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The 60-cycle breakdown strength of good quality transformer oil has been shown to depend upon electrode size for a physical area ratio of 20 to 1 and for an additional ratio of 400 to 1 by means of a “group-of-minima” inference (1). The extremal nature of the breakdown distribution found to exist by these tests was also found to apply to impulse breakdown (2).
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An experimental technique, known as the gassing of oils; has been used to study the mechanism of the degradation of insulating oils under electrical stress. The apparatus consists of a cell which contains electrodes which are completely immersed in insulating oil. As a result of the application of an ac voltage to the electrodes, gaseous decomposition products are formed. The cell is enclosed in a...
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In a previous paper (1), the necessary experimental and theoretical basis was established for a discussion of the effect of electrode area on the 60-cycle breakdown of transformer oil. A study of 1600 observations established the extremal nature (2) of such breakdown distributions and yielded an equation relating area and voltage which agreed closely with test results. This equation is: VA1 − VA2...
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As shown in Figure 1, the positive impulse breakdown voltage of SF6 in non-uniform fields can be lower than the positive direct voltage breakdown over a pressure region. Also, in this pressure region, the positive direct voltage required for breakdown goes through a maximum. It is believed that these two characteristics (impulse ratio <1; a maximum in the direct voltage vs. pressure curve) are...
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The low-frequency dielectric dispersion of tolyl xylyl sulfone, pentachlorobiphenyl and hexachlorobiphenyl follow the Davidson-Cole equation for “skewed arc”: ε∗ = ε∞ + ε0 − ε∞/(1+i∞τ0)β (1) rather than the Cole-Cole equation for “semicircular arc with depressed center”. β lay in the range 0.50–0.65. This behavior (with β in the range 0.55–0.65) had previously been observed only for glycerol and propylene...
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The dielectric constant and loss of sintered disks of some stannates and cerates were measured against frequency and temperature in the ranges 100 cps −1 Mc and −196°C to about 400°C. The values of K and tan 5 measured on several specimens of each oxide at room temperature and 1 Mc are given in the following-table, with no correction for ceramic density.
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Discs of borosilicate glass of 1,75 cm thickness were irradiated with 2 MEV electrons. Total intercepted charge was 4, 6 × 10−6 coul/cm2. Irradiation (a) produces formation of color centers and (b) introduces a layer of high negative charge, which is situated in the glass at a depth corresponding to the range of the electrons (0,3 cm). The existence of the charge is put into evidence by an experiment,...
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About a year and a half ago work was initiated at Mica Insulator Co. to compare the thermal stability of a new flexible micaceous insulation (Isomica∗∗∗) with other standard commercially available materials. The aim was a simple screening test, of the type described by Mathes1 as the “first rung on the ladder of insulation evaluation”, which could indicate the relative stabilities of simple insulations...
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Silicone fluids have numerous electrical applications in both energy conversion equipment and communications type apparatus. To aid engineers in the application of these fluids and to determine the general characteristics of these dielectrics, a study was made of their electrical properties. The measurements were made on fluids of commercial purity.
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Most available dielectric strength data for polyethylene apply to laboratory-prepared specimens that have been tested with very special electrodes under highly artificial test conditions. This paper reports certain of the results of extensive impulse tests performed on commercial polyethylene with plane electrodes that approximate the conditions found in practice.
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The paper presents some breakdown phenomena which take place in a uniform field gap (Rogowski gap) with an auxiliary electrode. This constitutes a part on an investigation to find a new method for determining the voltage distribution in the gap immediately prior to breakdown.
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Polychlorotrifluoroethylene is a linear straight-chain polymeric material with the chemical structure (Cl -C F — F C- F)n. It is well known that this material can be crystallized and that the melting point of the crystals is close to 215°C(1,2). Above this temperature, the polymer is in the liquid or “rubbery” state. On quenching the material very rapidly from the liquid state, crystallization can...
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It has been found that a number of commercial waxes exhibit singularities in the temperature variation of the D.C. conductivity at temperatures below the freezing point. In particular Acrawax C show a discontinuity between 58°C and 65°C and an anomolous change of slop at 73°C Fig. 1 shows the logarithm of the D.C. conductivity plotted against 1/T.
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When capacitance-bridge measurements in the audio-frequency range are made on most dielectrics, it will be found that, as the measuring frequency is carried below 1 kc, both capacitance and dissipation factor begin to increase. Even at a frequency of 10 c, which is about as low as bridge measurements are carried, both capacitance and dissipation factor are increasing at an undiminished rate. Now that...
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Several investigators (1)(2)(3) have attempted to correlate the breakdown strength of liquid hydrocarbons with their molecular structure. These attempts have been based on a mechanism in which the field accelerated electrons lose energy by excitation of the vibrational modes of the molecule.
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Examination of the leakage current resulting from a gradual or stepwise increase in d-c test voltage permits significant conclusions in the determination of the service suitability of rotating machine insulation. Dielectric absorption effects have an appreciable influence on the leakage behavior. Therefore, in attempting to use the results of such tests there is considerable merit in conducting the...
