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Kraft paper is used for insulation in oil-immersed equipment such as transformers. The reason is that Kraft paper has given satisfactory performance on electrical and mechanical properties. As for life, however, thermal degradation occurs due to temperature rise in operation. Examples of these kinds of the Kraft paper are cyanoethylated Kraft paper and amine modified Kraft paper. Kraft paper containing morpholine, oxyalkylamine, aromatic polyamine, para-toluene-sulfone, etc., are well known. This paper describes a method for thermally upgrading Kraft paper using carbamate compound. The results obtained are summarized as follows. 1) n-Butyl carbamate is effective for thermally upgrading Kraft paper. 2) n-Butyl carbamate modified paper does not contaminate transformer oil.

A rapid measurement of thermal deterioration of oil-immersed papers by mass spectrometer and gas chromatograph is described. A special assembly was developed using a mercury diffusion pump and automatic Topler pump to extract a very small quantity of generated gas from the oil and to collect it in a reservoir. The gas collected by the above assembly is analyzed by mass spectrometer and gas chromatograph. Logarithm of the generation rate of (CO + CO2) [log (CO + CO2)] is proportional to the reciprocal of absolute temperature (1/T). This linear relationship can be explained by chemical reaction-rate theory and it is shown that the thermal endurance and the thermal life of insulating papers in oil can be estimated by the generation rate of (CO + CO2) in a few weeks as was reported previously for air and vacuum. But the change of the slope of the line log (CO + CO2) versus 1/T at the temperature near 130°C was observed. This is a new mechanism of thermal deterioration of insulating paper in oil, which is apparently unexplained.

This paper describes the characteristics and properties of a new insulation system currently used to manufacture high-voltage bushings at voltage ratings of 15 through 69 kV for transformers and switchgear apparatus. The system is compared with other forms of insulation such as phenolic-paper, oil-paper, mineral-filled epoxy, and electrical porcelain. Its application in high-voltage condenser bushings is also discussed. With this new insulation, epoxy resin is used to totally impregnate an electrical grade of kraft paper resulting in a resin-rich (approximately 70 percent) material having unusual properties. Its predominant characteristic is the complete absence of voids resulting in high-corona-inception voltages and high-dielectric strength. It also has excellent physical properties which, coupled with a flexible manufacturing process. open up numerous new design possibilities in high-voltage engineering.

The objective of the work reported in this paper was to study the influence of the thickness of the paper mass and of the inner temperature distribution on the vacuum drying process; particularly to study the ways in which these two factors affect the quantitative distribution of residual moisture perpendicular to the layers of the paper. The quantitative distributions of residual moisture perpendicular to the layers in soft paper masses 0.3 to 2.4 cm thick were determined. The results show that both the magnitude and distribution of temperature throughout the paper affect the degree of drying. Neither a low vacuum nor a prolonged drying time will compensate for an unfavorable temperature distribution. The magnitude and distribution of temperature predominate over the effect of paper thickness. With unfavorable temperature conditions, paper only 0.6 cm thick will retain an excessive amount of moisture. With favorable temperature conditions, paper 2.4 cm thick can be dried under vacuum to an acceptable moisture content. The upper limit of residual moisture content for transformer insulating paper that can reasonably be expected to be achieved is about 0.5 percent, not 0.1 percent or less.

Based on measurements, the strength of synthetic oil impregnated capacitor paper (Askarel) to dc and ac voltage 1.2/50-?s voltage surges and switching surges of various wavefront duration are discussed. The results of measurements are compared with the performance figures of mineral oil impregnated capacitor paper. The diminishing effect of the number of lightning and switching surges and their amplitude upon electric strength and of initial voltage partial discharge is investigated.

