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The degradation of electrical kraft paper under carefully controlled laboratory experiments is being examined in order to find correlations between the change in dielectric and mechanical properties and the increase in concentration of furan compounds in the oil. These correlations could be valuable for assessing the extent of degradation of paper insulation in operating HV equipment. Preliminary investigation has shown that the concentration of furan compounds in mineral oil can be determined relatively easily using high-performance liquid chromatography (HPLC). While some of these compounds are themselves unstable, three have been found which are worth monitoring to correlate with the tensile and dielectric properties of the paper. Useful trends have been obtained, and further investigation is taking place to obtain clear cut correlation of practical value for monitoring the condition of actual transformers. >

Biaxially oriented polypropylene laminated paper (OPPL), composed of biaxially oriented polypropylene and kraft paper, has superior electrical and mechanical properties. 275 kV OPPL oil-filled cable has already been installed and OPPL POF (pipe type oil filled) cable is under development. By using OPPL, the insulation thickness can be reduced by 3 mm. Therefore, the conductor size of the POF cable can be increased without any change of cable core diameter, which makes it possible to increase the transmission capacity of the POF cable within a limited pipe size. >

Specimens of an aromatic polyamide (Aramid) paper, which were subjected to various degrees of moisture adsorption as well as water immersion, were investigated in order to estimate how much the adsorbed moisture and the stored water affect the dielectric and thermoanalytical behavior. In the humid specimens the dielectric properties did not change much and there were no endothermic and exothermic peaks. In the soaked specimens the dielectric properties changed remarkably and there were endothermic and exothermic peaks due to free water recognized at approximately 0 degrees C. It is concluded that the dielectric properties of Aramid paper are rather stable to moisture adsorption because its polyamide linkage bonds tightly with the adsorbed water which is in the shape of nonfreezing bound water, but when the total water content increases over the saturated moisture value of 6.8%wt, increases in the dielectric properties take place due to the free water in the fiber structure. In the bound water region, Aramid paper has higher DC resistivity values compared with other synthetic papers such as polyester paper. >

Conduction currents and thermally stimulated discharge (TSD) currents were studied in aramid paper at various field strengths and temperatures. At low field strengths the conductivity is ohmic, and at higher field strengths (up to 30 MV/m), ionic conduction is a possible mechanism. The activation energy of low field conduction is determined and at high electric fields the hopping distance for ions is in the range of 4.4 to 8.0 nm. TSD currents are also measured at various poling fields, formation and storage times, and rates of heating. The TSD current shows a peak at 135 degrees C for a rate of heating of 1 K/min and shifts to a higher value when the rate of heating is increased. The peak current and the charge released to the external circuit is proportional to the poling field. The observed peak is attributed to the depolarization of permanent dipoles with a room temperature relaxation time of 5.3*10/sup 5/ s. >

It is pointed out that the Bernard estimator is very accurate, with respect to other median or mean approximated ranks, confirming results presented by J.C. Fothergill in the above-titled paper (see ibid., vol.25, p.489-92, 1990). However, it is argued that the weighted Blom seems the most accurate estimator, among graphical ones, for normally sized samples and a reliable one for small sample sizes, although the Bernard and Filliben estimators are better than Weibull and Blom for small sample sizes. Statistical and graphical estimators are compared by plotting the percent deviations of the values of the Weibull parameters alpha and beta (scale and shape parameters, respectively) estimated by weighted Blom, Bernard, Weibull, maximum likelihood estimation (MLE), and Bain-Engelhardt (BE), with respect to the true values, as well as the estimates obtained by MLE and BE, as functions of the sample size. In replying, Fothergill states that for small sample sizes, the advantages of more sophisticated techniques do not seem warranted. >

In order to clarify basic surface flashover phenomena in transformer oil under switching impulse voltage, the flashover characteristics on four kinds of large insulating cylinders are studied for positive lightning impulse of 1.2/50 mu s, positive switching impulse of 500/3600 mu s, and 50 Hz alternating voltage. The insulating cylinders are made from material containing about 40 wt.% phenol resin and 60 wt.% kraft paper. A singular surface flashover path is observed under switching impulse voltage which includes a partial jump of the discharge channel around the HV electrode. The peak value of the flashover voltage for the switching impulse is nearly equal to that of a 50 Hz voltage; this was not expected before the experiment, because breakdown voltage of switching impulse for oil-immersed insulation is generally intermediate between the lightning implulse and AC voltage. The singular flashover path of the switching impulse is explained by an electric field analysis taking account of the remaining charge of 100 pC/mm/sup 3/ which has the same polarity as the applied voltage and is spread out along about 10 to 40% of the distance between the HV electrode to the grounded electrode. >

Using both an exact technique and Monte Carlo simulations to estimate the cumulative probability of failure of the ith of n identically stressed samples, it is shown that the approximation F(i,n)=(i-0.3)/(n+0.4) is considerably more accurate than other commonly used approximations. The significance of this in the interpretation of electrical breakdown tests is demonstrated. >

Kraft paper is used for insulation in oil-immersed equipment such as transformers. While it has given satisfactory performance on electrical and mechanical properties, 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. This report indicates the ability of insulating paper immersed in new synthetic insulating oil, alkyl-naphthalene oil, to withstand somewhat higher temperature than in conventional mineral transformer oil.

