VOL. XXI OBER, 1934 No. 10 THE BROWN BOVERI REVI EW EDITED BY BROWN, BOVERI & COMPANY, LIMITED, BADEN (SWITZERLAND)

OFFICINE ELETTRICHE TICINESI S. A„ BODIO (SWITZERLAND). MONTE PIOTTINO POWER STATION. 2 three-phase alternators each for 23,000 kVA, 8200 V, 630/750 r. p. m., 42/50 cycles.

CONTENTS: Page Page Centralized heating and power plants and the 410-H.P. Diesel-electric motor coaches for the economics of heat recuperation 175 Madrid-Saragossa-Alicante Railway .... 185 The corrosion of copper-zinc alloys 180 The Albbruck-Dogern Power Station .... 186 Notes: Brown Boveri turbo-blowers for copper Compensation of the earth current in a cable and nickel Converters in Canada 184 system 187

Printed in Switzerland. BROWN BOVERI MOTORS FOR MACHINE-TOOL DRIVES

CIRCULAR SAW FOR EDGING BOARDS, FOR CUTTING DEPTHS UP TO 200 mm. With circular-saw motor Type KK, 5-5 kW, 3000 r.p.m. (Protecting hood and table cover removed.)

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VOL. XXI OCTOBER, 1934 No. 10

The Brown Boveri Review is issued monthly. Single numbers: 1.20 francs. Annual subscription: 12 francs, postage and packing extra. Reproduction of articles or illustrations is permitted subject to füll acknowledgment.

CENTRALIZED HEATING AND POWER PLANTS AND THE ECONOMICS OF HEAT RECUPERATION. STEAM OR WATER AS THE HEATING MEDIUM?

Decimal index 621. 311. 22 : 697. F late increasing attention has been paid to the cause the difference in the volume of steam before O question of the recuperation of waste heat in and after the turbine is then not so great; the gain power plants and its use for heating purposes. The in power is, however, comparatively small. Even then object of this article, therefore, is to derive funda- it would first be necessary to investigate whether the mental principles for determining the Output which initial cost of the boiler, the superheating of the can be produced, and also to describe some note- exhaust steam and the small blade lengths which worthy solutions evolved by Brown, Boveri & Co. result are admissible. If, for example, the steam con- The great advantages of these combined plants are ditions are increased from 30 kg/cm2 abs, 400° C, at once obvious when it is considered that with the to 50 kg/cm2 abs, 450° C, the increase in the heat simplest means and in the smallest plants a thermal drop is little more than 20 kcal. If, however, the efficiency can be attained three times as high as that heat is distributed by means of warm water at 60 0 C of complicated and expensive super power stations. or 80 0 C instead of by back-pressure steam at say 2 The first question which arises is whether it is 4 kg/cm abs, then the increase in the heat drop possible, by suitably choosing the conditions, to prod- amounts to 110 kcal or 93 °/o, and 77 kcal or 65 % uce in the combined type of plant under consider- respectively of the total heat drop. In other words, ation the füll Output required by the power consumers with warm water operation 93 % or 65 % more connected to the network. In order to approach this Output can be obtained for a given volume of steam. ideal case it is endeavoured to distribute the heat at Should this increase in Output not cover the power the lowest temperature at which it can be utilized, requirements, warm water operation will still, in most even if the cost of the heat-consuming machines is cases, be more advantageous than using an extraction thereby increased somewhat. The exhaust heat of a turbine with steam distribution, even although part steam-driven prime mover can be given up either in of the warm water may be lost. Quite apart from the form of steam or warm water. Previously, when the fact that an extraction turbine is a comparativ- low-pressure boilers were used only for heating pur- ely expensive machine because it is equivalent to a poses, it was quite in order to distribute and utilize combined back-pressure and condensing turbine, it the heat by means of low-pressure steam. Heat inter- also possesses the disadvantage that the steam which changers and heating apparatus, e. g., calenders for flows into the condenser, and of which the latent the textile and paper-making industries, were accord- heat is lost, is throttled by the by-pass valves and ingly also developed for steam heating. The time has is not used with the thermo-dynamic efficiency ob- now arrived, however, when this type of apparatus tained in a condensing turbine. should be modernized and reconstructed for warm In order to show clearly the increase in Output water heating, which offers a much more economic or saving in coal which can be obtained with heat heat utilization in combined plants, as will be shown distribution by means of warm water when compared in this article. with steam, the steam consumption diagrams for both As regards the live-steam conditions, if it is solutions are given in Figs. 1 and 2. A turbine with an output of 1000 kW, using live steam at 24 kg/cm2 considered desirable to use inexpensive single-cylinder 0 turbines in combined plants, the pressure and temper- abs, 350 C, was chosen. ature should not exceed 30 kg/cm2 abs and 400° C (a) refers to heat distribution by extraction steam at respectively. These can be chosen somewhat higher, 3 kg/cm2 abs (dotted lines). referring to the same weight of steam, if back-pressure (b) refers to heat distribution by warm water, in turbines are used instead of condensing turbines, be- Fig. 1 at 50° C, in Fig. 2 at 800 C. PAGE 1 THE BROWN BOVERI REVIEW OBER, 1934

To simplify matters a constant thermo-dynamic This thermostat regulation in accordance with efficiency of 75% has been assumed for both so- the well-known Brown Boveri design has no rods; lutions because here only a comparison of the gain the impulses are transmitted by oil under pressure obtained with a heat plant in cases (a) and (b) is which can be taken from the oil pressure system of made. According to what has already been said it the turbine. The tubes in the heating condensers follows that in case (b) the turbine is actually not 9000 only much cheaper, but also has a better thermal kg/h efficiency, so that the saving in coal will be even greater than that calculated here. In case (a) the 8000 kg/h toH' 7000 7000 PfA / b • / 3 6000 • / T/50m yh s / 6000 / s / / • / r ' / / • 5000 'r • f / y V • 5000 / V / ' ' /40'f / •/ //. f• S / n K /' ' SmV / • s / * • s y S 4000 y $ / s / 4000 /. / / o. / • / / 'ÄV ' // /' y • <' / / / ' y 50s S 3000 ' / • V s 3000 / / / / y / s s s / . / • J V / /' / / , / / s s f20 2000 'f / 30' 2000

