Publication: American Institute Of Electrical Engineers
New York, NY, United States
(continued from article 2500)
MR. LINCOLN:—There is one point upon which possibly my discussion is not quite clear, and that is in regard to the transposition of the power circuits. I have stated that it is not of very great importance. My reason possibly is not very clear in the paper, and it is this—a telephone line ought to be so made that it will at all times operate, even if one leg of the power line is dead grounded. If it is made to operate under these conditions it will operate under the normal conditions of an untransposed power line, because the normal static potential induced in the telephone line by an untransposed power line is vastly less than that which will be induced in the telephone line by grounding the power line; so that if the telephone line is installed so as to run under grounded conditions, that is, with the power line grounded, it will certainly run with an untransposed power line.
VICE-PRESIDENT SHELDON:—The subject is now open for general discussion.
MR. RALPH D. MERSHON:—The truth of the observations of paragraph (6) is not clearly evident to me. The relation between the telephone circuit and the power circuit is, as stated in paragraph (4) such that in their electromagnetic relations they constitute a transformer—an air-core transformer and one having very poor regulation. This transformer has in its primary —the power wires—a current unaffected in value by any current in the telephone wires and which may therefore for any given condition of power load be considered as a constant current. If the telephones took any appreciable amount of current, an amount comparable to the power current, we should have a tendency toward constant current in the telephones. If, as is the case, the telephones take a current very much less than the power current, we will have a tendency for constant voltage.
Electrostatically, each telephone wire is a plate of a condenser, the power wires being the other plate. These two condensers are in series through the telephones and have impressed upon them a constant e.m.f. The condition here is similar to that for the electromagnetic action, in that if the current in the telephones be such as will convey a charge approximating that which the impressed e.m.f. can impose upon the condensers, there will be a tendency toward constant current ; if not, as seems to me is usually the case, there will be a tendency towards constant voltage. In short, it seems to me that the conditions are about the same in the case of the electromagnetic and electrostatic disturbances. This is on the assumption that the reactance of the telephone is low relative to its resistance. If this is not the case, and the reactance is comparable to the condensance concerned in the telephone circuit, then the current conditions may be almost anything, depending upon the relations between these two quantities Further, if by telephones of various resistances it is meant to designate telephones of various numbers of turns in the transmitter, then it seems to me the statement in regard to the disturbing ampere-turns is incorrect. The resistance of a coil varies as the square of the number of turns (assuming the same space available in each case for copper). The ampere-turns on constant e.m.f. vary, therefore, inversely as the square root of the resistance. It would seem, therefore, that if, as stated above, the electromagnetic and electrostatic effects are constant poten - tial in their nature, telephones should be wound for as high resistance as possible. This seems reasonable, as it is equivalent to saying that the telephonic e.m.f. should be kept as high as possible, which in turn is equivalent to saying that the disturbing e.m.f. should be as small a percentage of the telephonic e.m.f. as possible. In order to solve some of these questions, it is necessary to know the current and e.m.f. of telephones. I have no such data and have not been able so far to obtain them.
(7) It does not seem as though the objection given for the use of the series telephone were a valid one, since one ought not to introduce a telephone at B, Fig. 2, any more than one would connect a bridging telephone at this point. In other words, telephones, whether series or shunt instruments, should be connected at even transpositions in order to obtain the best results.
(14) It is not quite clear to me why the series telephone should
not be a satisfactory instrument. As has already been stated. the reason mentioned in (7) seems hardly a valid one for condemning it.
(15) There is another method of accomplishing this desired end of having the telephone lines as nearly as possible at the same potential as the earth, which seems preferable for the use of a grounded wire ; first, on the score of simplicity, and secondly, because it may also be a means of protection against loss of life or fire. This method is that of using autotransformers connected across the telephone line at a number of points, preferably at each of the telephone stations, each having its middle point connected to ground. Each transformer should be designed so as to take a very small amount of the telephonic current, but should have wires sufficiently heavy to enable it to take, in case of a cross with a high voltage wire, a current heavy enough to operate the circuit-opening devices in the power station or else to blow a suitable fuse in the telephone circuit itself. Such a device would protect the users of the telephones from disagreable or dangerous shocks, whether due to crosses, leakage or electrostatic induction, and would also help to minimize disturbances due to grounds, etc.
MR. P. H. THOMAS:—I think that the subject of telephones on long distance power transmission lines is perhaps the most important subject we have to-night. Mr. Lincoln has given a very good statement of the fundamental principles underlying the difficulties that have been found in many cases. By exchanging experiences and making suggestions, we can probably base on these fundamental principles improved methods by which the present service can be very much benefitted. As Mr. Lincoln concludes, I think there is no doubt that the trouble is chiefly due to the electrostatic induction from the normal voltage which tends to raise both telephone wires above the earth, either positively or negatively. There will also be a momentary disturbance whenever we have a charge of a lightning arrester, or any static discharge, in the neighborhood, but that will not give much trouble as regards clearness of communication, because it is over quickly. The transposition of the transmission line is, I think, an important point practically in high-voltage transmission. As Mr. Lincoln states, a telephone circuit should be built to work under all conditions; for instance, when one line is dead grounded. But if it is impossible to make it work at such times, and if you can make it work smoothly when not grounded by transposing the power line, you had better transpose it, and that is the actual condition of practice.
