Showing papers on "Pressboard published in 1996"

Time domain measurements of dielectric response in oil-paper insulation systems

Abstract: The results of dielectric response measurements in the time and frequency domain on homogeneous impregnated pressboard and on series arrangements of pressboard sheets and oil ducts are presented. A simple equivalent circuit is proposed to model oil-paper insulation systems for single impregnated pressboard sheets as well as for multi-layer arrangements. This equivalent circuit which is derived from relaxation current measurements is then used to calculate recovery voltages as well as the frequency-dependent permittivity. Finally it is shown that the same procedure can be applied to quantify the dielectric response properties on high voltage power transformers.

Flow electrification measurements of transformer insulation using a Couette flow facility

Abstract: A rotating cylindrical electrode apparatus, which provided cylindrical Couette flow, was used to simulate now electrification in an electric power transformer. The apparatus had Shell Diala A transformer oil filling the annulus between coaxial cylindrical stainless steel electrodes that were either bare metal, or covered by a thin copper sheet and/or EHV-Weidmann HiVal pressboard insulation. Extensive experiments characterized the time transient and steady state behaviour of the electrification through measurements of the volume charge density, the terminal voltage, and the terminal current as the system was driven out of equilibrium by changes in the flow rate (inner cylinder rotation rates of 100 to 1400 rpm, Reynolds numbers of 5/spl times/10/sup 3/ to 5/spl times/10/sup 5/), temperature (15 to 70/spl deg/C), insulation moisture content (0.5 to 20 ppm in the oil), applied voltage (0 to 2 kV dc), and concentration of the non-ionizable antistatic additive 1,2,3 benzotriazole (BTA, 0 to 60 ppm). Generally, the electrification increased with flow rate and temperature but the BTA appeared to cause competing effects: it decreased the volume charge density on the liquid side of the interface by a factor of 4, which reduces the electrification, but also decreased the oil conductivity by a factor of 10, which enhances the electrification. A critical oil BTA concentration of 5 to 8 ppm minimized the electrification. The volume charge density was sensitive to the current flowing through the interface and surface charge accumulation. With pressboard covering the cylinders, the electrification effects of the temperature and moisture were decoupled during the transient following step reductions in the temperature. The oil moisture content did not affect the oil conductivity significantly, but the volume charge density varied inversely with the moisture content during an initial equilibration period just after the oil had been added to the system. The highest electrification was observed during this equilibration period.

Dielectric measurements of semi-insulating liquids and solids

Abstract: Sinusoidal steady state and transient decay measurement techniques were used to monitor the dielectric properties of electrical insulation. Impedance measurements of an air-gap capacitor, immersed in Shell Diala A transformer oil and driven with a 1 V peak sinusoidal signal (5 mHz-10 kHz), showed that the oil conductivity increased with temperature (15-70/spl deg/C) and aging in air but was independent of the oil moisture content (2-30 ppm). Interfacial double layer effects were also observed. Surface charge effects were studied in a cylindrical electrode apparatus with EHV-Weidmann Hi-Val pressboard on the inner cylinder and transformer oil filling the gap. The pressboard dielectric properties were obtained from time transient decay measurements of the open-circuit voltage after disconnecting a DC voltage source from across the cylinders. These measurements indicate that the conduction mechanism through the pressboard follows a drift dominated unipolar conduction law rather than ohmic conduction. Further frequency sweep dielectrometry measurements using a three-wavelength interdigital electrode sensor confirm the dispersive properties of oil-impregnated pressboard. This sensor was also used to measure the time and space distributions of moisture diffusion into oil-impregnated pressboard.

Electrical strength of pressboard components for transformer insulations

Abstract: 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.

