of Electrical Engineers

Charles Pieter Marais transferred fromStudent to Associate Member.

The Secr^tary announced that an application for membership was received fromMr. George Herbert Bowden, that theCouncil had graded him as an AssociateMember, and that he would stand for election at the Monthly General Meeting inJune. That the Council had admitted thefollowing cadidates to membership of thefollowing candidates torn embership of theInstitute:—As an Associate, Mr. JamesThompson Pyott; as Students: Messrs.Fouchd Joseph and Mr. Johannes PetrusAlbertus Lochner. And that an application for transfer was received from Mr.Johnnie Cohen (Student Member), and theCouncil had graded him as an Associate.

GENERAL BUSINESS.Welcome to Visitors.

Thei President: Under "General Business," I would like to welcome all visitorshere this evening, and, in particular Mr.Sibson, of Bulawayo, and Mr. WallaceKing, Electricity Department, S.A.R. &H.,; Durban. This is the first meeting thatthey have been able to attend.

There being nothing further under thisheading I have much pleasure in askingMr. Lehman to present his paper.

Mr. C. A. Marshall (President) was inthe chair, and there were present 75' members, visitors and the Secretary.

OBITUARY.The President: Before starting with the

general business this evening, I regret tohave to announce the death of Mr. S.Pearson, Life Member. Mr. Pearson joinedthis Institute in 1914 and died on the 27thApril, 1941. (As a mark of respect to thedeceased all present stood in silence.)

MINUTES.The President: I regret that, owing to

unavoidable circumstances, the Minutesof the last two Monthly Meetings have notyet been circulated. We are sorry aboutthis, and would ask your permission thatthe confirmation of these Minutes be leftover until the next Monthly Meeting. TheMarch Journal will be out next week, andthe April Journal the week after. Agreed.

MEMBERSHIP.Messrs. G. A. Dalton and C. J. Monk

were appointed scrutineers of the ballot,after which the President said he hadmuch pleasure in declaring Mr. HerholdtMurray Sebastian Muller transferred fromAssociate Member to Member, and Mr.

29th May, 1941, at 8 p.m.

PROCEEDINGS AT THE THREE HUNDRED AND TWENTY-SECONDMONTHLY GENERAL MEETING.

Part 5MAY, 1941Vol. XXXII

Incorporated—December, 1909A. EVERETT, B.Sc. (ENG.)

Founded—June, 1909Editor

The Transactions of the South African Institute

of Electrical Engineers

' ROBI[RANDW

WEST RAND ,

**

ROUTE MILEAGE 95.TRACK MILEAGE =306.

FIG. I.

S.A.R. 8. H.

ELECTRIFICATION TRANSVAAL.

power requirements involved. Operatingexperience in connection with the conversionand distribution equipment is given and inconclusion the additional arrangements nowbeing carried out to meet the abnormalincrease in traffic are discussed.

No attempt is made to describe the basicoperating features or details of the rectifierequipment, this being fully covered in a

Mr. President, Gentlemen,

Summary.

The intention of this paper is to outlinethe experience in connection with the provision of power for the Reef SuburbanElectrification Scheme. This includes abrief description of the scheme in general,thus allowing a better appreciation of the

By W. W. LEHMAN (Visitor).

OPERATING EXPERIENCE IN CONNECTION WITH THECONVERSION AND DISTRIBUTION OF POWER FOR THEREEF 3,000 VOLT D.C. SUBURBAN ELECTRIFICATION.

[May, 1941.The Transactions of the S.A. Institute of Electrical Engineers.158

coach trains. As the majority of trainsoperate with two and three motor coaches,it will be appreciated that with colour lightsignalling allowing a minimum time interval between trains, the sub-station loadingparticularly in the central area is fairlyheavy.

The total number of 1,500 kW rectifierunits initially installed for the completeReef Scheme including the Pretoria lineand which are at present supplying therequired output, is twenty-three, with atotal continuous rating of 34,500 kW.

This results in a total number of journeysof 39-4 million and 41-6 million for 1939 and1940 respectively, of which at least 75jjier cent, are hy electrically propelled trains.

24-89-96-9

1940

... 22-89-76-9

1939

Third Class ...Second ClassFirst Class ...

(c)(b)(a)

During 1939 and 1940 the approximatenumber of passenger journeys for the Reefarea was as follows, the figures being givenin millions of journeys.

FIG. 2.

• PINEOENE.

—10-9\\^• PRETORIA.

WEST.

& TIE STATIONS .

PRETORIA

SUBSTATIONS

TRACK MILEACE= 306.ROUTE MILEAGE - 95 .

EXISTINC TIE STATIONS . ,EXISTINC SUBSTATIONS .

EXISTINGREEF ELECTRIFICATION.

This gives some idea of the magnitude ofthe electrification scheme, and it will beobserved that approximately 60 per cent,of the journeys were for third class nativetraffic.

Trains are arranged with one, two andthree motor coaches, the three motor coachtrains having a maximum of eleven vehicles.During acceleration with the motors connected in parallel the average current per motorcoach is approximately 500 amps withpeaks of 600, which results in a maximumcurrent of 1,800 amps for the three motor

previous paper on this subject. It is,however, considered desirable to give briefdescriptions where necessary when discussing the operating experience.

General.The major portion of the Reef Electri

fication Scheme was in operation at the endof 1937 and the present electrified area isas shown on Figure I, the non-electrifiedsections being shown by a broken line. Thetotal route mileage at present is 95 and theapproximate track mileage 306.

tie-stations were fitted with the necessaryselector telephones, which are also requiredfor communication with the central controloffice when the equipment has to be takenoff load and disconnected for service purposes. As certain of the new tie-stationsto be installed have no adjacent signalcabins with the necessary 24-hour attendance, they have been arranged for automatic operation, details of which will begiven later.

Maintenance of certain sub-station equipment in the central area which makes itnecessary to take a rectifier off load can onlybe started at annroximatelv 8.45 a.m. after

which can be seen on the desk in Figure 3.Portion of the operating diagram whichshows complete details of the switchingand sectioning arrangements is also visible.This operating arrangement, although notas satisfactory as direct operation by thecontrol office through one of the acceptedmethods of remote control, has generallyproved suitable for the Reef Electrificationservice, and has the advantage of lowinitial cost.

Minor troubles are experienced with newsignalmen or when they are transferredand have to operate control equipment ofdifferent manufacture with altered operating

Figure 3.

characteristics. It has therefore been foundadvisable that as far as possible the generalfeatures of the control equipment such asindication, audible alarm and the cancellation thereof should be common throughoutthe system, in order to minimise confusionand resulting delays.

Multi-core cables are used for the operation of the switching arrangements from thesignal cabins with provision for disconnection and local operation if required. It hason a few occasions been necessary to resortto local operation from the sub-stations dueto damaged control cables etc. With thispossibility in view, all sub-stations and

Three Tie-stations are also in operation andFigure 2 indicates the electrified sectionsincluding the sub-stations and tie-stationsnow in operation. The sub-stations areindicated by circles and the tie-stations bysquares.

Operation of Equipment.Operation of the sub-station and tie-

station equipment is by means of suitablecontrol switches situated in the respectiveadjacent signal cabins and operated by thesignalmen on instructions from a centralcontrol office. Communication between thecontrol office and the various signal cabinsis arranged by means of a selector telephone


The rectifiers have no compounding whichresults in a plain shunt characteristic, gridcontrol being used for protective purposesonly. The rating of the existing rectifiersis as follows :—•

(a)Continuous rating...1,500 kW.(b)Two hour rating...2,250 kW.(c)30 minute rating...3,000 kW.(d)10 second rating...5,250 kW.

It is found that the type of load variesin the different areas, this generally resultingin the load approaching any one of the aboveratings with, however, the remaining valuesreasonably near the maximum permissibleloads. Thus the loading approximates the

are serviced at night when the loading ismuch reduced.

Sub-Stations.(a) General.

The incoming supply at the various substations is three phase A.C. with a normalvoltage of 10, 20 or 40 kV, and this isconverted by means of steel tank rectifiersto the required 3,000 volts D.C. All therectifier transformers have star connectedprimary windings and six phase forkconnected rectifier secondaries. In additionto normal ratio, the transformers also haveplus and minus 2^ and 5 per cent, manually

Figure 4.

operated tap switches. As previously pointed out these have been found useful forreducing load on any sub-station with adefective rectifier unit, this being necessaryunder the present heavy loading.

