Tennessee Valley Authority's 500-ky System - Communications

IEEE TRANSACTIONS ON POWER APPARATUS AND SYSTEMS

Tennessee Valley Authority's 500-ky System-

CommunicationsT. M. SWINGLE, SENIOR MEMBER, IEEE, AND H. I. DOBSON, MEMBER, IEEE

Abstract-Basic factors in planning communication facilities forthe Tennessee Valley Authority (TVA) new 500-kV system arediscussed and descriptions given of broadband carrier couplingtechniques, coupling equipment, and tests for power-line carriermeasurements. Single-sideband power-line carrier equipment isevaluated in the light of its role in providing the bulk of communica-tion services between terminals of TVA's first 500-kV line. Expecta-tions that insulated ground wires will perform as communicationscircuits and the line's radio noise evaluation program are other itemsdiscussed.

INTRODUCTION

IOMMUNICATION REQUIREMENTS, althoughlargely overshadowed by 60-c/s requirements in such

respects as size, quantity, and cost of equipment, were in-cluded in the early planning stages of the 500-kV system,of which they constitute a vital part. In general, communi-cation requirements for operating the power system willshow little outward change with the addition of 500-kVtransmission, although power-line carrier and protectiverelaying will be influenced to some extent.

In its communication system, TVA uses microwaveradio; carrier on power lines, insulated ground wires, andtelephone lines; physical voice circuits on open-wire linesand cable; fixed and mobile space radio; and leasedfacilities. With the exception that power-line carrier is pre-ferred for protective relaying and interchange telemetering,no rigid policy governs the selection of media for a givenproject or given function. Rather, the requirements foreach new project are evaluated specifically on the basis ofreliability and cost.Comparative cost studies for the first two 500-kV lines

showed that the required new communication services canbe provided most economically by carrier on the trans-mission lines themselves. The high reliability of power-linecarrier on lower voltage lines was a major factor in decidingto use this medium on the 500-kV system, particularly forprotective relaying channels. Microwave radio would havebeen more costly; where only a few channels are required,the basic common radio equipment makes the cost perchannel comparatively high.The unknown characteristics of the 500-kV line as a

carrier medium prompted early studies of propagation andattenuation. These studies were both aided and en-couraged by an increasing amount of technical literature on

Paper 31 TP 65-14, recommended and approved by the Trans-mission and Distribution Committee of the IEEE Power Group forpresentation at the IEEE Winter Power Meeting, New York, N. Y.,January 31-February 5, 1965. Manuscript submitted October 1,1964; made available for printing November 16, 1964.The authors are with the Tennessee Valley Authority, Chat-

tanooga, Tenn.

47

various aspects of this and related subjects, including in-sulated ground wires and radio interference (RI). In-sulated ground wires are now accepted as an auxiliarycommunication medium on lower voltage lines but are stillsomewhat experimental. The approach of 500-kV trans-mission has focused attention sharply on RI character-istics of power lines. Previous experience in this area waslimited because TVA's 161-kV transmission system has notcreated significant noise problems. A testing program isbeing prepared which will permit a thorough evaluationof RI performance of the first several 500-kV lines and sub-stations.

POWER-LINE CARRIER ON 500-KV LINES

Primary consideration is being given to methods ofcarrier coupling which will avoid service outages of 500-kVtransmission lines. The first requirement is reliability;the second, versatility, i.e., initial communicationi require-ments must be met in a manner which will permit futureexpansion or modifications without the necessity of retun-ing or replacing components in coupling apparatus.

Reliability requires rigid specifications and tests forsuitability. To achieve versatility, broadband couplingtechniques will be used extensively. One or more carrierpaths, capable of efficiently propagating carrier signalswithin a sizable portion of the carrier-frequency spectrum,will be established on each 500-kV line. Each path will berouted into a communication room at each end of thetransmission line, via coaxial cable, so that its full band-width will be available. Frequency separation componentsfor all carrier circuits will be located in the communicationroom.

Frequency SpectrumTVA will continue to limit its power-line carrier spec-

trum to frequencies below 200 kc/s. While frequencies aslow as 30 kc/s are being used, and will continue to be used,on existing lower voltage lines, the requirements of suit-able bandwidth for the broadband channels on 500-kVlines make the lower frequencies undesirable. Frequencyassignments for 500-kV application will be confined,tentatively, to the 70- to 200-kc/s range.

Coupling EquipmentEach line trap in the first 500-kV line is rated at 2000

amperes, has an inductance of 440 ,uH, and provides ablocking impedance of 600 ohms or more within the 71-to 132-kc/s band. The 2-second thermal short-circuitrating is 39 000 amperes rms. The mechanical currentrating is 107 000 amperes peak.

