« SebelumnyaLanjutkan »
left in circuit the destruction of the arrester would put it out of operation anyway.
It is obvious, now, that no lightning arrester that functions by short-circuiting the system for the rest of the half wave, during which a discharge occurs, can take care of and protect against a recurrent surge, since the proper functioning of the arrester, with a recurrent surge, represents a permanent shortcircuit on the system over the arrester, and so a destruction of the arrester, no matter whose make it may have been, and shutdown of the system.
3. To take care of a recurrent surge, a protective device would thus be required that does not short-circuit the system even for one half wave, which never allows the normal voltage of the system to pass a current over the arrester, but acts as a short-circuit for any excess voltage above the normal voltage. The possibility of such a device we can understand by considering the effect that a storage battery would have in a direct-current circuit when shunted between the circuit and the ground. Assuming, for instance, in a 500-volt trolley circuit, a 500-volt storage battery of very high capacity--that is, negligible internal resistance-permanently connected between line and ground. With the normal line potential of 500 volts, no current would pass over the battery to ground, except the very small current required to maintain the battery charge. No rise of voltage, however, could occur in the system from lightning or any other cause, since any voltage above 500 volts, the counter-electromotive force of the battery, would be short-circuited to ground through the battery, so such a battery would give perfect protection against any high-voltage disturbances in the system. In case of a recurrent surge, the current discharging over the battery would be the short-circuit current of the excess voltage; that is, the surge potential; and the heating effect of this current is negligible, since high-potential high-frequency phenomena are of limited power, and especially of limited current, as condenser discharges.
A storage battery, obviously, is not suitable for alternating current, and would not be practical in any case, as it requires a cell for every two volts. The same effect, however, is produced at a much higher voltage, in an alternating-current circuit, by the aluminum cell. If such a cell, consisting of two aluminum plates in certain electrolytes, is exposed to an alternating voltage, a film forms on the aluminum plates, which holds back the impressed voltage, that is, acts like a counter-electromotive force equal to the impressed electromotive force, so that practically no current passes through the cell, or only the small current required to maintain the film, of a magnitude of about 0.01 ampere per square inch plate surface, while for any sudden rise of voltage the cell acts as a short-circuit for the excess voltage. Over the storage battery, the aluminum cell has the advantage of higher voltage: a single cell can take care of 500 or even 600 volts. It also does not have a fixed counterelectromotive force, but a counter-electromotive force that adjusts itself to equality with the impressed voltage, at any value 11p to about 600 volts per cell. Assuming, for instance, an aluminum cell connected across an alternating electromotive force of 300 volts. With the film formed, a negligible current passes through the cell—for instance, 0.25 ampere—maintaining the integrity of the film. If, now, the voltage is suddenly raised to 330 volts, in the first moment the cell acts as a short-circuit of the excess voltage—in this case, 30 volts—and for an instant a very large current, possibly hundreds of amperes if the supply source is capable of giving such current, rushes through the cell. This current very rapidly decreases, by the film of the aluminum plates forming for higher voltage, so that in a few seconds the current is already small, and in a few minutes again the normal current of 0.25 ampere passes, but now at 330 volts impressed, and the film formed to a counter-electromotive force equal to this higher voltage, probably has thickened. If, now, we again lower the voltage suddenly to 300, in the first moment the current in the cell practically disappears, and then gradually rises again, and after a few minutes is again normal at 0.25 ampere; that is, the film has built down again to 300 volts. In this manner the aluminum cell adjusts its counter-electromotive force to changes of impressed voltage by the film building up or building down. This adjustment, for moderate voltage variation, as may be expected when varying the generator voltage of the system, is quite rapid, most of the change occurring within less than a second, but still is extremely slow compared with the rapidity of lightning phenomena, and for lightning phenomena the aluminum cell acts as a short-circuit of the excess voltage
above the normal machine voltage. Thus the recurrent surge, with a system of aluminum cells in series with each other connected directly across the circuit, can not produce any rise of voltage, but the excess voltage over the normal, or the surge potential, is short-circuited through the aluminum cell, thus causing a small increase of the current in the cells, by the superposition of the high-frequency surge current over the normal leakage current of the cell, but no rise of voltage. Since the recurrent oscillations are intermittent, obviously the film of the aluminum cells can not build up to their voltage, but remains corresponding to the machine voltage; that is, the aluminum cell can permanently discharge a recurrent surge without any short-circuit of the main voltage, or any disturbance on the system.
