电力变压器介质响应的测量与分析(英文)

This paper discusses approaches to measure moisture in power transformers by analyzing the dielectric response of the main insulation. Dielectric diagnostic methods deduce moisture in the solid insulation from dielectric properties like polarisation depolarisation currents and dissipation factor vs. frequency. A new instrument "Dirana" combines time domain (PDC) and frequency domain (FDS) measurements and thus substantially shortens the measurement duration. New software was developed which bases on a new data pool,measured at new and aged pressboard samples with various moisture contents and oil impregnation. Its analysis algorithm compares measurements from a transformer to modelled dielectric responses,obtained from the so-called XY-model. To avoid overestimation of moisture for aged transformers,the analysis algorithm compensates for the influence of conductive aging by-products. Moisture especially increases the losses in the low frequency range of the dielectric response of pressboard. Thus,data on the left-hand side of the area dominated by interfacial polarization (insulation geometry) are required for reliable moisture determination. Case studies illustrate the application of moisture determination by dielectric response methods.     

油老化状态不同时油纸绝缘的空间电荷特性(英文)

Oil-paper compound insulation has been widely used in power transformers for quite a long time because of its good performances. The insulation gradually degrades under combined thermal, electrical and chemical stresses during routine operations, mainly because of space charges inside. This work investigated the space charge characteristics in oil-paper insulation under oil aging circumstance. New trans- former oil samples are thermally aged to obtain different aging states, and their physical and chemical properties are analyzed. New Kraft papers are dried in vacuum and fully immersed in these different aged oil samples, and three kinds of oil-paper samples are obtained. We use the pulsed electro-acoustic (PEA) method to measure space charge under both DC voltage-on and voltage-off conditions at room temperature. The effect of oil aging state on characteristics of space charge injection, accumulation, and decay is analyzed and discussed. The results show that comparing with the DC voltage-off condition, more charges are injected into samples at the interface of electrode and dielectric when DC voltage is on. When the oil-aged state gets worse, more charges are induced at both cathode and anode, more space charges are accumulated in the bulk, the area of negative charges is larger, and local electric field is distorted more seriously. Moreover, for the voltage-off condition, aged oil is good for space charge decay, and trapped positive space charges decay faster than trapped negative charges.     

油纸绝缘状态评价模型的改进(英文)

Identification of the aging condition and the failure probability of oil-paper insulation in transformer is important for improving the reliability of electric power transmission system and applying life cycle cost (LCC) management to electrical equipment. Based on data obtained in a series of multi-accelerated-aging experiments, two approaches for calculating failure probability of oil-paper insulation were compared in aspects of degree of polymerization (DP) and condition ranking. In the experiments, mineral oil and cellulose paper are sub- jected to electrical and thermal stresses, and several parameters, including dissolved gases’ volume fraction, furfural content, moisture content, and degree of polymerization, are measured after the aging process. Results show that weight of carbon oxide, which has a close relationship with cellulose paper degradation, is much higher in DP model than in condition ranking model. Moreover, it is concluded that DP model is more practically accurate than condition ranking model, because aging of cellulose paper rather than mineral oil is the key and critical factor of oil-paper insulation aging.     

交直流复合电压下油纸绝缘气隙局部放电与产气特性分析(英文)

Oil-paper insulation is used within most of high voltage direct current(HVDC) converter transformers.Partial discharge(PD) in oil-paper insulation is a major reason for the development of internal faults in HVDC converter transformer,while PDs in oil-paper insulation mainly occur in terms of gas cavity discharges.There are few study results on the development characteristics of partial discharges in oil-paper insulation and dissolved gases in oils of HVDC converter transformers.Based on a gas cavity discharge model of oil-paper insulation,the present study investigates the development characteristics of partial discharges under AC-DC combined voltages and emphatically examines the variation characteristics of dissolved gases in the discharge development process.Experiment and analysis results show that the characteristics of partial discharges and dissolved gases in oils under AC-DC combined voltages are appreciably different with those under AC voltage.These results provide theoretical support for further research on partial discharges and on dissolved gases of other internal insulating defects and are helpful for the fault diagnosis of HVDC converter transformers.     

