Showing papers in "IEEE Transactions on Dielectrics and Electrical Insulation in 2015"

Evaluation of polypropylene/polyolefin elastomer blends for potential recyclable HVDC cable insulation applications

Abstract: This paper evaluates the microstructure and properties of polypropylene/polyolefin elastomer (PP/POE) blends for potential recyclable HVDC cable insulation applications. PP/POE blends with different POE content were prepared by melt mixing. The introduction of POE results in a slight decrease of the melting points but improves the flexibility of PP. Compared with PP, the volume resistivity of the blends shows a decrease at low loading of POE and starts to increase when the POE loading is higher than 15 wt%. After the introduction of POE, the DC breakdown strength is slightly decreased and the hetero space charge accumulation is enhanced. Although the electrical properties of the PP/POE blends are inferior to those of the pure PP, the enhanced flexibility, high volume resistivity, high breakdown strength as well as the excellent thermal properties make the PP/POE blends have the potential for HVDC cable application. The hetero space charge accumulation is still an issue, and further modification of the blends should be considered for suppressing the space charges.

Flashover of insulators in vacuum: the last twenty years

Abstract: This paper reviews the last twenty years of work concerning surface flashover of insulators in vacuum. It complements an earlier review paper [H.C. Miller, Flashover of Insulators in Vacuum, IEEE Trans. Electr .Insul., Vol. 28, pp. 512-527, 1993]. The surface flashover voltage of insulators in vacuum depends upon many parameters, such as material, geometry, surface finish, and attachments to electrodes, but also on the applied voltage waveform, duration, single pulse or repetitive, and on the process history of the insulator, operating environment, and previous applications of voltage. Suggestions are made for choosing the material, geometry, and processing when selecting an insulator for a particular application. Some specific techniques for improving the holdoff voltage of insulators are recommended.

The energy distribution of trapped charges in polymers based on isothermal surface potential decay model

Abstract: Space charge formation in polymeric materials can cause some serious concern in real operation, because it has significant influence on the performance of polymers. For example, space charge in some insulating materials can severely distort the electric field, even lead to materials degradation. On the contrary, in the case of its applications, space charge stored in electrets can greatly improve their properties. It is therefore important to understand trapped charge distribution in materials as it is considered to be a novel indicator for effective evaluation of aging status and electric withstanding strength of insulating materials. In this paper, a model based on isothermal surface potential decay (ISPD) is proposed to study the distribution of trapped charges by considering the physical mechanism of the detrapping process. By measuring the ISPD characteristics of polymeric materials and fitting the data according to the assumption of shallow and deep traps, the distribution of trapped charges is obtained, which may be related to the change of aggregation structure of polymers. In order to verify the model, it is used to analyze different ISPD decay curves of polypropylene (PP) and low density polyethylene (LDPE), as well as the ISPD data of PP electrets with/without pressure expanding treatment. The results show that the proposed ISPD model is effective and convenient. Two peaks are observed on the curve of the trapped charge density versus the trap level. The obtained distribution of the trapped charges in polymers can reveal the different nature of electron/hole traps and the different transportation behavior of hole/electron carriers, i.e., the electron-type traps show an inter-chain character while the character of hole-type traps is intra-chain. In addition, the distribution of trapped charge is further related to aggregation structure of PP and LDPE, as well as PP electrets with/without pressure expanding treatment.

A comprehensive comparative study of DGA based transformer fault diagnosis using fuzzy logic and ANFIS models

Abstract: In this work, Dissolved gas Analysis (DGA) has been implemented using softcomputing models namely fuzzy logic and Adaptive Neuro fuzzy inference system (ANFIS). DGA has developed as an effective tool for the identification of transformer incipient faults. A number of standards and procedures have evolved over the years making DGA more reliable and user friendly. A comparative study of the two models has been developed based on their ability to circumvent the limitations of the IEC 599 standard, Rogers ratio method and Doernenburg’s method. The models have been tested using a reported fault database for their diagnostic capability. Results presented in this paper clearly indicate the superiority of the ANFIS model over the fuzzy system. The ANFIS model presents a reliable system with relative high degree accuracy. ANFIS model being very simple to develop can obviate the limitations of conventional methods of transformer fault diagnosis using DGA.

