Showing papers on "Pressboard published in 2014"

Understanding the impact of moisture and ageing of transformer insulation on frequency domain spectroscopy

Abstract: Moisture is one of the most significant parameters that can accelerate ageing in paper/pressboard insulation. In order to understand the impact of moisture and ageing on Frequency Domain Spectroscopy (FDS) measurements, oil impregnated pressboard samples of different moisture contents are prepared and aged at 105°C temperature. FDS measurements on pressboard samples are carried out at different ageing time. Other measurements such as moisture, degree of polymerization (DP), Furan and Dissolved Gas Analysis (DGA) are also performed on oil and pressboard samples. It is observed that FDS measurements on pressboard samples are mostly influenced by the moisture contents. Several other pressboard samples are prepared under different conditions to determine how FDS measurements are affected by moisture, aged oil and ageing of pressboard sample considering each of the parameter individually as well as in different combinations. Results suggest that FDS measurements are sensitive to the change in moisture, ageing products and ageing of pressboard (DP) sample. However, impact of ageing products and ageing of pressboard sample (DP) on FDS measurements are smaller in comparison to moisture.

Assessment of water content in an impregnated pressboard based on DC conductivity measurements theoretical assumptions

Abstract: The paper presents results of investigations of the effect of moisture content and temperature on DC conductivity of the impregnated pressboard. It has been found that the conductivity of the impregnated pressboard is a function of activation energy, moisture content and temperature. It has been found that moisture content ranging from 1.4% to 6.4% do not cause any changes in the conductivity activation energy whose value is ΔWav = (0.853+0.071) eV. On the other hand water content strongly influences the value of DC conductivity. It has been also found that the conductivity is determined by electron hopping between the closest located potential wells formed by water molecules in the oil-impregnated cellulose structure. For the experimentally obtained dependences of conductivity vs. moisture content and temperature theoretical ones have been determined to describe the experimental results. Based on the research results, a localization radius of the water molecule valence electron that coincides with the O2- ion radius has been determined. The obtained results concerning DC conductivity vs. moisture level and temperature dependence of an electric insulation pressboard can make a basis for the identification of the amount of moisture accumulated in solid elements of a power transformer paper-oil insulation.

Creeping discharge developing on vegetable-based oil / pressboard interface under AC voltage

Abstract: The behavior of creeping discharges developing on the pressboard surface was investigated experimentally in palm fatty acid ester oil, natural rapeseed oil and commercial mineral oil under variable ac voltages with 60 Hz, respectively. The oilimpregnated pressboard was used as the solid dielectric and it was immersed completely into the same oil. The tungsten needle electrode was installed in the one side of pressboard surface without the counter electrode as a high voltage electrode and a grounded fine copper rod was adhered tightly in the other side of pressboard as a back side electrode. The discharge shape, streamer extension, streamer velocity and tracking at the oil/pressboard interface were observed as a function of the time and voltage by using a still camera equipped with a night viewer. Simultaneously, the discharge current and dissipated energy were measured. The gas components dissolved in oils by the creeping discharge were also examined by the dissolved gas-in-oil analysis (DGA). In this paper, it is presented that there are distinctive events due to the occurrence of creeping discharges.

Nanoparticle effects on creeping flashover characteristics of oil/pressboard interface

Abstract: Surface creeping discharge at solid-liquid interface is regarded as one of the reasons for oil-immersed transformers failure. In order to restrain the surface creeping discharge, we investigated the effects of TiO2 nanoparticles on creeping discharge and flashover characteristics of the oil/pressboard interface under AC and impulse voltages. Partial discharge and creeping flashover tests of oil/pressboard (OPs) and nanofluid/- pressboard (NPs) were carried out. The results show that TiO2 nanoparticles can improve both partial discharge inception voltage (PDIV) and creeping flashover strength of oil-impregnated pressboards. Thermally Stimulated Depolarization Current (TSDC) and Electrostatic probe techniques were used to measure space charge behaviors of oil/pressboard before and after modification of TiO2 nanoparticles. It reveals that the space charge dissipation rate at the surface of NPs is much faster than that of OPs. Moreover, TiO2 nanoparticles can greatly increase the shallow trap density and lower the shallow trap energy level of oil-impregnated pressboard. It is believed that electrons may be repeatedly trapped and de-trapped by shallow traps in NPs, which can reduce the accumulation of space charges and result in improving the creeping flashover strength of oil/pressboard interface.

