Showing papers on "Pressboard published in 2009"

Ester fluids as alternative for mineral oil: The difference in streamer velocity and LI breakdown voltage

Abstract: Mineral oil has predominantly been used as the insulation and cooling liquid in transformer industry for more than 100 years. However, with the concern of fire safety and environmental impact alternative liquids such as ester fluids are of interest. In fact, these liquids are biodegradable, less flammable and have been used in thousands of distribution transformers world-wide. A trend has been seen in recent years that these fluids have been introduced in power transformers. With the aim at qualifying the dielectric performance of ester fluids for use in medium and large power transformers, the authors' group has carried out research work during the last 5 years. Part of results is reported in this paper. In the investigation, needle/plane geometry was used. A streamer was generated at the needle tip and propagated across the oil gap or along an oil/pressboard interface. Breakdown voltage, polarity effect, streamer velocity and acceleration voltages for transition to fast event were studied. Findings show that the streamer fast event feature in ester fluids was quite different from that in mineral oil especially at positive polarity, where significantly lower breakdown voltage values were observed in ester fluids than in mineral oil over long electrode gap distances, and that the transition voltages to fast event in ester fluids were much lower than those in mineral oil and that fast event streamer velocity in ester fluids increased much more steeply at over-voltages than in mineral oil. Electrode gap distances of 25 mm, 100 mm and 200 mm were tested at LI (lightning impulse) voltages of 1.2/50 μs. Streamer velocity up to 120 km/s was recorded. Results are summarized and analyzed. The work contributes to the understanding of the dielectric properties of ester fluids and mineral oil for the insulation system design of medium and large power transformers.

Partial discharge characteristics over differently aged oil/pressboard interfaces

Abstract: The choice of the materials used as the dielectric media in large transformers continues to be a combination of pressboard and mineral oil. However, a number of failures in the field have been attributed to the surface discharge or creeping discharge over the oil/pressboard interface. This paper reports on an experiment to study surface discharge over oil/pressboard. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD) from inception to flashover. The max PD charge, pattern, frequency and current pulse were measured during the flashover process. Four oil/pressboard specimens with different aging degrees were used and their influence on the flashover process over oil/paper interface was studied in this experiment. The results indicate that the max PD charge and PD frequency increase with the increase of applied voltage. The oil aging has great influence while the pressboard aging has little influence on PD characteristic over oil/pressboard. The PD pattern and current pulse shape show great difference for various applied voltages. The results therefore suggest that the PD detection on the oil/pressboard can be used effectively to implement the early monitor of surface discharge thereby to avoid the formation of surface flashover.

Lightning impulse testing of natural ester fluid gaps and insulation interfaces

Abstract: A significant amount of lightning impulse breakdown and withstand testing has been accomplished in mineral oil to understand this important electrical characteristic for transformer insulation system design. To be considered as viable insulating fluids for high voltage equipment, alternative dielectric liquids should have similar lightning impulse characteristics as compared to mineral oil to provide the clearances necessary for common dielectric design. This paper reviews the testing for establishing the lightning impulse breakdown characteristics of natural ester fluids relative to mineral oil test results. The key variables of the testing included: various oil gap and solid insulation creep distances, electrode configurations, electrical stress characteristics, and solid insulation surfaces. The fluid gaps ranged from 3 mm to 55 mm. The electrode configurations included quasi-uniform in oil gaps, with some attached to pressboard, and nonhomogeneous contacts in oil gaps combined with a phenolic interface. The range in gap distance was selected to reflect the range commonly used in liquid insulated transformer core/coil designs. The electrical stresses used included 1.2 × 50 ?s lightning impulses of both positive and negative polarity. The solid insulation materials consisted of high density pressboard, Kraft paper and a high density phenolic composite. It is concluded that the impulse breakdown voltage of the natural ester fluid is similar to mineral oil for the oil gaps and electrode configurations tested.

