A Oaj5er read at the i58th Meeting oj the American-Institute of Electrical Engineers, New Yorkand Chicago, Arovewber 22, IgOI.

DISTRIBUTION OF ELECTRICAL ENERGY INLARGE CITIES.

BY LOUIS A. FERGUSON.

With the trend of modern times towards consolidationi oflighting, power and transportation properties, the system ofdistribution of electrical energy, inx large cities especially, isdaily becomning a more important question, involving manyperplexing problems and considerations. Were one given theproblem of determining the system in an imaginary city fullydeveloped in its business and residential portions, its growthmatured and freed from the pioneering, and bad engineering ofthe promotion days of electricity, the question would be a com-paratively easy one. Bl3t as our only living ex-president hassaid, it is a condition and not a tlheory wvhich confronts us.

Nearly every city has granted franchises to many companieshaving varying degrees of financial and engineering ability, sothat those who are ultimately engaged in consolidating, thephysical properties usually find it advisable to adapt their en-gineering to existinig conditions abandoning as little as possible ofthe investtnent made by those which they have absorbed. Notonly must be considered what has been done in the past, but alsothe possible developulent of the future, so that the reinodellingshall niot necessarily be that which might appeal most stronglyto the conditions of the momDent, but of the future; for our in-vestments, let us hope, are made not only for the present, butfor the future.The-developments of the last few years have determined con-

clusively that the old method of installing a multiplicity of steam813

814 FERGUSON ON [Nov. 22

generating stations scattered thlrouighout the city and furnishingelectricity for lighting and power in the surrounding district is nolonger advisable. We filud the more progressive comipanies aban-doning their smaller plants as generating stations, converting someinto sub-stations and centr.alizing the generation of electricalenergy in one or two large power stations located preferably ona water front and having direct communicatiorn with twIo or norerailroads bringing coal from different sections of the country.

There are several instanees in 'Europe, as well as in America,of ylighting and railway cornpanies providing for their futuregrowth by the erection of large central power-houses of from50,000 to 100,000 horse-power ultimate capacity.. In some casesthese new power-houses supplant entirely the older stations, and inother-s the older stations are retained intact, and are used duringthe peak hours of the winter mnonths as auxiliary generatingstations. This latter method is usually the more econorhical,since it saves,a large additional investment which would have tobe made for use during only a few hours daily in two or tlireewinter irionths. Although the cost of generation per kilowatthour in these old stations will be very high as compared withthat of ti e new stationi, it is still wise to uise them as auxiliariesto the main station or sub-station even though they be non-con-densing plants, because their output being a small percentageof the total, the excess cost of their operating will be less thanthe interest and depreciation on the investment required in newstation and sub-station equipaient and transmnission ]ines to sup-plant them.The choice of a site for the main power-house (water and coal

facilities being equal in each case) will depend upon the comn-bined cost of the land and the transmission of the total eapacityto the sub-stations with the advantage in favor of the site nearestto the point of greatest delivered output; that is, the prefer-ence should be giveni to that site where the value of the landformxs a larger proportion of the total than does the cost of thetransmission. The less the cost of the transmissioni the less itslepgth, and therefore the less cable and conduit to maintain.Since the territory to be suipplied in any large city from a cen-tral power-house for general distributionj of electricity for all pur-poses is so extensive, the adoption of a high-pressure alternatingcurrenit is necessary for transmission. Tihere may be cases inwhich it is possible to oi)tain a site sufficiently large for a prin-

190t.] ELECTRICAL DISTRIBUTION. 815

cipal station near enough to the center of greatest load to war-rant the use of double current generators if direct current formsthe basis for lighting and power distribuition, and the energysupplied for railway purposes is relatively small. In any newprineipal station the generators should be of the same frequencyexven if it is advisable owing to local conditions that one or morebe double currenit machines. It is always conducive to economy,

