Coil-less cold-cathode arc chutes for high-speedd.c. circuit breakers for use on traction systems

J.S. Morton, B.Sc, Sen. Mem. I.E.E.E., C.Eng., F.I.E.E.

Indexing terms: Arcing, Circuit breakers

Abstract: The paper describes the principles of d.c. arc extinction using cold-cathode arc chutes, a typeoriginally designed for use with a.c. circuit breakers. Design details are given for.the coil-less cold-cathode arc-chute circuit breaker, illustrating the fundamental requirements of the arc chute, contacts, mechanism andwithdrawable circuit breaker in its metal enclosed cubicle for use up to 3 kV d.c. The application of the d.c.circuit breaker to the various types of traction circuits is described, and particular reference is given to therectifier and feeder circuits. The various types of basic protective devices, inherent in high-speed d.c. circuitbreakers, are the direct-acting electromagnetic overcurrent releases, which are described in detail. Coverageis given to the testing of d.c. circuit breakers to meet the onerous conditions of a traction application, andthese are supported by details of test and service experience.

1 Introduction

The paper describes the development and application of atype of arc chute used on air-break circuit breakers, uni-versally accepted for use on low-voltage a.c. systems andup to 6*6 kV a.c. by a specialist manufacturer, for use ond.c. systems up to 3kV which has proved to be ideallysuited to traction application.

The cold-cathode (or metal-splitter-plate) arc chute, withthe development of suitable techniques in arc control andits subsequent interruption, together with a knowledge ofapplication to traction systems, has enabled the develop-ment of a product, because of its predictable and consistentcharacteristics, as reliable as conventionally acceptedequipments which have been in service for many years.

It has been recognised for many years that d.c. inter-ruption of traction circuits is a specialised subject, andcircuit breakers tested to comply with internationalstandards for normal applications may not be satisfactoryfor use on traction systems. For this reason, the papermakes reference to some of the special conditions associatedwith traction and how they relate to the circuit breaker'sswitching-performance characteristics.

2 Principles of d.c. arc extinction

A circuit is interrupted in a d.c. circuit breaker by parting aset of circuit-breaker contacts and controlling the resultingarc, so that it is established inside an arc chute. Here theresistance of the arc is increased, which automaticallyincreases the impedance of the circuit, and hence thecurrent being interrupted is reduced. As the resistance ofthe arc increases, the voltage drop across the arc alsoincreases. This process continues until the arc voltage hasbecome greater than the applied voltage of the system, andthen the arc must extinguish as it can no longer bemaintained by the system voltage. At the instant immedi-ately before arc extinction and at the nominal current zeroat which arc extinction occurs, the arc power loss alwaysexceeds the power input to the residual arc path, becausethe latter decreases as the arc voltage falls to the systemvoltage. When the current reaches its nominal current zero,the residual arc path still has a low resistance, which thenstarts to increase very rapidly after arc extinction. Fig. 1shows a typical oscillogram of a d.c. current interruption;

Paper 496B, first received 13th July 1978 and in revised form 19thOctober 1979Mr. Morton is with Whipp & Bourne (1975) Ltd., Switchgear Works,Castleton, Rochdale, OLl 1 2SS, England

34

superimposed on it is a plot of the arc resistance. Thenominal current zero has a finite value, which is too smallto be measured on a normal oscillogram; the current thendecreases to zero, and is inversely proportional to theincreasing residual arc resistance.

arc resistance

recovery voltage

arc extinction

contact separation

Fig. 1 D.C. arc extinction

longitudinalinsulatedsplitterplates

arc chute

externaliron circuit

Fig. 2 Final arcing in insulated-splitter-plate arc chute

3 Types of d.c. arc chutes

3.1 General

The function of an arc chute is to increase the resistance ofthe arc, and hence the arc voltage. This is achieved byvarious ways, dependent on the following:

(a) increasing the length of the arc(bi) cooling the arc(c) splitting the arc into a number of series arcs.

Most arc chutes use at least two of these principles andapply them in various ways. There are basically two typesof arc chute, as follows:

IEEPROC, Vol. 127, Pt. B, No. 1, JANUAR Y1980

0143-7038/80/010034 +12 $01-50/0

(a) the insulated plate type that relies on stretching andcooling by means of refractory plates

(b) the bare-metal-plate or cold-cathode type that splitsthe arc into a number of series arcs, and also cools byconduction into the plates.In all arc chutes, the controlling forces that direct the arcinto the arc chute are provided by the natural electro-magnetic and thermal forces of the arc, supplemented by astrong magnetic field that is generally provided by:

(a) an external iron circuit around the arc chute, usuallyenergised by one of more turns of a coil carrying thecurrent being interrupted

(b) an internal iron circuit in the form of speciallyshaped steel plates which are energised by the arc itself.

Fig. 3a shows schematically the arc in its final arcingposition in a typical cold-cathode arc chute. The controlof the arc from the point of initiation to the position offinal extinction is by means of the runner system, whichis designed to move the arc rapidly along the runners andalways in the correct direction. The two main runners areat their closest points at the bottom of the arc chute withinthe iron circuit, and diverge at an angle up into the arcchute until they become vertical and extend beyond thenotch of the arc chute plate.

At arc initiation, one arc root is on one of the mainrunners, and the other root has to be transferred across thearc chute to the other main runner by means of a tailrunner fitted to the top of the moving arcing contact.

3.2 Insulated-splitter-plate arc chute

The majority of d.c. circuit breakers use arc chutes of thelongitudinal insulated-splitter-plate type, equipped with anexternal iron circuit energised by a coil or coils, either series-connected, shunt connected or both.

Fig. 2 shows schematically the arc on its final arcingposition in a typical insulated-splitter-plate arc chute, inwhich the arc roots are at the top of the runners and thearc is sandwiched between two insulated plates. Arcresistance is produced and increased by stretching the arcbetween the two vertical runners and by cooling into theadjacent refractory plates. The magnetic field whichprovides the driving force on the arc to direct it upwardsinto the arc chute is shown as being provided by:

(a) the series coil, which energises an external U-shapedpole piece of the magnetic circuit

(b) the shunt coils, connected to the same iron circuit,but arranged in series with the runner system in the arcchute.

Series coils produce a magnetic field in the arc chutebefore the contacts open, and act immediately on the arc assoon as it is formed. Shunt coils have to be switched in bythe arc itself as the arc roots run up the runners, andassociated with this will be inherent time delays because ofthe inductance of the switched-coil circuits. Series coils areideal, but because they are always in circuit they carry thenormal circuit current, which for large current ratings limitsthe number of turns which can be provided. High-speedcircuit breakers of this type usually employ series coilsbecause their speed of operation generally requires that thecontacts separate within 0005 s, which leaves little time forshunt coils, connected, say, in series with an arcing contact,to be immediately effective.

Very often, arc chutes of this type obtain increased arcstretching by allowing the arc to bow out of the top of thearc chute. Proximity of earthed metal above the arc chutemay be a problem with such an arrangement because theremay be a large amount of ionised gas generated.

