Showing papers on "Pressboard published in 2017"

Studying the loss of life of natural-ester-filled transformer insulation: impact of moisture on the aging rate of paper

Abstract: Moisture is one of the most harmful agents for transformer solid insulation. The catalyzing effect of moisture on the degradation process of paper and pressboard has been studied by several authors, who developed models which they used to estimate how the aging process would be accelerated in the presence of water [1]–[5]. In [2] and [3] Lelekakis et al. reported the results of an experimental study in which samples with different moisture contents were aged in the presence of mineral oil (MO). In [3] an equation to determine the reduction of service lifetime of insulation subjected to increased temperature and with increased moisture content is proposed.

A review of the techniques used by utilities to measure the water content of transformer insulation paper

Abstract: Cellulosic insulation materials are very widely used in power transformers, in designs up to 1,500 MVA [1]. Cellulosic insulation is formed of either wood, pressboard, or Kraft paper. The wood and thick sections of pressboard are used to manufacture the base of the transformer; pressboard is also used for the spacers and barriers of the windings, and the paper is wrapped around the current carrying conductors. The water content of this cellulosic insulation must be monitored, because if it becomes too high, a transformer can fail on overload, and will reach the end of its functional life sooner because the paper insulation will age faster [2]. Eventually, the paper will become brittle, and will either tear or lift off the conductor. The water content of paper (WCP) is the ratio of the mass of adsorbed water to the mass of dry paper, usually expressed as a percentage. Water can migrate out of the wood and pressboard and around the system, eventually increasing the WCP. If the WCP becomes too high, the transformer may be dried out by the relevant utility [3].

Fabrication of Al₂O₃ Nano-Structure Functional Film on a Cellulose Insulation Polymer Surface and Its Space Charge Suppression Effect.

Abstract: Cellulose insulation polymer (paper/pressboard) has been widely used in high voltage direct current (HVDC) transformers. One of the most challenging issues in the insulation material used for HVDC equipment is the space charge accumulation. Effective ways to suppress the space charge injection/accumulation in insulation material is currently a popular research topic. In this study, an aluminium oxide functional film was deposited on a cellulose insulation pressboard surface using reactive radio frequency (RF) magnetron sputtering. The sputtered thin film was characterized by the scanning electron microscopy/energy dispersive spectrometer (SEM/EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The influence of the deposited functional film on the dielectric properties and the space charge injection/accumulation behaviour was investigated. A preliminary exploration of the space charge suppression effect is discussed. SEM/EDS, XPS, and XRD results show that the nano-structured Al₂O₃ film with amorphous phase was successfully fabricated onto the fibre surface. The cellulose insulation pressboard surface sputtered by Al₂O₃ film has lower permittivity, conductivity, and dissipation factor values in the lower frequency (<10³ Hz) region. The oil-impregnated sputtered pressboard presents an apparent space-charge suppression effect. Compared with the pressboard sputtered with Al₂O₃ film for 90 min, the pressboard sputtered with Al₂O₃ film for 60 min had a better space charge suppression effect. Ultra-small Al₂O₃ particles (<10 nm) grew on the surface of the larger nanoparticles. The nano-structured Al₂O₃ film sputtered on the fibre surface could act as a functional barrier layer for suppression of the charge injection and accumulation. This study offers a new perspective in favour of the application of insulation pressboard with a nano-structured function surface against space charge injection/accumulation in HVDC equipment.

Partial discharge development in needle-plane configuration of oil-paper insulation under AC voltage

Abstract: Condition of insulation materials can be obtained by analyzing partial discharge (PD) characteristics since features of PD are closely related to it. In this paper, electrical deterioration caused by partial discharge of oil-paper insulation under long-term AC voltage is studied. Feature characteristics of PD, such as phase-resolved partial discharge analysis and equivalent time-frequency, were recorded during the test process. Electrical deterioration is divided into four stages: initial, develop, stagnation and erupt stage. Four stages are separated by pulse repetition rate and discharge magnitude of PD. In different deterioration stage, statistical characteristics of PD change significantly. Surface properties of pressboard in different deterioration stage were studied. Test results indicate that the depolarization of cellulose in pressboard is the main reason for development of partial discharge, and discharges occur in different position of pressboard in different deterioration stages.