VOL. PAS-85, NO. I JANUARY) 1966

I48I E E TRANSACTIONS ON POWER APPARATUS AND SYSTEMS

The bandwidth of a 440-/.tH broadband line trap rangesfrom approximately 60 kc/s in the lower frequencies (i.e.,70 to 130 kc/s) to slightly more than 110 kc/s at thehigher frequencies. For comparison, the 600-ohm band-widths of the more commonly available 265-,uH traps areapproximately 26 and 87 kc/s in the low- and high-frequency ranges, respectively. For most of TVA's 500-kVapplications, adequate line-tuning unit bandwidth willrequire no impedance compromise. Coupling capacitors,having capacitances of 0.0036 ,F or more, provide band-widths equal to or wider than that of 440-,OH broadbandline traps.

Coupling capacitors for the first 500-kV line are com-bined with high-accuracy potential devices and are equippedwith low-voltage capacitance taps to facilitate record-ings of line transients. Complete specifications are given inTable I.

TABLE ICOUPLING CAPACITOR ELECTRICAL CHARACTERISTICS

Minimum nominal capacitance 0. 0036 ;LFVoltage rating, continuous service 550 kV rms, phase-to-phasePotential withstand:One minute, dry 900 kV rms, to groundTen seconds, wet 825 kV rms, to groundExtended period (1 hour), dry 380 kV rms, to ground

RIVWith 350 kV rms, 60 c/s applied 500 ,.V, at 1000 kc/s

Potential device:Burden 200 VAAccuracy 0.3 percentRatios 300 000/68.2/118Capacitance tap 118 volts, nominal

Extensive tests for engineering information are plannedfor the first line. All three phases are equipped with linetraps, coupling capacitors, and line-tuning units. Thebroadband tuning units have special impedance-matchingtransformers which will facilitate test connections forvarious methods, including phase-to-ground and phase-to-phase carrier coupling, and a close approximation of eachnatural mode of carrier propagation.

Engineering and design of facilities for subsequent linesmust proceed before results from these tests are available.Present designs are based on experience with lower volt-age lines and published data from studies and experience ofothers. Studies by Adams [1], Barthold [2], and othersdealing with natural modes of carrier propagation, particu-larly as applied to single-circuit untransposed power lines,have largely clarified the comparative efficiencies of variousmethods of carrier coupling. Information is availablefrom carrier propagation tests on an actual line, which hasthe same voltage and similar configuration as TVA's hori-zontal arrangement [3 ]. Several significant facts are appar-ent. Phase-to-ground carrier is most efficient whencoupled on the center phase. Phase-to-phase carrier on ad-jacent phases is more efficient than on outer phases.Grounding unused phases increases carrier propagationefficiency.

Present plans are for most of the 500-kV line terminalsto have coupling capacitor-potential devices on all threephases. Some of them will have only one or two line traps.Coupling capacitors on untrapped phases will be equippedwith line-tuning inductors, resonated at the geometric-mean frequency of the carrier channel, and grounded.In addition to providing some measure of increasedefficiency, this practice will reduce cross-coupling of carriersignals between lines, a characteristic that may easilybecome one of the most important in the future networkof carrier on the 500-kV system.

Attenuation and Noise Levels

The first 500-kV line from TVA's Johnsonville steamplant to Arkansas Power and Light Company's WestMemphis substation is approximately 160 miles long.Calculations of attenuation by analytical methods weremade in the early stages of preliminary design. The resultswere quite different from more pessimistic estimates basedon published empirical data of attenuation as a functionof conductor spacing-to-height ratio. These values differedby as much as 2 to 1, as indicated in Fig. 1. Althoughthese higher values were believed to be quite extreme, theselection of equipment and power levels for the power-line carrier circuits was approached with caution.

It is estimated that noise in power-line carrier circuits on500-kV lines will be in the order of 5 to 10 dB higherthan noise on 161-kV lines. Only a limited amount ofliterature is available which has a direct bearing on thissubject. Most technical articles dealing with noise on high-voltage lines have been more concerned with the higherfrequencies generally associated with RI problems thanwith power-line carrier application.Based on these estimates of noise, an audio signal-to-

noise ratio of 25 to 30 dB for a typical single-sidebandtelephone circuit, operating at 100 kc/s, will requireapproximately 1 watt of transmitter power for a channelwith a 35-dB maximum wet-weather attenuation, 10 wattsfor a 45-dB maximum, and so forth.

