Publication: The Journal of Electricity, Power and Gas
San Francisco, CA, United States
In the early days of power transmission practice, glass insulators were more largely used than those of any other material. This was no doubt due to the fact that suitable types and sizes were more readily obtained in glass. No commercial factories for the exclusive manufacture of porcelain insulators had been established, and the few porcelain insulators which were manufactured were made in potteries devoted more to the manufacture of low voltage porcelain ware such as knobs, tubes, cleats, etc., and not calling for the highest grade of porcelain nor perfection in manufacture. Hence it was that a great deal of trouble developed in the early porcelain insulators, whereas at the voltages used upon power transmission lines, glass insulators gave fairly efficient service.
At the period referred to (some twelve to fourteen years ago), the general commercial voltages were in the neighborhood of 20,000 volts, although there were some few instances of higher voltages in successful operation. At that time, glass insulators of the triple petticoat type from five and one-half to six inches in diameter were successfully used at 20,000 volts. This type was succeeded by glass insulators seven inches in diameter, which proved fairly serviceable at as high as 40,000 volts under suitable conditions. With one or two exceptions, the seven-inch diameter glass insulator has been the largest commercially manufactured and used. In the early development of the art, it was claimed that glass insulators possessed numerous advantages over porcelain. One of the principal advantages mentioned was that of being able to determine the existence of flaws or imperfections by visual inspection. High voltage testing apparatus had not been sufficiently introduced to make the universal testing of insulators practicable, and as it was considered that a porcelain insulator must be tested before use, their general use was discouraged.
As transmission lines began to increase in length and the demand for high voltages increased, it became necessary to enlarge the size of the insulators. It was soon found that there were decided limits to the size to which glass insulators could be extended; the reasons being inherent in the very nature of the glass itself, and its process of manufacture; that is, increasing the size of glass insulators necessitated the manufacture of masses of glass which could not be readily annealed, and even though annealed according to the best practice of the glass factories, extreme changes in temperature after erection on the poles set up strains within the glass due to expansion and contraction which often resulted in failures. This not only applies to the larger types of glass insulators but has been a frequent cause of trouble upon small glass insulators subjected to a wide variation in temperature.
Although the general practice has practically driven glass insulators from the field upon any new high voltage transmission work, yet the glass insulator still has its place and can be used successfully at moderate voltages; that is, up to 25,000 to 30,000 volts, under suitable climatic conditions, with success almost equal to porcelain. At the present time the only valid reason for using glass insulators in place of porcelain, seems to be the question of initial cost. Generally speaking, the average price of glass insulators should not be over fifty per cent of that of porcelain of the same general type.
Turning now to the question of porcelain insulators, it would appear that an engineer to properly design a high tension insulator should first be conversant in a general way with the process of manufacture in order to know the limits of shape, size, thickness, anal other problems of manufacture which of necessity influence the design of the commercial insulator. Therefore, a short description of the process of manufacture will probably not be out of order.
Porcelain for electrical purposes is a mixture of ground flint or silicon dioxide, and feldspar or potassium aluminum silicate, raised to a vitrifying temperature; that is, to a temperature sufficiently high to melt the feldspar, and from it unite the parts of flint into a perfect homogeneous body of uniform electrical and mechanical strength. The production of electrical porcelain differs from the ordinary pottery product in that in addition to presenting a symmetrical and flawless exterior it must possess inherent electrical and mechanical strength.
Though very finely ground and apparently in the last stage of subdivision, the clays after having been properly mixed as to quantity of each, are placed, together with sufficient water to form a liquid mass, in a ball mill or large iron tub lined with porcelain brick and half Idled with quartz pebbles selected from the glacial drifts of Iceland. The tub is caused to rotate on its supporting axle and the quartz pebbles still further reduce the clays in matter of fineness and thoroughness of mixture. From the ball mill, the mixture of clay and water is run through a lawn or 150-mesh screen to a cistern from which it is drawn by means of a power pump and forced into filter presses. Filter presses for this class of work are of the type used in many potteries, and consist of a series of cast iron rings separated by sheets of canvas pierced by a three-inch hole to admit the liquid clay. The canvas sheets so arranged form a series of pockets from which water may readily leak, but in which the clay is retained—thus forming "leaves" about thirty inches in diameter. As the pockets become filled with clay, the pressure steadily rises until no more can, with safety, be applied. The clay is then ready for working, but to ensure its being homogeneous and thoroughly pliable it is put through a pug mill or large sausage machine, which forces it through a die under pressure from whence it emerges as a long "sausage" about four inches in diameter. In this state it is delivered to the potter, who is thus furnished with a perfectly pliable and reliable clay upon which to work.