Moisture dynamic measurements of transformer board using a three-wavelength dielectrometry sensor

Abstract: The spatial distribution of dielectric permittivity and conductivity of an insulating medium affects the quasistatic distribution of the electric field. Multiple wavelength interdigital sensors using the imposed frequency-wavenumber (/spl omega/-k) dielectrometry technique can measure complex permittivity distributions in dielectrics. Measurements of oil-free and oil-impregnated pressboard transformer insulation using a three-wavelength sensor with changes in ambient moisture concentration are presented. With a calibrated mapping relating conductivity and permittivity to moisture concentration in oil-impregnated pressboard, the spatial distribution of moisture can be measured.

Positive and negative streamers in oil gaps with and without pressboard interfaces

Abstract: Inception and propagation of streamers in a point-plane gap, with and without pressboard interfaces parallel to the field, have been studied. Current and light-emission have been recorded. Positive streamers propagate at about 2-4000 m/s at voltages up to two times the minimum breakdown voltage. Above this voltage a fast event (>100 km/s) may occur, preceded by streamers propagating with velocities in the 10-20 km/s range. The negative streamers have about twice as high a breakdown voltage as the positive ones. Also negative streamers may develop into a fast event. Solid field parallel pressboard does not change the breakdown voltage, but makes inception of fast events easier. The breakdown process in oil shows several similarities to a gas breakdown.

Effect of streaming potential on heat transfer in transformer oil

Abstract: 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 in power transformers: experimental study and modelling of a pressboard submitted to an impinging oil jet

Abstract: For the last twenty years, static electrification has been suspected as being responsible for failure in power transformers. As a matter of fact internal parts show evidence of electrical discharges: erosion of the pressboard surface (electric tree paths), "worms holes", presence of carbon, etc. The phenomena consists of 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. EPRI started an important research program involving universities, manufacturers and utilities to solve the problems or to minimize the effects on operating apparatus and to define new specifications for the future devices. If on the one hand, all the parameters of influence seem to be clearly identified, on the other hand their critical combination has not been verified. As part of the research program of Electricite de France, an experiment at the university of Poitiers, associating a numerical modeling method developed at the R&D center of EDF with the computational fluid dynamic software ESTET, has been carried out to study the influence of a 3D oil flow geometry encountered in transformers. These investigations should make it possible to determine a critical context which might explain the observed incidents.

Electrical conduction phenomena through impregnated pressboard submitted to a thermal gradient

Abstract: For the last twenty years, flow electrification has been suspected to be responsible of failures in high power transformers. In order to understand the phenomenon and to avoid incidents, several research programs have been undertaken in different countries. Our laboratory is involved in one of this program in collaboration with the French Electricity Company (E.D.F.). From this research in terms of velocity, pipe geometry, temperature and moisture content it seems that the transverse thermal gradient play a fundamental role in the process.

A study of phase angle distribution of partial discharges in oil pressboard insulation systems

Abstract: In electrical power transformers, composite insulation consisting of oil-pressboard insulation is widely used. For optimum design of such insulation systems, partial discharge (PD) behaviour is an important design criterion. A computer-aided PD measuring system which permits a time sequential data acquisition and phase resolved pulse height analysis has been developed. Using this system, PD in a representative oil-pressboard system has been measured. The effect of stress level, their duration and the test procedure on the discharges have been studied. The discharge data is analysed using Gumbel and Weibull statistics. An attempt is made to correlate the Weibull and derived PD parameters of each phase with their counter-parts in the cycle.

A contribution to modelling and experimental studies of flow electrification in transformers

Abstract: As part of the research program of Electricite de France on static electrification in power transformers, on one hand an experiment has been carried out at the University of Poitiers to simulate the oil path in a transformer and on the other hand the 3D Computational Fluid Dynamics software ESTET developed at the R&D centre of Electricite de France has been extended to the modelling of the phenomena. The experimental devices has enabled us to study in a pressboard duct the wall current response to a heat transfer step produced from the outer side of the duct and to calibrate the interfacial charge transfer laws used in our simulation. So we can characterise the influence of parameters such as velocity, temperature, moisture, transient and steady states on the phenomena. This paper shows that some parameters sets lead to important dielectric stresses in the pressboard which might be the cause of the hazardous discharges observed in power transformers.

