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MINIATURE PRECISION INSTRUMENTS
For Direct Current
The same degree of mechanical and electrical excellence which has won pre-eminence for the larger Weston Models, is embodied in these wonderful little masterpieces of the instrument maker's art. The finest watch is a less striking example of perfection in refined workmanship. They are accurate, dead-beat, extremely sensitive. They are shielded against external electrical and magnetic influences. Despite their great refinement in workmanship, they are very substantially constructed and have the longest scale ever provided in instruments of similar size. In short, they are the finest development of small instruments of the pivoted moving coil, permanent magnet type. And the prices are low. • The several models and ranges offer a selection from over 300 different combinations. The switchboard instruments are listed in bulletin No. 20 and portable instruments in bulletin No. 501. They will be mailed on request.
Weston Electrical Instrument Co.
33 Weston Ave., Newark, N. J.
THE FRANKLIN INSTITUTE DEVOTED TO SCIENCE AND THE MECHANIC ARTS
CONTROL AND PROTECTION OF ELECTRIC
Member of the Institute
When the first commercial electric circuit issued from a station the problem of control and of protection arose. It was a simple problem at first: an ammeter and voltmeter to measure current and voltage; a knife-blade switch to send the current into the desired path, or withdraw it; the fuse to open the circuit in emergencies, and if the wires became crossed and fuse and switch failed, generator and engines stopped, and not much harm was done.
With the extension of the circuits into the suburbs some lightning troubles were felt and led to the introduction of lightning arresters—in the early days based mainly on hope and trust in Providence rather, as very little was known of lightning phenomena.
Since these days, less than a generation ago, enormous changes have taken place, and the electric systems have increased in size, in voltage, and in extension.
Where 100-horse-power machines were large once, now steam turbine alternators of 40,000 horse-power and more are in com
* Presented at the joint meeting of the Electrical Section and the Philadelphila Section, American Institute of Electrical Engineers, held April 15, 1915.
(Note.—The Franklin Institute is not responsible for the statements and opinions advanced by contributors to the JOURNAL.)
Copyright, 1915, by THE FRANKLIN INSTITUTE.
mercial operation. The steam engine has made room for the steam turbine, and the steam turbine does not stop when the wires are crossed and a short circuit occurs, and the momentum of the turbine disks, revolving at velocities of 300 to 400 miles per hour, can supply ample energy for the destruction of any part of the system.
Feeders of 10,000 horse-power or more, generators of 40,000 horse-power have to be controlled by switching: an attempt to open such a circuit by the knife-blade switch of old would lead to the destruction of the switch—and probably its operator.
Instead of small machines operating separately on independent circuits, huge generators now feed in parallel into the system of busbars, on which is concentrated all the power of the station or the group of stations which are tied together. Numerous stations and systems of interconnected stations of 100,000 to a quarter million horse-power and over are in operation, and the half-million horse-power mark has been reached.
Anywhere on the busbars of the station or in the feeders near the station there is available, destructively in case of an accident, as a short circuit, not only the entire power of the station, of perhaps half a million horse-power, but the far greater power which the station generators can give momentarily.
Short-circuit currents of forty to fifty times normal fullload current may momentarily flow from some turbo-alternators, representing ten and more times full-load power.
Such a station, or group of closely interconnected stations, of half a million horse-power full-load capacity, may momentarily send into a short circuit at the busbars over five million horsepower. This is the power of Niagara : for Niagara is estimated variously at from 5,000,000 to 15,000,000 horse-power.
It is obvious that no switch or circuit breaker can be built to safely open such power, to suddenly stop Niagara, especially when considering that many hundreds of feeders issue from the busbars, that in any one of these feeders a short circuit in the cable may let loose the power of the entire system, and every feeder thus requires such a circuit breaker.
With half a million horse-power station capacity, a momentary overload capacity ten times as high, assuming that we could build a circuit breaker to open this short-circuit power as quickly as in three to four cycles, or one-eighth second: this would require to dissipate in the circuit breaker the energy of over 200,000,000 foot-pounds—the destructive energy of 1000 tons dropping from a height of 100 feet. This is about the energy of a projectile of 2000 pounds weight leaving the cannon at the velocity of 2500 feet per second. It is the destructive energy of two heavy railway trains, of 400 tons each, going at sixty miles per hour, and meeting in head-on collision. It is the energy of the explosion of thirty pounds of dynamite.
Equally great has been the increase of voltage: where once 2000 volts were high-voltage distribution, in circuits of a few miles length, now circuits of hundreds of miles length are in operation at voltages of 100,000 to 150,000. Such voltages jump toward any object for over a foot distance, and will maintain arcs of practically unlimited distance; that is, with 100,000 volts and practically unlimited power back of it, an arc can extend for hundreds of feet. Thus no simple switch will open such voltages under power.
Transmission systems at high voltages have been interconnected with each other into networks, which spread and extend, and already to-day often represent thousands of miles of interconnected high-voltage lines, covering tens of thousands of square miles, and picking up every lightning, every atmospheric disturbance within this entire area.
Thus the lightning protection also has become a far larger problem than in the small circuits of old.
But far greater than the energy of any lightning stroke is the energy stored as magnetic field surrounding the conductors, as dielectric field radiating from the conductors of these big transmission systems, and if this internal energy of the system is set surging, its effects are far more destructive than those of lightning, and the effects may not be merely momentary, as those of lightning, but continual, as machine energy continually replaces the stored internal energy, which causes the destructive surge.
And, in addition hereto, far greater reliability and continuity of service is to-day demanded from the electric systems than was in the early days, and that at a lower cost of electric energy, and it must be remembered that, with the increasing cost of living, electricity is one of the few commodities which has steadily decreased in price.
The foremost problem of control of electric systems thus is
that of controlling enormous powers; the foremost problem of protection is that against self-destruction by its own power.
Current and voltage have grown beyond the values for which instruments can be built, and current transformers and voltmeter transformers are interposed between the circuit and the instruments measuring it. With the general introduction of parallel operation, power factor indicators are required to insure the division of load without excessive waste currents; frequency indicators and synchronizing devices to safely connect machines into the system.
With hundreds of feeders radiating from the generating station, the office of the load dispatcher has become essential, and the necessity of keeping exact records of all operations and of all accidents and incidents is of the greatest importance. Automatic recording devices thus have been developed, as the multirecorder, to record, within fractional seconds, all important events, as opening and closing of switches, starting and stopping of generators, surges, lightning disturbances, etc. Such automatic devices afford a valuable check on the operating staff, but more important still is their record in emergencies, where a number of things happen almost at once, where the attention of the operators is detracted from accurate observation by the necessity of action, and the record thus could be made only afterwards from memory, which is not very accurate in such a period of excitement. It is just in such abnormal conditions where the most complete and accurate record is of greatest importance, to enable the engineers to determine with certainty what happened and why it happened, so as to take steps to guard against its recurrence.
Oil circuit breakers have been developed, which can safely and without disturbance close and open the feeder circuits of over 10,000 horse-power, the generator circuits of 40,000 horse-power and more, with an ample margin of overload capacity. In these the circuit is opened under oil with such mechanical arrangement of contacts and oil vessel that in the moment of circuit opening the current is extinguished at the end of a half wave by the rapid expansion and chilling of the oil vapor which is produced by the opening arc, and which in the first moment is under high compression, due to the momentum of the oil, which has to be set in motion.