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Electrical Insulation and Dielectric Phenomena, 2000 Annual Report Conference on

Date 15-18 Oct. 2000

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  • On the development of transfer function method for fault identification in large power transformers on load

    Page(s): 747 - 751 vol.2
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    Frequency Response Analysis (FRA) is now established as a proven tool in identifying some typical mechanical faults in power transformers. However, all FRA measurements so far have been off-line. An on-line technique is very much desired by the utilities, as this will prevent continued operation of the transformer while it is carrying a fault to failure stage. In this paper, challenges and some progress made so far toward the development of an on-line FRA technique has been presented. The paper also reports on the development of a high frequency magnetic coupler for this purpose. Results on some sensitivity analysis on a large reactor coil showing significant but characteristic changes on the FRA signatures to parameter variations. Observations of results obtained show that an on-line FRA tool is achievable but still there are some issues that need to be addressed. View full abstract»

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  • The lev-vection particle concentrator: some operational characteristics

    Page(s): 752 - 755 vol.2
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    One or more thermal convection patterns can be produced in a body of liquid by maintaining a temperature differential between base and top (Rayleigh-Benard or Benard-Marangoni convection). An electric field applied across an array of interdigitated electrodes on the base can supply the necessary heat, assuming the liquid to be lossy. At the same time, the field can cause levitation of weakly-polarizable, non-buoyant particles, which tend to become entrained in the convective pattern: the effect has been termed lev-vection. In the simplest case of a slow, single, toroidal convection pattern, the particles become concentrated and trapped in a particular and repeatable region just above the electrode plane. A desire to scale-up the apparatus has led us to examine the effects of varying the various parameters (size, voltage, viscosity etc.) on the speed and stability of the induced circulation. View full abstract»

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  • Ageing of composite machine insulation with a non-linear response

    Page(s): 756 - 759 vol.2
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    Samples of a novel design of machine conductor insulation comprising of commercial mica-paper and glass cloth impregnated with an epoxy resin filled with 20% ZnO have been investigated. Measurements of the transient currents, DC resistivity and thermally stimulated discharge currents (TSDC) have been performed on both unaged material and samples which have been exposed to corona discharge for periods up to 306 hours. A variation in resistivity and dielectric loss with time was observed in unaged samples which could be attributed to continuing curing of the epoxy. An increase in the dielectric loss in the corona-aged samples could be attributed to damage to the microstructure of the epoxy resin. View full abstract»

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  • Space charge behavior near LDPE/LDPE interface

    Page(s): 800 - 803 vol.2
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    We investigated the space charge behavior near the interface between different low-density polyethylene (LDPE). Charge carriers were mainly injected from the semiconductive electrode in a specimen of Al/LDPE/LDPE/semiconductive layer and they moved through the interface to the counter Al electrode. Charge carriers moving from a lower density LDPE to a higher density one were accumulated near the interface to form space charge, while there was no space charge accumulation for carriers moving in the opposite direction. The mobilities of charge carriers were estimated from the change in space charge profile with time. Charge carriers are more mobile in the lower density LDPE than in the higher density one. Positive carriers were more mobile in LDPE than negative ones. The interface between different LDPE's greatly affected space charge behavior and discharging currents. View full abstract»

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  • Investigation on discharge development phenomena on D.C. high voltage insulator of two grooves

    Page(s): 804 - 807 vol.2
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    The local discharge propagation direction towards flashover was investigated using an HV laboratory insulator of two grooves with the same resistance per unit channel length. The experimental results revealed that the presence of a metallic sphere in one channel influenced the discharge propagation direction. In addition to the gradient of potential and current in the pollution, we deduced that the electric field strength around the discharge tip has an effect on the discharge development direction. View full abstract»

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  • Creeping discharge on solid/liquid insulating interface under AC and DC voltages

    Page(s): 784 - 787 vol.2
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    This paper deals with the propagation of discharges on liquid/solid interface under ac and dc voltages using a point-plane electrode arrangement. The tests are carried out on different samples of Bakelite immersed in mineral oil. The final length of surface discharges and the associated currents are analysed as a function of the shape and the polarity of the voltage. It is shown that the final discharge length increases with the voltage and decreases when a hydrostatic pressure is applied. The number of creepage discharge branches, the associated current pulses and emitted light are also reduced when the pressure is increased. View full abstract»

