US1866345A - Current transformer with primary parallel resistance and flux leakage path - Google Patents

Description

A. CALLSEN 1,866,345

CURRENT TRANSFORMER WITH PRIMARY PARALLEL RESISTANCE AND FLUX LEAKAGE PATH A July 5, 1932.

Filed Dec. 12, 1950 Magnetic F lux Intensity May/wart Relucfance Maynelizz'ny Force l l l l 9 f 2 M f n m r mm k w mm RL rh 0. JC w a P /0% Fansformer Lbad Current I00 II'III" -mmm- INVENTOR 14 berz Cal/s en AT ORNEY WITNESSES:

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Patented July 5, 1932 UNITED STATES PATENT QFFICE ALBERT CALLSEN, OF STUTTGART, GERMANY, ASSIGNOR TO WESTINGHOUSE ELECTRIC 5 MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA QURRENI. TRANSFORMER WITH PRIMARY PARALLEL RESISTANCE AND FLUX LmlKAGE PATH Application filed December 12, 1930, Serial No. 501,852, and in Germany Ilecember 19, 1929.

Thi invention relates to current transforu .1: and has particular rel; tion to meth- Dds of eliminating or neutralizing the, phase :1u-"'lc and tinnsiformation errors which are rent therein. I

"Turnout iransfornwrs of conventional or ne consisting, as they do, of priand ondary windings linked by a on i ne-tic core, are subject to errors a transfl'n'nicr so constructed may be made capable of accurately maintaining the desired 180 phase relation between the current in the transformer secondary and the current in the power-circuit line in which the transformer primary is connected. Liliewise, by this method the transformation error may be. made substantially uniform. ll will be recognized that such errorfree characteristics are highly desirable in cur rent transformers for all applications, and a e particularh essential where extremely accuracy of operation is required.

is, accordingly, an object of my inventi n to provide a current transformer in whh h it) phase-angle error between the prinu: and the secondary currents is substantiall x eliminated throughout the entire load range of the transformer and in which the transhu'uiation error is made substantially uniform for all loads.

ii. is 2: further object of my invention to provide a current transformer in which both the phase-angle and the transforn'iation e rors are controllable to the extent that c angle and transformation,

they may be substantially neutralized throughout the entire load range.

More SPQClilClll) stated. it is the object of my invention to provide, in a current transformer, a shunting resistance and a magneticlealiage path of su h proportion that the phaseangle error of the transformer will be substantially eliminated and the tran formation error mad:- practically uniform throughout the entire load 191 Cl o attain these and other objects, the coil magnetizing the transformer shunted by a resistance and is coupled with at least one leakage member in which the point of lowest magnetic reluctance lies in the range of low transformer loads.

The invention, itself, both as to its organization and method of operation, together with additional objects and advantages thereof. will best he understood from the following; description of specific embodiments, when read in conjunction with the accompanying drawing. in which Figure 1 is a diagrammatic View of a current transformer constructed in accordance with my invention.

i2 a sectional VlQW of a current transformer of another type also embodying my invention.

F 3 is a vector diagram of the currents and the voltages acting in a transformer of my invention.

Fig: 4e comprises curves which illustrate the magnetic characteristics of iron-core structures such are used in current transformers.

Fig is a group of curves illustrating various characteristics of the transformer of my invention, and

, Fig. (i is a magi'iilied vector diagram showing the relative voltag s and currents in the, transformer of my invention at one-tenth and full-load values. respectively.

Referring to the drawing. particularly Figs. 1 and 2 thereof. inuncrals i l and 11 designate the currcnt-transformer terminals to which the power line. the current if which is to be measured. may be connected. In both figures, the primary windiuo' or maynetizing conductor of the transformer is shown at 12, the secondary winding at 13, the main magnetic circuit, coupling these two windings, at 14, a primary shunting resistance at 15, and a primaryfluX-leakage memher at 16, the embodiment of Fig. 1 utilizing one member 16 and that on Fig. 2, two members as shown.

It will be apparent that the elements enumerated are embodied in the transformers of both Figs. 1 and 2, though in a somewhat different form. The respective functions, however, are similar in both cases, as will be seen.

The vector diagram of Fig. 3 applies to both of the transformers just described, and is intended to illustrate the various current and voltage relations which obtain, at some given load for which complete compensation for phase-angle error is had.

Since it is difficult to represent clearly, on the same vector diagram, quantities differing greatly in magnitude, 1 have considered, in Fig. 3, a transformer having a ratio of 1 t 1 to permit greater clarity. It will be recognized that this is the equivalent of letting the vectors represent ampere turns and voltage per turn instead of current and total voltage. Likewise, the exciting-current vector is greatly increased in scale to better show the conditions.

In this diagram, a vertically drawn vector 17 represents, at some particular instant, a given value of secondary current. Vector 18 shows the exciting current required for this condition, and vector 19 the corresponding primary current. From the construction of this portion of the diagram, which will be recognized as being typical of all current transformers, it is apparent that the secondary current 17, flowing in winding 13, differs substantially 180 from the phase position of primary current 19, flowing in winding 12, except for the efiect of the exciting current 18. This effect introduces the phase-angle error denoted by theta in the diagram.

