US1955317A - Compensated current transformer - Google Patents

Description

April 17, 1934. E c w z 1,955,317

COIPENSATED CURRENT TRANSFORMER Filed Nov. 12, 1931- WITNESSES:

INVENTOR Edward C. Wenfz.

BY 5' 6'; M ATTORNEY Patented Apr. 17, 1934 UNITED STATES PATENT OFFICE COMPENSATED CURRENT TRANSFORMER.

Application November 12, 1931, Serial No. 574,557

8 Claims.

My invention relates to current transformers and has particular relation to methods for eliminating or neutralizing the phase-angle errors which are inherent in transformers of this type.

5 Current transformers of conventional or known design comprise primary and secondary windings linked with a common magnetic core, and are subject to errors of both phase angle and transformation ratio, which errors result from the effect of the exciting current required to magnetize and overcome the losses in the iron core. In practice, the magnitude of these errors is found to be sufficient to necessitate the use of compensating means in order to obtain the 5 high degrees of accuracy which many instrument-transformer applications require. It is to the provision of an improved form of phaseangle-error compensating means that my invention is directed.

An object of my invention is to provide compensating means for current transformers which is effective in substantially eliminating the phaseangle error between the transformer primary and the secondary. currents.

Another object of my invention is to provide compensating means of the type described which is readily applicable to current transformers of known design and which, when applied, do not materially increase the cost of the complete transformer.

An additional object of my invention is to provide a compensating means for current transformers that is simple in design, and the performance of which maybe predetermined with reasonable ease and accuracy.

A still further object of my invention is to provide means whereby the degree of phase-angle error compensation may be modified in such manner that it will be practically constant over the entire load range of the transformer or may be made to vary with load in some desired predetermined manner- In a preferred form of my invention, the compensating means is a closed-circulated auxiliary winding which links the total transformer flux and thus acts, by introducing a component of exciting current that is represented by a time vector at substantially right angles to that of the secondary current, to correct or compensate for the phase angle error of the transformer. In

the practice of my invention such a compensating winding may, in its simplest form, comprise a single short-circuited turn disposed around both the primary and secondary windings in the case of a wound type transformer, and around the secondary winding carried by the core member in the through-type of transformer.

I have discovered that the most effective compensating results are obtained when this closedcircuited compensating winding links the total 00 transformer flux rather than only the working flux. Such total-flux linkage can best be attained, in a wound type of transformer, when the secondary winding is disposed intermediate the magnetic'core and the primary winding, around 55 which primary winding the compensating winding of my invention is placed.

In order to permit of adjustment in the degree and nature of the compensation throughout a wide load range, I provide, in certain embodiments of my invention, an auxiliary core member in the transformer, which member is linked by the primary winding and which, together with the main core member and the secondary winding, is enclosed by the closed-circuited compensating wind- 7; ing. By changing the magnetic characteristics of this auxiliary core its saturation may be modifled to give a wide range of compensation adjustment, and, furthermore, adjustment in the shape of the phase-angle error with load curve for the go transformer is also permitted thereby.

The organization and operation of my invention, together with additional objects and advantages thereof, will best be understood from the following description of specific embodiments when read in conjunction with the accompanying drawing in which:

Figure 1 is a diagrammatic view illustrating an application of the compensating winding of my invention to a wound-type of current transformer.

Fig. 2 is a view, partly in side elevation and partly in section, of the current transformer shown in Fig. l in which the physical shape of the windings is more completely shown in order that the magnetic flux paths may better be depicted.

Figs. 3, 4, 5 and 6 are vector diagrams illustrating the relative magnitudes and phase positions of the fluxes, voltages, and currents acting in the transformer of Figs. 1 and 2.

Fig. "l is a diagrammatic view illustrating an application of the compensating winding of my invention to a through-type current transformer.

Fig. 8 is a diagrammatic view illustrating a modified form of the invention employing an auxiliary or compensation-control core applied 10 to a wound-type of current transformer and,

Fig. 9 similarly illustrates the application of the auxiliary core and compensating winding of my invention to a through type of current transformer. no

Referring to the drawing, and particularly to Figs. 1 and 2 thereof, reference character C designates the magnetic core of a wound-type current transformer. Around one portion of the core C, which is represented as being of laminated iron construction, is disposed a secondary winding S which in turn is surrounded by a primary winding P. In Fig. 1, a single turn only of reindings P and S is represented while in Fig. 2 the winding representation is such as to more clearly indicate the physical shapes. It will be understood that the primary winding P is adapted for connection in the circuit whose current is to be measured, while the secondary winding S connects with measuring or other instruments to which it supplies 'a current that is proportional to that flowing in the primarywinding circuit.

