Showing papers on "Pressboard published in 2015"

Experimental studies of influence of DC and AC electric fields on bridging in contaminated transformer oil

Abstract: Analysis of real operating condition revealed that HVDC transformers experience combined effect of DC biased AC electric field. The dynamics of pressboard particle in contaminated transformer oil under the influence of DC, AC and DC biased AC electric field has been investigated in this paper. Different levels of particle concentrations are tested at different applied voltages. Optical images of the particles accumulation together with conduction current have been recorded during the experiments. A complete bridge between the electrodes of cellulose particles were observed for all the tests carried out under DC and DC biased AC electric field. Opposite to that, for AC experiments, pressboard particles accumulated on surfaces of both electrodes but did not create a full bridge between the electrodes. It is concluded that a combination of DC and AC voltages in a HVDC transformer could lead to a bridge formation within the equipment which could cause failure.

Formation of water nanodrops in cellulose impregnated with insulating oil

Abstract: This article presents results on the DC conductivity of moist pressboard impregnated with insulating oil and its dependence on the moisture and temperature of the samples. The transfer of charges was found to take place because of electron hopping between potential wells produced by water nanodrops. The average dimension of the nanodrops was around 2.32 nm, forming approximately 220 water molecules.

An analysis of AC conductivity in moist oil-impregnated insulation pressboard

Abstract: The paper presents research results of testing AC conductivity in a moist oilimpregnated electric insulation pressboard, depending on the moisture content level and temperature of the samples. A new method has been elaborated for converting experimentally obtained dependences from frequency-domain spectroscopy (FDS) to the reference temperature of 293 K (20 /dpl deg/C). Activation energy values have been determined for the conduction and relaxation time. It has been found the conductivity is realized by electron tunnelling between potential well formed by water molecules contained in the structure of moist cellulose.

Space charge behavior in thick oil-impregnated pressboard under HVDC stresses

Abstract: The space charge accumulation can affect the dielectric performance of oilimpregnated pressboard in convertor transformers. In this paper, the space charge behaviour in 1mm thick impregnated pressboards is investigated by the means of pulsed electroacoustic method. The dried pressboards are separately impregnated in fresh oil and service aged oil as comparison. The space charge distributions are investigated under ±15 kV/mm and ±25 kV/mm at room temperature. The homocharge injection can be observed for all the samples, which is much more significant in the aged oil impregnated samples. The amount of injected charges and the depth of the injected charges accumulation in the pressboard bulk are closely related to the oil properties, amplitude and polarity of the applied voltages. The service aged oil shows the dominant effects on the local electric field distortion, about 48% enhanced in the aged oil impregnated samples while about 15% enhanced in the fresh oil impregnated samples. The estimations of the apparent charge mobility and trap depth also indicate the great impacts caused by the service aged oil.

Experimental analyses and molecular simulation of the thermal aging of transformer insulation paper

Abstract: Thermal aging of insulation paper occurs during transformer operation, with the water produced during aging accelerating the thermal aging process. Insulation paper aging largely determines the operational life of a transformer pressboard; accordingly, research pertaining to insulation paper enhancement is significant with respect to the functional safety of transformers. Within molecular research, particularly that focusing on molecular simulation, appropriate force field selection is paramount in order to acquire accurate results, in addition to appropriately reflecting cellulose thermal stability and associated degradation mechanisms. In the current study, two diverse cellulose models (water only; water, dicyandiamide, melamine and polyacrylamide) are developed and analyzed. Numerous model variables, including the diffusion coefficient of water and cellulose chain distance, are collated, in order to examine the effects of amines on the thermal stability of cellulose. Additionally, a novel insulation paper is formulated by combining traditional insulation paper with three diverse amines; an accelerated aging experiment is thus undertaken, comparing traditional and modified insulation paper samples at 110 °C, together with periodic assessment of both microwater content and the degree of polymerization (DP). Results indicate that during transformer operation, a higher degree of elasticity is associated with the modified insulation paper, thus significantly increasing its toughness and extensibility, in parallel with decreasing it brittleness. Results also show that the DP associated with cellulose comprising the modified insulation paper decreases at a reduced rate, with measured water content significantly less than that of traditional insulation paper. Simulation results are consistent with experimental findings. Therefore, it may be concluded that the thermal aging properties of cellulose significantly improve upon addition of dicyandiamide, melamine, and polyacrylamide as thermal stabilizers.

