Showing papers in "IEEE Transactions on Dielectrics and Electrical Insulation in 2012"
TL;DR: In this paper, a gas chamber and four typical types of artificial defects were designed to simulate the SF6 decomposition phenomenon under partial discharge in GIS, and a gas chromatography system was established to detect the decomposition products.
Abstract: Partial discharge (PD) is usually aroused before the failure of gas insulated switchgear (GIS) caused by defects, which results in the decomposition of the SF6 used as insulating gas. Analyzing SF6 decomposition products can aid in PD detection. Before recognizing PD types by analyzing SF6 decomposition products, the decomposition characteristics of SF6 under different types of PD should be investigated first. In this paper, a gas chamber and four typical types of artificial defects were designed to simulate the SF6 decomposition phenomenon under PD in GIS. A gas chromatography system was established to detect SF6 decomposition products. By selecting the chromatographic column, SF6 decomposition products were successfully separated and their concentrations were measured. Numerous SF6 decomposition experiments under four kinds of PD generated by four kinds of artificial defects were carried out and the SF6 decomposition products produced by each experiment were detected. The decomposition characteristics of SF6 under the four defects were different in terms of decomposition amount, generation rate, and concentration ratio, among others. It is feasible to recognize the type of PD by analyzing SF6 decomposition products.
211 citations
TL;DR: In this paper, a TiO2 semiconductive nanoparticles with a large relaxation time constant is added into transformer oil to form semiconductorive nanofluids (SNFs), with the aim of enhancing insulating characteristics.
Abstract: In this paper, TiO2 semiconductive nanoparticles with a large relaxation time constant is added into transformer oil to form semiconductive nanofluids (SNFs), with the aim of enhancing insulating characteristics. ac, dc and lightning impulse breakdown voltage and partial discharge (PD) characteristics of oil samples before and after modification were measured according to ASTM standard methods. It was found that SNFs have ac, dc and lightning impulse breakdown voltage up to 1.2 times compared with pure oil. Meanwhile, the partial discharge resistance of SNFs was also dramatically improved. Charge trap and transportation characteristics of both samples have been measured by thermally stimulated current method (TSC) and pulse electroacoustic technique (PEA). It was found that electron shallow trap density and charge decay rate are greatly increased in semiconductive nanoparticles modified transformer oil. It is proposed that electron trapping and de-trapping processes in the shallow traps could be one of the main charge transport processes in dielectric liquids.
191 citations
TL;DR: In this article, the conduction mechanism was investigated and the conductivities near breakdown were inferred through the use of a thermal runaway criterion and the measured breakdown strength at various temperatures, and it was shown that conduction is by hopping with a thermal activation energy that is about 0.76 eV and is field independent.
Abstract: Although biaxially-oriented polypropylene thin film is a common dielectric for many high voltage pulsed power capacitor applications, the electrical conductivity under high field at elevated temperature is mostly unknown. Such knowledge is valuable not only for better understanding of the origin and transport mechanisms of the charge species at high field, but also for gaining insight for improving breakdown strength and electro-thermal modeling of high energy density, pulsed power capacitors. In this work, the conduction mechanism was investigated and the conductivities near breakdown were inferred through the use of a thermal runaway criterion and the measured breakdown strength at various temperatures. Results suggested that conduction is by hopping with a thermal activation energy that is about 0.76 eV and is field-independent. Such finding allows the use of an Arrhenius term and a field-dependent term to describe the field-dependent conductivity up to breakdown.
175 citations
TL;DR: In this article, a novel approach using Gene Expression Programming (GEP) is introduced to help in standardizing DGA interpretation techniques, identify transformer critical ranking based on DGA results and propose a proper maintenance action.
Abstract: Dissolved gas analysis (DGA) of transformer oil is one of the most effective power transformer condition monitoring tools. There are many interpretation techniques for DGA results. However, all of these techniques rely on personnel experience more than standard mathematical formulation and significant number of DGA results fall outside the proposed codes of the current methods and cannot be diagnosed by these methods. To overcome these limitations, this paper introduces a novel approach using Gene Expression Programming (GEP) to help in standardizing DGA interpretation techniques, identify transformer critical ranking based on DGA results and propose a proper maintenance action. DGA has been performed on 338 oil samples that have been collected from different transformers of different rating and different life span. Traditional DGA interpretation techniques are used in analyzing the results to measure its consistency. These data are then used to develop the new GEP model. Results show that all current traditional techniques do not necessarily lead to the same conclusion for the same oil sample. The new approach using GEP is easy to implement and it does not call for any expert personnel to interpret the DGA results and to provide a proper asset management decision on the transformer based on DGA analysis.