Assuming both telephone wires are going to reach high potential above the earth, there is only one thing to do to get service at all times, and that is to eliminate the baneful effects of the high potential.
I will make a suggestion for accomplishing this purpose, which may not have much practical value, but should be worth trying. I hope that some of the engineers who have opportunities for experimenting at hand will try it for their own and the general good. For instance, it is possible we may be able to insulate the telephone wires, perhaps for 30,000 or 40,000 volts in an extreme case, and at the end of the line put the primary of a transformer and connect the telephone to the secondary, thus making very high insulation between the primary and secondary. By this means it would be possible to protect the operator, and since static disturbances do not induce potential between the two wires, it should not disturb the speech. The same result may possibly be accomplished with condensers, by connecting two condensers in series between the two pair of wires and putting the telephone in between the condensers, not connected with the line. In this case it will probably be necessary to put a choke-coil between the condensers and ground its middle point. The charging current of the condensers will be neutralized by going through the two halves of the coil in opposite directions and a telephone winding could be taken from the same core.
Mr. Lincoln suggested carrying ground wires in close proximity to the telephone wires. This should help much and would be a good method to try, but it would probably be necessary to use insulated wire for the telephone circuit, otherwise there would be trouble from repeated grounding. One method, which would be effective, but perhaps not practicable, would be to use for each side of the telephone wire a twisted pair; one wire of the pair for the telephone circuit and the other of the pair grounded. This method would make a large capacity between the telephone wire and ground, but would not actually ground the telephone wire itself.
There is another interesting possibility for those who like to speculate. Can we not use the power transmission wires themselves for sending signals ? If not for telephoning, at least for general signaling. For instance, as a suggestion, we might connect a high resistance between each wire and a common point, and connect this point to ground, and similarly at the other end; then put a high frequency generator between the ground and the common connection at one end of the line, and some kind of receiving apparatus in the other. The high frequency would prevent the transformers from taking too much current, and you might be able to signal when power was on the lines. The same thing might be done with condensers or choke-coils. The advantage of using all the wires in parallel for signaling is that if you have three or four wires burnt off, there might yet be one wire you could signal through.
Another point is, it would be possible to make temporary arrangements for signaling in times of shut-down, until the power comes. That is, when voltages go off, it would be possible to use the dead wires to make arrangements to start up again. Things are in a critical state when the power is off, and the telephone lines down, and such a system of signaling might easily be arranged.
Another thing is very important, and that is the protection of the man using the telephone. One or two fatal accidents have recently occurred to operators on the telephone circuits. They should either be insulated in a booth, or the circuits should be dead grounded, or protected through an air-gap small enough to be safe for the operators.
In regard to Mr. Lincoln's statement that telephone wires may reach as high a voltage as 20,000 volts. It occurs to me that as perhaps his statement is based on theoretical considerations, I can emphasize it by stating that I have seen this voltage in fact, in Mr. Gerry's plant. I have seen sparks from telephone line to ground which were something like half an inch long, indicating perhaps 20,000 volts, perhaps higher than that. The possibility which Mr. Lincoln describes is not at all imaginary; it is very real.
MR. C. E. SKINNER: — I wish to emphasize the last point made by Mr. Thomas, that is, the protection of the workman. The insulation of the instruments in the building has been mentioned by Mr. Lincoln. The insulation of the workman is even more important, and it is not difficult to make arrangements so that he will be well enough insulated, so that even a cross between a high potential line and a telephone line will not do serious harm. It is usually painful to receive a shock of this character, but not particularly dangerous. It has been my observation that men handling telephones on high-tension transmission lines, soon learn to be very cautious and are usually compelled to watch the surrounding material, walls, etc., when telephoning. This could be easily avoided by proper arrangements, so that the man need not touch anything which would connect him to the ground.