3.3 Cold-cathode arc chutes

These arc chutes comprise a number of bare metal platesarranged at right angles to the length of the arc chute, withspacers between the plates to allow the arc to be split upinto a number of series arcs.1 The anode and cathode rootdrop of an arc on steel plates when carrying high short-circuit currents is approximately 30 V, and for N plates, themean arc voltage in the arc chute if the arc splits completelyis approximately 30 TV V.

steel splitter plates

arc runners

currentmoving arcingcontact inopen position

top of notch.

force on arc-col umn

field producedby arc column

insulated coatingon both sides ofsteel plate

-steel arc-chuteplate in cold-cathodearc chute

I current

arc column

insulatedplatesshieldinglegs ofarc-chuteplates

Fig. 3

a Final arcing in cold-cathode arc chuteb Effect of sucker loop on arc column

IEEPROC, Vol. 127, Pt. B, No. 1, JANUARY 1980 35

The arc-chute plates shown in Fig. 3b are shaped from arectangular plate of steel with a notch extending from thebottom of the plate into the arc chute. The two legs ofthe plate extend down from the notch and are just farenough apart to allow room for the arcing contacts andrunners. To prevent the arc rooting to the legs of the platesbefore the arc reaches the top of the notch, an arc-resistinginsulated plate is fitted across the length of the arc chuteshielding the legs of the steel plates, forming a tunnel forthe arcing contacts and runners. The shape of the steel plateacts as a sucker loop on the arc, drawing it up into the arcchute towards the top of the notch, where furthermovement of the arc splits it up into a number of seriesarcs.

The series arcs are free to move up the steel plates, butcannot emerge from the arc chute because of the insulatedcoating at the top of the plates which limits further travelof the arc roots. Exhaust gases, however, escape throughthe plate spacings, and exhaust splitters fitted in the ventchamber prevent the escaping ionised gases flashing over thetop of the arc chute. The vent chamber finally releases theexhaust gas into the atmosphere, sufficiently deionised tobe harmless.

4 D.C.-traction interrupting performance characteristics

4.1 Normal d.c. system parameters

In general, the selection of a d.c. circuit breaker for aparticular application concerns the type of short circuit tobe interrupted and the speed of interruption required. Forthis purpose, three general classifications are available:

(a) General-purpose circuit breakers: these are circuitbreakers that may or may not interrupt quickly enough tolimit the magnitude of the fault current.

(b) Semi-high-speed circuit breakers: these are circuitbreakers that limit the magnitude of the fault current sothat the cutoff current is reached within 0 03 s from thestart of the short circuit.

(c) High-speed circuit breakers: these are circuit breakersthat limit the magnitude of the fault current so that thecutoff current is reached within 0-01 s from the start of theshort circuit.

Fig. 4 shows typical exponential current/time curves ofperformance for d.c. circuit breakers of the three classes,indicating the magnitude of the initial rate of rise of current(di/dt) likely to be seen by the circuit breaker and thelimiting'effect on the current.

In general, specifications for d.c' circuit breakers con-centrate on the short-circuit requirements only, withoutindicating the requirements for lower values of current. Abattery d.c. system behaves in exactly the opposite mannerto an a.c. system in that the highest short-circuit current,which occurs at the terminals of the battery, is usuallyentirely resistive. The equivalent on an a.c. system isinductive since short circuits on the system usually havepower factors less than 0-2. For a transformer/rectifiersystem, a fault on the d.c. side of the rectifier is inductiveto the same extent as for a fault on the a.c. side; if therectifier is considered to have a negligible impedance, bothsides have the same circuit time constant. For lower valuesof fault current which occur out on the system, the circuitgenerally becomes more inductive on a traction systembecause the impedance of the track is added into thecircuit.

Specifications tend to specify the maximum short-circuit requirements and load-switching requirements,leaving out all values between the two, and it is assumedthat they have the same circuit time constant as themaximum short-circuit current. This is the case with theIEC157 and the BS4752 documents, where the timeconstant is between 001 and 0-015 s.

ANSI specifications, however, recognise that this isincorrect for d.c. application and specify, for general-purpose circuit breakers, an intermediate value of short-circuit current which is more inductive than the maximumshort-circuit current. Semi-high-speed and high-speed circuitbreakers are expected to interrupt their maximum short-circuit currents at two values of inductance. These valuesare usually expressed by inference in the circuit parameterdi/dt, whereby semi-high-speed circuit breakers mustinterrupt fault currents having a di/dt in the range 1-7 to5-0A//US and high-speed circuit breakers in the range 5 to15 A//us- The di/dt of a circuit is always that which occursat the instant t — 0.

4.2 Traction d.c. track parameters

Fig. 5 shows the line diagram for a typical traction sub-station with two rectifiers supplying power to four feeders,each supplying a section of line. There are three positions

200 H

, r a t e of r i s e of cu r ren td i / d t =15x10 6 A / s

1 0 0 -

30 40t i m e , ms

Fig. 4 Interrtrptingperformance curves for d.c. circuit breakers

a General-purpose circuit breakerb Semi-high-speed circuit breakersc High-speed circuit breaker

-rectifier circuitbreakers

-, 1 . T —i main positivef busbar

A /•' / A AK track-feeder

bus section circuit breakerscircuit breaker

Fig. 5 Line diagram of typical traction substation positive switch-board

36 IEEPROC, Vol. 127, Pt. B, No. 1, JANUAR Y1980

in general where fault calculations should be taken intoaccount:

(a) position A: a fault on the d.c. side of the rectifierwhere the rectifier circuit breaker has to clear the faultcurrent fed via the other rectifier in the substation

(b) position B: a fault at the outgoing terminal of thefeeder circuit breaker which is fed by the maximum numberof connected rectifiers

(c) position C: a fault on the track itself, coveringpositions from adjacent to the substation right out to thedistant fault position that the feeder circuit breaker isrequired to protect.

For position A, the current fed through its own rectifier isprotected by the a.c. circuit breaker on the upstream side,whereas the backfeed into the rectifier on the d.c. side isprotected by means of reverse current protection on therectifier circuit breaker. Faults of this type have a magnitudedetermined by the maximum number of remaining rectifierswhich can be connected together at any one time. Becausethis fault is close to the terminals of the rectifier, it is asinductive as the fault on the a.c. input to the rectifier andhas a high di/dt.

For position B, the fault is protected by means of theshort-circuit protection on the feeder circuit breaker. Thefault current here is the maximum fault level of the systemand, again because it is close to the rectifiers, di/dt has itsmaximum value.

