the subject. The main point of the dis-cussion is the use of neutralizer reactorswith saturable iron cores. If all the zero-sequence quantities of the generator systemare known and appraised, it might appear,as in the case of the installations described,that an air-core reactor can be applied withthe same assurance that series resonantvoltages will not be excessive; hence, thequestion of air versus iron cores resolves toone of economics.

On a unit-connected system, it is question-able if the surge voltages which are trans-ferred electromagnetically from the trans-mission system to the generator system willrequire the use of surge protective capaci-tors. If such capacitors are used, however,an analysis can be made to predeterminethe extent of the neutral displacement, inthe event that an open circuit occurs in acapacitor unit.

In the detection of grounds within the

generator windings, it is true that the amn-plified zero-sequence voltage across theneutralizer during normal fault-free opera-tion may be exactly matched by a faultvoltage at some spot on the generator wind-ing. The author is unaware of any methodof detection where the normal voltage(or current) in the grounding device maynot be exactly matched in magnitude atsome spot in the generator winding by afault voltage (or current).

Telecommunication Equipment for Power

Systems: Developments and

Application in Sweden

U. HECHTNONMEMBER AIEE

S. RODHENONMEMBER AIEE

Structure and Organization of theSwedish Power System and ItsTelecommunication Requirements

SWEDEN IS highly industrialized;the power consumption in 1952 was

about 2,800 kilowatt-hours per head.The main centers of consumption lie inthe south and the middle, while the waterpower, representing more than 90 per centof the power supply, is concentrated inthe north. 1 The production and con-sumption centers are therefore linked by400-kv, 230-kv, and some 138-kv lines,forming the backbone of the nationaldistribution network, see Fig. 1. Thepower stations, lines, and transformerstations are owned in part by the StatePower Administration (producing about40 per cent of the total power) and in partby municipal and private undertakings.These are associated in a joint-operationpower system which supplies almost 100per cent of the counltry's needs.The central planning is carried out by

the Central Operating Management, anorganization established by voluntaryagreement between the parties. Thetechnical handling of the network restswith the operating department of theState Power Board, which is responsiblefor load dispatching, switching, etc. Inagreement witlh the other parties, theoperating department lays down day-by-day operation plans for the associatedpower stations. If the basic assumptionsare suddenly changed, the parties reachagreement on plan revisions by telephone.This method of co-ordination is based on

H. J. B. NEVITTMEMBER AIEE

good will and assumes ex post facto ac-counting for energy interchange. Auto-matic load control to ensure constantenergy interchange between differentundertakings is adopted only in excep-tional cases. The network is operatedinstead as though it belonged to a singleauthority. The control of the variouspower stations and transformer stationsremains, however, with the separate own-ers.The operating department for the whole

network needs to know:

1. The power production in the largerpower stations in order to change the loaddispatching if necessary;2. The load on the tie lines, to check thatthe joint operating plan is being followed;3. The load or phase-angle difference, orboth, on important transmission lines, todeterinine the stability and losses;4. The frequency and voltage at key points

Paper 53-282, recommended by the AIEE CarrierCurrent Committee and approved by the AIEECommittee on Technical Operations for presenta-tion at the AIEE Summer General Meeting, Atlan-tic City, N. J., June 15-19, 1953. Manuscript sub-mitted March 4, 1953; made available for printingApril 22, 1953.

U. HECHT is with the Allmanna Svenska ElektriskaAktiebolhget, Vasteras, Sweden, S. RODHsE is withthe Telefonaktiebolaget L. M. Ericsson, Stock-holm, Sweden, and H. J. B. NEVITT is with theEricsson Telephone Sales Corporation, New York,N. Y.

The methods and equipment described in thispaper have been developed in the Swedish telephoneand power industry in co-operation with the largerpower undertakings connected to the Swedish jointpower system. The authors wish to express theirgratitude to these undertakings and to their col-leagues for the valuable assistance they always havereceived and to G. Jancke, Swedish State PowerBoard, for his criticism and advice in the prepara-tion of this paper.

in the system, so that a disconnected net-work can be rapidly reconnected;

5. The state of the key circuit breakers inthe network to determine the extent of asystem fault.

The operating departmnent for an in-dividual power-supply undertaking needs:

6. To know the production of its own sta-tions and the state of the most importantcircuit breakers;

7. To be able to reach by telephone theother load dispatch offices to settle operatingplans, and to reach its own power and trans-former stations for exchange of order andreports.

- 400-KV LINES---400-KV LINES UNDER

CONSTRUCTION230-KV LINES

Fig. 1. Schematic plan of the Swedish extra-high voltage system

Hecht, Rodhe, Nevitt-Swedish Power TelecommunicationOCTOBER 1953 961

An interruption of power supply islimited in its effect, shortened or evenavoided by:

8. Supplementing the power-line protec-tion equipment with carrier relaying.

The map shows that the lines are ofconsiderable length. Carrier-frequencytransmission on the power lines has shownitself to be the most economic method oftransmission.

Other features are also characteristic ofproduction and distribution in a highlyindustrialized country with low popula-tion density. In Sweden there is ageneral shortage of labor, and the wagerate is high. Many transformer andwater-power stations are therefore oper-ated without permanent attendants; theyare automatized, and remote operatedfrom neighbouring attended stations.The transmission range is usually lessthan 10 miles, a distance for which phys-ical circuits, either pilot cables or open-wire lines, are most suitable. To usethese effectively a number of d-c circuitshave been developed to permit more in-formation to be transmitted simultane-ously in both directions.

