Percent efficiency.

loads or overloads are to be carried. The distribution of load on any large system is constantly changing as new customers are connected, and in large Edison stations certain feeders often become so overloaded that it is found necessary for their safety to put

100

40

10

20

30

40

60

Percent of maximum load.

70

resistance in series with them to force some of the current to other feeders. The possibility of overloading any individual feeder beyond its share is absolutely provided against in the system described.

To make the arrangement of this system perfectly clear it is illustrated diagrammatically in Figure 2.

In Figure are shown four curves of station distribution efficiency. No. 1 is that of a representative Edison three-wire, direct-current station. No. 2 is the new alternating-current system described. No. 3 is the usual alternating-current system employing separate transformers for each customer or for adjacent customers, the best modern transformers being used. No. 4 shows the same system, but with the average transformers found on the lines of most existing stations. These curves were plotted from the following data obtained from actual practice

The efficiencies of distribution at assumed average

loads are shown to be as follows:

At maximum load it will be noticed that the effi

ciency of alternating current No. 2 is ten per cent

higher than No. 1, the direct current. This means that the same energy can be delivered with ten per cent less capacity in generators, engines and boilers,

FIG. 2.-SINGIE PHASE A.C. SYSTEM
with-High pressure Feeders and low pressure distribution-
Independent Feeder for each Transformer or Group.

or a difference of ten per cent in the first cost of generating plant. It is pertinent to remark here that alternating-current generators also have a much

greater margin for overload for short periods than direct-current generators. The efficiency at average load, or the all-day efficiency of the No. 2 alternatingcurrent system, is also shown to be appreciably higher than the direct-current. In large central stations the average load is generally found in practice to vary from about fifteen to twenty per cent of the maximum, and it will be noted that from twelve per cent average load up, No. 2 curve for the special system of alternating-current distribution shows a higher efficiency than the direct-current.

It may be asked what effect the aging or fatigue of the transformer iron will have on these figures. I answer, none appreciable. Transformer manufacturers are willing to guarantee as low as one per cent increase per year in iron loss. This means one per cent of the actual amount of energy lost in the iron, so that, if this increase continued at the same rate, it would take one hundred years to double the iron loss. Experiments tend to show, however, that this effect rapidly approaches a maximum, and that, therefore, the increase goes on at a constantly decreasing rate. The entire effect may be eliminated, if desired, about once in five years by reannealing the transformer iron at a very small expense.

It has been the practice in some European stations to cut out transformers during periods of light load, but it will be seen that cutting out transformers in the system described would only decrease the efficiency; in fact, the efficiency is higher at about the average load, and the system was designed throughout to accomplish this result.

The less we say about curves Nos. 3 and 4 the better, as at small loads the efficiency is shown to be appallingly low.

EXTENSIONS OF DIRECT-CURRENT SYSTEM BY ALTER

NATING CURRENT

Several large three-wire, direct-current Edison stations are beginning to employ alternating current to extend their lighting territory beyond that possible or profitable with low-tension current. This is a recognition of alternating current which would not have been considered for a moment a few years ago. They now propose to use alternating current to transmit their energy at high potential to a distant substation, where it will be transformed to a lower pressure and then again transformed by means of rotary transformers to direct current, which is in turn distributed over the three-wire Edison system as if generated in the ordinary way. This is a very beautiful and instructive application of alternating currents, and ingeniously designed machinery. It gives the manufacturers a chance to sell additional machinery, swells the company's real estate investment and gives work to the unemployed to operate the sub-stations.

The loss in the conversion to alternating current and back is about fifteen per cent in addition to the loss in static transformers and lines. This distribution might be accomplished without the additional machinery, wire, real estate, labor, and loss in efficiency. It is hardly to be expected, however, that those who have grown up under the protection or influence of direct current would use alternating current any further than absolutely necessary until they have grown more accustomed to the new mode of travel. After looking across the stream to coveted. territory for many years, they are willing to use the new bridge and to ride across at about the highest speed provided, but when they get on the other side.