The thermal oxidation of low-density polyethylene and poly(ethylene-co-vinyl acetate) both cross-linked with triallyl isocyanurate (respectively denoted XLPE and EVA) was studied at 165 °C in air by Fourier transform infrared (FTIR) spectroscopy, dielectric spectroscopy and uniaxial tension. Monitoring the carbonyl index throughout the exposure allowed showing that these two polymers have similar oxidation reactions, taking place on their ethylene monomers, and therefore similar oxidation kinetics. In addition, in EVA, the acetate side groups thermally decompose into several degradation products during the exposure, such as: trans-vinylene double bond, acetic acid, saturated and unsaturated γ-lactone, and methane, but without interacting with the oxidation reaction. As oxidation is diffusion controlled, it leads to the development of oxidation profiles across the sheets of about 1 mm thick of these two polymers. In addition, as the oxygen transport properties (both solubility and diffusivity) are higher in EVA than in XLPE, oxidation affects the center of the EVA sheets from the early periods of exposure, whereas it is necessary to wait for high conversion ratios of the oxidation reaction to observe the same types of chain-shaped profiles in XLPE sheets. Oxidation results in dramatic changes in functional properties (both electrical and mechanical properties) for both polymers. Before thermal aging, EVA is already a poor insulating material compared to XLPE, due to the presence of polar side groups (i.e. acetates). In contrast, EVA exhibits a higher ductility, which justifies its use as nanofillers in PE-based insulating materials in order to improve their mechanical properties. The establishment of correlations between the changes in functional properties and the progress of oxidation allows the proposal of structural, electrical and mechanical end-of-life criteria for both polymers, corresponding to critical values of carbonyl index, dielectric constant, and elongation at break, respectively.

Surface flashover is a common breakdown phenomenon on material surfaces for which surface charge migration property, determined by surface composition and molecular chain structure, is crucial. Precise modulation of charge migration property by simple and efficient methods to improve surface flashover voltage is the goal in industry. Here, in-plane molecular chain orientation (MCO) modulation by uniaxial stretching was proposed to achieve this goal and investigate the intrinsic mechanism of charge migration on flashover. Flashover voltage and accompanying leakage current, performed with electrodes oriented at varying angles to the MCO direction, show a consistent trend, which skillfully reveals that the facilitated charge migration is favorable for improving flashover voltage. When the stretching ratio is 3.5, the flashover voltage along the stretching direction (SD) increases by up to 48.7%, while the in-plane minimum flashover voltage remains essentially unchanged with the change in stretching ratio. Molecular chain segment motion properties along different directions further elucidate that the surprising improvement of flashover voltage along SD is primarily due to the MCO that promotes intra-chain charge migration. This work provides a new perspective on anti-flashover modification of polymeric dielectric and will promote the development of surface flashover mechanisms.

The long-term operation of motors induces substantial alterations in the surface conductivity and nonlinear coefficient of anti-corona paint, diminishing its efficacy and jeopardizing the longevity of large motors. Hence, the development of high-performance anti-corona paint holds paramount importance in ensuring motor safety. In this study, we integrate two nano-fillers, namely silicon carbide (SiC) and organic montmorillonite (O-MMT), into a composite matrix comprising micron silicon carbide and epoxy resin (SiC/EP). Subsequently, three distinct types of anti-corona paint are formulated: SiC/EP, Nano-SiC/EP, and O-MMT/SiC/EP. Remarkably, O-MMT/SiC/EP exhibits a glass transition temperature about 25 °C higher than that of SiC/EP, underscoring its superior thermal properties. Moreover, the introduction of nano-fillers markedly augments the surface conductivity of the anti-corona paint. Aging tests, conducted across varying temperatures, unveil a notable reduction in the fluctuation range of surface conductivity post-aging. Initially, the nonlinear coefficients exhibit a declining trend, succeeded by an ascending trajectory. The O-MMT/SiC/EP composite displays a maximum nonlinearity coefficient of 1.465 and a minimum of 1.382. Furthermore, the incorporation of nanofillers amplifies the dielectric thermal stability of epoxy resin composites, with O-MMT/SiC/EP showcasing the pinnacle of thermal endurance. Overall, our findings elucidate the efficacy of nano-fillers in enhancing the performance and longevity of anti-corona paint, particularly highlighting the exceptional attributes of the O-MMT/SiC/EP composite in bolstering motor safety through improved thermal stability and electrical properties.