Dispersion behavior and breakdown strength of transformer oil filled with TiO 2 nanoparticles

Abstract: Recently, the study of transformer oil-based nanofluids became of great interest, due to their prospective properties as a dielectric and cooling medium. However, agglomeration of nanoparticles limits the beneficial properties that can be obtained from nanofluids. So, the work presented in this paper aims to get enhanced dispersion behavior of nanoparticles within transformer oil-based nanofluids. Then, breakdown strength with the enhanced dispersion behavior is evaluated. In order to get enhanced dispersion behavior, nanofluids were prepared using nanoparticles with a surfactant. The surfactant plays a role in the stabilization of nanoparticles and maintaining suspension stability. The considered type of nanoparticles is TiO2 nanoparticles. Two series of nanofluid samples were prepared. Through the first series, the effect of surfactant concentration on dispersion behavior and agglomerate size was studied. The dispersion of nanoparticles was characterized using three different techniques. These techniques are optical microscope analysis, transmission electron microscope (TEM) analysis and zeta potential measurements. Based on these techniques, the suitable concentration of surfactant was identified. The second series of nanofluid samples was prepared with different weight percents of nanoparticles for assessing the breakdown strength. Weibull distribution was used to calculate the breakdown probability for the base oil as well as nanofluid samples. The results showed an enhancement in the breakdown strength by about 27% in comparison to the base oil. Based on the obtained results, mechanisms behind the dispersion behavior and breakdown strength were proposed and discussed.

Effects of conductivity and permittivity of nanoparticle on transformer oil insulation performance: experiment and theory

Abstract: The mechanisms by which conductive and dielectric nanoparticles (NPs) trap electrons are explained by the potential well distribution caused by induced or polarized charges on NPs. Thus, the distributions of surface and saturation charges on conductive and dielectric NPs are determined. Given conductive Fe3O4, semiconductive TiO2, and dielectric Al2O3 NPs, insulation performance tests are conducted and ionization models of nanofluids (NFs) based on transformer oil are developed. These models are compared with those of NFs based on pure oil. The NP whose conductivity or permittivity does not match that of the dielectric liquid has a potential well and an increased amount of saturation charges on its interface. This NP influences streamer development strongly and enhances the breakdown of oilbased NF.

Improved power transformer winding fault detection using FRA diagnostics – part 1: axial displacement simulation

Abstract: Frequency response analysis (FRA) has become a widely accepted tool to detect power transformer winding deformation due to the development of FRA test equipment. Because FRA relies on graphical analysis, interpretation of its signature is a very specialized area that calls for skilled personnel, as so far, there is no reliable standard code for FRA signature identification and quantification. Many researchers investigated the impact of various mechanical winding deformations on the transformer FRA signature using simulation analysis by altering particular electrical parameters of the transformer equivalent electrical circuit. None of them however, investigated the impact of various physical fault levels on the corresponding change in the equivalent circuit parameters. In this paper, the physical geometrical dimension of a single-phase transformer is simulated using 3D finite element analysis to emulate the real transformer operation. A physical axial displacement of different fault levels is simulated in both low voltage and high voltage windings. The impact of each fault level on the electrical parameters of the equivalent circuit is investigated. A key contribution of this paper is the charts it introduces to correlate various axial displacement levels with the percentage change of all transformer equivalent circuit parameters due to the axial displacement fault. In contrary with other researchers who only considered mutual inductance between low voltage and high voltage windings, simulation results shown in this paper reveal that other circuit parameters should be changed by a particular percentage to accurately simulate particular fault level of transformer winding axial displacement. Results of this paper aid to precisely simulating winding axial displacement using transformer equivalent circuit that facilitates accurate qualitative and quantitative analysis of transformer FRA signatures.