Measurement and simulation of partial discharge in oil impregnated pressboard with an electrical aging process

Abstract: The continuous test on oil impregnated pressboard insulation with internal void defect was developed and the phase resolved partial discharge (PRPD) pattern of partial discharge (PD) signals during the electrical aging process was measured. Two different void structures which have different void volume were used in this experiment. It shows that the PD pattern could be classified into five stages and a great diversity in the first four stages is observed. The larger void volume leads to larger PD magnitude. The computer numerical simulation model which is based on a physical discharge process was used and the causes of PD pattern change were interpreted by comparison with computer numerical simulation results. The initial values and change tendency of gas pressure and surface conductivity were determined through experiment. The model parameters in different stages have been studied as well as the insight into the physical changes in the void during electrical aging. The results provide rules for the identification of the electrical aging stage through the partial discharge measurements.

Electrical Double Layer's Development Analysis: Application to Flow Electrification in Power Transformers

Abstract: A charge zone known as the electrical double layer exists at a solid-liquid interface. The liquid flow induces a phenomenon called flow electrification: it generates a streaming current (caused by charge convection) and a rise in the solid's potential (if it is insulated from the ground). These potentials may reach values high enough to produce electrical discharges and cause accidents. Although this phenomenon was identified a long time ago, its physical description remains unknown (i.e., production and displacement of charges, equilibrium, etc.). We have modeled flow electrification phenomena occurring when transformer oil flows through a rectangular pressboard duct. The results of a parametric study made with this model are presented in this paper. The duct's geometry and the materials were selected to compare some of the numerical results to experimental ones. The facility used to obtain these experimental results was developed some years ago as a part of the research program of Electricite de France and the University of Poitiers. Another facility will be designed in the near future with the aim of reproducing surface electrical discharges. The results of this parametric study will be useful for its design.

Space charge behavior in palm oil fatty acid ester (PFAE) by electro-optic field measurement

Abstract: For power transformer insulating oil, we focused on palm oil fatty acid ester (PFAE). It has satisfactory insulating performance, excellent cooling ability and superior biodegradability. Recently, investigations for application of PFAE to environment-friendly power transformers installations have been started. In this paper, the space charge behavior in PFAE was investigated from the measurement results of the electric field by using Kerr electro-optic method under dc voltages applications. For the field measurement, the Kerr constant of PFAE was identified first. Then, the electric field strength and its temporal change were measured in several PFAE / pressboard (PB) insulation systems. The electric field in PFAE was determined by the capacitive distribution at the moment of the voltage application, and decreased rapidly with time and reached the steady state determined by the resistive distribution. The transition time of the capacitive distribution to the resistive one and the strength of electric field in PFAE were much different from those in mineral oil. The influence of oil flow on the time transition of the electric field was also measured. Their differences were discussed with the charge behavior in liquids, based on electro-chemical properties of the oil. We discussed the difference of the flow electrification characteristics between PFAE and mineral oil, and its mechanism was discussed based on the charge behavior in oils.

Breakdown characteristics of converter transformer insulation under composite AC and DC voltage

Abstract: In HVDC transmission systems, the oil and oil-impregnated pressboard insulation of converter transformers is subjected to composite AC and DC voltage. Therefore, a study on breakdown characteristics of this insulation under AC, DC and composite AC and DC voltage is essential. In this paper, breakdown tests on transformer oil and oil-impregnated pressboard insulation under all kinds of those voltages have been completed and the relationship between breakdown peak voltage and AC ratio is discussed. It is shown that the breakdown field strength of transformer oil decreases with the increases of oil gap under any waveform. Breakdown peak voltage of a given oil gap has little change with the variation of AC ratio, but evident inflection point occurs in their relation curve. Breakdown peak voltage of a given oil-impregnated pressboard decreases remarkably with the increases of AC ratio.