Power-Line Carrier Terminal EquipmentTVA's history of AM and FM carrier telephone equip-

ment is similar to that of many other users of carrier inthe United States. Amplitude-modulated equipment wasused almost exclusively until about 1949, when it began togive way to the newer FM principle. In 1955, TVAbegan a study of single-sideband equipment. This in-terest stemmed mostly from the need for frequency con-servation. There were also expectations of less powerrequirements and less distortion caused by poor qualitycarrier transmission paths. Except for the extension ofexisting circuits, all new carrier telephone equipmentpurchased since the completion of the study has beensingle sideband. Experience at 161-kV during this periodhas been so favorable that it is proposed to use single-side-band carrier telephone equipment exclusively on 500-kVtransmission lines.The type of carrier terminal equipment for the first 500-

48 JANUAR'Y

SWINGLE AND DOBSON: TVA 500-KV SYSTEM COMMUNICATIONS

VOICE

PERMISSIVE 1TRIP

20 DUAL-CHANNELTRANSFER TRIP

Fig. 1. Estimates of loss on 160-mile 500-ky line including coupling.(a) From empirical data based on spacing to height ratio. (b)Calculated by digital programs.

kV line was largely determined by the protective relayingchannel requirements. The decision to use "permissive trip-ping" and dual-channel transfer trip in each directionmade a combination of voice and superimposed tone chan-nels a reasonable approach.Two carrier relaying schemes, each capable of independ-

ently protecting the line, will be in full-time service. Onescheme is conventional directional-comparison carrier pilotrelaying, using standard 10-watt transmitter-receiverequipment. The other system is a permissive underreachtransfer trip scheme, requiring one audio tone in eachdirection.

For differential protection and for coordination of cir-cuit-breaker operating sequences, a dual-channel transfertrip scheme is to be used in each direction. Completealternate facilities are to be provided for this function, re-

quiring a total of four tone transmitters and four tonereceivers at each end of the line.The channel equipment for relay functions will be con-

ventional audio-frequency tone units of the frequency-shifttype with 340-c/s spacing. This spacing permits sufficientbandwidth for high-speed transmission of trip signals.Actual operating speeds of the relay channels will be in theorder of 12 to 14 ms.

In addition to telephone and relaying, there will be a

telemeter channel, with full-time alternate, from WestMIemphis to Johnsonville. Also a "half-duplex" printertelegraph circuit will be provided between the two ter-minals. The telemeter and printer telegraph channels willuse conventional frequency-shift tone equipment with 120-c/s spacing.Two single-sideband terminals, each with 100-watt

capability, will provide all services except the directional-comparison pilot relaying. Figure 2 shows how the audio-tone services are combined with the single-sideband voice-frequency circuits. The high-power capability of thesingle-sideband terminals will prevent overmodulationwhen the individual tones are given reasonable power.

As an example, only 1 watt of output power is produced

ALTERNATEDUAL-CHANNELTRANSFER TRIP

TELEMETERING

ALTERNATETELEMETERING

Iff<1

OR-E. cLFUJ <1

{Er---

SSBTRANSMITTER-

RECEIVER

S+TFILTERS

PRINTER IhT-TELEGRAPH 3 e{ ,,

Fig. 2. Audio-tone arrangement at Johnsonville 500-kV terminal.

by a 100-watt transmitter with 10-percent modulation. Ifinitial tests indicate that the entire power capabilities are

not required, step attenuators are provided to reduce thenet output.A power-boost feature is incorporated in the circuit for

the benefit of the permissive trip relay function. Tofacilitate transmission of a carrier trip signal througha power-line fault, it is possible to transfer the entire powercapability of the 100-watt transmitter to the trip signalalone. It is also possible, as an option, merely to reducethe levels of other services and increase the trip signalproportionately.The exclusive use of frequency-shift audio tones for

transfer trip and telemetering on this first line is not to beinterpreted as a pattern for TVA's 500-kV system. Otherlines will use frequency-shift carrier equipment for bothof these functions. It is too early to predict the relativeproportions by which these facilities will be used in thelater growth of the transmission system.

INSULATED GROUND-WIRE COMMUNICATIONInsulated overhead ground wires on TVA's 161-kV

transmission lines are being used successfully for carriercommunication. On the basis of this experience and otherconsiderations, it is anticipated that ground wires on 500-kV lines will also be suitable for this purpose. The groundwires on at least the first two lines will be insulated andtested. On the first line, provisions are made so that thesingle-sideband power-line carrier telephone circuit, withits superimposed tone channels, can be switched from its

50

40

a 300

U)cn° 20

10

SSBTRANSMITTER-

RECEIVER

I'll (b)

50 100

FREQUENCY- KO/3159

1966 49


normal route on the power conductors to the insulatedground-wire circuit.

