XII Electricity From Central Stations
Large dynamos generate electricity very much more cheaply than small machines can, and machines which have a full load continually produce the current very much more cheaply than those which run upon very light load part of the time. The largest central stations with load evenly distributed for the whole day could furnish electricity profitably at four cents per kilowatt hour. There are many small electric lighting plants which furnish current from sundown to midnight only at fifteen cents per kilowatt hour, with little profit. The transformer (Fig. 127) makes it possible to gather all this generation of electricity for sparsely settled districts into large central stations, located sometimes far away from the consumer perhaps, where there is abundant power in some water-fall, thus saving the expense of coal for running the dynamos.
A few years ago there were no central stations for this purpose. Now according to the latest census reports there are in the United States about 30,000 plants, including those which belong to certain cities, that generate electricity for sale, and there are twice as many more isolated plants to furnish light and power in factories, hotels, etc.
The money invested in central station business now exceeds six billion dollars, and the annual output of electric current is sufficient to keep eight billion 16-candle-power carbon filament electric lights burning continuously night and day. All this has more than doubled in the last five years. Central stations are now furnishing about five times as much current for heating, cooking, and charging automobiles as they did five years ago. About one third of all the central stations depend on water power.
We might take as the type of hydro-electric central station, that is, one which generates electricity by water-power, the Glenwood Station of the Central Colorado Power Company. This station has two 9000 horse-power water turbines. Each water-wheel drives an alternating-current generator which develops 4000 volts of e. m. f. These water wheels and generators are shown in Fig. 129. The penstocks are to be seen coming through the back wall of the building. They bring water at 170 foot head, or about seventy-five pounds per square inch static (standing) pressure. Three huge transformers, each weighing twenty-six tons, step up the e. m. f. from 4000 to 100,000 volts. These are the cylinders shown in Fig. 130. They simply contain a great many coils of copper wire with a vast amount of iron at the centre. They accomplish in a large way what our spark coil does in a lesser degree. But why go to all this expense to produce such a dangerous and troublesome voltage? The answer briefly is, that while it is dangerous and troublesome the expense is not so great as it would be to supply by any other method the electric current required. Denver and numerous other places, large and small, require electric current. From one to two hundred miles away on the Grande River, there is vast power running to waste. We have to choose on the one hand between buying power in the shape of coal and distributing power plants to those various localities where electricity is needed, and on the other using this water-power, which is now running to waste, to generate electricity which we may transmit and distribute throughout the one hundred and eighty-five miles to Denver, Leadville, Boulder, Dillon, Idaho Springs, etc. But electric energy transmitted a long distance suffers great loss.
Suppose, for instance, I needed to supply fifty amperes at one hundred-volt pressure ten miles distant from the generator, and had a conductor the size of a trolley wire to bring the current. The resistance of the trolley wire is one ohm for every two miles, or five ohms. The drop in voltage is found by multiplying the amperes of current by the ohms of resistance. Ten miles from the central station, therefore, the drop on fifty amperes would be 50 × 5 = 250 volts. It would, therefore, be necessary to maintain a pressure of 350 volts at the generator to deliver the fifty amperes at 100 volts. The energy supplied by the generator is 350 volts × 50 amperes = 17,500 watts = 17.5 K. W. The energy delivered to the consumer is 100 volts × 50 amperes = 5000 watts = 5 K. W. In order to deliver fifty cents’ worth of electricity per hour to the consumer it would, in this case, be necessary to generate $1.75 worth of electricity at the central station. That is, about seventy per cent. of the energy generated would be wasted in transmission. If now we decide to generate this electrical energy at ten times as high voltage it will be necessary to transmit only one tenth as many amperes, or five. In this case the drop in voltage would be 5 amperes × 5 ohms = 25 volts. It would be necessary to maintain 1025 volts of pressure at the generator to deliver to the consumer the five amperes at 1000 volts = 5000 watts. That is, to deliver 5000 watts in this case we must generate 1025 volts × 5 amperes = 5125 watts, and less than 2½ per cent. of the energy generated would be lost in transmission.
If now the consumer must have his energy delivered at 100 volts, we must introduce a step-down transformer at his end of the line which may give him 50 amperes at 100 volts = 5000 watts. This transformer, being small, will cause a loss of 15 or 20 per cent., but if there were a very large amount to transform it might be done with a loss of only 4 per cent.