AC V-t and impulse V-N characteristics of shell-form transformer insulation model

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.

A study of partial discharge characteristics in oil impregnated pressboard insulation

Abstract: An attempt has been made to study the effect of time and test procedure on the behaviour of partial discharge (PD) pulses causing failure of oil-pressboard system under power frequency voltages using circular disc shaped samples and uniform field electrodes. Weibull statistics have been used to handle the large amount of PD data. The PD phenomena has been found to be stress and time dependent. On the basis of stress level, three different regions are identified and in one of the regions, the rate of deterioration of the sample is at a maximum. The work presents some interesting features of Weibull parameters as related to the condition of insulation studied in addition to its usual PD characteristics.

Cellular-type hollow bamboo pressboard

Abstract: The utility model provides a honeycomb-shaped hollow bamboo pressboard which comprises a bamboo pressboard of an upper layer and a bamboo pressboard of a lower layer and also comprises a hollow structure arranged between the bamboo pressboard of the upper layer and the bamboo pressboard of the lower layer, wherein the hollow structure is composed of a plurality of hexagonal hollow cylinder bodies which are mutually bonded to form a honeycomb-shaped hollow structure, and the stress distribution is of the hexagonal hollow cylinder bodies is optimum and rational; or a plurality of circular hollow cylinder bodies of which the stress distribution is good and the shape approximates to a hexagon are mutually bonded to form a honeycomb-shaped hollow structure. The bamboo pressboard made from the utility model has the advantages that the strength of the pressboard is not affected, the dead weight of the pressboard is lightened, and the manufacture cost and the processing strength of the pressboard are reduced. The utility model has the advantage of practicality.

Kerr electro-optic field measurement in transformer oil/solid dielectrics composite insulation system

Abstract: We measured the electric field in transformer oil/solid dielectrics composite insulation system under dc voltage application using Kerr electro-optic measurement system. When a pressboard was inserted between two parallel-plane electrodes, the electric field at cathode side of pressboard decreased with time. On the other hand, the electric field at anode side increased initially with time, reached a maximum value and then fell down. Next, we measured the electric field after polarity reversal of dc voltage. The results revealed that the electric field reached a magnitude of 2.7 times as much as the average dc applied field. We interpreted the above results in terms of a charge dynamic model considering the difference in charge accumulation speed on pressboard between positive and negative charges.

Prebreakdown behavior of a composite liquid-solid insulation system under impulse conditions

Abstract: This paper deals with the prebreakdown and breakdown phenomena of composite liquid-solid insulation systems in strongly non-uniform (point-point geometry) and weakly non-uniform fields (rod-rod geometry) at impulse voltages up to 500 kV. The investigations were carried out in oil-pressboard-oil barrier configurations with electrode gaps up to 22 mm. Streamer inception and propagation were visualized via high-speed shadow-graph photography. Compared with purely liquid insulation systems, the predischarges show identical propagation behavior, i.e. they appear either as primary or secondary streamers depending on the field. However, the propagation ceases at the liquid-solid interface and a luminous micro discharge (MD) without subsequent breakdown occurs. The light intensity of this MD becomes stronger with increasing voltage levels. Further, the MDs lead to surface discharges on the pressboard and, beyond a critical voltage level, Lichtenberg-type figures can be observed on the pressboard surface.

Effect of pressboard thickness and wall surface conditions on flow charging of oil [power transformers]

Abstract: A number of power utilities have reported high failures of power transformers due to flow electrification phenomena. Despite extensive work for more than two decades the problem still remains unsolved because of its complexity. The paper presents the outcome of the investigation carried out on several aspects of flow electrification, viz. effect of pressboard composition, thickness, type, surface finish on the charging behaviour of the oil.

