supply. From this standpoint this analysispresents an excellent insight into the prob-lems confronting utility system engineers inplanning the power systems of the future.

It may be interesting to compare theirspecific transmission economic results withthe results of a recent general study.' Fig.12 taken from reference 1 shows the pro-jected 230-kv line loadings plotted in termsof surge impedance loading. The rela-tively heavy 230-kv line loadings which arefeasible on a large integrated power systemprovide low-cost energy transmission. Thenormal and summer peak line loadings arein relatively close agreement with the ap-proximate economic power limits forstraightaway bulk power transmission shownby the curve in the illustration. On theother hand, if 345-kv transmission had beenselected, the per-unit line loadings at 345kv would be less than 50 per cent of thoseobtained at 230 kv, and therefore would beconsiderably below the curve of Fig. 12.Thus, as the authors point out, the econo-mies obtainable at 345 kv with straightawaybulk-power distance transmission would notbe realized in their system because of therelatively light loadings in terms of the cir-cuit capability at the higher voltage.The results of recent conductor econom-

ics studies undertaken by the discussersagree substantially with the conclusion ofthe authors. That is, the larger conductorsize (1,272,000 CM) is more economical at230 kv for heavy line loadings. Specifi-cally, for 75-mile 230-kv transmission, andwith energy evaluated at 4 mills per kilo-watt-hour and demand charges at $22.50per kw per year, the cost differential favor-ing 1,272,000 CM ACSR over 795,060 CMACSR was found to be about twice thatshown in the paper. These results obtainedwith higher demand and energy costs, inconibination with those of the authors,indicate that larger conductor sizes aregenerally warranted under today's economicconditions.

The authors have presented an interest-ing summary of the results of their exten-sive study. It is hoped that they will pre-sent additional details of their engineeringand economic analyses in the future as anaid to other system planning engineers.

REFERENCE1. AN ECONOMIC STUDY Op HIGH-VOLTAG}3 TRANS-MISSION, J. M. Henderson, A. J. Wood. AIEETransactions, vol. 75, pt. III. Aug. 1956, pp. 695-706.

W. A. Morgan, G. R. George, R. C. Guse,and M. F. Hatch: The discussion addssignificantly to the value of the paper incomparing it with the findings of otherstudies based upon different methods andassumptions.As a check upon our detailed economic

comparison of an all-345-kv transmissionsystem with a part-230-kv-part-345-kvsystem, Mr. Wood and Mr. Henderson havetaken the peak loads we presented for eachof the 230-kv lines and have plotted thesedata in terms of surge-impedance loadingson the curve of their reference. This illus-trates very clearly and simply what we hadfound by more detailed analysis, that is:Because the power flows in several directionsfrom the power plants, the loadings per lineare reasonable for 230 kv, and 345 kv is toohigh a voltage except for one line.

It is also interesting that the discussersfound a cost differential about twice that inour paper when they compared 1,272,000CM ACSR and 795,000 CM ACSR withlosses evaluated at about twice that in ourpaper. As the rate for power and energythey used might correspond to areas whereonly fuel-steam-electric plants are available,this would indicate that the justification forlarge conductors is not related specificallyto hydroelectric power.Another factor that might have a bearing

upon conductor size is the assumption re-garding fixed charges. We used 8 per cent

5 TRANSMISSION LINEIf,4.0 LOADINGScvi

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Fig. 12. 230-kv transmission line loadingsfor fixed charges in our paper because of thespecial financing planned for the PacificNorthwest Power Company as a generatingcompany. Therefore, it may be of interestto add that our calculations indicate thatthe point at which it is more economical touse 1,272,000 CM ACSR than 795,000 CMACSR shifts from 150 mw to 225 mw annualpeak load if 14 per cent instead of 8 per centis used for fixed charges.

Design and construction of the 230-kvSpokane-Lewiston line segment of the over-all transmission plan has started since thepresentation of the paper because of thenecessity to serve increasing loads in theLewiston area prior to completion of theproposed power plants. It may be of inter-est that this line will be constructed of all-aluminum 1,272,000-CM strand conductor,rather than the 1,272,000-CM ACSR indi-cated in the paper. The less costly all-aluminum conductor is being used becausethis line is in an area where relatively easyconstruction permits the use of wood-polestructures and short spans, and all-alum-inum conductor has adequate strength forthis application.