MR. C. O. MAILLOUX: — I would like to mention a phenomenon in connection with the charging of lines which, while not exactly within the scope of this paper, is still interesting, especially as it is a phenomenon which has puzzled me and which has puzzled others to whom I have mentioned it. I have observed the fact that the transmission line will become spontaneously charged electrostatically without being connected to any generating machinery. About a year ago, in Arizona, we repeatedly observed that the three wires of a 25 or 26 mile line would become charged spontaneously to a considerable potential under various atmospheric conditions. In Phoenix, Arizona, at this time of the year, the weather is very much the same as it would be here in June of July, except that there is no rain. The weather is usually very pleasant, quite warm in the day time—one can wear summer clothes—and it cools but very little in the evening. There are occasionally slight winds, especially over the deserts, over a portion of which the line runs. I have repeatedly observed, but during the day time only, or until early evening, that the three lines became charged electrostatically. They could be discharged by making a connection to earth, but if left alone they would soon charge again (in 10 to 15 seconds). The three wires would always charge at approximately the same potential with respect to earth, but this potential varied, as might be expected. It depended on the weather apparently, and varied, of course, with the length of time allowed for recharging. On one occasion, when there was a heavy wind preceding a rain, the potential was so high that we got sparks an inch and a half between the lines and the ground, through spark gaps, I have repeatedly observed cases where the spark was 1/16 to 1/8 of an inch between sparking points. There were no indications of lightning or storm. There have been cases where the phenomenon was observed when it was raining, (at the distant end, which is near the mountains, where it rains occasionally), or shortly after it had rained; but the phenomenon was never so marked at such times; it was repeatedly observed when there was no indication of rain whatsoever, the sun shining brightly, but there was then always some wind or slight breeze somewhere along the line. My own theory was that the charge was either caused or translated by the wind, and taken up by the wire surfaces acting as condensers. I have mentioned the matter to several physicists, but my theory was rejected, as moisture in the air is considered indispensable, and it was lacking in this case. I have not succeeded in getting any satisfactory explanation. I hope there is someone here this evening who may be able to give it.
Another interesting phenomenon which I have observed in the same climate is the fact that the lightning arrester, which is located at each end of the line, has generally a tendency to frying discharge, which is more pronounced between the lightning arrester gap of one line than of the others. It was not always the same line, but changed from one line to another. I could not determine with the facilities we had what that was due to, but 1 was tempted to ascribe it to some sort of electrostatic action. The three line wires were systematically transposed in building this line, so as to bring an equal length of each of the three wires in the same relation with the surface of the ground.
MR. LINCOLN:—The point has been brought up about the use of series telephone versus the bridge telephone. I know by bitter experience that the series telephone does not give very good satisfaction. I once had charge of a line on the poles of which was run a telephone line, and on that telephone line we had about fifty series 'phones scattered along over twenty-five miles, about half a mile apart. We were never able to get successful service from that system. I ascribed most of the difficulty to the fact that the talking current had to go through so many loose contacts, so many jacks, and it is almost impossible to keep so many contacts in good shape. With the bridge 'phone it is necessary for the speaking current to go through only one pair of loose contacts or jacks at each 'phone. That constitutes one great advantage of the bridge over the series telephone.
The suggestion has been made that the potential between telephone wires and ground can be reduced by introducing between them autotransformers or condensers or resistances and connecting the middle points of these autotransformers, etc., to ground.
The objection to that method, it occurs to me, is that it takes off the charging current at concentrated points. The current is induced in these telephone wires as a distributed effect, distributed along the whole length; and if you try to take it off at bunched points, there will be a flow of current in the telephone wires which will introduce an e.m.f. and probably make disturbance in the telephone. The remedy which I proposed is to run a ground wire along the whole length of the telephone wire, producing a distributed capacity which will take care of the distributed induced effect most efficiently.
MR. MERSHON:—With regard to the use of condensers on telephone lines, it seems to me we want to keep the capacity of the telephone lines as low as possible to get good operation of the telephone.
If any one wanted to signal with the power lines, it would be better to signal from neutral to ground, and receive messages at the corresponding place at the end of the line.
I do not think the objection Mr. Lincoln makes, relative to the use of autotransformers on telephone lines, that the current would be drawn off at certain points, whereas it is introduced uniformly along the telephone line, would hold, as the current is flowing in the same direction in both telephone wires, and the effects of the two currents will neutralize each other so far as the telephones are concerned.
VICE-PRESIDENT SHELDON :—We will now proceed to the consideration of the paper on " Burning of Wooden Pins on High Tension Transmission Lines," by Mr. C. C. Chesney.
Mr. Chesney presented his paper (see page 18) and read the following contribution by Mr. M. H. Gerry, Jr.
MR. M. H. GERRY, JR:—On a certain number of high-tension transmission lines there has been burning of the wooden pins. On other transmission lines of high voltage, there has been no such burning. Where burning has occurred, it has been due to leakage of current from the surface of the insulators, coupled with resistance conditions in the pin, such that sufficient heat was developed to char the wood.
When thoroughly dry, wood is one of the best of insulating materials, and one of the poorest when containing sap or moisture. The greatest objection to it is its unreliability, due to the difficulty of removing the last traces of sap or moisture. A thoroughly dry wooden pin, fifteen inches in length, will stand indefinitely 100,000 volts pressure, while a green pin of the same length, containing sap, will break down very quickly under 1,000 volts pressure. Paraffining the pins on the outside, or coating with asphaltum or linseed oil, is of no value. If the pin are thoroughly dry, the material in which they are dipped can be made to impregnate the entire body of the wood, thus producing a pin of high insulation. Such a pin is of value, as it reduces the static strains on the insulator and decreases the amount of leakage to ground.
In order to prevent burning of pins they should have either very high or very low resistance. With insulators having a large amount of surface leakage, such as those illustrated by Mr. Chesney, an iron pin is perhaps the only solution of the difficulty.
