Showing papers on "Pressboard published in 2012"

Power transformer insulation – history, technology and design

Abstract: This paper reviews briefly the history of paper and pressboard insulation being used for electric insulation of oil immersed power transformers. An introduction to today's insulation technique is given. Important details of materials selection, as well as drying, clamping and oil impregnation procedures required to achieve short-circuit resistant windings are explained. Aging effects are described, as they gradually weaken the solid insulation mechanically and electrically. The value of post-mortem analysis of transformers before scrapping is shown with examples. A list of factors important for the manufacturing and operation of reliable transformers, yet often neglected, completes this article.

Quantitative analysis of ageing condition of oil-paper insulation by frequency domain spectroscopy

Abstract: Frequency domain spectroscopy (FDS) has been used to assess the ageing condition of oil-paper insulation used in transformers. To further understand the ageing process, the reduction in degree of polymerization (DP) of cellulosic paper in itself on the dielectric response was investigated first. It has been found that the reduction in DP itself can alter the dielectric characteristics. The oil-paper insulation was thermally accelerated aged at 110 °C for up to 154 days according to the procedure described in IEEE Guide. Then dielectric characteristics of the thermally aged samples were obtained at different measurement temperatures. The results reveal that the relative permittivity (er') plots of oil impregnated pressboards increase with the reduction in cellulose polymer chain length. er' and tanδ values of oil impregnated pressboards shifts upwards in the lower frequency range (10-3 Hz -10-1 Hz) with different ageing condition, indicating the possibility of utilizing the characteristics of er' and tanδ values at lower frequencies (10-3 Hz-10-1 Hz) to quantitatively characterize ageing condition of oil impregnated pressboard. The parameters including DP, er' and tanδ values at the three characteristic frequencies (10-3 Hz, 10-2 Hz, 10-1 Hz) have been analyzed as a function of the ageing time, showing an exponential relationship based on the best fit. In addition, the er' and tanδ curves of aged oil impregnated pressboard have been observed to be shifted upwards to higher values at lower frequencies when the measurement temperature is increased. A shifting factor αT which describes the relationship between frequency and temperature has been defined. It allows one to shift the master curve of impregnated pressboard with DP obtained at reference temperature to other testing temperatures.

Characteristics of moisture diffusion in vegetable oil-paper insulation

Abstract: Vegetable insulating oils are fire-resistant and environment-friendly liquid dielectrics. The current paper presents an experiment system to simulate moisture diffusion in the insulation system consisting of vegetable insulating oil and insulation paper. The results show the coefficient of moisture diffusion in vegetable oil-paper insulation. The present paper also discusses a new numerical method of solving the diffusion equation. Using the numerical method and the diffusion coefficient, the moisture contents in the insulation paper can be predicted. A practical and significant equilibrium time constant is proposed. Based on defining the equilibrium time constant, the relationships between equilibrium time constant and temperature, initial moisture content, and thickness of paper/pressboard are simulated. The moisture diffusion characteristics of vegetable oil-paper and that of mineral oilpaper are compared as well. The experiment and analysis results show that the diffusion rate of moisture in vegetable oil-paper insulation is about half of that in mineral oil-paper insulation.

Experimental evaluation on dielectric and thermal characteristics of nano filler added transformer oil

Abstract: Liquid dielectrics play a major role in the insulation system used in high voltage equipments. The most commonly used liquid dielectric is transformer oil which serves the dual purpose as insulation and a coolant. It also acts as insulation between the conductingparts and as an arc quenching medium. Moreover the breakdown strength of the paper and the pressboard insulation depends on the properties of the oil in which it is impregnated. Therefore investigations have been attracted towards the development of nano fluids with better thermal and dielectric properties. In this work, ceramic nano particles are chosen because of their electrical insulating property. Because of higher relative permittivity Zirconia(ZrO2) and Titania(TiO2) nano particles are used here. The dielectric and thermal properties are measured for different concentrations of nanofillers. AC and Impulse breakdown voltage measurements were performed as per IS 6792:1972 and IS 11697:1986 respectively. The specific resistivity and the dielectric dissipation factor measurement were carried with reference to IS 6103:1971 and IS 6262:1971. The kinematic viscosity and flash point are measured as per IS 1448 Part-25:1970 and IS 1448 Part-21:1970 respectively.

