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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 2 • Date April 2010

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Displaying Results 1 - 25 of 43
  • IEEE Transactions on Dielectrics and Electrical Insulation [Front cover]

    Page(s): c1
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    Freely Available from IEEE
  • IEEE Transactions on Dielectrics and Electrical Insulation [Staff]

    Page(s): c2
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    Freely Available from IEEE
  • Outdoor high voltage insulation [Editorial]

    Page(s): 325
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    The 37 papers in this special issue focus on outdoor high voltage insulation. The content is broad, covering aspects of insulator performance and testing, selection and evaluation of materials, including evaluation of nanocomposites, development of new testing and diagnostic methods. View full abstract»

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  • Ageing of silicone rubber insulators in coastal and inland tropical environment

    Page(s): 326 - 333
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    This paper presents investigations on the performance of 33 kV silicone rubber insulators characterized by different creepage lengths, which aimed to find out whether the insulators could permanently work when electrically stressed beyond the recommended limits in polluted and clean tropical environments. The study was performed under natural field and laboratory conditions. The insulators tested included eight types of silicone rubber composite insulators, one type of hybrid silicone-ceramic insulator and one semi-conducting glazed porcelain insulator, while ordinary porcelain and glass insulators were used as reference. During the field investigation, two sets of the insulators were separately installed and energized in coastal and inland parts of Sri Lanka, being by that exposed to marine and clean tropical environments. Their performances were periodically evaluated by visual inspections and measurements of hydrophobicity class. After five years of field exposure, the insulator performances were evaluated in laboratory by measurements of leakage currents under clean fog conditions and of wet flashover voltage. A third set of the insulators was aged in laboratory for 1000 hours inside a salt fog chamber where the insulators were continuously energized and daily sprayed with salt solution for eight hours and left to rest for remaining 16 hours. This treatment represented conditions similar as those in the field i.e. insulators exposed to salt sprays during monsoons. The insulator performances were investigated by measurements of leakage currents and classifying their patterns into different categories, i.e. capacitive, resistive, non-linear, discharge and strong discharge types, by means of fast Fourier transform and short time Fourier transform analyses. It was found that the long-term field exposure yielded weaker insulator deterioration than the salt fog chamber ageing, which indicated for a possibility to increase the electric stress on silicone rubber insulat- - ors to levels higher than the ones used today on glass and porcelain counterparts. View full abstract»

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  • Prediction of electrical performance of medium voltage epoxy insulated equipment

    Page(s): 334 - 342
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    The paper presents a theoretical model based on the 2-parameter Weibull Distribution to predict the performance of cycloaliphatic based medium voltage (< 100 kV) insulation equipment. The model combines two prominent modes of degradation, namely surface damage (tracking and erosion) and flashover. New samples and samples that were aged in service and in outdoor test sites were evaluated. The flashover model is based on evaluating surface resistance under wet conditions. The degradation model is based on evaluating the tracking and erosion resistance in the Inclined Plane (ASTM D2303) test. The predictions of the model are shown to be in good agreement with experimental results. A total of 20 insulators and instrument transformers (new and field-aged) were evaluated. View full abstract»

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  • A new approach to anti-fog design for polymeric insulators

    Page(s): 343 - 350
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    Electric field strength and leakage current density in the shank region of a polluted polymeric insulator may cause local dry-banding, with a risk of long-term degradation from partial-arc discharges. In a novel approach to dry-band control, the characteristics of silicone rubber surfaces with a textured finish are investigated and, depending on the geometry chosen for the texture, two useful objectives can be achieved. First, the increase of surface area can both reduce the leakage current density in the vulnerable shank region, and also increase the longitudinal creepage distance. Secondly, the damage arising from surface discharges can be mitigated. The principles of the proposed anti-fog design are described in detail. While complete prototype insulators are now being specified for fabrication, this paper reports preliminary results from tests using rectangular samples, which show an improvement in performance. The tests are of two kinds: a) inclined-plane, to assess erosion and tracking on rectangular samples of insulation materials. Such samples have been prepared both with a conventional plane surface and with a textured surface for comparative tests; b) clean-fog tests that use an initially dry pollution layer of known salinity. Here, plane and textured samples have been used for comparative tests, where leakage current and discharge activity are monitored during the generation of the fog. View full abstract»