交直流复合电压油纸绝缘的沿面局部放电特性(英文)

The valve side windings of converter transformers bear AC, DC, impulse, and reversal-polarity voltages during operation, which could result in serious insulation problems of the equipment. By performing experiments with surface discharge model of oil-paper insula- tion at 80 ℃ under combined AC-DC voltage for 200 h, we studied the spectrums and statistical parameters of partial discharges at different discharge stages. Furthermore, some fingerprint parameters were calculated in order to estimate the development situation of par- tial discharge, while the characteristic gases dissolved in the transformer oil were measured by gas chromatography. The surface discharges in the experiments were observed using a high speed camera, and a full discharge process could be marked off into four stages as follows. ①The elementary stage. When a partial discharge occurs near electrodes, electrical charges are injected into the region near electrodes and causing bubble generation. ②Due to their high resistivity and low dielectric constant, the bubbles would bare the major part of the voltage applied to samples. Therefore, discharge happens inside the small bubbles, and it emits a lot of light. ③Micromolecules of gas are produced in discharge, and further ionization in the transformer oil takes place simultaneously when high-energy electrons collide with oil molecules. ④The carrier charge moves forward to electrodes driven by the applied electric field, till they neutralize with the charge from electrodes, and hence discharge channels are formed subsequently.     

交直流复合电压下的油纸绝缘局部放电统计图谱研究(英文)

The converter transformer is one of the most critical equipments in high-voltage direct current transmission systems.Valve-side windings have to withstand complex stresses combined of AC,DC,and pulsed voltages.Partial discharges(PDs) can thus easily occur in oil-impregnated paper insulation.The current paper presents the statistical phase-resolved distributions of partial discharges in oil-paper insulation under combined AC and DC voltage stress.First,the voltages in the converter transformer were analyzed.In the experiments,four artificial insulation defect models were designed to generate PD signals under AC-DC combined voltage stress detected by a Rogowski coil sensor.Histograms including the φ-q-n data of these PD signals were created.The variations of φ-q-n data generated by PDs in different insulation defect models were analyzed while increasing the test voltage.Experimental results showed that the PD phase-resolved distributions of different insulation defect models were different from one another.The findings of the current study are useful in further research on the mechanism and pattern recognition of PDs in converter transformers.     

基于统计分析和模糊推理的运行交联聚乙烯电力电缆绝缘老化诊断(英文)

Cables that have been in service for over 20 years in Shanghai, a city with abundant surface water, failed more frequently and induced different cable accidents. This necessitates researches on the insulation aging state of cables working in special circumstances. We performed multi-parameter tests with samples from about 300 cable lines in Shanghai. The tests included water tree investigation, tensile test, dielectric spectroscopy test, thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FTIR), and electrical aging test. Then, we carried out regression analysis between every two test parameters. Moreover, through two-sample t-Test and analysis of va- riance (ANOVA) of each test parameter, we analyzed the influences of cable-laying method and sampling section on the degradation of cable insulation respectively. Furthermore, the test parameters which have strong correlation in the regression analysis or significant differ- ences in the t-Test or ANOVA analysis were determined to be the ones identifying the XLPE cable insulation aging state. The thresholds for distinguishing insulation aging states had been also obtained with the aid of statistical analysis and fuzzy clustering. Based on the fuzzy in- ference, we established a cable insulation aging diagnosis model using the intensity transfer method. The results of regression analysis indicate that the degradation of cable insulation accelerates as the degree of in-service aging increases. This validates the rule that the in- crease of microscopic imperfections in solid material enhances the dielectric breakdown strength. The results of the two-sample t-Test and the ANOVA indicate that the direct-buried cables are more sensitive to insulation degradation than duct cables. This confirms that the tensile strength and breakdown strength are reliable functional parameters in cable insulation evaluations. A case study further indicates that the proposed diagnosis model based on the fuzzy inference can reflect the comprehensive aging state of cable insulation well, and that the cable service time has no correlation with the insulation aging state.     