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flexibility of operation and minimuim initial investment that allthe generators be arranged for parallel operation. The installa-tion of both 60 and 25-cycle genierators in the samle stationi in alarge city should be avoided, as the greatest economly is onlypossible in a station generating by one class of apparatus.The determination of the class of current wee shall generate

depends entirely on the class of enrrent we distribLute.' Consid-

816 PERGUSON ON [Nov. 22

ering the question for a large city fromh the broadest standpoint,and ju(dging by experience with the advantages and limitationsof both forms of distribution, IL would recommnend direct currentfor the liglhting and power work of the strictly business and cityresidential districts, anid alternating current for the less closelybuilt residential districts with their local business ineidentthereto, as well as for the actual suburban territory of the city.An analysis of the lighting and power output (not including

railways) of Chicago central station comnpanies where this methodof distribution is followed may be interesting. Referring toFig. 1, it will be seen that of a total maxiinum output of 22,500I<. w, 23.4 per cent. is by 60-cycle alternating distribution overa territory of approximately 58 square miles, and 79.5 per cent.is by direct currenit distribution over a territory of only ten squareniles.The output shown in Fig. 1 as 25-eycle low tension distribu-

tion is largely generated at present by direct current and doublecurrent machines, as the principal generating station is onlythree-fifths of a mile from the largest sub-station. A considera-ble portion of this output will probably continuie to be generatedbv the present low tension direct current and double currentmachines, even after a large 25-cycle alternating station is put inoperation; but this curve is presented to show what portion ofthe entire load would be supplied by 25-cycle alternators androtarv converter sub-stations, if an entirely niew system werebeing constructed, the condition toward which we are develop-Ing.'The growth of the business of the comlpaniy (except in the

Southern District, where a modern 60-cycle station operates) isbeing provided for by 25-cycle rotary sub-stations anid all directm.urrent generating apparatus will be superseded as it wears outor becomes obselete, by 25-clyele rotary sub-station distribution inone low tension district. As the outlying residence districts be-come more general consumers of electricity, they are changedover to low tension direct current and all new development inthe Nortlhern and Western Districts is supplied by miotor gen-erator sets receiving 15-cycle current from the main generatingstatioin avid delivering 60-cycle current to the distribution lines.

Reference to Fig,. 2 will show how the maximuni load of thelow telnsion- direct current distribution wmas pi-ovided for last win-ter. It will be noticed that the rotary converter output formed

1901.J]; ELECTRICAL DISTRIBUTIO0. 817

a very small portion of the total, but in this winter's maximumwb expect to turn out about three times the amount of rotaryoutput, while the direct current delivered to the distributionmains directly from the continuous current generators will showonly a slight increase. The storage battery discharge will showabout 100 per cent. increase. The growth in the rotary and battery

