Showing papers on "Pressboard published in 2007"

Dielectric response of mineral oil impregnated cellulose and the impact of aging

Abstract: Dielectric response is a non-invasive diagnostic method with a potential to give information about the status of the solid insulation in a power transformer. To investigate what conditions the method can and cannot reveal, frequency domain spectroscopy has been applied to oil-impregnated paper and pressboard with and without moisture and/or acids added, and to paper at various stages of aging. Water and carboxylic acids are important aging indicators. It is found that moisture is clearly detectable. Also low molecular weight carboxylic acids will affect the dielectric response in a way similar to moisture, but high concentrations are needed for a contribution comparable to that of moisture. It is difficult to tell moisture and acids apart from the responses obtained. High molecular weight carboxylic acids make little or no contribution to the dielectric response of the solid insulation in a paper/pressboard - oil system. It was also observed that keeping paper or pressboard at elevated temperatures had a conditioning effect on the response

Identification of a chemical indicator of the rupture of 1,4-β-glycosidic bonds of cellulose in an oil-impregnated insulating paper system

Abstract: In this study, headspace gas chromatography/mass spectrometry has been used to assess the volatile by-products generated by the ageing of oil-impregnated paper insulation of power transformers. Sealed-glass ampoules were used to age under oxidative conditions 0.5-g specimens of insulating paper in 9 mL of inhibited mineral oil in a temperature range of 60–120 °C and moisture of 0.5, 1 and 2% (w/w). A linear relationship between one of the oil-soluble degradation by-products, i.e. methanol, and the number of ruptured 1,4-β-glycosidic bonds of cellulose, regardless of the type of paper (ordinary Kraft or thermally-upgraded (TU) Kraft paper), was established for the first time in this field. Ageing at 130 °C of model compounds of the Kraft paper constituents (α-cellulose, hemicellulose and lignin) and two cellulosic breakdown by-products (D-(+)-glucose and 1,6-anhydro-β-d-glucopyranose) confirmed that the α-cellulose degradation was mostly responsible for the presence of this molecule in the system. Furthermore, additional 130 °C-tests with six different papers and pressboard samples under a tight control of initial moisture indicated that at least one molecule of methanol is formed for each rupture of 1,4-β-glucosidic bond of the molecular chains. Stability tests showed that the ageing indicator is stable under the oxygen and temperature conditions of open-breathing transformers. The presence of methanol was detected in 94% of oil samples collected from over than 900 in-service pieces of equipment, confirming the potential for this application. Lastly, the tests have shown that oil-oxidation by-products and TU-nitrogenous agents modify the methanol partitioning coefficients in the paper/oil/air system, which makes their study essential over a range of field conditions encountered by power transformers. Results are presented and discussed in comparison with 2-furfuraldehyde, which is the current reference in the domain.

A Comparative Study of the Dielectric Strength of Ester Impregnated Cellulose for Use in Large Power Transformers

Abstract: It is essential to ensure that the dielectric strength of an oil-cellulose insulation structure is not reduced when using esters to replace mineral oil in power transformers. One advantage proposed to using esters is that their higher dielectric constants, when compared to mineral oil, provide a better match to the dielectric constant of the impregnated cellulose. This results in the electric stress being reduced in the oil and increased in the cellulose, beneficial as the oil tends to have the lower dielectric strength. This paper reports the ac and impulse dielectric strengths of ester impregnated paper and pressboard. It was found that the ac withstand voltages of cellulose impregnated with ester are at least equal, or better, than cellulose impregnated with mineral oil. The lightning impulse dielectric strengths of cellulose impregnated with ester were found to be comparable to mineral oil. An analysis is given on the breakdown process of cellulose, as cellulose can fail due to either a direct puncture or discharges created in the oil wedge.

