US2573249A - Saturable reactor impedance relay - Google Patents

Description

Patented Oct. 30, 1951 SATURABLE REACTOR IMPEDAN CE RELAY Fredrik Dahlgren, Djursholm, Sweden,

assigner to Allmnna Svenska Electriska Aktiebolaget,

Vasteras, Sweden, a Swedish corporation Application August 23, 1947, Serial No. 770,250 In Sweden May 3, 1944 Section 1, Public Law 690, August 8, 1946 Patent expires May 3, 1964 Claims.

A frequently employed protective connection for power installations is, as is Well known, based on the principle that on certain points of the installation the proportion between voltage and current is measured, said proportion corresponding on theoccasion of a short-circuit to the impedance between the points in question and the point of short-circuit, and that an automatic disconnection is made at the points, where the said proportion is lowest, i. e. next to the short-circuit point. The measuring of the proportion between voltage and current has generally been made in special relays, called impedance relays, which directly or indirectly have released circuit breakers. It has, however, been found that such relays, if they shall be suiciently accurate and selective, will be rather complicated. Further it has been found that the so-called return ratio of most mechanical relays is rather low so that the value of the releasing quantity, for which the relay has acted, must vary rather considerably, before the relay ceases to act. This may often cause the release of a larger number of circuit breakers than those actuated by the relays which are most adjacent to the point of short-circuit.

The present invention relates to a protective connection for power installations, operating on the aforesaid impedance principles, which connection makes it possible to use very simple relays and further gives a considerably better proportion between the return current and the release current than hitherto employed connections for the same purpose. According to the invention a transductor (a direct current saturable reactor) is fed on the D. C. side by one current and on the A. C. side by another current determined from outside, of which currents one depends on the voltage and the other on the current in the point of the installation, where the protective operation is to be released, while a voltage across the transductor is caused to act on a protective device, generally on a relay releasing a circuit breaker. It is especially advisable to feed the transductor on the D. C. side by a current, dependent on the voltage, and on the A. C. side by a current, depending on the main current, whichmay for instance be obtained by means of a current transformer. In this case it can be arranged with least possible loss of power so, that the voltage acrossV the transductor is made to rise rapidly at such a proportion between the voltage and current of the line, which gives equal ampere turn numbers on the D. C. and A. C. sides of the transductor, so that even a relay, which is rather little sensitive in itself, only reacts for a certain value of the aforesaid proportion.

A form of the invention is diagrammatically i1- lustrated in Fig. 1 of the accompanying drawing, while Fig. 2 shows a diagram of the manner of operation of the transductor employed. Figs. 3 to 6 show diagrammatically other modiiications of the invention.

In Fig. 1 the primary winding of a current transformer I' is connected in a line 2 to be protected, while the secondary winding is connected to the A. C. winding 3 of the transductor, illustrated in the conventional way. The primary winding of a voltage transformer 4 is connected between the conductors of the line, While its secondary winding is connected through an adjusting impedance 5 and a rectiiier 6, which may be provided with smoothening members, to the D. C. winding l of the transductor. The voltage across the A. C. winding 3 is impressed on the relay 8, which for instance may act to release a circuit breaker.

The voltage conditions of a transductor, the traversing alternating current of which is kept constant by external means, are illustrated by the diagram of Fig. 2. In this diagram the abscissae designate the direct current IL and the ordinates the voltage Er across the A. C. winding of the transductor. The different curves IAI, IAz, correspond to different values of the alternating current traversing the transductor.

As long as the magnetizing direct current Iz. assuming the same number of turns considerably exceeds the crest value of the traversing alternatng current, the transductor absorbs practically no voltage, since its core is permanently saturated. When the transductor is employed as a. line protective device according to Fig. l, this is always the case in normal operation. If, on the contrary, the direct current sinks below the crest value of the alternating current, the voltage Er across the transductor rises rapidly, because the iron core will then during a larger or smaller part of the cycle be more or less desaturated, so that a voltage can be induced. For a rather small reduction of the direct current the voltage rises to a value, corresponding to a complete reversal of the magnetisation during one cycle, and then it can rise only very little, substantially corresponding to the leakage ilux.