Thermoplastic cable insulation comprising a blend of isotactic polypropylene and a propylene-ethylene copolymer

Abstract: There is much interest in the development of replacement materials for crosslinked polyethylene (XLPE) that are both recyclable (i.e. thermoplastic) and capable of high temperature operation. Thermally, polypropylene is the ideal choice, although its stiffness and low electrical breakdown strength make for a challenging materials design problem. We report here on the compositional optimization of a propylene homopolymer/propylene-ethylene copolymer blend in terms of its dynamic mechanical properties and thin film electrical breakdown strength. The extrusion of a trial minicable using the optimized blend is also discussed, which is shown to exhibit a significantly improved electrical performance, as gauged by its DC breakdown strength, than an XLPE-insulated reference.

Determination of charge-trapping sites in saturated and aromatic polymers by quantum chemical calculation

Abstract: We studied the trapping of positive and negative charges in the chemical structures of polymers under a high electric field using a space charge measurement system. Positive charges accumulated in low-density polyethylene (LDPE), whereas positive and negative charges accumulated in polyimide (Kapton) and also in ethylene tetrafluoroethylene (ETFE) subjected to electron beam irradiation. To determine the charge-trapping sites in the chemical structures, a quantum chemical calculation was carried out using Density Function Theory (DFT) with Gaussian 09. The relationship between the energy band and the isosurface of orbital electrons at various energy levels was obtained. A threedimensional (3D) electrostatic potential distribution map was obtained for positively and negatively charged polymers to determine the relationship between a trapping site and the charge accumulation center in the 3D potential distribution map. Positive and negative charges in Kapton and ETFE films are trapped in trapping sites in chemical structures and the positive charges in an LDPE film are trapped in physical defects.

Development of a new graphical technique for dissolved gas analysis in power transformers based on the five combustible gases

Abstract: Several methods have been proposed for dissolved gas analysis (DGA) of mineral oil in power transformers. One of the simple and most widely used methods is Duval triangle. However, Duval triangle does not consider the concentrations of two combustible gases, namely ethane (C2H6) and hydrogen (H2). As a result, Duval triangle exhibits a lower accuracy in diagnosing certain fault types, for which these gases are the key gases, such as low overheating and corona discharge. Accordingly, this paper proposes a novel graphical technique for DGA based on all the five combustible gases. The proposed graphical technique is developed in the form of a pentagon shape. The pentagon heads represents the percentage concentration of each individual gas to the total combustible gases. The corresponding point for a certain faulty case is determined by the center of mass of all pentagon heads. The knowledge extracted from previous DGA methods and field experiences are used to estimate the preliminary fault regions within the pentagon. The exact boundaries between fault regions are then specified using actual DGA data with corresponding fault types collected from the Egyptian Electricity Network and published cases. The overall performance of the proposed pentagon has been evaluated using a set of fault cases and it is revealed that the proposed pentagon has higher diagnostic accuracy compared to other methods including Duval triangles and IEC standard 60599.

Surface modification of polymethyl-methacrylate using atmospheric pressure argon plasma jets to improve surface flashover performance in vacuum

Abstract: Hydrophilic modification of polymethyl methacrylate (PMMA) surface is performed by atmospheric pressure plasma jet (APPJ) in Ar gas for improving the PMMA surface flashover performance in vacuum. In the experiments, APPJ is driven by a microsecond-duration pulsed generator, which has voltages of 0-30 kV, a rise time of 300 ns and a full width at half maximum of 2μs. Characteristics of the APPJ are analyzed according to its voltage and current waveform, discharge image and optical emission spectrum. Furthermore, surface properties of the PMMA surface before and after the treatment are characterized by water contact angle measurements and morphology observations. Results show that the main species of the plasma jet are composed of N2, Ar, OH, and O, among which such polar groups as OH and O enhance the hydrophilic property of the PMMA surface. The water contact angle decreases from 68° to a minimum value (16°) after the treatment. In addition, all the surface flashover voltages in vacuum for the PMMA samples treated by APPJ are higher than those for the untreated PMMA samples.