Very high frequency radio circuits arebeing used more and more, both for main-tenance service between fixed and mobilestations, and as links between fixed sta-tions. The choice of links is deterninedby distance, network structure, and anyshortage of available carrier frequencieson the power network. They supplementthe other types of channel and are ofteninterconnected to them. Equipmentparticularly suited for power-networkoperations has been designed.2

These systems will not be described indetail. The paper will be confined tocarrier-frequency transmission on powerlines and the telemetering, supervisorycontrol, and carrier-relaying equipmentwhich is used in connection with the car-rier channels.

The Swedish Carrier Network

The Swedish carrier network began tobe built up with modem equipment in1938, and now about 4,000 miles of powerlines, mainly at 400, 230, 138 and 80 kvare equipped with carrier circuits. Somepower-line sections are used for more thanone carrier circuit. The first two carriersections at 400 kv, each 300 miles long,were put into operation in the spring of1952. The longest carrier sections in the230-kv network are about 200 miles. Thelongest distance for a normal carrier-frequency circuit connected through sev-eral sections in tandem is about 750 miles.Many lines have high noise level in bad

weather, because the operating voltage isfairly near the critical corona voltage.Ice deposition is rare in Sweden, but rainand snow contribute to increase noise.The transmitted power for the longestcircuits has therefore been raised abovethe normal level to ensure a satisfactorysignal-to-noise ratio.The long distances have made phase-to-

phase connection to the power lines al-most universal in the Swedish carriernetwork because the attenuation is lowerthan with phase-to-ground connection.For the 220-kv lines, the attenuation isabout O.I decibel permile at lOOkc. Thecircuits are also more reliable in opera-tion as they can often be used even duringa ground fault on the lines. The reducedradiation from a phase-to-phase circuit isalso advantageous, as it reduces the dis-turbance caused to other carrier circuitson near-by power and telephone lines andto neighboring broadcast receivers.

Fig. 2. Line trap, 0.15 millihenry, 700-ampere rated cufrent, with and without cover.The tuning unit, including adjustable capacitorassemblies, is accessible on raising the lower

cover section

The frequency range 40 to 160 kc hasbeen used in Sweden until now, but ashortage of frequencies will probablymake an extension up to 320 kc neces-sary. However, gaps will need to be leftin the frequency coverage to avoid power-ful long-wave broadcasting stations. Thehigher frequency range is expected to beused for shorter circuits. The impedanceof the line itself, phase to phase, is 600 to800 ohms in this frequency range. Powerapparatus, such as transformers, circuitbreakers, etc., usually adds shunt capac-itance, while short branch lines have an

impedance varying, with frequency, be-tween capacitive and inductive. Dan-gerous resonances may therefore occurbetween the line traps which limit thecircuit and these impedances, if the trapsare unsuitably tuned. The circuit at-tenuation is then increased.

Line Equipment

The line trap normally used, see Fig. 2,has a main inductor of 0.15 millihenry.This is connected in parallel with a tuningunit consisting of three capacitor assem-blies, an inductor, and a resistor. Thetrap is tuned to the wanted frequencybands in the range 40 to 320 kc by choiceof connections in the capacitor assemblies.In the range 40 to 160 kc, the trap isnormally tuned for both bands of a car-rier circuit. The parallel-resonance fre-quencies are normally placed at the re-spective band edges, and the capacitivereactances above resonance are used fortrapping. Individual tuning is replacedin the 160- to 320-ke band by wide-bandoperation in one of three bands. Theresistor in the tuning unit is then con-nected and the tuning arranged so thatthe trap impedance is mainly resistive atthe lower frequencies in the band and iscapacitive at the higher frequencies. Ifthe network impedance is inductive, sucha 2-band or wide-band trap must often bedetuned. It is also necessary to considerthe possibility of resonance between twoor more traps connected in series.The effect of any resonances on the cir-

cuit attenuation is limited, if the trapimpedance has a sufficiently large resistivecomponent in the working band. This isused in the all-wave trap, which coversthe complete frequency range 40 to 320kc. The main inductor is 2 millihenrysand is combined with a capacitor andanother inductor to form a high-passfilter terminated in a resistor. The resis-tive component of the trap impedance isthen nearly constant (about 600 ohms)over the frequency range. All-wave trapsare used mainly in power lines with alarge number of carrier circuits.

Traps are designed for current ratingsof 250, 500, 700, and 1,000 amperes. Inthe 230-kv network, 700-ampere trapsare most common; in the 400-kv lines1,000-ampere traps are used and 1,600-ampere traps will be used in the future.These relatively high current ratings havebeen chosen because the lines will be fittedwith series capacitors. The short-circuitpower will be limited by the long lines sothat the short-circuit current (asymmetri-cal crest value) for which a 1600-amperetrap, for example, must be designed, need

Hecht, Rodhe, Nevitt-Swedish Power Telecommunication OCTOBER 1953962

niot be greater than 55,000 amperes.The tuning unit is protected by a light-

ning arrester, including a spark gap andnonlinear resistor. The residual voltagefor steep surges is co-ordinated with thedielectric strength of the tuning elements.The cut-off voltage is chosen in relation tothe voltage drop across the trap undershort-circuit conditions.The trap impedances are too low to

prevent carrier signtals passing from onesection to a neighboring one. To reducethe crosstalk in the network, resonantcircuits consisting of tuned coupling ca-pacitors sometimes are used betweenphase and ground at bus bar. The intro-duction of trap filters to separate the car-rier network groups is also under consider-ation to simplify the problem of carrier-frequency allocation.The carrier terminals are connected to

the power line through connecting filtersin which the coupling capacitors are serieselements. The capacitance of the cou-pling capacitor determines the filter band-width. The Swedish network lias beenfitted with coupling capacitors of about0.007 microfarad. The connecting filterscan then be used without retuning overthe whole band, 40 to 320 kc, a greatadvantage compared with line equipment

