electrical clearances for transmission line gesign at the higher voltages
TRANSCRIPT
AGREAT deal of study has beengiven by transmission engineers to
the problem of electrical clearances inline design. Because of the complexity ofthe problem and the uncertainties in-volved, until recently the approach hasbeen based partly on experience, coupledhowever in large measure with empiricism.With the advent of higher trailsmissionvoltages the economic urge for rationaliza-tion in this field made itself felt to a pointthat further research and studies becameinevitable. Manv have been engaged inthe work. This paper relates to part ofthe over-all effort in course during the past5 years or more and is presented as a basisfor further constructive thought and ac-tion in this important field.
Conditions of Service
The three voltage conditions to con-sider in determining the insulation andelectrical-clearance requirements for thedesign of high-voltage transmission linesare: 1. 60-cycle voltages; 2. switchingand similar transient overvoltages; 3.lightning.To secure the most economical and ef-
fective design, these conditions must beevaluated quantitatively with referenceto the principal factors influencing opera-tion of the line, which comprise:
1. The action of the wind in reducingclearances, resulting from the swinging ofthe conductors together and against thetower structure.
2. Reduced clearances to ground and inter-ference of foreign or adjacent objects,requiring proper measures in addition toadequate clearances (right-of-way clearing,etc.).3. Upswing of conductors due to verticalwind and convection effects, galloping and
Paper 54-203, 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, Los Angeles, Calif., June 21-25,1954. Manuscript submitted March 1, 1954; madeavailable for printing April 5, 1954.
P. L. BELLASCHI is a consulting engineer, Portland,Oreg.
In the preparation of this paper the author hasdrawn in good measure from his background andassociation as consultant to the Bonneville PowerAdministration on high-voltage transmission de-velopments, and he gratefully acknowledges theuse of various illustrative material presented.
dancing of conductors resulting from un-loading of sleet and snow, and similarfactors present in certain geographicalregions and under certain topographicalconditions, which affect spacing betweenconductors, especially at midspan.4. Application of suitable shields whenrequired, to grade stresses and suppresscorona on insulator assembly and hardware,so designed as to secure optimum clearancesto tower structure under all significantpositions of sideswing.a. Safety requirements, including con-sideration of national and state electricalcodes.
6. "Hot-line" maintenance.7. Effect of spacing of conductors on theimpedances and capacitances of the line;likewise, effect of spacing on radio influenceand corona.
S. The application of rapid-clearing andreclosing breakers as a factor in transmis-sion design, conducive to the acceptance ofreduced clearances for unusual conditionsof operation such as may result frommaximum sideswing of conductors andinsulator assemblies under high wind andother abnormal conditions.
In this paper the electrical clearance re-quirements from insulator assembly andline conductor to the tower will be con-sidered in detail first, and then the otherfactors listed in the foregoing will beexamined in their broader aspect in rela-tion to the over-all problem.
Permissible Electrical Clearances atTower, as Determined fromSwitching Overvoltages
Inasmuch as switching overvoltages area consideration of prime importance indetermining the electrical clearance re-quirements at the tower structures, this isa factor that should be examined in thefirst place. System studies and experi-ence all point to the criterion that for aninterconnected system of modern design,operating with the neutral solidlygrounded and provided with breakersessentially free of restrike, overvoltagesfrom line to ground on the energizedlines or parts of the energized system re-sulting from switching or similar effectsseldom attain 2.5 line-to-neutral crestvoltage.1-' (Experience on the 230-kvsystem of the Bonneville Power Adminis-tration indicates that for normal system
arrangements and with circuit breakersof good behavior, switching overvoltagesexceeding 2.5 times line-to-ground volt-age seldom occur.) On this basis the ex-pectancy distribution curve for switchingov7ervoltages in Table I has been con-structed for use in determining the corre-sponding permissible electrical clearancesrequired at the tower.A second important factor in determin-
ing the clearances from insulator assemblyto tower structure is the strength of air toswitching overvoltages. Based on theexperimental data obtained in recentyears,56 the strength of air to switchingsurges in relation to the impulse strengthis in the ratio of 9 to 10.The clearances for 2.5 times line-to-
neutral switching overvoltages are derivedin Table II. Column 1 gives the ratedvoltage and, in parentheses, the maximumoperating voltage nominally set at 5 percent more than rated voltage. Theswitching overvoltages in column 2 arebased on maximum operating voltage incolumn 1. Column 2 multiplied by 10/9gives the corresponding full-wave im-pulse voltage equivalent in column 3.
In calculating the required strength, anallowance of 10 per cent (1.10 factor) ismade between withstand voltage and crit-ical (50-per-cent) spark-over of air. Anadditional 10 per cent (1.10 factor) isallowed for nonstandard atmosphericconditions and similar effects. Andfinally a 15-per-cent margin (1.15 factorof safety) is applied. Thus column 3 ismultiplied by 1.10 by 1.10 bv 1.15 to ob-tain column 4. On this basis the elec-trical clearances required to tower for agiven sideswing of the insulator assemblyare given in column 5. The clearancescorrespond to point-to-point electrodedata and positive polarity of the volt-age.7'8The clearances in Table II assume that
switching overvoltages in excess of 2.5times line-to-neutral are a rare possibilityand they refer to operating conditionsthat may obtain, especially in a moderninterconnected system with no-restrikecircuit breakers and which is equipped tolimit 60-cycle dynamic overvoltages. Inthe event that system conditions are notso favorable and are conducive to higher
Table 1. Expectancy Distribution oF Switch-ing Overvoltages For Modern High-Voltage
System
Line-to-Neutral Crest, Per-CentMultiple Distribution
2. 5, maximum . .......... Practically none2.5 to 2.0 ........... 52.0 to 1.5............................ 10Less than 1.5, >1.25 ...... 85
Bellaschi-Clearances for Trans. Line Design at Higher Voltages
Electrical Clearances ror Transmission-Line Design at the Higher Voltages
P. L. BELLASCHIFELLOW AIEE
1192 OCTOBE-R 1954
Table IL. Electrical Clearances Required to Withstand Switching Overvoltages
Switching Full Wave Design Strength PermissibleOvervoltages Impulse Full Wave Clearances
Voltage 2.5 X Voltage (1 1/2x40 ConductorClass, 60-Cycle Crest, Equivalent, Microseconds), to Tower,Kv Kv Kv Kv Inches
230 (242) .. 495. 550 .765.... 48288(302) .620 .690.960... 61345 (362) .. 740 ...... 8253. 15150 .............4...4
switching overvoltages, correspondinglygreater clearances would be necessary.Table III gives the electrical clearancesrequired for switching overvoltages cor-responding to a maximum of 2.75 and 3.0times line-to-ground voltage. Theseclearances were derived in the same man-ner and by the same procedure as thevalues in Table II for 2.5 times line-to-ground voltage.Inasmuch as the spacing permissible
from line conductor to tower depends onthe probability of simultaneous occur-rence of maximum switching overvoltagesand maximum sideswing of the insulatorstring, the selection of effective and eco-nomical spacings rests very largelv upon arealistic evaluation of these two factors.In well-balanced designs, the two factorscombined should at no time or, if at all,only rarely encroach unduly on thestrength of the air from conductor totower. Therefore, an appraisal of per-missible minimum spacings requires con-sideration of probable maximum swing ofinsulator assembly encountered on high-voltage transmission lines. This factorwill be considered next.