Dielectric properties measurements of transformer oil, paper and pressboard with the effect of moisture and ageing

Abstract: Dissipation factor (tan delta) and capacitance measurement at power frequency (50 Hz) has been commonly used to diagnose the insulation integrity and condition of transformers. The main drawback of this technique is that the measurement results obtained represent the status of the "complete" transformer insulation systems - a combination of the bulk of oil, paper insulation and pressboard. The contribution of each individual insulation material to the end result is not well known, and the degradation status of the whole insulation structure and each material are difficult to separate. In order to improve comprehension of the measurement results of this technique, the basic relationship between tan delta and each insulating material has to be established first. Hence, in this paper, experimental investigation has been done by studying the dielectric properties changes (tan delta and dielectric constant, epsivtau) of mineral oil, cellulose paper and pressboard with the affecting parameters, namely moisture and ageing. A simulation model of the combined transformer insulation has been used to correlate the tan delta of each insulating material to the measured combined value of a transformer.

Water Absorption of Cellulosic Insulating Materials used in Power Transformers

Abstract: The combination of cellulosic paper and pressboard and mineral oil is still the most efficient and economic way to insulate power transformers. One of the drawbacks of cellulose is its highly hygroscopic nature. The hygroscopicity is caused by the cellulose molecule itself, whereas the rate of water absorption is influenced by many parameters. Thin, low-density papers exposed to a warm and humid climate show a much higher rate of water absorption than thick high-density pressboards exposed to a cold and dry environment. Impregnating cellulosic insulation materials with oil slows down the rate of water absorption, yet the moisture content at equilibrium is the same as for non-oiled materials. Tests for the determination of the sorption isotherms showed that data published in 1960 could be matched very well. But the tests also showed that the raw materials have a significant impact on the sorption curves.

Pre-breakdown characteristics of contaminated power transformer oil

Abstract: In this paper we have studied pre-breakdown characteristics of transformer oil in the presence of different levels of contamination. The contaminant is fibrous dust from pressboard insulation used for high voltage transformers. The contamination level investigated in the present study ranges from 0.0025 to 0.0075% by weight. The conduction current under dc voltages were recorded and bridging effect was monitored using optical images. It has been found that the conduction current typically increases with the applied voltage, the duration of the voltage application and the level of contamination. Optical images revealed that bridging occurs when oil is subjected to the voltage and bridge dimension increases with the contamination level and the duration. The current flowing between the two spherical electrodes is closely related to the bridging dynamics.

Effect of Dielectric and Magnetic Material Characteristics on Frequency Response of Power Transformers

Abstract: Frequency response analysis (FRA) technique has become increasingly popular for the assessment of mechanical integrity in power transformers. This method is based on the fact that the shape of the frequency response at higher frequencies is uniquely determined by the geometrical construction of the transformer. However, characteristics of the insulation system (pressboard, paper and oil) and the laminated ferromagnetic core may influence the distributed inductances and capacitances, resulting in deviations of the frequency response. The main goal of this thesis is to investigate the influence of properties of the core and insulation materials on the frequency response of power transformers. To be able to realize this goal, a small-signal high frequency model based on lumped circuit approach, of which all the circuit parameters are calculated by considering the frequency dependent phenomena related to transformer insulation and core materials, is developed. Capacitances and conductances are calculated based on analytical expressions derived from approximated geometrical parameters and measured frequency dependent permittivities of paper and pressboard. Hierarchical multi-scale modelling approach is adapted for the purpose of calculating inductances and resistances, including anisotropic properties of the core and diamagnetic properties of the windings. Complex permeability of silicon steel laminations, which is a vital input parameter in inductance calculations, is estimated using an optimization algorithm as well as by means of a single sheet tester. The developed circuit model is implemented in MATLAB as a state space model, which is used to quantify the influence of insulation condition on the frequency response and to explore a possibility of using FRA technique for characterization of the transformer insulation. According to the model prediction, the moisture content in transformer insulation can influence the response most, since it modifies both the real part of dielectric permittivity and the losses. Measurements were performed on two transformers at different magnetization levels to show effects of remanent magnetization in the core due to removal of 3-phase supply, relaxation demagnetization and DC flux on the frequency response of winding impedance. It was found that the low frequency response (<10 kHz) is affected by magnetic viscosity yielding relaxation of permeability over a long period of time. Therefore, to avoid the effects of core magnetization, diagnostic FRA measurements on power transformers should not be performed directly after disconnection from the network.