The resistivity of oil and oil-impregnated pressboard varies with temperature and electric field strength

Abstract: The mineral insulating oil and pressboard are important insulation dielectrics in AC-DC converter transformers. Thus, it is very significant to understand the dielectric properties of the oil and oil-impregnated pressboard. In this paper, the influences of the temperature, field strength and aging on the resistivity of oil and oil-impregnated pressboard have been investigated. Samples used in the experiments are new oil, aged oil, pressboard immersed in new oil and aged oil respectively. The measuring device of resistivity was full of oil and isolated from outside. All samples were measured at temperature range of 25-80 °C under the electric field strength range of 2-10 kV/mm for oil and 3-30 kV/mm for oil-impregnated pressboard. The results showed that the resistivity of oil-impregnated pressboard decreased along with the increase of field strength and temperature. For all oil specimens and pressboard immersed in aged oil, the resistivity decreasing rate (|∂1gρ / E∂|) along with electric field strength showed a downtrend with the increase of temperature; while, for pressboard immersed in new oil, the decreasing rate showed an uptrend with the increase of temperature. For oil, the resistivity reduction rate with aging (r) decreased along with the increase of field strength and temperature. For pressboard immersed in oil, the reduction rate with aging (r) showed an uptrend with the increase of temperature.

Understanding surface discharge activity in copper sulphide diffused oil impregnated pressboard under AC voltages

Abstract: The surface discharge injected current were measured by using High Frequency Current Transformer (HFCT). It is observed that the rise time of the current pulse is about 800ps. In addition, the rise time of the current pulses has not altered in copper sulphide (CuS) diffused Oil Impregnated Pressboard (OIP) material, but the width of the current signal has increased with the increase in the amount of copper sulphide diffusion into the pressboard. It is confirmed that the injected current pulse radiates Ultra High Frequency (UHF) signal and the dominant frequency of the UHF signal formed due to the surface discharge injected current is about 0.9 GHz. The characteristic frequency contents of the UHF signal formed due to surface discharge activity is the same with the virgin and copper sulphide diffused pressboard material. The surface discharge inception voltage is high with the virgin specimen compared with copper sulphide diffused oil impregnated paper insulating material and is the same irrespective of the amount of diffusion of copper sulphide to it. The Phase Resolved Partial Discharge (PRPD) analysis indicates that at the point of surface discharge inception, the discharge occurs at the peak of the applied AC voltage and when the applied voltage magnitude is increased, the discharge occurs at the rising portion of the applied AC voltage. The characteristics of the surface discharges formed are the same with the copper sulphide diffused oil impregnated pressboard material. It is noticed that with increase in ageing time, a reduction in partial discharge magnitude is observed. Attenuated total reflectance Fourier transform infra-red spectroscopy (ATR-FTIR) analysis was carried out to understand the characteristic changes that occur in the degraded zone formed due to surface discharges. The measurement of surface charge indicates that ageing of oil impregnated paper material increases the surface charge accumulation. Energy Dispersive Spectroscopy (EDS) confirms the presence of copper and sulphur in the aged Oil Impregnated Pressboard material. The Atomic Absorption Spectroscopy (AAS) results indicate that the amount of copper content in pressboard material have increased with ageing time.

A comparison of the voltage withstand properties of ester and mineral oils

Abstract: EO has proved to be a viable substitute for MO. Its dielectric performance is always comparable with, if not superior to that of MO, at least when new, dried, and highly purified. The smaller permittivity mismatch between pressboard and EO, compared to that between pressboard and MO, leads to a more uniform field distribution at the oil/pressboard interface. We have observed that EO has higher PDIVs than MO for corona and creepage discharges. However, our understanding of the oil/pressboard interface is incomplete. There is a lack of standardization in PDIV measurements, particularly in the case of creepage discharges. Because industry is striving to provide new insulating oils for use at increased transmission voltages, improving electrical testing techniques and the understanding of the oil/pressboard interface is important.

Equilibrium charts for moisture in paper and pressboard insulations in mineral and natural ester transformer oils

Abstract: This article presents experimentally obtained moisture equilibrium charts for paper and pressboard insulations in mineral and natural ester transformer oils.