Experimental Study on Influence of Trap Parameters on Dielectric Characteristics of Nano-Modified Insulation Pressboard.

Abstract: In order to study the influence of trap parameters on dielectric characteristics of nano-modified pressboards, pressboards were made using the nano doping method with different nanoparticle components. The dielectric characteristics of the modified pressboards were measured, and the trap parameters were investigated using the thermally stimulated current (TSC) method. The test results indicated that the conductivity initially declined and then rose with the increase of nano-Al₂O₃ content, whereas it solely rose with the increase of nano-SiC content. Moreover, the conductivity exhibited nonlinear characteristics with the enhancement of electric field stress at high nanoparticle content. The relative permittivity of modified pressboard declines initially and then rises with the increase of nanoparticle content. In addition, the breakdown strength of modified pressboards exhibited a pattern of incline followed by decline with the increase of nano-Al₂O₃ content, while it always declined with the increase of nano-SiC content. The analysis based on the energy band theory on trap parameters of the constructed multi-core model concludes that the nanoparticle components added in pressboard altered both the depth and density of traps. It is therefore concluded that trap parameters have significant influence on the dielectric characteristics of nano-modified insulation pressboard.

Thermal aging of cellulosic pressboard material and its surface discharge and chemical characterization

Abstract: Cellulosic pressboard is a key insulation material in oil filled transformers. Surface discharge or partial discharge degrades the insulation material and reduces the life time of the transformer. This study is aimed at understanding the effect of thermal aging of pressboard on the variation of its surface discharge and chemical characteristics. Thermal degradation of pressboard was carried out at 180 °C for 500 h in presence of ester oil under simulated aging conditions. The variation of salient electrical characteristics like surface discharge inception voltage (SDIV), charge measurement and optical emission spectra of the discharges were assessed at different aging periods. A significant reduction in SDIV was observed due to thermal aging. The charge measurement indicated that total charge deposition on pressboard increased with aging duration. Optical emission spectroscopy yielded valuable information regarding the elements evolved during discharge activity. Chemical characterization of the virgin and aged pressboard samples was carried out using analytical pyrolysis coupled with gas chromatograph-mass spectrometric technique (Py-GC/MS). The production of anhydrosugars, furan derivatives and oxygenates from the pressboard increased with aging time, while the esters in the oil were degraded to carboxylic acids with aging. Owing to the weakening of the hydrogen bonding network in oil impregnated pressboard with thermal ageing, high yield of end-chain depropagation products like anhydrosugars was observed during pyrolysis. The weak sites in cellulosic pressboard are expected to act as charge deposition sites.

Transformer Insulation Materials and Ageing

Abstract: Sufficient insulation between different active parts of the transformer is necessary for its safe operation. The insulation system in a transformer can be categorized as follows: major insulation, and minor insulation. The solid insulation materials widely used in the transformer are paper, pressboard, and transformer board, which are formed from the cellulose found in plants. The main concern with using dry paper as an insulating material is that it is very hygroscopic. The presence of cellulose insulation affects the voltage distribution in heterogeneous dielectric systems: it contributes to the dielectric loss; determines the short???term dielectric strength; and, to a major extent, the life of the total insulated system. Oil is an equally important part of a transformer's overall insulation. The insulation system of a transformer undergoes ageing due to transformer operating conditions. This chapter provides an overview of the key concepts discussed in the subsequent chapters of this book.

PD detection and analysis of oil-pressboard insulation under pulsed DC voltage

Abstract: HVDC transmission has been adopted widely for its large energy transport capacity, high stability and more economical in long-distance power transmission. Converter transformer is the key equipment in HVDC transmission system. The insulation of converter transformer valve side endures a composite voltage including AC, DC and pulsating component etc. Partial discharge (PD) occurring in a void surrounded by insulating material is believed to be one of the main causes of insulation breakdown and it is the main factor of electrical aging of materials. Therefore, it is important to recognize the prebreakdown phenomena and to evaluate the lifetime of insulating materials. Much research has been carried out to study the partial discharge characteristics under pure AC or DC voltage. Only a few investigations were focused on composite voltage. In order to obtain more understanding of PD characteristics under composite voltage, the PD detection and analysis of oil-pressboard insulation under pulsating DC voltage waveform are studied in this paper. The voltage waveform of converter transformer is analyzed through computer simulation. A 6-pulse DC voltage generator and PD detection system are set up. Influence of the DC component and pulsating component to the PD characteristics is studied in lab test. The experimental results show the DC component and pulsating component affect the PD inception voltage, but the pulsating component has more significant influence to PD characteristics than DC component. The study provides an experimental foundation for the converter transformer insulation breakdown mechanism analysis.