Advancing new techniques for UHF PDdetection and localization in the power transformers in the factory tests

Abstract: Partial discharge (PD) detection in the ultra-high frequency (UHF) range is state of the art. One of the most critical points in this detection method is adopting proper and suitable antenna design. In this paper, a new design of UHF antennas is introduced which is suitable to be used on the transformers in the HV laboratories of the manufacturers. To ensure the applicability of the new antenna, experimental UHF PD measurements are carried out on a transformer tank model as well as on the real power transformers. Installing several UHF antennas on the transformer tank to perform defect localization is also an important issue. There are rigorous limitations and lack of the places to install the antennas. This problem is almost solved by using the mentioned new antenna design. The proper positioning of the antennas is the other critical point in the UHF method. Placing the antennas in improper positions will damage the localization accuracy. In this paper, an automatic algorithm is introduced to determine the best installation positions for the antennas to enhance the localization accuracy. The efficacy of this algorithm is investigated by using the results of experimental measurements on power transformers.

Correlating conductivity and space charge measurements in multi-dielectrics under various electrical and thermal stresses

Abstract: The design of transmission systems requires electric field distribution estimation, which, in case of HVDC application is strongly sensitive to thermal and electrical configuration as well as to the nature of dielectric materials being used owing to the resistive field distribution. In this paper, the field distribution in a dielectric bilayer of XLPE and rubber materials, as representative of cable junctions, is estimated based on experimental data on field and temperature dependencies of conductivity. Through space charge measurements on bi-layer dielectrics, it is shown that the space charge density and electric field distributions are to a first order estimation consistent with data issued from conductivity measurements. Most notably, the interface charge building up between the two dielectrics changes sign, depending on field and temperature. However, in the high field range (order of 20 kV/mm), charge build-up in the bulk of dielectric materials introduces further distortion to field distribution.

Influence of functionalized MgO nanoparticles on electrical properties of polyethylene nanocomposites

Abstract: This experimental study reported electrical properties of linear low density polyethylene (LLDPE)/MgO nanocomposites, which were prepared by melt blending methods. The effects of surface modified MgO nanoparticles on the microstructure, space charge distribution, thermally stimulated current and DC breakdown strength of the nanocomposites were investigated. The addition of surface modified nanoparticles increases the amount of spherulites and decreases their sizes. It is found that the LDPE/MgO interface shows significant influence on electrical properties of nanocomposites. The addition of MgO nanoparticles is available to suppress the production of space charges and enhance the DC breakdown strength, depending on the loading levels of nanoparticles. Thermally stimulated currents of nanocomposites reveal strong correlation between the traps and electrical properties of nanocomposites. It is believed that this study would provide important hint to design and develop advanced polymer nanocomposites for dielectric applications, in particular the HVDC applications.

Improved power transformer winding fault detection using FRA diagnostics – part 2: radial deformation simulation

Abstract: Frequency response analysis (FRA) is proven to be a powerful tool to detect winding deformation within power transformers. Although the FRA test along with the equipment are well developed, interpretation of FRA signature is still a challenge and it needs skilled personnel to identify and quantify the fault type if exists as at this stage, there is no reliable standard code for FRA signature classification and quantification. As it is very hard to implement faults on physical transformer without damaging it, researchers investigated the impact of various mechanical winding deformations on the transformer FRA signature by randomly changing the value of particular electrical parameters of the transformer equivalent electrical circuit. None of them however, precisely investigated the correlation between physical fault level and the percentage change in each parameter. In this paper, the physical geometrical dimension of a singlephase transformer is simulated using 3D finite element analysis to emulate the real transformer operation. A physical radial deformation of different fault levels is simulated on both low voltage and high voltage windings. The impact of each fault level on the electrical parameters of the equivalent circuit is investigated and the correlation between the fault level and the percentage change in each parameter of the equivalent circuit is provided. This will facilitate precise fault simulation using transformer equivalent electrical circuit and ease the quantification analysis of FRA signature.