There is nothing especially mysterious about the burning of wooden pins on high-tension lines, as it is merely a matter of total resistance to ground and the relative resistances of the insulators, pins, cross-arms and poles. Wherever burning occurs, it can be remedied by altering the design, material or dimensions of the insulators and improving the quality of the pins.
MR. H. W. BUCK:—Referring to the point which Mr. Chesney spoke of in regard to the so-called "digesting" of pins, I have seen many pins taken from the Niagara-Buffalo transmission line where such disintegration had occurred, the top of the pins having crumbled into a white powder. We have recently had some of this powder analyzed by a chemist and it was found to be a nitrate salt. This would look as if nitric acid had been formed in the presence of a static discharge inside of the insulator by the well-known atmospheric reaction and had attacked the wood, forming the nitrogen salt in combination with the vegetable matter of the pin.
In this connection I would like to say that about six months ago we built an experimental single-phase line at Niagara, about two miles in length, and operated it continuously for nearly four months at approximately 75,000 volts. The conductors of this line were galvanized iron wire, tied to the insulators with copper tie wires. At the end of the experimental run, the galvanized iron wire had turned black to a considerable depth throughout its length. The copper tie wires had also been attacked, though not so much as the iron. This surface disintegration was not due to general atmospheric influence, for iron wire in the same place but not charged electrically retained its original bright condition. I believe that this action is also due in some way to the influence of nitric acid formed by the brush discharge around the conductor. It indicates that some trouble may be experienced at such excessively high voltages where static discharge from the line is active. This discharge probably causes a combination of the oxygen and nitrogen of the air which, with the moisture of the atmosphere, forms a film of dilute nitric acid surrounding and attacking the metallic conductor.
MR. LINCOLN:—Mention has been made by Mr. Chesney and in the communication of Mr. Gerry as to the treatment of pins. I think the treatment of pins should be with a view to making them durable rather than making them good insulators. The pins should not be relied upon as a part of the insulation. As long as they are dry and as long as the weather is perfectly dry. they may be most perfect insulators, but as soon as rain comes and the pins are wet on the surface even, they become practically useless as insulators and the entire insulation strain on the line falls on the insulators. We should treat the pins, therefore, with a view to preserving them rather than making insulators of them.
MR. MERSHON:—It does not seem that, as Mr. Lincoln stated a few minutes ago, when the arms and pins get wet, the insulation is all in the insulators; because if this were the case, I do not quite see how the sides of the pins next to the sea should be burned, and the sides away from the sea should not be burned. It seems to me it would be the other way. If the insulator controls the current, the lower the resistance of the pin, the less burning, and when the pin is charred all over there should be less heat generated on the surface, and consequently less tendency to burn. But if the pin does to some extent control the current, the lower the resistance the greater the current over the surface, and the more likely it is to burn, especially over any part of it which has had its resistance lowered.
The path of the leakage current from wire to wire of a power transmission line may be considered as a high resistance electric circuit, derived from a constant potential source. The total resistance of this derived circuit is the series of the resistance of the three elements, insulators, pins and cross-arms. The resistance of each of these elements is that resulting from two resistances in multiple ; namely, the resistance of the element through its substance and the resistance over its surface. The substance resistance of all the elements is usually high, so high in a well constructed line that it need not be considered.
The surface resistance of the three elements may or may not be high, depending upon the surface conditions as regards moisture, dust, dirt or other deposits. Suppose the surface conditions of all the elements is such as to allow considerable leakage. No harm will result to the insulator unless the leakage becomes great enough to start an arc. This is not the case, however, with the cross-arms and pins. The leakage over their surfaces, if great enough, will char all the surface over which it passes. Pins are more likely to char than cross-arms, since their surface is less and their surface resistance, therefore, higher than the cross-arms ; the result being, for a given leakage current, more loss per unit area of pin surface than of cross-arm surface. Any protected portion of the pin is especially liable to charring. For, if the cross-arms and pins have their exposed surfaces pretty thoroughly wet or dirty, so that the current passes over them with little resistance, the wet or dirty portions may be little affected; but if in the course of its path the current encounters any small portion of the pin which is not wet or which for any reason has a higher resistance per unit of length, the wood may be charred at this point. Now, this is what happens when the pins burn. The insulator, the lower part of the pin, and the cross-arm have their surface resistance lowered by moisture or otherwise, but the upper part of the pin being protected by the insulator does not have its surface resistance so much decreased; the consequence is burning of the protected surface. In some cases the inner surface of the insulator next the pin and the pin itself are so well protected by the insulator that the current, instead of leaking over the surface of the insulator until it reaches a point where the insulator and pin are in contact, jumps in a brush-discharge from the edge of the petticoat to the pin rather than follow the higher resistance of the protected surface. As a result, the pin is burnt at the point where the brush discharge strikes it instead of at or near the thread. There are apparent three possible remedies for the trouble due to charring pins.
1. Make the design and size of the insulator such that for all conditions its surface resistance will be so high as to control the leakage and keep it below a point which can harm the pin.