For position C, the fault calculations are very complexbecause the source impedance is on the a.c. side of therectifier and the load impedance is on the d.c. side. Thecurrent decreases as the fault point moves away fromposition B, at the same time becoming more inductive, andin general starts to become severe once the fault position is1 to 2 km from the substation, particularly for third-rail

deionised gases

vent chamber

top ofmsulat ionon plates

li mit of arcingtop of notch

bottom ofplates

arc position 3

contactseparation

release oftrip catch

arc position 1

splitter forexhaust gas

2 and 3

air puffersupply

prospective short-circuit current

arc voltage

actual current interruption

system recovery voltage

arc position 2

f inal arc extinction

2 5810 15time . ms

Fig. 6 Typical cold-cathode arc chute

a Schematic section view, showing arc transferb Typical oscillogram explaining arc control during short-circuit

interruption

systems. In this region and beyond, the inductance of thecircuit is mainly due to that of the track itself. However,the short-circuit current can be quite low because of theamount of track resistance in the circuit. The short-circuitcurrent for these distant faults depends on the size andextent of the system, with particular reference to thereturn paths for the current. Normally this is via therunning rails and is therefore dependent on the way inwhich a number of tracks can be paralleled to keep thereturn-rail resistance to a minimum.

5 Design details of cold-cathode arc chutes and circuitbreaker

5.1 Cold-cathode arc chutes and contacts

Section 3.3 dealt with the theory of operation of the cold-cathode arc chute, with particular reference to the shape ofthe plates themselves and the final arcing. This Section isconcerned with the design aspects of the arc chute and itsassociated contacts, explaining the principle of arc controlfrom initiation to extinction, and the effects that thedesign of the arc chute and contacts have on this process.

Fig. 6a shows a schematic section view of a typical arcchute, indicating the design features associated with theplates, runners and contacts. Superimposed on this view isthe arc transfer sequence of a typical short-circuit currentinterruption, which is shown on an oscillogram in Fig. 6bto illustrate the arc-interruption process.

5.1.1 Contacts: For high-speed circuit breakers, the contactsmust separate within 0-005 s after the inception of theshort circuit if the high-speed-interruption classification isto be achieved. The contacts illustrated are of the butt type;they have the shortest time to contact separation becauseof the minimum of mechanical travel. Again, for high-speedoperation this contact has the dual role of main and arcingcontact, thus eliminating the need to transfer current frommains to arcing contacts, with the associated time delays, toachieve a clean transfer.

The combined main and arcing contacts must havesufficient conductivity, even with burnt contacts, to carrythe rated normal current of the circuit breaker, and also tohave a capability of withstanding the erosion associatedwith arc initiation for normal service duties, includingseveral short-circuit interruptions, without detriment totheir basic functions. This can be achieved by the use of asilver/tungsten insert into the butt contact face.

5.1.2 Arc initiation: The arcing contacts are situatedbetween the legs of the metal plates, positioned so that, atarc initiation, the arc is immediately within the magneticfield, so that the arc is drawn upwards into the arc chute(illustrated in Fig. 3b). Because the contacts are of the butttype, they are ideally suited to being designed to blow offfrom the tight loop of current which passes through thesecontacts at arc initiation. This automatically directs the arcup into the arc chute, independent of the sucker-loopeffect from the magnetic circuit. The force on this arcand the moving contacts is in theory proportional to i,2

so that, as the short-circuit current increases, and at veryhigh short-circuit current interruptions, the arc at itsinitiation is directed immediately upwards before its ownarc roots have had time to move onto the runner system.Position 1 of Fig. 6a illustrates this; in practice, the arcvoltage produced by this initiating arc is too great for the

IEEPROC, Vol. 127, Pt. B, No. 1, JANUAR Y 1980 37

contact gap to withstand, so that multiple restriking takesplace until a sufficient contact gap is established. It generallytakes between 0-003 s and 0-005 s to clear this part of theoperation to position 2 and then to position 3 with the arcroots established up in the arc chute.

5.1.3 Arc running: Once the arc roots have moved off thearcing contacts, which are themselves designed to havedivergent tail runners, they must transfer to the runnersystem and then proceed up into the arc chute. To ensurethat transfer is swift, the transfer inductance must be keptto a minimum by keeping the paths in the transfer loop alsoto a minimum, assisted by the electrical connectionbetween the runners and the moving contacts. The directionof current passing through the runners should always besuch that the force acting on the arc root is directedupwards along the runner towards the final arcing positionon the runner. Position 2 in Fig. 6a illustates an intermediatestage in the process, and the direction of the force on thearc near its root is always towards the end of the runner.The angle of the runners and the arc-chute geometry arederived empirically; they are designed to give space for thearc to travel up into the arc chute without causing down-strikes to the runner system.

5.1.4 Final arcing: Position 3 in Fig. 6a illustrates finalarcing in the arc chute; it is at this instant that the arcpenetrates all the metal plates producing the maximum arcvoltage.

As previously explained in Section 3.3, the arc voltage isa function of both the number of series arcs produced inthe arc chute and the voltage drop across the arc rootsthemselves. The voltage drop across the arc root is alsoa variable and is dependent on its current density; thus thehigher the current density, the higher is the voltage drop.The current density is kept high as long as the arc rootis fast moving, and therefore a stationary arc on the metalplates must not be allowed.

Once the arc reaches the top of the notch in the metalplates and actually roots on to the plates themselves, thereis no longer a magnetic field from the plates trying todrive the arc upwards further into the arc chute. Thus thearc penetrates the plates at a speed associated with theimpetus received from the arc-running phenomenon, so thatit is at the instant when full penetration is achieved, andwhile the arc roots are moving at their maximum speed onthe plates that the maximum arc voltage is also produced.However, once the arcs are in their final arcing position,these short arcs are free to move independently of eachother, with a separate existence, influenced mainly by thefollowing effects:

(a) the loop effect of the current in the runner systemproviding a resultant driving force generally upwards intothe arc chute

(b) the arc's own thermal forces providing an upwardsdriving force

(c) the interaction of adjacent arcs whose current is fedto their arc roots through the plates themselves, which actas individual runner systems. Thus, once the arcs are out ofstep with each other in the direction across the arc chute,the interaction forces tend to move the arcs further out ofstep. The phenomenon produced as a result of this is tomake the arcs move up and down and from side to side asthe arcing process continues, doing so in an apparent

38

random manner but with the desired effect of keeping thearcs on the move.

The effect described above in (c) causes the arc voltageto have a drooping characteristic after the final arcingposition is reached. This is considered to be due to theslowing down of the arcs themselves, associated with inter-mittent shorting out of arcs between the plates with newlyestablished arcs at the top of the notch. The latter effect iscaused by an arc between the plates moving downwards andout into the notch and then transferring its arc roots on toadjacent plates. However, the magnetic field is immediatelyre-established by the sucker-loop effect and the arc is drawnback into the metal plates. High-speed cine pictures showthis phenomenon taking place; as time increases, thefrequency of the shorting out of the plates by the arcs alsoincreases, giving rise to a continuing drooping arc-voltagecharacteristic which races the falling current.

5.1.5 Arc extinction: Arc extinction is achieved by control-ling the arc within the arc chute to its final arcing position,always keeping the arc voltage greater than the systemvoltage, so that the resistance of the arc is always increasinguntil the current is forced to zero. Once the arc is in itsfinal arcing position in the arc chute, the arc chute shouldthen be virtually restrike free if the cold-cathode design isbeing used, because the maximum stress across the arcchute occurred at the instant the maximum arc voltage wasproduced, i.e. the instant it reached its final arcing position.From here onwards, if a restrike does occur, it is associatedeither with excessive ionised-gas production in the platesthemselves, or lack of arc control, and would indicate anapproach to a limit of performance with the particularcircuit parameters in use.

