Transmission Line Construction in CrossingMountain RangesBY M. T. CRAWFORD

Fellow, A. I. E. E.Superintendent Distribution, Puget Sound Power and Light Company

Review of the Subject.-In crossing mountain ranges with struction are discussed, which were designed to more efficientlya recent transmission line in the Pacific Northwest severe climatic stress the materials under intermittent strains, providing an in-conditions were encountered. Snow lies from ten to thirty feet creased flexibility by the substitution of suspension for strain formsdeep, wind attains high velocities and the temperature ranges of wire support.between wide limits. A strongly built steel tower line successfully Where line construction fails from excessive dead loading amet these conditions. At one point however, a somewhat unusual stronger design throughout is no doubt the proper remedy. Ifformation of frozen fog was found to successively build up and drop however this loading is only excessive in one span at a time it mayoff of wires and towers producing stresses greater than it would have be relieved by longitudinal flexibility. While not submitting thebeen reasonable to design a line to withstand, and as a result failures principle of the elimination of strain forms as universally applicableoccurred. in transmission line construction, it is hoped that this paper will

Conditions in the extreme loading area are outlined and the elicit a discussion from authorities on the subject that will defineassumption made that the most severe stresses were largely of an its range of application.intermittent nature. The subsequent modifications in the con- * * * * *

A MONG recent extensions of the Puget Sound with conductors pulled to a tension which wouldA Power and Light Company is a 120-mile 110-kv stress the wire to its elastic limit of 4250 lb. at 0

transmission line from the White River gener- deg. fahr., 8 lb. wind pressure and one-half inch ofating station to Wenatchee. This line was built up ice coating.the Green River valley, over the Cascade Rangenear Stampede Pass at about 4000 feet elevation,down through the Yakima River valley and over theWenatchee Range at an elevation of nearly 6000 feet,and down into the Columbia River valley to theCity of Wenatchee. The engineering features of

FIG. 1-LIGHT FROST FORMATIoN EARLY IN THE WINTER ONTOWER No. S25

this line were worked out with admirable care andskill, and the construction embodies exceptionalmechanical sturdiness throughout.On account of the heavy snows and inaccessibility

of sections of the line over the two mountain ranges,additional mechanical strength was provided wherethe elevation exceeded 3000 feet. Type S. A. 55-footsteel towers were used for line supports, 4/0 B. & S.hard-drawn seven-strand copper cables for conductors,with 0. B. No. 25620 cap and pin-type insulators,six-unit strings in suspension and seven-unit stringsin strain. Spans were normally from 400 to 700 feet, _______1_L_______ l______

Presentled at the Pacific Coast Convention of the A. I. E. E., FIG. 2-HEAVY FROZEN FOG FORMATIION LATER IN WINTER,Del Monte, Cal.. October 2-5, 192 3. TOW ER No. S23

970

October 1923 CRAWFORD: TRANSMISSION LINE CONSTRUCTION 971

Although the Wenatchee Range crossing was at a at from 5 to 10 lb. per cubic foot. Observations werehigher elevation, the climatic and topographic con- taken from time to time during the first winter andditions over the Cascade Range were much more severe advice sought from officials of the Weather Bureau.from the standpoint of line operation. The summit The indications were that the conditions bringingof this range marks a radical change in climatic condi- about this formation are apt to recur a number oftions, from the moderate temperatures and damp times per year, but usually within a limited area,winters of the Puget Sound area to the wider range in as the contact with cold air soon turns the fog cloudstemperature and dry climate of Eastern Washington. into snow. This formation has been found to occurIn crossing the Cascade Range at Stampede Pass, itwas necessary to locate the line at a bare exposed

Fio. 4-HEAVY FROZEN FOG FORMATION LATER IN WINTER,TOWER No. S26

along only about one mile of line near the summit, fromobservations made so far. The photographs wereobtained after the atmosphere cleared and the formationhad dropped off of the wires.One break occured in the line during the first winter,

which is considered too many for trunk line operationby an organization that has high standards of service.A careful study was therefore made of all the factors

FIG 3HEAY FROZEN FOG FORMATION LATER IN WI ER,TOWER No. S23

FIG. 5-CLOSE-UP OF FORMATION ON TOWER No. S26point, where the summit was clearly defined. It wasfound that at times during the fall and winter months involved in order to plan reinforcement work. Asa heavy frost formation builds up on exposed surfaces shown in the photographs and profile, the south wirenear this summit, due to the wind carrying moisture- was pulled in two in the second span east of the summit,laden clouds of fog up from lower elevations, which but no insulator strings failed on this wire, although thefreezes as it strikes the cold surfaces, building out wire slipped through the clamp at the next dead endrapidly toward the wind. Formations up to a maxi- east of the break. The other two wires were notmum of about two feet in thickness in one direction broken, but three insulator strings pulled in two andwere observed. As soon as the weight of the frosty one clamp slipped at the dead end points adjoining.mass becomes great enough it falls off and the building- It should be especially noted that there were severalup process starts again. The weight was estimated suspension towers in between these failures, but no

