GB2050070A

GB2050070A - Current dividers for measuring transformers

Info

Publication number: GB2050070A
Application number: GB8014594A
Authority: GB
Original assignee: Landis and Gyr AG; LGZ Landis and Gyr Zug AG
Current assignee: Siemens Building Technologies AG; Landis and Gyr AG
Priority date: 1979-05-31
Filing date: 1980-05-02
Publication date: 1980-12-31
Also published as: SE436229B; CH643954A5; DE3008308A1; GB2050070B; DE3008308C2; FR2458136A1; ATA279180A; FR2458136B1; NL8003180A; SE8004068L

Abstract

A current divider is used in a measuring transformer for potential- free measurement of currents and comprises a metal strip 1 which is originally flat and from which a web portion 4 and two strip portions to form shunts 6 and 7 are cut. The shunts 6 and 7 are shaped to form at least one loop with limb portions 12, 13 and 14 and the web portion 4 forms, as the measuring conductor, a circular ring 8 and two supply conductors 9. Two current rails 10 and 11 of an electricity meter carry the measuring current to the measuring conductor 4 and the parallel shunts 6 and 7. The current flowing in the measuring conductor 4 is evaluated by the measuring transformer for the purposes of determining the total measuring current. The ratio between the cross-sections of the measuring conductor 4 and the shunts 6 and 7 determines the measuring range, with the measuring transformer remaining the same. Calibration can be effected by removing material from the current divider. <IMAGE>

Description

SPECIFICATION Current dividers for measuring transformers This invention relates to current dividers for measuring transformers.

A measuring transformer for the potentialfree measurement of currents is known (DAS 27 34 729) wherein the current to be measured is divided by means of a current divider into a measuring current and a shunt current, and only the current flowing through the measuring current path is evaluated in the actual measuring transformer.

According to the present invention there is provided a current divider for measuring transformers for the potential-free measurment of currents, the current divider comprising a current conductor which is formed by a bandshaped metal strip, which carries the total current to be measured, and which is divided into a measuring conductor and a shunt, in the direction of current flow upstream of a measuring location, said measuring conductor and said shunt being re-united downstream of the measuring location to reform the original current conductor, and wherein in use the measuring conductor passes through a magnetic circuit forming the measuring system of the measuring transformer, said metal strip comprising a web portion extending in the longitudinal direction of said metal strip, and two strip portions, said web portion and said strip portions being joined again at end re regions at the two narrow ends of said metal strip, to restore the original configuration of said metal strip; wherein said web portion serves as said measuring conductor and said strip portions form said shunt and for this purpose said web portion on the one hand and said strip portions on the other hand are so shaped that they are displaced in different directions; and wherein in addition said strip portions form at least one U-shaped bend with limb portions which lie closely one beside the other.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like references refer to like elements, and in which: Figure 1 shows a plan view of a metal strip before it is shaped to form a current divider; and the following figures show perspective views of three embodiments, in which: Figure 2 shows a first embodiment; Figures 3 and 4 show two different views of a second embodiment; and Figure 5 shows a third embodiment, partly cut-away.

In Fig. 1 is shown a band-shaped metal strip 1 which preferably comprises copper, in its starting condition before a current divider is formed therefrom. It comprises a web portion 4, extending in the longitudinal direction, and two strip portions 6 and 7 which are joined together again to restore the original configuration of the metal strip 1, at end regions 2 and 3 of the two narrow ends of the metal strip 1. In the present example, the web portion 4 is formed by punching or stamping out two slots 5, and strip portions 6 and 7.

However, the web portion 4 could also be cut out directly by shearing away metal in the bending operation described hereinafter, that is to say, without first punching or stamping out the slots 5. The web portion 4 serves as a measuring conductor, that is to say, the current component which flows through the web portion 4 is detected by the magnetic circuit of a measuring transformer (not shown), while the strip portions 6 and 7 serve as a shunt means. In the bending operation, the metal strip 1 is shaped in such a way that the web portion 4 on the one hand and the two strip portions 6 and 7 on the other hand are displaced forwardly in different directions, although the longitudinal axes of the strip portions 6 and 7 and the longitudinal axis of the web portion 4 always remain in planes which are normal to the plane of the original metal strip 1.

The strip portions 6 and 7 are bent into a U-shape with limb portions which lie one beside the other and very close to each other, whereby the leakage fields produced by the shunt current virtually cancel each other out.

In Fig. 2, the plane of symmetry which extends between the two limb portions of the bent strip portions 6 and 7 is also the plane of symmetry of the measuring conductor which is formed from the web portion 4. The latter forms a circular ring 8 with two parallel feed conductors 9 which extend closely one beside the other. The circular ring 8 itself forms the primary winding of the measuring transformer (not shown in the drawings) which magnetically evaluates the current flowing through the circular ring 8.

The total current supplied to the illustrated arrangement is supplied by way of two current connecting rails 10 and 11 which are shown in dash-dotted lines in Figs. 2 and 4 and which are conductingly connected by known methods, for example by hard-soldering or brazing, to the end regions 2 and 3 which are still disposed in one plane when the metal strip 1 is in the bent condition.

By virtue of the common plane of symmetry, shaping the current divider in the arrangement shown in Fig. 2 requires a relatively low tooling cost, but on the other hand and for the same reason it requires a substantial installation depth.