Because of the narrow spacings between the plates,which also assists with deionising of the gas, the amount ofionised gas produced is considerably less in the cold-cathode arc chute than in the insulated-plate arc chute.Thus a minimal amount of ionised gas is produced abovethe arc chute, and by means of a suitable vent chamber,designed to minimise back pressure in the arc chute, theresulting exhaust gas can be made harmless. Because thearc chute is virtually restrike free, a minimal amount ofionised gas is produced at the bottom of the arc chute,mainly associated with good arc initiation, which if it doesnot cause a restrike across the contact gap is unlikely tostrike elsewhere if the clearances are not less than those ofthe contacts.

During the final arcing period, the manner in which thearc is reduced to zero depends on two parameters:

(a) the resistance of the arc being introduced into thecircuit

(b) the inductance of the circuit.With the high short circuits associated with traction systems,the inductance in the circuit is provided mostly by thesource itself, having a relatively low time constant, in the0-01 s to 0 02 s region, with a very high di/dt; for d.c. inter-ruption purposes this is considered to be nearer the resistiveend of the current-interruption spectrum. The maximumarc voltage in these conditions is rarely the maximum ob-tainable from the arc chute, because arc extinction isusually achieved before the arc has had time to reach itsfinal arcing position. Sufficient arc voltage, and hence arcresistance, is introduced into the circuit to make thecurrent reduce very rapidly to zero with the small amountof inductance in the circuit. With short circuits associatedwith distant faults, particularly on third-rail systems where

IEEPROC, Vol. 127, Pt. B, No. 1, JANUARY 1980

the time constants may be of the order of 0 1 s, theinductance in the circuit is provided mainly by the track,and the drooping arc-voltage characteristic is clearlydistinguishable on an oscillogram. The arc chute producesits maximum arc voltage and then must continuallymaintain the drooping arc voltage greater than the systemvoltage for extinction to be achieved. It is only when thecurrent has actually fallen to approximately 100 A that anincrease in arc voltage is seen; this is in line with thecharacteristics of arcs in this region of current.

The interruption of short circuits which are highlyinductive will eventually approach a limit of performanceof the arc chute as the inductance is increased.

On traction systems, the introduction of inductance isalways associated with resistance, so that the most severecondition usually occurs when the track inductance swampsthe source inductance, which for most third-rail systems isusually between 1 to 2 km along the track. The arc chutemust be designed to operate under these conditions, and foran optimum design to be obtained it is necessary todetermine by tests the correct number of plates to be usedin the arc chute to meet this condition, and then at the fullshort-circuit current to ensure that the arc voltage which isproduced does not cause a restrike.

5.1.6 Air puffer: At very low values of current, when themagnetic field from the iron circuit is very weak, it isdesirable that the movement of the arc up into the arcchute should be assisted by external means, usually in theform of an air puffer. This is the accepted practice with a.c.air-break circuit breakers, and works equally well on d.c.Low currents which produce long arc durations are in theregion of 10—200 A. Such currents can occur on tractionsystems associated with the auxiliaries of a stationary train.A particularly severe interrupting condition can occur withtrack-paralleling circuit breakers and feeders under certaincircumstances, namely those in which the circuit breakercan interrupt bidirectional currents. The worst case is theinterruption of a low current by a circuit breaker whichpreviously interrupted a high current in the oppositedirection. The residual magnetic field in the iron circuit ofthe arc chute from the high-current interruption produces aforce on the subsequent low-current arc in the downwarddirection, which could result in a failure to clear. The airpuffer must be capable of satisfactory operation in thesecircumstances.

5.2 Circuit breaker on a withdrawable truck

Fig. 7 shows a side elevation of a single-pole 750 V high-speed d.c. circuit breaker fitted with a cold-cathode arcchute. The circuit breaker is on a withdrawable truck whosepedestal supports a panel-mounted circuit breaker. On thefront of the panel is the mechanism, contacts and arc chute;at the rear are mounted the direct-acting short-circuitrelease and the main isolating contacts. The release isarranged with a tripping rod through to the front where thethe main trip catch is situated. Also illustrated is the meansfor manual closing of the mechanism for emergency andmaintenance purposes; this feature is available for closingon to a live circuit, since no blow-off forces are taken onthe closing handle for this particular design.

The circuit breaker employs a live-frame mechanism, sothat all secondary connections to the mechanism, i.e. coils,auxiliary switches etc., must be insulated to the fullwithstand voltage. An advantage of this type of design is

the complete removal of any earthed metal from the regionimmediately below the arc chute.

For ease of maintenance, the arc chute is arranged to tiltbackwards for contact examination, and the front of thecircuit breaker is kept clear of all secondary controls, relaysetc., giving easy access to all moving parts.

Fig. 7 Withdrawable high-speed d.c. circuit breaker

front contactrear contact

closing andtripping spring

return toclose contacts

spring charging &"latch resettingforce

tripping force

Fig. 8 High-speed tripping mechanism linkage with latch and tripcatch

5.3 Closing and tripping mechanism

Fig. 8 shows a schematic illustration of the high-speedtripping mechanism with latch and trip catch.

The circuit breaker has two moving contacts, identifiedas front and rear, and uses a combined closing and trippingspring. The effect of closing the circuit breaker is first toenergise the solenoid, which charges the closing spring,resets the front contact together with the latch and the trip

IEEPROC, Vol. 127, Pt. B, No. 1, JANUAR Y 1980 39

catch and at the same time moves back the rear contact inpreparation for closure. When the solenoid de-energises andresets, the rear contact follows the return of the solenoidunder the action of the closing spring, and the contactsclose. The tripping action first releases the trip catch, whichin turn releases the latch. The closing springs now becometripping springs and pull the front contact away from therear contact at a high speed. The rear contact is preventedfrom following the front contact by means of a stop in themechanism. High-speed opening is achieved because thehigh spring tension moves only the relatively light mass ofthe front contact. Because no toggles are employed in thetrip linkage, there are no time delays associated withcollapsing toggles, and a mechanical opening time of 0-003 sis achieved. This must be added to the operating time of theshort-circuit release, which is series connected and actsdirect onto the trip catch. For short circuits having valuesgreater than 10 times the release setting, the releaseoperating time can be as short as 0-002 s, giving a circuit -breaker opening time of 0 005 s.

5.4 Cubicle

Fig. 9 shows a side view of the cubicle which takes thewithdrawable circuit breaker described in Section 5.2.

The cubicle is of the metal enclosed type, having separatecompartments for each of the following:

(a) the circuit breaker(b) the positive busbar(c) the circuit connections(d) the front box, with mountings for relays, indication,

and control switches(c) the rear box with mountings for special relays,

contactors etc.Because the circuit breaker fitted with a cold-cathode arcchute produces a minimal amount of ionised gas emitted in aharmless manner, the design is ideally suited to its use in ametal enclosure for voltages up to 3 kVd.c. Cubicle sizes of500 mm widths have been tested and are in service forvoltages up to l-5kVd.c, where the normal current ratingof the circuit breaker is 4000 A. For 3 kV applications, thewidth is increased to 600 mm to meet specified increasedclearances.