Fig. 3. Blockschematic of thecarrier terminal TELEMETERINfor telephony, SUPERVISORY COIt e I e m e tering, TRANSMITTERSsupervisory con-trol, and protec-

tive relaying

AD=pilot filter,rectifier, and AUTOMATIC D-C

amplifier TELEPHONED = hybrid trdns- EXCHANGE

former SPEECHHCB = matching la

transformerHD=high - fre-quency demodu-lator with inter-mediate - fre-quency band fil-

terHM=high - fre-quency modu-lator with band TELEMETERING

filter SUPERVISORY COIHO=high - fre- RECEIVERSquency oscillatorIR=voice - fre-quency receiverIT=voice-frequency transmitterLIM=speech-peak limiterLR=automatic level regulatorMM = intermedidte-frequency modulator with barMO = intermediate-frequency oscillatorMON= speech-monitoring equipment

having multiple tuning of the capacitorsfor the individual circuit frequencies.Capacitors with this capacitance cost verylittle more in relation to installation cost,compared with smaller capacitors, that is,0.002 microfarad, at lower line voltages.At high line voltages, capacitance voltagedividers are usually fitted for measure-ment, and these can also be used as carriercoupling capacitors.The connecting filter is protected

against over-voltage by a spark gap anda carrier-current drain coil. The filter ismounted with the overvoltage protectingdevice in a box fitted near the couplingcapacitors. A cable connects filter andcarrier terminal.

Modulation System and FrequencyBand

Swedish developments have gone be-yond the use of double side-band ampli-tude modulation, although a few equip-ments of this type are still in use. The ex-tension of the network rapidly led to a needto economize in frequencies, and in 1943 asingle side-band system was introduced,using only half as much frequency space,as compared with the older system. Inthis way carrier channels could be placed

JG AND-_---NTROL_ _

at 4-kc spacing throughout the whole fre-quency band. At the same time, thesignal-to-noise ratio was improved. Fre-quency modulation has not been adopted,in spite of its good noise characteristicsand simple automatic level control, be-cause of its excessive demands in band-width.

In the Swedish carrier network, thecarrier is used as a signal for protectivecarrier relaying, and it is therefore trans-mitted continuously over the circuit,though at a much lower level than in anormal amplitude-modulation system.In addition, the carrier acts as a pilot forautomatic level control in the receiverand is used for demodulation.The latest model of the carrier system

has a channel bandwidth of 3,100 cycles.Normally the channel is used for teleph-ony, telemetering, supervisory control,and similar purposes. The speech bandoccupies 300-2,400 cycles. Calling anddialling signals for the telephone circuitare transmitted on a voice-frequency (v-f)circuit at 2,580 cycles and the telemeter-ing and control signals are transmitted infour channels at 2,700, 2,820, 2,940, and3,060 cycles.

Carrier relaying requires a continuouscircuit between the terminals in bothdirections, and telemetering and super-visory control require a circuit which iscontinuous in at least one direction. Thecarrier circuit must therefore operate withtwo carrier frequencies continuously

LINEt 4 HCB * * ~~EQUIP-N\ENT

.44* _ _I

P=carrier-relaying equipmentRA = receiver amplifier with intermediate-frequency detectorRF= receiver directional band filterSF=separation filter for the speech bandTA=transmitter amplifierTF = transmitter directional filter

Hecht, Rodhe, Nevitt-Swedish Power TelecommunicationOCTOBER 1953 963

Fig. 4 (above). Terminal bay for power-line carrier system, with andwithout dust covers

Fig. 5 (right). Carrier terminal bay. Unit removed for inspection,and connected to the bay by extension cables

transmitted. This requirement tlltis ex-cludes the use of a single-frequency sys-tem and of a switched 2-frequenci sys-tern.

If the need for v-f channels exceeds thefour in each direction which can be pro-vided at the same time as the telephonechannel, one or more additional carrierchannels are provided exclusively for v-fcircuits. The maximum number of cir-cuits in a channel depends on the noiselevel. Normally it is eight, but in carrierchannels with low noise level it can be in-creased to 12 or more. The v-f subearriersare odd multiples of 60 cycles, spaced by120 cycles, and with the lowest frequency660 cycles. This choice of frequency re-duces the intermodulation between v-fcircuits.

Carrier Terminals

As shown in Fig. 3, double modulationis used in the transmitter. The low-fre-quency signals modulate the 15-kc inter-mediate frequency in the first modulatorMM. The lower side band, 11.9 to 14.7kc, and the carrier, 15 kc, are translatedto the final carrier circuit band in themodulator HM supplied with a higher

carrier frequency f,,,. The carrier signalsare filtered after the second modulator,amplified and then pass through thetransmitting directional filter TF to theline equipment, and thence to the powerline. In the receiver, two stages are alsoused for demodulation. The first de-modulation, in the high-frequency de-modulator HD, moves the received car-rier signals down to the same intermedi-ate-frequency band as in the transmitter,after which a second demodulation pro-duces the low frequencies.The carrier band in one direction is

normally the upper side band produced inthe high-frequency modulation, while thelower side band is sent in the oppositedirection. The carrier frequencies arethen separated from each other by twicethe intermediate frequency, or 30 kc, andthe same high frequency fm from a com-mon oscillator HO is used for modulationand demodulation. Separate high-fre-quency oscillators are used in transmitterand receiver if the carrier circuits musthave a separation different from 30 kc.The ratio of side band and carrier levels

