Showing papers on "Pressboard published in 2008"

Dielectric spectroscopic measurements on transformer oil-paper insulation under controlled laboratory conditions

Abstract: For reliable operation of power transformers, the condition of the insulation system is essential. This paper reports on a detailed study of the effect of ageing, temperature and moisture on frequency and time domain spectroscopic measurements carried out on oil-impregnated pressboard samples as well as on a distribution transformer under controlled laboratory conditions. Because field measurements are generally performed after de-energizing the transformer, extreme care is required in interpreting the results due to inherent temperature instabilities. To avoid large thermal variations that may affect the results, a customized adiabatic room was built around the transformer for measurements above the ambient. Capacitance ratio and direct current conductivity deduced from the spectroscopic measurements, helped to interpret the data. Because, low frequency measurements techniques are time consuming, alternative to a transfer of time domain data into frequency domain data was investigated.

Thermal aging micro-scale analysis of power transformer pressboard

Abstract: In order to analyze the thermal aging mechanism of the insulation paper inside the power transformer, a series of accelerated thermal aging tests were performed on pressboard. Subsequently, the atomic force microscope (AFM) together with scanning electron microscope (SEM) and X-ray diffraction (X-RD) were utilized to observe the micro surface of the thermal-aged pressboard. The experiments and analysis indicate that either the links among the D-glucopyranose units or the hexagonal mesh structures of the D-glucopyranose units were broken under thermal stress; the number of D-glucopyranose units after 6 weeks of aging was 0.8-1 per nm2, only about one third of un-aged value. The wall of a cellulose cell was deteriorated and thinned by thermal stress. At the same time, the cracks expanded gradually on the surface of the cellulose, which shortened the average width of cellulose fiber from about 40 mu of un-aged sample to about 25 mu after 6 weeks of aging. Meanwhile, the relative crystallinity and the size of the crystallite in the pressboard decreased nonlinearly with the thermal aging time.

A Comparison of the Impregnation of Cellulose Insulation by Ester and Mineral oil

Abstract: Ester oils have been successfully used in distribution transformers for many years as satisfactory substitutes for mineral oil. In high voltage transformers however their application has been much slower partly due to the fact that the high viscosity of ester oil and hence uncertainties in efficient impregnation of cellulose insulation. In this paper the characteristics of the impregnation of cellulose insulation by ester oils are assessed and compared with mineral oil. Studies are carried out on thick laminated pressboard blocks, typical of support insulation structures in high voltage transformers. The degree of oil impregnation with time at various temperatures is examined. It is shown that the factors governing the impregnation processes with ester and mineral oils are very similar, i.e. mainly viscosities and surface tensions, and if a higher impregnation temperature is used for ester oils the same degree of oil impregnation can be achieved as with mineral oils.

A fast and reliable dielectric diagnostic method to determine moisture in power transformers

Abstract: This paper describes a novel dielectric diagnostic method ldquoDIRANArdquo which features a fast measurement methodology and an improved analysis algorithm for determining moisture in pressboard and paper of oil-paper-insulated transformers. Dielectric response methods provide substantial advantages compared to moisture determination by conventional equilibrium diagrams, for example it is unnecessary to wait for moisture equilibrium. For the new approach, combination of time and frequency domain measurements substantially shortens the measurement time, typically to 25 % compared to a pure frequency domain measurement. The data analysis algorithm compares the measured dielectric properties of the actual power transformer to modeled dielectric properties. The software features a compensation for the influences of conductive aging byproducts. Conductive aging byproducts have similar dielectric properties as water; without compensation the analyzed moisture content will be too high, leading to unnecessary actions like drying. This paper also includes examples of practical onsite measurements in comparison to other measurement methods and a guide for assessing the obtained moisture content.