Diffusion coefficient in transformer pressboard insulation part 2: mineral oil impregnated

Abstract: Moisture is one of the most influencing parameters which accelerates solid insulation degradation process in transformers. Correct estimation of the moisture within solid insulation of transformer is still a challenge due to temperature driven complex moisture dynamics between oil and paper insulation. Although some authors have proposed coefficients for Kraft paper as well as for non-impregnated pressboard much less work has been done on oil-impregnated pressboard insulations. In this paper, the determination of the diffusion coefficient of moisture in transformer pressboard impregnated with mineral oil is tackled. Studies were carried out on different samples thickness of pressboard which are typical in power transformer. The experimental conditions were simulated using a mathematical model based on the Fick?s second law and an expression for the diffusion coefficient is proposed and validated experimentally.

Influence of harmonic AC voltage on surface discharge formation in transformer insulation

Abstract: The Ultra High Frequency (UHF) signal radiated during the surface discharge process under harmonic AC voltages with different Total Harmonic Distortion (THD) shows a dominant frequency at 0.8 GHz. The surface discharge inception voltage decreases with an increase in harmonics and THD as observed in the present work. The peak factor of the harmonic voltage and the surface discharge inception factor show inverse relationship. The rise time of injected current pulse due to surface discharges measured using High Frequency Current Transformer (HFCT) is about 600 ps and 900 ps for the positive and negative current signal, respectively. Cumulative energy spectra and Ternary plot confirms that UHF signal generated due to surface discharge process does not significantly vary under harmonic AC voltage with different THD's. The magnitude of UHF signal formed due to surface discharge under 50 Hz AC voltage is higher than that formed under the harmonic AC voltage. Surface charge measurement after corona charging process with the oil impregnated pressboard material indicates that the amount of charge deposited is high with the specimen damaged under harmonic AC voltage. Degraded zone of oil impregnated pressboard insulation due to surface discharges was analyzed using Attenuated Total Reflectance Fourier Transform Infra-red (ATR-FTIR) spectroscopy. Phase resolved partial discharge (PRPD) studies indicate that surface discharge activity predominates when the rate of rise of voltage is high.

Space charge behaviour in oil and impregnated pressboard combined insulation system

Abstract: Dielectric performance of oil-pressboard insulation system used in high voltage direct current (HVDC) convertor transformers can be affected by the presence of space charge. In this paper, the space charge behaviors in a 0.5mm thick oil film combined with a 1mm thick impregnated pressboard have been investigated by the pulsed electroacoustic (PEA) technique under 12kV/mm and 20kV/mm at room temperature. Two types of oil with different aging status were used for comparison. The results show that a charge peak is quickly formed at the interface between oil and pressboard with the same polarity as the electrode at the oil film side. However, the dynamics of the interfacial charges are very different for fresh oil and aged oil samples. The maximum electric field occurs in the middle of the pressboard, which is significantly enhanced in the aged oil samples. Difference in the electric field between those obtained from measurement results and those calculated from Maxwell-Wagner theory has been discussed

DC field distribution in HVDC transformer considering the effects of space charge and temperature due to presence of oil immersed pressboard

Abstract: HVDC transformers are one of the main components for long distance transmission, for which many investigations have been done all over the world in order to increase their efficiency and reliability. Regarding their insulation, oil immersed pressboards are widely used, representing one of the most important elements playing a role in transformer behaviors. Under DC operation, space charges accumulating between the oil and pressboard have been reported as the influential cause possibly giving rise to unexpected service failures under the transient voltage from the grid. However, it might be possible to presume that the accumulation of space charge could be facilitated inside the multilayered pressboard causing insulation breakdown in a different way. In addition, other parameters could be potential issues to investigate: non homogeneous temperature distribution along the oil circulation path and the DC field intensity. The former could explain the conductivity variation causing the non-uniform DC field distribution, while the latter plays a role in the variation of conductivity. With these in mind, three parameters have mainly been put under experimental investigation. For this purpose, a DC voltage is applied to the oil immersed pressboard, and then the polarization and leakage current are measured and analyzed. The mobility of the space charge is calculated from the former as a function of the number of layers, while the conductivity is obtained from the latter.