at the transmitter output corresponds to200-per-cent modulation at full output.This large modulation depth is reduced in

a network preceding the receiver square-law detector, which is arranged as aspecial balanced circuit to eliminatedangerous intermodulation products. Inconsequence, the distortion at the low-frequency side is very small. A signal oftwice the intermediate frequency is alsorecovered from the detector and, afterrectification, this is used as a control forthe automatic level regulator. Theregulator is a bridge circuit, the balanceof which is varied by the control current.There is often some attenuation distor-tion in the received carrier band becauseof the irregularities of the line trap im-pedances and line attenuation; the re-ceiver is therefore provided with anequalizer which is adjusted when thechannel is put into service.

All modulators and demodulators areof the balanced rectifier type, with theexception of the intermediate-frequencydetector previously mentioned, whichuses a double triode. The high-frequencyoscillator is crystal controlled and thusgives a very constant frequency. Theintermediate-frequency osciUator isbridge-stabilized. As the carrier is trans-mitted and used for demodulation, thereis not the same need for frequency sta-

Hecht, Rodhe, Nevitt-Swedish Power Telecommunication964 OCTOBER 1953

bility as in systems using suppressed car-rier.The release signal for protective carrier

relaying is transmitted as a break in car-rier voltage by blocking the intermediate-frequency oscillator. In the receiver, arelay is normally held, operated by recti-fied 30-kc current from the detector. Thisrelay is de-energized if the carrier dis-appears and provides the release signalfor the line-protection equipment.At full modulation, the transmitter

effective peak power is about 10 watts, ofwhich the carrier represents about I watt.For very long lines an additional poweramplifier is used, usually giving an effec-tive peak power of 50 watts. The low-frequency equipment for the telephonecircuit includes a limiter to allow themaximum speech modulation of the trans-mitter to be kept at about 100 per cent.For each superimposed v-f channel, thedepth of modulation is set to 25 per cent.The total high-frequency attenuation be-tween two terminals may be up to 50decibels, if noise conditions are favorable.The telephone sets at the terminal sta-

tions are normally connected on a 2-wirebasis through a manual or automaticswitchboard, and the terminal is thereforeprovided with a hybrid transformer andassociated equipment. For speech over anumber of sections the interconnection atintermediate stations should, for stability,be four wire, and the hybrids are discon-nected by relays. Direct speech over thecarrier circuit without passing throughthe switchboard is provided by a micro-telephone connected for listening, calling,and speech, which forms part of themonitoring equipment.The v-f senders are amplitude modu-

lated in electronic relays controlled byd-c imnpulses from the switchboard, andfrom the telemetering and supervisorycontrol equipment. The v-f receivershave individual automatic level regula-tion for a range of about 46 decibels.The bandwidth is sufficient for teleprinteroperation. Special v-f channels are usedfor telemetering of the power frequencyand phase angle.

Fig. 4 is an illustration of a carrierterminal. The different apparatus isdivided into units consisting either ofstandard 19-inch panels which mount ona U-iron rack, or of boxes carried onshelves on the rack. Shelf mounting isadopted for those units which should beeasily accessible for inspection or replace-ment. Extension cables, with plug andsocket fittings, enable the boxes to beoperated away from the shelves so thatthey can be tested with the channel inoperation, see Fig. 5. The panels are

mostly connected by plug and socketfittings to the bay cabling, which connectsthe units together. Test jacks on theshelves enable measuring apparatus to beconnected for checking the equipment inoperation. Instruments are also providedin the equipment, and meter switchesenable these to be connected for theroutine check of the important currentsand voltages. At the bottom of the bayfront, the mains supply unit is mounted.A connecting panel at the back of thebay provides terminations for all incom-ing cables.

Telephone Exchanges

The telephone circuits of the Swedishcarrier network are fully automatic, anda special automatic exchange has beendesigned for the purpose. Up to ten linescan be connected from telephone sets andfrom the carrier equipment. The ex-change is normally used for toll trafficover the carrier circuit between the coII-nected telephone sets and for throughtraffic between carrier sections, but it canalso be used as a local exchange for callsbetween telephone sets in the same sta-tion. Important calls are not delayed bybusy lines. Since the calls are nonsecret,it is always possible to break in on a busycircuit and to request that the call beterminated. If an exchange capacity istoo small, a secondary exchange can beadded.The received impulses from the carrier

circuit are regenerated in the exchangeand then retransmitted free from distor-tion for through traffic. A circuit whichhas been seized by false impulses, that is,by line noise, is automatically discoii-nected.

Telemetering

Nonelectrical magnitudes are convertedby conventional techniques to electricalquantities by potentiometer, resistancebridge, selsyn, and.tachometer systems.For transmission over longer distances,these quantities are amphfied in measur-ing amplifiers or converted to variable-frequency impulses. The water level is aquantity of particular importance in thecontrol of large water-storage reservoirs.To achieve high accuracy, float-operatedlevel transmitters have been designedusing twin potentiometers, one for coarsemeasurement and the second, with 1-to-10gearing, for fine. This arrangement re-duces the electrical error by a factor ofnearly 10. In some places a pressure-sensitive transmitter is used with apotentiometer which controls a follow-up

potentiometer over the transmission cir-cuits; in turn this operates a recorder atthe receiver,The telemetering of electrical magni-

