WO1991019305A1

WO1991019305A1 - A method and a shield for shielding a current transformer as well as a current transformer including such a shield

Info

Publication number: WO1991019305A1
Application number: PCT/DK1991/000148
Authority: WO
Inventors: Thomas Erlang Hansen; Thomas Tingleff
Original assignee: Lk Lavspaending A/S
Priority date: 1990-06-01
Filing date: 1991-05-31
Publication date: 1991-12-12
Also published as: DK136790D0; AU7966591A; DK169008B1; DK136790A

Abstract

A current transformer comprising a circumferential core (12) and a through-going conductor (14) forming the primary winding of said transformer, and a secondary winding (16) wound on defined portions of the core (12) whereas the remaining portion of the core (12) is not covered by windings an provides a yoke (18), should be protected against exposure to environmental fields, especially when it forms part of a three-phase system with closely arranged conductors involving strong current intensities. According to the invention, the yoke or yokes exposed to foreign magnetic fields are completely or partially shielded by means of one or more shields (50) of a ferro or ferrimagnetic material neither extending over nor covering any windings or portions of windings. By foreign magnetic fields is meant fields beyond the field caused by a current in the primary winding of the transformer. In this manner the current transformer can meet the desired measurement accuracy.

Description

K91/00148

Title: A Method and a Shield for Shielding a Current Transformer as well as a Current Transformer Including Such a Shield-

Technical Field

The invention relates to a method of shielding a current transformer comprising a circumferential core accomodating a through- oing primary conductor forming the primary winding of said transformer, and a number of secondary windings wound on defined portions of the core whereas the remaining portions of the core are not covered by windings and provide yokes .

By a circumferential core is meant a core formed as a closed loop, which -is preferably circular, rectangular, square or a combination thereof.

Background Art

Known transformers are usually structured as single-phase transformers. Calculations have been made concerning loss of iron and copper, transformation ratio error and phase displacement in accordance with guidelines well-known to persons skilled in the art. The latter also applies to transformers to be used as current transformers, where it is not desired to transfer energy, but merely to achieve

^• a measurement of the current in a conductor, cf. for instance S. Vørts: "Maletransformatorer" ("Measuring transformers"). Standard requirements have been laid for current transformers, said standards appearing from IEC 185 for various classes 0.2, 0.5, 1 and 3 as well as 5pl0 and lOplO, concerning the measuring accuracy (expressed by the accepted transformation ratio error/phase displace- ment in percentages) and excess current numbers (ALF) .

Transformation ratio error and phase displacement can for any transformer for instance be measured in a bridge circuit by comparing with a very accurate "standard" inserted in said bridge circuit, which is a well-known procedure to persons skilled in the art.

When the transformers are to be installed in three-phase systems, a tendency applies to arrange the transformers closely side by side due to the frequent demand for struct¬ uring a compact unit not taking up unnecessary room.

Tests have, however, revealed that a current transformer operating satisfactorily in a single-phase test network, i.e. having^'a suitably low transformation ratio error and phase displacement, doe^s not operate quite as satisfactori¬ ly when it is installed in a multiphase conductor system.

The problem appears when the primary current is increased. At currents exceeding 1000 Amp, errors may in unfortunate cases exceed the acceptable level stipulated according to the classification of the transformer in question. Further¬ more, the problem is more distinct in connection with low voltage installations where the voltages are not high and consequently do not force the manufacturer to inter¬ space the components so as to ensure a suitable insulation between the high voltage components.

A similar problem arises when a transformer is placed in an iron casing or adjacent an iron frame of a casing.

Some of the above problems can be solved by enlarging the core in such a manner that the reluctance to the magnetic flux in the circuit is reduced with the effect that the tendency of the field to propagate in other directions is dampened.

The above solution is not always applicable. For a current transformer it is desirable that the transformer is unable to transfer an excess current during a fault condition. Such an excess current might damage the measuring equipment coupled to the secondary circuit. In some cases, the core has intentionally been dimensioned so small that it satu- rates when the current exceeds the maximally allowed level. Accordingly, a demand exists for a solution of the problem without increasing the dimensions of the core.

Several types of magnetic shielding of transformers are already known.