'10/ 1000 1000 // //V/

40485-1 500 750 kW 1000 500 750 kW 1000 Fig. 1. Comparison of two waste-heat recuperation plants in Fig. 2. - Comparison of two waste-heat recuperation plants in which which water is heated from 15 to 50° C. water is heated to 80° C. 2 = heated by extracted steam of 3 kg/cm abs, = heated by extracted steam of 3 kg/cm2 abs, vacuum 0*05 kg/cm* abs. vacuum 0-05 kg/cm2 abs. 2 . = heated by a heating condenser plant, live steam of 24 kg/cm • : heated by a heating condenser plant, live steam of 24 kg/cm2 abs, 350° C. abs, 350° C. water is heated by extracted steam at 3 kg/cm2 abs are expanded at both ends, and according to a Brown from 15° C to 50° C or 80 0 C, whilst the remain- Boveri patent (pending) are given a slight curvature ing steam, which is required to produce the desired to allow for free expansion. In this way the tubes output, is Condensed in a Standard condenser at a remain perfectly tight even during maximum variations constant vacuum of 0-05 kg/cm2 abs. In case (b), in temperature. A comparison of the live steam con- however, the whole of the steam flows into the con- sumption figures for a load of 750 kW and a desired denser and the quantity is regulated by the governor. warm water quantity of 50 m;l/h at 500 C (Fig. 1) The condenser is built in the form of a heating or 800 C (Fig. 2) shows :— condenser and allows the water to be heated directly Fig. 1 Fig. 2 from 15° C to 50° C or 80° C. This is possible Case (a) point A 5200 kg/h 6800 kg/h as a result of a special thermostat regulation evolved Case (b) point B 4050 kg/h 5890 kg/h by Brown Boveri and shown in Fig. 3. According to Saving in fuel % 22 13-4 3 this system, the quantity of water flowing through is Fig. 1, point B, shows 61 m /h of hot water always regulated by the thermostats to suit the load, instead of only 50 m8/h which are exactly required. ; so that the water temperature has a maximum Vari- Fig. 2, point B, on the contrary, shows 44 m '/h of ation of + 10 C as proved by numerous plants. hot water generated from the 5300 kg/h of steam OCTOBER, 1934 THE BROWN BOVERI REVIEW PAGE 177

Fig. 4. — Steam turbine with waste-heat recuperation and indepen- dent power generation. A. Turbine. 1. Live steam main. Fig. 3. — Turbine with circulating-water regulation to constant outlet B. Condenser. 2. Heating pipe. temperature. C. Valve for make-up live steam. 3. Hot water pipe. 4. Steam pipe to A. Turbine. Fi. Relay for the thermostat. D. Servo-piston for C. pressure regulator. B. Condenser. G. Adjusting screw. E. Valve for make-up heating steam. 5. Pressure oil pipe to C. C. Regulating valve. H. Water-jet air ejector. F. Servo-piston for E. 6. Pressure oil pipe to J. D. Servo-piston for 1. Air discharge pipe. G. Water circulating valve. 7. Pressure oil pipe to E and G. regulating valve. 2. Pressure-oil pipe. H. Servo-piston for G. 8. Oil regulating valve. E. Circulating water pump. 3. Oil drain pipe. J. Thermostat. 9. Oil drain pipe from C. F. Thermostat. 4. Live-steam main. K. Non-return valve. 10. Oil drain pipe from E. L. Water-jet air ejector. 11. Oil drain pipe from G. flowing through the turbine. In the latter case, 590 M. Motor-driven gear oil pump. 12. Diaphragm. N. Pressure regulator. 13. Air discharge pipe. kg/h of live throttled steam must be added in the condenser. In such cases the control system shown as warm water at constant temperature in the heat- in Fig. 4 is used. The thermostats automatically en- ing condenser. The thermostat and the automatic sure the supplementary steam supply. With a large steam throttle valves C and E receive pressure oil load and a momentarily reduced demand for warm from a special auxiliary oil pump M. water the reverse case can occur, namely, when the The following considerations reveal clearly the quantity of warm water produced is too great if limits within which the Solution (b) results in a saving a temperature of 50 0 or 80 0 C is to be maintained. in fuel when compared with Solution (a). According to this system, however, the thermostat allows the surplus warm water to flow through the In case (a) :— valve G to the by-pass. This surplus water can be he = effective heat drop in the extracted steam, stored up in a reservoir until such time as the demand hc = effective heat drop in the remaining steam, for warm water increases, or it can of course be quantity of extracted steam allowed to run to waste within certain Iimits, although GL total steam quantity. even then there is a saving in coal when compared By extracted steam is meant that quantity of with (a). Solution steam which is just sufficient to produce the required The plant shown in Fig. 4 is somewhat com- quantity of warm water. plicated because an extraction turbine is provided Specific steam consumption (kg/kWh) which can not only deliver warm water but also heating steam at a definite pressure. The supple- 860 (1) mentary steam is taken from the extraction pipe 2 a • he + (1 — a) — hc and flows into the condenser if the exhaust steam In case (b) :— from the turbine is not adequate. The plant is so designed that after the exhaust-steam valve has been hw = effective heat drop in steam closed when the turbine is shut down, throttled live 860 Db = steam can be produced in the heating pipe as well hIwT PAGE THE BROWN BOVERI REVIEW OBER, 1934

Percentage saving in fuel in case (b)

Da — Db 1 100 Da q h — [cc • h + (1 — a) h ] 220 w e c 100 200 For the example under consideration the follow- ing results are obtained :— 180 // Fi . l Fig. 2 g 160 80 74 = 0-4 hc 74 = 0-4 hc 185 185 140 70 / 164 = 0-885 hc 124 = 0-67 hc 120 60 0-6 a — 0-115 0-6 et —0-33 y, 100 50 0-885 0-67 A Both solutions a and b are thermally equal when the 80 40 X numerator = 0, namely:— 60 30 / for Fig. 1 o = = 19 % 40 20 0-6 0-33 20 10 for Fig. 2 55 % / 0-6 0 10 20 30 40 50 60 70 80 90 100a% If the extraction is greater, a saving is obtained with Fig. 5. — Comparison of a plant in which water is heated by heating steam of 3 kg/cm2 abs, with a plant in which water is heated in a warm water distribution, namely:— heating condenser. for Fig. la: 19 30 50 70 100% 1. Saving in fuel with water heated to 50 °C. 2. Saving in fuel with water heated to 80 C. I: 0 7-3 21 34-5 66% 3. Warm water produced per kWh at 50" C. for Fig. 2 a: 55 70 80 90 100% 4. Warm water produced per kWh at 80° C. q. Weight of warm water in kg per kWh. 1: 0 13-5 22 33 40% e. Percentage saving in fuel.

