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Nonuniformity in insulating paper has drawn our particular interest as an indispensable factor in order to clarify the dielectric breakdown mechanism in oil-impregnated paper.

1. The 60 Hertz and impulse electrical strength of oil impregnated polyamide paper is as good as that of high quality electrical grade kraft paper and better than that of rope-kraft paper used as conductor insulated. 2. The 60 Hertz dissipation factor of polyamide paper in oil increases moderately with increasing temperature. 3. Based on burst strength and tensile strength retention, the thermal stability of the polyamide paper is substantially superior to that of cellulosic paper when aged in oil or askarel at 150°C and 175°C. 4. Polyamide paper showed little or no degradation after extended life tests in distribution transformers. 5. Polyamide paper with its superior mechanical properties can better withstand manufacturing operations in production than can cellulosic papers. 6. Based on life test results the polyamide paper will permit the design of transformers with higher operating temperatures and greater overload capacities than can be obtained using cellulosic paper.

It is common practice to retain a portion of the aromatic constituents originally present in mineral oils in order to minimize gas evolution in cable-insulating oils under intense field conditions. However, the retention of aromatic fractions not only lowers the oxidation resistance of the oils, but has been also observed1 to influence adversely the magnitude of their dielectric loss. In cognizance of the latter effect, it was considered desirable to study the influence of aromatic fractions in oils upon the over-all loss mechanism in oil-impregnated paper insulations of the type employed in high-voltage power cables. An investigation was therefore undertaken on cable models constructed with kraft paper impregnated first with oils having different aromatic contents but equal viscosities and second with oils having equal aromatic contents but different viscosities.

Insulation paper is primarily used in electric motors as slot cells, coil separators and phase insulation in integral horsepower equipment. The slot cell being defined as the basic ground insulation that provides mechanical and dielectric barriers from the magnet wire to the stator core. This definition emphasizes that the insulation paper forming the slot cell is mainly ground insulation.

Improved functional properties and process simplification now can be attained through the use of solventless, resin-filled Mica Mat tapes used for high voltage ground insulation. Previously, all B-staged resin-filled tapes composed of either flake mica or mica paper have been manufactured with resin/solvent solutions because the solvents were necessary to reduce the viscosity of the resin for impregnation and to soften the tapes for application. However, when solvents are used, removal of them is difficult, costly, and complete removal is often virtually impossible.

A partially acetylated fiber having an acetyl content of 18–24% and adequate inherent fiber strength has been developed. Paper prepared from this acetylated fiber exhibits significant advantages over the non-acetylated parent fiber for moisture resistant properties. These advantages are reflected in improved electrical qualities. There are also definite advantages for dimensional stability and for thermal stability both in air and in oil.

Consideration is directed toward electromagnetic compatibility as viewed from the control aspect. Emphasis is placed upon an inter-system approach to the improvement of the over-all electromagnetic environment.

In the past few years it has become well established that presently available carbon black papers when used to screen the conductor or insulation of a high voltage cable increase the initial dielectric loss of that cable. Kitagawa1 developed a formula which relates this increase to the resistivity of the carbon black paper. This formula can be expressed to a first approximation, as Δ tan δ = ω CR where C is the capacitance of the insulation and R is the series resistivity of the carbon black layer. In the work discussed last year, we2 showed that the resistivity formula did not provide an adequate explanation for the increase in dielectric loss due to carbon black paper.

A microwave method for the measurement of fiber orientation in paper, is described. The method uses a dual mode microwave cavity where the modes have equal resonant frequencies and mutually perpendicular electric fields. The paper is inserted through the cavity so that the electric fields of both modes are in the plane of the paper. If the orientation of the fibers in the paper is not random the resonant frequencies will differ due to the dielectric anisotropy of the paper. This difference is measured and can be calibrated to give the amount of fiber orientation. Moisture and thickness variations in paper can cause calibration errors. However, if the variations are within certain limits, these errors can be eliminated by a suitable selection of the resonator bandwidths.

