Publication: Western Electrician
Chicago, IL, United States
Power Transmission on High-Tension Lines.
DISCUSSION OF P. M. LINCOLN'S PAPER.
M. H. Gerry, Jr.: There is no especial difficulty in installing a successful telephone circuit, which will give satisfactory service and reasonable safety in operation, on a pole line carrying from 50,000 to 60,000 volts.
Mr. Lincoln: 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.
Mr. Mershon: 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 electromotive force. 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 electromotive force can impose upon the condensers, there will be a tendency towards constant current; if not, as seems to me is usually the case, there will be a tendency toward constant voltage.
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 second, because it may also be a means of protection against loss of life or fire. This method is that of using auto-transformers connected across the telephone line at a number of points, preferably at each of the telephone stations, each having its middle point connected at 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 disagreeable or dangerous shocks whether due to crosses, leakage, or in electrostatic induction and would also help to minimize disturbances due to grounds, etc.
P. H. Thomas, New York: 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 may be worth trying. 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 from the telephone the secondary, making very high insulation between the primary and secondary. Thus, 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 the 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?
C. E. Skinner, Pittsburg: The insulation of the workman is important, and it is not difficult to make arrangements so that he will be as well 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.
C. O. Mailloux, New York: 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 represents 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 operating machinery. About a year ago, in Arizona, we were starting a 25 or 26-mile line, and repeatedly observed that the three wires would become charged to a considerable potential under various conditions. In Phoenix, Ariz., at this time of the year, the weather is very much the same as it would be here in June or July, except that there is no rain. The weather is usually very pleasant, quite warm in the daytime — you can wear your summer clothes — and it cools but very little in the evening. I have repeatedly observed during the daytime that the three lines became charged electrostatically. They could be discharged by making a connection to earth, but if you left them alone they would charge again. They would always charge at approximately the same potential with respect to earth, and the potential varied. On one occasion the potential was so high that we got sparks half an inch in the ground through spark gaps. I have repeatedly observed cases where the spark was one-sixteenth to one-eighth 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 time, or shortly after it had rained, somewhere on the line; but the phenomenon was repeatedly observed when there was no indication of rain whatsoever, the sun shining brightly, but there was some wind or slight breeze somewhere on the line. I have mentioned the matter to several physicists, but have not succeeded in getting any satisfactory explanation.
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. Another, point that has been brought up is to reduce the potential between telephone wires and ground, and. to introduce there auto-transformers, or regular transformers, or condensers, or something of that kind, or possibly resistance, although the latter was not mentioned directly. The objection to that method, it occurs to me, is that it takes off the charging current at points. The remedy which I propose is to produce a distributed capacity along the whole length of the line, to run a ground wire along the whole length of the telephone wire, producing a distributed capacity which will take off the distributed induced effect most effectively.
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 in regard to the signaling with the power lines themselves, if anyone wanted to signal with the power lines, it would be better to signal from neutral to ground, and receive messages at corresponding places at the end of the line. If you have an auto-transformer for each telephone, it is bound to equalize any trouble at the telephone by putting the telephone wires in ground. The objection Mr. Lincoln made that the current would be drawn off at certain points, whereas it is introduced uniformly along the telephone line, would not hold, as the current is flowing in the same direction in both lines, and the electromotive force induced would be induced in each wire, and neutralize the current as far as the telephone is concerned.
THE TESTING OF INSULATORS.
BY F. O. BLACKWELL.
Insulators must be obtained which will not fail in service and this can only be assured by the thorough testing of each one that goes on the electric lines. The potential that can be employed safely for the transmission of power is now limited by the pressure the insulators will bear. Transformers that are reliable and not excessive in cost can be built for twice the voltage that any line yet constructed will withstand. As the distance over which power can be transmitted with a fixed cost of conductor varies with the potential, the length of transmission lines is to a great extent limited by the insulator.
The design of new and improved types of insulators is, therefore, most important, and these can only be developed by experiment with adequate testing facilities. In order to ascertain the value of such insulators, no method of testing can equal a practical trial under conditions of actual service. Placing new insulators on power-transmission lines in commercial operation is impracticable in most cases and should only be permitted after they have successfully withstood tests to demonstrate their ability to stand operating conditions. These tests should duplicate as nearly as possible the electrical and mechanical strains set up in the insulators under the most severe conditions that would ever be met with on a transmission line.