Maximum Swing of InsulatorAssembly
To determine on an engineering basisprobable maximum swing angles encoun-tered in practice, a 165-mile single-circuitsteel-tower line designed for 288 kv butscheduled to operate initially at 230 kvwas selected as a reference for specificstudy. The line chosen traverses moun-tainous terrain as well as open countryand follows a valley for many miles, thuscovering a wide variety of topographicaland climatic conditions. The load datafor the line correspond to a wind of8 pounds per square foot (nominal 60miles per hour) and to 1/2-inch ice at 0degrees Fahrenheit, which is the criterionof design in determining maximum swing(limited, not to exceed 45 degrees). Spotchecks were made which show that for an8-pound wind swing angles for the con-ductor unloaded will not exceed swingangles corresponding to ice-loading condi-tions.
Thus, taking this reference case studywhich is typical of others, the following ob-servations are deduced in regard to maxi-mum swing angles corresponding to a 60-mile wind transverse to the line for the 758suspension structures examined:
1. The tower structures in which swingangles come within the limits of 40 to 45degrees are only 4.2 per cent.
2. Structures in which the swing angleis more than 35 degrees comprise about8.6 per cent of the total.
3. Swing angles of more than 35 degreesare scattered over the entire length of theline (165 miles), with some tendency tocluster in certain sections of the line, wherepresumably prevailing topography, terrain,and other factors in the design contributeto the large swing angles.4. The average swing angle for the entireline is in the order of 30 degrees.
Probability of SimultaneousOccurrences of MaximumSwing and MaximumSwitching Overvoltages
The 165-mile line previously mentioned(Fig. 1) as a case stud-, extends fromeastern Washington to western Montana.It was estimated from this and other stud-ies that but a small per cent of linewould be affected annuallx by- wind veloc-ities approaching 60 miles per hour.Furthermore, direction of wind is un-likely to be uniformly transverse to a linefor any great length. From this, it wasdeduced that only a limited number of in-sulator strings could be subjected to swingangles as great as 40 or 45 degrees at anyone time and then only for relatively shortperiods, i.e., a matter of minutes.
It is apropos to mention here that,based on a recent survey,9 strongestmaximum wind velocities (5 minutes) upto 60 miles per hour are seldom reached inthe United States, except on the seacoastand at other unusual locations. (TableIV).A 165-mile line such as that just de-
scribed may experience annually some 100switching overvoltages above about 1.25times line-to-ground crest. On the basisof Table I, overvoltages in the order of2.5 times line-to-ground would be limited
to a few (possibly a very few) assumingthe presence of circuit breakers of goodperformance and satisfactory operatingpractices. It is recognized that severesystem disturbances and abnormalmeterological conditions bear a certaincorrelation to each other. However, inview of the infrequency of occurrence andthe short times involved, the probabilityof simultaneous maximum swing andmaximum switching overvoltages is verysmall. On the strength of these con-siderations, it was estimated that the fre-quency at which a maximum swing (40 to45 degrees) and maximum switching over-voltages (above 2.25 line-to-ground)might occur simultaneously, would behardly more than once or twice per yearfor the entire 165-mile line studied.
Selection of Swing Angle in High-Voltage Transmission Design
From the foregoing, it is apparent thatin high-voltage transmission-line designpermissible spacings can be based realis-tically on a 35-degree swing angle. Fol-lowing this procedure the air clearancesfrom conductor (clamp or grading shield)to tower at 35-degree sideswing are pro-portioned to maximum switching over-voltage (Table III). In the event of a45-degree swing and maximum switchingovervoltage occurring simultaneously,the strength of the air would be en-croached upon, but in view of the infre-quency of occurrence it seems good en-gineering to accept the risk and securethe economic gain resulting from this pro-cedure in design. The probable risk ofspark-over from conductor to tower at45-degree swing angle becomes even moreacceptable and the consequences are fur-ther attenuated through the use of fastclearing and reclosing circuit breakers.
Since this proposal for a 35-degreeswing angle in transmission line design wasadvanced (in 1950), other investigatorssimilarly engaged in high-voltage trans-mission developments, both here3 andabroad,'0"' have come essentially to thesame criterion of design based on their
Table l1l.Switching
Times
Electrical Clearances Required forOvervoltages 2.5, 2.75, and 3.0Line-to-Neutral Voltage (ELN)
Clearances for SwitchingVoltage Overvottages, InchesClass, 2.5 X 2.75 X 3.0 XKV ELN ELN ELN
230 (242).. 48. 53. 58288 (302).. 61. 67. 74345 (362).. 74. 82. 90
Bellaschi-Clearances for Trans. Line Design at Higher VoltagesOCTOBER 1954 1193
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the Continent indicate similar trends inthe use of approximately a 35-degreeswing angle as a criterion in high-voltagetransmission-line design.)
Pyramiding of Margins and SafetyFactors in Design
The electrical clearances in Table IIIare realistic for the conditions of serviceand system operation stated. They are
valid for altitudes not exceeding about1,500 to 2,500 feet. Higher altitudesmay require additional allowance."2-14These understandably are electrical clear-ances with no allowance- for mechanicalvariations or similar design requirements.
