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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 6 • Date December 2013

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  • IEEE Transactions on Dielectrics and Electrical Insulation [Table of contents]

    Publication Year: 2013 , Page(s): c1
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  • IEEE Dielectrics and Electrical Insulation Society - Executive committee officers

    Publication Year: 2013 , Page(s): c2
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  • Dear Reviewer [Editor-in-Chief's thank you]

    Publication Year: 2013 , Page(s): i - iii
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  • Condition monitoring and diagnostics [Editorial]

    Publication Year: 2013 , Page(s): 1961
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  • Transformer frequency response analysis: mathematical and practical approach to interpret mid-frequency oscillations

    Publication Year: 2013 , Page(s): 1962 - 1970
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1068 KB) |  | HTML iconHTML  

    Frequency Response Analysis (FRA) has been utilized as an off-line diagnosis test since last decade to investigate mechanical integrity of transformer. FRA data typically reports as a spectrum in Bode diagram over determined frequency band. To evaluate FRA data, determined frequency band may be conveniently divided into three bands, namely low-, medium- and high- frequency bands. It is well-known low-frequency band dominated by core, mid-frequency region dominated by winding structure and high-frequency band influenced by measurement connection leads. This study has concentrated on midfrequency oscillations of transformer winding frequency response spectrum presented in Bode diagram. Mathematical approach using travelling wave theory is employed to explore frequency response trace behavior. Practical studies on a prepared glassy transformer as well as two 66 kV, 25 MVA continuous and interleaved disc windings have been performed to validate mathematical calculation. In addition, two 245 kV, 45 MVA and 66 MVA power transformers have been examined to study mid-frequency oscillations and compare the result with mathematical evaluation. View full abstract»

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  • Aging behavior of cellulosic materials in presence of mineral oil and ester liquids under various conditions

    Publication Year: 2013 , Page(s): 1971 - 1976
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    This paper is aimed at the study of aging behavior of cellulosic materials in different dielectric liquids under air and nitrogen atmospheres. Paper for copper conductor and high density transformer board were aged at 130°C up to 1128 h in sealed vessels in presence of mineral oil (MO), natural ester (VO) and synthetic ester (SE). The influence of thermal aging on cellulose and oil characteristics are investigated and compared. It's observed that the paper degradation is associated with the generation of CO2 and CO gases. Whatever the type of oil, this generation is more important with vessel under air atmosphere than that under nitrogen gap indicating a protective effect of nitrogen. In mineral oil, the degree of polymerization (DPv) reaches the end of life criteria at around 200 whereas it remains around 400 with ester oils. This is mainly due to the higher water solubility of esters compared to mineral oil. The measured degree of polymerization in MO is comparable to that calculated through 2-FAL (Furanic compounds) content using Vuarchex equation whereas for esters oil, there is no agreement between the measured and calculated DPv through 2-FAL content. This can constitute a limitation in the use of 2-FAL for the diagnosis of ester-filled transformers. Overall, the insulation cellulose/oil behavior seems better under nitrogen atmosphere. View full abstract»

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  • Optimized models for overload monitoring of power transformers in real time moisture migration model

    Publication Year: 2013 , Page(s): 1977 - 1983
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    In this paper a mathematical model, based on experimentation, to estimate the axial moisture distribution in power transformer windings was developed. The model considers the variables of moisture content in oil, top oil temperature, as well as the transformer's design parameters and ageing of the insulation system. The model was validated using a small-scale experimental setup, which represents a 75 MVA transformer. The developed model proved to be more accurate than existing models. It considers the axial moisture distribution in paper, in 4 thermal zones equidistantly distributed along the height of the winding. This paper compares the results of the proposed model with those provided by other authors. View full abstract»

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  • Determination of the thermal aging factor for life expectancy of 550 kV transformers with a preventive test