Clearances inside cubicles for traction applications aregenerally very generous, and very often these are specified

front rela\

B000000

0] ° ° GO

a

D »©

r box

\ :\ |

c0

(

[

vent

r — T

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-

±7fi

JrL

•

chamber

rear relay box

^ , rear auxiliarychamber

- shutters^^ busbar

chamber

^-" circuitchamber

withdrawable^^~ circuit breaker

Fig. 9 Metal enclosed cubicle for high-speed d.c. circuit breaker

by the railway authorities in the region of 75—100 mm for750 V operation. Historically, these are figures found togive satisfactory operation, derived from experience withstretching-arc type of arc-chute circuit breakers, of whichsome designs, particularly those with open-top arc chutes,do emit large amounts of ionised gas. Unfortunately, norecognition is given by railway authorities and their con-sultants to the special advantages of the cold-cathode arcchute to allow the switchgear manufacturer to reduce thesefigures, particularly in areas unaffected by ionised gases, toproduce compact equipment on the lines adopted with a.c.switchgear.

In the United States, one manufacturer, whose circuitbreaker uses a cold-cathode arc chute at 750 V, mounts it ina totally enclosed compartment, confirming that anothermanufacturer has made use of the advantages described inthis paper.2

In many parts of the world there are railway authoritiescompletely prejudiced against the use of d.c. cubicleswitchgear; their past experience has led them to thisconclusion. In general, one finds that 1-5 and 3kV equip-ments are of the open-cell design, very often with open-type rear connections, the operator being protected by adead front cover mounted on a very large removable truck.There are other users of 750 V d.c. equipments who will notentertain anything other than panel-mounted circuitbreakers of the back-of-board type. Others operate theircircuit breakers in the local position only through 10 m ofumbilical cord with the control switch on the end.

In the United States in particular, it is standard practiceto mount all cubicle d.c. switchgear on insulated floorsand apply earth-leakage detection equipment to the switch-boards or to sections of the switchboard, and in some casesto individual panels on the switchboard.

If the cubicle equipment is fully tested for the knownsevere interrupting conditions, and during these tests thecubicle earth metal is stressed by the system voltage and arcvoltage in just the same manner as with a.c. circuit breakertesting, then the need for insulating the switchboard fromearth should only be for the same reasons as with a.c.switchgear. A.C. switchgear is rarely insulated at thesevoltage levels. In the UK, the practice is to earth all metalenclosures solidly for d.c. traction.

6 Applications to types of circuits

6.1 Rectifier of main circuit breakers

The rectifier circuit breaker should, in general, be rated inaccordance with the output of the transformer/rectifierfeeding it, for normal, overload and short circuit.

Traction transformer/rectifiers have special overloadcharacteristics to cater for the stopping and startingconditions of trains during rush-hour periods, and for thecontingency of one transformer/rectifier being out ofservice. Typical US requirements would be for one alreadyrunning at full load to be capable of increasing to 150% offull-load current for two hours, during which are super-imposed cyclic overloads consisting of five periods of 300%full-load current, each of one minute duration, equallyspaced throughout the two-hour period, and followed byone period of 450% full-load current for 15 s at the end ofthe two-hour period.

The overload I21 is equivalent to approximately at 160%full-load current for two hours, and so this calculation canbe used for determining the rectifier-circuit-breaker normal

40 IEEPROC, Vol. 127, Pt. B, No. 1, JANUARY 1980

current rating, taking into account the termal time constantof the circuit breaker itself. In practice, because the thermaltime constant of the circuit breaker is fairly short (say45min), an approximate rating of 150% full-load currentis a good guide to the selection of the rectifier-circuit-breaker normal current rating.

Rectifier circuit breakers connect the d.c. supply to themain busbars. They are not required to discriminate withthe track-feeder circuit breaker and are therefore not fittedwith short-circuit protection in the forward directions fortwo reasons:

(a) Because both the rectifier and feeder circuit breakersare of the series trip type, they are unable to discriminatewith each other on short circuit currents.

(b) Because only the positive busbar is mounted in theswitchgear, the negative being mounted elsewhere, thebusbars are normally considered to be in a fault-free zone asfar as high short circuits are concerned.

Thus the positive busbar, the d.c .-rectifier circuit breaker,the rectifier and its transformer are all protected by thea.c. circuit breaker in the forward direction. To meet such arequirement, it is usual to assign to the rectifier circuitbreaker a making and. short-time current rating the same asthat of the rectifier itself. A making peak of 20 times therectifier full-load current and a short-time steady-statecurrent-carrying rating of 12 times the rectifier full-loadcurrent for a time of 0-25 s should be sufficient. However,rectifier circuit breakers are fitted with reverse-currentprotection (to cater for faults on the rectifier itself) whichwill be fed by the other rectifiers and feeders connectedto the busbars. The release need only be set at 50% of thecircuit-breaker normal current rating, for the reasondescribed in Section 7.2. The fitting of reverse currentprotection is a hangover from the days of mercury-arcrectifiers, which were prone to backfiring. However, withmodern silicon rectifiers, which have a reliable servicerecord, the need for reverse-current protection is nowadayssometimes questioned. If it is not required, then the use ofa rectifier circuit breaker is also questioned, since if noprotection is fitted and remote control is not needed, thenthe use of offload isolators can be considered. Tractionsystems of French origin use this method and thereby lose acertain amount of flexibility in control.

6.2 Track-feeder circuit breakers

Track-feeder circuit breakers should be rated in accordancewith the overload characteristics of the track itself. Thecomplexity of the running conditions is associated withstarting trains close to the substation and at the far end ofthe track, together with the known number of trainssupplied by a track feeder at any one time and thefrequency of service. All these factors are taken into accountin assigning the normal current rating.

Normally, the short-circuit protection is provided by abidirectional direct-acting overload release withch gives thehigh-speed operation. These releases, in general, have a 2:1calibration range, of either 100 to 200% or 200 to 400% ofthe circuit-breaker normal current rating set at a valuegreater than the starting currents of the trains, and arebasically required for short-circuit protection only.

The fitting of a bidirectional release is technicallyincorrect, because the feeder circuit breaker should berequired to trip in the forward direction only. Any tripresulting from carrying current in the reverse direction mustbe due to the supply of fault current into an adjacent track

IEEPR0C, Vol. 127, Pt. B, No. 1, JANUARY 1980

fault. A unidirectional release would prevent this and leavethe healthy track still in service. In practice, the componentof fault current fed from an adjacent substation is usuallynot high enough to operate the release in the reversedirection; hence the almost universal use of the bidirectionalrelease, with its inherent simplicity.

