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.     

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.     

Increase of static electrification in an aged oil‐immersed transformer

Increase of static electrification activity and discharge generation was experienced during the service operation of an aged power transformer. The transformer oil had high sulfide and sulfoxide contents and was liable to show high ECT value after aging. However, there was no apparent increase in standard ECT (electrostatic charging tendency) measurement of the insulating oil and, therefore, deterioration of pressboard was suspected as the cause of the increase. The ECT tests with aged oil and pressboard sampled from the aged transformer were insufficient to demonstrate the increase in static electrification for the aged transformer. On the other hand, the results suggested a possibility that increase in static electrification can be caused by adsorption of some deterioration compound in the aged oil to the pressboard. In order to make more effective evaluation to the transformer insulation, potential measurement on the pressboard in the oil duct was carried out. The experimental results demonstrated strong increase of the accumulated charge by the aged pressboard, which suggested effectiveness of potential measurement for the assessment of static electrification in transformers. The device of potential measurement for practical use was designed by reducing the necessary amount of oil sampling. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(3): 10–19, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20789     

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     

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     

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     

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     

Flow electrification characteristics of transformer oil by rotatingelectrode systems

Flow electrification has been found to be the principal cause of a number of failures of forced oil cooled power transformers. Flow charging characteristics of oil/cellulose system with factors like electrode configuration, electrode material, presence of Benzotiriazole (BTA), metallic contaminants and copper coils were investigated for paraffinic oil by employing a rotating electrode system. A few hydrodynamic parameters (viz. Reynolds number, boundary layer thickness and friction factor) were correlated with flow charging characteristics of oil for varying temperatures concentrations of BTA. With concentrations of BTA in oil (viz. 10 ppm and 25 ppm) a marginal reduction in flow charging of oil was noticed, but about 40% reduction was observed with 150 ppm of BTA. A significant reduction in the flow charging characteristics of untreated and BTA treated oils was also observed in the presence of copper coils and metallic particle contaminants     

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     

电导率对大型电力变压器油流带电影响的研究

分析了油流带电的产生及危害，利用循环管道模型研究了变压器电导率对大型电力变压器油流带电的影响。其结论为解决大型电力变压器油流带电问题提供了必要的参考依据。     

太赫兹时域光谱技术在绝缘纸板微水含量检测中的应用分析

水分是电力设备油纸绝缘寿命的首要劣化因素。将太赫兹时域光谱系统（THz-TDS）引入到油纸绝缘微水含量测试中,利用太赫兹时域光谱系统对不同微水含量绝缘纸板样品进行测试,得到了不同太赫兹时域及频域响应信号结果并进行拟合,从而获得了可以初步估计纸板中水分含量的计算公式;最后结合太赫兹成像技术对纸板平面的水分不均匀分布情况进行了检测。由于水分子中的氢键在太赫兹波照射下会产生强烈的共振与吸收,在太赫兹透射波中产生明显的反馈作用,基于此原理可以将太赫兹检测技术运用于对绝缘纸板水分含量与分布的评估中。     

Power systems and harmonic factors

Harmonic currents and distortions in power systems are a natural progression in society's use of electrical equipment. A proliferation of harmonics can cause problems-some severe. However, the benefits we receive from electrification far outweighs the downside to this problem. There is an increased awareness among equipment designers, manufacturers, and users for the need to reduce power system harmonics. However, much work still needs to be done. This paper describes how harmonics arise from nonlinear loads and their effects on the power system. The problems that arise from harmonic current flow are straightforward and simple. The two main problems are abnormal heating of components and disruption of operation. The paper also describe mitigation of harmonic effects which take on two strategies: one is to reduce the harmonic effect; the other is to reduce or control the harmonic content     

Electrical strength of pressboard components for transformerinsulations

More and more, the insulation systems used in oil-filled transformers consist of oil-impregnated pressboard placed around the low and high-voltage windings and supported by the iron core. This system provides high mechanical stability both in the main and end insulation areas. The thin oil layers within the multilayer pressboard structure result in high electrical strength, which increases with pressboard density. In addition, the oil passing between the individual pressboard components of the insulation structure allows for heat dissipation, so that the necessary electrical and thermal performance may be achieved provided the insulation is sized correctly     

Electrical and chemical diagnostics of transformers insulation. B.Accelerated aged insulation samples

This paper describes the analysis of accelerated aged insulation samples to investigate the degradation processes observed in the insulation from aged power transformers. Short-term accelerated ageing experiments were performed on paper-wrapped insulated conductors and on pressboard samples. The condition of aged insulation samples was investigated by two relatively new diagnostic techniques: (a) measurements of interfacial polarization spectra by a DC method; and (b) measurements of molecular weight and its distribution by gel permeation chromatography (GPC). Several other electrical properties of the paper/pressboard samples were also studied. Possible correlations have been investigated among the different measured properties. The GPC results have been used to predict how power transformer insulation molecular weights change with temperature and time     

Flow Electrification Process: the Physicochemical Corroding Model Revisited

Though the phenomenon of flow electrification has been observed for several decades, the physicochemical process appearing at the solid/liquid interface creating the double layer is not yet totally understood. In particular, returns from experiments made with oil and pressboard seems to show that the wall current density at the interface for a diffuse layer under development is not only a function of the chemical behaviour of the interface but also of the flow wall shearing stress. The present work concerns analysis of experiments made with heptane flows through a stainless steel capillary of varying length. In that case also, even if the wall material is conductive and not porous, divergences appear with the classical physicochemical model predictions, while a model taking into account the effect of the wall shearing stress on the interfacial process seems to have a much better agreement with the experiments, especially, for high laminar Reynolds numbers.     

Kerr electro-optic field measurement and charge dynamics intransformer-oil/solid composite insulation systems

We measured the electric field in transformer oil/solid composite insulation system under dc voltage application using a Kerr electro-optic measurement system. When a pressboard was inserted at the center between two parallel-plane electrodes, the electric field at the mid-point between the pressboard and the cathode decreased with time. On the other hand, the electric field at the mid-point between the pressboard and the anode increased initially with time, reached a maximum value and then fell down. We measured also the electric field before and after polarity reversal of the dc voltage. The results revealed that the electric field after polarity reversal reached a magnitude 2.7× as much as the average dc applied field. We interpreted the above results on the time transition of the electric field in terms of a charge dynamic model, considering the difference in charge accumulation speed on the pressboard between positive and negative charges. This model was verified to be applicable to the time transition of the electric field in oil/solid composite system and BTA added oil/solid composite system as well     

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     

Electrical and chemical diagnostics of transformers insulation. A.Aged transformer samples

This paper describes the application of two relatively new diagnostic techniques for the determination of insulation condition in aged transformers. The techniques are: (a) measurements of interfacial polarization spectra by a DC method; and (b) measurements of molecular weight and its distribution by gel permeation chromatography. Several other electrical properties of the cellulose polymer were also investigated. Samples were obtained from a retired power transformer and they were analysed by the developed techniques. Six distribution transformers were also tested with the interfacial polarization spectra measurement technique, and the molecular weight of paper/pressboard samples from these transformers were also measured by the gel permeation chromatography. The variation of the results through different locations in a power transformer is discussed in this paper. The possible correlation between different measured properties was investigated and discussed in this paper     

大型变压器中油流带电特性的研究

在了解和掌握国内外有关油流带电研究现状的基础上,研制出圆筒式油流带电模型,用于模拟大型变压器中的油流带电过程。用该装置对油流带电的转速特性,温度特性,油流带电的表征参数——泄漏电流与油的电气性能参数之间的关系进行了研究。