Showing papers in "IEEE Transactions on Electrical Insulation in 1990"

Water treeing in polyethylene cables

Abstract: Water tree growth in polyethylene cable insulation is discussed. The characteristics of water trees, the effect of aging parameters on water tree growth, and the possible mechanisms of growth are considered, emphasizing vented tree development in polyethylene insulating materials. The morphology of water trees, the characteristics of the tree-infested dielectric cable, and test methods and measures to reduce water treeing are discussed. >

Fundamental processes of SF/sub 6/ decomposition and oxidation in glow and corona discharges

Abstract: It is known that sulfurhexafluoride (SF/sub 6/), used as an insulating gas in HV apparatus, will oxide in electrical discharges in the presence of oxygen or water vapor to form various reactive and stable by-products. In order to interpret experimental data on rates of oxidation and by product formation in discharges, meaningfully, it is necessary to apply theoretical chemical kinetics models that utilize rates for numerous gas-phase processes as functions of gas temperature and/or electric-field-to-gas-density ratio. Current knowledge about the fundamental collision processes involving electrons, ions, free radicals, and molecules needed to understand the gas-phase discharge chemistry in SF/sub 6/ is reviewed. Implications of the fundamental rate data reviewed here for recently proposed chemical-kinetics models of corona and glow-type discharges in SF/sub 6/ are discussed. >

Topics in electronic ceramics

Abstract: Four topics in the field of electronic ceramics are reviewed: multilayer ceramic capacitors, piezoelectric ceramics, electrooptic ceramics, and ferroelectric thin films. They all are related to ferroelectric ceramic materials. The scope of this review is limited to oxide compounds, because oxide ferroelectrics, for the most part, exhibit the most useful properties and have found the most applications. The underlying physical concepts, ceramic and/or thin-film fabrication, structure/property relationships, specific applications, and future prospects in each of the four electronic ceramic application areas are discussed. >

Models for insulation aging under electrical and thermal multistress

Abstract: A review of the methods and models used in lifetime studies of solid insulators under single- and multiple-stress conditions is presented. The statistical methods used with this type of data are described briefly, including the two-parameter Weibull distribution and the log-normal distribution, which are the two distributions most frequently used in aging studies. Lifetime models under multiple stresses are discussed. In particular, several models under combined electrical and thermal stresses are presented and their applicability is analyzed and discussed. These include multistress models proposed by L. Simoni (1981, 1984) and T.S. Ramu (1985, 1987), both employing the inverse power law for electrical aging, the exponential model by B. Fallou (1979), the probabilistic model by G.C. Montanari (1989), and the physical model by J.-P. Crine (1989). Trends in the development of these models are discussed. >

Estimating the cumulative probability of failure data points to be plotted on Weibull and other probability paper

Abstract: Using both an exact technique and Monte Carlo simulations to estimate the cumulative probability of failure of the ith of n identically stressed samples, it is shown that the approximation F(i,n)=(i-0.3)/(n+0.4) is considerably more accurate than other commonly used approximations. The significance of this in the interpretation of electrical breakdown tests is demonstrated. >

Degradation of electrical insulating paper monitored with high performance liquid chromatography

Abstract: The degradation of electrical kraft paper under carefully controlled laboratory experiments is being examined in order to find correlations between the change in dielectric and mechanical properties and the increase in concentration of furan compounds in the oil. These correlations could be valuable for assessing the extent of degradation of paper insulation in operating HV equipment. Preliminary investigation has shown that the concentration of furan compounds in mineral oil can be determined relatively easily using high-performance liquid chromatography (HPLC). While some of these compounds are themselves unstable, three have been found which are worth monitoring to correlate with the tensile and dielectric properties of the paper. Useful trends have been obtained, and further investigation is taking place to obtain clear cut correlation of practical value for monitoring the condition of actual transformers. >

Measurements of partial discharges by computer and analysis of partial discharge distribution by the Monte Carlo method