Aging characterization of high temperature vulcanized silicone rubber housing material used for outdoor insulation

Abstract: During the field service of composite insulators, high temperature vulcanized (HTV) silicone rubber (SIR) material used for insulator’s sheath and sheds is gradually aging, and thus how to effectively evaluate its aging state has become an inevitable issue. In this paper, artificially corona-aged and naturally site-aged SIR materials are employed as samples, and their properties such as hydrophobicity, leakage current, trap density/energy level, surface microstructure and chemical composition are investigated for comparative study. With the increasing of corona aging intensity, the contact angle of SIR samples drops gradually and recovers more slowly. The hydrophobicity of site-aged insulators also declines, from HC1 for 2-year service to HC5 for 15-year service. The leakage current of corona-aged and site-aged samples both increase with aging, which indicates that aging induces the increase of surface conductivity of SIR material. The peak trap density of corona-aged and site-aged samples increases with the aging duration or service duration remarkably. The changes of these properties are attributed to the changes of micro-structures and compositions in the surface layer of SIR. The scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analysis results reflect that a hardened inorganic silica-like (SiO2) layer with many polar chemical groups and distributed micro-pores is formed on the surface of SIR material after corona aging. Because of this silica-like layer, the hydrophobicity decreases while surface conductivity and trap density of SIR material increases. Besides the traditional properties like hydrophobicity and surface conductivity, the trap density is expected to be a novel parameter for effective evaluation of aging state of HTV silicone rubber material.

The effect of magnetite, graphene oxide and silicone oxide nanoparticles on dielectric withstand characteristics of mineral oil

Abstract: In this work, we compare the dielectric withstand performance of nanofluids prepared using Shell Diala D as a base fluid, and magnetite, graphene oxide and silicone dioxide as nanoadditives. The ac withstand capability of the nanofluids was investigated as well as partial discharge inception voltage, partial discharge and repetition rate under ac, dc positive and dc negative voltages. The results indicate that, for all nanofluids, nanoparticle concentrations around 0.2 g/l enhance dielectric withstand properties under quasi uniform fields. Under divergent fields, partial discharge characteristics are improved under ac conditions. Under dc conditions silica nanofluid performs better than mineral oil, but the other two nanofluids do not perform well.

Simulation of the electric field strength in the vicinity of metallization edges on dielectric substrates

Abstract: High electric field strengths at the edge of the metallization of insulated gate bipolar transistor (IGBT) power modules are, besides defects in the substrate or the potting gel, the main reason for partial discharge. These critical electric field strengths occur at the energized contact where it is bordered by the insulating ceramic and the cover (mostly silicone gel). The reduction of high electric field strengths for increasing the threshold voltage for partial discharge has been studied in several publications based on experiments as well as on simulations. Simulations allow the localization of the critical spots and the quantification of the maximum electric field strength. However, a systematic study of the singularities of the electric field strength at the sharp edges is lacking. Such singularities are investigated in this article. The calculation of an absolute electric field strength value is only possible for finite edge radii. For sharp edges, however, the maximum electric field strength returned by simulation depends on the grid size: Through the finite grid size a virtual edge radius is induced that suppresses the singularity at the edge. To get around this problem, a mesh-independent evaluation procedure is introduced. With this procedure it is possible to quantitatively evaluate the electric field strength in the vicinity of the sharp edge. As an example, the influence of the offset between the top and bottom metallization layer on the maximum electric field strength is studied. Moreover, the influence of the thickness of the involved layers and of the shape of the electrodes is discussed. Also, the impact of the material properties of the involved dielectrics is examined. In addition to electrostatic simulations we have carried out electric transient simulations, which show that the ratio of the conductivities of the involved dielectric materials plays a major role for determining the maximum electric field strength.

