US2276888A - Flared-ratio differential relay and auxiliary transformer therefor - Google Patents

Description

March 17, W 2 w. K. SONNEMANN FLARED-RATIO DIFFERENTIAL RELAY AND AUXILIARY TRANSFORMER THEREFOR F iled Dec. 28, 1939 4 Sheets-Sheet l INVENTOR WzZZzam if 5077726777077.

WITNESSES:

ATTORN EY March 17, 1.942. K g M NN 2,276,888

FLARED-RATIO DIFFERENTIAL RELAY AND AUXILIARY TRANSFORMER THEREFOR Filed Dec. 28, 1939 4 Sheets-Sheet 2 6 0 3 0 0 3 0 6 0 :0 Leading [7) Phase flizg/e Lagyz'rzg n I I w 12 (1} z'fi Amps.

90 6 6 3 0 0 3 0 I 6 0 20 WITNESSES: ([2 Mad? 1}) ([2 6799 H9 j INVENTOR Phase-fingle WzZ/Zam d. San/76020072. I a. M44 44 ATTORN EY March 17, 1942., SQNNEMANN 2,276,888

FLARED-RATIO DIFFERENTIAL RELAY AND AUXILIARY TRANSFORMER THEREFOR Filed Dec. 28, 1939 4 Sheets-Sheet 3 lagging: j} by 90 I lead?? I; by 90 2'; 2'72 phase 501272 B L M422 2 H A N n w: m

[ 272 amps.

ATTORNEY latented Mar. 17, 1942 UNITED STATES PATENT ()FFICE FLAREDRATIO DIFFERENTIAL RELAY AND AUXILIARY TRANSFORMER THEREFOR of Pennsylvania Application December 28, 1939, Serial No. 311,377

15 Claims.

My invention relates to high-speed ratio-differential relays, that is, to high-speed relays which operate when the apparent difference between the currents entering and leaving a piece of apparatus exceeds a predetermined ratio or proportion of the apparent sum of said currents, thus operating on a ratio or proportion, independently of the size or magnitude of the difference-currents. More particularly, my present invention relates to means for obtaining a flared operating characteristic in such a relay, that is, a characteristic such that a larger ratio or percentage is required under certain circumstances, such as when the currents involved are abnormally large, or when the difference-currents are greatly out of phase with the additive currents, or when there is an asymmetrical currentwave clue to the presence of a direct-current component.

The need for a flaring-ratio characteristic, when the currents are abnormally large, is apparent from the fact that the currents are measured by current-transformers which are usually of a certain order of commercial accuracy. Then a large number of commercial transformers of nominally the same rating and characteristics are manufactured, there is inevitably a certain amount of difference in the individual characteristics of different transformers, particularly under heavy overload conditions when the transformers become saturated, and when their various degrees of saturation may vary considerably, rom transformer to transformer. In actual commercial installations, it frequently happens, also, that current-transformers of different makes, or of different ratings, are utilized to respond to the currents in the incoming and out going leads of an apparatus to be diiferentially protected, and in these cases, the difference between the transformer-characteristics is, of course, very much magnified. It is usually desirable to be able to detect even weak faultcurrents when there is an internal fault in the differentiall protected apparatus, which may be the winding of a generator or a power-transformer. It is also invariably desired that there shall not be a response of the differential relay to the apparent difference-currents which result from differences in the individual calibration or performance of the respective current-transform- A To prevent faulty operations of the ratio-differential relay as a result of these, sometimes large, differences between the secondary currents of two. current-transformers, the primaries of which are traversed by the same current entering and leaving the protected apparatus, a flaredratio characteristic has been previously sought, and obtained, either by reducing the effectiveness of the relay-operating currents, or differencecurrents, or by increasing the effectiveness of the relay-restraining currents, or summation-currents, during high-current conditions as compared to low-current conditions. I have found, however, that it is not enough, simply to cause the ratio-characteristic to become flared under heavy-current conditions, when the differencecurrent is substantially in phase With the summation-currents, and when the current-waves are substantially symmetrical.