Estimated Attenuation and Noise LevelLoss measurements on insulated ground wires composed

of three No. 6 Alumoweld strands on several 161-kVtransmission lines have shown that dry-weather attenua-tion is only slightly higher than that on phase-to-groundpower-line carrier circuits; i.e., slightly more than 0.1dB/mi at 100 kc/s. Furthermore, a well-balanced ground-wire circuit has less noise than an average phase-to-ground carrier channel, in some instances, as much as 10dB less.

Rigorous attenuation calculations have not been madefor 500-kV ground wires. It is estimated that little netdifference will be observed from that of 161-kV circuits.Two factors tend to cause opposite conditions. A largerconductor size, composed of seven No. 9 vs. three No. 6Alumoweld strands, favors lower losses. A larger spacing-to-height ratio tends to cause higher losses. Average spac-ing and heights at tower are 34 and 87 feet, respectively,for 161 kV and 66.5 and 105 feet, respectively, for 500 kV.The estimated atteinuation at 100 kc/s on the 160-mile

line from Johnsonville to West Memphis is approximately20 dB, plus coupling losses.As with power-line carrier, the noise level on 500-kV

ground wires is expected to be 5 to 10 dB higher than oncorresponding 161-kV circuits.

TranspositionsIn general, a large number of closely spaced transposi-

tions is not necessary to accomplish current balance in thetwo ground wires. However, if intervals are excessivelylong, the buildup of induced longitudinal voltage willcause a high voltage to ground at the transposition loca-tions. In one sample calculation, the ground-wire potentialat a transposition tower 13.5 miles from a groundedterminal was about 1200 volts above ground with a power-line loading of approximately 1000 MW. TVA's first 500-kV line has about 30 transpositions in the ground wireswithin the 160-mile length. The maximum ground-wirepotential at any transposition tower is not expected toexceed 600 volts with 1000-MW loading.

Drainage CurrentExperience on single-circuit 161-kV transmission lines

has shown that, with reasonable care in locating trans-position points, the resultant drainage current through theinsulated ground wires will be quite low-usually less than5 amperes. Digital-computer calculations also indicate thatthe drainage current on 500-kY ground wires will besmall. One calculation indicates that ground-wire currentwith no transpositions is in the order of 28 to 34 amperes ineach wire, largely circulatory, with 1000-MW power-line loading. With transpositions, this may be reduced to

total of approximately 4 amperes to be drained at thetermination.

This current-balancing effect of transpositions in single-circuit lines has created appreciable interest in power-saving possibilities. In one sample calculation [4], a hypo-thetical 185-mile 500-kV transmission line, carrying 1000MW, may save approximately 500 kW in transmissionlosses if the ground wires are insulated and transposed.

RI INVESTIGATIONSRadio interference (RI) is a problem not unique to any

one phase of a high-voltage transmission system [5].While it is of major concern in the design of substations [6and transmission lines [7], it is also considered withinTVA as a communication problem.An elaborate research program, dealing with both prac-

tical and theoretical aspects of the RI problem, wouldadd comparatively little to the mass of statistical dataalready collected from extensive explorations on severaltest lines throughout the country. TVA's objective will be,more simply, to evaluate the performance of its 500-kVlines and substations and to determine, wherever possible,the amount of control that various components may haveover RI.Apparatus specifications issued by TVA contain limits

of radio noise influence voltage (RIV) which follow Ameri-can Standards Association (ASA) recommendations wherepractical standards exist. Many older ASA recommenda-tions, apparently extrapolated from lower voltage experi-ence, seem unduly pessimistic. For example, line traps andcoupling capacitors each have an ASA specified RIV maxi-mum of 5000 ,uV when energized at rated 60-c/s potentialabove ground. It is reasonable to expect lower values.

In addition to corona and RIV tests on insulators,transmission-line hardware, and substation apparatus,performed mostly by the respective manufacturers, TVA'stesting program will include a comprehensive fair-weatherRI survey, RIV measurements at all pertinent frequencieson 500-kV and lower voltage buses, and an all-weatherRI monitoring program in transmission line vicinities.

Fair-Weather RI (Radio Noise Field Strength)A survey of typical noise levels on several existing 161-

kV lines has already been made. The transmission linesinvolved in these measurements represent a wide variety oftypes and conditions: new and old conductors of varioussizes, single-circuit and double-circuit configurations, steeltowers, and wood poles. The maximum surface voltagegradient on the various conductors ranges from about 12to 20 kV(rms)/cm.