Abstract: The mechanism of partial discharge (PD) occurring in the CIGRE Method II (CM-II) electrode system, which is a representative closed-void model system is discussed in the context of a computer-aided PD measuring system. Measurements of PD are made for the CM-II electrode system. Effects of the pressure and gas inside the void on the PD are examined. Taking into account the experimental results, a model for the PD mechanism is proposed. This model assumes that the statistical time lag of discharge depends on the overvoltage and that the residual voltage depends on the PD magnitude. A Monte Carlo simulation of the PD distribution is made on the basis of this model. The computed results agree well with the experimental data and the appearance of swarming pulsive micro discharges. The physics of the model are discussed. >

Basic physics of gaseous dielectrics

Abstract: Basic physical processes involved in the prebreakdown stages of gaseous dielectrics are discussed, with particular emphasis on electron attachment, electron-molecule scattering, and electron impact ionization. Particular emphasis is placed on the dielectric properties of gases at moderately high pressures (few kilopascals to few atmospheres) and under uniform DC conditions for which the electrons generated in the gas quickly reach a steady state, and thus electron-molecule interactions leading to electrical breakdown can be addressed assuming steady-state conditions even though the breakdown itself is a transient, nonequilibrium phenomenon. Utilization of the understanding of these basic processes for tailoring gases/mixtures with optimum dielectric strengths is indicated. >

Calculation of DC and AC flashover voltage of polluted insulators

Abstract: The phenomena and the propagation processes of DC and AC arcs on the surface of polluted insulators were observed and investigated by means of a high-speed camera. Time-resolved photographs indicate that there is an obvious distinction between DC and AC flashover processes. Based on the alternating character of an AC arc on the polluted surface, the relationship between its reignition and recovery conditions is analyzed. The most important factor is the recovery rather than the reignition condition. A formula expressing the resistance of the pollution layer on the insulator is derived. From this, the flashover voltage of a polluted insulator of complex shape can be calculated. A computer program has been developed to calculate the flashover under both DC and AC. Some arc phenomena, such as arc bridging between the sheds or ribs of insulators, the drift of the arc from the insulator surface, and the number of arcs in series before flashover, are taken into account. Much experimentation has been done to determine the flashover voltages of polluted insulators with complex shapes under both DC and AC. Experimental data are in good agreement with the calculated results. >

Polymeric outdoor insulation

Abstract: The need for resistance to surface tracking and erosion partially differentiates outdoor insulation from indoor insulation. A review of the current status of development of standards for testing these properties worldwide on materials and components is presented. Of special interest is the self-restoration of the hydrophobicity of some materials, most notably silicones, after the hydrophobicity has been removed by surface arcing. Since the accelerated tests are continuous, the restoration is not permitted, while in real life it is. This is important because silicones are among the best candidates for outdoor insulation in severely polluted areas. This means that new and different test methods must be developed. It is emphasized that it is important to integrate this information with the old in order to plan future work. Other requirements of outdoor insulation are discussed. >

The influence of morphology on electrical tree initiation in polyethylene under AC and impulse voltages

Abstract: AC and impulse voltages were applied to needle electrode specimens modeling the interface of the semiconducting layer and insulation of XLPE (cross-linked polyethylene) cables. TEM (transmission electron microscope) photographs of minute trees ( >

Detection of partial discharges (Corona) in electrical apparatus

Abstract: The development of the various partial-discharge test methods as applied to cables, capacitors transformers, and rotating machines is examined in an historical context. Special emphasis is placed on various discharge pulse analysis techniques, although it is pointed out that integrated charge measurements should receive greater attention, in particular as concerns their capability to respond to glow and pseudoglow discharges. >

Determination of electric field distribution in oil using the Kerr-effect technique after application of DC voltage

Abstract: A technique based on the Kerr electrooptic effect is used for the measurement of electric field strength in dielectric liquids such as transformer oil. An elliptically polarized laser beam is used as incident light and the applied DC voltage is modulated with an AC voltage. Using this technique, low-level electric fields are measured in liquids with small Kerr constants using a short electrode length. The change of electric field distribution with respect to time after application of a DC step voltage and the nonuniformity of the electric field in water-saturated transformer oil are also studied. The electric field in water-saturated transformer oil is shown to be distorted, and the charge density is shown to be about 1 nC/cm/sup 3/ when the applied step DC voltage is 2 kV. However, electric field distribution is found to be uniform in dry transformer oil, dry silicone oil, and water-saturated silicone oil. >