Comparison analysis to thermal aging properties of vegetable and mineral insulating oils

Abstract: The paper presents a comparison investigation to thermal aging behaviors between vegetable and mineral oil-paper insulation. Insulation pressboard samples were aged at 130 °C for 80 days in presence of either vegetable or mineral insulating oils, with copper wires together in well-sealed stainless vessels. The vessels contain some air in their vacant space at the beginning of the thermal aging experiments. The aged oil properties, including moisture content, acid number, viscosity, furfural concentration, dielectric loss, and AC breakdown voltage of oil samples, were measured at each interval of thermal aging experiment. The vegetable has a higher moisture content than mineral oil. Dielectric loss increases marginally with aging time in mineral oil while the increase is drastic for vegetable oil.

Diffusion coefficient in transformer pressboard insulation part 1: non impregnated pressboard

Abstract: Water is harmful for transformers; the dynamic behavior of moisture influences the dielectric integrity and insulation aging in a transformer on operation. In this paper, the determination of the diffusion coefficient of moisture in transformer pressboard is tackled. This work has been focused in analyzing non oil-impregnated pressboard, so the obtained coefficient is useful for studying the processes of transformer drying in factory. Drying experiments were developed in a thermogravimetric analyzer on nonimpregnated samples of pressboard subjected to different drying temperatures. The experimental conditions were simulated by means of a diffusion model implemented by means of Finite Element Method. The comparison between estimated and measured values allows drawing conclusions about the accuracy of the proposed coefficient.

Electrical–thermal aging characteristic research of polymer materials by infrared spectroscopy,

Abstract: Traditional insulation paper (pressboard) used in power transformers has weaknesses such as poor thermostability, low breakdown voltage, and high permittivity, which leads to its degradation or even breakdown over time. For this paper, in order to judge if they could be used as new insulation materials for transformers, polycarbonate and polyester films were selected for electrical–thermal aging tests in temperatures of 110°C and 130°C in comparison with the insulation paper. Several infrared spectral tests were carried out on the three materials under a scanning electron microscope to analyze their electrical–thermal aging characteristics, mechanical behaviors, and degrees of polymerization. The functional group whose absorption peak intensity decreased drastically with aging was referred to as the characteristic functional group, and its peak absorption intensity was used to reflect its aging level. This paper found that the polycarbonate had a better aging resistance than both the insulation paper and PET. Copyright © 2014 John Wiley & Sons, Ltd.

Effect of Different Shapes of Electrodes on Bridging in Contaminated Transformer Oil

Abstract: Contaminated transformer oil has been tested under non uniform electric fields and the effect of different electrode systems presented in this paper. Three different electric fields were examined i.e. DC, AC and DC biased AC. These experiments revealed that with all the different electrodes arrangements, contaminated particles were always formed bridges between electrodes under DC electric field. AC field does not induce any bridging. Combination of AC and DC enhances the bridging dynamics. The bridges were thicker or more particles attracted with more uniform electric field (spherical electrode) than diverse electric field (needle-plane). During the operation oil is in contact with metal, iron core and pressboard insulation. Contaminants such as metal filings or cellulosic residue can be formed in the oil, especially for transformers with aged paper insulation. Non-uniform fields are present within the transformer. These contaminants tend to move towards high field regions due to dielectrophoresis (DEP) forces (2). Once in the high field regions, these particles may acquire a charge when in contact with different parts of the transformer and contribute to a leakage current (3). They also could form a bridge over a period of time. The bridge may potentially act as a conducting path between two different potentials within the transformer structure, leading to partial discharges or insulation failure. We have reported previously the bridging with bare spherical electrode under different combinations AC and DC electric fields as well as mathematical model of the bridging (2-6). This paper focuses on the effects of strong non-uniformity of an electric field in the vicinity of sharp edges and how it affects the bridging. The experiments have been carried out to investigate the particles' accumulation between two different shapes of electrodes under different conditions: AC, DC and AC biased with DC voltages. The paper presents the details of the experimental setup and explains the observed results. Three different levels of AC and DC voltages were investigated for these experiments for needle-plane and spherical sets of electrodes. Different levels of contaminations were also examined. Optical images of bridge formation process were taken and conduction current was also recorded simultaneously during all the tests. Pressboard fibers were produced by rubbing an unused pressboard typically used in high voltage transformer by different sizes of metal hand files. Different sizes of sieves were used to separate the fibers by width rather than length i.e. 250-500 μm, 150-250 μm, 63-150 μm and less than 63 μm. All the four sizes of particles were tested under DC electric field. Only the results from 150-250 μm and 250-500 μm tests are discussed in this paper. Only 63-150 μm size particles were investigated for AC and DC biased AC experiments. The contamination levels for each size of particles were 0.001%, 0.002%, 0.003%, 0.004%, 0.006%, 0.008%, 0.016% and 0.024% by weight. For DC experiments only 0.001% to 0.004% contamination levels were investigated whereas for AC all the above contamination levels were used. In the case of DC biased AC, only 0.024% which was the highest contamination level was tested along with a pure 3 kV DC voltage.

