IE863301L - Current transformer - Google Patents

Description

£.9537 2 The invention relates to a current transformer arrangement, and in particular to such an arrangement wnich is suitable for static electricity meters., The measurement of high currents for ascertaining energy '•» consumption by means of static electricity meters requires the use of ->. current transformers, the output signals of. which must be suitable for further processing in electronic measuring assemblies. The currents to be measured are of values of more than 100 anps, which must be ascertained in the milliamp measuring range, with low levels of deviation from linearity.

Such arrangements must be substantially insensitive to dc components in the 10 current to be measured. In addition, the level of consumption of auxiliary energy required for operating the arrangement is to be as low as possible.

In addition, the requirements set forth in I EC publication 521, in particular galvanic separation with a high level of insulation strength, ability to withstand short-circuiting, insensitivity in relation to 15 external interference magnetic fields and restricting the influence of frequency, must be met.

The arrangement disclosed in DE-B-1 079 192, which is in the form of a magnetic voltmeter canprises tvro series-connected secondary coils which embrace a current bar. The secondary winding portions are short-20 circuited at their ends with magnetic material. That provides a closed magnetic circuit (Rogowski coil) which behaves astatically in relation to external fields insofar as the turns density of the winding portions is sufficiently high and the distribution of turns is uniform. With high levels of current density in the primary conductor, the secondary coils 25 must be at a certain spacing* therefrom in order to permit satisfactory integration of the partial voltages from the irregularly distributed winding.

The known current transformer arrangement is provided with an electronic integration stage, wherein the frequency characteristic of the 30 integration stage, which is inversely proportional to the output signal, in relation to its input signal, compensates for the proportional frequency dependency of the voltage induced in the secondary winding by the current to be measured, while it turns the input signal through a phase angle of 90° with respect to the current to be measured, to provide for a phase-35 opposition position in respect of its output signal. The measurement signal at the output of the integration stage is independent of the measurement frequency, by virtue of the effect of that stage, and is in phase opposition to the current to be measured, with direct proportionality between the amplitudes. 40 A disadvantage in the known current transformer arrangement is that the requirement for substantial insensitivity in relation to external interference magnetic fields, due to the use of magnetic material, is not entirely met. In addition the known arrangement delivers very low output 3 signals as there is only a slight coupling effect between the fields of the primary conductor and the secondary coils. That means that that process is not suitable for measuring current strengths below about 1 kiloanp.

WD 83/01535 discloses an active current sensor with primary 5 reducing winding, in which error corrpensation is effected by means of an indicator winding in such a way that a current is produced by means of an amplifier in the secondary winding, which eliminates induction in the magnetic core. A serious disadvantage of that arrangement lies in the use of magnetic core material for producing a sufficient magnetic coupling 10 effect as the magnetic material results in considerable errors in relation to dc components in the current to be measured. A reduction in magnetomotive force on the primary side is achieved by means of a primary winding which is wound in opposite directions from two conductors, in order to prevent saturation of the core by direct-current magnetomotive force.

The invention is based on the problem of so developing a current transformer arrangement of the kind set forth in the opening part of this specification that the influence of external interference iragnetic fields is further reduced and it is possible to have a high output signal on the secondary side, with the arrangement being of a compact construction in 20 three-dimensional terms as well as at the same time using inexpensive components.

Accordingly, the invention provides a. current transformer arrangement, in particular for a static electricity meter, comprising a primary conductor carrying the 25 alternating current "to be measured, and a secondary winding comprising at least two series-connected electrically identical coils of an astatic configuration, the output voltage of which is applied to an electronic integration stage on the output side 30 thereof, to produce a measurement signal which is independent of frequency, wherein to produce a maximum magnetic field strength the primary conductor is formed into at least one current loop, the turns of at least one of the secondary coils, for maximum magnetic 35 coupling, are arranged at the smallest possible spacing relative to the corresponding current loop and engage as completely as possible the magnetic flux produced by the current loop, the magnetic coupling is effected without magnetic materials, the secondary coils extend 4 in their axial direction over the smallest possible part of the length of the magnetic field lines produced by the current in the primary conductor, and the secondary coils are arranged in as closely adjacent ^ juxtaposed relationship as possible for optimum external field compensation, in that arrangement, the secondary coil carrier has a permeability which is substantially independent of the magnetic field of the primary conductor. The secondary winding comprises two series-connected secondary coils, the axes of which 10 extend parallel to each other. The direction of winding of the secondary coils corresponds to that of a solenoid which is bent over through 180° at its middle. That astatic arrangement of the secondary coils results in a secondary winding which is independent in relation to external homogenous interference ac magnetic fields as the partial voltages induced by the 15 interference fields in the two secondary coils cancel each other out.