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  • Effect of hydrophobicity coating on insulator icing and DC flashover performance of iced insulators

    Page(s): 351 - 359
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    How to decrease the ice on insulators and increase the flashover voltage of iced insulator are hot topics of power system in the world. Many tests have been carried out to investigate whether the permanent room temperature vulcanized (PRTV) coating can change the electrical characteristics of iced insulators both in laboratory and field. Firstly, the test to investigate the icing and electrical characteristic of dc insulator with PRTV coating was carried out. Moreover, comparison has been made between artificial polluted insulator and field polluted insulators. Results indicate that the PRTV coating can not decrease the ice accretion on insulators. Instead, it results in accumulating more ice at the beginning of icing regime, but the influence of PRTV coating will not be remarkable to accumulate ice when there is heavy ice on insulators. . In addition, the PRTV coating can decrease the flashover voltage by 10%. It is also revealed that the flashover voltage for the artificial polluted insulators is lower than that of the field polluted insulators due to the non-uniform of the natural pollution. View full abstract»

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  • Models of the long-term mechanical strength of long rod composite insulators

    Page(s): 360 - 367
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    Different mathematical models of long-term fatigue strength of long-rod insulators were evaluated by studying the effects of superimposed cyclic and constant loads. It has been shown that the strength of composite insulators can well be represented by the power type behavior of the fatigue characteristics and that the time to fatigue failure of a composite insulator strongly depends of the cyclic load amplitude and the maximum level of the total load. It has also been shown that the diameter of the core of composite insulator with crimp-mounted end fittings is the major factor determining the insulator's fatigue and static strength. View full abstract»

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  • Surface micro-crack initiated brittle fracture in fiber reinforced plastic rod of composite insulator

    Page(s): 368 - 374
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    Previous studies on brittle fracture of composite insulator were concluded, and unanswered questions were analyzed. Almost all the previous studies focus on the source of corrosion medium (the acid) and the mechanical load in brittle fracture, but the stress concentration of stress corrosion fracture was neglected. Sand paper polishing in the manufacturing process of composite insulator may lead to surface micro-cracks with the depth deeper than 30 ¿m-40 ¿m on the fiber reinforced plastic (FRP) rod surface. Stress concentration caused by surface micro-crack in FRP rod and its influence on brittle fracture was noticed in experimental study. The importance of the surface micro-crack in FRP rod was verified by a series of experimental studies. An explanation of the randomness of brittle fracture in composite insulator was put forward. Finally, the actions that should be taken to prevent brittle fracture of composite insulator were emphasized. View full abstract»

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  • Electrical and material characterization of field-aged 400 kV silicone rubber composite insulators

    Page(s): 375 - 383
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    This paper presents the electrical and material characteristics of field-aged silicone rubber composite insulators, which have been deployed for 15 years on a 400 kV transmission line in a coastal region of the UK. There were no indications of reduced performance in service. Observations indicate non-uniform aging of the insulators on the different surfaces of the insulator sheds and core, along the insulator string length and in the different compass orientations. A uniquely large number of contact angle measurements, made on each of the insulators' different surfaces, confirm the visual aspect of nonuniform aging. Electrical investigations of the insulators have been performed in terms of leakage current analysis, ac flashover / withstand and switching-surge impulse flashover. A correlation is seen between the ac leakage current and the hydrophobicity measurements. Energy dispersive X-ray (EDX) and Fourier transform infrared spectroscopy (FTIR) analysis of the changes of the materials' surface chemistry is presented. The role of solar radiation and organic growth appears critical to the observed non-uniform aging and must be considered if the longer term operation of the insulator is to be forecast. It is proposed that a reduction of the difference between positive and negative wet-flashover voltages may be a good indicator of early insulator aging. View full abstract»