Thus, I have ascertained that many faulty operations of differential relays have been obtained, in the past, under conditions in which the apparent difference-currents, or the differences between the secondary currents of two current-transformers whose primaries are identically excited, are frequently considerably out of phase with the through-currents flowing through the primaries, due to an external fault; and this difference is largely unpredictable, depending upon which one of the current-transformers is saturated first, or has a more rapidly drooping characteristic than the other one. I have found this difficulty, due to out-of-phase conditions, even with differential-relays which had a decidedly flared characteristic for in-phase currents. In designing a generally acceptable flared-ratio differential relay, I have found it to be necessary, therefore, to cause the relay to have just as much of a flared characteristic for outcf-phase operating and restraining currents, as for in-phase currents, and it is even desirable to have a greater flaring, for out-of-phase conditions, because true fau1t-currents are likely to be more in phase with the restraining currents which are impressed upon the differential relays as a result of any through-load which is carried by the faulted protected apparatus, such as a winding of a generator or a power-transformer, at the time Of the fault.

I have also discovered that asymmetrical faultcurrents have frequently been the cause of faulty operation of previous ratio-differential relays, either of the straight-ratio type, or of the socalled flared-ratio type. The reason for this, is that an asymmetrical fault-current contains a direct-current component, which is like a halfwave of non-sinusoidal pulsating current of a half-wave length extending over a large number of cycles of the line-frequency current, usually 60 cycles. In an asymmetrical current-wave, the line-frequency, or 60-cycle, wave-form of the current is superimposed over this direct-current component, making the resultant pulsations of the wave-form asymmetrical with respect to the zero-current axis, hence the term asymmetrical. When such directcu1rent components appear in the primary currents which are passed through the two current-transformers at the entrance and exit terminals of the protected apparatus, the direct-current component almost invariably saturates the current-transformers, and usuall saturates them initially at different times and each for a different length of time.

My investigations have led me to believe that the resultant efiect of asymmetrical-current saturation, upon the difference-currents derived from the two current-transformers, is that it results in a very large difference-current with a considerable direct-current component, and of relatively brief duration, as compared with the duration of the direct-current component in the asymmetrical primary-current waves. As these difference-currents are utilized as the operating currents of the relay, I believe that this unidirectional-current component of the asymmetrical wave-form of the difference-currents has frequently been a source of erroneous operation of ratio-diiferential relays, where asymmetrical fault-currents have been encountered. The seriousness of this difiiculty will be realized when it is remembered that whenever a short-circuit happens to occur at a point which is relatively close to a large generating source, or where the time-constant is long, and at the moment when the impressed voltage-wave is at zero, or anywhere close to a minimum or zero-value, the r sultant fault-current wave will be considerably distorted or asymmetrical with respect to the zero-current axis. In other words, relays must be designed to operate properly, with asymmetrical fault-currents.

With the foregoing considerations in View, the principal object of my invention is to provide a novel type and arrangement of auxiliary transformer which is utilized in combination with a ratio-differential relay, to give the relay a ratiocharacteristic which is flared or increased, either when the operating current is large, or when it is greatly out of phase with the restraining current, or when it has a direct-current component.

More specifically, it is an object of my invention to provide an auxiliary transformer built on a three-legged core having a central leg on which the primary and secondary restraint-current windings are disposed, and having two outer legs on which the operating-current primary and secondary windings are disposed, the design being such that the operating-winding currents produce a flux circulating in the two outer legs and saturating said legs during certain portions of the cycle, thereby in effect removing the returnflux portions of the magnetic circuit from the center leg which carries the restraint-windings, and thus producing the desirable characteristics which I have mentioned.

With the foregoing objects in view, and other more detailed objects which will be apparent as the description proceeds, my invention consists in the parts, elements, combinations, systems and methods hereinafter described and claimed, and

illustrated wherein:

Figure l is a diagrammatic view of circuits and apparatus illustrating my invention in a preferred form of embodiment, and illustrating its mode of application to a protected apparatus;

Fig. 2 is an enlarged fragment of some of the current-wave forms which are obtained in my apparatus;

Figs. 3 and 4 are curve-diagrams illustrating the relation between the value of the operating current which is necessary to cause a relay-operation for various phase-angles of the operating current with respect to the restraining current, and for various values of the restraining current;

Fig. 5 is a curve-diagram illustrating the flaredratio characteristic of my relay, as compared to a constant-ratio relay, both with in-phase currents and with out-of-phase currents;

Fig. 6 is a copy of some of the traces obtained with a multiple-element oscillograph, showing the performance of my relay in the presence of asymmetrical fault-currents; and

Figs. 7 and 8 are copies of some of the traces obtained with a multiple-element oscillograph, illustrating the performance of my improved relay during in-phase conditions and out-of-phase conditions, respectively.

