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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 1 • Date Feb 2000

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Displaying Results 1 - 20 of 20
  • Power frequency dependence of space charge behavior and partial discharge characteristics in SF6 gas

    Page(s): 152 - 160
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    The partial discharge (PD) and breakdown (BD) characteristics in SF6 gas under commercial and higher frequency (∼600 Hz) ac voltage applications were investigated using high-speed electrical and optical measuring techniques with phase gate control method. Experimental results revealed that 400 Hz BD voltage at a certain gas pressure range was higher than that for 60 Hz and PD characteristics especially at the positive PD inception phase were much influenced by the applied power frequency. From these results, we clarified the dependence of space charge behavior on the applied power frequency and discussed the physical mechanism of PD and BD in SF6 gas with consideration of the space charge behavior generated by PD in the previous half cycle of ac voltage. View full abstract»

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  • Wavelet analysis for classification of multi-source PD patterns

    Page(s): 40 - 47
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    Multi-resolution signal decomposition (MSD) technique of wavelet transforms has interesting properties of capturing the embedded horizontal, vertical and diagonal variations within an image in a separable form. This feature was exploited to identify individual partial discharge (PD) sources present in multi-source PD patterns, usually encountered during practical PD measurements. Employing the Daubechies wavelet, features were extracted from the third level decomposed and reconstructed horizontal and vertical component images. These features were found to contain the necessary discriminating information corresponding to the individual PD sources. Suitability of these extracted features for classification was further verified using a radial basis function neural network (NN). Successful recognition was achieved, even when the constituent sources produced partially and fully overlapping patterns, thus demonstrating the applicability of the proposed novel approach for the task of multi-source PD classification View full abstract»

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  • Fundamental aspects of PD patterns of on-line measurements on turbogenerators

    Page(s): 59 - 70
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    This paper discusses partial discharge (PD) measurements performed on turbogenerators using the VHF PD detection technique. With this technique it is possible to perform PD measurements on turbogenerators without interruption of their operation. In addition to PD in any particular phase, the recorded patterns can show crosstalk from other phases and disturbances. The membership of each signal source (phase's own PD pattern, crosstalk, disturbances) to the recorded pattern depends on the selected center frequency. By examination of some typical examples, the relationship between the center frequency and recorded patterns is discussed. Furthermore, PD measurements performed over time on the same generator are studied to discuss possible changes in the spectral responses of particular phases of a turbogenerator. By visual comparison of PD patterns as well as computer-based discrimination tools it was found that several center frequencies are suitable to tune the VHF PD detection technique. However, these center frequencies can change in the course of time for a particular phase of a turbogenerator. As a result, in order to avoid a misjudgment on the insulation condition of turbogenerators, the analysis of on-line recorded PD should not be restricted to just one single center frequency View full abstract»

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  • A PD measuring and evaluation system based on digital signal processing

    Page(s): 21 - 29
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    Many current partial discharge (PD) fault diagnosing systems are based on conventional PD measurements with limited bandwidth. In this contribution, a modular structured system is introduced, which analyzes the characteristics of measured broadband PD pulses and their distortion on the way to the measuring system by using digital signal processing algorithms. Future requirements of PD monitoring systems are discussed View full abstract»

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  • Interpretation and classification of PD in a HV cryogenic cable termination

    Page(s): 71 - 77
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    A description of the experimental and theoretical investigations, carried out for the realization of a cryogenic cable termination, is presented. Identification and classification of the partial discharge (PD) patterns obtained on an HV vacuum bushing and on two types of cable model insulation was attempted. It is shown how a methodological and proper interpretation of experimental data can provide information on the design of cryogenic devices as well as indications on their optimal operating conditions View full abstract»

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  • Effect of dielectric surfaces on the nature of partial discharges