Investigation of the impregnation of cellulosic insulations by ester fluids

Abstract: This paper confronts the practical problems associated with ester fluids, due to their higher viscosities, for the impregnation of cellulosic insulation systems used in power transformers. The impregnation mechanism by an insulating fluid was studied using an analytical model and then laboratory tests were carried out to investigate the impregnation process of cellulosic materials in the forms of paper, pressboard and laminated pressboard blocks by various insulating fluids. The impregnation of both papers and pressboard barriers brings no practical problems due to their thinness; however the impregnation procedure for thick laminated pressboard blocks by esters needs to be established. It was shown that by increasing the impregnation temperature to 60degC, the viscosities and the capillary action of esters can achieve a similar impregnation to that of mineral oil at room temperature and consequently a full impregnation can be achieved for the ester filled power transformers in the same length of time.

Kerr electro-optic field measurement in palm oil fatty acid ester transformer insulation system

Abstract: A palm oil fatty acid ester (PFAE) has good insulation performance, excellent cooling ability and superior biodegradability. Recently, investigations for application of PFAE to power transformers have been started. In this paper, the electric field in PFAE was measured in situ by using Kerr electro-optic method under dc voltages application. At first, Ken-constant of PFAE was identified. Then, electric field strength E and its temporal change were measured in several PFAE / pressboard (PB) insulation systems. Electric field E in PFAE showed the capacitive distribution at the moment of dc voltage application, and decreased rapidly with time and finally the resistive field distribution was obtained. The transition time of the capacitive distribution to the resistive one and the value of E in PFAE were different from those in mineral oil. Their differences were discussed with the charge behavior in liquids, based on electro-chemical properties of the oil.

Novel charge generation model for simulation of streaming current based on shearing stress at the oil/pressboard interface

Abstract: Although the streaming electrification phenomenon in oil-Ailed transformers has been studied intensively since the 1970's, no simulation model or quantitative computation method is yet available for the streaming current, i.e. the electrical current caused by streaming electrification. We propose a novel charge generation model which makes it possible to numerically calculate the streaming current. In our model, oil flow velocity at the interface is approximated by the shearing stress at the oil/pressboard interface, which is an essential parameter in fluid dynamics. Combining the approximated velocity of the oil with the Debye model for positive charge distribution, we derive that the local streaming current is proportional to the shearing stress. Measured streaming currents were found to be proportional to the calculated shearing force for different duct structures and for different oil flow velocities, showing that the proposed model can be used to estimate the streaming current for a given duct structure in an oil-filled transformer.

Increase of Static Electrification in Aged Transformers

Abstract: Increase in static electrification and discharge generation was experienced during the service operation of aged power transformers of both shell and core-form. However, the electrical characteristics of the oil did not demonstrate increase in static electrification. The investigation on the aged oil by measurement of electrostatic charging tendency suggested the possibility that some deteriorated compound in oil was adsorbed to the pressboard and increased the static electrification. However, the measurement of electrostatic charging tendency with the aged pressboard did not demonstrate enough increase, whereas accumulated charge measurement of the aged pressboard confirmed strong increase in static electrification. On the other hand, the measurement of accumulated charge by potential measurement confirmed strong increase of charge accumulation by the pressboard aging. It was suggested that measurement of accumulated charge was suitable for the assessment of static electrification in aged transformers.

Creep stress failure in high voltage transformer interwinding insulation

Abstract: Three phase transformers have an insulation system which is a complex three dimensional structure of paper and pressboard surrounding the conductors immersed in a large volume of mineral oil. In core form transformers, a key insulation area is the region between adjacent phases which is often reinforced with vertical pressboard barriers. At the higher voltages, these pressboard barriers provide a crucial element in the insulation structure between the phases. The barriers are subjected to large electrical and thermal stresses. The pressboard barriers sit in the time varying electric field pattern generated from the transformer winding coils which are operating with 120deg phase difference. The electric field can be resolved along the surfaces of the pressboard and this is termed creep stress or tangential stress. This electric stress can lead to surface tracking along the oil-pressboard boundary layer. A large scale test facility has been developed to investigate the effects of creep stress with the goal of understanding the onset of surface tracking on the pressboard. This paper details the main features of the test platform and control equipment. Finally, the paper includes some initial results of experiments conducted to detect partial discharge under two temperature conditions.