The embodiment shown in Figs. 3 and 4 is better from the point of view of space. In this embodiment, the strip portions 6 and 7 are twice bent into a U-shaped configuration and form an approximately closed loop, wherein both U-shapes each have a common limb portion 1 2 while the second limb portions 1 3 and 14 thereof are each passed to the end regions 2 and 3 of the metal strip 1 by virtue of a respective right-angles bent portion 1 5 and 16. The plane of symmetry which extends between the limb portions 12, and 1 3 and 14 is normal to the plane of symmetry of the measuring conductor which is of the same form as in Fig. 2. The end regions 2 and 3 are also soldered to current connecting rails 10 and 11 respectively.

The embodiment shown in Fig. 5 again represents an improvement insofar as the metal strip 1, besides being subdivided into the web portion 4 and the strip portions 6 and 7, also has at the end regions 2 and 3 a respective additional bent portion 17, the directions of bending of the bent portions 17 being in the same direction of rotation as in the case of the first bent portions 1 5 and 1 6 respectively. The outermost parts of the end regions 2 and 3 form a plane parallel to the plane of symmetry which extends between the limbs 12, and 1 3 and 14 respectively. By virtue of this arrangement, the end regions 2 and 3 can be soldered to the ends of the current connecting rails 10 and 11 which lie directly one beside the other.The distance thereof is then only governed by the electrical insulation required, which has a favourable effect with regard to compensation of the leakage field which is produced by the current in the current connecting rails 10 and 11.

The above-described current dividers are advantageously used in measuring transformers for static electricity meters. In that case, they can be used in addition both for establishing the measuring range and also for calibrating the electricity meter. Indeed, with an electronic measuring device which is the same in each case, it is possible to achieve different measuring ranges in respect of the electricity meter, with current dividers of different sizes.

The measuring ranges are defined by suitable selection of the ratio between the cross-sectional areas of the web portion 4 with respect to the strip portions 6 and 7, the cross-section of the web portion 4 only ever being a fraction of the cross-section of the two strip portions 6 and 7. Advantageously, the crosssectional ratio associated with each current divider type is initially shifted slightly in favour of the cross-section of the strips 6 and 7, in other words, the cross-section thereof is somewhat over-size. In order to compensate for inevitable inaccuracies, it is then possible to provide for individual calibration of each individual meter by removing a small amount of material from the strip portions 6 and 7.

This permits adjustment of the meter in a manner which is advantageous from the cost point of view.

The above-described arrangement of a current divider makes it possible for the circular ring 8 which forms the primary winding of the measuring transformer to be easily arranged at a sufficiently larger distance from the two strip portions 6 and 7 which serve as shunt means. There is thus sufficient space for the measuring transformer and the screening thereof from extraneous fields, and the influence of the leakage field of the part of the current flowing through the strip portions 6 and 7 is slight.

Instead of connecting the end regions 2 and 3 of the current divider to the current connecting rails 10 and 11, it would also be possible for the end regions 2 and 3 to be formed directly into connection terminals.

Claims

1. A current divider for measuring transformers for the potential-free measurement of currents, the current divider comprising a current conductor which is formed by a bandshaped metal strip, which carries the total current to be measured, and which is divided into a measuring conductor and a shunt, in the direction of current flow upstream of a measuring location, said measuring conductor and said shunt being re-united downstream of the measuring location to reform the original current conductor, and wherein in use the measuring conductor passes through a magnetic circuit forming the measuring system of the measuring transformer, said metal strip comprising a web portion extending in the longitudinal direction of said metal strip, and two strip portions, said web portion and said strip portions being joined again at end regions at the two narrow ends of said metal strip, to restore the original configuration of said metal strip; wherein said web portion serves as said measuring conductor and said strip portions form said shunt and for this purpose said web portion on the one hand and said strip portions on the other hand are so shaped that they are displaced in different directions; and wherein in addition said strip portions form at least one U-shaped bend with limb portions which lie closely one beside the other.

2. A current divider according to claim 1 wherein said web portion is arranged symmetrically with respect to said strip portions, with regard to said metal strip prior to bending.

3. A current divider according to claim 2 wherein said plane of symmetry which extends between the two limb portions of said bent strip portions is also the plane of symmetry of said measuring conductor which is formed from said web portion.

4. A current divider according to claim 1 wherein said strip portions are bent twice in a U-shaped configuration and form an approximately closed loop the U-shapes of which each have a respective common limb portion while their second limb portions are passed to said end regions of said metal strip by a respective right-angled bent portion, and wherein the plane of symmetry which passes between the said limb portions is normal to the plane of symmetry of the measuring conductor formed from said web portion.

5. A current divider according to claim 4 wherein said metal strip has at said end regions, besides being subdivided into said web portion and said strip portions, a respective additional bent portion, the directions of bending of said bent portions being in the same direction of rotation as in he case of the first bent portions, and wherein the outermost parts of said end regions form a plane parallel to the plane of symmetry which extends between said limb portions.

6. A current divider according to any one of the preceding claims for use in measuring transformers for static electricity meters the measuring ranges of which are variable by suitable selection of the ratio between the cross-sectional areas of said web portion and said strip portions, with the respective electronic measuring apparatus remaining the same.

7. A current divider according to claim 6 wherein said current divider itself serves for individual calibration of each individual electricity meter by the removal of material from said strip portions.

8. A current substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.

9. a current divider substantially as hereinbefore described with reference to Figs. 3 and 4 of the accompanying drawings.

10. A current divider substantially as herein before described with reference to Fig. 5 of the accompanying drawings.