Publication: The Journal of Electricity
San Francisco, CA, United States
INSULATORS FOR TRANSMISSION LINES.
BY JOHN MARTIN.
ONE of the most important questions which presents itself to the engineering fraternity who are exerting their efforts to utilize the vast water powers in this State and elsewhere, that have been for ages wasting their energies in wearing away rocks and washing mud and sand to the lower levels, is the problem of insulating the line wires of a transmission circuit so as to withstand the severe strain of the higher potentials which are now being used in order to bring nature's bounteous energy from the distant mountain ranges to the assistance of struggling humanity, and to enable them to cope more successfully with the ever-present problem of economics that is being forced upon them by the gigantic march of progress throughout civilization. The generation of high potentials has been brought to a state of perfection, and while it is a question of insulation, the conditions encountered arc perhaps more favorable than the insulation of a wire which is exposed to the elements.
In the early days of electric transmission, when the power house was always located as near the center of distribution as possible, the ordinary glass insulator, which had been designed for the low potential service of telegraph work, was found all sufficient to give proper insulation to the fairly well insulated wire used in constructing these circuits. When, however, the potential of the circuit is raised into the thousands of volts, the wire with ordinary line material becomes a case of "paving too much for the whistle," and the money expended in paying for a complete coating of insulating material al the price of copper per pound had better be expended in a higher grade of supporting insulator which will protect the line at its most exposed points.
We are living in an age of progress, and intercommunication with the old stage coach, while it still exists here in this State and elsewhere as a matter of necessity, is no longer the only means of bringing the population of widely separated sections in touch with each other. The extension of our railroads, with the constantly increasing train speed and the vast increase in traffic, made it desirable and necessary to communicate rapidly with distant sections of the country. In order to accomplish this, it was found necessary to increase the working potential of telegraph lines, and in its train came the demand for a better class of insulators.
During the battle between the demands of progress and the efforts of humanity to meet the imposed requirements, Mr. M. Locke was employed as a telegraph operator on a division of the New York Central & Hudson River Railroad, and also on the Pennsylvania Railroad Company. During wet and foggy weather he experienced difficulty in keeping the instruments which operated on the long lines in adjustment. On several occasions he was reported and censured for not answering his call. On one particular occasion Harrisburgh started to call him and he opened his key to answer, but found on closing it again that Harrisburgh was still calling. This puzzled him, as there was but one wire and that grounded at each end. Being of an inquisitive nature, he set out to determine the cause of such conditions, and found that the insulators were completely coated with a film of moisture, resulting from a fog. As this condition existed on a large number of insulators, the greater portion of the current returned to Harrisburgh through the shorter circuit by leakage over the surface of the insulators, and the opening of Mr. Locke's key did not interrupt Harrisburgh's circuit.
It naturally occurred to him that by increasing the surface length between the point of attachment of the wire and the cross arm, as well as the proper designing of the insulating surface, the tendency to surface leakage would be reduced. He started experimenting in a rather novel way with various kinds of insulators tied to the main line by the regular wire groove and a wire attached to the inner side of the pin hole, using his tongue as a galvanometer. After receiving several severe shocks from insulators with short surfaces, he was more careful in the selection of an insulator with a longer surface between the wire groove and the pin hole. This led up to the idea of increasing the number of petticoats and also increasing the diameter and lowering the height. The lowering of the height had the advantage of bringing the dripping edge closer to the cross arm and the increase in diameter brought it further away from the pin. For further protection the middle petticoat was made to extend below the outer edge so as to further protect the inner petticoat and the pin. Patents were taken out by Mr. Locke covering the various improvements and the credit must be given him for the pioneer work in the development of a multi-petticoat insulator, which has since been adopted for all high potential transmission circuits.
Dr. Louis Bell is quoted as having said that the best insulator is an ordinary five cent coffee cup, and in correspondence on the subject with Prof. Perrine, of Stanford University, he was informed that if the same amount of money had been expended in the perfection of the High Potential Porcelain Insulator, that had been expended in the perfection of manufacture of the five cent coffee cup, the product would now be more perfect and better suited to our needs.
Porcelain was used in the manufacture of insulators in order to get greater strength and to overcome the hygroscopic nature of glass insulators produced at that time. The production of a good porcelain was not well understood by our American manufacturers, and the first porcelain insulators produced were not as good as glass as far as insulating properties were concerned. Improvements have been made in the methods of manufacture and the following process has been found to give the best results.
The mixture is made up from a high grade of spar, which has been calcined and ground to a fine powder, finely ground flint of the best quality, and a quantity of the finest plastic white clay. This is thoroughly mixed together in wooden tanks with water, and then run over bolting cloth screens of a very fine mesh, to take out any coarse particles; it is then strained through a cloth to separate the "slip," as it is called; from the water. The slip is then dried to the proper degree and formed into insulators; then set away to dry sufficiently so they can he handled without destroying their shape. They are then placed in the upper part of the kiln and baked just enough to dry off the moisture. In this shape it is called a soft biscuit; this biscuit is then immersed in a mixture composed of the same material as the body of the insulator, but the proportions are varied by the use of a greater quantity of spar. After the biscuit has been immersed in this solution they are allowed to dry fur a short time, and then put into the kiln and fired at a high heat. At this high temperature the water of absorption and crystallization is driven off and the greater quantity of spar in the surface mixture produces a perfect glaze of practically the same material as the body. An insulator produced in this way will not crackle or "craze" when exposed to variations of temperature. Great care must be used in annealing all insulators, as a sudden change from the high heat of the kiln would crack the glaze of the best porcelain. Insulators coated with a glazing similar to soft glass may pass test and be put on the but after a year or so the surface may be found full of cracks, or crazed, and the accumulation of dust and dirt result in serious surface leakage.
A porcelain insulator should he vitreous through the entire body and the co-efficient of expansion of the surface material should he the same as that of the body. It is an open question as to the relative advantages of a porcelain insulator properly made, against a properly constructed glass insulator which has been properly annealed. It is claimed that the difference between the hygroscopic properties of porcelain and glass cannot be measured, and there seems to be good ground for this assertion.
After analyzing the great difficulties which are encountered in the manufacture of porcelain insulators, and the inefficiency of visual inspection, the glass insulator presents a better front in two particulars: it will not puncture, and defects are readily discernible with the eye. By the proper annealing of glass, the objection of extreme brittleness is removed. The question of leakage (if in excess of porcelain) can be overcome by proper cross arm and pin installation.