There are certain facts which must be considered if correct deductions are to be made from insulator tests. For instance, we cannot test each insulator with a given number of volts continuously as it would be in service. As is well known, all insulating materials are most apt to break down on long applied electric stress. The prepared cloth wrappings used on the windings of electrical machinery will stand instantaneously two or three times the potential that they will carry continuously. Glass and porcelain are not affected by time to the same extent as organic materials, but we know that both are punctured by long continued applications of lower pressures than they have withstood in test.
The shape of the potential wave also has a pronounced effect in breaking down insulation. Insulating materials being more affected by time than air, show in their ability to resist puncture that the average potential of the wave is more important than the maximum.
It is not safe to assume the potential either by the voltmeter or air gap as the true potential for determining the insulating value as it is somewhere between the two. Moisture in the atmosphere also effects the arcing distance. In steam, a given potential will jump twice as far and in a fog 25 per cent. farther than under ordinary conditions. Of course, if the altitude is high and the air more rarefied, the arc will also jump a greater distance.
The alternators generally used for long-distance transmission plants give very nearly a sine wave and therefore the testing generator should be one which will give a sine wave under all conditions. It is not sufficient to do so at full potential and no load, as tests are made with all degrees of excitation and with both leading and lagging currents.
The armature reaction should be as small as possible, which means that the generator should be much larger than would ordinarily be thought necessary. It is also desirable to have a high reluctance in the magnetic circuit to secure stability when running with weak fields and permit of control with a reasonable amount of field resistance.
There should be but one transformer used to step up to the highest potential required and its reactance should be as low as possible. A number of transformers in series is particularly bad, as it gives poor regulation and leads to great uncertainty as to the actual potential to which an insulator is being subjected.
Assuming that insulators are to be passed upon for a specific transmission plant, they should first be inspected to see that they are free from cracks, bubbles or pits that will impair their strength or in which moisture can lodge. If of porcelain, the glaze should cover all the outer surfaces. The glaze is of no insulating value in itself, but dirt sticks to unglazed surfaces. Experience has shown that porcelain insulators which are not absolutely non-absorbent are worthless. The best porcelain shows a polished fracture like glass. If there is any doubt about the quality of the porcelain in this respect, it should be broken into small pieces, kept in a hot dry place for some time, weighed and immersed in water for a day. When taken out of the water the weight should be the same as at first. A puncture test should be made by setting the insulator in a cup of salt water, filling the pin hole also with water and slowly increasing the potential between the top and bottom until the desired test potential is reached or the insulator either punctures or arcs over the surface.
If an insulator is built up of several parts, each part should be able to withstand a pressure greater than it will have to sustain when the complete insulator is tested. If it is to be tested for 100,000 volts and is made in two parts, each part might, for instance, be tested with 70,000 volts. The object of this is to have the weak parts rejected before they are assembled. A fair puncture test for an insulator is twice the potential for which it is to be employed, applied between the head and the interior for one minute. In a wet arcing test, a stream of water from a sprinkler nozzle under a pressure of at least 50 pounds to the inch should be played on the insulator at an angle of say 30 degrees from the horizontal. This will be similar to the condition which exists in a rain and windstorm. The insulator should not arc over from the wire to the pin at less than the potential which will exist in service between any two conductors.
In no case should wooden pins be relied on for insulation, as their value is only temporary. All wooden pins in time become dirty and absorb moisture. Eventually they burn off unless the insulator is good enough to be used with an iron pin. If an insulator is going to fail, it is better to have it do so at the start and not interrupt the service, by breaking down perhaps years afterwards.
In addition to the electrical tests, it is well (if the insulator is of a type that seems to require it) to try samples for mechanical strength. When mounted on pins the insulator should stand a side strain of at least 10 times the pressure exerted by the air on the conductor with a wind velocity of, say, 100 miles an hour. It should also be able to slip the conductor through the tie wire should the former break.
These tests are particularly desirable with built-up insulators in order to be certain that the parts will not separate. With such insulators, it would also be well to test them in tension along the axis of the pin. In transmission lines crossing depressions, such an upward pull is not infrequently exerted on the insulator.
M. H. Gerry: Insulators are tested for two purposes: First, to determine, the design, shape, material and dimensions best suited for a given voltage and set of conditions; secondly, insulators are tested as a matter of routine, to determine whether manufacturers have complied with specifications regarding material and workmanship.
