Showing papers on "Pressboard published in 1997"

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

Abstract: 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.

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

Abstract: 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.

A chemical reaction-based boundary condition for flow electrification

Abstract: A physical model is developed for the charge transfer boundary condition in semi-insulating liquids. The boundary condition is based upon interfacial chemical reactions and extends established relations for the interface by including the effects of interfacial surface charge and charge desorption at the interface. A steady state model for flow electrification in a rotating cylindrical electrode apparatus incorporated this boundary condition and described polarity changes in the open-circuit voltage and short-circuit current as a function of the fluid velocity, the volume charge density dependence an the terminal constraints, and the charge density dependence on applied dc voltages. Previously used boundary conditions are shown to be special cases of the chemical reaction rate boundary condition. A general methodology is developed for combining the volume charge density and voltage/current terminal measurements to estimate the parameters describing the interfacial charge transfer kinetics. Volume charge densities /spl rho//sup w/ on the liquid side of the interfaces of 1 to 20 mC/m/sup 3/ were estimated from the open-circuited electrode measurements, with the stainless steel /spl rho//sup w/ typically larger than that of copper but smaller than that of transformer pressboard. Activation energies for an Arrhenius temperature dependence of /spl sim/0.16 eV for pressboard, 0.25 eV for stainless steel and 0.28 eV for copper were obtained. Interfacial adsorption reaction velocities, estimated to be 10/sup -5/ m/s, were not large enough to make the terminal current transport limited which contradicts the often used assumption that the reaction velocities can be considered 'infinite'. Estimated surface reaction rates at a 70/spl deg/C stainless steel/oil interface of /spl sim/20 /spl mu/m/s for adsorption and /spl sim/0.5 s/sup -1/ for desorption were obtained. The additive BTA reduced the /spl rho//sup w/ for pressboard and stainless steel at concentrations >8 ppm in transformer oil.

Application of relaxation current measurements to on-site diagnosis of power transformers

Abstract: Results of relaxation current measurements on impregnated pressboard samples and on multi-layer arrangements of pressboards in series with oil ducts are presented. The measurements have been performed dependent on the material properties of oil and pressboard and the volume ratio of these materials. Calculated values of related dielectric quantities such as tan/spl delta/ versus frequency, based on equivalent circuits obtained from relaxation currents are also shown. The relaxation currents of multi-layer test objects are calculated from the dielectric properties of the individual components and their geometrical capacitances. Finally, it is shown that relaxation currents of power transformers can easily be measured and their evolution can be described well by basic relations of linear dielectric theory.

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

Abstract: 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/spl times/ 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.

Dielectrometry measurements of effects of moisture and anti-static additive on transformer board

Abstract: Moisture in solid insulation has detrimental effects on transformer life. When the system is not in equilibrium laboratory measurements with multiple wavelength interdigital sensors using the imposed frequency-wavenumber (/spl omega/-k) dielectrometry technique have proved to be an effective way to measure complex permittivity distribution in solid insulation which is directly related to the moisture distribution. Measurements of oil-free and oil-impregnated pressboard using interdigital sensors with changes in ambient moisture concentration are presented. A comparison between existing moisture equilibrium curves for the oil-paper system and experimental results is given. Measurements are also performed for anti-static additive in oil and the dielectrometry effects are discussed.

Experimental study and modeling of static electrification in power transformers

Abstract: 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.

Optical measurement of electric field in transformer oil/pressboard composite insulation system and discussion on charge dynamics

Abstract: We measured an electric field in transformer oil/pressboard composite insulation system under dc voltage application using Kerr effect in the oil. When a pressboard was inserted between two parallel-plane electrodes, the electric field between the pressboard and the cathode decreased monotonously with time. On the other hand, the electric field between the pressboard and the anode increased with time, reached a maximum value and then fell down. We discussed this phenomenon by a charge accumulation and potential distribution model considering the difference between the charge accumulation rate on the pressboard of positive and negative charges. Discussion results were also described based on experiment with pressboards changing its thickness and location in the oil. Charge dynamics was also discussed when the polarity of dc voltage was reversed.