Feasibility study for detecting copper contaminants in transformer insulation using laser-induced breakdown spectroscopy

Abstract: In recent times, copper sulphide (Cu2S) diffusion in the transformer insulation is a major problem reducing the life of transformers. It is therefore essential to identify a simple methodology to understand the diffusion of Cu2S into the solid insulation [oil impregnated pressboard (OIP)]. In the present work, laser-induced breakdown spectroscopy (LIBS) was adopted to study the diffusion of Cu2S into the pressboard insulation and to determine the depth of diffusion. The diffusion of Cu2S in pressboard was confirmed by electrical discharge studies. In general, flashover voltage and increase in ageing duration of pressboard insulation/Cu concentration had inverse relationship. The characteristic emission lines were also studied through optical emission spectroscopy. Based on LIBS studies with Cu powder dispersed pressboard samples, Cu I emission lines were found to be resolvable up to a lowest concentration of 5 μg/cm2. The LIBS intensity ratio of Cu I–Ca II emission lines were found to increase with increase in the ageing duration of the OIP sample. LIBS studies with OIP samples showed an increase in the optical emission lifetime. LIBS results were in agreement with the electrical discharge studies.

Influence of oil property on the space charge characteristics and electric field distortion in oil-paper insulation

Abstract: The formation of space charge in oil/paper insulation system can lead to material degradation in the high electric field region and affect system reliability. It is important to understand factors that affect space charge formation in oil/paper insulation system. All the researches on the space charge characteristics of oil-paper insulation are all based on the oil impregnated insulation paper. In this paper, the space charge dynamic behavior of the oil impregnated insulation pressboard and the mixed insulation with a combination of oil gap and oil impregnated insulation pressboard were both studied. The oil property effect on the charge and electric field distribution behavior were discussed. The injected charge in aged oil impregnated insulation pressboard has a higher migration rate than that in new oil impregnated insulation pressboard. Compared with the oil impregnated insulation pressboard, a lot of space charges accumulate at the interface between oil and pressboard, which leads to obvious electric field distortion in oil gap and in oil impregnated insulation pressboard. The charge injected in new oil-paper insulation is more difficult to dissipation than that in aged oil-paper insulation.

Analysis of the micromechanical deformation in pressboard performed by X-ray microtomography

Abstract: A large number of electrical insulation components are produced in paper-based materials. Paper combines good insulating properties with the necessary mechanical and chemical stability. Paper consists of a system of fibers binding to each other creating a strong network. The presence of large open pores allows for impregnability of the material but also causes mechanical weakness in particular in the out-of-plane direction of the material. This aspect is important for pressboard components, where the resistance to compression stress is relevant for e.g. transformer windings. It is therefore relevant to understand the mechanisms that underlay the out-of-plane deformation of pressboard. In order to get a clear picture of the deformation patterns within the material, X-ray micro-computed tomography was used. Pressboard test pieces were subjected to in-situ out-of-plane compressive loading. 3D images of the sample could be captured before, during and after the loading sequence. Image analysis allowed for the definition of strain fields. The results revealed a strong correlation between the density variation within the sample and the strain calculated from the 3D images.