In their derivation, unnecessary com-
pounding of margins and safety factorshas been avoided since such procedurewould be the antithesis of good engineer-ing approach and economic design. It iswell to point out that variations and un-certainties inherent in the respective fac-tors offset each other in a measure andtherefore are not accumulative or addi-tive. That is, when all the factors (ratioof impulse to switching, ratio of with-stand to critical spark-over, nonstandardatmospheric conditions, etc.) are com-
bined together in the derivation (seeTable II) their respective inherent varia-tions are not effective at the same timeand do not add simultaneously.Where a greater degree of conservatism
is imposed on the designer by virtue of cir-cumstance and other considerations, thensome compounding of margins and factorsof safety is possibly a proper recourse forjustification. In such instance, for exam-ple, the ratio of withstand strength tocritical spark-over may be increased to1.15 to narrow down further the proba-bility of breakdown.15 Likewise, whenit is recognized that a more completeknowledge of the switching-surge strengthof air is desired, the ratio of impulse toswitching may be increased to 100/85,again with a view toward reducing proba-ble risk thereby, however, further com-pounding the over-all factor of safety indesign.The strength of air is affected by the
temperature, the barometric pressure, andthe absolute humidity. Considering theextent to which the three effects offseteach other,"3.'4 an allowance of 10 percent (1.10 factor) for nonstandard atmos-pheric conditions is realistic for the ap-plication under consideration. It may beadded that rain also affects spark-over,possibly as much as 10 per cent underheavy precipitation.''16 The practical
Bellaschi-Clearances for Trans. Line Design at Higher Voltages
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Table V. Comparison of Proposed Clearances and Present NESC Rules
Proposed Clearances, Table II Present NESC Rules
Swing SwingVoltage Class, Minimum Angle, Minimum Angle,
Kv Clearances Degrees Clearances Degrees
230 (242) ... .. 4 feet ...........35.5 feet 1 inch.4..........54288 (302) ..... 5 feet 1 inch .......... 35 .6 feet 4 inches.......... 45345 (362) ..... 6 feet 2 inches........ 5..3 7 feet 7 inches .......... 45
significance of this condition, like allothers, again must be weighed in terms ofprobabilities, i.e., the probability of simul-taneous occurrence of maximum switch-ing overvoltages, maximum insulatorswing, and heavy rain precipitation. Inaddition to these factors, for very longlines some consideration may need begiven also to the Ferranti effect.
Therefore, where a greater measure ofconservatism is the set aim, clearancesas much as 10 per cent higher than thosein Table III may find justification.Higher clearances beyond this level wouldhardly seem warranted.
Influence of Lightning onClearances to Tower
Overhead ground wires on high-voltagetransmission lines are frequently omittedin regions where the incidence of lightningis low, especially if ice in combinationwith the overhead ground wire becomes acontributing factor to hazard. This is theusual practice in the Northwest. In thiscase, the electrical clearances required totower are determined from switching over-voltage considerations.
Because of the more severe conditionsof lightning and its effect on line perform-ance, overhead ground-wire protectionremains the practice throughout easternUnited States and Canada. Likewise,analysis of this application will show thatfor the higher transmission voltages thereis no valid justification in proportioningclearances to tower above the require-ments for switching overvoltages. On thebasis of a 35-degree swing angle andswritching overvoltages 2.5 times line-to-ground as the criterion of design, the im-pulse strength of the insulator stringeither for a 16-unit or an 18-unit assemblyessentially is co-ordinated to the air clear-ances from conductor to tower up to 20-degree swing angle. These are the num-ber of elements (53/4 spacing by 10-inchdiameter) in an insulator string thatwould be required normally for transmis-sion voltages of 288 kv and 345 kv re-spectively. For a line design where clear-ances are proportioned up to switchingovervoltages 2.75 times or more line-to-
ground, co-ordination between the insula-tor string and the clearances to towerwould extend to swing angles well beyond20 degrees. The probability of simul-taneous occurrence of a severe lightning-current discharge (in excess of 50,000 am-peres) in a tower footing in combinationwith high tower-footing resistance and ofswing angles much in excess of 20 degreesis too rare practicallv to require greaterclearances, merelIv to improve further thelightning performance of transmission linein the higher voltage classes. Thereforethe practical fact is that, in view of thehigh insulation levels and the inherentlysuperior performance to lightning of trans-mission-line designs in the higher voltageclasses, the clearances from conductor totower as determined from switching-overvoltage considerations also ade-quately fulfill, or more than fulfill, thelightning requirements.
Other Factors in Design
Grading shields suitably designed tosecure optimum clearances can effectworth-while economies in tower and linedesign. A total reduction of 15 per centin the spacing of conductors reduces theline reactance about 3 per cent and raisesthe capacitance the same amount.Corona losses and radio influence are af-fected but not appreciably so.
MIinimum mid-span spacings betweenconductors depend on span and tower de-sign, weather conditions, and geographi-cal location. Experience with 230-kvlines in similar locations provides a goodcriterion for design, and there is no goodreason whv minimum clearances at mid-span for 288 kv and 345 kv need be muchgreater than for 230 kv.
In setting up design clearances attowers, special attention must be given totransposition towers, and also to anchorand angle towers.The application of fast circuit breakers
is definitely a factor conducive to im-proved operation and to economies in de-sign. The application of rapidly reclos-ing breakers to remove nonpermanentfaults permits the use of reduced clear-ances, within the limits indicated.
Likewise, hot-line maintenance imposesa need for clearances at the higher volt-ages which are reasonably low, so mencan handle their equipment efficientlyand with safety.
State and National Safety Codes
These studies show that the clearancesand swing angle specified in the presentrules of the National Electrical SafetyCode (NESC) are too high for applica-tion to line design at the higher transmis-sion voltages. The rules were developedfor low and medium voltage transmission.Apparently they do not differentiate forthe conditions at the higher voltages inwhich large conductors are used and svs-tems invariably operate with neutralsolidly grounded. A companson is givenin Table V.
