Publication: Western Electrician
Chicago, IL, United States
By C. E. DELAFIELD.
The science of properly and safely insulating line voltages of high potential has not kept pace with the demands of transmission engineers, and today we are face to face with the problem of successfully transmitting potentials in excess of 100,000 volts. In California power is transmitted at 60,000 volts more than 200 miles, but great line losses are suffered and the investment in copper is heavy. An increase in voltage from 60,000 to 150,000 would make it possible to deliver this power with reasonable losses and in much greater quantity. As an illustration of the possibilities of delivering power at 150,000 volts, it would be possible to deliver the power generated at Niagara Falls economically to Boston, New York or Philadelphia, and, apparently, the principal hindrance to this consummation at the present time is in the fact that there is not on the market what might be termed a successful insulator for this enormous voltage, although the merits of a number of different types of insulators are at the present time being advocated for this purpose.
The design of an insulator for high voltages (quoting the words of Gerry) should involve a consideration of all of the effects of electrical tension on the dielectrics in the vicinity of the conductors. In the case of a line insulator, air is always a dielectric in combination with wood, porcelain, glass or other materials, and, wherever there is a difference of electrical potential, there exists in the surrounding media a certain state of strain called an electrostatic field. This state of strain is the result of electrical stress applied to the insulating material. It frequently happens, when several dielectric materials are subjected to the same electrostatic field, that one or more of the materials will be strained beyond the limit and fail, although the others will stand the electrical tension. Air adjacent to powerful dielectrics frequently fails in this manner, thus giving rise to the well-known brush discharge.
The structural failure of air from an engineering standpoint has been studied by a number of investigators, including Dr. C. P. Steinmetz, Dr. F. A. C. Perrine, Prof. Harris J. Ryan, Mr. M. H. Gerry and others, and, as the result of their published investigations, it is well known that air at the ordinary pressures and temperatures has a much lower dielectric value and strength than the common insulating materials. Air in thin films, adjacent to solid bodies, has greater strength than in bulk, and is still inferior to such substances as glass and porcelain. The dielectric strength of air is affected by its physical condition and varies directly as the pressure and inversely as the absolute temperature. Under uniform conditions dielectrics rupture at definite applied tensions, Professor Ryan having shown that there exists for each dielectric material a certain strength of electrostatic field, which will cause a rupture.
These being the fundamental rules by which designing engineers formulate their plans for the manufacture of insulators for varying potentials, it can well be seen that the various forms of insulators on the market today are the results of working out these rules by different individuals looking at the same thing from a different standpoint. That is to say, climatic and geographical conditions exert considerable influence in the design of an insulator. Insulators suitable for dry atmospheric conditions would not be suitable for a condition where sea fogs and dust exist, and there is no question that a correct solution will soon be forthcoming for a standard insulator for voltages of 75,000 and upward. In fact, as noted before, the different manufacturers are now experimenting toward that end.
Looking over the history of high-tension transmission, it is only about 15 years since we looked with wonder on the Frankfort-Lauffen transmission line in Germany, of 30,000 volts over 100 miles. Today there are thousands of miles of long-distance transmission at voltages ranging from 11,000 to 65,000 volts, and great credit is due the engineers who have designed and carried out this work in the face of almost insurmountable obstacles. The progress of high-tension transmission has been very rapid, and in all of the various branches, with the exception of the line insulators, it is now possible to handle voltages in excess of 75,000 volts, there being no difficulty whatever in the designing and manufacturing of successful transformers and switchboard apparatus for these high potentials. It should be taken into consideration, in the future designing and laying out of transmission lines, the possibility of increasing the present voltage to the voltage that may be possible a year or more from now, so that large quantities of power may be economically distributed over long distances.
It has been demonstrated by practice that very large generating units can be successfully operated and that both steam-turbine and hydro-electric plants can be operated successfully, and the one question to be decided is, how can large powers that are so successfully generated be distributed over long distances economically, taking into consideration the high price of copper and aluminum. The answer to this question is, by high voltages only. Up to the present time the commonly accepted form of insulator is what is known as the pin type, meaning by that an insulator having for its resting place a pin embedded in, or fastened to, a cross-arm, this pin being of wood or metal. Present practice has demonstrated that wood can be safely accepted for insulator pins up to 25,000 or 30,000 volts. Beyond that it is advisable, for mechanical reasons, to use malleable iron, but the so-called pin type of insulator has reached such dimensions, in the endeavor to meet the requirements for higher voltages, that it seems to be the consensus of opinion of the leading high-tension engineers that this type of insulator has reached the limit of good line construction, and, when one stops to think of the dimensions of an insulator used on a 60,000-volt transmission, one is inclined to think that the engineers are correct.
