Publication: Electrical World
New York, NY, United States
Progress in Transmission
By Ralph D. Mershon
THE past year has not been marked by any startling advance in the art of high-voltage power transmission, though there has been that evolutionary progress that might be expected in so active a line of work. A number of high-voltage plants, 100,000 volts or over, some of them of voltages considerably higher than any used heretofore, have been put into commission, and a number of others are in course of construction or have been projected. A considerable number of outdoor high-voltage substations have been put in, both switching and transforming stations. There has been a thorough try-out of a system devised a year or so ago for removing short-circuits or grounds caused on transmission lines by flash-over of the insulators. This is the system which suppresses such disturbances by automatically producing a short-circuit at the generating station and immediately afterward removing it. As shown by practical experience, it effectually removes or suppresses line disturbances of the kind mentioned with practically no interruption to the service of the system.
Increase in Voltage
Transmission voltages are gradually creeping up to higher and higher values. In some cases higher voltages have been used than would seem to be economically necessary. Presumably this is in some measure due to the desire of the engineers having the plants in hand to be in the vanguard of progress. But even though in some cases a lower voltage might, from an economical standpoint, be equally satisfactory with, or even more satisfactory than, the high one adopted, the art of power transmission is undoubtedly indebted to these enterprises and to their engineers for the adoption of the higher voltages, by reason of the aid which such adoption and the carrying out of the enterprises lend to the progress of the art. There is no doubt that so far as our present knowledge goes there is no physical limitation to the transmission voltage which may be employed. The limitation, as with most other matters, is a purely economic one. It is purely a question of cost and the return that can be derived therefrom.
The highest transmission voltage put into practical operation during the last year is 140,000 volts. This is the voltage now regularly employed on the lines of the Au Sable Electric Company in Michigan. The highest projected voltage is 150,000 volts, for which the lines of the Pacific Light & Power Corporation are being constructed. Although the value of voltage, in common with other considerations affecting the transmission line, is usually fixed by economic considerations, there is one element in connection with all transmission lines, whether high voltage or low voltage, not ordinarily considered as being strictly subject to economic limitations alone. This is the matter of interruptions due to lightning. Strictly speaking, this question is also determined by economics. Because if we chose to spend a sufficient amount of money on an installation it would be possible to make it immune to atmospheric disturbances. To do this in the case of a long high-voltage transmission would mean such an enormous first cost as to be entirely prohibitive. So it might be considered, in view of the enormous cost involved, that these costs themselves compel the classing of the problem as a physical one instead of an economic one. That is, it may be considered that such drawbacks as still exist in the art of transmission due to atmospheric disturbances are drawbacks which from practical considerations must necessarily be overcome, not by an enormous increase in first cost, but by some solution having to do more directly with physical conditions—by some solution having mainly to do with the working out of a problem in the arrangement of transmission lines, or the insulation of transmission lines, or both—and which is, therefore, more immediately physical in its nature and does not greatly involve the relation between cost and the result obtained.
Insulation of Line
While this physical problem of procuring for transmission lines practical immunity against atmospheric disturbances involves in some measure the arrangement and disposition of the lines, the present tendency is to consider it more a problem of insulation and its accessories. This is shown in the investigations that