用多参数研究变压器油的热老化

以常用的老化参数（介质损耗因数、酸值、电导率、表面张力）和油流带电电荷密度这个新参数研究变压器油的热老化。试验结果表明,这些参数与油的热老化程度有很大的关系。而且油流带电电荷密度与介质损耗因数和电导率有一定的相关性。     

变压器油流带电数学模型的研究及其应用 第一部分：数学模型的推导

由油流带电引发的变压器事故或故障引起了各国电力部门的广泛关注，许多国家纷纷开展变压器油流带电的研究，大量的油流带电的机理模型也就应运而生。本文在国外学者提出的流动作用机理和电动作用机理两种机理模型的基础上，推导出变压器油流带电的数学模型，与目前存在的几种典型的油流带电计算模型相比，本模型的特点在于它可综合反映起电的流动作用和电动作用，并可基本反映油流流速、油品电导率、外加交流电场强度和系统温度等因素对油流带电的影响。关于本模型的应用实例及与实验的比较见第二部分。     

变压器油道内冲流电流的一种计算方法

对紊流态下变压器油道内冲流电流的计算进行了研究。描述油内电荷分布的输运方程是一个非线性方程,不存在精确的解析解。应用文中所给定的绝缘油/纸板界面的边界条件并经过合理的简化,推出了该方程的近似解析解和冲流电流的计算公式。公式能描述油流速度、电导率、油温和外加交流电场对变压器油流带电的影响,所引入的修正量可通过实验数据确定。实验证明,计算公式可以用于预测冲流电流的大小,从而避免了繁重的数值计算,为变压器油流带电的计算提供了一种方便的手段。     

Development and calibration of a network-based streamingelectrification model for core-form transformer structures

Streaming electrification occurs in large power transformers when oil circulated for cooling purposes acts to separate charges from the Debye layer on the cellulose insulation surfaces. This phenomenon is believed to have caused several catastrophic transformer failures and remains to be satisfactorily understood in realistic geometries. A network-based streaming electrification model is presented which utilizes single winding ducts as the basic element to investigate a core-form structure. The network is first analyzed using hydraulic duct parameters to calculate the oil flow distributions within the structure. This data is then provided to the electrification solver which predicts the charge densities at each duct outlet. The charge density results are ultimately used to determine the streaming potentials at each node based on an a suitable matrix of leakage resistances. Development of the network model is presented along with empirically-based calibration data for a realistic section of core-form transformer geometry     

Charge and mass flux in the radial electric field of an evaporatingcharged water droplet: an experimental analysis

In the electrostatic application of pesticide sprays, charge retention by evaporating droplets in transit to deposit surfaces is a limiting problem. The possible charge loss via evaporative mass transfer and the possible alteration of evaporation rate caused by the presence of the droplet surface charge were experimentally investigated. For studies of evaporation from a charged 3 mm diameter water droplet conducted as a function of droplet potential (-1, -2, and -3 kV) and atmospheric relative humidity (22, 64, and 100%), the current flowing in the radial electric field of the charged evaporating was compared with that flowing in the radial field of a nonevaporating metal sphere to determine if the departing water vapor was electrified. It is concluded that electric charge on evaporating liquid droplets comprising agricultural chemical sprays neither alters the mass transfer rate nor is dissipated by the evaporation     

Direct Observation of the Collection Process for Dust Particles from an Air Stream by a Charged Water Droplet

One of the most effective precipitation methods for submicrometer particles is the charged droplet scrubber. The dust collection in the charged droplet scrubber is considered to consist of some processes with respect to the interaction between charged dust particles and a charged water droplet. That is, the charged dust particles are attracted to, collide with, adhere to, penetrate into, rebound from, accumulate on, and reentrain from a charged water droplet. The direct observation of the collection process for dust particles by a charged water droplet was conducted by using the experimental setup designed by the authors. It was found from the experiments that particles with good wettability were captured inside the water droplet, particles with poor wettability floated at regular intervals on the droplet surface, and particles with medium wettability were trapped on or inside the water droplet. When particles were trapped on the droplet surface the growth of dendritic depositions was found, and their behavior was very distinct from those obtained on the fibrous filter due to the fluidity of the droplet surface. The deposition mechanisms of charged particles on the charged water droplet are discussed qualitatively.     

Relationship between static electrification of transformer oils with turbidity and spectrophotometry measurements

The paper presents the research results on electrostatic charging tendency (ECT) of transformer oil in a spinning disk system. The measurements were performed at different aging severity. Changes in static electrification were related to some classical aging indexes (conductivity, dissipation factor, water content, resistivity, etc.). Fast, inexpensive and reliable laboratory testing procedures developed by ASTM (D 6802 and D 6181) were also used to monitor the decay products as trace impurities. The obtained results show that static electrification currents increase with temperature, oil flow velocity, coating disk material properties and oil’s aging byproducts. The polarity, the amplitude and the time constant of the streaming electrification currents are also affected. This contribution is not only intended to provide a fresh review in this domain of research, but also contains a substantial amount of new material with a view of closing some gaps in the present state of knowledge of transformer oil streaming electrification phenomenon.     

