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Showing papers in "IEEE Transactions on Dielectrics and Electrical Insulation in 2002"


Journal ArticleDOI
Ray Bartnikas1
TL;DR: Different partial discharge detection and measurement procedures suitable for use on cables, capacitors, transformers and rotating machines are examined and compared in this paper, with particular attention given in regard to their suitability to different types of electrical apparatus and cable specimens under test as well as their applicability to discharge site location and their capability to detect different forms of PD.
Abstract: Different partial discharge (PD) detection and measurement procedures suitable for use on cables, capacitors, transformers and rotating machines are examined and compared. Both narrow and wide bandwidth PD detectors are considered; particular attention is given in regard to their suitability to different types of electrical apparatus and cable specimens under test as well as their applicability to discharge site location and their capability to detect different forms of PD. A rather substantial portion of the discussion is devoted to the use of intelligent machines as applied to PD pattern recognition in terms of either PD pulse-height/discharge epoch (phase) distributions or discharge pulse shape attributes.

489 citations


Journal ArticleDOI
TL;DR: In this article, a fuzzy classifier is used for the analysis of the acquired PD-pulse shape signals, and the result of the fuzzy classification is a cluster of signals homogeneous in terms of stochastic features of PD pulses.
Abstract: This paper deals with digital acquisition, classification and analysis of the stochastic features of random pulse signals generated by partial discharge (PD) phenomena. Focus is made on a new measuring system for the digital acquisition of PD-pulse signals, which operates at a sampling rate high enough to avoid the frequency aliasing, but that provides an amount of PD pulses which enables PD stochastic analysis. A separation and classification method, based on a fuzzy classifier, is developed for the analysis of the acquired PD-pulse shape signals. The result of the fuzzy classification is a cluster of signals homogeneous in terms of stochastic features of PD pulses. The classification efficiency is evaluated resorting to the PD-pulse height and phase distributions analysis. The instrumentation, and the associated classification methodology, are applied to measure and analyze PD data recorded for mica-insulated stator bars and coils, where typical defects, occurring during normal operations, were simulated. It is shown that the proposed procedure enables PD-source identification to solve the identification problems which arise, in particular, when different sources of PD are simultaneously active. In addition fuzzy classification provides an efficient noise-rejection tool.

335 citations


Journal ArticleDOI
TL;DR: A review of the history of electrets and pyroelectricity in non-uniform space charge electrets is given in this paper, where the most relevant charging and measuring techniques for piezo and pyroproelectric polymer-foam electrets are introduced.
Abstract: The review begins with a very brief history of electrets and of piezo and pyroelectricity in nonuniform space charge electrets. The development of cellular propylene as an electret-transducer material since the mid-eighties is described, and the accompanying research into the origins of (quasi-)ferro-, pyro- and piezoelectric effects in voided polymer electrets is discussed in some detail. In this context, the most relevant charging and measuring techniques for piezo and pyroelectric polymer-foam electrets are introduced. Porous fluoropolymer electrets and their investigation as single or multiple-layer piezoelectrics are considered in view of their better thermal and temporal stabilities. Finally, some proposed or demonstrated applications of voided polymer electrets are briefly mentioned.

285 citations


Journal ArticleDOI
TL;DR: A review of recent developments made in the understanding of the electrical tree mechanism is presented and the chaotic nature of the tree propagation mechanism is discussed both through experimental data and the results of a completely deterministic theoretical model.
Abstract: A review of recent developments made in the understanding of the electrical tree mechanism is presented. The life of the tree is covered from initiation, through propagation, to long-term changes in shape. The initiation process is examined in terms of the injection of space charge and its ability to transfer energy to the polymer to create damage. Theoretical models for the processes involved are assessed in terms of the experimental data and an outline for the sequence of events in tree initiation developed. The inter-relationship between tree discharges, tree propagation, and tree shape is discussed. Theoretical models for these processes are evaluated in terms of their ability to reproduce experimental data, especially tree shapes and discharge sequences in time and space. The chaotic nature of the tree propagation mechanism is discussed both through experimental data and the results of a completely deterministic theoretical model. Some special features of electrical trees such as the existence of conducting trees, acceleration at long times and slow growth in thick insulation are briefly touched upon. Finally a summary of the state of the art is presented.