972 CRAWFORD: TRANSMISSION LINE CONSTRUCTION Transaetions A. I. E. E.

evidences of insulator damage or wire slipping were would not have equalled this ultimate strength withfound at suspension points. Two of the insulator the largest formation that evidence could be found offailures were of a character typical of the failure of adhering to the wires. This formation builds outcap and pin-type insulators under extreme tension, almost horizontally toward the wind from the con-the porcelain parting well up under the cap with afracture at right angles to the tensile stress. The lthird failure was in the lip of the cap c-asting. Thewire failure was typical of a tensile break, six of thestrands being drawn out to a reduced diameter beforeparting. The seventh strand had a brazed lap jointat this point which was put in at the time of manu-facture and apparently parted somewhat more easilythan the other strands. The effect of this was probably

FIG. 7-LiNFE DAMIAGE, SUMMIT ANGLE, TOWER No. S25,LooKr.lG NORTH

Fio. 6-LINE DAMAGE, TOWER No. S26, LooKING NORTHEASTSH-OWING BRoKEN WIRE

to reduce the strength of the cable at this point afew per cent, which accounts for the wire parting andnot the insulators on this conductor.The towers were not damaged by the frost formation.

Later in the winter heavy snow falls packed down to .~a depth of eight to ten feet, very hard on top. Thespring melting underneath and the settling of thispacked crust damaged the tower members badly in FiG. S-LINE DAMAGE, TOWER No. S292, LOOKING NoRTHWESTplaces near the ground and sheared off bolts.The ultimate strength of this conductor is about ductor, forming in a nar-row elliptical cross-section up to

8500 lb. and of some of the insulator disks probably about 3 by 8 inches before dropping off, weighingslightly less. Approximate computations indicate that possibly one and one-half to two pounds per lineala quiescent dead load on the wires in these spans foot. It is believed that the building up and falling

October 1923 CRAWFORD: TRANSMISSION LINE CONSTRUCTION 973

off of the heavy frost formation on the conductors 3. Additional strength in tower members near theresulted in intermittent strains and jerks of a very base.severe character near dead end points which exceeded It is believed that the substitution of the suspensionthe strength of the wire, and that where the insulator form of support for all dead ends has taken care of thestrings failed or clamps slipped at dead end points, first two requirements. The insulator string underthis tension was relieved thereby, but where the in- suspension conditions carries only the resultant of thesulators held rigidly the conductor was broken. Itshould be noted that the damage along the two wiresthat stayed up through the winter was such as tovirtually convert the construction to a suspensionform through the loaded area.

In the heavily timbered Puget Sound region it hasbeen found expedient to build some less important l _lines with only a narrow swath cut through the timber,the taAi firs towering above the wires on each side.Branches and sometimes entire trees will often beandthblown across the line, producing a suddenly appliedextreme loading. Many years of experience with thissort of trouble has shown that the most flexible types

FIG. 10Angle strain tower No. S25 at Summit, with onew irechanged to sus-

pension form of supportand two yet to be changed. Wire as hung infree running sheaves for over a mile before placing inclatnps to insurebalanced stresses at supports.

respective wire stresses in each direction, and thedead end loading is largely shifted from porcelain tocopper, reducing insulator tensions without increasingthe number of insulators on the line. The suspensionunit installed in places of foirmer strain construction

FIG. 9-LiNE DAMAGE, TOWIER No. 821, LOOKING NORTHWEST

of line construction are the least damaged. Forexample, one 110-ky. line of this class is built on asingle wood pole line with light crossarms and suspen-sion construction with few dead ends. Trees andbranches will often fall on the wires without breakingthem, although they may be carried to the ground.The tension is relieved by pulling slack out of spanssome distance on each side, with possibly a few brokencrossarms. On lines where wires are rigidly held thedamage is much more severe.