There can be no complete set of tests to cover the first purpose, as it is not only a matter of experiment, but of skill and judgment in properly interpreting the results of many tests in relation to service conditions. The testing of insulators for the second purpose is comparatively simple.
P. M. Lincoln, Pittsburg: I would take issue with Mr; Gerry, who made a statement that glass insulators did not need testing beyond visual inspection. I know of a certain line which was put up, the insulators of which were not given a voltage test, but were simply tapped with a mallet in order to eliminate, if possible, any which had undue internal strain, but after the insulators were up and the normal voltage applied, quite a number of them broke down in actual service, which probably would not have occurred if they had been tested with the voltage strain before being erected. I do not think there is much danger of breaking down of an insulator from lightning strains which have double normal potential of steady potential.
Mr. Mershon: As between glass and porcelain, it seems to me that about the only advantage, other things being equal, that porcelain has over glass for transmission work is that of mechanical strength. Sometimes it has that advantage and sometimes it has not. It is a fact, however, that in some forms of insulator — in some complex forms — it is possible to get the porcelain insulator cheaper than the glass ones. The losses of insulators from high voltages show practically no difference that could be detected between glass and porcelain in any measurements I have made.
T. A. W. Shock, York, Pa.: I had the pleasure of building a line this last year in the state of Pennsylvania, in which I used an insulator which tested 75,000 volts. The ordinary pressure will be 24,000 volts — that gives a factor of safety of over two. I think I coincide with Mr. Blackwell that in testing insulators for a line of high pressure that the testing pressure should be at least three times the ordinary line pressure. My idea in adopting that style of testing is due to the fact that the insulator was built for a line on the Pacific Coast, for 40,000 volts, and I adopted the insulator to give me an extra factor of safety on the line.
F. N. Waterman, New York: Mr. Blackwell's reference to testing a double insulator — an insulator primarily of two parts — that each should be separately tested, seems to me practically to bar out double insulators and does not seem to be logically founded.
Mr. Thomas: Mr. Gerry and Mr. Lincoln have referred to one point in the testing of insulators that deserves a good deal more consideration, and that is the point of initial strains, the annealing of the glass. Mr. Mershon also spoke of the difficulty of getting glass well annealed. The breakage of glass insulators on the line during rain may more often be due to too much sun on one side of a poorly annealed insulator than to electrical operation.
C. C. Chesney, Pittsfield, Mass.: I think, in general that the glass insulator will give as good satisfaction as porcelain insulators, except the annealing strains which, when the sun strikes them unequally, simply break out on all sides. I had some experience in some of the California work, in which very large insulators are used, and after very cool nights, when the sun strikes them on the side only, they will drop off.
Mr. Mershon: I cannot entirely agree with the author in regard to the use of one transformer for testing purposes. A series of transformers need not have a bad regulation or seriously distort the wave form, and such a testing set has a very great advantage that a transformer breakdown is much less serious than in the case of a single testing transformer. For very accurate results in any case a step-down voltmeter transformer is desirable.
Neither can I agree that insulators should necessarily be tested on iron pins. As I have stated before, an iron pin is likely to be the means of putting upon the insulator a mechanical stress all out of proportion to any which it would meet in practice if installed upon a wooden pin. An insulator which will stand up well under a salt-water test will often break down quickly when tested on a metal pin. While I do not believe in depending upon the wooden pin for insulation, I see no good reason for condemning it in general where there will be no trouble from burning.
I agree thoroughly that any new type of insulators should be given a rain test; that is, a voltage test while water is being sprayed upon the insulator in imitation of rain. If a wooden pin is used in this test it should be covered with tinfoil from a point inside the petticoat nearest the pin to a point two or three inches below. The voltage should be applied between the foil and the wire around the neck of the insulator, or preferably, between the tinfoil and a piece of wire representing the line wire, and tied to the insulator as it will be in practice. It has been my practice to endeavor, to adjust the spray for a precipitation of one inch in five minutes.
W. L. Waters, Milwaukee, Wis.: The charging current in a test on insulators on an overhead line generally conforms more or less to the potential wave form of the alternator, and whether the current is lagging or leading, the distorting effect on the wave form is slight, and with all ordinary excitation and loads on the alternator the effect on the insulator test will be inappreciable.
I think there is only one really satisfactory way of measuring high voltages, and that is, by means of a voltmeter transformer connected straight across the insulator being tested. The above remarks are strictly only applicable to voltages up to 50,000, as I have had no experience with higher voltages.