US1861076A - Current transformer - Google Patents

Description

May 31, 1932. A. o. AUSTIN CURRENT TRANSFORMER Filed Nov. 11, 1929 2 Sheets-Sheet m 1 5 N T0 1e big/0M7.

Ar BY ATTORNEY May 31, 1932. Q ug-rm 1,861,076

CURRENT TRANSFORMER Filed Nov. 11, 1929 2 Sheets-Sheet 2 R Ya INVENTOR ATTi k zb Patented May 31, 1932 UNITED STATES PATENT OFFICE ARTHUR 0. AUSTIN, OF NEAR BARBERTON, OHIO, ASSIGNOR, BY MESNE ASSIGNMENTS,

TO THE OHIO BRASS COMPANY, OF MANSFIELD, OHIO, A CORPORATION OF NEW JERSEY CURRENT TRANSFORMER Application filed November 11, 1929. Serial No. 406,284.

This invention relates to current transformers and has for one of its objects the provision of 'a current transformer which shall have a substantially constant transfor- "i mat-ion ratio, and in which the currents in the primary and secondary shall be substantially in step with each other throughout the range of operation of the transformer.

Other objects and advantages will appear from the following description.

The invention is exemplified in the combination and arrangement of parts shown in the accompanying drawings and described in the following specification, and it is more particularly pointed out in the appended claims.

In the drawings:

Fig. 1 is a diagrammatic view showing one embodiment of the present invention.

Fig.2 is a fragmentary wiring diagram showing a slightly modified arrangement of a portion of the circuit shown in Fig. 1.

Figs. 8, 4 and 5 are views similar to Fig. 1 showing different arrangements of the invention.

Fig. 6 is a wiring diagram illustrating one feature of the invention.

Fi 7 is a vector diagram illustrating the relation of the currents in the primary and secondary circuits of the current transformer.

Fig. 8 is a vector diagram illustrating the effect of a correction factor.

Fig. 9 is a wiring diagram showing a slightly different arrangement of the corrective circuit.

In current transformers, it is desirable to have the current in the secondary maintain a definite ratio to the current in the primary winding, and it is further desirable that the current in the secondary be in phase with the current in the primary. The difficulty of maintaining a definite or constant ratio in the same phase relation over a wide range of current in the primary is well recognized in current transformer practice. Where current transformers are applied to very high voltage conductors, the difficulty is even greater than where the primary has a moderate voltage only. In order to improve the characteristics of the current transformer, I have applied an improved method for correctlng either phase angle displacement, or ratio, or both. The method is applicable, not only to high voltage current transformers, but to lower voltage current transformers where an improved accuracy is desired over a very considerable range. While the method is applicable to the'lower voltage current transformers, the more difiicult application of current transformers to very high voltage conductors will be discussed.

1n the application of the invention shown in Fig. 1, the primary conductor 10 passes through a bushing housing composed of members 11 and 12, equipped with a mounting flange 13. The primary conductor entering the bushing at 14 and leaving at 15 may consist of a single conductor or may have several turns passing around a magnetic core, as shown in my prior Patent No. 1,723,000. The bushing is equipped with a secondary winding 17. If the conductor 10 passes directly through the bushing, the usual mag netic core about which the secondary is wound is used, as shown in my prior Patent No. 1,699,342, or a magnetic core is placed outside of the bushing and the secondary wound about this in the usual manner.

It is understood that this improvement applies to any one of these various arrangements as well as to low voltage current transformers where the primary consists of a single turn or a number of turns. It is well known that where the current in the primary varies over a considerable range, the transforma tion ratio will be different for very small currents from the ratio for larger currents. For very small currents, the ratio between primary and secondary currents will be high er than for full load current upon the primary. For small loads, the current in the secondary will be less than it should be. It is possible to correct this error by reducing the impedance in the secondary circuit for the lower currents in the primary or by introducing impedance in the secondary circuit for the higher values for the current in the primary. Another method is to change the ratio of turns between primary and secondary, and still another method is to apply a corrective potential or current.