Progress in the field of electric properties of dielectric liquids

Abstract: It is noted that the past three decades have seen significant changes in the types of materials used for electrical insulation. These changes were brought about by the realization that some of the previously used materials were detrimental to the environment and to the people who came in contact with them. The use of polychlorinated biphenyls was prohibited, and replacement materials had to be developed. Paper was replaced to a large extent by polymer film, and paraffinic oils were finally used as replacement for oils derived from naphthene. At the same time progress was made in the understanding of the nature of electrical charges and how they move through dielectric fluids under the influence of externally applied electrical fields. The impact of these developments on the field of liquid insulating materials is discussed. Attention is also given to the information obtained with the latest ultra-high-speed electrooptical equipment. The generation and propagation of charge carriers are examined in detail, and their relation to electrical breakdown is analyzed. A brief survey of the special instruments used in these studies is presented, and the advantages and disadvantages of the various measuring techniques are discussed. >

The statistics of electrical tree inception

Abstract: A theoretical model is proposed to explain the statistics of electrical tree inception based on the geometry of the initiating electrode and the field variation around its surface. Numerical calculations show that the model is sufficient to describe qualitatively the observed features of both the time to inception distribution and the distribution of inception voltages (fields) obtained in ramp experiments. The implications of the model for the analysis of such experiments are discussed. >

Detection of Joule heating before dielectric breakdown in polyethylene films

Abstract: Local Joule heating in polyethylene films was detected at room temperature before dielectric breakdown under DC field. When a high-temperature point appeared in a film, the conduction current increased with time. The final breakdown occurred at the point of the highest temperature. This concurrence of the breakdown point and the Joule heating point suggests that a thermal process takes part in the DC electrical breakdown of polyethylene films at room temperature and that there are weak points leading to the breakdown in the film where the current density is above the average. Almost the same results were observed for ethylene-vinyl acetate copolymer films, although the local Joule heating and the electrical breakdown occurred at lower electric fields than those for polyethylene films. >

Surface electric strength of processed pressboard under composite AC and DC and conventional stresses

Abstract: In DC transmission systems the converter transformers operate under a superimposed DC bias. A knowledge of the surface strength of processed pressboard in mineral oil under superimposed AC and DC stresses is therefore essential. In this study, the surface strength has been determined experimentally for cylindrical specimens and for three gaps under the influence of superimposed stress in the range of ripple factor from 0.4 to 2.2. The surface strength has also been found under application of AC, DC, lightning and switching impulse voltages. The influence of the interface on the breakdown value of the gap has been found for each of the above types of stresses. It is shown that the surface strength is greatest under lightning impulse voltage, followed by switching impulse, DC and AC voltages. Under DC voltage with ripple the surface exceeds the DC value in the ripple range considered for the 10- and 25-mm gaps. The presence of the interface lowers the breakdown voltage except when either DC or superimposed stresses are applied. For the test configuration considered, the impulse ratio increases with gap length especially under application of lightning impulse voltages. >

The effect of pressure on streamer inception and propagation in liquid hydrocarbons

Abstract: High-speed electrooptical techniques are used to study prebreakdown behavior in n-hexane and toluene at pressures between 0.1 and 5 MPa. The results show that pressure inhibits the formation of low-density regions and increases prebreakdown inception voltage. The observed pressure effects suggest that the low-density region is the result of liquid vaporization near the electrode. Results obtained with a chopped voltage pulse indicate that the shape of the applied voltage is not critical to streamer growth. Streamers were observed growing after the voltage pulse had been removed, which suggests that the streamer growth process is: (1) to some extent locally controlled by the presence of injected charge, or (2) controlled by thermal expansion of the low-density region. It was also observed that the streamer would initiate, grow, and then disappear under impulse voltage conditions. >