Understanding and analysis on frequency dielectric parameter for quantitative diagnosis of moisture content in paper–oil insulation system

Abstract: The ageing rate of cellulose insulation materials (paper/pressboard) in oil-impregnated transformers is largely determined by moisture. In this paper, several experiments were conducted under controlled laboratory conditions to quantitatively diagnose the amount of moisture in cellulose insulation using frequency domain spectroscopy (FDS) technique, which has been proven to be an effective tool to predict the insulation condition in oil-impregnated power equipment. The frequency dielectric response in oil-impregnated pressboards, moisture content as well as test temperature, was monitored carefully, and a new dielectric characteristic parameter for the diagnosis of moisture content was proposed. In addition, the master curve technique was introduced to calibrate the temperature effect on FDS measurement results. These experiments revealed that a strong correlation exists between frequency dielectric parameter and moisture content in cellulose insulation. Such correlation can be utilised for better prediction of moisture content in paper–oil insulation. A modified assessment method for the diagnosis of moisture content in paper–oil insulation was investigated and its effectiveness was also demonstrated using a case.

The influences of solid surface on the propagation of creepage discharge in insulating liquids

Abstract: The influences of solid surface on the propagation of creepage discharge in liquids were investigated under ac divergent electric field. Synchronized phase-resolved partial discharge (PD) detection, wideband current measurement and streamer channel visualization were employed to study the differences of discharge patterns between in open gap and on solid surface and to explore the mechanisms behind from the linkages between consecutive discharges. It is found that Perspex, glass and pressboard would all promote the propagation of discharges, especially negative discharges, and enable more discharges to occur at lower instantaneous voltages. It is experimentally verified that this promotion effect of solid surface on discharges is caused by the memory effect of solid surface for space charges and residual low density channels.

Understanding Moisture Dynamics and Its Effect on the Dielectric Response of Transformer Insulation

Abstract: Dielectric response measurement has recently been adopted by utilities for evaluating moisture content in cellulose insulation (paper and pressboard) of transformers. Moisture distribution is highly dependent on temperature. Since the temperature inside a transformer may change during the dielectric response measurement, the moisture in the transformer’s cellulose and oil insulation can hardly attain an equilibrium state. Instead, moisture dynamics exist inside the transformer: 1) cellulose absorbs (desorbs) moisture from (to) oil with the changes in temperature and 2) moisture migrates inside cellulose due to a moisture gradient. This paper investigates moisture dynamics and its effect on the dielectric response of a transformer’s cellulose insulation. It proposes a distributed parameter model to reveal the correlation between moisture distribution (under nonequilibrium conditions due to thermal transients) and dielectric response parameters (dielectric losses and permittivity) of cellulose insulation. It then estimates these parameters under moisture nonequilibrium conditions. The accelerated ageing and moisture diffusion experiments are conducted on a prototype transformer to verify the proposed model. The methodology developed in this paper can help the proper interpretation of dielectric response measurement of field transformers under thermal transients.

Compare the performance of natural ester with synthetic ester as transformer insulating oil

Abstract: Natural and synthetic esters are considered as potential candidates to replace mineral oil in power transformers. However, there is comparatively less experience on usage of ester-based insulating liquids in large power transformers. The high reliability and safety level required for large power transformers lead to careful assessment of the use of ester as the insulating liquid in these units. Therefore, this study assesses the ageing behavior of the dry pressboard insulation immersed in a commercially available natural ester and a synthetic ester through accelerated ageing experiment. At different stages of ageing, oil conditions are analyzing by measuring acidity value, 2-furaldehyde (2-FAL) concentration and kinematic viscosity. Dissolved Gas Analysis (DGA) is also performed to assess the degradation of both pressboard and oil. Moreover, the change of AC breakdown strength of natural and synthetic ester due to mixing with mineral oil (new and aged) is analyzed. This paper reveals that pressboard ageing in both types of oils associates with dissolved 2-FAL and CO 2 +CO concentration in oil. It has been observed that there is different gassing behavior between natural and synthetic ester due to thermal ageing. This paper reveals that, the AC breakdown strength of both types of ester used in this study decline due to mixing with mineral oil.