All measurements in this preliminary survey were madewith a Stoddart NM 20B noise meter. Quasi-peak meas-

urements, for which the NM 20B has 1x600-ms time con-

stants, were made at all sites. Average values of noise werealso recorded at a large number of measuring locations. Amodification was made to the noise meter which enables

2 amperes or less in the same direction in each wire, a

50 JANUARY

quasi-peak measurements with IOx6O-ms, as well as Ix6OO-


ms time constants. Comparative quasi-peak measurementswere made at all sites with both time constants duringapproximately the last half of the survey.

Quasi-peak noise levels were found to range from 10 to80 ,uV/m at a distance of 100 feet from the outer con-ductor. The highest field strength, 80 MV/m, was measuredat a wood-pole line which had been upgraded from 110 to161 kY. In general, with equal conductors, wood-polelines were noisier than steel-tower lines. The average RI,measured 100 feet from the outer conductor of a single-cir-cuit steel-tower line, was about 14 ,uV/m in fair weather.No accepted standards have been established for per-

missible levels of RI fields in the vicinity of transmissionlines and substations. As a tentative goal, TVA has set 50,V/nm quasi-peak as a maximum field strength, measuredat a frequency of 1000 kc/s at 100 feet lateral distancefrom the outer conductor of the 500-kV line during fairweather.

Calculations have indicated that the surface voltagegradient on TVA's 3-conductor bundle will not exceedapproximately 18 kV(rms)/cm for a line voltage as highas 550 kV.A 3-part survey of RI levels will be associated with each

500-kV line. The first part will precede line construction.In this step, background or ambient noise levels will bemeasured along the transmission line right of way and inthe vicinity of the terminals. Also in this step, the noiselevels associated with any lower voltage lines adjacentto the right of way or any power lines that cross the rightof way will be investigated. The final part of this step con-sists of measuring broadcast radio, FM radio, and televisionsignal strengths at representative points along the right ofway.

Part 2 will consist of a limited number of measurements,similar to those in part 1, made after the line is built butnot energized. The purpose of these measurements will beto study the effect of the dead line on existing radio-fre-quency signal and interference patterns, and to obtain arecheck of high-noise locations observed in part 1.

Part 3 will consist of a comprehensive series of RImeasurements to establish typical longitudinal profiles at100-foot lateral distances from the outer conductor withthe line energized and carrying normal load. Sample read-ings of lateral profiles and other measurements of generalinterest will be included.

RIV on Station BusAfter the substations are energized, a series of RIV

measurements, beginning at the lower carrier frequenciesand extending through the broadcast radio band, will bemade. Data from these tests are expected to be valuablein future carrier communication applications, and willcontribute toward correlating the additive effects of allapparatus within the substation. It is expected that someRIV information will be available on almost every pieceof equipment, either from manufacturers, laboratorymeasurements, or other sources.

All-Weather RI Monitor

Two field stations will be equipped with noise measuringand recording equipment and placed at appropriate loca-tions along the 500-kV line to monitor quasi-peak noise fora period of at least one year. On the first line, the twolocations are centered within sections which are equippedwith different types of insulators. This will determine ifthe two types of insulators show significant differencesunder varying weather conditions. It will provide maxi-mum, minimum, and statistical time distribution of noiselevels.At the end of approximately one year's operation, the

monitoring equipment will be transferred to another 500-kV line for a similar series of measurements. This will con-tinue, possibly augmented by additional instrumentationif the need arises, until the accumulated informationrepresents realistically the RI performance of these lines.

CONCLUSIONSThis paper describes the basic factors considered in

planning the communication facilities for TVA's 500-kVsystem. The study led to four main conclusions whichguided the initial 500-kV projects.

1) Power-line carrier is the economical choice for provid-ing communication services between line terminals. Single-sideband carrier telephone equipment will be used basedon calculations and experience on the 161-kV system.

2) The ground wires can likely be used for carrier com-munication by insulating them from the towers and pro-viding suitable transpositions. Calculations indicate thatthe saving in transmission losses is appreciable due totransposing the ground wires and isolating them from thesteel towers.

3) Maximum dry weather RI levels of 50 AV/m at 100-foot lateral distance from the outer phase conductor of thetransmission line appears to be reasonable and has beenestablished as a goal for both urban and rural areas.

4) Due to the limited knowledge of 500-kV operation,extensive tests will be made on the first line. Other con-clusions must await results of these tests.

REFERENCES[1] G. E. Adams, "Wave propagation along unbalanced high-

voltage transmission lines," Trans. AIEE (Power Apparatusand Systems), vol. 78, pp. 639-646, August 1959.

[2] L. 0. Barthold, "Radio-frequency propagation on polyphaselines," IEEE Trans. on Power Apparatus and Systems, vol. 83,pp. 665-671, July 1964.

