Part 8

The following example will show the method pursued. The figures resulting from the calculations are placed immediately below the sections to which they refer in Fig. 84. The rails are assumed to be 70 pound to the yard. These have a resistance of about .018 ohms per mile. Adding one-sixth for additional resistance of bonds gives .021 and since the track is composed of two rails the resistance of the track will be one-half of this or .0105 ohms per mile.

The maximum drop in any section occurs when the car is farthest from the power house. Each car is assumed to take 50 amperes and the feeders are to be so designed as to allow a 10 per cent or 60 volts drop.

The current in the two miles of track nearest the power house is 150 amperes, in the next section 100 amperes, and in the last section 50 amperes. The drop in each section is as shown. The drop in the trolley which is 00 wire is, in each section, 20.5 volts. Subtracting from 60 volts the drop in the return circuit and trolley, gives the allowable drop in the feeder.

The resistance of each feeder can be calculated, since the current in each one is 50 amperes. The first feeder is one mile long, the second 3 miles and the third 5 miles, and with these figures the feet per ohm can be computed. The size of wire may be obtained by reference to a table of copper wire resistances.

BLOCK SIGNALS FOR ELECTRIC RAILWAYS.

The simplest block signal used by electric roads is a hand-operated one constructed on the principle shown in the diagram Fig. 85. A double throw switch is placed at each terminal of the section of track that is to be protected.

[Illustration: Fig. 85.]

The switches have no central position, the knife blade always making contact with one or the other of the terminals shown. If the lamps are lighted, throwing either one of the switches will put them out. If they are not burning, they will be lighted by throwing either one of the switches.

A motorman on reaching a section of track finding the lamps not burning throws the switch. Lamps now burn in each switch box and show that the section is in use. On arriving at the other terminal of the block the switch is thrown, extinguishing the lights and showing that the block is clear.

Automatic signal systems have been devised on the same principle, in which magnets, operated by contacts made by the passage of the trolley wheel, cause the lamps to be lighted and extinguished automatically.

ELECTROLYSIS.

Much has been said about the possibilities of electrolysis of underground metal by the action of the return current of electric railways, when such railways are operated with grounded circuits, as they usually are. If electric current is passed through a liquid from one metal electrode to another, electrolysis will take place; that is, metal will be deposited on the negative pole, and the positive pole or electrode will be dissolved by becoming oxidized from the action of the oxygen collecting at that pole.

[Illustration: Fig. 86. Showing Electrolytic Action.]

In an electric-railway return circuit, there is necessarily a difference of potential between the rails at outlying parts of the system and the rails and other buried pieces of metal located near the power house. Just what this total difference of potential is, depends on the loss of voltage in the return circuit. Thus, suppose there is 25 volts drop in the return circuit between a certain point on the system and the power station. There is, therefore, a pressure of 25 volts tending to force the current through the moist earth from the rails at distant portions of the line, to the rails, water pipes, and other connected metallic structures located in the earth near the power station. The amount of current that will thus flow to earth in preference to remaining in the rails, depends on the relative resistance of the rails, the earth, and the other paths offered to the current to return to the power house.

To take a very simple case, let us suppose a single-track road, Fig. 86, with a power house at one end, and a parallel line of water pipe on the same street passing the power house. If the positive terminals of the generators are connected to the trolley wire, the current passes, as indicated by the arrows, out over the trolley wire through the cars and to the rails. When it has reached the rails it has the choice of two paths back to the power house. One is through the rails and bonding; the other is through the moist earth to the line of water pipe and back to the power house, leaving the pipe for the rails, at the power house. Should the bonding of the rails be very defective, considerable current might pass through the earth to the water pipe.

Remembering now the principles of electrolysis, we see that the oxidizing action of this flow of current from the rails to the water pipes at the distant portion of the road will tend to destroy the rails, but will not harm the water pipe at that point, as it will tend to deposit metal upon it. When, however, the current arrives at the power house, it must in some way leave this water pipe to get back to the rails, and so to the negative terminals of the generators.

Here we see that there is a chance for electrolysis of the water pipe, because at this point the water pipe forms the positive electrode, which is the one likely to be oxidized and destroyed. This very simple case is taken merely for illustration. In actual practice the conditions are never so simple as this, for there are various pipes located in the ground running in various directions, which complicate the case very much; but we can see from this simple example that the principal place electrolysis of water pipe is to be feared is at points where a large volume of current is leaving the water pipe to take to some other conductor.