166 citations
TL;DR: In this article, the authors have concentrated on issues arising while on-line transformer winding deformation diagnosis is going to be applied on transformers with various kinds of techniques, such as frequency response analysis (FRA), short circuit impedance measurement and transfer function measurement.
Abstract: On-line monitoring and diagnosis of transformers have been investigated and discussed significantly in last decade. This study has concentrated on issues arising while on-line transformer winding deformation diagnosis is going to be applied on transformers with various kinds of techniques. From technical perspective, before replacing off-line methods by on-line methods and eventually by intelligent approaches, practical challenges must be addressed and overcome. Hence, available off-line transformer winding deformation diagnosis methods are discussed precisely. Mathematical calculation in on-line short circuit impedance measurement is investigated. On-line transformer transfer function measurement setup is presented. A profound insight to the problems pertaining on-line transformer winding deformation recognition methods, characterizes existing online methods, explains the concepts behind online measurements and striving to open the discussion doors towards challenges are discussed. In the end a 400 MVA step up transformer has been taken as a case in order to clarify the capability of Frequency Response Analysis (FRA) method in fault detection while short circuit impedance could only demonstrate some rough understanding about transformer condition.
164 citations
TL;DR: In this article, three different methods of acquiring the time differences of arrival (TDOA) from the PD waveforms are discussed, including the first peak of the signal as the arrival instant, or from the cross-correlation of the PD signal waveforms, or by applying the similarity function to the plots of PD signals cumulative energy.
Abstract: The location of a partial discharge (PD) source inside a transformer can be determined from the time differences of arrival (TDOA) between signals that are captured by an array of UHF sensors. The TDOA can be acquired from the received PD waveforms. In this paper, three different methods of acquiring the TDOA from the PD waveforms are discussed. The time difference can be calculated either by taking the first peak of the signal as the arrival instant, or from the cross-correlation of the PD waveforms, or by applying the similarity function to the plots of the PD signals cumulative energy. Computation algorithms for determining the TDOA automatically are introduced so that possible bias from human interpretation is avoided. The presence of noise and its effect on the accuracy of the PD localization will also be presented. Experimental results show the first-peak method has higher accuracy than the two other methods. The application of signal denoising further improves the localization accuracy.
133 citations
TL;DR: In this article, the effect of surface charges on dc flashover characteristics of a composite polymeric insulator is studied by means of experiments and theoretical calculations, and it was revealed that negative deposited surface charges led to an enhancement of the flashover performance whereas positive ones reduced the voltage level.
Abstract: Effect of surface charges on dc flashover characteristics of a composite polymeric insulator is studied by means of experiments and theoretical calculations The considered insulator consisted of a glass fiber reinforced epoxy core covered with a layer of silicone rubber and terminated by metallic electrodes with rounded smooth edges In the experiments, the insulator surface was charged by external corona while keeping the electrodes grounded and different charging levels were realized by varying its intensity A series of disruptive discharge tests were carried out on the charged insulator under negative dc voltages It was revealed that negative deposited surface charges led to an enhancement of the flashover performance whereas positive ones reduced the flashover voltage level A theoretical model has been developed and utilized for analyzing the experimental results In the model, surface charge density profiles deduced from measured surface potential distributions were used as boundary conditions for calculations of electric fields The measured and calculated flashover voltages were found to be in agreement indicating that the observed variations in the flashover characteristics could be attributed to the modifications of the electric field produced by the surface charges
126 citations
TL;DR: In this article, the ion flow field problem is used to calculate the gas conductivity arising from ions produced by natural radiation, and its role is compared to spacer bulk conductivities.