[3] B. Bozoki and D. E. Jones, "Power-line carrier coupling in-vestigations on a 500-kV line," IEEE Trans. on Power Apparatusand Systems, vol. PAS-84, pp. 197-200, March 1965.

[4] H. Holley, D. Coleman, and R. B. Shipley, "UntransposedEHV line computations," IEEE Trans. on Power Apparatusand Systems, vol. 83, pp. 291-296, March 1964.

[5] F. Chambers, 0. S. C. Hammer, and L. Edwards, "TennesseeValley Authority's 500-.kV system-System plans and con-siderations," this issue, pp. 22-28.

[6] R. M. Milton, H. H. Leech, and R. C. St. Clair, "TennesseeValley Authority's 500-kV system-Step-down substationdesign," this issue, pp. 36-46.

[7] A. C. Pfitzer and G. Wilhoite, "Tennessee Valley Authority's500-kV system-Transmission line design," this issue, pp.28-35.

1966 51


DiscussionJ. E. O'Neil (Westinghouse Electric Corporation, East Pittsburgh,Pa.):' To better understand the data used by the authors in arrivingat their 500-kV design criteria, especially in the RI section, I havea few questions. Many measurements were made on existing linesfor both average field-intensity and quasi-peak values of RI. Insummarizing these data, the fair-weather average RI is given asabout 14 ,uV/m. Is this the field intensity or the average of manyquasi-peak readings? The authors also use two different time con-stants in recording line quasi-peak RI data. It would be interestingto see a plot of this comparison.

If the 14-pV level is the field intensity, the quasi-peak level is4 to 10 times higher on most 3-phase lines. The quasi-peak rangegiven earlier would indicate the foregoing to be a quasi-peak reading.With a 22.9-dB average quasi-peak level for existing lines, ifthe 500-kV line is 5 to 10 dB noisier, as indicated, then an averagevalue as high as 32.9 dB would result. Maximum fair-weatherRI levels would then approach those for foul-weather. Assumingonly two-thirds of the foul-weather levels indicate that another12 dB should be added in order to get maximum fair-weather levelsof 44.9 dB or 175 uV/m. Using data obtained at other locationsas a basis for this same conductor analysis could bring RI levelsas much as 12 to 15 dB higher. It would be helpful if the authorswould indicate the method of estimating the maximum RI level oftheir conductors.The authors indicated a reasonable expectation of lower RIV

values than those given by ASA Standards. What is the desirabilityof lower levels? With transmission line average fair-weather RIVbetween 10 000 and 40 000 AV, and with much lower shunt imped-ances of substation equipment, the noise levels indicated will con-tribute nothing to either general RI levels, or to improved signal-to-noise ratios for carrier applications. While it may be relatively easy,in some cases, to obtain lower levels, in others, it can add to overallequipment costs. Would the authors comment on the desirabilityof these lower levels.

Jack R. Elwood (Bonneville Power Administration, Portland, Ore.):2Mr. Swingle and Mr. Dobson have done a commendable job indelineating subjects that must be considered in planning a multi-purpose power-line carrier system for a 500-kV transmission line.Although their paper is not elaborately detailed, it presents impor-tant general and specific information of interest to those in the powercommunications and protection field.TVA's plan to enhance the reliability of the proposed carrier

system by achieving optimum mode power line coupling is interest-ing. Analytical proof and some test data have been presented byothers on this improved coupling method, but final judgment mustrest on more experience. How effectively this technique works atthis installation, therefore, will be closely followed by power systemcommunication engineers throughout the world.The authors estimate the noise level for 500-kV ac lines to be

5 to 10 dB higher than that of 161-kV lines. This is a matterof great concern to those planning to use carrier on EHV lines.Some preliminary tests by others indicate optimistic noise levelsbut, during severe weather, we have observed that noise levels on345-kV lines exceed those on 230-kV lines by 10 to 15 dB. Forthis reason, results of tests to further determine RI and carrierattenuation characteristics of both the 500-kV line and insulatedoverhead ground wire will be most valuable to the power industry.

D. E. Hedman and L. 0. Barthold (General Electric Company,Schenectady, N. Y.):3 While the authors describe various aspectsof the communication problem for the 500-kV system, this discussion

'Manuscript received February 10, 1965.2Manuscript received February 19, 1965.3 Manuscript received February 19, 1965.

will be limited to the power-line coupling and propagation problemand ground-wire communication.The authors discuss briefly the methods of coupling to EHV lines.

