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 in high power transformers: BTA effect on pressboard degraded by electrical discharges

The flow electrification process occurring at the oil-pressboard interface in high power transformers seems to cause electrical discharge incidents and may cause failures. The goal of this experimental study is to measure and to compare the streaming current obtained from unused pressboard, pressboard artificially degraded by electrical discharges, and pressboard obtained from a damaged power transformer. The influence of additive 1,2,3 Benzotriazole (BTA) in oil is also analyzed. The results show that flow electrification is greatly increased by the degraded pressboards compared with the new one. On the other hand, adding of BTA significantly reduces flow electrification.     

Effect of streaming potential on heat transfer in transformer oil

The results of an experimental study of the effects of streaming electrification on the heat transfer of transformer oil (Voltesso 35) in Plexiglas and paper/pressboard pipe models are reported. The magnitude and the polarity of the charge in the oil entering the test section was varied by using two different filters (Millipore cellulose and Sartorius fiber glass) in the upstream of the oil flow. An enhancement in heat transfer was observed with both types of filters in the Plexiglas model and with cellulose filter in the paper/pressboard model; whereas a reduction in heat transfer was observed with fiber glass filter in paper/pressboard model under similar flow rates and temperatures. With uncharged oil, heat transfer was either inhibited or no enhancement was noticed in both models. The results have been discussed considering the temperature gradients, the flow conditions and the electrical forces due to static charge in the liquid     

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.     

Experimental study and modeling of static electrification in powertransformers

For the last 20 years, static electrification has been suspected to he responsible for failures in power transformers (electric “tree” paths, “worm holes,” etc.). The phenomenon consists in a preferential adsorption of negative ions from the oil (impurities) into the pressboard. This yields, on the one hand, a space charge in the oil which can relax in contact with grounded metallic walls and, on the other hand, a space charge in the pressboard which can accumulate depending on the leakage paths. As part of a research program by Electricite de France in the field of static electrification in transformers, an experiment has been carried out, at the Universite de Poitiers, Poitier, France, to study the most constrained parts in transformers with regard to this phenomenon: the insulated pressboards close to the oil inlet. For this, the experimental loop consists in impinging an immersed oil jet on a pressboard target in which annular insulated electrodes have been inserted to determine the charge in the pressboard. Considering the scale effect, the equipment involves taking oil from an operating transformer and making it possible to study the influence of the flow velocity and the length of the free jet. From a first modeling, compared to the authors' experimental results, it seems that the local wall current coming from the pressboard is proportional to the local wall shearing stress. Considering the magnitude of the process, it finally turned out that this new configuration increases greatly the charge generation process in comparison to a flow parallel to the pressboard     

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.     

提取杂质后的老化油浸绝缘纸板介电响应特性研究

首先制备老化绝缘纸板样品,并制定了提取老化绝缘纸板水分及分解产物的方法,然后在去除油纸绝缘的杂质及变压器油后,分别针对不同老化状态的油浸纸板开展介电响应实验。分析排除杂质后的老化油浸绝缘纸板的介电响应特性,以及纸板自身老化与油纸绝缘老化对其绝缘状态的影响差异。结果表明:老化时间和激励电压对频域介电谱(FDS)的影响主要集中在中低频段,在高频段介质损耗因数(tanδ)较为稳定;温度变化会导致FDS曲线发生平移,在高频段,温度越高tanδ越小,在低频段,温度越高tanδ越大,且频率越低tanδ的增长趋势越明显。     