270 citations


Journal ArticleDOI
TL;DR: The paper demonstrates that the wavelet based denoising method proposed in the paper can be employed in separating PD pulses from electrical noise successfully and can be used in pulse propagation studies of partial discharge in distributed impedance plant to provide enhanced information and further infer the original site of the PD pulse.
Abstract: The objective of the paper is to discuss a tool which is proving extremely efficient in partial discharge measurement studies. Though the technique itself is not new, its application to partial discharge studies is. It will be demonstrated in this paper that it has tremendous power and this accounts for its rapid growth as an application in this field. The paper begins with the description of the fundamentals of wavelet analysis, wavelet categories and the properties of the associated wavelet transforms. PD pulses as acquired from detectors composed of different detection circuits are investigated and numerically simulated, and a method on how to select optimally the wavelet corresponding to the representative forms of PD pulse is then presented. Finally, applications of wavelet analysis to partial discharge studies are explored. The paper demonstrates that the wavelet based denoising method proposed in the paper can be employed in separating PD pulses from electrical noise successfully and can be used in pulse propagation studies of partial discharge in distributed impedance plant to provide enhanced information and further infer the original site of the PD pulse.

260 citations


Journal ArticleDOI
TL;DR: A review of the current state of understanding of dielectric mixture properties, and approaches to use numerical calculations for their modeling are presented in this article, where it is shown that interfacial polarization can yield different non-Debye dielectrics responses depending on the properties of the constituents, their concentrations and geometrical arrangements.
Abstract: A review of the current state of understanding of dielectric mixture properties, and approaches to use numerical calculations for their modeling are presented. It is shown that interfacial polarization can yield different non-Debye dielectric responses depending on the properties of the constituents, their concentrations and geometrical arrangements. Future challenges on the subject are also discussed.

259 citations


Journal ArticleDOI
TL;DR: In this article, the authors highlight the achievements during the last decades in life modeling under electrothermal stress, considering both Phenomenological and physical models, considering also a statistical approach, with applications to experimental results.
Abstract: This paper has the purpose of highlighting the achievements during the last decades in life modeling under electrothermal stress. Phenomenological and physical models are examined, considering also a statistical approach, with applications to experimental results.

177 citations


Journal ArticleDOI
TL;DR: In this paper, the capacitive coupler, acoustic emission (AE) sensor and radio frequency current transducer (RFCT) have been used to detect partial discharge (PD) activity within a 132 kV prefabricated cable joint containing a known defect.
Abstract: The capacitive coupler, acoustic emission (AE) sensor and radio frequency current transducer (RFCT) have been used to detect partial discharge (PD) activity within a 132 kV prefabricated cable joint containing a known defect. Although each of the three methods has been applied individually under different situations, a comprehensive investigation and comparison of these three methods has not yet been made. Results obtained were compared to measurements made using the conventional electrical detection method. The quantification of the capacitive coupler measured signal in mV with a discharge apparent quantity in pC has been investigated and an on-line quantification method proposed. The capacitive coupler has good detection sensitivity and PD location can be realised by studying the time of Right between signals from two sensors. The difference in pulse shape, spectrum and time of flight between an internal discharge and external interference has been investigated. The AE approach has the advantage of being free from electrical interference. However, investigation indicated that AE signals were significantly attenuated within the cable joint. RFCTs were used to detect the discharge current flowing through the cable sheath. Where the detection sensitivity was low, a wavelet de-noising method was applied to process the RFCT signals and proved to tie effective in increasing the measurement signal to noise ratio.