IN 1954, the Bureau of Reclamationstarted construction of a double-cir-

cuit 230-kv transmission line 204.8 mileslong from Big Bend on the Missouri Riverin South Dakota to Granite Falls, Minn.The 130.3-mile section of line from BigBend to Watertown, S. Dak., was com-pleted, with one circuit strung, late in1955. One circuit on the 74.5-mileWatertown to Granite Falls section wasscheduled to be completed in the fall of1956. This transmission line is a partof the network of lines being constructedby the Bureau of Reclamation to deliver

power to wholesale customers within theeastern part of the Missouri River basin.This line will transmit power to loads ineastern South Dakota and southern Min-nesota. The power will be generated inplants being constructed by the Corps ofEngineers, Department of the Army, atGarrison, Oahe, Fort Randall. and GavinsPoint dams, on the Missouri River.To lower construction costs, several

modifications in previous designs weremade for this transmission line. In-sulation was reduced from 16 to 14 disksper suspension string to permit smaller

co-ordinating clearances, which reducesthe size of the steel towers. The load-ing assumptions for the standard suspen-sion-type towers were reduced to an un-balanced longitudinal tension of 5,300pounds at each of two conductor posi-tions, or 5,300 pounds at one conductorposition and 3,000 pounds at the over-head ground wire position on the sameface of the tower. Only one overheadground wire was used instead of two.

Paper 56-708, recommended by the AIEE Trans-mission and Distribution Committee and approvedby the AIEE Committee on Technical Operationsfor presentation at the AIEE Summer and PacificGeneral Meeting, San Francisco, Calif., June 25-29,1956. Manuscript submitted March 9, 1956;made available for printing September 12, 1956.

TROMAS M. AUSTIN is with the Bureau of Reclama-tion, Denver, Colo.

The author wishes to acknowledge the assistance ofhis associates and to express his appreciation for de-sign information presented herein, especially toReclamation Bureau engineers S. Judd, D. M. Robin-son, R. S. Dorcas, and H. Brenman for informationon the steel tower and foundation designs.

AFustin-Big Bend-Granite Falls 230-Kv Transmission Line

Big Bend-Granite Falls 230-KvTransmission Line

THOMAS M. AUSTINMEMBER AIEE

FEBRUARY 1957 1305

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Fig. 1. Bureau oF Reclamation 230-kv system in the Missouri River basin Fig. 3. Suspension-type tower outline andclearance diagram

Fig. 2. Double-circuit suspension-type tower

The maximum tension in the conductorsunder heavy loading conditions was m-creased from 12,000 to 13,000 pounds,which results in slightly shorter towerswithout adding appreciably to the towerweight for the dead-end-type towers.

East of Watertown, the line route runsapproximately straight east. The areais farmed intensively in quarter- andhalf-section farms. The farmers desiredthat towers be kept on or near the edgesof fields to minimize interference withmechanized farming. To accomplish thisspan lengths of 1,320 feet, four per mile,were adopted as standard, with a longerspan on one side and a shorter span on theother side of each section line to maintainclearance from the structure to roadsalong the section lines. Cost estimatesindicated lower cost per milefor 1,320-footspans than for the 1,050-foot spans usedon earlier designs; therefore, it wasdecided to use the 1,320-foot ruling spanfor the entire line from Big Bend toGranite Falls, even though the longerspans may increase the danger of outagesfrom galloping conductors. The design

changes have resulted in an estimated sav-ing of about 25 per cent in the cost of thistransmission line, in comparison to thecost of similar lines of a previous designdescribed in an earlier paper.'The location of the line is shown

in Fig. 1. Single-circuit dead-end-typetowers are used adjacent to each substa-tion to permit reducing the tension in theapproach span and to aid in changing theconductors from vertical to horizontalconfiguration. A typical double-circuittower is shown in Fig. 2. Studies of theinterconnected power system on theBureau of Reclamation a-c network ana-lyzer indicates that the peak power flowfrom Big Bend to Watertown may be 155megavolt-amperes per circuit, and 115megavolt-amperes per circuit from Water-town to Granite Falls, with load factorsof 67 per cent. Double-circuit towershave been erected with 954,000-circular-mil steel-reinforced aluminum conductorsinstalled on the first circuit. One 1/2-inch galvanized steel overhead groundwire is used for the entire length of theline, except that two overhead ground

Austin-Big Bend-Granite Falls 230-Kv Transmission Line FEB3RUARY 19571306

Fig. 4. Galloping conductor ellipses. Half-sag at ruling span

wires are used on each single circuitapproach span at the substations.

ductor after suspension insulator stringswings into the ruling span with a brokenconductor in an adjacent span, at minus40 F (degrees Fahrenheit) with no ice andno wind: 5,300 pounds.Overhead ground wire tension at minus40 F with no ice and no wind: 3,000pounds. No static tension reduction per-missible because of short length of over-head ground wire suspension link.