In Fig. 1, impedance is introduced into the secondar circuit as the current is increased, so that the ratio may be maintained from a low value to a high value. The secondary 17 of the current transformer is equipped with leads 18 and 19. The full current or a shunted current, asdesired, is caused to pass around the solenoids of suitable relays 20 and 21.

The shunting impedances 22 and 23 may be omitted. These impedances, however, can be used to control the amount of current and oint of operation of the relays 20 and 21. If necessary, capacitance 24 may be applied to correct the displacement caused by the inductive reactance of the relay. For small currents, the shunting switches 25 and 26 permit the current in the secondary leads 18 and 19 to flow directly to the ammeter, relay or other load 27 As the current increases in the primary, the ratio of the current in the primary to that in the secondary will decrease, so that the current flowing to the ammeter or load 27 will be relatively larger than it should be compared to that when the current was small in the primary. If a con- .stant ratio is to be maintainedflt will, therefore, be necessary to reduce-the current in the secondary. This can readily be done by the insertion of a resistance or a suitable impedance in the secondary circuit. As the in creased current is caused to flow around the solenoid 20, it opens the switch 25, inserting the resistance or impedance 28. This reduces the current in the secondary so as to restore the ratio established at the low current value.

It is evident that to obtain a constant ratio, a continuous correction would have to be made, depending upon the current. In practice, however, even a singlecorrective step will frequently be of material value. In Fig. 1 two corrective steps are shown which may, of course, be changed for any given condition, depending upon the results desired.

Where the range of current is such that the correction provided by the insertion of the impedance or resistance 28 is not suiiicient, a further correction may be provided by inserting another resistance 29 through the opening of the switch 26 by the relay 21. It is evident that any number of steps may be used, and the number of steps may be controlled by asingle relay of suitable design, if desired. It is evident that the above will apply equally well if the current transformer is interposed in the leads 18 and 19 connected directly to the load 27 as shown in Fig. 1, or through a transformer 30 as shown in Fig. 2.

In Fig. 3, in place of inserting an impedance III the secondary circuit, as in Fig. 1, the number of turns in the secondary of the current transformer is changed to correct the ratio. The secondary winding is equipped with leads 31, 32, 33 and 34. The secondary winding is shown running to another current transformer 5, but it-may go directly to the instrument 36 if desired.

Where it is desired to equip the bushing with both a capacitance coupling and a current transformer, the corrective means make it possible to maintain a much more accurate ratio than would otherwise be possible, so that the current transformer 35, which would normally be required as an insulating trans former, would have little effect upon the final result. For the low value of current, it is necessary to reduce the number of turns in the secondary in order to increase the current, so that the ratio will be approximately the same for the higher current values. For the low values, the lead 32 taps the secondary at the point 37. \Vhen the current reaches a predetermined value in the relay 38, the switch 39 is caused to operate so that the number of turns in the secondary is increased. This switch may be equipped with double contacts or any other suitable means whereby the current in the relay 38 is not interrupted long enough to cause the switch to tend to drop back into its original position. This is readily taken care of by a switch having double contacts so that contact with the lead 40 is made before contact with 32 is broken. While this arrangement will tend to shortcircuit momentarily a small portion of the secondary, the duration will, in general, be too small to make any material difference. If desired, an impedance or reactance may be placed between the two contacts of the switch, as in the case of ordinary tap changers to cut down the short-circuit current. It is evident that a further correction may be made by switching from the tap 33'to 34 through the action of the relay or solenoid 41. The action in this case is similar to the first-mentioned correction. It is evident that any number of taps may be used, depending upon results desired.

In general, however, one or two corrective steps will greatly improve the characteristics of the current transformer. This is particularly important where it is desired to meter current at high voltage. lVhcre this metering takes place through interconnected transmission systems, where the flow of current may cover a wide range or where the flow of power may change at different times, accurate metering is difficult except by some corrective means. Under conditions of this kind, the flow of power may be small at times, hence the ratio may be important. It is evident that this method may be applied in connection with my previous Patent No. 1,723,000, where the number of turns in the primary are changed.