Modelling of Oil-Paper Insulation Layers in the Frequency Domain with Cole-Cole-Functions

Abstract: The measurement of oil-paper insulation systems in the frequency domain provides novel diagnostic methods for quality control of materials and high- voltage power transformers. The first task is to measure and to analyse the dielectric behaviour of oil-paper insulation systems and their dependency on material quality, moisture content and ageing products in oil, paper and pressboard. Furthermore, the geometrical layout of power transformers and the type of modelling are very important for the diagnostic. Every medium has its own special complex capacitance CMedium in frequency domain that can be simulated with a Cole-Cole function with conductivity. The behaviour of the power transformer can be calculated from the parallel connexion and the series connexion of the individual microscopic capacitances CMedium. The quantitative usage of the insulation layer model requires small parasitics on the boundaries, homogeneous materials of the individual layers and a uniform field. The paper describes the models of oil, paper and press- board with different values of parameters like temperature and moisture. This is the basic model for the power transformer. It describes a method of generating the different power transformer models with parameters like temperature, moisture etc. and a technique to control and show the error of measurement.

Internal temperature pressure measurement device of artificial board and thermal pressure board flan

Abstract: An internal temperature, pressure measuring equipment for man-made hot pressboard, belongs to timber industrial technical field. At present, two kinds of equipment are used to measure the internal temperature, pressure of somewhere in the hot pressboard. The utility model comprises a pressure sensor (6), a temperature sensor (4), a connecting link (3), a pressure-directing capillary (2), a signal-processing and data-collecting box (7), and a microcomputer (8), wherein the back end of the connecting link (3) is connected with the pressure sensor (6) while the front end of the connecting link (3) is connected with the pressure-directing capillary (2), and the pressure sensor (6), the temperature sensor (4) are respectively connected with the signal-processing and data-collecting box (7) which is connected with the microcomputer (8) to collect the temperature, pressure information. The utility model assembles the pressure sensor and the temperature sensor together to realize the accurate simultaneous measurement of the temperature, pressure of somewhere in the board plank, and the utility model is convenient to operate, maintain and repair.

A simulation method for streaming electrification by direct current field analysis

Abstract: A simulation method for evaluating potential and electric field distribution in an oil-filled transformer was developed. Potential distribution around the entrance of the oil duct should be carefully considered in view of the risk of static discharge. A large potential may be built up by negative charge current through highly resistive insulators due to charge separation caused by streaming electrification at the pressboard (PB) surface. At steady state in terms of both fluid dynamics and electrical conduction, a flow of the negative charge can be regarded as a constant current source distributed over the PB surface that flow into grounded conductors through resistive materials such as the solid/liquid insulation of PB and oil. As will be presented in another paper in this conference by the authors, the local streaming current can be obtained from the local shearing stress calculated by a fluid dynamics simulator. Using the wall current distribution calculated from the shearing stress model as a distributed current source, the electric field and potential distribution were calculated by a direct current field simulator.

Flat hoister of hot pressed board and overturn combined hoisting tool

Abstract: The utility model relates to a hanging tool of flat plate type, particularly relates to a combined hanging tool used for flat hanging and turning over of hot pressboard. In past days, we need to drill hanging technical screw holes on two sides of hot pressboard and then hang it directly with cable wire, which sways to-and-fro, thereby it is very dangerous and apt to make damages to processed surfaces. The technical plan of the utility model has changed original hanging with soft cable wire into hanging with steel frame structure, which consists of arch board, beam, flat hanging board, hanging hook, turning board and channel steel. It makes use of frame structure, hanging hole centre of turning board which is lower to gravity center of hot pressboard, and structure of arc board to easily carry out flat hanging and turning over of hot pressboard. The utility model is characterized in that the structure of the tool is simple, it is easy to be manufactured, the versatility is good, it can be used smoothly, safely and reliably, it can prevent hot pressboard from distortion because of being pressed, and from damaging processed working surface.