Final tension at 60 F, no wind:Conductor: 6,123 pounds.Overhead ground wire: 2,647 pounds.

Final wire sag, no wind, at ruling span:Conductors: 44.09 feet at 60 F, 48.0 feetat 120 F.Overhead ground wire: 42.95 feet at 60 F,45.61 feet at 120 F.

Tower outline, conductor configuration, andclearance to structure: Fig. 3.Midspan separation, top conductor to over-head ground wire at 60 F for ruling span:

Vertical: 27.22 feet.Resultant: 31.75 feet.

Isokeraunic level: 35.Assumed tower footing resistance: 15 ohms.Probable outages per annum due to light-ning: two per circuit.Galloping conductor ellipses: Fig. 4.Conductor clearance above:

Uncultivated ground: 31 feet.Cultivated ground: 33 feet.Railroads: 41 feet.

Electrical and General Design Data Design Details

A summary of electrical and general de-sign is given in the following.

Voltage: 230 kvLength of line: 204.8 miles total.

Big Bend-Watertown-130.3 miles.WVatertown-Granite Falls: 74.5 miles.

Type of structures: Double-circuit steeltowers. Single-circuit dead-end steel towersadjacent to substations.

Design loading: National Electrical SafetyCode2 (fifth edition) heavy loading plusprovision for a load due to 1 inch of ice and4 pounds per square foot wind pressure ap-plied only for the intact condition (nobroken wires).Ruling span: 1,320 feet.Insulators: Standard 10-inch diameter by53/4-inch spacing, 15,000-pound units.

Suspension: Single strings of 14 units.Dead ends: Double strings of 15 unitsper string.

Conductors:Line: Six 954,000-circular-mil steel-rein-forced aluminum cables (54 aluminum-7steel), with ultimate strength of 34,200pounds. Only one circuit of three con-ductors is being strung initially.Overhead ground wire: One 1/2-inch 7-wire high-strength class-A weldless gal-vanized-steel strand, ultimate strength18,800 pounds.

Maximum tensions at National ElectricalSafety Code heavy loading conditions:

Conductors: 13,000 pounds, 38 per centof ultimate strength.Overhead ground wires: 7,000 pounds,37.2 per cent of ultimate strength.Allowable reduced static tension in con-

A study of weather reports for manyyears indicated that ice loading heavierthan specified for National ElectricalSafety Code (fifth edition) heavy load-ing conditions occurs occasionally; there-fore, the towers were designed to supportthe conductors and overhead ground wireloaded by 1-inch radial thickness of iceand 4 pounds per square foot wind pres-sure at 0 F when all wires are intact.The conductors on the Big Bend-Gran-

ite Falls line are heavy and spans arelong. Under these conditions, accordingto A. E. Davison3'4 and others,5 it is safeto assume that the amplitude of gallopingwill not exceed approximately half the

maximum conductor sag. Half-sag el-lipses are shown in Fig. 4.The 3-phase fair-weather corona loss,

calculated by the method of Carroll andRockwell,6 is 0.5 kw per circuit per mile.The average elevation is 1,600 feet abovesea level and the conductor diameter is1.196 inches; therefore, radio interferenceis not expected to be objectionable.7

