Showing papers in "IEEE Transactions on Electrical Insulation in 1989"

Surface flashover of insulators

Abstract: The author reviews surface flashover (i.e. voltage breakdown along the surface of insulators), primarily in vacuum. He discusses theories and models relating to surface flashover and pertinent experimental results. Surface flashover of insulators in vacuum generally is initiated by the emission of electrons from the cathode triple junction (the region where the electrode, insulator, and vacuum meet). These electrons usually then multiply as they traverse the insulator surface, either as a surface secondary-electron-emission avalanche or as an electron cascade in a thin surface layer, causing desorption of gas which had been adsorbed on the insulator surface. This desorbed gas is then ionized, which leads to surface flashover of the insulator. The theory and modeling of this phenomena and experimental studies of surface charging, the applied voltage waveform, prestressing, conditioning, discharge delay and speed, insulator geometry AMD material, surface treatment, surface gases, temperature, and pressure are reviewed. Some suggestions are made regarding how to choose the material geometry and processing when selecting an insulator for a particular application. >

A review of the charge simulation method and its applications

Abstract: The author provides a comprehensive review of the basic charge simulation method and its various modified versions reported in the literature. Applications of the charge simulation method, alone as well as in combination with other methods, are considered. Different aspects of this method are critically examined and its potentials and limitations are identified. Possible areas of future research are outlined in order to improve the overall effectiveness of computer-aided analysis and design of high-voltage insulation devices by using the charge simulation method. >

Piezoelectricity and pyroelectricity in polymers

Abstract: Piezoelectric properties of polymeric substances arise from either the poling-induced orientation of molecular dipoles (class 1) or the uniaxial orientation of chiral chain molecules (class 2). The class 1 polymers include ferroelectric polymers, polar polymers with frozen-in dipole orientation, and the composites of polymers with ferroelectric ceramics. Details of the poling procedure in representative materials are described, and optimum poling conditions are suggested. Resulting piezoelectric and pyroelectric activities are specified as a function of remnant polarization, and underlying basic mechanisms are summarized. The class 2 polymers consist of drawn optically active polymers and exhibit face-shear piezoelectricity appearing in proportion to the orientation coefficient of chain axes. The spontaneous polarization of ferroelectric liquid-crystalline polymers is attributable to this type of piezoelectricity coupled with the spontaneous strain. >

Partial discharges in ellipsoidal and spheroidal voids

Abstract: Transients associated with partial discharges in voids can be described in terms of the charges induced on the terminal electrodes of the system. The relationship between the induced charge and the properties which are usually measured is discussed. The method is illustrated by applying it to a spheroidal void located in a simple disk-type spacer. >

The mechanism of leader breakdown in electronegative gases

Abstract: A survey is given of the present state of understanding of leader breakdown of electronegative gases in nonuniform field gaps and under fast-rising voltage waveforms. The basic physical processes involved are the formation of a pulsed streamer corona, the transformation of one of the streamers into a leader step, the temporal development of the leader channel, and the stepped propagation of the leader through the gap. The growth and structural characteristics of the corona are modeled and the results obtained are used to derive simple approximations for the spatial corona extension and the corona charge. Two leader inception mechanisms are discussed, namely, the stem and the precursor mechanisms. Inception criteria and characteristic time scales are derived for both of them in terms of the experimental parameters. A simplified model, which also predicts the random aspects of leader propagation through the gap, is presented. Some typical gas-insulation design problems are treated as examples. >

On the electrical breakdown of gaseous dielectrics-an engineering approach

Abstract: Following an examination of the basic gas-discharge concepts, the Townsend breakdown criterion is derived for nonuniform fields and a reformulation of the streamer criterion is undertaken. A direct application of the Townsend criterion to a practical situation is hardly possible, whereas the streamer criterion leads to applicable criteria for the onset of breakdown in air as well as in strongly electronegative gases. In this approach, a knowledge of the relevant ionization coefficients is not required, because these are replaced by parameters obtained from uniform-field Paschen curve data. In addition, the criteria developed do not contain any arbitrary constants. By introducing the electrode surface mean curvature, it is shown that the calculation of breakdown data for widely different electrode shapes can be addressed through a single formula. It is emphasized that the inherent roughness of practical electrode surface demands that caution be exercised in all design criteria. >

Degradation of polymeric insulation due to photoemission caused by high electric fields