Increase of Static Electrification in an Aged Oil-Immersed Transformer

Abstract: Increase of static electrification activity and discharge generation was experienced during the service operation of an aged power transformer. The transformer oil had high sulfide and sulfoxide contents and was liable to show high ECT value after aging. However, there was no apparent increase in standard ECT (electrostatic charging tendency) measurement of the insulating oil and, therefore, deterioration of pressboard was suspected for the cause of the increase. The ECT tests with aged oil and pressboard sampled from the aged transformer were insufficient to demonstrate the increase in static electrification for the aged transformer. On the other hand, the results suggested a possibility that increase in static electrification can be caused by adsorption of some deterioration compound in the aged oil to the pressboard. In order to make more effective evaluation on the transformer insulation, potential measurement on the pressboard in the oil duct was carried out. The experimental results demonstrated strong increase of the accumulated charge by the aged pressboard, which suggested effectiveness of potential measurement for the assessment of static electrification in transformers. The device of potential measurement for practical use was designed by reducing the necessary amount of oil sampling.

Press type multilayer gum-dipping chipboard production device

Abstract: The utility model relates to a press type multilayer gum-dipping chipboard production device, which comprises a frame, a hydraulic unit, an upper laminated board, a middle laminated board, a connector, a lower laminated board, a mother plate, a buffer and a hot pressboard. A plurality of laminated boards are provided on the frame, wherein the lifting and pressing movement of these laminated boards is controlled by the hydraulic unit. The upper laminated board comprises the hot pressboard, the buffer and the mother plate. The buffer and the mother plate are arranged under the hot pressboard sequentially. The middle laminated board comprises the hot pressboard, the buffer provided upon the hot pressboard and the mother plate provided under the hot pressboard. The upper laminated board and the middle laminated board are capable of being driven to do lifting movement by the force from the hydraulic unit. The lower laminated board is provided on the bottom of the frame. At least a middle laminated board is provided between the upper laminated board and the lower laminated board, wherein the upper laminated board is connected to the middle laminated board via the conector. The utility model improves working efficiency, greatly decreases energy consumption and cuts down produce cost.

Effect of oil or pressboard paper barriers on negative corona discharge

Abstract: In this work some results related to the effect of insulating barriers on corona discharge are reported. It was clearly established that both the amplitude and the shape of the currents are strongly affected by the presence of organic barriers. The insulating barriers used in this work were constituted by layers of oil or pressboard paper with different thickness. The experiments were carried out in negative point- to-plane corona. The aim of the dc voltage, used in this work, was an examination of the I-V characteristics in order to determine the modifications in the discharge which can be related to electrochemical interaction involving activated species.

Optical projecting system

Abstract: The invention discloses optical projection system. It includes one carriage, one bracket, one vision modulator, one circuit board, one pressboard, and one deformable member. The bracket is set many connecting members each of which respectively has the first and second ends. The vision modulator is set between carriage and the bracket, has the back, forms electrical property connection with each the first end. The pressboard has one undersurface used to fix the circuit board on the carriage. The deformable member is stuck at the undersurface of the pressboard. When the pressboard fixes the circuit board on the carriage, the deformable member is deformable to make circuit board fully contact with the second end of the connecting member to form electrical property connection.

AComparative StudyoftheDielectric Strength ofEster Impregnated Cellulose forUsein LargePowerTransformers

Abstract: Itisessential toensure thatthedielectric strength ofanoil-cellulose insulation structure isnot reduced whenusing esters toreplace mineral oil inpower transformers. Oneadvantage proposed tousing esters is that their higher dielectric constants, whencompared to mineral oil, provide a better matchtothedielectric constant oftheimpregnated cellulose. Thisresults inthe electric stress being reduced intheoilandincreased in thecellulose, beneficial astheoiltends tohavethelower dielectric strength. Thispaper reports theacandimpulse dielectric strengths ofesterimpregnated paperand pressboard. Itwasfound that theacwithstand voltages of cellulose impregnated withester areatleast equal, or better, thancellulose impregnated withmineral oil. The lightning impulse dielectric strengths of cellulose impregnated withester werefoundtobecomparable to mineral oil. An analysis isgivenonthebreakdown process ofcellulose, ascellulose canfail duetoeither a direct puncture ordischarges created intheoilwedge.