Breakdown strength of solid dielectrics in liquid nitrogen

Abstract: The objective of this research was to determine if a relationship could be found between dielectric strength and other properties of electrical insulating materials in the medium of liquid nitrogen on an empirical basis by using variables predicted by basic theory. A simple equation of the form E=A+Blog (rhov/xir tandelta) to predict the dielectric strength of a solid insulating material in liquid nitrogen medium has been proposed. The equation requires the values of volume resistivity (rhov), relative permittivity (xir) and loss tangent (tandelta) in the medium of liquid nitrogen, which may be obtained easily by low voltage non-destructive measurements. The values of electric strength calculated using this equation for Crepe paper, Kraft paper, varnished paper, pressboard, presspahn, mica, bakelite and asbestos are quite in agreement with the experimentally measured values. It is expected that the equations obtained will help the designers as a handy tool for quick estimation of breakdown strength of solid dielectrics dipped in liquid nitrogen.

Moisture and Aging Effect on the Creepage Discharge Characteristics at the Oil/Transformer-board Interface under Divergent Field

Abstract: The creepage discharge on pressboard barriers is regarded as one of the failure modes for power transformers. In literature, extensive research has been carried out previously to investigate the nature of creepage discharges, while little work was done to study the creepage discharge on the aged transformer board. A literature survey will be given first to summarise the understanding of creepage discharges on cellulose/oil interfaces. The effects of the aging products like moisture and absorbed sediments on the characteristics of creepage discharges will be investigated.

A new approach to the study of surface discharge on the oil-pressboard interface

Abstract: The materials used as the dielectric media of choice in large transformers continue to be a combination of pressboard and mineral oil. These materials offer a cost effective solution and are backed by a historical knowledge base of performance data and lifetime characteristics. A proven method to prevent inter-phase flashover is to use successive layers of pressboard and oil gaps. However, a number of failures in the field have been attributed to surface discharge or creeping discharge over the pressboard surface. This paper reports on an experiment to study surface discharge on pressboard in oil. A new technique for the generation of repeatable surface discharge is outlined. The paper explains the differences between surface tracking, and surface flash-over. The results indicate that the tracking is the result of surface discharge and that surface discharge and surface flash-over are two distinct phenomena.

On electric stresses at wedge-shaped oil gaps in power transformers with application to surface discharge and breakdown

Abstract: The problem of a high-voltage applied to a wedge-shaped oil gap in power transformers is considered as far as induced distortions and tangential electric stresses are concerned Near-tip distortions and electric stress concentration occur due to the essentially difference in the relative permittivity between the insulating oil and the pressboard When the magnitude of tangential electric stress intensity reaches a critical threshold value, surface discharge and breakdown occurs at the oil/pressboard interface In this work, a simplified computation model is developed for the quantitative analysis of the distribution of electric stress and the breakdown electric field strength in the wedge-shaped oil gap in power transformers The detailed computation results on the effects of pressboard permittivity and thickness on the surface discharge inception voltage are presented It shows that the surface discharge inception voltage is a function of pressboard permittivity and thickness; and the value of surface discharge inception voltage decreases with increasing dielectric permittivity

Partial discharge patterns in oil-impregnated paper and pressboard insulation at high temperature

Abstract: The power transformer is the most expensive and arguably the most important equipment item in a power system. Insulation failure in transformers can cause long term interruption to supply and loss of revenue and the condition assessment of the insulation is thus an important maintenance procedure. Oil-impregnated transformer insulation consists of mainly organic materials including mineral oil and cellulose-base paper and pressboard. The operating life of cellulose-based insulation, as with most organic insulation, depends heavily on its operating temperature rise above ambient. This paper reports results of a laboratory-based experimental investigation of partial discharge activity at high temperature in oil-impregnated insulation. The experiments reported here are part an on-going programme aimed at investigating the way in which insulation deterioration can be monitored and quantified by use of partial discharge diagnostics. Partial discharge patterns were recorded and analyzed during increasing and decreasing phases of the temperature. The effect of ageing of the insulation on the PD patterns in oil-impregnated insulation is also considered.