While, in the case of the known secondary coils consisting of winding portions, the winding portions are always combined together to form a closed integration path corresponding to a Rogowski secondary coil, in the case of the present invention, in order to achieve snail dimensions, the 20 secondary coils each extend only over a part of the length of less than 50% of the magnetic field lines produced by the current in the primary conductor so that no closed integration path is formed. In that arrangement the second secondary coil serves primarily to compensate for the influence of external fields. With regard to optimum ccrpensation, the 25 two secondary coils are of snail dimensions in three-dimensional terms and are arranged in as closely juxtaposed relationship as possible.

The secondary coils may be in the form of cylindrical or flat coils with their axes parallel to each other, wherein at least one of the two secondary coils is disposed at a location at which the primary current produces a field strength which is as high as possible. The high field strength required for a high output signal on the secondary side of the arrangement is achieved by the primary conductor being put into the 5 configuration of a current loop. In that way the secondary coil portions pick up the magnetic field of the prirrary conductor only in a locally point-wise form, with the sun of the voltages induced in the two secondary coils being proportional to the primary current to be detected.

A particular feature of the novel current transformer arrangement 10 is the high level of magnetic coupling thereof between the primary conductor and the secondary coil, thus giving high output signals on the secondary side, which makes it possible to use the arrangement for the linear detection of currents with current strengths down to a few mi 11 i amps. That is achieved without using nagnetic rraterial. That 15 configuration provides a compact construction in three-dimensional terms, which permits inexpensive rranufacture.

In a preferred embodiment the primary conductor which is in the form of a secondary loop completely embraces one secondary coil in the peripheral direction thereof. As in that case the secondary coil is 20 arranged within a primary conductor which is in the form of an eye, that provides for optimum-magnetic coupling with correspondingly high output signals.

In an advantageous entxxiiment two current loops are connected in series, with each current loop embracing a secondary coil. It is however 25 a3 so possible for two current loops to be connected in mitually parallel relationship and for each current loop to embrace a secondary coil. In that case the primary current to be measured is divided to two turns so that, in the case of a primary conductor of rectangular cross-section, which is preferably stamped out of copper, folding can be avoided where the 30 conductor portions cross over each other.

A highly desirable embodiment is characterised by the features of claim 5. In that arrangement the primary conductor is folded about a transverse axis through an angle of 180° so that the go-and-return conductors are disposed at a snail spacing one above the other. That 35 spacing nay be of such a configuration at least in portions thereof that the space formed thereby is suitable for disposing the secondary winding therein. In that construction the influence of the interference magnetic field on the measurement result is practically eliminated. Fully autaratic production is made possible in a simple manner by virtue of the shape and 40 the snail dimensions of the arrangement.

It is advantageous if the openings each extend in mutually opposite directions approximately from the centre line to the edge of the flat conductor. By virtue of that configuration, the electrical current in the longitudinal direction of the primary flat conductor is deflected 6 towards the middle of the primary conductor so that the current paths are formed into a loop.

A further embodiment provides that the mutually oppositely disposed conductor portions of the flat conductor each have two nutually 5 oppositely directed openings which are arranged in parallel displaced relationship, thereby forming two portions of the current loop, which are arranged side-by-side in the longitudinal direction of the flat conductor. In that arrangement the secondary coils are desirably disposed between the flat conductor portions. As the fact that the flat conductor is shaped as 10 two juxtaposed portions of the current loop, the axes of which are respectively formed by the mutually facing ends of the openings, each secondary coil may be associated with a respective primary winding, thus providing optimum flux interlinking.

Another advantageous embodiment provides that the secondary coils, 15 being produced by the planar technique, are disposed on a substrate in one or more layers and possibly also on both sides, in the form of spirals. That plate-like substrate rtay be enclosed between the spaced-apart conductor portions. The substrate with both secondary coils may also be arranged outside the space between the conductor portions over the 20 effective winding surfaces of the primary flat conductor.

It is also possible for the substrate to include further electronic components of the electricity meter. They may be for example the electronic components of the integration stage and the nultiplication stage.