Conclusions reached involved a redesign of certainfeatures of the construction through the exceptional FIG. 11loading area, to provide: Straight Strain Tower No. S22, with one wire changed to suspension

1. A reduction in working tension on insulator form and two yet to be changed.strings which will safeguard against failure, as well asimprove the electrical reliability, was made up of two strings of insulators in parallel

2. A flexibility in wire supporting mechanism to with strain yokes at each end, which reduced theallow some longitudinal movement of wires at all tension duty in the porcelain by half. This yokedsupports, which will permit the easing off of temporary double string unit was installed to hang approximatelyover stresses from one span into adjoining spans. in vertical suspension, and the wire so pulled as to

974 CRAWFORD: TRANSMISSION LINE CONSTRUCTION Transactions A. I. E. E.

give normally equal tensions on each side. At such prevented by the installation of additional pieces oftimes as abnormal loads come on one side these sus- heavier angle stock alongside existing members, withpension units will swing out into the position of strain the corner of the angle facing upwards to form a roofunits. The necessary spacing out was provided at effect and provide a cutting edge to break the snow.tower fastening and wire clamp to allow this with If still more severe winters are encountered in theproper clearances. future which this construction will not withstand,

Computations indicate that a slight increase in the a stronger conductor with more insulator stringslength of arc resulting from this swinging out of sus- yoked in parallel and possibly additional towers maypension units makes a very material increase in sag be needed, but the principles of mechanical design areand -corresponding reduction in tension. Under ex- submitted as obtaining the best balance in the stressingtremely unbalanced conditions the conductor might of materiaLs under such extreme loading conditions.

1 /tI ' / ~~~~~~~~~~~PROFILEoF 1.E\

FIG. 12-TRANSMISSION LINE CROSSING CASCADE RAN-GE, NEAR STAMPEDEC PASSShowing details of damage by heavy frost formation.

even sag down until it touched the snow in the center Discussionof the span. This movement is restricted, however, 110.KV. TRANSMISSION LINE FOR OAK GROVEby the amount of sag that can be pulled out of the DEVELOPMENT OF PORTLAND RAILWAY, LIGHTseveral adjoining spans in each direction, and an AND POWER COMPANY'equilibrium is usually reached after a slight movement (WAKEMAN AND LINES),which equalizes tensions. This action is very common TRANSMISSION LINE CONSTRUCTION INon lines of suspension construction with wet snow DEL MONTE, CAL., OCTOBER 3, 1923.alternately sticking and falling off of wire spans, R. J. C. Wood: We have encountered some little trouble inresulting in short circuits where vertical wire arrange- crossing mountain ranges in snow countries, supposedly due toments are used. the loss of the snow or sleet load from a span and the conductor

In replacing strain forms with suspension forms on coming in contact with the one above it. On the double-circuitsteel tower line, in question, the conductors are offset a foot and along steep slopes where it is desired to have wire half from the vertical plane of the one above it. We have had

stresses unequal on each side of towers, it would be considerable discussion as to just what happens to cause thesenecessary to install additional tie down strings on the conductors to come together. One theory being that the sleetup-hill side, offsetting the point of clamping the sus- falls off the one span and still remaining on the adjacent span,pension and tie down strings to wire several feet. the lighterspanispulledupintotheoneaboveitThis provides for necessary clearances, and gives some Another theory is that the sleet falls off a portion only of alongitudinal flexibility. Had there been any sharp long span and that gives it a deformed shape sufficient to permitlongitudinal flexibility. Had thzere been any shzarp the two conductors to come together.

line angles in this section, the flexible constru'ction tetocnutr ocm oehrThe third theory is that when the sleet falls off it sets up wavescould have been provided by installing additional of sufficient amplitude to produce contact.towers so as to make a round turn with short spans, It will be interesting to know if anybody else has madethe suspension strings swinging out nearly horizontal, observations on this subject and can give us information on justThis form of corner construction has been successfully what happens when the sleet falls off.used on suspension insulator lines along county Je P. Jollyman: WVe do not cross any high ranges at high

ralrad whr .hih fwytrso altitudes or any locations subject to extremely severe climaticroads and r1rasweetergtowatunona conditions.wide radius. It involves the use of bracket supports W aehdsm le rul.I a engnrlytogtfor inside insulator strings to obtain clearances, though not actually observed, that the sag in the conductors isThe snow damage to tower leg members will be greatly interfered with by the inequality in the loading of sleet.