    Page(s): 146 - 151
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    A mathematical model for a short gap breakdown in air was employed to examine the nature of the initiating discharge on the dielectric surface of a dielectric/metallic electrode gap of 0.500 mm separation in a uniform electric field at atmospheric pressure. The calculations demonstrate that in the absence of field enhancing surface asperities, the incipient discharge, occurring on a dielectric surface, exhibits the pronounced breakdown channel expansion characteristic of pulseless glow discharges View full abstract»

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  • Fuzzy logic applied to PD pattern classification

    Page(s): 118 - 123
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    A procedure is described for the application of fuzzy logic systems to the classification of partial discharge pulse patterns in terms of cavity or void size. The features employed in the pattern recognition task are those related to the form or shape of the partial discharge pulses and their associated apparent charge transfer. Preliminary results, obtained with the fuzzy logic system on simple partial discharge sources, indicate a performance approaching that attainable with artificial neural networks View full abstract»

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  • Partial and complete electrical breakdown in simulated high temperature superconducting coils

    Page(s): 78 - 86
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    The partial breakdown (PBD) and complete breakdown (BD) phenomena in a composite insulation system of glass fiber reinforced plastic (GFRP) and liquid nitrogen are investigated to find the PBD and BD characteristics in high temperature superconducting (HTS) coils at quench. The electrode system used is made from a coaxial spiral coil-to-cylindrical electrode with an insulation barrier and spacers, and is immersed in liquid nitrogen. A heater is mounted inside the coil electrode to generate boiling which occurs on quenched superconducting coils. The experimental results show that (1) the polarity of PBD initiation under 60 Hz ac applied voltage depends on the width lc of the cooling channel, (2) PBD appears through a gas bubble locked between the coil and concentric insulation barrier due to the electrical gradient and the spacer, (3) breakdown voltages are affected severely by the risetime of the applied voltage and the current amplitude associated with PBD, (4) two kinds of BD mechanisms are found depending on the shape of the spacer, length of cooling channel and heater power View full abstract»

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  • PD recognition by means of statistical and fractal parameters and a neural network

    Page(s): 87 - 94
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    A novel partial discharge (PD) defect identification method is described. Starting with PD data on different families of specimens, a suitable set of parameters are determined and then used as input variables to a neural network for the purpose of identifying the defects within the insulation. In this procedure the statistical Weibull analysis is performed on PD pulse amplitude histograms to obtain the scale parameter α and the shape parameter β. Thereafter, the two statistical operators (skewness and kurtosis) and two fractal parameters (fractal dimension and lacunarity) are evaluated from the PD phase on the discharge epoch histogram and from the 3 dimensional (pulse amplitude/phase/discharge rate) histogram, respectively. Following the exposition of the basic mathematical concepts regarding the above parameters, experimental results are reported on the recognition capability of the method in defining the defect category in a number of different specimens View full abstract»

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  • Importance of bandwidth in PD measurement in operating motors and generators

    Page(s): 6 - 11
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    It is well known that partial discharge (PD) events produce electromagnetic radiation with frequencies from de to sometimes hundreds of megahertz. In addition, in complex structures such as stator windings, the initial pulses are significantly distorted and attenuated as they propagate through a winding. The detection of the PD pulses needs to account for these effects. In particular, the bandwidth of the detection system will influence the sensitivity of the PD measurement, as well as in what portions of the winding PD can be detected. This paper summarizes over 20 years of research into these effects in stator windings, and discusses the implications for off-line and on-line PD testing, and the calibration of the PD quantities into apparent charge View full abstract»

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  • The use of the CIGRE data format for PD diagnosis applications

    Page(s): 102 - 112
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    A standardized format for partial discharge (PD) data recorded in gas-insulated switchgear (GIS) was proposed recently by a CIGRE Task Force (15.03.08). Being an open structure, the PD CIGRE data format can be used for all HV apparatus and equipment. Such a format is necessary to process PD measurements from any PD acquisition equipment by different PD analysis programs or expert systems. Furthermore, this format can decouple PD acquisition and PD evaluation. For easy handling, the PD data in the standardized format are encoded as plain ASCII text which can be processed by a text editor or a commercial spreadsheet program. In addition to an Electra publication where solely the structure of the data format is defined, this approach investigates the use of this format in practical PD applications. It discusses the options of the format and compares the diagnostic capability of different evaluation tools and gives examples. This discussion thus can assist a user to choose between the different graphs which are contemplated in the format View full abstract»