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outputs and the practical st' ndstill in direct culrrent genleiatingoutput gives somne conception of how we are working, towards theidea of 25-cycle alternating supply from one large station to anut ber of comlbination rotary and battery sulb-stations. To e:ffecta change o)f this kind without sacrificing the value and use of ourpresent invrest,ment in direct currenlt generatinlg stations will takeseveral years.:

~~~~~~. M. P.M

FEIW USON ON [Nov. 22

In determining what shall be the general system of distribution,a study of the present conditions and futuLre possibilities of elec-.trical consumption must irst be made, and the system chosenshould be one that will best suit these conditions. Fig. 3 showsthe relative quantities of light and power consumed monthly bythe customers of the two Chicago compa-nies. These curves rep-resenit only the actual powoer business for elevators and smyiall fac-tory use, and other general power for a large number of relativelysmall consumers, and do not include any street or elevated railway

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work whatever. It should be remembered, however, that anysystem of generating or supply which does not take into con-sideration this factor is not comprehensive enough to provide for,the future conditions of our American cities, which will un-doubtedly involve the supplyinig of electrical energy for lightiig,,general power and local transportation work from one or norelarge generating plants.

With the percentage of power as high as 30 per cert. (shownin Fig. 3), even when general power only is included, the great

1901.] ELECTRICAL DISTRIBUTION. 819

importance of selecting a system of generation and distribution,which is well adapted for power as well as lighting supply, willbe thoroughly appreciated. It is also important to note that thepower curve shows no month of the year higher thani another, asthe light does, and also that a daily power curve shows practicallya straight line from 8 A. M. to d P. M. every business day of theyear, while the light load shows a sharp peak of only from one totwo hours' width every winter day. The importance of the powerbusiness leads us then to adopt the three-wire direct current sys-tem for distribution in the business and city residential sectionsshown on the map of the city as the "1 Central District." Thefeasibility of applying the direct current tmotor to general powerwork is much superior to that of the alte'rnating current motor,and the distributing ef eets on li:ne pressures are less. The initialcost of the direct current motors themselves and their attendantequipinent is very much less; in fact, at the present time theselling prices of three-phase motors in America are far in excessof being reasonable. The direct current is also very mueh betteradapted to electric elevator work than is alternating current; atthe present tile there is nio alternating current elevator equip-mxent whielh could be con-sidered fitted for first-class high dutyservice, and the progress in developmient of an econom-lical alter-nating current elevator for any service is slow. The imnportanceof the electric elevator to the central station company is shown.by the diagrammatical representation of outputs. There it willbe seen that the consumption of electrieal energy by elevatorsalone is 12W per cent. of the total output of the comnpanies. Ourability to install an economical and sueessfully operating elevator,equipment is frequently the deciding featuire in the question in-volving the choice of central station service in preference to theinstallation of a private plant, and with its aid we are enabled ina large percentage of irnstances to show to the owners of largemercantile establishmnents and other buildings a decided savingby purchasing elecetrical service from the central stationi eompanyfor liglhting, elevator and generIal power service when comparedwith the cost of operati:ng an isolated plant using hydraulicelevators.The development of the automrobile and its extendded use in

the large cities has rendered necessary the installation of chargingequipments at convenient points throuighout the city. With directc-urr-ent the, problemn of charging is simplicity itself, requiring

820 FERG JSON OJV [Nov. 22

mnerely an outlet in one's stable or in the sidewalk or a clhargingpost on the street corner. Where only alternating mains are foundthe installation of a charging equipment is not only very expen-sive in its first cost, requiring transformers, Inotor genierator andsuitable housing, but also expensive to operate, requiring a com-petent attendant to start and stop the apparatus; so that the netrevenue to the central station company will at best be small, andoften times the results of the operation will represen-t an actualloss.The use of alternating current in the distribution system pro-

hibits the use of storage batteries. Even assuming thatstorage batteries do not improve the economy of operating, stillthey are desirable wherever the service of the central stationcompany is important. Thev mlay be adapted for use in var-ious ways; in the central station itself when it is located atthe electrical center; at the center of distribution for dischargingduring the peak; as auxiliaries in the rotary converter sub-stationsfor diseharge use at the time of maximum load in the main cen-tral station, or at any time of break down of the transmission linefeeditng the sub-station. They may also be uLsed in sub-stations,in special districts where the load factor of the district is sinall,or midway between rotary sub-stations for purposes of regula-tion.The greatest valule of the storage battery, however, is its ability