Classification of Acoustic Emission Based Partial Discharge in Oil Pressboard Insulation System Using Wavelet Analysis

Abstract: Insulation used in transformer is mostly oil pressboard insulation. Insulation failure is one of the major causes of catastrophic failure of transformers. It is established that partial discharges (PD) cause insulation degradation and premature failure of insulation. Online monitoring of PDs can reduce the risk of catastrophic failure of transformers. There are different techniques of partial discharge measurement like, electrical, optical, acoustic, opto-acoustic and ultra high frequency (UHF). Being non invasive and non interference prone, acoustic emission technique is advantageous for online PD measurement. Acoustic detection of p.d. is based on the retrieval and analysis of mechanical or pressure signals produced by partial discharges. Partial discharges are classified according to the origin of discharges. Their effects on insulation deterioration are different for different types. This paper reports experimental results and analysis for classification of partial discharges using acoustic emission signal of laboratory simulated partial discharges in oil pressboard insulation system using three different electrode systems. Acoustic emission signal produced by PD are detected by sensors mounted on the experimental tank surface, stored on an oscilloscope and fed to computer for further analysis. The measured AE signals are analyzed using discrete wavelet transform analysis and wavelet packet analysis. Energy distribution in different frequency bands of discrete wavelet decomposed signal and wavelet packet decomposed signal is calculated. These analyses show a distinct feature useful for PD classification. Wavelet packet analysis can sort out any misclassification arising out of DWT in most cases. Keywords—Acoustic Emission, Discrete Wavelet Transform, Partial Discharge, Wavelet Packet Analysis

Improved Moisture Analysis of Power Transformers Using Dielectric Response Methods

Abstract: This paper starts with a systematic benchmark test of the three commercialised dielectric diagnostic methods RVM (recovery voltage method), PDC (polarisation and depolarisation currents) and FDS (frequency domain spectroscopy), performed on a large transformer model. Starting from the explored strengths and weaknesses a more sophisticated instrument with corresponding software was developed. The new instrument called "Dirana" combines time and frequency domain measurements resulting in an essentially decreased measurement time. The corresponding software bases on a new data pool, measured at new and aged pressboard with various moisture contents, temperatures and oil impregnation.The analysis algorithm works in frequency domain and features weighting of low frequency data and compensation for conductive aging by products. An onsite moisture evaluation compares the new system to other measurement methods.

Electric Field Characteristic Calculation on Typical Oil-paper Combination Insulation Under Polarity Reversal

Abstract: Polarity reversal (PR) is a special working condition occurring during the operation of a converter transformer. Due to the action of spatial charge under polarity reversal an extremely high dangerous electric filed will appear in the insulation oil. By use of ANSYS10.0 finite element method (FEM), the electric field strengths of several typical oil-paper combination insulations, such as plate-plate oil-paper insulation, coaxial oil-paper insulation, rod-plate oil-paper insulation and local model of converter transformer’s winding end at valve side, are calculated while polarity reversal occurs, and the relation curves of the maximum filed strength in insulation oil vs. the parameters such as the thickness of insulation pressboard, conductivity ratio of oil to pressboard, the time of polarity reversal and so on, are plotted. In plate-plate oil-paper combination insulation, increasing the thickness of insulation pressboard can reduce the field strength in oil and pressboard. However, in coaxial oil-paper combination insulation when the thickness of insulation exceeds a certain value the field strength within the pressboard under polarity reversal will increase instead. In rod-plate oil-paper combination insulation, more indeterminacy exists in the relation among the field strength under polarity reversal, the size and position of pressboard. Elongating the time of polarity reversal can reduce the field strength under polarity reversal. Field strength under polarity reversal increases with the increasing of the conductivity ratio of oil to pressboard and the field strength in oil and pressboard under polarity reversal will be saturated finally.

Identification and Localization of Multi-source Partial Discharges by Acoustic Measurements in Oil-pressboard Insulation System

Abstract: Partial discharges (PD) deteriorate insulation of power apparatus and lead to final failure. Early detection and localization of PD can avoid unwanted failure of power apparatus. There are different techniques for PD detection. Acoustic emission technique for partial discharge detection is advantageous for online detection and source location. Normally, acoustic PD detection and location is based on the assumption of a single PD source. But in practice, more than one source of PD is present and they may be active simultaneously in a power apparatus. This paper addresses the identification and localization of two simultaneous PD sources employing acoustic emission technique in oil pressboard insulation system. The PDAE signals from two simultaneous PD sources are analyzed using wavelet analysis with time frequency resolution and Independent Component Analysis for identification and localization.