As an indication of how much current is likely to be leaving the water pipes at various points, it is customary to measure the voltage between the water pipes and the electric railway track and rails. When this voltage is high, it does not necessarily mean that a large volume of current is leaving the water pipes at the point where these pipes are several volts positive with reference to the rails; but such voltage readings indicate that, if there is a path of sufficiently low resistance through the earth, and if the moisture in the earth is sufficiently impregnated with salts or acids, there will be trouble from an electrolytic action due to a large flow of current. There is obviously no method of measuring exactly the amount of current leaving a water pipe at any given point, since the pipe is buried in the earth. Voltmeter readings between pipes and rails simply serve to give an indication as to where there is likely to be trouble from electrolysis. The danger to underground pipes and other metallic structures from electrolysis has been much overestimated by some people, as the trouble can be overcome by proper care and attention to the return circuit. Trouble from electrolysis, however, is sure to occur unless such care is given.

=Prevention of Electrolysis.= Remedies for electrolysis may be classified under two heads—general and specific. The general remedy is obviously to make the resistance of the circuit through the rails and supplementary return feeders so low that there will be but little tendency for the current to seek other conductors, such as water and gas pipes and the lead covering of underground cables. This remedy consists in heavy bonding, in ample connections, around switches and special work where the bonding is especially liable to injury, and in additional return conductors at points near the power house to supplement the conductivity of the rails.

It is important that all rail bonds be tested at intervals of six months to one year in order that defective bonds may be located and renewed, as a few defective bonds can greatly lower the efficiency of an otherwise low-resistance circuit.

The specific remedy for electrolysis which may be applied to reduce electrolytic action at certain specific points, consists in connecting the water pipe at the point where electrolysis is taking place, with the rail or other conductor to which the current is flowing. Thus, for example, if it is found that a large amount of current is leaving a water pipe and flowing to the rails or to the negative return feeders at the power house, the electrolytic action at this point can obviously be stopped by connecting the water pipe with the rails by means of a low-resistance copper wire or cable, thereby short-circuiting the points between which electrolytic action is taking place. There are certain cases in which it is advisable to adopt such a specific remedy. It should be remembered, however, that a low-resistance connection of this kind, while it reduces electrolysis at points near the power house, is an added inducement to the current to take to the water pipes at points distant from the power house, because of the decrease in resistance of the water-pipe path to the power house resulting from the introduction of the connection between the water pipe and the negative return feeder at the power house. With the water pipes connected to the return feeders in the vicinity of the power house, the current which flows from the rails to the water pipes at points distant from the power house will obviously cause electrolysis of the rails but not of the water pipes, since the current is passing from the earth to the pipe, and the pipe is negative to the earth. In this case the principal danger is that the high resistance of the joints between the lengths of water pipe will cause current to flow through the earth around each joint, as indicated on some of the joints, Fig. 86, and will cause electrolytic action at each joint. It is evident, however, that the conditions of the track circuit and bonding must be very bad if current would flow over a line of water pipe, with its high-resistance joints, in sufficient volume to cause electrolysis, in preference to the rail-return circuit, especially since ordinarily the resistance offered to the flow of current over the water pipes back to the power house must include the resistance of the earth between the tracks and water pipes.

It is usually considered inadvisable to connect tracks and water pipes at points distant from the power house, because of the danger of electrolysis at water-pipe joints, as just explained.

Methods of testing rail bonds in the track will be explained under the head of “Tests.”

POWER SUPPLY AND DISTRIBUTION.

=Direct-Current Feeding.= As already explained, the majority of electric railways are operated on a 500-volt constant-potential direct-current system with a ground return. A constant potential of 450 to 550 volts is maintained between the trolley wire and track. Where the trolley wire is not sufficient, additional feeders are run from the power house and connected to the trolley wire, the number of feeders depending on the distance from the power house and the traffic.