Additional protection, known as rate-of-rise protection,is sometimes provided to cater for special fault conditionsassociated with those that occur at a distance down thetrack. The need for such a protective system occurs whenthe distant fault condition has a value of current comparableto or less than the highest train-starting current on thesystem, and the short-circuit current release cannot be setto distinguish between the two. In general, once substationsare spaced more than 2-5 km apart on third-rail systems,one can encounter this problem, particularly during peakperiods where maximum loading conditions apply.3 It is ofinterest to note that rate-of-rise protective systems must beunidirectional for discrimination purposes, because of theirlower levels of operating current.

6.3 Bus sections

Normally these are only provided on switchboards wherethere are more than two rectifiers, so that additional safe-guards and flexibility can be achieved. Splitting the busbarsalso reduces the fault level on the system, and when therectifier is out of service the closing of a bus section cangive a near-normal running condition. The bus section alsoallows one half of the switchboard to be shut down duringthe operating period, for maintenance purposes. Becausemost rapid-transit systems are not required to provide acontinuous service, maintenance on the busbars is doneduring the night when the trains are not running, or intimes of light loading when the tracks can be energised bythe feeders at the far end.

Bus sections are normally offload isolators, but some-times are nonautomatic circuit breakers when remotecontrol is considered necessary.

6.4 Track-paralleling circuit breakers

These are circuit breakers normally equipped in a verysimilar manner to the track-feeder circuit breaker, but areused to parallel sets of tracks. Sometimes they are housedin a substation situated at the junction between two sets oftracks, or they may be sited in the main substation toprovide an alternative supply to a track when its own trackfeeder is out of service.

Careful consideration must be given to the type of over-load release fitted to the circuit breakers. In most instanceswhere they are sited at a track paralleling substation,between main substations, the release should be uni-directional to avoid tripping all the circuit breakers at thetrack-paralleling substation because of a fault on one ofthe feeders.

7 Overcurrent releases

7.1 Bidirectional short-circuit current release

The basic short-circuit current release fitted to most feeder-circuit-breaker applications is a bidirectional device in itssimplest form (see Fig. 10a), comprising an attracted-armature magnetic circuit around the main current-carryingpath on the circuit breaker. Connected directly to thearmature is an operating rod which acts directly on the

41

circuit-breaker trip catch to give instantaneous high-speedoperation. The armature is held open by means of acalibration spring whose force is overcome when theprimary current reaches its setting. Usually the spring canbe adjusted to give the release a 2 to 1 calibration range,which is suitable for most practical applications. Whenhigh-current calibrations are required, the ampere-turnsavailable to operate the magnet can be excessive, and it isthen common practice to fit a bypass connection across theprimary circuit of the release to shunt most of the highcurrent away from the magnet (see Fig. 106). When thismethod is used, a magnetic choke is usually fitted aroundthe bypass connection to prevent all the short-circuitcurrent taking the bypass route when the fault current has ahigh di/dt. Releases of this type can be made sensitive todi/dt by adjusting the airgap in the choke; the less sensitivethey are to the value of di/dt, the slower the releaseoperates on short circuit. It is quite often normal practiceto determine the airgap setting of the choke by means offield tests.

cali brat ionspring

\

attractedarmature

magnet

primary conductor

t r ip rod

overcurrent release choke w11 hfixed ; ir gap

bypass connectionin para llel withrelease connection

• trip rod

Fig. 10 Bidirectional overcurrent release

a Basic deviceb Device for high current settings

7.2 Unidirectional release

The basic unidirectional release is the reverse-current releaseused on most rectifier-circuit-breaker applications. Fig. 11shows one type of design for this application. Essentially,the magnetic circuit comprises two U-shaped magnetswhose open ends face each other and an armature issituated between the pole faces. Both magnets are fittedwith polarising coils, connected in series so that the polepieces facing each other are of opposite polarity. Aroundthe armature is fitted a hairpin primary conductor, whosenormal current direction gives the armature pole pieces apolarity biased in the direction against the stop, onto whichthe armature is held by means of the calibration spring.When the primary conductor carries current in the oppositedirection, the polarity of the armature pole pieces willreverse and the armature will be attracted away from thestop at a predetermined value of current and will trip thecircuit breaker.

When used as a reverse-current release, settings of theorder of 10% of the circuit-breaker normal current ratingcan be obtained with this design. Because a reverse-currentrelease must not operate with currents in the forwarddirection, the design must cater for this condition. Thehigher the reverse-current setting, the more reliable therelease, because the ratio of forward current to releasesetting is lower. A setting of 50% is generally satisfactoryfor use with silicon rectifiers, and a forward-to-reverseratio of 26-7 times would meet the requirements.

An external supply is necessary for use with the release,and it is common practice on 750 V systems to use the750Vd.c. system voltage as the supply for the polarisingcoils. However, if this voltage varies, the setting of therelease also varies and can give an increased setting for afalling voltage. Normally this is of little concern when usedfor reverse-current protection. This particular design willstill operate even if the polarising voltage is removedcompletely, in which case it reverts to a bidirectionaloverload in a failsafe mode.

When using this system as a unidirectional short-circuit-current release, it is normal to connect the polarising coilsto a guaranteed stabilised d.c. supply to maintain theoverload settings.

For high-normal-current-rating circuit breakers fittedwith reverse-current releases, the bypass connection canalso be used in the same manner as described in Section 7.1.

-polarising coilsd.c.supply n

stop - j l

N N

hairpinprimaryconductor

trip rod

Fig. 11 Unidirectional release for reverse-current protection

7.3 Falling voltage unidirectional overload release

This type of release has the advantage that the setting ofthe release decreases as the voltage to the polarising coildecreases, as opposed to the case of the release described inSection 7.2. Its principle of operation can be seen in theillustration in Fig. \2a. The overload comprises a U-shapedmagnet with an armature held on to the pole pieces by theflux produced from the ampere-turns in the polarisingcoil. A calibration spring applies tension to pull the armatureoff the pole pieces and thereby trip the circuit breakerdirect. The hairpin primary conductor is inserted into thewindow of the magnet, arranged such that when carryingcurrent in the forward direction the flux from the hairpinconductor opposes the flux from the polarising coil in thearmature, diverting it through the path with the airgapbetween the hairpin, so that at the required setting thespring releases the armature and the circuit breaker istripped. For currents in the reverse direction, the flux fromthe current in the limb nearest the armature assists inholding the armature in position, and tripping is restrained.

Because the armature is electrically held, the cancellationeffect from the primary conductor must reduce when the

42 IEEPROC, Vol. 127, Pt. B, No. 1, JANUARY 1980

polarising voltage is reduced. The characteristic curvesshown in Fig. 126 are typical for such a release. This over-load also acts as an undervoltage release, as in all suchelectrically held devices.

The polarising voltage is always drawn from the d.c.system voltage, so that if, for operational reasons, thevoltage on the track is reduced, an automatic compensationis obtained by the reduction in the overload setting; this isjust the requirement wanted when the voltage does drop. Afurther advantage with such a device is that circuit breakersact as track-tie or track-paralleling duties at locationswhere there are no rectifier infeeds or where rectifierincomers are out of service. In these cases, when a distantfault occurs, the fault current could pass through twocircuit breakers in series. Because of the voltage gradientout to the fault, the circuit breaker nearest the fault seesthe lower voltage and hence the lower setting. If bothcircuit breakers have the same release settings then thatnearest the fault will trip first.

