Fig. 65.—Area of Electrical Distribution at Montreal.

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Fig. 66.

Fig. 67.

Fig. 68.

In some instances, however, transmission and distribution lines are joined without the intervention of a sub-station, where regulation of voltage can be accomplished, though this practice has little to recommend it aside from the savings in first cost of installation and subsequent cost of operation. These savings are more apparent than real if fairly constant pressure is to be maintained at the lamps, because what is gained by the omission of sub-stations will be offset, in part at least, by additional outlays on the lines if good regulation is to be maintained. This fact may be illustrated by reference to [Figs. 66], [67], and [68], in each of which D represents a generating station and A, B, and C towns or cities where energy from the station is to be distributed. In the case of each figure it is assumed that the distance between the generating station and each of the cities or towns is such that distributing lines with a loss of, say, not more than two per cent in voltage at full load cannot be provided between the generating station and each city or town because of the cost of conductors. This being so, one or more centres of distribution must be located in each town, and the transmission lines must join the distribution lines at these centres either on poles or in sub-stations. If several of these towns are in the same general direction from the generating plant so as to be reached by the same transmission line, as A, B, and C in [Fig. 66], this one line will be all that is necessary with a sub-station in each town. Where sub-stations are not employed a separate transmission circuit must be provided between the generating plant and each town for reasons that will appear presently. The percentage of voltage variation in a transmission line under changing loads will be frequently from five to ten, and is thus far beyond the allowable variations at incandescent lamps. To give good lighting service the centre of distribution, where the transmission line joins the distribution circuits, must be maintained at very nearly constant voltage if no sub-station is located there. Regulation at a generating station will compensate for the changing loss of pressure in a line under varying loads so as to maintain a nearly constant voltage at any one point thereon. No plan of station regulation, however, can maintain constant voltages at several points on the same transmission line when there is a varying load at each. The result is that even though the several towns served are in the same general direction from the generating station, as in [Fig. 67], yet each town should have its separate transmission line where no sub-stations in the towns are provided. In the case illustrated by [Fig. 68], where the towns served are in very different directions from the generating station, there should be a separate transmission line to each, regardless of whether there is a sub-station or only a centre of distribution there.

Even in the case illustrated by [Fig. 68], as in each of the others, there is a large saving effected in the cost of distribution lines by the employment of a sub-station at the point where these lines join the transmission circuit, provided that the variation of pressure at lamp terminals is to be kept within one volt either way from the standard. With the variations of loads the loss of pressure in the distribution lines will range from zero to its maximum amount and the connected lamps will be subjected to the change of voltage represented by this total loss, unless the distribution start from a sub-station where the loss in distribution lines can be compensated for by regulation. To give good service the distribution lines should be limited to a loss of one per cent at full load if there is no sub-station where they join transmission lines. With opportunity for regulation at a sub-station the maximum loss in distribution lines may easily be doubled, thus reducing their weight by one-half in comparison with that required where there is no sub-station.