Dielectric materials with high surface electric insulation strength are in great demand in a high-power space solar cell array (SSCA). A moderately conductive surface is favorable to inhibit charge accumulation and mitigate electric field distortion, thus improving the surface flashover voltage. Although numerous modification methods have been proposed to achieve this goal, the facile, efficient, scalable, and environmentally friendly modification strategy remains a critical challenge to date. Considering the excellent charge modulation ability of ZnO and its mild preparation conditions, a facile and economical hydrothermal strategy was proposed to fabricate in situ a durable poly(ether imide)/zinc oxide (PEI/ZnO) coating with a high charge decay rate. The blooming flower-like ZnO in the coating is proved to play a key role in enhancing lateral charge dissipation on the surface of PEI, thereby suppressing surface charge accumulation. It was also shown that the shielding effect of ZnO on high-energy photons during flashover and the catalytic effect of Zn2+ on PEI molecular chains during hydrothermal treatment had a facilitating and suppressing effect on outgassing, respectively, and consequently affected the flashover. Excitingly, the synergistic effects of both accelerated charge dissipation and suppressed outgassing helped to improve the flashover voltage of PEI by up to 36.7%. The strategy selected here is efficient, scalable, and facile, and the coating is durable, which makes sense for commercial promotion.

Polyethylene (PE) has numerous applications in electrical and electronic products. However, PE insulation materials have a short service life, which poses a safety risk during power system operation. To address this issue, a molecular model of polyethylene-montmorillonite (PE-MMT) nanocomposites is developed to simulate and explore the microscopic mechanisms influencing their breakdown characteristics. PE-MMT nanocomposites loaded with 0, 3.3, 4.0, or 5.1 wt% organically modified MMT are obtained via disordered doping. X-ray diffraction and scanning electron microscopy experimentally demonstrate the effects of modifying these nanofillers and dispersing them in PE, while radial distribution function, interaction energy, and fractional free volume studies reveal the microscale characteristics of the nanocomposites. Hydrogen bonds form in the PE-MMT nanocomposites, and the nanocomposite with 4.0 wt% nanofillers exhibits better breakdown properties than pure PE. The simulation results are in agreement with the data collected from experimental analogs, which confirms the accuracy and effectiveness of the PE-MMT nanocomposite models described herein. The findings of this study thus provide both a model for studying breakdown in PE and a modification procedure for improving PE as an insulating material.

为了研究在极端低温和油介质共同作用下丁腈橡胶(NBR)密封件的可靠性,对极寒地区服役高压变压器中的密封圈进行力学性能和微观结构的研究.通过拉伸实验、微米压痕、扫描电镜(SEM)、红外光谱(FTIR)、差示扫描热量分析(DSC),测试极寒环境中服役1年后密封圈的力学性能、形貌变化、化学结构特性及热响应特性.结果表明:在极寒环境中服役的NBR密封圈的弹性模量和硬度均发生下降,进而导致密封作用丧失.这是由于在低温和油介质共同作用下,密封圈表面发生了溶胀,出现了微裂纹.油介质浸入NBR使其分子链间距增大,NBR分子主链柔顺性提高,同时NBR中的氰基发生水解反应,分子结构发生变化.

There is an urgent need for high-temperature capacitors films materials in many applications. In the present study, poly (4-methyl-1-pentene) (PMP) was chosen as the matrix because of its high-temperature resistance. The matrix was then enhanced with boron nitride nanosheets (BNNSs) to improve the breakdown strength (BDS) and the consequent energy storage density. Here we proposed a simple yet cost-effective approach, a liquid phase-assisted blending approach to improving the dispersion of BNNSs within the PMP matrix. The homogeneous distribution of nanosheets would refrain from the degradation in properties by agglomeration and enhance the energy storage property. The maximum energy density (Ue) of the resultant nanocomposites was enhanced to 1.2 J/cm3 at 120 ºC, which was around 42% higher compared to the undoped PMP films. The dielectric loss of the obtained BNNS/PMP nanocomposites was as low as 10-5 until 120 °C. Such results indicated that BNNS/PMP was very promising as the dielectric material for high-temperature applications.