tudes, in particular those of power, reac-tive power, current, and voltage, overshort lines suitable for d-c working is car-ried out using measuring rectifiers or d-ctorque-balance transmitters. For carriertransmission, the impulse-frequency sys-tem is used in Sweden. The transmitteris an electricity meter provided withphotoelectric-pulse generation. The re-ceiver converts the impulses to directcurrent by charging and dischargingmeasuring capacitors. This current oper-ates the receiving instrument.The impulse-frequency range has been

chosen as 2 to 6 cycles. The minimumfrequency is given at no load when poweris transmitted in one direction only, andat maximum reverse load when powercan be transmitted in both directions.This provides continuous supervision ofthe system. The meter therefore rotatesat 2 cycles for the no-load or maximum-reverse-load condition. The base speedis determined by a measuring system inthe meter, fed from a separate base-cur-rent device, which is ahmost independentof voltage and frequency. Relatively fastresponse and simple convertibility to dif-ferent ranges have been sought. In spiteof the relatively low frequency, whichhas been chosen to permit simultaneoustransmission of several meter readingsand other information in both directions,using simple multiplex circuits, the re-sponse time to 97 per cent of final readingis only 3 seconds. This has been achievedby impulse division in the meter amplifier,so that the outgoing impulse ratio isexactly 0.5. The base speed of the

Fig. 6. Constant-voltage device, output 80volts, 0.5 ampere direct currentj an exampleoF a unit assembly For insertion in telemetering

receiver cabinet

Hecht, Rodhe, Nevitt-Swedish Power TelecommunicationOCToBBR 1953 965

iPRmXI GROUP I6NAISSEtLECTION1 UNIT SELECTION SIGNALS I FINAL SIGNALS;1, 234;1112 2 334455667788i PERAN INDICATIONSIGNALS 91001i12233 'RELEASE ELYSIGNALS

_ W~aba bababababababcibaba bab jSIGNALPREFIX 6ROUPWl6NAiSELECTIONj UNIT SELECTION SIGNALS IFINAL SIGNALS!I SIGNAL I CATnS; I1 2 3411 1 2 2 3 3 4 4 5 56 6 7 7 8 8 9 91001111221313! RELEASEI

La ababab abababo abababababS6NALS

Fig. 7. Register selector system for supervisory control-impulse series transmitted during an operating cycle. The prolonged group and unit selec-tion signals determine the tripping of unit 2 in selector group 1

A=initiating impulse series B=reply impulse series

meter can be altered by a resistance ad-justment in the base-current device. Thewanted meter range can be chosen bymeans of taps on the current coils of themeter. The receiver is suitable for usewith indicating and recording movingcoil and crossed coil instruments withoutan amplifier being required, and for thecontrol of a totalizing circuit without reac-tion on the individual values. Automaticdaily reversal of the current direction ex-tends the life of the charging-relay con-tacts.For frequency and phase-angle meas-

urement the method used is different.The wanted frequency is transmittedover one of the v-f channels describedearlier, and a conventional frequency-measuring instrument is connected to theoutput amplifier of the v-f receiver. Ifthe frequency is to be used for phase-angle comparison, the channel phaseshift is corrected at the receiver. The dif-ference between two phase angles is con-verted in a device to a proportional directcurrent, 0 to 10 milliamperes in a maxi-mum resistance of 5,000 ohms, and thiswill operate indicating and recordinginstruments or a regulator. Phase-dif-ference measurement is based on compari-son of the cross-over epochs of two volt-ages. If the phase-angle difference is con-stant, a constant reading is obtained on aconnected instrument; but if the voltagesare not synchronous, for example, due toa power-network fault, the instrumentpointer oscillates at the beat-frequencyrate. An auxiliary device indicates whichof the two voltages has the higher fre-quency.The receiving equipment for the various

telemetering systems has been split upinto standard assembly units, such as theimpulse-frequency system receiver, con-stant-voltage device, and phase-angle dif-ference device, see Fig. 6. The dimen-sions of these are multiples of 8 by 8inches, and they are provided with at-tached cabling and terminal blocks. The

units can, therefore, easily be assembled,supplemented, or changed when mountedin a cabinet with standard shelving.

Remote Operation and Indication

Remote-indication equipment, whichprovides the operating department withinformation about circuit-breaker posi-tions, fault signals, etc., usually also pro-vides remote operation. The ability tocontrol is particularly needed in the super-vision of unattended power stations andtransformer stations, but load dispatchersin larger networks have also found thisfacility valuable. In this way orders ofstandard form can be transmitted to theoperating staffs in the stations. The loaddispatchers use remote ordering mainly toselect telemeters of immediate interest inthe various power stations and substa-tions for alternative transmission on acommon telemetering channel.On physical circuits, multiwire systems

are often used, chiefly the duplex systemknown in telegraph technique. This hasbeen modified so that short circuits andbreaks in the lines do not cause faultyoperation. If the distance is increased, aline combination system or a synchronousselector system is used.A universal code system, the register

selector system, often associated withcarrier channels, has been developed forindication in larger networks and forsupervisory control of unattended sta-tions. Operating and indicating signalsare checked back by impulse series sentback to the transmitter, see Fig. 7. Toshorten the operating time, the reply-impulse series is transmitted almostsimultaneously with the initiating im-pulses. This implies that the impulsegenerators at the two stations must runat the same speed, a result achieved bymaking the impulses reaching the controlstation determine the speed of its impulsegenerator. The equipment has a maxi-mum capacity of 28 selector groups, each

with 13 controlled units; a total of 364units. These may be distributed amonga number of substations connected to thesame channel. Simultaneous operationand simultaneous indication of all unitsin a selector group is possible. In thisway, for example, the indication of anumber of breakers can be speeded up inthe event of power-system failures, or forchecking the equipment. Thus 100breakers can be indicated in about 110seconds. The condition of the equipmentand the transmission circuit is monitoredcontinuously and automatically, and de-fective units or faulty transmission cir-cuits are selectively disconnected. Anysuch disconnections are indicated at thecontrol station.A selector-system cabinet, Fig. 8, con-

tains standard plug-in units, consisting ofselectors, relays, resistors, capacitors,