Because of the magnetic and other losses which are necessarily present in all current transformers, the ratio between the primary and secondary currents will, in the absence of compensating means, be something less than that given by the relative number of turns'in these windings, andthe phase position of the secondary current will be displaced fromthe desired 180 or completely reversed primary current vector relation. 7

In order. to compensate for this phase-angle error, I provide a compensating winding T which links the total transformer fiux and thereby acts, in a manner to be more completely explained, to introduce a component of current the time vector of which is at substantially right angles to that of the secondary winding current. In Figs. 1 and 2 this compensating winding T is illustrated as comprising a single short-circuited tum. It will be apparent, however, that, should it be desired in certain cases, more than one turn may be utilized, or an external impedance may be introduced into the circuit of the winding.

When disposed in the manner shown in Figs. 1 and 2, compensating winding T encloses the total flux of the transformer, which includes the working flux and the leakage flux. In a current transformer, as is known, the total leakage flux is frequently greater than the working flux due to the design proportions which are found to be most practical for current transformers. The vector diagram of Fig. 3 shows typical relative magnitudes of the fluxes acting in a current transformer designed in accordance with present-approved practice.

The diagram of Fig. 3 may be considered as applying to the current transformer of Figs. 1 and 2 when operating at some particular load and supplying a given burden. The load is such that a secondary current Is flows in the secondary circuit when there is induced in the secondary winding a voltage designated by E5, the value of the angle alpha between vectors Es and Is being determined by the reactance characteristics of the burden.

It should-be noted that vectors Es and Is have been drawn in directions reversed from their actual phase positions with respect to, the other quantities represented. This has been done to simplify the vectorial representations and descriptions to follow. The voltage Ea is induced in the secondary winding, S by a working flux F5 which is displaced frdm the voltage by the angle of indicated in Fig. 3.

The primary current sets up a leakage flux F1 which is in phase with the primary current, which produces it and which has some such relative magnitude with respect to the working flux F5 as indicated in Fig. 3. The total flux Fp which the primary winding P must supply is the vector sum of F5 and F1. The compensating turn T encloses this total flux and thus has induced in it a voltage Er which is displaced from Fp by 90 as shown. If the compensating winding T in cludes only resistance in its circuit, a current will flow therein that is also displaced from the 'total flux F by 90.

To produce the flux Fe in the core, in Fig. 4, an exciting current Im is required. This exciting current must be supplied by the primary winding so that to produce the secondary current Is the primary current must have a value, indicated by vector 1 in Fig. 5, which is the vector sum of L and Im.

It should be noted that in the vector diagrams, the ratio of the transformer is assumed to be 1:1 in order to permit greater clarity of representation. 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 vectors are increased in scale to better show the conditions.

The vector diagram of Fig. 5; which is drawn for a current transformer without phase-angle compensating means, indicates that an appreciable phase displacement designated by the angle theta, 0, is normally introduced between the primary and secondary'currents, and that the exciting current similarly introduces a transformation error. This latter type of error may quite readily be compensated for by suitably changing the turn ratio and hence, it in itself presents a much less serious problem than that of phaseangle compensation, to a solution of which latter problem my invention is directed.

The vector diagram of Fig. 6 illustrates the compensating action afforded by the addition of the winding T in the transformer of Figs. 1 and 2. The voltage Er acting in this winding causes a current to flow therein designated by It in Fig. 6, which current, for the preferred resistive type of winding circuit, will be in phase with the voltage and hence will have the phase position shown. This current also must be supplied by the primary winding with the result that the total primary current will thereby be changed from that designated by Ip in Fig. ,5 to that given by 11) in Fig. 6. It will be observed that for the conditions assumed, the primary current has, by action of the compensating means of my invention, been shifted from a position in which it lags the secondary current by angle theta (Fig. 5) to a position in which is slightly leads the secondary current, as is indicated in Fig. 6.

It will be apparent that, for any given set of load and burden conditions, the degree of compensation is readily controllable by merely varying the electrical resistance of compensating winding T. Thus, increasing the resistance of the winding circuit will act to shorten vector It while decreasing the resistance, for the same number of winding turns, will act to lengthen this vector and effect corresponding changes in the phase relation of the transformer primary and secondary currents.

In the matter of design of the compensating winding, the instrument transformer art is now advanced to the stage in which calculation of flux, induced voltage and current in this winding is a relatively simpleqmatter. Hence, the compensating capabilities of the winding may be readily predetermined within reasonable degrees of accuracy and the practicability of this scheme is thereby greatly enhanced over other phase-angle compensating schemes, the operation of which cannot be so readily analyzed. Furthermore, it will be apparent that the cost of construction of such a compensating winding is very small and that the space required by it is correspondingly slight.

As indicated in Fig. '7, my invention may likewise be applied to through-type current transformers as well as to the wound variety already described. In Fig. '7 the primary conductor P is surrounded by the core member C which carries the secondary winding S around which compensating winding Tis placed. It will be apparent that the action of this winding in the transformer of Fig. '7 is very similar to that already described for the transformer of Figs. 1 and. 2, since the winding T is acted upon by the total flux produced by the current in conductor P.