It is not thought to be advisable to raise the voltage at the generator higher than 4000. This will not suffice to supply large working currents to a greater distance than about six or eight miles. For a distance of 10 miles 6000 volts are desirable; for 50 miles 30,000 volts; for 100 miles 60,000 volts; for 165 miles 100,000 volts; and for 200 miles 120,000 volts. Notice that in this table the voltage rises at the rate of 600 per mile. Since it is not desirable for the generator itself to produce a higher voltage than 4000, we must depend upon transformers to produce these high voltages. Let us then consider, a little more in detail, the construction of a transformer. I have here some drawings of one which I propose that we make in the machine shop, and use in our central station equipment in the future. We will procure the thinnest and softest sheet iron possible and cut out of it a lot of pieces shaped like the letter H with the dimensions shown in Fig. 131. These are to be piled one upon another, with strips of paper between, until the pile is 1½ inches thick. Then four pieces of board are to be bolted to the sides of these (Fig. 132). The dimensions of each of the four blocks, is to be 7½ inches long by 3 inches wide by 1½ inches thick. Upon the cross bar of the H we will wind 400 turns of No. 12 double cotton-covered copper wire, bringing out the ends for future attachments, and then wind on 1200 turns of No. 10 double cotton-covered copper wire. The wire will fill the space between the blocks as indicated by the diagram in Fig. 133. We will then cut strips of the sheet iron 6 inches long by 1¼ inches wide, and bridge across the ends of the H, prying open the leaves of sheet iron and tucking them in between as shown in Fig. 134. We shall then drill a hole at each corner and bolt them in place. Binding posts will be placed at a, b, c, and d (Fig. 134), and the two ends of the No. 12 wire will be brought to a and b and those of the No. 18 wire will be brought to c and d. Going through all this detail of construction has probably made you lose sight of the essential features of this transformer. Let us for a moment turn back and see what they are. We have a large coil of wire 3 inches long and 7½ inches in diameter. It is composed of a coarse winding and a fine winding, which we may designate as the primary and secondary coils, if we choose. Of course the only reason for having different sizes of wire is so that we may send larger currents through one than the other. The coil has a laminated iron core, that is, it is composed of layers of sheet iron. These layers are insulated from one another. This is essential, although we cannot explain why now. But perhaps the most essential feature of the transformer is that iron extends clear around from one pole of this electro-magnet to the other. Fig. 135 represents a section through the coil made in the plane of e f g (Fig. 134). The core of the magnet is represented as heavily shaded. The magnetic circuit is said to be closed from one pole of this magnet to the other through the strips of iron which pass across the ends and down the sides of the coil. The arrows show the path of the magnetic circuit. The dotted portion shows where the copper wire may be supposed to have been cut across. Inasmuch as the electric current is induced in the secondary circuit by continually varying the strength of the magnetic field as much as possible, the alternating current is the most desirable to use in the primary. If the direct current were used an interrupter would be necessary, which would of course produce too much sparking when any but low tension currents are used in the primary circuit. The most interesting and curious fact about the transformer is that the voltages of the primary and secondary currents are in exact proportion to the number of turns in the wire of the two circuits.
In our transformer the number of turns in the coil between the binding posts a and b is 400 and the number of turns between c and d is 1200. If now we connect a 112-volt alternating current with the binding posts a and b, a volt meter connected across between c and d will show 336 volts, and if b and c be connected by a short wire, bringing in 1600 turns into the secondary circuit, a volt meter connected across between a and d will show a voltage of 448. Or if, leaving b and c still connected by a short wire, we connect the 112-volt alternating current to a and d a volt meter connected across between a and b will show 28 volts, or if connected between c and d it will show 84 volts, and if finally the 112-volt current is connected to c and d the pressure between a and b will be 37⅓.
The story, then, of the central station which we have chosen as a type is briefly this: Falling water makes dynamos revolve, generating a 4000-volt alternating current. This current is sent through the primary windings of transformers. The secondary windings of these transformers have twenty-five times as many turns as the primary coils. This steps up the voltage from 4000 to 100,000, making it necessary to send only one twenty-fifth as many amperes over the lines as would be required at 4000 volts, and reduces the loss in transmission to nearly one twenty-fifth. At the other end of the line the current traverses the secondary windings of transformers, and the consumer receives his current from primary coils which may step the e. m. f. down to any required volts of pressure, generally 110.
Now I shall be glad to have you consider whether this suggests any practicable problems for us here in Millville.
The sun is nearly setting and I suppose the family is expecting me home.