Rectifiers for railway-traction substationsF. T. Bennell

Indexing terms: Railways, Rectifier substations, Solid-state rectifiers, Traction

Abstract: This is a general review of the present state of development of silicon rectifier equipment for thesupply of d.c. for railway traction. Current general practice is described and reference is made to the latestdevelopments, including compensators for paralleling double-bridge rectifiers, and capsule diodes.

1 Introduction

D.C. supplies to railways are provided by rectifier equip-ment in substations distributed along the track. The basicrequirements have remained the same, dating back to themercury-arc era,1 but, particularly since the advent ofsilicon-diode rectifiers, there has been, and continues to be,substantial progress in rectifier equipment design.

The direct voltages commonly in use are nominally750 V, 1500 V and 3000 V. The voltage level affects thebalance of factors that determine the optimum basic design.The 750 V level is the most widely used and this paper iswritten generally for this, with references to variationsarising from the use of higher voltages.

The simplest form of circuit for this duty is a 3-phasebridge, giving 6-pulse rectification. Its main disadvantageis the relatively high ripple in the d.c. at six times funda-mental frequency, increasing from about 4% r.m.s. at noload to, typically, 6% at full load and higher for overloads,due to increasing overlap. This can cause interference withsignalling circuits which are running in parallel in closeproximity along the same route. The 5th and 7th harmoniccurrents drawn from the supply by 6-pulse rectifiers arealso undesirable as, in addition to other effects, they cancause unbalance in 12-pulse rectifiers.1'3

2 12-pulse rectification by parallel bridges

6-pulse ripple and 5th and 7th harmonics can be avoided byhaving two phase displaced 6-pulse rectifiers operatingtogether, the combined effect corresponding to 12-pulserectifier equipment, provided of course both sets of equip-ment are always in operation together. This can be devel-oped further by combining the equipment into a commonbank and the transformers into a common tank, or twosecondary windings on a common transformer.

A combined rectifier giving 12-pulse operation can haveanother important effect. If a common transformer isused, with two secondary windings, one star and one deltafor a 30° phase displacement, and these secondary windingsare closely coupled, for the same direct voltage regulationthe short-circuit current is halved. This merits carefulexplanation as it is a point that has been brought up in IEEdiscussions, apparently without generally understoodclarification.

Considering that part of the d.c. regulation (the mainpart) due to reactance, a rectifier is required to have a 5%regulation, for example 750 V on full load and 789 V on noload. An individual 6-pulse rectifier would require a re-

Paper T298 P, first received 14th September and in revised form 26thOctober 1978.Mr. Bennell is with Foster Transformers Ltd., The Path, MordenRoad, London, SW19 3BN, England.

22

actance of 10%, and the resultant short-circuit currentwould be a nominal ten times full load.2

With a common transformer and two secondary windingsthe required reactance is 10% for the load of each second-ary, the other bridge circuit involving the other secondarynot commutating at the same time. This gives a nominal20%* if the load on this one secondary is increased untilfull primary current is taken, this being the way reactanceis measured for the determination of d.c. regulation. Hence,the relationship that the percentage regulation is a nominal0-25* times the percentage reactance for 12-pulse operationcompared with 0-5 times for 6-pulse.

Short circuit involves both secondary windings, henceshort-circuit current depends upon the total reactance. Ifthe secondary windings are not magnetically coupled, i.e.do not share the same flux, the reactance with both second-aries short-circuited decreases nominally to a half, due todoubling the flux linkages between primary and secondaryby involving the second secondary winding, making the re-actance a nominal 10%. If, however, the secondary windingsare closely coupled they occupy the same envelope volumewhether one or both are involved and the reactance is thesame for both conditions, i.e. nominally 20%, and the short-circuit current is nominally only 5 times full load, as shownin Fig. 1.

This reduction in short-circuit current and its resultantrate of rise are a considerable help in enabling the rectifierdiodes and their fuses to withstand short circuits. It alsohelps the d.c. circuit breakers and reduces the stress on theequipment and the damage where the short circuit occurs.

The point is that this reduction in short-circuit currentis not achieved by all 12-pulse rectifier arrangements but

Fig. 1 Output characteristics of 6-pulse and 12-pulse rectifiers

a 12-pulseb 6-pulse

Strictly speaking, 19-3% and 0-259, respectively, as the cancel-lation of the 5th and 7th harmonics in the primary winding reducethe primary current and kVA on which the percentage reactanceis based, by 3-4%.

ELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. 1

0140-1327/79/010022 + 05 $01-50/0

only when the two transformer secondary windings areclosely coupled in a common flux circuit.

Associated with this desirable effect is, however, adesign difficulty. To the extent that the leakage flux andtherefore reactance is common to the two secondarywindings it does nothing towards influencing currentsharing between them. The current sharing is determinedonly by that part of the reactance that is individual to therespective secondary windings and by their individualresistances. It may, therefore, be substantially out ofbalance and readily upset by supply harmonics. Carefuldesign is necessary to keep these effects within satisfactorylimits.4

3 Current balancing compensator

A recent development is the introduction to parallel bridgerectifier circuits of a compensator to ensure equal currentsharing,4 as shown in Fig. 2. The compensator has thefollowing virtues:

(a) In otherwise unfavourable conditions for goodcurrent sharing the current is shared equally between thetwo rectifier bridges over the complete current range.

(b) The interphase transformer, which is in the d.c.circuit and therefore normally fitted in the rectifier cubicleassembly, is an awkward component to accommodate, andtends to be a nuisance in respect of the noise it makes,operating at six times the fundamental frequency. Thenecessity for an interphase transformer is avoided by theuse of the compensator.

(c) The compensator, being in the a.c. circuit, goesunder oil in the main transformer tank.

(d)12-pulse operation and current waveforms areobtained in the individual bridges. The 5th and 7th har-monics normally present in the transformer secondarywindings are eliminated, reducing the total r.m.s. current

rflAtransformer

compensator

•fcM

L tVL

• * •

•£+

Fig. 2 Compensator in a parallel-bridge rectifier

by 3-4%. The transformer secondary kVA and transformerlosses are correspondingly reduced.

(e) The conducting period in each rectifier arm isincreased from 120° plus overlap to 150° plus overlap, thusreducing the form factor of the current and that part of thediode losses associated with the slope resistance (see typicalmanufacturer's diode-rating curves).

The compensator itself is somewhat larger in ratingthan the interphase transformer it displaces. However,there is usually an appreciable depth of oil in the trans-former tank between the top of the transformer and theoil level and the compensator can go there without addingappreciably to the tank dimensions.

If there is an individual transformer for each rectifierbridge, or if the transformer secondary windings are end toend and therefore have no appreciable flux coupling, it ispossible to omit the interphase transformer without addinga compensator. This also avoids the problem of criticalconditions for current sharing. It may be practical in somecases to do this and indeed it is done. However, the short-circuit current is then about double what it would be forthe close coupling.

4 Cooling

The diodes are mounted on heat sinks, which are generallyof extruded aluminium and invariably air cooled. Naturalconvection is preferable as the equipment is then completelystatic, and no warning or tripping circuits are necessary asa protection against fan failure.

Sometimes, however, it is left open to the manufacturerto use either forced or natural cooling, and the contractgoes to the lowest bidder. This is undesirable as, althoughthere is not a great difference in cost, a forced-air-cooledrectifier may be marginally cheaper. A minor difference inprice should not outweigh a major difference in simplicityand reliability.

When there is sufficient diode capacity to deal withshort-circuit conditions, cooling is not then a difficultproblem and the saving in heat sinks and size of equipmentis largely offset by the cost of fans and their protectioncircuits.

Natural cooling is now normal in substation applications.

5 Diodes

Diode development has kept up with the requirements forsubstation duties. In respect of voltage rating, peak ratingsof 2kV are adequate for bridge-connected rectifiers in750 V d.c. systems and 4kV peak ratings for 1500 Vsystems. Series connections of diodes are used for highervoltages than these, or 1500 V conversion rectifiers wheremercury-arc rectifiers were originally used in single-wayconnection.

As diode-voltage ratings increase, thicker silicon slicesare necessary; the forward diode losses increase and thefault-current ratings are reduced. In this situation there is atemptation to use lower voltage diodes than are reallydesirable. This is something that should be controlled bynational standards or user specifications. USA specificationsANSI C34 and NEMA R9 appear to require, for this duty,a voltage safety factor of 2-5, without being sufficientlyclear about it as to ensure that it is complied with. NoBritish Standard covers this point. 2-5 is a figure that, to a

ELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. I 23

degree, is arbitrary; perhaps further experience since thepublication of the US specifications would justify thisbeing reduced to 2-25. These remarks assume adequateconventional capacitance/resistance transient overvoltageprotection.