Figure 4 shows the general layout of theoutdoor equipment. Two rectifier transformers and an oil circuit breaker can beseen in the foreground, the equipment beingprotected by a suitable wire enclosure.The two smaller transformers in the background are for local 6-6 kV distribution,this being arranged at a number of tractionsub-stations for general railway powerrequirements.

the morning peak. At midday there isvery little extra loading, this being acharacteristic of the Reef traffic, and it istherefore only necessary to place all rectifiersback on load at about 4.30 p.m., when theevening peak starts. With the additionalsub-stations and tie-stations now beingarranged to cater for the abnormal increasein traffic, it will be possible to have rectifiers off load during peak periods. Atpresent this can only be done by suitableadjustment of the rectifier transformertappings, thus reducing the load on theparticular sub-station in which plant hasbecome defective. The present tie-stations

lei

the greatest number being ten at theBraamfontein sub-station.

(b) Rectifier Equipment.Two makes of equipment are in use on the

Reef electrification, the major differencesof which will be briefly described and theoperating experience outlined. Figure 5shows the general layout of one make ofrectifier equipment. A characteristic ofthis equipment is the large ribbed anodecoolers filled with water and each housinga temperature operated valve for controllingthe flow of cooling water between the hollowanode stem and the cooler, thus regulating

from the particular sub-station which isoverloaded, but no detrimental effect hasbeen observed in connection with theoperation of the service.

Each rectifier has a protective polarisedhigh speed circuit breaker which operatesonly on reverse current during a back-fire,etc. This is necessary when it is considered that in the case of a back-fire andthe flow of current from anode to anode theapplication of a suitable negative gridpotential will result in the arc beingextinguished when the current in the lastanode passes through zero, whereas theD.C. current flowing between cathode and

Figure 5,

anode fed in from the adjacent rectifiers canonly be interrupted by means of a reversecurrent breaker.

In the majority of cases each track circuitis fed through a high speed circuit breakersuitably calibrated and adjusted for theparticular circuit to be protected. Thishigh speed protection has resulted in a minimum of damage to the traction motors underflash-over conditions, so that very littletrouble is experienced in this direction, andthe motors generally remain in operation.

The number of track circuit breakersvaries considerably at each sub-station,

rectifier ratings reasonably well with noparticular rating becoming a definite limitingfactor. It has been observed that the gridoverload protection operates on occasionsat some of the sub-stations, this beinginstantaneous at four times the continuousloading of the plant compared with theshort time rating of 3-j times continuousload for ten seconds, but without furtherinvestigation it cannot be said that highershort time ratings are necessary. Theoperation of the grid protection (under theabove conditions) results in a momentarycutting off of the 3,000 volt D.C. supply


case of reverse current, to high speed operation through suitable anode lead currenttransformers. This gives additional protection against anode to anode back-firesand momentary overloads, the normalpositive potential being applied after afraction of a second when the rectifier goesback into operation. Grid control voltageis obtained from a suitable D.C. generatordriven from the motor which operates thelow vacuum pump.

Figure 6 shows the other make of rectifierequipment as used on the Reef electrification. It will be seen that the anodes haveno cooling arrangements worthy of mention, it even having been found necessary

experienced on open type interlock contacts,particularly during windy periods and wheremine dumps are present. All tie-stationequipment now on order for the additionalpower arrangements has totally enclosedsteel control panels, which house the complete control equipment, thus ensuring aminimum of contact trouble from dust anddirt. The other sub-station equipmentalso has suitable protection by means ofpartially enclosed steel panels and individualprotection of the relays, etc., by meansof suitable dust proof metal cases. Thisprotection against dust and dirt is desirableparticularly with the Reef electrificationscheme, where dust from mine dumps in

Figure 6.

addition to dirt from steam engines whichstill operate the majority of goods andthrough passenger trains has been found tocause contact failures.

Trouble was initially experienced withload sharing of these rectifiers in the samesub-station, and it was found that a permanent positive potential has to be appliedto the grids to ensure satisfactory paralleloperation. The application of a suitablenegative potential for protective purposeswas retained and a further improvementeffected by transferring the operation ofthe grid protection from the main D.C.output which gave protection only in the

the temperature of the anode. Troublehas been experienced with the valves sticking due to sludge formation which resultsin overheating of the anodes and it hasbeen found necessary to clean out theanode coolers and check and adjust thevalves.

This plant has a continuous running lowvacuum pump, whereas the other make hasone with intermittent operation controlledby the degree of vacuum in the inter-chamber, thus resulting in the pumpremaining idle for considerable periods whichis found to be preferable.

A certain amount of trouble has been

163The Transactions of the S.A. Institute of Electrical Engineers.May, 1911.]

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In the case of Braamfontein unit No. 3and Germiston unit No. 1, which are ofthe water cooled anode type, persistent backfiring started and finally the rectifiers couldnot be switched on to the A.C. supplybecause of an immediate back-fire. Onopening up these two rectifiers it was foundthat the proper return of the condensedmercury to the cathode was prevented bythe collection of considerable quantities ofgraphite in and about the drainage holes,which normally allow the mercury to returnto the cathode pool. A metallic deposit oncertain of the anode insulators inside theshields, with definite indications of currentflow between the anode and shield in thisvicinity was also discovered and removed.Further evidence of this current flow wasthe exterior appearance of these anodeshields which had the typical dull greyappearance resulting from an arc in vacuum.

From Figure 7 it will be observed thatBraamfontein unit No. 3 since being openedup in July 1940 has had as many as twelveback-fires in October 1940, which is threemonths later. However, the figures forthe subsequent four months are reasonablylow and it would appear that this rectifierhas now become stable.

Rectifier unit No. 1 at Germiston hasshown no abnormal tendency to back-firesince it was opened up in June, 1940.

It has been found advisable to close thebuilding ventilators in winter as temperatures as low as 5C. have been recorded insome sub-stations, which resulted in anabnormal number of back-fires. It willbe observed from Figure 7 that in spite ofthis, the monthly number of back-firesis still higher during the winter period.

(d) Rectifier Transformer Operation andProtection.

The transformers in general have given acertain amount of trouble due to loosewindings which appears to be the result ofheavy stresses under fault conditions. Itis considered that the transformers shouldbe examined periodically and the windingclamps tightened if necessary.

When considering the numerous faults onan electrification system due to tractionmotor flash-overs, etc., compared withordinary distribution systems where severe

to arrange heating due to the very lowtemperatures experienced in winter.

Suitable anode reactors were subsequentlyfitted to increase the reactance and thusreduce the short circuit stresses, the transformers for this equipment having a reactance of 3 per cent, compared with 5 percent, for the other equipment.

Trouble was experienced with insulationtransformers for separating certain A.C.auxiliary supplies, required to work at3.000 volt D.C. This has, however, beeneliminated by the use of transformers for11 kV normal operation. Grid negativepotential is obtained by dry plate rectifiers,this being permanently applied and superimposed by an A.C. voltage to permit anodefiring. This arrangement ensures rapidapplication of the grid protection in thecase of anode back-fires, the operation beingby means of a high speed relay controllingthe superimposed A.C. supply and operatedby suitably connected current transformerson the primary side of the rectifier transformer.

Additional overload protection is arrangedby means of an adjustable time relayoperated by the output current and adjustedfor a high current value which must howeverbe of short duration to obviate damage tothe transformer, etc. This also operateson the grid control thus blocking the anodes.

The general protection which operates themain oil circuit breaker is discussed underrectifier transformer operation and protection.

(c) Rectifier Back-fires.

Back-fires have been experienced with theReef equipment and Figure 7 gives recentdetails for a period of twelve months. Itwill be observed that the individual rectifiers vary considerably in their behaviourwith respect to back-fires. In the case ofBraamfontein sub-station, rectifier unit No.3 had 78 back-fires during the twelvemonths, whereas unit No. 2 only had onefor the same period, thus indicating that theheavier loading of this sub-station which isin the central area, is not responsible for thelarge number of back-fires. The sameconsiderable difference in the number ofback-fires is evident in the case of Pretoriaand Germiston two unit sub-stations, thefigures being 62-4 and 22-3 respectively.