Abstract: It seems generally accepted that HVDC technology is to become increasingly relevant in power systems. With this expected future advance of HVDC, the use of gas insulated switchgear or lines for dc application are getting increasingly interesting. As in the case of dc the electric fields are determined resistively in contrast to the capacitive fields in the case of ac, gas insulated technology has to be revisited. In many previous works surface charging of model spacers was investigated. However, the ion flow problem to properly account for the electric conduction in the gas has not yet been solved. Further, no combination with inhomogeneous electric conductivity determined for example by temperature profiles from ohmic heating in nominal condition has been performed so far. In this paper, the gas conductivity arising from ions produced by natural radiation is computed by solving the ion flow field problem, and its role is compared to spacer bulk conductivities. Further, the impact of temperature profiles on spacer charging is investigated. The results indicate that for actual disc spacers, the spacer bulk conductivity is more important than that from the gas ionized by natural radiation. Especially in the case of polarity reversal, thermally enhanced spacer charging can result in considerable increase of tangential field strengths.
125 citations
TL;DR: In this article, a fuzzy c-means clustering algorithm was adopted to assess the performance of the two types of characteristic quantities, which was based on the data of SF6 decomposition products under the four kinds of partial discharges in Part 1.
Abstract: The decomposition characteristics of the SF6 under the different kinds of partial discharges (PD) should be understood first when recognizing PD by analyzing SF6 decomposition products in gas insulated switchgear (GIS). Moreover, the characteristic quantities used for recognition must be found. In this paper, the concentration and concentration ratio of SF6 decomposition products were each selected as characteristic quantities. Fuzzy c-means clustering algorithm was adopted to assess the performance of the two types of characteristic quantities, which was based on the data of SF6 decomposition products under the four kinds of PD in Part 1. Concentration ratio had better performance than concentration as a characteristic quantity in PD recognition. The concentration ratio method for PD recognition was established based on the decision tree theory, in which the three concentration ratios, namely c(SOF2)/c(SO2F2), c(CF4)/c(CO2), and c(SOF2 +SO2F2)/c(CO2+CF4), were used as characteristic quantities. The physical significance of the three concentration ratios was also analyzed. Finally, the concentration ratio method was applied to test the performance of PD recognition. The method has a good performance and can successfully recognize different kinds of PD.
124 citations
TL;DR: In this article, the use of nonlinear resistive field grading to mitigate the electric field at the edges of the substrate metallization increases accordingly and can exceed the dielectric strength of the gel commonly used as encapsulation leading to partial discharges or even breakdown of the insulation.
Abstract: The trend towards increasing blocking voltage of high power Insulated Gate Bipolar Transistor (IGBT) modules is challenging for their electric insulation system. In particular, the electric field at the edges of the substrate metallization increases accordingly and can exceed the dielectric strength of the gel commonly used as encapsulation, leading to partial discharges or even breakdown of the insulation. This paper reports on the use of nonlinear resistive field grading to mitigate the electric field within power electronic modules. Finite element simulations show how the field enhancements can be significantly reduced by applying a functional coating with suitable nonlinear resistive characteristic. Coatings made of polyimide filled with ZnO microvaristors were manufactured and tested.
112 citations
TL;DR: In this article, the unique characteristics of the oil-paper insulation system utilized in power transformers, their influence on the dielectric response and means for proper modeling of these characteristics are described.
Abstract: This paper describes the unique characteristics of the oil-paper insulation system utilized in power transformers, their influence on the dielectric response and means for proper modeling of these characteristics. Dielectric response measurement between windings of a power transformer is used to estimate the water content of the solid insulation. By measuring the dielectric response over a wide frequency range different properties of the insulation materials can be determined. These properties can be modeled in a laboratory so that measured response can be compared to modeled response with known properties. This comparison enables the estimation of properties of the materials such as water content and conductivity. The influences of temperature, aging and insulation geometry are described. Case studies are presented which demonstrate the validity of the test method. The influence of frequency range, temperature, aging byproducts, oil conductivity and insulation material type on the accuracy of the moisture estimation is discussed.
TL;DR: In this paper, the stationary resistive field distribution on the gas-solid interface along epoxy resin insulators is theoretically and experimentally investigated, and the results are compared to simulations, in which the electric conductivity of the gas is assumed to be constant.
Abstract: The stationary resistive field distribution on the gas-solid interface along epoxy resin insulators is theoretically and experimentally investigated. Due to the phenomenon that the charge carriers necessary for the conduction process originate from natural ionization and a constant generation rate is assumed, the resulting electric conductivity of gases under dc voltage stress is nonlinearly depending on the field strength. A simulation model which considers the relevant characteristic is used to calculate the electric field in gas-solid insulation systems. The influence of the ion pair generation rate of the gas, the electric field strength and the surface conductivity of an insulator on the resistive field distribution is investigated. The results are compared to simulations, in which the electric conductivity of the gas is assumed to be constant. The simulations are verified by measurements of the surface potential along an epoxy resin insulator under dc voltage.