Dissolved gas analysis of overheating fault in bio-degradable transformer oil

Abstract: This paper presents results of experiments performed on laboratory to simulate overheating fault on bio-degradable transformer insulation oil Dissolved hydrocarbon gas products generated by such a fault in three oil conditions (normal, dried and wet) and with and without presence of pressboard are analyzed For comparison, the tests are also performed on conventional mineral Results obtained indicate that existing DGA technique which were developed for monitoring overheating faults in mineral oil need to be modified when used for insulation assessment of bio-degradable oil

Remaining life assessment of power transformers

Abstract: The concentration of furans in the mineral insulating oil of a power transformer is a good indicator of the condition of the cellulosic materials such as paper, pressboard, and cotton materials used both as electrical insulation and as mechanical support. The condition of the cellulosic materials is vital, because these materials are the limiting factor on the life of a transformer. Over the last three years, the Liquid Dielectrics Laboratory at CPRI conducted a massive survey of the concentration of furans in the mineral insulating oils of more than eight hundred operating transformers. Remaining life assessment of transformers is attempted based on the percentile population of furans.

Behavior of Acoustic Partial Discharge Signal in Oil-Pressboard Insulation System

Abstract: Online monitoring of partial discharges (PD) can reduce the risk of catastrophic failure of power apparatus. Acoustic emission technique is advantageous for online PD measurement. PDs are classified according to the origin of discharges and their effects on insulation deterioration are also different. Acoustic emission (AE) signal produced by PD are detected by sensors mounted on the experimental tank surface, stored on an oscilloscope and fed to computer for analysis. PDAE signal propagates from source to sensor through oil and pressboard insulation. This paper discusses the effects of different thicknesses of pressboard insulation barrier on the propagation path of partial discharge acoustic emission signal. PD may occur at surface of insulation having different thickness at different voltages. This paper also discusses the effect of insulation thickness on surface discharge. The behavior of PDAE signal at different voltage level is also discussed. The AE signals are analyzed using FFT and DWT. The parameters calculated are frequency at peak magnitude & median frequency from FFT data and percentage signal energy in different frequency bands from DWT data. Effect of applied voltage beyond PD inception voltage is also examined. Acoustic PD magnitude (mV) and electrical PD magnitude (pC) relation for different electrode system is investigated.

Oil-filled electric device

Abstract: An oil immersed electrical apparatus such as an oil immersed transformer that can detect copper sulfide precipitated on coil insulating paper is provided. In the oil immersed transformer in which a coil with its surface covered by insulating paper is arranged in a container filled with insulating oil, a detection member is prepared by providing two electrodes on a surface of a plate-like pressboard formed of cellulose, which is a same material as that of the insulating paper, this detection member is arranged in contact with the insulating oil, and generation of copper sulfide is detected from reduction in a surface resistance between the electrodes. The temperature of the detection member is set higher than the temperature of the coil so that copper sulfide is generated thereon prior to generation of copper sulfide on a coil unit.

Study on Life Prediction Method of Oil Immersed Power Transformer

Abstract: Moisture and degradation of oil-immersed power transformer dielectric material are the key factors for its failures. Therefore, in this paper moisture content, aging degree and their influence on transformer dielectric material are discussed. Chemical analyses can provide direct information on parameters, however, most chemical analyses must be performed under laboratory conditions and for some chemical analyses paper samples are needed. To overcome these disadvantages, dielectric response measurements have been introduced for state assessment of transformer dielectric material, especially for the evaluation of water content in transformer pressboard. Voltage response measurement is one of the effective diagnosis and life assessment methods. In this paper application of voltage response measurement, reliability of power transformer life assessment and reasonable explanation of polarization spectrum are introduced in detail. Investigations of several transformers in new and aged state of different designs show the voltage response measurement results can assess aging degree effectively. Therefore, it may be concluded that the method is one of the effective and credible life prediction methods of power transformer.