The best arrangement would appear to bebi-metal relays arranged with a heatingcharacteristic similar to that of the equipment, thus ensuring satisfactory overloadprotection of the plant. These shouldbe mounted in the transformer tank, whichwill result in the initial temperature conditions being similar, thus obtaining the properprotection, particularly at the heavier shorttime loadings. Instantaneous or inversetime relays would be required to operate inconjunction with the supply network protection under fault conditions. It is

1

rectifier connected in parallel. The D.C.current produces a strong magnetisingof the transformer core which in turnconsiderably increases the current from theA.C. side. Although the high speed circuitbreaker clears the D.C. current in approximately 0-015 of a second and the anodesare blocked in a shorter time by the application of a negative grid potential, the actualtime depending on the method of gridpotential application, it would appear thatsuitable bracing of the transformer windingsis of paramount importance.

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1500

2000

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SUBSTATION LOADING.

FIG. 8.REEF ELECTRFICATION.TYPICAL PEAK PEWIOO LOAD CURVES.

In addition to the grid protection whichhowever only interrupts the current flowfor a fraction of a second with lock out afterthree consecutive operations and only forcurrents in excess of the 10 second ratingof the rectifier, induction typo inverse timeoverload relays are fitted. These howevercannot be set to give the necessary protection for the numerous ratings of the equipment, due to the wide variations which arefrom the continuous rating to 3^ times fullload for ten seconds, as previously outlined.

fault conditions are comparatively rare, itis evident that the transformer duty isextremely severe. Further, when a rectifier back-fires the result is a flow of currentbetween two or more anodes, which ispractically a short circuit across the transformer secondary. At the same time, areverse current flows from the positive busbar via the cathode and one or moresecondary phases of the transformer to theneutral point and so back to the negativebus bar, this current being fed from the


equipment of -5 sq. inch with a resistance of•0875 ohms per mile. Tests conducted on96 lb. rails including bonds as used on theReef, show a resistance of -061 ohms permile for a single rail. No impedance bondsare used on the Reef, one rail being used fortrack circuits in connection with the colourlight signalling. This leaves one rail pertrack for the return circuit, with paralleltracks suitably cross-bonded.

In view of the fact that the rectifiers havea shunt characteristic and the three phasesupplies to the sub-stations were found tovary considerably in their regulation, itwas considered desirable to have the substation voltage regulation when checkingthe track breaker settings, so as to take intoaccount the correct voltage under the actualfault conditions. It must be pointed out,that during heavy load periods the settingof the track protective breakers is not socritical, as in addition to the fault currentthe presence of normal load current willensure the operation of the breakers. However, the limiting factor is during lightand no load periods when a defect in anadjacent sub-station, feeder cables, insulators, etc., will result in the current beingsimply that which is limited by the trackcircuit resistance and protection must beprovided for.

In order to arrive at the sub-station voltage which is a limiting factor in connectionwith the track breaker settings, the majorityof which do not at present exceed 2,000amps except for a few in the central area,suitable voltage regulation curves wereobtained. This was carried out for allsub-stations except two, where it wasknown that the track loading was light,resulting in a big margin between loadcurrent and minimum track fault current,with the result that these breakers couldbe set at a low value with the assurancethat fault tripping would occur even withlow sub-station voltage. Typical substation voltage regulation curves are shownin Figure 9. The D.C. voltage and loadcurrent values were taken simultaneouslyfrom which the voltage regulation curveswere obtained, this being for the totalrectifier equipment installed and wjth ofiprectifier out of commission- When plo^tingthese curves it was observed that thevoltage variation was to a cert^jp extentindependent of the loading, due jib l

understood that this is to be arranged forthe additional sub-station equipment onorder.

From the foregoing it will be seen that theratings of this plant for traction purposes,requires the proper grading of the protectiveequipment.

Effect of Sub-Station Regulation onTrack Breaker Settings and Sub-

Station Load Sharing.

Figure 8 shows typical sub-station andsingle track load curves. It will be observed that the track load curve consistsessentially of very high peak currents ofshort duration with a low average currentvalue. Thus, whereas in the case of theusual distribution system the high standardof supply regulation results in a wide marginbetween load and short circuit fault current,the high peak load currents in the caseof traction circuits become the limitingfactor, it being necessary to discriminatebetween these and the minimum faultcurrent. The required track circuit regulation is much lower than that required forthe usual distribution system, the averagevalue being approximately 10 per cent,with a maximum of 20 per cent, for theReef type of service.

To arrive at the minimum fault currentfor a track circuit it is necessary to considerthe maximum resistance of the overheadconductors and rail return. This is thecase when a short circuit takes place atthe end of the circuit remote from thebreaker for which the fault current settingis desired, making a suitable allowance forthe additional arc resistance that mightbe present at the fault, and reduction incontact wire area due to wear. Certaindifficulties in the case of a flash-over oftrack insulators to the steel structures whichare not directly connected to the rail return,have been experienced on .the Reef electrification due to the high circuit resistanceand this will be discussed later.

The various track circuit resistances werecalculated, practically ail of the overheadequipment having a total cross-sectionalarea of -4 sq. inch and a resistance of -1094ohms per mile at 60F. As a check,resistance tests on some sections were alsotaken. A section of double track betweenCanada and Village âin has overhead

i• •

rating of the two rectifiers at Roodepoort,which is 3,500 amps and this is generallythe case on the Reef scheme.

The regulation of Roodepoort sub-stationis definitely above average as will beobserved in Figure 10 which shows the D.C.regulation for all the Reef sub-stationsexcept Booysens and Welgedacht. In thecase of the outlying sub-stations such asSelection, Birchleigh, Pinedene and Robin-

for one unit will result in a sub-station output voltage of approximately 2,540 andwith the two unit load of 3,500 amps avoltage of 2,420.

Considering one of the single trackcircuits between Roodepoort and Langlaagtetie-station with a resistance of 1-112 ohmsand assuming the operating voltage of 3,000under fault conditions, the minimum fault3urrent would be 2,710 amps. However,

AMPS.4000300020001000

K.W.UNIT

1800 .

2200.

2600.

3000.

FIG. 9.

ROODEPOORT TRACTION SUBSTATION.D.C. REGULATION .VOLTS.

the sub-station as indicated on the regulation curve for two units, the breaker tripsetting is only 2,390 amps, which is a reduction of 320, the voltage being 2,640. Withone of the two available units out of operation there would be a further reduction to2,150 amps which is 560 lower than if theoperating voltage of 3,000 was availableunder fault current conditions, the actualvoltage in this case being 2,380. Theabove fault currents are in no way abnormaland are even well within the 10 second

fluctuation from external causes, this beingquite appreciable in some cases, and thefinal curves represent average conditionstaking into account any consistently lowvalues.

Figure 9 is for Eoodepoort two unit substation which results in curves for one andtwo 1,500 kW rectifier units. The A.C.line regulation is also included from theknown shunt characteristic of the rectifierplant. Thus it will be seen that in thiscase the ten second loading of 1,750 amps


the general case, the heavier track breakerloading in the central area having beenreduced by means of tie-stations, the substation voltage varies from 2,975 in the caseof Braamfontein to 2,290 volts with Robinson sub-station, this representing a differenceof 685 volts.

Considering adjacent sub-stations, theoutput voltage of which has an importantbearing on the load sharing, the followingdetails show the voltage difference for thesections in the main area.

Thus it has been found that to ensuretripping of the track breakers under allfault conditions it has been necessary totake into account the fact, that an appreciable reduction of the trip setting resultsfrom the lower sub-station voltage underthese conditions. This has resulted indifficulty being experienced particularlywith increased loading, in obtaining breakersettings sufficiently above the peak loadcurrents, with the result that numerousunnecessary track breaker trips are ex-

IN.

IP.

perienced due to normal loading. Further,the margin between the breaker settingsand the minimum fault current is notconsidered sufficient to ensure positivebreaker operation under these conditions,particularly as fluctuations of the substation supply are encountered which mightresult in a somewhat lower voltage at thetime when a fault occurs.

From Figure 10 it will be observed thatin the case of a track breaker setting of 2,000amps which as previously pointed out is

shown, the track breaker settings to ensuresatisfactory operation under fault conditionsare particularly reduced compared with thecase of sub-station voltages at or near thenormal operating value of 3,000 volts. Thetrack breaker setting at Robinson for thesection Robinson-Roodepoort with the twotracks coupled in parallel is reduced from2,540 amps with a sub-station voltage of3,000 to 1,965 amps, the voltage being2,320 and both rectifier units on load. Thisis a reduction of 575 amps and with onlyone'unit on load this will be further reduced.

maximum demand values were reversed,whereas the loading is such that Braamfontein should be the higher of the two,there being three 1500 kW rectifiers forthis reason, whereas Germiston has onlytwo 1500 kW units. This alteration of2^ per cent, represents approximately 75volts increase on the D.C. output voltageat all loads, from which it is possible toobtain an approximate idea of the effectwith the voltage differences of adjacentsub-stations as previously outlined.