    Publication Year: 2013 , Page(s): 1984 - 1991
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    It is feasible to survey actual performances of degradation for 550 kV transformers by a preventive test. By means of condition monitoring, it is possible to forecast the transformers' lifetime. Such data, particularly in an application where few or no failures are expected, can provide considerably more information than the failure-time data from a traditional accelerated life test. In order to develop a life model for 550 kV transformers, the effect of temperature on a thermal aging factor is taken into account since it is associated with various degradation performances of a transformer. With the aid of measurement, the thermal aging factor is analysed by the probability plot and the least-squares fitting of data so that all unknown parameters of the life model are estimated. The method will extrapolate the lifetime of large power transformers from a preventive test in practice. View full abstract»

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  • Electrical aging lifetime model of oil-impregnated paper under pulsating DC voltage influenced by temperature

    Publication Year: 2013 , Page(s): 1992 - 1997
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    Main insulation of windings connected with valves in high voltage direct current (HVDC) convert transformers withstands pulsating DC voltages. This paper presents breakdown properties of oil-impregnated paper under pulsating DC voltages at different temperatures. Progressive stress tests were carried out to obtain breakdown voltages of oil-impregnated paper with thickness of 0.2 mm under pulsating DC voltages at different temperatures. Constant stress tests were executed to measure time to breakdown of the oil-paper specimens. Both types of the experiments were also undertaken under either AC or DC voltages. An electrical aging lifetime model was proposed for the oil-impregnated paper, and parameters of the electrical aging lifetime model were presented. Experiments and analysis results show that the breakdown voltages and time to breakdown of oil-paper insulation are influenced by temperature significantly. View full abstract»

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  • Reliability assessment of insulation system for dry type transformers

    Publication Year: 2013 , Page(s): 1998 - 2008
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    The insulation system of a dry-type transformer under the combined thermal, electrical, mechanical, and environmental stresses undergoes gradual deterioration, which ultimately leads to transformer failure. It is hence important to test specimen insulation of transformer coil so as to explore the aging law and the associated characteristic parameters related to the insulation condition. This paper proposes a method to evaluate reliability of the insulation system of a dry-type transformer based on the three models of degradation, regression and expert judgment. The three models are developed based on the quality parameters (QPs) from the lab tests. The method is illustrated by estimating the reliability of the insulation system of a mining flameproof dry-type transformer. The accelerated aging tests on the Nomex paper (i.e. the turn-to-turn insulation of the transformer) are conducted. The QPs of the Nomex samples of degree of polymerization, partial discharges, and photomicrograph were measured periodically during aging test. The effectiveness of the proposed method was proved through the lab experiments. The results show that the parameters from the aging test can be used to represent the condition of the insulation system. View full abstract»

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  • Impact of high voltage harmonics on interpretation of partial discharge patterns

    Publication Year: 2013 , Page(s): 2009 - 2016
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    Typically, in laboratory and diagnostics tests focused on measuring partial discharges, a pure sinusoidal voltage waveform is assumed. However, in practice the spectral content of the applied voltage is rarely so ideal and additional spectral components have a significant impact on the discharge behavior in electrical insulation systems. In this paper the influence of voltage harmonics on partial discharge behavior and pattern evolution is analyzed. The presented experiments were conducted on a transformer installed in a municipal network and on specimens representing defects in a transformer insulation system, in the form of a micro blade protrusion on a winding conductor and an embedded gas inclusion. The experimental results showed that various harmonic compositions superimposed on the fundamental sinusoidal waveform have a significant impact on PD intensity and maximum charge. In consequence, PD pattern phase and amplitude distributions are distorted and calculated statistical parameters are changed. The rough parameters reflecting the waveform spectral quality, such as THD, are rather superficial and do not provide a unique and reliable signature of the applied voltage in terms of phase angle variations. In certain environments, neglecting harmonic content in the voltage may lead to a misleading interpretation and assessment of partial discharge severity. A complete knowledge of harmonic content is essential and critical in conducting a proper assessment of the impact of PD, including acceptability criteria and compatibility with the standards. View full abstract»

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  • Hilbert fractal antenna for UHF detection of partial discharges in transformers