Effect of nanoparticles on charge transport in nanofluid-impregnated pressboard

Abstract: Transformer pressboard impregnated in mineral oil modified by nanoparticles (nanofluid) exhibits substantially higher AC and DC breakdown voltage than that of the pure oil-impregnated pressboard (OP). Charge transport and decay characteristics of both pressboards were measured by pulse electroacoustic technique. It reveals that nanofluid-impregnated pressboard (NP) has a more uniform internal electric field and a higher charge decay rate compared to OP, which is caused by an increase of shallow trap density in NP related to the difference of internal structures, based on the test results of thermally stimulated current and Fourier transformed infra-red spectroscopy.

Influence of natural ester on frequency dielectric response of impregnated insulation pressboard

Abstract: It is important to investigate the dielectric behaviour of natural ester-paper insulation, as the number of transformers choosing natural ester as their insulation oil is increasing. A comparative study of the pressboard with different conditions immersed in natural ester and mineral oil was conducted. X–Y model simulation was used to indicate the difference between the frequency-domain spectroscopy characteristics of transformer main insulation system using natural ester-pressboard insulation (NEPI) system and mineral oil-pressboard insulation (MOPI) system, which have the same moisture content or experience the same ageing period. Compared with the MOPI with low moisture content, the NEPI with the same low moisture content has higher ɛ′r and tanδ values in the lower frequency region at various temperatures; although no significant differences exist in the ɛ′r and tanδ curves of these two kinds of oil-impregnated pressboard, when its moisture content is high. The X–Y model simulation results also show that the dielectric loss difference between the NEPI system and the MOPI system with low moisture content is more obvious at higher temperature. Besides, the pressboard aged in natural ester has higher ɛ′r and tanδ values than that aged in mineral oil after ageing for the same time. X–Y model simulation results show that the tanδ values in the lower and middle frequency regions could be used as the ‘fingerprint’ of the ageing condition of the NEPI system and the mineral-oil pressboard insulation system.

Effect of nanoparticles on electrical characteristics of transformer oil-based nanofluids impregnated pressboard

Abstract: In order to improve the electrical characteristics of transformer oil-impregnated pressboard, transformer oil-based nanofluids were used to impregnate pressboards. It is found that nanofluid-impregnated pressboard (NP) exhibits substantially higher AC and DC breakdown voltage than that of pure oil-impregnated pressboard (OP). Pulse electroacoustic measurement (PEA) was carried out to study the difference of space charge distribution and decay characteristic in OP and NP. It has demonstrated that NP has a more uniform internal electric field, which is caused by a higher charge transport rate compared to OP, resulting in the improved electrical insulating properties.

Dean-Stark vs FDS and KFT methods in moisture content recognition of transformers

Abstract: Transformer is one of the crucial equipments in electric power network. Moisture in paper insulation acts as a catalyst in transformer insulation aging. Experience has shown that humidity will affect electrical strength of paper and pressboard as well as other insulation system materials by reducing degree of polymerization and accelerated aging. Thus, it is essential to determine the extent of moisture content within insulation system before exciting the transformer and also during normal operation. In this regard, chemical tests on oil have been carried out for many years. Also, electrical tests for assessing the transformer condition have been developed and improved in recent years while most introduced methods aim to extract moisture from transformer insulation system. In this study, the results of practically applicable methods in oil sampling (KFT and capacitance sensors), paper sampling (Dean-Stark method) and also dielectric response (RVM, PDC, FDS) are examined in determining insulation system humidity. Test results through various methods are compared to analyze the sensitivity of different approaches. Crucial practical recommendations as well as advantages and disadvantages of each method are discussed. At last, to recommend an accurate and reliable method for practical implementations, practical tests have been performed simultaneously on three distribution transformers and the results of them are discussed.

Influenceof density on the out-of-plane mechanical properties of pressboard

Abstract: A large number of power transformers rely on the mechanical properties of oil-impregnated cellulose. In particular, the deformation of windings in power transformer depends on the out-of-plane mechanical properties of pressboard insulation. Compressive loads are applied via clamping rings onto insulating paper and spacers. It is therefore relevant to define the out-of-plane stress-strain curves in compression for cellulose based materials. From a mere structural point of view pressboard can be regarded as a porous material in which the load bearing matrix consists of randomly connected fibers. It is expected that, by increasing the density, larger fiber-to-fiber contact areas are created and free span between fibers decreases. The combination of these two phenomena results in a material which is both stiffer and stronger. Furthermore cellulose is a polymer and its mechanical behavior also depends on temperature and moisture content. The present work investigates the relation between density and stiffness in the thickness direction, at different temperatures and moisture contents. Laboratory experiments were performed on a custom built set-up that allowed continuous measurements of load and deformation. The temperature and moisture content of the tested material were carefully controlled during the performing of the experiments.