TL;DR: In this paper, high aspect ratio fillers are used to increase the dielectric constant of polymer composites more efficiently than spherical fillers according to the rule of mixtures.
Abstract: High aspect ratio fillers are predicted to increase the dielectric constant of polymer composites more efficiently than spherical fillers according to the rule of mixtures. Using high aspect ratio fillers is a promising route for creating high dielectric constant, low loss materials at a low filler volume fraction, for use as capacitor and electric field grading materials. In this work, two high aspect ratio fillers were mixed into a polymer matrix, and the dielectric properties of composites were studied. Barium titanate fibers were synthesized by electrospinning a sol-gel, followed by a heat treatment to obtain a perovskite crystal structure. The heat treatment conditions were found to be crucial for obtaining tetragonal barium titanate fibers with high dielectric constant. Graphene platelets were prepared by a thermal shock method, which was found to result in a larger dielectric constant. A combination of barium titanate and graphene platelets yielded the highest dielectric constant when used in a polydimethyl siloxane matrix. The increase in dielectric loss over the pure matrix was small when the volume fraction was below the percolation threshold of graphene platelets. Electric flux density-electric field (D-E) measurements showed a linear dielectric constant in barium titanate filled composites and higher loss when graphene was added. The ac breakdown strength was reduced compared to the neat polymer and was affected by filler aspect ratio. The mechanisms that lead to the observed phenomena are discussed.
TL;DR: In this paper, the effect of inorganic nanoparticle fillers on the electrical properties of low density polyethylene (LDPE), ZnO nanocomposites were prepared with a variety of filler loadings by melt blending method and the corresponding electrical properties are investigated.
Abstract: In order to investigate the effect of inorganic nanoparticle fillers on the electrical properties of low density polyethylene (LDPE), LDPE/ZnO nanocomposites were prepared with a variety of filler loadings by melt blending method and the corresponding electrical properties are investigated. Experiments show that, the trap density is increased by 3-5 times in the nanocomposite as compared with LDPE. The conduction current of the nanocomposite is decreased to 0.5 - 0.25 of the value of LDPE both at low and high electric fields. The electrical breakdown strength is increased at low filler contents and decreased at high contents (>;1 wt%). The electrical treeing life time is elongated 50 times at most in the nanocomposites. Both homo- and hetero-space charges are remarkably suppressed in the nanocomposite with 0.5 and 7 wt% contents. The study indicates that the improvements on the electrical properties, especially the charge transport and space charge suppression in the nanocomposites are closely related to the significant increase of deep trap density. The effect of UV light absorption and electric field homogenization by ZnO can also contribute to the enhancement of electrical breakdown strength and elongation of electrical treeing life.
TL;DR: In this paper, a two-arm equiangular spiral antenna is designed and optimized in order to match the impedance of the sensor and the coaxial transmission line, a kind of micro strip impedance transformer which is of an exponentially gradual type is designed, the parameters of which are defined through simulation.
Abstract: A partial discharge (PD) sensor based on two-arm equiangular spiral antenna is designed and optimized in this research, and key parameters which mainly influence the performance of sensor are optimized. Through simulation and contrastive study, the key parameters' values are confirmed, which improves the sensor's performance to a great extent. To match the impedance of the sensor and the coaxial transmission line, a kind of micro strip impedance transformer which is of an exponentially gradual type is designed, the parameters of which are defined through simulation. Two improvement schemes for the designed transformer are also proposed to make an easy package and installation for the sensor and the transformer. Then, the physical model is made and tested. The results of simulation and test show good agreement, and verify that the sensors designed can meet the GIS internal partial discharge monitoring requirements well in bandwidth, performance and size, etc. The designed impedance transformer (IT) achieves impedance matching within the sensor and transmission line in ultra wideband range, and also enhances the sensitivity of the sensor.
TL;DR: In this article, a study on the dielectric spectroscopy of epoxy-based nanocomposites filled with different types of particles, such as Al2O3, AlN, MgO, SiO2 and BN, is presented.