According to the above, a • Da represents the quantity of steam extracted for each kilowatt hour. From Fig. 5 it can be seen that the higher the Furthermore, since each kilogram of steam produces temperature selected for the warmed water, the Ai heat units (== 672 — 55 or 672 — 85 kcal)1 and smaller must be the surplus which can be allowed each kilogram of water requires At heat units (in above the average daily warm water consumption, this case 35 or 65 kcal), then the quantity of warm if a saving in fuel is to be achieved. But even when water produced per kWh is the quantity of warm water is less than the above corresponding to 19 % extraction at 50 0 C or 55 % Ai n at 80 0 C, the Solution (b) is much superior to (a), when q = a Da * ~~~ At the installation shown in its simplest form in Fig. 6 is If Da is replaced by formula (1) and the values sub- stituted, then 82 tof r rigP- . 1 q ' « 1 — 0 • 6 a 42 • a P and for Fi«?. 2 q = M 1-0-6« from which the following values are obtained for q :— ß 1 -N -M a: 0 0-1 0-3 0-5 0-7 0-9 for Fig. 1 R q: 0 8-75 30 58-5 99 158 205 kg/kWh -«I ©c for Fig. 2 q: 0 4-45 15-3 30 50-5 82 105 kg/kWh 7 When it is desired to find the saving e for a e j L load L and a total quantity of water Q, then calculate Fig. 6. — Independent turbine plant used alternatively for water heating* and as a purely condensing plant. Q = and the value for e can be read off in the A. Turbine. H. Warm water accumulator. <1 ^T B. Heating condenser. J. Water supply. C. Circulating-water pump. L. Water outlet. diagram shown in Fig. 5. D. Three-way valve. M,N. Float contacts. 1 E. Jet condenser. P. Warm-water distributing i — 672 cal of heat contained in the exhaust steam, F. Thermostat. pump. 55° and 85° temperature of condensate. G. Regulating valve. R. Condensate pump. OCTOBER, 1934 THE BROWN BOVERI REVIEW PAGE 179

realized in practice. The plant operates here alternately case in all quarters of a town. If not only heating in two ways, either producing warm water which is but also power is required, then there is increased regulated by the thermostats F and G and fed into the economy because the sale of electric current relieves reservoir H, or operating on ordinary vacuum, whereby the capital charges on the plant. the cooling water is allowed to flow freely into L In accordance with the foregoing remarks, water until the reservoir is empty. In order to change over and not steam must be taken as the heat carrier from one form of operation to the other it is only when considered from the point of view of power necessary to move the three-way cock D (Fig. 6). production, but this question will now also be exam- This arrangement produces the maximum thermal ined from the point of view of a central-heating plant. efficiency obtainable under the circumstances. If C represents the quantity of heat to be dis- Vacuum operation requires a many times larger tributed per second, then for quantity of cooling water than heating operation, Steam Water and impure water is often used in this case, whilst f f' = cross-section of pipe w t velocity of heat carrier for heating operation clean and perhaps chemically w = t specific weight of heat carrier treated water is used. In such cases Brown Boveri Y Y = employ a condenser with a double tube nest. This Ai Ai' = heat given up per kilogram enables each nest of tubes to be adapted better to and C = f • w • Y • Ai = f • w' • y' ' Ai' suit the quantity of cooling water passing through. or the ratio of the cross-sections of the pipes is The foregoing explanations show that Brown f y' w' Ai' w Boveri are in a position to supply condensing turbo- f' y Ai sets for heating purposes, with complete automatic Substituting the usual values control to enable the heat in the exhaust steam to f_ 1000 2 20 __ 40 be fully utilized without back pressure or extraction f' ~~ 1 40 ' 550 ~ 22 turbines and whereby, without parallel operation with i. e., with this assumption the steam requires a pipe a network, the desired Output can at any moment be obtained. These sets are particularly safe in ser- nearly twice as big. vice and have been put to many different uses, such This formula is very convenient for enabling as in the textile industry. If, in addition to warm some idea to be obtained of the relative sizes of water, steam is also required, then an extraction the pipes for definite conditions. turbine with heat condensation can be used. This example will now be examined even more closely by taking the pressure drop into consideration There are plants which have been in existence for the various diameters of piping. for a number of years and where one industrial heat- Case (a): With steam as the heat carrier an average ing set serves as the source of energy for supplying 3 specific weight of y — 1 kg/m is assumed, the factory with current and heat. Such an indus- that is, a pressure of about 2 kg/cm3 abs. trial heating plant has been in continuous service in Case (b): With water as the heat carrier, a difference the largest Italian wool-combing factory, the Lani- in temperature of At = 20 0 C is assumed ficio Marzotta, Mortara, since 1929. Industrial heat- between entry and exit. ing plants without doubt afford the best Solution as regards central-heating installations. The problem of Pipe diameter . 100 200 300 400 mm central-heating plants will, therefore, be considered in more detail in the following lines:— Solution . a b a b a b a b

The advantages of the combined heating and Heating- power power plant for central-heating purposes have long in millions kcal 0-62 1-4 2-5 4-6 5-68 9-94 10 17 been recognized, and at first sight it is, therefore, remarkable that modern towns should still make use Pressure loss kg/cm2 of stoves and self-contained heating installations in- per kilometre . 1 0-8 0-3 0-2 abs. 30 13 8 5-5 metres stead of relieving the housewife of the unpleasant of water and dusty work entailed by the carrying of coal 1 and ashes inside the house. Experience and calcul- Power loss 46 152 318 515 kW ation show, however, that under normal conditions a Pump power 9-5 16 3 22-4 27-6 kW central-heating plant is unfortunately only economical when an annual quantity of heat equivalent to 8 to 1 This loss is calculated from the loss in heat drop com- pared with the production of warm water at 90" C in the heating 10 million heat units per metre length of heating condenser with an assumed turbine efficiency of 70 piping can be sold. This implies a fairly thickly po- pulated district which, however, is not always the (MS 860) (To be concluded.) J. Broggi. (E. J. B.) PAGE 180

THE CORROSION OF COPPER-ZINC ALLOYS.

Decimal index 669. 355.1.