Abstract: The mechanism of electrical tree initiation in polyethylene subjected at AC, DC, and half-rectified AC voltages is studied by an optical technique which is at least two orders of magnitude more sensitive than partial discharge detection. For DC voltage up to 46 kV, no light emission or tree inception was detected in the polymer containing the normal and decreased concentrations of air in its free volume. For AC and half-rectified AC voltages, tree inception only occurred if light, in the visible and ultraviolet ranges, was detected. Photodegradation of the polymeric insulation by ultraviolet radiation can occur when the voltage applied to the polymer exceeds the threshold voltage of light inception such as during switching and lightning overvoltages. This can account for the gradual degradation of the polymeric insulation which ultimately leads to the formation of an electrical tree. By comparing the light emitted from the polymer subjected to AC voltage having half-cycles of different amplitudes to DC superimposed on AC it is shown that the space charge injected into the polymer causes the zero-crossing of the local field in the polymer. This is required for light emission and subsequent tree inception. >

Dielectric effects of moisture in polyimide

Abstract: Measurements of the complex permittivity of polyimide (PI) films in a temperature range from 80 to 325 K are reported. Two loss peaks show the pronounced influence of absorbed water on this part of the relaxation spectrum. Since these loss peaks can be removed by drying the film, it is concluded that the water dipole causes this relaxation process and not an intrinsic dipole of the PI chain. Different sites in neighboring PI chains are proposed to explain the two loss peaks and the temperature dependence of the relaxation strength. The activation energies expected for the proposed sites agree well with the activation energies determined by the measurements. The relaxational strength calculated by assuming noninteracting water molecules which relax as point defects is in good agreement with the measurements. A simple model, the hindered rotation of the water dipoles, is able to explain all the features of the measured loss curves without requiring complicated movements of chain segments. >

Dynamics of polarization growth and reversal in PVDF films

Abstract: Measurements are presented of the time development of the dielectric displacement and the remanent polarization in polyvinylidene fluoride (PVDF) for poling times ranging from 1 mu s to 1000 s and poling fields between 0.8 and 2.0 MV/cm. For longer times (0.1 to 1000 s), the time dependence of the polarization distribution across the film thickness is also determined. After application of a steep rectangular high-voltage pulse, the sample is shorted to zero voltage. The remanent polarization under the short-circuit conditions is compared to the maximum dielectric displacement under the external poling field. A significant time delay of the buildup of the remanent polarization was observed as compared to the dielectric displacement under field. This time delay depends significantly on the applied field strength and the crystallinity of the films. In the case of polarization reversal, a 'flipping back' of the polarization was observed for shorter poling times of up to 200 mu s. Under these conditions, a large part of the polarization is reversed under the field, but after the removal of the field, most of the polarization returns to the original direction. The results can be explained by the ferroelectric cooperative coupling of oriented crystallite dipoles to charges trapped at the surface of polarized crystallites. >

Application of digital filtering techniques to the determination of partial discharge location in transformers

Abstract: The capacitative network representation of transformer windings has long been used for the study of pulse distribution along the windings. However, the author's test results and those published in the literature show that theoretical analysis based on this equivalent network may not be applicable. It was found that the capacitive network approximations were valid only within certain ranges of frequency. The frequency ranges were determined through the analysis of terminal measurements. The capacitatively transferred components of the input pulse along the windings in the appropriate frequency range were then extracted by the aid of digital filtering techniques, and good agreement between measured and theoretical distributions was obtained. These techniques, in conjunction with a new straight-line interpolation method, were employed to locate simulated partial discharges in a 66 and 132 kV transformer winding. Measurement results showed that an accuracy of better than 5% of winding length was obtained. >

Semiconductor-based electret sensors for sound and pressure

Abstract: The theory and experimental results for integrated electret-based silicon sensors for the detection of sound and pressure are presented. A silicon electret microphone for use in hearing-aids is described. It has an experimentally determined sensitivity of 19 mV/Pa in the frequency range of 50 Hz to 10 kHz. The realization of a pressure sensor for application in catheter-tip blood-pressure measurements is discussed. This type of electret sensor appeared to have a pressure sensitivity of 1.6 mu V/Pa in the required frequency range of 0 to 100 Hz. >