Abstract: A study on the dielectric spectroscopy of epoxy-based nanocomposites filled with different types of particles, such as Al2O3, AlN, MgO, SiO2 and BN, is presented. The surface of the nanoparticles was modified with a silane coupling agent, in order to make them compatible with the organic host and create a system with homogeneously dispersed filler material. Morphological characterizations of individual particles and fabricated composites were performed by means of transmission and scanning electron microscopy. The present research addresses an analysis of the complex permittivity. The relative permittivity of nanocomposites shows an unusual behaviour. Introduction of a low percentage of high permittivity filler results in a decrease of the permittivity of the bulk polymer material. We propose a qualitative explanation for the reduction of the relative permittivity, compared to the reference samples. The interface layer of surface modified particles plays a more important role than the nature of the particles themselves. The immobilization caused by the surface treatment of the nanoparticles seems to be the main factor determining the relative permittivity of the composites with fillgrade below 5 wt.%. The imaginary part of the complex permittivity, which represents the dielectric losses in the system, does not change significantly with addition of nanofiller up to 5 wt.%.
TL;DR: In this paper, a fractured 500 kV ac composite insulator, whose fracture characteristics differ significantly from those of brittle fracture, is analyzed in order to identify the aging mechanisms involved and their causes.
Abstract: In this paper, a fractured 500 kV ac composite insulator, whose fracture characteristics differ significantly from those of brittle fracture, is analyzed in order to identify the aging mechanisms involved and their causes. Several sheath punctures occurred along the insulator which apparently originated from inside the insulator. Removal of the sheath revealed weak adhesion as well as tracking along the sheath-core interface. Weak microscopic as well as macroscopic interfaces were indicated by the insulator's weak performance in the water diffusion and dye penetration test according to IEC 62217. The core material was analyzed by means of FTIR identifying glass corrosion, ion exchange and hydrolysis as major degradation mechanisms. Based on the results of analysis, a water induced aging process is proposed and recommendations for online monitoring of composite insulators with respect to interfacial aging are given.
TL;DR: In this article, the authors used an inclined plane (IP) tracking and erosion test under both positive and negative dc voltages for silicone rubber samples filled with micron and nano-sized particles to understand the phenomena occurring during such tests.
Abstract: In order to improve the tracking and erosion performance of outdoor polymeric silicone rubber (SR) insulators used in HV power transmission lines, micron sized inorganic fillers are usually added to the base SR matrix. In addition, insulators used in high voltage dc transmission lines are designed to have increased creepage distance to mitigate the tracking and erosion problems. ASTM D2303 standard gives a procedure for finding the tracking and erosion resistance of outdoor polymeric insulator weathershed material samples under laboratory conditions for ac voltages. In this paper, inclined plane (IP) tracking and erosion tests similar to ASTM D2303 were conducted under both positive and negative dc voltages for silicone rubber samples filled with micron and nano sized particles to understand the phenomena occurring during such tests. Micron sized Alumina Trihydrate (ATH) and nano sized alumina fillers were added to silicone rubber matrix to improve the resistance to tracking and erosion. The leakage current during the tests and the eroded mass at the end of the tests were monitored. Scanning Electron Microscopy (SEM) and Energy dispersive Xray (EDX) studies were conducted to understand the filler dispersion and the changes in surface morphology in both nanocomposite and microcomposite samples. The results suggest that nanocomposites performed better than microcomposites even for a small filler loading (4%) for both positive and negative dc stresses. It was also seen that the tracking and erosion performance of silicone rubber is better under negative dc as compared to positive dc voltage. EDX studies showed migration of different ions onto the surface of the sample during the IP test under positive dc which has led to an inferior performance as compared to the performance under negative dc.
TL;DR: In this paper, a new method for assessing the ageing condition of the oil-paper insulation in terms of the depolarization charge quantity was proposed, which can also be used to assess the condition of new insulation system using natural ester in transformers accurately.