Study on dielectric breakdown at DC polarity reversal in oil / pressboard-composite insulation system

Abstract: By using Kerr electro-optic method, charge behavior in oil / pressboard (PB) composite insulation system, which is the basic insulation structure of a transformer, was investigated under DC voltage. Right after the DC voltage application, electric field became the capacitive distribution, and had a strong value which was determined by permittivity of materials. The field strength decreased gradually with time as charge in oil was accumulated on PB. When the voltage polarity was reversed, the applied electric field was superimposed by the electric field formed by the accumulated charge on PB, and very strong field appeared right after the polarity reversal. This could cause the discharge in the oil gap even when the independent applied voltage was insufficient to initiate discharge. It was found out that the critical value of the discharge initiation at the polarity reversal was determined by the breakdown field strength of the oil gap.

Impact of high temperature on partial discharges in oil-impregnated insulation

Abstract: Insulation within an oil-impregnated insulation consists of mainly organic materials including mineral oil, cellulose paper and pressboard. Temperature is a major cause of degradation of the cellulose insulation. Increased temperature accelerates chemical change in the insulation thus reducing its lifetime. In addition to normal temperature operation, transformers are sometimes subject to sudden thermal overload and the effect of this on the insulation has not been fully investigated. In particular the partial discharge (PD) activity has not been well documented. This paper investigates the thermal dependence of PD activity in new and aged paper insulation, using temperatures up to 145 °C. At these temperatures there is substantial gas bubble formation and these bubbles have a significant impact on the PD activity. In addition, the level of PD activity is also affected directly by the temperature itself. The paper provides results of PD monitoring at a range of temperatures with and without bubble formation and discusses these effects on the interpretation of PD levels obtained in either separate source or on-line PD tests of transformers. The physical effect of ageing on oil-impregnated insulation is also considered in this paper.

Temperature and thermal aging effects on the Frequency Domain Spectroscopy measurement of oil-paper insulation

Abstract: Transformer oil-paper insulation system is continuously subjected to thermal stress which is a key factor inducing its deterioration. Dielectric response of oil-paper insulation in frequency domain is being gradually used by power utilities for assessing the condition of transformer oil-paper insulation system, and the Frequency Domain Spectroscopy (FDS) measurement is a promising method. It is important to understand the effects of temperature and thermal aging on FDS results for accurately assessing the condition of oil-paper insulation system. In this paper, the accelerated thermal aging experiments are performed on an oil impregnated pressboard sample at high temperature. Dielectric responses of sample in frequency domain are investigated during the aging process, and at the same aging condition, FDS measurements are repeated at different temperatures. Experiment results show that measurement temperature and thermal aging process significantly impact the FDS results of the oil impregnated pressboard sample in lower frequencies. The dissipation factors (tanδ), real capacitance and imaginary capacitance all increase with the increment of measurement temperature and thermal aging time, especially in lower frequencies.

Fundamental study on partial discharge induced acoustic wave propagation in simulated transformer composite insulation system

Abstract: We aim to establish the advanced diagnostic technique for electric power transformer using online partial discharge measurement. This paper deals with propagation characteristic of acoustic waves emitted by partial discharges, or PDs in a simulated transformer which was composed of pressboard and mineral oil. PDs were generated by a rod-insulator-rod electrode immersed in mineral oil. The acoustic waves were measured with three AE sensors installed on the outside surface of the simulated transformer. PD-induced acoustic waves were also measured through pressboard, with attenuation characteristic of the acoustic waves considered. Numerical simulation allows us to understand the propagation characteristics of acoustic waves.

Heat press plate for hot press

Abstract: The present invention discloses a hot pressboard used in a hot-press. The hot pressboard comprises a board body. A drainage which drains moisture in wet plate to the outside of the board body is arranged in the board body. Concretely, the hot pressboard is provided with a drainage passage at the inside, and a drain hole is arranged between the drainage passage and a working face of the board body, thereby reducing the dehydrating distance, ensuring that the board surface is dehydrated uniformly and that the moisture in the wet plate is drained uniformly during the hot pressing process, and the board has flat surface, causes no warp and is not required the preserving procedure. The hot pressboard saves time and work and improves the production efficiency.