A further embodiment is provided in that the one conductor portion has two mutually oppositely directed openings which extend to the edge of the primary conductor on a cum on axis and in parallel relationship with which is arranged an opening, which does not extend as far as the edges, on the other conductor portion. With that arrangement of the openings, the 30 current paths are such that two turns are formed, which are connected in parallel and which are each linked to the magnetic flux of a respective secondary coil.

The invention is described in greater detail hereinafter by means of embodiments illustrated in the drawing. The known integration circuit 35 is not shown therein. In the drawing: Figure 1 is an end view of two secondary coils which are of an astatic configuration and of which one is enclosed by a primary conductor.

Figure 2 is an end view of two coils which are of an astatic configuration and which are enclosed by series-connected turns of the 40 primary conductor, Figure 3 shows an arrangement of the secondary coils as illustrated in Figure 2 but with parallel-connected turns of the primary conductor.

Figure 4 is a perspective view of a primary conductor in the .form of a flat conductor, wherein. one of the secondary coils which are of an astatic configuration is arranged between oppositely disposed portions of the flat conductor and the other secondary coil is arranged outside the flat conductor, Figure 5 is a perspective view of a primary conductor in an efrbodirrent which is modified in relation to Figure 4, Figure 6 is a perspective view of the coils of an astatic configuration of the secondary winding with a baseplate which can be introduced into the prinery conductor as shown in Figure 5, Figure 7 is a cross-sectional view of the arrangement shown in Figures 5 and 6 on a reduced scale, in the operative condition.

Figure 8 is a perspective view of an embodiment of the primary conductor, which is modified in comparison with Figures 4 and 5, Figure 9 is a perspective view of flat coils of an astatic configuration as a secondary winding on a baseplate which can be introduced into the primary conductor as shown in Figure 8, Figure 10 is a perspective view of a primary conductor which^ is ccnparable to that shown in Figure 8, with coils of the secondary winding arranged therein.

Figure II is a plan view of a primary conductor in the form of a flat conductor, in an opened-out condition.

Figure 12 is a plan view of the primary conductor of Figure 11 in the folded condition.

Figure 13 is a plan view of a baseplate with flat coils of an astatic configuration, of a construction ccnparable to that shown in Figure 9, Figure 14 is a plan view of the folded primary conductor shown in Figure 11, viewing onto the opposite side from that shown in Figure 12, and Figure 15 is a view in cross-section of the arrangement shown in Figure 14.

Figure 1 shows two spaced-apart cylindrical coils 1 and 2, which are of an astatic configuration, of a secondary winding 3. The coils 1 and 8 2 which are held in position by way of a spacer 4 are geometrically and electrically identical and are disposed with the axes of their cylindrical configurations extending parallel to each other. The coils 1 and 2 are arranged in insulating cylinders 5 and 6. The coil 1 is embraced by a turn 5 7a of the primary conductor 7 through which the current I., to be measured flows in the direction of the arrows indicated. The voltages induced in the coils 1 and 2 by the magnetic field of the alternating current flowing in the primary conductor 7 are added to provide a signal which is proportional to the alternating current 1^ to be measured. Voltages 10 induced by external homogenous interference ac fields, because of the astatic arrangement of the coils 1 and 2, are of different signs and cancel each other out in the sun. By virtue of that arrangement, the influence of external ac magnetic fields on correct operation of the current transformer arrangement is substantially eliminated. The influence of external fields 15 can be further reduced by magnetic screening material surrounding the coils.

In Figure 2 the coils 8 and 9 correspond to those shown in Figure 1. The secondary coils 8 and 9 are successively embraced by the cannon primary conductor lO. That series connection of the primary windings 10a 20 and 10b results in a measurement signal which is of greater magnitude, in comparison with the arrangement of Figure 1.

In Figure 3 the coils 11 and 12 correspond to the coils 8 and 9 in Figure 2. The primary conductor 13 is branched to give two conductor portions which are each formed into a respective turn 13a and 13b embracing 25 the respective coils 11 and 12. The current I. is branched onto the conductor portions with the turns 13a and 13b, with the sun of the voltages induced in the coils 11 and 12 being proportional to the current 1^ to be measured. The advantage of the arrangement shown in Figure 3 over the construction shown in Figure 2 is that, with the primary conductor 13 which 30 is preferably stamped frcm copper, of rectangular cross-section (flat conductor), folds can be avoided where the conductor portions cross over.