These values are for a load current of2,000 amps which is a general short timepeak value, except in the case of Braamfontein, which has a heavier loading , andit will be observed that the difference inadjacent sub-station voltage is quite appreciable.

Figure 11 indicates the one hour maximum demand curves for the two adjacentsub-stations, Braamfontein and Germiston.Here it will be seen that an alteration of2^ per cent, by^means of the rectifier

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FIG. II.REEF ELECTRIFICATION.ONE HOUR MAX. DEMAND CURVES FORBRAAMFONTEIN AND CERMI5TON SUBS.

K>00

2000

3000

4000-

5000

K.W.6000

transformer tap switches during May, 1939,resulted in a reduction of Braamfonteinloading, the extent being approximately15 per cent, with a corresponding increase ofGermiston load. The total loading of thetwo sub-stations, however, remainedapproximately the same with 8150 kWduring the alteration and 8180 kW as thehighest one hour maximum demand forthe rest of May, 1939. Thus with thisincrease in 'voltage at Germiston, the

AdjacentVoltageSub-Stations. Difference,

(a) Robinson-Roodepoort ... 390(6) Roodepoort-Braamfontein 295(c)Roodepoort-Orlando... 155(d)Orlando-Braamfontein ... 135(e)Braamfontein-Germiston ... 265(/) Germiston-Apex ... ... 180(g) Apex-Selection 310(h) Germiston-Birchleigh ... 270


supply from sub-stations with higher supplyvoltage.

When investigating the electrification of asection of track to be fed from one end onlyby the Pretoria sub-station and requiringa minimum fault current of approximately3,000 amps to guarantee a suitable marginabove the anticipated load currents, thefollowing was found to be the case :—

Considering overhead conductors of atotal cross-sectional area of -4 sq. inchresulted in a maximum resistance of #978

vicinity of Springs. The difference of 310volts in this case would be minimised bysuitable adjustment of the transformertappings, as the slope of the regulationcurves is approximately the same, but thevoltage difference due to supply fluctuationcannot be overcome. In addition, whenconsidering the general position there isalso the difficulty of regulation differences.

The additional sub-stations being installedbecause of increased loading, were alsoconsidered with a view to lower rail poten-

AMPS.

1

PRETORIA TRACTION SUBSTATIONVOLTS

tials and consequent reduced electrolysisdanger. Thus, with an additional substation between two existing ones and thesame track loading, the maximum railpotential will be reduced to one quarterof the previous value. This assumes equalvoltage at each sub-station and it wouldappear that this benefit will be appreciablyreduced under the present supply conditionson the Reef, particularly with the shortersections between sub-stations. This resultsin a lower track circuit resistance thusallowing a greater proportion of distant load

Another disadvantage of such high voltagedifferences between adjacent sub-stationsis that one will supply loads at greaterdistances away than under normal conditions, when supply would be effected by theadjacent sub-station. This results in higherrail potentials and increased electrolysisdanger.

During electrolysis investigation it wasfound that Apex sub-station was supplyingan appreciable portion of a single trainloading even when this was beyond theadjacent sub-station Selection and in the

171me ^.a., institute of ^lectrical Engineers.

traffic and the resulting extremely lowcircuit resistance, particularly in the caseof third rail systems.

High Speed Breaker Calibration.

No heavy current equipment is as yetavailable for the purpose of calibrating andchecking the high speed breaker settings,although investigations have been made inthis connection. One of the difficulties isto obtain equipment which can be transported to the various sub-stations, thisfeature being considered desirable.

The one make of rectifier equipment hasa heavy current bake out winding on themain transformer, which for bake outpurposes is connected to the anodes, theoutput of the rectifier being shorted throughthe smoothing reactor and the currentadjusted by grid control. This is usedwhen necessary for checking breaker calibration, the only disadvantage being thatthe rectifier connections have to be arrangedas for bake out and the current is limitedto just over 2,000 amps. When requiredto calibrate a breaker at 2,800 amps, itwas found that this current value couldonly be obtained by shorting out the smoothing reactor. However, comparative teststaken under both conditions showed thatwithout the reactor in circuit, the tripcurrent as indicated on a moving coilammeter was reduced to 80 per cent, of thevalue with the reactor in circuit which wouldappear to be due to the following. Firstly,due to the high operating speed of thebreaker, which enabled tripping to takeplace during peak current values whichwere higher than the value indicated by theammeter. These peak current values wouldbe due to the omission of the smoothingreactor and aggravated by grid control.Secondly, when considering the trippingarrangements of the high speed breaker itwill be seen that any current of a pulsatingnature will result in tripping at a lowercurrent value. This tripping takes placedue to a series coil acting against a polarisedfield system of fixed strength, this ensuringrelease of the holding armature by means ofpowerful springs at a definite currentvalue, the moving contact being attachedto the holding armature. An inductive Ishunt is connected in parallel with the j

ohms under fault conditions, the currentbeing 3,060 amps assuming 3,000 volts.However, due to the sub-station regulationas shown in Figure 12, the current wouldonly be 2,560 amps the voltage being2,520, although the no load voltage was3,180. Considering overhead conductors of•625 sq. inch cross-section with a circuitresistance of -7015 ohms, resulted in a substation voltage of 2,330 with a fault currentof 3,300 amps, which is approximatelythat which was required. This howeverrequires an extra -225 sq. inch of copperconductor for the seven miles of track circuit,the negative return being two 96 lb. rails.

Comparing the increased fault currentwhen considering additional overhead conductors under constant 3,000 volt conditionswith the actual supply conditions, shows thatin the latter case this is only 61 per cent, ofthat under the constant voltage conditionsdue to supply regulation.

From the foregoing it would appear, thatoverall level compounding of the rectifiersup to a certain load after which the normalshunt characteristic would come into operation, would be of considerable value, whentaking into account the effect that the shuntcharacteristic plus supply regulation has ontrack breaker settings, load distribution andtrack circuit conductor sectional area.Further, the acceleration of trains is adversely affected by low supply voltage andalthough this can be noticed in certainlocalities, no investigations have as yetbeen made to determine the effect duringpeak periods, when it is necessary to havetrains making numerous stops with a minimum scheduled running time.

The final shunt characteristic is necessaryto ensure protection of the plant, particularly the transformer from heavy shortcircuit stresses under severe fault conditionswhich are fairly frequent. This would alsoensure the proper shifting of load to adjacentsub-stations during heavy local peak loadsin excess of the level compounded values,although this characteristic is perhaps notso important in the case of the Reef type ofscheme, with overhead conductors of sufficient resistance to limit any tendencytowards peak overloading of the local substation. The shunt characteristic coupledwit.^ nlnsfilv recmlated sub-station sunolies


Figure 13.

generally -75 square inch sectional areacan be seen on the right, leading up to thecable end boxes adjacent to which thenecessary disconnecting switches arearranged. These are fitted with arcinghorns, thus allowing for operation withcurrent interruption if required. Theswitches for coupling tracks in parallel toa common feed, are on the left top of thestructure. All switches are operated frombelow by suitable insulated and earthedoperating rods. The operating mechanismand locking arrangements for both open andclosed positions are suitably placed for convenient operation and clearly marked foridentification purposes when operating oninstructions from the Control Office.

In most cases each track is fed from ahigh speed circuit breaker at both ends,although experience has shown that twoparellel tracks fed from a single breaker(at each end) is quite satisfactory on certainsections. In this case it is necessary to

typical structure which accommodates therequired switching facilities, is shown inFigure 13. The feeder cables which are

of rise of current will result in the flow ofmore than the normal proportion of currentthrough the trip coil than under steadyconditions or slow rates of increase, thetrip coil having a lower inductance.

This reduction of the tripping value withthe reactor short circuited was found to beconsistent throughout the current range andas the trip setting of the particular breakerwas not critical, a suitable allowance wasmade and the breaker set to trip at approximately 2,800 amps.