Easy-to-operate bag sealer

Abstract: The item is an easy-to-operate bag sealer that can inflate as well as vacuum, for normal household use, to keep foodstuff fresh. In the front portion of the surface of the body is the bottom frictional pressboard bar that has a frictional surface on one side. Under the cover, facing the frictional surface of the bottom frictional bar is fitted with the upper frictional pressboard bar for vacuuming or inflating. The diagonal frictional force produced by the frictional surface of the bottom frictional pressboard bar and that of the upper frictional pressboard bar is enough to enable the cover to stay closed during the process of realizing the vacuuming and inflating functions. A heat wire is located in parallel to and in the front of the bottom frictional pressboard bar and under the cover is fitted with another coordinating pressboard bar for bag sealing. Also included is an indirect switch to be pressed down to activate the vacuuming and inflating functions control, placed underneath the body, through a hole opened on the body at a corner position formed by the bottom frictional pressboard bar and the surface of the body. Under the cover, at the exact position facing the hole on the body through which the indirect switch is protruding, a cave-like opening is made on the upper frictional pressboard bar or wherever deemed appropriate. The coordinating factor of this cave-like opening and the indirect switch has a procedural function to control the activation of vacuuming and inflating.

Research on streaming electrification of insulation mineral oil

Abstract: As we know, streaming electrification due to oil flow is an important phenomenon in power transformers particularly on those high rated voltage ones, and the study around it is still a challenge. In this paper, the streaming electrification characteristic of insulation mineral oil from Nynas was studied by using an improved mini-static tester to measure the electrostatic charging tendency (ECT) of oils with different aging conditions. The oil samples were aged under 120°C in different periods up to 1350 hours, and together with different contents of oxygen and a certain amount of copper and pressboard. A comparatively better linear correlation is shown between ECT and volume resistivity (ρ v ), dissipation factor (tanδ) and water content, The tanδ and water content are increasing gradually with the aging time prolonged, while ρ v and carboxyl decrease slowly, the acid number does not present evident fluctuation during the whole aging periods.

The relationship between surface partial discharge and carbon trace in oil-impregnated paper insulation

Abstract: In this paper, high voltage, which was raised step by step, was taken on an oil wedge model. Partial discharge (PD) signal was measured by normal IEC 60270 method. And the carbon trace on the pressboard was checked. Using Weibull function, the changes on the type of the PDs, were analyzed. Furthermore, the relationship between the carbon trace and the change of PDs was though about. From the results of the experiments, the PDs changed their type during the high voltage increasing. At the beginning, the PDs were small and produced dim carbon trace. This situation kept on for a long time and the carbon trace gradually extended its range and depth. Then, new type of PDs with very big amplitude appeared just before the breakdown, which we believed produced the breakdown point and due to the breakdown.

Moisture content assessment of transformer solid insulation using return voltage spectrum

Abstract: Dielectric properties of oil-paper insulation transformer are influenced intensively by moisture; therefore, it is imperative to assess integral condition of oil-paper insulation for assuring normal operation of power transformers. As a non-destructive diagnostic technique, return voltage measurements have been introduced for the assessment of water content in transformer pressboard. This paper further investigates the possibilities of assessing moisture contents of power transformer by return voltage method. Field test results performed on several transformers and its interpretation are given. The derived integral moisture and ageing assessment procedures using the return voltage method are discussed. Moreover, the influence of on-the-spot factors on the test result are also detailed and analyzed in this paper, and the approaches to eliminating them were given accordingly. Finally, reliability of the method was assessed by comparing derived moisture contents based on return voltage measurements with dielectric loss. The test results show that this method is reliable and practical.

Measurement and Simulation of Partial Discharge in Disc-void of Oil-pressboard Insulation With Electrical Degradation Process

Abstract: The continuous test on oil-pressboard insulation with internal disc-void was developed and the phase resolved partial discharge pattern (PRPD) of partial discharge (PD) signals during electrical degradation process was measured. It shows that the PD pattern could be classified into five stages and great diversity in first four stages is observed. The computer numerical simulation model which is based on a physical discharge process was established. The causes of PD pattern change were interpreted by comparison to computer numerical simulation results. The model parameters in different stage have been studied and insight into the physical changes in the void during electrical degradation. The results provide rules for the identification of the electrical degradation stage by the partial discharge measurements.