Current ratings are basically determined by silicon-slice diameters. Those in service for this duty go up to50mm diameter and 76mm diameter is available. Diodeswith 38 mm silicon diameter are in general use in Britishequipment. These are base mounting, and therefore single-side cooled, and are about the maximum size that can beusefully used with single-side cooling, although base-mounting diodes with 50 mm silicon have been produced.

6 Capsule diodes

Capsule-type diodes for clamping between two heat sinkshave many advantages:

(a) Obviously there are two heat sinks per diode insteadof one.

(b) Less obviously but perhaps even more important, thejunction to case internal thermal resistance is halved for thesame current and power, as shown in Fig. 3.

(c) The variety of mounting bases and top terminalconnections is eliminated and the various makes are physi-cally interchangeable.

(d) Capsule diodes are the same for either polarity.There are two factors that have delayed the more general

use of capsules. One is that they are not so easy to mountas base-mounting diodes, the clamping requirements beingmore critical and the spring loading has to be external outsidethe heat sinks. The second is that the optimum economicsof manufacture will not be achieved until they are requiredin the same numbers as the common base-mounting diodes,and production is as well developed and established. How-ever, the design advantages become overriding for the largersilicon diodes now available.

7 Capsule diode rectifier in service

Equipment with double-sided cooled capsule diodes hasnow been in service in the UK for several years. It is shownin Fig. 4 and incorporates several novel features designed to

junction155°C

base100°C

630A, 850W

base mounting diodesingle-sided cooled

pole peices100t

junction128°C

100 C

155C

630A,2xA25W

1000A, 2x85OWcapsule type diodesdouble sided cooled

Fig. 3 Comparison of diode temperatures for single-sided anddouble-sided cooling for the same mounting surface temperature

Arrows indicate heat flow

overcome difficulties in the satisfactory application ofcapsule diodes:

(a) The diodes are clamped between pairs of heat sinksin banks of six. This gives a high degree of compactness.

(b) The disc spring clamping assemblies project from themain rectifier enclosure and are accessible for visualchecking of pressure and adjustment without opening therectifier enclosure.

(c) The series clamping gives equal clamping pressure forall the diodes in the stack.

(d) The diodes are carried in removable cards which onunclamping can be removed like books from a bookshelf.

(e) The diode cards are designed to completely enclosethe diodes for dust protection and externally to provide abarrier between the poles.

(/) There is no need to remove heat sinks to replace adiode.

(g) Connection is inherent in the clamping, so there is noundoing of connections involved, as such.

The only problem during assembly was that, since diodereplacement was so easy, it was difficult to prevent itscontinual demonstration. However, the balance of advan-tage was restored when in test it was discovered that alldiodes had been put in the wrong way round and it tookonly half an hour to put this right. The design now incor-porates locating features such that the diodes can only becorrectly fitted.

Fig. 4 1500kW 12-pulse rectifier equipment with readily checkedand removed capsule diodes with double-sided cooling

Reproduced by courtesy of London Transport

8 Single-diode design

A further consequence of the capsule development is theability to use just one large diode in each rectifier arm.Single diodes are available for rectifiers up to about1500 kW. While cooling is the most difficult aspect, such arectifier has been made and is now in service. It is 12-pulse,comprising parallel bridges and has a rating of 1500 kW (seeFig- 5).

This development radically changes the design consider-ations. When diodes are used in parallel there is a current-sharing problem. Their forward voltage drops must liewithin close limits and a substantial out-of-balance currentis allowed for in their rating. If a diode fails, and is taken

24 ELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. 1

out of operation by its fuse, this is not directly apparentand an indicating system is therefore required.

Diode fuses are used to take out any diode which fails,enabling the remainder to carry on working. A failed diodeshort-circuits the transformer and there is a possibility thatin unfused applications a diode may explode if it cannotcarry the fault current until it is cleared by the a.c. switch-gear.

Fig. 5 1500kW 12-pulse rectifier equipment with single diode perrectifier arm

Reproduced by courtesy of London Transport

The diodes used in the single-diode rectifier referred toare rated to carry the short-circuit current until the a.c.switchgear opens. There is therefore no need for diodefuses, and while the rectifier is operational there is no needto check whether any diode has failed.