Fig. 8. Register selector cabinet for controlstation, front view. General equipment andindividual relay sets for 69 controlled units,

at the rear, 100 additional relay sets

Hecht, Rodhe, Nevitt-Swedish Power Telecommunication966 OCTOBER 1953

Fig. 9. Part ofsupervisory con-trol board, lengthabout 4 Feet)fitted with tele-metering instru-ments, 121 con-trol switches forpower units, 23Fault indicatorswitches, and 23calling switchesfor request tele-metering. Frontplate section, 12by 24 inches, is

removable

rectifiers, etc. The individual relays foreach unit are combined into relay sets.The cabinet cabling is standardized andthe most elaborate version allows for theconnection of all types of individual relaysets. The plug-in principle makes addi-tions and changes simple. The relays andselectors are of standard telephone cir-cuit type and operating-coil circuits ofthe high-voltage switchgear and trans-mission circuits are insulated from thecentral part of the equipment.

Supervisory Control Boards

The design of control boards is impor-tant to provide flexible handling of tele-metering and supervisory control instal-lations. Space saving and the possibilityof making changes have been soughtabove all. The size has been kept small,mainly by the use of small controlswitches and fault indicators. The con-trol switches (knob diameter 7/8 inch,projected area about 11/4 by 11/4 inches)give"on-off" operation, "on-off" acknowl-edgment on the discrepancy principle,and an auxiliary operation for specialpurposes. The fault indicators (frontdiameter 3/8 inch, projected area 5/8 by7/8 inch) also operate on theacknowledge-ment principle. A board carryingswitches for many hundreds of powerunits is thus only some yards in length,and a separate operating desk is not re-

quired in addition to the board. Indicat-ing instruments are normally placed inthe mimic diagram, see Fig. 9. In somecontrol rooms, however, the instrumentsare separated from the mimic diagram,and the board is supplemented by a con-trol or acknowledgment desk so that theoperator will not need to leave his writingtable.To make it possible to change and ex-

tend mimic diagrams easily, the panelsconsist of a rear mounting plate and afront plate divided into sections. A sec-tion of the front plate can easily be re-moved for modification while the appa-ratus remains fixed to the back plate.Operation can continue almost undis-turbed while changes are being made.The control switches can easily be movedor inserted with their associated cabling,since this is not fixed to the board.

Carrier Relaying

In Sweden, the indirect-comparisonprinciple has been used mainly. Normalline-protection equipment in the form ofdistance relays and zero-sequence currentrelays form the main and backup protec-tion, and in the event of a network fault,provide signalling impulses to the carrierterminal. Normally the carrier is trans-mitted and received, and the release signalis a short break. This closed-circuit prin-ciple means that the transmission channel

is monitored continuously and that thecarrier-relaying equipment can be auto-matically disconnected if the carrier dis-appears for longer than some hundredmilliseconds.Two variants of the indirect comparison

system have been used in Sweden. Inboth, the carrier-relaying supplements thephase-to-phase and phase-to-ground pro-tective equipment and provides a success-ful system for the high-speed reclosing oftripped circuit breakers.

Before the 230-kv network was directlygrounded, and the first 400-kv lines wereput into operation, a circuit was used inwhich instantaneous tripping assumed theexistence of a local criterion and a releasesignal from the corresponding station.This variant had the disadvantage thattripping was delayed by the transmissiontime of the carrier channel (about 10milliseconds) even for faults in the middleof the section, where the main protectionwould have operated without help fromthe carrier relaying. The same carrierchannel was used for the release signalsfrom the short-circuit and the ground-fault relays. The latter operated moreslowly than the former.The direct grounding of the 230-kv net-

work required faster ground fault protec-tion. At the same time, changes weremade in the release principle for multi-phase short circuits. In this variant, thetransfer tripping principle, the short-

Hecht, Rodhe, Nevitt-Swedish Power TelecommunicationOCTOBER 1953 967

circuit or ground-fault protection in astation is in a condition to trip the localbreaker instantaneously. The trippingstation sends a release signal to the corre-sponding station, where the local protec-tion is now operated in such a mannerthat instantaneous tripping is obtained,even if the local protection by itself wouldrequire a longer time. Distance relaysand zero-sequence relays send releasesignals over the same carrier channel; insome cases, simultaneously.

Comprehensive tests have been carriedout in the Swedish power network toascertain to what extent the overvoltagesurges caused by short circuits andbreaker operations will imitate or sup-press a release signal.3 The tests showedthat a release signal could be receivedover a faulty line section; in some cases,however, imitation could occur in neigh-boring lines owing to short-durationblocking of the receivers by disturbances.At present, this tendency to blocking hasbeen eliminated by changes in the re-ceiver.Another application of the carrier

system which may be mentioned in thisconnection is directed towards maintain-ing the stability of the parallel-workingfault-free transmission lines when a faultysection is disconnected. When a breakerin an important line is tripped, a signal issent over the v-f channels to trip generatorcircuit breakers at the appropriate powerstation. In this way, the intact power

lines will not be required to carry excessload.