Assessing insulating oil degradation by means of turbidity and UV/VIS spectrophotometry measurements

Abstract: Oil is a vital part of the transformer body and (similarly to blood in a human being body) keeps responsibility for the condition of the entire organism. Oil is particularly responsible for functional serviceability of the entire insulation system. The insulating oil must be kept in pristine condition, since its condition can be a decisive factor, which determines the life span of the transformer. Fields and laboratory experiences have shown that transformer oil contains a vast amount of information. Oil analyses can be extremely useful in monitoring the condition of power transformers. To meet pressing needs of power industries, fast, inexpensive and reliable laboratory testing procedures are necessary. To ensure long-term reliability of oil filled power transformers, it is important to identify early sign of degradation of the insulating oil. In this paper, oil degradation was monitored with various ASTM test methods. Investigations were performed on service-aged oil samples as well as on oil samples aged in laboratory conditions. Many key parameters actually used to monitor the condition of transformer oil relative to oxidation/degradation were investigated. From the obtained results, correlations were found between some of them. The results indicate that Dissolved Decay Products (DDP) and turbidity, which change with a higher rate than interfacial tension (IFT) and Acid Number (AN) values, can be possibly used as an effective index for insulating oil degradation assessment. Limits are suggested which provide a “picture” of the fluid condition.

Localization of multiple insulators by orientation angle detection and binary shape prior knowledge

Abstract: Failure of insulators leads to failure of transmission system and causes heavy loss to the power sector. This has necessitated helicopter inspection on transmission line. Therefore, localizing insulators in collected aerial image is an essential step for automatic fault detection. This paper proposes a localization method of multiple insulators with different angles in complex aerial image based on Orientation Angle Detection and Binary Shape Prior Knowledge (OAD-BSPK). Detect possible orientation angles of insulators preliminarily. For each possible angle, reserve insulator by its binary shape prior knowledge. Traverse all the possible angles, and multiple insulators can be localized in aerial image finally. This paper uses a large number of real aerial captured images as experimental images. The results show that this method can localize multiple insulators with different orientation angles in complex aerial image, and has higher positioning precision, lower computational complexity and lower time consuming compared with existing methods. This method can meet the requirements of insulators’ real-time and accurate localization.

Numerical analysis of space charge accumulation and conduction properties in LDPE nanodielectrics

Abstract: LDPE nanodielectrics show good space charge suppression performances, reducing the electric field distortions and improving the electric strengths. The decrease of space charge accumulation of LDPE nanodielectrics with increasing the nanoparticle loadings can be explained by the reduction of charge injection, the enhancement of conduction, and so on. However, the phenomena that the conductivities of LDPE nanodielectrics decrease firstly and then may increase with increasing the nanoparticle loadings has not been fully understood. A bipolar charge transport model consisting of charge injection, charge migration, and charge trapping, detrapping, recombination dynamics is used to investigate the space charge accumulation and conduction properties of LDPE nanodielectrics. Based on simulation results and existing experimental results, we discuss the influencing factors for space charge accumulation and conduction properties of LDPE nanodielectrics. It is found that the heightening of injection barrier plays a more important role in the suppression of space charges and the reduction of conductivities of LDPE nanodielectrics. Whereas, the variation of trap density and trap energy will regulate the nanoparticle loading dependent conduction properties.

Design and numerical investigation of A HVDC vacuum switch based on artificial current zero

Abstract: With the fast development of multi-terminal high-voltage direct-current (HVDC) transmission technology, there is an urgent demand for the HVDC interruption technique. In this paper, an interruption scheme for HVDC based on artificial current zero is proposed with its main circuit breaker consisting of modularized vacuum switch in series. A triggered sphere gap is adopted as the commutation switch to achieve bidirectional DC interruption. The interruption process, particularly, the most important two commutation processes, is simulated based on a circuit model. The residual current and its influence after current commutation, which might exist due to the weak arc extinguishing capacity of the sphere gap, are analyzed. It is proposed to use back-up circuit breakers to interrupt the residual current. Furthermore, the influence of the frequency of the countercurrent on the interruption process, particularly, on the integral of i2t in the triggered sphere gap due to the residual current, is discussed.