The foregoing indicates the importance ofchecking high speed breakers under thecorrect current conditions and it wouldappear that the best arrangement in thiscase is to have a portable induction regulatorand transformer making use of one of thesub-station rectifiers and its smoothing reactor for checking breaker trip settings, asuitable 3-phase auxiliary supply beingavailable from each rectifier transformer.A portable transformer with inductionregulator and three phase oxide type rectifier was under consideration, but from theforegoing it would appear that suitablesmoothing of the current will be necessaryand consideration might have to be givento six phase rectification. The tie-stationhigh speed breakers have a calibration coilfitted, consisting of a considerable numberof turns of suitable gauge wire which actsin the same way as the normal series tripcoil of a few turns. A tabulated scale isfitted, showing the necessary current tobe passed through the calibration coil forthe various main current trip values, thefigure being approximately 20 amps fora main current value of 2,000 amps. Thenecessary current is ohtained from theauxiliary 110 volt battery and this arrangement has proved very satisfactory forsetting or checking the breakers.

Track Feeding Arrangements.

In view of the fact that the sub-stationsare normally unattended and to facilitatefeeding track circuits under emergencyconditions, means are provided outside thesuh-stations, for disconnecting from thenormal feed and coupling in parallel withadjacent tracks. These switching arrangements which are locked against unauthorisedmanipulation, can be performed by thesignalman on instructions from the CentralControl Office when the need arises. A

in this vicinity on the other track. Thisarrangement has been carried out, as wellas coupling the same track through at asub-station by means of the coupling switch,but this is not advisable in view of the lossof protection and is only done under extremeemergency conditions. The centre diagram shows the same double track fed fromadjacent sub-stations, except that a tie-station has been interposed with the trackssectioned at this point and coupled to a

inductors for each track at the sub-station,arranged by running them parallel for

te span length but insulated, so that themtographs fitted to the motor coaches^ss from one to the fother without inter-ption of the power supply. The switchingrangements for disconnecting one trackould its feeder cables or protective breakercome defective and connecting same in

•rallel with the adjacent track are shown^grammatically. Under these conditions,

I. HIGH-SPEED BREAKER .2.TRACK SWITCH •3.COUPLING SWITCH.

ITIE STATION.

2222TIE STATION.

SUBSTATION.SUBSTATION.

FIG. 14.REEF ELECTRIFICATION.TRACK FEEDING ARRANGEMENTS.

both sub-stations can be set to twice thecurrent for one track or whatever the substation regulation will allow. In the caseof both tracks being fairly well loaded, thisis necessary to ensure the breakers carryingthe load and not tripping. Closing onecoupling switch only will result in thetracks being coupled in series, which reducesthe minimum fault current to approximatelyhalf that of the single track. This will notensure proper protection, when fed from thebreaker in the sub-station where thecoupling switch remains open, with a fault

ave the two track circuits insulated and:d through suitable disconnecting andjupling switches, so that in the eventf a defective track, it can be disconnectedad the other track used. Figure 14 showsrpical track feeding arrangements as useda the Reef electrification.

The top diagram indicates two tracksstween adjacent sub-stations, fed through;parate protective breakers at each endi are also the tracks beyond these sub-âtions. The sectioning of the overhead


station arrangements are altered. Here thetracks are not sectioned at the tie-station,but simply connected together by means ofbreakers to the tie-station bus bar, thusreducing the individual sub-station breakerloadings. This also provides for a suitablefeed for branch lines at this point, but allowsno increase in the breaker settings. This isthe arrangement at the additional tie-station to be erected at Driehoek, where theshortness of the section between sub-stationsdoes not require increasing of the breakersettings. The reduction of breaker load,due to the coupling of the five tracks at thispoint is all that is necessary, in addition to

other three breakers at the tie-station remaininoperative due to their current flow beingin the opposite direction whereas the otherbreaker trips with current flow in the normaldirection. The second advantage, is thatwith heavy loading on one track only whichis typical of most suburban electrificationschemes and generally the case on the Reefsections, the current previously fed by thetwo breakers at the sub-stations withoutan intermediate tie - station is now considerably reduced. This is due to the factthat portion of the track loading is now fedthrough the lightly loaded track and its

Figure 15.

sub-station breakers via the tie-station bubar and breakers.

Thus it will be seen, that with a tiestation of this description the breakeisettings are considerably increased, inaddition to which the breaker loadings arereduced, thus allowing extra loading of thesection with the same overhead conductors.In addition, the voltage regulation isimproved and a substantial reduction of theoverhead copper losses is affected.

The lower diagram of Figure 14 indicatesa track feeding arrangement similar to theone previously described except that the tie

coupling switches are provided the sameas at the sub-stations. The effect of thetie station is two-fold, the first advantagebeing that the tracks have been halved inlength and therefore in resistance. Thusfrom a protection point of view the breakersfeeding these tracks at the sub-stations,can be set up to approximately twice theirprevious value and still ensure proper faultprotection. This also results in only halfa track section being de-energised during afault, which in this case means the trippingof the breaker feeding the half section oftrack from the sub-station and the corresponding breaker at the tie-station. The

^ 7 8 9 10 II MILES.

single one of 1,000 amps. In this case it wasfound that contrary to the usual approximately uniform loading throughout a sectionheavy loading was encountered betweenBraamfontein and George Goch duringcertain periods, due to the overlapping of thetrains from each end of the Reef, whenarriving at this central area. The loadcentre was in the vicinity of Jeppe which isindicated in the diagram, the Braamfonteinbreaker loadings therefore being the most

Tie Stations.a) General.

The tie-stations consist essentially ofligh speed track breakers and the necessarysontrol equipment for coupling the various•rack sections together, to a common bus)ar and housed in a suitable building asihown in Figure 15. On the left hand wallsan be seen the feeder cables which connecthe track breakers with the switchingsquipment on the switch structure and then

250

500

750

AMPS.1000

FIG. 16.REEF ELECTRIFICATION.

TRACK BREAKER LOADINBRAAMFONTEIN AND CECONSIDERING VARIOUS TIPOSITIONS AS INDICATED

250

500

750-

1000AMPS. m

The main circuit arrangements of thetie-stations and a brief summary of theiradvantages have been detailed in theprevious section. However, when decidingthe position of the tie-station on theBraamfontein-Germiston section, the curvesas shown in Figure 16 were drawn, whichshows the possible reduction of trackbreaker loading.

This is a four track section with portionof a fifth track as shown in the track diagramin the right bottom corner, the load forcomparative purposes being taken as a

feed point with proper breaker protectionir the Alherton-Wattles sections. Theictioning of the tracks at Driehoek wouldiean abnormally short sections, whichicreases the danger of trains running fromlive to a dead section when the breakers

:eding the latter have tripped. With this[•rangement it will be observed, that undermlt conditions on one track, its respectiveieder breakers at the two sub-stations andue only at the tie - station trip. This^sults in the one complete track only beinge-energised.

[May, 1941.The Transactions of the S.A. Institute of Electrical Engineers.

and finally burnt down the overhead equipment. It can be imagined what this meansduring the peak morning period, particularlywhen taking place on a number of occasions.

Prior to these tie-stations going intocommission in the central area, the weeklybreaker trips amounted to seventy-nine,which subsequently dropped to nine, thesefew being mainly due to motor flash-overs,etc. Figure 17 shows the reduction ofcurrent for one of the Braamfontein substation track breakers, with Langlaagte

TOO 6.40 6.20 AM.

MAXIMUM RECORDED = 1000 A.

1rtfminilirlit

r

IKi111HiIIÎI

i imi

1,1.3

1

this point, the current supplied by theBraamfontein breaker with the 1,000 ampload at Jeppe would be reduced from 685to 560. A further reduction to 360 ampswould result with the tie-station at GeorgeGoch, as shown by the lower left hand curve,thus indicating the extent of the breakerload reduction and the variation thereof byshifting of the tie point. Further investigation, taking into account the actualposition of trains under conditions of trackbreaker tripping due to loading, revealed

AFTER.

500

1000

1500

AMPS .2000

• i i7.00 6.40 6.20 Ah,

MAXIMUM RECORDED = 1900A.

IîlHkl ••IIUUIJillI'll II' 1

II Ii111 i,i|l

11 1

ji.i ill mmII IIJillllLUI

•ii nuniMRiiiiru.il ilIMUITJHIl

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—i

BEFORE.

FIG. 17LOADING OF ORLANDO DOWN BREAKERAT BRAAMFONTEIN SUBSTATION SHOWINGTHE EFFECT OF LANGLAAGTE TIE STATION.

that of the two possible alternative positions,George Goch gave the maximum generalreduction of current for both Germiston andBraamfontein breakers.

