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 are now beginning to be actively taken up with relation to the effect on insulators of such electrostatic stresses as may be met as the result of lightning; in the study that is being made as to the relative effectiveness of insulators when subjected to voltages at ordinary commercial frequencies and to the same voltages at frequencies approximating those which we are led to believe exist ill the case of many lightning disturbances.
It has long been known that insulators which would flash over at ordinary frequencies, rather than puncture, are often punctured on a transmission line by lightning disturbances without any evidence of accompanying flash-over. In other words, there seems to be no question that there is some very considerable difference in the condition which exists when an insulator is, on the one hand, subjected to a given voltage at a commercial frequency and when, on the other hand, it is subjected to the same voltage at an enormously greater frequency. Or, to put the matter in a little more exact language, the behavior of insulators when subjected to a difference of potential whose rate of change, or steepness of wave front, approximate those which are met with in commercial work is quite different from the behavior of the same insulator when subjected to a difference of potential whose rate of change is enormously greater than that normally impressed upon the line.
Need of Better Insulators
The results obtained so far seem to point to the possibility of obtaining insulators which will meet that condition usually considered most desirable for enabling them to withstand lightning disturbances; that is, that of always withstanding the tendency to puncture to such an extent as will force the abnormal voltage to flash over the insulator instead of going through it. If this condition of affairs could always be insured, we might have a line practically immune to every condition save that of malicious interference. Because, if we could be sure that the insulator would always flash over and never puncture, it is comparatively easy, at relatively small additional expense, to make such provision as will insure the insulator not being harmed by the power arc following such flash-over.
The direction in which the greatest progress to-day can be made in the art of high-voltage power transmission is that indicated above. Undoubtedly the greatest desideratum at the present time is an insulator which will always flash over, rather than puncture, under any voltage to which it may be subjected, and will not be damaged by a flash-over. As a secondary matter, it is desirable that the power arc following a flash-over will not interrupt the service rendered by the transmission system. But, as previously mentioned, means are now available by which such interruption can be almost entirely insured against. The need, therefore, is only for an insulator which will never puncture and which will not be injured by the power arc following a flash-over during such period of time as may elapse from the time the power arc starts to the end of the very short period of time during which the auxiliary apparatus referred to has had an opportunity to suppress the arc.
The tendency is more and more toward installing high-voltage apparatus outdoors. Undoubtedly the time will come when all high-voltage apparatus whatsoever will be so installed. There is no good reason, either logical or financial, for going to the expense of bringing a high-voltage terminal through the metallic case of a transformer or switch of which it is a part and then going to a similar expense to bring the same terminal out through the wall or roof of the building housing the apparatus. A terminal built to withstand the weather might just as well be put on the metallic tank in the first place and the apparatus placed out of doors, thus saving not only the insulating bushing but practically all of the building as well, since with such an arrangement no protection is necessary save that, for switchboard panels and operators.
Operation of Transmission Systems
The tendency at the present time as regards the operation of transmission systems is undoubtedly more and more toward the creation of enormous networks extending over large areas of country, into which is fed energy from a plurality of sources. The desirability of such combinations is unquestionably indicated by all the economic considerations bearing upon this class of work. Such networks not only benefit by the diversity factor of the various customers they supply, but their utility is also strongly indicated by the additional insurance of continuity of service resulting from energy supply from a number of sources instead of from one. We may continue to expect the amalgamation of transmission systems into transmission networks wherever such systems are within economically reachable distances of one another. We may expect in the not distant future such development in the insulation and construction of transmission lines as will guarantee to them practical immunity from any interruption save that due to malicious interference.