Showing papers on "Pressboard published in 2016"

Understanding the ageing aspects of natural ester based insulation liquid in power transformer

Abstract: Cellulose based insulation materials and mineral oils have widely been used in liquid filled transformers. Environmental and fire-safety concerns have accelerated the use of Natural Esters (NE) as a sustainable alternative to mineral oil. There is a rapid growth of their application for distribution level transformers. To ensure a safe and long-term operation of the vegetable oil filled transformers, it is necessary to clarify the knowledge gaps with respect to the ageing of vegetable oil impregnated cellulose and insulation diagnostic methods. The condition monitoring of mineral oil filled transformers has been well characterised, due to the numerous laboratory and field studies conducted during the past several decades. The application of these available condition monitoring techniques for NE filled transformers is yet to be validated as the use of NE for power transformers is in the evaluation stage. In this paper the performance of NE as insulation in transformers by evaluating the ageing of NE impregnated cellulose pressboard is presented. For this purpose, an accelerated ageing experiment has been carried out in sealed tubes at 120 °C. The ageing of oil impregnated pressboard has been characterised by decrease in the degree of polymerisation (DP) of pressboard, and the formation of both 2-furfuraldehyde and dissolved carbon dioxide (CO2) in the oil. The structural changes of cellulose have been supported by Fourier Transform Infrared (FTIR) spectra. Moreover, the results have been compared with cellulose insulation samples aged under similar conditions in conventional mineral oil. Acidity value, dissipation factor and viscosity measurements have been used to compare the stability of NE in a transformer operating environment to mineral oil.

Permittivity of a composite of cellulose, mineral oil, and water nanoparticles: theoretical assumptions

Abstract: The paper presents results of testing permittivity of an oil-impregnated electric pressboard containing water nanoparticles depending on AC frequency, moisture content level and the temperature of samples. A new method has been developed for converting experimentally obtained frequency dependences of permittivity determined with the frequency-domain spectroscopy (FDS) to the reference temperature of 293 K (20 °C) using exponential dependence of the relaxation time versus temperature. Activation energy of the permittivity relaxation time has been determined for a moist oil-impregnated electric insulation pressboard. It has been established that variations of the moisture content level in a composite of cellulose, mineral oil, and water nanoparticles do not cause any changes in the relaxation time activation energy value, neither in energy states of electrons in potential wells nor in the structure of water nanoparticles. It has been also found that the conversion of experimentally obtained permittivity versus frequency dependences determined with the FDS method to the reference temperature of 293 K (20 °C) eliminates temperature dependences that occur in characteristics based directly on measurement results. Once the relative permittivity is converted to the reference temperature it is only its dependence on the moisture content level that remains.

Moisture-Dependent Thermal Modelling of Power Transformer

Abstract: Hot-spot temperature (HST) is recognized as one of the key factors affecting the life expectancy of a transformer. In this study, a moisture-dependent thermal model (MDTM) for estimating the transformer hot-spot temperature is proposed. In this model, nonlinear thermal resistance is formulated by considering oil and cellulose (paper and pressboard) of the transformer. Especially, the effect of moisture concentration and temperature on the thermal resistance of cellulose is taken into account. The proposed MDTM is verified by using historical data, including moisture-in-oil measurements, temperature measurements, and loading profiles, which were collected from two in-service power transformers, that is, a vegetable oil-filled transformer and a mineral oil-filled transformer. Comparisons between the proposed MDTM and a number of existing thermal models were performed on the basis of adequacy and accuracy metrics.

Condition diagnosis of transformer oil-paper insulation using dielectric response fingerprint characteristics

Abstract: The life expectancy of a transformer is closely related to the condition of its oil-paper insulation. In the present study, oil impregnated pressboard samples with different aging states and water contents were prepared under controlled laboratory conditions to analyze the oil-paper insulation condition of the power transformer by the dielectric response method. Dielectric response measurements, i.e., polarization and depolarization currents (PDC) and frequency domain spectroscopy (FDS), were conducted on the pressboard samples at different temperatures. Results show that the dielectric responses of oil impregnated pressboards are highly sensitive to aging, water, and temperature, but the influence of the temperature on FDS can be effectively eliminated by using the master curve technique. Meanwhile, dielectric characteristic parameters, which are sensitive to aging state and water content, were extracted to form a fingerprint database to indicate the various states of the oil impregnated pressboard. A grey correlation diagnosis model was first introduced to assess the aging state and water content of oil-paper insulation. Finally, FDS measurements were performed on a field transformer to verify the effectiveness of the new evaluation technique proposed in this paper for the condition assessment of the transformer oil-paper insulation.

