US 1696177 A
Description (OCR text may contain errors)
Dec. 18, 1928. I 1,696,177
E. R. EVANS SYSTEM OF DISTRIBUTION Filed March 11 1921 IIIIIII II "'HUI.
/N\/EN TOR MK a Patented Dec. 18, 1928.
UNITED STATES 1,696,177 PATENT OFFICE.
EARL R. EVANS, OF GREAT BARRINGTON, MASSACHUSETTS.
SYSTEM OF DISTRIBUTION.
Application filed March 11, 1921. Serial 1C0. 451,637.
My invention relates to electrical systems,
and it has particular reference to electrical circuits containing reactance or impedance for limiting the flow of current therein under certain conditions.
It has been customary to insert reactance in certain types of alternating current circuits to limit the current which flows in case of short-circuit or other abnormal condition. For instance, in power generating and distributing systems, where it is of the utmost importance that uninterrupted service be maintained and that the generating apparatus be protected from abnormal surges of current resulting from short-circuit-or other cause, it has been common practice to insert a current-limiting reactor in the circuit or to design the generator to obtain a high internal reactance.
As the method of increasing the internal reactance of the generator has inherent limitations and obvious disadvantages, external reactors are generally used. These reactors comprise simply a number of turns or coils of wire, whereby the self-inductance of the circuit is increased. .The underlying principles of both methods of increasing the reactance of the circuit are, in fact, simple and well understood and need vnot be described in detail. External reactors of the air-core type are used where large currents are involved on account of the excessively large iron core required in order that ma etic saturation with consequent decrease selfinductance may not occur.
The insertion of external reactance into a power circuit is undesirable, as it reduces the power factor. If the current wave is nonsinusoidal and contains harmonics of widely different frequencies, even a smallreactance will produce distortion of the wave which it is sometimes desirable to avoid. Under normal operating conditions therefore the reactance should be as small as possible, though a relatively large reactance is desirable when abnormal conditions obtain. The size of the reactance used heretofore is thus a compromise between that fixed by one consideration and that fixed by the other, with the result that the reactance chosen was not large enough to limit the short-circuit current to a reasonably safe value thou h it was large enough to have an apprecia le effect on the power factor of the circuit.
The object of my invention is to provide an external reactor which will have less reactance for a certain predetermined current than for currents in excess of this normal value. In more general terms, it is the object of my invention to automatically change the impedance of a portion of an electrical circuit carrying alternating current from a smallvalue while normal current flows in said circuit to a much larger value when an abnormal current flows therein; It is apparent that such a variable impedance maybe so proportioned as to limit the abnormal current to a safe value and yet not appreciably decrease the power factor of the circuit or cause prohibitive distortion of the current wave under normal conditions.
The principle of operation of my invention and the mode of carrying it into effect will be clear from the following detailed description taken in connection with the accompanying drawings, wherein Fig. 1 is a of, I
, Fig. 2 is a diagrammatic view of another form of reactor embodying my invention;
Fig. 3 is a similar view of a third modification of my invention; and
Fig. 4 is a similar view of an embodiment of my-invention which is particularly adapted to small currents and high frequencies.
Referring to Fig. 1,1 and 2 are feeders which supply power from the alternator 3 to the load. In one of the feeders is connected a reactor, designated generally by 4, and comprising a main winding 5 in series with the vline and an auxiliary winding 6. These windings are shown as placed on an iron core 7, but this core may be omitted if desired-for instance if the current in the winding 5 be very largethe essential requirement being solely that the windings 5 and 6 be closely mutually-inductively related. The passage of current through the winding 5 is opposed by the reactance of the winding. This reactance, for a given frequency, is proportional to the amount of magnetic flux linked with the turns of the winding. Therefore by decreasing the flux interlinkages the reactance of the Winding 5 may be reduced to any desired value. To decrease the flux inter-linkages I prefer to use an auxiliary winding of opposing magneto-motive force and in which the current is controlled in response to the conditions in the main circuit so as to vary the reactance of the main winding as desire In Fig. 1, the auxiliary winding is shortdiagrammatic view of one embodiment there circuited by the circuit-breaker 8. The current induced in the winding 6 by the flux from the main winding 5 is in such a direction as to oppose the flux and, if the winding 6 is of low resistance, the flux threading the core 7 and the windings is only a small fraction of that which exists with the winding 6 opened. The circuit-breaker 8 is shown sche matically as an overload breaker adapted to automatically interrupt the circuit when the current in winding 6 exceeds a predetermined value, whereby the reactance of the winding 5 will be automatically greatly increased.
In Fig. 2 isshown a modified form of reactor 4 in which the core 7 isprovided with a restricted le 9 on which the auxiliary winding 6 is placed, said winding preferably consisting of a solid ring or sheath of copper.
The core leg 9 is of such limited cross section as to be nearly saturated with the small amount of flux threading it under normal conditions. Under such conditions the current in the short-circuited winding 6 reduces the self-inductance of the main winding 5 to a negligible value, as has been explained with reference to Fig. 1. But an increase of current in the main winding, particularly if of a hi h frequency, produces a large leakage flux because of the saturation of the core leg 9, and the leakage reactance of the reactor will limit the abnormal current to a safe value. Preferably a leakage path 10 of relatively low reluctance is provided.