Measuring the Pressboard Water Content of Transformers Using Cellulose Isotherms and the Frequency Components of Water Migration

Abstract: The measurement of water within power transformer insulation is necessary for the unit's reliable operation. Probes can be inserted into the oil of a transformer to measure water activity, and then this water activity is used to determine the water content of transformer cellulosic insulation. The migration of water between the insulation media is driven by differences in water vapor pressure around the insulation system. Over time, this water reaches an equilibrium partition when cellulosic adsorption isotherms are used to calculate the concentration of water absorbed by the cellulose. However, the insulation of a normally operating transformer is highly unlikely to reach this state, as the migration of water between the insulation media is much slower than the rate of change of temperature caused by the varying load of the transformer. This leads to inaccuracies in analyzing water vapor pressure, which is a function of both oil water content and temperature. The frequency spectrum of the oil's water activity data was used to determine whether very slow changes in dissolved water, attributed to long diffusion time constants, could be separated by a filter from fast changes caused by temperature, leading to a better measurement technique. This filter was then used on data from three operating power transformers, which had their insulation water content measured using dielectric response. The results are discussed in this paper.

Comparative analysis of air-gap PD characteristics: Vegetable oil/pressboard and mineral oil/pressboard

Abstract: Partial Discharge (PD) is considered to be one of the main reasons for aging and degradation of the oil/pressboard insulation system in power transformers. Vegetable oils which own excellent dielectric performance are introduced as potential insulation liquids substituting traditional mineral oil. In this paper, an air-gap PD model was adopted to investigate the PD characteristics of refined rapeseed oil and Karamay 25# mineral oil that are both currently employed in liquid filled power transformers. The PD current pulse waveform analysis (PD-CPWA) method was used to investigate PD mechanisms of two different insulation systems: mineral oil/pressboard and vegetable oil/pressboard. For both insulation systems, phase resolved partial discharge (PRPD) patterns throughout the accelerated deterioration experiments were compared. The extracted ϕ-q-n plots with respect to various PD times were analyzed. It is found that, the air-gap PD stage characteristics of vegetable oil/pressboard are more notable. It has fewer double-peak pulses, smaller inception phase angle, lower charge amplitude, higher repetition rate, and more remarkable ‘rabbit-ear’ patterns than mineral oil/pressboard. The PD development process of both insulation systems can be characterized by four stages: initial discharge stage, weak developing stage, discharge burst stage and pre-breakdown stage.

Degradation characteristics of oil-immersed pressboard samples induced by partial discharges under DC voltage

Abstract: To investigate partial discharge-induced degradation characteristics of oil-immersed pressboard under DC voltage, an experimental platform with needle-plane oil-pressboard insulation model is established. The surface condition and DC breakdown voltage of pressboard samples after PD pretreatment are observed and measured. In addition, a fitting formula to evaluate the degradation extent of pressboard samples is proposed. The results show that under constant PD pretreatments, surface roughness of pressboard samples decreases first and increases afterwards. The changing rate is positively correlated with PD intensity. During PD pretreatment, a “whitish-colored circular region” appears at samples' surface and grows with PD intensity. For PD pretreatments of 500 pC and 1000 pC, DC breakdown voltage of pressboard samples remains unchanged. The breakdown spot has no clear carbonization nor creepage trace. For PD pretreatments of 2000 pC and 3000 pC, DC breakdown voltage decreases. The breakdown spot is obviously carbonized and creepage paths emerge around spots. Further analysis indicates that degradation of oil-immersed pressboard samples due to DC PDs is the result of motion activation process induced by electrical and thermal effects under DC electric field.