In Fig. l, I illustrate my invention as being utilized to protect (or to detect internal faults in) a single-phase generator G having a currentinput terminal 9 and a current-output terminal Hi, the input and output currents being derived for relaying purposes, by current-transformers II and I2, respectively, having secondary currents which are indicated as Is and I1, respectively. For convenience of reference, in the following description, I shall visualize the secondary current I1 as being the larger current, so that there will be a difference-current 12:11-13. Of course, it will be understood that either I1 or I3 may be the larger, and that the foregoing convention or designation is adopted merely for convenience. It will also be understood that either one of the terminals 9 or It! may be the input terminal, the particular designation being again merely for convenience.

My invention is particularly related to highin the accompanying drawings,

speed ratio-differential relays, and I have illustrated the invention, in Fig. 1, in combination with a balanced-beam type of instantaneous ratio-differential relay which comprises, typically or in general, a balanced beam E3, of magnetizable material, which is weighted at N at its back end, and which has electrical contacts 15 at its front end, for controlling a relay-circuit It which may be utilized for any desired purpose, either for indication or for automatic operation or control or correction of an internal fault-condition in the protected generator G. The beam I3 is intermediately pivoted on a pivot l1. Its front end is attracted by an operating winding 0 which is wound on a magnetic structure I8, while its rear end is attracted by two restraint-windings R1 and R2 which are mounted upon a multiple-legged magnetic structure l3. Two restraint-windings are utilized, by preference, so that one of said windings, R1 as illustrated, can be connected in series with a resistor 2 I, while the other one, R2 is connected in series with a capacitor 22, so as to produce two out-of-phase pulsating restraining forces on the beam l3, so that the restraining force never passes through zero.

In the normal use of a differential relay such as that which is shown in Fig. 1, the operating winding is normally energized in accordance with I2=(I1I2) while the restraint-winding or windings R1 and R2 are energized in accordance with (I 1+I3). When the ratio-differential relay is directly energized in accordance with these socalled additive and differential currents, respectively, the relay has a substantially straight-line characteristic, such as is represented by the line 24 in Fig. 5, when the restraint-current is in phase with the operating-current. I have discovered, however, that such a relay has in herently a much lower ratio-characteristic when the restraint-current is considerably out of phase with the operating-current, because in such a case, the maximum force developed by the operating winding 0 occurs at a time when the force developed by the restraint-winding or windings R1 and R2 is at a minimum. This reducedratio out-of-phase characteristic of a straightline or constant-ratio relay is indicated by the line 25 in Fig. 5.

In accordance with my invention, I provide means for causing the ratio-differential relay to depart from the constant-ratio characteristic just described. In the preferred form of embodiment which is illustrated in Fig. 1, I utilize, for this purpose, an auxiliary transformer having a three-legged iron core 21 having a central leg 28 and two outer legs 29 and 39. I pass the summation-current (Ii-l-Ia) through a primary winding 32 which is disposed on the central leg 28, accomplishing this purpose by connecting the outer terminal of the first current-transformer I2 to the terminal I of the auxiliary transformer, which terminal is connected to one end of the primary winding 32, and connecting the outer terminal of the second current-transformer l l to the terminal 3 of the auxiliary transformer, the latter terminal being connected to the end of the primary winding 32, while the inner terminals of other end two current-transformers l I and [2 are connected together as indicated at 33. This primary winding 32 is utilized for the purpose of energizing the restraint windings R1 and R2 of the ratio-differential relay, a highvoltage secondary winding 34 being disposed on the aforesaid central leg 29, for this purpose, as illustrated in Fig. 1.