绝缘纸板老化对油纸绝缘沿面放电发展规律的影响

为研究绝缘纸板老化对油纸绝缘沿面放电发展规律的影响,采用加热老化方法制备了不同老化程度的绝缘纸板试样,设计了柱板放电模型,采用恒压法对各老化程度绝缘纸板试样进行长时间加压实验,测量并采集了局放信号,统计了不同老化程度绝缘纸板沿面放电各阶段总放电量、最大放电量、放电次数等变化规律,总结了绝缘纸板老化对其沿面放电发展规律的影响。将理论分析与仿真相结合,分析了产生该影响的主要原因。实验结果表明:放电前期,放电主要由处在高场强区的变压器油产生,此时绝缘纸板老化程度对沿面放电总放电量、放电次数、最大放电量等无明显影响;随着放电的发展,绝缘纸板内由老化产生的孔隙中填充的变压器油分解产生气体,加剧了放电程度,此时绝缘纸板老化程度对其沿面放电影响较大,老化程度越高,总放电量、放电次数、最大放电量等增加速度越快,数值也越大。     

电力变压器中的静电起电

对变压器中绝缘油流动带电现象进行了研究，给出了一些大型变压器运行故障实例，分析了变压器静电起电的原因和条件，介绍了静电起电的监视技术。     

采用纳米改性纸板的换流变压器出线装置电场分布

针对换流变压器出线装置在直流及极性反转电压作用下电场分布不均的问题,探究了非线性纳米Si C对绝缘纸板进行改性的方法,制备了纳米改性绝缘纸板试样,并测量了改性纸板的介电特性;结合换流变压器出线装置的实际结构建立了电场分布的仿真模型,分析了纸板改性对出线装置电场分布的均化作用。试验结果表明:纳米改性绝缘纸板的相对介电常数随纳米Si C掺杂浓度的增加而增加,但增加的幅度不大,随电场强度的增加呈略微上升的趋势;其电导率随着纳米掺杂浓度的增加而增加,当掺杂浓度达到一定值时,电导率随场强增加表现出明显的非线性特征。仿真结果表明:在直流和极性反转电压下,纸板改性可降低绝缘纸板中的场强,提高变压器油中的场强,使油纸绝缘结构中的电场分布更趋于均匀。     

Flow electrification in transformers: relevance of ECT and leakage current measurements

The accumulated charge Q on a well insulated pressboard part being probably the most relevant parameter for flow electrification evolution inside transformers but clearly not measurable in situ, we compare in this paper this parameter with others possible to measure. The two most commonly used are oil parameters: ECT (electrostatic charging tendency) and, to a smaller extent, the continuous aging test for tan /spl delta/. Another is not so often used because of measurements difficulties, but was considered to be one of the most relevant: the leakage current. Thus in this paper we compare on two facilities (specially made for that) the accumulated charge Q to the ECT and leakage current. We find that, even if sometimes a correct correlation exists between these parameters, several cases show a bad correlation. Thus monitoring transformers by analyzing only ECT or leakage current seems to be hazardous.     

变压器油的静电性能及变压器油流静电试验方法

本介绍了采用“过滤式”带电度测试装置对国产变压器油的带电度及有关静电带电性能进行试验研究的结果，分析了变压器油流带电过程及静电放电特点。此外，初步试验研究苯并三唑对油中静电的抑制作用及其影响，在此基础上，探讨了检测变压器中油流静电的试验方法。     

Streaming electrification of thin insulating pipes under electricfield

Streaming electrification under dc and ac electric fields was investigated by using test pipes and oil circulation apparatus. We used two different test pipes, one made of pressboard, and the other of insulating paper. At first, the amount of static charges which flowed into the test pipes was measured without electric field. For the experiment using an insulating paper pipe, when the oil temperature increased, the measured current curve shows a peak. When the velocity increased, the current increased proportionally from the 2nd to 4th power of velocity. For the experiment using a pressboard pipe, the current increased proportionally from the 3rd to 5th power of temperature, and by the 1st power of velocity. Then, we applied ac and dc electric fields on the test pipes and the current from a relaxation tank to ground was measured. The ions in oil carry the charges, so an electric field affects ions in the oil. It makes the mathematical approach to streaming electrification difficult. We know that as the electric field intensity increased, the current due to selective adsorption of the test pipe was increased. The current vs. velocity curve under ac electric field had a peak point in velocity     

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。     

油纸绝缘系统中含高水分绝缘纸板的沿面爬电特性

水分是导致沿面爬电的重要原因之一,为研究高水分质量分数绝缘纸板的沿面爬电特性,分析了实际变压器中绝缘纸板可能达到的高水分质量分数值,建立了绝缘纸板沿面爬电特性试验装置,制备了相应高水分质量分数的绝缘纸板,对其沿面爬电过程进行了观测。研究结果表明：油中水分质量分数在30×l0^－6“时,变压器中绝缘纸板的局部水分质量分数...     