Not only is it a difficult matter, from a mechanical viewpoint, to find a pin that will take the necessary stress incident to an insulator of this large size and weight, but the problem of manufacture, from the viewpoint of the pottery, is one that is exceedingly difficult, so that, apparently, it is necessary to make a radical departure from the present practice of pin insulation in order to take care of the various difficulties that are encountered in the construction of insulators for the higher voltages, and it is the belief of the writer, and also of other engineers, that a suspended form of insulator will be the type which will be used, it being, from a mechanical standpoint, a comparatively simple, matter to suspend any desired weight, and, from an electrical standpoint, it seems possible so to design an insulator that it will be mechanically strong, and a good dielectric as well.
The suspended type of insulator would have the advantage that ample arcing distance could be provided without making the insulator topheavy and difficult to manufacture. It should be so designed that arcing cannot occur until the voltage is sufficient to rupture the air and cause the current to arc from end to end, this feature being of great importance where the insulators are mounted on steel towers, which is conceded to be the best engineering practice. On high-voltage lines where steel towers are used, the pin type of insulator for 100,000 volts or higher would seemingly be almost an impossibility, owing to the size necessary to take care of the surges and other line disturbances and owing to the fact that the earth potential is carried into the head of the insulator by the steel pin and through the metal towers.
An ideal insulator for all conditions of high-voltage stress should be one that would take care of climatic conditions, such as fogs, dust deposits, salt spray, etc., and should have as few still-air spaces as possible. That is to say, it should expose a large part of its surface to the wind and should have a long leakage distance of small area. In the designing of a type of porcelain insulator for this class of work, it should be borne in mind that cemented parts, if there are any, should be under compression and not under tension, owing to the strains to which it may be subjected to from expansion and contraction. There should be as few still-air spaces as possible, to avoid the accumulation of dust, insects, etc., and there should be nothing but porcelain, well vitrified, between the points of opposite potential. Engineers are now at work along these lines, and, as the result, a number of plans have been proposed embodying more or less of these ideas, and it is only a question of a short time before the successful insulator will be evolved for these higher tensions, if it has not already come to pass.
Leaving for the time being the open question of extraordinary potentials, we will take up the question of high-tension transmission as it exists today. Continuous operation of a transmission system is an absolute essential, and depends to a large extent on the effectiveness of the insulator used. In this paper the writer only aims to discuss the qualities of porcelain insulators, as it is now generally conceded that porcelain is superior to glass for the manufacture of high-tension insulators, and, in fact, supersedes glass wherever the question of cost is not a paramount problem.
In the designing of an insulator for any given voltage, and especially for the higher voltages, there are three considerations of primary importance: First, electrical design; second, the mechanical strength; and third, the quality of the material.
In the electrical design consideration must be taken of the dielectric strength of the adjacent air, so that sufficient distance be allowed between the points at which the line voltage is impressed that it will not arc over to the pin or cross-arm under ordinary working conditions. In other words, make the potential gradient as gradual as possible from line wire to ground. These points of impressed voltage may vary greatly in an insulator of poor manufacture, although of the same electrical design. For instance, in two insulators of the same general design and different manufacture, the one having the greatest electrostatic capacity, and, therefore, the greatest electrostatic field, will suffer from brush discharge and arcing-over sooner than one having less electrostatic capacity and, therefore, less electrostatic field. In another case, two insulators of the same design but of different manufacture, the one possessing a body of greatest density and which is the most vitreous, will carry ordinary working voltages and line disturbances with less trouble than an insulator that does not possess these qualifications.
Due care must be exercised in the manufacture of porcelain insulators to secure the necessary dielectric strength between the tie wire or top groove and the point inside the insulator which is in closest proximity to the head of the pin, providing that pin is made of iron, which is usually the case in voltages in excess of 30,000, and in many cases misfortune has come to the engineer who depended to a large extent on the pin and cross-arm for additional insulating qualities. Practice dictates the fact that on the insulator alone should be the reliance of the engineer for his insulation, and all insulators, whether of porcelain or glass, should be tested with approximately three times the full line voltage brought to the inside of the insulator head. The entire burden of correct and sufficient insulation should be placed on the insulator itself; thus a large number of line troubles would be prevented.