EP publication No. 0 Oil 590 discloses a completely insu¬ lated, metal-enclosed three-phase high-voltage switchboard with a current measuring transformer for a through passing conductor in each phase. Three conductors extend through a metal cap of a circular cross section as well as their respective closed iron core. Each iron core comprises a secondary winding, a shield of a ferromagnetic and/or a conducting material extending substantially parallel to the conductors, where the shields have been mutually short- circuited at both ends.

GB-PS No. 1,191,756 discloses a current measuring device almost completely surrounded by and enclosed in one or more shields, where it has been necessary to provide a short-circuiting winding around the shield in order to compensate for an undesired flux induced in the shield by the primary conductor extending through said shield.

GB Patent Application No. 2 073 499 discloses a winding support or coil form for a measuring transformer. Electro¬ magnetic shields shield the transformer against fields ex- citated by the motor, said transformer measuring motor current.

US-PS No. A, 749, 979 discloses a transformer with a magnetic shield surrounding an E-I laminated core with windings around the central limb. The shield comprises two circum¬ ferential metal strips extending almost parallel to one another almost the entire way around the laminated core at an as constant as possible mutual distance and perpen- dicularly to the direction of the laminates. The object of the shield is to protect the electronic circuits of a Compact Disc player against magnetic fields excitated by the transformer supplying the circuits with power.

None of the known types of shielding are suited for trans- formers operating with currents in the kA class (from 1000 Amp and more).

Disclosure of Invention

The object of the present invention is to provide a simple and efficient method of shielding a current transformer of the above type against the effects of environmental fields from the outside (i.e. magnetic fields other than the fields excitated by the current to be measured) , especially in connection with coupling thereof to a compact multiphase heavy current system with short distances between the phases, and to produce a current transformer and a shield for a current transformer suitable for current intensities in the kAmp class of for instance about 4000 Amp .

According to the present invention, the shielding problems are solved by the yoke(s) exposed to environmental magnetic fields, i.e. magnetic fields other than the fields exci¬ tated by a current through the primary winding of the transformer, being completely or partially shielded by a ferro or ferrimagnetic material neither extending over nor covering any windings or portions of windings.

The shield is preferably an iron plate bent substantially like a U in such a manner that it can cover the three surfaces of the yoke of the current transformer. The iron plate is preferably of a thickness of about 2 mm.

As a result, the environmental magnetic fields excitated by the neighboring conductor follow the shield instead of interfering with the core of the current transformer.

The shield should not be in conducting contact with the core of the current transformer, but it may be placed close to said core.

Brief Description of the Drawing.

The invention is described in greater detail below by way of examples of embodiments with reference to the accompany¬ ing drawings , in which

Fig. lA illustrates a typical non-enclosed current trans¬ former with a rectangular core,

Fig. IB illustrates a typical current transformer enclosed in a plastic housing,

Fig. 2 is a sectional view of a three-phase transformer arrangement for measuring current,

Fig. 3 is a top view of a measuring circuit,

Fig. 4 illustrates curves of the transformation ratio error as function of the primary current,

Fig. 5 illustrates curves of phase displacement as function of the primary current,

Fig. 6 illustrates curves of the transformation ratio error as function of the primary current, Fig. 7 illustrates curves of phase displacement as function of the primary current,

Fig. 8 illustrates curves of the transformation ratio error as function of the primary current,

Fig. 9 illustrates curves of phase displacement as function of the primary current,

Fig. 10A is a sectional view of a preferred embodiment of a shield according to the invention for a transformer,

Fig. 10B is a view of the embodiment of Fig. 10A in the unfolded state,

Fig. 11 illustrates a preferred embodiment of a trans¬ former ,

Fig. 12 illustrates an alternative shielding,

Fig. 13 illustrates an example of a current rail, and

Fig. 14 show sectional views of various embodiments of shields according to the invention.

Best Mode for Carrying Out the Invention

A typical current transformer 10 appears from Figs. 1A and IB, and it comprises a substantially rectangular core 12 enclosing a primary conductor in form of a current rail 14, or in form of a rail assembly including several current rails 14. On two sides, the core 12 is provided with a secondary winding 16 connected to a measuring instrument not shown. Two yokes 18 interconnect the wound portions of the core. The above portions are known, and it is furthermore known to embed the transformer in plast¬ ics as shown in Fig. IB. Two or three current transformers 10, 20, 30 are often placed side by side in a three-phase system R, S, T, cf. Fig. 2, and all portions are often enclosed in a casing 40. The casing is typically a case of the type used in connection with switchboards, and it is usually made of conventional, magnetizable iron plate influencing the fields because part of the magnetic field will find a path via the case and thereby possibly cause errors in measurements .