In the embodiment shown in Figure 4 a primary conductor 14 is in the form of a flat conductor of rectangular cross-section, which is folded in such a way that oppositely disposed conductor portions 14a and 14b 35 produce a square hollow space 15. Outside the hollow space 15, oppositely disposed portions of the primary conductor 14 are separated from each other by an insulating layer 16. The conductor portion 14a has a slit-like opening 17 which extends frcm approximately the middle to the edge. An opening 18 which extends to the edge in the opposite direction is provided 40 in the oppositely disposed conductor portion 14b. The openings 17 and 18 influence the geometrical position of the current paths of the current to be measured, which is represented by the arrows 19 and 20, in such a way that a turn is formed for the primary current. Arranged in the nagnetic field of that turn is the secondary coil 21 which is shown in broken lines. 45 A second secondary coil 22 is disposed outside the prixrary conductor to compensate for external magnetic fields. 9 The primary conductor 23 shown in Figure 5 differs from the construction shown in Figure 4 in that two mutually oppositely directed slit-like openings 24 and 25, and 26 and 27 respectively, are provided in each of the oppositely disposed conductor portions 23a and 23b. The 5 laterally open openings 24 to 27 each extend approximately as far as the middle of the conductor portions 23a and 23b. The openings 24 and 26 are disposed in the same plane perpendicularly to the primary conductor 23 when same is folded through 180° in the condition for operation thereof, that is to say the conductor portions 23a and 23b extend parallel to each other. 10 The openings 25 and 27 are arranged in the same manner in a common plane perpendicularly to the primary conductor 23.

By virtue of the above configuration of the openings 24 to 27 the primary current flows along current paths which are identified by arrows 29a to 29g. By virtue of that arrangement, series-connected primary turns 15 are formed in the planes of the conductor portions 23a and 23b, and secondary coils may be arranged in the magnetic field of the primary turns.

Two astatically arranged secondary coils 31 and 32 are disposed on the baseplate 30 shown in Figure 6. In the operative condition the baseplate 30 is disposed with the coils 31 and 32 between the conductor 20 portions 23a and 23b of the primary conductor shown in Figure 5. The position of the coil 31 in Figure 6 is identified on the conductor portion 23b in Figure 5 by the broken-line circle 33. Similarly, the coil 32 in Figure 6 is disposed in a region identified by the broken-line circle 60 in Figure 5.

Figure 7 shows the current transformer arrangement with the primary-side portion of Figure 5 and the secondary-side portion of Figure 6, in the operative condition of the arrangement. In that case the upper and lower portions of the primary conductor 23 are separated frcm each other by an insulating layer 61.

The primary conductor 34 in Figure 8 is comparable to the primary conductor 23 in Figure 5. Only the spacing between the upper conductor portion 34a and the lower conductor portion 34b is smaller and corresponds to the thickness of the insulating layer 35.

The baseplate 36 shown in Figure 9 with the secondary coils 38 and 35 39 arranged thereon is disposed between the conductor portions 34a and 34b of the primary conductor 34 shown in Figure 8, in the operative condition of the current transformer arrangement. The coils 38 and 39 in Figure 9 are of a spiral configuration and are produced using the planar technique so that the small amount of space between the conductor portions 34a and 40 34b as shown in Figure 8 is sufficient. In the operative condition the middle point of the coil 38 is approximately at the end, which is at the middle, of the slit-like opening 40 in Figure 8. Similarly the middle point of the coil 39 and the end of the opening 41, which is at the middle are arranged approximately in alignment.

Figure 10 shows a primary conductor 42 which substantially corresponds to the primary conductor 34 in Figure 8. However in this case the openings 44 and 45 are in the form of holes at their ends which are towards the middle of the primary conductor 42, with secondary coils 46 and 5 47 of an astatic configuration being mounted in the holes.

In the embodiment shown in Figure 11 a primary conductor 48 is shown in an open condition, that is to say before being folded about a line 49. In the folded condition one conductor portion 48a is disposed above a conductor portion 48b. The conductor portion 48b has openings 50 and 51 10 which are directed in opposite relationship to each other and which extend on a cannon longitudinal axis parallel to the fold line 49. Provided in the conductor portion 48a is an opening 52 which extends only in the middle region of the conductor portion 48 and which is at the same spacing relative to the fold line 49 as the openings 50 and 51. The locations for 15 the secondary coils are shown by the broken-line circles 53 and 54. The corresponding base surfaces 55 and 56, for the secondary coils, are disposed in mirror image symmetry in relation thereto, with respect to the fold line 49.