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  • Surface flashover characteristics of polymeric rods with temperature gradient

    Page(s): 808 - 811 vol.2
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    Surface flashover characteristics of polymeric rods were investigated experimentally by artificially creating a temperature gradient along the rod. Two types of temperature gradient were studied. In one case, the temperature gradient was obtained by cooling a portion of the rod with liquid nitrogen, because such temperature gradients are known to occur along electrical insulating materials of superconducting power apparatus. In the other case, part of the rod was warmed with a heater since such temperature gradients along an insulating material may occur at a contact point with a conductor of power apparatus operating at ambient temperature. In both cases, the flashover voltages were reduced in the case of higher temperature gradients. View full abstract»

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  • The performance of electrically stressed droplets on insulating surfaces observed with an optical measuring system

    Page(s): 792 - 795 vol.2
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    The observation of the behaviour of a single droplet on an insulating surface is a basic step to understand the ageing phenomenon in a very early stage of ageing. This paper presents results of experimental work with the use of a contact angle measuring system. The performance of single droplets is observed on an horizontally arranged epoxy resin surface under electrical stress. View full abstract»

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  • Correlation between numerical field computation and experimental investigations on the parameters governing the flashover behavior of solid/liquid interfaces

    Page(s): 788 - 791 vol.2
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    This paper compares experimental investigations dealing with the ac breakdown and flashover strength of liquid insulated gaps without and with a parallel solid/liquid interface in a non-uniform electrical field with results obtained from field calculations obtained using the finite element method. The liquids investigated were a mineral based transformer oil and an ester liquid, the solid insulating materials were laminated paper, glass-fiber reinforced epoxy resin, and polyethylene. View full abstract»

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  • The effect of residual crosslinking by-products on the local dielectric strength of HV extruded cables

    Page(s): 743 - 746 vol.2
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    Crosslinked polyethylene (XLPE) is generally obtained by the activation of a chemical agent like dicumyl peroxide (DCP). Many crosslinking by-products are produced by the decomposition of DCP. These by-products are always present in the XLPE matrix after crosslinking and manufacturing. Typical crosslinking by-products from DCP have been measured for five HV extruded cables at five radial positions in the insulation bulk. Measured by-products consisted in acetophenone and cumyl alcohol which are the most significant in HV cables. Measurement method consisted in Fourier Transform Infrared Spectroscopy (FTIR) carried out on peeled ribbons. In order to avoid by-product loss by diffusion and evaporation, ac breakdown strength measurements have been carried out immediately after FTIR. FTIR measurements show that crosslinking by-products content is higher in the bulk of the insulation than near the conductor and insulation shields. In addition, residual amount of cumyl alcohol is generally higher than for acetophenone. No important effect of both by-products on the ac dielectric breakdown strength was observed. View full abstract»

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  • Radiation effects in polyimide foils subjected to a simulated space environment

    Page(s): 768 - 771 vol.2
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    Samples of polyimide have been aged under a simulated space environment under various stressing conditions for periods of up to 48 months. Changes in the polymer have been monitored by measuring TSDC spectra. The behaviour of activation energies derived from the TSDC spectra as a function of polarisation voltage and ageing conditions after 24 months are presented in this paper. View full abstract»

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  • Surface flashover dynamics in metallized polymer film capacitors

    Page(s): 816 - 819 vol.2
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    Flashover discharges are one of main failure mechanisms in metallized polymer film capacitors. To study the basic characteristics of surface discharges and their dynamics, an equivalent electric circuit model is developed for metallized polymer film capacitors with the surface discharge modelled with a time-dependent resistor. The discharge resistance is related to the thermal dynamic equation, and as such the heat generated within the capacitor unit can be estimated and subsequently used to assess changes in material properties due to temperature rise. Also discussed are the conditions for propagation of surface discharges and its elongation. For different combination of key system parameters, surface discharges are simulated and this provides insights into factors that influence the growth of surface flashover in metallized polymer film capacitors. View full abstract»

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  • Frequency dependence of interfacial space charge formed in laminated dielectrics under AC voltage application conditions