Lightning protection is furnished byone 1/2-inch galvanized-steel overheadground wire. Each tower leg is groundedby one copperweld ground rod drivenunder each concrete footing. Lightningoutages, estimated by the AIEE com-mittee method,8 are 2.0 per circuit peryear.The insulating value of wet porcelain

insulators, with recommended safety fac-tors applied, for the peak switching surgevoltage is approximately 0.70 of the criti-cal impulse voltage.9 On this basis, 14insulators are ample for expected switch-ing surges. Structures are located tomaintain the conductor side swing, with7.6 pounds per square foot wind pressureon the bare conductor, such that theminimum clearance between the conduc-tor and the tower is 5 feet. At approxi-mately 30 degree conductor side swing,the maximum expected under lightningconditions, the clearance from the con-ductor to the tower is 6 feet, 6 inches.There are no transpositions in the line,

but in order to help maintain a balancedsystem, the conductors are transposed inthe Watertown substation.To minimize damage from vibration

fatigue, preformed galvanized-steel armorrods are used at all suspension points onthe overhead ground wire, and aluminum-alloy armor rods, either tapered or pre-formed, are installed at all suspensionpoints on the conductors. Stockbridge-type vibration dampers are used at bothends of all spans on the conductors andoverhead ground wire.The estimated saving in energy losses

Table 1. Steel Tower Design Data

MaximumUplift

Maximum Distance ConductorMaximum Sum of Between Low Points, Feet or Over-

Adjacent Spans, Feet Maximum headSingle Overhead Ground

Tower Line Angle, Minimum Maximum Span Ground Wire,Type Degrees Angle Angle Feet Conductor Wire Pounds

DSH........ 0-5 ...... 3,000... 2,800... 1,500... 1,600... 2,000DSAIH........ 5-20 ...... 3,400... 2,800... 1,700... 2,000... 2,500... 1,500*DSAH-1

suspension.......0o 5. ..... 3,600... 3,400... 1,800... 2,000... 3,000... 1,500*DSAH-1

tension........ 0-20 ...... 3,600... 2,800... 1,800... 2,000... 3,000... 1,500DTH........ 0-40 ...... 4,000... 3,000... 2,000... 2,500... 3,000... 1,500TAH. 40-90 ... 3,000 ... 3,000.. 1,500 ... 2,500 ... 3,000.. 1,500

*The type DSAH-1 tower is the same as the DSAH tower except that it has symmetrical crossarms andmay be used as either a suspension- or tension-type tower.

Austin-Big Bend-Granite Falls 230-Kv Transmission Line 13017FEBRUARY 1957

in each conductor suspension clamp byusing nonmagnetic clamps is 49 cents peryear; therefore, aluminum clamps areused.

Structure Design

The following are the design criteria forthe steel towers and foundations.

STEEL TOWERS

Procurement specifications, drawings,and text were prepared on the basis ofinviting bids on the bidder's own design.The design loads were established con-

sistent with the steel tower data shownunder "Steel Tower Design Require-ments." Longitudinal components ofconductor tension for suspension towersunder unbalanced loading assumptionsare based on the statically reduced valueswhen a conductor is broken in an adja-cent span. The basic over-all factor ofsafety for all tower types is 1.5, based onthe yield point of the steel. Whereverrequired to assure meeting any morecritical loading combinations and varyingsafety factors specified by the NationalElectrical Safety Code, loads were ad-justed upward or a separate and in-dependent load schedule was shown. Anindependent load schedule was estab-lished on all towers for the intact condi-tion (no broken wires) with 1 inch of ice,4 pounds per square foot wind pressure, at0 F, for a safety factor of 1.23. Thetowers furnished under the supply con-tract, designed and approved on the fore-going basis, successfully passed full-scaletests with very few and only minorchanges in member sizes or details.

TOWER FOOTINGS

A study of foundation conditions andthe nature of soils found along this lineled to the adoption of four types of foot-ings, all employing stub angles embeddedin reinforced concrete, as follows:

1. Auger type, undercut or belled at thebottom, with concrete placed directly in theexcavation and a concrete cap formedabove ground level.

2. Pad type with undercut, in hand excava-

tion, with pad concrete placed directly inthe excavation and a formed stem placedabove the pad. Designs of this type wereprovided for use in tight soils either as analternative to the auger type where pres-ence of boulders made augering impossible,or where foundation loading exceeded thecapacity of any reasonably proportionedauger type.

3. Pad type without undercut, in machineexcavation, with formed pad and stem.Designs of this type were provided for usewherever the nature of soil encountered pre-cluded the use of types 1 or 2.

4. Pile type with formed cap and stem overdriven timber piles. Designs of this typewere provided for use in areas such as swampsand soils of very low bearing capacity.

All of these types were designed to re-sist the most critical combination ofthrust or uplift accompanied by shearsimposed by the supported tower underits design loading with the safety factorat least equivalent to that of the tower.

Steel Tower Design Requirements

The following criteria were set up forthe Big Bend-Granite Falls line.

