Ans. It is a method of arranging the connections of a bank of three delta connected transformers when one becomes disabled as in fig. 2,171.
Change of Frequency.—There are numerous instances where it is desirable to change from one frequency to another, as for instance to join two systems of different frequency which may supply the same or adjacent territory, or, in the case of a low frequency installation, in order to operate incandescent lights satisfactorily it would be desirable to increase the frequency for such circuits. This is done by motor generator sets, the motor taking its current from the low frequency circuit.
Synchronous motors are generally used for such service as the frequency is not disturbed by load changes; it also makes it possible to use the set in the reverse order, that is, taking power from the high frequency mains and delivering energy at low frequency.
Fig. 2,172.—Course of the Schaghticoke-Schenectady transmission line of the Schenectady Power Co. This transmission line carries practically the entire output of the Schaghticoke power house to Schenectady, N. Y., a distance of approximately 21 miles. The line consists of two separate three phase, 40 cycle, 32,000 volt circuits, each of 6,000 kw. normal capacity. These circuits start from opposite ends of the power house, and, after crossing the Hoosic River, are transferred by means of two terminal towers, fig. 2,173, to a single line of transmission towers. The two circuits are carried on these on opposite ends of the cross arms, the three phases being superimposed. The power house ends of the line are held by six short quadrangular steel lattice work anchor poles with their bases firmly embedded in concrete, the cables being dead ended by General Electric disc strain insulators. This equipment, together with the lightning arrester horn gaps and the heavy line outlet insulators mounted on the roof of the power house, is shown in fig. 2,174. While each circuit carries only 6,000 kw. under normal conditions, either is capable of carrying the entire output of the station; in this case, however, the line losses are necessarily augmented. This feature prevents any interruption of the service from the failure of one of the circuits. There are altogether 197 transmission towers, comprising several distinct types.
Ques. In the parallel operations of frequency changing sets what is necessary to secure equal division of the load?
Ans. The relative angular position of the rotating elements of motor and generator must be the same respectively in each set.
Fig. 2,173.—Beginning of Schaghticoke-Schenectady transmission line; view showing transfer towers with power house in background.