2. Make the pins fireproof, but non-conducting.
3. Make the pins conducting.
The remedy recommended in the introduction to this discussion is (3). It is recommended that an iron pin be used. This certainly would do away with the trouble of charring pins, but whether or not it will introduce other and more serious trouble remains to be seen. It seems to me there is a very likely chance of trouble from the use of iron pins, due to the unyielding character of both iron and porcelain or glass and their widely different coefficients of expansion. Insulating material under mechanical stress will generally break down under a lower voltage than when not strained. An insulator or an iron pin might, when installed, be put under a considerable mechanical stress or one which when first installed has comparatively little stress upon it may, due to changes of temperature, be much strained; the result in either case is increased liability to puncture.
The endeavor may be made to get around this trouble by using a wooden thread upon the iron pins, but as shown by one of the cuts in the introduction, the charring trouble may still remain if this course is adopted. If a wooden cross-arm is used with the iron pin, the seat of the charring trouble may be transferred to the cross-arm unless the pins be connected by a conductor. It would seem better to adopt the first remedy and make the design of the insulator such as to protect the pins.
MR. DE MURALT:—It may possibly interest you to know that while the general practice in America is evidently to use wooden pins, in Europe it is just the opposite. Practically all the high potential installations use iron pins, and more than that, while here very often the whole pole is treated with as little iron as possible, in Europe there are quite frequently poles constructed entirely of iron, with iron cross-arms and iron pins, and the only insulation relied upon is the insulation proper. I believe this does away with the burning of pins, cross-arms and poles. I do not think there is very much difficulty in the way of avoiding mechanical strains, which have been alluded to several times to-night, with regard to fixing the insulator in the pin. One way to get around that is to fix the pin into the insulator by means of a cement which will take up any such strain, and in a great many installations that I know of a cement made of a mixture of litharge and glycerine has been used with, as far as I know, very good results. It seems to me that it is a very fair scheme thus to lay the entire insulation into the insulator, and then let the rest of the pole take care of itself. I know of one installation, where they are operating at 26,000 volts, using American glass insulators and iron pins and iron poles; and of another one which is using 25,000 volts, and has porcelain insulators, with iron pins on wooden poles part of the way, and iron pins in walls and on any kind of a support on the other portion of the road. Neither of these installations has given any trouble whatsoever and they are amongst some of the best high-voltage installations in Europe.
MR. PHILIP TORCHIO:—I want to suggest an explanation of the burning of the wood between the iron pin and the porcelain base in the Locke insulator shown in Fig. 3 of Mr. Chesney's paper. I wish to call attention to the fact that, if two plates are maintained at a certain difference of potential, acting as condensers and spaced at such a distance that there will be no discharge between the plates, but set near the limit at which the discharge would begin and then there is inserted in the middle a plate of vulcanized rubber, which has a higher dielectric resistance than air, right away the discharge takes place between the plates. Now, that is contrary to what might have been expected. The explanation is that before the insertion of the vulcanized rubber plate the fall of potential between the two plates of the condenser is a uniform straight line, but when we introduce the vulcanized rubber plate we alter the conditions, as we have then three condensers in series, which will distribute the total fall of e.m.f. in inverse ratio to their capacities. Therefore, this plate of vulcanized rubber acting as a condenser with a larger capacity than the same amount of air which it displaces will be charged at a smaller fall of e.m.f. between its faces than existed before, and the e.m.f. between the outer condensers will be increased and then the discharge begins. Now, it seems to me that in the Locke insulator, with double porcelain petticoats and an oak thread between iron pin and porcelain, there are present the conditions of several condensers in series which might give rise to a lack of uniformity in the distribution of e.m.f. between line wire and iron pin and cause the charring of the wood at the base of the insulator.
MR. C. E. SKINNER:—I understand that the pins used on one transmission line were selected with the utmost care, and were most carefully treated, and that they have had practically no trouble whatever in more than a year's run with a potential of over 50,000 volts. These pins are protected from the weather by glass sleeves. We should keep in mind that this is in a different climate from many other installations, and that a cure for these troubles in one climate may not be a cure in other climates.
MR. W. N. SMITH:—In the matter of wood and iron pins, it seems to me that along with various other elements the question of cost will govern. Iron poles in this country at this time cost anywhere from $30 up, and a wooden pole of suitable size runs from perhaps $7 to $20, according to size and where it can be obtained. The size of cross-arms may be governed to some extent by the size selected for the butt of the pin. If you determine first on the size of the shank of the pin that enters into the cross-arm, that in a measure determines the thickness of the cross-arm, if of wood and larger than usual. That may mean quite an additional percentage to the number of feet of lumber to be bought to provide cross-arms for a long pole line. Lumber is higher than it used to be, so that there are considerations, commercial as well as technical, that these various elements of design all enter into. The cost of selecting some particular pin because it looks a little better may thus run into some thousands of dollars on a long pole line.
VICE-PRESIDENT SHELDON:—We will now give Mr. Chesney an opportunity to close the discussion on his paper.