Ester oils and flow electrification hazards in power transformers

Abstract: In large power transformers where forced convection of the insulating liquid is the predominant heat transfer mean, the electrification risk can appear. This phenomenon has been suspected to be responsible for several failures. Studies have shown, in addition to the transformer design (shell or core type), that the type of liquid used is quite important in limiting these hazards. The goal of this paper is to present an analysis of ester oil behavior versus electrostatic hazard due to flow electrification. Two experimental studies were conducted on different ester oils: the first one is for measuring the Electrostatic Charging Tendency (ECT) by using ministatic charge tester, and the second one is made by mean of a test loop simulating the oil flow as in a real transformer and equipped with a capacitive sensor. This sensor is able to measure both the charge accumulation at the pressboard surface and the generating current. All these results are compared with those of mineral oils which are actually used for large power transformers. It was shown that even if ester oils increase the charge generation in comparison with mineral oil, the charge accumulation on the solid surface is not excessive thus limiting the potential at the solid/liquid interface and associated electrification hazards. One can assume that the higher conductivity of esters would enable a better evacuation of generated charges.

Numerical simulations of bridging phenomena in contaminated transformer oil

Abstract: Experiments were carried out on transformer oil contaminated by cellulose fibre After the high voltage was applied to the electrodes, the pressboard particles accumulated in the highest electric field region and eventually formed a bridge We have developed a mathematical model to simulate the bridging dynamics The dielectrophoretic and drag forces are considered for simulation of the particles motion and accumulation The predicted bridge formation rate and the conduction current between the electrodes showed similar trend to the experimental results The model is a successful means of predicting bridge formation phenomenon compared with the experimental data

Impact of Cellulose Aging on Moisture Equilibrium of Oil-pressboard Insulation

Abstract: To research the impact of cellulose aging on moisture equilibrium of oil-pressboard insulation,cellulose structure and its moisture absorption mechanism were analyzed,various aged pressboard samples were prepared,cellulose micro structure and specific surface area of various aged pressboards were observed,and stable moisture and moisture mobility in temperature decreasing process of various aged pressboards were measured.The results show that stable moisture and moisture mobility in temperature decreasing process of aged pressboard is lower than un-aged pressboard;aged pressboards have lower moisture absorption capacity,which is related to the decreasing of cellulose diameter and specific surface area;Oommen’s moisture equilibrium curves are not fit for aged pressboards,27% moisture decreasing correction for 20 years equivalent old pressboard and 35.5% moisture decreasing correction for 40 years equivalent old pressboard are necessary.

Application of common transformers faults diagnosis methods on biodegradable oil-filled transformers

Abstract: Due to environmental concerns regarding the use of mineral oil, biodegradable oil is increasingly being used as an alternative dielectric fluid. This paper presents results of experiments performed in the laboratory on the use of biodegradable oils for transformer application. The investigations cover two important diagnostic techniques for insulation assessment: (i) an investigation of partial discharge (PD) activity and characteristics in such oils and (ii) an investigation of the standard hydrocarbon dissolved gas products produced due to transformer faults. The fundamental aim of the investigation was to provide information as to whether the existing analysis techniques of PD fault pattern recognition and dissolved gas analysis methods, developed for mineral oil, are valid when used for insulation assessment with biodegradable transformer oil. The experiments were conducted on test samples to simulate three common types of transformer faults: low-energy PDs, high-energy arcing/sparking, and overheating. For the purpose of comparison, the same tests were performed using a mineral oil (Shell Diala MX) and a biodegradable oil (Envirotemp FR3). Also, each oil type was tested at three different moisture levels and with or without the presence of solid insulating materials (pressboard). PD activity was monitored using the standard IEC60270 phase-resolved analysis method. The fault gases produced were extracted and analysed by standard gas chromatography methods. Test results indicate that the PD phase resolved patterns are, in general, similar for the two oil types and thus existing PD pattern interpretations can be used to distinguish different types of PD faults, e.g. corona versus surface discharges. However, the values of various discharge quantities (PD magnitudes, repetition rate, current, etc) are very different under the same test condition. The quantities and the trend of dissolved gases for faults in biodegradable oil are substantially different compared with mineral oil. For the PD fault, biodegradable oil was found to release only a limited number of gases. For arcing and overheating faults, the main key gas produced is different between the two oils. Also, the presence of cellulosic materials such as pressboard influences the extent of hydrocarbon gases dissolved in the oil. It was found that for biodegradable oil, fewer amounts of gases are produced as the oil gets wetter.