    Publication Year: 2013 , Page(s): 2017 - 2025
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    Electromagnetic waves generated due to partial discharge (PD) can be captured by ultra-high-frequency (UHF) antennas. The size and frequency band of an UHF antenna for PD online monitoring are critical factors for practical installation inside a transformer. This paper presents a compact multi-band UHF Hilbert fractal antenna with wide frequency band and suitable size for easy installation. Design criteria of a PD antenna were proposed and the Hilbert fractal antenna was selected as an appropriate candidate for PD detection. Principle of Hilbert fractal antenna was discussed and a fourth order Hilbert fractal antenna was optimized for detecting PD signals. Actual PD experiments have been carried out for two typically artificial insulation defect models while the antenna was used for the PD measurement. More simulation studies and actual PD experiments have been done to further examine the effects of electromagnetic wave refraction and reflection by transformer components. The results show that the proposed UHF antenna is qualified for PD online monitoring. View full abstract»

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  • A new method for purposes of failure diagnostics and FRA interpretation applicable to power transformers

    Publication Year: 2013 , Page(s): 2026 - 2034
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (922 KB) |  | HTML iconHTML  

    To diagnose electrical and mechanical failures on the active part of power transformers efficiently, different state-of-the-art testing methods are recommended to be performed since transformers are complex devices. Measurement results can be equivalent electrical parameters relating to the windings (resistances, leakage/zero-sequence inductances and capacitances), core magnetic property (via induced voltages and exciting currents) or terminal frequency responses (standard FRA tests). Consequently, it is not easy for normal users to implement all necessary measurements, which require much time and training skills due to use of different specialized testing devices. To help users for fast and efficient diagnostics, the paper presents a new method in determining most electrical parameters of the transformer active part based on only the driving-point impedance tests performed by means of a vector-network analyzer. More importantly, the method can be applied to give a FRA interpretation based on physical electrical parameters, which is currently requested from relevant standards. The physical FRA interpretation is found to be useful in calculating immeasurable series capacitances of windings in transformer bulk, which is still problematic until now. Investigation on a large distribution transformer shows that the new method is simple, less time consuming but efficient. View full abstract»

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  • PD properties when varying the smoothness of synthesized waveforms

    Publication Year: 2013 , Page(s): 2035 - 2041
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1045 KB) |  | HTML iconHTML  

    The increased use of power electronic components in power systems makes it important to understand how rapidly rising voltages affect insulation systems. One vital aspect of this challenge is the measurement of partial discharges, PDs, which are considered as being a sign of weakness and can affect the life of insulation considerably. In this paper an approach is presented to measure PDs in a dielectrically insulated cavity when exposed to pulse width modulated (PWM) voltage shapes with different degree of smoothness. This is a continuation of our earlier investigations on the different behavior of PDs where voltages characterized by different rise times were applied. The present investigation shows that the PD amplitude decreases significantly already at a moderate level of PWM voltage smoothness to a magnitude that is about the same as for sinusoidal voltage shape. For the phase resolved PD (PRPD) pattern to become similar to the normal AC pattern it is required that the remains from PWM steps are lower than the extinction voltage. This work elucidates how PDs are affected by synthesized waveforms and limits for a sufficient smoothing level are found, which is of importance when designing insulation systems exposed to fast transients. View full abstract»

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  • Robust measurement, monitoring and analysis of partial discharges in transformers and other HV apparatus

    Publication Year: 2013 , Page(s): 2043 - 2051
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1417 KB) |  | HTML iconHTML  

    This paper describes and practically illustrates modern methods for robust, and at the same time sensitive measurement of partial discharges; applied in the laboratory, onsite and during on-line monitoring of HV apparatus. For conventional, electrical measurements, the principle of dynamic noise gating is illustrated. Above this, amplitude-based and spectrum-based impulse correlations allow for discrimination between noise and partial discharges, but also for separation of several discharge sources. Automatic pattern recognition then classifies the separated sources and provides basic risk assessment. Acoustic waves of partial discharge events serve for PD localization, but, due to their high damping, often require combination with electrical or electro-magnetic methods. Measurements in the UHF range, well-known from gas-insulated equipment, are of particular benefit for noise-immune analysis of oil-filled equipment, too and compared to the traditional, electrical method in this paper. View full abstract»