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  • Pollution tests for polymeric insulators made of hydrophobicity transfer materials

    Page(s): 384 - 393
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    There are no standardized methods available at present for artificial pollution testing of polymeric insulators. Modified Salt Fog and Solid Layer tests based on procedures described in IEC 60507, intended for ceramic insulators, are thus often applied. Among methods evaluated for polymeric insulators made of hydrophobicity transfer materials (HTM), a modified Solid Layer test with preconditioning by dry powder is considered as being representative and most promising for future standardization. Further, the proposed procedure allows for testing at different hydrophobic states of the polluted insulators via the introduction of defined time between the application of pollution and voltage test. Finally, if the 50% flashover voltage is determined using e.g. the up-and-down method, results of the tests can be directly applied for insulator selection and dimensioning by means of statistical or deterministic methods. View full abstract»

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  • A proposal for intensified inspection and acceptance tests of composite insulators as an addition to the guidelines of IEC 61109 Ed. 2: 2008 and IEC 61952 Ed. 2: 2008

    Page(s): 394 - 401
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    The paper provides an example of a test programme, which aims to add to the evaluation of composite insulators after they have being supplied. It is to be considered as a tool to prove the product quality versus the submitted reports from international standards. The proposed tests are not selected to discriminate against any particular material or manufacturer but rather to consider well accepted and recently developed test philosophies, in order to safeguard the quality of the supplied insulators, in particular for bulk volume contracts. View full abstract»

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  • Detecting defects in outdoor non-ceramic insulators using near-field microwave non-destructive testing

    Page(s): 402 - 407
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    This paper presents a novel near-field microwave nondestructive testing technique for defect detection in non-ceramic insulators (NCI). In this work, distribution class 33 kV NCI samples with no defects, air voids in silicone rubber, cracks in the fiberglass core and small metallic inclusion between the fiber core and shank were inspected. The microwave inspection system utilizes an open-ended rectangular waveguide sensor operating in the near-field at a frequency of 24 GHz. The used inspection system is simple, safe and relatively inexpensive. A data acquisition system was used to record the measured data. The results showed that all defects were repeatedly detected with high sensitivity. Line scans of the samples were obtained revealing the presence of different defects and their location. The technique also demonstrated ability to detect thickness variations in the silicon rubber shank. View full abstract»

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  • Fluid influence on dynamic characteristics of transformer-bushing system using fluid structure interaction (FSI) approach

    Page(s): 408 - 416
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    Currently used procedures for seismic qualifications of HV bushings are based on the analytical equations, and very often are verified by shake table tests. As many specialists claim this approach does not fully reflect the real behavior of the bushing coupled with the transformer tank during seismic loads. In the paper a new methodology for seismic analysis of the transformer/bushing system has been presented using complex numerical simulations. In the proposed approach Fluid Structure Interaction is involved. The methodology has been experimentally verified and can provide useful information for proper design of transformer tank and bushings with high resistance to seismic loads. View full abstract»

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  • Partial-arc and spark models of the flashover of lightly polluted insulators

    Page(s): 417 - 424
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    The established model of the flashover of an insulator, whose surface is covered with a moist, conducting layer of pollution, envisages that it develops from the formation and bridging of a dry band by a partial-arc pre-discharge. This pre-discharge is assumed to possess arc characteristics, so that the electric field across the dry band will fall as the partial arc current increases. Observations of insulators which are only lightly polluted, however, show that pre-discharges are of low luminosity and rich in ultraviolet, and resemble the spark leader inception and extension sequence in air gaps. This is because the current limitation by low values of surface-layer conductance can inhibit the transition to an arc at the pre-discharge stage. An alternative model using simplified voltage-current equations is developed to represent these spark properties rather than an arc characteristic. The predictions of the partial-arc model and this new approach are both tested here, using published experimental data from light-pollution fog tests in the authors' laboratory. For this purpose, the partial-arc theory is further developed in order to reveal its implied prediction for the increase of pre-discharge length with applied voltage before flashover. A corresponding relationship is obtained for the increase of spark leader length using the new model. It is found that for light pollution, the dry-band spark model better represents the test data than the partial-arc model. Consequent predictions are made for the variation of flashover voltage with pollution severity in this regime, which may account for anomalous insulation failures. View full abstract»