Reclaiming aged transformer oil with activated bentonite and enhancing reclaimed and fresh transformer oils with antioxidants

Abstract: Transformers are the vital units of power system and the major dielectric used in transformer is petroleum based mineral oil, which acts both as an insulating medium and potential coolant. This work focuses on reclaiming aged mineral oil using activated bentonite and enhancing the critical characteristics of reclaimed mineral oil and fresh transformer oil using antioxidants. The reclamation is carried by treating 500 ml aged mineral oil with 5 g activated bentonite quantity. Then the treated sample is mixed with different combinations of natural and synthetic antioxidants. The enhancement of fresh transformer oil is carried by adding multiple combinations of antioxidants with different mechanisms like free radical scavenging, metal chelating and synergism. The critical parameters like breakdown voltage, viscosity, flash point and fire point of the samples are calculated using ASTM and IEC specifications before and after adding antioxidants. The recorded parameters prove that the reclaimed aged mineral oil after added with high performance antioxidant combinations can perform superior than base fluids and can be reused in power transformers. Moreover the enhanced fresh transformer oil with antioxidants shows greater improvement and this technique could be possibly employed for reclamation (regenerating) and enhancement of waste insulating oils.

Partial discharge pattern recognition via sparse representation and ANN

Abstract: In this study, seventeen samples were created for classifying internal, surface, and corona partial discharges (PDs) in a high voltage lab Next, PDs were measured experimentally to provide a dictionary comprising the types Due to the huge size of the recorded dataset, a new and straightforward preprocessing method based on signal norms was used to extract the appropriate features of various samples The new sparse representation classifier (SRC) was computed using l1 and stable l1-norm minimization by means of Primal-Dual Interior Point (PDIP) and Basis Pursuit De-noise (BPDN) algorithms, respectively The pattern recognition was also performed with an artificial neural network (ANN) and compared with the sparse method It is shown that both methods have comparable performance if training process, tuning options, and other tasks for finding the best result from ANN are not taken into account Even with this assumption, it is shown that SRC still performs better than ANN in some cases In addition, the SRC technique presented in this paper converges to a fixed result, while the results after training the ANN vary with every run due to random initial weights

A new technique to measure interfacial tension of transformer oil using UV-Vis spectroscopy

Abstract: Interfacial tension (IFT) and acid numbers of insulating oil are correlated with the number of years that a transformer has been in service and are used as a signal for transformer oil reclamation. Oil sampling for IFT measurement calls for extra precautions due to its high sensitivity to various oil parameters and environmental conditions. The current used technique to measure IFT of transformer oil is relatively expensive, requires an expert to conduct the test and it takes long time since the extraction of oil sample, sending it to external laboratory and getting the results back. This paper introduces a new technique to estimate the IFT of transformer oil using ultraviolet-to-visible (UV-Vis) spectroscopy. UV-Vis spectral response of transformer oil can be measured instantly with relatively cheap equipment, does not need an expert person to conduct the test and has the potential to be implemented online. Results show that there is a good correlation between oil spectral response and its IFT value. Artificial neural network (ANN) approach is proposed to model this correlation.