=Booster Feeding.= Boosters are sometimes used on long feeder lines where there is a heavy load only a small portion of the time. These boosters are direct-current dynamos that are connected in series with the feeder upon which the voltage is to be raised above the regular power-house voltage. The booster may be driven either by a small steam engine or by an electric motor. The simplest form of booster is a series-wound dynamo. A booster armature must, of course, be of sufficient current capacity to pass all the current that will be required on its feeder. The voltage yielded by this dynamo, plus the power-station voltage, is the voltage of the boosted feeder as it leaves the power house. Supposing that a series-wound booster will give 125 volts at full load; it is obvious that being series-wound it will give no voltage at no load. The voltage will increase approximately as the load on the feeder increases; and since the drop in voltage on the feeder for which the booster is to compensate also varies with the load, the action of the booster is simply to add sufficient voltage to its feeder at any instant to compensate for the line loss upon that feeder and to maintain approximately constant potential at the far end of the feeder. Boosters raising the power-station voltage of a feeder more than 250 volts above the normal power-station voltage, are not common, though cases are on record where a feeder has been boosted as high as 1,100 volts above the power-station voltage. Since all the power used in driving a booster is wasted in line loss, this method of feeding is not economical; but where used only a few days out of the year it is sometimes to be preferred to a heavy investment in feeders. The investment in feeders might involve more interest charges than the cost of power wasted in booster feeding would amount to.

=Alternating-Current Transmission.= High-tension alternating-current transmission _to_ substations, with direct-current distribution _from_ substations, is extensively used on long interurban roads, and on large city street-railway systems where power is to be distributed over a wide area. In such cases the power house is equipped with alternating-current dynamos supplying high-tension three-phase alternating current to high-tension transmission lines or feeders. These high-tension feeders are taken to substations located at various points on the road, where the voltage is reduced by step-down transformers; and these transformers supply current to operate rotary converters, which convert from alternating to direct current for use on the trolley.

The advantage of this system of high-tension distribution is that, owing to the high transmission voltage, there is but a small loss in the high-tension lines, which lines can be made very small, and will thus involve but little copper investment. The substations can be located at frequent intervals, so that the distance the 500-volt direct-current must be conducted to supply the cars is not great. Current from one power house can thus be distributed over a very large system in cases where, if the 500-volt direct-current system of distribution were used, the cost of feeders for distributing such a low-voltage current would be prohibitive. Were the alternating-current high-tension scheme of distribution not used, it would be necessary to have a number of small power houses at various points on the system instead of one large power house. The cost of operation of several small power plants per kilowatt output, is likely to be much greater than that of one large power plant. The first cost of the alternating-current distributing system, including power house and substations, is likely to be considerably higher than would be the cost of a number of small power houses; but in cases where alternating-current distribution has been installed, it has been figured that the cost of operation of the central power house with alternating-current distribution would be sufficiently low as compared with several small ones to pay more than the interest on this extra investment.

[Illustration: Fig. 87. Diagram of Distributing System.]

=A System of Distribution for an Interurban Railway.= The typical features of a high tension system of distribution for an extensive interurban railway system are shown in Fig. 87, which represents the electrical transmission and distribution system of the Indiana Union Traction Company. The central power station at Anderson feeds into thirteen rotary converter substations from 7 to 65 miles distant from the power house. The substations east of Indianapolis are fed at 16,000 volts and are placed about 11 miles apart. The substations due north of Indianapolis are located at intervals of about 17 miles and are fed at 30,000 volts.

The power station at Anderson has a total capacity of 5,000 K. W. The substations vary in capacity from 250 to 1,500 K. W.

=Efficiency of Transmission Systems.= The average efficiency of a high tension transmission system for a certain interurban electric railway system are given below. Current was generated at 380 volts. The step-up transformers raised it to a potential of 16,000 volts at which pressure it was transmitted to eight substations at distances from 10 to 40 miles from the power station. It was then stepped down to 380 volts and converted to direct current by a rotary converter. The tests extended over a period of three days. The efficiency of the step-up transformers was 95 per cent; of the high tension line 92.9 per cent; of the step-down transformers 95 per cent; and of the rotary converters 88 per cent; giving a total efficiency of the transmission system of 73.5 per cent.

=Power House Location.= A power house is usually located where coal and water supply can be cheaply obtained. For this reason it is placed either on some line of railroad or where coal can be taken to it over the electric railway.