Effects of thermal aging on creepage discharge in oil- impregnated pressboard under combined AC-DC voltage

Abstract: In this study, we investigate the characteristics of creepage discharge on oilimpregnated pressboard under combined AC–DC voltage conditions using a sphereplane electrode configuration. In our experiments, we use four oil-impregnated pressboard samples subjected to different thermal aging time. Experimental results show that the creepage discharge inception voltage (CDIV) and creepage discharge flashover voltage (CDFV) are lowest under pure AC voltage and gradually increase with higher DC voltage components. Longer aging time may contribute to higher CDIV with low DC voltage components; however, with higher DC voltage components, the effect of aging time is less pronounced. The CDFV is not significantly influenced by aging time. The length of carbonized tracks and the development velocity of the creepage discharge increase and the endurance time decreases with longer aging time and higher DC voltage components. We divide the development of creepage discharge in aged pressboard into four stages according to changes in the discharge characteristic parameters. With longer aging time, characteristic parameters exhibit less fluctuation and development velocity exhibits greater sensitivity to the DC voltage component. Our experimental results demonstrate that the characteristics of creepage discharge are significantly influenced by the age of the pressboard and by DC voltage components.

Moisture diffusion coefficients of transformer pressboard insulation impregnated with natural esters

Abstract: In previous studies moisture diffusion coefficients for transformer pressboard insulation impregnated with mineral oil have been proposed. In this paper diffusion coefficients are proposed for transformer pressboard insulation impregnated with different natural esters. The coefficients were validated with experimental data and compared with those of mineral oil. In the determination of the moisture coefficients drying experiments at different temperatures were performed on pressboard samples of several thicknesses. An optimization process based on particle swarm and a finite element model were used to derive the coefficients from experimental data.

Behavior of the charge accumulation at the pressboard/oil interface under DC external electric field stress

Abstract: Mineral oils are commonly used for cooling and electrical insulation purposes in electrical power transformers. The physicochemical process appearing at the Pressboard/Oil interface, leads to the formation of an Electrical Double Layer (EDL). Therefore the interface is polarized by the EDL, the charge of one sign is generated within the solid medium and the opposite charge remains in the liquid medium. The process of flow electrification is due to the convection of the diffuse layer inside the liquid. The test loop was recently optimized with the addition of an energized wrapped copper electrode in the sensor. This modification allows studying the effect of an external electric field on the charge production and the distribution of the EDL : in the liquid via the streaming current (global charge production) and in the pressboard via the capacitive current. This capacitive current allows quantifying the accumulated charge within the pressboard plate (local charge production). Therefore, the dynamics of the accumulated charge refer to two EDL affected in the same way by the DC electric field. This paper proposes to show the influence of an external DC electric field on the accumulated charge within the pressboard as well as on the streaming current.

Creeping discharges over pressboard immersed in jatropha curcas methyl ester and mineral oils

Abstract: Due to their high biodegradability, non-toxicity and higher fire safety guarantee, vegetable-based oils are considered today as a potential substitute for mineral and synthetic liquids for electrical insulation and especially in high voltage power transformers. However most of known vegetable oils are derived from food materials (rape-seeds, sunflower, palm, olive). And one has to be vigilant to the fact that the replacement of mineral oil which is a petroleum-based product by natural ester fluids based on “renewably sourced” vegetable oils, does not create new problems as this could cause global food crisis due to diversion of food. An interesting product can be jatropha curcas oil extracted from jatropha curcas plants (“miracle tree”) which is non-food crops. This paper presents an experimental study of creeping discharges propagating over pressboard of different thicknesses immersed in jatropha curcas methyl ester oil (JMEO) and mineral oil (MO) under positive and negative lightning impulse voltages (1.2/50 µs), using two divergent electrode configurations (electrode point perpendicular and tangential to pressboard). It is shown that the thickness of pressboard significantly influences the characteristics of creeping discharge and especially the stopping (final) length L f and the density of branches. For a given thickness, L f increases with the voltage and decreases when the thickness increases. L f is longer when the point is positive than with a negative point. For a given voltage and thickness of pressboard, the values of L f in mineral oil and JMEO are very close

Understanding the surface discharge characteristics of thermally aged copper sulphide diffused oil impregnated pressboard material