Influence of ambient medium on thermal ageing of pressboard in transformer oil containing dibenzyl bisulphide (DBDS)

Abstract: Impact of thermal ageing of pressboard material in transformer oil containing dibenzyl disulphide (DBDS) under different ambience, such as air, N2, He and vacuum has been studied to understand the level of copper sulphide diffusion. The characteristic variations in electrical and thermal properties of copper sulphide diffused oil impregnated pressboard (OIP) material have been measured under different ageing conditions. Surface charge accumulation studies with AC and ±DC voltages showed that surface charge decay time was longer in air aged OIP material. Surface discharge studies by adopting ultra-high frequency (UHF) sensing indicated a reduction in the SDIV with the level of ageing although the characteristic variation in flashover voltage (FOV) under lightning impulse (LI) voltage is almost the same. Also the SDIV reduced with increase in frequency of supply voltage. Thermo-gravimetric analysis (TGA) indicated that maximum weight loss temperature (Tmax) of the cellulose constituent of OIP material decreased with ageing. Laser induced breakdown spectroscopy (LIBS) was adopted to quantify the diffused copper sulphide contaminant in OIP material and the results were compared with the inductively coupled plasma (ICP) spectroscopy based analysis. Based on these studies, deteriorating effect of ageing medium on several properties of OIP material was understood.

Influence of moisture dependency of pressboard on transformer winding clamping pressure

Abstract: Short circuit (SC) strength of a power transformer is influenced by its winding clamping pressure. The required level of clamping pressure is selected at the design stage based on the maximum allowable SC current level of the transformer. Progressive loss of clamping pressure is a common problem in power transformers. Shrinkage and fading of the stiffness of solid insulation due to thermal degradation are the most common causes of this problem. It has also been identified that changes in the moisture content and repetitive compression cycles on pressboard during high current faults tend to change the winding clamping pressure. This study aims to investigate the effects of moisture and compression cycles on winding clamping pressure through a set of laboratory experiments and finite element analyses. Thickness change and compressive stress-strain curves of pressboard were measured under different moisture conditions. The results were then used to calculate changes in winding geometry and clamping pressure of a 100 kVA three phase, disc type transformer winding.

A mathematical model to measure instantaneous moisture content in transformer insulation cellulose

Abstract: Solid transformer insulation is mainly composed of cellulosic materials, in the form of pressboard, Kraft paper and wood. Oil is used to impregnate voids in the cellulose. The water content of cellulose usually increases over the life of the transformer, caused by both moisture ingress and the gradual degradation of cellulose, which produces moisture as an aging by-product. Equilibrium curves are used to estimate the moisture content of cellulosic insulation using the vapor pressure of water dissolved in the oil. However, a very long duration is required for water to attain this equilibrium partition, much longer than the usual daily cyclic temperature profile of a transformer, and thus it is very unlikely for the insulation to reach this state. In this paper a novel method to estimate the moisture in cellulose during non-equilibrium conditions is presented. The water adsorbed by the cellulose migrates in and out the oil, and the diffusivity depends on the temperature and the concentration of moisture in the cellulosic insulation. This diffusion is mainly governed by Fick's law. For a transformer, the change in moisture content of oil in a small duration is mainly due to the moisture content in the cellulosic insulation. This paper presents a method to estimate the moisture content of cellulosic insulation of the transformer at that instant from the change in the moisture content of the oil.

Aging assesment of transformer pressboard insulation by micro-strip ring resonator at GHz frequencies

Abstract: Condition assessment of insulation is widely used to identify the state of aging of power transformers. Traditionally, condition assessment is done using dielectric response measurements conducted at low frequencies. However, due to long testing time required for these methods, attention has now been paid towards measurements at microwave (above 1 GHz) frequencies. In this paper, we establish a correlation between dielectric properties and the state of aging of transformer pressboard samples. This is useful as it paves way to develop techniques to assess the state of aging by measuring dielectric properties at high frequencies. Dielectric measurements are made by using a microstrip ring resonator. The state of aging is determined by measuring the tensile index. Tests were carried out on dry and wet oil-impregnated pressboard samples thermally aged at laboratory conditions for durations up to 45 days. The results were compared with field aged samples collected from a 33 kV sealed type distribution transformer of 18 years in service. It was found that the permittivity values had a good correlation with respective tensile index values whereas loss tangent values showed limited correlation for both dry and wet samples. The results of field aged samples matched well with the established correlation curves. It can be concluded that the permittivity of pressboard samples estimated from the ring resonator can be effectively used to evaluate the state of aging of transformer insulation.