It has been the unfortunate habit of some engineers to consider the cost of the insulator of paramount importance, and when one takes into consideration the importance of the insulators to the construction of a line, one is always led to wonder why, by the additional cost of a few cents to each insulator, a reasonable factor of safety is not obtained. It is, however, pleasing to note that many engineers are profiting by the sad experience of their brothers and are securing their insulators based on specifications that insure a reasonable factor of safety, and, in fact, are in some cases going to the other extreme and not only require the manufacturer to guarantee their insulators to stand a rigid test, both before and after erection, but in one case which the writer recently noticed, an additional clause was inserted requesting the manufacturers to guarantee that the railroad would not break them in transit, which, we will have to agree, was rather a severe test.
Reverting once more to the electrical design, it is necessary, in the design of an insulator, that the factor of safety be sufficiently large so that the abnormal electrical strains that may be, and are, occasionaly brought to bear, will not cause a puncture and consequently a shutdown of the line. For instance, an insulator designed to carry 50,000 volts, should stand a dry test of approximately 150,000 volts, thus giving a fair factor of safety to enable it to withstand the possible surge voltages caused by short-circuits, etc. This very fact of requiring a reasonable factor of safety in the electrical and mechanical design of an insulator has decided the limiting possibilities of the pin type of insulator as approximately 60,000 volts line voltage, as, to secure a factor of safety of three, it would be necessary to build an insulator of mammoth proportions and uncertain body, having a weight that is almost prohibitive to pin work. This brings us again to the conclusion that the only method of securing a proper factor of safety on the higher voltages would be to use a suspended type of insulator.
Having discussed the engineering design of a high-tension porcelain insulator, the writer thinks it might be of interest to take up its composition and the difficulties of manufacture, concluding with the tests to which all insulators should be submitted before being placed on the line.
The porcelain of which a high-tension insulator is made is composed of certain proportions of English ball, China clay, some domestic clay commonly called Tennessee, and some feldspar and quartz. The clay forms the body and gives the proper mechanical strength, while the function of the feldspar and quartz is to act as a flux and thoroughly permeate all the parts of the insulator, thus making a thoroughly vitreous mass when subjected to a sufficient heat. This mixture, having gone through the various steps of grinding, is forced through a filter of copper or silk cloth of 110 mesh to remove all impurities, and is then formed in the various shapes and designs suitable for the purpose for which it is to be used and placed in the drying room.
After a sufficient amount of moisture has been removed in this way the insulators are dipped in the glaze solution and are again placed in the drying room, after which they are placed in the kilns and subjected to a heat approximating 2,700 degrees Fahrenheit. The function of the glaze is to give the insulator the necessary, color and also a smooth, even surface, in order that dust and rain may be easily dispelled. Ordinary unglazed porcelain would come from the kilns a pure white color with a comparatively rough surface, which would hold the dirt and moisture, so that it is necessary to glaze the insulator in order that a smooth, glassy surface may be obtained, as well as the desired color. If, in baking, the insulator is not subjected to the proper amount of heat, the body of the clay will not be thoroughly vitrified. If, on the other hand, the insulator is subjected to too much heat the body of the clay will be porous. Therefore, it is necessary that the insulator be subjected to the exact amount of heat necessary to secure the desired results.
In determining the exact amount of heat to which these insulators shall be subjected, ordinary thermometers are not used, but a small, carefully prepared cone of predetermined composition is utilized, which is placed at intervals in the kilns and is observed through small orifices by the attendant. These cones assume an erect position under any degree of heat below the desired one. As soon as the predetermined point has been reached, however, the cones melt and form glaze, and at this point the heat is turned off and the ovens allowed to slowly cool, so that the ware may be thoroughly annealed. This process cannot be hastened, but takes a certain well-defined time for its operation. When removed from the kiln the insulators are then subjected to a most rigid electrical and mechanical test to determine their qualifications for the work which they are designed to do, and all those not coming up to the specifications should be rejected at this time. A certain number of all insulators coming from the kilns are subjected to two electrical tests, one a dry test to determine the dielectric strength of the porcelain body, and the other a precipitation test to determine as nearly as possible the action of the insulator under most severe climatic conditions.
The only objection that has been offered to porcelain as the composition of which the insulator should be made is the fact that in no other way than the above described can a porcelain insulator be properly tested, whereas the defects that might occur in glass can be detected by the eye. Unless an insulator is thoroughly vitreous and is practically non-absorbent and shows a fracture similar to glass, it is unfit for use under high-potential stress. An ideal insulator would be one having a minimum amount of electrostatic capacity with a maximum amount of mechanical strength, but in the pin type of insulator a strange phenomenon exists, inasmuch as it is necessary to sacrifice one condition to some extent to obtain the other good points, and an insulator of successful design is one which exhibits a happy medium.
1. A paper read at the annual convention of the Canadian Electrical Association, Montreal, September 12, 1907. The author is manager of the high-tension division of the Ohio Brass Company.