Showing papers on "Pressboard published in 1998"

Propagation of positive and negative streamers in oil with and without pressboard interfaces

Abstract: Inception and propagation of streamers in a point-plane gap, with and without pressboard interface parallel to the field, have been studied. Current and light emission have been recorded. Positive streamers propagate at /spl sim/2 to 4 mm//spl mu/s at voltages to 2/spl times/ the minimum breakdown voltage. Above this voltage, a fast event (>100 mm//spl mu/s) may occur, preceded by streamers propagating with velocities in the 10 to 20 mm//spl mu/s range. The negative streamers have /spl sim/2/spl times/ the breakdown voltage as the positive ones. Even negative streamers may develop into a slower kind of fast event. Solid pressboard parallel to the field 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.

Measurement of moisture diffusion as a function of temperature and moisture concentration in transformer pressboard

Abstract: The moisture diffusion process is measured for oil-free transformer pressboard using a flexible interdigital dielectrometry three-wavelength sensor. Experiments are performed for five different temperatures at various moisture levels. The diffusion coefficient as a function of temperature and moisture concentration is estimated and compared with literature values. The numerical methods of solving the diffusion equation are discussed.

On-site diagnosis of power transformers by means of relaxation current measurements

Abstract: The purpose of this paper is to show how the relaxation currents of a power transformer can be calculated, when the geometrical configuration of the main insulation and the dielectric properties of its components, oil and pressboard, are known. To model this insulation a simple equivalent circuit is presented, which is derived from basic relations of linear dielectric theory. The dielectric characteristics of oil and pressboard necessary for modelling are determined from dielectric response measurements on samples. Therefore some results of relaxation current measurements of pressboard samples in dependence on moisture content and temperature are presented. Finally, some examples of on-site measurements of relaxation currents on hitherto unused power transformers are shown which are used here to demonstrate how the moisture content of pressboard of these transformers can be quantified from evaluation of their relaxation currents.

Moisture and space charge in oil-impregnated pressboard under HVDC

Abstract: Transformer oil and oil-impregnated pressboard are major insulating materials in a HVDC (high voltage direct current) converter transformer which provides electrical power transformation between an AC and a DC system. However, the dc component on the valve side of a HVDC converter transformer gives rise to special conditions, regarding space charge for example, for the insulating materials. In this paper, the relationship between moisture and space charge in oil-impregnated pressboard was investigated under dc electrical fields at room temperature. Formation, accumulation and distribution of space charge were studied using a PWP (pressure wave propagation) technique with the moisture concentration varying between 0.3% and 1.7% by weight. Experiments have shown that moisture influences seriously space charge distribution and thus electric field distribution in oil-impregnated pressboard. The higher the moisture concentration, the faster the establishment of space charge and the deeper the charge penetration into the insulation. The influence of moisture on the distribution of space charge should be taken into consideration in the design of an insulating system under HVDC.

Static electrification of pressboard/oil interface and transient phenomena

Abstract: 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 /spl sim/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.

And space charge in oil-impregnated pressboard under hvdc

Abstract: I Transformer oil a d oil-impregnated pressboard are major insulati g materials in a HVDC (high voltage direct cum nt) converter transformer which provides electrical power transformation between an AC and a DC system. However, the dc component on the alve side of a HVDC converter transformer gives rise to special conditions, regarding space charge for example, for the insulating materialr i .

Streaming electrification of thin insulating pipes under electric field

Abstract: 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.

Construction component used in domestic and commercial buildings

Abstract: The component (1) serves a static and insulation function and includes a plant fibre (9) as well as a filler material (11) which are bonded together under the formation of a solid component with the help of a binding element. The fibre may be provided from straw, reeds and similar plant. At least one of the components' surfaces (3,4) is made from that fibre and at least one of its longitudinal sides (2) is covered by a wooden plate (6) e.g. plywood, pressboard etc..

Measurement of moisture spatial profiles in transformer pressboard

Abstract: Moisture concentration was measured in a 1.5 mm thick transformer pressboard as a function of time and position during moisture diffusion process. A flexible interdigital three-wavelength sensor was used to provide the necessary information regarding pressboard dielectric properties at different depths from the sensor-pressboard interface. Results were obtained by processing the signals from the three-wavelength sensor, air humidity sensor, and thermocouple installed in the experimental chamber. Air-pressboard equilibrium curves were used to provide boundary conditions and the relative moisture level magnitude. The proposed technology has significant potential for measurement of diffusion processes in various dielectrics.

