US2639357A

US2639357A - Current limiting apparatus

Info

Publication number: US2639357A
Application number: US259993A
Authority: US
Inventors: Kesselring Fritz
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-08-07
Filing date: 1951-12-05
Publication date: 1953-05-19
Anticipated expiration: 1970-05-19
Also published as: BE467116A; CH282851A; GB632794A; DE862470C; NL186333B; GB632843A; CH262898A; FR931542A

Description

May 19, 1953 F." KESSELRING CURRENT LIMITING APPARATUS Original Filed Aug. 1, 1946 5a 5a 5a 53 53 ?l ,l l 51 {I v .r'

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2 INVENTOR FRITZ KESSELRIN a Patented May 19, 1953 UNITED STATES PATENT OFFICE Original application August 1, 1946, Serial No.

687,625, now Patent No. 2,619,563, dated November 25, 1952.

Divided and this application December 5, 1951, Serial No. 259,993. In Switzerland August '7, 1945 6 Claims.

This is a division from my copending patent application Serial No. 637,625, filed August 1, 1946; for Electromagnetic Control Device, now Patent No. 2,619,568.

My present invention relates to current limiting apparatus for direct-current and alternoting-current power circuits.

It is an object of my invention to devise current limiting apparatus capable of reducing or limiting excessive overloads, such as short-circuit currents in power lines, at a velocity of current reducing operation superior to that heretofore obtainable.

Another object is to provide current limiting apparatus affording a selective disconnection or sectionalization of overloaded portions in meshed power-line networks.

A current limiting apparatus according to the invention has at least one electromagnetically actuated switch element or armature whose switching gaps are paralleled by an electric impedance. Each switch element is an elastically suspended rigid body of at most 1 cm. length in the direction of the magnetic field and has a switching travel between closed and tion are preferably provided with a plurality of switch members or armatures arranged electrically in series but magnetically in parallel relation to each other, each armature having an impedance means connected parallel thereto. The current limiting apparatus, as a rule, do not have to completely interrupt the current but may merely reduce the current to a residual value readily controllable by a separate disconnect switch. The impedances may be dimensioned to reduce the residual current at least to the rated current value of the circuit interrupting switch, or the impedances values may be large enough to reduce the current to such a. value that the are between the parting contact surface of the switch extinguishes by itself. The impedances may be purely ohmic resistors, free of self-induction, made of thin wires or of banda first maximum shaped conducting material with a positive temperature resistance coeflicient, such as pure iron or tungsten. These resistors are preferably arranged in an insulating and cooling liquid within the housing of the current limiting apparatus. Under certain conditions it is also possible to use capacitive impedances which automatically, a few hundredths of a second after their insertion across the contacts, reduce their capacity to a fraction of its original value, for instance, by means of increasing the distances between the capacitor electrodes.

These and other objects and features of my invention will be apparent from the following description in conjunction with the drawing in which Fig. 1 shows schematically and in section a current limiting apparatus according to the invention, and Fig. 2 is an explanatory coordinate diagram of current and voltage curves.

The apparatus illustrated in Fig. 1 has 'a magnetic structure with a field coil 51. lhe structure 50 has two parallel pole faces between which a plurality of armatures 52 are pivotally mounted on torsional springs 49 which bias each armature against stationary contacts 53. The housing of the apparatus forms a tank 54 beneath the structure 5c and is equipped with

insulating bushings

55, 56 for two respective line terminals. A resistor 57 made of fiat iron strips is immersed in insulating oil '58 contained in the tank 54. The line terminals are connected by leads 59 and 6!] with the two outermost stationary contacts 53. The resistor 51 has taps connected by leads 6! with the armatures 52 and with the intermediate stationary contacts 53. Relative to the line terminals, the armatures 52 are electrically series arranged. Relative to the magnet field, the armatures lie parallel to one another for simultaneous operations.

When the current limiter is traversed by load current, this current flows from the terminal of bushing 56 through lead 6i! and serially through all stationary contacts 53 and armatures 5.2 to lead 59 and the terminal of bushing 55. sufiiciently, energized to move the armatures against their bias. However, when a heavy overload such as a short circuit occurs, the field of coil til causes all armatures to move to the open positions. in the most common case, the excitation current of field coil 54 is provided by the short-circuit current itself, in which case the excitation current is tapped from across a gap in lead 59 as shown by dotted lines. It is Normally, the coil 5! is not, or not also possible to use auxiliary devices for controlling the switching of the resistors in ac-' cordance with the special character of the shortcircuit current. The ohmic value of this resistor is dimensioned to produce an intense heating effect so that, due to the high positive temperature coefiicient of resistance, the ohmic value increases considerably. By employing chemically pure iron as material for the resistor, an increase of 5 to times its rated value becomes effective. By this means a further increase of the overload current is prevented.

This operation of the limiter will be more fully understood from the following description of a typical performance represented by Fig. 2.

