Publication: Western Electrician
Chicago, IL, United States
POWER TRANSMISSION ON HIGH-TENSION LINES.
At the regular meeting of the American Institute of Electrical Engineers in New York city on March 27th the subject discussed was power transmission on high-tension lines. The meeting was very well attended and was called to order by President Charles F. Scott. Several short papers were presented, and the contributions to the discussion that were sent in and the discussion which took place at the meeting were valuable and comprehensive. Much credit for the success of the meeting is due to Ralph D. Mershon, the chairman of the committee on high-tension transmission, which had charge of the meeting. President Scott introduced the subject and reviewed briefly the wonderful progress that had been made in the electrical transmission of power. The last 10 or a dozen years had witnessed a steady advance in the voltages in commercial use. He spoke of the purpose of the transmission committee of the Institute and mentioned how the progress in high-tension transmission was indicated by the transformer statistics of one of the large manufacturing companies, whose output for the last two years had exceeded that of the preceding 10 years.
Ralph D. Mershon then presented his paper which had been prepared to get discussions sufficiently full on which to base recommendations to the standardizing committee of the Institute on a standard pin. Mr. Mershon's paper follows:
MECHANICAL SPECIFICATIONS OF A PROPOSED
STANDARD INSULATOR PIN.
BY RALPH D. MERSHON.
At present no general standard exists in the matter of insulator pins. As a result, there is often confusion and dissatisfaction in ordering and obtaining pins. This discussion of a proposed standard pin is intended to lead up to a general specification covering wooden pins, and, so far as it may, metal ones.
Theory. — The expression for the extreme fiber stress at any point of a beam of circular section fixed at one end and loaded at the other, as in Fig. 1 is
where (assuming inches and pounds as our units) P is the pull or weight in pounds; x is the distance in inches from the point of application of P to any point (a) of the beam; d is the diameter in inches at the point (a); s is the extreme fiber stress in pounds per square inch, i. e., s is the stress on the extreme fibers at the top and bottom of the beam at the point (a).
This equation shows that for a given pull P the fiber stress at any point (a) at a distance x from the point of application of P varies directly as x and inversely as the cube of d. It is possible, therefore, to design a beam of circular section whose diameter in passing from the point of application of P to the point of support shall vary in such a way that s will have the same value all the way along the beam. Such a beam will be of uniform strength throughout its length. The value which, in such a beam, d must have at any point distant x from the outer end may be found by assuming s and P constant in equation (1) and solving for d in terms of x. This gives
where K is a constant whose value must be determined from the extreme fiber stress allowable with a given pull P. Equation (2) shows that in order to have the beam of uniform strength throughout its length, its diameter must vary as the cube root of the distance from the point of application of its load.
An insulator pin is the case of a beam of circular section fixed at one end and with a load (any side pull which may come upon it) applied at or near the other end. There is no object in having an insulator pin any stronger at any one point than at another. It should, therefore, in its capacity as a beam be tapered as nearly as practicable in such a way that s will be constant throughout, that is, so that equation (2) will apply to it.
The point where pins usually break; their weakest point, is just at the cross-arm. The wooden pin most generally in use is one having a diameter of about 1 1/2 inches in the cross-arm and a length such that the wire is from five to six inches from the cross-arm. Let us obtain the value of K in (2) on the assumption of d = 1 1/2 inches and x = five inches. This gives the value of K as .877, so that, substituting, (2) becomes
From (3) we may find the diameter required at any point in any length of pin, the pin to be of uniform strength throughout. Substituting various values of x we have a table which shows that for a pin having upon it a pull one inch above the cross-arm, the diameter at the cross-arm must be, 0.877 inch; that one having a pull upon it 10 inches above the cross-arm must have a diameter at the cross-arm of 1.88 inches, etc. Fig. 2 is a sketch of such a theoretical pin drawn by platting the above values to a scale one-quarter of full size. Fig. 2 represents all sizes of .pins up to and including one, the pull upon which is applied 21 inches above the cross-arm. That is, if we want a theoretical six-inch pin, we must cut six inches off the end of Fig. 2 and use that; for a 10-inch pin we must cut off 10 inches, etc.
