Insulators for Electric Power Transmissions

INSULATORS FOR ELECTRIC POWER TRANSMISSIONS.

BY T. A. W. SHOCK.

(A paper read before the Santa Cruz Convention of the Pacific Coast Electric Transmission Association, August 17, 1897.)

The advent of electric transmission of power into the electrical field has made the question of line insulation considerable of a study. The possibilities of a shut down, especially in instances where the plant is delivering a twenty-four hour service, are worthy of serious consideration and the object of every manager is to reduce these possibilities to a minimum. The high voltages at which transmission lines are now being run and the still higher voltages at which they will soon be operated, makes the insulator a very important factor in the successful insulation and operation of electrical transmission plants.

The climatic conditions must be considered in every Case. In countries where snow and ice are unknown, an insulator might stand heavy rains, but it might not stand the heavy strains to be brought upon it by the atmospheric rigors of other localities. Where snow and ice are prevalent for four or five months of the year, the question of proper line insulation becomes a serious problem, for the insulator has not only to resist the passage of. current through the ground, but must be sufficiently homogenious to withstand the extra weight of ice, sleet and snow that would adhere to the wire. The heavy snow and- sleet: storms prevelent in the East instance the great strain that insulators might be subjected to.

In California these conditions do not exist except in cases where the source of power is located above the snow line. This is a condition to be considered, as the future of the transmission of power in California lies to a certain extent in the utilization of streams that have their sources well up in the mountains where snows prevail each winter. In localities subject to heavy rain, it is necessary to use an insulator that will shed the rain quickly, and at the same time have an outside petticoat sufficiently large to protect the pin from moisture.

On July 16th, 1894, the Sacramento Electric, Gas & Railway Company, with which the writer was connected, commenced transmitting electric power from. Folsom to Sacramento, a distance of twenty-two and one-half mile. During the succeeding winter the rains were quite heavy and some difficulty was experienced from punctured insulators and burned pins. The line was equipped with two styles of porcelain insulators, viz: A triple, vertical petticoat, and a later type consisting of a triple flare petticoat. The former gave us all the trouble the first year and the latter most of the trouble the second year of operation.

In most of the cases the pin was burned at the lower edge of the inner petticoat and only in a very few cases was the pin burned at the top. Sometimes the insulator was punctured at the top, but in most of the insulators the trouble that developed seemed to be at the lower edge of the thinner petticoat close to the pin where the glazing had been removed in baking.

In examining the insulators taken off the line I found in some cases that although the puncture showed at the top it did not appear to carry all the way down. Generally cracks were shown at the top of the insulator which followed all the way dawn to the bottom, thus showing a lack of vitrification in moulding the insulator, as the cracks were, in some cases, simply covered by the glazing. Invariably the glazing was off where the insulator rested when baking, and this was also in evidence on many of the new insulators. Both styles of these insulators are now in use on the transmission line between Folsom and Sacramento. This experience appears to show that it is not so much in the shape of the insulator as it is in the manner in which it is made. A dozen china Locke insulators were placed on the line last winter and so far as is known they have given quite good satisfaction.

In an insulator recently taken from the lines of the Sacramento Electric, Gas & Railway Company and tested as to its porosity by immersing it in red ink, the writer found that the ink worked its way through the material to its surface where the glazing was worn off showing that when the glazing was removed there was sufficient porosity to allow leakage through the material to the inner petticoat and thus to the pin. In most cases where the insulators were taken off the line they were removed front portions where bare copper conductors were used and these insulators showed that the glazing was worn off where the wire came in contact with it by the working of the wire during winds or heavy storms. The glazing having been worn off, the current easily found its way to the pin, which meantime was also burned. This experience also appears to show that an extra heavy glazed surface is required and that as perfect homogeneity in the material is necessary in order to prevent leakage.

In designing an insulator the object is to get the longest insulating surface in the smallest number of square inches and to secure the cheapest insulator that can be manufactured to meet the existing conditions. The cost of insulators is a very important factor in the installation of electric transmission lines. The first cost has to be considered as against possible interruptions that might occur to the line.

Many experiments have been and are being made on glass insulators for high potential currents, and to my knowledge no tests in actual service have as yet been reported. In my opinion the Locke two-part insulator meets the conditions for high insulation as well as any insulator on the market. In this the outer shell is of china, which presents a high insulating surface, as well as mechanical and electrical strength and the inner or center shell is of glass or china, to afford further insulation. This makes it possible to obtain a more thorough vitrification than would be obtained in one solid insulator and in addition, affords four thicknesses of glazing in further insulation. The cost of this insulator, of course, will be a factor, but this is certainly offset by its higher insulating properties.