Safety is an important factor in all high-voltage design and the question of safetynaturally should always receive foremostattention. However, a realistic andpractical approach to this problem in thelight of present knowledge and require-ments is paramount if cost of transmis-sion is not to be penalized unnecessarily.There is a pressing need therefore to re-examine present NESC rules to ensuretheir continuing to fulfill safety require-ments without hampering or restrictingprogress in the power-transmission field.
Economies Attained Applying theCriteria of Design Proposed
An example will be considered to illus-trate relative economies attained througha rationalized design, i.e., for proportion-ing clearances to actual requirements.According to the present NESC rules,
the minimum clearance to tower requiredfor 288 kv is 6 feet 4 inches, correspondingto a 45-degree swing, compared to 5 feet1 inch and a 33-degree swing. For an in-sulator string of 16 units, normally usedfor 288 kv, a reduction in the width oftower crossarm of about 8 feet for single-tower design can be effected. The netsaving in weight of tower varies depend-ing on the design, but as an estimate mayrange from 5 to 10 per cent or possiblymore.
In addition, there are other benefits,the reduced impedance of the line, forinstance. Thus, the economies attaina-ble through rationalization constitute animportant factor in design not only inthemselves but as an essential basis forfurther development and progress in thefield of power transmission at the highervoltages.
Bellaschi-Clearances for Trans. Line Design at Higher Voltages1196 OCTOBER 1954
References
1. POWER SYSTEM OVERVOLTAGES PRODUCED BYFAULTS AND SWITCHING OPERATIONS, AIEE Com-mittee Report. AIEE Tr-antsactions, vol. 67, pt.II, 1948, pp. 912-21.
2. EXPERIENCES WITH 230-KV ON THE BONNE-VILLE POWER ADMINISTRATION, A. A. Osipovich,H. L. Rorden. Midwest Power Conference,Chicago, Ill., 1952.
3. THE 300/315 KV EXTRA-HIGH-VOLTAGETRANSMISSION SYSTEM OF THE AMERICAN GAS ANDELECTRIC COMPANY, Philip Sporn, E. L. Peterson,I. W. Gross, H. P. St. Clair. AIEE Transactions,vol. 70, pt. I, 1951, pp. 64-72.
4. EXPERIENCE GAINED WITH THE SWEDISH400-Kv POWER TIRANSMISSION AND THE NOVELFEATURES OF THE SYSTEM, B. G. Rathsman, G.Jancke. AIEE Transactions, vol. 72, pt. III,Dec. 1953, pp. 1089-1100.
5. DIELECTRIC STRENGTH OF STATION AND LINEINSULATION TO SWITCHING SURGES, P. L. Bellaschi,L. B. Rademacher. AIEE Transactions, vol. 65,1946, pp. 1047-54.
6. FLASHOVER VOLTAGE OF INSULATORS ANDSPARK GAPS IN THE INTERMEDIATE RANGE BE-TWEEN IMPULSE-VOLTAGE TESTING AT OPERATINGFREQUENCY, W. Wanger, H. Huber. BrownBoveri Review, Baden, Switzerland, vol. 27, Dec.1940.
Discussion
S. G. Pann (Department of Water andPowver, Los Angeles, Calif.): Mr. Bellaschi'sinteresting paper discusses two basic factorswhich are important to the design of high-voltage transmission lines: 1. swing angles ofinsulator assemblies under wind conditions;and 2. permissible electrical clearances be-tween conductor and tower at various con-ductor positions, system conditions, andmeterological phenomena. Both of thesefactors assume an even greater economic as-
pect in transmission lines of 230 kv andhigher providing that adherence to the pres-ent provisions of the NESC Code is desiredor required. Industry experience and re-
cent investigations, both here and abroad.indicate that the maximum swing angle of 45degrees and the minimum conductor totower clearances provided for in the N'ESCCode are not realistic for transmission linesat the higher voltages. This is especiallytrue, as Mr. Bellaschi points out, if the prob-ability factor of simultaneous occurrence ofswing angle and overvoltage is taken intoconsideration.
In attempting to set up permissible clear-ances between conductor and tower, it is ad-visable to consider not only the insulationstrength of air but that of the insulatorstring as well. Thus, a proper balance andco-ordination between porcelain and air in-sulation strengths will be achieved.
There is a definite and painful lack of en-
gineering information on swing angles of in-sulator assemblies under wind conditions.Wind tunnel and field tests conducted inEurope during the last few years with bothsingle and bundle conductors show thatfurther studies and investigations are
needed on this subject.Winds, especially at the higher velocities,
are seldom constant in time and space bothhorizontally and vertically. Furthermore,they are influenced by climatic and topo-graphic conditions. In transmission lines
7. FLASHOVER CHARACTERISTICS OF ROD GAPSAND INSULATORS, Joint EEI-NEMA CommitteeReport. AIEE Transactions (Electrical Engi-neering), vol. 56, June 1937, pp. 712-14.
8. MEASUREMENT OF TEST VOLTAGE IN DI-ELECTRIC TESTS. AIEE Standard No. 4, Jan.1953 revision.
9. WIND EXTREMES AS DESIGN FACTORS, ArnoldCourt. Journal, Franklin Institute, Philadelphia,Pa., July 1953, pp. 39-56.
10. ACTION DES VENTS DE GRANDE VITESSE SURLES CONDUCTEURS DES LIGNES AERIENNES, R.Poyart. Bulletin, Societe Francaise des Elec-triciens, Paris, France, Nov. 1953.
11. DESIGN FEATURES OF THE BRITISH 275 KVTRANSMISSION SYSTEM, F. J. Lane, A. J. Gibbons.Report No. 229. CIGRE, Paris, France, 1952.
12. EFFECT OF ALTITUDE ON IMPULSE AND 60-CYCLE STRENGTH OP ELECTRICAL APPARATUS,P. L. Bellaschi, Paul Evans. AIEE Transactions(Electrical Engineering), vol. 63, May 1944, pp.236-41.
13. REFERENCE VALUES FOR TEMPERATURE,PRESSURE AND HUMIDITY, P. L. Bellaschi, P. H.McAuley. AIEE Transactions, vol. 59, Dec.1940, pp. 669-75.