Dielectric small-signal response by protons in amorphous insulators

Abstract: A model is proposed in which protons fluctuate between neighboring atomic shells either by thermal activation or by a tunneling process. The transition probability of the protons depends on the distance between the adjacent atoms. The characteristic pair distribution function of interatomic distances in the amorphous state is a consequence of locally minimizing the potential energy of the atoms during condensation, as has been shown by F.F. Abraham (1980). This general principle of minimizing the energy leads to similar distribution functions in different materials. Therefore, different types of insulators have a similar relaxational behavior. Experiments carried out with thin films of aluminum oxide and polyimide confirm quantitatively the proposed model. >

Electrical properties of electrode/polyethylene/electrode structures

Abstract: The pressure-wave propagation (PWP) method allows for the nondestructive measurement of charge distributions in dielectric materials. This method has been used to study the electrical properties of electrode/polyethylene/electrode structures such as those involved in high-voltage cables. The influence of the composition of the insulating resin itself and that of the electrodes are analyzed in polyethylene samples. According to the chosen combination, charge transfer at the interfaces, migration of ionizable impurities, or a strong decrease of both is observed. This application of the PWP method is of particular interest, since it allows for a suitable choice of the materials and structures involved in insulator/conductor interfaces. >

A probabilistic life model for insulating materials showing electrical thresholds

Abstract: A probabilistic life model for insulating materials showing lifelines for combined thermal-electrical stresses which tend towards electrical thresholds is proposed. The model is obtained by starting from the two-parameter Weibull distribution of times to failure, and explaining scale and shape parameters so that time-to-failure percentiles agree with life curves showing an upward curvature and a tendency to infinite life. The parameters of the probabilistic life model are explained as a function of stresses, thus providing a complete characterization of the combined-stress endurance and, hence, estimation of electrical threshold at each selected temperature, including service stresses. The validity of the model is verified by means of experimental results from a test cycle carried out on XLPE (cross-linked polyethylene)-insulated cable models. >

An assessment of the physical basis for the application of design criteria for dielectric structures

Abstract: The use of design criteria for assessing the stresses to which composite dielectric structures are subjected implies careful and informed application. The convention required in the application of these criteria is examined in the light of the physical processes expected in an oil/cellulose transformer structure and related to an equivalent stressed-oil-volume theory of breakdown. The magnitude of the stresses implied by such criteria has been gauged from an examination of a number of case studies in which finite-element analysis has been used to determine the cumulative stress characteristic. >

Field-enhancing defects in polymeric insulators causing dielectric aging

Abstract: Defects that produce strong local field enhancement are discussed. They are especially harmful because they do not produce partial discharges detectable in usual factory control tests. Typical examples for such defects are electrode protrusions, floating conducting particles, and the tips of electrical trees. Charge injection from field enhancement tips is shown to be the first step in a series of pretreeing events. In model experiments using a needle/plate arrangement, a field threshold for injection is observed, separating a low-field regime of 'infinite' lifetime for the dielectric from a high-field regime where treeing will almost inevitably set in after some incubation time. This incubation time is shown to depend very sensitively on how much the applied voltage exceeds the value at which the critical field is reached locally. The value of this threshold field is in the range of megavolts per centimeter for pure or filled homogeneous insulators and about two orders of magnitude lower in compound insulators containing (bad) interfaces. >

Hygroscopic properties of pollutants on HV insulators

Abstract: The mechanisms of moisture absorption by an insulator surface contaminated with different salts is described A list of some salts depending on their hygroscopicity is given Measurements of surface conductivity and leakage current on flat insulator models at different relative humidities and with different types of salt were carried out At natural atmospheric conditions, the flashover voltage of cap and pin insulators subjected to 250/2500 mu s switching impulses was evaluated It is shown that the intensity of moisture absorption on insulator surfaces depends upon the chemical constitution of the pollution The effect of intense and continuous moisture absorption can cause long-lasting surface discharges at operating voltage or can be a reason for flashovers a short time after the voltage has been switched on to a line >

Effect of crystallinity on charge storage in polypropylene and polyethylene

Abstract: The improvement of charge storage with annealing, the use of different cooling rates, and stretching in polypropylene (PP) and polyethylene (PE) is discussed. In annealed PP and PE storage increases linearly with both crystallinity and crystallite size. The half-value charge decay temperature can be used as a measure of the latter parameters. Annealing and cooling rate affect the rate of crystalline growth. Changes occur in the physical and/or chemical nature, the concentration of defects in the crystalline region, and the concentration of traps at chain fold-amorphous interfaces. Annealing 4:1 stretched PP film at 140 degrees C gives a half-value charge decay temperature of 152 degrees C, approximately=70 degrees C higher than that for unannealed, unstretched film. Stretching increases amorphous content creating new boundaries, decreases crystallite size, and creates defect traps. >

