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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. >

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. >

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. >

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. >

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.

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.

Electrical breakdown strength of paper-oil insulation samples was meas ured under dc, ac and pulsating voltages. The latter was obtained by superimposing ac upon dc voltage and provides an attractive method for a simultaneous testing and assessment of the state of insulation of the various parts of HV apparatus in service. The measurements were carried out over a wide range of the pulsation ratio defined as p=Eac/Edc. The results obtained under pulsating voltages follow closely an expression which relates the breakdown strength to the sum of arctangent and arccotangent function of the parameter p. The study was carried out using dry paper as well as paper containing various degrees of moisture. The presence of moisture showed a pronounced effect upon the breakdown strength which varied with the pulsation parameter p.

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.

The dielectric breakdown of oil-impregnated paper insulation was studied. Fast-front step voltages and standard lightning impulses were used to determine the breakdown volt-time characteristics of the insulation. Breakdown time lags in the submicrosecond range were obtained with the fast-front voltages. A model was proposed for the breakdown time lag probabilities. The model relies on charge injection from the cathode followed by the generation of intense local fields arising from structural fluctuations within the dielectric. The parameters for the probability functions were estimated from the fast-front measurements, and the limiting probability envelopes were derived for the standard lightning impulses. Satisfactory agreement was observed between the calculated probability envelopes and measured data, thus indicating that the physical processes leading to breakdown do not change substantially over the time regime investigated. >

Results are reported for the measurement of the electrical breakdown location in the vicinity of an oil-paper interface over the temperature range from room temperature to 150 degrees C. The data indicated that the electrical breakdown occurred at the interface from 15% to 43% of the time, depending on the details of the particular set of measurements. A theoretical analysis shows that this experimental result is consistent with the electric-field enhancement, the area over which the enhancement occurs, and the spread in the breakdown voltages for nominally identical tests. >

As in many other electrotechnical fields, the research work and the early realizations in the cable field during the second half of the last century were the germ of most of the techniques developed in the 20th century. For more than 70 years, paper impregnated with oilresin compounds or simply with oil, prevailed as an insulating material in cable manufacturing, though rubber insulated cables were also used to some extent. The OF cable technique is going on its road towards very high voltages and ratings. It was in the 1960's that the use of extruded polymeric materials, either thermoplastic or thermosetting, made rapid progress. Since then, the most prominent material has been polyethylene, either pure or cured, but the use of ethylene-propylene rubber is also developing. Cables with extruded synthetic insulation are now on the point of meeting all the current needs, from low voltage to the higher existing transmission voltages, with an unequalled ease of installation and operation and a reliability that can already be regarded as quite satisfactory.

Charge or RIV techniques for corona (partial discharge)have been used in factory tests of high-voltage oil-filled equipment.Such tests have also been used in the field. More recently, measurementsof evolved gas have been made as well.In the laboratory, the effect of corona in oil has been determinedlargely by measuring gas evolution using the Pirelli test, in whichdischarges take place in oil vapor. A new technique?the floating-electrodecell?has been developed to obtain controlled discharges in oil. Tomonitor the discharges, histograms of the number of pulses versusthe pulse magnitude are obtained periodically. Provision is made todetermine the amount and nature of evolved gas with both the"floating-electrode" and Pirelli cells. When paper or pressboard as wellas oil is used, the development of x wax has been observed with scanningelectron microscopy (SEM).Results are reported for conventional transforner oil and dodecylbenzeneand a naphthenic white oil from which all aromatic componentshave been removed.

Results obtained in paper insulation tests have led to a new definition of the damage as D = K (Vn t)a, when the appearance of partial discharges with 25 pC of apparent charge is used aa a failure criteration. It is suggested that the proposed expression for damage can be generalized to the aging of soild insulators if the criterion of total failur of specimens is adopted.