Abstract: Polarization and Depolarization Current (PDC) technique is an effective tool to assess the condition of oil-paper insulation system in power transformers. So far the PDC behaviors of mineral oil-paper insulation have been widely investigated. However, with the increasing number of transformer choosing natural ester as its insulation oil, it is important to investigate the PDC characteristics of natural ester-paper insulation to see whether the PDC technique can also be used to assess the condition of new insulation system using natural ester in transformers accurately. In this research, natural esterpaper insulation sample and mineral oil-paper insulation sample were subjected to thermally accelerated ageing experiment at 110°C for 120 days. The PDC characteristics of natural ester-paper insulation sample and mineral oil-paper insulation sample were compared over the ageing process. A new method for assessing the ageing condition of the oil-paper insulation in terms of the depolarization charge quantity was proposed. Results show that the polarization/depolarization current of natural ester-paper insulation sample is higher than that of mineral oil-paper insulation sample with the same ageing intervals. The depolarization charge quantity of both kinds of oil-paper insulation sample is very sensitive to their ageing conditions. There is an exponential relation between the stable depolarization charge quantity of both kinds of oil-paper insulation sample and the degree of polymerization (DP) of paper. The depolarization charge quantity can be used to predict the ageing condition of oil-paper insulation providing the measurement temperature is kept the same.
TL;DR: In this paper, the degradation rate of vegetable oil can be compared to that of mineral oil in terms of chemical, mechanical and electrical properties of the two oils by the aging time.
Abstract: The usage of vegetable oil for power transformer insulation has increased, compared to mineral oil, for its better environmental characteristics. However, although the temperature rise depending on load difference and the relationship between hottest spot temperature and life time depending on long-term operation for mineral oil immersed transformer has been studied, there is being established for vegetable oil immersed transformer. Therefore, the MV power transformers either filled with conventional mineral oil or with vegetable oil are thermally aged and the transformer oils are sampled regularly. With these samples, the chemical, mechanical and electrical characteristics of the two oils are investigated by the aging time. According to our experiment, the degradation rate of vegetable oil can be compared to that of mineral oil.
TL;DR: In this paper, an approach to high field control, particularly in the areas near the high voltage (HV) and ground terminals of an outdoor insulator, is proposed using a nonlinear grading material; Zinc Oxide (ZnO) microvaristors compounded with other polymeric materials to obtain the required properties and allow easy application.
Abstract: An approach to high field control, particularly in the areas near the high voltage (HV) and ground terminals of an outdoor insulator, is proposed using a nonlinear grading material; Zinc Oxide (ZnO) microvaristors compounded with other polymeric materials to obtain the required properties and allow easy application. The electrical properties of the microvaristor compounds are characterised by a nonlinear field-dependent conductivity. This paper describes the principles of the proposed field-control solution and demonstrates the effectiveness of the proposed approach in controlling the electric field along insulator profiles. A case study is carried out for a typical 11 kV polymeric insulator design to highlight the merits of the grading approach. Analysis of electric potential and field distributions on the insulator surface is described under dry clean and uniformly contaminated surface conditions for both standard and microvaristor-graded insulators. The grading and optimisation principles to allow better performance are investigated to improve the performance of the insulator both under steady state operation and under surge conditions. Furthermore, the dissipated power and associated heat are derived to examine surface heating and losses in the grading regions and for the complete insulator. Preliminary tests on inhouse prototype insulators have confirmed better flashover performance of the proposed graded insulator with a 21 % increase in flashover voltage.
TL;DR: In this article, the authors used a special test rig to compare the ageing rate to previous work and to determine the ageing effect of paper by combining temperature, water content of paper and oxygen content of the oil.
Abstract: A substantial quantity of testing has been performed on the degradation of paper insulation over the decades. The aim being to better educate the electrical industry on how to best operate expensive assets, such as transformers. The longevity of paper insulation is frequently tested using accelerated ageing experiments, where the effects of temperature and the chemical environment on paper lifespan can be studied. Such research has resulted in paper life expectancy curves being published by the IEEE and the IEC. The investigations tend to use sealed vessels. However, the disadvantage of using this method is that the water content of the paper changes during the ageing process which then changes the ageing rate. In these ageing experiments the water and oxygen content was controlled using a special test rig to compare the ageing rate to previous work and to determine the ageing effect of paper by combining temperature, water content of paper and oxygen content of the oil. We found that the rate of paper ageing can be more accurately determined by controlling the water and oxygen during the experiment which then produced noticeable changes in predicting life expectancy.
TL;DR: A review of the history of paper and pressboard insulation for electric insulation of oil immersed power transformers is given in this paper, along with a list of factors important for the manufacturing and operation of reliable transformers.