HE problem of the destruction of metals and alloys few selected conclusions will be reproduced here. Fig. 1 Tby corrosion has been the object of intensive scientific shows the constitutional diagram of the copper-zinc research for decades. Various hypotheses and theories alloys. This shows that copper-zinc alloys with more have been utilized in the attempt to classify the diversi- than 68 °/o of copper fied phenomena which come under the head of cor- are composed of rosion. At first, it was thought possible that an ex- crystals of the pure planation would be found by applying the purely a phase. In the alloy chemical laws governing Solution and the mutual reactions ränge of 68—61 °/o of the materials in Solution. But this basis had, soon, of copper, a mixture to be enlarged to include certain electro-chemical prin- of a and ß phases ciples, without which much of the phenomena and can occur, accord- processes were inexplicable. This enlarged basis of ing to the thermal research permitted of establishing the real basic laws treatment and, if which could be generally applied. It remained to the the copper com- most recent research work, however, to give a com- ponent is further prehensive explanation of the phenomena of corrosion. reduced, only the The phenomena in question are those processes grouped mixture of a and ß under the term of topochemical reactions. Kohlschütter1, phases is en- who introduced this method of approaching the problem countered. Alloys into modern research, understands under the term those with less than 54°/o processes in which the morphological elements of the of copper are system reacted on play the paramount part and, further, composed exclus- in which the reaction takes place at a determined centre. ively of ß phase If the centre of reaction and the conditions of reaction crystals, etc. are of great importance chiefly as regards the formation A certain school of experts are of the opinion that of material, these causes are no less important in the only a copper-zinc alloy consisting of crystals of purely disintegration of certain systems through corrosive in- a phase should be used for condenser tubes and that the fluences. To explain corrosive processes, therefore, not copper content must not be less than 70°/o. In Order only the chemical relations of the various reactive com- to make the material very resistant to corrosion, a ponents must be known but, chiefly, the centre of certain size of grain is stipulated as being necessary, reaction to which the corrosion is related must be this is asserted by Lasche 2. Köhler 3 states, on the examined and recognized. other hand, that there does not seem to be a simple In the following paragraphs, the new basic consi- functional relationship between the different grain sizes derations are applied to a special case of corrosion, caused by the annealing treatment and the resistance to namely to the destruction of copper-zinc alloys which corrosion of the alloys, Schumann reported at the occurs very frequently in steam-turbine condenser tubes, Corrosion Assembly in Berlin in 1933 that tests in for example. If this special example is chosen, it must service had shown tubes with an unbroken annealing be understood that it is no proof that the explanations skin and medium grain size were the most impervious furnished here are limited to the case in point; on to corrosion. Various deductive researches carried out the contrary, the results of the researches carried out by Naval Authorities and Electricity Boards led to can be applied quite generally to the corrosion of copper- the most contradictory conclusions. Up tili to-day, zinc alloys. the alloys with 63 °/o copper and 37% zinc besides Considering the capital importance which must be the so-called Admiralty Alloy of 70°/o copper, 29°/o attached to the destruction of condenser tubes both zinc and 1°/« tin or 70 "I« copper and 30°/o tin have in land and in marine plants, it is understandable that continued to hold own. Just lately, alloys containing the chief conditions of reaction in play here have been aluminium and those containing copper and nickel, etc. studied by a variety of authorities searching for effi- have been brought forward. These remarks will suffice cient means to eliminate the trouble in question. For 2 this reason there is a great deal of printed matter Konstruktion und Material im Bau von Dampf- available on the subject, from which, however, only a turbinen und Turbodynamo, Springer, 1920. 3 Zentralblatt der Hütten- und Walzwerke, vol. 32, 1 Helvetica Chimica Acta, vol. 12, year 1929. page 229, year 1928. OBER, 1934 THE BROWN BOVERI REVIEW PAGE 1 to show that the decisive factors which pertain to the In order to examine conditions experimentally, different corrosion of copper-zinc alloys are not entirely known series' of important tests were carried out in the Brown yet. At any rate, the vulnerability of copper and zinc Boveri laboratories in Baden. Only the most important alloys the structure of which is composed of a mixture results of tests carried out on alloys of 63 °/o copper of st and ß phases can be taken as an established fact. and 37 % zinc and of 60 % copper and 40% zinc are given, which, as shown in Fig. 1, belong to the transition ränge between pure a phase crystalsand a mixture of oc-| -ß phases. The cor- rosive fluids used were a 10 % Sol- ution of hydro- chloric acid, a 5 °/o Solution of ammoniac and a 5 % Solution of sodium cloride. It is true that these solutions are hardly ever en- countered as cor- rosive agents in practice but, with their help, it was possible to de- termine the basic processes in the destruction of copper-zinc alloys. 200 mm lengths of tube of 0 • 8 mm wall thick- ness were used as test pieces and these were com- pletely immersed in the solvent. In order to ascertain, at the same time, the influence exer- cized by the size of grain and, where necessary, that of mechanical tensions (cold- hardening ten- sions) the test pieces were ex- amined in four • soft annealed. b = 2 % drawn and recrystallized. hard drawn. d = 20 °/o drawn and recrystallized. different states:— the first state was Fig. 2. — Micrographs of the alloy with 63 °/o copper Fig. 3. Micrographs of the alloy with 60 °/o copper and 37 Vo zinc, enlarged 75 times. and 40 °/o zinc, enlarged 75 times. that of a tube PAGE 182 THE BROWN BOVERI REVIEW OCTOBER, 1934

properly treated thermally and soft annealed. The other 10"/» HCl 86 T 5% NaCI 273 T extreme was a hard-drawn tube, not reannealed in which more or less cold hardening tensions were present. Various sizes of grain were also attained by recrystallization of cold worked tubes having a degree of cold deformation of 2 and 20 °/o. Table 1 gives the composition and TABLE L Composition and mechanical properties of alloys tested.

630 » copper 60°'i copper Alloy n 37°/0 zinc 40 o zinc

C ü Mechanical lup I ? «SS E M 3 c 6 c properties Ii w 3 E E I O.Ü M .1

Soft annealed . 12 36 58 14 42 44 Hard drawn 49-5 57-5 9-5 47 58-5 12 2°/o cold worked 830° C; 15 min 650-660° C; andrecrystallized water - quenched; water - quenched temp. 580-600° C for 30 min. reheated during Mechanical values 30 h. as under soft an- Mechanical values as under soft an- nealed. nealed. = copper • = rupturing elongation 20 °/o cold worked As above As above ES3 = zinc |"J rupturing strength and recrystallized Fig. 4. — Summary of test results. Dissolving tests on two alloys in different solvents. Number of days. mechanical values. Figs. 2 and 3 give micrographic reproductions of the structure for the various conditions so-called basic salt. In such cases, the proposition of of both alloys under test. As is seen, the tubes of an copper-zinc in the salts deposited was determined so alloy of 63 °/o copper and 37 °/o zinc are of pure a that the tests might be complete; here Cu -)- Zn = 100 phase crystals after all the treatments to which they were was adopted. subjected while those of an alloy of 60 % copper and Fig. 4 summarizes the most important results of 40 °/o zinc are composed of a and ß phases. the tests with two alloys, just described; in the upper The corrosion of the test pieces was determined half of the figure the changes of the mechanical properties by ascertaining the weight of the material dissolved at are illustrated, that is to say the tensile strength, and 2 given intervals and transforming it into mg/cm surface. elongations at rupture. The different fields correspond It has, however, been well known for a long time that to the values ascertained, after a given testing time, the corrosion process cannot be determined correctly in percentage of the original tensile strength and in this way and such determinations are far from being elongations at rupture. The lower half of the figure gives accurate, although certain authors still repeat to-day total quantities of the alloys dissolved in the same 1 that they are reliable. Czochralski and Schmid already space of time, shown in the same manner. The black pointed out that in corrosion tests it is absolutely surfaces mean the elongation at rupture which could necessary that the changes in the mechanical properties be measured after the corresponding test time, and such as the rupturing strength and the elongation should the white surfaces show the rupturing strength. be taken into account. For this reason, the tests in In general, it must be recalled that the dissolution question comprised, apart from the measurement of processes of crystals can be compared with the phen- the material dissolved and quantitative analysis of the omena of growth; to a certain extent, the dissolution for each test, the determination of the ultimate Solution, is a negative growth. The anisotropic crystals will tensile strength as well and the rupturing elongation. therefore behave differently and according to the dif- It will be shown that it is precisely these combined ferent senses of growth during dissolution. The process tests which first produced a picture of what the cor- will progress more or less rapidly according to the rosive processes were, sufficiently correct to be made corresponding sense of the crystal surface-fluid. Further, use of. In certain corrosive fluids, salts are deposited, the edges and corners will dissolve more rapidly than 1 Zeitschrift für Metallkunde, vol. 20, page 1, year the flat surfaces, in accordance with modern knowledge 1928. on topochemical reactions. OBER, 1934 THE BROWN BOVERI REVIEW PAGE 13