Dielectric losses in the composite cellulose–mineral oil–water nanoparticles: theoretical assumptions

Abstract: The paper presents an analysis of test results by Frequency Domain Spectroscopy (FDS) of the loss tangent (tgδ) in electrotechnical pressboard impregnated with insulating oil and containing nanoparticles of water depending on the frequency of alternating current, sample temperature and degree of moisture. We found tgδ reduction in areas of low and ultra-low frequency. It is associated with relaxation due to the hopping conductivity (tunnelling) of electrons between moisture nanoparticles occurring in the cellulose impregnated with insulating oil. In frequency areas close to the local minimum of tgδ and higher, relaxation does not depend on the moisture content and is associated with other polar molecules which are part of the cellulose. We developed a new method converting experimental frequency dependence of the loss angle tangent, measured by the FDS method to the reference temperature of 293 K (20 °C), using the exponential dependence of the relaxation time on the temperature, was developed. The activation energy of the relaxation time was determined based on the loss angle tangent of moist electrotechnical pressboard impregnated with insulating oil. It was found that the variation in moisture content in the composite cellulose–mineral oil–water nanoparticles do not cause changes in the activation energy of the relaxation time. The conversion of experimental frequency dependence of the loss angle tangent determined by the FDS method to the reference temperature of 293 K (20 °C) eliminates the temperature dependence in the areas of ultra-low and low frequencies occurring in the runs made directly on the basis of the measurement results. After calculating the frequency dependence of the loss angle tangent to the reference temperature, all that remains is its dependence on the moisture content.

Space charge characteristics in oil and oil-impregnated pressboard and electric field distortion after polarity reversal

Abstract: The formation of space charge in the oil/paper insulation under HVDC condition could influence the electric field distribution. In the case of HVDC converter transformer, the distortion of the electric field may affect its performance. It is, therefore, important to analyze factors that can affect space charge formation and dissipation characteristics such as temperature, moisture, electric stress and ageing process. This paper mainly focuses on the effect of electric stress and oil ageing on space charge characteristics in oil/paper insulation system. The pulsed electroacoustic technique (PEA) method was used to monitor space charge dynamics. This data was further incorporated into a model established using COMSOL software to estimate space charge impact on electric field distribution, with an emphasis on the electric field after polarity reversal operation. The results show that the electric field arising from the space charge formation is much higher compared to the average electric field after the polarity reversal operation. Moreover, the electric field caused by space charge decreases faster for the aged oil-impregnated pressboard compared to the fresh one, which could be related to different space charge dissipation characteristic related to the condition of the oil.

Performance of natural ester as a transformer oil in moisture-rich environments

Abstract: Interest has risen among utilities in using natural ester (NE) insulating oils in transformers as a substitute for conventional mineral oil. However, present understanding on aging behaviour of NE-paper composite insulation system and knowledge on application of existing condition monitoring tools for NE-based insulation are inadequate. This limits the cost effective and reliable field applications of NE insulating oil. To pave the way the application of NE-based insulation in transformers, a systematic study has been performed to compare the aging behaviour of transformer grade pressboard (PB) impregnated in NE and conventional mineral oil. Applicability of a number of chemical and physical parameters, including acidity value, dielectric dissipation factor (DDF), viscosity, and colour for assessing the quality of NE insulating oil is also discussed in this paper. Comparisons are made based on the limiting values provided in the related IEEE Standard and properties of mineral oil under similar aging conditions.