Charge behavior in Palm Fatty Acid Ester Oil (PFAE) / pressboard composite insulation system under voltage application

Abstract: Palm Fatty Acid Ester Oil (PFAE) is one of the most promising environment-friendly biodegradable insulating oils for transformers. PFAE has significant advantages compared to other vegetable origin insulating oils and mineral oil, e.g. not only good insulation performance, high permittivity and high moisture tolerance, but also excellent cooling ability due to low kinematic viscosity, high thermal and chemical stability and high productivity. However, since the molecular content of PFAE is quite different from mineral oil, the fundamental charge behavior in PFAE has not been clarified yet. To clarify the temporal and spatial charge behavior in PFAE, we directly measured the electric field in oil duct model with PFAE / pressboard (PB) composite insulation system under dc voltage application and charged flow condition by using Kerr electrooptic field measurement technique. By the comparison of electric field and charge behavior characteristics with fresh and degraded mineral oil, we revealed that the essential mechanism of charge behavior is similar, but the difference in temporal and spatial characteristics is attributed to the higher intrinsic ion density and higher permittivity of PFAE than that of mineral oil.

Modern insulation condition assessment for instrument transformers

Abstract: Instrument transformers in service are exposed to the harming effects of water and ageing. The increasing age is leading to a higher risk of explosions, which can damage surrounding parts and cause high follow-up costs. This paper introduces a new approach to use the dielectric response analysis for condition assessment of instrument transformers. The measurement of the dielectric response in a wide frequency range provides information about the insulation condition and especially for oil-paper/pressboard insulations about the water content in the solid insulation. The measured curve is formed by the superposition of the dielectric response of the cellulosic material and the oil. The curve can be used for the assessment of the insulation condition in various ways. Qualitative comparisons, e.g. between instrument transformers of the same type and age, might give general information which of these is in better condition. The dissipation factor and the capacitance trend can also deliver valuable data. Furthermore the water content in the solid insulation can be assessed using an automatic analysis algorithm. To confirm the validity of this assessment, measurements on several instrument transformers in service were performed. Dielectric response curves and the trend of the capacitances for new and aged instrument transformers with different water contents are discussed. Selected examples presented in this paper show, that the dielectric response is well modeled, the water content can be derived and ageing effects can be observed.

Ester Insulating Liquids for Power Transformers

Abstract: The properties of ester insulating liquids, including both synthetic and natural esters, offer the potential for transformers with lower fire risk and greater environmental compatibility when compared with those filled with mineral oil. This potential has led to intensive studies of the electrical and thermal performance of ester liquids in recent years.A collective research effort was made to study ester liquids by a number of companies within the UK electric power industry particularly in respect of their application to large power transformers. With a liquid provider and a transformer manufacturer, extensive research has been carried out at a university into the dielectric aspects of the insulating liquids to prove the feasibility of using an ester liquid in place of mineral oil in large high voltage power transformers.This paper sets out the results of this research focusing mainly on the electrical performance of ester liquids. Experimental tests in both quasi-uniform and non-uniform electric fields including discharge initiation, propagation and breakdown were carried out under both AC and lightning impulse voltages. In addition to open oil gaps, the influence of pressboard in parallel to the oil gap was also investigated. The comparison between the performance of ester liquids and mineral oil, allows conclusions to be made regarding the application of ester liquids in power transformers.