The electro-chemical behaviour of copper-zinc al- its capacity to expand suffers more than that of the loys has been studied lately by Bauer, Vollenbruck and other test pieces. Schikorr.1 These researches showed that the potential The 60% copper and 40 % zinc Solution behaves of the pure oc phase crystal increases only slightly with quite differently. As shown in the structure in Fig. 3, the approach to the transition to a -(- ß phase. With the apparation of the ß phase crystal the difference of potential increases suddenly. The difference of potential of the pure ß phase crystal is then, itself, constant through the whole ränge of its maintenance. The greater the difference of potential of a material the greater is its electrolytic dissolvent tension. This fact, however, does not suffice, as will be shown, to explain the phenomena which appear when copper-zinc alloys corrode. Fig. 4 shows that the 63 % copper and 37 % zinc alloy, composed of pure a phase crystals, dissolves relatively easily in a 10% Solution of hydrochloric acid, and this in proposition to the component ratio of the alloy treated, that is to say without any predisposition of one or other of the two components of the alloy. If compared to Fig. 2, it will be seen that the^tubes, which were first 20 % cold hardened and then recrys- Fig. 5. — Lengths of tubing made up of a + ß solid crystalline Solution tallized, have the biggest crystals, while the hard-drawn from which the zinc has been dissolved away. tubes have small crystals without well defined structural The ß solid crystalline Solution is replaced by a porous deposid of copper. parts. According to this, the first mentioned test piece dis- solves considerably better than the hard one. The soft oc -f- ß phases are present in all cases. Dissolution is annealed test pieces and those cold-hardened 2 % and considerably less in a 10% hydrochloric Solution than then recrystallized occupy intermediate positions. In is the case for the other alloy and, which is most impor- spite of the relatively advanced dissolution, the decrease tant, there is no more dissolution proportional to the in rupturing strength and the elongation at rupture component ratio of the alloy; practically, only zinc is are not very different one from another, in the different dissolved. The ß phase crystal loses its zinc completely stages, exception being made of the hard drawn pieces. there remains only a finely perforated structure. In the Although dissolvability in conjunction with the structure place of the ß phase crystal there is a small-pore copper is smallest in this case, the tubes tested are very brittle, deposit, which is seen from Fig. 5. Malleability is al- the elongation is nearly entirely suppressed. If a 5% most completely exhausted, after the test time, and the elongations at rupture in all states diminished. The Solution of ammoniac is used as a corrosive fluid, the different tubes behave quite similarly as regards di- character of the ß structure parts has a greater influence minution of strength, although the attack by the solvent than that of the cold hardened tensions. Once the ß is weaker than that of hydrochloric acid, the dissolution part is completely dissolved the x phase crystals dis- again takes place in the proportion of the component solve in proportion to the component ratio of the alloy. ratio of the alloy and, in the salt mixture deposited, As a result of the considerable reduction in malle- the proportions of copper and zinc remain the same. ability and, above all, of the great porosity which was The bigger the grain, the smaller the influence on the shown to accompany the loss of zinc, a tube of this rupturing strength and elongation at rupture, the hard kind is already useless long before this. The cold drawn tube, however, looses its malleability despite the hardening tensions are again very apparent in the dight dissolution it is subject to, and becomes very ammoniac and sodium chloride solutions, while the brittle. With the help of the so-called strip test (removal elongation in both solutions is practically completely of a strip 5 mm wide and 80 mm long), proper stresses lost, with hard-drawn tubes. In this case, as well, only of about 7 kg/mm2 were revealed in the hard-drawn zinc is found in the Solution up tili the complete tubes of this alloy. This test, therefore, confirms the destruction of the ß part. This dissolution of zinc and old experience that ammoniac is very dangerous for the formation of the porous copper deposit combined copper-zinc alloys in which exist cold hardening tensions. therewith form a typical topochemical reaction. It has already been said that the difference of In a 5 % Solution of sodium chloride the hard-drawn tube is again the least stable, despite its slight dissolution, potential of the ß crystals is higher than that of the oc crystals. To complete the series of tests, some were 1 Korrosion, VDI-Verlag, Berlin 1932. carried out with an alloy of about 52% copper and PAGE THE BROWN BOVERI REVIEW OBER, 1934