120 citations


Journal ArticleDOI
TL;DR: Aging of polymeric and composite materials is reviewed mainly from the standpoint of their interfacial performance in this article, where the degradation by tracking and erosion and the loss of hydrophobicity in case of surface properties, and with PD, electrical treeing, water treeing and combined phenomena.
Abstract: Aging of polymeric and composite materials is reviewed mainly from the standpoint of their interfacial performance. Insulating materials can be divided into simple polymers and composites. Polymers for power cables, transformers, insulators and rotating machines consist of thermoplastics such as PE, PET and PPS, elastomers such as silicone, EPR and EPDM, and thermosets such as epoxy. Composites for GIS, rotating machines and insulators comprise epoxy/glass, epoxy/silica/alumina, and epoxy/mica systems. Aging processes are complicated in general, and take place under simultaneous multiple stresses such as electrical, thermal, mechanical and environmental stresses. Some of the phenomena covered in this paper are associated with the degradation by tracking and erosion and the loss of hydrophobicity in case of surface properties, and with PD, electrical treeing, water treeing and combined phenomena. The quality of cable insulation such as XLPE has been improved extensively from the standpoint of design electric strength. Interfacial problems will emerge for modification of cable joints. It is expected that polymers for outdoor use and filled epoxy resin systems should be improved from their environmental stability and from their design stress enhancement, respectively.

116 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present a review of the conducting properties of semi-crystalline polyethylene (PE) and conclude that the fundamental characteristics of the lamellar PE crystal are still the determining factors.
Abstract: Fifty years ago, the concept of an intrinsic electric strength for insulating solids was fashionable. It was supported by the sophisticated theoretical models of von Hippel, Frohlich and others for the interaction of electrons with crystal lattices in an electric field. The models came to be applied, rather implausibly perhaps, to polymers such as semi-crystalline polyethylene (PE). In that climate, the work of van Roggen on the high-field conductivity of single lamellar crystals of PE was notable. Since then, the picture of the insulating properties of PE has had to be painted with an increasingly complex palette in which the electrical elements have had to be supplemented by mechanical, chemical, and optical ones. The present review of the conducting properties of the polymer considers these other features, but concludes that the fundamental characteristics of the lamellar PE crystal are still the determining factors.

116 citations


Journal ArticleDOI
TL;DR: In this article, the authors explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities, and show that the number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire.
Abstract: Streamer discharges in tap water and distilled water have been generated by applying a voltage pulse from 120 to 175 kV and 500 ns duration to a wire-to-electrode configuration. Electrical and optical diagnostics were used to explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities. With the wire serving as anode, multiple, parallel streamer discharges were generated. The number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire. The dependence of the streamer density on electric field indicates the role of field enhancement at inhomogeneous microstructures along the wire as streamer initiation mechanism. The appearance of the discharge was different for tap and distilled water. However, the measured average streamer propagation velocity from the positive wire to the grounded plane electrode, of 32 mm//spl mu/s, was independent of the water conductivity and the applied voltage. This suggests the existence of a self-sustained electric field at the streamer head. With the wire serving as cathode, only a weak light emission from the area close to the wire was observed, and streamers did not appear for the same voltage amplitude as with the positive polarity. This suggests that an ionic current flowing in the water is not dominant in the streamer propagation process.

Journal ArticleDOI
TL;DR: In this article, the authors reviewed the relevant phenomena during the period of the recovery of the dielectric strength of vacuum interrupter (VI) and found that metal vapor, residual charge and the effects of molten contact surfaces reduce the breakdown strength after the interruption of high currents.
Abstract: Relevant phenomena during the period of the recovery of the dielectric strength of vacuum interrupter (VI) are reviewed. Metal vapor, residual charge and the effects of molten contact surfaces reduce the breakdown strength after the interruption of high currents. Metal droplets seem to play only a secondary role during the recovery phase. Instability of liquid protrusions is suspected to initiate breakdown if the electric field strength exceeds 10 kV/mm at the molten contact surfaces (undisturbed field). Key parameters are surface temperature and vapor density. Breakdown during recovery of the dielectric strength depends not only on the value of the voltage applied but also on the duration of the high voltage stress. A spontaneous triggering event in the presence of metal vapor appears to initiate breakdown at conditions below the Paschen breakdown limit. This type of breakdown events limits the interruption performance of VI's.