    Page(s): 796 - 799 vol.2
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    The frequency dependence of interfacial space charge was experimentally obtained for LDPE/EVA laminated dielectrics in the frequency range of dc to 50 Hz. It was found that there are high, low and intermediate transition frequency regions in this performance. Interfacial space charge might be governed in principle by the Maxwell-Wagner type polarization, but modified by a semi-blocking contact of the interface in the low frequency region, and by a limited number of carriers in the high frequency region. Increased voltage induces carriers in the interface to become less mobile or more trapped. Both phenomena occur in the transition region. View full abstract»

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  • Surface flashover of alumina insulators in vacuum under impulse voltage (the influence of additives)

    Page(s): 812 - 815 vol.2
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    This paper investigates flashover performance of alumina insulators prepared with different additives in vacuum under an impulse voltage. The four kinds of alumina insulator studied are prepared under the same sintering conditions with different additives. The surface flashover performance of these samples is tested in vacuum. The microstructure of the alumina samples is also observed using scanning electron microscopy (SEM). Based on the experimental results, the characteristics of flashover are different from the test samples. The analysis shows that the influence of additives may be attributed to the change of microstructure. View full abstract»

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  • Dielectric property measurement of nonlinear grading materials

    Page(s): 764 - 767 vol.2
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    Programs are being developed which can solve the electric field distribution in systems which include nonlinear dielectric materials, i.e., materials for which the dielectric constant and/or electrical conductivity is a function of field parameters such as electric field and, possibly, temperature. However, measurement of input parameters for such programs, i.e., the dielectric properties as a function of field and temperature, is problematic. This paper considers the options for such measurements. View full abstract»

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  • Surface flashover of a channel filled with an electrolyte

    Page(s): 776 - 779 vol.2
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    We observed the extension of a discharge on an electrolyte surface either with an electronic camera or by using an original set-up permitting simultaneous measurements of optical and electrical parameters on the same flashover. The corresponding results are presented here. They show two zones of different luminosity. The development of them are very different. Zone 2, the darker of the two, is nearer the ground electrode, whereas the zone 1, the brighter one, is behind it starting from the HV electrode and has a better defined outline. At the onset of the flashover in its first phase, the zone 2 spreads faster than the zone 1. Later, in a shorter second phase, the zone 1 develops more quickly and progressively overruns the zone 2. Thereafter, suddenly the flashover is completed. We comment on these results and suggest an explanation. View full abstract»

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  • Investigation of the performance of field aged HMWPE and XLPE cables

    Page(s): 760 - 763 vol.2
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    A field-aged, cable AC breakdown program was performed to assist in the management of Pacific Gas and Electric Company's underground assets. The performance of cross-linked polyethylene, XLPE, and high-molecular weight polyethylene insulation, HMWPE was determined and fitted inverse power curves for both insulation types predict AC breakdown strengths in excess of operating stress at thirty years life. Testing entire runs of field-aged cables showed that an AC breakdown test on a single section is most likely to produce the average and not the lowest value. The testing demonstrates that a better diagnostic is needed to help prioritize cable replacement. Attempts to convert the field-aged XLPE and HMWPE cable AC breakdown data to single cumulative distribution functions were only successful for very limited cases. The field history could not be incorporated into the transformation. A replacement program based on one manufacturer's distribution function whose cables were the oldest submitted for test and failed the earliest would be very conservative. View full abstract»

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  • Dielectric properties of PTFE-based composite insulating materials: interfaces effects

    Page(s): 772 - 775 vol.2
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    This paper relates to the study of the dielectric behavior of polymer-based insulating materials used in high voltage equipment. The materials tested are microheterogeneous, basically made out of a semicrystalline polymeric matrix (polytetrafluoroethylene) with mineral particles (calcium fluoride) dispersed in it. The dielectric properties are determined in a wide frequency (or time) range by a combination of two techniques: the short time response is assessed by measuring the complex permittivity in the frequency domain (20 Hz-1 MHz), and the long term response is studied by measuring the polarization current in the time domain (1s-600s) thus covering more than 8 decades in frequency or time. The modelling used to describe microheterogeneous dielectrics is shown to fit the experimental data. However, exposure to moisture is shown to induce strong relaxational effects. These effects vanish in a reversible way by drying treatment (i.e. removal of water molecules). These additional relaxations are discussed in term of conductivity effects occurring at the interface matrix-filler particles. View full abstract»

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  • Behaviour of the metal-oxide-fluid-interface

