[Trade Journal]
Publication: American Institute Of Electrical Engineers
New York, NY, United States
p. 389-409, col. 1
SOME DIFFICULTIES IN HIGH-TENSION TRANSMISSION AND METHODS MITIGATING THEM.
BY J. F. KELLY AND A. C. BUNKER.
In discussing the conditions which affect and limit the constants and operation of high-tension lines, pressures of over 30,000 volts and lines of over 50 miles in length only will be considered.
The usual relations between voltage and length of line, namely, "1000 volts per mile," or "the pressure in thousands of volts equals one-third the number of miles," cannot be applied generally until all sources of interruptions are taken into account, so that the length of transmission does not altogether determine the voltage to be used, for a voltage as high as possible will be used and its value be determined from local and climatic conditions.
These will be the principal factors in the design of any line, as all the other constants, except, perhaps, the kind of conductor, are interdependent upon them. Since there is always some doubt as to the successful maximum operating voltage which these conditions will permit, and just how the line will be affected, it is well so to design the step-up and step-down apparatus that, without seriously affecting its capacity, several voltages. say 30, 40, 50, and 60 kilovolts, or even higher, can be obtained at will. This arrangement will permit the power to be transmitted with the highest possible voltage, and the causes which prevent the use of the next higher pressure can be studied and overcome if possible. As a new plant is usually started with a load less than its capacity, there will be no serious decrease in the efficiency by this method of experimenting.
The principal causes of interruption of the supply of power in the past and at the present time are: Open-circuit, grounds, short-circuits, and other circuit changes which produce oscillations.
These are directly and indirectly traceable to weak insulators, lightning, defective pins, burning of poles at the ground, storms, and to a class which might be called unexpected sources. All the above may not be common to one locality, but all may exist on a single system. It may be said that where the quality and design of the apparatus and accessories for the generating and sub-stations have been selected with regard to their requirements, and where such are afterward intelligently handled, almost the entire list of troubles which are at the present time affecting the continuity of power service, may be credited to the line.
In designing a transmission line, experience has shown that the. most careful study of local and climatic conditions should be made in order that all the facts and data bearing on these and their probable effects may be obtained.
It has been demonstrated that one transmission line for voltages. over 30,000 will not give continuous service except when ideal climatic conditions exist. There is one, and possibly two, plants that have given continuous service for more than a year, with two circuits each, and with climatic oonditions better than generally exist. It is believed that, in some localities, even with duplicate lines, the best insulators obtainable at present, and with perfect circuit-breakers, the maximum voltage which would permit continuous operation or delivery of power would be 40,000 volts, or possibly 50,000 volts with the utmost care and diligence.
The selection of the line-insulators depends entirely upon the voltage, mechanical strength required, and the localities through which the line passes, more particularly the latter, as lines have been operated at 45,000 volts with two or three types and sizes of insulators in as many different sections. The design of the insulators should be such as to give the smallest amount of still-air space and the greatest accessibility for wiping by hand. Fog occurring at the same time or intermittent with soil, factory, or car-dust is one of the surest causes of trouble, and reduces probably, to the greatest extent, the effective commercial size and value of insulators. Upon examining a large number of insulators which had to be removed, it was found that the dust, with which they were coated, was thickest in the still-air spaces, and was as thick on the vertical as on the horizontal surfaces. It has been found that where insulators were subjected to fogs or dust alone (except sea-fog), the same number of troubles did not occur as when both appeared together. Where insulators are covered with dust parts of each year, it has been necessary to shut down the circuit from one to three times during the dust season to wipe them clean. This can be done while in position and without disturbing them unless they are found to be damaged.
The fact that insulators are successfully tested for high voltage before they are put up does not necessarily prove that they will not cause any trouble when on the line. Insulators which were tested for 120,000 volts water test for one minute have given trouble in less than a month after being placed on a 40,000-volt line. Other types which had stood 40,000 volts water test for five minutes have been known to be unsatisfactory for 13,000-volt city (overhead) service, though this would not hold in every city. The greatest value of electrical best for insulators before being used is to determine whether the various parts are homogeneous and whether they have been properly cemented together.
If an insulator was made up of three separate pieces, each having been tested for my 80,000 volts before cementing, it does not follow that the completed insulator will stand 240,000 volts or even 120,000 volts. The striking distance of the completed insulator, together with the quality and manner of cementing, determines very largely the final test voltage, even though they have sufficient creeping surface for a higher voltage.
In cementing a large number of insulators together, it was noted that the percentage broken down under test could be reduced almost one-half by a little more care in the method of cementing. When insulators are glazed together at the factory, a uniform insulator should be obtained. The conditions for transmission are very good, if, for continuous use, 1 per cent of the insulators does not have to be replaced each year. Taking a circuit having 12,000 insulators installed, there would be at least 120 renewals each year. Each poor insulator is liable to cause a disturbance or interruption, and the system might be subjected to an average of 10 per month.
If some seasons of the year are more severe on insulators than others, there may be more than 30 cases of trouble per month. It has been observed that, where insulators were giving trouble on a line operating at 40,000 volts, reducing the pressure to 30,000 volts did not produce a like or immediate decrease in the number of insulators broken per month. The total number remaining seemed to be in a more or less weakened condition, and would continue to break down after the line-pressure was reduced, though after a certain period of time, the breakage per month was less.