Assuming a certain stage of development has been attained, it will be possible in the future to use cables with superconductors if they are reliable and economical.1, 2 In addition to the development of superconductors, the cryogenic envelope, and cooling equipment, it is necessary to investigate feasible dielectrics at a few degrees K. This paper is intended to give information regarding the experimental determination of the breakdown strength of paper wrappings impregnated with supercritical helium which could be used in direct voltage cables with superconductors.

Research and development in synthetic fiber papers, started by the authors' company in 1955, has been directed for the past four years toward flexible sheet insulation for use in electrical equipment. This paper is a progress report on the work done to date.

For voltage levels of 300 kV and above,1,2 oil-impregnated cellulose paper becomes less and less efficient as insulation in underground transmission cables because of the increasing dielectric losses. Above 500 kV cellulose paper will not be applied as a result of prohibitive dielectric loss heat as well as of excessive growth of insulation thickness, so that maintaining both sufficiently low thermal resistance and acceptable bending properties becomes difficult.

It has been recognized for some time, that oil impregnated cellulose paper will not be a useful insulating material for cable carrying in excess of 500 kV because the thickness of the insulation layer would become excessive and the dielectric losses associated with such thick structures would be prohibitive. Also, removal of the considerable heat generated in such a structure would become very difficult if not impossible. To overcome this problem, it has been suggested that the cellulose paper be replaced by some synthetic material possessing better dielectric properties than does cellulose paper. Paper-like structures obtained from such polymeric materials as polyphenylene oxides1 and polyolefins2 have shown considerable promise as replacements of cellulose paper in cable insulation. While polyphenylene oxides exhibit many attractive properties for the contemplated use in oil filled cables, they are not yet available as an industrial product with fixed specifications. Polyolefins are commercially available in large quantities and at relatively low cost. In view of these considerations and the encouraging results reported by Buehler et al.2 a more detailed study of polypropylene paper has been undertaken and the results are reported in the following sections.

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 system 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 three meters, “pre-fab” 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 “pre-fab” type condenser bushing of the 500 kV class was trially 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 D.C. converters using thyristors, and draw-out type condenser bushing for disconnecting devices used in the compact type substation.

It is a well-known fact that when some insulants under dc voltage are subjected to a polarity reversal, the breakdown immediately after the reversal may appear at a substantially lower voltage than the breakdown without polarity reversal. Since complete discharging of the sample does not occur after the polarity reversal, the voltage due to the charges remaining in the sample will add to the reverse voltage applied and will, in effect, increase the value of maximum stress in the sample. References 1, 2, and 3 consider the transient-stress distribution at the instant of polarity reversal in a cylindrical sample of oil-paper insulation subjected to temperature gradients. In their considerations, the influence of polarization on the maximum stress occurring immediately after a rapid polarity reversal is not taken into account and, as will be indicated later, this leads to substantial errors. The purpose of this paper is to establish the electrical strength of oil-paper insulation under rapid polarity reversals, with all space charges present in the insulation taken into account.

For many years, design engineers have been faced with perplexing problems in finding suitable high-temperature electrical insulating materials to meet specific application requirements. Because these materials are widely used as supporting insulation for resistance wires in high-heat applications such as toaster elements, etc., they are more commonly referred to in the industry as heater plates. Many types of heater plates are being offered today which include materials such as: mechanically fastened mica splittings; organic bonded mica and mica paper plates, i.e., plates bonded with fugitive, easily volatilized resins, treated asbestos boards, glass bonded mica plates, inorganic bonded mica and mica paper plates, certain ceramics, and semi-inorganic silicone bonded mica paper heater plate. Obviously, some of these materials possess individual property or economic advantages over the others, but none is ideal, and selecting the optimum material to meet one's particular needs is not easy.

This paper is concerned only with the effect of the dissipation factor of the liquid upon the overall dissipation factor of impregnated paper insulation.

The growth of the mica paper business from small beginnings in the early 1950's to the large and important segment of the electrical insulation market which it is today has been marked by great activity and progress in many areas of endeavor concerned with mica and mica paper. This progress ranges all the way from pure research on the structure of mica, through an elucidation of many processes for producing mica papers to the practical problem of applying mica mat tapes to obtain the best overall results.1 A measure of this activity is the fact that in Chemical Abstracts alone, which cover this period, there are well over one hundred pertinent articles on mica and mica paper.

