At sub-stations it is desirable to have a group of transformers for each transmission circuit, and it may be necessary to subdivide the transformer capacity still further in order to keep transformers in operation at nearly full load, or to provide a group of transformers for each sort of service or for each distribution circuit. All of the transformers at a sub-station should have a total capacity at least equal to that of the generators whose energy they are to receive, minus the losses in step-up transformers and the line. Transformers at sub-stations do not necessarily correspond in number or individual capacity with those at generating stations, and the number of sub-station transformers bears no necessary relation to the number of generators by which they are fed.
Two transmission circuits extend from Cañon Ferry to a sub-station at Butte, and in that sub-station there are six transformers divided into two groups for three-phase operation, each transformer being rated at 950 kilowatts. This sub-station equipment thus corresponds to only the six 950-kilowatt transformers in the generating station, because the four groups of smaller transformers there are used to supply the transmission line to Helena.
In the sub-station at St. Paul that receives the entire output of the plant on Apple River, where the six transformers of 500 kilowatts each are located, ten transformers receive energy from two three-phase transmission circuits. Six of these transformers are rated at 300 kilowatts each. The 300-kilowatt transformers are connected in two groups of three each, and the 200-kilowatt in two groups of two each, transforming current from three-phase to two-phase. The aggregate capacity of the sub-station transformers is thus 2,600 kilowatts, while that of transformers at the generating station is 3,000 kilowatts. With four generators at the water-power plant there are ten transformers at the sub-station, where all the energy, minus losses, is delivered.
At Watervliet, where one of the several sub-stations of the system with its larger generating plant at Spier Falls is located, the capacity of each transformer is 1,000 kilowatts, though each transformer at Spier Falls has a rating below this figure.