A number of points which have been accepted practicein the design of rectifiers for this duty need rethinking.When there are a number of diodes in parallel it is quiteeasy and economic to have one extra in each arm, so thatthe rectifier can carry on working with one failure. Butwith a rectifier designed to operate with only one diode perarm, an extra diode doubles the rectifier capacity.

In this respect this is a return to the situation withmercury-arc rectifiers. Because they were substantial items,an extra bulb was not normally included to take care of thepossibility of a failure. There may have been extra completerectifier equipment to provide spare capacity, but wherethis was the practice with mercury-arc rectifiers a similarnumber of silicon rectifiers are now installed.

In this, there are the factors of reliability and the needfor maintenance. Mercury-arc rectifiers gave good servicefor many years but they had a tendency to blow their fuseswhen suddenly taking a heavy load in cold weather and thebulbs or tanks occasionally needed replacing or reprocessing.Silicon diodes do not really need any maintenance, replace-ment or reprocessing. Experience has shown that, providedthere is nothing wrong with the basic design of siliconrectifier equipment, failure of silicon diodes is virtuallyunknown.

The point is that with the use of silicon diodes insteadof mercury-arc rectifiers the need for standby features isnot increased but reduced, and spare diodes, when there areonly one two or three in parallel, are not justified.

9 Diode failure indication

Early silicon rectifier equipment had many protectivefeatures, some of which have been found by experiencenot to be necessary, but are still specified. A case in pointis provision for a warning to be given in the event of onediode failure, and tripping if two diodes fail in any arm.This, of course, is quite inappropriate in the case of tractionrectifier equipment since to cope with a heavy load allavailable rectifiers should be kept operational. A warningmay be helpful so that the number of trains and thenumber of operational rectifiers may be kept compatible.

However, as rectifier diodes for this duty are very un-likely to fail, such systems are superfluous and only reducethe reliability of the complete equipment. The majorBritish users have from the beginning kept to a simplevisual indication of diode-fuse operation and their ex-perience has proved this to be sound.

10 Series bridges

For 1500V equipment it becomes reasonable to havephase-displaced rectifier bridges in series for 12-pulseoperation. No interphase transformer is necessary and thetwo bridges being in series carry equal currents. The onlydisadvantage is that compared with parallel bridges thereare twice as many diodes, heat sinks and fuses. However,the diodes and fuses are of lower voltage and the lossesper diode are less. Fig. 6 illustrates a 1500 V rectifierincorporating series bridges.

11 Conclusions

It is gratifying to note that the UK has played a majorpart in the development of rectifier equipment for thisduty, both at home and abroad. The following featureshave been actively promoted by British manufacturers:

(a) natural current sharing between diodes wherepreviously forced current sharing by compensators hadbeen specified

Fig. 6 1500kW 1500 V 12-pulse rectifiers comprising seriesbridges. 3 equipments being installed at Gosforth substation, Tyneand Wear Metro

Reproduced by courtesy of Tyne and Wear PTE

ELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. 1 25

(b) natural convection cooling 12 References(c) 12-pulse operation with reduced short circuit currants , , . . . _ . n t , , i w i x t~.o n Ar. „ t w) ,\ • i • , . i. x- /• J- , c • 1 MARTI, O. K., and WINOGRAD, H.: Mercury arc power(d) simple visual indication of diode fuse operation rectifiers' (McGraw-Hill, 1930)

instead of microswitches, resistors, transistor circuits and 2 SCHAEFER, J.: 'Rectifier circuits' (Wiley, 1965)small wiring in the main circuit rectifier assembly 3 READ, J. C: 'Calculation of rectifier and inverter performance

(e) capsule diodes with double-sided cooling, culmi- characteristics',/./^, 1945, 92, Pt. II ,P P 495-509x. • • i J. j x-f 4 BENNELL, F. T.: 'Current balance in 12-pulse rectifiers com-

nating in a single diode per rectifier arm p r i s i n g p a r a M b r i d g e s ,5 in T o w e r e i e c t r o n i c s _ P o w e r s e m i-( / ) compensators for parallel bridge circuits, without conductors and their applications', IEE Conf. Publ. 154, 1977,

interphase transformers (protected by patent). pp. 66-69

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