Very great difficulties had to be overcome in the development of the aluminum arrester, and great credit is due Professor Creighton for this work For instance: while very mãny electrolytes are available for an aluminum cell intended for temporary service, it was difficult to find an electrolyte where permanence of the aluminum cell during continuous operation for a long time is required. When connected permanently in circuit, that is, without spark gap-as necessary to give effective protection—a small leading current continuously passes through the cell, and the heat produced by this current must be dissipated, thus requiring an entirely different construction from an aluminum cell for intermittent service. By not connecting the aluminum cells continuously into circuit, but connecting them through a single spark gap, or a multi-gap, the arrester can be made very much smaller, and so cheaper, since it carries current only during the discharge. In this case, however, the film, following its characteristic of adjusting itself to the impressed voltage, which in the case of series spark gaps is zero, gradually dissolves and the first discharge then finds the aluminum cell practically without film; that is, the discharge is a half-wave short-circuit, just as in the multi-gap, so that the arrester is usually destroyed by a recurrent surge, hence offers no particular superiority over the standard multi-gap arrester.
It is interesting to observe that in an electrical system the aluminum cell exerts a similar effect on the voltage as that exerted by the flywheel on the speed in a mechanical system. The aluminum cell neither lowers nor raises the voltage, but adjusts
itself to any voltage given by the generator; but it does not permit any rapid change or sudden pulsation of the voltage, but evens out all fluctuations, just as a flywheel does not allow any sudden change or rapid pulsation in speed, but gradually follows whatever speed the prime mover tends to give.
Since the aluminum cells exert on the voltage of a system a similar action to that of the flywheel on the speed of a mechanical system, just as the flywheel is located at the prime mover, the aluminum cell finds its proper place at the 'bus-bars, low-potential as well as high-potential, of the generating station, and in very long transmission lines also at the receiving station, and further at points on the system where sudden voltage rises are especially dangerous, as at cables interposed between overhead lines, and so forth, while the multi-gap lightning arrester should be used for general protection throughout the system.
To conclude, then, of the three types of lightning arresters:
The single-gap arrester protects by short-circuiting the system for a considerable time, and thereby shutting it down.
The multi-gap arrester short-circuits for one half wave, and, if without series resistance, protects without shutting down the system, for all disturbances and high-potential phenomena except recurrent surges, as arcing grounds.
The aluminum arrester short-circuits only the excess voltage, but not the normal system voltage, and so protects against recurrent surges, but continuously consumes a small amount of power and finds its proper place at the station 'bus-bars, in addition to the multi-gap arresters on the system.
MR. FERGUSON : I move a most hearty vote of thanks to Dr. Steinmetz for the very instructive and interesting lecture that he has given us.
(The motion was seconded by Mr. Dow, and was carried.) The meeting then adjourned to executive session.
In the executive session the report of the secretary and treasurer was placed upon the minutes, as if read.
The following gentlemen were elected as a nominating committee, to nominate officers for the ensuing year: Messrs. John F. Gilchrist, Chicago; W. W. Freeman, Brooklyn, N. Y.; Percy Ingalls, Newark, N. J.; Henry L. Doherty, New York city; J. E. Montague, Niagara Falls, N. Y.
The meeting then adjourned until Thursday morning.
ORDER OF BUSINESS
THURSDAY, June 6, 1907
FIFTH SESSION, IO A. M. Paper-Gas Engines and Producers for Central Stations. By
ROBERT T. LOZIER 2 Report-Committee on Amendments to Freight Classification.
ERNEST H. Davis, Chairman 3 Paper-Recent Steam Turbine Developments. By W. L. R.
EMMET 4 Paper-Balancers versus Three-Wire Dynamos. By BUDD
FRANKENFIELD 5 Paper-The Effect of Frosting Incandescent Lamps. By
EDWARD P. HYDE and F. E. CADY 6 Paper-Some Electric Power Experience. By SARAH M.
SHERIDAN 7 Executive Session
SIXTH SESSION, 8 P. M. Report-Committee on Membership. J. ROBERT CROUSE,
Chairman 2 Paper-Opportunities for the Sale of Current for Charging
Electric Automobiles. By HERBERT H. RICE 3 Report-Committee on Relations with Local Associations.
S. R. BRADLEY, Chairman 4 Report-Committee on Grounding of Alternating-Current
Circuits. W. H. BLOOD, JR., Chairman 5 Announcements 6 Report-Committee on Public Policy. EVERETT W. BURDETT,