Lengths of flat copper conductor insulated with aromatic polyamid (aramid) tape or cellulose tape were bent into rings and secured around the edge of discs. Two discs were placed edge-to-edge in one plane and breakdown voltages were measured between the conductors. The conductor assemblies were dried under vacuum, impregnated and immersed in several low-flammability liquids at temperatures up to 180 degrees C under air. Comparisons were made with mineral oil which was tested up to 130 degrees C under nitrogen. With the discs set to provide a 1-mm liquid gap between the insulated conductors, the variation of breakdown voltage with temperature is limited to a few percent and values tend to their highest at 80 degrees C for both AC and lightning impulse. With the conductors in contact, alternating breakdown voltages vary little, whereas impulse voltages fall appreciably with temperature rise, especially for cellulose. >

New composite materials such as laminated tapes were applied to high voltage cables. Plastic films laminated with kraft papers were developed for the high voltage oil-filled cables to improve the dielectric properties. The 500 kV oil-filled cables insulated with these laminated papers were also developed and tested with good results. Lead foil, laminated with plastic film, was developed for the water-impervious layer of XLPE cable to improve its reliability. It was confirmed that bow-tie trees were observed in the conventional XLPE cable after six months of water-immersion, but the water impervious XLPE cable retained its original performance.

The breakdown voltage of a rod to plate gap is largely affected by the insertion of an insulating paper in the gap and this is known as the barrier effect. The complicated characteristics of the breakdown were studied experimentally for dc, ac, lightning and switching impulse voltages. The explanation of these effects were attempted by considering the space charge distribution on the insulating paper. The trial to measure the space charge distribution was also made using an optical method.

The degradation behavior of various dielectric systems for power capacitor use is discussed. Accelerated life testing of prototype capacitors and their constituent dielectric materials are described in detail. The composite polypropy lene/kraft paper system was estimated to have a life time of more than 30 years in service and its degradation behavior was predominantly determined by that of the kraft paper. Moreover, the all-film type system has a longer lifetime than that of the composite system.

The location of electrical breakdown in a composite insulating system has been measured. In these experiments, a paper sample was mounted so that it connected the two electrodes. Electrode structures ranging from plane-plane to sphere sphere were used. The electrode-paper system was tested in oil to determine the properties of an oil paper interface. The data indicated that, in a carefully prepared system, the breakdown will not necessarily occur at the interface. In addition, it was found that breakdown voltages were not significantly lower for those breakdowns which occurred at the interface than for those which did not. It was noted that if the paper was not dried or if many gaseous voids were left in or on the paper, the breakdown regularly occurred at the interface and at a lower voltage.

Impulse and sixty-hertz breakdown tests were conducted on stationary and circulating 50 centistoke dimethyl silicone using bare and insulated uinform field elect-rodes. The electrode insulation materials used were polyamide paper, and Kraft paper thermally upgraded with dicyandiamide. The test results show that insulating the electrodes, significantly increases the sixty-hertz and impulse dielectric strengths of silicone. Under most conditions, the uniform field insulating characteristics of dimethyl silicone are comparable to those of conventional transformer oils; however, the bare electrode sixty-hertz strength of silicone is approximately 20% lower than that of transformer oil.

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.

A novel method is proposed and validated experimentally to increase the breakdown voltage of an insulating system for applications in the electrical insulation of large power equipment. The method relies on controlling the electric field within a multilayer insulation system. The insulation system is constructed from n layers of insulation and m very thin conducting strips placed between successive insulating layers. The conducting strip at each layer is symmetrically shifted by a constant distance determined from a shifting factor which is calculated to maximize the withstand voltage of the insulation. The potential distribution within the composite insulation is determined from the solution of the Laplace equation for different design parameters, and it is shown that the field can be reduced for a proper choice of the dimensions of the system. A composite insulation system based on the proposed method has been fabricated and tested for ac (60 Hz) and impulse voltages. Substantial improvements in the withstand voltage are obtained over an identically sized system without the conducting strips, thus validating the analysis presented in this paper.

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.

The impregnation of HV components is important if not critical for reliable operation. Fine windings, porous insulation such as kraft paper and pressboard, and similar elements need to be impregnated for good thermal conductivity, mechanical strength and, in the case of insulation, to minimize internal discharges. Although widely used, there appears to be little systematic work done to explain the important factors governing the basic process when a filled, thermosetting compound such as a filled epoxy is used in a pressure impregnation. In this paper we have derived an expression for the depth of penetration of a thermosetting compound into a porous substrate such as kraft pressboard. The equation is extended to the case of a filled thermoset where filtration of the filler occurs at the substrate surface. The expression derived relates filter cake permeability, substrate permeability, applied pressure and the viscosity-time relationship of the impregnating. filtered compound. Data are presented showing good agreement with the derived expression.