The difficulties of taking care of lightning discharges increase much more rapidly than the line-pressure, for the reason that any disturbance or change in circuit conditions, produced by getting rid of, or dissipating, a charge in a circuit having high voltage and high inductive and capacity reactances, may set up oscillations which, if not serious to apparatus, are disastrous to regulation and service. Various combinations and multiples of low-voltage types of arresters have been used, but where these have not had the proper addition of a resistance, they have seldom failed to be completely destroyed when the particular stroke or circuit change occurred. It has been clearly shown that the same arrester could not, without special adjustments, be used on all parts of the circuit, and that arresters performing their function for a lightning stroke, or taking the kick-back from a short-circuit opening, when a given number of generators, length of line, or transformers were in circuit, would not so operate for another number of generators, transformers, or length of line. This may have been due to the increased inductance in circuit obtained from a smaller number of generators or transformers, to a longer length of line, to the nature and duration of the arc in opening the short-circuit, or to any number together of these conditions. Where a resistance is used with any type of arrester, in order to keep the value of current which would flow over the arresters, to a given percentage of the load-current, the amount should be such that five or six times the normal impressed voltage can be taken care of.
A modified form of the Siemens' arrester has been used on circuits up to 50,000 volts with a fair degree of success when they were correctly adjusted for the different positions of the circuit, and, where a resistance was placed in series, the voltmeter cards were not painted badly when lightning or a short-circuit occurred. This design can be greatly improved, and no doubt would give very good results and thoroughly protect connected transformers. Their low cost, ease of construction, and their outdoor serviceability are points in their favor.
Perhaps the most reliable arrester is one consisting of an inductance and condensance in parallel, so that any frequency variation. from the normal would cause a certain value of current to flow. This type, immersed in oil, would be rugged, and could easily be adjusted for any position of the circuit. One of the best means of dissipating an induced charge or stored energy in a line is by having a distributed load along the circuit. If this load has a grounded neutral, the effect of a lightning stroke will be greatly reduced and more easily taken care of by the regular arresters. The star-connected line with grounded neutral has, however, some disadvantages of equal importance, which should be carefully considered before being adopted.
In practice, next to troubles from lightning, short-circuits on long lines of low ohmic and high inductive and condensive reactances produce the most serious consequences. It is, therefore, necessary to use accessory apparatus which will discharge the circuit between wires, as well as between circuit and ground. This point should not be lost sight of in the selection of arresters, and in their connection to the circuit. When a line is short-circuited from any cause, there is a rush of current, the value of which depends upon the impressed voltage and the impedance of the circuit up to the point of the short-circuit. When this current is suddenly interrupted, the voltage induced depends upon the constants of the circuit and increases in value with the length of circuit. distance between wires, the amount of inductance of the connected apparatus, the inductance of the rupturing arc and its duration, the impressed voltage, and the instantaneous value of the current when the short-circuit is opened. This induced voltage will be small or of little importance if the short-circuit is opened at or near the zero value of the current. In operation, induced voltages have been observed u-hen opening a 40,000-volt 100-mile line when short-circuited, of from 2-1/2 to 6 times the normal voltage, as measured by the length of air-gap broken down by the kick-back. In the more severe cases, some point of the system usually suffers; that is, there will be a discharge or arc across some point of the line or transformer terminals, a puncturing of transformer coils, break-down of insulators, the destruction of lightning-arresters, or some other like effect. In nearly every case the circuit is put out of service unless efficient arresters are used. The fact that there is not mare damage done than would seem likely from the voltages observed is, no doubt, due to the property of solid dielectrics of withstanding momentarily very high voltages and which would be punctured in an interval of time. Air having the property of breaking down immediately upon the application of the proper voltage for the gap is the probable reason why these manifestations more commonly occur in air, from terminals, and around other dielectrics.
The troubles from the charring of wooden pins were due to the continual leakage of current over dust-coated insulators. In some localities, pins would last only from one to three months. This was entirely corrected by placing a metal short-circuit around the pin. Molding at the thread, which is often noticed where the line passes through a marsh, can be prevented only by the use of a metal pin. Several lines have now been equipped with steel pins and no new troubles have developed, but, on the contrary, a decided decrease. It would seem that for large high-pressure lines steel pins should be used exclusively. Their initial cost is from one and a half times to twice the price of wooden pins, though cheaper in the end. Soft lead gives better results for the thread than any composition. The moulds should be made so that enough lead can be used to extend a little way below the bottom of the thread, as this will give a good bearing to the •insulator over and above that obtained from the thread. This will greatly add to the mechanical strength of the insulator and of the line, as, with the ordinary pin, the insulator is the weakest element of the line_ Precaution should be taken to have the thread portion short enough so as not to come in contact with the top of the insulator. This will prevent the tops being forced off when the insulators are put on the pins, and will allow a firm seat at the other end of the thread.
The service given by wooden pole-line construction is subjected to interruptions from falling and burning poles, due to decay, freshets, forest or grass fires, the large number of insulated points. and from the necessary short length of the poles. The decay of poles can be greatly lessened by continual inspection and care after they are up. The idea that poles of the right kind of wood for the soil can be placed in the ground and last for 10 or 20 years has been the cause of many and costly repairs. One 6-year-old redwood line, with butts treated before raising, had to have 33 per cent stubbs. Another redwood line, untreated, had to have 10 per cent stubbs in three years. Another line of untreated cedar poles required 35 per cent stubbs in six years. In long lines and even in some short ones, soils may be found that have an entirely different effect upon the life of the same wood.