Study of the influence of injected charge on streamingelectrification

In this paper, the influence of injected charge on streaming electrification of transformer oil has been investigated using a combined model. The experimental results reveal that there are relations between the injected charge, the charge density and the leakage current, and the polarity effect of injected charge exist in the influence on streaming electrification. The influence of injected charge is attributed to the interaction between the electric double layer and the injected charge. The effect of oil velocity on injected charge has been analyzed also     

Charge to Mass Distributions in Electrostatic Sprays

The dependence of the charge to mass ratio or specific charge of the spray stream on fluid and automization parameters in electrostatic spraying of various solvents and paints has been investigated. In addition, atomized paint particle sizes have been analyzed. It has been found that an optimum value of fluid conductivity exists at 2 ?ho/cm where the specific charge of the spray stream exhibits a maximum. From the particle size distribution and from the dependence of the specific charge of the spray stream on conductivity it has been concluded that the basic spray droplet charging mechanism is due to the breakup of a charged liquid surface, while corona discharge does not appear to play a significant role in the charging of the particles.     

Computation of Streaming Current in Oil Pipes

The calculation of steaming current in oil pipes is involved in solving the charge conservation equation. It is a nonlinear equation without accurate analytical solution under turbulent-flow condition. Based on some reasonable simplifications, an approximate analytical expression for the equation and a due formula for streaming current are derived. It is proved with experimental results that the formula can accurately predict the streaming current, thus providing a method that is able to reliably and conveniently compute streaming electrification in oil pipe.     

A parametric study of streaming electrification in a full-scalecore-form transformer winding using a network-based model

Streaming electrification remains a problem for large power transformers which require forced-oil cooling. Oil flowing against cellulose insulation surfaces results in the separation of charges from the electrical double layer at the interface. This process can generate hazardous static potentials and volume charge densities in the transformer dielectric system. While streaming electrification has been studied in great detail using small-scale laboratory models, extension of these results to full-scale transformers is rather difficult. This issue has been addressed through the development of a network-based analytical approach which is calibrated against empirical data at the elemental duct level. The model has been utilized to perform a parametric study of streaming electrification in the full-scale high-voltage winding of a core-form transformer. Volume charge densities exiting from each duct in the structure and the local surface potentials are predicted as a function of influent charge, oil volume flow rate, temperature and applied voltage     

Resonance phenomena of a single water droplet located on a hydrophobic sheet under AC electric field

Resonance vibration of a water droplet located on the surface of a hydrophobic sheet under an AC electric field was investigated experimentally. In order to confirm the effect of the surface property of hydrophobic materials on the resonance phenomena of a water droplet, three kinds of hydrophobic materials, silicone rubber (SR), polytetrafluoroethylene (PTFE), and polydimethylsiloxane-grafted fluoropolymer (PGF) sheet were used. A single water droplet with a volume ranging from 3 to 30 /spl mu/L was placed between parallel electrodes, where a tangential electric field was formed along the surface of the hydrophobic sheet. A water droplet under the AC field vibrates strongly only at a particular frequency range. The resonance frequency, when the water droplet vibrates, decreases with increasing the volume of the water droplet. The resonance frequency on the SR sheet is lower than that on the PTFE and the PGF sheet. The resonance frequency and the magnitude of the deformation of the water droplet depend on the surface properties of the hydrophobic material.     

变压器油流带电现象

为了抑制油流带电造成的危害同时最大程度地减少热效应对绝缘的影响,延长变压器的使用寿命,通过对变压器油流带电现象的分析,得出变压器内的绝缘击穿通常发生在上油箱内,因此变压器设计时,应采取有效措施降低上游带电油流对上油箱内局部放电的影响;同时提出应对冷却油泵进行智能控制,使其在流速和温度最佳点运行。在对现有变压器简单油道内油流计算分析的基础上,提出芯式变压器复杂油道内油流带电的计算可通过以下方式实现,即将油道近似等效为简单直管连接而成的管网,然后利用基尔霍夫电流定律和简单直管内的油流带电计算模型求得油道内任意一点的油流带电情况,这为实际变压器油道内的油流带电计算提供了一种新方法。     