October 1923 CRAWFORD: TRANSMISSION LINE CONSTRUCTION 975

It is now believed that the conductors load up uniformly and considerable degree. We expect in the future to complete thiswhen the sleet starts to melt the trouble begins. off-setting in order to eliminate the chances of failure in anyWe had one interesting report where sleet accumulation on locality throughout the length of the line. On account of the

spans was found to slide to the lower part of the span and pile design of our steel tower being such that it greatly weakens theup in very great weight at the bottom of the span. We had one same if we lengthen the middle crossarm or reduces the clearanceor two cases of trouble which indicated that such an event had to ground at the tower should we shorten the arm, we havehappened. The only precaution that has been taken in country followed the scheme in use on the Pacific Gas & Electric Com-known to be subject to sleet is to employ the horizontal con- pany's lines, using an additional string of insulators on thefiguration, or to employ the displaced conductor arrangements of middle arm to pull the conductor towards the tower.the vertical circuit. It has been our experience thus far in We are not certain whether our three-foot off-set is sufficientpoints not subject to very heavy loading, that a 20-foot vertical and would like to ask Mr. Jollyman, if, in his opinion, it is enoughdisplacement seems to be free from trouble, whereas, a 10-foot is for operation at 130-kv.?not. In a configuration using three conductors normally R. H. Halpenny: On one tower line of The Southernspaced ten feet apart vertically, the center conductor is displaced Sierras Power Company, some trouble resulfing from sleet oneither outward or inward two or three feet and thus far it has conductors has been experienced, particularly at a point wherebeen satisfactory. the line crosses a mountain range at an elevation of approxi-

R. J. C. Wood: Referring to the 20-foot vertical spacing, mately 4000 feet. No trouble of this nature has been noticed,what length of span is that? however, which could be traced to the swinging together of

J. P. Jollyman: We would not ordinarily regard the hori- conductors when loaded with sleet, this being due possibly to thezontal displacement as effective in spans of, over 500 or 600 arrangement of conductors, which offsets the middle conductorfeet. Where we have longer spans, and where we have any of each circuit five feet from the vertical plane in which the topidea that sleet conditions will occur, we would feel it necessary and bottom conductors lie, with a resultant normal verticalto change from a vertical to a horizontal configuration, to be separation of twenty feet between top and bottom conductors.entirely safe from the difficulties attending on the unloading The longest span in this section ofline is approximately 1400 feet.of sleet. The most severe kind of trouble in this locality has been towerW. S. Jennens: Sleet does not occur on our lines on the failure. Line conductors have failed due to sleet loading in a few

plateau or high points in the Rocky Mountain district, in which instances but an excessive dead load of this nature has ordinarilyour system is located, but mostly in the low hills adjacent to the resulted in the deformation of the cross arms or some otherBear River and at those points where the cold air from the other tower member.side of the hills strikes the moisture-laden air. The conditionknown as sleet by most of us, is not in the form of solid ice, but This trouble has been overcome to a great extent by additionalchiefly in a compact form of frost. Sometimes, of course, we bracing of tower members showing a tendency to fail under suchhave damp or extremely wet snow which, in fa,lling from the loading, also in some cases by the addition of towers at pointsconductors, gives us the same condition as you gentlemen know liable to be most heavily loaded.as "sleet-jump." Fortunately the condition prevails mostly John B. Fisken: This discussion takes me back to thein short sections of the line and if the condition can be discovered Vancouver Convention in 1913, at which convention the samein time, lines mostly concerned are taken out of service long subject was discussed. The question has been asked, and theenough to permit the maintenance crews to dislodge the wet desire has been expressed for actual testimony, as to what occurs.snow by jarring the conductors. From the observation of I can't of my own knowledge give any such testimony, and,these crews, we have coneluded that there is no definite way in as a general rule, I don't always believe all that the averagewhich the conductors act when the sleet jumps before the snow farmer says, but we have one farmer in our region who I believeactually falls. We will find adjacent spans having entirely told us the truth. He gave us, some years ago, a very fulldifferent appearances, one being loaded with snow, the con- description of what he actually saw. The conditions wereductors will sag down and the other span without snow, the vertical arrangement of conductors on a two-circuit tower line,conductors will be stretched tight and consequently rise. If, the wires in vertical planes having horizontal separations of abouttherefore, no equal loading of the conductors in the same span six inches. He said that while he was passing the line one day,prevails, it will readily be seen that the conductors will come in and this was after we had all been driven nearly crazy with thecontact with one another without the sleet having jumped. shutdowns, he saw the frost drop off one wire and that wire flyOn the other hand, when the sleet does not let go through any up and strike the one above it. This is not sleet loading butunbalance of adjacent spans, conductors may rise in accordance frost that forms and drops off when the day gets warmer.with their previous loaded condition, and come in contact with After that we separated the vertical planes of the wires hori-each other in the third or even in the fourth span, particularly zontally by moving the top conductor in towards the tower,if the latter be below on a hillside where the tendancy of the and lengthening the middle crossarm and moving the conductorconductor is to run down towards the bottom of the slope. from the tower. This gave horizontal separations betweenIn some cases there will be a wave motion of the conductors, vertical planes through the conductors of top to center, 5 feet,aws pointed out by Mr. Wood, due to snow falling in one spa,n and 10 inches, center to bottom 3 feet, 5 inches and top to bottom,its jarring effect communicated to the conductors of other spans. 2 feet, 5 inehes. Since then we have had no trouble at all and IThe result of change of tension in the various contiguous spans am satisfied that all of our troubles were due to the same thing,causes the wave of the conductor to run along the line and if two the unloading of the conductor in one span and the excessiveor more conductors do not aet similarly, they will come in weight of the adjoining spans causing it to hit the wires above it.contact with each other. Where they are out of step, so far as Harold Michener: Referring again to the line of whichthe wave motion is concerned, it is evident that on 130-ky. line Mr. Wood spoke-I was over it on Saturday and Sunday, last,that contact made thus will quickly burn the conductor through. trying to determine what we could do to it. We selected a spanWe find that the conductors burn chiefly in the center of the span. about 890-ft. long that was held by suspension insulators at eachEach year's condition has become worse and worse, and end, calculated the load for the span with half an inch of ice, and

