The lower valleys of the Sacramento and the San Joaquin rivers, which are crossed by these California systems, as well as the shores of San Francisco Bay, have as much annual precipitation and as moist an atmosphere as most parts of the United States and Canada. Therefore there seems to be no good reason to prevent the use of 60,000 volts elsewhere.
The distance over which energy may be transmitted at a given rate, with a fixed percentage of loss and a constant weight of copper, goes up directly with the voltage employed. This rule follows because, while the weight of conductors to transmit energy at a given rate, with a certain percentage of loss and constant voltage, increases as the square of the distance, the weight of conductors decreases as the square of the voltage when all the other factors are constant.
Applying these principles to the 150-mile transmission, it is evident that an increase of the voltage to 60,000 will allow the weight of conductors to remain exactly where it was for the transmission of 100 miles, the rate of working and the line loss being equal for the two cases.
The only additional item of expense in the 150-mile transmission, on the basis of 60,000 volts, is the $35,000 for pole line. Allowing 15 per cent on the $35,000 to cover interest, depreciation, and maintenance, as before, makes a total yearly increase in the costs of transmission of $5,250 over that found for the transmission of 100 miles. This last sum amounts to 0.0175 cent per horse-power hour of the delivered energy.
The cost of transmission is thus raised to 0.307 + 0.0175 = 0.324 cent per horse-power hour of delivered energy on the 150-mile system with 60,000 volts.
Existing transmission lines not only illustrate the relations of the factors named above to the cost and weight of conductors, but also show marked variations of practice, corresponding to the opinions of different engineers. In order to bring out the facts on these points, the data of a number of transmission lines are here presented. On these lines the distance of transmission varies between 5 and 142 miles, the voltage from 5,000 to 50,000, and the maximum rate of work from a few hundred to some thousands of horse-power. For each transmission the single length and total weight of conductors, the voltage, and the capacity of the generating equipment that supplies the line is recorded. From these data the volts per mile of line, weight and cost of conductors per kilowatt capacity of generating equipment, and the weight of conductors per mile for each kilowatt of capacity in the generating equipment are calculated. In each case the length of line given is the distance from the generating to the receiving station. The capacity given for generating equipment in each case is that of the main dynamos, where their entire output goes to the transmission line in question, but where the dynamos supply energy for other purposes also, the rating of the transformers that feed only the particular transmission line is given as the capacity of generating equipment.
Distance and Voltage of
Electrical Transmission.
| Distance in Miles. | Volts. | Volts per Mile. | ||||
|---|---|---|---|---|---|---|
| Colgate to Oakland, Cal. | 142 | 60,000 | 422 | |||
| Cañon Ferry to Butte, Mont. | 65 | 50,000 | 769 | |||
| Santa Ana River to Los Angeles | 83 | 33,000 | 397 | |||
| Ogden to Salt Lake City, Utah | 36 | .5 | 16,000 | 438 | ||
| Madrid to Bland, N. M. | 32 | 20,000 | 625 | |||
| Welland Canal to Hamilton, Can. | - | 35 | 22,500 | 643 | ||
| 37 | ||||||
| San Gabriel Cañon to Los Angeles | 23 | 16,000 | 695 | |||
| Cañon City to Cripple Creek, Colo. | 23 | .5 | 20,000 | 851 | ||
| Apple River to St. Paul, Minn. | 25 | 25,000 | 1,000 | |||
| Yadkin River to Salem, N. C. | 14 | .5 | 12,000 | 827 | ||
| Into Victor, Colo. | 8 | 12,600 | 1,575 | |||
| Montmorency Falls to Quebec | 7 | 5,500 | 785 | |||
| Farmington River to Hartford | 11 | 10,000 | 909 | |||
| Sewall’s Falls to R.R. shops Concord | 5 | .5 | 10,000 | 1,818 | ||
| Wilbraham to Ludlow Mills | 4 | .5 | 11,500 | 2,555 | ||
| To Dales, Ore. | 27 | 22,000 | 814 | |||
The transmission systems here considered have been selected because it was possible to obtain the desired data as to each, and it may be presumed that they fairly illustrate present practice. It may be noted at once that in general the line voltage is increased with the length of the transmission. Thus, the transmission for the Ludlow Mills over a distance of 4.5 miles is carried out at 11,500 volts. On the other hand, the transmission between Cañon Ferry and Butte, a distance of 65 miles, employs 50,000 volts and represents recent practice. The system from Colgate to Oakland, a distance of 142 miles, the longest here considered, now has 60,000 volts on its lines. In spite of the general resort to high pressures with greater distances of transmission, the rise in voltage has not kept pace with the increasing length of line. For the Wilbraham-Ludlow transmission the total pressure amounts to 2,555 volts per mile, while the line from Colgate to Oakland with 31.5 times the length of the former operates at an average of only 422 volts per mile. Of the fifteen transmissions considered, six are over distances of less than 15 miles, and for four of the six the voltage is more than 900 per mile. Eight transmissions range from 23 to 83 miles in length, with voltages that average between 1,000 volts per mile at 25 miles and only 397 per mile on the 83-mile line. The volts per mile are 6 times as great in the Ludlow as in the Oakland transmission.