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.

The discussers separately address questions raised in the above-titled paper by M. Nishida et al. (ibid., vol.27, p.1050-1053, 1982) concerning tracking test procedures. The three discussers agree that there is a need to revise IEC 587. Nishida replies that the above paper did not intend to evaluate the tracking resistance of the electrical insulating materials, or to offer any information for a revision of IEC 587. >

In the paper, life models, are collected valid for solid insulating materials subjected to electrical and multiple thermal-electrical stresses whose life lines show a tendency to reach an electrical threshold. Their fit is investigated with reference to several sets of data derived from accelerated life tests performed on different insulating materials (XLPE, composite Nomex-Mylar-Nomex, epoxy bisphenolic resin, polyimide). In particular, two phenomenological life models, characterized by four parameters and based on exponential and inverse-power relationships, are compared with three-parameter life models taken from the literature and derived considering different aging and breakdown mechanisms. Discussion of the ability of these models to provide significant parameters for insulation endurance evaluation and electrical system design closes the paper.

The author presents a bibliography covering the field of electrical discharges in vacuum, comprising both electrical breakdown in vacuum and vacuum arcs. A brief section lists some review papers which would be helpful to the novice in this field. The bulk of the presentation consists of bibliographic listings arranged by year of publication and alphabetically by author within each year. An author index refers one to all papers authored or coauthored by a particular person. >

Investigations to determine the relevant electric and dielectric properties of certain insulating fluids which appeared most suitable from the viewpoint of environmental compatibility and inflammability are discussed. Two of these are silicone fluids based on linear polydimethyl siloxane (kinematic viscosity at 25 degrees C is 50 mm/sup 2//s) and an ester fluid based on penthaerythrithol tetraester. After a short description of the insulating fluids, the influence of aging on the breakdown and dissipation factor is determined. Furthermore, a comparison of the breakdown and aging behavior of fluid-paper composite dielectrics is made. The influence of partial discharge prestressing on the breakdown of the fluid-paper composite dielectric is shown. >

The 50-Hz AC partial discharge (PD) triggered by a lightning impulse voltage of 1.2/50 mu s and breakdown characteristics under superimposed voltage conditions are studied for several modes of oil-immersed transformer insulation, representing typical combinations of mineral oil and oil-impregnated paper. It is concluded that (1) no AC PD phenomena are triggered by lightning impulse voltage when the preapplied 50-Hz voltage is less than 50% of the RMS value of the AC PD inception voltage for all combinations of polarities, and (2) peak breakdown values of the superimposed voltage are reduced by 10 to 19% for a single applied lightning impulse with a low solid-to-liquid dielectric packing fraction (SLPF) for oil-impregnated paper in the intercoil model, but the difference in breakdown voltage between the superimposed voltage and a single lightning impulse is very small for a high SLPF for oil-impregnated paper, i.e. above about 30%. >

This bibliography covers the field of electrical discharges in vacuum, comprising both electrical breakdown in vacuum and vacuum arcs. A brief review section lists some review papers which would be helpful to the novice in this field. The bulk of the paper consists of bibliographic listings, arranged by year of publication and alphabetically by first author within each year. An extensive author index is given. >

The commenters raise some questions regarding the measurements reported in the above titled paper (ibid., vol.27, pp.1026-39, October 1992) and their meaning. J.K. Nelson answers some of the questions in terms of the paper's objectives. >

For original paper by S.M. Gubanski see ibid., vol.25, p.331-40 (1990). The commenter raises the question of how long silicone rubber can preserve its hydrophobicity despite pollution, in the context of the interpretation in the above-titled paper of merry-go-round (MGR) test results. The commenter also raises the issue of the validity of the MGR test in relation to polymers containing each different quantities of filler. Gubanski discusses the first issue in some detail, and the second briefly. Gubanski concludes that the MGR test leads, in many cases, to contradictory results and that more work has to be done before it could be applied for material assessment for HV outdoor insulation. >