    Page(s): 780 - 783 vol.2
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    Surface phenomena at electrodes play a major role in streamer inception. Metal-electrodes are always covered with oxide-layers, changing the electrical and chemical properties of the metal-surface. Semiconducting metal-oxide structures arise at the surface. For experiments, oxide covered metal electrodes are dipped into the liquid. A double layer-structure with an inner and outer Helmholtz plane and a subsequent diffuse layer are established at the metal-oxide-liquid interface. Charge transport from and into the liquid is controlled by the electrical properties of the metal-oxide-double-layer structure. Streamer inception modifies the structure of these surface layers and leads to restructuring at the interface and in the liquid bulk. A model for the behaviour of the metal-oxide-liquid interface using percolation theory concepts is presented as well as experimental investigations of surface structures of electrodes used in experiments. Oxide layers found on brass electrodes as well as the impact of streamer inception on electrode surfaces is presented. View full abstract»

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  • Model specimens for testing interfacial electrical insulating properties in EHV extruded cable splices and preliminary results

    Page(s): 820 - 823 vol.2
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    Model specimens to evaluate the insulating properties at the solid-solid interface have been developed. The specimens are easy to fabricate and fulfil the requirements discussed in the 1996 CIGRE Paris meeting. With the developed specimens we can control the electric field direction at the interface. Using different kinds of EPR material we can distinguish their different PD behavior. We can also examine the effects of electric field direction and of the existence of silicone oil or various defects. View full abstract»

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  • 2000 Annual Report Conference On Ielectrical Insulation And Dielectric Phenomena [front matter]

    Page(s): i - xiii
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    Freely Available from IEEE
  • Modeling ferroelectric properties in systems of induced and permanent dipoles

    Page(s): 514 - 517 vol.2
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    In earlier studies we reported on Monte Carlo simulations of dielectric properties in systems of permanent or induced dipoles considering electrostatic interactions. The calculations are expanded now to systems composed of induced and permanent dipoles with the final purpose to model ferroelectric properties in structures like BaTiO3 . In a first step a dipole chain with permanent dipoles (Ti-ions moving in double well potentials) and induced dipoles (O-ions) in alternating order is considered referring to the [001] crystal direction in BaTiO3. The calculation yields a hysteresis loop of the polarization, a Curie-Weiss law, a temperature dependent remanent polarization, and a spontaneous polarization. A second system refers to the (200) plane of BaTiO3. Besides a hysteresis of the polarization with a temperature dependent remanent polarization, we find spontaneous ferroelectric domains with an uniform polarization. Typical ferroelectric properties arise if permanent and induced dipoles are involved in the interaction process View full abstract»

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  • On investigation of PD aging of epoxy resin under distorted voltage

    Page(s): 573 - 576 vol.2
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    The first results of a research program carried out in Palermo University, dealing with the degradation mechanisms of epoxy resins due to partial discharges (PD) under 50 Hz distorted voltage, are presented. Accelerated life tests were performed until breakdown, measuring PD evolution with time in an electrical grade epoxy resin and making use of a standard Cigre' II cell. The tests were performed on samples of five specimens and using an harmonic voltage superimposed to the fundamental 50 Hz component. The long term goal of the research is to scan either different effective voltage amplitudes and different frequencies and phase values of harmonics having different amplitude percentage, in the range of those usually recorded in electrical plants, e.g. up to 11th harmonic. Actually, a test session of 10 kV effective is going to be completed in terms of sinusoidal and distorted voltages. Either reduction or increase of lifetime, with respect to the sinusoidal conditions, were detected dependently on the way as the harmonic voltage is summed up in consequence of its phase angle value. In particular, significant life shortening was obtained in the cases when voltage maximum is increased. Further comments on the subject are finally presented. View full abstract»

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  • Author index

    Page(s): 827 - 831
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    Freely Available from IEEE
  • Aging of ceramic materials used in power electronic substrates

    Page(s): 604 - 607 vol.2
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    In power electronics, the development of new IGBT power modules allows them to switch kV and kA during μs. However, such components need to be cooled and ceramic materials like AlN and Al2O3 are used as an interface between the electronic devices (the chips) and the cooling. Such materials have to present very well known electrical and thermal characteristics since the reliability of the inverters these components are used in is strongly depending on them. In order to increase the knowledge of the physical mechanisms involved during their aging, different experiments are performed (lifetime, breakdown, PD and Space charge measurements...). Electrofracture mechanism is claimed to be responsible of the observed behaviors View full abstract»

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