between both bare metal electrodes and plastic insulation coated electrodes. The effects of the condition of the electrode surfaces and the area of the electrodes were essential in determining the breakdown voltage in a short gap of high density cryogenic helium.     

Thermally stimulated discharge current and DC resistivity of ethylene propylene rubberwith various Pb concentrations

DC resistivity of extruded ethylene propylene rubber (EPR) samples with various Pb concentration have been measured under wet conditions as a function of electrical field at selected temperatures in a range from 20 to 100°C. The temperature and electrical field coefficients of resistivity have been calculated. Thermally stimulated discharge current (TSDC) has also been measured and a broad positive peak has been observed for three EPR samples. It has been found that the resistivity of EPR is not sensitive to the Pb concentration within the range of 0 to 5 parts per hundred base resin (phr). The results show that the resistivity of EPR varies non-linearly with both temperature and electrical field. The temperature coefficient of resistivity α of EPR has been measured to be ∼0.1 K⁻¹ for all the samples with various Pb concentration. The electrical field coefficient of resistivity β of EPR at room temperature is small and increases with temperature. Increasing Pb content increases slightly the electrical field coefficient β of resistivity. Based on a space charge limited conduction model, the trap depth of EPR has been estimated. TSDC measurements indicate that doping with Pb increases both the density of charge carriers and the number of deep traps simultaneously. The broad TSDC peak reveals that there must be a distribution rather than just a single value of the trap depth.     

PTCR barium titanate ceramics obtained from oxalate-derived powders with varying crystallinity

Barium titanate powders with average crystallite sizes of 68–2000 nm have been prepared by the calcination of barium titanyl oxalate (BTO) at temperatures of 700–1150 °C. The morphology and recrystallization kinetics of the powders have been studied using the SEM and X-ray methods. Samples of PTCR (BaCaPb)TiO₃ ceramics have been made from these powders and their microstructure and electrical properties have been investigated. It has been found that the increase of the crystallinity of the starting powders suppresses recrystallization of the ceramics, leading to growth in resistivity and significantly influencing on the resistance jump and breakdown strength of the ceramics. An optimal temperature range for the calcination of BTO has been found to ensure maximum breakdown strength of the PTC thermistors with the resistance of 31 Ω. At this temperature range the barium titanate powders had crystallite sizes of ~200 nm.     

Numerical study on self-field losses of 30 m BSCCO HTS transmission cable

The self-field losses of the one phase of high-T_C superconducting (HTS) transmission cable are calculated by the electric circuit (EC) model. The one phase of HTS cable is constructed by the former of fine-strands copper rod, HTS conductor with four superconducting layers, the insulation made by polypropylene laminated paper, and HTS shielding with two superconducting layers, which was fabricated by Sumitomo Electric Industries (SEI). The length of the cable is 30 m. Each HTS layer comprises BSCCO tapes. The current-dependent resistance of HTS layers in EC model is estimated on the base of Norris expressions for ellipse. The calculated losses are compared with the experimental results measured by 4-terminal method by SEI. The calculation of alternating current (AC) losses, a summation of the self-field losses in HTS layers and the eddy-current losses in the former, is almost equal to the measurement at wide transport-current range below the lowest value of the layer critical current. This result indicates that the numerical calculation by EC model is quite reliable. The minimum AC loss is also calculated by obtaining the optimum helical-pitch lengths of HTS layers at transporting 1 kA_rms. The minimum loss is 36% lower than the loss of HTS cable designed by SEI at the transport current value. In HTS cable with the optimum helical-pitch lengths, the calculation of the layer currents are not uniform in HTS conductor but are almost uniform in HTS shielding, which is contradict to SEI’s one. It is considered that the numerical calculation by EC model is useful to obtain the optimum helical-pitch lengths in HTS cable with the minimum AC loss.     

Review of tape materials for cable insulation at liquid nitrogen temperatures

This Paper reviews the electrical breakdown, partial discharge and loss angle characteristics of taped cable insulation impregnated with liquid nitrogen. Also life expectancy and the effects of semiconducting screens are surveyed. With the recent announcement of a superconductor at liquid nitrogen temperatures a review of cable insulation impregnated with liquid nitrogen is felt to be appropriate.