The first two tie-stations were installed atLanglaagte and George Goch during 1939,these being necessary to increase thetrack breaker settings in the central area.Numerous track breaker trips were beingexperienced, particularly during peakperiods when on a number of occasions thisresulted in trains under power running froma live to a dead section, with the resultthat the arc thus created was maintained

reduction. These are represented by thethree left-hand curves, the top curve beingfor the load of 1,000 amps in the case of atrack fed at each end only from the substations Braamfontein and Germiston.Under this condition, the current throughthe Braamfontein breaker with the loadat this point would be 1,000 amps asindicated, which drops to 685 at Jeppeand 500 at the mid point, when each substation supplies half the current assumingequal sub-station voltage. However, witha tie station at Cleveland as indicated bythe second curve, and all tracks coupled at

IllMiunsactions oj tlie IS.A. Institute of Electrical Engineers-

if adopted, would result in a simplificationof the 3,000 volt circuits and more convenient checking of the breaker trip settings.It was considered that suitable reduction ofthe trip settings under fault conditions,could be obtained if necessary by inductiveshunt adjustment, particularly with theshorter track sections of low inductance andhigh rate of current rise. This as previously pointed' out, makes it possible totrip the breakers at a current considerablybelow the steady value or rates of rise metunder motor coach operation. The inductive shunt is adjusted by altering the number of laminated iron discs surrounding thesingle bar shunt.

Tests were finally carried out at Canadatie-station where temporary breakers havebeen installed, fed from the 110 voltauxiliary battery supply and this has provedsatisfactory. It was therefore decided todispense with the 3,000 volt holding coilsupply arrangement, which from calculatedbreaker trip settings was also considered asnot justified.

Tie-stations at Oakmore, Lyttelton andWitpoortje which amongst others are nowbeing provided, are to be arranged withautomatic reclosing of the breakers, theremainder having the usual operation fromthe adjacent signal cabins. The systemadopted was that of closing the breakers bymeans of the auxiliary 110 volt battery andthe necessary control equipment, operatedby relays suitably connected to the respective overhead sections normally fed by thebreakers. Thus, when a sub-station trackbreaker and the corresponding tie-stationbreaker are tripped due to a fault in thesection, the usual closing of the sub-stationbreaker will result in the automatic closingof the tie-station breaker, which closes whenthe section is energised from the sub-stationend. A suitable time delay has beenarranged so that the tie-station breakeronly closes if the fault has cleared, in whichcase the sub-station breaker will remainclosed thus permanently energising thetrack section, which allows the tie-stationbreaker to close.

(6) Protection.

The standard practice of connecting allsteel work, etc., in the tie-stations to anearthing bar and thence to an efficient earth

tie-station in operation. In this case, themaximum peak current which was requiredto be reduced, dropped from a maximumrecorded value of 1,900 amps to 1,000.From the left-hand load curve it will heobserved that the current has numeroushigh peak values and drops to zero onseveral occasions. The right-hand curvetaken after the Langlaagte tie-station cameinto operation, clearly shows the reductionof the peak values as a result of the tracksbeing connected together, with consequentload distribution. This is also supportedby the fact that the curve now has a fairlyconsistent minimum value which would bethe case with all the breakers, thus accounting for the reduced individual peak values.

The previous maximum permissiblebreaker setting for Braamfontein was in theregion of 2,000 amps, whereas the tie-station resulted in an increase to approximately 7,000 amps. In the case of theLanglaagte tie-station, three sub-stationswere affected, there being four tracks fromthe Braamfontein sub-station as far asLanglaagte tie-station where they separatedwith two to Orlando and two to Roodepoortsub-station. The relief to the trackbreakers at Orlando and Roodepoort wasnot as much as that already given for theBraamfontein breaker, because of the factthat Langlaagte was the only suitableplace for the tie-station in view of theseparation of the tracks beyond this point,and it was nearer to Braamfontein, thusaffording greater relief to these breakers.

The high speed breakers for these twotie-stations have the polarised magneticsystem commonly known as the " holdingcoil," against which the series trip coilacts, suitably energised from the 3,000 voltline instead of the usual constant 110 voltauxiliary battery supply. Thus it will beseen that under fault conditions, thereduction of the 3,000 volt supply, automatically results in a lowering of the breakertrip value due to a weakening of the polarisedmagnetic trip system, which also assistsin the proper discrimination of breakeroperation under fault conditions.

Before ordering tie-station equipment forthe additional power requirements, thepossibility of utilising breakers with theholding coils energised from the fixed 110

The Transactions of the S.A. Institute of Electrical Engineers.[May, 1941.178

emergency trip button in the tie-station aswell as the signal cabin, thus ensuring rapidopening of all the high speed breakersshould the necessity arise. It is consideredthat this arrangement has definite possibilities if utilised for protection againsttrack insulator flash-overs. These faultswhich as previously mentioned, have resulted in unsatisfactory protection, due to thefault current being insufficient to operatethe protective breakers. The presentarrangement consists of connecting all thesteel structures which support the overhead

laagte tie-station confirmed this, wherepeak leakage currents of. 70 amps wereobserved.

An arrangement as indicated on Figure18 was therefore adopted. This consists of aheavy duty contactor shown on the right,which is normally open, thus insulating therails from the tie-station earth. A voltagecoil is arranged, so that in the event of apotential difference of approximately 120or more between the rails and tie-stationearth, as in the case of a 3,000 volt flash-over, the contactor is released and closes

TRACTION RAIL NEGATIVE RETURN.

EARTHING CONTACTOR .

EMERGENCY TRIP.BREAKER TRIP RELAY.

TRIP^11""

CIRCUITS, i

AUXILIARYSUPPLY.

FIG. 18.REEF ELECTRIFICATION.EARTH LEAKAGE & EMERGENCY.PROTECTION FOR TIE STATIONS.

the fault circuit to the rail return. Thisensures the operation of the respectivetrack breakers, thus clearing the fault.Operation of the contactor is suitablyindicated in the adjacent signal cabin, theresetting being by hand. Further, shouldthe current be in excess of the normalleakage value due to a flash-over, etc., thiswill result in tripping of all the local tie-station breakers, by means of the breakertrip relay indicated on the left of Figure18. This is also possible by means of an

resistance of approximately one and a halfohms, which is however too high, thusresulting in insufficient current to trip thebreakers should a 3,000 volt flash-overoccur in the tie-station. It was not considered desirable to connect the rails atthese mid-section points to the earthingsystem, as this would allow abnormalleakage of current from the rail via the tie-station earth and the various cables connected thereto, thus causing increased electrolysis troubles. Tests carried out at Lang-

negative rail return at the sub-stations, inwhich connection tests are at present beingcarried out from an electrolysis point ofview, it would appear that suitable protection could be obtained in the case of the substations, by series earth return currentrelays only.

Should any of the foregoing arrangementsprove successful, it might be possible to doaway with the present spark gaps which are

any abnormal potential difference betweenthis earth wire and the negative rail due toinsulator flash over, would result in closingof the earthing contactors. The resultinghigh return current would trip all the substation breakers at each end of the defectivesection by means of the breaker trip relays.It would then simply be necessary to closethe breakers one at a time, when thedefective track would be disclosed. An

each end through separate earthing contactors and breaker tripping relays, in whichcase only the breakers feeding the tracksin the defective section would be affectedunder fault conditions.

Tests conducted on the Natal electrification reveal a current ratio of approximatelyseven to one between the earth returncurrent under fault conditions and normalconditions, which indicates that suitablediscrimination can be obtained. Should itnot prove advantageous to insulate the

;quipment by means of a steel wirelaving a resistance of about 4 ohms pernile. At intervals of approximately one-ifth of a mile this earth wire is connected;o the rail return through suitable sparkâps. This arrangement has on certainiccasions resulted in the inclusion of suffi-;ient earth wire, to limit the fault current:o a value which was insufficient for the^ripping of the protective breakers. Bylonnecting the earthing contactors to theiverhead earth wire at the sub-stations,


Additional Equipment to Meet theAbnormal Increase op Traffic.

Due to the rapid development of the Reefand the hauling of certain goods trains byelectric locomotives which is to be extended,it was found that the power requirementsshowed a rapid increase as indicated inFigure 19. From this it will be observedthat whereas the monthly unit consumptionwas 3 million units at the end of 1937, whenthe major portion of the electrification wasin operation, it has now risen-to approxi-

sub-station output voltage is dissipated inthe overhead and rail return with a corresponding high voltage on the contactor coil.This actually resulted in the operation of theearthing contactor during breaker trips,under fault conditions, which was howevereliminated by including sufficient time delayin the operation of the contactor. Further,it was necessary to guard against openingor chattering of the contactor under heavyfault currents, due to the resulting magneticfield acting against the closing spring.Before installing this equipment, an earthing

FIG. 20.REEF ELECTRIFICATION

contactor was tested by applying 3,000 voltsthrough a track breaker. The coil operatedthe contactor, which closed without anymarked burning of the contacts. Previoustests under similar conditions, when anoscillograph was used to check the current,showed a peak value of 10,400 amps.