8 Mechanical and electrical design requirements

8.1 Mechanical and electrical endurance

Most specifications recognise the need for mechanicalendurance testing of a complete circuit-breaker equipmentand for circuit breakers in the normal current-rating rangeof 2000 to 6000 A. They would be expected to be typetested for at least 8000 mechanical operations as for theANSI specification. The mechanism described in Section 5.3has been type tested for 10000 mechanical-endurance

trip rod

rj-v'W'—cal i brationspring

t

//

ulairpm primaryconductor

\

1 ^"^*-^"• polarisingcoil supply

800-

600

2 400-

200

10

up to 750 Vd.c:

up to 1500 Vd.c:up to 3000 V d.c:

Ffg. 12 Falling-voltage unidirectional overcurrent release

a Schematic diagramb Characteristic curves

operations to ensure an adequate safety factor for each ofthe frame sizes.

Electrical endurance is the switching of rated normalcurrent for 250 times, again as specified by ANSI. Becausethese tests require rated current and rated voltage at thesame time, one must resort to field tests or testing stationsfor this information. A 750 V 4000 A circuit breaker hasbeen tested in service for 200 operations at 10 times thefull load current, which is equivalent to a duty well inexcess of these requirements.

8.2 Normal current

British and IEC standards do not put a temperature-risefigure on contacts for this requirement, other than that itshall not cause damage to adjacent parts. For this reason,the circuit breakers have been assigned their normal currentratings to the ANSI standards where an 85 °C rise is statedfor silver-plated contacts, joints and conductors.

8.3 Withstand voltage

For equipment rated up to 3 kV d.c no impulse-test require-ments are specified in any of the standards. Only an a.cwithstand voltage level is specified, and these are generallyas follows:

withstand voltage 4-5 kVa.c BritishRail (SR)withstand voltage 5 -4 kV a.c. (ANSI)withstand voltage 8-8 kV a.c. (ANSI)

Equipment connected to a catenary system can be sub-jected to lightning strikes, and, for this reason, surgearresters would be fitted to the feeder connection to protectthe switchgear and rectifiers.

8.4 Short-circuit currents

The high-speed d.c. circuit breaker should be capable ofswitching all values of current on a traction system, i.e.from switching off a stationary train running the minimumof auxiliaries (the low current) up to a bolted fault as-sociated with the rated short-circuit current. Discriminationmust be made between third-rail and catenary systems; thetime constants of the former can be very severe. Nostandards exist which specify these parameters, and onlya few traction authorities give any indication of the re-quirements. Short-circuit tests for fault conditions on thetrack are dealt with separately in Section 9, and additionalrequirements are stated below.

Low-current switching occurs in practice when switchinga lightly loaded track, usually associated with a depotfeeder, where trains may be standing idle with theirauxiliaries as the only load. Air-break circuit breakersgenerally find 10-200 A to be the currents which give thelongest arc durations, and this is of increasing importancefor the higher-voltage systems where train load currents cancome into this range.

At the rated short-circuit current, high-speed circuitbreakers do not have to meet a make-and-latch or a short-time current-carrying requirement, because they are seriestripped. Only a rectifier circuit breaker requires a short-time current-carrying rating because its series trip is in-operative in the forward direction. This rating need only bebased on the rectifier output, which is a duty of eight timesthe circuit-breaker normal current rating for 0-25 s. Thistest should be performed with forward current, and thereverse-current release set at 50%. The peak-to-steady-state

IEEPROC, Vol. 127, Pt. B, No. 1, JANUARY 1980 43

current ratio should be 1-66, for the reasons stated inSection 6.1. All frame sizes of circuit breakers have beentested to show compliance with this recommended re-quirement.

Because the mechanism described in Section 5.3 mustlatch before the contacts can actually touch,its 'close-open'performance is identical to its 'open' performance whenfitted with an attracted-armature release. This makes itvery much easier to perform short-circuit tests without theneed to do the 'close' part of the duty.

9 Short-circuit testing of high-speed circuit breakersfor traction application

From the previous Sections it is clear that the interruptingrequirements of high-speed d.c. circuit breakers used fortraction applications are very dependent on the system inwhich the circuit breaker is being applied. Therefore, todetermine a standard on which a series of proving testsshould be based has been very difficult. Existing standards,e.g. IEC 157 and BS4752 for low-voltage circuit breakers,recognise this, and they state that special considerationmust be given to traction applications. In fact, circuitbreakers tested to these standards alone may not be com-pletely suitable, because the time constant specified is only0-01 to 0-015 s. For this reason, traction authoritiesthroughout the world have found that their only methodof accepting equipment has been to try that equipment ontheir system. The more knowledgable authorities, usuallythose with large systems, have generally required andspecified an extensive series of proving tests. These are splitinto two categories:

(a) short-circuit tests at testing stations(b) field tests

9.1 Short-circuit tests at testing stations

Short-circuit-testing stations have the best facilities forperforming short-circuit tests, because they are able to varythe parameters, obtain the correct settings, recordaccurately, control, provide independent certification ofcompliance, and finally to test in safety. Unfortunately,such d.c. testing stations are few and far between particularlywhere high fault levels are required. Test made at testingstations can also determine safety factors to prove maximumdesign figures for the system. Normally, if tests are made at

the calculated figures, these in themselves give safetyfactors, because certain losses are ignored in calculations.

9.2 Field tests

These are made on the traction authority's system, at asubstation which gives the maximum severity. During thesetests, the system is put at risk if a failure of a test apparatusoccurs.

These tests usually take the form of applied short circuits,starting just outside the substation, and then applied atintervals out along the track to the most distant point thatthe circuit breaker is required to protect. At each point outfrom the substation the fault current becomes less, but thecircuit becomes more inductive, until the circuit timeconstant approaches that of the track itself.

The problem usually encountered in field testing relatesto the oscillographic recording of the tests. The sophisticatedcontrol equipment available in testing stations is tooexpensive to use on, say, a one-off operation. The siteconditions, namely system earthing, location of primaryequipment and point of fault, are so spread out and themeasurement leads, so prone to pick up that some of thevalues of the measurements can be suspect. However,whether the circuit breaker operated and cleared the circuitis not difficult to determine.

If the traction authorities could accurately predict theparameters of their system, and specify these values, thereis no reason why all proving tests should not be performedin a testing station.

9.3 Recommended testing procedure

A complete proving series can be made in a testing station ifthe following data is known:

(a) The maximum short-circuit level of the system: sincemost modern systems use solid-state rectifiers, there will bea peak short-circuit current followed by a steady-stateshort-circuit current. The time constant for the latter mustbe known to establish the source values.

(b) The track constants must be known, in resistance/kmand inductance/km, together with the maximum length ofprotected track.