I utilize two identical primary windings 35 and 36 for developing the flux which produces the secondary current Io energizing the operating winding 0 of the ratio-differential relay,

these two operating-current primary windings 35 and 38 being respectively disposed on the two outer legs 29 and 39 of the auxiliary transformer. The two operating-current primary windings 35 and 36 are connected in series with each other, between the central point 31 of the restraintcurrent primary winding 32 and the aforesaid common return-point 33 of the two currenttransformers II and I2, so that the two operating-current primary windings 35 and 36 are traversed by the difference between the currenttransformer currents I1 and I3, said differencecurrent being designated I2. Each of the outer legs 29 and 39, where the operating-coil primary windings 35 and 35 are disposed, is provided with an operating-current secondary winding, these two secondary windings being designated by the numerals 39 and 40, respectively. These two operating-current secondary windings 39 and All are connected in series with each other, and are utilized to energize the operating coil 0 of the ratio-differential relay.

The two operating-current primary windings 35 and 36 produce fluxes which both circulate around the two outer legs 29 and 39 in the same direction, so that, because of the balanced construction, there is substantially no magnetomotive force tending to divert any of said fluxes through the center leg 23, assuming that the relative conditions of saturation of the two outer legs 29 and 39 remain substantially identical. Consequently, the operating-current fiux 0 is opposed by the restraint-current flux m, in one of the outer legs, such as the leg 39, as indicated by the arrow (01/2R), while, in the other leg, such as the leg 29, the operating-current flux is augmented by the restraint-current flux, as indicated by the arrow (o+ /2R), representing conditions prevailing at some assumed instant within the auxiliary transformer.

The manner of operation of the apparatus which has just been described, with reference to Fig. 1, will perhaps best be understood with reference to some tests which I have made on a particular form of embodiment of the auxiliary transformer, the iron core of which is indicated, full scale, at 21 in Fig. 1. While I am not limited to any particular size or proportions of either the magnetic or electrical circuits of this transformer, it may be helpful, to fix our ideas on some one particular design, to mention the fact that the particular tested transformer to which I shall refer had a restraining-current primary winding 32 having four turns, two operating-current primary windings 35 and 35 having turns each, a restraint-current secondary winding 34 having 1200 turns, and two operatingcurrent secondary windings 39 and 49 having 15 turns each.

Fig. 7 shows the results of oscillographic tests which were made upon such a transformer, in order to study its performance. Asubstantially sinusoidal (SO-ampere, GO-cycle, current I3 was caused to flow through the terminal 3, with other connections such as to apply a substantially sinusoidal Gil-cycle voltage tending to produce a differential current-flow I2. The first two traces, in Fig. 7, show these currents I3 and I2 respectively. The third and fourth traces respectively show the relay-restraining voltage Ea which was developed in the secondary winding 34, and the relay-operating current Io delivered by the two serially connected secondary windings 39 and 49. The last two traces, in Fig. '7, show the wave-form of the currents flowing in the respective restraint-windings R1 and R2, the former having the resistor 2| connected in series with it, while the latter had the capacitor 22 connected in series with it.

For a first analysis, reference will be made to Fig, 2, in which I have reproduced the waveforms of I2, 12 Io, as derived from the first, second and fourth traces of Fig. 7. I explain the operation of the auxiliary transformer as follows. During the short period from a to b, the auxiliary transformer is unsaturated, thus tending to limit the differential-current I2 to a small value which would have reached a peak value as indicated by dotted lines at e in Fig. 2. During this time, from a to b, the auxiliary transformer was performing with its true ratio, so that its secondary current In was increasing normally from a to b, and would have reached a peak value as indicated by dotted lines at f, if the transformer-ratio had not broken down tremendously.

However, at the point b the iron became saturated, producing a sudden rise of the differentialcurrent I2, to the value 9. visualize this sudden rise in the differential current is as being caused by the fact that the operating-current primary-coils 355 and 35 of the transformer were unable to resist the voltage impressed thereon, so that a high peak value of current resulted, as indicated at g. At the same time, since the outer legs 29 and of the core were badly saturated, the operatin -current secondary windings 39 and as could not function in a normal manner, so that the secondary current I began to decay rapidly from the point 29 to the point 0.