Engineers are many times prevented from buying poles at the proper time to have them cut, on account of the interest chargeon the cost of the poles and erection, from the time the poles are paid for to the time when the wires are strung. The freight and hauling charges on from one-fourth to one-third more weight will offset a large amount of the interest charge. The result is that the poles are put in green and, unless they are afterward treated, decay will begin in a short time. If it is possible to obtain seasoned poles, their life will be much increased by thoroughly treating the butts before raising than by any subsequent single treatment. When green poles are used, no treatment should be given before raising, but the butts should be treated after the first dry season, and retreated every second or third season after, this depending upon the material used and condition in which the pole is found. There are on the mark-et several kinds of treating material which are showing good results. This after-treatment consists in digging away the earth from the pole for about 18 ins. below ground-line and treating this surface, together with that 18 ins. above ground-line, after the decay and earth have been cleaned off. The old ground-line should then be changed by banking earth up around the pole. The cost of this treatment varies from $0.60 to $1.00 per pole, depending upon the location of the poles and the kind of material used.
At the same time the butts are treated, the pole-tops gain cracks, and ends of the arms under the dead circuit should be painted. This is the best-known method whereby wooden construction, as a general thing, can be made to last the so stated 20 or 25 years.
The burning of the poles at the ground has been the cause of interruptions even where the line was patrolled twice a day, but the remedy is simply a question of persistence and expense in keeping the right of way cleared of all growth. It might be noted here that, even with a generous right of way kept cleared, the wind may carry the heat from a fire toward the line. Two cases are on record where the heat from a forest fire along a pole-line was not great enough to harm in any way the wood arms or poles, but did cause large numbers of glass insulators to crack and fall to the ground. Porcelain is less affected by heat than glass, and probably would not have caused as much breakage.
Some of the unexpected sources of trouble show how detailed must be the design and care of a line, and what insignificant and harmless-looking objects and occurrences may cause the complete shut-down of a circuit. After everything imaginable has been considered and provided for, there may still be accidents. One case is known where some dried hay was carried up into a 40,000-volt line, with the result that it was set on fire and produced an arc that shut off the power. The burning hay, being carried on by the wind, did considerable damage. On another line, a flock of pelicans flew into the telephone circuit which was strung several feet below the power wires. The span was something over 600 ft., with a sag of 19 ft. The telephone wires were struck so hard as to wrap them around the power-circuit.
Another case was where a long piece of light bark was blown several rods across a 42-in. line, with the usual result. On the same line, during one season, there were three interruptions, in one locality, caused by large birds getting across two of the wires.
The falling out of step of synchronous apparatus, while not frequent, does happen and, unless the breakers operate promptly, other apparatus may add to the trouble and the circuit be opened. On the other hand, with proper attention to field strengths, synchronous motors have several times been known to keep in step during temporary short-circuits on their connected direct-current generators, the direct-current breakers being purposely set at a high current or tied in.
The question as to whether wooden poles or steel towers should be used for a given transmission will be determined by the advantages of one over the other for the conditions to be met.
In countries where wooden poles are plentiful and inexpensive, it is probable that every expedient will be resorted to before steel towers are used.
One of the principal advantages of wooden construction is, that, in case an insulator is broken, allowing the wire to come against the arm or pole, the burning which takes place almost immediately in most oases may continue for several minutes before a blaze is started which will short the circuit. Several times it has been observed that from 20 to 30 minutes elapsed from the time trouble was first noted by the ammeters or telephone until it was necessary to shut off the circuit. In one case a 40,000-volt (grounded neutral) wire lay on a dry cross-arm for several hours before the circuit could be shut off. and at the end of the time the arm was not badly charred. With a duplicate line, ample time would in most cases be given for changing from one circuit to the other, or to cut out the affected circuit, providing the telephone line was operative or the men at both ends recognized the difficulty. For the past four years, engineers have tried to adopt, where possible, steel towers, instead of wooden poles, as a means of correcting a large number of line troubles.
At first thought, towers would seem to solve all difficulties previously experienced and certainly do eliminate a great many. The spans can be increased, so that as few as eight towers per mile can be used with safety. This would greatly reduce the number of insulators which can be larger, and the means for their attachment to the towers can be quite elaborate without exceeding the cost of the other construction. The height of towers can be greater, which will decrease troubles from wires, branches, and other material being thrown or blown across the circuit and reduce the breakage of insulators from the heat of forest or grass fires. If galvanized, or painted, occasionally, their life would be greater than could be expected of wooden construction.
Towers can be erected in places even more difficult of access, since they can be taken apart in pieces of lighter weight than a wooden pole. They would offer a more or less good lightning path to ground which would help to prevent the injury to connected apparatus. but will no doubt subject each insulator to greater strains. Any leakage around, or puncturing of, an insulator will mean the immediate shut-down of the circuit, and, in order to prevent the shut-down of the entire system, overload and reverse circuit-breakers of the best possible design will have to be used.
Auxiliary insulation of sufficient mechanical strength could be used to reinforce the insulators carrying the conductors, as the towers would be able to carry considerably more weight than wooden poles for the same cost per mile.
The most economical design of a tower is bet suitable for a good many places where the line would have to be erected, and could only be universally used on a private right of way. On railroad rights of way, narrow county roads, village streets, etc., the spreading base would not be allowed, and resort would have to be made to steel poles, which for the same strain and height would be more expensive.
The distance between wires is usually determined from the highest voltage which can reasonably be expected as a limit, as determined above. The rule that the distance between wires in inches equals one and one-half times the number of thousands of volts is safe so far as the striking, or repeating, distance is concerned, though to correct for arcs holding on for a time after once established would be impossible. Where the cost of erected poles is high, or the right of way expensive, two circuits per pole-line should be used, and, with good wooden construction, mechanical difficulties would limit the distance between wires to at most 60 ins., which would allow a line voltage of say 50,000 or 60,000. This distance between wires is for spans not over 150 ft. to 200 ft. The size of wire is determined from the load, voltage, length of line, losses allowable, etc. Five per cent energy loss per 50 miles with 60-cycle frequency gives a line which can be taken care of, but a smaller loss should be obtained where important lighting service is had in connection with a fluctuating load. On account of the distance and pressure, a charging current, at no load, is required of the plant, which at 60 cycles and one line 100 miles long, or 30 cycles and two lines, would require a generator as large as 2000 kilowatts, so that, unless more than this capacity had to be delivered as load, the system would not be economical. In order to be perfectly flexible, this amount of power would have to be carried over one circuit. The wire would, therefore, be large enough for mechanical reasons, and the energy loss per insulator, or per unit length, would be negligible, except, perhaps, for voltages over 60,000.