As it is always desirable to operate the engines in connection with condensers, on account of the saving in fuel, which is approximately 20 per cent with condensers, power stations are located, when possible, near rivers and ponds from which a large supply of cold water for condensation of exhaust steam can be obtained. Where no such natural water supply is available, it has become customary to provide means for artificially cooling a sufficiently large supply of water for condensation. One method is to erect a number of towers, so constructed that the water when pumped to the top will fall through a structure that breaks the water up into fine spray as it falls, thus allowing it to cool by evaporation so that it can be used again for the condensers when it arrives at the bottom of the tower. Where more room is available, ponds are sometimes excavated near the power house, and the water is made to flow back and forth through a series of troughs located above the pond, and it is thus cooled.

Where a power station is of the direct-current type, operating at 500 to 600 volts, it is desirable to have it as near the center of electrical distribution as possible, in order to keep down the amount of investment in the feed wire; but it is more important to have it located near a cheap coal and water supply than exactly at the center of distribution.

It is also desirable to have the station located where there is room for coal storage, on account of the chances for interruption of the coal supply by strikes, railroad blockades, and other causes beyond the company’s control. The continuity of the coal supply is also another argument against placing the station where dependence must be placed upon wagons or inadequate railroad facilities.

Coal handling, after the coal has reached the station, is done by hand in the smaller power stations; but in larger power stations it has come to be the general practice to do as much of the handling as possible by means of automatic coal conveyors. The most elaborate power stations have means for dumping coal from cars into hoppers, from which it is conveyed by an endless chain provided with buckets, called a _coal conveyor_, to storage bins. Coal conveyors also take the coal from the storage bins, and deposit it in the hoppers of mechanical stokers in front of the boilers. Ashes are conveyed from under the boilers by the same kind of conveyors, and are dumped into hoppers, whence they are drawn into cars or wagons to be hauled away. The coal, having been deposited in hoppers at the boiler front, is automatically fed into the furnaces by automatic stokers. One type of automatic stoker in common use is of the chain-grate or link-belt type, which is constructed like an endless sprocket chain, with links composed of heavy cast-iron blocks that serve as grate bars. This link belt or chain is kept in constant, slow motion by a small stoker engine or motor which operates all the stokers of a line of boilers. The coal is fed from the hopper on to the chain grate, and the chain is slowly moved under the boilers. As the coal on that part of the grate under the boilers is on fire, the fresh coal as it enters the furnaces is soon ignited. The grate is run at such a rate, and the thickness of the coal is so adjusted, that the coal is burned to an ash by the time it has traveled to the back of the furnace. There the grate turns down over a sprocket wheel, and the ashes are dumped into the ash pit as the grate revolves.

The boilers in most common use in large American electric-railway power houses are of the water-tube type, in which water is contained inside of a bank of tubes, the ends of these tubes being connected to drums or headers. The horizontal return-tubular type of boiler is used in many of the smaller power stations, and vertical boilers are also in use.

The engines in the larger and more economical stations are generally of the Corliss compound-condensing type, running at speeds of from 60 to 120 revolutions per minute, according to the size of the unit. The smaller the unit, the higher the speed. In the smaller and older stations, simple Corliss engines belted to generators are frequently found, and high-speed engines also are used. It is the almost universal custom now, to place the generator directly on the engine shaft, making a direct-connected unit.

Steam turbines, in which the steam acts in jets against the blades of a turbine wheel, are beginning to come into use at the present time. These turbines rotate at very high speed, the largest and slowest speed-units running 600 r.p.m., and others at higher rates. As the output of any generator varies directly according to its speed, a very much smaller generator can be used when coupled to a high-speed steam turbine, to obtain a given output, than if the generator must be coupled to a Corliss steam engine which revolves at very low speed. The economy of the steam turbine at full load is about that of a compound-condensing Corliss engine, but is better on light loads than the engine. The turbine requires less building space and a much less expensive foundation.

[Illustration: Fig. 88. Plan of Power House.]

Railway generators or dynamos for direct current are usually built with compound-wound fields, so that, as the load increases, they will automatically raise the voltage at their terminals to compensate for the drop in the feeders and to maintain a constant potential at the cars. Thus, if the line loss on a system is 10 per cent, or 50 volts at full load, the generators will be provided with shunt fields of sufficient strength to give 500 volts at no load, and with series field coils which will add to the field strength enough to give 550 volts at full load. The amount of “compounding”—which is the term applied to this method of increasing voltage—may be any amount within reasonable limits. The pressure maintained at different companies’ electric-railway power houses varies, but is usually between 500 and 600 volts.