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Rectifiers for railway-traction substations F. 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 the supply of d.c. for railway traction. Current general practice is described and reference is made to the latest developments, including compensators for paralleling double-bridge rectifiers, and capsule diodes.1 Introduction actance of 10%, and the resultant short-circuit currentD.C. supplies to railways are provided by rectifier equip- would be a nominal ten times full load.2ment in substations distributed along the track. The basic With a common transformer and two secondary windingsrequirements have remained the same, dating back to the the required reactance is 10% for the load of each second-mercury-arc era,1 but, particularly since the advent of ary, the other bridge circuit involving the other secondarysilicon-diode rectifiers, there has been, and continues to be, not commutating at the same time. This gives a nominalsubstantial progress in rectifier equipment design. 20%* if the load on this one secondary is increased until full primary current is taken, this being the way reactance The direct voltages commonly in use are nominally is measured for the determination of d.c. regulation. Hence,750 V, 1500 V and 3000 V. The voltage level affects the the relationship that the percentage regulation is a nominalbalance of factors that determine the optimum basic design. 0-25* times the percentage reactance for 12-pulse operationThe 750 V level is the most widely used and this paper is compared with 0-5 times for 6-pulse.written generally for this, with references to variationsarising from the use of higher voltages. Short circuit involves both secondary windings, hence The simplest form of circuit for this duty is a 3-phase short-circuit current depends upon the total reactance. Ifbridge, giving 6-pulse rectification. Its main disadvantage the secondary windings are not magnetically coupled, i.e.is the relatively high ripple in the d.c. at six times funda- do not share the same flux, the reactance with both second-mental frequency, increasing from about 4% r.m.s. at no aries short-circuited decreases nominally to a half, due toload to, typically, 6% at full load and higher for overloads, doubling the flux linkages between primary and secondarydue to increasing overlap. This can cause interference with by involving the second secondary winding, making the re-signalling circuits which are running in parallel in close actance a nominal 10%. If, however, the secondary windingsproximity along the same route. The 5th and 7th harmonic are closely coupled they occupy the same envelope volume whether one or both are involved and the reactance is thecurrents drawn from the supply by 6-pulse rectifiers are same for both conditions, i.e. nominally 20%, and the short-also undesirable as, in addition to other effects, they can circuit current is nominally only 5 times full load, as showncause unbalance in 12-pulse rectifiers.13 in Fig. 1.2 12-pulse rectification by parallel bridges This reduction in short-circuit current and its resultant rate of rise are a considerable help in enabling the rectifier6-pulse ripple and 5th and 7th harmonics can be avoided by diodes and their fuses to withstand short circuits. It alsohaving two phase displaced 6-pulse rectifiers operating helps the d.c. circuit breakers and reduces the stress on thetogether, the combined effect corresponding to 12-pulse equipment and the damage where the short circuit occurs.rectifier equipment, provided of course both sets of equip- The point is that this reduction in short-circuit currentment are always in operation together. This can be devel- is not achieved by all 12-pulse rectifier arrangements butoped 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% Fig. 1 Output characteristics of 6-pulse and 12-pulse rectifiersregulation, for example 750 V on full load and 789 V on no a 12-pulseload. An individual 6-pulse rectifier would require a re- b 6-pulsePaper T298 P, first received 14th September and in revised form 26th Strictly speaking, 19-3% and 0-259, respectively, as the cancel-October 1978. lation of the 5th and 7th harmonics in the primary winding reduceMr. Bennell is with Foster Transformers Ltd., The Path, Morden the primary current and kVA on which the percentage reactanceRoad, London, SW19 3BN, England. is based, by 3-4%.22 ELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. 10140-1327/79/010022 + 05 $01-50/0
only when the two transformer secondary windings are by 3-4%. The transformer secondary kVA and transformerclosely coupled in a common flux circuit. losses are correspondingly reduced. Associated with this desirable effect is, however, a (e) The conducting period in each rectifier arm isdesign difficulty. To the extent that the leakage flux and increased from 120° plus overlap to 150° plus overlap, thustherefore reactance is common to the two secondary reducing the form factor of the current and that part of thewindings it does nothing towards influencing current diode losses associated with the slope resistance (see typicalsharing between them. The current sharing is determined manufacturers diode-rating curves).only by that part of the reactance that is individual to the The compensator itself is somewhat larger in ratingrespective secondary windings and by their individual than the interphase transformer it displaces. However,resistances. It may, therefore, be substantially out of there is usually an appreciable depth of oil in the trans-balance