The process of drying and firing of electrical porcelain causes it to shrink about fifteen per cent, and accordingly the model of the insulator from which all moulds are made is fifteen per cent larger than the finished piece of ware. Such models are usually turned from blocks of plaster of paris, their surfaces oiled, and the moulds cast from them in one, two or three parts as occasion may require.
Having been properly dried, the moulds are filled with just enough clay by the potter's assistant, and are placed by the potter upon his wheel where the operation of shaping the inner side of the particular piece under consideration is carried on by the aid of the hands and a properly shaped former, which is held in place mechanically, and thus produces a perfectly uniform shape inside. The outside of course is determined by the shape of the mould. The mould with its clay is now set aside for some hours, during which time the plaster performs its functions of absorbing the water of the clay immediately adjacent to it. The clay has now become dry enough to handle, and is removed from the mould and carried to the finisher, who, by means of a revolving table, sets the partially dry ware to rotating, a wet sponge, and if necessary, a sharp knife being used to remove any irregularities and produce a smooth exterior. The piece thus finished is now set away and allowed to dry completely, after which it is clipped in a silicate solution, some of which is absorbed in the ware, thus forming a thin glaze or glasslike surface, whose only function is to produce a smooth finish as well as impart color to the insulator. At present a dark brown color is usually employed because of its supposed inconspicuousness compared to white; however, it is possible to produce any desired color by placing in the glaze the proper coloring matter.
In some of the smaller sized insulators the glaze accomplishes a double purpose, for besides covering the surface of the ware, it is made to serve as a cement for fastening the parts together, forming a neat and strong joint, for the glaze liquefies at the temperature at which the body vitrifies.
The kilns in which the ware thus prepared is fired are cylindrical, being about eighteen feet in diameter and sixteen feet high, lined with firebrick, and arranged with fire bags around the base, the fire from which is drawn over into the kiln and down through the floor and thence to the chimney. In order to protect the insulators during firing, they are placed in fire-clay receptacles called "saggers," which are piled one on the other until the inside of the kiln is completely filled, after which the entrance to the kiln is bricked up and sealed with fire-clay mortar. There remains now but to gradually raise the temperature to the required degree, which point is made evident by the fusing of small porcelain cones placed at regular intervals about the, kiln, an opening being left so that ready access may be had to them. Before each opening are placed four cones which fuse at various heats, and in firing a kiln, the temperature is raised until three of each set of cones are fused and the fourth very nearly so. In watching the cones the heat is so intense that a man cannot face it unprotected, and the light is so dazzling that a colored hand-glass is necessary to soften it so that the eyes can endure it. The heat is raised with such regularity that no unnecessary strains are created in the ware by sudden variations in temperature. The heat within some parts of the kiln is so intense as to fuse the firebrick with which the sides are lined. The proper heat having been attained, the fires are allowed to cool down and the annealing process carried on until an insulator absolutely free from internal strains is assured. The door is broken down and the finished insulators pass from the ceramic to the electrical test and shipping departments.
It has been found that the economical limit of size in a single piece of porcelain is about fifteen to sixteen inches in diameter, although larger pieces have been manufactured; notably the 75,000-volt insulators of the Kern line of the Edison Electric Company, which have a top diameter of eighteen inches.
The nature of the process of manufacture puts a decided limit upon the shape and size of the various parts of a porcelain insulator, and owing to the difficulties in burning, to say nothing of the expense entailed, all pieces of a large diameter, and at the same time of any considerable height, should be avoided. It is also well known that owing to the difficulties encountered in burning, all sharp angles should be avoided, and further, that each piece of ware should have a uniform thickness in all its parts, for the reason that if there is any considerable variation, the evaporation will be more rapid in the thin portions and cracks will be developed in drying, which will be aggravated in the burning.
It was formerly believed that the glaze upon the exterior of the porcelain insulator possessed greater insulating qualities than the body of the insulator. This idea no doubt became prevalent, due to the fact that the early porcelain insulators were made of inferior material, and oftentimes the porcelain was more or less porous. The glaze performed the function of filling up the pores and forming a glass-like surface of high dielectric strength, at the same time keeping the porcelain from absorbing moisture and becoming clogged with small particles of foreign matter which would eventually serve as a pathway for current leakage.
At the present time, the porcelain body of high tension insulators does not depend upon the glaze as an insulating medium as the porcelain itself, while clean, will stand the same test potential as with the glaze. The glaze, however, performs the important function of smoothing over the natural imperfections of the body of the porcelain and preventing the collection of foreign matter which would serve as a leakage path.