Safety factors were 1.5 based on theyield point of the steel at National Elec-trical Safety Code heavy loading condi-tions, unbalanced loading as specified:1.25 based on the yield point of the steel,1-inch radial thickness of ice on conduc-tors and overhead ground wire, and 4pounds per square foot wind pressurewith all conductors and overhead groundwire intact.Unbalanced longitudinal loading as-

sumptions were as follows:

1. DSH standard suspension structure,line angles 0 to 5 degrees-5,300 pounds un-balance at each of any two conductor posi-tions, or 5,300 pounds unbalance at one con-ductor position and 3,000 pounds unbalanceat the overhead ground wire position on thesame face of the tower; the portion of thetower above the bend line is designed towithstand 9,600 pounds unbalance at oneconductor position or 7,000 pounds un-balance at the overhead ground wire posi-tion.2. DSAH and DSAH-1, structures,medium-angle line angles 0 to 20 degrees-13,000 pounds at each of three conductorpositions, or 13,000 pounds at each of any

two conductor positions and 7,000 poundsat the overhead ground wire position on thesame face of the tower.3. DTH and TAH structure, large anglesand dead ends-All conductors and theoverhead ground wire broken on the sameface of the tower at maximum loaded ten-sions.

Body and leg extension heights wereprovided as follows:

1. DSH type-55-foot body with 5- to 25-foot legs; 70-foot body witn 10- to 30-footlegs; 70-foot body with 15-foot body ex-tension and 15- to 35-foot legs.

2. DSAHand DSAH-1-55-foot body with5- to 25-foot legs; 70-foot body with 10-to 30-foot legs.

3. DTH and TAH-50-foot body with 10-to 30-foot legs; 70-foot body with 10- to25-foot legs.

Type TAH is a single-circuit vertical-configuration tower for line angles from40 to 90 degrees.

Further data are given in Table I.

References

1. BUREAU oF RECLAMATION 230-KV TRANSMIS-SION LINES IN NORTH DAKOTA, Thomas M. Austin.AIEE Transactions, vol. 74, pt. III, Dec. 1953, pp.1147-51.

2. NATIONAL ELECTRICAL SAFETY CODE. "Part 2.Rules for the Installation and Maintenance of Elec-tric Supply and Communication Lines," pp. 77-241. "Sec. 9. Rules Covering Methods of Pro-tective Grounding of Circuits, Equipment, andLightning Arresters for Stations, Lines and Utiliza-tion Equipment," pp. 13-29. Handbook H32,National Bureau of Standards, Washington, D. C.,fifth edition, 1941.

3. DANCING CONDUCTORS, A. E. Davison. AIEETransactions, vol. 49, Oct. 1930, pp. 1444-49.

4. DESIGN FOR MINIMIZED TRANSMISSION WIRECONTACTS, A. E. Davison. Bulletin, Hydro-Elec-tric Power Commission of Ontario, Toronto, Ont.,Canada, vol. 29, Jan. 1942, pp. 16-23.

5. GALLOPING CONDUCTORS AND A METHOD OFSTUDYING THEM, E. L. Tornquist, C. Becker. AIEETransactions, vol. 66, 1947, pp. 1154-61.

6. EMPIRICAL METHOD OF CALCULATING CORONALoss FROm HIGH-VOLTAGE TRANSMISSION LINES.Joseph S. Carroll, Mabel MacFerran Rockwell.Ibid. (Electrical Engineering), vol. 56, May 1937,pp. 558-65.

7. INVESTIGArION OF RADIO NOISE AS IT PERTAINSTO THE DESIGN OF HIGH-VOLTAGE TRANSMISSIONLINES, H. L. Rorden, R. S. Gens. AIEE Trans-actions, vol. 71, pt. III, 1952, Jan. 466-81.

8. A METHOD OF ESTIMATING LIGHTNING PER-FORMANCE OF TRANSMISSION LINES, AIEE CommitteeReport. Ibid., vol. 69, pt. II, 1950, pp. 1187-96.

9. ELECTRICAL CLEARANCES FOR TRANSMISSION-LINE3 DESIGN AT THHE HIGHER VOLTAGES, P. L.Bellaschi. Ibid., vol. 73, pt. III, 1954, pp. 1192-1200.

Austin-Big Bend-Granite Falls 230-Kv Transmission Line FEBRUARY 19571308