An investigation of thermal-oxidative degradation of cross-linked polyethylene (XLPE) has been carried out in this paper. As the key characteristics, dielectric properties are important for assessing the insulation state of cables. XLPE specimens were thermally aged at 165 °C and removed at regular intervals for analysis. The dielectric properties were investigated through the Frequency Domain Spectroscopy (FDS) and the structural changes were analyzed with the Fourier transform infrared (FT-IR) spectroscopy. And the Haviliak-Negami dielectric model is applied to extract the characteristic parameters. The experiment and fitting results show that the dipolar polarization strength Δεα, the DC conductivity σdc, the relative permittivity at high frequency ε∞, and the thermal expansion coefficient β increase gradually with the aging time. It is found that the quantitative analysis of the frequency domain spectroscopy can be well applied to the state assessment of XLPE cables.

为了研究低温环境下变压器密封圈的失效机理,收集了极寒地区变压器中失效的橡胶密封圈试样,并进行了微观结构和力学性能研究:利用扫描电子显微镜表征由于低温和机械应力共同作用导致的微观形貌变化,利用傅里叶变换红外光谱和差示扫描量热仪表征橡胶试样分子链结构和热运动的变化,通过无损微米压痕测试表征橡胶试样微米压痕硬度和简约杨氏模量的变化.结果表明:由于低温、绝缘油和机械压力的长期共同作用,丁腈橡胶(NBR)的分子链结构和排布均发生了变化,导致橡胶的微米压痕硬度和简约杨氏模量下降明显,密封作用丧失.

Investigations into surface flashover characteristics and underlying mechanisms in the range from atmospheric to near vacuum pressure are significant for developing high-power equipment based on the application of gases used for flashover prevention. In this work, polyetherimide (PEI) films with controllable outgassing properties were prepared. Then, the gas-related factors, namely, gas pressure, the type of gases, and outgassing were clarified by investigating the DC surface flashover of films in a wide pressure range (10−4–105 Pa) in different gas atmospheres. In all conditions, the variation of flashover voltage was found to be divided into three regions with pressure increasing, while its deeper reason is that the number density of molecules of the gas layer is affected by outgassing and ambient gas pressure, which affects the collision of electrons. In region I, the surface flashover voltage is closely related to outgassing and shows the highest value. In region II, the surface flashover is jointly affected by outgassing and ambient gas, along with a rapid decrease of flashover voltage with increasing pressure. In region III, the surface flashover is dominated by ambient gas, and the surface flashover voltage shows a U-shaped curve with increasing pressure. Transition points of the three regions shift significantly with the change of outgassing properties. In region I, the desorption of adsorbed gas has little effect on the surface flashover voltage, while the escape of dissolved gas dominates. Finally, a model of the diverse effects of gases on surface flashover in a wide pressure range was established.

氧化铝掺杂环氧树脂复合材料在电力绝缘设备中应用广泛,然而人们对其在纳秒脉冲下的绝缘性能研究较少,这限制了它在指导脉冲功率装置中的应用.为探究其在纳秒脉冲下的沿面绝缘性能,对氧化铝掺杂环氧树脂复合材料在前沿数十ns快脉冲电压下的闪络特性进行了研究,结果显示,其闪络电场较纯环氧有较大提高,闪络电压符合韦伯分布.实验表明,闪络电压随电压上升率的增加而显著增加,从5.8 kV/ns时的108 kV上升到20.5 kV/ns时的226 kV,增幅超过1倍.闪络时延随电压上升率的上升呈现"先快速下降、后趋于平缓"的趋势.在试样闪络通道表面观测到明显的碳化现象,说明实验中的闪络放电对复合材料有破坏性影响.