In the foregoing discussion, one means of correcting the ratio by changing the number of turns between the primary and secondary of the current transformer has been shown. While this method is very effective in maintaining fairly uniform ratio over a wide range of current in the primary, the method does not correct the phase angle between the current in the primary and secondary. Where it is desired to correct both the phase angle and ratio, it is possible to correct the phase angle by obtaining a source of energy from one of the other phases. In general, a

phase relation may be chosen such that the component. will be nearly at right angles to the current. In this case, the phase angle correction will have little effect upon the ratio. The phase angle error is greatest at small currents and comparatively, slight at the higher values of current. Therefore, a correction made for the small value of currents will have but little effect upon the large values. Hence, a single step of correction is usually all that is necessary. If desired, more steps may be used by equipping the solenoids or relays with additional contacts for making these corrections.

Fig. 6 shows the general method of applying the corrections for ratio and phase angle. In Fig. 7 OP represents the current in the primary and OS represents the current in the secondary times the normal ratio. The angle POS represents the phase displacement. The difference SC in the length of the vectors OS and OP is a correction factor which must be applied to maintain a constant ratio. As previously explained, this correction may be obtained by changing the number of turns in the secondary so that the ratio times the increase in the current will equal the difference SC. Cutting out impedance from the secondary circuit will also provide the necessary correction factor. The phase angle of the current OS without correction, and OC, will change but little with the corrections. It is desirable, however, for metering and other purposes, that the angle POS or POC should be zero, that is, the current in the secondary and primary should be in step. In order to bring the current in the primary and secondary into step, a correction factor approximately at 90 degrees with the secondary current may be introduced to change the phase angle without materially changing the current value. In simple diagrammatic form, this is illustrated in Figs. 6 and 7 of which the current in phase A, Fig. 6, is represented by OP, Fig. 7, in which figure OS represents the current in the secondary, times the ratio, where no correction for ratio is provided. OC represents a corrected value,

obtained b one of the previously described schemes. rom Fig. 7 it is evident that the correction factor OP should be approximately at right angles to either OP or 00.

It. is possible to obtain a definite component for a correction factor from a potential transformer across the phases B and C which will be approximately at right angles to the current in phase A. The winding for this factor is shown at CP in Fig. 6 and at 42 in Fig. 5. While a shift in the current at phase A will produce an error, this is relatively small 7 and may usually be neglected. The potential transformer having a secondary OP can be applied so that its corrective component will be equal .to the value represented by vector CP in magnitude. It is evident that this component may be made either leading or lagging with respect to the primary current by simply reversing the terminals of the correction potential transformer. Any suitable method may be used for applying the corrective current for the phase angle. This component may be placed across a resistance in series with the secondary current or may be applied as a tap or as a separate winding to a current transformer in the secondary circuit or to the measuring instruments.

Fig. 5 shows a diagrammatic arrangement where the correction is applied by a winding 43 placed in a relation to modify the current in the secondary 37 of the main current transformer. The component thus introduced may be adjusted by adjusting the resistance or impedance 44.

Another manner of applying the correction is to cause the charging current flowing between primary 10 and secondary 37 of the current transformer or the current supplied by a capacitance coupling 45 in a bushing having a current transformer, to flow over a corrective winding 46 in the external transformer or to be ap[plied across an impedance or re sistance. n general, the current in the capacitance coupling will lead the current in the primary circuit by nearly 90 degrees. If a step-down transformer is placed in the circuit, as shown in my prior Patent No. 1,709,826, the phaseange difference will still be nearly 90 degrees. Since the current obtainable from the secondary of this capacitance coupling transformer will probably not vary more than 15 degrees from being 90 degrees out of phase with the current to be corrected. it is evident that although this current is leading, reversing the leads will change the component from leading to lagging.