There is one plant in operation which, if the energy loss per insulator, or unit length, was as much as calculated from experiments, it would not be able to deliver load.
In stringing the conductors. especially if they are of aluminum, attention must be given to the temperature at the time the wires are tied in. This might seem to many to be a useless and tedious process; but a set of curves showing the sags for given spans and temperatures, in the hands of a careful line foreman, will give a line good in appearance, and at all times safe from overstrains. It is not so important to know what the maximum sag for maximum temperature will be, as the maximum strain at lowest temperature, with sleet, if any, taken into account. Aluminum cables are made which are as strong as copper for the same conductivity. When conductors are given the proper sag, a given safe tension, mu be maintained for longer spans than would ordinarily be used in transmission work. There are a number of spans over 600 ft. in length, and have been in operation for two or three years. These have been closely watched during wind-storms, to see what deflection would be given to the wires. Three aluminum cables 7/8 in. diameter, 600 ft. span, 19 ft. sag, were deflected from 30 degs. to 45 degs. from the vertical by a wind that was estimated to be 70 miles per hour. All three conductors kept their relative position when deflected. and there were no perceptible waves or vibrations in the cables.
It is claimed by some who have had the opportunity to notice, that in longer spans there is less tremor, vibrations, or waves passing over the span when there is a wind than when there is none.
All observations of the writers show that, for spans of 600 ft. at least, there is no tendency of the wires to swing together in ordinary storms. Tornadoes would no doubt twist the wires to-get h er, but that would not be the worst damage done.
The height of poles or towers would depend upon the sag and whether or not a telephone circuit was strung underneath. With spans of 660 ft., the sag for aluminum would be about 20 ft., and with a telephone circuit 6 ft. below, a 65-ft. tower would give a clearance below the telephone wires of 29 ft.
A clearance of 35 ft. below the lowest power-wires is little enough for places where a house or derrick is liable to be taken under.
The frequency to be adopted depends upon whether the power is to be supplied to already installed apparatus of a given frequency. For long lines, a frequency of over 60 cycles will give a regulation difficult to allow for. The lower the frequency, the better will be the regulation of a line for a given load, the smaller will be the generator capacity required to charge the line, and the voltage drop will more nearly approach the IR of the circuit. For a given line, there is only one particular value of current where the condensance of the line will be neutralized by the inductance; so that this fact also decreases in importance. The swing of the power-factor at the power-house will not be
With the general use of A. C. railway motors, 15 cycles or less may be advisable.
The power-wires of a single- or double-circuit line should be transposed with reference to the power-taps arid talking-points.
Experiment has shown that transpositions at stated distances need not be made and may not give as good results as the first method. With two circuits, one should be transposed in the opposite direction to the other; although there is one double-circuit line operating satisfactorily as fax as the telephone is concerned, with one of the circuits run straight through. Experiments made with a power-line without transpositions and a telephone transposed every fifth mile placed 5 ft. below the power-wires, gave a pressure to ground of from 2100 volts to 2800 volts when the line-pressure was 40,000 volts. With 30,000 volts, the telephone voltage to ground was reduced in the same ratio.
By giving the power-wires two-thirds of a rotation between power-taps and talking-points, this voltage was not readable on a Weston or hot-wire 150-volt voltmeter. The induced voltage was due to capacity, and in none of the tests was there any measure-able electromagnetically-induced voltage.
The large number of fatal accidents, which have occurred in the past from the telephone circuit being placed on the same poles with and under the power-wires, would warrant a separate pole-line, even if the service were no better.
A telephone is most needed at times of line disturbances, and at such times it is rarely of service. The induced voltage on a telephone circuit, even where power-line transpositions are made, when one or more of the power-wires are out or grounded, is high enough to be dangerous to life and to set fire to adjacent woodwork. The distance between the two circuits should be at least 6 ft., and 8 ft. would be better. In stringing the telephone wires, the same sag should be given as to the power-wires. For lines over 50 miles in length, copper or aluminum should be used instead of the regulation No. 9 BB.
The question of high-tension switches and circuit-breakers is one of the most important in the operation of a system. They should be of the most approved design only, and placed at both ends of a circuit and at intermediate, or cross-over, paints. All poles of a three-phase switch, or breaker, should work together and not singly. A switch which tests satisfactorily in the shop may not operate in service; so that it should be placed in position and opened 10 or more times under the most severe conditions with which it is likely to meet, before it is pronounced safe. All breakers and switches should be provided with cut-put switches on each side, so that they can be taken out of a live circuit for repairs.
DISCUSSION.
MR. BUNKER: There was a statement made yesterday in the discussion that there was no ground for argument on the advantages of iron over wooden pins. There are some localities where wooden pins have no doubt been entirely successful; but as a general case, I would like to take exception to that statement, because there are plants now operating where they have a great deal of trouble with pins, and some of the pins only last from one to six months, until they begin to burn or mould, while in some cases they burn entirely off within that time. There was also another statement made that the burning of pins was due to the brush discharge and charging current of the pin. Isuppose the charging current was due to the electrostatic capacity of the insulator itself. It was not stated what the brush discharge was due to, but if there was a brush discharge around the insulator it was either due to the fact that the insulator was not large enough for the voltage under normal conditions, or else that the insulator was covered with some dust or dirt. Now, in a given line, the pins are subjected to the same static pressure at all times. Some of them lasted two years and over, and have not been changed yet, being apparently as good as they ever were, while in other localities the pins have been changed as many as three times in one season. These were wooden pins, and I might say were made with as great care as possible. The sap was boiled out of them, and they were then treated in oil at about 100 degrees Centigrade, so that the insulation of the pin, when new, was perfect. You could subject a pin along its length to 60,000 volts without any effect and could leave it there as long as desired. The pins were of eucalyptus wood.