Abstract: An attempt has been made to understand the impact of thermal aging with copper sulphide diffused oil impregnated pressboard (OIP) material for surface discharge inception voltage (SDIV) variation. The SDIV of Copper sulphide diffused OIP material reduces with increase in thermal aging. The UHF signals generated due to surface discharges with copper sulphide diffused OIP at the point of inception have frequency content in the range 0.7-1.5 GHz with its dominant frequency at 0.9GHz. HFCT measurement indicated that the rise time of injected current pulse due to surface discharge activity in the negative and positive half cycle of the AC voltage are about 0.8ns and 1ns respectively. It is also found that increase in thermal aging of copper sulphide diffused OIP, the amplitude and width of the current pulse increases. The magnitude of surface charge and its mean life is high with thermal aging temperature of copper sulphide diffused OIP material. Polarity of surface charge acquired by OIP due to AC corona charging depends on the surface condition of the pressboard material and its charge decay rate is lower than under DC voltage. Atomic Absorption Spectroscopy (AAS) and Energy Dispersive Spectrometry (EDS) study confirms the presence of copper sulphide on the surface of OIP and is found to be increased for copper sulfide diffused OIP material compare to unused OIP material. Scanning Electron Microscope (SEM) analysis indicates formation of cracks with thermally aged OIP material. Phase Resolved Partial Discharge (PRPD) study indicates that the surface discharge activity occurs near the peaks of the applied AC voltage at inception and at the rising portion of the applied AC voltage at higher voltages. It is found that the tensile and flexural strength reduces drastically for thermally aged copper sulphide diffused OIP material compare to unused specimen.

Space charge dynamics in oil-impregnated pressboard under AC electric field

Abstract: Space charge has been a general concern related with the reliability and the life-time of HVDC components. In this present work, the space charge profiles in oil-impregnated pressboards are investigated by the means of the pulsed electroacoustic (PEA) technique under AC voltage with 50 Hz at the room temperature. It has been observed that the ageing status of the mineral oil and the presence of the oil gap can influence the charge accumulation significantly. In the fresh oil impregnated pressboard, considerable negative charges are accumulated near the aluminium electrode. However, only little or no space charge can be detected in the aged oil impregnated pressboard and the oil gap and pressboard composite insulation system. The results may suggest that the amount of the AC space charge is highly dependent on the charge mobility within the dielectrics. The higher charge mobility can result in a smaller amount of AC space charge.

Development of a novel dual temperature model for insulation aging in oil immersed transformers

Abstract: Power transformer is one of the most expensive equipments in a high voltage AC power system. Adequate monitoring system for power transformers can help in reducing the failure rate and thereby enhancing system reliability and economic efficiency. The failure of power transformer has serious consequences on system reliability. Long term and comprehensive testing cannot be done in working transformer, so prorated model of transformer is preferred. In the proposed work, a new dual temperature test cell is developed based on IEC 62332 and IEEE C57.100. The cell provides for a current carrying conductor wrapped with insulating paper and further insulation is provided with high-density pressboard as per arrangements in power transformer. In this paper a new mathematical model is developed using activation energy and preexponential constant calculated from experimental results at different temperatures. The expected life based on degree of polymerization (DP) value, shows nonlinear aging of insulation based on investigation carried out on prorated model.

Research on some related mechanisms of corrosive sulfur problems in mineral oils

Abstract: Since a number of failures have happened on transformers and reactors due to the corrosive sulfur problem, especially for Dibenzyle-disulfide (DBDS) in mineral insulation oils, many researchers have carried out a series of studies on such issue. In this paper, some related mechanisms of corrosive sulfur problems in mineral insulation oils are introduced in detail. Such as the mechanism of DBDS how to change into copper sulfide (Cu 2 S), and which results are proved and checked by gas chromatography equipped with mass spectrometry (GC-MS), when there is copper sulfide deposited, what the surface color and microstructure of copper and insulation paper would be change are shown via scanning electron microscope (SEM), and the mechanism of Cu2S how to influence the dielectric properties of oil immersed insulation pressboard, together with the changing tendency of dissipation factor, permittivity and breakdown strength etc are introduced. Besides, the mechanism that how oxygen in oil to influence the Cu2S migrating from the copper surface into oil or insulation paper is given, and the activation energy of DBDS reacting with copper is obtained by Arrhenius equation, which is about 43.73 kJ/mol in air condition and 98.61 kJ/mol in nitrogen condition, at last, the mechanisms that how to inhibit such corrosive phenomena in mineral insulation oil via adding some passivators are introduced as well.