Space charge behavior in oil gap and impregnated pressboard combined system under HVDC stresses

Abstract: The reliability 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 0.5 mm thick oil film combined with a 1mm thick impregnated pressboard have been investigated by the pulsed electroacoustic (PEA) technique under 12 kV/mm and 20 kV/mm at room temperature for both short term and long time tests. 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. Compared with the Maxwell-Wagner (MW) polarization, more space charges are formed in the insulation system. It needs much longer time to reach steady state, depending on the status of the oil. The interfacial and bulk effects result in a significant electric field enhancement in the middle of pressboard, especially in aged oil samples.

PD characteristics of oil-pressboard insulation under AC and DC mixed voltage

Abstract: The insulation problem of converter transformers has become one of the core issues in HVDC system, which is receiving more and more attention. A converter transformer has complex structures and requires high insulation level. The voltage waveform which the converter transformer insulation withstands is different from AC transformers. In the valve side of the converter transformer, the insulation endures a mixed voltage of AC voltage, DC voltage, harmonics and the pulsating component etc. Partial discharge (PD) occurring in oil-pressboard insulation is the one of the main causes of insulation breakdown, and it is the main factor of electrical aging of materials. Even though PD characteristics under pure AC or DC voltage waveform have been studied by many researchers, little research has been done with the mixed voltage. In this paper, the AC + DC mixed voltage experiment system is set up to study the PD characteristics under mixed voltage waveforms. The study results indicate that the pressboard withstands most of the DC component and void withstands most of the AC component when applied mixed voltage. For PD inception voltage, it is decided by both the AC component and DC component. For PD development, AC component has more significant influence than DC component. However, the DC component is found to promote PD activity under AC half cycle with the same DC polarity and inhibit PD activity under AC half cycle with the opposite DC polarity. The results in this paper provide an experimental foundation for the converter transformer insulation breakdown mechanism and analysis.

Streamer Propagation and Breakdown in a Very Small Point-Insulating Plate Gap in Mineral Oil and Ester Liquids at Positive Lightning Impulse Voltage

Abstract: This article presents the results of comparative studies on streamer propagation and breakdown in a point-insulating plate electrode system in mineral oil and two ester liquids. The studies were performed for a 10-mm gap and a positive standard lightning impulse. The work was focused on the comparison of light waveforms registered using the photomultiplier technique. The results indicated that both esters demonstrate a lower resistance against the appearance of fast energetic streamers than mineral oil. The reason for such a conclusion is that the number of lightning impulses supplied to the electrode system for which the above-mentioned fast streamers appeared at a given voltage level was always higher in the case of ester liquids than mineral oil. In terms of breakdown, the esters tested were assessed as more susceptible to the appearance of breakdown in the investigated electrode system. The number of breakdowns recorded in the case of esters was always greater than the corresponding number of breakdowns in mineral oil. This may be supposed on the basis of the obtained results that imply that, in both synthetic and natural ester, the formed breakdown channel, which bridged the gap through the surface of pressboard plate, is characterized by higher energy than in the case of mineral oil.

Creeping Discharge Characteristics of Nanofluid-Impregnated Pressboards Under AC Stress

Abstract: This paper presents experimental research on creeping discharge characteristics of TiO2 nanofluid-impregnated pressboards (NPs) under sustained ac voltage. It was found that NPs possess higher resistance to creeping discharge than that of pure oil-impregnated pressboard. The results of permittivity and surface charge accumulation and decay characteristics reveal that the modification of nanoparticles can not only make the electric field more uniformly distributed at the interface of nanofluid/pressboard due to a better matching in permittivity but also significantly increase the surface charge decay rate via a trapping and detrapping process in shallower traps. Thus, the creeping discharge characteristics of NPs are greatly improved by the modification of TiO2 nanoparticles.