MR. E. KILBURN SCOTT: When you say "burning," what do you mean?
MR. BUNKER: When you put an insulator on a line, using wooden pins, and everything is new and clean, there is no discharge or sound from it; but after it stands awhile, in certain sections you begin to hear a discharge, and if you watch the insulator at night, you will see, up in the thread portion underneath the inside petticoat, a discharge taking place.
This gradually begins to burn the wood and after a while burns the pin entirely off at the bottom of the thread. Pin holes are at first made but after the char becomes general, it keeps getting deeper and deeper as a regular burn. There is only one remedy for moulding and that is a metal pin. I am not able to state what would take place where a line crosses a fresh-water marsh, but I do know in a salt-water marsh that a pin that lasts six months is considered to be doing well. That rot or mould has a white appearance and is very soft. When you take the insulator off, you can very easily rub the threads off with your thumb. The treated pin appears to mould as rapidly as the untreated pin. The pins are treated with boiled linseed oil, after the sap is boiled out and drying, then being subjected to the hot oil, but not in a vacuum.
DR. Louis BELL: In this suggestion of treating pins, I asked whether they were treated by vacuum treatment, and I regard that as of fundamental importance. You can boil even a thing so porous as a coil of wire, in insulating material,— for instance, melted paraffine — until you get black in the face and give up in despair, and then take it out and the insulation will not have thoroughly penetrated. Put that same coil of wire, in vacuo, in hot insulating material, and you see the gases rush out of the thing, and the whole surface of it foams for minutes. After that is over, the insulation has a chance to creep in. I therefore should ascribe some troubles to which Mr. Bunker refers, to the fact that although the pins were thoroughly treated, apparently, there was no small amount of material which the insulation did not fully penetrate, so that while it would hold the voltage for a while, the remaining air and moisture would sooner or later get in their work, the air helping the oxidation and the moisture gradually working itself through the structure. I should like to see the thing tried with pins which had been very carefully and thoroughly dried, to see whether the time effect would take place to the same degree.
MR. BUNKER: On the other hand, you take a metal pin and put it in the same locality and you would not have any trouble at all.
DR. BELL.: Save perhaps in puncturing the insulators. I approve of metal pins from a mechanical standpoint, but when we are fighting this high voltage I think if we can get any insulation strength below the main insulator on which we depend, we are so much better off in the desperate fight against creeping due to atmospheric moisture and to dirt accumulating on the insulator. If we could have a porcelain pole, in other words, we wouldn't have very much trouble in protecting insulators. The more insulating material we get in series, the lower potential gradient we have, and the less trouble we are likely to have. So that if it prove to be possible, as I hope it may, to use some absolutely nonmetallic material for the pins, we shall be vastly better off than if carrying our ground to within an inch or half or three-quarters of an inch of sixty or eighty thousand volts. When we do that, we pin all our faith on the insulators, and insulators, as we see from this paper, sometimes fail; they do so much oftener than we like to have them.
MR. E. KILBURN SCOTT: How would you stop the moulding of the pin?
DR. BELL: I do not believe, with a properly designed insulator, the moulding of the pin, which is due largely to the brush discharge, as far as we have been able to ascertain, is going to take place, and I think in an iron-pin line, particularly with iron towers, you are depending too much on the insulator. Anything that happens to that insulator means just one thing — a complete shut-down, because you have grounded the whole circuit. As very properly noted here, you can have some troubles on a wooden pole line without causing that. And while eventually we may, and probably will, use both steel pins and steel towers very largely, that being a matter which has to be treated symptomatically, still I do not believe that an attempt to get an insulating pin should be abandoned at the present time, and I do not think that with proper treatment of the pins, and with a properly designed insulator — in other words, an insulator which will hold back; as far as possible, the brush discharge —the matter of burning the pin, which in some places has been very serious, is going to take place to anything like the same extent. At least it is to be hoped so.
MR. BUNKER: There is one thing I would again like to bring up in that connection, and that is that when the insulator and when the pin are new, when they are both clean, there is no brush discharge that you can detect, either by sight or sound; the brush discharge only occurs later on, as the insulator becomes coated with either fog or dust. And it has been my experience with all high-pressure discharges of a static nature, where they were produced from transformers, that they immediately set fire to combustible material.
DR. BELL: There is no doubt that treated wood has insulating properties of a fairly good quality. The question is whether they are permanent. With many transmission lines, they have been using pins under conditions which would lead one to expect trouble, and yet the trouble has not occurred. Of course, if the insulators are allowed to get dirty, you will get dynamic discharges anyhow, after a certain point, particularly if subjected to salt fog or anything of that kind. But it seems to me that throwing away the insulation of properly treated wood, is not a thing which should be done without due cause, and I do not think that the burning trouble has been sufficiently general as yet, to make one feel that it should be thrown away without any further attempt to improve the question of insulating the wooden pins. We have had wooden pins described on several lines — for instance, Mr. Gerry's — where they have been in absolutely successful use, as far as we can find out, and it strikes me that these brush discharges are due very largely to an imperfect design of insulator. Of course, where you have dust storms, us in the case of some of the plants west of us, which coat the insulator with mud, or with moisture which is more or less dusty, it is very hard to keep up the insulation in any way. But in the face of the fact that some of the very large high-voltage plants are using wooden pins successfully, it does seem to me that throwing up the game and depending on the insulation strength of insulators alone — which is great, of course, but still is subject to failure—is an unwise proceeding. I think we want to exhaust the possibilities of an insulating backing for our lines before we absolutely throw it aside. I hold no brief for wooden pins at all; am perfectly willing to use the steel ones when I can get them combined with insulators that will meet the requirements. But anything in the way of additional precaution seems to me justifiable.