14. RECOMMENDATIONS FOR HIGH-VOLTAGE TEST-ING, Joint EEI-NEMA Committee Report. AIEETransactions (Electrical Engineering), vol. 59, Oct.1940, pp. 598-602.
the swing angle of an insulator assembly isproduced by an effective wind in which thecombined effects of wind velocity and direc-tion on the span length are integrated into atransverse force at the suspension point ofthe conductor.Assuming for simplicity that a wind has
constant direction, uniform front, andvariable speed, the question arises: Shouldthe extreme (fastest mile) or the strongestmaximum (5-minute average) wind velocitybe used for calculating swing angles? Thepaper indicates that Mr. Bellaschi favors thelatter value. However, calculations withlarge-size conductors indicate that maximumswing angle can be attained in a matter ofseconds.The Department of Water and Power re-
cently undertook to develop a simple and in-expensive type of instrumentation tomeasure maximum swing angles of insulatorassemblies. A few of these instruments arealready in operation at various locations onthe Department's transmission system. Inaddition, we are developing a more elaborateinstrumentation technique which will enableus to record continuously the swing angle ofan insulator assembly and the wind velocitycomponent normal to the line. For thispurpose, we have obtained a set of windinstruments which incorporate special fea-tures for continuously resolving wind speedsinto X and Y components with respect tothe line. This instrumentation will be usedon full-scale field installations employingboth single and bundle conductors. Thepurpose of these tests will be to obtain infor-mation on swing angles under naturalwinds and actual line conditions.
E. V. Sayles and C. E. Waits (ConsumersPower Company, Jackson, Mich.): In thisfine paper, the author has presented a logicalmethod of determining electrical designclearances for higher voltage transmissionlines. The method is particularly applica-ble to line designs above 230 kv where im-
15. THE SELECTION OF POWER STATION INSU-LATORS TO MEET SPECIFIED IMPULSE VOLTAGEPERFORMANCE, P. M. Ross. Report No. 2009,CIGRE, Paris, France, 1946.
16. FLASHOVER CHARACTERISTICS OF INSULATION,P. H. McAuley. Electric Journal, Pittsburgh, Pa.,July 1938.
17. A FORMULA FOR ESTIMATING TOWER WEIGHTSAND ITS APPLICATION TO THE ECONOMICS OFTRANSMISSION LINE DESIGN, William S. Peterson.CIGRE, Paris, France, 1950.
18. CO-ORDINATION OF INSULATION AND SPACINGOF TRANSMISSION LINE CONDUCTORS, W. W.Lewis. AIEE Transactions (Electr-ical Engineer-ing), vol. 65, Oct. 1946, pp. 690-94.19. LIGHTNING PERFORMANCE3 OF 200-KV TRANS-MISSION LINES-IT, AIEE Committee Report.AIEE Transactions (Electrical Eingineering), vol.65, Feb. 1948, pp. 70-76.20. SWITCHING SURGE VOLTAGES IN HIGH-VOLTAGESTATIONS, I. B. Johnson, A. J. Schultz. AIEETransactions, vol. 73, pt. III, Feb. 1954, pp. 69-79.
21. ELECTRICAL TRANSMISSION AND DISTRIBU-TION REFERENCE BOOK (book). WestinghouseElectric Corporation, E. Pittsburgh, Pa., 4th ed.,chap. 15, "Power System Voltages and CurrentsDuring Abnormal Conditions," 1930.22. WIND EXTREMES AS DESIGN FACTORS,Arnold Court. Journal, Franklin Institute, Phila-delphia, Pa., July 1953, p. 49.
pulse insulation is not necessarily thecritical insulation path in the structure de-sign. The author's basic assumptions ap-pear valid and his reasoning sound. How-ever, with the limited experience with volt-ages as high as 345 kv, he has, very properly,applied his method to 230-kv and 288-kv de-signs, where there is some 20 to 30 years ofexperience, to test the sounidness of themethod. It may be interesting to extendthis test to the 138-kv class of line.Applying Mr. Bellaschi's method to the
138-kv class, the values for this class inTable II would be:
Voltage class: 138 kv (145 kv).Switching overvoltages 2.5aX60-cycle crest:300 kv.
Full wave impulse voltage equivalent:330 kv.
Design strength, full wave: 460 kv.Permissible clearances, conductor to tower:
27 inches.
In Table III the 138-kv values would be:
Voltage class: 138 kv (145 kv).Clearances for switching overvoltages:
2.5X60-cycle crest: 27 inches2.75X60-cycle crest: 30 inches3.0X60-cycle crest: 33 inches
Based on the 45 years of experience thatConsumers Power Company in Michigan hashad with 138-kv line design and operation,the values just given are reasonable. In-cluded in the Consumers 138-kv system are300 miles of single wood-pole line insulatedwith seven units of either 3-inch by 10-inchor 53/,-inch by 10-inch suspension insula-tors. The use of seven units is dictated byleakage requirements and six units wouldwithstand switching surges. The wet flash-over of six 5-inch by 10-inch units is 235 kv,and the equivalent gap distance is 27 inchesThe seven 5-inch by 10-inch unit string ac-tually used has a wet flashover of 270 kv andan equivalent gap distance of 31 inches.
Bellaschi-Clearances for Trans. Line Design at Higher VoltagesOCTOBER 1954 1197
These values agree quite well with those ofthe author.The Transmission and Distribution Com-
mittee has proposed using the 60-cycle-per-second wet flashover voltage as the funda-mental basis for the insulation of transmis-sion lines to withstand power frequency andswitching surges. Applying this methodbut using the author's basic assumptions of5-per-cent allowance for overvoltage, switch-ing overvoltages 2.5 times crest, 10-per-centnonstandard atmospheric conditions, and a15-per-cent margin for safety, the followingclearance will result:
138 kv, 31 inches230 kv, 51 inches288 kv, 64 inches345 kv, 78 inches
These values are slightly higher than thosegiven by Mr. Bellaschi's method.He has also made a strong case for revising
the requirements of the NESC Fifth Editionpertaining to clearances from conductors tosupports (Rule 235 A 3). He particularlychallenges the requirement that clearancesbe maintained with an insulator swing 45degrees on steel structures. He proposed aswing value of 35 degrees as more realistic.At voltages above 230 kv the angle of in-sulator swing can profoundly affect line cost,and with the large conductors used on highervoltage lines a 45-degree angle is certainlynot realistic and a 35-degree angle may beunnecessarily severe.On a tangent structure with no line deflec-
tion angle, an insulator string is acted uponby three significant moments. The firstmoment due to wind on the conductor tendsto swing the string toward the tower. Thesecond and third moments due to the weightof conductor and the weight of insulatorstring tend to restrain the string from swing-ing toward the tower. If a very commoncondition of equal wind and weight spans isassumed and the weight of the insulatorstrinig neglected, insulator swing angles for1.6-inch expanded steel-reinforced aluminumcable will be 16 degrees for a 4-pound windon bare conductor, 30 degrees for an 8-pound wind on bare conductor, and 15 de-grees for a 4-pound wind on conductor with1/2-inch radial ice. For a large conven-tional steel-reinforced aluminum-cable con-ductor such as 1,590,000 circular mils (diam-eter 1.545 inches), the angles are all some-what smaller and the same is true of largecopper conductors such as 750,000 circularmils, type Hedernheim (diameter 1.622inches).