Since the early days of electricity supply (1890), impregnated paper based cables have been used for transmission and distribution purposes.1 Besides a long record of cable reliability, paper insulation has unique characteristics such as a ruggedness which provides easy taping and subsequent bending of the finished cable and a porous matrix which allows easy flow of the impregnant assuring a virtually void free composition with a high breakdown strength. However, the inherent electrical properties of the oil impregnated paper composition give rise to a limited power transmission capability for paper-oil insulated power cables at extra high voltage. This limitation has stimulated research on new transmission systems and new materials. This report is concerned with one of these efforts, namely, synthetic paper insulation.

Because of the low power factor available with non-polar polymers considerable interest has arisen in the possible use of polymers for oil-impregnated EHV cables.1–3 It is anticipated that the use of a synthetic paper for such an application will combine the best features of both polymer film and cellulose paper without the disadvantages of either. Developmental sheets of a synthetic paper made from P 3 O polymer [Poly-2,6-diphenyl-1, 4-phenylene ether)] have been examined for such application. This study describes the dielectric loss characteristics of the dry (i.e., unimpregnated) paper, and some of the factors involved in maintaining the lowest possible dielectric loss when P 3 O paper was impregnated with polybutene oils.

The voltage endurance of liquid impregnated paper insulation is important in both test and service for high voltage equipment such as transformers, capacitors, and cable. It has been recognized for a long time that if electrical discharges occur, they may limit the life. Especially with cable, corona studies have been carried out for many years using both dielectric loss measurements (i. e., Schering Bridge) and several types of amplified band pass techniques for more direct measurement of corona pulses.

The use of capacitors in electric networks is becoming more and more common. The capacitor remains under the voltage of the network, and in addition it is subjected to various over-voltages appearing in the network. The latter will originate from lightning strokes and more frequently from switching operations. Due to the incompleteness of data on the capacitor dielectric strength under switching surges, the problem became of interest. The lightning and switching surges may cause a weakening of the dielectric strength of many types of equipment. Power capacitors, with their impregnated paper insulation, belong to this category of equipment.

1. “Resiwrap” offers a solid insulation system with good thermal stability, high dynamie and static strengths, both through and between laminations, good temperature vs. power factor performance, relatively flat voltage vs. power factor curve and high di electric strength . 2. This resin-rich epoxy-impregnated paper material offers one of the few solid insulation systems that is void-free, has high corona inception voltages, low moisture absorption and high impermeability to gases and liquids. 3. The manufacturing process offers flexibility in application such as its use in condenser bushings. 4. All testing to date also demonstrates a long and dependable life. This material has had over five years of field experience in Europe, plus considerable development testing.

Mica paper was invented at almost the same time in the United States and in France. It was during the war and that is probably the principal reason why its development was delayed.

The effect of switching surges on the dielectric strength of Askarel impregnated capacitor paper is less than for standard impulses and approximately the same for A.C. From the practical point of view the small variation in breakdown strength resulting from a change in the wave front duration from 35/110 µ sec to 600/2000 µ sec can be neglected in the design of capacitors.

In an oil-impregnated paper cable the resistivity varies exponentially with temperature and very nearly exponentially with stress, so that the true dc stress-distribution equation cannot be solved analytically. If the effect of stress is ignored, the equation presented by Bragin applies E(r) = [Vβ(r/r o )β-1]/[r 0 (μβ-1)] where E(r) is the stress at radius r, V the applied voltage, r 0 the conductor radius, r 1 the sheath radius, μ = r 1 /r 0 , β = bKW/2π, b is the temperature coefficient of resistivity, K the thermal resistivity, and W the conductor dissipation.

One of the problems encountered when evaluating the dielectric constant and power factor of either impregnated or unimpregnated paper is the variation in apparent density of the sheets. It is important to know whether measured differences in the electrical properties are due to differences in sheet density or to differences in the fibers themselves.