MR. N. J. NEALL: I should like to ask whether you have had any use of glass shields for pins?
MR. BUNKER: No, I have never had any experience with glass shields. The only shield we did have was a small sleeve at the base of the pin. This cracked off, having simply allowed the dust to collect around the pin and prevented the rain cleaning it off.
DR. BELL: The glass shield was practically a pretty deep petticoat that Mr. Gerry used, but it simply protects from these brush discharges. Under the existing working circumstances of the line there is no trouble from that cause. The pin and insulator, whether steel or wood, must be treated as a single structure. The support of the line depends on the electrical and mechanical strength of both those elements, and that is generally the weakest point in a line, from both standpoints. But it seems to me that Mr. Gerry's immunity from trouble with pins is to be ascribed to his very successful and careful insulator design more than anything else.
MR. BUNKER: I think it due to climatic conditions more than anything. Secretary BELL: Possibly.
MR. NEALL: I think the insulator, mechanically, which Mr. Bunker has in line, would appear to you as being the same as Mr. Gerry's; because the latter has simply a sleeve on which the insulator rests, while the former has a long sleeve attached to the insulator and the space below this, where the pin could be exposed, has been covered with a small porcelain sleeve.
MR. BUNKER: They simply allowed the dust to get in, and there was no way to clean it out.
DR. BELL: The absolute difference in experience between Mr. Gerry and yourself must have some basis. Both insulating systems were un-questionably built with skill and care. The difference may be purely climatic. The fact remains, however, that Mr. Gerry, on a very high-tension line, has been using wooden pins with complete success, so far as we can find out from him.
MR. BUNKER: The result to be obtained is the smallest number of shutdowns possible. Now, in a fog section we have had as many as twenty-six shutdowns in a month from broken insulators and wooden pins. We have changed as many as 600 wooden pins in a month. When we got on the steel pin the number of line troubles was greatly reduced. The voltage is 40,000, but even at 30,000 volts, if you can get better service with a steel pin, that is what you want to use.
MR. N. A. ECKART: I would like to ask Dr. Bell if, with pins treated by the vacuum process, he would expect the trouble to arise from moisture still in the pin or due from outside conditions, from atmospheric conditions.
DR. BELL: I should expect the trouble would largely come at first from the fact the insulating material had not worked thoroughly into the pin; in other words, had left it only partially filled. Second, from the fact that the presence of moisture and air remaining would gradually tend to damage the insulating material which had worked in. In other words, I shouldn't think it anything remarkable if some of the moisture, under stress of heat and cold and diffusion in time actually got through; so as to damage the insulating properties which had been obtained initially. I have never had any chance to compare, on a large scale the vacuum-treated pin with one that is merely boiled, but I know, from considerable experience in forcing insulation into material in general, including wood, that the vacuum process is the only way of getting all the moist air out of the pin.
MR. BUNKER: There is another thing I would like to mention in regard to that point, and that is the burning and moulding takes place inside of the insulator above the lowest contact of the insulator with the wood, so that you get very little oxidation action from the air. In fact the greatest moulding is at the top of the pin.
DR. BELL: Mainly on the thread where the fibers are cut crosswise?
MR. BUNKER: Yes, but it is away from the air.
Secretary BELL: Well, partially away from the air.
MR. BUNKER: But at that point, the threads, of course, are the most saturated with oil.
Secretary BELL: Maybe.
MR. BUNKER: There is no question of that. We sawed several pins through to see.
Chairman SCOTT: A gentleman who in recent construction has concentrated himself along the wooden idea, both in poles, pins, cross-arms. braces and everything else, so that everything is wood and no metal at all. is Mr. Nunn. If he can add something to our discussion now we will let him have the opportunity for a final word on this question.
MR. P. N. NUNN: The experiences of the Telluride Power Company seem to show that wooden pins are all right when rightly treated. The 40,000-volt Utah transmission was put into service in 1897, when 16,000 was the highest voltage elsewhere used. This was an advance at one step from 16.000 to 40,000 volts,— nearly thrice. That transmission has now been in operation for seven years, has been entirely successful, and is in operation to-day. The same pins and insulators used at the start are still in use — paraffined locust pins and Provo type glass insulators. These have since been used everywhere, and in no known case have pins been burned or replaced, except on account of broken insulators or the severest salt storms. The insulator has been criticised in all quarters, and its undeniable success has been attributed to the paraffined pin. Now that pin is said to be bad. The Provo insulator is certainly inferior to those now generally used for 40,000 volts. It was designed in the day of 16,000 volts maximum. These later and better insulators represent the advance of seven years in insulator development. The Provo insulator was known to be inadequate to use with metal pins; hence they were used with wooden pins impregnated with paraffine by the following method, previously devised and since used:
Clear, straight-grained locust pins are stirred for six to twelve hours in vats of hot paraffine at 150deg C. and then kept submerged during slow cooling. If the pins are green, the boiling must begin at a low temperature, be slowly raised, and be continued much longer than if dry; but no matter how dry they may be, water vapor will be freely liberated for some hours, this part of the treatment being little more than a method of kiln drying. While slowly cooling, however, the condensation of water vapor remaining in the wood provides a most perfect " vacuum process " which sucks in the still liquid paraffine. If a sliver be removed from the center of a pin treated in this manner, it will be found well filled with paraffine.