Space charge dynamics in oil and thick pressboard combined system under polarity reversal voltage

Abstract: In this study, the space charge dynamics in an oil and oil impregnated pressboard combined insulation system are investigated by the means of pulsed electroacoustic (PEA) technique under voltage polarity reversal conditions. To be closer to the real case in convertor transformers, a 1mm thick pressboard covered with a 0.5mm thick oil film were used as the testing sample. For comparison, three types of oil with different aging stages were used: fresh oil, medium aged oil and severely aged oil. The external electric field was firstly applied +10kV/mm for 1hour, then reversed to -10kV/mm in 1min period. After that, the -10kV/mm was applied for 1 hour. The results have clearly shown the impact of the oil film and the interface between oil and pressboard on charge dynamics under polarity reversal conditions. The electric field enhancement and the time constant of the 'mirror image effect' have been analyzed for different cases.

Analysis of ageing characteristics of solid insulation in mixed insulating fluids

Abstract: The practical stability of solid insulation used in the transformer depends on liquid insulation, where periodic assessment of solid and liquid insulation is mandatory. The usage of mineral oil in the transformer system cause rapid deterioration of solid insulation and which in turn weakens the strength of liquid insulation. Instead of traditional mineral oil, combinations of natural esters and mineral oil have been formed and these mixed natural ester insulating fluid samples are compared against the commercial mineral oil. The ageing tendency of Kraft paper and Pressboard are evaluated by treating the test pieces with samples of mixed insulating fluid groups `A', `B', `C' and `D' by accelerated thermal ageing test. Samples and test piece (Kraft paper and Pressboard) were subjected to acidity and water content test before and after accelerated thermal ageing test as per IEC and IEEE standards. By visual observation, it is agreed that the solid insulation in the 100% composition of natural esters samples ages at a much slower rate than sample of 100% mineral oil. Also the ageing rate increases rapidly with the increasing proportion of mineral oil in the samples.

Modeling and experimental validation of the mechanical behavior of pressboard

Abstract: High density (HD) pressboard is an essential element in power transformers combining good electrical insulation properties with effective mechanical characteristics that well suit design requiremen ...

Effect of Kraft Paper Barriers on Bridging in Contaminated Transformer Oil

Abstract: The paper investigates effects of Kraft Paper to prevent bridging in pressboard fibre contaminated transformer oil. There are three different configuration of Kraft paper tested. These are 1) covered electrode with loose bond, 2) tight bond and 3) bare electrode with a paper barrier in the middle. There were no bridges formed with loosely bonded tests with DC electric fields. For the other two cases, there was always a complete bridge formed.

Interaction of pressboard with impregnating fluids in HVDC insulation systems

Abstract: Pressboard is one of the key components in insulation systems for HVDC (high voltage direct current) equipment. The conductivity of pressboard is one of the most important properties of these systems. It is dependent on various parameters, e. g. time, temperature, electrical field strength, water content and impregnating fluid, which are varied in this paper. Pressboard manufacturing is described and results of conductivity measurements of pressboard according to the PDC method (step response measurement in time domain) are presented. In order to come to a physical understanding of pressboard conductivity in transformer insulation systems, conductivity of dry pressboard is examined under vacuum. Low electric field strength leads to a restriction in parameter variation (Paschen's law) and therefore, pressboard impregnated with dry air is investigated. No significant influence of impregnating air on pressboard conductivity is found. Moreover, pressboard is impregnated with mineral oil and the change in pressboard conductivity by impregnating with different fluids is shown. Polarization and conduction processes in insulation systems are taken into account. With an understanding of these processes, transformer design can be improved.

Electrical properties of oil-paper insulation affected by water conductivity during paper-making process

Abstract: Developments of ultra-high voltage grid in China require better insulation materials, and that forces the transformer presspaper and pressboard manufactures to pay more attention to the papermaking process to enhance their product qualities. In this study, pure water and three kinds of sodium chloride solutions with conductivities of 100 μS/cm, 500 μS/cm and 1000 μS/cm were respectively used to prepare presspaper samples. By vacuum impregnation, four kinds of oil-paper samples were acquired. Test results showed that relative permittivity and tand of oil-paper specimens first varied little then increased a lot with the increasing of water conductivity; DC breakdown field decreased while AC breakdown field changed little. To confirm the differences were due to water conductivity, ion chromatography was used to determine the sodium and chloride ion contents of the presspaper samples, we found that the ion contents increased with electrical conductivity. This indicates that water conductivity does have effects on most electrical properties of oil-paper insulation and lower conductivity can bring better insulation performances.