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  • Influence of low atmospheric pressure on AC pollution flashover performance of various types of insulators

    Page(s): 425 - 433
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    The electrical performance of outdoor insulators decreases under low air pressure conditions. The ac pollution flashover performance of various types of insulators are investigated in the multifunction artificial climate chamber in the State Key Laboratory of Power Transmission Equipment and System Security and New Technology, Chongqing University. The field investigation of the ac pollution flashover voltage performance are carried out at three different high altitudes including Wangkun station (altitude of 4484 m), Nachitai station (altitude of 3575 m) along the Qingzang railway and Geermu urban (altitude of 2820 m). The test results indicate that the characteristic exponent n describing the influence degree of air pressure on pollution flashover voltage, which has influence on the pollution flashover voltage of insulators, is variable, and the value of n is related to the pollution, insulator type etc. Under ac voltage, n is 0.45-0.85. The composite insulators are superior in external insulation selection in high altitude and heavy polluted areas. The relative error between the field test results and laboratory test results is less than 8%, so the research results obtained in the laboratory can be applied to engineering design. View full abstract»

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  • Icing flashover characteristics and discharge process of 500 kV AC transmission line suspension insulator strings

    Page(s): 434 - 442
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    The icing flashover characteristics and discharge processes of 500 kV ac transmission line long insulator strings energized in the ice accretion were studied. The influence of pollution and grading rings (including simulated conductor) on heavy ice-covered insulators¿ flashover voltage were considered in this paper. Based on the artificial icing test results, it was found that 1) under the severe icing conditions and three salt deposit density (SDD) levels of PSDD=0.1, 0.05, 0.025 mg/cm2, compared with 500 kV transmission line rated phase to ground voltage, the flashover voltages of 28 units XWP2-160 porcelain double-shed insulator are 20.5% lower, 13.1% lower and 2.3% higher respectively; and the values of the FXBW4-500/160 composite insulator are 18.7% lower, 12.0% lower and 4.3% higher respectively. 2) equipped with the given grading ring and simulated conductor the icing flashover stress of 28 units XWP2-160 is lower than those without grading ring and simulated conductor under the three pollution degrees. The discharge process of the icing flashover and relevant factors were investigated also, which shows that 1) the locations of the air gaps on icing insulators are not completely random. 2) arcs on long insulator strings require more time to reach a critical length, hence, the arc propagation might be easily influenced by the decreasing of melted water conductivity and ice shedding than short insulator strings. View full abstract»

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  • Monitoring leakage current of ice-covered station post insulators using artificial neural networks

    Page(s): 443 - 450
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    This paper presents the analysis of leakage current evolution of an ice-covered station post insulator during a melting period using artificial neural network (ANN) models. The tests, carried out under wet-grown ice regime for different experimental conditions, showed that the permanent establishment of white arcs, identified as ¿permanent regime¿ led to flashover in the large majority of the cases,. Based on these observations, the development of a monitoring methodology aimed at forewarning the approach of the leading white arc during melting periods is proposed. The monitoring methodology uses different ANNs in order to predict the appearance of the white arc based on the identification, classification and analysis of the occurrence frequency of electric discharges. The results show that the ANN monitoring model developed is able to predict the onset of permanent regime under various experimental conditions. Hence, it was found that the delay between the permanent regime onset prediction delivered by the ANN model and its realization is 9 minutes on average. These results confirm that the proposed ANN model could be used as part of a monitoring system for post insulators during icing events for protection against potential flashover hazards. View full abstract»

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  • Application of identification methods for predicting the flashover voltage of contaminated insulators covered with ice