The practical pin must be a modification of the theoretical pin. The end must be square and a portion of the small end must be threaded. The pin must also have a shoulder just above the cross-arm. It will be noticed that, except near the end, the sides of the theoretical pin are practically straight. It will suffice, therefore, if in designing a pin we fix the diameter at the lower end of the thread portion and the diameter just above the cross-arm and make the contour between these points a straight line.
Threaded End. — It is proposed to make the diameter of the small end of the pin one inch; the length of the threaded portion 2 1/2 inches, and the diameter at the lower end of the threaded portion 1.25 inches, so that the threaded portion will taper from 1.25 inches to one inch in a length of 2 1/2 inches. The threaded portion of the insulator should have the same dimensions and taper as that of the pin.
Shoulder. — It is proposed to make the shoulder three-sixteenth inch on all pins. That is, the diameter of the pin just above the cross-arm will be three-eighths inch greater than the nominal diameter of that portion of the pin in the cross-arm; it is proposed to carry this diameter one-fourth inch above the cross-arm before tapering the pin.
Dimensions in Cross-arm. — It is proposed to make the diameter of that portion of the pin in the cross-arm, just below the shoulder, one thirty-second inch less than the diameter of the hole in the cross-arm and at the lower end of the pin one-sixteenth inch less than the diameter of the hole in the cross-arm. It is proposed, also, to designate this portion of the pin as having a nominal diameter equal to that of the hole in the cross-arm into which the pin fits. Therefore, that portion of a pin which is to fit a 1 1/2-inch hole in a cross-arm will have a nominal diameter of 1 1/2 inch, but will have an actual diameter just below the shoulder of one fifteen-thirty-second inch, and at the lower end of the pin of one seven-sixteenth inch.
Thread. — It is proposed to use on all pins a thread having a pitch of one-fourth inch, or four threads to the inch, the form of thread to be that shown in Fig. 3. As there shown, the angle between the faces of the thread is 90°, and the top of the thread is flattened by cutting off, from the form the thread would have if not flattened, one-fourth its unflattened depth. The form of the thread in the insulator should be the same as that on the pin. If this is done it will insure the bearing surface being always on the sides of the threads and never on the edges.
Designation. — It is proposed to designate that portion of the pin above the cross-arm as the "stem" of the pin. That portion in the cross-arm as the "shank" of the pin. It is proposed to designate a pin by the length of its stem, i. e., a pin whose stem is five inches long will be designated as a "five"-inch pin, one six inches long as a "six"-inch pin, etc.
Dimensions of Standard Pins. — In accordance with the above, the following table has been prepared, giving a number of sizes of pins, and their dimensions, which it is proposed to make standard. The diameter of the shank has in each case been fixed by making it approximately equal to (slightly larger than) the diameter of the theoretical pin corresponding to the length of the Stem of the pin in question.
The headings of the columns of the table refer to lettering of Fig. 4. Fig. 4. is a full-size unthreaded five-inch (proposed standard) pin.
M. H. Gerry, Jr., of the Missouri River Power Company: Mr. Mershon has suggested the adoption as standard of certain forms of wooden pins, the designs of which conform to common construction. Most engineers of experience, however, would be in favor of a diameter greater than one inch at the top of the pin, and a greater length of thread than 2 1/2 inches. Wooden threads most frequently fail by shearing, and the strength in this particular is greatly increased by a larger diameter of pin, and a greater length of thread. The longer thread is an advantage also, in preventing the insulator from tipping out of line, when the fitting is loose between the thread of the pin and that of the insulator. The shoulder of the pin, as suggested by Mr. Mershon, has certain disadvantages for heavy construction. On account of the rounding off of the top of the cross-arm, this form of shoulder does not bear well, and the sharp corner at the point of greatest strain introduces an element of weakness. A form of shoulder which has been used by the writer with considerable success tapers at an angle of 60°, and fits into a counter-bore, in the top of the cross-arm. With this design the pins may be driven to a firm bearing at the top of the cross-arm, which has the effect of steadying the pin and increasing the strength of the construction at this point.