Prebreakdown currents: Basic interpretation and time-resolved measurements

Abstract: The relation between the motion of charges in an electrode gap and the current induced in the external circuit is described on the basis of energy considerations and on the basis of the Ramo-Shockley theory. It is shown that both approaches are applicable, and mutually consistent, not only for gas-filled gaps but, if applied properly, also when the gap is partially filled with a solid dielectric. Energy considerations are especially useful for two-electrode gaps. If more than two electrodes are involved, the Ramo-Shockley theory has to be invoked. Based on the theoretical description, fast measuring systems with a subdivided measuring electrode can be constructed (typically 400 MHz bandwidth, 1 ns risetime). Fundamental and electrotechnical bandwidth limitations are discussed. The derived techniques and insights are applied to perform and interpret time-resolved measurements of currents induced by avalanches in homogeneous fields corona discharges in inhomogeneous fields and partial discharges in voids. >

Statistical properties of electrical breakdown in vacuum

Abstract: The breakdown probability distribution function was determined for DC and impulse voltages, for conditions of gap pressure of 10, 0.1, and 10/sup -4/ Pa and gap length of 0.1 mm. It was found that DC and impulse voltages are associated with different types of probability distribution function due to different initiation mechanisms. The statistical influence of the number of previous breakdowns on the probability distribution of the breakdown voltage was investigated. By applying the U test for analysis of measured data, it was found that at lower gap pressure, the breakdown voltage probability changes after a smaller number of breakdowns than it does at higher gap pressure. >

A study on the profile of HVDC insulators-DC flashover performance

Abstract: The results of flashover tests on insulator models with various geometrical configurations under DC field are presented. Dimensional parameters are used to describe the influence of the insulator profile on the flashover performance. The effects of both the surface contamination level and the geometrical shape on the arc propagation characteristics are investigated. It is shown that the flashover voltage of an insulator model under DC field is significantly influenced by the relative width and depth of the insulator underrib. It is also shown that the arc path under DC field is a function of both the geometry and surface contamination distribution. >

Charging, long-term stability, and TSD measurements of SiO/sub 2/ electrets

Abstract: SiO/sub 2/ layers with thicknesses of 150 nm, 1 mu m, and 2.5 mu m were either thermally grown or made by chemical vapor deposition on silicon wafers. Negative and positive charging of the free SiO/sub 2/ surface was performed by means of liquid-contact, corona, and electron-beam methods. Partially penetrating electrons were used to charge the volume of 2.5- mu m-thick SiO/sub 2/ layers. The good charge stability of negatively charged SiO/sub 2/ is demonstrated in isothermal measurements at room temperature and at 350 degrees C as well as in TSD experiments. In the isothermal measurements, positively charged samples showed a somewhat faster decay than negatively charged ones. The decay curves of negatively charged samples at elevated temperature can be interpreted with a simple theory based on carrier drift and compensation by conductivity. In the measured TSD spectra, the difference between differently charged samples is that the main peak of a positively charged sample appears at a much lower temperature than the peak of a negatively charged one. Samples positively charged with the corona or liquid contact method also show lower peak performance temperatures than samples positively charged with the electron-beam method. >

Aging of multistressed polymeric insulators

Abstract: Presents a comprehensive bibliographic survey pertaining to multi-stress-related degradation and aging of polymers with principal focus on their use as HV (high-voltage) insulators. The papers in the survey are arranged in the order of the year of publication, with most recent first and data as far back as 1948. However, the literature search for the 1980s was more intense than that for the earlier years. This is because most older literature is always referred to in the recent papers. The survey listings do not discriminate between simultaneously and sequentially applied stresses, or do they preclude papers in which only one externally applied stress induces other stress factors. The survey is intended to help the researchers in academia and industry, both in making an assessment of the current status of the particular research area(s) of their interest and in seeking directions in which to proceed. >

Subminiature silicon integrated electret capacitor microphone

Abstract: Microphones were prepared that use one silicon wafer to support a thin polyester diaphragm and a second to carry a Teflon or silicon dioxide electret. Subassemblies are diced from the wafer and bonded together to form complete microphones. The preamplifier circuit can be carried on either subassembly. Microphones with reasonable signal-to-noise ratios can be obtained with edge dimensions of 3 mm or less. The electroacoustic properties of prototype units are described. >