Journal ArticleDOI
TL;DR: In this article, the authors discuss the motivation for online monitoring, discusses the transition to today's technology, and describes the variety of methods now in use for rotor winding and stator winding monitoring.
Abstract: Electrical insulation plays a critical role in the operation of motor and generator rotor and stator windings. Premature failure of the insulation can cost millions of dollars per day. With advancements in electronics, sensors, computers and software, tremendous progress has been made in the past 25 yr which has transformed on-line insulation monitoring from a rarely used and expensive tool, to the point where 50% of large utility generators in North America are now equipped for such monitoring. This review paper outlines the motivation for online monitoring, discusses the transition to today's technology, and describes the variety of methods now in use for rotor winding and stator winding monitoring.

Journal ArticleDOI
TL;DR: In this article, a study of streamer inception in mineral transformer oil, in point-plane and rod-plane geometry under impulse voltage, was performed with points and rods over a wide range of tip radii, showing a marked decrease of initiation fields when the electrode radius is increased.
Abstract: Presents a study of streamer inception in mineral transformer oil, in point-plane and rod-plane geometry under impulse voltage. The measurements performed with points and rods over a wide range of tip radii show a marked decrease of initiation fields when the electrode radius is increased. The initiation field is divided by 30 over the investigated range (from 1 /spl mu/m points up to 2 cm rods). This effect compares fairly well with the "surface effect" known for breakdown voltages under uniform field with large electrodes. Plotting these results together shows the decrease of streamer initiation fields over a very wide electrode surface range (12 decades), proportional to S/sup -0.17/ (S: electrode surface area, cm/sup 2/). These results suggest the influence of electrode surface defects on streamer initiation under impulse voltage.

Journal ArticleDOI
TL;DR: In this paper, a multi-conductor transmission line is modeled and simulated by using multichannel transmission line theory, which takes into account the main phenomena occurring along the lines, i.e., propagation and the reflection, together with the time dispersion introduced by the losses, eventually dependent on the frequency.
Abstract: The numerical evaluation of the electrical stress in the line-end coil of the stator winding of a medium voltage motor fed by a pulsed width modulated (PWM) inverter seems to be indispensable for a rational design of the machine. In order to fulfil such a task, the system, composed of a feeder cable and a stator winding, is modelled and simulated by using multi-conductor transmission line theory. The model can take into account the main phenomena occurring along the lines, i.e. the propagation and the reflection, together with the time dispersion introduced by the losses, eventually dependent on the frequency. The multi-conductor transmission line is solved in the time domain by adopting a technique based on the perturbation theory of the spectrum of symmetric matrices, which sensibly decreases the computational effort with respect to the analysis in the frequency domain. Furthermore, an accurate calculation of the characteristic matrices, which contain the cross-sectional information of the line, is performed by means of a FEM package, so taking into account the effective field distribution in the region of interest. The influence of the accurate evaluation of the capacitance and inductance matrices is considered by comparing the numerical results of the proposed model with those obtained by a simple equivalent circuit, frequently adopted in the literature. In order to validate the proposed model, the simulated results are compared with experimental data.