Installations dimensioned to for instance about 1000 Amp are typically equipped with conductors or rails of a cross section of about 10 mm times 100 mm, and the conductor must be of a well conducting material, preferably copper. The conductors 14 may be interspaced for instance 50 mm. Sometimes, it is only necessary to measure the current in one or two of the phase conductors (R,S,T) , and accordingly current transformers are only placed about the phase conductors where a measuring is desired.

In order to test the actual importance of the placing of the transformers and a possible shielding relative to their closest surroundings, transformation ratio error and phase displacement were measured under laboratory condi¬ tions as described below.

In a flexible measuring arrangement including three dis- placeable, horizontal rails 14, i.e. one in each phase R, S, T, as shown in Fig. 3, the measuring was carried out on transformers placed in either phase R, S or T, and the following parameters were varied one by one:

1) d. representing the distance, i.e. the spacing, between the rails 14 measured in mm, typically 55 mm, 2) f. representing the displacement in the direction of the conductors of the intermediary transformer 20 relative to the two outer transformers 10, 30, i.e. f, corresponds to the infinite in the examples shown in Figs. 4 to 9 , and f. equals zero in Fig. 3,

3) a, representing the distance to the case 40, typically the rear wall and/or the front wall of a casing measured in cm,

4) the primary current in percentages of rated current ,

5) the load, i.e. the load of the transformer on the secondary side,

6) the thickness of a shield between the trans- formers .

The curves indicated in Figs. 4 to 9 show a number of typical results of such measurements on a class 0.25 trans¬ formers .

According to the standards, the transformation ratio error for the above transformer class must not for example exceed 0.2% for currents on the primary side from 20% to 120% of the rated current. The same applies to other classes.

The curves 5B of Figs. 4 and 5 are measured for varying loads on the secondary circuit, 2.5 VA, 5 VA, 7.5 VA, 10 VA and 17.5 VA. The distance d_, between the phase conductors 14 was 55 mm. In comparison therewith, two curves are indi¬ cated, viz. Ref IB 2.5 VA and Ref IB 17.5 VA, measured with the distance d. corresponding to 250 mm between the phase conductors, i.e. with such a large distance between the conductors that the mutual influence can be ignored. It appears that the curves 5B highly deviate from the reference curves IB when the primary current exceeds about 100% of the rated current, and especially when the secon¬ dary side is heavily loaded.

It appears furthermore, that the transformer only meets class 0.2 for a load of up to 7.5 VA.

The curves of Figs. 6 and 7 are based on measurements with varying distances to the front wall and the rear wall of the casing (100/100, 90/90, 80/80, 70/70 and 60/60) and with a permanent distance to the side walls.

The shielding is provided by means of a 1.5 mm thick iron plate folded into a U and placed such that the yokes of the transformers are covered.

The distance d. between the conductors was maintained at 55 mm, and the displacement f. was infinite as only one trans¬ former was placed in the intermediary phase. The secondary side was loaded by 10 VA.

A number of measurements resulting from measurings made under the same conditions were chosen as reference, but without the casing on a transformer 20 in the intermediary phase S, where d, equals 55 mm between the conductors, and the load is 10 VA.

As demonstrated previously, the error increases heavily when the primary current exceeds about 100% of the rated current, and the error is larger when the case or cabinet is close to the transformer. In connection with the used shielding, only the measurement 29B with a casing distance of 60/60, i.e. 60 mm between the transformer and the rear/- front wall, exceeds the allowed error. It appears from the curves, that the phase displacement increases in almost the same manner as the transformation ratio error, but when seen in relation to the standards, the transformation ratio error is the most critical error.

A great number of similar measurements have revealed that the measurement accuracy of the current transformer can be considerably improved by placing a plate of iron 50 or of a corresponding magnetizable material between the two neighboring transformers 10, 20 or 20, 30, cf. Fig. 3, or between a transformer 20 and the neighboring conductors 14 in such a manner that the transformer yokes are shield¬ ed. The plate is preferably bent such that it is of a U- shaped cross section or as shown in Fig. 11.

Figs. 8 and 9 show curves indicating the transformation ratio error and the phase displacement as function of the primary current in percentages of the rated current, and in connection with three different thicknesses of the plate .