In Fig. 2, the normal, rated current of the limiter is indicated by a wave 2' Curve z' denotes a surge of a short-circuit current as it might appear when using conventional overload breakers. Curve i denotes the current through a parallel connected impedance according to the invention, and curve Uw represents the transient voltage drop across this impedance and hence the restriking potential. A few 10 see. after the initial occurrence of overload current (instant to.) the armature contacts are opened so that the parallel connected impedances (resistor 51 in Fig. 1) are inserted into the load circuit. The initial voltage drop in the parallel connected impedance must not amount to more than volts, in order that an are between the contact shall either not occur at all or shall at once be extinguished. Due to heatin of the resistor 51, the effective resistance increases. Hence, the voltage drop also increases. The ccntact separation must increase to the same extent in order to avoid a return ignition. Finally, almost the whole voltage of the circuit is impressed across the impedance, and

the current is reduced to a fraction of its normal value and can be readily interrupted by a disconnect switch. By suitably dimensioning the impedances, it is possible to limit the amplitude of the current z',,,, for example, to 1 /2 or twice the value of the rated current of the limiter (instant in). The movement of the contacts, as explained, is so rapid that the dielectric strength between the contact surfaces is sufficient to withstand the maximum of the restriking voltage (instant is). At about the instant is the disconnect switch may be opened to fully interrupt the current 2' at the instant te. Indeed, such a limiter can readily be made to begin opening its contacts not later than about 10 sec. after the occurrence of .a surge front (instant ta) and to pass through because it was found that arcs are extinguished on the line when the current is reduced to a certain minimum value and when the full magnitude of the above-mentioned short-circuit current is prevented from flowing through this arc. The are on the line is then extinguished at about 'the moment is (Fig.- 2) whereupon the current limiter at once returns to its closed position. In this case, the trouble is eliminated in the shortest possible time without complete interruption of the line current, and the consumer may not even be aware of a disturbance on his line.

In case of a metallic short circuit all current limiters in the short circuit current path will be affected and therefore insert their resistors. If the system of the power network is laid out to operate the limiter at the location where the voltage decreases to zero, a selective isolation of the short circuit is established at the instant ha and at the same time a short-circuit current of full magnitude is prevented.

Generators, transformers and the like devices subject to damage by overloads may be equipped from the outset with a current limiting according to the invention. The general use of such devices equipped with such current limiters, when the already existing line switches can be used for the normally required control operations, permits a considerable simplification in the design of power line networks, since thermal as well as dynamic damages due to short-circuit currents are virtually eliminated.

It should be understood that the illustrated embodiment is presented for exemplification rather than limitation and may be varied and modified without departing from the essential features of my invention and within the scope of the claims annexed hereto.

I claim:

1. Current limiting apparatus for direct-current or alternating-current power circuits, comprising terminals for connection in a circuit whose current is to be limited, a magnetic circuit having a magnet with a coil and having at least one armature disposed in the field of said magnet and movable between two given positions. spring means connected with said armature and having at said armature a force opposed to and normally stronger than that of said field so as to normally hold said armature in one of said positions so that said magnet field is effective to move said armature to said other position only when said current has an excess value, stationary contact means engaging said armature in said one position and being connected through said armature and in series with said coil across said terminals, said armature having in the direction of said field a maximum length of one centimeter and having a maximum travel between said positions of one millimeter and a maximum travel period of 10- seconds, impedance means connected between said stationary contact means across said armature, said magnetic circuit having a time constant below 10* seconds so that the armature-moving variation of excitation of said magnetic circuit takes place at the latest a few 10- seconds after the initiation of the building-up of said excess current value, whereby contact separation occurs at most 10* seconds after said initiation and at least half said travel is traversed before the restriking potential corresponding to the voltage drop at said impedance means attains its first maximum value.

2. In apparatus according to claim 1, said impedance means being substantially free of inductance and having a positive coefiicient of resistance to increase the resistance value during circuit opening performance.

3. In apparatus according to claim. 2, said impedance means being composed of chemically substantially pure iron.

l. In apparatus according to claim 1, said impedance means having a maximum voltage drop of 20 volts at the moment of contact separation.

5. Current limiting apparatus, comprising two mutually insulated line terminals, an electromagnet having two pole faces spaced from each other .8 to provide a magnetic field, a plurality of annatures disposed between said pole faces in magnetic parallel relation to each other and being individually biased to one position and magnetically movable due to said field to another position, stationary contact means engageable by said armatures and connecting said armatures in series relation to each other across said terminals in said one position of said armatures, each of said armatures having substantially in the direction of said field a maximum length in order of one centimeter and having a maximum travel distance between said two positions in the order of one millimeter and a maximum travel interval between said positions in the order of 10- seconds, and impedance means connected across each gap of interruption between said armatures and said contact means, said impedance means having a maximum voltage drop of about 20 volts across each of said gaps during said interval.

6. Apparatus according to claim 1, comprising a tank with cooling liquid, said impedance means consisting of ohmic resistors and having a posi- 1c tive temperature coeiiicient of resistance, and

said resistors being immersed in said liquid.

FRITZ KESSELRING.

N 0 references cited.