and readily upset by supply harmonics. Careful former tank between the top of the transformer and thedesign is necessary to keep these effects within satisfactory oil level and the compensator can go there without addinglimits.4 appreciably to the tank dimensions. If there is an individual transformer for each rectifier3 Current balancing compensator bridge, or if the transformer secondary windings are end to end and therefore have no appreciable flux coupling, it isA recent development is the introduction to parallel bridge possible to omit the interphase transformer without addingrectifier circuits of a compensator to ensure equal current a compensator. This also avoids the problem of criticalsharing,4 as shown in Fig. 2. The compensator has the conditions for current sharing. It may be practical in somefollowing virtues: cases to do this and indeed it is done. However, the short- (a) In otherwise unfavourable conditions for good circuit current is then about double what it would be forcurrent sharing the current is shared equally between the the close coupling.two 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 cubicle 4 Coolingassembly, is an awkward component to accommodate, andtends to be a nuisance in respect of the noise it makes, The diodes are mounted on heat sinks, which are generallyoperating at six times the fundamental frequency. The of extruded aluminium and invariably air cooled. Naturalnecessity for an interphase transformer is avoided by the convection is preferable as the equipment is then completelyuse of the compensator. static, and no warning or tripping circuits are necessary as (c) The compensator, being in the a.c. circuit, goes a protection against fan failure.under oil in the main transformer tank. Sometimes, however, it is left open to the manufacturer (d)12-pulse operation and current waveforms are to use either forced or natural cooling, and the contractobtained in the individual bridges. The 5th and 7th har- goes to the lowest bidder. This is undesirable as, althoughmonics normally present in the transformer secondary there is not a great difference in cost, a forced-air-cooledwindings are eliminated, reducing the total r.m.s. current rectifier may be marginally cheaper. A minor difference in price should not outweigh a major difference in simplicity and reliability. A When there is sufficient diode capacity to deal with short-circuit conditions, cooling is not then a difficult problem and the saving in heat sinks and size of equipment is largely offset by the cost of fans and their protection rfl circuits. transformer V Natural cooling is now normal in substation applications. t 5 Diodescompensator L L Diode development has kept up with the requirements for substation duties. In respect of voltage rating, peak ratings of 2kV are adequate for bridge-connected rectifiers in 750 V d.c. systems and 4kV peak ratings for 1500 V systems. Series connections of diodes are used for higher voltages than these, or 1500 V conversion rectifiers where mercury-arc rectifiers were originally used in single-way connection. •fcM As diode-voltage ratings increase, thicker silicon slices • * • are necessary; the forward diode losses increase and the fault-current ratings are reduced. In this situation there is a •£+ temptation to use lower voltage diodes than are really desirable. This is something that should be controlled by national standards or user specifications. USA specifications ANSI C34 and NEMA R9 appear to require, for this duty, a voltage safety factor of 2-5, without being sufficiently clear about it as to ensure that it is complied with. NoFig. 2 Compensator in a parallel-bridge rectifier British Standard covers this point. 2-5 is a figure that, to aELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. I 23
degree, is arbitrary; perhaps further experience since the overcome difficulties in the satisfactory application ofpublication of the US specifications would justify this capsule diodes:being reduced to 2-25. These remarks assume adequate (a) The diodes are clamped between pairs of heat sinksconventional capacitance/resistance transient overvoltage in banks of six. This gives a high degree of compactness.protection. (b) The disc spring clamping assemblies project from the Current ratings are basically determined by silicon- main rectifier enclosure and are accessible for visualslice diameters. Those in service for this duty go up to checking of pressure and adjustment without opening the50mm diameter and 76mm diameter is available. Diodes rectifier enclosure.with 38 mm silicon diameter are in general use in British (c) The series clamping gives equal clamping pressure forequipment. These are base mounting, and therefore single- all the diodes in the stack.side cooled, and are about the maximum size that can be (d) The diodes are carried in removable cards which onusefully used with single-side cooling, although base- unclamping can be removed like books from a bookshelf.mounting diodes with 50 mm silicon have been produced. (e) The diode cards are designed to completely enclose the diodes for dust protection and externally to provide a6 Capsule diodes barrier between the poles. (/) There is no need to remove heat sinks to replace a Capsule-type diodes for clamping between two heat sinks diode. have many advantages: (g) Connection is inherent in the clamping, so there is no (a) Obviously there are two heat sinks per diode instead undoing of connections involved, as such. of one. The only problem during assembly was that, since diode (b) Less