During a flash-over of the 3,000 voltequipment at the Langlaagte tie-station,the protective arrangements as outlined,operated quite successfully.

season, when they become shorted due tosurge discharge currents, after which currentfrom the track leaks away, thus increasingthe electrolysis danger.

The earthing contactor operating coil,although normally subjected to the rail dropvoltage only, has to withstand high potentials for short periods under fault conditions.Considering the case of a short circuit ator near the tie-station, it will be seen thatuntil the protective breaker opens, the full


This matter was carefully investigated,taking into account the extra track sectionloadings, electrolysis mitigation, voltageregulation and general supply flexibility,which is important for the Reef type ofssrvice. As a result of these investigations,it was decided not to install the extra plantat the existing sub-stations, but to interposeadditional ones.

Figure 21 shows the existing arrangement,in addition to which the extra sub-stations 1

be as shown for the half hour and two hourratings of the other unit. Although theload would be reduced somewhat due tothe lower output voltage with one unit onload, it will be seen, that the load curvefor the full three hours is very much inexcess of the two hour rating and even morethan the permissible loading for half anhour. The Braamfontein curve shows asimilar overload condition, although with+Lô oîT,d+ci+ir>Ti ^+. ia nnlir +hp twn hmir

Ô•D

FIG. 21.REEF ELECTRIFICATION.ADDITIONAL SUBSTATIONS 6, TIE STATIONS •

with one defective unit and only the remaining two to carry the load. It will beobserved that although the Braamfonteiiisub-station carried a heavy load from 7.45to 8.00 a.m., there is no indication of anytendency for Germiston to assist with thispeak value.

With the foregoing unsatisfactory state ofaffairs, and taking into account increasedloading due to more electric goods workingand extra passenger trains, it was evidentthat additional sub-station plant was

necessary.

mately 9 million units per month, with aâlue of almost 10 million during last year'swinter peak.

Tests taken of the sub-station loadingduring the morning peak, showed that in thecentral area in particular, it was not possibleto switch off any rectifier plant. Figure 20indicates the load curves obtained from theD.C. kW hour meters, with readings takenevery fifteen minutes. The top curve showsthe Germiston loading with two rectifiersat this sub-station. With the failure ofone unit, the permissible loadings would

[May, 1941.The Transactions of the S.A. Institute of Electrical Engineers182

sub-stations, as mentioned by the author,is essential.

Mr. E'. T. Price intended personally proposing a vote of thanks to the author, unfortunately he is indisposed, but he hassubmitted his remarks in writing, and Ishall be very pleased if Air. Trelease willread it for him.

Mr. J. S. Trelease (Vice-president): Air.President, in amplification of your remarks,Mr. Price asked me to state that he is notat all well and is under the doctor's care.He was going to venture out to-night, butthe doctor had other views on the matter,and won't allow him even to go out in theday time. So there was no possibility ofhis coming out on a night such as this.However, he has submitted these notes,which I will now read on his behalf:—

Air. President and gentlemen, whenseconding the vote of thanks to Air. Cunlifffor his paper on "Mercury Arc RectifierSub-stations," read at our March meeting,I referred to his as being the first of threepapers dealing with the use of mercury arcrectifiers for railway traction purposes,and gavei it as my opinion that these threepapers taken together, would form a valuable addition to the Proceedings of thisInstitute. Now that you have heard Mr.Lehman's paper, the second of the series,I feel certain that you will be in full agreement with that opinion.

The paper is the result of an enormousamount of work on the part of the author,not only in its preparation, but more particularly in the collection of the very interesting and valuable operating data itcontains. He is to be congratulated on theclear and concise manner in which thepaper has been presented.

Mr. Lehman's reference to certain majordifferences between the two makes ofmercury arc rectifier plant in use on theReef brings to light the fact, at least in myopinion, that the technique of mercury arcrectifier design has not yet reached thatdegree of certainty associated with thedesign of other types of converting plant,such as motor generators and rotary converters. I refer particularly to the problemof anode cooling and the prevention ofbackfires.

With regard to anode cooling, it will benoted that rectifiers of the one make em-

alteration to the existing plant was that ofadding converting equipment to GeorgeGoch tie-station, thus making it a substation. Remote operation for the Elands-fontein sub-station requires a control cableof considerable length. This is necessaryto ensure the required track breaker settingsby a suitable position of the sub-stationwhich is approximately one and a quartermiles from the signal cabin. Each additional sub-station will be provided with onerectifier of 2,500 kW continuous rating andcorresponding other ratings, this being anincrease in capacity compared with theexisting 1,500 kW units.

Conclusion and Acknowledgments.

It will be appreciated that it is extremelydifficult to deal adequately with all theproblems and troubles experienced, and atthe same time ensure general interest in apaper of this nature. It was thereforedecided to discuss the major points only,taking into consideration those of generalinterest.

In conclusion I wish to thank the SouthAfrican Railways for permission to publishsuch information as was obtained duringmaintenance and investigations in connection with the sub-station and tie-stationequipment. Also the Electricity SupplyCommission for permission to publish information on the sub-station equipment.

The President: By your applause, it isevident that Mr. Lehman's paper has beenmuch appreciated. The paper contains agreat deal of information, which we willfind great pleasure in digesting at ourleisure.

I intend contributing to the discussionat a later date, at this stage, however, Imight mention that at a point where theelectrified system passes through the EastRand Proprietary Alines property, we findthere is at times, a potential difference ofup to 200' volts between the track and anear-by water main.

By connecting an ammeter between therailway track and the water main, a current up to 8001 amperes has been registered.

It would appear that on account of loading conditions, and the high track resistance encountered, the installation of additional feeding stations between the present

183

Experience over a number of years onthe three makes of rectifier plant in use inNatal and on the Reef has satisfied us thatbackfiring is an inherent characteristic ofall mercury arc rectifiers, for 3,000 voltD.C. working in any case. In practice itis found that, provided the rectifier transformers are designed to take care of thesevere stresses involved, and suitable quick-acting arc suppression equipment is installed, occasional backfires have no harmful effect on the plant and cause no interruption to railway service.

The position does, however, become serious when the frequency of backfiringreaches the order of that in the two casesspecially referred to by Mr. Lehman as,owing to the formation of a conductingdeposit on the anode insulators, the condition is finally reached when the rectifiercannot be energised without backfiringimmediately.

In the design of the new type of air-cooled anode I have already referred to,special shielding is provided to prevent theformation of any conducting deposit on theanode insulators.

That portion of Mr. Lehman's paperdealing With track feeding arrangementsand tie stations is, in my opinion, of particular value. One point which occurs tome as being of considerable interest is thereduction in the number of track feederbreaker trips from 79 to 9 per week afterthe installation of the tie stations at Lang-laagte and George Goch.

With regard to Mr. Lehman's referenceto the possible use of level compounding onthe rectifiers on the Reef, whilst there maybe advantages from the operating point ofview, there is the possibility of the introduction of considerable inductive interference in communication circuits adjacent tothe track and to H.T. power lines. Members will better appreciate this point whenthe third paper of the series dealing withinductive interference from rectifiers hasbeen presented to the Institute.

There is one little request I would liketo make, and that is, will Mr. Lehman begood enough, before the paper is printed inthe Journal, to tie up the two makes ofplant referred to by him with the samemakes of plant referred to in the first paper~^ ^m, o^.^0 K,7 \Tr r.nnliff This will be

ploy large ribbed anode coolers filled •withwater, whilst those of the other make, inthe words of the author, " have no coolingarrangements worthy of mention." It willbe of interest to members to learn thatthe makers of the water-cooled anode typeof rectifier have recently decided to abandon water cooling in favour of air coolingfor the anodes of all rectifiers for workingvoltages of 3,000 or over. Consequently,all mercury arc rectifiers of this make nowon order by the Electricity Supply Commission, including the 2,500 kW units forthe Reef referred to by Mr. Lehman, willbe equipped with anodes of the air-cooled

type.

On the question of prevention of backfires, it will be clear from the followingremarks that, at least, for 3,000 volt Working, makers were just as much " at seaas on the question of anode cooling.