A duty cycle should be made, at each value of shortcircuit, to represent the fault conditions for various tracklengths up to the track length which gives the maximumtime constant.

Table 1: Test schedule of circuit values for d.c.-traction circuit breakers

Maximum

designvoltage

V800800800800

160016001600

320032003200

System

Third rail

catenary

catenary

Prospective values of short-circuit current

Peak

kA200__

-

100_

-

50_

-

Steady-state di/dt(60Hz) di/dt (50 Hz)

kA120

801910

6021

9

3024

9

A/MS36-8

1-70-20-1

18-41-20-4

9-22-80-35

A/MS30-7

1-40-20-1

15-41 00-4

7-72-20-33

Time const.

ms•

50-100

108

_ •

2122

»

2026

Added values of track to circuit

Length

km—

0-11 02 0

_

1 03 0

_

1 01 0 0

Resistance Inductance

—3-5

3570

._

50150

_

25250

mH—0-448

_

1-123-36

_

0-757-5

Time const.

ms- ( i )115115115

- ( i i )22-522-5

- ( i i i )3030

Note:- Above values are based on 8mW of source power at p.f. = 0-16 on 60 Hz and 50 Hz systems*Time constants cannot be measured for zero track length

(i) British Rail (Southern Region)(ii) Tyne and Wear PTE

(iii) South African Railways

44 IEEPROC, Vol. 127, Pt. B, No. 1, JANUARY 1980

The duty cycle should be either o-3M-co-3M-co or theautorelease duty cycle of the system.

Test figures for typical systems are shown in Table 1,where recognition is given to the basic types of systems(namely third rail or catenary systems) because the charac-teristics of each are quite different. The Table is meant asa guideline, and the basis of the values are indicated.

10 Service and field-test experience

The cold-cathode arc chute of the design described in thispaper has been in service on a.c. systems since 1963 at3-3kV a.c. and since 1965 at 6-6kV a.c. Over 1000 are inservice and no problems have occurred which can beattributed to the design of the arc chute. The d.c. versionsof the arc chute are almost identical, differing only indetail concerning the fixings, the supports and the con-nections to the arc runners. The high-speed circuit breakerusing the mechanism described in Section 5.3 was designedin 1936 as a pedestal-mounted circuit breaker, when it wasfitted with an insulated-plate arc chute. In 1957, a l-5kVcubicle version was supplied with the circuit breaker on awithdrawable truck.

In 1970, experimental tests using the cold-cathode arcchute on d.c. circuits were first tried out at the British Rail(Southern Region) substation at Wimbledon, clearing faultsup to 70 kA at 750 V.

In 1974, the cold-cathode arc chute replaced theinsulated-plate arc chute with its series and shunt blow-outcoils. This resulted in a considerable reduction in cubicledimensions, for the reasons mentioned in Section 5-4.

In 1975, short-circuit tests were made at the KEMAtesting station in Holland at its maximum outputs of 84 kAat 1 -5 kV and 142 kA at 750 V.

First, service experience was gained on the 1 -5 kV Tyneand Wear PTE test track, on which valuable serviceexperience was gained for the full-scale Metro. Because ofthe low power rating at the test track, short-circuit testingwas confined to the range 600 A-to 15 kA. Low-currentswitching tests have also been made on the system, withswitching current values from 15 A up to 400 A. Currents inthis range were from the auxiliaries, the starting and therunning currents of one- and two-train conditions. The finaldesigns of the contacts and air puffers were evaluatedduring these tests.

During the period 1975—79, extensive field tests havebeen made by British Rail (Southern Region) at varioussubstations in their region to cover all types of faults, fromclose-up to distant faults.

These were in the range lOkA to 50 kA at 750 V whentested at the Croydon substation and 7kA to 12 kA at750 V at the Epsom substation. The latter tests were madeto cover the long-distance fault requirements where timeconstants up to 012 s were achieved.

In 1975, the London Transport Executive tested thecircuit breaker at their Tower Hill substation for shortcircuits in the range lOkA to 40kA at 660V, and sub-sequently a large number of circuit breakers was used toreplace very old stock throughtout the system.

British Rail have been testing a circuit breaker at FinsburyCircus to evaluate its capability of handling a large numberof short-circuit operations. To date, nearly 1000 shortcircuits of approximately 40 kA prospective have beeninterrupted by the circuit breaker, with the contacts andthe arc-chute liners generally changed after every 200operations.

In 1977, a 3 kV version was tested on the South AfricanRailways for the maximum fault condition of 37 kA at4000 V, followed by service experience as a feeder-circuitbreaker which was still in service in 1979.

In 1978, short-circuit tests were made at the Gould Inc.Electrical Testing Laboratory, Pa., USA, on three framesizes of circuit breakers complete with their cubicles —2000 A, 4000 A and 6000 A - for an interrupting rate of201 -7 kA peak, di/dt = 368 A/ns at 800 V d.c.4

The equipment described in the paper is currentlyinstalled or being installed at the following locations:

Tyne and Wear PTE test track and final MetroIllinois Central Gulf Railroad, USABritish Railways BoardLondon Transport ExecutiveMexico City SubwayWashington WMATANetherlands Railways, HollandSouth African Railways

11 Conclusions

The paper shows that, in applying the principles of operationand design of cold-cathode arc chutes used in modern a.c.air-break circuit breakers to d.c. circuit breakers equippedwith a high-speed mechanism, a development has beenachieved for their use on all traction systems. Because theamount of ionised gas generated by circuit interruption isminimal compared with that produced by the stretching-arc and the insulated-plate types, the design of metalenclosed switchgear useful up to 3kVd.c. has beenachieved. A range of switchgear whose design andconstruction possess all the features which are accepted asnormal practice on good a.c. equipment has therefore beendeveloped and is giving satisfactory service.

The production of such a design has been made possibleby an understanding of the d.c. traction systems themselvesand by the co-operation of the traction authorities whosewillingness to allow field tests on their systems has allowedthe author to collect the necessary data.

12 Acknowledgments

The author is indebted to the directors of Whipp and Bounre(1975) Ltd. for permission to publish the paper; to manycolleagues for their contributions, particularly J. Stillie,Chief Development Engineer, who has been responsible formuch of the design work; the Foster Transformers Ltd.,and many traction authorities and their consultants both inthe UK and the USA for their valuable advice and criticism.

13 References

1 FLURCHEIM. C.H.: 'Power circuit breaker theory and design'(Peter Peregrinus Ltd.) ch. 5, p. 189 I

2 BRANDT, T.F., and NETZEL, P.C.: 'New circuit breakers formodern d.c. traction power systems' ITE Power EquipmentPubl. SPC-MON-3, pp. 605-610

3 WATKINS, S.S., and REGAN, M.E.: 'DC power supplies andisolation of faults on electric transit systems —part II: Isolationof dc faults', Proc. Am. Inst. Elec. Eng., 1951, 70, (T), pp. 1-91

4 'High speed circuit breakers meet US standards', Int. Railway. J.,1978, November, pp. 48-49

IEEPROC, Vol. 127, Pt. B, No. 1, JANUARY 1980 45