油纸绝缘状态对变压器油油流带电的影响(英文)

This contribution is not only intended to provide a fresh review for researches on static electrification, but also contains a sub- stantial amount of new material with a view of closing some gaps in the present state of knowledge of oil streaming electrification. In this paper, the Electrostatic Charging Tendency (ECT) of fresh (unaged) and aged transformer oils were investigated in a spinning disk system. Static electrification measurement at different aging degrees was carried out under laboratory controlled conditions. Changes in static elec- trification were compared to some classical aging indexes (conductivity, dissipation factor, water content, resistivity, etc.). Fast, inexpensive and reliable laboratory testing procedures developed by American Society for Testing Materials (D 6802 and D 6181) were also used to monitor decay products as trace impurities. The obtained results show that static electrification currents increase with temperature, fluid mo- tion velocity, surface properties and the aging byproducts of the oil. The polarity, the amplitude and the time constant of the electrification currents are also affected. The results show that static electrification has relationship with thermal aging and is sensitive to turbidimetric and spectrophotometric measurements.     

基于脉冲电压法的10kVXLPE电缆绝缘诊断非线性分析(英文)

The diagnosis of water trees of cable insulation is of great importance as the water-treeing is a primary cause of aging breakdown for the middle voltage cables. In this paper, it is described how the water-tree-aged 10 kV XLPE cables were diagnosed. The cables were subjected to electrical stress of 5.9 kV/mm and a thermal load cycle in a curved water-filled tube for 3, 6 and 12 months of aging in accor- dance with the accelerated water-tree test method. The aged cables were used as the samples for water-tree diagnosis. First, the water-tree degraded cable, was charged by a DC voltage, and then the cable was grounded while a pulse voltage was applied to it for releasing the space charge trapped in the water trees. The amount of the space charge, which corresponds to the deterioration degree of the water trees, was calculated. The effects of DC voltage amplitude, pulse voltage repetition rate and aging conditions on the amount of the space charge were studied. Obtained results show that the amount of the space charge has a positive correlation with the applied DC voltage and the ag- ing time of the cables, and that a peak value of space charge appears with the increase of the pulse voltage repetition rate. An optimum pulse voltage repetition rate under which the space charge can be released rapidly is obtained. Furthermore, the releasing mechanism of space charge by the pulse voltage is discussed. Accumulated results show that the presented method has a high resolution for the diagnosis of water tree degradation degree and is expected to be applied in practice in future.     

植物绝缘油对绝缘纸热老化速率的影响及其机理(英文)

With the development of new insulation materials,vegetable oil-the best substitute for mineral oil-has gradually been widely used in the liquid insulation of transformers.To investigate the influence of vegetable oil on the thermal aging rate of oil paper,Kraft paper impregnated with mineral oil and vegetable oil have been underwent thermally accelerated aging at three different temperatures.The degree of polymerization(DP) of Kraft paper was measured to indicate the aging degree of insulation paper.The aging rate of Kraft paper in mineral oil and vegetable oil was compared quantitatively,and results showed that vegetable oil retarded paper’s degradation rate and extended its useful lifetime.The reasons contributing to such phenomena were analyzed using X-ray Photoelectron Spectroscopy(XPS) and molecular simulation software.Kraft paper in vegetable oil had larger activation energy.Due to the larger interaction force between water and natural ester molecules,water molecules were easily bonded with natural ester,weakening the hydrolysis process of cellulose.Cellulose was chemically modified by natural ester during the thermal aging process,and the reactive-OH(hydroxyl) groups on the cellulose became esterified with fatty acid esters.Water molecules were firmly bounded to the ester groups on glucose produced by esterification.The long-chain fatty acid esterified to cellulose was parallel with cellulose chains and acted as a "water barrier" to further weaken the hydrolysis process.     

电场诱导产生的聚低密度聚乙烯和取向排列蒙脱土的纳米复合材料的特性(英文)

In order to make montmorillonite (MMT) nanosheets disperse in low-density polyethylene (LDPE) with highly homogeneous orientation, alternating voltage is applied to molten LDPE with MMT nanosheets. The effect of electric field on the dispersion of MMT in the solidified LDPE is studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses suggest that the MMT nano- sheets are aligned with high anisotropy to the electric field direction, which is perpendicular to the LDPE film plane. Differential scanning calorimetry (DSC) results reveal that the crystallization degree of the oriented LDPE/MMT composite increases. Moreover, through a broadband dielectric spectroscopy analyzer, it is found that MMT manifests a significantly influence in the dielectric property of the oriented composite: the dielectric constant and loss tangent of the composite both become larger. Analysis shows that the electric field-induced torque caused by the polarization of MMT flakes is the main force inducing the orientation of the MMT flakes.     