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  • Condition assessment of power transformers using a synthetic analysis method based on association rule and variable weight coefficients

    Publication Year: 2013 , Page(s): 2052 - 2060
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    Condition assessment for power transformer requires not only integrating the known artificial intelligence (AI) technology, but also exploiting the interrelation of the measured data. According to the association rule of information data and the variable weight synthesizing theory of factor spaces, an assessment method of transformer condition was proposed in this paper. Via analyzing the interrelation of the independent status parameters (ISP) and transformer fault types, the set of synthetic status parameters (SSP) can be built up. For avoiding interference from subjective experience, association rule theory was used to calculate the constant weight coefficients (CWC) of the ISPs. Since the true transformer condition may not always be accurately reflected under the condition of CWCs of a few SSPs, the method of variable weight synthesizing was used for computing the variable weight coefficients (VWC) of the SSPs. Then, combining with the existing maintenance procedures, a preferable condition assessing system of power transformer was proposed. Operational example proved the condition assessing system may reflect the real operation condition of power transformer. View full abstract»

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  • An efficient diagnosis method for data mining on single PD pulses of transformer insulation defect models

    Publication Year: 2013 , Page(s): 2061 - 2072
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2263 KB) |  | HTML iconHTML  

    Reviewing the various Partial Discharges (PD data mining researches which have been reported so far, this study compares the performance of different feature spaces and different classifiers employed for PD classification in insulation condition monitoring of power transformers. In this process, first a knowledge basis is developed through construction of 4 different types of PD models in the high voltage laboratory. Background noise is considered as one class in this knowledge basis. The high frequency time domain current signals of high voltage equipment are captured over one power frequency cycle. The single PD activities within this captured signal are extracted by application of a threshold-based method. Four popular feature extraction methods i.e. Statistical, texture, FFT and Cepstral features are applied on these recorded extracted PD signals. To distinguish the different PD types, three conceptually different classifier types, Neural Network, Decision Tree, and k-nearest neighbours, are applied on the recorded feature spaces. Using Bayesian theory, a performance analysis is carried out to find whether the classifiers are over-fitted or not. Although, the most reliable data mining tool found to be a combination of a Cepstral feature space, and neural network classifier however, since the statistical features can be computed very fast it is employed in this work. Next, it is proposed to use a cascade PD identifier to find whether the detected signal is noise or not. And if it is PD, employing Cepstral feature space knowledge-basis, its type is identified. View full abstract»

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  • Condition monitoring and diagnostics of motor and stator windings – A review

    Publication Year: 2013 , Page(s): 2073 - 2080
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (431 KB) |  | HTML iconHTML  

    Condition-based maintenance (also called predictive maintenance) requires diagnostic tools to determine when significant insulation aging has occurred, and thus when maintenance is necessary to avoid an in-service failure. Over the years many off-line diagnostic tests and on-line monitoring systems have been proposed and implemented, especially on critical motors and generators. In the past decade, most of the research has concentrated on improving existing off-line diagnostic tests and on-line monitoring such as magnetic flux, partial discharge, temperature, endwinding vibration, etc. However, some newer tests such as polarization/depolarization current, dielectric spectroscopy and on-line leakage current monitoring have been introduced. These tests and monitoring systems are reviewed. Tests and monitoring systems for rotor winding insulation and stator winding insulation are addressed separately. View full abstract»

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  • Condition assessment of stator insulation during drying, wetting and electrical ageing

    Publication Year: 2013 , Page(s): 2081 - 2090
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (414 KB) |  | HTML iconHTML  