Radio Program Transmission OverPower Lines

The field strength from the broadcast-ing transmitters is low in some of theleast populated districts. In consequence,it is found that any corona on the powerlines can cause serious disturbances toradio receivers in the vicinity. To im-prove the signal-to-noise ratio, the powerlines are used as radiating transmissionlines for the broadcast program. In thiscase, a broadcast transmitter is connectedto the line through coupling capacitors.4

Power Supplies for TelecommunicationEquipment

All carrier transmission equipment andtelemetering equipment, and most super-visory control equipment, is providedwith built-in power supply units. Theseare normally fed from the a-c network,and in the event of failure, emergencyequipment is used. The advantage ofindividual supplies is that each equip-ment can have the a-c and d-c supplies itrequires without the user needing toinstall a large set of batteries giving dif-ferent voltages. A further advantage isthat anv circuits which must be insulatedfrom each other can be fed by separatetransformer windings. This is important

for remote transmission, when it is notdesirable to connect the line circuits tothe local circuits.

Various types of emergency supplyequipment have been designed. In thesimpler cases, rotary converters are usedto convert d-c to a-c supplies, and thesehave a starting time of about 1 second.For installations requiring uninterruptedsupplies, for example in carrier-relayingapplications, continuously running 2- or3-machine sets are used. A 3-machineset consists of an a-c generator which isnormally driven by an a-c motor, butwhich, in the event of a network failure,is driven by a d-c motor. The emergencyequipment now is often provided with analternating voltage regulator. The im-proved voltage stability increases thesafety margin of all the telecommunica-tion equipment and reduces service re-quirements.

References

1. THE SWEDISH 380-KV SYSTEM, Ake Rusck,Bo G. Rathsman. Electrical Engineering, vol. 68,December 1949, pp. 1025-29.

2. RADIO LINKS FOR POWER STATIONS, L. Persson.Ericsson Review, Stockholm, Sweden, vol. 28, 1951,pp. 42-47.3. EXPERIMENTAL INVESTIGATION OF TRANSIENTINTERFERENCE FROM POWER LINE FAULTS ONCARRIER CURRENT EQUIPMENT WITH REFERENCETO RELAYING, H. Elmlund, S. Engstri5m, A. Hollner.Paper No. 303, CIGRt, Paris, France, 1952.

4. RADIO TRANSMISSION ON 220-KV LINES INSWEDEN, S. Parding, C. A. Enstrom. PaperNo. 323, CIGRfE, Paris, France, 1952.

DiscussionS. C. Bartlett (American Gas and ElectricService Corporation, New York, N. Y.):This paper contains a number of very in-teresting features, particularly the problemof providing carrier channels on a systemhaving line sections of 200 and 300 mileswith tandem channels as long as 750 miles.This points up a basic problem entirely dif-ferent than that in this country with trans-mission systems more of a network char-acter, with the various carrier channels run-

ning in all directions rather than in parallel.This has led to the almost universal ap-

plication of single-purpose carrier devices.If the problem here had been similar to thatin Sweden, it is likely that we would havecome out with a solution much the same as

that expounded by the authors.The subject of line traps is most important

and we are interested to observe that devicesmore or less similar in character are in use.

The authors, however, then proceed to dis-cuss an all-wave trap in the form of a high-pass filter built around a main inductor of 2millihenries so arranged as to produce a re-sistive component of about 600 ohmsthroughout the carrier range. This item isnew to us and it would be most interestingto know how it is accomplished, both elec-

trically and mechanically. At first sight, itwould seem that having a main inductor of 2millihenrys this alone would have a mini-mum impedance of 500 ohms at 40 kc and,therefore, should be completely effectivewithout tuning.

It is interesting to note the very distincttrend from amplitude modulation to singleside band. In this country most channelsare still operated using amplitude modula-tion, with a definite trend toward frequencymodulation. For some reason single side-band has never made much progress. Aninteresting sidelight here, which may meritconcern, came to light in a recent attempt toinstall an extensive frequency-modulationphone channel on 115 ke. It was found thatintolerable distortion resulted, although theearlier amplitude-modulation system hadworked with entire satisfaction. Studyingthe system characteristics, it was found thatfrequencies below 115 were very severelyattenuated, although from 115 to about 118there existed a flat-top plateau. Evidentlythe amplitude-modulation system was notdegraded by the elimination of one side bandbut the frequency modulation was renderedalmost unusable.Although American practice does not per-

mit or promise the spectrum economy pos-sible with single-side-band equipment thetrend is in the direction of using multiple-

section front-end filters, which will permitchannels to be operated possibly 8 or 10 kcapart, even where they are coupled to thesame capacitor.

It is interesting to note that the system ofmodulation and demodulation using com-mon oscillators is very similar to that used inthis country for a number of microwavesystems.

In the section entitled "Carrier Terminals"is discussed a scheme starting with a 15-kcoscillator with another oscillator on the finalfrequency which provides a simple means ofseparating the upper and lower sidebandsso that filters can suppress the unwantedone. We wonder, however, how the twoside bands are separated after modulationof the 15-kc oscillator. Is this accomplishedsimply by a multiple-section filter?The subject of interference from power

arcs has concerned us for sotne time and, al-though we are convinced that the magnitudeis low compared with such a disturbance as acharging current arc, there is still some con-cern as to whether or not occasional false re-lay operations might be attributed to thiscause. The paper mentions that the troublewas encountered and overcome throughchanges in the receiver. It would be inter-esting to know just how this was accom-plished.