Mechanism of increase of electrostatic charging tendency in insulating oil for oil‐immersed transformer

The mechanism of the increase in the electrostatic charging tendency (ECT) of insulating oil and causative compounds were investigated by accelerated deterioration tests with the addition of various compounds. Although the ECT of the insulating oil was almost constant when only sulfoxide compounds were added, a marked increase was observed when either hydrochloric acid or moisture, which was considered to be generated by the aging of insulating oil, was also added to oil containing sulfoxide compounds. It is assumed that the sulfonium ion, which is generated by the reaction between sulfoxide compounds and hydrogen ion, is the compound that directly contributes to the increase in the ECT. Hydrogen ions can be supplied from organic acids generated by the oxidation of hydrocarbons in aging insulating oil. It is considered that the increase in the ECT of insulating oil is caused by the generation of sulfoxides by the oxidation of sulfide, which are present in fresh oil (originating compounds), and the generation of sulfonium ions by a reaction between sulfoxide and hydrogen ions, which are formed during aging. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 176(4): 26–33, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21113     

Flow electrification in power transformers. Explanation of thewall-current measurements

The aim of our work is to understand better the mechanisms of flow electrification due to oil circulating inside a rectangular pressboard channel. This research has been done because of technical incidents in power transformers in which oil is flowing past pressboard with a velocity in the range of several meters per second. A possible explanation of the wall current observed along the channel is proposed     

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     

极性反转时典型油纸复合绝缘的电场特性

极性反转是换流变压器一种特殊的工作状态,因极性反转时空间电荷的作用,绝缘油中会出现极高的危险场强。文中采用ANSYS10.0有限元仿真软件计算了极性反转时板-板、同轴、棒-板及换流变压器阀侧绕组端部局部模型等几种典型油纸复合绝缘电场的场强,绘制了绝缘油及绝缘纸板中最大场强与绝缘纸板厚度、油纸电导比、极性反转时间等参数的关系曲线。在板-板油纸复合绝缘中,增加绝缘纸板的厚度可以降低极性反转时油中及纸中场强。但在同轴油纸复合绝缘中,当绝缘厚度超过一定值后,纸中极性反转场强反而增加。在棒-板油纸复合绝缘中,极性反转场强与绝缘纸板的尺寸及位置关系具有较大的不确定性。延长极性反转时间可降低极性反转时的场强。极性反转场强随油纸电导比增加而增加,且存在饱和现象。     

Influence of pressboard physico-chemical composition on static electrification in power transformers

As part of a research program in the field of flow electrification in power transformers, some fundamental studies have been undertaken on the physical chemistry of this phenomenon. The two main constituents involved in the process of charge generations are oil and pressboard. As oil composition is difficult to deal with, the choice has been made to understand the overall chemistry through the role played by the pressboard, whose composition is better mastered. In order to identify the chemical species in the pressboard responsible for the charge generation, in view of perfecting additives to neutralize the phenomenon, the French Institute of Pulp and Paper carried out modifications on commercial pressboards, and manufactured some others. Charge accumulation, measured from capacitive currents on insulated electrodes facing the pressboard ducts, has been observed for modified and manufactured pressboards with regard to oil flow rate and temperature. All the experiments have led to various behaviors in terms of kinetics and magnitude of the charge accumulation. The obtained results tend to prove that chemical composition of pressboards, with regard to their content in chemical functional groups (hydroxyl or carboxyl groups) or radical nature (lignin), as well as board manufacturing parameters (pulp refining, layer composition) and surface properties seem to be at the origin of the electrification phenomenon.