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  • Influence of bulk dielectric polarization upon partial discharge transients effect of heterogeneous dielectric geometry

    Page(s): 124 - 132
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    A physically valid theory of partial discharge (PD) transients is based upon the concept of the charge induced upon the detecting electrode by the PD. This induced charge consists of two components. One is associated with the actual space charge in the void, while the other is related to changes in the polarization of the bulk dielectric. These changes are brought about by the field produced by the space charge. The magnitude of the induced charge and its components are examined for several heterogeneous dielectric systems. It is demonstrated that, in relation to a homogeneous dielectric system, the magnitude of the induced charge either increases or decreases depending on the ratio of the dielectric permittivities and within which dielectric the void is located. It is shown that this behavior is directly related to the magnitude and polarity of the polarization component of the induced charge. Furthermore, we demonstrate that the geometry of the dielectric system and the physical dimensions of the different dielectrics influence in a similar manner the magnitude of the induced charge, although to a lesser degree View full abstract»

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  • Fundamentals in computer aided PD processing, PD pattern recognition and automated diagnosis in GIS

    Page(s): 12 - 20
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    The partial discharge (PD) measurement is a sensitive method for testing and monitoring the insulation condition of HV apparatus. Digital PD measurements have become state of the art. Computer-based PD diagnostic systems nowadays can be used to increase the reliability of modern gas-insulated system (GIS). Also on-line monitoring of HV equipment is becoming increasingly important to enable prevention of a sudden breakdown of PD affected components. On-line monitoring must be done on-site, but an important problem of on-site PD measurement is the superposition of PD data and interference. Furthermore, reliable recognition of the type or nature of the insulation defect is necessary for any risk assessment. Traditionally, this is achieved with reference to expert knowledge. An alternative approach is presented in this paper, which treats the fundamentals of automated computer-based PD diagnosis, taking into account that successful noise suppression is essential for a successful on-site diagnosis View full abstract»

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  • Random sampling and data processing for PD-pulse height and shape analysis

    Page(s): 30 - 39
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    A measuring system for the digital acquisition of partial discharge (PD) pulse signals, based on the last generation of oscilloscopes, has been developed in order to perform both PD pulse shape and PD pulse height analysis. Wide-band, fast sampling rate and individually-triggerable memory blocks are available for the acquisition of PD pulse signals. However, a problem of sampling the population of PD pulses arises from the limited available on-line storage memory. Four sampling techniques are investigated and evaluated in order to record an amount of PD pulses which enables PD stochastic inference for a minimum of the on-line memory use. Some statistical indexes based on the pulse-height and pulse-phase distributions are used to compare performances of the different techniques. It is shown that a technique based on the Poisson law provides the most accurate sampling of PD pulses, while minimizing the memory, particularly in the presence of two simultaneously active PD phenomena. The developed procedure enables an accurate analysis of the shape of a large number of PD signals, but also stochastic processing of height distributions, which is becoming a reference for PD pattern analysis View full abstract»

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  • PD source recognition by Weibull processing of pulse height distributions

    Page(s): 48 - 58
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    The problem of identification of partial discharge (PD) phenomena occurring in an insulation system is addressed in this paper. PD distributions coming from different sources, such as internal voids, as well as surface and corona discharges, are compared. In particular, the investigation focuses on the Weibull probability function applied to pulse charge-height distribution. It is shown that the different discharge sources can be identified on the basis of the value of the shape parameter of the Weibull distribution and that identification holds even when two PD sources are combined, i.e. are active simultaneously. In this case, the application of the 5-parameter Weibull function permits separation of PD phenomena, and recognition by means of the shape parameter value associated to each phenomenon. Finally, the proposed procedure is applied to practical objects, i.e. insulation systems of rotating machines, with real insulation defects, showing promising on-field application prospective View full abstract»