    Stator insulation failure due to moisture ingress is one of the common reasons for generator failures. Non-destructive tests such as DC ramp, insulation resistance (IR) and frequency domain spectroscopy (FDS) are usually conducted to assess the conditions of such insulations. This paper presents a laboratory investigation of generator spare windings under drying, wetting and cyclic electrical ageing conditions. First Dc ramp, IR and FDS tests were conducted on asphalt-mica and epoxy-mica insulated spare windings to obtain initial finger print values. Then the drying effects was investigated by DC ramp and FDS measurements conducted on asphalt-mica insulated winding samples by drying them up to three days at 80°C. The wetting effect was studied by wetting the polyester-mica insulated winding samples in a water bath up to one week at 25°C. Further a correlation has been established between the moisture content and FDS results by drying and wetting of two asphalt-mica insulated samples at 70°C and the room temperature respectively. The ageing effects was obtained by electrically stressing two asphaltmica and two epoxy-mica insulated winding samples at 300% of the rated voltage up to 9 weeks in a cyclic manner. For all cases, conductivity (σ) and parameters representing the response function were estimated by the Davidson-Cole and inverse power dependence functions. The estimated parameters under DC ramp, IR and FDS tests were in similar range for new samples. During ageing and drying, a clear variation was noted only on the σ values and FDS results. It was found that FDS results and estimated parameters provide useful information about the condition of generator insulation. View full abstract»

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  • Slot discharge pattern of 10 kV induction motor stator coils under condition of insulation degradation

    Publication Year: 2013 , Page(s): 2091 - 2098
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    In the manufacturing process of the 10 kV induction motors, a semi-conductive layer with certain resistance covering the linear part of the stator coil is often used to suppress the partial discharges (PDs) happened in the slot. However, due to electromagnetic force while the motor is running, slot discharge is still one of the main causes of stator insulation degradation. In this paper, an adjustable width mockup core slot is designed using silicon steel sheet. Several 10 kV real machine stator coils subjected to varying aging degrees are adopted as test coils. The phase resolved partial discharge (PRPD) patterns of slot discharge are analyzed using a PRPD analyzer and the variation law of slot discharge patterns with voltage variation is illustrated. The investigation result has been applied to the online monitoring and pattern recognition of the insulation for a 10 kV induction motor in a coal mine and it has great application significance. View full abstract»

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  • Precise location of the excessive temperature points in polymer insulated cables

    Publication Year: 2013 , Page(s): 2099 - 2106
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    This paper demonstrates that it is possible to locate very precisely a point showing a high temperature in a polymer insulated cable. The method is based on frequency domain reflectometry and inverse fast Fourier transform. The cables tested are a coaxial communication cable with a length of 32 m insulated by low density polyethylene and a flat in-house electric cord with a length of 21 m insulated by polyvinyl chloride. The cable or cord was heated at different positions for different lengths. The ratio between the powers of electromagnetic waves incident to and reflected from the cable was measured using a network analyzer in a frequency range from one to several hundred MHz or 1.5 GHz. The spectra obtained by the measurements were then analyzed by inverse Fourier transform. As a result, the position exhibiting a temperature higher than the adjacent points can be located with a spatial resolution as short as 2.5 cm. It was also confirmed that the sensitivity or spatial resolution can be improved by an increase of the highest measurement frequency. View full abstract»

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  • Degradation distribution in insulation materials of cables by accelerated thermal and radiation ageing

    Publication Year: 2013 , Page(s): 2107 - 2116
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    The degradation of cables by the accelerated thermal and radiation ageing for nuclear power plant was studied. The thermal oxidation of crosslinked polyethylene (XLPE) as a cable insulation showed the heterogeneous features along the depth of sheet specimens. The degree of oxidation was closely related to the distribution of antioxidant content due to the decay of antioxidant content by the evaporation and radiation decomposition during accelerated ageing. The specific relations were observed between the tensile properties and the yield of oxidation products and also between the oxidation products and antioxidant content in XLPE specimens. The yields and the content and their distribution were detected by FTIR spectra. The oxidation of XLPE containing antioxidant of enough content proceeded proportionally with ageing time or radiation dose at any ageing conditions. When the antioxidant content decreased less than a critical content by thermal and/or radiation ageing, the oxidation rate by thermal ageing increased sharply with ageing time. Therefore, the oxidation at surface of XLPE was much accelerated when the antioxidant decreased to result the heterogeneous oxidation. The mechanical properties depend closely on the degree of oxidation at the surface. The behavior of oxidation was also affected by the types of antioxidant. View full abstract»