    Page(s): 451 - 457
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    The arc characteristics on contaminated insulator surfaces covered with ice were investigated. The arc constants (A, n), as well as the ac re-ignition parameters (K, b) were determined using a simplified cylindrical physical model. Based on identification methods, a new approach for calculating the residual voltage is introduced. These results will serve to extend the previous mathematical model developed at CIGELE for its application to predicting the flashover voltage of contaminated insulators covered with ice. The model was successfully applied to a five-unit IEEE standard insulator string, and the calculated results were in good agreement with the experiments. View full abstract»

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  • Experimental investigation of the process of Arc propagation over an ice surface

    Page(s): 458 - 464
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    In this paper, some of the characteristics of an arc propagating over an ice-covered insulator surface were investigated. Special attention was paid to the arc root and channel characteristics. All experiments were performed on a simplified physical model to simulate real ice-covered insulators. From the obtained results, based on a set of time-resolved measurements of a surface discharge, some important characteristics, namely arc foot geometry and channel diameter were observed. Possible mechanisms controlling these variations are discussed. An empirical model, accounting for arc propagation pattern on an ice surface, is proposed. The paper contains a substantial amount of new material with a view of closing some major gaps in the present state of knowledge of ice surface flashover. View full abstract»

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  • Recurrent plot analysis of leakage current on flashover performance of rime-iced composite insulator

    Page(s): 465 - 472
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    Due to the wide application of composite insulators in the power industry, the insulator performance is challenged by various environments. To determine the flashover performance of rime-iced composite insulator, laboratory investigation was carried out in an artificial climate chamber to simulate different rime-ice morphology on the insulator surface. The configuration and characteristics of the rime-ice were demonstrated to establish the relationship between the rime-ice parameters and the flashover performance. In accordance with the discharge phenomena, the transition of leakage current (LC) until the flashover was analyzed by using a recurrent plot approach. After extracting the high frequency components by using a wavelet transform technique, the LC just before the flashover was extended to m dimensional phase space based on a phase space reconstructed method. The recurrent plot was obtained to reveal the non-linear characteristics of LC for identifying the dynamic behaviors on the insulator surface. It is shown that the propagation and properties of the discharges can be graphically projected on the topological structure of recurrent plot as a function of the rime-ice parameters. The process and underlying mechanism of flashover performance of rime-iced composite insulator can be visually reflected by the recurrent plot and the quantitative indicators of LC. View full abstract»

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  • Increased energy in stable dry-band arcs due to length compression

    Page(s): 473 - 480
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    The occurrence of dry-band arcs on outdoor composite insulators can degrade the polymeric materials¿ surface and ultimately may lead to insulator failure. The degradation processes are generally considered as aging effects occurring over long periods of time, from years to decades. In this paper, it is shown that if a stable dryband arc is physically compressed in length by external forces, such as electrolyte deformation due to wind or gravity, the arcing activities will become more severe. This in turn, may accelerate the degradation into a short time-frame hazard. A series of experiments are carried out to investigate the electrical characteristics of the arcs as they become compressed. In this case experiments are performed on silicone rod insulators at controlled angles to the horizontal. Rapid aging is observed after such events. Measurements of arcing period, peak current, and arc resistance during the arcing compression process are analyzed. Based on the experiments, a `Double Sinusoidal Model¿ is developed to simulate the current-voltage characteristics of dryband arcing during its compression. Both experiment and simulation show that arc power, arc energy and corresponding energy density will dramatically increase if arc compression occurs, which may lead to more rapid and serious damage on composite insulator surfaces than is experienced otherwise. It is suggested that aggressive erosion events may occur in short periods of time within extended test regimes or entire service histories. View full abstract»

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  • Evaluating the safety condition of porcelain insulators by the time and frequency characteristics of LC based on artificial pollution tests