The electrostatic discharge of gas is a key issue in the industry of hazardous, such as petroleum, chemical, powder and so on. To explore feasible methods for inhibiting excessive electrostatic discharge voltage (ESD), in this work, the electrostatic breakdown characteristics of typical gases was studied. Two methods of temperature-control and pressure-control were adopted to conduct ESD experiment of air and carbon dioxide in different environment. The electrostatic breakdown energy also was calculated. Results indicated that as temperature decreases, or pressure increases, the electrostatic breakdown voltage increases. This is due to the increase of electron collision coefficient. And calculation results of electrostatic breakdown voltage energy unveiled that energy of air is lower than carbon dioxide in same condition. This work also discussed the process of charge transport in gaseous medium. Through the research in this paper, it provides data support and theoretical basis for the selection of protective gas for industrial production that requires electrostatic protection.

At present, the insulation structures of many gas insulated switchgear (GIS) and gas insulated metal enclosed transmission line (GIL) are mainly disc insulator cooperating with SF6 gas, and the disc insulator has the function of isolating air chamber, supporting conductor and insulating it to the ground. For expending the application of disc insulators, thermal characteristic and dielectric response of materials for disc insulators were measured. The differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) were employed to measure the melting endotherm process and the thermal stability, and dynamic thermos-mechanical analysis (DMA) was used to record the response of the samples to temperature under shear stress. The surface resistivity was measured by 6517A electrometer. The dielectric response under broadband frequency, especially in the radio frequency range, was measured to analyze the material properties. The experimental results were analyzed comprehensively to discuss the application feasibility of disc insulators in pulse power technology.

The dynamic mechanism of breakdown in polypropylene (PP) were represented based on the discharge-avalanche theory. PP is a typical semi-crystalline thermoplastic polymer with low loss and high thermal stability, which had broad application prospects in recyclable and environment-friendly insulation. Dielectric strength was one of the important properties for polymer insulation, hence research of breakdown mechanism was of great significance for insulation reliability increase and advanced materials development. In this paper, the dielectric strength of PP was enhanced by nano-silica filling, and mechanism of enhancement was discussed based on the change of activation energy measured by thermally stimulated depolarization currents (TSDC).

The development of power system requires power capacitors to have higher energy density and smaller size, so to improve the breakdown strength (BDS) of polypropylene film, which is the dielectric material of capacitors, has drawn much attention. Nanocomposite is a promising way to achieve higher breakdown strength, but the poor compatibility of polymer matrix and nanoparticles has always been a problem. In the present study, we doped nano-ZrO2 into PP matrix, and together introduced maleic anhydride grafted polypropylene (PP-g-MAH) into the nanocomposite to form ternary system. Comparing with PP only doped with nano-ZrO2, the ternary system sample showed better dispersion of nanofillers. Besides, PP-g-MAH could help build stronger force between nanofillers and PP which was evidenced by the Fourier infrared test and differential scanning calorimetry test. Furthermore, we performed dynamic mechanical analysis test at different frequency and calculated the activation energy of molecular segment motions of the ternary samples, which was about 10% higher than the binary samples. As a result, the DC breakdown strength of ternary samples was about 9% higher than the sample only doped with nano-ZrO2, while the highest value of 387.7 kV/mm was obtained.

Nano-dielectrics are sensitive to humidity and easily degraded in damp environment because of the high surface energy of nanoparticles. In order to study the effect of humidity on the dielectric properties of nano-dielectric, polypropylene (PP) was modified by polyolefin elastomer (POE) and nano-SiO2, and the samples with obvious filling concentration were pre-selected by breakdown strength for damp aging. The aging experiments were carried out in different relative humidity. The dielectric properties of new, hygroscopic saturation and samples after drying were measured and analyzed. It is found that the breakdown strength of hygroscopic saturation nano-dielectrics decreased obviously compared with new samples, and it was difficult to recover after drying. The damp degradation resulted in different changing trends of permittivity of PP and nano-dielectric, but there were relaxation loss peaks of water in both of them. The influence of damp degradation on the trap distribution was studied by thermally stimulated depolarization currents (TSDC), and it was found that the traps level introduced by water molecules was different in PP and nano-dielectrics. All experiment results showed that the performance of nano-dielectrics degraded obviously in humid environment, and it was difficult to recover even after complete drying because of the existence of bounded water molecules in nano-dielectrics.