Characteristics of leakage current during surface disharge at the oil-pressboard interface

Abstract: Surface discharge at the oil-pressboard interface leads to the development of a conducting path that is generally accepted as white marks due to localized heating. The visible white marks are believed to be due to partial discharge (PD) energy that is high enough to dry out the pressboard through vaporizing the moisture and breaking oil molecules to generate gases in the pressboard pores. A long period of electrical stress applied to a surface discharge experiment using a needle-bar electrode configuration has led to the formation of full-gap white marks on the pressboard surface that bridged the needle tip and earth bar. During the surface discharge, leakage current may occur intermittently that sometimes is visible in the form of full discharge of arcing at the oil-pressboard interface bridging the needle tip and earth bar. This condition is expected to be similar to that of dry-band arcing from inclined-plane tracking experiments for outdoor insulation. This paper explains the surface discharge experiment and details the obtained leakage current characteristics in terms of waveforms and frequency spectra. Correlation between leakage current measurements using a shunt resistor and PD signal detection has been undertaken in order to identify the useful information for condition monitoring of surface discharge in a large transformer. An equivalent circuit to represent the leakage current occurrence at the oil-pressboard interface without considering the superimposed pulses due to individual PD events is also proposed. The effect of moisture on the experimental results is also discussed

Oil Gap dependence of partial discharge starting point in the oil/pressboard composite insulation system

Abstract: This paper deals with the impulse PD (partial discharge) characteristics in oil/pressboard composite insulation systems. We examined the PD starting point in test samples with the oil gap length varied from 1.6 mm to 9.6 mm, which simulated the oil duct in an oil-immersed transformer. Consequently, we found that when the oil gap length was below 4.8 mm, the PD starting point shifted from the oil gap to creepage of a spacer when the oil gap length increased. On the other hand, when the oil gap length was over 4.8 mm, the PD starting point shifted from creepage of a spacer to the oil gap as the oil gap length increased. These results of experiments can be explained by considering the volume effect of the transformer oil and assuming that the fibrous protrusions on the surface of the pressboard had an influence.

Thermal aging of paper and pressboard in mineral and ester oils under air and nitrogen atmospheres

Abstract: The purpose of this paper is to simulate an accelerated ageing of paper and high density pressboard type B.3.1 in three different liquids (mineral oil MO, natural ester VO and synthetic ester SE) for different periods of ageing at 130°C. The materials were aged in sealed vessels under two different atmospheres: air and nitrogen gaps. The effects of aging on physicochemical properties of oil and cellulose are analyzed and compared. It is shown that ageing rate of pressboard and paper are comparable in each type of oil. Nevertheless, it can be underlined that cellulosic materials are less affected after aging in ester oils evidencing a higher degree of polymerization (DPv). The use of 2-FAL in ester oils remains questionable. The reduction of oxygen content in mineral oil minimizes ageing of oil more than solid insulating materials.

Impact of moisture and ageing on the dielectric response of transformer insulation

Abstract: Moisture accelerates the ageing process in the oil-paper insulation of the transformer. In order to understand an influence of moisture on the ageing process of transformer insulation, ageing experiments are carried out on oil-impregnated pressboard sample of different moisture contents, and influence of moisture and ageing on the dielectric response is studied. In this paper, variation of PDC and FDS responses of oil-impregnated pressboard samples with ageing is discussed. Further to that, change in DP value of pressboard sample with ageing is presented for different ageing periods.

Analysis of frequency domain spectroscopy measurements on power transformers by the use of a Finite Element based model

Abstract: Dielectric spectroscopy (FDS) is widely used to assess the condition of transformer oil-paper insulation systems Typically the frequency dependent complex capacitance C HL between the high- and low-voltage windings is measured This capacitance can be modeled analytically using simple RC-equivalent circuits, due to the homogenous electrical field distribution in the main insulation duct (XY or Pancake-model) [2] A curve fitting of this model to the measured data enables extraction of dielectric parameters like oil conductivity and moisture content in pressboard Apart from C HL it is also possible to measure the complex capacitance between HV windings and tank (C H ) where we also have an oil-paper insulation system The corresponding measurement data can't however be fitted to the conventional Pancake-model, because of the inhomogeneous field distribution between HV windings and tank Hence a Finite Elements Analysis (FEA) based model which describes the dielectric behavior of the transformers isolation system between HV windings and tank was developed in this paper This numerical model was combined with an optimization algorithm to determine the unknown dielectric parameters from dielectric spectroscopy data for C H FDS measurements of C H and C HL from three power transformers were analyzed by the use of the conventional Pancake-model for C HL and the new FEA-model for C H The results showed good agreement in dielectric parameters of oil and pressboard what leads to a strong verification of dielectric diagnosis results for a transformer by two independent models and measurements

Surface flashover development on transformer oil-pressboard interface

Abstract: Highly insulating pressboard immersed in transformer oil applies a polarization force on charge carrying streamers, proportional to the permittivity difference between the oil and the pressboard. If the pressboard permittivity is greater than the oil permittivity, the attractive force turns the streamer into a surface flashover. On the other hand, a low permittivity pressboard interface repels the approaching streamer and reduces the streamer velocity. Theoretical analysis of electric field propagation through the interfacial surfaces, oriented in either parallel with or perpendicular to the streamer propagation direction, is presented. Pressboard relative permittivities of 1.1 and 4.4 have been studied while the oil relative permittivity is assumed 2.2.