MR. CHESNEY:—I was particularly interested in Mr. Buck's information concerning the cause of the "digesting" of the pin. This has bothered me on a number of transmission lines. I attributed this trouble to the formation of ozone. If it is due to the formation of nitric acid, I am glad to know it. As far as I know, on the particular line on which the Locke iron pins with wooden threads were used, the burning was not serious. The thread was punctured at one point but was in no other way injured. In order to relieve the mechanical strain between the iron pin and the insulator, I think it is quite possible to use a lead thread. Litharge and glycerine have been used to some extent in this country to cement iron pins in porcelain insulators, but lately Portland cement has been used with quite as good results. I understand that one of the largest new transmission lines in Mexico is to be built with iron pins and porcelain insulators. The pins will be cemented in the insulators with Portland cement. Instead of iron poles, iron towers will be used, placed 400 or 500 feet apart.
A NOTE ON LINE INSL LATION FOR HIGH VOLTAGE.
[COMMUNICATED BY M. H. GERRY, JR.]
The maximum practicable limit of pressure on transmission lines has been frequently stated as fixed at a certain voltage, but this limit has as frequently been extended, with good results. At the present time, no considerable difficulty should be experienced with 100,000 volts, and there is no good reason to fix the limit at that figure.
The problems of insulation are becoming better understood, but there is still much to learn. The capacity and the surface effects of line insulators have received but little attention from engineers and many of the failures are due to this fact. The form of the insulator and the material have not, in general, received the proper treatment. A desirable insulator for high-tension is not merely a piece of glass or porcelain arranged to shed rain, and of sufficient thickness to resist puncture.
The materials for construction of insulators are not so limited as assumed in the past. Glass and porcelain have been used almost exclusively, but from the experiments of the writer the material of greatest promise for high-tension insulators is prepared paper. Organic material, such as paper, has great advantage and is well suited for this purpose. Compound insulators in which the petticoats and water-sheds are made of metal, and the core of glass, porcelain, paper or other insulating material, are also feasible.
For moderate tensions, up to perhaps 30,000 volts, insulators having metal tops and outer petticoats are not only perfectly feasible, but are very desirable, and can be made very strong and practically indestructible, and much superior to the common glass or porcelain types now in use. For high voltages, the entire insulator can be made of prepared paper, or of a combination of paper with glass, porcelain or other insulating materials. Insulators on these lines may be designed for almost any desired pressure obtainable with commercial transformers, provided that all the conditions are properly understood in advance.
The writer has tested and experimented with nearly every type of insulator manufactured and with many special forms and constructions, and his conclusions, as stated above, are based on this experience, coupled with that gained from the practical operation of the highest voltage transmission in commercial service to-day.
[COMMUNICATION AFTER ADJOURNMENT BY W. N. SMITH.]
An important matter that has not been touched upon in this discussion is the design of the pole-top pin, which, on a single three-phase transmission line, is of equal importance with the cross-arm pins. As in other details of line construction a variety of methods has been followed, of which some are doubtless better than others as regards their mechanical features. In the construction that has come under my observation, either the top of the pole has a hole bored vertically to receive a bolt or the shank of a wooden pin, or else a so-called " ridge iron " has been lag-bolted to the pole top, with the usual wood or porcelain fittings for carrying the insulator. Sometimes an ordinary oak bracket is framed into the top of the pole, the roof of which is shaped to accommodate it.
Without entering into a discussion of the relative merits of these or other methods, it seems to me that there is enough difference between all the methods in vogue to warrant an attempt at standardization. This subject would, therefore, seem to be a proper one for the careful consideration of the Committee on High-Tension Transmission.
[COMMUNICATION AFTER ADJOURNMENT BY J. R. ARMSTRONG.]
Relative to the discussion on "Insulator Pins for High-Tension Transmission Lines," the iron pin seemed to be spoken of favorably by a great many present, but to me this iron pin has one great disadvantage (leaving the difference of coefficients of expansion of glass and iron out of the question) nearly every pin has a burr on the end, due to the way in which the ordinary pin is manufactured.
Now, there is a tendency to a continual discharge between the line and this burr or sharp point on the other end of the pin. This, after a time, cuts through the glazed finish of the insulator, and consequently causes the breaking down of the insulator.
Also in the same discussion, one of the objections raised to the use of wooden pins was that of the corroding at the ends and sides.
I would just like to raise the question: if nitric acid is formed as was suggested, could not some base be used, which would form a neutral salt with nitric acid, the pin being treated in some way with this base.
[COMMUNICATION AFTER ADJOURNMENT BY F. S. WOODWARD.]