Considering the magnitude of theseangles, it does not seem possible to assign anarbitrary angle of 45 degrees or even 35degrees without unduly penalizing the de-sign. A specified wind pressure such as 4pounds per square foot on the bare conduc-tor is certainly more realistic. For 25 yearsConsumers in Michigan has used a 4-poundwind on the bare conductor as a design fac-tor in determining clearances to provide thedesired impulse insulation value. The ex-perience has been good and it would seemthat this method would be suitable at thehigher voltages where switching surgesrather than lightning determine clearances.
Richard F. Stevens (Bonneville PowerAdministration, Portland, Oreg.): Mr. Bel-
laschi's fine paper is a much needed contri-bution.
Implied in his conclusions, of course, is theneed of a modernization of our code for elec-trical clearances. Some inertia must beovercome in order to do so, and much patientwork and effort will be necessary to resolveconflicting opinions and to agree on newvalues.
Therefore, we should first ask whether thepossible savings justify this effort. If theNESC formula is applied to the nominalvoltages of 230, 288, and 345 kv, values of58.7, 74.7, and 87 inches are obtained, whichare in remarkably close agreement with theright-hand column of Table III, the clear-ances derived by Mr. Bellaschi for a threetimes normal switching surge. Hence, onthis basis little could be gained by coderevision.
If, on the other hand, it is possible to de-sign basically for a two and one-half timesnormal switching surge, (and with manymodern systems this should be reasonable)then fairly worth-while savings in clearancecan be realized. (It should be emphasizedthat even with design on this basis and iflarger switching surges should occur, it islikely that many and perhaps most of thesewill fail to cause flashovers, as the sum ofthe other adverse factors will, during mostof the time, usually be less than that uponwhich the design is based.)
Calculations made by the BonnevillePower Administration check quite closelywith Mr. Bellaschi's estimate that savingsfrom the lowered clearances which he pro-poses would be of the order of 5 to 10 percent of the cost and weight of the transmis-sion towers. In terms of total line cost ex-clusive of right of way and clearing, thiswould probably be of the order of 3 to 6 percent.
If this percentage is disappointing, it mustbe remembered that a decreased electricalclearance requirement permits shorter arms,but has no important effect on the height ofsingle-circuit structures, or the horizontalstresses. Hence the lower part of the tower,where most of the weight is, is loaded aboutthe same as before.
Nevertheless, even 3 to 6 per cent on, say,a 5,000,000-dollar transmission line, wouldrepresent a $150,000 to $300,000 saving,which cannot be dismissed as of no account.
Next, if it worth while to change the code,what form should the new code take? Inderiving minimum clearances should alti-tude factors suitable for an 8,000-foot highRocky Mountain pass be used, and imposedon a system operating in Indiana? Shouldswitching surges be estimated on the basisof the most badly restriking breaker in serv-ice in the country, and the result imposedon a system using improved modern switch-gear? It seems to me that the answer is adefinite no. Mr. Bellaschi, in effect, saysthe same thing.
I believe we need a code which can bedirectly applied to the large majority ofwell-designed modern systems with modernequipment, and with no unusual adverseconditions as, for instance, extreme altitude.It would be necessary for our proposed newcode to define the systems closely whichwould fall into this preferred risk category.We might say that our basic rules would
apply under 3,000 feet, just as we say a stand-ard bushing is good under 3,000 feet. Wemight perhaps require that breakers be of a
design which limits the number of restrikesto one or less. Systems less favorablysituated would then apply additional safetyfactors, just as an engineer designing a high-altitude substation installation might callfor special bushings.However, if corrections are to be applied
intelligently to the new code values, it willbe necessary to know the assumptions andthe philosophy upon which the new basiccode clearances are founded. A statementon this should, I believe, be included in anynew code, or at least widely publicized andmade readily available. Thus the designerwould have a basis from which to start whennonstandard conditions cOnfront him.Mr. Bellaschi presents such a derivation
and philosophy, which I believe could beexpanded a little. For instance, nonstand-ard atmosphere covers temperature, humid-ity, and barometric pressure, which in turnare related to altitude. What range ofvariation is expected in each of these, andwhy was the particular figure of 10 per centselected? How should an allowance bemade for Ferranti effect? It might beprofitable to explore a little more thoroughlythe probability and the extent of the pyra-miding of adverse factors. Further dis-cussion of this, which represents the key tothe whole problem, would be helpful.
Finally, how serious are the consequencesto a system of a rare flashover, at the timewhen it is most likely to occur, namely, whencharging current on a line is being inter-rupted, and at a point where it can be mademost likely to occur, namely, a gap at theline terminal?