Journal ArticleDOI
TL;DR: In this article, a general methodology that allows failure statistics to be derived from a given breakdown mechanism is presented, and a number of case studies are used to illustrate the way in which mechanistic features can be related to the parameters of the failure statistic.
Abstract: Breakdown theories yield a specified time to breakdown for fields exceeding a critical value, whereas experiment shows that times-to-breakdown and breakdown fields are different on a sample-to-sample basis. This has forced the use of a statistical approach to failure, often without a clear physical understanding of the pertinent parameters. A general methodology that allows failure statistics to be derived from a given breakdown mechanism is presented. A number of case studies are used to illustrate the way in which mechanistic features can be related to the parameters of the failure statistic. In particular it is shown that the Gumbel statistic and not the Weibull function is the one appropriate to failure initiation by random defects. Both the formative stage and initiation stage of breakdown are considered, and it is shown that features can be identified in the failure statistic related to known thresholds in the active breakdown mechanism. Aging is examined through the three main control features: activation energy, field enhancement factor, and threshold factor. It is shown that distributions in each of these parameters could give rise to the observed statistics above the estimated characteristic threshold field. However, distributions in the material variables that define the local threshold value allow some failures to occur in the sample set, even at fields below the characteristic threshold level, whereas distributions in activation energy do not. The possibility of predicting the lifetime of individual specimens is briefly addressed in closing.

Journal ArticleDOI
TL;DR: In this paper, an expert system based on re-confirmative method for the diagnosis of minor insulation failures involving small number of turns in transformers during impulse tests is presented.
Abstract: The presence of insulation failure in the transformer winding is detected using the voltage and current oscillograms recorded during the impulse test. Fault diagnosis in transformers has several parameters such as the severity of fault, the kind of fault and the location of the fault. Detection of major faults involving a large section of the coils have never been a big issue and several visual and computational methods have already been proposed by several researchers. The present paper describes an expert system based on re-confirmative method for the diagnosis of minor insulation failures involving small number of turns in transformers during impulse tests. The proposed expert system imitates the performance of an experienced testing personnel. To identify and locate a fault, an inference engine is developed to perform deductive reasoning based on the rules in the knowledge base and different statistical techniques. The expert system includes both the time-domain and frequency-domain analyses for fault diagnosis. The basic aim of the expert system is to provide a non-expert with the necessary information and interaction in order to make fault diagnosis in a friendly windowed environment. The rules for fault diagnosis have been so designed that these are valid for the range of power transformers used in practice up to a voltage level of 33 kV. The fault diagnosis algorithm has been tested using experimental results obtained for a 3 MVA transformer and simulation results obtained for 5 and 7 MVA transformers.

Journal ArticleDOI
TL;DR: In this paper, the properties of streamers traveling over the surface of oil-immersed solid dielectrics were experimentally studied under lightning impulse conditions, and the potential drop inside the streamer channel was measured as a function of normalized streamer length.
Abstract: The properties of streamers traveling over the surface of oil-immersed solid dielectrics were experimentally studied under lightning impulse conditions. Streamer polarity and the position of a grounded side electrode significantly affected the relationship between the streamer extension length and the applied voltage. Solid surface charging also had a large effect on the streamer propagation. However, the streamer propagation properties showed a consistent dependence on the potential at the solid-liquid interfaces. In addition, the potential drop inside the streamer channel was measured as a function of normalized streamer length. The curve revealed that the potential drop increased drastically within the region of /spl sim/20% from the streamer tip. The streamer appeared to progress with a constant mean velocity.

Journal ArticleDOI
TL;DR: In this article, the authors report on AC breakdown (puncture and/or flashover), and impulse breakdown of solid materials in either vacuum or in liquid nitrogen, including fiberglass reinforced plastics, epoxies with and without filler, and polymeric tape.
Abstract: The discovery of high temperature superconductors (HTS) has triggered renewed interest in the study of dielectric materials at cryogenic temperatures. While considerable work was done in the 1970s and 1980s on dielectrics immersed in liquid helium for low temperature superconducting applications, there remains a need for dielectric research at liquid nitrogen temperature for HTS applications, requiring experimental data oriented toward practical situations. We report on AC breakdown (puncture and/or flashover), and impulse breakdown of solid materials in either vacuum or in liquid nitrogen. Solid materials which we examined, include fiberglass reinforced plastics, epoxies with and without filler, and polymeric tape. Combinations of some of these materials have also been studied at low temperatures. Additionally we have measured permittivity and dissipation factor for materials for which these parameters are not available at 77 K. Finally, we also discuss specific applications for HTS cables including breakdown and aging studies on model cables, with lapped tape electrical insulation, immersed in liquid nitrogen.

