G00048 85°C 0.0040 00032 0.0024 po 00016 15018 Average voltage stress(kv/cm FIGURE 55.4 Loss characteristics of mineral oil-impregnated paper. with applied voltage and temperature [as is apparent from the nature of Eqs.(55. 12)and (55. 13)]. The exact frequency value at which the space charge loss exhibits its maximum is contingent upon the value of the relaxation time T. Figure 55.4 depicts typical tan8 versus applied voltage characteristics for an oil-impregnated paper-insulated cable model at two different temperatures, in which the loss behavior is primarily governed by ionic conduction and space charge effects. The monotonically rising dissipation factor with increasing applied oltage at room temperature is indicative of the predominating ionic loss mechanism, while at 85C, an incipient decrease in tan8 is suggestive of space charge effects 55.3 Dielectric Breakdown As the voltage is increased across a dielectric material, a point is ultimately reached beyond which the insulation will no longer be capable of sustaining any further rise in voltage and breakdown will ensue, causing a short to develop between the electrodes. If the dielectric consists of a gas or liquid medium, the breakdown will be self-healing in the sense that the gas or liquid will support anew a reapplication of voltage until another breakdown recurs In a solid dielectric, however, the initial breakdown will result in a formation of a permanent conductive channel, which cannot support a reapplication of voltage. The dielectric breakdown processes are distinctly different for the three states of matter. In the case of solid dielectrics the breakdown is dependent not only upon the molecular structure and morphology of the solid but also upon extraneous variables such as the geometry of the material, the temper ture,and the ambient environment. Since breakdown often occurs along some fault of the material, the breakdown voltage displays a readily perceptible decrease with area and thickness of the specimen due to increased incidence of faults over larger volumes. This is indeed part of the reason why thin-film inorganic dielectrics, which are normally evaluated using small-diameter dot counter electrodes, exhibit exceptionally high dielectric strengths. With large organic dielectric specimens, recessed electrodes are used to minimize electrode edge effects, leading to greatly elevated breakdown strengths in the order of 10 to 10 kv cm,a range of values which is considered to represent the ultimate breakdown strength of the material or its intrinsic breakdown strength; as the intrinsic breakdown occurs in approximately 10-to 10-5s, an electronic mechanism The breakdown strength under dc and impulse conditions tends to exceed that at ac fields, thereby suggesting ne ac breakdown process may be partially of a thermal nature. An additional factor, which may lower the ac breakdown strength, is that associated with the occurrence of partial discharges either in void inclusions or at the electrode edges; this leads to breakdown values very much less than the intrinsic value. In practice, the breakdowns are generally of an extrinsic nature, and the intrinsic values are useful conceptually insofar as they provide an idea of an upper value that can be attained only under ideal conditions. The intrinsic breakdown theories were essentially developed for crystalline dielectrics, for which it was assumed that a very small number c 2000 by CRC Press LLC© 2000 by CRC Press LLC with applied voltage and temperature [as is apparent from the nature of Eqs. (55.12) and (55.13)]. The exact frequency value at which the space charge loss exhibits its maximum is contingent upon the value of the relaxation time t. Figure 55.4 depicts typical tand versus applied voltage characteristics for an oil-impregnated paper-insulated cable model at two different temperatures, in which the loss behavior is primarily governed by ionic conduction and space charge effects. The monotonically rising dissipation factor with increasing applied voltage at room temperature is indicative of the predominating ionic loss mechanism, while at 85°C, an incipient decrease in tand is suggestive of space charge effects. 55.3 Dielectric Breakdown As the voltage is increased across a dielectric material, a point is ultimately reached beyond which the insulation will no longer be capable of sustaining any further rise in voltage and breakdown will ensue, causing a short to develop between the electrodes. If the dielectric consists of a gas or liquid medium, the breakdown will be self-healing in the sense that the gas or liquid will support anew a reapplication of voltage until another breakdown recurs. In a solid dielectric, however, the initial breakdown will result in a formation of a permanent conductive channel, which cannot support a reapplication of voltage. The dielectric breakdown processes are distinctly different for the three states of matter. In the case of solid dielectrics the breakdown is dependent not only upon the molecular structure and morphology of the solid but also upon extraneous variables such as the geometry of the material, the temper￾ature, and the ambient environment. Since breakdown often occurs along some fault of the material, the breakdown voltage displays a readily perceptible decrease with area and thickness of the specimen due to increased incidence of faults over larger volumes. This is indeed part of the reason why thin-film inorganic dielectrics, which are normally evaluated using small-diameter dot counter electrodes, exhibit exceptionally high dielectric strengths. With large organic dielectric specimens, recessed electrodes are used to minimize electrode edge effects, leading to greatly elevated breakdown strengths in the order of 106 to 107 kV cm–1, a range of values which is considered to represent the ultimate breakdown strength of the material or its intrinsic breakdown strength; as the intrinsic breakdown occurs in approximately 10–8 to 10–6 s, an electronic mechanism is implicated. The breakdown strength under dc and impulse conditions tends to exceed that at ac fields, thereby suggesting the ac breakdown process may be partially of a thermal nature. An additional factor, which may lower the ac breakdown strength, is that associated with the occurrence of partial discharges either in void inclusions or at the electrode edges; this leads to breakdown values very much less than the intrinsic value. In practice, the breakdowns are generally of an extrinsic nature, and the intrinsic values are useful conceptually insofar as they provide an idea of an upper value that can be attained only under ideal conditions. The intrinsic breakdown theories were essentially developed for crystalline dielectrics, for which it was assumed that a very small number FIGURE 55.4 Loss characteristics of mineral oil-impregnated paper