The dielectric response of an oil-impregnated-kraft paper system has been examined in the temperature range from 25-85°C at frequencies down to 10-2 Hz. Single and double sheet oil-paper specimens were studied. With single sheet paper specimens the long range charge carrier migration process was found to follow the Eyring behavior over the main test temperature region and was characterized by dc and ac activation energies ranging from 0.40 to 0.47 eV and from 0.37 to 0.48 eV, respectively. The involvement of mobile ions, possibly hydrogen, appeared to be a likely cause for the observed conduction process. Due to appreciable interfacial polarization effects, the dielectric behavior of the two paper sheet specimens exhibited strong deviations from the Eyring behavior. No definite physical significance could thus be attached to the derived values of activation energy over the restricted linear regions of the ln Rac versus 1/T plots.

This paper is about the applications of an epoxy-impregnated paper insulation to ultra-high-voltage systems. An epoxy-impregnated paper insulation is a resin-rich void-free structure, and therefore has extremely superior voltage withstand characteristics. It also has outstanding resistances to thermal cycle and mechanical vibration. Consequently, such insulation systems can be applied easily to condenser bushings. From the standpoint of the production facilities, techniques, and economical considerations, in items with an overall length of more than 3 m, "prefab"-type condenser bushings, consisting of several condenser cores, are desirable. In cases when two-unit condenser cores are assembled leaving a simple conical-shaped gap between them, it is extremely effective to adopt a "potential-expanding"-type gap designed so that the equipotential surface in the gap will be in the vertical position to the cone surfaces as far as possible. The prefab-type condenser bushing of the 500-kV class was experimentally manufactured using this arrangement; it displayed all the desired properties. This paper also describes the other examples of applications of epoxy impregnated paper insulation: CT bushings and pulse transformers in the magnetic firing system for high-voltage dcconverters using thyristors, and draw-out-type condenser bushing for disconnecting devices used in the compact-type substation.

Experiments have been carried out to determine the partial discharge performance of laminated Kraft paper/oil dielectric (1.3 mm thick). As with conventional corona pulse detection equipment, the pulses were measured on a radio interference (RI) meter which proved well suited for this purpose. The type of insulation investigated has high long-term electric strength if correctly assembled, even when small voids are present in the dielectric. If discharges do occur, a certain recovery interval is required to regain almost full electrical strength, because of the time needed to absorb gases formed by the discharge. The data obtained on the test specimens were confirmed by long-term tests carried out on oil-filled instrument transformers.

New synthetic resin insulation systems have been established in Japan, as in Europe and the United States [2], and have a history of almost fifteen years. The research work of authors, initiated in 1949, led to the completion of the unsaturated polyester insulation system called SLS insulation. Outstanding performance was obtained as of 1957 when this new insulation system was applied to large rotary-type machinery. In the meantime, a trial manufacture study was conducted on epoxy-resin insulation in view of its excellent electrical and mechanical properties. As a result, an epoxy insulating system of quality distinctly superior to the former system was successfully put on a mass-production basis. This paper describes the fundamental insulation procedure, the dielectric properties, and the service experience of this system.

Knowing that oil is a weak link in a composite or impregnated dielectric system with reference to both dielectric strength and ease of contamination, many investigators have devoted great efforts to try to ascertain the best types of oil for various operating conditions. However, different investigators seem to have different opinions as to the best-suited type of oil for this purpose. It is the object of this study to involve a wide variety of oils and test conditions to shed some light on this controversy. Ten original oils and three blended samples selected from an array of over 50 original specimens tested have been chosen to illustrate the tenacity of this problem. The samples include both mineral and synthetic oils, coming from domestic sources as well as from abroad. Their sources or ethnic groupings seem to indicate no preference, but the care in refining these samples seems to be a factor. In this investigation, two criteria have been chosen to indicate the service quality of the oils: gas-absorption and liberation characteristics and electrical deterioration under moderate electrical stress. In the presence of carbon dioxide, no oil has been found to liberate gases. In the presence of air, nitrogen, or oxygen, although gas liberation has been observed in some of the European oils at some stages of aging, there has been no sharp rise in dissipation factor or indication of internal discharge or failure.