Short-circuit resistant power transformers - prerequisite for reliable supply of electrical energy

Abstract: SUMMARY The ability to withstand short-circuit events ("thr ough faults") in the transmission system is an important feature of power transformers. This paper summarizes several important issues related to the construction and operation of transformers in this respect. General information is given about the selection of suitable pressboard insulation. The su rface of the pressboard can be mechanically treated for obtaining optimum characteristics. Processing t he transformer active part in the right way in the factory is necessary for obtaining robust, well cla mped windings. The processing steps, i.e. the sizin g of the windings, the drying, the clamping and the s ubsequent oil impregnation, are discussed by means of practical examples. Finally, some basic consider ations are given regarding the maintenance of transformers in operation.

Plywood hot-pressing buffer and processing method thereof

Abstract: The invention provides a veneer hot-pressing cushion and a method for processing the same, which belong to veneering manufacturing and particularly relate to a hot-pressing cushion used between a hot pressboard of a press and a pressboard in a veneer hot-pressing process and a method for processing the same. The cushion is characterized by being formed by mixing and weaving metal wires and elastic polymer synthetic fibers. The invention provides an ideal veneer hot-pressing cushion which has excellent heat conductivity and heat resistance, good elasticity and high strength, provides excellent strain recovery capacity and adaptability under a hot-pressing condition to make sure no gap exists between a pressed blank body and the pressboard, and gives full play to the pressure and heat provided by the press. Meanwhile, the invention also provides a method for processing the veneer hot-pressing cushion, which allows for simple equipment, convenient operation and low investment. The invention successfully solves the problem that the prior plastic gasket or felted wood plate cushion has low strain recovery capacity and poor heat conductivity and leads to undesirable quality of the veneer products.

Heat conducting oil heating press

Abstract: The utility model discloses a heat-conducting oil hot press which comprises a stand (1); a hollow hot pressboard (2) is arranged in the stand (1); the hot pressboard (2) can moves up and down in the stand (1); an oil delivery pipe (3) is connected on one side of the hot pressboard (2); an oil return pipe (4) is arranged on the other side of the hot pressboard (2); and the oil delivery pipe (3) and the oil return pipe (4) are respectively connected with a heat-conducting oil temperature machine (5). The hot press uses the oil pipes to deliver heat-conducting oil to the hot pressboard and heat the hot heat-conducting oil, and the heat-conducting oil conducts thermal circulation in the heat-conducting oil temperature machine, so that traditional steam heating is replaced. And the hot press has the advantages of quick heating up, even temperature distribution, simple structure, easy operation and the like.

Insulating paper board bending machine for transformer

Abstract: The utility model provides an insulated pressboard bender for a transformer, belonging to the manufacture technical field of transformer processing equipment. The insulated pressboard bender mainly comprises a bracket (4) with a fixed plate (2), a regulating plate (3) fixedly connected on the fixed plate (2) through a location-limited bolt (5), a pressure plate (8) with a handle (1) arranged at the upper part of the fixed plate (2), a location-limited plate (7) arranged between the pressure plate (8) and the fixed plate (2), a regulating groove (10) arranged on the regulating plate (3), a graduated scale (6), and the like. The utility model has reasonable whole structure design, and can efficiently enhance the product quality and the working efficiency. A processed product is used for an insulating part of the transformer such as a phase partition, an iron core pulling plate, and the like. Through practical application, the utility model can save the processing time by more than 50 percent.

Electric field interaction in the inter-phase region of transformers

Abstract: Large three-phase power transformers normally consist of three cylindrical coil sets (phase windings) assembled side by side in line. The high voltage winding of each phase is generally the outermost coil. The space between the phase windings is called the inter-phase region. Insulating sheets of pressboard, are placed between the phase windings to form dielectric barriers to prevent flashover between the phases. The insulation design is often a multi-layer arrangement of boards and oil gaps. A small number of transformers have been affected by the phenomenon of creeping discharge on the surface of the pressboard barriers. The mechanism of this phenomenon is not well understood. The paper describes the test facility which has been developed to model the inter-phase barrier region. Two independent electric fields are used to re-create the electric field profile in the barrier region. A discharge source is introduced into the resultant electric field and applied onto the pressboard surface to explore the effect of the electric field, both along and perpendicular to the barrier, on the discharge. Initial results show that the magnitude and density of discharge is affected by the phase relationship between the interacting fields.