Influence of thermally aged barrier on corona discharge activity in transformer oil under AC voltages

Abstract: Corona Inception voltage (CIV) / breakdown voltage increases when the barrier is placed anywhere from needle tip to the 20% of the needle-plane electrode gap above which a marginal reduction in breakdown voltage is observed but its value is much higher than the breakdown strength of the electrode gap without barrier. Thermally aged oil impregnated pressboard insulation, as barrier, have high impact on CIV and breakdown voltage. The injected current due to corona activity, its rise time, measured using HFCT, in the negative and positive half cycle of the AC voltage measured to be 600 ps and 500 ps respectively. The shape of the current pulse is the same with the aged pressboard insulation barrier in the electrode gap. The corona inception voltage was measured by using UHF sensor and the signal has frequency contents between 0.5 -2.2 GHz. The surface charge measurements indicates that the amount of charge accumulated is high with thermally aged oil impregnated pressboard (OIP) than the unused material. Polarity of surface charge acquired by OIP due to corona charging depends on the surface condition of the material. To understand the mechanical behavior of the aged specimen, tensile test and flexural test were carried out and could conclude that ageing of material reduces the fundamental mechanical properties of the material. SEM analysis indicates that scission of fibers occurs due to thermal ageing at high temperatures. Partial Discharge studies on corona activity indicate, on corona inception, discharges occur near peak of the applied AC voltage, irrespective of position of barrier. The phase width of the PD generated due to corona discharge activity is slightly high with thermally aged specimen compared with unused specimen.

Investigations on the flashover characteristics of ester oil impregnated pressboard under the influence of nanoparticles

Abstract: Surface discharge is the kind of partial discharge that occurs along the surface of the solid insulation in contact with the gas or liquid insulation It degrades the solid insulating materials and reduces the life time of the equipment In order to prevent the surface discharge, the influence of the SiO 2 nanoparticleson the partial discharge characteristics of the oil impregnated pressboard insulation is to be investigated And now a days, the natural esters are preferred as the alternative for the mineral oil To employ natural ester in the transformer, we have to investigate the properties and changes in the characteristics of the solid insulation with the natural esters In this work the flashover characteristics of pressboard samples with pure transformer oil, Rice bran oil (RBO) are also to be analyzed The experiments are carried out on the 1 mm, 3 mm thickness pressboard samples by using sphere to sphere electrode configuration under the AC voltage The result shows that nano ester fluid improves the partial discharge inception voltage (PDIV) and flashover characteristics of oil impregnated pressboards

Ageing analysis of mineral insulating oils using CCTO Nanofluids

Abstract: Power Transformers are one of the most critical component of power system in which mineral oil is used for both insulation and cooling purposes. It acts a insulating medium between solid insulations like kraft paper, pressboard etc. In transformer, dielectric strength of insulation materials mainly depends on the properties of the transformer oil. In this work, a comparative analysis can be made with pure and aged mineral insulating oil using high dielectric permittivity CCTO(CaCu3Ti4O12) nanopowder and its electrical and physical properties are investigated. Nanopowder is added with mineral insulating oil under various particle volume fractions to form nanofluids (NF's). The electrical and physical properties like breakdown voltage, loss tangent, viscosity, interfacial tension, flash point were measured at room temperature under IS standards. It was observed that breakdown voltage and flash point of pure and aged mineral oil shows improved results after the addition of nanopowder with respect to increase in particle volume fractions and temperature.

Investigation on static electrification phenomenon of ester liquids and mineral oil

Abstract: This paper aims at the comparison of the Electrostatic Charging Tendency (ECT) of mineral, vegetable and synthetic oils using a spinning disk system (CIGRE Cell) covered on its both sides with different types of pressboards and papers. The ECT of insulating oils generated by the charge concentration gradient at the oil - pressboard/paper interfaces are analyzed in function of the rotating speed of the disk. It is shown that the generated charge is slightly higher with vegetable oil than that with mineral and synthetic ester oils. This may be due to differences in the molecular structures of these oils, whose interaction with the pressboard/paper is different; the phenomena occurring at the double-layer (interface) are not identical. However, the differences observed do not constitute an obstacle to the use of vegetable oil as a substitute for mineral or synthetic oils, with respect to the problems and risks associated with static electrification phenomena.