to earry us through anr emergency which conmes as a rule in anelectric lighting or power plant with very little warning. Thefully chiarged battery kzept constantly floating on the systemnbecomes the watehdog of the companiy's service, responding to acall automatieally, keeping the pressure chart perfect despiteblown oLut cylinder heads in the central station or failures in sub-station transmission lines.The storage battery has a very distinct value, whlih is seldom

recognized and employed to its full advantage, when located atthe central distributing point of a system with feeders radiatirngto various points in the network. The battery nay be providedwith two or more enid eell switches, so arranged that they rnay beconnected in multiple and feed into the main distributing bus, orthey imay connect to one or more auxiliary bus bars with a differ-ent iunumber of cells in series, feeding into each bus and thus pro-viding two or more potentials at the center of distribution. -itwill be fo-and that when only oine pressure is maintained, at the

1901.] ELECTRICAL DISTRIBUTJOJV, 821

center of distribuition during the timie of maximum load of a largedistriet the pressure at the ends of the short feeders will be somle-what higher than the standard and they will be overloaded, andat the enid of the long feeders the pressure will be lower and the

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feeders underloaded, althouglh the pressure variation at othertimes may be negligible. Under such conditions the storage bat-tery becomes exceedingly valuiable, for by arranging it to operateat two or miore potentials, the long feeders may be connected

822 PERGUSON 0N [Nov. 22

during the time of maximum load to the auxiliary bus or bussesand additional current forced over them, utilizing their fullcapaeity and mnaintaining a uniform feeder end pressure bymeans of an inivestment in end cells very slight as compared withthe investment in additional feeders and mains to accomplish thesame resuilt.

Fig. 4 shows three curves, representing two extremes of dailyoutput, one being the Sunday curve and the other the yearlymaximuni curve, and also a third curve, representing the form ofthe companvys curve for about two-thirds of the year.The rectangular crosshatehed portio'n shows the capacity of the

batteries at the one hour rate of discharge and might be placedanywhere on the curve.The upper crosshatched portion shows the use of the battery in

cutting off the peak, thus saving an otherwise necessary invest-ment in generating planit.

Occasions have arisen where through the bursting of a boilertube the storage battery has saved the direct current system fromnshut doWn by carrying the entire load for about an hour, allowingtime for cutting out the disabled boiler. It is in possible in thigcurve to show the value of the battery in maintainin6 evenpressure throughout the system, but it is on-e of the very im-portant uses of the battery.The battery capacity of the Chicago Edison company is dis-

tributed over the entire low tension distri t. In addition to thethree batteries in the' Adams street sub-station, having a combinedcapacity at the eight-hour rate of 66000 ampere lhours, there aresmaller batteries in three of the rotary sub-stations in the resi-dence districts.

These batteries in the residenice districts, of course, cannoteconomically cut off a residence load peak, because the latter isabout four hours wide, but by diselarging at the time of thedown-town peak, whieh is only between one and two hours inwidth, they are able to cutt down the demand for three-phasecurrent at the generating station. In this way they are as valu-able on peak work as though they were located in the down-towndistriet, except that they do not save any investment in trans-mnission line or sub-station apparatus.

Fig. 5 shows clearly the value of the storage battery in a motorgenerator sub-station operating an interurban railway. Its mainfunction here is as a regulating battery used to take up the

1901.] ELECTRICAL DISTRIBUTiON. 823

fluctuations of the railway and keep a nearly constanit load onthe motor generators.

Having determined that direct-current distribution is the inostdesirable for the central district of the city, arid having shownthat of the total maximum output of the lighting companies ofthe city, 79.5 per cent. is by direct current even when thepromising sections of the whlole citv are well covered by mains,it then becornes natural to choose a systerm of generation for theprincipal stationls that is best adapted to the direct current dis-tribbution, as it represents such a large percentage of the totaloutplt. We, thlerefore, originally adopted in Chicago for trans-