Using the three-parameter Weibull distribution in assessment of threshold strength of pressboard impregnated by different liquid dielectrics

Abstract: This study presents the results of the measurements of electrical strength of pressboard specimens impregnated with three different liquid dielectrics: mineral oil, natural ester and synthetic ester. In the experiment carried out in accordance with IEC 60243 standard, two types of pressboard specimens of thicknesses 0.5 and 1 mm, respectively, were considered. The data obtained during the measurements in the form of breakdown voltages were used for calculation of the electrical strength. Both standard-based approach where median value was taken into account in calculations and statistical approach using three-parameter Weibull distribution function where location parameter V 0 was considered were applied. The results obtained on the basis of both approaches showed that both the specimens impregnated with mineral oil and ester liquids gave the satisfactory results within the electrical strength determined, which in all cases was higher than minimum required for the impregnated pressboards. The differences indicating slightly better impregnation properties of mineral oil, when the same impregnation conditions are used, are noticeable especially in the case of location parameter of Weibull distribution function. However, these differences are small and in relation to the requirements for the impregnated pressboards may be treated as negligible.

Efficiency of ageing markers for different transformer insulation systems

Abstract: The lifetime of a transformer is driven by the lifetime of its solid insulation, namely cellulosic materials (impregnated with liquid). Indeed, this later cannot be easily replaced as the insulating liquid. The most important parameter to assess cellulose aging status in transformer is to measure its degree of polymerization (DPv), giving data on its mechanical behavior. Nevertheless, the drawback of this method is the non-accessibility of cellulosic samples in service. This is the reason why the monitoring of cellulose insulation is generally made through oil sampling followed by laboratory analysis. Some of main markers, dissolved in the oil, and giving such evaluation are: water content, 2-FAL furanic compound and more recently Methanol. These chemical tracers are currently used in mineral oil and still under study for natural ester (vegetable oil). This paper presents humidity equilibrium charts developed for natural ester/cellulose systems, including Kraft paper and pressboard. Another part is dedicated to ageing studies realized on different transformer insulation systems (mineral oil / vegetable oil / Kraft paper / TUP) and discusses the efficiency of Methanol and 2-FAL ageing markers.

Seasonal influence on moisture interpretation for transformer aging assessment

Abstract: Aging of power transformers is one of the biggest challenges electrical utilities face. As transformers are commissioned, their insulation system composed of liquid (oil) and solid (paper and pressboard) insulations will deteriorate over time, jeopardizing longevity and reliability of transformers. As a means for optimizing capital expenditure without undermining the reliability of electricity supply, transformer aging or condition assessment has been a key practice for utilities. In most cases, it stems from interpreting the measurements of different parameters that can be tested from transformer oil. One such parameter is moisture in oil, which is one of the most commonly measured parameter by the UK utilities [1].

Interface charge polarity effect based analysis model for electric field in oil-pressboard insulation under DC voltage

Abstract: The Resistive-Capacitive (RC) model has been widely adopted for analysis of the electric field in oil-pressboard insulation under DC voltage. However, studies in this regard have indicated that the charge polarity effect in oil-pressboard insulation, namely the stronger adsorption of negative charges by pressboard, could not be adequately captured by the RC model. In order to analyze the electric field with due consideration of charge polarity effect, the present paper adopted the Kerr electro-optic techniques to have measured the electric field in the oil spacing under DC voltage for two experiments models with Model I having two oil spacing divided by a pressboard at the spacing ratio of 1:1, and the Model II at the spacing ratio of 1:2. As Model I is concerned, the strength of electric field in the upper oil spacing reached 1.61 times of the initial one under positive voltage and 1.60 times under negative voltage, while the field in the lower oil spacing registered as one eighth of the initial one under both positive and negative voltages. For Model II, the electric field strength in the upper oil spacing reached 2.3 times of the initial field under positive voltage and one fourth of that under negative voltage, showing much stronger charge polarity effect compared to that in Model I. In order to analyze the effect of interface charge on electric field, an electric field analysis model based on interface charge, named Interface Charge Polarity Effect (ICPE) Model, is proposed in this paper. Referring to the different adhesive abilities of positive and negative charges on pressboard, the influence of interface charge on electric field can be analyzed by the proposed model, which could make the contribution to the insulating design and proofread for the converter transformer.