Influence of contamination on the electrical performance of power transformer oil

Abstract: Transformer failure statistics from all over the world showed that almost 30% of them were due to insulation Large amount of those failures were due to particles in transformer oil Main focus of this research is to effects of contamination on electrical performance of transformer oil A literature review of major causes of transformer failure, breakdown mechanisms of transformer oil has been conducted The experimental setup and results from the pressboard-dust-contaminated transformer mineral oil test are also discussed Several experiments have been carried out with cellulose particle contaminated transformer mineral oil The experiments of bridge formation are conducted under the influence of DC, AC, and DC biased AC voltages Samples with several levels of contaminant are tested under different voltage levels The influence of different electrode systems is also tested ie bare electrode, covered electrode, bare electrodes with paper barrier, spherical and needle – plane electrodes These experiments revealed that the bridges are always formed under the influence of DC voltages The particles are attracted towards high electric field due to Dielectrophoretic (DEP) force and become charged once in contact with the electrode surface Long fiber particles were attached to the electrodes and aligned parallel to electric field towards the other electrode More particles attached to the initial fibers and the process continued until a full bridge formed between the electrodes The conduction current increased with contamination levels as the bridge thickened with increment of contaminations There is no complete bridge formed under AC electric field The particles were attracted to the high electric field and attached to the electrodes but the particles are not been able to charge before the polarity of AC electric field alters with spherical electrodes The current for AC remained unchanged with the increment of particle contamination levels as there is no bridge formed However, when the DC biased AC signal is applied, the bridge is formed for all the three voltage levels tested DC and AC breakdown tests were also conducted for several contamination levels Experiments with kraft paper covered spherical electrodes confirmed that a tightly bonded cover does not stop the bridge; only a loose bonded cover stopped the bridging Another test with a paper barrier between bare electrodes also did not stop the bridging Partial discharge (PD) and breakdown test of the contaminated transformer oil is also measured but the results were not conclusive An initial mathematical model of pressboard dust accumulation using Finite Element Analysis (FEA) software, COMSOL multiphysics has been conducted The result of the simulation model for charging-discharging and bridging showed similar trend as experimental results There are a number of changes that can result in improved simulations There are several variables affecting the simulation ie the pressboard dust particle shape, size, conductivity of impregnated pressboard fiber etc

Parameter Influences on HVDC Transformer Insulation and Its Link to Conduction Processes

Abstract: HVDC (high voltage direct current) transmission is an effective way to transport electrical power over long distances by using direct current. The insulation system of HVDC components has to be designed for both AC and DC field distributions. It consists of mineral oil and pressboard. The used insulation material behavior has not been completely understood under DC stress. In this paper, electrical conductivity of pressboard is evaluated and conduction processes are considered. Therefore, conductivity measurements are linked to a number of RC network elements in order to achieve a network model representative for the insulation material behavior under DC stress. Time-constants are determined under different temperatures and different electric field strengths. In addition, the performed simulations provide evaluation of parameters affecting the dielectric behavior of the insulation system oil and pressboard, which are not measured or even cannot be determined by measurements.

Space charge dynamics in pressboard-oilpressboard multilayer system under DC voltages

Abstract: Converter transformers play a critical role in HVDC power transmission system. Space charge is a general concern for dielectric system under DC field. In this paper, the space charge behavior in a sandwiched oil-pressboard insulation system has been investigated using the pulsed electro-acoustic (PEA) method. The presence of homo-charges in the pressboards bulk and hetero-charges at the two interfaces between oil gap and pressboards leads to the electric field enhancement in the pressboards but reduction in the oil gap. The electric field enhancement is much larger than the value estimated by the Maxwell-Wagner polarization.

