These two transformers, with the connections above indicated, will change the 500-volt, two-phase current to 20,000 volts, three-phase.
At one of the water-power stations supplying energy for use in Hartford, four transformers of 300 kilowatts each change 500-volt, two-phase current from the generators to 10,000-volt, three-phase, for the transmission line.
In the Niagara water-power station the generators deliver two-phase current at 2,200 volts, and 975-kilowatt transformers are connected in pairs to change the pressure to 22,000 volts, three-phase, for transmission to Buffalo.
A transformer is used in some cases to raise the voltage and compensate for the loss in a transmission line. For this purpose the secondary of a transformer giving the number of volts by which the line pressure is to be increased is connected in series with the line. The primary winding of this transformer may be supplied from the line boosted or from another source.
Transformers ranging in capacity from 100 to 1,000 kilowatts each, such as are commonly used for transmission work, have efficiencies of 96 to 98 per cent at full loads, when of first-class construction. Efficiency increases slowly with transformer capacity within the limits named, and 98 per cent can be fairly expected in only the larger sizes. In any given transformer the efficiency may be expected to fall a little, say one or two per cent, between full load and half load, and another one per cent between half load and quarter load. These figures for efficiencies at partial loads vary somewhat with the design and make of transformers. In general, it may be said that step-up or step-down transformers will cost approximately $7.50 per kilowatt capacity, or about one-half of the like cost of low-voltage dynamos. If dynamos of voltage sufficiently high for the transmission line can be had at a figure below the combined cost of low-volt dynamos and raising transformers, it will usually pay to avoid the latter and develop the line voltage in the armature coils. This plan avoids the loss in one set of transformers.
+-------------------------------+----------+---------+---------+ | | Trans- | Trans- | Genera- | | | formers | formers | tors at | | |at Power- |at Sub- | Power- | |Transmission System. |stations. |stations.|stations.| | +----+-----+---+-----+---+-----+ | |No. | Kw. |No.| Kw. |No.| Kw. | | | |Each.| |Each.| |Each.| +-------------------------------+----+-----+---+-----+---+-----+ |Cañon Ferry to Butte | 12| 325 |[A]| [A] |...| ... | | | 6| 950 | 6| 950 | 10| 750 | |Apple River to St. Paul | ...| ... | 6| 300 |...| ... | | | 6| 500 | 4| 200 | 4| 750 | |White River to Dales | 3| 400 | 3| 375 | 2| 500 | |Farmington River to Hartford | 4| 300 |...| ... | 2| 600 | |Ogden to Salt Lake |[B]9| 250 |...| ... | 5| 750 | |Colgate to Oakland | ...| 700 |...| ... | { 3|1125 | | | | | | | { 4|2250 | |Presumpscot River to Portland | ...| ... | { 6| 200 |...| ... | | | | | { 3| 150 | 4| 500 | | | | | | | { 1| 180 | | | | | | | { 3| 300 | |Four water-powers to Manchester| ...| ... | 21| 200 | { 1| 450 | | | | | | | { 4| 650 | | | | | | | { 1|1200 | +-------------------------------+----+-----+---+-----+---+-----+ [A] Other transformers at Helena sub-stations. [B] Part of energy distributed directly from generators.