48 % zinc which according to the Constitution diagram currents, etc. Coming into play, the composition of the Fig. 1 is composed of pure ß solid crystalline Solution. alloy only has a certain influence. As long as a uniform The dissolution in a 10°/o hydrochloric acid Solution structure persists, as for example, pure a crystals, the was practically similar to that of the a structure, that resistance to corrosion opposed by the different alloys is to say, it is in accordance with the component ratio in this ränge is the same, allowing for proper heat of the alloy. The only difference was that with this treatment, in all cases. The corrosion is then a process concentration of solvent, the total quantity dissolved of dissolution according to the alloy. If, however, a was smaller than that with the a crystals, though it defective heat treatment has produced an a -f- ß struc- must be said here that only structures of different grain ture, the state of constraint of the ß portion causes sizes could be compared so that this assertion does not greater dissolution of the zinc alone accompanied by form a basic difference. This shows that it is not the the formation of a very fragile porous structure. The ß structure in itself which is unstable but that the so- purely ß structure, considered in itself, cannot be said called transition structure is in a constrained state to be more liable to corrosion. Alloys of this nature which reinforces the properties of ß. also dissolve like those of the pure a structure in pro- It is a known fact that transition structures are very portion to the ratio of the alloy. The size of the grain unstable as a consequence of their State of constraint. has an influence on the phenomenon as large grain It should be added, to complete the subject, that alloys dissolve easier in strong solvents. As the latter other series' of tests were carried out with tubes of an are, usually, not met with in practice, this point is not alloy of 70 °/o copper and 30 °/o zinc which, according important for service conditions. Alloys having cold hardening tensions even in structures of pure crystalline to the Constitution diagram, are of pure a crystals. In this case, as well, there was no basic difference in Solution are much more liable to corrosion although behaviour of the pieces tested as compared to the other the dissolution is slight, the tubes loose their malleability tubes tested of 63 % copper and 37 % zinc, which completely in a very short time. In contact with cor- are themselves, as shown by Fig. 1, also of pure a rosive solutions they may become brittle and disag- crystals. The results of these tests will be reported on gregate completely. A uniform crystalline structure elsewhere. In the present article only the limit ränge must, therefore, be the object aimed at combined to between the a and a -f- ß structures are described. The one without cold-hardening tensions. If the latter tensions results, however, allow of arriving at some fundamental are present in parts, resulting from the metal having conclusions. been worked on when cold, it is possible that local corrosion may be set up in the parts in question. If copper-zinc alloys are subjected to corrosive (MS 847) Dr. H. Stäger. J. Biert. (Mo.) influences without any external influences such as stray

3 NOTES. m /min sets, each complete with starters and switchgear. The main technical data of the 5 sets is as follows :

Brown Boveri turbo-blowers for copper and nickel New Plant Existing sets Converters in Canada. sets

3 Decimal index 621 633. 5. (71). Suction volume 1130 710 1130 m /min Delivery pressure . 0-915' 1-125 1-195 kg/cm2gauge THE International Nickel Company of Canada has its Input, measured at main workings at Copper Cliff, Sudbury, Ontario. This motor terminals . 2020 1730 2385 kW Company extended its plant in 1929 and installed two 1 710 ms/min and one 1130 m3/min motor-driven, geared This blower is being modified to deliver at a final pres- sure of 1-195 kg/cm2. turbo-blowers. All three sets were supplied complete, each with its own Starter and switchgear. The three- Each blower is governed by an automatically operated phase synchronous motors were designed for a supply at suction-throttle valve, which maintains the final pressure 25 cycles, 2300 V. of the blower constant. The two new blowers will be This plant has been running continuously since its similarly equipped, so that, for parallel Operation, each installation, to the füll satisfaction of the owners. This is blower can be adjusted to deliver at the same final pressure. proved by the fact that the Company recently placed a A special feature in the design of these turbo-blower repeat order with Brown Boveri for two further 1130 sets is the provision of motors having ample capacity to OBER, 1934 THE BROWN BOVERI REVIEW PAGE 1

410-H. P. Diesel-electric motor coaches for the Madrid-Saragossa-Alicante Rail- way. Decimal index 621. 335.4. 033. 44 (46). RAILWAY managements are showing increasing interest in the utilization of Diesel-electric motor coaches for passenger service on non-electrified main and second- ary lines. The object followed is to im- prove and to speed up traffic conditions by Splitting up trains and bettering existing con- nections. At the beginning of this year, the Madrid - Saragossa-Alicante Railway (Compania de los Ferrocarriles de Madrid a Zaragoza y a Alicante MZA) placed an order for four Diesel-electric motor coaches with the Compania Auxiliar de Ferrocarriles in Beasain. Brown, Boveri & Co., Ltd., Baden were entrusted with the electrical equip- Fig. l. International Nickel Co. of Canada. Converter turbo-blower handling 710 m3/min, free air, delivery pressure 1-125 kg cm2 gauge. ment of the coaches in question. These coaches are to handle express and cope with the demand for air in winter, when the air ordinary passenger traffic on the very hilly Madrid-Aranjuez- density is much greater, due to the intense cold. During Cuenca line section. According to the number of passengers, these periods the thermometer drops to — 400 C and the motor coaches will be operated as Single units or with a lower. This means that, for the same volume handled, trailer car having a driver's cabin at one end and which the delivery pressure remaining unchanged, the weight acts as a control coach. Further, multiple unit operation of air compressed is greater, and the power input is in- of two motor coaches or of trains made up of one motor creased in inverse ratio to the absolute temperature of the coach and one to two trailer cars will be feasible. indrawn air. The following traction conditions as laid down in the The advantages of turbo-blowers are becoming more MZA specification must be met by these coaches:— widely recognized in the copper industry, as well, and this Maximum speed of a single motor coach unit com- type of machine seems to be steadily supplanting reci- pletely loaded procating blowers. 100 km/h on the level (MS 855) A. R. Knowler. 70 km/h on the steepest gradient:— 17°/oo.

Fig. 1. — 410 - H. P. Diesel-electric motor coach for the Compania de los Ferrocarriles de Madrid a Zaragoza y a Alicante. Mechanical part by the Compania Auxiliar de Ferrocarriles in Beasain; coachwork in light-weight design, according to the Waggonfabrik Uerdingen. Electrical equipment by Brown, Boveri & Co., Ltd., Baden, and Diesel engine by Maybach Motorenbau G.m.b.H., Friedrichshafen. PAGE THE BROWN BOVERI REVIEW OBER, 1934