Analysis of frequency domain dielectric response of pressboard insulation impregnated with different insulating liquids

Abstract: This study investigates the influence of moisture, temperature and diverse oil properties on the frequency domain response of pressboard insulation samples, which are impregnated with four different insulating liquids including two commercially available natural esters, one synthetic ester and mineral oil. Measurements are performed over a wide frequency range from 0.1 mHz to 1 kHz at variable moisture contents and temperatures (35, 55 and 75°C). It has been found that dielectric response is extremely sensitive to moisture content regardless of the type of impregnated liquid. Furthermore, dielectric response behaviour of ester impregnated pressboard insulation is qualitatively resembles to that of mineral oil impregnated pressboard which contains two distinct relaxation processes including Quasi-DC (q-dc) conduction features at low frequencies, and loss peak behaviour at mid frequency range. This paper proposes that the loss peak arises due to cooperative microscopic motion in water cellulose mixed phase dipole clusters together with the microscopic level interfacial polarization occurring at the interface of oil filled cavities. It is also discussed the q-dc process in pressboard insulation due to the charge transport between water and oil cavity based charge clusters through a percolation network of water molecules and hydroxyl groups along the cellulose fiber wall. In order to provide better understanding on charge transport and polarization phenomena in oil impregnated pressboard insulation, this paper also proposes an equivalent circuit, which is mainly derived based on Dissado- Hill (DH) cluster framework theory. Furthermore, this study has used activation energy to characterize the temperature dependent dielectric response of pressboard impregnated in mineral oil, natural esters and synthetic esters.

Moisture estimation of transformer pressboard by micro-strip ring resonator at GHz frequencies

Abstract: Increased moisture content (MC) is considered as one of the main factors for the failure of transformer insulation. This paper presents a method of estimating the MC of a transformer pressboard insulation based on the relative permittivity and loss tangent measurements performed at GHz using a multi-layer micro-strip ring resonator. It was fabricated on a material of known dielectric properties and the sample to be tested (i.e. pressboard) was placed on the ring structure. The dielectric properties, namely loss tangent and permittivity, of the sample were then estimated by measuring the S 21 transmission characteristics of the ring structure using a vector network analyzer. The technique was validated using a test sample of known dielectric properties. Additionally the results were compared with traditional frequency domain dielectric spectroscopy (FDS) measurements conducted at 1 mHz −1 kHz. The measurements were performed on oil impregnated and non-impregnated pressboard samples having different MCs conditioned by drying and/or wetting processes. The actual MC of the pressboard samples were obtained by weight measurements. It was found that the resonance frequency increased when the MC was reduced. Furthermore, a positive non-linear relation could be observed between the MC and both the relative permittivity and the loss tangent. It can be concluded that the proposed method could be used to estimate the MC of pressboards.

Terahertz Detection of Wavelength-Size Metal Particles in Pressboard Samples

Abstract: Results of a theoretical and experimental investigation of THz detection of randomly-oriented metal particles smaller than 1 mm embedded in pressboard are reported and critically compared with X-ray scanner results. A theoretical description based on Gaussian beam theory and a transfer function approach enabled simulation of signals transmitted around and reflected from real particles and was able to predict signal detectability. Simulated signals and detection limits were compared with experimental transmission and reflection time domain spectroscopy (TDS) raster scanning data. Simulated and measured pulse amplitude and shape were in agreement. Reflection mode was superior to transmission mode in terms of both particle visibility and hit ratio. Imaging of particles was also possible in “reflection in transmission” mode, which can be used to complement classical transmission mode. X-ray images did not detect the smallest particles sufficiently and, therefore, could not be used as reference data for comparison with THz data. The results demonstrate that TDS reflection scanning is able to determine the position of small metal particles and the technique could be possibly used for quality control of pressboard insulation.

Methods to reduce errors for DC electric field measurement in oil-pressboard insulation based on Kerr-effect

Abstract: Kerr-effect based methods show great potential for the measurement of electric field in liquid dielectric. However, the accuracy of the Kerr-effect based measurement over the DC electric field in oil-pressboard insulation remains to be a challenge. With the establishment of a Kerr-effect based non-contact and real-time electric field measurement system, the present paper analyzes the factors causing the measurement errors and thus proposes the countermeasures accordingly. It is indicated that the measurement errors resulted from the edge effect generated by electrode and pressboard could be reduced to 1.3% by lengthening the electrode, shortening the spacing distance between the electrode and the test chamber, and providing the electrode covering. The errors caused by non-ideal characteristics of the optical devices could be adjusted through the calibration of its parameters. The errors caused by the noise from laser source could be reduced to 0.86% for DC electric field and 1.6% for AC electric field through suitable selection of AC modulation frequency as 1310 Hz, which could also improve the signal-to-noise ratio. The test results conclude that the measurement system established under the present research could accurately measure DC electric field of 20 V/mm with the error being smaller than 5.4%.