0. D. Evans (Oklahoma Gas and ElectricCompany, Oklahoma City, Okla).: Thispaper cites further proof of the need for re-examining the NESC, particularly in re-gard to clearances for the higher voltagetransmission lines. Already under con-sideration is the seemingly excessive clear-ance to ground required by the Code for thehigher voltage transmission lines. At thetime of the present revision of the Code,probably little attention was given thesehigher voltage lines since they constitutedan insignificant part of the transmission gridof this country. The rapid increase in de-mand for electrical power in the last 12years has resulted in the construction ofseveral of these lines, and the future trend iscertainly in that direction for all companies.Also such things as the upper level of switch-ing overvoltages and the strength of air toswitching and impulse surges have becomebetter understood since the last revision ofthe Code. The paper seems to cover all thefactors that should be considered in the elec-trical clearance requirements between in-sulator assemblies and towers.The economies to be attained in the higher
voltage lines, as pointed out in the paper,certainly justify the use of as small clearancesas conditions permit. On shielded lines,the saving may be more pronounced, since ashorter arm for the conductor will require ashorter arm for the shield wire to maintain aparticular shield angle.The NESC rule 235 A 3(b) now specifies
a 30-degree swing angle for determiningclearances from line conductors to supportsof wood pole lines although a 45-degreeswing angle is specified for steel tower lines.The logic behind this difference in specifica-
Bellaschi-Clearances for Trans. Line Design at Higher Voltages1198 OCTOBER 1954
(A) (B)
Fig. 2. 220-ky tangent tower. Comparison between standard and cross-catenary suspension towers
A Standard rotated tower. Spdn 1,100 feet 0 inch; sag 33 feet 0 inch at 80 degrees Fahrenheit. Total width including 35-degree conductorand insulator swing is 99 feet 0 inch
B-Cross-catenary tower. Span 1,100 feet 0 inch or 1,200 feet 0 inch; sag 33 feet 0 inch or 40 feet 6 inches, both at 80 degrees Fahrenheit.Total width, including 35-degree conductor swing, is 84 feet for 1,100-foot span and 88 feet for 1,200-foot span
tion is not apparent. It seems to me thatif the 30-degree swing angle is satisfactoryfor wood pole lines, then certainly the 35-degree angle should be satisfactory for steeltower lines.A re-examination of the present NESC is
an expensive and time-consuming process,requiring much committee work. Never-theless, it seems justified because of the re-sulting economies as more and more highervoltage transmission lines are constructed.As far as safety is concerned, the higher volt-age lines would be less hazardous with the re-duced clearances shown in the paper thanwith present NESC clearances, since lessrigging would be required for hot-line main-tenance. With shielded lines, armor rods,and fast reclosing breakers, the danger thata transmission line will burn down is remoteeven though a flashover to a tower shouldoccur, and it is almost inconceivable thathot-line maintenance will be undertakenin winds required to produce even a 35-de-gree insulator swing.
A. S. Runciman (The Shawinigan Water &Power Company, Montreal, Quebec,Canada): A discussion on phase wireclearances to ground has been long overdueand Mr. Bellaschi is to be commended for
his frank and clear paper on the subject.All discussions on single-circuit high-volt-
age line supports deal with standard types.It may be an opportune time for study ofnonstandard proposals, such as the designknown as cross catenary, as shown in Fig.2. This radical change in construction,fundamentally, permits less phase spacingbecause of reduction or elimination of in-sulators in suspension. With all of theinsulators in the cross span, no swing ispossible at the point of conductor supportand hence the phase spacing may be re-duced by the amount of the horizontalswing plus the width of the support itself.
There appears to be a very real reductionin right of way required by as much as one-third of that normally necessary. Becausetwo-thirds of most line material is in thetangent supports, and because of the largeand heavy conductors nothing worth whilecan be altered in the dead-end or anchorsupports, the decreased weight of the inter-mediate supports may be a worth-whilesubject for analysis, taking into account newtypes of design, such as the cross catenaryjust mentioned.
F. 0. Wollaston (B. C. Electric CompanyLtd., Vancouver, B. C., Canada): The com-
pany with which I am associated has de-signed two 345-kv lines, one of which is inoperation' at 230 kv. The other is underconstruction. My discussion will compareour design data with the author's proposals.The author derives permissible clearancesfrom conductor to tower in Table II by ap-plying a series of multiplying factors to thenominal system voltage. We consider thatthis process determines the minimum num-ber of insulators required, rather than theelectrical clearance. To obtain the fullvalue of the insulators, the clearance flash-over should exceed the string flashover.Accordingly, we feel that the clearancesgiven in Table II are too small.A comparison of the factors used by us
with those used by the author to obtain therequired insulation strength to withstandswitching surges is given in Table VI. Wediffer in several respects. With regard toitem 1, we think it possible that the maxi-mum operating voltage of our nominal 345-kv system may reach 380 kv, rather than362 kv, as suggested by the author. Forthe system we envision, we think a switchingsurge magnitude of 3.0 rather than 2.5 isproper. We have used a factor of 1.15 in-stead of the newer factor of 1.11 for theratio "critical impulse to switching surge."We have used the more conservative figure
Bellaschi-Clearances for Trans. Line Design at Higher V7oltagesOCTOBER 1954 1199
Table VI. Comparison of Pro
Bellaschi
1. Maximum operatingvoltage (timesnominal) ............. 1.05
2. Crest factor ............ 1.4143. Switching surge
(time 60-cycle crest) .2.2.54. Ratio critical impulse
to switching surge..... 1.115. Ratio impulse with-
stand to impulseflashover ............. 1.10
6. Nonstandard atmos-phere ................ 1.10
7. Contaminated atmos-phere ....................
8. Factor of safety. 1... 1.15
Over-all factor. .. 5.759. Design strength (full
wave positive 11/2X40 microseconds) ...... 1,150 kv
Table VIl. Comparison of Pro
Bellaschi
9. Design strength (asper Table VI), kv.... 1,150
10. Minimum number ofstandard 53/4 by10-inch units toequal item 9 ......... 12.6
11. Item 10 rounded off.... 1312. Impulse critical
flashover for stringlength of item 11. 1, 183
13. Rod gap to equalitem 12, inches....... 76
14. Rod gap to exceeditem 12 by 10 percent, inches .......... 84
15. Wet 60-cycle flash-over for stringlength of item 11,kv...........2....... 525
16. Rod gap to equalitem 15, inches....... 62
17. Rod gap to exceeditem 15 by 10 percent. inches .......... 68
cedures 21-unit strings. Two extra units wereadded to ensure successful operation with
B.C. one or two defective insulators in the string.Electric I find no discussion in the paper of 60-Company cycle strength requirements, although this is
item 1 of the author's list of conditions whichdetermine the insulation and clearance re-quirements of high-voltage lines. In our
1.10 designs, the 60-cvcle conditions were found to... 1 414 require less string length than switching*.o surges. Using the longer string required
by switching surges, the clearance to steel1 1e for wet 60-cycle flashover is found to be less
than is required for switching surges (Table. . . 1 l.) VII, items 13, 14, 16, and 17).