If OP, Fig. 8. represented the current in the primary, then OD would represent the current in the secondary of a step-down transformer of a capacitance tap applied to the same phase. It is evident that by reversing the terminals ofthe step-down transformer it would rotate the component 180 degrees, or the resulting factor would be OE. \Vhere the angle POD is less than 90 degrees, the correction would have an appreciable component. in the direction of the primary current OP. Where the correction factor is applied so as to correct the phase angle for small currents. the phase angle correction may introduce a ratio component OF. Where the angle POD int or EOF is 90 degrees, this correction will be zero. If the angle becomes less than 90 degrees, the component. OF will increase. This is readily taken care of either by over-com pensating for the ratio or by the ratio correc tion means previously described.

A capacitance coupling for correcting the phase angle may be obtained from one of the other phases, if desired, through any combination of capacitance taps and step-down transformers. It is evident that this corrective scheme applies not only to high voltage current transformers but to low voltage current transformers as well where a high degree of accuracy is desired over a large range of current in the primary. The scheme includes applying corrective components for both ratio and phase angle. In general, the ratio correction is obtained through a change in impedance in the secondary circuit or in the number of turns in the current transformer attached to the particular phase. The base angle correction, however, may be obtained from a capacitance tap or coupling attached to the same phase, as shown in Fig. 4, or from a potential transformer giving the proper phase relation, as shown in Fig. 5. A potential transformer whosehigh voltage side is connected to the other ,two phases of ath rec-phase system will give a component approximately 90 degrees to the current in the transformer whose phase angle it is desired to correct. A correcting factor obtained in this manner will tend to change the ratio but little. Fortunately, the phase angle correction required is relatively large for small currents in the primary and very small for heavy currents. A correction factor therefore applied for light currents will have relatively little effect where the currents are large and the phase angle correction is small. If desired, the phase angle may be corrected by steps controlled through the same relays or solenoids which are used to control the ratio ing transformer where a capacitance cou pling of of a bushing is used, or the component may be applied by means of a second magnetic coupling or transformer placed in series or multiple, s desired.

Vhile all corrective factors as to phase angle have been shown as obtainable by means of capacitance couplings or potential transformers, it is evident that components may be obtained from current transformers applied to other phases under some conditions. A construction, however, has been shown which will make it possible to correct not only phase angle but ratio if desired.

ln'the arrangement shown in Fig. 9, the

corrective circuit 50 is energized from transformer windings 51 and inductively related to the current in the phases of the polyphase circuit other than the one which energizes the secondary 53 of the current transformer. provided with a plurality of taps so that the points of connection between the windings and the corrective circuit may be adjusted thus making it possible to vary both the phase angle and the amount of current flowing in the corrective circuit.

\Vith this arrangement, it is possible to secure practically any phase angle and current value required in the corrective circuit for overcoming variations from the constant transformation ratio, and from synchronism with the current in the primary of the current transformer.

I claim:

1. The combination with a current transformer, of adjustable means for correcting variations in the ratio between the current in the primary and secondary of said transformer and means dependent upon the current in the primary of said transformer for regulating said adjustable means.

2. The combination with a current transformer, of means for introducing a correction component into the secondary of said transformer out of step with the current induced in said secondary by the primary'of said current transformer for correcting phase displacement of the current in the secondary of said currenttransformer and means other than the primary circuitof said transformer for supplying the energy for said correction component.

3. The combination with a current transformer, of means energized from a source other than the primary of said transformer for introducing a component factor in the secondary of said transformer at approximately 90 degrees to the component induced in said secondary by the primary of said current transformer for correcting phase displacement of the current in said secondary.

1. The combination with a current transformer, of means for introducing a component factor in the secondary circuit of said transformer out of step with the component induced in said secondary circuit by the primary of said current transformer for correcting phase displacement of the current in the secondary of said current transformer, and means for modifying the amount of current flowing in said secondary to compen- The windings 51 and 52 are sate for variation in the transformation ratio of said current transformer.