Optical registration of transformer oil absorption processes in electrical pressboard nano-capillaries

Abstract: The paper presents designed and built measurement system for measuring electrical pressboard oil absorption speed. During measurements the test bench allows for automatic record of oil level, time, temperature, humidity and atmospheric pressure. Measurements of Nytro Taurus oil, produced by Nynas company, absorption by pressboard, produced by Weidman company, were made. It has been observed, that the absorption of oil takes place in three stages, duration of which are respectively τ1≈0,66 h, τ2≈109 h and τ3≈524 h. It was established that the time required to complete the oil absorption process takes more than 46 days. This means that at least as much time should take oil impregnation process of pressboard samples made for electrical measurements.

Effects of DC prestressing on partial discharge in oil-impregnated pressboard insulation

Abstract: The AC (50 Hz) partial discharge (PD) characteristics of oil-impregnated pressboard (OIP) insulation are severely influenced by the DC voltage component in the converter transformer. Therefore, this study investigates the effect of different DC prestressing voltage (DCPV) amplitudes, prestressing time, and repetitions times of the applied AC voltage on the OIP insulation AC PD characteristics. Meanwhile, the space charge characteristics of distribution and dissipation are measured by pulsed electro-acoustic method. The experimental results show that the PD inception voltage (PDIV) and PD extinction voltage (PDEV) after DCPV were lower than that under AC voltage without DCPV. The PDIV decrease with higher DCPV at prestressing time from 30 to 60 min. But it increase with higher DCPV at prestressing time from 90 to 120 min. However, PDIV initially increase then decrease with the increasing repetition times of the applied AC voltage. The space charge distribution in 1 mm OIP insulation takes 60 min to reach a balanced condition. The residual space charge is relatively higher at a longer prestressing voltage time and higher DC voltage when the DC voltage was turn off. The PDIV and PDEV after DCPV is improved by Joule heating with longer prestressing time and higher DCPV. The space charge had negative effect on them.

Thermal aging properties of pressboard in mineral oil and natural ester

Abstract: This article aims to contrastively study the thermal aging characters under different temperature between natural ester and mineral oil-pressboard insulation system. The thermal aging experiment was conducted with copper flat bars together in a sealed stainless vessel at 130 °C for 80 days, and at 100 °C for 220 days separately. Some basic physicochemical and electric characteristics of oil and cellulose varied with aging time, including moisture content, acid value, viscosity, furfural content, dielectric loss, AC breakdown voltage of oil, and the degree of polymerization were tested at each scheduled aging time. Results showed that the value of moisture content and dielectric loss in natural ester were higher than that of mineral oil, but it had a better insulation strength and service life.

Particle tracing modelling on moisture dynamics of oil-impregnated transformer

Abstract: An accurate estimation of moisture in oil-impregnated cellulose of a transformer is difficult due to the complex moisture dynamics inside the transformer, which is highly influenced by the temperature and dimension of the transformer. In this study, a novel particle tracing method is proposed for estimating the moisture in transformers. Different from the conventional approaches using Fick's diffusion law, the proposed method formulates moisture diffusion from a microscopic view of water particles’ motion. Especially, the transmission probability of water particles is employed to correlate the microscopic particles’ motion with the macroscopic moisture distribution. The proposed particle tracing method is applied to model moisture dynamics in both pressboard specimens and a prototype transformer. Extensive moisture diffusion and ageing experiments are carried out to verify the proposed method.