In Fig. 3 is shown an arran ement Whereby the reactance may be completely neutralized for a predetermined value of current, usually the normal 0 rating current. This result is accomplishe by circulating current through the auxiliary wlnding 6 from a suitable source of current 11, which may be a small alternator driven synchronously with machine 3. A regulating device, such as a variable impedance coil 12, is used to control the current. The phase of the current in winding 6 may be adjusted for the average power factor of tlg load by the mechanical coupling between the alternators. By making the load on machine 11 almost wholly 1nductive, the power required to drive the machine is reduced to a negligible value. The reactor 4: is similar in most other respects to that shown in Fig. 2, but is preferably desi ed for a very high leakage reactance. T' e leakage path 10 may be an unbroken core leg as shown, the current in winding 6 being increased aecordingl to the value required to eliminate all the ux threading the winding 5. An abnormal current in the feeder 2, caused perhaps by a short-circuit in the dis tribution system, induces a voltage in the auxiliary winding 6. However on account of the leakage path 10, this voltage is not proportional to the increase in current in windln'g5. Furthermore on account of the imquiredl pedance of the alternator 11 and the coil 12 in series with the winding 6, the increase of current in said winding is still further reduced, and the ma neto-motive force of the winding no longer balances that of the main winding 5. The current in the main winding therefore establishes a flux and is limited by the self-inductance resulting therefrom. Thus it is seen that the reactance of the reactor 4 to current flowing in the circuit 1, 2 changes instantaneously from zero to a rela tively large value when an abnormal current flows through the circuit. It will be readily apparent that the alternator 11 may supply current for a number of recators 4 disposed in separate feeders from the alternator 3 or from machines in synchronism therewith, where the power factors of the loads in the respective circuits are about the same. Furthermore, while I have for the sake of simplicity illustrated my invention as applied to single phase alternating current circuits, it will be evident to those skilled in the art how my invention may be applied to polyphase circuits. My invention is not limited to sinusoidal alternating current but may be used with non-sinusoidal variable currents. In F ig. 4 is shown an embodiment of my invention which is particularly ada ted for small, high frequency, nonsinusoida currents. 1 and 2 represent as be fore the line wires of a circuit in which a source of power and a translating device, not
shown, may be connected. A reactor 4 having windings 5 and 6 comprising many turns, preferably interleaved to increase the coefiicient of mutual induction, is connected in the circuit. An iron core may be used if desired, depending on the amount of reactance re- The auxiliary winding 6 is provided as in Fig. 3 with current of proper magnitude and phase relation from a source of limited capacity. This source is in this case a threeelectrode vacuum tube 13, also termed an electron tube, thermionic tube, or electric-discharge device. This tube may be connected in various ways to obtain a current in the winding 6 in phase with and opposing the current in the main Winding 5. As shown, the input electrode is connected across a noninductive resistance 14 which is in series with the main circuit. The variation of grid potential is therefore in phase with the current in the main circuit. Accordingly the current in the plate circuit including the winding 6 is in phase with the current in the winding 5. The plate battery 15 and the resistance 16 are adjusted so that the current in the winding 6 exactly neutralizes the inductive effects of the winding 5. Prefer ably also the grid battery 17 and the filament current are so chosen that the maximum normal variation of current in the line 2 forces the tube 13 to operate at full capacity; i. e., carries the grid potential over practically the entire unsaturated portion of the, grid characteristic. An abnormal current will then produce no proportionate effect in the plate circuit, and the unbalancing of the currents in windings 5 and 6 will cause an increase in the reactance of the circuit including the winding 5 as has been explained with reference to the other modifications of my invention. A voltage will be induced in the winding 6 but on account of the high resistance of the plate circuit, the resulting increase of current will be slight. Since the reactance of the winding 5 can be very accurately-and completely neutralized by this arrangement, there will be no distortion of non-sinusoidal currents under normal operation.
By my invention I have provided simple and effective means for protecting elect-ri. al apparatus in a circuit subject to abnormal currents. The current-limiting means described herein possesses the advantages of the prior rcactance devices but is more effe tive for the purpose intended. Furthermore it does not decrease the power factor of the circuit in which it is used nor distort the current wave as does the reactor heretofore used. Accordingly it is capable of many applications where these defects of prior reactancc dcvices prohibited their use. The variation of impedance, except in the first modification illustrated, is effected instantaneously so that the full protective effect of the reactor .is available for very sudden surges of current.
Since my invention is not limited to circuits carrying sinusoidal current, as I have specifically stated, I desire that the term alternating current where it is used in the following claims be construed to cover 'both sinusoidal and non-sinusoidal currents.
What I claim as new and desire to secure by Letters Patent of the United States, is:
1. A system of alternating current distribution comprising a source of alternating current, a. line connected to said source, and reactive means in said line for instantaneously varying the reactance of said line from substantially zero to-a relatively large value when an abnormal current flows therein.
2. In combination, two inductively coupled circuits containing sources of alternating current having a definite phase relation, one of said circuits being subject to abnormal surges of current, and means whereby the current in the other circuit is limited to substantially the normal value.