Glazes are of two types, namely, soft and hard. The soft glaze is merely a soft lead glass with sufficient clay present to prevent its settling until placed on the ware. Such a glaze is naturally very unreliable, not only because of the tendency of lead to volatilize under heat, but owing to its glassy nature, it is not able to expand and contract equally with the body of the ware, and the result is crazing. By "crazing" is meant that cracked condition of the glaze so commonly noted in art and table ware, and caused by sudden variations in temperature, making the mass expand or contract. Soft glazes being a species of glass, their power of expansion is not equal to that of the body to which they are attached; hence when a change in temperature occurs, the strain becomes too great for the brittle glaze, and breaks occur, leaving the glaze in small individual pieces on the surface of the ware. Crazing may not appear for some considerable time after the insulator is placed in service, but the elements will eventually cause this to occur.
Hard glaze—which-is the type used upon all high grade porcelain insulators—is the true porcelain glaze, and its essential features are:
First. That it shall be composed of practically non-volatile substances (which excludes lead).
Second. That it shall contain only enough flux to fuse the matrix of silica and clay, and shall not flow or run.
Third. That the body shall be sufficiently absorbent when the glaze is applied so that the glaze may attack it and thus make a perfect union under the fire action.
Fourth. That the body and glaze shall have the same coefficient of expansion and shall mature at the same temperature.
It is much more difficult to produce a smooth and uniform surface where lead is not used, which may account for the wide difference in beauty of finish of insulators of different makes.
Any desired coloring can be obtained by the use of the proper colorants. The most generally accepted color at the present time is brown, although slate, or a neutral tint, has been extensively used. It is claimed that either brown or slate is much more inconspicuous upon the pole line than white, but no glaze which has vet been observed by the present writer has been found sufficiently inconspicuous to make its selection of any particular advantage in preventing damage to insulators by miscreants.
The matter of glazing joints does not usually come up for discussion in the consideration of insulators of the higher voltages, that is, 60,000 and over, but there is yet some considerable difference of opinion regarding glazed joints versus cemented joints for the moderate voltages, say from 25,000 to 35,000 volts. An insulator made with glazed joints, after having successfully passed the various mechanical and electrical tests, and having been placed upon the line in good condition, is undoubtedly as good an insulator as one made with cemented joints. When it is understood, however, that an insulator with glazed joints must be fired as one piece in the kiln, it will readily be seen that in the failure of any one piece, the entire insulator becomes worthless. This necessitates adding the loss of a higher percentage of parts to the factory cost, which the consumer is expected to pay. It will also be noted that unless a hard glaze is used in glazing the joints together, a loosening of the joints is liable to occur in the course of time, due to variation in temperature, for reasons above explained, namely, the different co-efficient of expansion of the porcelain and the glaze. The method of testing high voltage insulators is so well known that a description will not be attempted here. Owing to the nature of the ware it is generally conceded that all high voltage porcelain must be tested by wet test according to one of the usual methods in order to determine if any flaws which are not perceptible to the eye exist. Many of the imperfections in the body of the porcelain ware are microscopic and others are entirely concealed by the glaze so that the electrical test is absolutely essential.
In general, the electrical test applied to the various parts of multi-part insulators is kept very near to the arcing-over point, but there are limits to this test as described later. The number of failures under test is usually from two per cent to three per cent of the number tested. Once punctured, the porcelain is ruined and can by no known means be recovered.
The parts of multi-part insulators are united by means of pure Portland cement mixed with water only. After the cement has obtained the initial set, the insulators are subjected to an assembled electrical test of double line potential for a period of time sufficient to remove all doubt as to the electrical strength of the insulators, usually from one to five minutes.
Design of High Voltage Insulators.
Insulators for the lower voltages call merely for the introduction of sufficient good material between line and supporting structure to prevent destructive leakage. For voltages up to 20,000 little or no difficulty is experienced in securing such insulation, but above that voltage it becomes not only necessary to have good material in plenty, but it must he properly distributed in order that danger of puncture and severe leakage he reduced to a minimum.
The manufacture of good porcelain from a mechanical standpoint forbids that it be made so vitreous as to resist a puncture test of 65,000 to 70,000 volts through 1/2-inch or 5/8-inch thickness. It is useless to attempt to gain greater dielectric strength by increased thickness since individual cracks in thick pieces really reduces the effective strength to that of 1/2-inch or 5/8-inch porcelain. Accordingly it has come to be recognized as best practice to make no attempt to manufacture high voltage insulators in a single piece, but rather to secure greater electrical strength by multiplicity of parts. Though entirely possible to construct 30,000 volt insulators of one piece, and still apply a double potential test, experience has demonstrated that a multi-part insulator is much less liable to fail entirely when struck by stones or bullets than large single-piece insulators,