A study of the sweep frequency impedance method and its application in the detection of internal winding short circuit faults in power transformers

Abstract: Undetected short circuit faults are a significant problem in power transformers and can eventually develop into catastrophic faults. At present, frequency response analysis (FRA) is one of the well-recognized diagnostic tools for the detection of winding faults, but it has some limitations, such as a low signal-to-noise ratio (SNR) and instability caused by changes in the measuring voltage. In this paper, a novel method called sweep frequency impedance (SFI) is proposed to address the difficulties that arise from FRA. Based on the mechanism of this new method, a nondestructive testing system was established to demonstrate the advantages of SFI measurements. The SFI test system has a better stability, repeatability, and SNR by comparing it with the FRA test system. Moreover, FRA and SFI curves obtained under the same conditions was symmetrical about a specific straight line above 10 kHz, and the SFI value at 50 Hz is equivalent to the short circuit impedance (SCI) value of a transformer. These results indicate that the existing criteria of FRA and SCI methods can be used in the SFI method to detect transformer faults. Finally, the experiments on a special oil-immersed testing transformer demonstrate that the SFI detection system is feasible, sufficiently sensitive to detect short circuit faults and able to quantify the level of the fault.

Local degradation diagnosis for cable insulation based on broadband impedance spectroscopy

Abstract: The diagnosis of portions in a cable that are severely degraded before breakdown occurs is of prime importance. This paper presents a novel method based on the broadband impedance spectroscopy (BIS) to precisely locate local degradations in a cable and determine their insulation conditions. Firstly, an integral transform (IT) algorithm is developed to transform the frequency-dependent BIS to a spatial domain function, which characterizes the changes of propagation constant along the cable and then degradations can be located. Next, a procedure based on the particle swarm optimization (PSO) is worked out to minimize the error between the measured impedance of the cable and that calculated by a model, and then the complex dielectric permittivity for each of the degradations is figured out to assess the insulation condition. Simulation results show that portions with loss tangent (tan δ) higher than the adjacent regions can be clearly located with a spatial resolution as short as 0.01 m even if the cable length is 500 m. After the PSO procedure, tan δ of the degraded portions can be obtained to indicate the severity of the degradation. The validity of the method is also verified by experiments performed on a 50 m long cable.

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

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

High thermal conductivity transformer oil filled with BN nanoparticles

Abstract: Transformer oil is widely used in electric transformers serving the dual functions as insulating medium and cooling material. However, safety and stability of power system is interrupted due to the operation failure of electric transformers in certain case. Therefore efforts should be focused on improving the property of transformer oil for the stable operation of insulating system. The purpose of this paper is to obtain a new type of nanomodified transformer oil with improved dielectric and thermal properties. Boron nitride (BN) nanoparticles with high thermal conductivity were dispersed into transformer oil to form the nanooil. Dielectric and thermal properties of the samples before and after modification were measured. Dielectric property experiments were performed in accordance with IEC standard. Discharge phenomenon in breakdown process was captured by a high speed CCD camera. In order to investigate the effect of BN nanoparticles on thermal property, temperature change and heat distribution of the samples in heat transfer process were measured by an infrared thermal imager and a temperature sensor. It is found that the nanooil has higher dielectric breakdown strength and lower dissipation factor which indicate better dielectric property compared with nonmodified oil. Obtained results show significant improvement in heat transfer process with increasing the concentration of BN nanoparticles. It is proposed that the nanoparticles create an interfacial region which contributes to the enhancement of dielectric strength. For thermal property the ballistic phonon transport of the nanoparticles acts as the key factor in the remarkable improvement. Furthermore, the improved thermal property of the nanooil has an inhibitory effect on the breakdown process according to the bubble theory.

Partial discharge analysis of gas insulated systems at high voltage AC and DC

Abstract: Partial discharge (PD) measurements are commonly used for monitoring and diagnostics of AC gas insulated systems (GIS). Because of the growing importance of DC GIS, a research project was started focused on the measurement and interpretation of PD signals obtained under DC stress. Results are presented using IEC60270 and unconventional methods through UHF antenna and high frequency current transformer (HFCT) on three different PD generating defects in a GIS chamber. The results of the PD analysis of the defects are compared for AC and DC stress. Phase resolved partial discharge patterns (PRPD) are used for the HVAC analysis, whereas time resolved partial discharge patterns (TRPD) are used for HVDC analysis. Finally, relevant characteristics of PD patterns in HVAC and HVDC are presented and related with the physical phenomena of the discharges.