Thus in the two water-power plants connected with the electrical supply system of Hartford, Conn., the alternators operate at 500 volts with transformers that put the line voltage up to 10,000. In the station on Apple River that supplies the lighting system of St. Paul, Minn., the alternators operate at 800 volts, and this is raised to 25,000 volts for the line. At Cañon Ferry the alternator voltage of 500 is multiplied by 100 in the transformers giving 50,000 on the line.
[Illustration: FIG. 52.--Transmission Line of New Hampshire Traction Company.]
Where the generating station of a transmission system is located close to a part of its load the alternators are given a voltage suitable for distribution, say about 2,400, and any desired pressure on the line is then obtained by means of step-up transformers. Two of the Niagara Falls plants are an illustration of this practice, the voltage of all the alternators there being 2,200, which is raised to 22,000 for the transmission of a part of the energy to Buffalo. A similar practice is followed in the water-power plant at Ogden, where the generators furnish current at 2,300 volts for local distribution, and transformers raise the pressure to 26,000 volts for the transmission to Salt Lake City. In the 32,000-kilowatt plant at Sault Ste. Marie, Mich., the alternators operate at 2,400 volts and a large part of their load is local, but this voltage will no doubt be raised by transformers when transmission lines are operated.
For generating stations that carry little or no local loads the cost of transformers can be saved if the generators develop the voltage required on the transmission lines. This possible saving has led to the development of alternators that generate voltages as high as 15,000 in their armature coils. Such alternators have stationary armatures in all cases and are of either the revolving magnet or inductor type.
At the present time many transmission systems in the United States operating at 10,000 or more volts develop these pressures in the armature coils of their alternators, and the number of such systems is rapidly increasing. It is now the rule rather than the exception to dispense with step-up transformers on new work where the line voltage is anything under 15,000. Perhaps the longest transmission line now in regular operation with current from the armature coils of an alternator is that at 13,200 volts between the generating station at Portsmouth and one of the sub-stations of the New Hampshire Traction system at Pelham, a distance of forty-two miles.