Vector 20 represents the negative induced voltage in the primary winding 12, due to the flux produced in the transformer core 14 by the exciting current 18, and has a phase position differing from that of vector 18 by 90. Vector 21 is the ohmic voltage drop in the primary winding, set up by the flow of primary current 19 through the resistance of that winding.

Due to primary leakage member 16, which provides an additional path for magnetic flux linking the primary winding 12 only, an additional voltage is induced in the rimary winding by this leakage flux which 1s represented, in Fig. 3, by a vector 22 which bears a 90 phase relation to the primarycurrent vector 19.

The voltage across the primary terminals 10 and 11 is indicated by vector 23 and represents the sum of voltage vectors 20, 21 and 22, previously mentioned. This terminal voltage is impressed directly across parallel-connected resistor 15, and causes a current to flow therein which is represented by vector 24.- having an in-phase relation to the total primary-voltage vector 23.

The total currentflowing in the power-line circuit, or between transformer terminals 10 and 11, is the sum of primary winding and shunting-resistor currents represented by vectors 19 and 24, respectively, and is so shown in Fig. 3 by vector 25.

The particular conditions for which this vector diagram has been drawn are such, as will be seen, that the power-line current 25 bears an exact 180 phase relation to the secondary current 17 of thecurrent transformer, and represents a complete elimination of, or compensation for, phase-angle error.

Thus, as is shown in Fig. 3, it is possible to overcome the angle error for a certain load by the use of parallel resistance and a magnetic-lealrage member. The transformation ratio of that load can be compensated for by a suitable variation in the number of secondary turns, in a known manner.

Attempts by the means already outlined to compensate for angle errors throughout the entire load range of the transformer are successful only when the magnetic characteristics of the leakage member 16 are properly proportioned with respect to the other elements of the transformer, and my invention is likewise directed to such proper proportioning, in a manner to be explained.

Curve 26, of Fig. 4, for a magnetic circuit made up of typical transformer iron, shows the manner in which the magnetic flux intensity varies with the magnetizing force acting on that circuit. Curve 27, derived from curve 26, in a known manner, shows that the magnetic reluctance of such circuit is different for different values of flux intensities, such reluctance being relatively high for the extremely low values of flux, decreasing to a minimum value at some intermediate flux intensity, and again increasing as the flux intensity rises above that intermediate value.

Current transformers are preferably so designed, as is known, that, for the normal range of load currents, the magnetic density in the core is sufliciently far below the point of saturation that the exciting current required will be reasonably small. In the curve of Fig. 4, such range would be included by the portion of the curves to the left of the dotted line 28. Curve 29 of Fig. 5 illustrates the manner in which the transformerexciting current varies with the load current for the conditions just outlined.

If the magnetic-leakage member 16 is so proportioned that the operating flux densities in it are comparable to those of the main core member 14, the angle error, shown as being completely compensated for in the vector diagram of Fig. 3, does not disappear for load values other ,than the particular one chosen, the reason being that, with a decrease in the primary current, the magnetizing cur rent 18 does not decrease proportionately, but, as can be seen from curve 29, decreases much more slowly, while all the other primary vectors of the diagram become proportionately smaller. The result is that the angle and the transformation errors of the transformer thereby increase at the smaller loads. Hence, as regards the characteristics of the transformer at different loads, little is gained by the use of this arrangement unless the elements are properly proportioned in the manner to be explained.

I have discovered that such an objection may be overcome by so designing the leakage member 16 that its point of lowest magnetic reluctance lies in the range of the lowest transformer loads, especially below one-tenth load. Curve 30 of Fig. 5, shows the manner in which the leakage-member reluctance varies with the transformer load when the member is so designed.

It has already been seen that these leakage members increase artificially the voltage drop in the primary winding or conductor 12 of the transformer and, by so arranging that the minimum magnetic reluctance of the leakage member occurs at about one-tenthload, the leakage voltage will increase in this range much more with the primary current than in the higher ranges of load on the transformer.

The current traversing the parallel resistance 15 is always proportional, ashas been seen, to the sum of the voltage drops in the primary winding. As the total voltage drop in the winding 12 is greater, for the reason just pointed out, at low loads, than at the high loads, relative to the primary current, the value of the current traversing the parallel connected resistance 15 is also greater, in percent, at the lower loads than at the higher loads.

The vector diagram of Fig. 6 shows portions of the diagram of Fig. 3 on an enlarged scale. similarly designated, and assumed to correspond to normal full-loads for the current transformer of my invention. Fig. 6 shows, also, by the dotted lines. vectors corresponding to one-tenth load which, according to a well known method, are shown magnified ten times for more convenient comparison with the full-load vectors.