聚丙烯(PP)由于具有优异的介电性能和耐热性,而成为热塑性高压直流电缆绝缘材料研发关注的热点.为此利用纳米Si02粒子填充协同聚烯烃类弹性体(POE)共混的方法,调控PP的结晶结构,改善其电、热、机械等宏观性能.利用扫描电子显微镜(SEM)、偏光显微镜(PLM)、交/直流击穿系统、电-声脉冲测试系统(PEA)、差示扫描量热仪(DSC)和电子万能实验机分别对复合材料的形貌、击穿场强、空间电荷分布、结晶行为和力学性能进行了分析和表征,结果表明纳米Si02粒子填充协同POE共混有效提高了PP的直流击穿场强,抑制空间电荷注入,改善其机械性能.相较于纯PP试样,PP-POE-纳米Si02构成的复合材料,击穿场强提高约25％,断裂伸长率提高约30％.机械与介电性能的改善,有利于PP在高压直流电缆绝缘中的应用.

Nuclear cables were one of the key electrical facilities, which were used not only in severe environment, but also needed to provide high security. The evaluation of aging state for the nuclear cables insulation, was of great significance for the normal operation of the whole nuclear power system. In order to evaluate the thermal-oxidative aging properties of nuclear cable insulation, the artificial accelerated aging was performed at polyolefin used in nuclear cables, and the chemical and dielectric properties in different aging stage were compared. The thermal-oxidative aging process was carried out at 165□. The changes of infrared spectra, volume resistivity, breakdown strength, dielectric constant and loss were measured at aging time of 0h, 48h, 96h, 168h, 336h, 504h, 672h and 840h. The thermo-oxidative reaction caused chemical and physical changes of samples. The chemical aging was mainly characterized by cracking and branching of internal molecular chains. Physical aging was characterized by chain arrangement, which reduced free volume and increased the density.

Polypropylene (PP), with high breakdown strength, low dissipation and good processibility, is one of the most widely used dielectric material for power equipment, especially in power capacitors and power cables. The improvement of PP-based dielectric material can benefit the properties enhancement of power capacitors and cables, and thus to meet with the rapid development of the power system. Nanocomposite provided a promising orientation to reach the target and recent research approaches of PP nanocomposite for power equipment were reviewed in this paper. In this paper, we linked the nanofillers to the improved properties of PP nanocomposite, and categorised the research works into nanoclay/PP composites, metal oxide/PP nanocomposite, conductive particles/PP nanocomposite, and PP core–shell nanocomposites chronologically, corresponding to the enhanced thermal and mechanical property, breakdown strength property and energy storage property, respectively. Based on the achieved approaches, prospective for future research was proposed, providing a worth-considering direction for the future work.

Polypropylene (PP) contains promising application prospects in thermoplastic cables for high voltage direct current (HVDC) power transmission because of its outstanding thermal and dielectric properties. However, the problem of poor toughness and space charge has restricted the application of pure PP in HVDC cables. In this paper, polyolefin elastomer (POE) and nano-silica were blended thoroughly and added into a PP mixture by a melting method. Scanning electron microscopy (SEM) was employed to observe the dispersion of POE and nanoparticles. Thermal properties were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Mechanical properties were evaluated by tensile tests. The elastomeric properties of composites were improved as the dispersed POE could transfer and homogenize external mechanical forces. DC breakdown results showed that the fail strength of composite with 10 phr POE and 1 phr nano-silica was obviously enhanced. The pulsed electro-acoustic (PEA) results showed that the injection and accumulation of space charge was increased by the introduction of POE, while it was restrained by the collective effect caused by nano-silica filling. X-ray diffraction (XRD) spectrograms showed that secondary ordered structures existed in the composites of PP, POE, and nano-silica, and that the ordered structure around the nanoparticles contributed to the enhancement of breakdown strength. The mechanical and dielectric properties were modified synergistically, which made the modified PP a propitious insulation material for HVDC cables.