The Correlation between Surface Tracking and Partial Discharge Characteristics on Pressboard Surface Immersed in MIDEL eN

Abstract: This paper presents the investigation of the surface tracking on pressboard surface immersed in MIDEL eN oil. In this work, the development of surface discharge was analyzed by correlating the visual records of surface tracking on impregnated pressboard and the partial discharge (PD) activities. The PD activities during the surface tracking process were analyzed in terms of Phase Resolved Partial Discharge (PRPD) patterns. Throughout the experiment, surface discharge is found as the development of tree-like patterns in the form of white marks occurring on the oil-pressboard interface. This phenomenon is generally accepted as the drying out process that involves evaporation and decomposition of the oil molecules in the pressboard pores due to the surface discharge activities on the pressboard surface layer. The development of surface discharge on the pressboard surface can continue from minutes to months or even years until failure. Thus, condition monitoring system is important to characterize this type of faulty condition. The experimental results show that there is the decreasing trend of PD magnitude during the development of white mark hallway of a gap distance which is eventually suffered from an unexpected fault.

Investigation of electrification and breakdown strength about transformer oil/pressboard

Abstract: Forced convection of insulation oil is the main heat transfer mode in large power and ultra-high voltage (UHV) transformers. The electrification risk caused by friction between mineral oil and pressboard has been suspected to be responsible for several failures. The electrification measurement was carried out in a rotation system. The influences of the oil temperature, rotation speed, time, and applied electric field of fresh and dry transformer oil have been investigated by a measurement system, respectively. A comparison between these results has been made, and the related theory can be used to explain the physical and chemical processes which take place on the interface between the oil and pressboard. The results from the rotation system indicate that interface charge density increases with the increase of the pressboard thickness, rotation time and speed, but decreases with the increase of the temperature. The electrification phenomenon can lead to reduction of the oil/pressboard breakdown strength. It is concluded that some effective measures should be taken to restrain the possible harm caused by the streaming electrification in UHV converter transformer.

Thermal degradation of transformer pressboard impregnated with magnetic nanofluid based on transformer oil

Abstract: In this paper, the thermal ageing of pressboard impregnated with two kinds of insulation liquids is studied by means of frequency domain spectroscopy which involves the measurement of real and imaginary part of complex permittivity as function of frequency. The magnetic nanofluid was composed by mineral transformer oil as carrier liquid, nanoparticles of magnetite and surfactant. The thermal aging test was performed at the temperature 90°C for 1000 hours. Each sample had two variations, with and without copper, which has function catalyst. Then frequency domain spectroscopy was investigated at 40°C for restriction of moisture influence from surrounding. From the data and curves, it can be shown that the undried samples are dried with the aging time and cooper considerably getting worse the imaginary part of complex permittivity. In case of magnetic nanofluid, the colloidal suspension was unstable after 800 hours of the thermal ageing. There was found much more resistance of moisture at the nanofluids than in case of mineral oil impregnated pressboard.

Effect of cellulose impurities on partial discharges in oil-pressboard insulation under DC voltage

Abstract: Previous studies indicate that cellulose bridges in oil-pressboard insulation are self-generating, and can influence the characteristics of partial discharges (PDs) under DC voltage. In this letter, further experiments based on an optical-electric experimental platform were conducted to determine the generating sequence between cellulose bridges and PDs, and to provide visual evidence of bridges' effects on PD process. The results show that the generation of cellulose bridges can be divided into three stages, which can be finished at a DC voltage lower than PD inception voltage (Updiv). With the increase of the applied DC voltage, the generating time of the first bridge decreases. In addition, by using shadow photographic method, a bridge-triggered discharge in oil was observed. Further analysis indicates that cellulose bridges generate gaseous paths to discharges in oil due to its high conductivity.