    Page(s): 481 - 489
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    Leakage current (LC) measurement is one of the effective methods for analysis of polluted insulators. But the traditional evaluation methods do not combine well the time-domain with frequency-domain characteristics of LC. In order to evaluate the safety condition of polluted insulators more effectively, an artificial neutral network (ANN) based method was developed in this paper. Firstly, a large number of artificial pollution tests for IEC standard suspension insulators were investigated under different relative humidity (RH) and salt deposit density (SDD). Then, based on the experimental data, the characteristics of LC were analyzed in both time-domain and frequency-domain. The results showed that the peak values of the LC (Ih ) has no relation with SDD when the RH is low ,but when the RH is high, the Ih increases with the increase of SDD; the phase difference (¿) between LC and applied voltage decreases with the increase of RH, the LC becomes inductive when strong local arc occurs; the total harmonic distortion (THD) of LC increases slowly with the increase of RH when the pollution is light, but decreases firstly, then increases with the increase of RH when the pollution is middle or heavy level, the value reaches to minimum when the RH is about 80%. Consequently, Ih, ¿ and THD were proposed as characteristic parameters to evaluate the safety condition of polluted insulators. Finally, an ANN with fuzzy output was developed to evaluate the safety condition of polluted insulators. The input parameters of the ANN were Ih, ¿ and THD, while the output parameters were fuzzified into four fuzzy subsets, the capability of ANN was validated by 8 pairs of testing samples, and the effects of SDD and RH on the performance of insulators were discussed. View full abstract»

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  • Use of leakage currents of insulators to determine the stage characteristics of the flashover process and contamination level prediction

    Page(s): 490 - 501
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    In order to improve the reliability of power transmission lines, one of the key issues is to reduce the hazard of contamination flashovers. Presently, the most efficient way is to clean (or replace) the heavily polluted insulators. The leakage current is the critical online quantity that can be detected. A number of laboratory experiments on 35 kV voltage class ceramic and glass insulators show that the leakage current fully reflects the entire process of contamination flashover development. The test results reveal that the process can be classified into three stages, i.e., security stage, forecast stage and danger stage. The results, that were duplicated several times, are based on three characteristics of the leakage current, i.e., the root-mean-square value, waveforms, and power spectrum estimation. In addition, the boundaries of the three stages in both time domain and power spectrum domain are also determined. All these can be used for the stage pre-warning of contamination flashovers. The security stage is most important since it precedes the contamination flashover sufficiently. The three characteristics of the leakage current in the security stage are proposed as the inputs of a neural network model together with the operating voltage, and the relative humidity in order to determine the equivalent salt deposit density (ESDD) of the insulators. The comparison of the simulated and actual (measured) results demonstrates that the ESDD prediction model has a very low relative error if the training data and the testing data both come from the security stage. The application of this research results in (1) optimal ESDD prediction inputs and (2) sufficient pre-warning time before the ultimate contamination flashover. View full abstract»

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  • Determining the probability of flashover occurrence in composite insulators by using leakage current harmonic components

    Page(s): 502 - 512
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    Flashover of contaminated insulators in polluted areas is one of the most important factors influencing the operation of transmission and distribution lines and substations. Hence predicting an approaching flashover is of a great importance. In this paper, a new method and criteria have been proposed to predict the flashover occurrence of SIR composite insulators and its probability by using leakage current harmonic component. The tests were performed on various profiles of artificially polluted SIR insulators under clean fog condition. It is proposed that an important index is the ratio of 5th harmonic component to the 3rd for which the operational and critical limits have been determined. The probability of flashover occurrence has also been calculated by geometric distribution. Only the measurements of a minimal set of parameters are required. The presented criteria can be used to optimally schedule overhead lines maintenance teams and to develop proper Condition-Based Monitoring Systems (CBMS). Experimental results illustrate high performance of this approach. View full abstract»

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Aims & Scope

IEEE Transactions on Dielectrics and Electrical Insulation contains topics concerned with dielectric phenomena and measurements with development and characterization of gaseous, vacuum, liquid and solid electrical insulating materials and systems.

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Editor-in-Chief
Reuben Hackam