Ratio hydraulic valve and displacement sensor combined slab thickness control device

Abstract: The utility model relates to a thickness control device which combines a hydraulic proportional valve and a displacement sensor, comprising a hydraulic proportional valve which can control an upper hot pressboard to move and a displacement sensor which can measure the distance between the upper hot pressboard and the lower hot pressboard The utility model can control the thickness of the finished plate and the precision during pressing the artificial board by press, and also has the advantages of improving the quality of the artificial board and lowering the manufacturing cost

A fast and reliable dielectric diagnostic method to determine moisture in power transformers

Abstract: This paper describes a novel dielectric diagnostic method ldquoDIRANArdquo which features a fast measurement technique and an improved analysis algorithm for determining moisture in pressboard and paper of oil-paper-insulated transformers. Dielectric response methods provide substantial advantages compared to moisture determination by oil sampling and conventional equilibrium diagrams, for example the influence of aging can be compensated and it is unnecessary to wait for moisture equilibrium. For the new instrument, the novel combination of time and frequency domain measurements substantially shortens the measurement time. The data analysis algorithm compares the measured dielectric properties of the actual power transformer to modeled dielectric properties. The software features a compensation for the influences of conductive aging by-products, which have similar dielectric properties as water. Without compensation the analyzed moisture content will be too high and may lead to unnecessary actions like drying. After moisture analysis, the new software categorizes the moisture concentration according to IEC 60422.

Application of modern diagnostic techniques to assess the condition of oil and pressboard

Abstract: Aging of oil-paper insulation was investigated under controlled laboratory conditions. Traditional and modern diagnostic tools and techniques are discussed. Insulation condition testing includes common chemical diagnostics such as acidity tests and several new chemical techniques such as determination of relative content of dissolved decay products by spectrophotometry, solid suspension by turbidimetry, Fourier transform infrared spectroscopy, and electrical diagnostic techniques that have gained exceptional importance for utility professionals, such as polarisation/depolarisation current and frequency domain dielectric spectroscopy measurements. Based on the preliminary results, the suitability of each method analyzed along with its interpretation scheme. Finally, recommendations for further investigation are presented.

Partial discharge characteristics in UHF-band of composite insulation system including various artificial defects in transformer

Abstract: Experiments of partial discharge (PD) current pulse and phase resolved PD characteristics were performed using various artificial defects in oil/pressboard composite insulation system in oil-filled transformer. As artificial defects, we used barrier insulation model-in-oil, oil wedge model, air void model, floating electrode model, and oil gap model. As a result, it was found that the rise time, fall time, and pulse width of each PD current pulse differed depending on each model. Moreover, we confirmed the existence of PD-radiated electromagnetic wave with frequency UHF band for the defects. It was shown from these results that UHF method is applicable for detecting PD originating from the defects and identifying the defect types in oil-filled transformer.

Tensile strength of thermally degraded resin-coated papers and their electrical breakdown strength in composite insulation system with insulation oil

Abstract: Two types of resin-coated papers were investigated; kraft papers and heat-resistant kraft papers partially covered with epoxy resin; those were laminated to some thickness. They were thermally degraded at 120degC for 240 to 1320 hours, and their mechanical strength, i.e. tensile strength was measured. The tensile strength of the resin-coated kraft paper decreased with the heating time and became smaller than that of the pressboard after 500 hours while the tensile strength of the resin-coated heat-resistant kraft paper almost retained the initial value after 1320 hours of the heating time. Then, electrical breakdown characteristics of composite insulation systems with a resin-coated paper and insulation oil were investigated. In the system, an oil-filled gap was artificially introduced between the resin-coated paper and a plane electrode to induce partial discharge (PD) initiation at the same location. PDs occurred before breakdowns and their PD inception electric field strength was almost as high as pressboards and the effect of the heating time was negligible. Characteristics of electrical breakdown field strength were similar to the PD inception field strength; negligible effects of the type of the resin-coated papers and the heating time. Electrical breakdown occurred at the oil-filled gap and the edge of a high voltage electrode.

Influence of the Relative Amount of Paper on Aged Mineral Oil Decay Products

Abstract: Aging of different oil-pressboard insulation ratios was investigated under controlled laboratory conditions. Modern diagnostic tools and techniques were used to assess the degree of influence of the relative amount of cellulosic materials on aged mineral oil decay products. Determination of relative content of dissolved decay products by spectrophotometry, solid suspension by turbidimetry, Fourier transform infrared spectroscopy, and electrical diagnostic techniques were used to judge the most affected oil parameters by oil/pressboard proportion and thus the most effective way to choose the best proportion that can reduce solid insulation degradation and increase power transformers lifespan. A quantitative study of the effect caused by the ratio of oil and paper can help to a better modeling of solid insulation end of life.