When calling for tenders for the originalmercury arc recifier plant, tenderers wereinvited to state the probable number ofbackfires to be expected over a given period.No firm answered this question directlybut simply made statements on the following lines:—

" Backfires are practically unknown onour plant, and on the rare occasionswhen they have occurred they havebeen found on investigation to bedue to causes not attributable to anyfeature of the rectifier design.

" During the twelve months prior totendering only one backfire has beenexperienced on all rectifiers of ourmanufacture."

I well remember a conversation I had onthe subject of backfires with the rectifierexpert sent out by the factory to supervisethe erection and setting to work of one ofthe makes of rectifier plant supplied to theReef. To my question: "What aboutbackfires on your plant? " he replied: " Nobackfires can occur on our plant," andseemed surprised that I should have askedhim such a question. A few days later,when the plant was put into service severalbackfires occurred within the first fewhours of operation, and, notwithstandingsubsequent modifications made by manufacturers, these rectifiers are still subject


rather than the spoken word, and this willundoubtedly give added zest to the paperin printed form.

Though he has dealt with his subject soconcisely and succinctly, I feel that he hasalso set himself out adroitly to tempt theenquiring mind, and I shall indeed be surprised if this paper does not arouse considerable discussion.

Mr. Lehman has certainly succeeded inproviding us with a contribution of valueand interest, which will whet the appetitein anticipation of the other paper which hehas now in preparation covering the functioning of the mercury arc sub-stations inNatal.

I have much pleasure in seconding thevote of thanks so ably proposed by Mr.Price.

The President: Thank you, Mr. Dalton.On behalf of members, I wish to thank Mr.Lehman for his very interesting, instructive and valuable paper.

Does any other member wish to discussthe paper this evening?

There being no further discussions, themeeting terminated at 9.25 p.m.

of particular value when the three papersare later reprinted and bound in onevolume.

^lr. President, gentlemen, it affords megreat pleasure to propose a hearty vote ofthanks to Mr. Lehman for his very interesting and valuable paper.

The- President: Thank you, Mr. Trelease.I will now ask Mr. Dalton to second thevote of thanks.

Mr. G. A. Dalton (Member) : Mr. President and gentlemen, at the outset I wouldlike to compliment Mr. Lehman on themanner in which he has set out his paper.Obviously he has not had to resort to teething troubles of a lesser nature to providevolume, and he has dealt with the mainproblems met with in operation in a concise and explicit way, which T feel surewill appeal to those interested in the subject, and who are in search of such information.

There is much in Mr Lehman's paperwhich is the result of personal investigations. I refer particularly to the tie-stations.

It will be noted that he has covered information in the form of graphs, etc.,

185

Neilson, B. A.Parrott, B. S.Pearce, A. G. TJ.Quinn, E. A. S.Bink, H. E.Bumble, B. V.Vivian, B. J.Watson, 0. E.West, C. T.Wood, E. B.

STUDENTS:

Hills, L. G.Hogg, D.Jack, G. B.Joubert, J. G.Marshall, L. M.Martin, C. A.McMaster, H. S.Middlecote, A. A.Munks, E. L.

Barnes, B.Barrow, M.Browne, J. H.Carr, J. D.Charlton, L.Christie, B. V.Dawe, H. J.Flekser, N.Harmer, J. B.

Morley, E.Neale-Shutte, L. M.Pryce, H. E.Bice, E. C.Boss, C. G. (Jun.)Sabatier, F. A.Saretzky, B.Simpson, B. W.Wheeler, H. A.White, H. Ei.Wood, V. F.

ASSOCIATES:

Edwards, E. A.Gillespie, M. L.Graul, H. W.Healey, G. T.Heaton, G. CHolden, G. E.Horrell, H. G.Horrell, W. L. M.Hughes, B. C. E.Joubert, G. J.Kirk, G. J. McC.Marshall, H. W. S.

Absolom, N. G.Andrews, E. E. G.Bampfield-Duggan, T. C.Boyee, H.Boyle, G. H.Breeze-Carr, A.Cowling, E. G.Curt in, A. G.Denison, S.Dewar, B. W.Easterbrook, J. L.

Bichfield, L. S.Bitchie, C.Schoch, W. E.Spaan, J.Stanbridge, C. H.Tyler, F. H.

ASSOCIATE MEMBERS:

Home-Bigg, J. B.Hutton, H. W.Jobling, H. B.Johnson, E. N.Nattrass, H.Philips, S. M.Price, B. B.

Campbell, A. B.Coles, D. E.Dannenbaum, A.Dinwoodie, J. D.Freemantle, A. W.Goetz, M.Gyngell, A. H.

ltendell, E. F.Sclionland, Prof. B.F. J.

MEMBERS:

Michell, J. A. F.Privett, G. J.Proctor, L. B.

Cotton, G. H.Ewer, G. G.Harvey, A. Q.

MEMBERS ON ACTIVE SERVICE

The South African Institute of Electrical Engineers

[May, 1941.The Transactions of the S.A. Institute of Electrical Engineers.L86

Where symbols or abbreviations are considered necessary, it is desirable thatdefinite standards should be used.

Attention is drawn to the publications ofThe British Standards Institution dealingWith " terms " and graphical symbols usedin electrical engineering, with the suggestion that these be employed.

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LETTERS OF DESIGNATION.The attention of members is drawn to

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The authorised letters designating theclass in the Institute to which a memberbelongs shall be as follows: —

AWARDS.The Council is empowered each year at

its discretion to award the Gold Medal ofthe Institute, together with a certificateand, if considered desirable, a premium notexceeding 10 10s., to a member of anygrade for an original paper of outstandingmerit submitted during the year.

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The Institute is indebted to the Management of the Victoria Falls and TransvaalPower Company, Limited, for an annualdonation of 25 for the purpose of awarding premiums, not exceeding 10 10s. invalue for any one award, for the purchaseof scientific and technical books or instruments for papers and contributions to thediscussion of papers, which, in the opinionof the Council, merit special recognition.

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The Institute is also indebted to the SouthAfrican Cable Makers' Association for anannual donation of 5 5s. for the purpose ofawarding one or more premiums each yearto the total value of 5 5s.

The conditions governing this Award arethe same as those for V.F.P. Awards, withthe exception that all Papers and Contributions read before the Institute are eligiblefor the Award irrespective of whether theauthors are members of the Institute or not.

PAPERS.Members presenting papers before the

Institute are requested to forward a copyof their paper, in final form, to theSecretary at least one month before thedate of the meeting at which it is to beread.

INSTITUTE ANNOUNCEMENTS

187Tlie Transactions of the S.A. Institute of Electrical Engineers.May, 1941.]

BOOK-PLATES.Book-plates for publications purchased

from Award Premiums may be obtainedfrom the Secretary of the Institute.

TECHNICAL PUBLICATIONS.By the kindness of Doctor H. J. van der

Bijl, the chairman of the Electricity SupplyCommission, the Institute is favoured withcopies of the weekly library extracts ofcurrent technical literature prepared by theLibrarian of the Commission.

The Council desires to announce that theExtracts are available for reference bymembers on application to the Secretary,and a copy is also available on the bookshelves in the lounge of Kelvin House.

RECIPROCITY ARRANGEMENTS.Beciprocity arrangements exist between

the South African Institute of ElectricalEngineers and the Institution of ElectricalEngineers, London, the American Institute of Electrical Engineers. It is hopedthat members proceeding overseas will availthemselves of the facilities offered. Fullparticulars are obtainable from theSecretary.

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Members are invited to send in anyclippings from the Technical Press whichmay have bearing on papers up for discussion, or may be of general interest.Please state source. These • clippings willbe submitted to the responsible editor, who—"- - --l ^

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The Minutes of the above Institutionare regularly received by the Institute, and

TO COUNTRY MEMBERS.The Council is particularly desirous of

receiving written contributions from members resident in the country on any of thepapers up for discussion as set forth in theAgenda.

Members who are unable to attend theOrdinary General Monthly Meetings shouldavail themselves of this opportunity.

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Kindly address all contributions, etc.,to the Secretary, P.O. Box 5907,Johannesburg.

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•L. W. Card.H. CorkL. Graham.S. Hampton.N. E. Herberg.

ADDRESSES WANTED.Addresses are wanted for the following:-

BRITISH STANDARDSSPECIFICATIONS.

H.M. Acting Senior Trade Commissioner,Johannesburg, has advised the Institutethat he has received from the BritishStandards Institution, of London, a complete set of British Standards Specifications.

These Specifications are filed in hisoffice, and may be consulted there whenrequired.

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