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Proceedings of the IEE - Part IIA: Insulating Materials

Issue 3 • Date March 1953

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Displaying Results 1 - 25 of 44
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  • Opening address

    Page(s): 3 - 4
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    First Page of the Article
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  • A spectrometer for millimetre wavelengths

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    A free-space method for the measurement of permittivity at millimetre wavelengths is described. This method utilizes the microwave analogue of the optical spectrometer, electromagnetic horns with lenses taking the place of the collimator and telescope of the optical instrument. At these wavelengths aperture dimensions are necessarily comparable with the radiated wavelength, and diffraction effects are pronounced. The design of the spectrometer and possible aperture illuminations are discussed using the concept of an angular spectrum of plane waves. Permittivities are measured using sheets of material on the spectrometer and studying the reflectivity with angle of incidence for different polarizations. Minima at which destructive interference occurs together with the Brewster angle are determined, and accurate values permittivity deduced. The effects of dielectric loss on the minima are investigated and are found to be small for normal values of loss tangent. The loss in the dielectric is assessed from the shape of the minima and by measurements of transmission coefficients. Diffraction effects on the measurement are discussed, whence it appears that the method should give identical results in both Fresnel and Fraunhofer regions of diffraction. Results for a whole range of specimen sizes and spacings in the instrument substantiate these conclusions, and show that the measurements of permittivity are accurate within ±0.5% in both regions of diffraction. An extension of the method to liquids is also described. View full abstract»

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  • Dielectric behaviour of granular semi-conducting aggregates, with special reference to some magnesium ferrites

    Page(s): 15 - 22
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    Aggregated granular semi-conductors can display high permittivities and dispersion effects which vary with temperature and voltage. This behaviour need not be characteristic of the granule material: can be a consequence of its conductivity and of a particular kind inhomogeneity of the aggregate arising from the contact structure the intergranular boundaries. The dielectric properties of certain sintered magnesium ferrites can be accounted for in this way. View full abstract»

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  • The dielectric losses in some representative insulating materials

    Page(s): 23 - 37
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    A detailed study is made of the permittivity and power factor of a number of insulating materials over the whole range of conditions encountered in electrical practice, including frequencies from 10 c/s to 24 000 Mc/s and temperatures from ¿35° C to +240° C in appropriate cases. The materials fall into the following groups: (a) Silicones and other silicon-oxygen structures including mica and glass. (b) Non-polar plastics including polystyrene, polyethylene, polytetrafluorethylene, etc. (c) Pure synthetic resins of the phenol-formaldehyde and anilineformaldehyde classes. (d) The polar plastic Kel F, a chlorinated derivative of the nonpolar polytetrafluorethylene. The main features of the dielectric behaviour of these groups are explained in terms of known features of their structures. View full abstract»

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  • Variation of capacitance with temperature in metallized-mica capacitors

    Page(s): 38 - 40
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    The temperature coefficient of capacitance of a metallized-mica capacitor varies with frequency as a result of relaxation processes, and is therefore correlated with the power factor of the mica. The temperature coefficient of commercial capacitors ranges at least from ¿40 to +100 parts in a million per degree centigrade at 1 kc/s; it may be more negative, by about 40 parts in a million per degree centigrade, at 1 Mc/s. There is usually a non-cyclic variation as well, which may be of the order of 0.1%. View full abstract»

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  • The dielectric properties of rubber

    Page(s): 41 - 45
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    The paper describes the results of measurements of the power factor and relative permittivity of purified, uncompounded, natural rubber within the temperature range from ¿50 to +40° C and within the frequency range from 50c/s to 1 Mc/s. The effect of exposure to water vapour on the dielectric properties is contrasted with the effect of a similar exposure on the properties of crude rubber. The methods of purification of the rubber and preparation of the test specimens are described, and the dielectric properties of vulcanized purified rubber are briefly indicated. The Introduction describes the nature of the rubber hydrocarbon and its natural impurities, and mentions the methods of compounding and vulcanizing rubber for electrical purposes. View full abstract»