To obtain the full insulation value of the... 1.10 insulator string, the clearance to steel should... 1.10 have about 10 per cent higher flashover
.1.1 strength (Table II, Items 14 and 17). On- our first line we provided the full 120-inch
..8.e~ clearance derived in Table II (item 14) atmaximum insulator swing. This was re-
... 1,700 kv considered for our second line. On thatline the clearance is 8 feet at maximum in-sulator swing. As will be seen from Table
cedures II, item 17, this will withstand the 60-cyclevoltage that would flash the string.
B.C. The maximum angle of swing is calculatedElectric for an actual wind velocity of 40 mph and isCompany 26 degrees. If the maximum switching
surge voltage should occur when the conduc-tors are at maximum swing, the line would
1,700 flash to the tower. We consider it highlyunlikely that this will occur, for reasonssimilar to those given by the author. For a
... 19 4 wind velocity of 31 mph (actual) the clear-19 ance to steel will be 110 inches. From
Table II, item 13, this is approximately thel 663 equal of the impulse flashover of the string.
We expect this clearance to be satisfactory... 19 in operation.
120 REFERENCE
.. 740
1. THE WAHLEACH HYDROELECTRIc DEVELOP-MENT, AIEE Committee Report, AIEE Trans-actions, vol. 72, pt. III, Dec. 1953, pp. 1077-88.
88
... 97
of 1.15 for the ratio "impulse withstand toimpulse flashover." We have allowed afactor of 1.10 for contaminated atmos-phere; the author does not make any allow-ance. Our over-all factor comes out about50 per cent higher than the author's becauseof these differences. However, our designstrength of 1,700 kv is not converted di-rectly to air gap clearance to steel as is donein the paper. Our procedure is illustratedin Table VII and, for comparison, the datafrom the paper are similarly treated.
Item 11 indicates that a 13-unit string isapparently enough to satisfy the author'scriteria for switching surge strength of a 345-kv line. Possibly this is an unwarrantedconclusion, since he says in the section"Influence of Lightning on Clearance toTower" that an 18-unit string would nor-mally be required for a 345-kv line. How-ever, the paper does not give any reasonswhy an 18-unit string should be selected.In our designs, a minimum length of stringequal to 19 standard 53/4-by-10 inch unitswas derived. The actual installations have
P. L. Bellaschi: The discussions, both oraland written, are gratifying. Almost unan-imously they center attention on the needof re-examining the NESC, in particular,the requirements for electrical clearancesin transmission line design. It is reassur-ing that the majority are in accord with thebasic approach and the principal conclu-sions of the paper.
In regard to the pressing need for en-gineering data on swing angles of insulatorassemblies under wind conditions, I concurwith Mr. Pann. The investigation pro-posed by the Los Angeles Department ofWater and Power is indeed a step in theright direction. Although field data arestill limited, the findings from recent in-vestigations abroad (see reference 10 of thepaper) substantiate the use of reduced swingangles as a criterion of design. Whetherextreme velocity (fastest mile) or strongestmaximum (5-minute average) shall be theaccepted basis of design, is also a questionthat can be resolved through further studiesin the laboratory and in the field and is onewhich calls for a realistic evaluation of theprobability of simultaneous occurrence ofmaximum switching overvoltages in relationto these contingencies.
The experience at 138 kv reported by Mr.Sayles and Mr. Waits provides valuabledata on clearances for the lower range oftransmission voltages and also a useful refer-ence for correlation with the higher voltages.With 60-cycle flashover (wet) as the basisfor derivation, somewhat higher clearancesare naturally expected. The discussionclearly gives support to correspondingly re-duced swing angles where light loadingconditions prevail, as borne out by experi-ence, compared to a 30- or 35-degree anglefor heavy-loading design (8-pound wind persquare foot, 1/2-inch ice, 0 degrees Fahren-heit).The justification for revision of the Code
and the basic approach in the proposed re-vision are so well presented by Mr. Stevensthat little need be added. In regard to thefactor for nonstandard atmospheric condi-tions (temperature, barometric pressure,and absolute humidity) the figure of 10 percent is based on experience in testing out-doors and indoors over two decades, and onan analysis of the conditions in the UnitedStates (see, for example, references 12, 13,14, 15, and 16). The author concurs withMr. Stevens that in the proposed revisionof the Code, the basis for the respective fac-tors and their derivation must be clearlystated and spelled out in full.As set forth by Mr. Evans, the economies
that accrue from reduced clearances anid areduced swing angle are even greater for theshielded lines. The need of re-examiningthe Code is again amply apparent from hisdiscussion.The author is not in a position at this time
to assess critically the advantages and limi-tations of the cross-catenary tower designproposed in the interesting discussion by Mr.Runciman. Certainly, should this designprove feasible and economical it would elim-inate a number of vexing problems.The larger clearances adapted by Mr.
Wollaston are no doubt dictated by the par-ticular conditions encountered in the initialstage of system development for the first345-kv line. In the design of the secondline substantial reductions in the clearances(from 120 to 96 inches) were effected.The clearances and the number of insula-
tors recommended in the paper apply to amodern interconnected system, for the con-ditions usually encountered and discussedin the paper. The insulator units recom-mended (16 for 288 kv and 18 for 345 kv)are based on, and take into full considera-tion, 60-cycle voltages, switching overvolt-ages, and lightning. To illustrate, the num-ber of insulators required for 60-cycle opera-tion allow for system dynamic overvoltages,contaminated atmosphere, damaged or de-fective insulators, and hot-line maintenance,a more liberal factor of safety because of theuncertainties of creepage surfaces, in addi-tion to the factors already listed in the paperfor the air-jump clearances. The clear-ances and the number of insulators recom-mended provide a well-balanced, co-ordi-nated design.
Utilities in increasing number are adopt-ing 230-ky and the higher voltages to meetsystem-growth requirements more economi-cally. At this juncture, the measures andproposals conducive to more rational eco-nomical line design are certainly timely.
Bellaschi-Clearances for Trans. Line Design at Higher Voltages1200 OCTOBER 1954