During the discussion relative to the breaking down of insulators and the burning of high-tension insulator pins, one possible cause of the trouble was not stated. It may sometimes be due to the method of fastening the tie-wire to the insulator. In many cases I have known linemen in making what is known as a pigtail tie, after the wire was finally twisted, to bend down the end of this pigtail so that it came in contact with the surface of the insulator at a point near the lower rim; the distance between the edge of the rim on the insulator and the end of the tie, depending, of course, upon the tie's length. This would reduce the amount of creepage surface between the pin and the tie-wire, which partakes of the line potential. In this connection it might be well to state that in some eases spun-yarn, thoroughly saturated in tar or asphalt or in P. & B. paint, makes a good substitute for tie-wire, the coating practically protecting the spun yarn against weather effects. It might not, however, be serviceable upon a line under the conditions of voltage as described by Mr. Buck, where the surface of the line wire and of the ties showed signs of reaction due to the formation of nitric acid, which would probably affect the vegetable fibre of the spun yarn in the manner indicated in the case of the thread of the insulator pin. I regret that Mr. Buck did not state the size of the iron wire used on their experimental 75,000 volt line. I recall the paper published in the TRANSACTIONS on the "Dielectric Strength of Air," by Mr. Chas. P. Steinmetz, in which he gave an account of a number of experiments on the sparking distance between sharp points, between spheres of various sizes and cylinders of various sizes. The lower portions of the curves, as I recall, departed more and more from the straight line effect as the voltage was reduced and radius increased. It would be interesting, in this connection, to follow out these experiments and see whether a change in the diameter of the wire (practically being a continuous cylinder) would stop sparking or brush discharge a the desired voltage. For instance, if the wire in Mr. Buck's experiment was a No. 8 and the wire in the second experiment was a No. 1 or a No. 2, if the increased radius would so modify the curve that the brush discharge and the probable formation of nitrogen would be prevented.
As a sequel to the discussion on pins and insulators, it would be a very desirable thing to take up and standardize the cross-arm to which these pins are attached. Also that the distance between wires and the most desirable method of spacing same should be outlined in the report of the Transmission Committee.
COMMUNICATION AFTER ADJOURNMENT BY HENRY FLOY.
Fearing that the remarks of some of the speakers may have left an erroneous impression as to the potential of telephone circuits carried on the poles of high-tension transmission lines, I desire to state that some measurements made by a Weston voltmeter between the conductors of a telephone circuit placed five feet below the conductors of a 25,000 volt overhead circuit and ground, showed the potential to be only from 140 to 160 volts. Similar measurements on a telephone circuit three feet below a 10,000 volt line showed only about 95 volts to ground and, naturally, no difference of potential between the telephone conductors. It seems to me that the voltage of a telephone circuit given as 20,000 by Mr. Thomas cannot be such potential as would be indicated by a voltmeter or such as would cause particular damage, being, I assume, simply static potential.
Referring to the suggestions made as to signaling in case of partial breakdown of the telephone system, it has occurred to me that as a relay to the telephone circuit, a system of wireless telegraphy could be installed without large expense, which might advantageously be used in transmitting signals in case of trouble with the telephone circuit.
DISCUSSION AT MINNEAPOLIS, MINN., APRIL 3, 1903.
The Minnesota Branch held its 11th regular meeting Friday, April 3d, at the Electrical Building at the State University. Six members and 13 visitors attended.
The meeting was devoted to the four papers of the Transmission Committee. The papers were read, and produced considerable discussion. Prof. D. C. Jackson, of Wisconsin State University, and Dean F. S. Jones, of the Engineering Department of Minnesota State University, were present. The opinion of the members regarding the papers and new ideas brought forth were:
1st. That the proposed standardization of pins and pole construction must consider not only the transmission voltage, but particularly such local conditions as mist and dampness at inland lakes and from the ocean, of salt storms, the amount of lightning, etc.
2d. Regarding wooden pins, that trouble from same must be expected in time, say after fifteen years' service, when the pins have weakened mechanically. An iron pin would seem to be more permanent.
3d. That wooden pins should not have a shoulder just above the cross-arm. The use of a shoulder produces additional mechanical strains in the pin at the cross-arm or shoulder not considered in the formula or theoretical basis given by Mr. Mershon. The shoulder was considered a relic from telephone lines and not necessary or advisable where there are heavy mechanical strains.
4th. That in service, the great majority of the insulator failures were mechanical and were due to strains produced by a poor fit between the pin and the insulator. Manufacturers of porcelain and glass insulators in the States produced excellent insulation, but the threads were not of uniform size in each and every insulator, as in those made by foreign manufacturers. The best workmanship is also desired in cutting the iron or wooden threads of the pin.
5th. That transmission lines as a whole—the pole, arm, pins, insulators and power circuits—have many weak links, in a long line. It is advisable where there are two or more companies, possibly competing for the power business of a city, to have connecting circuits and even to operate their lines in parallel. A somewhat similar arrangement is common among steam railways. A competing road gives the use of its tracks to a rival during temporary trouble to roadbed or at a burned-out bridge. A working arrangement of this nature, i.e., to assist each other as far as possible in times of trouble, would help the reputation of power transmissions.
DISCUSSION AT SCHENECTADY, APRIL 7, 1903.
DR. F. A. C. PERRINE:—There is so much in these papers, that it is hard to enter upon a discussion of them. In regard to the paper by Mr. Mershon, I believe that there is one element in the strength of the pin which he has altogether neglected, which however, may possibly be neglected on account of the roofing or rounding of the cross-arm. I refer to the element of strength in the shoulder. The pin is discussed as a beam fixed at one end, and in consequence the ordinary parabolic section of the beam is brought out, because the fibers of the pin are considered to be in tension or compression. Now, as a matter of fact, if the shoulder is made pronounced and firmly fixed on the cross-arm, the pin is very much increased in strength; because there is an element of the stress applied to the end of the pin, which is transmitted parallel to the side of the pin and against the cross-arm.