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  • The dielectric properties of certain benzene derivatives in the solid state

    Page(s): 46 - 53
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    The paper gives the results of an investigation of the dielectric properties of two chlorine-methyl substituted benzenes, pentachlortoluene and 1,2,4-trimethyl 3,5,6-trichlorbenzene, and of pentaiodobenzene, over the frequency range 50¿4.8×1010c/s, and discusses the observed behaviour in terms of modern dielectric theory. It was thought possible that these materials would exhibit resonance absorption owing to rotational molecular oscillation at the upper end of this frequency range. This phenomenon was not observed, but study of the dipolar absorption in the chlorine substituted compounds showed this to be characterized by a very narrow band of relaxation times, approximating closely to a single relaxation time. A quantitative estimate of the breadth of the relaxation time distribution was made. Directional permittivities of crystalline pentachlortoluene made at low frequency and at optical frequencies showed this material to be strongly anisotropic. View full abstract»

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  • Some aspects of the deterioration of insulating fluids

    Page(s): 61 - 67
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    Acidity and resistivity measurements have been used to judge the condition of used oil and have revealed a few oils with conductivities which are abnormally high when compared with their low acidities. Examination of such oils by filtration, separation and chemical extraction, together with attempts to simulate the high conductivity by additions of pure substances to new oil, suggested that low resistivity was associated with high iron content. To determine the extent of the problem a survey was carried out on oil samples from Grid and generator transformers with ages ranging between one and 24 years. A statistical analysis of the resistivities, acidities and iron contents of these oils showed that the combined presence of iron compounds and acids, which is more often encountered generator transformers, can give rise to high conductivity. Synthetic insulating fluids, and particularly the chlorinated diphenyls, may be used where low fire risk or high operating temperatures are required. Brief mention is made of a laboratory investigation of these substances which gives some idea of the problem involved when hydrochloric acid is released by a spark occurring in them. View full abstract»

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  • The measurement of the power factors of insulating oils at 50 c/s

    Page(s): 68 - 72
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    A modified Schering bridge which, whilst simple in operation, will measure the power factors of insulating oils with a high order of accuracy has been designed and constructed. The power factors of insulating oils are often very low, e.g. 0.00005 or less at room temperature, and considerable care must therefore be taken to avoid small errors which may arise from deficiencies in the electrical circuit, the oil test cell and the measuring technique. An oil test cell available commercially has been modified for use in an oil bath, so that accurate temperature stabilization is rapidly attained, and a method of cleaning and filling the cell has been evolved. Details of cell corrections are given. The procedure described in the paper enables the 50-c/s power factor of an oil to be measured with an error of approximately ±3% ±0.000015. View full abstract»

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  • Effect of humid air on power loss in pure and impure paraffin oil

    Page(s): 73 - 80
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    Measurements were made over the frequency range 50 c/s¿30 Mc/s on a very pure paraffinic oil, and on the same oil containing a few parts per thousand of an ester of the type used in the plasticizer trade. Neither displayed a measurable power loss in the low-frequency region, the oil containing the ester showing the usual dipole loss in the megacycle region. Treatment with humid air had no effect on the pure oil, but produced a serious low-frequency power loss in the oil containing an ester. This loss was proportional to the square root of the ester content and increased rapidly as the voltage was raised from 50 to 250 volts on a film 0.025 cm thick (2 to 10 kV/cm). The conductivity corresponding to this power loss is roughly constant from 100 to 1000 c/s and falls rapidly to a low value around 3000 c/s. View full abstract»

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  • Dielectric properties of oil-soaked pressboard as affected by water