On one occasion during a severe storm following a long period of dry weather, partial grounds developed upon a section of line supplied with insulators from a certain shipment which had been improperly annealed. After the storm, over 50 broken insulators were removed, yet no interruption had occurred and few pins had been burned. According to the results of a laboratory test, published a few years ago by a prominent insulator manufacturer, the entire capacity of the Provo plant should not be sufficient to supply leakage current to half its lines in bad weather. Yet the facts are that leakage has never been appreciable. Wooden pins are said to burn with slightest leakage, yet brush discharge has rarely been visible, and then only when insulators and pins have been heavily coated by salt storms, and no difficulty has been met from burned pins. These salt storms are believed to be as severe as any sea-coast spray, and it does not seem probable that serious trouble would be met upon the coast with properly paraffined wooden pins.
MR. BUNKER: Just one thing I would like to mention in regard to the last remark, and that is that where we removed several wooden pins we put the same insulators back onto the steel ones without experiencing any trouble. My argument in regard to the iron over the wooden pins is simply as a general case. I agree with you and Mr. Gerry that in a great many localities wooden pins are all that could be desired, but in other localities something else will have to be done, either in the treatment of the pin or the use of steel.
MR. K. LANDTMANSON : I should like to ask if for all voltages wooden pins are used?
CHAIRMAN SCOTT: I think I am right in saying that both kinds of pins are used; that in general wooden pins in work that would be called transmission work; sometimes where the wires are heavy, iron pins are used. One difficulty with the pin on the higher voltage is that they need to be large and consequently the metal pin is especially desired on account of its strength, and in high-tension work the pole lines are out over the mountains and sometimes have longer spans, so that the difficulties of construction and inspection are greater than with the low-voltage lines which are not so long. I believe I am correct in saying that wooden pins are generally used for the lower voltage work where they can be used. That is the preference.
MR. LANDTMANSON : If you have a line of, say 50,000 volts and if an insulator broke down, have you found danger from touching a pole? I have heard that a man has been killed who touched the wire with a wet ladder, and I think if we have, say, 50,000 volts between two wires, and if an insulator breaks down and the wire then touches the wooden pins, that the leakage can be so great that a man who touches a pole can be killed by it.
NUNN: No one has ever been seriously injured in that way. A few poles have been carbonized along a streak down one side throughout their length. Leakage can be determined by feeling the pole near its bottom.
MR. E. KILBURN SCOTT: Where you have great depth of insulator, I think pins made of malleable iron are good; because they can be made with a good broad base to rest on the cross-arm. They might also have a vitrified surface. I have seen many articles of steel or other metal furnished with quite a thick coat of glaze or enamel and they could be dropped on the ground without breaking the glaze. I should think the glaze might be of value from the standpoint of insulation. Regarding wooden pins, I think I can safely say that there is no such thing in all Europe. We are quite satisfied with steel pins; but then, of course, we do not have your very high pressures. As I may not have an opportunity of referring to it again, I may mention that in some of the British colonies, there is great trouble with the white ant. If, in such places, a wooden pole were to be placed in the ground, all the inside wood would be eaten away; indeed they would think nothing of invading the cross-arms. The poles must, therefore, be of iron, or be composite; i. e., have an iron socket in the ground, and only the upper portion of wood, as at Cauvery Falls. To give an idea of what the white ant is capable, there is a story of an Anglo-Indian official who left his house in India for some considerable time. The white ants penetrated the legs of A table, and after they had cleaned them out and the table top, they crawled up and ate the inside of the family bible. When the official returned, everything seemed all right until he laid something on the bible, when it went right through.
MR. J. S. PECK: One thing struck me as rather interesting as showing the difference of opinion of eminent engineers on the same subject. Mr. Baum told us a couple of days ago that when you exceed 60,000 volts lightning protection need not be considered. Mr. Bunker says the difficulties due to lightning discharges increase much more rapidly than the line pressure. I would like to ask Mr. Bunker whether he has ever tried the arrangement he speaks of in his paper — that is, an inductance and condenser in parallel with the lightning arrester and air gaps?
MR. BUNKER: I should have stated that it has only been tried in laboratory experiments. That is Dr. Kelly's idea of an arrester, and it has never been put in practical operation. As regards lightning protection, when the voltage goes up, I think nearly everybody will agree that inasmuch as the impressed voltage is a function of your troubles, the trouble is going to increase. For instance, at 25,000 volts we would have very little trouble as compared with 40,000.
MR. R. S. HUTTON I think the proper construction of Mr. Baum's remarks is, that as lightning arresters had given considerable trouble at 40,000 volts, if you attempted to go higher it would be harder to make a successful lightning arrester. We know we have lightning arresters that are quite successful at ten, fifteen, twenty thousand volts. Some may have been made that are giving good service on even higher voltage; but it stands to reason that when 40,000 give trouble, and considerable trouble, that if you go to 60,000 you are going to have more. Now, Mr. Baum meant this: That with the particular conditions which we have on the Pacific Coast, severe lightning is very infrequent, and as it does not bother us a great deal, it is not necessary to have any elaborate system of lightning arresters. Therefore, the horn arrester has practically answered the purpose. As we increase the insulation on our whole system, which is necessary to be done, of course, with increasing voltages, I think we shall have less trouble from lightning, but at the same time it would be more difficult to make a lightning arrester to take care of it if you did attempt it.