Optical registration of the vacuum impregnation process of electrotechnical pressboard by transformer oil

Abstract: The paper presents a research stand developed to measure the kinetics of the vacuum impregnation process of electrotechnical pressboard with transformer oil. The object of the research was electrical pressboard made by Pucaro company impregnated by Nynas company Nytro Taurus oil. Studies were carried out in a vacuum chamber under reduced pressure. The results of the experiments allowed to determine the influence of reduced pressure on the impregnation process kinetics and to determine the diameter of the capillaries present in the electrotechnical pressboard.

Dielectric properties of mineral oil compared with natural ester

Abstract: In this paper, dielectric properties of the mineral oil, ELECTROL A, and the natural ester, FR3, are comparatively investigated. AC breakdown voltages, dielectric dissipation factor, relative permittivity, partial discharge inception voltage and partial discharge of the mineral oil and of the natural ester are tested. In order to verify the capability in practical use of the mineral oil and the natural ester for transformer insulation systems, the impregnation of such liquids with pressboard insulation is performed. The impregnated pressboards are tested in the laboratory to measure the AC breakdown voltages. It is found that the natural ester shows superior dielectric properties to the mineral oil except the dielectric dissipation factor.

AC breakdown and frequency dielectric response characteristics of the mixed oil-paper insulation with different moisture content

Abstract: Taking into account the advantageous or un-advantageous properties of mineral oil and natural ester, our team developed a new type of mixed insulating oil with good physical and electrical performances which were derived from mineral oil and natural ester. The moisture effect on the AC breakdown and frequency dielectric response characteristics of mineral oil/paper insulation and mixed insulating oil/paper insulation was compared. The new type of insulating oil has much better AC breakdown behavior than the mineral oil with nearly the same moisture content. The mixed insulating oil impregnated pressboard also shows a higher AC breakdown strength value than mineral oil impregnated pressboard with nearly the same moisture content. the mixed insulating oil impregnated pressboard has higher eί and tanS values in the 10−1Hz to 106Hz region.

Statistical analysis of transformer oil penetration speed through electrotechnical pressboard

Abstract: The paper presents a measuring stand used to measure oil penetration times of capillaries present in electrotechnical pressboard. Based on the measurements, the statistical distribution of capillaries occurring in cellulose insulation was determined. Based on the analysis of the obtained results, the presence of randomly distributed areas with reduced density was detected.

Transformer insulation structure for dielectric liquids with higher permittivity

Abstract: Insulation structures based on cellulosic materials and mineral oil have been the standard solution for transformers for the last hundred years, both in distribution and power transformers. These structures typically consist of a solid-liquid arrangement, aimed to divide larger voltage drops in smaller steps, creating small volumes of liquid with high dielectric insulating performance and reaching the most compact design. The total potential difference is borne by sequential voltage drops defined according to the geometry and the permittivity of the materials. Standard arrangements and configurations have been developed by transformer manufacturers according to the voltage levels, as the analytical solution of the distribution of the electrical field may be extremely complex. Even nowadays, where numerical calculation models, mostly based on the Finite Elements Method, are available, the use of standardized solutions is not uncommon, as the detailed calculation may be highly time consuming. When the dielectric liquid is changed, replacing the traditional mineral oil by natural ester liquid, the distribution of stress in the insulation system is no longer the same. The permittivity of natural ester liquid is around 40% higher than that of mineral oil, while the permittivity of pressboard and paper made from kraft pulp, impregnated with natural ester liquids is just slightly higher. The result is a different distribution of voltage drops in the insulation system, which must be taken into consideration when designing a natural ester filled transformer. At first glance, the effect is positive, since the critical parameter is typically the stress, the field strength, in the liquid insulation and the higher permittivity shifts the stress from the liquid into the solid insulation. This modification of electrical field distribution may allow, in some cases, increasing the average field strength and thus a more compact design, as there is a better balance between the stress in the solid and liquid portions. However, the modification of the electrical field distribution may lead also to new critical regions. Corners and labyrinth constructions may have higher superficial dielectric field gradients. Additional edge protection may be required in regions where they were not required with mineral oil, as the stress in the paper will be increased. Less specialized engineers tend to take the breakdown voltage as the only relevant parameter for the insulation design. A large quantity of test data has indicated that the breakdown voltage of natural ester liquid, excluding the undesirable highly divergent field (needle to sphere), is similar to mineral oil. But this does not mean that adjustments in the insulation system design would not be required. This paper will present some common configurations, potential optimizations and areas requiring attention for designing insulation systems using cellulose and natural ester liquids.