Journal ArticleDOI
TL;DR: In this paper, a new approach using the wavelet transforms to classify the patterns inherent in different fault currents is described, where wavelets are not only localized in frequency, but also in time.
Abstract: One of the tests carried out on a transformer after assembly is the lightning impulse test, for assessment of the integrity of its winding insulation. In the case of a fault, it has been well established that the pattern of the fault currents contains a typical signature of the nature and location of the insulation failure involved. This paper describes a new approach using the 'wavelet transforms' to classify the patterns inherent in different fault currents. Whereas conventional frequency-response analysis based techniques fail to identify the time-localization of a particular frequency component in a time-dependent signal, the wavelets are not only localized in frequency, but also in time. The 'time-frequency localization' feature of wavelet transform is employed for pattern classification of impulse fault currents of transformers. Results for simulated models of 3 5 and 7 MVA transformers are presented to illustrate the ability of this approach to classify insulation failures.

Journal ArticleDOI
TL;DR: In this paper, a study of the dependence of the leakage current and the charge on the flow rate, conductivity and pressure of the solution forming the fog is reported, which is consistent with those published in the literature with RTV coated fiber glass reinforced plastic (FRP) rods.
Abstract: Room temperature vulcanizing (RTV) silicone rubber coating has excellent leakage current suppression characteristics. It is used extensively on porcelain and glass insulators in pollution prone areas as an anti-pollution measure. A pollution aging chamber has been fabricated and an on line 4-channel personal computer (PC) based data acquisition system has been developed to conduct tests on RTV coated porcelain samples. A study of the dependence of the leakage current and the charge on the flow rate, conductivity and pressure of the solution forming the fog is reported. It was observed that a higher charge and a higher average leakage current were obtained at a higher flow rate and a higher conductivity. The results with coated HV insulators are consistent with those published in the literature with RTV coated fiber glass reinforced plastic (FRP) rods.

Journal ArticleDOI
TL;DR: In this article, a study of streamer propagation in transformer oil, with point-plane and semi-uniform geometry, is presented, and the effect varies widely according to the streamer type considered.
Abstract: Presents a study of streamer propagation in transformer oil, with point-plane and semi-uniform geometry. The latter is made of parallel plane electrodes, with a thin triggering point of calibrated size. By reducing the length of the point, it is possible to move progressively from a point-plane geometry to a quasi uniform geometry. The propagation of streamers is impeded by the presence of a metallic plane behind the triggering point, that reduces the field on propagating streamers. The effect varies widely according to the streamer type considered. The propagation of negative and fast positive streamers is nearly quenched, whereas slower filamentary positive streamers (usually responsible for breakdown in oil) are less affected. This shows that many results obtained in point-plane geometry can not be simply extrapolated to the more realistic case of uniform field.

Journal ArticleDOI
TL;DR: In this article, the results of an investigation concerning a mechanism of brittle fracture in glass reinforced plastic (GRP) rods used in non-ceramic insulators (NCI) are presented.
Abstract: The results of an investigation concerning a mechanism of brittle fracture in glass reinforced plastic (GRP) rods used in non-ceramic insulators (NCI) are presented. Commercial grade GRP rods and GRP rods from actual insulators were exposed to ultra-pure water (UPW) and acids while being subjected to mechanical stresses. The experimental results revealed that water has the potential of inducing stress corrosion cracking on the fibers and hence brittle fracture in the rods. It is observed that the fracture proceeded faster when the rods were exposed to UPW than when exposed to acids. Furthermore, a brittle fracture in an epoxy cross-arm, which was installed in a region where the formation of acids in the atmosphere can be neglected, is analyzed. Based on these evidences, it is postulated that the failure of in-service NCI in the brittle mode can occur under the influence of water and mechanical stresses, and that the failure is more likely to happen with water than with acids.