The concept [1], [2] of modes of propagation is a useful tool inthe analysis of this problem. On a 3-phase transmission line,three modes which propagate independently exist. For such a line,modes 1 and 2 effectively transmit the signal to the receiver whilemode 3, the ground mode, is completely attenuated before it reachesthe receiver on lines more than 20 miles in length. The slight differ-ence in attenuation and velocity of modes 1 and 2 also may producesignificant effects on the received signal [3].As pointed out, line-to-ground coupling on a horizontal con-

figuration on the center phase is most efficient. Specifically, cal-culations for a line over a lossy earth show that, for a 50-kc/s signalcoupled to the center phase, approximately 50 percent of the avail-able power is coupled into mode 1, while no power is coupled intomode 2 if the remaining phases are ungrounded. Coupling to oneof the outside phases with the remaining phases open-circuitedproduces only 10 percent of the power in mode 1, with approximately45 percent of the power in mode 2. For these two cases, the remainingpower is coupled to mode 3, which is attenuated greatly and doesnot contribute to the received signal. From the above couplingefficiency numbers it is evident that, for the unfaulted line, thecenter phase coupling should be more efficient.As to security, the previous conclusions may not be true. For

example, a center-phase line-to-ground fault near the transmitterwould eliminate essentially all mode-1 energy. Now, even thoughthe mode-2 attenuation is great, mode-2 signal, essentially, mustbe relied on at the receiver. This would suggest that, for the linefault considered, outer-phase line-to-ground coupling would be moreefficient. By the same argument, which is based on the modalconcept of propagation, the most critical fault locations, from asecurity viewpoint, are center phase-to-ground near the transmitteror on the receiver side of a transposition on long transmission lines.

This argument, of course, neglects any resonance which may occurfrom faults near the transmitter. Have the authors, in their analysis,evaluated the effect on carrier attenuation caused by faults on theline? Also, have the authors determined the location and type oftranspositions which will be used on their line? They have hadconsiderable experience in using insulated ground wires at 161 kV.One hazard of insulating ground wires is that the 60-c/s voltageappearing across the ground-wire insulation may be sufficient tosustain an arc that could be initiated by a stroke or disturbance,not sufficiently severe to cause the line to trip out itself. The seriousresult of a ground-wire burn down from this continuous arc dictatesthat this problem be completely eliminated.Recent computer programs and detailed transmission line constant

calculations [4] have solved many problems associated with insulat-ing ground wires. When grounded at both terminals, there is nomechanism to produce a zero sequence voltage across the ground-wire insulation except for nonuniformity in the longitudinal induct-ance or resistance of the earth path. Positive and negative sequencecurrents, circulating between ground wires, usually can be mini-mized although seldom eliminated by transpositions. Have theauthors made direct measurements of ground-wire voltage to verifytheir prediction of a 600-volt maximum at 500 kV?Some test results, together with theoretical arguments, show that

coupling onto one insulated ground wire with the other permanentlygrounded could be more efficient on long transmission lines thanpush-pull coupling to two insulated ground wires. Have the authorsexplored this prospect?

REFERENCES[1] D. E. Hedman, "Propagation on overhead transmission lines,

I-Modal theory of analysis," IEEE Trans. on Power Appa-ratus and Systems, vol. PAS-84, pp 200-205, March 1965.

[2] , "Propagation on overhead transmission lines, II-Earthconduction effects and practical results," IEEE Trans. on PowerApparatus and Systems, vol. PAS-84, pp 205-211, March 1965.

[3] B. Bozoki and D. E. Jones, "Power line carrier coupling investi-gations on a 500-kV line," IEEE Trans. on Power Apparatusand Systems, vol. PAS-84, pp. 197-200, March 1965.

52 JANUARY


[4] M. H. Hesse, "Electromagnetic and electrostatic transmission-line parameters by digital computers," IEEE Trans. on PowerApparatus and Systems, vol. 82, pp. 282-291, June 1963.

T. M. Swingle and H. I. Dobson: Mr. O'Neil is correct in assumingthat the average RI of about 14 uV/m is the average of many quasi-peak readings. For comparisons of time constants, we have includedtwo lateral profiles (RI) and a plot of laboratory data (RIV) on atest specimen, Fig. 3. In general, the field data show less differencethan the laboratory data between the 1x600-ms and 10x600-ms timeconstants. Quasi-peak measurements provide values which areindicative of both peak impulse amplitude and impulse repetitionrate. The division of control between these two characteristics is afunction of the relationship between noise-meter time constants andthe nature of the noise; i.e., the period and duration of the noiseimpulses. The repetition rate of the field noise impulses is high-acomposite of many sources energized with 3-phase voltage. Thelaboratory noise comes from a single specimen energized with single-phase voltage.With regard to noise estimates, the 5- to l0-dB difference in

noise on 500-kV lines, as compared with 161-kV lines, was a pre-liminary design figure for use in the power-line carrier frequencyrange of 200 kc/s and below. It was an estimate of performancemade solely from the study of limited available information. Nocalculations were made.The tentative goal,-50 ,uV/m at 1000 kc/s and a lateral distance of