Anomalous oil electrification phenomena in Couette flow between bare stainless steel electrodes

Abstract: Electrification, which occurs in transformers due to the entrainment of electrical double layer charge from the recirculatory flow of the oil over internal insulating pressboard surfaces, is simulated in an apparatus consisting of two coaxial cylinders filled with oil. The inner cylinder is rotated, resulting in a turbulent flow in the fluid region between the inner and outer cylinder. Measurements of oil charge density, open circuit voltage and short circuit current are made. Experiments using bare stainless steel cylinders demonstrate transient behavior having time constants which are much greater than known time constants of the oil dielectric. Anomalous and nonreproducible system responses occurring due to changes in rotational speed, as a result of electrical prestressing, and for no known reasons are observed.

Static electrification of the insulating materials used in power transformers

Abstract: It is shown that the electrostatic charge tendency (ECT) is a valuable indication allowing one to evaluate the levels of the charge density which can be reached by the oil in a power transformer. A correlation can be made between this parameter (ECT) and the physicochemical and electrical characteristics of the oil. It is shown that the water migration phenomena at the oil/pressboard interface play an important role in the charge separation. Hence we investigate the influence of the rugosity of the pressboard, the temperature gradients and the ageing of the oil. The effect of the antistatic additives on the ageing and the ECT is also considered.

Effect of pressboard thickness and wall surface conditions on flow charging of oil

Abstract: Anumber of powerutilitieshave reported high failures of power transformers due to flow electrification phenomena. Despite extensive work for more than two decades the problem still remains unsolved because of its complexity. The paper presents the out come of the investigation carried out on several aspects of flow electrification, viz. effect of pressboard composition, thickness, type, surface finish on the charging behaviour of the oil.

k Measurements of Transformer Board Usin Wave1 en g t lh i e I ct ct ro m et ry Sensor

Abstract: Ab~fr~~b.' The spatial distribution of dielectric permittivity and conductivity of a medium affects the quasistatic distribution of the electric field. Multiple wavelength interdigital sensors using the imposed frequency-wavenumber (0-k) dielectrometry technique can measure complex permittivity distributions in dielectrics. Measurements of oil-free and oil-impregnated pressboard using a three-wavelength sensor with changes in ambient moisture concentration are presented. With a calibrated mapping relating conductivity and permittivity to moisture concentration in &impregnated pressboard, the spatial distribution of moisture can be measured. The presence of moisture in transformer oil and oilimpregnated pressboard has a detrimental effect on insulation life by lowering electrical breakdown strciigtli and thermal endurance [l]. When the transformer is in operal.ion, the conventional way to estim,ate the moisture in the solid insulation is to measure the rnoisture content in the oil and to use moisture equilibrium curvcs to estiauate the iiioisturc level in the pressboard [2]. However, since it may take a long time for the system to reach equilibrium. using the curvcs under non-equilibrium conditions may result in errors. The thrccwavelength sensor is designed to monitor the dielectric rcsponse of the insulation in real time and to calculatc the moisture distribution in the pressboard by using a calibratcd mapping relating dielcctric permittivity and Conductivity to moisture coriccntratiort [a]. Preliniinary moisturc dynamic expcnments havc bccn donc with oil-free pressboard at room tempcraturc [4.5] We rcpcated the nicasurernciits alt 70°C to increase the moisture diffusion spccd Measurements of oil-impregnated pressboard at 70°C, a more typical case for the transfornicr systcni, are also presented

Influence of electrode temperature on the characteristics of a non uniform field system with a barrier

Abstract: Electrode temperature controlled AC corona discharge inception voltages (CIV) of a nonuniform field electrode system and the characteristics change of CIV in the presence of a barrier is investigated. Reduction in the breakdown voltages of the gap for negative standard impulse applications was observed as the point electrode temperature reached 200/spl deg/C and the characteristic occurrence was repeated in the presence of a pressboard barrier.