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.
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.