Mr. Mershon says that usually pins break off at the shank. This is generally the case where pins do not bear on their shoulder in the cross-arm. In some experiments made in the West with a number of pins, I found that if the pins were given a proper shoulder and made to bear in the cross-arm, they did not break at the shank, but broke diagonally from a point about at the end of the thread crossing the pin. The pins that were tested were approximately the same locust pins that were mentioned in the discussion. By giving these pins a proper bearing, the strength was found to be increased from 700 lbs. to 900 or 1,200 lbs., with approximately the same pin.
I notice that the pins designed by Mr. Mershon correspond very closely to the pin in Fig. 1 in Mr. Chesney's paper. Furthermore, I see that the pin in Fig. 1 in Mr. Chesney's paper is not given a bearing in the cross-arm, as the shoulder is filleted so that it does not come down to a solid bearing. The other pin in Fig. 2 is given a solid bearing, and this pin is very much stouter than the pin in Mr. Mershon's paper or the pin in Fig. 1. As you will notice, these pins are both for the same insulator. The pin in Fig. 1 is the pin used on the Standard Company's lines and in Fig. 2 the pin used on the Bay Counties lines. Mr. Hancock of the Bay Counties Company designed this pin after testing a number of pins and insulators. He found that the pin in Fig. 1 would almost invariably break before the insulator; that the pin in Fig. 2 would practically never break before the insulator, this pin having practically the same strength as the insulator. The material is eucalyptus. Since the discussion, Mr. Hancock has reported that the strength of this eucalyptus pin compared with a steel wagon-axle. The axle was broken at a strain that would not break the insulator, although the wood pin was of approximately the same strength as the insulator.
The observation made in the discussion that Mr. Mershon's pin is based on a uniform stress applied to the insulator, and that this is not a reasonable specification for the standard pin, is a point that is very well taken. With lines such as are proposed now, with spans of five or six hundred feet, the transverse stress on a wire from one-half to three-quarters of an inch in diameter, will be in excess of 600 lbs. With oak or locust, as specified by Mr. Mershon, the pin will not have a strength much in excess 600 lbs. With eucalyptus, it would have a greater strength, eucalyptus having approximately the strength of good hickory. In such spans of five or six hundred feet it would be necessary to install more than one of Mr. Mershon's pins to stand the strain from heavy cables.
In regard to the testing of glass insulators, so far as I am aware, having had experience with a good many thousands of glass insulators, the puncture of glass insulators by reason of breakage after the insulator had been inspected visually and tapped with a mallet, is very unimportant. The point that Mr. Blackwell makes of glass insulators breaking down, due to lack of annealing is, on the contrary, an exceedingly important one. One of the lines in California installed a type of glass insulator that had been well tried, but apparently it received a batch of unannealed insulators, for before the end of the year a large number of their insulators separated and broke down; the head of the insulator cracked off and let the wires drop. Such occurrences with glass insulators, are far more important and more likely to happen than punctures. A glass that will puncture at all, I believe to be a glass that is so bad that you could see the defects. No insulator should be installed which has a bubble between the wire and the pin, because these bubbles are vacuums and you might as well have just so much metal in the insulator.
Mr. Lincoln's paper, is I think, the first approaching a complete discussion of the telephone line transposition problem, and it is se complete that I cannot sit down without commending it, without saying that in my belief it contains the elements of the entire solution of this very important and difficult problem. Had this paper been printed four years ago, I believe that I know of more than one man's life that would have been saved by it. A very sad accident happened about a year ago: A patrolman starting out to work, went to the closed telephone box to report. In connecting this telephone to the line, he was killed. I believe the power line was grounded, but not to the telephone line. Since that, the company has observed the rule of seeing that the operator is insulated as well as the line.
The question of potential to earth is the only thing that Mr. Lincoln has not given an absolute statement of, and I am inclined to think that that is because there is something in it that we don't yet know. Mr. Lincoln writes of the difference of potential, but says that that represents so small a current as to be inappreciable. If the surface of the condenser that is produced by the power line and the telephone line and the condenser of the telephone line and the earth is calculated, where the line is one or two hundred miles long, the amount of current that is transmitted will not be by any means inappreciable, and will be enough to give a great deal of trouble. This is the only criticism that I would have to offer to this most excellent paper of Mr. Lincoln's.
(article is continued with 8086)
|Keywords:||Power Transmission : Problems : Pin : Telephone|
|Researcher notes:||The article used (and page numbers) was from a bound two-volume set of published AIEE articles from 1902-1904 owned by N. R. Woodward. The title of the books are "High-Tension Power Transmission", which were published in 1905 by the AIEE. The date for this article was the original publication date in the AIEE journal.|
|Supplemental information:||Articles: 2500, 8086|
|Date completed:||November 23, 2009 by: Elton Gish;|