Figure 12 shows the primary conductor 48 of Figure 11 in the form 20 in which it is folded together so that the conductor portions 48a and 48b are disposed one above the other. Accordingly only shown are the openings 50 and 51 with the locations for the secondary coils, which are indicated by circles 53 and 54.

Figure 13 shows the secondary coils 56 and 57 which are fixed on a 25 baseplate 55, of an astatic configuration. This arrangement which corresponds in principle to that shown in Figure 9 differs therefrom substantially in that the coils 56 and 57 in Figure 13 are at the same spacing from the fold line 49. The coils 56 and 57 may also be produced using the planar technique. In order for both coils to be exposed to the 30 corresponding primary magnetic flux, they may also be arranged outside the space between the folded conductor portions 48a and 49b shown in Figure 11 if the level of rregnetic coupling is sufficient for a high output signal. In that situation also the circles illustrated in Figures 11 and 12 are the corresponding locations for the secondary coils.

Figure 14 shows the primary conductor 48 of Figure 11 in the condition in which it is folded together, frcm the opposite side in ccnparison with Figure 12. Accordingly only the central opening 52 can be seen.

Figure 15 also shows the primary conductor 48 in the condition in 40 which it is folded together, with the spacing between the upper conductor portion 48a and the lower conductor portion 48b being determined by an insulating layer 57. The direction of the primary current to be measured is identified by the arrows 58 and 59. The baseplate 55 shown in Figure 13 is introduced into the space 70. 11

Claims (11)

CLAIMS:

1. A current transformer arrangement, in particular for a static electricity meter, comprising a primary conductor carrying the alternating current to be 5 measured, and a secondary winding comprising at least two series-connected electrically identical coils of an astatic configuration, the output voltage of which is applied to an electronic integration stage on the output side thereof, to produce a measurement signal 10 which is independent of frequency, wherein to produce a maximum magnetic field strength the primary conductor is formed into at least one current loop, the turns of at least one of the secondary coils, for maximum magnetic coupling, are arranged at the smallest 15 possible spacing relative to the corresponding current loop and engage as completely as possible the magnetic flux produced by the current loop, the magnetic coupling is effected without magnetic materials, the secondary coils extend in their axial direction over 20 the smallest possible part of the length of the magnetic field lines produced by the current in the primary conductor, and the secondary coils are arranged in as closely adjacent juxtaposed relationship as possible for optimum external field compensation. 25

2. A current transformer arrangement according to claim 1 wherein the current loop closely embraces the one secondary coil in the peripheral direction thereof.

3. A current transformer arrangement according to claim 1 or claim 2 wherein two current loops are 30 connected in series and each current loop embraces a secondary coil.

4. A current transformer arrangement according to claim 1 or claim 2 wherein two current hoops are connected in parallel with each other and each current loop embrances a seondary coil.

5. A current transformer arrangement according to one of the preceding claims wherein the primary conductor is in the form of a folded flat conductor withmutually oppositely disposed conductor protions which, by means of at least one opening, each form at least one protion of the current loop in the plane of the flat conductor.

6. A current transformer arrangement according to claim 5 wherein the openings extended in mutually opposite directions approximately from the centre line to the edge of the flat conductor.

7. A current transformer arrangement according to claim 6 wherein the oppositely disposed conductor portions of the flat conductor each have two mutually oppositely directed openings which are arranged in parallel displaced relationship and thereby form two portions of the current loop which are disposed side-by-side in the longitudinal direction of the flat conductor.

8. A current transformer arrangement according to one of claims 5 t o 7 wherein the secondary coils are arranged between the conductor portions of the flat conductor.

9. A current transformer arrangement according to claim 7 or claim 8 wherein the secondary coils, being produced by the planar technique, are disposed in one or more layers in the form of spirals on a substrate. 1 3

10. A current transformer arrangement according to claim 5, claim 8 or claim 9 wherein the one conductor portion has two mutually oppositely directed openings which extend to the edge of the flat conductor on a 5 common axis and arranged in parallel therewith on the other conductor portion is an opening which does not go to the edges.

11. A current transformer arrangement substantially as herein described with reference to any of Figures 1 to 10 5, or Figures 5, 6 and 7, or Figure 8, or Figures 8 and 9, or Figure 10, or Figures 11 to 15 of the accompanying drawings. F. R. KELLY & CO., AGENTS FOR THE APPICANTS.