The distance d. between the conductors was fixed at 55 mm, and the displacement f. was infinite as only one transformer was placed in the intermediary phase. The secondary side was loaded by 10 V .

A curve 14B is chosen as reference curve, said curve being based on a number of measurements carried out on a trans- former 20 in the intermediary phase S (with d. corresponding to 55 mm between the conductors and a load of 10 VA) under the same conditions, but without the casing, and with a 1.5 mm thick shield. The curves 42B , 43B and 44B of Figs. 8 and 9 are measured with a casing or case comprising a front wall 80 mm above the casing of the current trans¬ former and a rear wall 40 mm below said casing of the current transformer, cf. at the bottom of Fig. 8. Further¬ more, the transformer yokes were shielded by means of a U-shaped shield according to the invention of a thickness of 1.5, 2.0 and 2.5 mm, respectively.

The curves of Figs. 8 and 9 show that the effect of the shield is increased, i.e. the error being measured de- creases and the curves approach the reference curves 14B , when the thickness of the shield increases. The curves 14B and 44B meet the standard, the curve 43B is on the limit, and the curve 42B clearly exceeds the limit of 0.2%.. The latter curve is obtained with a thin shield of only 2.5 mm, and said shield turned out to be insuffi¬ cient in connection with the casing/case used, at least when the primary current approaches 120% of the rated current .

It turned out that the shield 50 is advantageously struc- tured as shown in Figs. 10 and 11. The shield is preferably made of iron plate, but other magnetizable materials, such as a sintered ferrimagnetic material, are within the scope of the invention. In the illustrated embodiment, the shield comprises a central portion 52 dimensioned almost identical with the width and the length of the yokes 18 of the current transformer. The shield comprises furthermore two side portions 54 providing an additional shielding, and they are advantageously provided with obliquely cut corners 56 and advantageously bent such that they are placed close to the transformer yoke 18, preferably at a distance optionally filled with an insulat¬ ing material, such as sheet or plastics, forming part of a coil^{^}form for the transformer.

The shield is advantageously embedded in a plastic enclo- sure for a current transformer.

Thereby a compact shield is achieved, which does not increase the dimensions of the transformer to a significant degree, but which provides the desired effect, i.e. prevent environmental fields from entering the core of the trans¬ former, cf. the curves shown in Figs. 4 to 9.

The shielding effect is obtained because the shield 50 "collects" the fields from the neighboring conductor 14.

In order to allow the advantageous effect of the shield, said shield should neither cover nor partially enclose the secondary winding because the shield in that case would magnetically shunt the magnetic field in the core. Such an unintentional structure implies that the current transformer is encumbered with a higher measurement error.

According to a second embodiment, the shield may be of a "top hat"-shaped cross section with additional side webs, cf. Fig. 14A or side webs, cf. Fig. 14B . The bottom of the U may be almost rectilinear, cf. Fig. 10A, or semicir- cular, cf . Fig. 14C. The legs of the U may extend obliquely to the sides as indicated in Figs. 14D and 14E. Finally, the shield may be made of a sintered ferrimagnetic , option¬ ally crescent-shaped material, cf. Fig. 14F.

A preferred embodiment of a current transformer with a shield according to the invention appears from Fig. 11. The embodiment of Fig. 11 avoids the sharp corners present¬ ed by the rectangular transformer, cf. Fig. 1. Such sharp corners may cause field concentrations in turn causing a spreading of the said field and consequently measuring errors.

The circumferential transformer core 72 is preferably wound by circumferential iron straps and preferably by a single continuous iron strap. The core comprises two opposing, substantially circular portions 75 carrying the secondary winding 76. The circular portions are intercon¬ nected by two substantially rectilinear portions forming yokes 78. The above expresses a compromise between the circular shape ideal in theory and the compact shape desired in practice and allowing a close arrangement of current transformer cores and phase conductors.

5 In order to avoid a high measurement error, i.e. transfor¬ mation ratio error and phase displacement, in connection with high currents, two U-shaped iron plate members 50 are according to the invention placed about the two recti¬ linear transformer yokes 78. The two iron plate members

10 50 are preferably dimensioned such relative to the core that they are exactly so wide that they can enclose a transformer yoke while allowing a slight clearance. Fur¬ thermore, the two iron plate members are arranged such that they preferably, cover completely the three surfaces

15 of the unwound transformer yoke 78 without extending over the secondary winding 76. Care should be taken, that the fields concentrating in the U-shaped shield cannot couple to the secondary winding.

