Niagara Falls Transmission Line

THE NIAGARA-BUFFALO TRANSMISSION LINE.

READ BEFORE THE NATIONAL ELECTRIC

LIGHT ASSOCIATION, JUNE 9, 1897

By J. G. White.

First cost within reasonable limits should always be considered secondary to good construction, so that you will probably concur in awarding first place as a factor in problems of this nature to the avoiding of interruptions. This is surely of primary importance in most lighting work, and it is easy to conceive of circumstances where the unexpected shutting off of power might have more serious or possibly more fatal results than could ever arise from sudden deprivation of light. Entering into this problem of sure and continuous operation, we have the same two factors entering which usually confront us in connection with any electrical installation; namely, insulation and mechanical strength. In low-tension plants of all kinds the insulation is usually accomplished with ease, and any probable defects are likely to be of minor importance; but on such a line as that under discussion the importance of these two factors is practically equal and they are mutually inter-dependent. This has been practically illustrated by the experience afforded by the present line. With units of an ordinary size a short-circuit on a line carrying 10,000 volts, even if through a defective insulator, a wooden cross-arm and a wooden pole, would make itself manifest at the power station by the opening of a circuit-breaker, the blowing of a fuse, or some similar method. With the huge generators which furnish power for the line the effects are different. Of what importance to a 5,000-horse-power dynamo is the current which will leak down a wood pole, even when wet? Nevertheless, this same current is sufficient to char or burn the pin under a defective insulator. During one night last Fall, while an attempt was being made to operate the line on temporary insulators, the best obtainable at the time, the ends of no less than five of the large cross-arms used on this line were burned entirely off, and this, too, without any manifestation having been made at the station that anything unusual had occurred. This naturally raises the questions whether it is possible to procure insulators which can be depended on to maintain the insulation on a circuit carrying 10,000 volts or over, and whether it is not practice to have fewer poles and, consequently, less weak spots in the form of insulators. There are two sides to this question, and both deserve serious consideration in designing any transmission line. Let us assume that poles are set 100 feet apart and allow a sag in wires, between supports, of 20 inches, or one-sixtieth of the length of the span; we find that the area of the wire in use on this line is 0.267 of one square inch, and that its tensile strength, even assuming a high value for soft copper, is about 10,000 pounds. Allowing the same deflection, one-sixtieth of the length of the span, this determines the maximum safe distance between poles as 178 feet, allowing a factor of safety of four, and shows that the cables might be expected to break if the span were lengthened to 712 feet, not allowing for wind pressure or extra load due to ice.

Assuming a tensile strength of 8,000 pounds per square inch for yellow pine, we find that the larger cross-arms used on the line, which are 12 feet long and nearly five inches wide by six inches high, would support a load of 270 pounds on each end without bracing, and of 360 pounds on each end with the steel angle braces used, and this, too, with a factor of safety of 10, an unnecessary margin when we consider that the arms are all specially selected heart long-leaf Georgia pine. Similarly these cross-arms would have the same factor of safety carrying three power cables on each side with spans 177 feet long if without braces, and 366 feet long with braces. Besides giving this added strength the braces used on this line prevent such vibration and oscillation as usually take place with the ordinary strap-iron braces, such oscillation being the cause of many of the petty troubles on ordinary lines. These braces were each made from a single piece of steel angle two and one-half inches by two inches by one-quarter inch, bent to shape and forming, when finished, a truss 18 inches deep and five feet in extreme length, their weight being a little over 20 pounds each. Assuming again that the poles are set 100 feet apart, we find that 12 wires with cross-arms, insulators, etc., would present an area to the wind aggregating about 67 square feet, and that, consequently, each pole would be subjected to a side strain, when wind pressure was 30 pounds per square foot, of about 2,010 pounds. A sound, 50-foot cedar pole, eight inches in diameter at top and 18 inches in diameter at butt, eight feet being held rigidly in the ground, would be capable of withstanding, before breaking, a side pressure near its top of only about 4,900 pounds, and of only about 3,400 pounds when a layer two inches thick had decayed around its circumference.

With spans of 100 feet the pole would therefore have a factor of safety of only about two and one-half when new, while the wires would have a factor of safety of about seven, and the cross-arms of about 26. The advantages of having cross-arms amply strong are so evident, and the possible reduction in cost such an insignificant part of any ordinary line carrying much copper, that it would seem foolish to reduce the size of these in order to bring their strength down to correspond with other parts of the line.