3. In combination, a source of alternating current. an inductive device comprising a main winding connected to said source and an auxiliary winding, and a second source of current connected to the auxiliary winding and having a definite phase relation to the first mentioned source, the relation of said windings and sources being such that the inductive device exerts substantially no effect upon the circuit of the main winding under normal conditions.
4. A reactor comprising a main winding and means for eliminating substantially all of the magnetic flux interlinked with said winding when normal current flows therethrough, said means being instantaneously rendered ineffective for abnormal currents so that impedance is offered to sudden surges of abnormal current.
5. In combination, a. reactance winding, an electron tube wherein the current flow is limited by the electron emission from a cathode and means including said electron tube for neutralizing the inductance of said winding for normal currents;
6. In combination, a reactance winding, and means including an electron tube having a current characteristic which becomes saturated for varying the inductance of said winding for abnormal currents in accordance with the degree of saturation.
7 In combination, a differentially wound coil, a source of current connectedto one winding of said coil, an electron tube having a cathode and an anode connected to the other winding of said coil and means whereby a characteristic of the cathode-anode current of said tube corresponds under certain conditions' only with a characteristic of the current in said first-mentioned winding.
8. In combination, a vacuum tube having a cathode, a plate and a grid, a differentially wound coil having a main winding and an auxiliary winding, the latter being in the plate circuit of said tube, and means for varying the potential of said grid in accordance with the current in the main winding.
9. In combination, a conductor, a non-inductive resistance in series with said conductor, a. differentially wound coil having one winding in series with said conductor and a three elect-rode vacuum tube, the grid circuit of said tube including said resistance and the plate circuit of said tube including the other winding of said coil.
10. In combination, a differentially wound coil. a circuit subject to abnormal current flow including a winding of said coil, an electron tube having electrodes connected to the other winding of said coil and means including said tube and said other windingfor'neutralizing the impedanceof said first winding to normal currents, the tube being operated above the point of saturation when abnormal currents flow in said circuit.
11. A conductor subject to abnormal surges of potential, and means for limiting the flow of current in said conductor comprising means for instantaneously increasing the reaetance of said conductor from a relatively smallvalue to a relatively large value when the current exceeds the normal value.
12. In combination, a reactor, an electron tube, and means including said tube for altering the reactance of said reactor from a normally negligible value.
13. In combination, a reactor, an evacuated valve device, and means including said device for altering the reactance of said reactor under conditions of excessive current therein.
14. In combination, a reactor, an electron tube, and means including said tube for altering the reactance of said reactor in accordance with the current flowing therethrough.
15. In combination, a reactor, an electron tube, and means including said tube for increasing the rcactance of said reactor when current in excess of a certain value traverses the same.
16. In combination, two electrical windings, an electron tube associated therewith, and means including said tube for altering the current in one of said windings depending uponthe current in the other windin 17. In combination, two inductively related electrical windings, an electron tube connected thereto, and means including said tube for causing a current to traverse one of said windings that has a definite relation to the current in the other winding up to but not in excess of a certain value.
18. Protective means for an electrical circuit comprising a,tube provided with elecon the system and another relation under predetermined abnormal current conditions.
20. An alternating-current distribution system comprising a source of alternating current, a line connected to said source and means including two inductively related windings for instantaneously varying the reactance of said line from a relatively small to a relatively large value when an abnormal current flows therein, only one of said wind-' ings being connected in said line.
21. In oomb1nat1on,a source of alternating current, a power distributlon conductor connected to said source, a reactor having a main winding connected in series relation with said conductor and an auxiliary winding inductively related to said main winding, and a second synchronous source of current connected to said auxiliary winding, said auxiliary winding and second source of current cooperating with the main winding to effect an increase of impedance in said power distribution conductor upon a predetermined increase of current in said conductor.
22. In combination, an alternating current electric power transmission system, a reactance device interposed in said system, connections including an electron tube for controlling said reactance device, and means electrically connected to said system for controlling said electron tube.
23. In combination, an electric power transmission system, a reactance device interposed in said system, means including an electron tube normally establishing a currentconducting path of comparatively low impedance across said device, and means called into action upon abnormal rise of current in said system for automatically governing said electron tube to increase the impedance of said current-conducting path.
24. In combination with an electrical distriliftion circuit subject to excessively large current flow under abnormal conditions, an electric discharge device com rising an envelope and two spaced electro es therein and an inductive device comprising two inductively related windings, one connected in series with said distribution circuit and the other to the said electrodes of the discharge device.
25. The combination of a circuit, a device comprising an impedance coil connected to said circuit, and means comprising a thermionic device arranged to regulate the impedance of said coil in response to variation in the electrical characteristics of said circuit.
26. The combination of a circuit, a device ion comprising an impedance coil connected to said circuit and means comprising an elec-' trical-discharge device for controlling the impedance of said coil in response to variation in the current traversing said circuit.
27. In combination, an electrical-discharge device provided with electrodes supporting an electric discharge, a reactor and means including said discharge device for controlling the effective impedance of said reactor.
EARL R. EVANS.