Abstract: This paper reviews briefly the history of paper and pressboard insulation being used for electric insulation of oil immersed power transformers. An introduction to today's insulation technique is given. Important details of materials selection, as well as drying, clamping and oil impregnation procedures required to achieve short-circuit resistant windings are explained. Aging effects are described, as they gradually weaken the solid insulation mechanically and electrically. The value of post-mortem analysis of transformers before scrapping is shown with examples. A list of factors important for the manufacturing and operation of reliable transformers, yet often neglected, completes this article.
TL;DR: In this article, a frequency domain spectroscopy (FDS) was used to assess the ageing condition of oil-paper insulation used in transformers, and the results reveal that the relative permittivity (er') plots of oil impregnated pressboards increase with the reduction in cellulose polymer chain length.
Abstract: Frequency domain spectroscopy (FDS) has been used to assess the ageing condition of oil-paper insulation used in transformers. To further understand the ageing process, the reduction in degree of polymerization (DP) of cellulosic paper in itself on the dielectric response was investigated first. It has been found that the reduction in DP itself can alter the dielectric characteristics. The oil-paper insulation was thermally accelerated aged at 110 °C for up to 154 days according to the procedure described in IEEE Guide. Then dielectric characteristics of the thermally aged samples were obtained at different measurement temperatures. The results reveal that the relative permittivity (er') plots of oil impregnated pressboards increase with the reduction in cellulose polymer chain length. er' and tanδ values of oil impregnated pressboards shifts upwards in the lower frequency range (10-3 Hz -10-1 Hz) with different ageing condition, indicating the possibility of utilizing the characteristics of er' and tanδ values at lower frequencies (10-3 Hz-10-1 Hz) to quantitatively characterize ageing condition of oil impregnated pressboard. The parameters including DP, er' and tanδ values at the three characteristic frequencies (10-3 Hz, 10-2 Hz, 10-1 Hz) have been analyzed as a function of the ageing time, showing an exponential relationship based on the best fit. In addition, the er' and tanδ curves of aged oil impregnated pressboard have been observed to be shifted upwards to higher values at lower frequencies when the measurement temperature is increased. A shifting factor αT which describes the relationship between frequency and temperature has been defined. It allows one to shift the master curve of impregnated pressboard with DP obtained at reference temperature to other testing temperatures.
TL;DR: In this paper, the authors present the experience of unconventional methods for partial discharge (PD) measurement to detect and localise PD sources in power transformers, which is usable as stand-alone measurement and as a supporting measurement for off-and on-line PD detection.
Abstract: The paper presents the experience of unconventional methods for partial discharge (PD) measurement to detect and localise PD sources in power transformers. The UHF PD measurement method is usable as stand-alone measurement and as a supporting measurement for off- and on-line PD detection. The sensitivity of UHF PD measurements is sufficient and is normally not affected by external disturbances. Especially in noisy surrounding it might be a very helpful method to support other PD measurement techniques for example dissolved gas analysis and acoustic location of PD. Propagation time of UHF signals can be used for geometrical PD location. The accuracy seems to be adequate to determine the phase limb where the PD is located. Additionally, different measurable UHF amplitudes support an estimation of the PD location. However, since transformers rarely possess more than three oil valves for installation of UHF probes, an additional acoustic measurement is usually required for location. Using the knowledge gained from the UHF location, acoustic sensors can be placed near to the PD source at the transformer tank. The paper explains first the fundamentals of PD measurements and PD source location and presents two case studies.
TL;DR: An improved wavelet de-noising approach for PD online measurement is presented, based on a genetic adaptive threshold estimation (GATE) scheme that generates significantly smaller waveform distortion and magnitude errors than the Donoho's soft threshold estimation.
Abstract: Wavelet shrinkage is efficient for de-noising the partial discharge (PD) detection. An improved wavelet de-noising approach for PD online measurement is presented. The wavelet de-noising approach is based on a genetic adaptive threshold estimation (GATE) scheme. The thresholding functions with continuous derivatives are used for the GATE scheme. A genetic algorithm is used to obtain global optimum thresholds of the GATE, and to improve the robustness and computation speed of the adaptive threshold estimation. De-noising experiments of simulative high-frequency PD signals, actual PD ultra-high-frequency (UHF) signals, and a field detected PD signal are presented. The GATE generates significantly smaller waveform distortion and magnitude errors than the Donoho's soft threshold estimation.
TL;DR: In this paper, the ageing rate of dry paper in air (excess oxygen) has been studied for thermally upgraded Kraft paper in medium and high oxygen with varying water content, and it has been shown that the paper with the same water content increased with oxygen content in the oil.