It is evident from the above that the weakest point of this line, mechanically, is the pole, in spite of the fact that, as advised by one of the prominent members of your executive committee, final decision was made in favor of spacing the poles 75 feet apart on straight, and proportionately closer on curved parts of the route. The only reasonably safe and practical method of decreasing the number of weak spots furnished by the insulators would be to use poles larger than eight-inch tops, or to brace the poles to withstand this wind strain. Fifty-foot poles having tops greater than eight inches are now hard to find, and, although 35-foot poles were used on a part of the company's private right of way, nevertheless, many 50-foot, and even a few 65-foot poles were required to avoid obstructions and for crossing railroads and highways. The only feasible plan, therefore, would seem to be to brace the poles laterally, which can readily be done if set in pairs, but which would be very difficult to accomplish in a satisfactory manner with a single line. This naturally brings us back to the question, "Are insulators such insurmountably weak links in such a chain, and is it not possible to get insulators which can be depended on even when supporting wires under a pressure of 10,000 volts?" This can be most satisfactorily answered by again narrating the experience with this line. During the past eight months insulators have been sent to Niagara Falls by four of the works which are amongst the first six in this country in the production of porcelain for electrical use. Of a sample lot of 10, received a few days ago for test from one of the factories, one had broken in transit, eight broke down under the strain of electrical pressure, varying from 16,000 to 36,000 volts, and the last broke down under 40,000 volts strain. This latter would seem to be a minimum safe test limit for any insulator expected to sustain a regular strain of 10,000 volts, and is a test which any mechanically good, well vitrified insulator of ordinary design will pass. As several previous lots from the same factory showed even poorer results, the manager of that company states that he hopes within a few weeks to be able to furnish insulators which will stand a 40,000-volt test. It is only fair to state that these insulators were of a smaller type than, and different design from, those in use in the Niagara line. Several lots of somewhat similar insulators from another factory gave about the same results.

Several thousand insulators of a diameter almost equal to the round type now on the line, but of a design somewhat different in details, were furnished by a third porcelain works. These were all supposed to have been tested and to have successfully withstood a pressure of 40,000 volts at the factory before shipment. When, however, these were tested at Niagara Falls, by Mr. Lincoln, the electrical superintendent of the Cataract Construction Company, it was found that a large majority of them broke down under a 40,000-volt test, illustrating that a dry test, such as had been previously made, is useless for practical purposes. The method of test used at Niagara Falls, was as follows: The insulators were set inverted in a shallow iron pan, in lots of about 20, the bottom of the pan being covered with an inch or two of water containing a little salt. A little of the same brine was poured into the pin-hole of each insulator, and into this was thrust a small piece of metal such as an ordinary iron spike or the small round zinc rod from an ordinary sal ammoniac battery, this being connected to one side of the testing circuit, the other being connected to the pan containing the insulators. After the metal rod had been placed in the brine in the pin-hole of an insulator, the primary circuit of the testing transformers, specially built for the purpose, was closed; and, if the insulator was weak, this was quickly manifested by a series of sparks through the punctured porcelain. Experiments made with pure water and with brine showed that there was no difference in the results; but that any weakness was manifested a little more quickly with brine, besides which the salt imparted the characteristic bright sodium color to sparks, otherwise almost colorless and difficult to detect.

As it was important that the line should be ready to deliver current by a specified date, the test was reduced on these insulators to 20,000 volts, and all which withstood this pressure were passed for temporary use. These insulators were later replaced by some of those now on the line, all of which successfully passed a 40,000-volt test, made as above described.

A lot of the temporary insulators illustrated the old saying that every rule has its exception, for when, after removal from the line, they were tested under 40,000-volts pressure, a solitary insulator from a total of 1,150 was able to pass muster.

Of the insulators shipped by the fourth factory and of the two types now on the line about 25 to 40 per cent were usually found to be defective, breaking down under 40,000 volts, this percentage decreasing in the last shipments received. It is, however, worthy of special note that since the last of the temporary insulators were removed from the line, there has not been one minute's suspension of current supply due to defective insulators. During this time, some three or four months, three insulators have been replaced, none of these being of the oval type. Two of these three had been broken while being put in place, and the third was broken by a stone or other missile. In all three cases only the outer petticoat was broken, and the insulator continued to do satisfactory service until such time as it could be replaced without affecting the operation of the line. Apparently, therefore, it has been demonstrated that it is possible to secure insulators which are reliable. That there have been no greater troubles in the past from defective insulators is probably due to the fact that most of the large transmission plants in operation under high voltage, up to within a few months, have been in the far West, where a climate prevails very different from that natural to this immediate region.

In the above, and other experiments with insulators, some interesting facts have developed and are worthy of note. The insulating strength of porcelain depends almost entirely on the thoroughness of its verification and very little on its thickness, a thin china teacup having successfully withstood a pressure of 60,000 volts, while a porous piece of porcelain, two inches thick, was readily pieced by 20,000 volts. It is, therefore, practically unnecessary to test electrically any insulator which, when broken, will not pass a good absorption test, using red ink or other fluid.

It is quite, if not entirely, impossible to puncture a glass insulator, even an ordinary pony telegraph insulator withstanding any pressure which can be applied, the last being determined by the pressure which will send an arc around the insulator. The objection to using glass insulators in the past has been due to the difficulty in getting a well annealed and mechanically strong insulator of such massive design as is needed for this work and to the hydroscopic property of glass, which is not shared by porcelain. The first can unquestionably be overcome by care in manufacture. The importance of the second has probably been exaggerated in most calculations made in the past, due to an inadequate appreciation of the static effects of 10,000 volts in warding off snow-flakes and drops of rain, and to a less extent of the rapidity with which water falling on such insulators is evaporated by the heat of the current leaking over the surface.

It is, consequently, reasonable to expect that the use of glass insulators for high-voltage lines will greatly increase with improved manufacture. Meantime any lines erected should have the best obtainable porcelain and every insulator should be subjected to test.