The ideal composition of fibers for insulating paper has been clarified by investigating the influence of the shape of fines on dielectric and tensile breakdown of oil-impregnated Kraft paper for oil filled (OF) cable at 105N/M oil. Furthermore, the pulp refining methods for producing such insulating paper have been investigated.

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 two-stage breakdown model for the fast-front breakdown voltage-time characteristics of oil-paper insulation is proposed and discussed. Several different breakdown probability curves have been obtained using this model. >

Non-linear losses in heterogeneous iusulation (e.g. impregnated paper) are often attributed to ions oscillating between insulating paper walls without penetrating or even adhering on them. To investigate this problem, we studied the physico-chemical properties of capacitor paper and of other insulating solids having the same blocking behavior. We have investigated the transport of electrolytes across paper impregnated with organic solvents due to an applied electric field or to a pressure gradient, the adsorption of electrolyte, and the net charge carried by paper by means of electrophoresis. It turns out that in the organic liquids tested, paper behaves like a semi-permeable barrier. Synthetic polymers also exhibit the blocking effect when they are made selective by grafted weak acid groups. We demonstrate that the ion blocking effect is of a physical and chemical nature and not of a mechanical one. The chemical nature of paper and impregnant modifies the amount of absorbed electrolyte and above all changes the sign of the net charge on the paper; on the other hand, the pore diameter modifies the concentration value for which selectivity no longer operates.

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.

A new synthetic paper MPSP having high breakdown properties has been developed. The synthetic paper is mainly comprised of polypropylene and is prepared using a stretching process, has a stratiform structure with many micro pores. The electrical and mechanical properties of the synthetic paper were measured and compared with those of cellulose paper and polypropylene film. The possibility of using this synthetic paper as an insulating material for electrical equipment was investigated, and the results indicate that the paper may be used as a substitute for cellulose paper.

The dielectric loss behavior of lapped dodecylbenzene-impregnated poly (2, 6-diphenyl-1, 4-phenylene oxide) paper insulation has been investigated. It was found that when this composite system is subjected to ac stress, some shoulders are observed in the curve obtained by plotting dissipation factor as a function of measuring stress. After chemical analysis of the impurities involved and after some calculations using Held's equations, it was found that the shoulder located at 0.1 kV/mm is associated with ionic motion in the liquid layer between paper sheets while the other at 1.2 kV/mm corresponds to ionic motion in the channel enclosed within the paper sheet itself. Furthermore, the dielectric deterioration of a model cable by ac stress application at elevated temperature is thought to result from the increase in ionic motion in the butt gaps. A nitrogen-containing compound is considered to be the source of these ions. These observations suggest that exhaustive removal of ionic impurities is essential for a successful operation of an oil impregnated synthetic polymer paper insulation.

The authors developed mica paper containing a small amount of aramid pulpous particles (fibrids), which were obtained by adding a precipitant into a polymer solution under a high shear condition. This mica paper is characterized by high dielectric strength, high tensile strength and high cut-through resistance. This mica paper is especially suitable for dry tape in conductor and coil insulation, because of the good workability and good resin impregnability, even in the case of small binder content. A significant improvement of insulation characteristics in thermal stability and mechanical fatigue resistance was attained by an application of this mica paper to rotating machinery coil insulation.

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.

Several new liquid impregnants for capacitors have been suggested as possible alternatives to PCB's which are being replaced in view of their harmful effects. The application of castor oil, a vegetable product, has been studied in the authors' laboratory with the same objective. The results of the investigations have shown that castor oil possesses most of the favorable properties of a good impregnant. This paper gives a detailed analysis of the dielectric behavior of the oil alone and all-paper capacitors impregnated with castor oil.

In view of its non-toxicity, and good dielectric properties, castor oil, a polar liquid dielectric of vegetable origin is suggested as a possible alternative to PCB's for capacitor applications. In this paper the dielectric properties (including partial discharge behavior), of all-polypropylene and paper-polypropylene capacitors with castor oil as impregnant, are reported. The paper also contains results of life studies conducted under accelerated electrical and thermal stresses when they are occurring both individually and combined. The data obtained have been statistically analyzed and approximate life of the system calculated by linear extrapolation.

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.