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insulating material used in these hightension cables consists of paper treated with a resinous compound, the thickness in the 30,000-volt cable being about onehalf inch. Over this is a lead sheath about 34-inch thick, to protect the cable against moisture and mechanical injury. These cables are drawn into ducts laid below the surface in the streets, as shown in Figs. 3 and 4.
One of the earliest instances of the use of a three-phase transmission to a substation, for conversion to direct current of an Edison system, was in Chicago,
EUctrical Aer, February, 1904, p. 6S.
stepped down again; and, after passing through the rotary converters, the current was fed into the direct-current system at 115 volts, one rotary being connected to each side of the Edison system. Such was the humble beginning of the very extensive system of high-tension transmission lines and substations which, at double the initial voltage of 4,500, is now in operation in Chicago.
In the same year there was installed in Brooklyn, N. Y., a similar transmission system with a rotary converter substation. Here the current was generated at 6.600 volts, 25 cycles, three-phase, at the
Fig. 20. A 500-volt 2-phase Generator.
phase 25-cycle 6.600-volt alternator. A 5,000-K. W. Curtis turbo-generator is being installed, and there is room for four more. This will make a total rated capacity of 75,000 K. W., all power being generated as alternating current for transmission to rotary converter substations, from which it will feed into the Edison three-wire direct-current system. In the splendid new Fisk Street station in Chicago, 9,000-volt 25-cycle threephase current only is generated, all by Curtis turbo-generators. In Fig. 28 is shown a 5,000-K. W. Curtis turbine with generator; and Fig. 29 shows a Westinghouse-Parsons unit of the same capacity. In Philadelphia the 5,500-volt twophase 60-cycle system, already referred to, is used for transmission to rotary converter substations, as well as for the alternating-current distribution. Fig. 30
shows a row of rotary converters of a substation located in the basement of a sky-scraper in the heart of the business district of Chicago. The switchboard of a combined rotary converter and battery substation is shown in Fig. 31 ; the rear view, in Fig. 32.
Where the load connected to transmission lines consists solely of substation converting apparatus, a low frequency is desirable because of the accompanying low inductive and capacity reactance of the lines, and also because of the slower speed of the synchronous motors and rotary converters which is had for a given number of field poles with a lower frequency. The higher the frequency, the greater the number of poles required, or the greater the speed. Because of the necessary number of commutator bars required between the brushes of a D. C. machine, and therefore also on a rotary converter, the distance between the centers of the pole pieces (that is, the pole ''pitch") cannot be less than a certain
ators, producing both direct and alternating current at the same time, form a very valuable element in a large station in which both these currents are generated, the direct current for general distribution, and the alternating current for transmission to substations. A frequency of 25 cycles is therefore generally accepted as the most desirable for straight
the alternating distribution, as distinguished from transmission to substations, is effected by 60-cycle current. The 25cycle transmission current is then converted to current of 60 cycles per second by means of motor generators or straight frequency-changer sets.
In Fig. 33 is shown such a motor-generator substation. The small units in the constant-potential arc lamps. What this new system meant to the neighborhood in which the old stations were located, will be appreciated when one remembers the noise and dirt and smoke of these old stations, and then views Fig. 34, a substation set in the rear of a lot in a fine residence section.
We have seen, then, how the highvoltage polyphase system has been evolved out of, and has unified, the mixed systems which were brought under one head during the era of consolidation. A study of some of the newest installations leads to the thought that perfection of system has almost been reached, and that further progress will be rather along the line of higher efficiency of apparatus at both ends of the system. When the true electrical era has arrived, when houses no longer have need of chimneys and all operations are performed electrically, then new problems will arise. How they will be met, none can now say; but they will