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  • Space charge behavior in SF6 gas and sequential generation of PD pulses

    Page(s): 141 - 145
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    We observed time-resolved partial discharge (PD) characteristics under ac and dc conditions in order to demonstrate the effects of space charge behavior and the corona stabilization effect in SF6 gas. From the experimental results, we found that the charge magnitude of the first PD in the positive half cycle of applied ac voltage depended only on the instantaneous voltage. Under dc conditions we found that the magnitude of the PD charge related closely to the time interval between PD pulses. Space charge behavior and the PD generation mechanism are interpreted diagrammatically, and we describe how positive ions reduce the magnitude of PD charge and activate the corona stabilization effect and that the negative ions shorten the time interval between PD pulses View full abstract»

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  • PD pattern analysis during induced test of large power transformers

    Page(s): 95 - 101
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    The partial discharge (PD) measurements on power transformers are studied with a view toward discrimination and classification of different discharge sources. For this purpose digital analysis is applied to power transformers and reactors going through acceptance testing. Furthermore, based on these measurements, PD databases have been developed to support the discharge evaluation during induced voltage tests of transformers. Using this technique, a clear distinction is possible between transformers showing different discharge behavior View full abstract»

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  • The requirements of a PD measuring system analyzed in the time domain

    Page(s): 2 - 5
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    As known, partial discharge (PD) measuring systems usually are classified according to their frequency band (wide or narrow), that also corresponds to the easiest verifiable parameter. However, the connection between the errors introduced by a given measuring system and its frequency response can be shown only qualitatively. On the contrary, by representing its behavior in the time domain, we can demonstrate in a quantitative way that to limit both the integration and the superposition errors, corresponds to contradictory requirements for the parameters of the transfer impedance View full abstract»

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  • Partial discharge tests using CIGRE method II

    Page(s): 133 - 140
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    In this paper, the results of an experimental project on insulating material aging, performed in both Denmark and Italy, are reported. This study was concerned with partial discharge (PD) behavior at temperatures between 30 and 80°C using CIGRE method II. The material tested was a commercial polymethylmethacrylate (PMMA) which was chosen not for its good dielectric properties but rather because much of its discharge resistance data at ambient temperature is already well documented. A description is given of the theoretical and experimental methodology followed in this work. Mixed Weibull analysis techniques in terms of the PD amplitude and phase distribution characteristics were employed to distinguish the presence of different aging mechanisms. Such a difference was observed at 30 and at 80°C. At 30°C the analysis inferred a single discharge aging process acting until breakdown, while at 80°C the results suggested the predominance of a single PD aging mechanism for the first half of the insulation lifetime; however, for the remainder of the insulation life time, an additional degradation mechanism was evinced View full abstract»

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  • Phase relationship of PD pulses in dielectric liquids under ac conditions

    Page(s): 113 - 117
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    Partial discharge pulse phase analysis carried out on a mineral oil and a perfluoropolyether with point-to-plane electrodes provided further and additional evidence that discharge epochs tend to be concentrated in the vicinity of the alternating voltage peaks at discharge inception in lieu of the voltage zeros characteristic of normal cavities occluded in solid and liquid-impregnated solid insulating systems. Relatively large positive polarity discharge pulses, having an associated charge transfer of ⩾15 pC with a recurrence rate of at least one per every ten consecutive cycles, appear initially on the positive half-cycle; with further increases in applied voltage both their number and magnitude continue to exceed those of negative polarity over the negative half-cycle. This behavior is to be distinguished from that commonly observed with high sensitivity measurements, which clearly indicate that early discharge onset in liquids is characterized by the occurrence of minute highly intermittent negative polarity pulses. The increase in charge transfer of the discharge pulses with applied voltage is suggestive of the development of longer more intense streamers in the dielectric liquids at the more elevated electrical fields. Also the results infer that discharges or streamers form and propagate more readily in the mineral oil than in the electronegative perfluoropolyether 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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