Effects of multiple parameters on static electrification and breakdown strength of transformer oil

Abstract: Forced convection of insulation oil is the main heat transfer mode in large-power and ultra-high-voltage (UHV) transformers. The electrification hazard caused by physicochemical phenomena between mineral oil and pressboard has been suspected to be responsible for several failures. The electrification measurement was carried out in an oil flow system. The influences of oil temperature, flow velocity, time and applied electric field of fresh and dry transformer oil have been investigated by a measurement system. The experimental results from the oil flow system show that the streaming current increases with the increase of the temperature when the temperature is below 50°C, while it reverses when the temperature is above 50°C. The streaming current increases with the increase of AC and DC electric fields under the different flow velocities, but the streaming current polarity reverses with DC electric field polarity under the 6.9 m/s flow velocity. The electrification phenomenon can lead to reduction of the breakdown strength of the oil. It is concluded that some effective measures should be taken to restrain the possible harm caused by the streaming electrification in UHV converter transformer.

Effect of the Thickness on the Water Mobility Inside Transformer Cellulosic Insulation

Abstract: Traditionally, water mobility inside the cellulosic insulation, as those used in power transformers, has been modeled using the so-called Fick's diffusion theory, considering that the water movement in this porous material only depends on local conditions, such as temperature and moisture concentration. However, recent experimental works have demonstrated that the moisture movement inside the cellulose insulation could be also influenced by a global condition, that is, the insulation's thickness. In this paper, the influence of the insulation thickness in the mobility of moisture inside the cellulosic materials, such as Kraft paper and pressboard, is analyzed. In addition, the improvement in moisture dynamics estimation obtained when this variable is included into the diffusion models has been proved.

Condition monitoring of biodegradable oil filled transformer

Abstract: Natural and synthetic ester insulating oils have higher fire points and excellent biodegradable characteristics. Therefore, in order to reduce the adverse environmental impact and to improve the fire safety of transformers, there is an increasing demand for natural and synthetic ester insulating liquids as a transformer insulating oil. However, present understanding on ageing behaviour of ester oil-paper composite insulation system and knowledge on application of existing condition monitoring tools for ester based insulation systems are inadequate. This impedes the cost effective and reliable field applications of ester insulating oils, particularly application of natural esters. To reduce this knowledge gap, series of controlled ageing experiments are performed in this research project to provide a better and comprehensive understanding on ageing behaviour of ester oil-paper insulation systems. Furthermore, applicability of existing chemical and electrical based condition monitoring techniques for ester oil-paper insulation systems is systematically investigated in this research project. In this thesis, ageing behaviour of dry pressboard insulation in mineral and three different ester insulating oils under simulated transformer operating environment is investigated. Moreover, the ageing behaviour of natural ester-pressboard composite insulation in moisture rich environment is also compared with that of mineral oil-pressboard system. Degree of polymerisation of pressboard samples measured at different ageing interval is used in this research to determine the ageing condition of pressboard. Moreover, applicability of oil related diagnostic parameters such as concentration of dissolved furanic compounds, acidity value, Dielectric Dissipation Factor (DDF), viscosity and colour to assess the degree of degradation of both ester and mineral insulating oils is thoroughly investigated in this research project. The potential of FTIR (Fourier Transform Infrared spectroscopy) techniques for characterising the degree of degradation of paper insulation is also discussed in this thesis. In addition, this thesis concentrates on characterisation of the charge dynamic in insulating oil through modelling of their dielectric responses. The comparison of gassing behaviour of ester and mineral insulating oils under two different low temperature faults and low energy electric discharge condition is also presented in this thesis. Dissolved Gas Analysis (DGA) results presented in this thesis depicts that faults gases detected in ester and mineral oil samples subjected to a similar fault are akin in type but quantitatively different. The quantity of fault gases produced in two different natural ester oils is also dissimilar. For example, soy-based natural ester produces a large quantity of ethane (C2H6) under low temperature overheating condition than sunflower oil based natural ester. Therefore, this research project investigates the applicability of well-established DGA interpretation schemes namely Duval II | P a g e triangle, IEC gas ratio and IEEE key gas method on ester based insulation systems in order to identify the possible faults based on DGA data. Investigation of impact of moisture, temperature and ageing on dielectric response of ester impregnated pressboard insulation by Frequency Domain Spectroscopy (FDS) and Polarisation Depolarisation Current (PDC) is a major contribution in this thesis. In order to study that, dielectric responses of well-controlled pressboard insulation samples which are impregnated with four different insulating liquids have been critically investigated. These measurements have been performed with two natural esters, one synthetic ester and one type of mineral oil under varying moisture (0.3%-8.7 %) and different temperature levels (35, 55, 75oC). In addition, the effect of ageing on dielectric response of natural ester impregnated pressboard insulation has been compared to that of mineral oil impregnated pressboard insulation. In this thesis, low frequency conductivity based model is proposed to determine the moisture content in transformer solid insulation based on FDS data of unaged pressboard insulation. Furthermore, the effect of ageing and temperature on the proposed model is quantitatively analysed. In order to interpret the FDS data of pressboard insulation in frequency range 10-4-10-3 Hz, a hierarchical equivalent circuit is derived based on Dissado-Hill’s cluster theory. The equivalent circuit model parameters are then used to explain the influence of diverse oil properties and moisture on microscopic level charge transport and polarisation phenomena in oil impregnated pressboard insulation. Furthermore, this research has used activation energy to characterise the temperature dependent dielectric response of pressboard insulation impregnated in mineral and ester insulating oils. The applicability of commercially available FDS based moisture diagnostic tool on ester based insulation system is also systematically investigated in this research project.