Space charges and initiation of electrical trees

Abstract: On the basis of experiments, a model is proposed for the initiation of electrical trees within voidless PMMA (polymethyl methacrylate) samples in a divergent field. Initiation of electrical trees under a DC applied voltage is due to the localized field intensity overcoming the intrinsic breakdown strength of dielectrics. However, for AC or pulsating voltages, as a result of accumulated charges, trees are initiated at lower voltages, and in general there is an incubation period before the trees appear. It is suggested that space charges play an important role in the initiation of trees under different conditions based on experiments of electron injection. It is concluded that all experimental results on tree initiation can be interpreted satisfactorily in accordance with the proposed model. >

Investigation of transformer oil electrification in a spinning disk system

Abstract: The author presents a novel measurement system for testing electrification of transformer oil when moving a solid phase in a liquid-phase environment. The system is based on hydrodynamic phenomena occurring for a rotating disc in liquid. It is concluded that the spinning disc system is useful for examining the electrification of transformer oil. The results obtained, particularly those concerning the effect of oil aging electrification, agree with the results obtained by means of A. Klinkenberg's (1958) system. >

Breakdown of alumina RF windows

Abstract: The breakdown mechanism of RF windows has been investigated. The localized multipactor electron bombardment, accompanied by rapidly growing alumina luminescence leading to the surface melting, were observed by the resonant ring test. The puncture is found to be induced by surface melting. TiN coating on the alumina disk was confirmed to suppress secondary electron emission and to prevent multipactor bombardment. >

Spark decomposition of SF/sub 6//H/sub 2/O mixtures

Abstract: The spark decomposition of mixtures of SF/sub 6/ (partial pressure from 100 to 500 kPa) and water (concentrations between 600 and 2000 VPM) has been studied. The sparks were generated between a point (stainless steel) and a plane (aluminum) either under 50-Hz AC voltage or by discharging a capacitor. Owing to the high water content the only stable gaseous oxyfluorides detected were, in order of decreasing importance, SOF/sub 2/ and SO/sub 2/F/sub 2/. The yields of these two byproducts have been measured as a function of the number of sparks, the SF/sub 6/ pressure, and the H/sub 2/O pressure. When the sparks additionally struck a solid insulator (polytetrafluoroethylene or biphenolic resin), CF/sub 4/ was detected. Its appearance was paralleled by a sharp increase in the yield of SOF/sub 2/, whereas that of SO/sub 2/F/sub 2/ remained unchanged. >

Shear stress detection in an elastic layer by a piezoelectric polymer tactile sensor

Abstract: A polyvinylidene fluoride (PVDF) sensor sandwiched between two elastic layers fixed to a rigid foundation is shown to be able to resolve a shear stress component produced by loading the upper layer's surface with a rigid indenter. When the measured charge response of the sensor is compared to that predicted by analytical solution of the corresponding axisymmetric elastic contact problem, a significant qualitative agreement between the two responses results. The experimentally determinated sensitivity, on the other hand, is found to be higher than that predicted analytically. Possible causes of this discrepancy are discussed. >

Electrohydrodynamic traveling-wave pumping of homogeneous semi-insulating liquids

Abstract: Charge separation near a liquid/solid interface is explored by the electrohydrodynamic pumping of Freon TF, transformer oil, and silicone oil using high-voltage traveling waves. Liquid head displacement measurements show strong dependences on peak voltage amplitude (1.0-2.5 kV), frequency (0.1-20 Hz), liquid viscosity (0.5-20 cS), and liquid conductivity (10/sup -13/-10/sup -9/ Omega /sup -1/ m/sup -1/). Reverse pumping, opposite to the traveling wave direction, occurs when the ratio of the voltage to the frequency is small but switches to forward pumping when the ratio is large. The largest measured head was approximately 7 cm, which can drive an average flow velocity of 18 cm/s. A charge transport analysis coupled to viscosity-dominated flow is used to describe the pumping process. The time-averaged coulombic force on the migrating charge can result in pumping in either direction. In the regime where ion migration dominates charge relaxation, a universal curve for the pumping dependence on traveling-wave voltage and frequency is deduced experimentally and predicted by the model. Measurements of average volume charge density show that undoped liquids have a positive net charge density, whereas additives used to increase the conductivity lead to bipolar injection and an augmentation of the injection as well as the conductivity. >