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mitting to rotary converter sub-stations 25-cycle three-phasealternating current geineraited by double current maehines at160 volts alternating arranged for stepping up t6 4,500 and 9,000volts. The reason for employing double-current generators inpreference to three-phase alternators generating the line voltagewas becatuse the principal station of the conmpany was only three-fifths of a mile from the point of greatest direct-current output.The double cLurrent maehine, therefore, could be worked at bestefficiency supplying the necessary three-phase current for theoutlying rotary sub-stations, the balance of capacity in the gen-erators being used to feed directly into the low tension networkthus working the generators at all times at full load.

824 FERGUSON ON [Nov. 22

There has julst been started in operation in this station thelargest double current machine which has yet been built. It hasa nominial capacity of 2,500 K. w, with overload limnit for peakwork to 3,200 K. W., generating 300 volts direct current and 180volts three-phase alternating. This machine is to run in parallelwith the other double-current and direct-current machines in thestation as well as in parallel with a new 3,500 K. w. three-phasealternator being erected beside it to gernerate 9,000 volts.As we have reached the limit of available space in our princi-

pal plants for the central district it is probable that alladditional generating plants will be by 25-cycle three-phasealternators generating directly 9,000 volts, the trarismission linevoltage.

It is almost universally conceded that 2a-eycles is the mostdesirable frequency for transmission and conversion by rotaryconverters. Sixty-cycle rotaries are considerably more expen-sive and much less satisfactory in their operation for the ordinarythree wire network, and still less satisfa-tory for 500-volt directcurrent supply for surface or elevated railway service. Also forhigh voltage underground transmission the lower frequency ispreferable, inasmuch as the energy loss in transmission is lessand its resultant strain on undergronnd cable insulation, when-ever there is a slight disturbance on the system, is less trouble-some. The kilowatt hour efficiency of transmission and trans-formation between three-phase generators and rotary direct-cur-rent outpult for thle entire sub-station system equals 82.2 per cent.The portionis of the city of Chicago in which 60-cycle alternatingcurrent is employed in the distribution system are shown on themap as Northern, Western and Southern districts. These terri-tories were, when acquiired, partially covered by single-phase1,000-volt 125-cycle circuits. [Se:e Fig. 6.]

For all these districts' a four wire three-phase -system from"Y " wound armatures supplied with the fourtlh terminal is used,the generator delivering a maximum of 4,150 volts, with 2,400volts between the neutral point and any one of the outside ter-minals. There is thus available a voItage best suited for thesing,le-phase linies in the immediate locality and the higher volt-age for four wire three-phase transuission to centers in moredistant localities.

In the Northern District there have been installed in the orig-inal generating station shown on the map, motor generator sets,

1901.] ELECTRICAL DISTRIBUTION. 825

the motor sides of which are fed by 25-cycle three-phase eeurreritat 9,000 volts from the principal generating station at Ilarrisonstreet, the generating side of the set being a three-phlase "Y"wound machine delivering 4,150 volts across its outsides, and2,400 volts between the neutral and any outside termninal. Itis the intention of the compatny to ultimately shlut down thesteam generating plant in this district and supply all the energyby motor generator sets similar to those now in operation.

Several years ago an extension was made from the NorthernaDistrict power station to Edgewater, a suburb lying four myilesnorth of th-is station, for the puirpose of taking over the businessin Edgewater, which was then supplied by a three-wire directcurrent local plan. The conversion was accomplished bv in-stalling in the stnall generating station at Edgewater two largetransformers with three-wire secondaries, which were connecteddirectly to the bus bars of the station after shutting down thedirect curretnt generators. The primnaries of these transformerswere fed by a 5,000-volt line from the Northern District station,customers in Edgewater theni receiving alternating current overthree-svire network with results which have been more satisfac-tory than when the local plant was operated. The regulation ofthe line is effected by means of induiction regulators in theNorthern District station.

In the Westerni District, where the four-wire three-phase sys-tetn is likewi'se employed, the sul-station is located as shovn onthe map, and current is furnished to the circuits emnanating fromthis sub-station by nmeans of motor generator sets, the motorsides of whclih receive three-plhase current at 9,000 volts from thebprincipal generating station at flarrison street.

In the Southern Distriet there ha3 been erected a principalgenerating station, the location of which is shown on the map asFifty-sixth street, and the territory supplied is 35 square mniles.This station supplanted six generatinig stations formerly operatedby as manly different companies. The system now, employed 'inthis district is also a four-wire three phase system, siingle-phaseinains provided with potential regulators being nsed ftor distribu-tion in the Englewood district, which territory adjoins this station.Four-wire thiree-phase lines transmit at present about one-half theoutput of the generating station 31 miles to Hyde Park, whiere adistributing sub-statiLn is established. The first story of thebuildino, is the sub-statiorn proper, while the secoTnd story isfittedup as an apartnent for the mnian in clharge of the sub-station. On