With a train composition of one motor coach and one utilizes the 12-km length of river from the Coblenz-Walds- trailer coach the maximum speed on the level is 90 km/h hut railway bridge down to Schwaderloch-Albbruck, that and 44 km/h on the 17°/oo gradient. The single motor is to say down to the backwash from the Laufenburg coach unit, completely loaded, must attain the füll speed power Station. The fall of the river on this length is about of 100 km/h on the level within 150 sec. The gross 1 %o so that the total fall available behind the dam is service weight of a motor coach is 52 t and that of a 12 m. The power Station is designed as a head-race channel trailer 31 t. type of plant. The dam is placed above the Swiss Federal Fig. 1 gives the main dimensions and general design Railway Station of Leibstadt; from this point a head-race of the motor coaches, which, as well as the trailer coaches, channel about 3 km long and 47-2 m sole width takes off, are of light construction, the external lines being designed on the German side of the Rhine. The power house proper to offer as little resistance to air as possible. The 410-H. P. is near the village of Albbruck and just on the mouth of Maybach - Diesel engine with accessories as well, as the the river Alb into the Rhine, so that a new bed about electric equipment are also designed to be as light as is 500 m away had to be created for the Alb, it being im- feasible. The net weight of a motor coach, with all driving possible to dam the said river, on account of the village equipment included, is 45 t. of Albbruck. The motor coaches are designed with a two-axle The power house ressembles that of Ryburg-Schwör- engine bogie in which the Diesel-generator set is lodged stadt with the difference that it holds three generating and a two-axle driving bogie containing two driving motors. sets instead of the four at Ryburg-Schwörstadt. The tur- The auxiliary services are supplied from an auxiliary gener- bines and generators are built to exactly the same conditions ator forming a single unit with the main generator.1 and, on the whole, are of similar design. The Diesel engine is started up by means of the main Reference is made here to the article in the Brown generator acting as a motor and supplied from an aecu- Boveri Review, year 1929, page 311. mulator battery. The main characteristics of a generator are recalled The regulating equipment which assures the desired below:— tractive effort-speed regulation of the coach is especially interesting. The driver has only control over the power 32,500 kVA developed by the Diesel engine, that is to say indirectly Voltage . . 10,500 V of the speed of the train. The voltage regulation of the Frequency 50 cycles main generator is automatic and carried out through the 75 r. p. m. agency of a servo field-rheostat. The latter is connected Weight of pole wheel with shaft abt. 245 tons to the feed-regulating rod of the Diesel. The servo field- Diameter of pole wheel . . . 9-5 m rheostat maintains a constant Diesel Output corresponding External stator diameter without to the Output set by the driver by means of the regulating sheet-metal housing .... abt. 11 metres Controller. The Output of the Diesel is modified by increasing Height of the machine from th e or decreasing its speed. Further, the servo field-rheostat coupling flange to the Upper edge is combined with a device which allows the Diesel to of the exciter abt. 9 metres develop the torque which the builder has specified as being Total weight abt. 550 tons. allowable for the speed it is working at. It may be interesting to note that 21 railway cars The great advantage of this method of regulation, were required to transport one generator alone. which we will describe in a latter article in more detail The four generators delivered to Ryburg-Schwörstadt than we are able to do here, is that constant maintenance have given an excellent account of themselves in the three of the Output is attained in absolute correlation with the years they have been in service and the putting to work electrical load developed and the charge delivered by the of the three Albbruck-Dogern machines was carried out fuel pump of the Diesel. Thus all irregularities resulting perfectly smoothly. The taking-over tests which took place from a main cylinder being out of action, etc. as well as afterwards not only showed that the machines came up fluctuations in the load of the auxiliary generator are to the guarantees given but, in part, that their Performance taken into due account. Thus, the Diesel engine can be was superior to the guarantees. Two of these generators utilized to the füll under all service conditions and it is were built in the Baden shops and one in the shops of efficiently protected against overloading. Brown, Boveri & Co. in Mannheim. (MS 836) A. E. Müller. (Mo.) Brown Boveri also supplied, with the machines, an eight-panel Switchboard for machine regulation and for the The Albbruck-Dogern Power Station. relays, containing all apparatus and Instruments requisite to the operation and protection of the generators. Decimal index 621.311.21 (43). This power Station does not contain any switchgear AT the end of the year 1933, a further and important proper for power distribution as each generator works direct step in the work of harnessing the water power of the on its own step-up transformer, there being no oil circuit Rhine was completed, namely: the putting into service breaker or bus-bars between them. From each transformer of the Albbruck-Dogern power Station. This power Station an outgoing line takes off, through circuit breakers and

1 See Fig. 62, No. 1/2, year 1934 of this Review, which shows the disconnecting links, carrying power to the Thiengen design of the generating set. (Schwarzwald) outdoor Station, 15 km away. OBER, 1934 THE BROWN BOVERI REVIEW PAGE 1

The three transformers are placed on a terrace running (1) When an earth fault occurs, the damaged cable along the wall of the building-, on the tail-race side. They should be cut out instantaneously. — Assuming that compen- are built for outdoor service with separately mounted oil sation of the earth current by means of the extinguishing conservators. The outer cooling surfaces are enlarged by coil can be made sufficiently accurate, that is to say that the addition of radiators ; the latter being connected up to the residual current can be kept low, it will usually be the air-tight oil tank on the two longitudinal sides of the possible to keep up Service with the damaged cable for latter by means of gate valves. Seif cooling suffices up to some little time, despite the earth fault, and to discover half load while, for heavier loads, artificial air cooling is the faulty section, make certain switching changes in the used which is effected by means of two fan sets mounted supply and finally cut out the faulty length of cable on the truck frame, which projects at one end, to this without more serious damage being done at the point purpose. where the earth has occurred, despite this delay. The loading capacity of each transformer corresponds Systematic tests carried out with Standard cables on to the Output of a generator. In order to be able to use the 30-kV cable system of the Berlin Municipal Electricity the transformers in as many ways as possible, and also to be Works1, which is equipped with extinction coils, showed able to adapt the voltage ratio to a variety of requirements, that service could be maintained for 48 seconds with a the low-voltage winding is so subdivided and equipped residual current of 10 A at 30 kV, without a short circuit with tappings that it is possible to work to the following between phases being set up. Of course, this space of voltage ratios:— time would not be sufficient to allow of finding out where kV 104/26-0 24-7 23-4 22-1 20-8 star/star the fault had occurred and of making the required service kV 104/13-0 11-7 10-4 star/star modifications. The test showed, however, that under very kV 104/ 5-85 star/star unfavourable current and voltage conditions the earth kV 104/15-0 14-25 13-50 12-75 12-0 star/delta contact did not extend to a short circuit between phases, during the time in question. The tappings of the low-voltage winding are Ied to terminals so that the connections can be changed above In another case, on the occasion of a service disturb- the cover of the oil tank. The high-voltage winding can be ance, it was shown that an earth current of 50 A occur- switched over from 104 to 116 kV by means of a change- ring on a 30-kV cable of Standard design only became a over switch, which has to be used when the transformer short circuit between phases after a minute had elapsed. is cut out. This data allows of concluding that, when the earth- The external dimensions of these 35,000 kVA Units current compensating device is sufficiently accurately tuned, are worthy of note. Each transformer can be moved about it is very probable that Standard cables (within the voltage as a whole and with its oil filling on a special railway limits of 5—20 kV) can be kept in service during the car belonging to the Rheinisch-Westphälische Elektrizitäts- time necessary to find the faulty section and make the werke either on the system of the Swiss Federal Railways necessary switching changes. Hochstädter cables (cables or on that of the German State Railway Company. In with metal clad insulation of each separate conductor), or order to transport a transformer it is only necessary to single-pole cables, allow of carrying out the necessary remove the oil conservator, insulator-bushings, safety valve, switching modifications without the earth fault going over radiators and truck. The total gross weight of a trans- to a short circuit between phases, this thanks to their former is about 108 tons. The possibility of being able to suitable design. move the transformers about as complete units on the railway system is a very welcome advantage as dismantling (2) The Insertion of an extinguishing coil on a and erection including drying out of the tanks are Oper- system does not prevent having to replace the section ations which take up a great deal of time and are ex- affected when a defect occurs on a cable. — The insulation pensive while being accompanied by a certain amount of the cable at the point where the earth occurs is des- of risk for units of this size. troyed ; the extinguishing coil, however, prevents the occur- rence of other faults at other points on the system as well (MS 856) C. Fisler. H. Rehsteiner. (Mo.) as the excess voltages resulting therefrom, which are otherwise created when earths occur in an uncompensated Compensation of the earth current in a cable system. system. It must also be noted that Service experience Decimal index 621.316.761:621.316.13. shows that earths very rarely occur on the cables proper; usually trouble of this nature is located in the end boxes THE earth currents created in a cable system when and in the connecting sleeves. The tests, mentioned above, an earth fault occurs are considerably greater than in an on the Berlin system also show that it often happens that overhead system of the same voltage and extent (about a defect in an end box or in a sleeve eliminates itself, 30 to 60 times greater, according to the type of cable once the arc has been extinguished, this because the cable used). Owing to the strength of these earth currents and mass Covers up the weak point again and thus insulates also to the methods in use, up tili lately, to eliminate a it from earth. fault, electricity works have adopted a very reserved attitude towards extinguishing coils for compensating (3) The extinguishing coil does not prevent the creation earth-fault currents on cable systems. of excess voltages due to switching in and out or those Some of the objections raised by clients and Brown due to atmospheric disturbances. — An excess-voltage surge Boveri's attitude thereto are given in the following para- graphs:— Elektrotechnische Zeitschrift, year 1924, Nos. 13 and 14. PAGE THE BROWN BOVERI REVIEW OBER, 1934