Now referring back to Fig. '7, it will be noted that, during the time from a to b, when the two outer legs 29 and of the auxiliary transformer were unsaturated, the center-leg primary and secondary windings and (Fig. 1) will behave relatively normally, that is, the secondary voltage ER was increasing rapidly, at the same time when the primary current I3 was increasing. Between the times indicated by the lines b and c, however, the two outer legs 2% and 3b, which constitute the return-path for the magnetic fiux qbR of the center leg 23, became so thoroughly saturated that the transformerwindings til-dd on the center leg have, in eifect, an air (or non-magnetic return-circuit for the center-leg flux (pa, even though the central core 28 may be unsaturated. During this saturated period from b to 0, therefore, the transformationratio of the center-leg transformer 3234 breaks down, so that the secondary voltage ER falls off rapidly, as indicated in Fig. '7,

The result of the foregoing considerations is that, on the outer legs 29 and 30, there is, in effect, a saturating operating-current transformer which reduces the magnitude of the operating current In when excessive difference-current conditions I2 exist, thus tending to produce the desired flared characteristic. Furthermore, on the center leg of the same iron assembly 2?, there is provided another transformer 32-43%, for exciting the restraint-windings R1 and R2, and this restraint current transformer 32-34 has its operation also modified in accordance with the condition of saturation of the outer legs 29 and 3!) resulting from the magnitude of the difference-current I2.

The exact nature of the eifcct of the difference-current saturation, in the outer legs 29 and 3t, upon the operating characteristics of the restraintcurrent transformer 3234 on the central leg will best be understood by reference to Fig. 8 in comparison with Fig. '7.

The films which are reproduced in Figs. 7 and 8 differ in that the Fig. 7 film was taken for conditions in which the differential-current I2 was approximately in phase with the through-current I3, whereas the Fig. 8 film was taken for the case in which the difierencewurrent I2 was leading the through-current I3 by approximately 90. A comparison of the two films will show that there is very little difference in the operatingcoil currents In which were obtained, in the two cases, the first one having a peak-to-peak Value of 11.8 amperes, and th second one having a peak-to-peak value of 10.4 amperes. This shows that the outer legs 29 and 30 of the auxiliary transformer wer operating approximately the same, regar less of the phase of the magnetizing currents on these legs, as compared with the phase of the magnetizing current for the central leg 28. When a comparison is made of the relay restraining voltages ER, however, a considerable difference will be discovered between Figs. 7 and 8. When the currents were in phase, as in Fig. 7, the peak-to-peak value of the restraint-voltage Ea was 200 volts, whereas, when the currents I2 and I3 were out of phase, the peak-to-peak value of the restraint-voltage Ea was 575 volts, indicating a much stronger restraint in the case of the out-of-phase currents.

I explain this increased restraint, when the primary currents I2 and I3 are 99 out of phase, by noting that, when the currents are iii-phase, as shown in Fig. '7, the unsaturated period a-b (as determined by the relatively flat part of the I2 wave) occurs at a time when the transformerflux due to the Is current is close to the zero point, whereas, when the currents are 90 out of phase, the outer legs of the transformer are unsaturated when the Is current, and hence its flux, are very close to their maximum values, on the central leg 23 of the auxiliary transformer, so that the central-leg transformer 3234 has its normal high transformation-ratio, inducing a high voltage ER in the secondary winding 24. Reference to the last two traces in each of Figs. 7 and 8 will show a considerable increase in the areas of the restraint-winding currents IE1 and Im in the Fig. 8 film representing the out-ofphase conditions. The difference in the relayconditions shown in Figs. '7 and 8 is witnessed also by the fact that the relay operated, under the conditions depicted in Fig. '7, whereas the relay did not operate, under the conditions depicted in Fig. 8.

It will be noted, in connection with the eX- planation of the saturating operation of the auxiliary transformer, that I utilize an outer-leg flux e0, which is at least several times larger than the central-leg flux 4m, so that, during saturated operating conditions, when the flux in one of the outer legs is (o1/2R) and the flux in the other outer leg is (0+1/2n) these two fluxes will both be nearly enough of the same value, because both are well out on the flat part of the magnetization-curve of the transformer, representing highly saturated conditions.