Journal ArticleDOI
TL;DR: In this paper, a new model that enables the estimation of life of polymeric insulation subjected to DC stress, both in the presence and in the absence of voltage polarity inversions, is presented.
Abstract: This paper illustrates a new model that enables the estimation of life of polymeric insulation subjected to DC stress, both in the presence and in the absence of voltage polarity inversions. The derivation of the model parameters is based on the results of space charge measurements and accelerated life tests realized at constant DC stress, which allows the characterization of the performance of insulating materials under DC electrical stress to be carried out in short times. The expected life of a cable in service can be evaluated by means of this model, provided that the rate of voltage inversions expected during its life is known or estimated. The model is applied here to results of tests carried out on polyethylene-based materials for cable insulation, and model parameters are calculated resorting to two different approaches. A satisfactory fitting of life test results is achieved, thus confirming the validity of the proposed model.

Journal ArticleDOI
TL;DR: In this paper, the laser induced pressure propagation (LIPP) technique was used to measure the space charge in polyethylene (XLPE) insulation under 50 Hz ac stress.
Abstract: Space charge is believed to play an important role in ac electrical ageing of polymeric insulation, particularly where electrical treeing is the dominant cause of premature failure. The majority of work to date has been on the space charge characteristics under dc electric stresses, whereas work on the dynamics and the role of space charge on electrical breakdown under 50 Hz ac conditions have only received limited attention. Consequently, a full understanding of space charge trapping and dynamics under ac operating conditions is required if ever more reliable polymer insulated cable systems are to be developed. The present paper reports on aspects of an on-going investigation into the measurement of space charge in XLPE insulation under 50 Hz ac stress using the laser induced pressure propagation (LIPP) technique. Some important features of the method are described such as, point on wave control of the laser and use of correction factors to offset variations in the laser power and ablation of the target material. Emphasis has also been placed on establishing a simple method for analysing data, termed "X-plots". Some preliminary results of the space charge and electric stress distribution of un-degassed and degassed crosslinked polyethylene (XLPE) plaques aged electrically under ac stress (30 kV/sub peak//mm) for 24 h are also presented.

Journal ArticleDOI
TL;DR: In this paper, a conducting ball immersed in a poorly conducting liquid between two horizontal plates acquires a charge when subjected to a dc voltage, when the applied voltage is high enough for the electric force to overcome gravity, the ball rises, moves up through the liquid and falls down as its charge leaks away.
Abstract: A conducting ball immersed in a poorly conducting liquid between two horizontal plates acquires a charge when subjected to a dc voltage. When the applied voltage is high enough for the electric force to overcome gravity, the ball rises, moves up through the liquid and falls down as its charge leaks away. The threshold of voltage, as well as the time of flight between contacts, depends on the charge acquired by the ball during the contact with the electrode. Experiments have been done using liquids of different conductivities and it was shown that the conductivity does not affect the lift-off voltage. When the ball gets close to the electrode a discharge occurs and a current pulse is registered by the external circuit. The charge carried by the pulse is an order of magnitude smaller than the charge transferred to the ball from the electrode. We have made a detailed study, with different balls and liquids, of the charge transferred to the external circuit. The dependence on the different parameters of the ratio charge transferred to the circuit-charge acquired by the ball is explained using the coefficients of capacity of the electrostatic system.

Journal ArticleDOI
TL;DR: In this article, the adoption of a field-theoretical approach to problems arising in the framework of electrical insulation is discussed with reference to six main topics, which have been addressed over the last 30 years.
Abstract: The adoption of a field-theoretical approach to problems arising in the framework of electrical insulation is discussed with reference to six main topics, which have been addressed over the last 30 years. These include uniform field electrodes, Green's differential equation, electrode surface roughness, induced charge, electrostatic probes, and partial discharge transients, together with several follow-on aspects. Each topic is introduced and thereafter the progress achieved through the use of a field-theoretical approach is reviewed. Because the topics cover a wide spectrum of conditions, it is amply demonstrated that such an approach can lead to significant progress in many areas of electrical insulation.