Statistical analysis of oil percolation through pressboard measured by optical recording

Abstract: The paper presents a measuring station used to measure the percolation of transformer oil by electrotechnical pressboard. Nytro Taurus insulating oil manufactured by Nynas company percolation rate by the Pucaro company pressboard investigation was made. Approximately 60 samples of Pucaro made pressboard, widely used for insulation of power transformers, was measured. Statistical analysis of oil percolation times were performed. The measurements made it possible to determine the distribution of capillary diameters occurring in the pressboard.

Investigation on the impregnation characteristics of a new GTL based synthetic insulating fluid

Abstract: The main goal of this research was the comparison of the impregnation behavior of a new oil, based on the gas-to-liquids process, with a mineral oil, which should be representative for standard mineral oils used in transformers today. The investigation and comparison were made by dielectric measurements with a flat pressboard with a thickness of 0.5 mm and 3 mm as a sample. Additionally wraps with a layer thickness of 6.25 mm, made from kraft paper in cylinders with a thickness of 0.15 mm and an aluminum cylinder as a rack, were investigated. Because of the different geometric shapes of the samples the investigations were made with two different types of electrode assemblies. The measurement of the dielectric dissipation factor was performed with the method of Frequency Domain Spectroscopy (FDS) as well as with the Polarization and Depolarization Current (PDC). The obtained results show that both oils have a similar behavior of the dissipation factor for different samples and temperatures and so they show a similar impregnation behavior. Therefore, it could be considered that the diagnostic methods for dielectric measurements could be also applied to the new GTL based oil.

Molecular dynamics study of moisture migration between transformer oil and pressboard under variation temperature

Abstract: Moisture has a significant effect on the insulation performance of the transformer. Some transformers have failed during rapid temperature ramp-up. When the temperature changes, the moisture will migrate between the oil and cellulose pressboard may form local high-moisture regions is considered to be a possible cause. But it has not been confirmed. It is difficult to validate this idea by experimenting because it is difficult to measure. In this paper, we tried to validate it by simulate the migration of moisture in oil and pressboard based on molecular dynamics. Molecular models were constructed to oil without water molecule and cellulose with 5% moisture by Materials Studio. The molecular dynamics simulations were carried out at a temperature ranging from 20°C to 80°C. The simulation results show that there are large amounts of water molecules accumulate at the interface of the oil-paper interface during the rapid temperature rise. This indicates that high moisture regions do occur at the oil-pressboard interface during a rapid rise in temperature for transformer.

Understanding the dielectric properties of pressboard material thermally aged in Dibenzyl Disulphide (DBDS) included transformer oil

Abstract: Surface Discharge Inception Voltage (SDIV) of unaged and copper sulphide diffused Oil Impregnated Pressboard (OIP) material was studied by adopting Ultra High Frequency (UHF) technique. Copper sulphide diffusion into OIP material was carried out by thermal aging of pressboard material in Dibenzyl Disulphide (DBDS) added transformer oil. The SDIV reduces with increase of thermal aging temperature. The current pulse injected in positive and negative half cycle during the surface discharge of the AC shows the rise time of 0.8ns and 1ns respectively. The FFT analysis of UHF signal measured during surface discharge process has shown frequency content in the range 0.7–1.2 GHz with its dominant frequency at 0.9 GHz. The life of the charge presented on the pressboard increases with thermal aged specimen. Energy dispersive spectroscopy (EDS) analysis of OIP material indicates presence of copper and sulphur content. Frequency Domain Dielectric Spectroscopy (FDDS) studies indicate the permittivity(er), dissipation factor (tan(δ)), conductivity (σdc) increases and relaxation time (τ), coefficient (α) decrease with OIP material (in air / Nitrogen) aged at higher temperature in DBDS added transformer oil. Cole-Cole double relaxation model was used to understand the dielectric parameters of copper sulphide diffused OIP material. The DC conductivity of pressboard material aged in DBDS included transformer oil follows Arrhenius law and the activation energy decreases with increase in temperature of thermal aging.