The modification of my invention just described is capable of phase-angle compensation over reasonably wide ranges of transformer operating load. However, because of the magnetic characteristics of the single magnetic core which is commonly utilized by the usual designs of current transformers illustrated in Figs. 1, 2 and '7, the exciting current does not vary directly with the load. Due to this non-uniform relation, the transformer errors which, as has been pointed out, are introduced by this exciting current, depend somewhat in magnitude upon the value of load at which the transformer is being operated. Complete compensation for these errors at one particular value of load therefore cannot be entirely effective throughout the entire load range.

In applications in which more than ordinary accuracy throughout the complete load range is required, I am able to provide such special compensation as may be required by utilizing the modification of my invention depicted in Figs. 8 and 9 in which an auxiliary core member C1 is combined with the elements previously discussed. The purpose of this auxiliary core member is, as has been pointed out, to modify the degree of compensation, and in operation it functions to attain this result by operating at a different degree of saturation than the main core member C with which it is associated.

In this modification, the primary winding P and the compensating winding T are both disposed to link both core sections C and Car, while the secondary winding S links only the main core section C. The result is that changes in the saturation of auxiliary member C1 afl'ect the primary and compensating winding currents without directly influencing the secondary winding current. It will be seen that, in addition to the exciting current for main core member C, the primary winding must supply an additional component of exciting current for auxiliary member Cx.

By so proportioning the magnetic circuits C and Cx that the changes from the straight line variation of exciting current with transformer load for auxiliary core member C; act to neutralize the changesfrom this straight line relation for main core member C, it is evident that in effect the two exciting currents may be made to combine to produce a substantially straight line variation with load for the total exciting current. Such a result may be obtained, as will be evident, by properly proportioning the relative degrees of magnetic saturation of these cores as, for example, by designing core C1 to operate at much higher flux densities than core C.

It will be evident that a current transformer having a straight-line excitation current will maintain substantially constant errors irrespective of the load. Thus, compensation for the phase angle error at one value of load will be permitted to effectively act at other values of load so that the desired results suggested may thereby be achieved. Furthermore, the combination, with the compensating winding, of an auxiliary core member permits of a wide variation in compensastion characteristics and thereby broadens the scope of my invention to applications in which predetermined non-uniform compensation for load may be required. The auxiliary core Cx may, for example, have an air gap introduced into it to modify its magnetic saturation current to give different corrective effects at different values of transformer current.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that mam modifications thereof are possible. -My invention, therefore, 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. In a current transformer including a magnetic core, a source of excitation for said core and a secondary winding disposed on said core, the combination of a compensating winding having a single short-circuited turn disposed to link the total flux produced by said source of excitation for producing a small exciting current in substantially quadrature phase relation to the total flux to compensate for phase angle error.

2. A current transformer comprising a magnetic core, a primary winding disposed to excite said core, a secondary winding disposedin inductive relation with said primary winding and core, and a short-circuited compensating winding disposed to link the total flux produced by said primary winding for producing a small exciting current in substantially quadrature phase relation to the total flux to compensate for phase angle error.

3. Means for compensating for the phaseangle error in a current transformer having a magnetic circuit and primary and secondary windings associated therewith, comprising a closed-circuited winding disposed to link the total flux set up by said primary winding, the impedance characteristics of said closed-circuited winding being fire dominantly resistive.

4. A current transformer having a magnetic core, a primary winding, a secondary winding, and a closed-circuited compensating winding which links the total flux produced by said primary winding, said compensating winding being so disposed that thecurrent which flows therein has a substantially quadrature phase relation with respect to the said total flux.

5. A current transformer having a magnetizable core, a primary winding and a secondary winding disposed on said core, and a closedcircuited winding disposed to link the total flux produced by said primary winding and thus set up a current which acts to compensate for the phase angle error between the primary and secondary currents of the transformer, said current having a phase position which is displaced from that of the said total flux by an angle of substantially 90.

6. A current transformer having a core, a secondary winding disposed about said core, a primary winding disposed about said secondary winding, and a closed-circuited winding disposed about said primary winding in such manner that it links the total flux produced by the primary winding.

7. A current transformer having a magnetic circuit comprising a main core member and an auxiliary core member, a secondary winding disposed about said main core member, a primary winding disposed about said secondary winding and said auxiliary core member, and a closedcircuited winding disposed about said primary winding in such manner that it links the total flux produced by the primary winding, said main and auxiliary core members being disposed to operate at different values of flux densities, the impedance characteristics of said closed-circuited winding being such that the current which flows therein is substantially in phase with the voltage induced in the winding by the said total flux.

8. A current transformer having a main core member and an auxiliary core member, a primary winding disposed to excite both of said members, a secondary winding disposed about said main core member, and a closed-oircuited compensating winding disposed about said secondary winding and said auxiliary core member in such manner that it links the total flux produced by the primary winding, said auxiliary member being proportioned to operate at flux densities within the saturating range for the core material, the impedance characteristics of said compensating winding being such that the current which flows therein is substantially in phase with the voltage induced in the winding by the said total flux.

EDWARD C. WENTZ.

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