obviously but perhaps even more important, the replacement was so easy, it was difficult to prevent itsjunction to case internal thermal resistance is halved for the continual demonstration. However, the balance of advan-same current and power, as shown in Fig. 3. tage was restored when in test it was discovered that all (c) The variety of mounting bases and top terminal diodes had been put in the wrong way round and it took connections is eliminated and the various makes are physi- only half an hour to put this right. The design now incor- cally interchangeable. porates locating features such that the diodes can only be (d) Capsule diodes are the same for either polarity. correctly fitted. There are two factors that have delayed the more general use of capsules. One is that they are not so easy to mount as base-mounting diodes, the clamping requirements being more critical and the spring loading has to be external outside the heat sinks. The second is that the optimum economics of manufacture will not be achieved until they are required in 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 larger silicon diodes now available.7 Capsule diode rectifier in serviceEquipment 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 junction 155°C pole peices 100t base Fig. 4 1500kW 12-pulse rectifier equipment with readily checked 100°C and removed capsule diodes with double-sided cooling junction Reproduced by courtesy of London Transport 128°C 8 Single-diode design 630A, 850W 630A,2xA25W A further consequence of the capsule development is the base mounting diode 100 C ability to use just one large diode in each rectifier arm. single-sided cooled Single diodes are available for rectifiers up to about 1500 kW. While cooling is the most difficult aspect, such a 155C rectifier has been made and is now in service. It is 12-pulse, comprising parallel bridges and has a rating of 1500 kW (see 1000A, 2x85OW Fig- 5). capsule type diodes This development radically changes the design consider- double sided cooled ations. When diodes are used in parallel there is a current-Fig. 3 Comparison of diode temperatures for single-sided and sharing problem. Their forward voltage drops must liedouble-sided cooling for the same mounting surface temperature within close limits and a substantial out-of-balance currentArrows indicate heat flow is allowed for in their rating. If a diode fails, and is taken24 ELECTRIC POWER APPLICATIONS, FEBRUARY 1979, Vol. 2, No. 1
out of operation by its fuse, this is not directly apparent 9 Diode failure indicationand an indicating system is therefore required. Diode fuses are used to take out any diode which fails, Early silicon rectifier equipment had many protectiveenabling the remainder to carry on working. A failed diode features, some of which have been found by experienceshort-circuits the transformer and there is a possibility that not to be necessary, but are still specified. A case in pointin unfused applications a diode may explode if it cannot is provision for a warning to be given in the event of onecarry the fault current until it is cleared by the a.c. switch- diode failure, and tripping if two diodes fail in any arm.gear. This, of course, is quite inappropriate in the case of traction rectifier equipment since to cope with a heavy load all available rectifiers should be kept operational. A warning may be helpful so that the number of trains and the number 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 reduce the reliability of the complete equipment. The major British users have from the beginning kept to a simple visual 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 have phase-displaced rectifier bridges in series for 12-pulse operation. No interphase transformer is necessary and the two bridges being in series carry equal currents. The only disadvantage is that compared with parallel bridges there are twice as many diodes, heat sinks and fuses. However, the diodes and fuses are of lower voltage and the losses per diode are less. Fig. 6 illustrates a 1500 V rectifierFig. 5 1500kW 12-pulse rectifier equipment with single diode per incorporating series bridges.rectifier armReproduced by courtesy of London Transport 11 Conclusions It is gratifying to note that the UK has played a major The diodes used in the single-diode rectifier referred to part in the development of rectifier equipment for thisare rated to carry the short-circuit current until the a.c. duty, both at home and abroad. The following featuresswitchgear opens. There is therefore no need for diode have been actively promoted by British manufacturers:fuses, and while the rectifier is operational there is no need (a) natural current sharing between diodes whereto check whether any diode has failed. previously forced current sharing by compensators had A number of points which have been accepted practice been specifiedin 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 instead Fig. 6 1500kW 1500 V 12-pulse rectifiers comprising seriesof mercury-arc rectifiers the need for standby features is bridges. 3 equipments being installed at Gosforth substation, Tynenot increased but reduced, and spare diodes, when there are and Wear Metroonly one two or three in parallel, are not justified. Reproduced by courtesy of Tyne and Wear PTEELECTRIC 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 . „ tw ) , • 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 495-509 x. • • 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. 66-6926 ELECTRICPOWER APPLICATIONS, FEBRVARY 1919, Vol. 2, No. 1