Space charge development measurements inside oil impregnated pressboard

Abstract: Electrification due to oil flow inside transformers induces charge accumulation at the pressboard interface. A part of this charge migrates inside the impregnated pressboard, which leads to a space charge development inside the pressboard. Then this accumulation may be responsible of electrical failure. In order to predict the phenomenon it is important to know the charge evolution and its variation with the temperature gradient and concentration of additives, considered to be used to prevent such failures. An experimental setup has been perfected in which an impregnated pressboard stack is submitted to a potential difference and a temperature gradient. From measurements of potential on special electrodes inserted in the pressboard stack it is possible to obtain the space charge density development. We have made experiments for several potential difference and temperature gradient. The evolution of the space charge density development is observed in terms of time. Then the effect of the Benzotriazol (BTA) concentration has been tested.

Direct measurement of electric field using Kerr electro-optic method in transformer oil/pressboard composite system at DC polarity reversal

Abstract: We measured an electric field in transformer oil/pressboard 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 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. In order to clarify the mechanism, we measured an electric field at DC polarity reversal and at different temperatures in transformer oil. For understanding of these phenomena, we proposed a charge accumulation model considering the difference in charge accumulation rate on the pressboard surface between positive and negative charge.

Influence of additives and pressboard degradation on flow electrification in high power transformers

Abstract: The aim of this work is a better understanding of the flow electrification process occurring in high power transformers. In order to characterize the effect of the pressboard degradation and the additive benzotriazol, the experiments consist in measuring the streaming currents generated by an oil flow through a pressboard sample. Two main results have been obtained: the addition of several ppm of benzotriazol in oil permits to reduce significantly the flow electrification process whereas the degradation of the pressboard by electrical discharges induces a high increase of the phenomenon.

Capacitance change during the aging of a combined system of a solid-liquid dielectric under switching impulse voltages

Abstract: This paper presents the result of an experimental investigation of the arising non-linear phenomena in bakelite and phenol impregnated pressboard specimens (pertinax), immersed in insulating oil, under the application of high voltage impulses. For this purpose, bakelite and pertinax samples were electrically stressed with 250/2500 /spl mu/s switching impulse voltages through a tip-plane electrode assembly. The total electric charge flow through the specimens and the overall capacitance C of the metal tip-solid dielectric-metal plane combination were measured in the high-field regime, the experimental results show an exponential increase of the overall capacitance which may be attributed to the excessive charge flow due to the electron avalanche effect. The aforementioned capacitance involved with the partial discharges depends on the environment temperature (partial discharges enhanced by temperature). Further theoretical consideration leads to a mathematical model which gives an expression for C. The theoretical expectations on the basis of this model are found to be in good agreement with the obtained experimental results.

Transport container for highly enriched uranium (options)

Abstract: FIELD: nuclear power engineering SUBSTANCE: container has drum provided with cover and is placed in this drum with cover locked on it Insulating material is inserted between container and drum Insulating material is made in the form of alternating first layer of fiber pressboard, first layer of plywood, fiber pressboard cylinder open on both ends and placed between container and drum, second layer of plywood, and second layer of fiber pressboard First layer of plywood and first layer of fiber pressboard cover drum bottom Inspection port is provided in container EFFECT: improved design 10 cl, 2 dwg

Static electrification as an evaluation method of thermal aging in transformer oil

Abstract: The experimental results of static electrification of several kinds of thermally aged transformer oil are presented in this paper A pure oil sample and four oil samples in presence of copper, pressboard, silicon steel sheet, copper+pressboard+silicon steel sheet respectively, have been aged at 110/spl deg/C under air condition Static electrification measurement at different aged degree was carried out in a system with the sintered metal filter The changes of static electrification are compared to classical aging indexes, such as loss factor (tan /spl delta/), conductivity (/spl gamma/) and acid number (AN) The experimental results show that static electrification has relationship with thermal aging and the sensitivity of classical indexes for thermal aging process is smaller than changes of static electrification

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 0005 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

Flow electrification phenomena employing co-axial cylinders

Abstract: To date, dozens of failures of large forced oil cooled power transformers have been attributed to flow induced electrification phenomena. The aim of the study is to evaluate the influence of various operational factors on the flow charging characteristics of the oil, by simulating some of the practical configurations. The investigation has revealed that the flow charging of the oil increases with flow rate, ageing and energisation. It also depends on the nature of the oil and the composition, type and surface conditions of the pressboard insulation employed.

Static Electrification Phenomena on Dielectric Materials of SF6 Gas-Insulated Transformer

Abstract: In Japan, large-capacity gas-insulated transformers (275kV, 300MVA) designed to be cooled by flowing SF6 gas have been developed and are operating in underground substations.1 They use insulating plastics with excellent heat resistance and insulation strength, in addition to cellulose materials such as insulating paper and pressboard used in conventional oil-immersed transformers. Such insulating plastics have a high surface resistance and can be electrified through contact with each other, delamination, or friction. Again, the resistivity of SF6 gas is so high that once the insulation material surface is electrified, charges are slow to leak out.