Showing papers on "Pressboard published in 2000"

Flow electrification in transformers: correlation between winding leakage current and pressboard charge accumulation

Abstract: As far as static electrification transformer diagnosis is concerned, the leakage current measurement is recommended on apparatus declared suspect following on the Electrical Charging Tendency values and, to a smaller amount, the continuous aging test for tan /spl delta/ test. As part of the research program of Electricite de France in the field of static electrification in power transformers, an experiment has been carried out, at the University of Poitiers to investigate the correlation between the leakage current and the charge accumulation in some insulated pressboards of the transformer. On one hand, the leakage current has been observed as the result of flow electrification in a duct composed of pressboards and wrapped conductor standing for the oil channels between windings in transformer. On the other hand, the charge accumulation is computed from capacitive currents on insulated electrodes laying on the pressboards of a second duct in the loop. These two currents have been analyzed in terms of flow velocity, oil temperature, conductivity and moisture content. It finally turns out that to define an alarm threshold based on leakage current seems rather difficult as great values can mean an important charge generation but a weak accumulation when small values can lead to a dangerous accumulation in the pressboard.

Investigations in order to improve the E.C.T. measurement protocol of large power transformer oil

Abstract: The streaming electrification in oil-immersed transformers is the cause of some failures in all the countries of the world. Thus, it is necessary to have reliable servicing and checking tools in order to perform steady and reproducible measurements. One investigation method consists of studying the oil/pressboard couples [Touchard G., and al. 1999]. Another method consists of improving the protocol used for electric characterization of the oil (mainly the protocol combined with the Ministatic Tester). This paper explains an enhancement of this measurement protocol.

Investigations on cavities inside liquid-immersed paper and pressboard insulation materials and their influence on the electrical behavior

Abstract: Although the process of impregnating cellulosic materials with liquid insulants is well known and has been in use for many decades sometimes problems with gas-filled cavities inside the impregnated material still arise. Therefore extensive experimental investigations on the effects of gas-filled cavities inside an impregnated pressboard, like they appear after an imperfect impregnation, have been performed. The filling of hidden voids with surrounding insulation liquid during and after impregnation process was examined and the results are presented in this contribution. Using defined failures inside a transformerboard also the influence of these defects on the electrical behavior of the insulation in a semi-uniform field was simulated.

Method to observe the change of charges in an electric double layer between pressboard and the insulating oil in a power transformer

Abstract: Space charge is one of the important causes of electrical breakdown. It has been necessary to sample the oil in an oil-cooling transformer to establish how charged it is. A method to detect the change in accumulated charges on a pressboard in an oil transformer is suggested. The change in the level of charge relates to the charges in the oil, so the charges in oil and on a pressboard can be observed without sampling the oil by using this method. An electrometer, a high input impedance voltage meter, and an oil circulation system were used to detect the change. When the oil went through the pressboard pipe, it was charged positively due to the streaming electrification. The voltage change on the surface of the pressboard was measured by a high input impedance voltmeter and the current from the surface by a micro-ammeter. From these data, the change in static charge is known.

Investigation of charge behavior in flowing transformer oil/pressboard composite insulation system by electro-optic field measurement

Abstract: Electrostatic charging may occur in oil immersed power transformers because of a flow electrification phenomenon. In order to discuss the charge behavior, we directly measured DC electric field in the oil/pressboard composite insulation system using Kerr electro-optic field measurement method under oil flowing conditions. As a result, even if no voltage was applied, the electric field emerged because of space charge in oil and charge accumulation on pressboard. We changed the flow rate of oil, the arrangement of pressboard barrier, and the applied voltage as parameters. We discussed experimental results from the viewpoint of charge behavior and generated field distribution.

Pressing machine for continuous prodn. of artificial wood

Abstract: A pressing machine for continuous prodn. Of artificial wood of the utility model forms a feeding system mainly by a machine frame, a hydraulic cylinder fixed to the front end of the machine frame, a drift pin. An upper pressboard, a lower pressboard and a bolt are fixed with a side guide and together form a mold cavity. The drift pin and the hydraulic cylinder are installed in the back end of the machine frame, by which to control the density of the materials in the cavity. A bracket is fixed to the machine frame top. The hydraulic cylinder installed on the cross beam of the bracket and the pressboard compose a movable bin. The hydraulic cylinder drive the drift pin to have back and forth movement to make the materials in the bin continuously squeeze in squeeze into the cavity. The materials are extruded after the curing in the channel. The device is mainly used for the production of imitation wood used for the xylon or non-xylon long fibrous structure.

Improving the insulation condition of power transformers with an innovative paper insulation demoisturing procedure

Abstract: Some aspects of improving the insulation conditions of power transformers by using a new method for drying the paper insulation during service are presented. The drying system is cost efficient, easy to adapt on every liquid and paper/pressboard insulated high voltage apparatus, does almost not affect neither the dissolved nor the undissolved gas analysis and does not require to take the apparatus out of service. The contribution reports about the principle of operation and the relations and time perspectives that have to be regarded for a practical application of the new system.