Relationship between the physicochemical properties of materials and the fractal dimension of creeping discharges propagating at solid/fluid interfaces

Abstract: The present paper is aimed at the main parameters that affect the fractal dimension D of creeping discharges propagating over different types of insulators immersed in gases at different pressures or in dielectric liquids, in a point-plane electrode arrangement. Especially, the dielectric constant, thickness of insulators, gas pressure, type of liquids (mineral and vegetable oils) and the type of voltage waveforms (lightning impulse voltage or DC) are analyzed. The considered insulators are circular samples of different thicknesses made of different materials (namely, glass, epoxy resin, PTFE, phenolplast resin and pressboard). The fractal dimension D of the observed discharge patterns is determined by the box counting method. It is shown that D depends on the thickness (e) and the dielectric constant of insulator (s), gas and its pressure or type of liquid. In a given gas or liquid, D decreases when e increases and it increases with s this dependency of D upon e and e indicates the important role of the electric field and capacitive effect in the propagation mechanism. Also, D decreases when the gas pressure is increased; and D is higher with lightning impulse voltage than with DC voltage. However, in liquids, D decreases when increasing the dielectric constant of liquid. These results evidence the existence of a relation between the fractal dimension and the physicochemical parameters of both materials constituting the insulating mixed structure.

Research of insulating oil percolation in electrical pressboard using optical registration

Abstract: The paper presents developed and built measuring system for oil percolation through electrical pressboard speed measurements. The system by using automatically controlled camera records the time of percolation. The rate of insulating oil Nytro Taurus, produced by Nynas company, penetration across electrical pressboard, produced by Weidman company, was investigated. The time dependence of insulating oil percolation on sample thickness has been designated. New method of pressboard capillaries radius determining on the basis of oil percolation speed experimental measurements has been developed. It was found that electrical pressboard capillary radius is from 45 nm to over 100 nm.

Study on space charge behavior of oil-paper insulation under AC-DC combined stress

Abstract: The space charge within oil-impregnated pressboard under AC and AC-DC combined stress were observed with a fast space charge detecting system based on the automatic equipartition phase shift (AEPS) principle. The phase resolution is about 1.79°, i. e. 201 equal division during whole period. Space charge accumulation is not distinct under AC stress at 15 kV/mm. The homocharge accumulates near ground electrode and the density decreases toward the top electrode. It may be associated with the charge injection from electrode. The amount of injected charge and the transfer distance are both depended on the electric field. Space charge accumulation is significant under AC-DC combined stress with the equal component at 15 kV/mm. The negative space charge distribution is dominant within oil-impregnated pressboard during whole period. It is mainly caused by the disruption of balance for electric field and polarization time under AC-DC stress. The polarity reversal, i.e. the voltage crossing 0 V for periodic stress, must be considered when the DC component exceeds a certain range. The space charge accumulated within oil-impregnated pressboard can distort the electric field distribution.