5. The combination with a current transformer, of means for introducing a component factor in the secondary circuit of said transformer out of step with the component induced in said secondary circuit by the primary of said current transformer for correcting phase displacement of the current in the secondary of said current transformer, and means for modifying the amount of current flowing in said secondary to compensate for variation in the transformation ratio of said current transformer due to said phase displacement compensation factor.

6. The combination with a current transformer, of means for modifying the current induced in the secondary of said current transformer by the primary of said transformer to compensate for variations in the ratio between the currents in said primary and said secondary, and means for modifying the phase relation between the currents in said primary and said secondary to bring said currents substantially into step with each other.

7. The combination with a current transformer, of means dependent upon the current flowing in said transformer for reducing the ratio between the current in the secondary to the,current in the primary for higher current values flowing in said transformer to compensate for variations in the transformation ratio due to increase in the current flowing in said transformer.

8. The combination with a current transformer, of means dependent upon an increase in the current flowing in said transformer for restricting flow of current in the secondary of said transformer to compensate for variations in the transformation ratio due to increase in current in said transformer.

9. The combination with a current transformer, of means for maintaining the transformation ratio of said transformer approximately constant, said means comprising a relay switch actuated by current flowing in one of the circuitsin said transformer, and means controlled by said switch for decreasing the current flowing in the secondary of said transformer when the current flowing in said relay switch reaches a predetermined value.

10. The combination with a current transformer, of means for maintaining the transformation ratio of said transformer approximately constant, said means comprising a current actuated device, and means controlled by said current actuated device for restricting the amount of current flowing in the secondary of said transformer as the currents in the circuits of said current transformer increase.

11.. The combination with a current transformer, of means dependent upon the current flowing in one of the circuits in said transformer for changing the ratio between the primary and secondary windings of said transformer as the current in said circuit increases to compensate for changes in the transformation ratio of said transformer due to increase in current.

12. The combination with a current transformer, of means dependent upon the current flowing in one of the circuits in said transformer for increasing the impedance in the secondary circuit of said transformer as the current in said transformer increases.

13. The combination with a current transformer having a capacitance tap, of means energized by the current flowing in said capacitance tap for influencing the phase of the current in the secondary of said transformer toward synchronism with the current in the primary of said transformer.

14. The combination with a polyphase cir cuit, of a current transformer energized by one phase of said circuit, and means energized by a different phase of said circuit for influencing the phase of the current in the secondary of said current transformer toward synchronism with the current in the primary of said current transformer.

15. The combination with a current transformer, of a corrective circuit energized from a source other than the primary of said transformer but in a definite phase relation but out of step with the energy of the secondary of said current transformer, said corrective circuit and sald secondary being operatively related to permit the energy of said corrective circuit to influence the current in said secondary toward synchronism with the current in the primary of said current transformer.

16. The combination with a current transformer, of a corrective circuit having current induced therein from a sourceof energy other than the primary of said transformer at approximately 90 degrees displacement from the current in the secondary of said current transformer, said corrective circuit and secondary being operatively related to permit the current in said corrective circuit to influence the current in said secondary toward synchronism with the current in the primary of said current transformer.

17. The combination with a current transformer energized by one phase of a threephase system and a circuit bridged across the other phases of said system, and operatively related with the secondary of said current transformer for correcting the phase relation of the current in said secondary to the current in the primary of said current transformer.

18. The combination with a current transformer, of means controlled by the current in one of the circuits of said transformer for introducing impedance into the secondary of said current transformer when the current flowing in said circuit is increased to a predetermined amount.

19. The combination with a current transformer, of a relay connected directly in one of the circuits of said transformer for relatively decreasing step by step the current in the secondary of said transformer as the load on said transformer increases.

20. The combination with a current transformer, of means controlled by the current flowing in one of the circuits in said transformer for increasing step by step the inipedance in the secondary circuit of said transformer as the load on said transformer increases.

In testimony whereof I have signed my name to this specification this 9th day of November, A. D. 1929.

' ARTHUR O. AUSTIN.

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