Composite hot pressing plate of hot press

Abstract: The utility model relates to a hot pressboard, in particular to a compound hot pressboard of a hot-press. The hot-press comprises a plurality of compound hot pressboards and each compound hot pressboard comprises an upper surface hot pressboard, a lower surface hot pressboard, a middle heat insulated mat and a connecting plate. The utility model is characterized in that the upper surface hot pressboard and the lower surface hot pressboard are connected with a heating device with different heating temperature respectively. The heating temperature of the upper surface hot pressboard and the lower surface hot pressboard can be controlled respectively, so the utility model can meet the requirement for pressing special finished plate.

Arc prevention anchor clamp

Abstract: A preventing arc hardware comprises an upper pressboard and a lower pressboard which are clasped with each other through a bolt, the matching surface of the upper pressboard and the lower pressboard is provided with puncture teeth, each puncture tooth is rabbetted in the upper pressboard or the lower pressboard to form a split rabbet puncture mechanism. As the puncture teeth and the upper pressboard or the lower pressboard of the utility model are the split rabbet, the puncture teeth can be processed into the ideal shape and size independently without the limitation of casting technique. The utility model with reasonable structure can greatly enhance the puncture ability and puncture an insulating layer made of various materials, and is very sufficient in conductive contact. What is more, the puncture teeth can be independently produced by using materials such as red copper, thus leading to better conductivity performance.

Charge behavior in Palm Fatty Acid Ester oil (PFAE) / pressboard insulation system under flow condition in power transformers

Abstract: From viewpoints of low environment impact, palm fatty acid ester oil (PFAE) is potent insulating liquid for power transformers because of its high biodegradation performance. Moreover, PFAE has high permittivity, low viscosity, high thermal and chemical stability and high productivity. These characteristics well satisfy the requirement of electric insulation performance in power transformers. In this paper, electric field distribution in a PFAE / pressboard insulation system was measured with a Kerr electro-optic technique at various oil flow conditions under dc voltages and the charge behavior was investigated systematically. 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.

Analysis of flow electrification parameters of power transformer oils

Abstract: As a part of a research program at EDF, University of Poitiers has developed a prototype of a sensor dedicated to the monitoring of oil flow electrification through transformer pressboard-made channels This phenomenon can be responsible for destructive breakdowns when the resulting charge accumulation leads to high electrical fields In order to identify transformers running electrostatic hazards, an experimental setup been has developed to measure the generation and the accumulation on pressboard surface of electric charges with regard to different oil flow conditions (velocity, temperature) Various oils have been characterized with the same reference pressboard: new oils and oils coming from operating transformers (healthy or suspected to develop the phenomenon) The analysis of the measured parameters makes it possible to derive some criteria with regard to the electrostatic hazard probability in transformers

Presser feet of sewing machine

Abstract: The utility model discloses a sartorius presser foot, which comprises a pressboard with pinhole. The utility model is characterized in that the lower surface of the pressboard is provided with a flange which extends along the pressboard material feeding direction. The flange is arranged in the position of the pinhole, where the flange breaks or forms a through hole. The sartorius presser foot enables the sartorius to stitch granular ornament on the products waiting to be adorned, and improves the stitching efficiency.

Amorphous alloy iron core whole hermetically-sealed transformer

Abstract: The utility model relates to a transformer, in particular to an amorphous alloy cores omni-sealed transformer, which comprises a core, an inner winding and an outer wingding which are winded outside the core, wherein the core comprises three cubic core units, the middle core unit is bigger than the core units on the two sides, a forming hard pressboard cylinder is sleeved outside the core, and the inner winding and the outer wingding which are of an ellipse shape are winded on the outboard of the forming hard pressboard cylinder. The utility model has the advantages of simple manufacture, and light weight, thereby being capable of reinforcing the mechanical strength of windings, and improving the anti-short circuit ability.