Thermal characteristics of a meta-aramid and cellulose insulated transformer at loads beyond nameplate

Abstract: Advancement in meta-aramid-a synthetic, aromatic polyamide in which the phenyl molecules are connected in the meta orientation-paper and pressboard technology has lead to the production of a high temperature insulation system that can be used in power transformers, replacing cellulose materials in the windings, thereby eliminating the cellulose temperature limitation. A medium-power mineral oil-immersed transformer was constructed with a hybrid insulation system comprised of meta-aramid paper and pressboard insulated windings and cellulose bulk insulation. Fiber-optic temperature sensors were installed for direct reading of oil and conductor temperatures at key points throughout the transformer. The transformer was tested and thermal constants were determined in accordance with PC57.119/D12 for both ONAN and ONAF cooling modes at load levels up to 137% of the maximum nameplate rating of an otherwise identically designed all cellulose insulated unit. Temperatures predicted by equations in the ANSI/IEEE C57.92 Loading Guide were compared with measured values. Anomalies in the data are investigated and explanations offered.

Evolution of the streaming current generated by an oil flow through a pressboard-application to flow electrification in power transformers

Abstract: In high power transformers, pressboard and oil are needed for electrical insulation. Added to that, the oil is using as a cooler fluid of the system. Furthermore, during this last decade, the phenomenon of flow electrification at the oil-pressboard interface region is suspected to be at the origin of several damages occurring in power transformers, as well in Europe, North America or Japan. Thus as these phenomena are strongly in correlation with the physico-chemical processes at the interface, the goal of this experimental study is to characterize the streaming current intensity in function of the electrochemical and morphological properties of the pressboard. For this, the streaming current and the mean space charge density generated by an oil flow through a non degraded pressboard and pressboards degraded by electrical discharges have been characterized. The results show that the electrochemical modifications of the oil-pressboard interface highly enhances the flow electrification of the oil.

Thermal characteristics of a meta-aramid and cellulose insulated transformer at loads beyond nameplate. Discussion. Authors' reply

Abstract: Advancement in meta-aramid - a synthetic, aromatic polyamide in which the phenyl molecules are connected in the meta orientation - paper and pressboard technology has lead to production of a high temperature insulation system that can be used in transformers, replacing cellulose materials in the windings, thereby eliminating the cellulose temperature limitation. A medium-power mineral oil-immersed transformer was constructed with a hybrid insulation system comprised of meta-aramid paper and pressboard insulated windings and cellulose bulk insulation. Fiber-optic temperature sensors were installed for direct reading of oil and conductor temperatures at key points throughout the transformer. The transformer was tested and thermal constants were determined in accordance with PC57.119/D12 [1] for both ONAN and ONAF cooling modes at load levels up to 137% of the maximum nameplate rating of an otherwise identically designed all cellulose insulated unit. Temperatures predicted by equations in the ANSI/IEEE C57.92 [2] Loading Guide were compared with measured values. Anomalies in the data are investigated and explanations offered.

Measurement of Stratified Distributions of Dielectric Properties and Dependent Physical Parameters

Abstract: Recent advances in ω-k (frequency-wavenumber) interdigital dielectrometry are described. Using this technology, information about the microstructure of dielectric materials is obtained by applying to the sensor-dielectric interface a spatially periodic electric potential swept in frequency from 0.005 Hz to 10,000 Hz. The penetration depth of the electric field is proportional to the spatial wavelength of the electric potential. Application of multi-wavelength electrode arrangements allows measurement of stratified distributions of complex dielectric permittivity. Calibration techniques serve to relate the distributed dielectric properties of materials to other physical variables, such as density, porosity, cracking, lamination, and diffusion of contaminants into the material. The specific problem treated in this paper is in the measurement of moisture concentration distribution in transformer pressboard during the diffusion of water molecules from ambient transformer oil. The output of interdigital sensors is strongly influenced by the microgranularity of the material’s surface. Although this dependence complicates interpretation of the measurements in some applications, the variation of the output may also be used to characterize the shape of the surface on the microscale.