In lieu of the form of thread proposed by Mr. Mershon, the writer suggests a design in which the square top of the thread is as wide as the groove, resulting in increased area of wood to withstand shearing strains.
William R. C. Corson, Hartford, Conn.: Mr. Mershon's method of developing the table of dimensions of the proposed pin is so rational that criticism of it is difficult. Briefly stated, of course, this method assumes the pin in general use as a basis, and mathematically determines the diameters of pins of different length which shall be capable of sustaining the same tension at their extremity as will this pin. In other words, the safe load that may be applied at the top of the proposed pins is of constant value in all. This value is not discussed, and, feeling that its determination would be of interest to me and might prove of value, I have made a series of tests in a Seller's machine to ascertain the value of "s" in equation (1) for the ordinary locust pins under the actual conditions of support suggested.
C. L. Cory of San Francisco: The mechanical strength of insulator pins for use on long-distance transmission lines has been given much consideration by electrical engineers in California during the last six years. For the most part the insulator pins in use are wood. On the 33,000-volt, 83-mile double circuit, three-phase transmission line of the Edison Electric Company, from its Santa Ana power house to the city of Los Angeles, iron pins with porcelain sleeves are used, and in this transmission is the most notable system in California at the present time using insulator pins other than wood.
Eucalyptus has been found to be perhaps the best wood to use for insulator pins. After being turned up and threaded, they are usually treated with hot linseed oil. This treatment is desirable more on account of the protection which such treatment gives the pin against weather than on account of the insulating qualities of the oil or oil treatment.
It should be understood in this connection that an insulator pin should be depended upon only to support the insulator. The insulator in turn should be depended upon to provide the necessary insulation for the line wires. No pin after being in use for a few years on a pole line can maintain to any marked degree the insulating qualities originally existing, due to such oil or paraffine treatment.
Some tests were made in the electrical laboratory of the University of California for the purpose of determining how near the pins generally in use in California conform to the proposed standard pins suggested by Mr. Mershon.
The average results, referring to the "real breaking load" of the tests of two pins are, respectively, 1,085 pounds for the 6 7/8-inch pin and 2,310 pounds for the 10 1/8-inch pin. The character of break in the two pins, however, is not the same. Almost without exception the 6 7/8-inch pins were broken approximately square off at the cross-arm. The larger pins, however, split in the stem, the beginning of the split being just at the bottom of the thread. The shank is the weakest part in the 6 7/8-inch pins, while the stem, and particularly the upper portion of the stem or thread, is the weakest part of the 10 1/8-inch pins. The variation of the "real breaking load" for the different 6 7/8-inch pins tested is from 705 pounds maximum to 1,360 pounds minimum. For the 10 1/8-inch pins this variation is from 1,475 pounds minimum to 3,190 pounds maximum.
For good construction, on lines using 30,000 to 60,000 volts, the larger pin must be used. It does not seem, however, that any good reason exists for a great number of different sizes of pins, as it would seem probable that the two sizes tested might be used to fulfill almost every requirement for transmission work where wooden pins are at all allowable.
In many respects an iron pin is better than one of wood. In the first place, to secure sufficient strength in the shank, the wooden pin must be of such a large diameter that the size of the cross-arm is necessarily increased. In addition, using an iron pin, the insulator can be held down on poles or supports where the tendency of the line wire is to raise or pull the pin out of the cross-arm. In using wooden pins, this is usually prevented by driving a nail through the cross-arm into the shank of the pin.
D. L. Huntington, electrical engineer of the Washington Power Company, Spokane, Wash.: Wooden pins are subject to so many uncertainties where used in connection with very high voltages, especially where the atmosphere contains salt, smoke or dust, that it seems desirable to abandon their use for such purposes, wherever possible, and to substitute a metallic pin, The construction of a long-distance line for 60,000 volts led the writer to make some investigation as to what could be done in this direction without excessive cost.