大型电机定子线棒主绝缘老化性能的介电响应研究现状

介电响应法(Dielectric Response Method,DRM)是一种近年蓬勃发展的绝缘体系无损检测技术,对确定发电设备绝缘体系老化状态具有十分重要的作用。目前利用极化/去极化电流法(PDC)、频域谱(FDS)、热刺激去极化电流(TSDC)等介电响应法研究电机绝缘老化状态已经成为热点方向。国内外学者利用介电响应法研究绝缘老化状态取得了较好的成果,为建立绝缘老化状态的介电响应无损检测方法、指导电力系统机组评估诊断绝缘老化状态奠定了理论基础。为提高我国制造业创新能力,掌握无损检测大型电机定子绝缘缺陷关键技术,国内需要在此方面深入开展研究工作。     

半绝缘砷化镓光电导开关表面闪络电场的实验研究(英文)

With its unique features, photoconductive semiconductor switch (PCSS) is generally recognized today as a promising power electronic device. However, a major limitation of PCSS is its surprisingly low voltage threshold of surface flashover (SF). In this paper, an experimental study of surface flashover of a back-triggered PCSS is presented. The PCSSs with electrode gap of 18 mm are fabricated from liquid encapsulated czochralski (LEC) semi-insulating gallium arsenide (SI-GaAs), and they are either un-coated, or partly coated, or en- tirely coated PCSSs with high-strength transparent insulation. The SF fields of the PCSSs are measured and discussed. According to the experimental results, the high-dielectric-strength coating is efficient in both reducing the gas desorption from semiconductor and increasing the SF field: a well-designed PCSS can resist a voltage up to 20 kV under the repetition frequency of 30 Hz. The physical mechanism of the PCSS SF is analyzed, and the conclusion is made that having a channel structure, the SF is the breakdown of the contaminated dielectric layer at the semiconductor-ambient dielectric interface. The non-uniform distribution of the surface field and the gas desorption due to thermal effects of semiconductor surface currents are key factors causing the SF field reduction.     

大气压氦气与氩气等离子体射流形成机理的对比研究(英文)

To compare the formation mechanisms of He and Ar atmospheric pressure plasma jets(APPJs),an intensified charge-coupled device(ICCD)are utilized to observe the dynamic process of APPJ.The experimental results show that,He APPJ is first ignited,which is independent of the dielectric barrier discharge(DBD)between the two wrapped electrodes when the high voltage placed at the downstream.The intensity and APPJ length under positive discharge pulses are bigger than that under negative discharge pulses due to the space charge effect.The He APPJ is formed by the DBD development when the high-voltage electrode placed at the upstream side of tube.However,the plasma plume in Ar APPJ is formed by the propagation of DBD whatever the high-voltage electrode is arranged on upstream or downstream side of ground electrode.The difference in formation mechanism between He and Ar APPJs is mainly caused by the gas properties.Moreover,during the discharges,Ar tends to lead to thermal instability and electron Maxwellian instability.     

基于不同烧蚀模型的脉冲等离子体推力器壁面-等离子体交互作用的机理(英文)

Ablation excited by current pulses is a very critical physical process in pulse plasma thrusters(PPT).Its effects on wall-plasma interaction directly determine the PPT performances.In order to reveal the process of the ablated wall interaction with the discharge plasma in PPT,ablation models formulated by three different boundary conditions at the wall-plasma interface are studied.These are the two widely used high-speed evaporation models(Model-L and Model-M),and the recently developed Keida-Zaghloul model(Model-K)of the Knudsen layer that takes into account the internal degrees of freedom on the energy flux conservation.First,fundamental mechanisms of the three ablation models are clarified by comparative analysis in order to gain a comprehensive understanding of the wall-plasma interaction.Then,the applicability of different ablation models with the numerical solutions of LES-6 PPT is investigated in detail using magnetohydrodynamic(MHD)modeling.Results show that Model-L and Model-M are actually special cases of Model-K when a simplified jump conditions limited by high velocity at the vapor/plasma interface is used;A ratio of ablation rate in Model-L to that in Model-M is about 0.8at the same wall surface temperature,while it rises to 1 at different surface temperature determined by Model-L and Model-M in PPT.Even though Model-K solution requires significant computational time,it shows more accurate ablation feature for the wall-plasma interaction and possesses better computing precision of impulse bit during post-pulse which is useful for future studies of the late time ablation.