Mica in the form of mica paper will soon make its appearance as a component, in electrical insulation. This revolutionary advancement in the use of mica will make available to the electrical manufacturer products which have heretofore been impossible to produce.

This session is devoted to the insulation probieros in the manufacture and use of transformers. These probieros resuit from the progress in delermining the life of equipment from many viewpoints. New matcrials and ways of using them, processes, quality control, and corona effects, are illustrated as problems in the papers being presented.

Cellulose papers expand and contract when the humidity of the atmosphere with which they are in equilibrium changes. This phenomenon — known as hygroexpansivity — plays a significant role in the manufacture of such electrical components as capacitors, cables and transformers due to the shrinkage and consequent mechanical instability which the units suffer on being dried before impregnation. While many investigations have been made of the phenomenon, most of the work has been hampered and made useless by the failure of the researchers to establish repeatable data. It was the purpose of this work to determine why the effect was not reproducible within close limits and from the results to understand the mechanism of hygroexpansivity.

Better insulation performance is persistently required for high voltage rotating machines with the trend of increasing capacity and rated voltage. Resistance to thermal shock, mechanical reliability and dielectric strength of insulation are especially important. As for the impregnating varnish, asphaltic compound has been replaced by synthetic resins with progress in the chemical industry. Unsaturated polyester and epoxy resins are the most popular resins, the latter becoming more important because of their overall suitability. On the other hand, the main barrier insulating material tends to move toward reconstituted mica from mica splittings. In this paper, electrical and therm omechanical performances of newly developed epoxy insulation systems based on mica splittings and reconstituted mica are compared with asphaltic compound and unsaturated polyester insulation systems based on mica splittings by extensive thermal cycle tests. Superior characteristics of epoxy insulation systems are proved.

In dielectric materials, measurements of the low frequency ac conductivity can often reveal certain features of the electrical conduction mechanism which are often not apparent from the data derived through dc or higher frequency ac measurements alone. In the not too distant past, low frequency measurements were difficult and tedious due to the rudimentary measuring techniques available and the associated instrumentation problems. However, the recent development of a low frequency bridge by Harris1 has changed the situation entirely; now low frequency measurements may be carried out with relative ease in the range from 10−3 to 1 Hz, using the Harris technique.

Actual defects, as well as areas of low resistance to the penetration of moisture, can be detected on coated insulating materials; and the spreading of these areas can be recorded by a modification of the electrographic printing method.

It has been shown1 that the distribution of the gradient in a dielectric under dc conditions generally depends on the rate of the voltage variation. This fact has already been studied in connection with a voltage polarity reversal1,2 in extra-high-voltage dc power cables, but it can also be of importance to define a correct testing procedure for the same cables. Conversely, an analysis of the transient behavior of the dielectric may also be useful from a physical point of view, in order to get a deeper knowledge of the phenomena involved.

In composite materials, the laminating industry has been built on the principle of locating things where they would do the most good. Way back in 1922 at General Electric Company I began to appreciate that the same base materials in the same proportions would give twice as much strength if they were disposed in laminate fashion than if they were randomly mixed. Throughout the years, these structures have been applied, originally to give greater strength and stiffness; but more recently through the whole gamut of qualities, to toughness and controlled permeation of films. Let me review some of those earlier days with you.

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.

Fiber Modified Isomica provides the electrical design engineer the tools needed to overcome many critical insulation problems at a reasonable cost. The unimpregnated nature of the tape coupled with its physical strength makes it suitable for many taping applications previously not considered for reconstituted mica products. The very high mica content suggests a practical way of obtaining the reliability of mica insulation in a wide variety of rotating electrical apparatus.

Immediately following the breakdown of a liquid dielectric a variety of dynamic events appear. Initially, a highly luminescent, thin, plasma column is formed followed by a radially expanding mechanical shock wave. In the process, a gaseous bubble composed of vaporized dielectric is created. The study described here is concerned with this bubble. Specifically, investigations were made into the effects of hydrostatic pressure upon the dynamics of the gas bubble.