Enhanced partial discharges due to temperature increase in the combined system of a solid-liquid dielectric

Abstract: Partial discharges which emanate from solid dielectrics immersed in insulating oil when high impulse voltages are applied under different environmental temperatures (20, 40 and 80 degrees C) are discussed. The solid dielectrics used for the measurements are phenol-impregnated pressboard and industrial bakelite. The samples are cut in wafers of 150 mm diameter and are 1 or 2 mm thick. The insulating oil is a typical transformer oil. The switching type of the impulse voltage used, 250/2500 mu s, corresponds to functional situations and is long enough to produce the effects that can be detected without the consideration of charge transfer phenomena due to fast rates of field changes. Experimental results show that the total charge transfer due to partial discharge is increased by temperature. A possible explanation of the undergoing physical process may be that in the low-field regime, temperature perturbation effects on the injected current are characterized by the conductivity changes in the volume of the dielectric, while as partial discharges start occurring, avalanche multiplication of conduction electrons appears to be the dominant phenomenon, characterizing the behavior of a metal/thick insulator/metal structure in the high-field regime. The circuits, measurement, and measuring procedures are discussed. >

Steady-state conduction in linear low-density polyethylene with Poole-lowered trap depth

Abstract: A theory of steady-state conduction of space-charge-limited carriers injected into linear low-density polyethylene with Poole-lowered trap depth is presented. Injected carriers are deeply trapped at crystalline-amorphous boundaries before the steady state is reached. At elevated temperatures, they are thermally excited to the conduction band in the crystalline regions, and then hop with field-independent mobility through the shallow band-tail states of the amorphous regions. A deep-trap-site separation of 2.8 nm, corresponding to 4.5*10/sup 19//cm/sup 3/, yields good agreement with experimental current-field characteristics for fields up to 5*10/sup 5/ V/cm in the temperature range 49 to 82.5 degrees C. The final equation for current I versus field F and temperature T equation shows that the activation energy of the detrapping and transport processes are additive on a semilog plot of I/T/sup 2/ versus 1/T. The total activation energy is 0.83 eV at 2*10/sup 5/ V/cm, in agreement with the results of other methods. >

Electrical properties of ceramic/polymer composites

Abstract: Three types of ceramic/polymer composites were prepared and their electrical, dielectric and pyroelectric properties analyzed. The composite PZT5/VDF-TrFE shows a significantly higher pyroelectric figure of merit than PZT (VDF=vinylidine fluoride, TrFE=trifluoroethylene). The dielectric loss of the ceramic/polymer composites is observed to be dominated by those of the polymer, whereas the ceramic phase has a significant contribution on the steady-state electrical conduction and low-frequency dielectric loss at high temperatures. >

The influence of morphology on the electrical breakdown strength of polypropylene film

Abstract: The relationship between the microscopic structure morphology and the electrical strength of polypropylene (PP) film used in PP-laminated paper (PPLP) in very high voltage oil-filled cable is studied by several methods including infrared spectrum, X-ray diffraction and dynamic mechanical spectra. It is found that the key to improve the breakdown strength, the tensile strength and the stability in oil for PP film is to increase its structure parameters, namely, its crystallinity, orientation and molecular weight. The interaction between PPLP and liquid dielectric is also studied. It is found that when it is immersed in dodecyl benzene at 100 degrees C, the strength of PP film decreases because the amorphous part dissolves and disorientation occurs. >

Decay of electrical charges on organic synthetic polymer surfaces

Abstract: Surface-charge decay of low-density polyethylene films is studied after corona charging of the polymer. Cross-over and other phenomena related to charge injection processes are not observed. The decay behavior may originate from an induced dielectric polarization in which the carriers are self-trapped or localized by the polarizing influence of their own charge. The equilibrium value of surface charge density can be established at long times by the classical laws of diffusion and field-independent drift processes. >

A short method of estimating lifetime of polypropylene film using step-stress tests