According to this method, O is the end of the secondary-current vector 17 which is to be considered as lengthening downwardly. B is the end of the primary-current vector 19 at full load; C is the end of the excitingcurrent vector 18, also at full load. and D is the end of parallel-resistor and power-linecurrent vectors 24 and 25, respectively, likewise at full load, and, similarly, E designates the ends of voltage vectors 22 and 23, respectively, for the full-load condition.

At one-tenth load, the exciting current has increased proportionately to some value, such that, when magnified ten times, it will be represented by vector OC which shifts the end of the primary-current vector to B, as indicated. If it is assumed, for the purpose of illustration, that the magnetic-flux densities in the leakage member 16 are similar to those in the main core member 14, the voltage drops in the primary winding 12 at onetenth load will substantially retain their proportions as at full load, and, when thevectors are magnified ten times, they will coincide with the full- load vectors 20, 21, 22 and 23. In such an event, the current flowing in parallel resistor 15 will, likewise, retain its fullload proportions at one-tenth load, and, when magnified ten times, will lie on the line B D parallel to vector 24.

For such a condition, the diagram of Fig. 6 shows that, while, at normal full load, the angle error has been corrected,'at one-tenth load there has been introduced an error corresponding to the distance K.

By so designing the leakage member 16 that its value of minimum magnetic reluctance occurs at approximately one-tenth load, as indicated by curve 30 of Fig. 5, it ispossible to proportionately increase the leakage-voltage drop at one-tenth load to some value such that, when magnified ten times, it will be indicated by vector 22, plus the additional length EF, thereby increasing the total voltage drop in the primary winding at one-tenth load to OF. The current in the parallel-connected resistance 15 will vary proportionately with this primary terminal voltage OF, and thus the vector B. G is obtained for the one-tenthload condition. G is, at the same time, the upper end of the vector of the primary total current at one-tenth load, magnified ten times, corresponding to vector 25 for the fullload condition.

A comparison of the diagrams at full load and at one-tenth load will show that, at onetenth load, a relatively small-angle error M only is obtained, which may be either positive or negative or even zero, according to the values of the parallel-connected resistance and the leakage members. The difference in the percentage of the errors in the transformation, denotedby N, at full load and at onetenth load is seen also to be very small.

In the diagram of Fig. 6, the phase-angle and the transformation errors are shown as not being fully compensated for, in order to make the diagram clearer. It will be understood that there exists the possibility of regulating the primary terminal voltages, as to value and phase, by varying the dimensions of the leakage members, and by varying the parallel resistance of controlling the ratio factor between the primary terminal voltage (vector 23) and the current (vector 24) flowing in the parallel resistance. It is thus possible to make the point for one-tenth load coincide with the corresponding point for a normal load, or, if such complete compensation is not desired, to place it anywhere within a reasonable range.

In those cases in which complete compensation for transformer errors is not necessary, through the use of my invention, the transformer can be built with a smaller number of primary ampere turns than is possible for other designs intended to operate within similar error limitations. It will be appreciated that this feature is especially important for through-type of single-conductor transformers adapted to handle relatively small current values.

Although I have shown and described certain specific embodiments of my'invention, I am fully aware that many modifications thereof are possible. My invention, there- V fore, is not to be restricted, except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A current transformer having a parallel connected resistance and characterized by magnetizing winding coupled with at least one leakage member, the point of lowest magnetic reluctance of which is within the lower ranges of load on the transformer.

2. In a current transformer, a primary shunting resistance and a primary magnetic leakage member so proportioned as to substantially neutralize the phase-angle error, introduced by the exciting current, throughout the entire load range of the transformer.

3. In a current transformer, a shunting resistance and one or more magnetic leakage members associated with the primary conductor or winding which magnetizes the transformer, to neutralize or control the phaseangle error, introduced by the exciting current, between the primary and secondary currents of the transformer.

4. A current transformer comprising, in combination. a primary-winding and a secondary winding coupled through a common magnetic circuit, a resistance shunting said primary winding and a magnetic-leakage member coupled therewith, said leakage member being so proportioned that its point of lowest magnetic reluctance is within the range of load on the transformer below 10% of normal full value.

5 A current transformer comprising, in com nation. a primary winding and a secondary winding coupled by a magnetic circuit common to both, a resistance arranged to shunt said primary winding and one or more magnetic-leakage members coupled therewith, said leakage members being so proportioned that their points of lowest magnetic reluctance are within the lower ranges of load for thetransformer.

6. A current transformer having a parallel connected resistance and comprising a winding disposed to magnetize a core with which is coupled a secondary winding, and one or more magnetic-leakage members coupled with said winding, the points of lowest magnetic reluctances of said paths being within the range of load on the transformer below 10% of normal full value.

7. A current transformercomprising a secondary winding linked with a magnetic core, a'primary winding for magnetizing said core, a resistance paralleling said primary winding and one or more magneticleakage members linked with said primary winding, said leakage members being so proportioned that their points of lowest magnetic reluctance are within the lower ranges of load on the transformer.

In testimony whereof, I have hereunto subscribed my name this 28 day of November ALBERT CALLSEN.

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