Thus, referring to Fig. 5, it will be seen that, for a through-terminal current I3 of 60 amperes, the minimum in-phase difference-current I2 necessary to cause a re1ay-operation is 12 amperes, which means that the 11 through-terminal current will be 60+12=72 amperes. The operatingcurrent transformer on the two outer legs 29 and as (Fig. 1), under these conditions, has a magnetomotive force corresponding to 12 240=2880 ampere-turns in the two -turn primary windings 35 and 38, whereas the restraint current transformer on the central leg 2% has a magnetomotive force corresponding to (60+72) X2 264 ampere-turns in the primary winding 32.

In the particular form of embodiment of my invention which is shown in Fig. 1, the restraining windin s R1 and R2 of the differential relay impose an extremely low volt-ampere burden on the ER secondary winding 34 on the central leg 23 of the auxiliary transformer, and it was found that this current-transformer burden was too low to prevent excessive saturation of the central leg 28, with the normal size of phase-shifting re sistance 2| and capacitance 22 which were normally utilized in other applications, as, for example, in impedance-relays. Rather than increase the burden of the restraint-coils R1 and R2, which would necessitate utilizing a larger (and hence more expensive) capacitor '22, and

additional development to determine the proper values of the new magnitudes, I have corrected this low-burden difliculty by adding an artificial burden in the form of a resistor 50, as shown in Fig. 1. Instead of using this shunt-connected artificial load or burden 5!], I could have redesigned the auxiliary transformer to utilize either a smaller number of primary turns 32 on the central leg 28, or I could have short-circuited a small percentage of the turns in the winding 34, so that the central leg 28 would not saturate before, say, to amperes, (or two or three times normal load-current) were passed through the primary winding 32, even with the low-burden restraint-windings R1 and R2 of the particular form of differential relay which I utilized.

The performance of my relay, with respect to various-phase-angles between the operating-current I2 and the restraint-current I3, is well shown in the test-curves reproduced in Figs. 3 and 4, which show the minimum operating-currents I2 necessary to cause a relay-operation, under different conditions of restraint. It will be noted that a fairly flat curve is obtained for a retraint of 13:5 amperes, corresponding to normal-load conditions of the current-transformers II and I2, and hence corresponding to normal full-load conditions of the protected apparatus G in Fig. 1. Thus, in Fig. 3, it will be seen that the minimum operating-current I2, necessary to trip the relay with normal-load restraint, varies only between the limits of .24 ampere and .36 ampere, no matter what the phase-angle between I2 and Is. On the other hand, as the restraint-current I3 increases, it will be seen, from Figs. 3 and 4, that the curves for the minimum values of the operating-current I2 slope sharply upwardly when the phase-angle departs greatly from the in-phase condition, producing the desired concave, or upwardly flared, curve representing the relation between the operating-current I2 and phase-angle. It will be noted that this is just the reverse of previous ratio-differential relayperformances, in which this curve had a convex form, of a type requiring something like three or four times as much operating-current I2 to operate the relay, when the currents I2 and I3 are in-phase, as when said currents are 90 out of phase.

The operational characteristics of my flaredratio relay are summarized in the curves 5|, 52 and 53 of Fig. 5, utilizing values which are taken from the test-values which are reproduced in Figs. 3 and 4. The difference in operation of my relay, as compared with an old constant-ratio relay, will be readily apparent, by comparison with the straight-line curves 24 and 25 of the same figure.

An important feature of my improved relay is also that it is much better able to resist faulty operations in response to asymmetrical faultcurrents, than previous relays. The tests which were recorded by the films reproduced in Figs. 7 and 8 were taken for artificial test-conditions utilizing substantially symmetrical currents such as I3. In actual applications of the relay, it is necessary for the relay to operate properly when the impressed currents have a strong asymmetrical, or direct-current, component.

In order to verify the operation under such conditions, a test, as recorded by the film which is reproduced in Fig. 6, was made, in which the primary current was made to have a directcurrent component, 54, as shown in the third trace in Fig. 6. It will be noted that, under these conditions, the difference-current 12 developed a strong asymmetrical, or direct-current, component, 55, which was relatively highly damped with respect to the primary-current asymmetrical component 54, as evidenced by the fact that the difference-current asymmetrical component 55 reduced substantially to zero value much more quickly.