during the past summer we ha.ve off-set the center conductor of concentrated one-fourth this load at the one-eighth and one-each circuit arranged in a vertical configuraJtion to a distance of fourth aWt the three-eights points of the spaJn. This pulled theapproximately three feet, only however in the locations where center wire down about one foot below the bottom wire andprevious observations have shown the sleet to have formed to any showed that if the bottom wire should be unloaded and the center

976 CO-TWRD. TP4N5rT> ON LINE CONSTRUCTION Transactions A. I. E. E.

wire loaded, the center wire would go au" . into the bottom wire along the conductors by means of weights, and then releasing thefor more than three-eights of the length of the span. That was weights and observing what took place. At that time Mr. Geiseronly an indication of what might happen in a territory where we wrote a paper on those experiments, which I think was given athad not assumed any sleet load in designing the line, and we are the Vancouver Convention in 1913.of the opinion that in the territory where the design assumptions H. T. Plumb: The Utah and Idaho power companies haveinclude a sleet load, and, for that reason, a lower stringing had some trouble with frost conditions; but along most of thetension, and where the spans are longer, the to-p wire would come lines it has caused no trouble. The frost referred to is a wonder-down to the bottom wire if the top wire were loaded in one-half ful formation; it happens only in a very still valley where nothe span and the bottom wire unloaded. The vertical distance wind is blowing; it builds up in the night, and disappears as soonbetween top and center and bottom wires at the towers, is six as the sun begins to shine when it drops off. It adds considerablefeet, three inches. weight to the conductors, and their diameter may become fourH. H. Schoolfield: We have observed a very peculiar or even six inches. An explanation of why it might form on

phenomena on one of our transmission lines which might be of copper conductors more than on iron conductors is that the ironinterest to the engineers present. I have never yet been able to is probably smaller in diameter and the amount of growth seemsfind a satisfactory explanation for it. Perhaps some of you to be dependent on the diameter of the thing to which thegentlemen can enlighten me. frost fastens itself.We have a 66,000-volt line, with suspension insulators, and Not long ago one of the smaller power companies in Utah had

conductors of three-strand copper clad steel. In connection difficulty with a very wet sleet bridging over insulators andwith this we have a seven-strand steel ground wire. There are causing them to ground, or flash across. It was a sleet so wetseveral places in the line where we get frost conditions similar to that it could hardly be called ice and yet it would stick andthose described by Mr. Fisken. I call it frozen fog. It builds build up on the lee side of an insulator.up on the copper clad conductors to a considerable extent and we M. T. Crawford: Experience with wires arranged in ahave experienced the same trouble of wires swinging together; vertical plane has been universally unsatisfactory in our terri-the lower wire flying up and hitting the upper wire. The peculiar tory through snow country and it should be noted that, in thefact is that the frost does not accumulate on the steel ground line described in this paper, a horizontal arrangement was used.wire. I don't know why it is and I can't find a satisfactory Longitudinal movements of the wire accompanying the ver-explanation for it, but it is a fact that the frost will build up to tical movements sufficient to cause short circuit with a ten-footconsiderable extent on the copper clad conductors, but does not vertical displacement would necessarily produce severe stressesbuild up at all on the steel ground wire. at dead end points, and it was to avoid damage from these

L. J. Pospisil: In addition to what Mr. Fisken has saidin stresses that the special form of suspension construction wasregard to his experience, I want to say that in the year 1910, employed. Since writing this paper the line has gone practicallyI believe it was, there was an effort made to reproduce the through another winter without a repetition of the failureconditions, of what occurred from actual experience, by con- described although ice and frost formations have been equallystructing two spans of a sample line and reproducing the loadings severe.