Journal ArticleDOI
TL;DR: In this article, the authors used partial discharge (PD) to study the development of leakage current (LC) on silicone rubber insulators in a salt-fog chamber and found that PD is a sensitive method for detecting the transition from corona from water droplets to dry-band arcing.
Abstract: The paper presents an investigation on the use of partial discharge (PD) as a means to study the development of leakage current (LC) on silicone rubber insulators in a salt-fog chamber. Simultaneous measurements of LC and PD on dead-end type silicone rubber insulators exposed to different combinations of electrical stress and salt-fog salinity are performed. The results show that for an electrical stress of 33 V/mm applied to the insulator, the average LC experiences gradual changes and reaches a critical level (around 1 mA) where dry-band arcing is observed after 20 minutes at 2300 /spl mu/S/cm, and after 25 minutes at 1800 /spl mu/S/cm. The corresponding PD levels reach hundreds of pC after 15 minutes at 2300 /spl mu/S/cm, and after 20 minutes at 1800 /spl mu/S/cm. For a salinity corresponding to 1800 /spl mu/S/cm and stress of 26 V/mm, the LC changes much more gradually and reaches the critical level only after approximately 60 minutes. The corresponding PD reaches hundreds of pC after 30 minutes and more than 1500 pC after 60 minutes. Pattern recognition techniques have been applied to investigate the transition from corona to dry-band arcing. The features extracted from the maximum and mean apparent charge trends are found to be useful to cluster and classify the two phenomena. Simultaneous visual observations of the discharges on the insulator surface support the results. The investigation, therefore, shows that PD is a sensitive method for detecting the transition from corona from water droplets to dry-band arcing and an alternative method for studying the early aging stage of silicone rubber insulators.

Journal ArticleDOI
TL;DR: In this paper, the effects of specimen thickness, testing conditions and preparation technique on the measured ramp-breakdown strength of 2 polyethylene systems have been investigated, and the obtained data have been found to be highly reproducible, enabling results under different test conditions to be correlated with one another.
Abstract: The effects of specimen thickness, testing conditions and preparation technique on the measured ramp-breakdown strength of 2 polyethylene systems have been investigated. The obtained data have been found to be highly reproducible, enabling results under different test conditions to be correlated with one another. In all cases, an increase in ramp rate results in an increase in the measured breakdown strength, as does a reduction in sample thickness. Irrespective of the test parameters, the short-term performance of a polyethylene blend containing a designed microstructure is always improved compared to that of the base low density polyethylene alone. Impulse testing leads to an identical conclusion. The effect of the blending method has been shown to have a negligible effect on the breakdown strength, despite consequent changes in nucleation density, implying that it is the local lamellar texture that is responsible for the enhanced properties of the blend.

Journal ArticleDOI
TL;DR: In this article, two types of sensors are designed, which can not only carry enough water for the contamination on the insulator surface to dissolve and cover the surface with conducting solution, but also measure the conductance of the solution.
Abstract: Equivalent salt deposit density (ESDD) is widely used to describe the state of contaminated insulators. In this paper, a new method of ESDD on-site measurement is studied. Two types of sensors are designed, which can not only carry enough water for the contamination on the insulator surface to dissolve and cover the insulator surface with conducting solution, but also measure the conductance of the solution. The design of the sensors and the determination of the desired parameter are introduced in detail. Experiments prove that the conductance of the contamination solution is linear with the square root of ESDD of the insulator. This relationship can also be deduced by Debye-Huckel-Onsager theory. According to this relationship, the conductance measured by the sensor can be converted to ESDD. Therefore, Measuring ESDD on-site can be realized.