大型换流变压器油流带电问题的分析

油流带电可能破坏换流变压器的油道绝缘性能,成为威胁超高压变压器甚至影响电网安全、稳定运行的重要因素之一,在特殊的绝缘结构和油冷却方式下,换流变压器中可能存在较多的油流带电问题。通过分析油流带电的产生机理、影响因素、抑制措施和换流变压器的结构特点,研究了换流变压器的油流带电问题。研究表明,降低油速和改善绝缘(特别是油道)设计是减少油流带电问题的最重要措施。开展全电压空载试验等可检验、监测换流变压器的油流带电问题是否严重,此外,向绝缘油中添加电导率调节剂或电荷抑制剂也可降低油流带电的倾向。     

Static electrification of pressboard/oil interface and transientphenomena

The static electrification phenomenon of insulating materials used in power transformers is investigated through two devices. The first is a cell with a rotating disk covered on both sides with a given pressboard and immersed in a metallic tank containing ~41 of oil; and second device enables us to measure the electrostatic charge tendency of oils. The electrostatic charge tendency (ECT) of insulating oils and the leakage current generated by the charge concentration gradient at the oil/pressboard interface are analyzed as function of temperature, water content and aging of oils, the nature of the pressboard, and the rotating speed of the disk. The influence of the surface roughness of the pressboard and antistatic additives on the aging and ECT also are considered. A correlation between ECT and the physico-chemical and electrical characteristics of the oil is established. It is shown that the temperature gradients and the water migration phenomena at the oil/pressboard interface play an important role in the charge separation. Transient currents are strongly affected by the presence of air in the pressboard and the aging of the oil     

Flow electrification of transformer oil effects of mixed fields

Static electrification in transformer oil is investigated experimentally using a laboratory synthetic closed cycle, where the oil is pumped in a coaxial electrode arrangement. The electrode system is electrically energized radially with dc and/or ac, and the electrostatic charging tendency (ECT) of the oil is quantified by measuring the streaming current. The results indicate that for the same voltage ratio (K=Vˆ/(Vˆ+V)), increasing the ac or the dc voltage component leads to higher ECT of oil, although the ECT under only ac field is much lower than that under dc field. The unenergized streaming current is inherently affected by the electrode material and configuration; where electrodes having a lower work function give higher positive ECT of oil at high temperature. The energized streaming current increases with oil temperature, oil velocity and electric field; where negative dc voltage application to the outer electrode gives both higher conduction and streaming currents. While the conduction current decreases with oil velocity, increasing the frequency of mixed ac voltage has no significant effect on streaming current. Moreover, the effect of combined radial electric (ac or dc) and axial magnetic (ac or dc) fields on ECT of oil is investigated, and the results reveal that the ECT is enhanced by the magnetic field while the radial conduction current decreases. Derived formulas for the streaming current for unenergized and ac energized cases are also presented together with expressions of the volume and surface charge densities for the coaxial electrode system     

A basic study on the effect of voltage stress on a water droplet on a silicone rubber surface

Observations and analyses have been conducted on the behavior of water droplets deposited on the surface of polymer insulators. As an index to characterize the repellency of water droplets on the polymer surface, the contact angle was measured for different situations. Experiments were conducted to see the behavior of water droplets of different volumes and their size distribution on silicone rubber. Electric field strength calculation was performed to analyze the effect on water droplets. Finally, effects of the Maxwell stress and the Coulomb?s force were compared, to conclude that, even if there is some true charge on the water droplet, the Maxwell stress is predominant.     

Calculations based on measurements of charge deposited by a streamer on a PTFE surface

Surface charge on an insulator's surface is studied using 3-dimensional finite element modelling. The calculations are based on measurements of surface charge deposited by a streamer on a PTFE surface. The surface electric field strengths associated with uniform surface charge density distributions over a 1 mm/sup 2/ region are calculated for a range of different measured charge densities. The effect of reducing the area of the surface charge whilst keeping the total amount of charge in the region constant is observed. The surface charge is then modelled with a Gaussian distribution of charge and compared to that of a uniform distribution. By assuming a maximum allowable surface electric field strength an estimation of actual charged path diameter is made.     

Charge behavior in flowing oil in oil/ pressboard insulation system by electro-optic field measurement

The electrical insulation structure of most power transformers is a combination of insulating oil and solid materials. One of the crucial problems we face in transformer operation is flow electrification, which occurs at a flowing oil/solid material interface. Thus, for the transformer insulation design, we need to clarify the now electrification phenomena. In this paper, we measured the electric field strength in an oil/pressboard composite insulation system using an electro-optic method of the Kerr effect. We obtained the time variation of electric field distribution in both flowing uncharged and charged oil. It is notable that we could quantitatively clarify the electric field distortion in flowing charged oil by flow electrification. Furthermore, we measured the leakage current from divided electrodes and derived a charge density distribution along the flow direction. Finally, we quantitatively discussed the electric field distribution and the charge behavior from the measurement results.