All in all, the paper contains much ma-

8Hecht, Rodhe, Nevitt-Swedish Power Telecommunication OCTOBER 1953968

terial for thought and many details whichmight well be fitted into our own require-ments, even though the over-all picture andof necessity the basic solutions in thiscountry are entirely different.

T. A. Cramer (General Electric Company,Schenectady, N. Y.): I would like to askthe authors four questions.

1. Regarding telephone, on the averagehow inany channels are used on each sectionfor voice communication?

2. On relaying for 3-terminal lines, how isthe carrier system arranged? It appearsthat the 2-frequency type of relaying systemwould become complicated and require ad-ditional carrier frequencies. Many of ourimportant transmission lines are 3-terminallines. Carrier relaying has a great appeal tothe system planner since it pernits him toadd a third terminal on an existing line andobtain good protective relaying.

3. The use of the transmitted carrier oilthis telephone system for protective relayingrequires the power to be supplied from thestation battery by means of a continuouslyoperating converter. Control equipment,regulating equipment, and a spare machineare required. At one time, all of our carrierrelay sets were operated in this maninier butas soon as suitable vacuum tubes wereavailable, the designs were changed fordirect operation from the station battery,thus eliminating an expensive item anid alsoa maintenance problem.

4. In the carrier relaying scheme using thetransferred-tripping principle, it is not statedhow this signal is actually transmitted. Isthis accomplished by using the break in thecarrier or by audio tones oni a voice chan-nel?

U. Hecht, S. Rodhe, and H. J. B. Nevitt:We are not entirely of the same opiniioIn asMr. Bartlett regarding single-purpose use ofcarrier circuits in meshed networks and forcombined operation on long parallel lines.In somne parts of Sweden the power iietworkis niore complex than is shown in the sim-plified plan in Fig. 1. There anld in othercountries single-side-band systems utilizing

the same carrier circuit for voice, telemeter-ing, and similar communications, and forcarrier relaying, has shown itself to be moreeconomical in frequency spectrum thansystems with separate channels for each pur-pose.As to the all-wave trap, we agree with Mr.

Bartlett that the description of the circuitas a high-pass filter may cause misunder-standings. Across the main conductor of thewave trap a capacitor is connected in serieswith a second inductor, and the latter isloaded by a resistor in parallel. If the maininductor should be used alone without tun-ing, the reactance would be high enough fortrapping with resistive and inductive im-pedances behind the trap. The risk of seriesresonances with capacitive impedanceswould, however, as discussed in the paper,in most cases render such an untuned all-wave trap inapplicable. The great stressesduring short-circuit conditions must be con-sidered in the mechanical design. Due tothe high inductance of the main inductor,all-wave traps have hitherto been manufac-tured only up to 500 amperes.

It is well known that frequency-mnodula-tion systems are particularly subject to dis-tortion due to inequality of and phase shiftbetween the side bands. Double-side-bandamplitude-modulation systems also suffertransmission impairment to a lesser degreeon account of changed phase angles andmagnitudes of the side band and carriervectors. On the other hand, single-side-band systems are relatively free from thissource of distortion.The method of modulation is such that

the upper side band, after modulation of15 kc by the audio frequencies, is removedby means of a filter in the first modulatorMM, Fig. 3. The lower side band with 15kc as a carrier modulates the frequency fnin the second modulator HM. Then one ofthe side bands related tofm is readily selectedfor transmnission by means of a filter in H.lsince these side bands are 30 kc apart.

In carrier relaying using the transferred-tripping principle we have to distinguish be-tween false relay operation on the faulty lineand on the neighbouring lines. As men-tioned in the paper, the release signal istransmitted as a break in the carrier voltage.

On the faulty line the strong disturbanicesfrom arcs caused by circuit-breaker opera-tions may simulate the carrier and thusneutralize the release signal. This is over-come by a proper timing of the release signal,as discussed in reference 3 of the paper. Onneighbouring lines it was found durinig pro-totype tests that some resistance-capaci-tance circuits in the receiver had too longtime constants which caused blocking of theamplifier during the disturbances andthereby siinulated a release signal. Theseresistance-capacitance circuits were modi-fied accordingly to eliminate false operatioin.We also agree with Mr. Bartlett that themagnitude of disturbances from steadyburning arcs is generally too low to causefalse relay operation.

This ainswers question 4 in Mr. Cramiier'sdiscussion. Tone channels may, however,be employed in addition to the carrier itselfwhen carrier relaying is used for two parallellines on a common channel.The answer to question 1 by Mr. Cramier

is that normally only one channel is used fortelephony on each section of a power line inSweden because there are generally severalpower lines on the same route and each oftheIn is equipped with carrier used not onlyfor voice but also for carrier relaying andtelemetering. In addition, separate carrierchannels on the same section may be usedfor modulated tone transmission. Undercertain congested conditions, however, thesame line may be equipped with severaltelephone channels. Radio links for thatpurpose are also being applied.

Question 2 is a very interesting one. Simicethe carrier channel which is used for carrierrelaying also transmits voice and continuoustelemetering from each terminal, separatecarrier frequencies are used on the sectionsin a 3-terminal arrangement.

Regarding question 3, carrier equipmentused for carrier relaying is normally suppliedfrom the a-c network by means of a 3-machine set. Only during breaks in thealternlating voltage does the station batterytake over and drive the converter. Since,however, the plate voltage in the equipmentis the same as the battery direct voltage, themethod of power supply discusse(d by Mr.Cramer can also be used.

IOe-cht, Rodhe, Nevitt- Swedish Power TelecommunicationOCTOB)ER 1953 969