The proportion between direct current and alternating current, at which the sudden rise of voltage occurs, corresponds in the connection according to Fig. 1 to a certain proportion between the line voltage and the line current, i. e. at a short-circuit, to the impedances between thel lay 8 will enter into action and release a protective operation as soon as this impedance sinks below a certain value. The adjustment can preferably be made by means of the impedance 5. In certain cases it may be advisable to make this impedance dependent on the voltage, for instance so that the current traversing it rises more rapidly than the voltage. Hereby certain imperfections of the connection illustrated may be compensated, such imperfections arising for instance from the fact that the secondary current of the current transformer I is divided between the transductor and the relay coil, whence the transductor current will not be exactly proportional to the main current.

Instead of simply connecting the relay 8 in parallel to a transductor fed by current and voltage, it may be connected in other ways, so as to depend on the voltage across the transductor, for instance as shown in Fig. 3 in a bridge between two opposite corners of a tetragon, in which two opposite sides consist of transductors having A. C. windings 3', 3" and D. C. windings 1', l and the two remaining ones of constant inductances 9. The two transductors may then have a common core or common cores, if each transductor have two cores in the very usual manner. The connection is preferably so' arranged that the bridge conductor normally will carry no current,vwhereby the action of the relay may be more sensitive than in the simple connection shown in Fig. 1,' or alternatively the sensitivity of the transductors for a certain proportion between direct and alternating current may be less. In employing one single transductor it is possible as shown in'Fig. 4; to apply to the relay only part of the transductor voltage instead of the entire voltage. In transductors having two cores and the two halves l0, I of the A. C. winding connected in series, as shown in Fig. 5,- it is also possible to utilize the voltage across third windings II, II' connected in the same manner as the D. C. windings I2, I2'. This voltage contains an essential second harmonic, which does not appear to an essential degreebefore the crest value of the alternating current exceeds the direct current.

It is also possible as shown in Fig. 6 to interchange the functions of the current and voltage transformers in Fig. 1, so that the alternating current of the transductor will depend on the voltage and its direct current on the main current at the point of the installation, where the protective action is to be released. The A. C. winding I3 should in such case be series-connected with an impedance. I4, which essentially determines the current therein. It has then full voltage in normal operation, but loses its voltage at a short-circuit through an impedance, corresponding to the adjustment of the protective connection, whence the relay should be 4 connected' in a corresponding manner so as to release the protective action, when its current sinks below a certain value.

I claim as my invention:

1. A protective connection for la power circuit, comprising a direct current saturable reactor, means for feeding the alternating current winding of said reactor directly, rectifier means feeding the direct current winding o'f said reactor, one of the winding currents being dependent on the voltage and the other on the current in said power circuit, a protective device for such circuit, vand means controlled by the voltage across the alternating current winding of said reactor to operate said protective device.

2. In a device as claimed in claim 1, the means for feeding the alternating current winding being dependent on the current in said circuit.

3. In a device as claimed in claim 1, said feeding means feeding the reactor windings with currents proportional to the voltage and current of said circuit.

4. In a device as claimed in claim l, the means feeding a current dependent on the voltage including a voltage-dependent impedance, whereby the current fed by such means is a non-linear function of the voltage in said circuit.

5. A protective connection for a power circuit comprising two direct current saturable reactors, means for feeding the alternating current windings of said reactors directly, rectier means feeding the direct current windings of said reactors, one of the winding currents being dependent on the voltage and the other on the current in said power circuit, means connecting the alternating current windings of said reactors in two opposite legs of a bridge, impedances in the other legs of the bridge, a protective device for said power circuit, and means controlled by the voltage from the bridge across the alternating current windings to operate said protective device.

FREDRIK DAHLGREN.

' REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date Re. 22,768 Lamm June 4, 1929 1,715,684 Thomas June 4, 1929 1,774,944 Petch Sept. 2, 1930 2,013,815 Wensley Sept. 10, 1935 2,067,143 Logan Jan. 5, 1937 2,149,092 Kettler Feb. 28, 1939 2,157,006 Oesinghaus' May 2, 1939 2,352,985 Washington July 4, 1944 2,381,527 Traver Aug. 7, 1945 FOREIGN PATENTS Number Country Date 566,232 Germany Dec. 13, 1932 66,170 Norway June 25, 1946

Images