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  • Autonomous classification of PD sources within three-phase 11 kV PILC cables

    Publication Year: 2013 , Page(s): 2117 - 2124
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (720 KB) |  | HTML iconHTML  

    To allow utilities to fulfill self-imposed and regulative performance targets that apply to them, the demand for new tools to help judge the health of modern power distribution networks has increased. The analysis of partial discharge (PD) signals has been identified as a potential diagnostic tool for the condition monitoring of HV plant. In order to investigate the PD activity produced by a range of defects within three-phase paper insulated lead covered (PILC) distribution cable under rated conditions, an experiment has been developed. The experiment incorporates a commercially available on-line PD measurement system employing a high frequency current transformer (HFCT) to record PD data in a manner that is currently in operation in the UK. By replicating field conditions and using realistic hardware to collect experiment data, that any findings or analysis tools developed during this investigation are directly transferable to use in the field. Four defective cable samples, each containing different imperfections that are known to reduce in-service plant life have been fabricated and extensively PD tested. The raw experiment data was processed to produce a dataset containing a range of features from individual PD pulses including time, frequency and time-frequency information. This data was used to optimize and train several support vector machine (SVM) models to perform automated pulse classification. Four SVM models were tested using different combinations of pulse features to identify which characteristics were most effective at transferring source dependent information for classification. The results of the automated algorithm validated the approach returning a classification accuracy of 91.1%. View full abstract»

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  • Prediction of mechanical properties of XLPE cable insulation under thermal aging: neural network approach

    Publication Year: 2013 , Page(s): 2125 - 2134
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (318 KB) |  | HTML iconHTML  

    The widespread use of Cross-linked Polyethylene (XLPE) as insulation in the manufacturing of medium and high voltage cables may be attributed to its outstanding mechanical and electrical properties. However, it is well known that degradation under service conditions is the major problem in the use of XLPE as insulation in cables. In order to reduce the aging experiments time, we have used Artificial Neural Networks (ANN) to predict the insulation properties. The proposed networks are supervised and non supervised neural networks. The supervised neural network was based on Radial Basis Function Gaussian (RBFG) and was trained with two algorithms: Backpropagation (BP) and Random Optimization Method (ROM). The non supervised neural network was based on the use of Kohonen Map. All these neural networks present good quality of prediction. View full abstract»

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  • Toward understanding the relationship between insulation recovery and micro structure in water tree degraded XLPE cables

    Publication Year: 2013 , Page(s): 2135 - 2142
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    In this paper, to understand the insulation recovery phenomenon of water-tree cables when an applied voltage is removed, the insulation recovery mechanism of the water tree cable is analyzed. A water-needle electrode is used in an accelerated water-tree aging experiment for XLPE (Cross Linked Polyethylene) cables. The dielectric loss factor (tanδ) increases with aging time, but it gradually returns to a lower level once the applied voltage is removed for some time, which means that the insulation recovery of the water tree cables possibly occurs during the process. Depending on the geometric size by micro observations of water-tree slices, a watertree model is constructed for electric field simulation, which includes a series of water-filled micro voids and interconnected channels. According to analysis of the electric field force and the mechanical properties of water tree region, the force generated by the elastic deformation of the molecular chains results in the shrinking of the channels. The water is gradually squeezed out of the channels, which causes the gradual decrease of conductivity and relative dielectric constant in channels. According to the computed results of tanδ at different states of water tree, the reduction of tanδ can be mainly caused by the decrease of conductivity and relative dielectric constant in channels. The time needed for insulation recovery may be determined by the internal electric field in the water tree, the mechanical relaxation time of XLPE, and the diffusion of water. 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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Meet Our Editors

Editor-in-Chief
Reuben Hackam