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Showing papers on "Pressboard published in 1995"


Journal ArticleDOI
TL;DR: In this paper, the authors measured the dielectric spectrum of EHV-Weidmann HIVAL pressboard impregnated with Shell Diala A transformer oil, as a function of temperature and water content.
Abstract: In oil-impregnated pressboard of medium and low humidity the complex dielectric spectrum's shape and amplitude do not change with variations in temperature and moisture content, but only shift in frequency. Thus it is possible to create a 'universal' curve, with appropriate temperature correction factors, which can be used to extract information about the moisture dynamics of solid transformer insulation from dielectrometry measurements. To create the universal curve, measurements are taken with the material placed in a parallel-plate structure whose complex impedance is measured. To measure spatial distributions of dielectric properties, imposed /spl omega/-k (frequency-wavenumber) dielectrometry is used, where a set of interdigitated electrodes on one surface of the material cause the electric field to have a limited depth of penetration into the dielectric, proportional to the electrode spacing. Measurements using the parallel-plate methodology establish a mapping of the dielectric spectrum of EHV-Weidmann HIVAL pressboard impregnated with Shell Diala A transformer oil, as a function of temperature and water content. This mapping is then used to determine spatial moisture profiles in pressboard in other experiments which make use of a three-wavelength interdigitated sensor. >

69 citations


01 May 1995
TL;DR: In this article, the authors examined the flow electrification process appearing in power transformer in which oil is circulating and focused on the physicochemical reaction occurring at the contact between the oil and the pressboard.
Abstract: In this paper we examine the flow electrification process appearing in power transformer in which oil is circulating. Our interest is focussed on the physicochemical reaction occurring at the contact between the oil and the pressboard. The study is divided in two sections. In the first one, we present two different models for the process of flow electrification when oil is flowing past the pressboard. The first model assumes that the ions provided by the pressboard are always in sufficient quantity to have a physicochemical reaction in equilibrium on the wall. In the second one we suppose that the wall shearing stress contribute to the production of these ions. In both cases we compute the evolution, in terms of the distance to the entry of the pipe, of the space charge density on the wall, of the total current and of the wall current density. The experimental work is divided into three parts. In the first one, we describe the general device on which we worked and the precautions we took to be sure to measure the flow electrification only. In the second part, we describe the experiments we did on duct 1 in order to get information on themore » migration of the charges from the interface oil-pressboard towards an earthed wall. In the third and last part of that experimental section, on duct 2 we study the phenomenon of charge of the pressboard when it is electrically isolated. Then we conclude with a comparison between the experimental and theoretical results.« less

17 citations


Proceedings ArticleDOI
22 Oct 1995
TL;DR: In this paper, the authors focus on the charge accumulation and movement in an transformer-oil-impregnated pressboard under DC polarity reversal at 60/spl deg/C. Using a pressure wave propagation (PWP) technique, the distribution of space charge in the sample was measured both at stationary and transient conditions.
Abstract: Space charge plays an important role in the breakdown and conduction processes of a high voltage direct current (HVDC) insulation system. Electrical breakdown due to charge accumulation in transformer oil and pressboard can cause failures of power transformers, especially at DC polarity reversal. In this paper, we focus our attention on the charge accumulation and movement in an transformer-oil-impregnated pressboard under DC polarity reversal at 60/spl deg/C. Using a pressure wave propagation (PWP) technique, the distribution of space charge in the sample was measured both at stationary and transient conditions.

8 citations


Patent
21 Jul 1995
TL;DR: In this article, a current-carrying heating element which is heated by application of voltage to a resistor composed chiefly of a composition wherein an electricaly conducting material is dispersed in an organic high polymer is made from polyether ketone (PEEK) and a resin whose melt viscosity is 10 -10 poise at a temperature of 380 deg.C and at a shear rate of 10 s.
Abstract: PURPOSE:To provide a self-temperature controlling, current-carrying heating element having a good temperature distribution with a heating temperature of 150 deg.C or higher and therefore a long load life by composing a resistor chiefly of a composition wherein an electrically conducting material is dispersed in an organic high polymer of a specific characteristic. CONSTITUTION:In a current-carrying heating element which is heated by application of voltage to a resistor composed chiefly of a composition wherein an electricaly conducting material is dispersed in an organic high polymer, the organic high polymer is made from polyether ketone (PEEK) and a resin whose melt viscosity is 10 -10 poise at a temperature of 380 deg.C and at a shear rate of 10 s . Polyphenylene sulfide(PPS), polyether imide, or polyether sulfone is used as such a resin. The electrically conducting material added to the organic polymer is carbon black, graphite, or preferably MnZn ferrite. The conducting material is kneaded and dispersed by 50vol.% in the organic high polymer (PEEK/PPS=6/4) to form a pressboard, and electrodes are bonded to both sides of the pressboard and voltages are applied to the electrodes for heating.

4 citations


Proceedings ArticleDOI
21 Nov 1995
TL;DR: In this paper, the influence of higher oil flow rate and upstream charge on flow electrification was analyzed by simulating an actual transformer internal structure, and it was noticed that there is a probability of partial discharge inception under higher oil circulation velocity.
Abstract: Electrical breakdown due to charge accumulation from transformer oil flow has caused many failures of large power transformers world wide. The problem is due to the entrainment of diffused electrical double layer charges into circulating transformer oil. As the charges accumulate on the surface of solid insulating materials and in volume oil, static potential builds up. If the rate of charge accumulation is greater than the rate of charge relaxation, harmful spark discharge may occur. By employing a pressboard pipe model, the study carried out revealed the influence of higher oil flow rate and upstream charge on flow electrification By simulating an actual transformer internal structure, it is noticed that there is a probability of partial discharge inception under higher oil circulation velocity. However, the upstream charge and dry zone can lead to a great increase of electric field strength, which may become important potential causes of partial discharge inception under the condition of relative low oil velocity.

2 citations


Journal ArticleDOI
TL;DR: In this article, the authors used a parallel electrodes system with a compound structure of pressboard barrier and oil gap to evaluate an appropriate level of ac test voltage for 1000-kV transformers, which is necessary to make an accurate estimation of ac V-t characteristics.
Abstract: To evaluate an appropriate level of ac test voltage for 1000-kV transformers, it is necessary to make an accurate estimation of ac V-t characteristics. For the level of impulse test voltage, V-N characteristics are important to evaluate effects of impulse stress repetition. Since both of the characteristics are dependent on insulation structures and manufacturing processes of transformers, it is important to make accurate simulations on transformer insulation for reliable insulation data. To obtain some of the basic data for 1000-kV transformer test levels, long-time ac V-t tests and impulse V-N tests of up to 1000 times voltage application were carried out on an insulation model simulating fundamental structure of shell-form transformers. The model is composed of a parallel electrodes system with a compound structure of pressboard barrier and oil gap. The tests showed the following characteristics: ac V-t characteristics of up to several ten days could be described by two lines, each representing short-time and long-time characteristics. V-N characteristics of lightning and switching impulse were expressed by relatively flat lines, which showed a decrease in breakdown voltage by less than 10 percent after 1000 times impulse application.

2 citations