Abstract: Polypropylene films were tested for lifetimes under high electric fields, using both constant and step-stress methods. Various time intervals for the step-stress test and voltages for the constant stress test were selected. The inverse power law was used in the analysis of the experimental data, with the value of the power exponent determined from the step-stress test. This exponent was found similar to that obtained from the constant stress test. The lifetimes of polypropylene were then estimated from the step-stress test and were found to show good agreement with the experimental results of the constant stress test. The Weibull distribution was employed in the analysis of the experimental data. >

Study of space charge effects in polyethylene by thermal-pulse current techniques

Abstract: Effects of oxidation on space-charge formation, carrier injection, and electrical conduction in polyethylene (PE) are studied by using thermal pulse current and thermally stimulated current (TSC) techniques Oxidation increases the conduction current by a few orders of magnitude and negative homo space charge is formed near the cathode The introduction of an oxidized PE layer on the surface of unoxidized PE enhances both electron and hole injection The enhanced electron injection predominates over the hole injection and plays an important role in the increase of conduction with oxidation The negative homo space charge is released at approximately=40 degrees C and gives rise to a TSC peak >

Electric field around parallel DC and multi-phase AC transmission lines

Abstract: A new method is described for calculating the electrostatic field values around parallel DC and multiphase AC transmission lines. The new method is based on the charge simulation technique combined with the method of imaging, where the number and locations of the simulating charges are no longer arbitrary. The surface charge on each subconductor is simulated by the images of the remaining subconductors. The superposition technique is employed, and the RMS value of the electric field is calculated at any point in the space around a transmission line of any number of subconductors. Several examples of field calculation were carried out by the new method, showing good agreement with previous calculations and measurements. The merits of this method are simplicity in programming and considerable saving in computer time compared to classical field calculation techniques. >

An overview of dielectric measurements

Abstract: It is noted that, approximately 25 years ago, commercial electronic instruments were brought on the market that heralded a new era in dielectric measurements; this electronic era is now rapidly drawing to a close, and is on the verge of being eclipsed by an era that could be called intelligent instruments. The development of dielectric measurement technology before and after the founding of the Dielectrics and Electrical Insulation Society is discussed, with the main emphasis on measurement of permittivity and related matters in the lower range of frequencies. Attention is given to the interpretation of results and the instrumental errors may occur. >

Basic physics of insulators

Abstract: A number of areas of basic insulator physics that have made considerable steps forward the last few years are reviewed, and it shown how these have affected the understanding of practical dielectrics and methods of data handling. The topics covered include mathematical techniques, advances in the computation of electric fields and the theory of dielectrics and space charges, novel experimental techniques for carrier mobility and charge location, and changes in the view of composite materials and breakdown brought about by the introduction of percolation theory and the methods of chemical analysis. Speculation on the most fruitful directions for the future is included. >

Pulse-count distribution as a possible diagnostic tool for assessing the level of degradation of rotating machine insulation

Abstract: A method is presented by which one can qualitatively assess the condition of epoxy mica insulation systems used in large rotating machines as a function of time. Experimental techniques for acquisition and analysis of failure data are presented. Experiments were conducted on actual rotating machine coils subjected to combined electrical and thermal stress, under simulated conditions, so as to reflect the service environment as far as possible. Diagnostic measurements such as incremental loss tangent, capacitance change, partial discharge pulse count/magnitude, and PD energy were performed along with destructive measurements (dielectric strength). There appeared to be an excellent correlation between pulse count/magnitude and dielectric strength. This fact can be used in the assessment of the degree of degeneration of the insulation. >

Modification of XLPE exposed to partial discharges at elevated temperature

Abstract: Extensions are presented of research on partial-discharge-induced surface modifications of XLPE (crosslinked polyethylene) wherein the XLPE sample surface during corona exposure is maintained at an elevated temperature ranging up to 160 degrees C. As for the ambient temperature case, circular degradation patterns are found, but their appearance and composition depend strongly on temperature. Furthermore, oxidation within a shallow (several micrometers) surface layer is found to increase sharply with rising temperature, an effect attributed to inward diffusion of corona-produced oxidizing species, particularly ozone and nitrogen oxides. >