Moisture Content Analysis of Oil-paper Insulation for Large Power Transformers Using Dielectric Response Methods

Abstract: The dielectric response technology is a nondestructive technology of moisture content diagnosis for oil-paper insulation by dielectric response curveTo promote the development and application for the dielectric response technology,the dielectric response characteristics of the actual large oil-paper insulation power transformer were studied on the basis of dielectric theory,and the test results show a strong regularity that frequency,temperature,and moisture content effect on the dielectric response curve of large power transformerThe qualitative analysis method of moisture content in oil-paper insulation for the large power transformer was proposedThe quantitative analysis method of moisture content in oil-paper insulation of the large power transformer was developed based on the equipment X-Y model and the complex permittivity database of oil impregnated pressboard under various moisture contents and temperatures,and the calculated values agree with the experimental data approximately

The influence of oil speed and temperature on PD phenomena in transformer insulation

Abstract: Identifying partial discharge (PD) phenomena in power transformers is the key to achieve proper maintenance actions. In order to tag a recorded PD pattern obtained from either on site testing or monitoring as, e.g., internal, surface or corona discharges, a diagnostic database has to be built first. This database would contain patterns and relevant statistic parameters recorded in the lab under controlled conditions. One of the limits of this approach is that several factors may influence the PD pattern or the partial discharge inception voltage and, thus, identification procedures. In this paper, focus will be on the effects of both temperature and oil flow velocity on surface PD in a pressboard layer at five different oil speeds and with static oil. Statistical analysis is presented to compare the PDIV values.

Characteristics of creepage discharges along ester-pressboard interfaces under ac stress

Abstract: Ester liquids including natural ester and synthetic ester are considered as potential substitutes for mineral oil, due to their good biodegradability and high fire points. Although these liquids have been widely used in distribution and traction transformers, research efforts are required for the purposes of design and manufacture of high voltage and large power transformers which are filled by esters. Indeed, it would be risky to apply esters in large power transformers without thorough understandings of their behaviours in large gaps and/or when combined with pressboard insulation. Therefore, investigations of creepage discharges along the surface of pressboard in esters are vitally important and their behaviours should be compared with those of mineral oils. This thesis is aimed to investigate the creepage discharges along pressboard in esters and mineral oil under ac divergent electric field. Apparent charges, current signals and images of streamer channels were obtained synchronously to identify whether and how the introduction of pressboard surface would influence the inception and propagation of discharges as compared to tests in open gap. When over-stressed by higher voltages, the surface tracking along the pressboard-ester interface, triggered by sustaining creepage discharges, was studied and the evolutions of accompanying creepage discharge patterns were investigated. In these experiments, both esters and mineral oil impregnated pressboards were comparatively studied. The test results indicated that at the inception stage, the presence of pressboard or any other solid types in different liquids under test do not influence the PD inception voltages; in the propagation stage, solid surface tends to promote the development of discharges, especially those occurring in negative half cycles, and shifts more discharges towards the zero-crossing phase angles. This discharge promotion effect is much more evident in esters than in mineral oil, probably because of higher discharge intensity in esters and higher viscosity of esters. The space charge effect and the residual low density channel effect are proved as the mechanisms best explaining the influences of solids on creepage discharges.Under higher voltages, it was found that the impregnated pressboard is susceptible to discharge erosion characterized by ?white and carbonized tree-shaped marks?, due to intense discharges occurring on or near the pressboard surface. The ?white mark? appears at a lower voltage and propagates more easily on ester impregnated pressboard. The gaseous ?white mark? channels will attract the subsequent discharges to follow the same discharge routes; the accumulative energy dissipation in these channels will then result in the carbonization of the channels. Once formed, the surface tree-shaped mark can continue to grow even under reduced voltage levels until it bridges the gap and causes the final flashover.