MR. PECK: I think the point you made last was the thing he had in mind —that the lightning effect is, in a sense, constant and that the factor of safety which you have in a high-tension plant is such that the lightning effect, added to the normal pressure, is not sufficient to break down the system. At least that is the argument I thought he advanced.
MR. HUTTON: Mr. Baum stated the other day when his paper was being discussed, that no poles, to his knowledge, had ever been struck by lightning. Just before Mr. Baum was connected with the company, the Sacramento-Colgate line was struck about ten miles, I think it was, from our sub-station_ The transformers were connected at the time at both ends on the high-tension side, but the low-tension sides were cut out and the line was not being used. Two poles were completely destroyed. The line is run along a county road which is fenced off with barbed wire, and it tore all the posts to pieces in the span between the two poles and pretty nearly consumed the barbed wire, but the line wires on the pole and the insulators were uninjured. The cross-arms were all split to pieces and lay in a tangled mass, about half up the pole. There was not the slightest kind of a burn on the line wire. Nobody knew anything about it until we tried to put current on the line. As the wires were together in contact, they did not get any chance to burn from an arc and when we sent a man out he found this mess.
MR. E. KILBURN SCOTT: Of course, the inside of the metal socket pole I referred to just now is filled with concrete. White ants never crawl outside of anything. Another difficulty which has to be considered in the East is the monkey difficulty. In some cases these animals will climb up the poles, and the only way to prevent them is to wind barbed wire around the poles. The ordinary spiked ring which deters a small boy or a native is of no use with a monkey. Perhaps some day we may be able to print danger notices in the Simian language.
MR. NEALL: Mr. Scott's remarks lead up to one conclusion that I think has been lost sight of, and it is this: If we could depend absolutely on the insulator, and use metal pole construction throughout, we should then know exactly the weak points of the line, and by making due allowance for the insulators and their effect on the line —such for example as their capacity effects at times of line disturbances — we could anticipate the troubles more closely and consequently have better service.
MR. NUNN: Without doubt metal pins will eventually be used with each successive transmission voltage, but they should be used only when that voltage and its insulator have passed their experimental stage. In pioneer work insulators are always likely to be worked to a very close margin, and then they should be supplemented with treated wooden pins.
CHAIRMAN SCOTT: In the remarks Mr. Nunn has just made he has struck the key-note of transmission work as it has been in the past, and while we are apt sometimes to consider that things are pretty well established, the same word I think will apply for many years to come —pioneer work. As we branch into new fields of high-voltage work, we encounter new experiences; new things, as well as matters which were of no concern before, come up to the first rank in importance. Take our whole discussion this morning and what has it been? It has been on the insulator pin, a thing which a man not familiar with the subject would think one of least consequence, but we have found that it is one of the vital points; that the different methods of construction and treatment, and the experience which in one place and by one man differs in many ways from those of others. Now, one of the pioneers in this work, a man who has already said in his remarks a few minutes ago what I intended to say at this time, a man who went ahead years ago and used a voltage three times that which was in common use, which sounded higher in those days than a hundred thousand volts sounds now, a man who went ahead with a plant of that kind and has made it work, and has been one of the leaders in power transmission work in the West, is Mr. Nunn. So far as I know, Mr. Nunn has never been before a technical society before with a paper on this or any other subject. I think that we are especially to be congratulated on having Mr. Nunn at this time present a history of this pioneer work. This Congress ought to deal somewhat with the past as well as the present and future.
MR. BUNKER (communicated after adjournment): There seems to be a prevailing idea among many engineers that a rainy or a wet season is something to be feared in the operation of a high-tension line. As a matter of fact, experience has shown that fewer line troubles occur in a wet than in a prolonged dry season, due to the cleaning effect of the rains. Forty to fifty thousand volts have been thrown during heavy rains onto long stretches of dead line with no more disturbance than under normal conditions. The first rain, however, after a duration of dry or dusty weather which has permitted the insulators to be covered with dirt, causes increased leakage due to the mud formed. With proper insulators, a wet season is to be preferred to a drought, so that wet pins have not actually proven to be a disadvantage.
A further cause of the burning of wooden pins other than leakage. is due to the fact that when the insulators are coated with a more or less conducting material, they become condensers of greater or less capacity which reduces the value of the pin as an insulator. The small contact area of the insulator with the pins, increases the density of current flow to an extent which produces heat enough to char the wood. Where this contact area is increased by using a metal pin, or a metal short around the wood pin no burning takes place. The use of insulating materials of various values in series has the same effect here as in other places, where it is more commonly known and breaks, or tends to break down, the insulation having the least dielectric strength. These small insulator condensers simply add to the capacity of the system, and if the small condenser currents can be prevented from causing burning action as by the use of metal connections to the supports, the insulation of the line is thrown back where it belongs, namely to the insulator.
MR. P. N. NUNN then read a paper on "Pioneer Work of the Telluride Power Company."
NOTE.- It is frequently stated by some engineers that a three-phase circuit should be strung with the base of the equilateral triangle on top in order to prevent more than one-phase being shorted by wires being thrown over the circuit and in order that synchronous motors will continue to operate until they can be thrown on to another circuit.
If a sketch is made of either a star or delta circuit, and a wire shown across two of the circuit wires, it will be seen that two of the phases instead of one will be shorted, and that what remains is a modified single-phase, with varying constants depending upon the resistance and the swing of the shorting wire.
It has been observed, under the conditions, that a synchronous motor will, if carrying load, immediately fall out of step. For mechanical ma-sons, it would be better to place the apex at the top in order to reduce the pull from the pole top, and for electrical reasons, it would be as well, since the men in charge of the line cannot be present to select the kind and length of wire that is to be thrown over the circuit.