Abstract: Utility asset managers and engineers are concerned about the life and reliability of their power transformers which depends on the continued life of the paper insulation. The ageing rate of the paper is affected by water, oxygen and acids. Traditionally, the ageing rate of paper has been studied in sealed vessels however this approach does not allow the possibility to assess the affect of oxygen on paper with different water content. The ageing rate of paper has been studied for dry paper in air (excess oxygen). In these experiments we studied the ageing rate of Kraft and thermally upgraded Kraft paper in medium and high oxygen with varying water content. Furthermore, the oxygen content of the oil in sealed vessels is low which represents only sealed transformers. The ageing rate of the paper has not been determined for free breathing transformers with medium or high oxygen content and for different wetness of paper. In these ageing experiments the water and oxygen content was controlled using a special test rig to compare the ageing rate to previous work and to determine the ageing effect of paper by combining temperature, water content of paper and oxygen content of the oil. We found that the ageing rate of paper with the same water content increased with oxygen content in the oil. Hence, new life curves were developed based on the water content of the paper and the oxygen content of the oil.
TL;DR: In this paper, a comparative study between mineral and various ester oils based on Dissolved Gas Analysis (DGA) was performed at laboratory scale, then gas formation as well as diagnosis methods were compared.
Abstract: This paper reports on a comparative study between mineral and various ester oils based on Dissolved Gas Analysis (DGA). Electrical (breakdown of low energy) and thermal (stray gassing) faults were realized at laboratory scale, then gas formation as well as diagnosis methods were compared. Two extraction methods, described in IEC 60567 Ed.3 and independent from partition/solubility coefficients were used: multi-cycle vacuum degassing and stripping. Three different diagnosis methods were applied: IEC gas ratios, Duval's triangle and key gases. For electrical faults and especially for low energy discharges, the same main gases (Hydrogen and Acetylene) are created in relatively same ratios, thus confirming the efficiency of key gases method for ester oils. It is also shown that classic Duval's triangle (triangle 1) is well applicable for this type of fault confirming the results reported in literature. For thermal faults and especially for stray gassing issue (low thermal faults), Ethane (associated with Hydrogen) constitutes the key gas of natural esters. The new Duval?s triangles can be applied: Triangle 4 for mineral oil and Triangle 6 for ester oils. For extraction methods and in both electrical and thermal faults cases, it can be noted that results are comparable for mineral oil whereas they are slightly underestimated with stripping method for ester oils. The higher viscosity of ester could interfere with the extraction.
TL;DR: In this paper, a simple theoretical analysis of the initial stage of a sub-nanosecond pulsed high-voltage breakdown in liquid dielectrics is presented, showing that a rapid breakdown is related to a rupture in the continuity of fluid induced by the electrostrictive forces in the inhomogeneous electric field in the vicinity of a needle electrode.
Abstract: This paper presents a simple theoretical analysis of the initial stage of a sub-nanosecond pulsed high-voltage breakdown in liquid dielectrics. We show that a rapid breakdown is related to a rupture in the continuity of fluid induced by the electrostrictive forces in the inhomogeneous electric field in the vicinity of a needle electrode.
TL;DR: The obtained results indicate that, the proposed methodology can be used to locate partial discharges in high voltage equipment where the optical signals due to discharges find a path to get radiated towards the outer surface.
Abstract: The present work represents a methodology to detect the location of single as well as multiple Partial Discharge (PD) sources by optical method and to investigate the performance of optical sensors for this purpose. An experimental setup has been arranged in the laboratory for generation of PDs, optical sensing and analysis of the recorded signals obtained from multiple optical sensors. The analysis results prove the effectiveness of the methodology using optical sensors to find whether PD is occurring at single location or multiple locations. For identification of PD locations pattern recognition technique has been utilized by considering the received optical energy as a feature. For feature selection and classification two techniques have been evaluated, viz. Gaussian Mixture Model (GMM) and Support Vector Machine (SVM), and both have shown promising performance. SVM in regression mode was used for identification of unknown PD location/locations. In this case average accuracy obtained was 92.6% when PD is occurring at one location and 80.1% when PD is occurring at two locations. The obtained results indicate that, the proposed methodology can be used to locate partial discharges in high voltage equipment where the optical signals due to discharges find a path to get radiated towards the outer surface.