Surface discharges within oil insulated apparatus

Abstract: Liquid insulation, mainly in the form of mineral oil, is a critical component of many different items of plant within electricity transmission and distribution systems-cables, bushings, switchgear, transformers. Usually it is in combination with solid insulation. This may take the form of paper impregnated with oil, such as in cables or bushings, resin impregnated components such as drive rods in tapchangers, or solid pressboard insulation to break up oil gaps, such as the barriers in transformers. Therefore, when one considers the effect of oil breakdown in oil filled equipment, one should consider the interactions which electrical discharges may have with solid surfaces. These are often the weak points of such systems. This paper is mainly concerned with the surfaces of barriers within transformers. (2 pages)

Light emission study in transformer oil and along oil/cellulose interface using a spark-gap system

Abstract: Interfaces between oil and oil-impregnated-pressboard are known to weaken insulation system strength. The work here describes differences in the observed discharges in transformer oils and along oil/oil-impregnated-pressboard interfaces. Oil breakdown in a uniform electric field was initiated by triggering a spark gap either at the anode or at the cathode using a liquid trigatron. Light emission during the breakdown processes was detected using a photomultiplier tube. Breakdown voltage, time to breakdown, streamer propagation speed and polarity effect were seen to differ in a transformer oil and along an oil/oil-impregnated pressboard interface.

Manufacture of duct piece of static inductive electric apparatus

Abstract: PROBLEM TO BE SOLVED: To manufacture a duct piece in a short time by unifying a material to a desired thickness of the duct piece and delivering it in a prescribed length for being compiled in one bunch followed by mechanical processing. SOLUTION: A long pressboard material 10 with the same width to a width measure W of a duct piece 5 is heaped to be bundled for setting this up the material supply part provided with a fixed measure feeding device. Next, this set length is set to a length L of the duct piece 5 and the pressboard material 10 is delivered by the length L of the pressboard material 10. Next, a tape T is strongly wound on this delivered terminal part by means of a taping machine. Later, this is cut by means of a circular saw blade so as to make a piece block PB. Next, dovetail processing is performed on the terminal face side to be provided with a dove-tail of this piece block PB by means of a milling cutter 21 further followed by the dovetail processing by means of an angular cutter 22. Later, terminal face processing is performed by means of a milling cutter 23.

A Chemical Reaction-based Condition for Flow Electrification

Abstract: A physical model is developed for the charge transfer boundary condition in semi-insulating liquids. The boundary condition is based upon interfacial chemical reactions and extends established relations for the interface by including the effects of interfacial surface charge and charge desorption at the interface. A steady state model for flow electrification in a rotating cylindrical electrode apparatus incorporated this boundary condition and described polarity changes in the open-circuit voltage and short-circuit current as a function of the fluid velocity, the volume charge density dependence on the terminal constraints, and the charge density dependence on applied dc voltages. Previously used boundary conditions are shown to be special cases of the chemical reaction rate boundary condition. A general methodology is developed for combining the volume charge density and voltage/current terminal measurements to estimate the parameters describing the interfacial charge transfer kinetics. Volume charge densities p" on the liquid side of the interfaces of 1 to 20 mC/m3 were estimated from the open-circuited electrode measurements, with the stainless steel p" typically larger than that of copper but smaller than that of transformer pressboard. Activation energies for an Arrhenius temperature dependence of -0.16 eV for pressboard, 0.25 eV for stainless steel and 0.28 eV for copper were obtained. Interfacial adsorption reaction velocities, estimated to be mls, were not large enough to make the terminal current transport limited which contradicts the often used assumption that the reaction velocities can be considered 'infinite'. Estimated surface reaction rates at a 70°C stainless steel/oil interface of -20 pmls for adsorption and -0.5 s-l for desorption were obtained. The additive BTA reduced the p" for pressboard and stainless steel at concentrations > 8 ppm in transformer oil.

Measurement of flow electrification in transformer models and its predetermination by theoretical approach

Abstract: The impact of flow electrification in transformers is highlighted, using experimental studies on pressboard pipe models and a multilayer sectional model of a 400 kV transformer. The need for an analytical approach to calculate the potential distribution under actual flow conditions is discussed. The development of a computer program and its application to predetermine the electric stresses in oil-paper interface are described.