100 feet from the outer conductor, was based on that which wouldprobably provide a safe signal-to-noise ratio in fringe radio-receptionareas. It was, in no sense, an estimate of performance. It bears nointentional relationship to the estimated noise below 200 kc/s.We agree with Mr. O'Neil that it is possible for the maximum fair-weather noise to exceed this figure.We have stated that it is reasonable to expect lower RIV levels

on substation apparatus than ASA limits. Numerous examples ofequipment tests show this is true. The question of whether it is alsoreasonable to require such lower levels is a moot point which hasbeen considered by the Radio Noise Subcommittee [1]. The apparentopinion of the majority of this group is that existing NEMA (Na-tional Electrical Manufacturers Association) RIV limits are con-servative. For RIV, NEMA references are more appropriate thanASA because of its more complete coverage. The desirability oflower limits might apply to any apparatus not physically locatedwithin the confines of a station. Lower limits serve as a safeguardagainst defects in equipment such as, for example, a power trans-former where RIV can be related to internal corona. We agree thatit is unrealistic to spend a great deal of money for reducing RIVstrictly from the noise contribution aspect. However, it will take aconsiderable length of time and experience to establish a sense ofsecurity for all concerned in this unfamiliar territory of 500 kV.Mr. Elwood's observations of comparative noise levels on 345-

and 230-kV transmission lines are interesting and also disturbing.We are awaiting our first opportunity to obtain comparisons.

In answer to the question by Mr. Hedman and Mr. Barthold,we have made no detailed analysis of various line fault effects oncarrier attenuation. The coupling on the first three lines will be phase-to-phase on adjacent conductors. This arrangement has severalimportant advantages, among which it is believed to be the leastvulnerable to faults of all coupling methods short of 3-phase faults.We have made simplified comparisons of center phase-to-ground

with outer phase-to-ground coupling regarding security againstline-to-ground faults. On the basis of a line-to-ground fault appliedto the coupled phase, center phase-to-ground is more efficient.We do not feel that a comparison of the two coupling methods, eachwith a center-phase line-to-ground fault is realistic unless followed

Manuscript received March 22, 1965.

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Fig. 3. Comparative noise levels. (a) Lateral profiles of RI underdouble-circuit lines near station. (b) Lateral profiles undersingle-circuit lines remote from station. (c) RIV on laboratorytest specimen.

by a similar comparison where each coupling method is evaluatedwith an outer-phase line-to-ground fault. There will be no trans-positions in the power conductors of the first three 500-kV lines.

Regarding voltage on the ground wire, to date we have made nocalculations or measurements which can directly verify the pre-dicted 600-volt maximum. This figure was based on a general cal-culation of a line of the same configuration over a uniform earth.It contains some safety factors to allow for inaccuracies in trans-positions and so forth. Measurements have been made on 161-kVlines but not in sufficient quantity or type to permit extrapolationof data for 500-kV application.The question of zero sequence voltage produced as a result of

nonuniform longitudinal earth impedance comes at an opportunetime. The Johnsonville-SCEC (South Central Electric Companies)line traverses two areas with earth resistivities of 500 and 10mQ, respectively. The transition between these areas is apparentlyfairly abrupt. When the line is energized and loaded, we plan tomake measurements at appropriate locations on the insulatedground wires. Meanwhile, time permitting, more detailed calcula-tions will be made.Our experience with carrier signals coupled to a single insulated

ground wire has been meager compared with the work of othersin this area. Tests on the first 100-mile circuit on a 161-kV trans-mission line included coupling to one wire while the other wasgrounded at each end, and with no transpositions. This, however,would not be the same condition as having the unused conductorgrounded at each tower. An analysis based on four modes of propaga-tion should prove very interesting. Our reluctance to pursue thistechnique has been based on theoretical expectations of smootherfrequency response, less interference, and lower noise onf the balancedpair. It is entirely possible that, as the discussers point out, one wirewould have lower losses in the presence of three power phases.

REFERENCE[1] IEEE Committee Report, "Transmission system radio in-

fluence," IEEE Trans. on Power Apparatus and Systems, vol.PAS-84, pp. 714-722, August 1965.

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Documents

Tennessee Valley Authority's 500-ky System - Communications