The transformer-operation, as depicted in Fig. 6, may be interpreted as follows. The directcurrent component 55 of the difference-current I2 saturates the two outer legs 29 and 30 (Fig. 1) of the auxiliary transformer with a unidirectional bias, and when this occurs, the operating current Io which is produced in the secondary windings 39 and 40 on the same legs is not very high, notwithstanding the fact that the primary currents I2 on the same leg have extremely high peaks. Toward the end of the film, it will be noted that the primary current I2 diminishes to a very small value, and yet there is a very material value indicated for the secondary current Io on the same legs of the transformer. This can be explained by the absence of the direct-current saturation during the latter portions of the film, which restores the normal transformation-ratio to the outer-leg transformer. It will be further noted that the central-leg secondary voltage ER is distorted, and somewhat limited, because of the direct-current component 55 in the I2 current during the early portions of the film, indicating that the return-path of the central-leg flux, through the two outer legs 29 and 36, is saturated at this time. As a matter of fact, the differential relay did not respond, during the test-conditions depicted in Fig. 6, notwithstanding the presence of very high operating-current peaks due to the presence of the direct-current component 55 in the I2-trace.

It will thus be seen that my auxiliary transformer, which is built on the three-legged core 21, provides a very simple means for giving a ratio-differential relay a flared ratio-characteristic which will enable it, not only to automatically increase the ratio between difference-current and through-current at which the relay will operate during excessive-current conditions when current-transformer saturation is inevitably obtained, but which will also enable it to have a still more steeply flared ratio-characteristic when the difference-current is greatly out-of-phase with respect to the through-current, which is normally the case when there is an external fault resulting in heavy through-currents, and which will also enable the relay to have a flared characteristic during asymmetrical current-conditions, so as to enable the relay to ride through severe asymmetrical current-conditions without a faulty relay-operation.

While I have illustrated my invention in accordance with a preferred form of embodiment, and while I have described the same in accordance with my best present understanding of its theory of operation, I do not wish to be limited, in either respect, to all of the details shown and described, as it should be obvious that various changes may be made, by those skilled in the art, without departing from the essential principles of my invention, particularly in its broader aspects. I desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language and the prior art.

I claim as my invention:

1. A flared-ratio differential relay assembly for use in connection with current-responsive input and output transforming-means for deriving additive and differential relaying-currents, respectively, from a differentially protected device, said differential-relay assembly comprising the combination, with said relaying-current transforming-means, of a differential relay element having an operating winding or windings and a restraining winding or windings, and an auxiliary transformer, said auxiliary transformer having a three-legged magnetizable core having a center-leg magnetic circuit and an outer-leg magnetic circuit, an additive-current primary winding-means and a restraining-current secondary winding-means both disposed upon one of the magnetic circuits of the auxiliary transformer, a differential-current primary windingmeans and an operating-current secondary winding-means both disposed upon the other magnetic circuit of the auxiliary transformer; the additive-current primary winding-means being electrically connected so as to circulate an additive-current flux around its magnetic circuit of the auxiliary transformer, the differentialcurrent primary winding-means being electrical-- ly connected so as to circulate a d erentialcurrent flux around its magnetic ci cult of the auxiliary transformer, the operating-current secondary winding-means being electrically connected so as to energize the operating winding or windings of the differential relay element, and the restraining-current secondary windingmeans being electrically connected so to energize the restraining winding or windings of the differential relay element.

2. The invention as defined in claim 1, characterized by relative proportions of the numbers of turns and loads such that the magnetic flux in one of said magnetic circuits is at least several times larger than the magne cc flux in the other of said magnetic circuits during the operative conditions of the device.

3. The invention as defined in claim 1, characterized by the fluxes and core-cross-sections of the auxiliary transformer being so related that one of said magnetic circuits saturates sooner than the other, during excessive-current operating-cdnditions of the device.

l. The invention as defined in claim 1, charac erized by the fluxes and core-cross-sections of the auxiliary transformer being so related that at least One of said magnetic circuits becomes badly saturated during excessive-current operating-conditions of the device.

5. The invention defined in claim 1, characterized by relative proportions of th numbers of turns and loads such that the differentialcurrent magnetic flux is at least several times larger than the additive-current magnetic flux during the operative conditions of the device.