It was decided that a drop-forged or a turned-steel pin would be so expensive as to exclude either, even if the time at our disposal would have permitted. Experiments were made with cast-iron pins almost identical in dimensions to that shown in Fig. 8 of Mr. Chesney's paper, except that it was cored out internally so as to make its weight about 10 pounds. The diameter of the shank was 2 1/8 inches. This pin sustained a load slightly in excess of 3,000 pounds, suspended from near the end of the threaded portion, before fracturing.
We have designed and adopted a pin 18 inches long, made of 1 1/8-inch round-milled steel bar, and having a shoulder and shank cast upon it. This pin we find will begin to bend when loaded with about 1,000 pounds at the upper end — the shank being rigidly supported. This is much lower than the results obtained by the cast-iron pin referred to above, but it is believed that it is more reliable and that it is sufficient for nearly all ordinary work. In addition, the steel bar will not snap off under sudden shock, but will support the insulator and line safely even when badly bent.
W. N. Smith, New York: In this table of dimensions for a standard pin, which is proposed; it seems to me that the dimensions given are all very well for a pin carrying a certain weight of wire, but it does not seem to me that a single standard table of dimensions could be laid down to cover anything like all the conditions of various power transmission lines all the way from a line of 5,000 or 6,000 volts, No. 4 wire, up to a line like the Niagara Falls power transmission plant, which has a 300,000-centimeter cable. It struck me the scheme of the tabulation should be enlarged and gone over carefully, so as to have a separate tabulation for a small range of weights of copper wire to cable.
P. H. Thomas, New York: Something should be said about the method of treating the pins to make them waterproof and to do it without destroying the mechanical properties of the pin. Another point is, there is trouble experienced in putting up a line where the pin comes to a bearing on the inside of the insulator.
Mr. Mershon: There was a remark made by Mr. Gerry to the effect that it is too early to adopt a standard pin. I think if we adopted a standard and changed it from year to year, we should not be any worse off than we are now, when we have no standard.
TRANSPOSITION AND RELATIVE LOCATION OF POWER
AND TELEPHONE WIRES.
BY. P. M. LINCOLN.
There are three ways in which disturbing current in telephone circuits may be caused by the high-tension circuit: (1) Electromagnetic induction. (2) Electrostatic induction. (3) Leakage.
It is the first two causes of disturbances which will claim particular attention in the following discussion.
Electromagnetic induction may be briefly described as a transformer action. In Fig. 5 let (a), (b) and (c) be the conductors of a three-phase line, and (m) and (n) the two wires of a paralleling telephone circuit; (a) and (b) may then be regarded as the primary and (m) and (n) as the secondary of a transformer. The electromotive force in circuit (m) (n) will depend, among other things, upon the amount and frequency of the current in the inducing circuit. By transposing (m) and (n) in the well-known manner, the electromotive forces set up in one part of the telephone circuit will be neutralized by equal and opposite electromotive forces set up in other parts. Thus, the electromagnetic effects between (m) and (n) may be entirely neutralized by transposing the telephone wires only, regardless of whether the transmission wires are transposed or not. It may be well to note, however, that while the electromotive force between the two telephone wires may be reduced to zero by properly transposing the telephone wires only, the electromotive force between the two telephone conductors considered as one side of a circuit and the earth as the other, can be reduced to zero only by transposing the power wires. This point is of little importance, however, as any electromagnetic electromotive force between the telephone wires and ground is entirely overshadowed by the electrostatic which will be considered later.
Electrostatic effects will also take place in (m), (n), due to transmission circuit (a), (b), (c). If conductor (a) has a minus charge, for instance, it will induce a certain plus charge on (m) and a smaller plus charge on (n), on account of (n's) greater distance from (a). If now the minus charge be removed from (a) current will flow from (m) to (n), proportional to the difference in the amounts of these charges. The electrostatic influence of (b), being opposite (a) in sign, will reinforce the action of (a). Transposition of the telephone wires will have the effect of neutralizing this tendency of setting up electrostatic currents between (m) and (n). It is important to note that a system of transpositions designed to correct