Streaming Electrification Dynamics in the Multilayer Sensor

Abstract: Streaming electrification dynamics is experimentally investigated by using a multilayer sensor in a closed loop and fresh transformer oil. This sensor consists of two identical rectangular oil ducts constructed by pressboard paper and separated by a middle copper plane electrode. The ground current from this middle electrode is measured. Two outer steel plane electrodes are connected together and used to measure the capacitive “accumulation” current. The leakage currents at the sensor inlet and outlet, as well as the streaming current are also measured. The waveforms of these five currents together and oil temperature at the sensor inlet are simultaneously recorded for laminar flow and at controlled operating conditions; namely, oil temperature, mean flow velocity, humidity, and pressure. The results reveal that the all-recorded currents nonlinearly augment with oil temperature and linearly with oil-flow velocity. These results are interpreted in terms of shear stress, physicochemical reaction, the effect of streaming/floating potential on the electrical double layer (EDL), temperature-dependent oil properties, and introducing an expression of the streaming current. Calculations of accumulated charges on steel electrodes and the loating potential of the copper electrode at different oil-flow velocities, and the fully developed wall space-charge density and physicochemical reaction coefficient are presented.

Analysis of the dielectric and breakdown characteristics of Nano MMT modified insulation pressboard

Abstract: In order to improve the breakdown voltage of oil/press board insulation structure in converter transformer, the MMT ( Montmorillonite) modified pressboard was developed by nano doping method. The conductivity, relative permittivity and breakdown strength of modified pressboard were measured. The measuring results show that the conductivity of modified pressboard increases at high nano doping ratio, and it's obviously influenced by electric field strength. The relative permittivity of modified pressboard decreases to the lowest at nano doping ratio of 1%, which is 14% lower than non-modified ones. Moreover, the breakdown strength of modified pressboard at nano doping ratio of 1% is higher than other ratios under DC and AC voltage. Based on the dielectric characteristics of modified pressboard, the homogenizing effect of the nano modified pressboard on electric field distribution was verified hy simulation. Finally, the breakdown voltages of oil/pressboard insulation structure with modified pressboard at different nano doping ratios were tested. The results show that the DC and AC breakdown voltage can be improved in varying degrees by using the modified pressboard.

Analysis of Electric Field Homogenization of Converter Transformer Barrier System Based on Nano Modification of Pressboard

Abstract: In order to solve the problem of non-uniform electric field distribution in converter transformer barrier system caused by conductivity difference between the transformer oil and pressboard, the SiC(silicon carbide) modified pressboard was developed by nano doping method. The conductivity of modified pressboard was measured. The measuring results show that the conductivity of modified pressboard increases exponentially with the increase of nano SiC doping ratio. The electric field strength has obvious influence on conductivity at high nano doping ratio, which shows great nonlinear characteristic. Based on the nonlinear conductivity characteristic of modified pressboard, the homogenizing effect of the nano modified pressboard on electric field distribution was verified by simulation. The simulation results show that the electric field distribution under DC and polarity reversal voltage in barrier system can be well homogenized by using the nonlinear characteristic of nano modified pressboard.

Characteristics of partial discharge in the oil gap separated by pressboard barrier in oil/pressboard composite insulation system

Abstract: This paper deals with the characteristics of partial discharge in oil/pressboard composite insulation system, which is adopted in oil-immersed power transformer. In the insulating structure of the transformer, large oil gaps are often subdivided by pressboard barriers into small ones. Once partial discharge occurs at the point where the electric field is enhanced and progresses, the electric field distribution may change from the static one. As a result, there are possibilities that partial discharge successively occurs at the oil gap across pressboard barrier, which might be the trigger of breakdown of the apparatus. In this study, the authors experimentally observed the partial discharge generation across the pressboard barrier in an element model and examined the mechanism of the above phenomenon.

Characteristics of discharge progress across barrier in the oil/pressboard composite insulation system under impulse voltage application

Abstract: It is crucial for reasonable transformer insulation design to understand the characteristics of discharge progress under the application of a standard lightning impulse voltage. This paper deals with the fundamental discharge characteristics of oil/pressboard (PB) composite insulation system. We obtained discharge images using a digital camera equipped with an image intensifier. In this research, we focused on the route of discharge progress along pressboard barrier. Furthermore, we evaluated the discharge velocity as well as the time required for the successive discharge generation across the barrier PB.