6. The invention as defined in claim 1, character zed by the fluxes and core-cross-sections of the auxiliary transformer being so related that the outer legs saturate sooner than the central leg during excessive-current operating-conditions of the device.

'7. The invention as defined in claim 1, characterized by the fluxes and core-cross-sections of the auxiliary transformer being so related that the outer legs badly saturate during excessivecurrent operating-conditions of the device.

8. A flared-ratio diflferential-relay assembly for use in connection with current-responsive input and output transforming-means for deriving additive and differential relaying-currents, re-

spectively, from a differentially protected device, said differential-relay assembly comprising the combination, with said relaying-current transforming-means, of a differential relay element having an operating winding or windings and a restraining winding or windings, and an auxiliary transformer, said auxiliary transformer having a three-legged magnetizable core, an additive-current primary winding-means and a restraining-current secondary winding-means both disposed upon the central leg of the auxiliary transformer, a differential-current primary winding-means and an operating-current secondary winding-means both disposed upon one of the outer legs of the auxiliary transformer, and a similar pair of dififerentiahcurrent primary winding-means and operating-current secondary winding-means both disposed on the other outer leg of the auxiliary transformer; the two differential-current primary winding-means being electrically connected so a to cumulatively circulate a differential-current flux around the two outer legs of the auxiliary transformer, the two operating-current secondary winding-means being electrically connected so as to energize the operating winding or windings of the differential relay element, and the restraining-current secondary winding-means being electrically connected so as to energize the restraining Winding or windings of the differential relay element.

9. The invention as defined in claim 8, characterized by relative proportions of the numbers of turns and loads such that the differential-current magnetic flux circulating around the two outer legs is at least several times larger than the additive-current magnetic flux in the central leg during the operative conditions of the device.

10. The invention as defined in claim 8, characterized by the fluxes and core-cross-sections of the auxiliary transformer being so related that the outer legs saturate sooner than the central l g during excessive-current operating-conditions of the device.

11. The invention as defined in claim 8, characterized by the fluxes and core-cross-sections of the auxiliary transformer being so related that the outer legs badly saturate during excessivecurrent operating-conditions of the device.

12. Electrical translating-apparatus comprising, in combination, two diverse input-circuits, two diverse output-circuits, and a transformer for couplin said output-circuits to the respective input circuits, said transformer hav'mg a threeleggcd magnetizable core, primary and secondary winding-means disposed on the central leg of the core, primary and secondary winding-means disposed on, and divided between, the two outer legs of the core so as to cumulatively circulate their magnetic flux around said outer legs, circuitconnections for energizing the respective centralleg and 0uter-leg primary winding-means from the respective input-circuits, and circuit-connections for energizing the respective output-circuits from the respective central-leg and outerleg secondary winding-means.

13. Cumulative and differential electricallyresponsive translating-apparatus comprising, in combination, two differently energized sourcecurrent circuits for said apparatus, circuit-connections for deriving a cumulative input-circuit and a differential input-circuit responsive, respectively, to the sum and difference of electrical quantities in said source-current circuits, and saturable transforming=neans for deriving cumulative and differential output-circuits in such manner as to be responsive to the respective cumulative and differential input-circuits and also to be responsive in some measure to each other, said transforming-means comprising a transformer having an at least partially saturable three-legged magnetizable core, primary and econdary Winding-means disposed on the central lee of the core, primary and secondary windingmeans disposed on, and divided between, the two outer legs of the core so as to cumulatively circulate their magnetic flux around said Outer legs, circuit-connections for energizing the respective central-leg and outer-leg primary Winding-means from th respective input-circuits, and circuitconnection for energizing the respective outputcircuits from the respective central-leg and outerleg secondary winding-means.

14. The invention as defined in claim 12, in combination with electrically responsive means having an operating winding-means energized from one of said output-circuits and a restraining winding-means energized from the other output-circuit.

15. The invention as defined in claim 13, in combination with electrically responsive means having an operating winding-mean energized from said differential output-circuit and a restraining winding-means energized from said cumulative output-circuit.

WILLIAM K. SONNEMANN.

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