    Page(s): 81 - 88
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    The effect of water, in amounts capable of being absorbed from a normal atmosphere, upon the dielectric properties of oil-soaked pressboard has been investigated over the temperature range from 20° to 90° C. Curves relating loss tangent, permittivity and electric breakdown strength of 0.2-in thick material with absorbed moisture and temperature have been plotted, and from these a final relationship between dielectric loss and breakdown strength has been obtained. With the data provided it is possible to make a fair estimate of the one-minute breakdown value of the material for any temperature between 20° and 90° C from a dielectric-loss measurement made at a small fraction of the breakdown voltage. Abnormalities in the loss-tangent/temperature characteristics resulting from the unequal distribution and subsequent migration of the absorbed water¿effects likely to be experienced when moisture is absorbed after oil impregnation¿are described. Brief reference is made to the implication of these findings in connection with the testing of the applied insulation. View full abstract»

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  • Dielectric loss of insulating oil

    Page(s): 89 - 93
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    Considerable discrepancies between the results of different laboratories when measuring the dielectric loss of the same insulating oils has been reported in co-operative tests for C.I.G.R.É., Second Series, 1949¿50. These can arise either from contamination of the oil sample with impurities or from the instability of the oil. Errors due to the first cause have been rendered negligible by evolving a suitable technique for cleaning and handling the commercially available test cells. Instability of an oil, during the course of a measurement, is not so readily controlled, depending as it does on the composition of the oil. It is shown that, with an oil complying with B.S. 148: 1951, oxidation can cause the dielectric loss to fall to a small fraction of its initial value during the time required by some test cells to attain thermal equilibrium at 109° C. This factor can render meaningless exact values of the dielectric loss of an oil at high temperatures in the neighbourhood of 100° C. View full abstract»

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  • An experimental investigation of factors influencing the electric strength of potassium chloride crystals

    Page(s): 105 - 110
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    Experiments with single crystals are described, showing that the electric strength of potassium chloride is not significantly affected by the choice of electrode material. It is, however, considerably increased by treatments that introduce the structural defects, associated with residual strains, caused by plastic deformation. This does not seem to have been appreciated in the past. Structural defects of this sort can be created very easily, and great care is therefore required to obtain measurements of electric strength for strain-free crystals. It is speculated that this has not yet been done for crystals of any of the alkali halides. The electric strengths of strain-free crystals are the values suitable for comparison with existing theoretical predictions, and they may be as much as 50% less than those usually quoted for the alkali halides at room temperature. View full abstract»

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  • Some mechanisms of insulation failure

    Page(s): 111 - 120
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    The paper reports recent investigations into mechanisms of insulation failure. After a review of recent progress emphasizing that failure depends on a number of distinct factors which can be separated by suitable techniques, experimental results are given covering some theories recently established. These include: (a) Electrochemical attack on capacitor dielectrics, with results of life tests on capacitors stabilized with a hydrogen acceptor. (b) Discharge inception in oil-impregnated paper insulation, with results showing that the inception voltage is raised by addition of a hydrogen acceptor to the oil. (c) Failure in thin polystyrene films due to the action of oily particles. (d) Local thermal instability in high-permittivity ceramics due to stress concentration at electrode edges. At least five general causes of failure are now clearly distinguishable, and there is strong evidence for two others which are under investigation. The conclusion is drawn that working stresses in insulation can usually be increased when the cause of failure is known in detail and the design modified accordingly. Attention is drawn to the need for close collaboration between laboratory workers investigating causes of failure and design or development staff. View full abstract»

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  • Electric discharges in air-gaps facing solid insulation in high-voltage equipment

    Page(s): 121 - 131
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    The performance of high-voltage alternators under conditions of over-voltage is materially improved by the application of an insulating sheathing to exposed parts of the bare earthed metal. The authors first derive the physical explanation for this observation from experiments using auto-photographic recording methods. These experiments are then expanded to cover other phenomena observed with discharges between insulated electrodes and lead to problems of discharges in voids. In particular, two hitherto unexplained phenomena are considered: the increase of discharge current with rising voltage, and the origin of the long discharge-free range in the decaying part of alternating voltages. Both these observations can be explained by the loss of some of the surface charges left from earlier discharges due to local reversals of the electric field. Some of the explanations of the probable mechanism by which charges are lost must still be considered as tentative, and more experimental evidence will be needed for a complete clarification of the processes involved. The results give some information on the effect of harmonics on internal discharges, which have been observed to lead to accelerated ageing of dielectrics. View full abstract»