Experimental Investigation and XY Model Simulation on the Frequency Domain Spectroscopy Characteristics of Transformer Oil-paper Insulation

Abstract: In order to achieve better field-diagnostic performance of frequency domain spectroscopy in diagnosing insulation status of oil-paper in transformers,the frequency domain spectroscopy characteristics of transformer oilpaper insulation system is investigated on the basis of XY model simulation associated with a transformer experimental model,which supports main insulation structure modification.Firstly,the equivalence between XY model simulation and field FDS measurement of transformer model is verified using the Concept 80broadband dielectric spectrometer and dielectric response analyzer apparatus DIRANA.Then,based on the complex permittivity spectroscopy data of pressboard,frequency domain spectroscopy characteristics of oil-paper insulation system under different conditions,such as different insulation structures,oil conductivity,water content of pressboard and aging status,are systematically discussed using XY model simulation.Results show that a"hump" can be observed in the frequency spectroscopy of tanδwhen the oil conductivity is relatively higher.The frequency relating to the maximum value of the"hump"shifts to higher with the increase of oil conductivity,but the maximum tanδvalues maintain the same.The frequency relating to the maximum value of the"hump"also shifts to higher values with the increase of X or Y value of the insulation structure,meanwhile the amplitude of the"hump" decreases.As the aging proceeds,the frequency spectroscopy of tanδshifts to higher frequency.Therefore,a new way of aging-status diagnosis for transformer utilizing FDS is provided.

Hot pressboard for hot press

Abstract: The utility model provides a hot pressboard for a hot press. The hot pressboard comprises a board core, a tensioning mechanism and a stainless steel board, wherein the board core is composed of circulating pipelines, an upper flat board and a lower flat board, square pipes are arranged to form the circulating pipelines, and the upper flat board and the lower flat board are connected with the square pipes to form an integration. The board core is tightly wrapped by the stainless steel board through the tensioning mechanism. The hot pressboard ensures the whole precision of the surface of the hot pressboard, shortens a preheating time of the hot pressboard, improves thermal efficiency, saves 70% energy, relieves 65% in weight compared with a steel board hot pressboard and saves steel products, and is high and even in a heating rate.

Comparative PD characteristics of pressboard/mineral oil and pressboard/vegetable oil insulating systems

Abstract: This paper is devoted to a comparative study of partial discharge (PD) characteristics initiated in pressboard/mineral oil and pressboard/vegetable oil insulating systems. We mainly analyze the average number N and maximum apparent charge q ap.max of discharges versus the voltage. The results are discussed with respect to the thickness of pressboard and type of oil. It's shown that for a given voltage and pressboard of 4 mm thickness, any significant difference is observed in the value of N and q ap.max in both mineral and vegetable oils while for a thickness of 2 mm N and q ap.max are much higher in vegetable oil than in mineral oil. As concerns the phase, the maximum of discharge activity moves to the left (i.e. towards the low phases) when increasing the voltage. This would indicate that the influence of the electric field and capacitive effects decrease when the thickness increases. As concerns the phase, the maximum of discharge activity moves to the left (i.e. towards the low phases) when increasing the voltage.

Characteristics of Oil-pressboard Insulation Under Polarity Reversal Voltage

Abstract: The failure often happened to the oil-pressboard insulation in converter transformer during the polarity reversal voltage testIn order to look for the causes,the polarity reversal characteristics of the oil-pressboard insulation were analyzed by the RC modelA three-electrode test model and a type of polarity reversal voltage experiment unit were designedThe characteristics of mean currents and breakdown voltages of the oil-pressboard insulation were experimentally studied under different polarity reversal waysThrough analysis on experiment results and comparison with analytical results,it is indicated that the relationship between residual currents received in test and applied voltages is linear scalingThe initial values of absorption currents increase along with the applied voltages in an exponential law,and the attenuation function is an exponential function with a power function variable of time,which is different from rules determined by the RC modelBreakdown voltages decrease clearly with the increasing of the polarity reversal speedThe accumulated charges also increase along with the increasing of applied voltages in an exponential lawBased on the results,it is considered that the differences of changing regulations of absorption currents are due to the changes of strength and polarity of the internal electric field formed by space chargesThe reason causing the breakdown happened easily to the oil-pressboard insulation by the polarity reversal is the rapid increase of electric field in oil immersed pressboard