Space is both an environment and an insulating medium. Space, viewed as an environment, is characterized by great temperature extremes, bursts of dangerous particulate and electromagnetic radiation, extremely low gas pressures, and swarms of particles and clouds of dust. Space as an insulating medium is characterized by a heat transfer process that is limited solely to radiation, the high dielectric strength associated with an essentially ion free environment, and the Jack of deterioration of materials caused by oxygen, water, and other similar terrestrial causes of corrosion and decomposition.

1. At stresses up to 100 volts per mil the three mica-containing materials have withstood corona at least 10 times longer than the organic insulations tested.

Some present developments in the area of laminated, low-loss and combined substrates for use in electronic equipment are discussed. The tailoring of these materials, particularly in light of their intended fabrication, to meet specific engineering needs is reviewed in terms of several concrete applications. Some of the properties which appear critical for the successful use of these materials are noted and used to provide insight concerning future trends in materials engineering in this area.

The major growth area within the printed circuit market is expected to be as a result of the development of flexible and multi-layer circuits. The continuing pressures on designers of electrical and electronic equipment for smaller and lighter packaging has greatly increased the demand for thin, multi-layer and flexible, wiring packages. In the automotive field, dashboard wiring is rapidly being replaced with flexible circuitry. Computer, appliance and meter manufacturers are among the many circuit users finding benefits in flexible multi-layer circuitry. Mechanical pattern generation methods lend themselves to the production of flexible circuits, where the insulating film to which the pattern is bonded is tough, flexible, and possesses good electrical properties.

According to Time magazine, the dreadful summer of 1955 will be remembered for a long time to come in the Eastern United States. Beginning in July, the region was withered by drought and a heat wave, the worst on record, with temperatures in the 90's for a large part of the month. Then Hurricanes Connie and Diane flooded the Northeast, killing more than 250 people and causing property damage well in excess of $1.5 billion (1).

The electrical properties of papers made from woodpulp prepared by different processes are known to differ. These differences have been attributed to differences in the quantities of residual pentosans, cooking salts, lignin and hemi-celluloses, as well as to the degree of attrition to which the fibers have been subjected. The study reported here is concerned with the variations in electrical properties observed on sheets of paper made from different lengths of fiber taken from the same pulp. It is concerned furthermore with differences in the electrical properties of sheets of paper made from the same size fibers taken from different pulps.

In a laminated, composite dielectric of polymer and paper, the paper-faced film of polymer behaves like a paper tape mechanically, while the cellulose protects the polymer against the direct effects of discharges which may occur in liquid filled cavities such as the butt spaces. The paper ply also facilitates initial impregnation and its maintenance under thermal cycling.

Five years ago, the author presented to this Conference a paper entitled “A New Approach to the Analysis of Insulating Liquids”. In addition to presenting theories concerning the behavior of insulating oils, this paper described the Doble Oxidation Test which furnishes information during an accelerated life test that is indicative of the oil quality and its probable service performance by determining the time to the first appearance of sludge or an acidity of .25 KOH/g. Published as a suggested method in the 1958 Annual Report of ASTM Committee D-9, it is currently under study by Committee D-27. A new equally pertinent test which measures power factor has been developed. The combination of both provides information of far greater value than does either test alone.

Measurements of temperature histories of epoxy/glass prepreg layers during multilayer printed wiring board lamination have been recently obtained. In the present paper, it is shown that the temperature histories may be predicted by an analysis of the transient heat flow process within the MLB. The analysis requires a knowledge of the press platen temperature history during laminations and the thermal resistance of the padding material. Theoretically predicted results show close agreement with the experimental data for both 5 layer and 15 layer MLB's using the "hot press" as well as the "cold press" cycles. The present method may be used in the design of MLB lay-ups, in particular the determination of the necessary padding thickness to achieve heating rates which give adequate timing "windows" for cure pressure applications. The present theory has been applied to study the effect of kraft paper padding on the prepreg temperatures. Results are shown for both 15 and 5 layer laminations employing the hot press as well as cold press cycles. The number of kraft paper sheets used is shown to have a significant bearing on the temperatures.