Fig. 2,142.—Single and twin circuit poles (Southern Power Co.). The twin circuit pole at the right is used for 11,000 volt circuits, while the single circuit poles at the left carry 44,000 volt conductors, being used on another division for 100,000 volt line.
Every case of electric transmission presents its own problem, and needs thorough engineering study to intelligently choose the system best adapted for the particular case.
Single Phase Systems.—There are various arrangements for transmission and distribution classed as single phase systems. Thus, single phase current may be conveyed to the various receiving units by the well known circuit arrangements known as series, parallel, series parallel, parallel series, connections previously described and illustrated in figs. 2,131 to 2,134.
Again single phase current may be transmitted by two wires and distributed by three wires. This is done in several ways, the simplest being shown in fig. 2,143.
Fig. 2,143.—Diagram illustrating single phase two wire transmission and three wire distribution. The simplified three wire arrangement at A, is not permissible except in cases of very little unbalancing. Where the difference between loads on each side of the neutral may be great some form of balancing as an auto-transformer or equivalent should be used, as at B.
Ques. Under what conditions is the arrangement shown in fig. 2,143 desirable?
Ans. This method of treating the neutral wire is only permissible where there is very little unbalancing, that is, where the load is kept practically the same on both sides of the neutral.
Ques. What advantage is obtained by three wire distribution?