826 FERG USON ON [Nov. 2?

the first floor there is also an office and store-room, where lampsand small operating supplies are kept on hand, the trouble manfor Hyde Park reporting at this station.The sub-station proper consists merely of a switchboard and

pote-ntial regulators, which are installed on each circuit, no step-down transformers being necessary. Each circuit is equippedwith a single-pole three-throw oil-balancing switch, by which thesingle-phase circuits are balanced on, the three-phase bus bars.From Fifty-sisxth street station a transmission line also runs 81miles to South Chicago, where another local center for distr'ibt -

tion is established.The syrstem of distribution in these outlying districts is suchi

tlhat series alterniating arc lights imiay be operated when desiredfrom the bus-bars of tthe generating stationi or sub-station.The method of generating low frequency alternating current

in onle or two large stations and transmitting the energy athigh voltage to sub-stations for conversion to direet current inthickly settled districts is now being adopted in some of thelarge and more progressive cities of Europe, notably in Berlinand Milan-.The general scheme of generation and distribution in Berlin is

similar to the system in use in the central district of Chicago,with the exception that in Berlin, motor generators are usedinstead of rotary converters. Their practice also differs from theAmerican in that the direct current network is not interconnected,as for somne reason they consider it dangerous to do so.

In addition to the former direct-current stations in the centraldistrict of Berlin, they have erected two large stations away fromnthe center of the city, each having an ultimate capacity of about40,000 horse-power. In these plants three-phase current isgenerated at 6,000 volts by direct coupled three-phase units, andis transmitted 'to the various siab-stations throughout the city.Onie of these sub-stations has at present a capacity of T,000 k. w.,current being taken in at 6,000 volts and reduced by means ofthree part transformers of 1,000 k. w. each to the proper work-ing pressure for operating motor generators of about the samnecapacity. The generating side of each is wound for 250 volts.It is the practice of the Berlin comnpany to use storage batteriesin connectiotn with sub-stations i much the same manner asis being done in American sub-stations.The general scheme of transmission and distribution in the

City of Milan involves the use of three-wire direct-current distri-

1901.] ELECTRICAL DISTRIBUTION. 827

bution in the central portion of the city, supplied by a motorgenerator station located near the Cathedral, with a 3,600-voltthree-phase primary distribution for the territory beyond thecentral district. Current is generated at, 13,500 volts with a fre-quency of 42 cycles per second by three-plhase alternators in thewater-power plant located at Paderno, 32 kilomnetres fronm thesteam station at Porto V\olta. At the latter point the voltage isreduced to 3,600 volts by step-down transformers and connectionsmade in parallel with the 3,600-volt generators in the stean sta-tion at Porta Volta. Froml this 3,600-volt bus, current is dis-tributed throughout the outlying portions of the city by mieansof primary mains which feed transformer houses located at con-venient points. From these transforner houses small secondarynetworks are built to supply the customners in the neiglhbor-hood.The sub station near the Cathedral receives its current at 3,600

volts from the main bus bar in the stationi at Porta Volta, themotor side of thie generators in the sub-station being wound for3,600 volts.The Milan Edison Company operates the street carls of the

City of Milan as well as doing the lighting and power work forthe city, the motor generators in the sub-stations being also usedfor operating the street cars.

Storage batteries are use(d in Milan for both railway and light-ing business, the railway battery. having a capacity to take careof the maximum. load of the railway for one hour, and the light-ing batteries being sufficient to take care of the full direct cur-rent lightinig loac at the tine of peak.The general principles and systems which have been here ad-

vocated as advisable for Chicago should be applicable to anylarge American city as the general conditions in all large citiesare approximately the same. This, however mnay not neces-sarily be true in the case of the smaller city or town as they areusually affected by some local conditions, and in some instanceseven the general conditions may not be such as to make the samesystem applicable.The aitm of every central station mnanager should be to strive

for sinplieity and interchangeability in his systemi of generationand distribution, avoiding as far as possible multiplicity and com-plexity of methods, building up a distribution system whichshall be universal, thus standardizing the customers' appliaucesand apparatus throughout. the city.