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be below the surface. In the early days where, in 1897, a 250-K. W. inverse of underground cables 5,000 volts was rotary converter, which converted direct thought very high pressure; to-day 25,- current of 250 volts to three-phase 25000 volts is not considered excessive; and cycle currents, was installed at the Hara recent article in an electrical journal* rison Street station. By means of step-up stated that an underground cable system transformers, this pressure was raised to will soon be installed, to be operated at 4,500 volts, the pressure of the transmis30,000 volts. Unfortunately the article sion. In the substation on Wabash Avedid not state where this would be. The nue near 27th Street, this voltage was
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 18-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,
Electrical Age, February, 1904, p. 68.
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
Union station, a similar system to that operated by the New York Edison Company. In the magnificent Waterside station of this latter company, there are at present eleven 5,500-H. P. vertical engines, each driving a 4,500-K. W. three
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
Fig. 26. 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
Fig. 28. A 5,000-K. W. Curtis TURBO-ALTERNATOR.
fixed limit; and therefore, for a given speed, the lower frequency requires a far simpler and cheaper construction. The higher-frequency machines are also more liable to "hunt," especially when the load varies considerably. Furthermore, double-current generators having a commutator connected to the armature wind
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