Another case in which synchronous motor-generators deliver power from a transmission line that carries both a lighting and a motor load is that of the Shawinigan sub-station in Montreal. At this sub-station the 85-mile transmission line from the generating plant at Shawinigan Falls terminates. As already pointed out, this line is composed of a single three-phase circuit of aluminum conductors, each of which has a cross section of 183,750 circular mils. In the Montreal sub-station the thirty-cycle, three-phase current from Shawinigan Falls is delivered to transformers that lower the voltage to 2,300. The current then goes to five synchronous motor-generators of 1,200 horse-power capacity each, and is there converted to sixty-three cycles per second, two-phase, at the same voltage. This converted current passes onto the distribution lines of the local electrical supply system in Montreal, which also draws energy from two other water-power plants, and is devoted to lighting, stationary motors, or to the street railway work, as may be required. Though separate local distribution circuits are devoted to these several loads, the fluctuations in the stationary and railway motor work necessarily react on the voltage of the transmission line and transformers at the sub-station. By the use of the synchronous motor-generators the lighting circuits are protected from these pressure variations.
As the numbers of sub-stations at different points on long transmission lines increase, and stationary motor and railway loads at each become more common, it is to be expected that the use of synchronous motor-generators for lighting service will be much more frequent than at present. With such use there will disappear one of the reasons for the multiplication of transmission circuits.
Fig. 78.—Transfer Switches at Saratoga Switch-house on Spier Falls Line.
Where several transmission circuits connect a generating plant with a single sub-station, or with several sub-stations in the same general direction, it is desirable to have switches so arranged that two or more circuits may be combined as one, or so that any circuit that ordinarily operates a certain load or sub-station may be devoted to another when occasion requires. For this purpose transfer switches on each circuit are necessary at generating plants, sub-stations, and often at switch-houses. These transfer switches will ordinarily be of the knife type, and intended for manual operation when the circuits to which they are connected are not in use. As such switches are exposed to the full voltage of transmission, the insulation of their conducting parts should be very high. In the extensive transmission system between the power-plants at Spier Falls and Mechanicsville and the sub-stations at Troy, Albany, and Schenectady, N. Y., a transfer switch of highly insulated construction has been much used. The two blades of this switch move independently of each other, but both are mounted between the same metal clips. Each blade is of two by one-quarter inch drawn copper rod, and the clips supporting the two blades are mounted on top of a circular metal cap four and three-quarter inches in outside diameter and two inches high, that is cemented over the top of a large, double petticoat, porcelain line insulator.
Fig. 79.—Cross Section of Schenectady Switch-house on Spier Falls Line.
Clips into which these copper blades are swung in closing the switch are also mounted in caps carried by insulators in the way just described. Each of these insulators is mounted on a large wooden pin, and these pins are secured in timbers at the points where the switches are wanted. This construction of switches gives ample insulation for the line voltage of 30,000 in this system. By means of the transfer switches just described, either of the transmission circuits leaving the Spier Falls power-plant may be connected to any one of the ten generators and ten groups of transformers there. At the Saratoga switch-house, any one of the twelve conductors, making up the four three-phase circuits from Spier Falls may be connected to any one of the eighteen conductors making up the six three-phase circuits that go south to Saratoga, Watervliet, and Schenectady sub-stations, in the way indicated by the drawing. So again at the Watervliet sub-station, where energy at 26,500 volts is received from Spier Falls and energy at 10,800 volts from Mechanicsville, any single conductor from either of these water-power plants may be connected, either directly or through a transformer, with either conductor running to the railway and lighting sub-stations about Albany and Troy. Where several transmission circuits are employed, this complete flexibility of connection evidently adds materially to the convenience and reliability of operation.
Circuits in Transmission Lines.
| Location of Lines. | Length in Miles. | Number of Circuits. | Number of Pole Lines. | Circular Mils per Wire. | Cycles per Second of Current. | |
|---|---|---|---|---|---|---|
| Electra to San Francisco | 147 | 1 | 1 | [[A]]471,034 | 60 | |
| Colgate to Oakland, Cal. | 142 | 2 | 2 | 133,100 | 60 | |
| [[A]]211,000 | ||||||
| Santa Ana River to Los Angeles | 83 | 2 | 1 | 83,690 | 60 | |
| Shawinigan Falls to Montreal | 85 | 1 | 1 | [[A]]183,750 | 30 | |
| Cañon Ferry to Butte | 65 | 2 | 2 | 106,500 | 60 | |
| Welland Canal to Hamilton | 35 | 1 | 1 | 83,690 | 60 | |
| Welland Canal to Hamilton | 37 | 1 | 1 | 133,100 | 60 | |
| Spier Falls to Schenectady | 30 | 2 | 1 | 105,600 | 40 | |
| 167,800 | ||||||
| Spier Falls to Watervliet, N. Y. | 35 | 2 | 1 | 167,800 | 40 | |
| Ogden to Salt Lake City | 36 | 2 | 1 | 83,690 | 60 | |
| Apple River Falls to St. Paul | 27 | 2 | 1 | 66,370 | 60 | |
| Niagara Falls to Buffalo | 23 | 2 | 1 | 350,000 | 25 | |
| Niagara Falls to Buffalo | 20 | 1 | 1 | [[A]]500,000 | 25 | |
| Farmington River to Hartford | 11 | 1 | 1 | [[A]]364,420 | 60 | |
| Niagara Falls to Toronto | 75 | 2 | 1 | [[B]] | 190,000 | 25 |
| [A] Aluminum conductor. | ||||||
| [B] Steel towers. | ||||||