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  • The electrical breakdown of air between insulators

    Page(s): 132 - 137
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    By the use of a cathode-ray tube it is shown that the a.c. breakdown current in a glass vessel with external electrodes, containing air at reduced pressures, consists of a number of separate pulses, the number in a half-cycle increasing with the excess voltage. The current flows in pulses because once a breakdown occurs in the gas, charges collect on the glass walls and produce reverse fields which stop the discharge. These wall-charge fields also determine the phase when the pulses occur. Ionization by collision of the molecules of the gas and secondary emission of electrons from the inner surfaces of the glass walls are the fundamental processes in the discharge. The breakdown conditions are shown to be similar to a d.c. discharge between metal electrodes. A theory is given explaining the shape of the pulses and their peak currents for different voltages and pressures. View full abstract»

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  • Electrical breakdown in vacuum

    Page(s): 138 - 140
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    Direct and impulse voltages were applied to an electrode system in a vacuum of the order of 10¿5 mm Hg. Of particular interest were the results obtained with a needle-and-sphere system, the breakdown voltage of which was considerably higher than that of a normal sphere-gap with the same electrode spacing, especially when a very fine needle was used. Oscillograms were taken of the current during a breakdown in vacuum, and of the collapse of voltage across the gap. With the fastest time-sweep available the current rise and voltage collapse were found to take place in 0.1 microsec. A longer sweep demonstrated that there is a minimum stable current below which a continuous breakdown will not occur; if circuit conditions limit the current to below this value, it will flow only intermittently. This may account for the difficulty experienced by some workers in making measurements of breakdown in vacuum with direct voltages. View full abstract»

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  • Electrical breakdown in organic liquids

    Page(s): 141 - 148
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    The dispersion in the measurements of the electric strength of pure hydrocarbon liquids using direct voltage is significantly reduced using a hydrogen thyratron by-passing circuit. The damage due to discharge, mainly produced on the anode, is much less, and many more measurements on a particular sample are possible. Experiments with n-hexane show that the breakdown is practically independent of both electrode material and external radiation uniform fields but that it is dependent on the cathode gradient when the field is non-uniform. The electric strengths of the paraffins, pentane to decane, at room temperature increase with chain length and with decreasing gap. The strength is higher at low temperatures but decreases rapidly near boiling point. Cyclohexane and benzene show a different temperature dependence. A possible mechanism of breakdown in these liquids, in accordance with the experimental results, is suggested. This involves the formation of an electron avalanche in the liquid and subsequent bombardment of the anode. The molecules of the liquid provide a vibrational barrier to acceleration of the electrons, this barrier being influenced by molecular orientation at very high fields. View full abstract»

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  • Breakdown of insulation by discharges

    Page(s): 149 - 158
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    The principal factors limiting the safe working electric strength of modern low-loss insulation are electrochemical deterioration and breakdown caused by internal or surface discharges. In the paper the mechanisms of deterioration and breakdown by discharges are discussed in detail. Special consideration is given to the effects of the amplitude, frequency and waveform of the applied voltage on the discharge sequence and on the rate of dielectric deterioration. Suggestions are made for supplementary industrial tests to assess the resistance of materials to internal and surface discharges. View full abstract»

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  • The a.c. breakdown of polythene and polystyrene

    Page(s): 159 - 162
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    The electric strength of polythene when subjected to 50-c/s alternating electric stress has been measured for a range of ambient temperatures from ¿196° C to 17° C, and that of polystyrene for temperatures between ¿196° C and 70° C. The a.c. breakdown strengths are found to be lower than the corresponding intrinsic electric strengths at room temperature and above, particularly with polythene, but no difference is observed at low temperatures. In measurements in which this lowering occurred, the results were dependent on the duration of the stress application. It is concluded that an increase in the internal temperature of the test specimens can explain these results. View full abstract»

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