Coming into a cable from outside, by way of the breakers Practical experience has shown that the presence of or transformers, is usually considerably weakened already. an extinguishing coil causes the earth arc to extinguish Excess voltages due to atmospheric disturbance have, and this even when the residual current is considerably practically, no deleterious effect on a cable system. stronger than the earth current which would be required to keep up the arc to earth in an uncompensated system. (4) In case of an earth fault, the voltage of the un- affected phases, with reference to earth, rises to that corres- Further, it should not be forgotten that the higher ponding to the composed voltage. — A voltage rise of this harmonics do not always appear with the same strength kind should, surely, be stood up to by a well insulated in the course of a day. Really disfavourable conditions cable system, in the voltage ränge of 5 to 30 kV, without would have to be assumed to suppose an earth fault damaging the cable. If this is not the case, the insulation occurring just at the very moment the voltage curve showed of the cable must be deemed insufficient. the greatest deviation from its pure sine wave form. It would mean a disregard of the manifold uses of an ex- (5) Insertion extinguishing coils may The of be tinguishinthe g coil and its influence on Service conditions, to cause resonance phenomena appearing. — A phenomenon of leave it out of the system simply on account of this one of this kind can only appear if the partial capacity of the possible disfavourable case. As the Said coil assures cer- three conductors in a cable as compared to earth are tain extinguishing of the earth arc with fundamental wave, dissymmetrical. Only in this case is it possible that a voltage even when the residual currents are strong, the frequency be created in ordinary service, between the neutral point of disturbances on the system is considerably reduced, and earth. As in a cable the three partial capacities of because the influence of the fundamental wave as com- the different phases with relation to earth are equal to pared with that of the higher harmonics is preponderant. one another, none of the dissymmetries due to the capacities Further, the coil also protects the substations and over- occur in a cable system and there is no considerable head transmission lines connected thereto, in which the displacement of the neutral point. occurrence of earth faults is the most common cause of (6) Another objection which, however, is less fre- disturbances. quently raised, as it applies to definite cases only, is given Hochstädter cables, as said before, can be cut out by herewith. — If the voltage curve of a cable system contains hand, before a short circuit between phases has arisen. higher harmonics of a certain amplitude, these may also Further, the coil prevents the formation of dangerous produce higher harmonics in the curve of the earthing excess voltages, because it suppresses intermittent arcs current. If, for examples, the fifth harmonic of the voltage which occur frequently on overhead lines. The coil fixes curve appears with an amplitude of 5°/o of the fundamen- the voltage of the system against earth and prevents the tal wave, it creates theoretically, in the earthing current, sudden rise of the recovery voltage after the extinction of an additional current of 25°/» of the fundamental current an arc and, thereby, the creation of oscillation phenomena wave. In practice, this additional current component is on the system. smaller, because the reactances of the earth current circuit The Electricity Works of the town of Geneva recently increases proportionately with the order of the harmonic, ordered from Brown Boveri an earth-current compensating it being nmL (n being the order of the harmonic) and equipment by means of an extinguishing coil, for the it, therefore, limits the strength of the earth current. It protection of an 18-kV distribution system. may happen, however, that the reactance n a) L of this current circuit is partly compensated by the capacities The equipment comprises :— A single-phase extinguishing coil of 2520 kVA with — „ so that the influence of the higher harmonics on not neutral-point transformer of the same output, both for the earthing current still remain strong. Now, the extin- outdoor erection. guishing coil is dimensioned so that, above all, the funda- The system to be protected includes both cables and mental wave of the earth current is compensated and, for overhead lines. The three-pole cables are of the Standard this reason, only a fraction of the higher harmonics as type and of the Hochstädter type. well. The residual current may, therefore, become relatively The total earthing current amounts to 202 A at strong despite the insertion of the extinguishing coil. With 19 kV phase voltage; according to calculation. As exten- the help of suitable chosen inductances and capacities, it sions of the system are planned, the coil is dimensioned is possible to compensate the additional current components, for 230 A. In order to be able to tune the coil to the which are created by the higher harmonics. different service conditions of the system, it is equipped It is, however, recommendable to first be content with 15 taps each for 6 A. By means of these taps, it is with compensating the fundamental wave of the earth possible to use the coil down to a minimum value of current and make observations on the influence of the current of 140 A. A tapping switch, to be moved when extinguishing coil in cases of disturbance. In most cases, the coil is dead, and which is built into the coil tank is it is probable that the improvements introduced to the used for adjusting the current taps. system by the compensation of the fundamental wave will The big Output for which the apparatus is designed prove to be quite sufficient so that it will not be necessary and the composition of the distribution system, make this to put in additional apparatus to compensate the influence order worthy of note. of higher harmonics. (MS 859) A. Maret. (Mo.)

Published by BROWN, BOVERI & COMPANY, LIMITED, BADEN (Switzerland). Printed by Kreis & Co., Basle (Switzerland).