Establishment and application for diffusion model of moisture in drying process of pressboard

Abstract: In this paper, a capacitive sensor is used to monitor the dielectric response of the pressboard and to calculate the moisture content in the pressboard by using calibrated expressions. The structure of the capacitive sensor and experiment system are introduced. The kinetics of the drying process is discussed. Models of moisture content diffusion and application of the drying kinetics of pressboard and of multilayer pressboard are presented, and the rate of the series convergence of models is high and accuracy is suitable, which provide a theoretic basis for the on-line moisture measurement in transformer industry. Based on the diffusion equations and their initial and boundary conditions, the distribution of moisture is calculated during the experiments, and the models developed for mass transfer inside the pressboard is validated for a one-dimensional case and a two-dimensional case. The result is helpful in reasonable design of production technology.

PD distribution in aged oil impregnated insulation using non sinusoidal voltage supply

Abstract: This paper presents an investigation into PD distributions of oil-impregnated aged pressboard insulation system using non-sinusoidal voltage supplies. The aim of this experimental investigation was to study the impact of the non-sinusoidal voltage conditions on the ageing rate of the insulation, particularly in aged material.

Influence of Temperature on Partial Discharge Properties in Disc-void of Oil/pressboard Insulation

Abstract: Partial discharge based risk assessment method has rapidly developed as an effective diagnosis method in oil/pressboard insulation system.The temperature in field power transformer is variable according to loading and the PD properties in insulation are also variable in different temperature.In order to assess the influence of temperature on partial discharge properties in oil/pressboard insulation system,a partial discharge testing system with temperature adjusting function was adopted in this paper.The partial discharge data obtained from the experimental work were presented in phase-resolved PD(PRPD) pattern analysis and pulse sequence analysis(PSA).The single pulse in time domain at different temperature is also analysed in this paper.

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 as 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 to 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. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(3): 10–19, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20789

Mechanisms and testing of moisture content measurement of transformer pressboard

Abstract: In this paper, the method to measure moisture content of oil-free pressboard with single frequency experimental method is studied and the paper focuses on the implementation of experimental studies based on dielectrometry. Accurately monitoring and controlling of moisture can be implemented by the measurement of the permittivity of pressboard. A capacitive sensor is used to monitor the dielectric response of the pressboard. The effects of frequency and temperature to the signal of capacitive sensor are analyzed. The models of effects of frequency and temperature on signal are developed with studying of the dielectric polarization and dielectric loss of pressboard. A universal expression relating permittivity and moisture content of oil-free pressboard is developed to calculate the moisture concentration in the pressboard by fitting the moisture data obtained from the drying experiments to the dielectric properties of pressboard. Real-time on-line measurement of the moisture diffusion process is achieved in this study and the moisture profile in transformer pressboard is non-destructively estimated.

Research on moisture content analysis of insulation pressboard in oil-paper transformer using polarization spectrum

Abstract: The insulation fault diagnosis of power transformers is important to guarantee the safe and stable operation of the power system.Moisture and aging degree of oil-paper insulated power transformer has a sure influence on its insulation property.The water content in insulation pressboard is considered as an important indicator of insulation quality and the aging condition.Although chemical analyses can provide direct information on parameters,most chemical analyses must be performed under laboratory conditions and for some chemical analyses paper samples are needed.As a non-destructive diagnostic technique,return voltage measurements based on dielectric response theory have been introduced for condition assessment of transformer insulation,especially for the evaluation of water content in transformer pressboard.The study presented in this paper aims of further investigating the reliability of using the return voltage method to assess solid insulation of power transformer.The field test results show that this method is practical,which can estimate the water content in insulation paper using its polarization spectrum and dominant time constant.