Hybrid Dissolved Gas-in-Oil Analysis Methods

Abstract: Dissolved gas analysis is the most widely used diagnostic test in power transformers. The aim of this paper is to introduce the dissolved gas analysis (DGA) methods able to diagnose the transformer conditions. The faults cause the transformer oil, pressboard, and other insulating materials to decompose and generate gases, some of which dissolve in the oil. The results of DGA must be accurate if faults are to be diagnosed reliably. There are different established methods used in industry for interpreting DGA results. We will compare the result of IEEE Key Gas Methods and Rogers’ Ratios. The transformer conditions are evaluated by the Key Gas Method with total combustible gas method (TCGM) and then verified by the Rogers’ Ratios. As result, the aging pattern and trend of the power transformer deterioration can be determined. The 30 sample data from IEEE with known faults and dissolved gas concentrations were used as the basis of comparison.

Dielectric frequency response of mineral oil impregnated pressboard

Abstract: The degradation processes of pressboard, paper or oil used in transformer are influenced by many factors. One of them is the moisture which is usually inhomogeneous distributed in oil-paper insulation, especially for power transformer. The dielectric properties of mineral oil impregnated pressboard with various moisture concentrations were measured. These measurements were made at different temperatures. The measured dielectric parameters were modeled by Debye model. Two different instruments: IDAX and LCR Meter with various frequency ranges for the measurements were used.

Influences of Ratios of AC and DC Voltages on Partial Discharge Characteristics of Gas Cavity in Oil-pressboard Insulation

Abstract: In order to realize the fault diagnosis for converter transformer, to study the influence of ratios of AC and DC voltages on partial discharge(PD) characteristics has practical engineering significance for diagnosis of converter transformer. A set of platform was built for PD detection under AC and DC voltage. The relationship between ratios of AC and DC voltage and PD characteristics was obtained by several tests and simulated by building physical model. The experimental measurement and theoretical simulation results are the same: the pulse repetition frequency and PD amount decrease with the DC voltage increasing. But the PD inception voltage increases when DC voltage becomes larger. In addition, the PD phase moves form two locations to just one part in negative half cycle. So for converter transformer, the diagnosis basis should distinguish with the insulation problem at different location.

Influential factor analysis and computation on electric field of converter transformer barrier

Abstract: Bushing and barrier system are the critical components in ultra high voltage (UHV) converter transformer for safe operation. Electric field distributions within bushing and barrier system are complicated in UHV converter transformer under combined AC and DC voltages, which, as a consequence, results in some technical difficulties in insulation structure design and manufacture. The paper presents the FEM electric field simulation of busing barrier structure under AC, DC and polarity reversal operation voltages. Firstly, the research results show that transformer oil with lower relative dielectric constant undertakes higher field strength under the AC voltage, whilst the insulating pressboard with higher resistivity bears higher field strength under the DC voltage, and the maximum field strength occurs in the insulating coating on the shielding ring surface. Moreover, extremely high field strength appears in transformer oil due to the space charge accumulation during voltage polarity reversal. Secondly, the research indicates that temperature has a big influence on electric field distribution of barrier system under DC voltage, specifically, with reduction of temperature the electric field distribution among oil and pressboard becomes increasingly uneven. Finally, the feasibility of applying nonlinear insulating pressboard to replace tradition pressboard in barrier system insulation structure is discussed and concludes that the nonlinear pressboard could improve electric field distribution dramatically.

Relationship between carbonized traces and space charges in oil-paper insulation

Abstract: Among all the types of transformer insulation deterioration, partial discharge is relatively a direct one that may lead to carbon traces which have negative effect on insulation. We believe that the relationship between the trace length and total appearance charge of partial discharges is a key to diagnose the oil paper insulation's remained life, even though the carbon trace in paper or pressboard cannot be seen directly like the transparent material such as XLPE. In this paper, we put forward an electrical method based on the hypothesis that the space charges just stay where the partial discharge happened before. Based on a distributed potential measuring system, we pay attention to carbon traces developed in a single electrical tree. After a series of high voltage experiments, we calculate the position of the space charges by their electric field and then we get the trace length by the farthest space charges. Finally, we get the result of experiment consisted with that of the theory calculations.

Nonlinear phenomena in the constant electric field in insulation pressboard

Abstract: The paper presents the results of DC conductivity dependence of a transformer oil impregnated pressboard, with moisture content of 2.9% by weight, measured for temperature from 20 °C to 80 °C and a field strength from 10 kV/m to 1000 kV/m. It was found that the non-linear phenomena, which presence cannot be explained by the theory of ion conduction of oil-paper insulation, is intensifying with increase of the electric field strength.