As an alternative, the same effect can be achieved by 20 directly shielding the neighboring conductor, such as by means of a U-shaped shield 60, cf. Fig. 12. Such a shield for the neighboring conductor may be shaped in^' the same manner as the shields shown in Figs. 14A, B, C, D and E. According to a particularly simple embodiment, the shield 25 is provided by a simple flat plate between the neighboring conductor and the transformer yoke. According to a par¬ ticularly advantageous embodiment, the shield is a combina¬ tion of a U-shaped rail 60 gripping about the conductor 14, and a plate 61 optionally welded to the U-shaped rail 30 60, cf. Fig. 13, and abutting the yoke not shown in Fig. 13.

A shielding of the above type may optionally be provided in connection with or as part^' of a fitting, such as a fitting for fastening current rails or transformers. Such an embodiment is within the scope of protection of the present application.

Claims

Claims .

1. A method of shielding a current transformer comprising a circumferential core accomodating a through-going primary conductor forming the primary winding of said transformer, and a number of secondary windings wound on defined por¬ tions of the core whereas the remaining portions of the core are not covered by windings and provide yokes, c h a r a c t e r i s e d in that the yoke(s) exposed to environmental magnetic fields, i.e. magnetic fields other than the fields excitated by a current through the primary winding of the transformer, is(are) completely or partially shielded by a ferro or ferrimagnetic material (50) neither extending over nor covering any windings or portions of windings .

2. A method as claimed in claim 1, where the current transformer accomodates a through-going a phase conductor (14) forming part of a multiphase system with several phase conductors arranged in parallel adjacent the yoke(s) of the transformer core, c h a r a c t e r i s e d in that the current transformer (20) is shielded by a plate of a ferro or ferrimagnetic material (50) placed between the yoke and the neighboring phase conductor (14) and/or the current transformer (10, 30) of said phase conductor which is connected with a phase other than the transformer (20) being shielded, and that the plate is being bent in such a manner that it substantially covers the three sides of the yoke facing outwards.

3. A method as claimed in claim 1 or 2 , where the current transformer accomodates a through-going phase conductor (14) forming part of a multiphase system with several phase conductors arranged in parallel adjacent the yoke(s) of the transformer core, c h a r a c t e r i s e d in that the current transformer (20) is shielded by a plate of a ferro or ferrimagnetic material (50) placed between the yoke and the neighboring phase conductor (14) and/or the current transformer (10, 30) of said phase conductor which is connected with a phase other than the transformer (20) being shielded, and that the plate is being bent in such a manner that it substantially covers and extends over and below at least a portion of the neighboring phase conductor (14) .

4. Shield for carrying out the method of claim 1, 2 or 3, c h a r a c t e r i s e d in that in the direction of the through-going conductor (14), the width of the shield (50) exceeds the width of the core (12, 72) in the same direction, and that in the direction perpendicular to the above direction, i.e. measured along the circum¬ ference of the circumferential core (12, 72), the shield (50) is of an extent exceeding the thickness of the through-going conductor and preferably almost corresponding to the length of the yoke, i.e. the non-wound portion of the core (12, 72), but does not extend over the portion of the core (12, 72) wound with the secondary winding.

5. Shield as claimed in claim 4, c h a r a c t e r ¬ i s e d in that the shield is of a U-shaped cross section with a flat or curved bottom, and that it is dimensioned so as to enclose the three outer surfaces of the yoke of the transformer and/or to cover and extend over and below at least.a portion of the neighboring phase conductor (14).

6. Shield as claimed in claim 4 or 5, c h a r a c ¬ t e r i s e d in that the shield is of a "top hat", i.e. U-shaped, cross section, with a plane or curved bottom and provided with side webs or "wings" (cf. Figs. 14A, B, C, D or E) .

7. Shield as claimed in claim 4, 5 or 6, c h a r a c¬ t e r i s e d in that it is made of a sheet of a ferromag¬ netic material, such as soft iron, and of a thickness of more than 0.5 mm, and especially from approximately 1.5 mm to approximately 2.5 mm.

8. Current transformer comprising a circumferential core and a through-going conductor forming the primary winding of said transformer, and a secondary winding wound on defined portions of the core whereas the remaining portion of the core is not covered by windings and provides a yoke, as well as with a shield protecting against expo¬ sure to fields from the outside, c h a r a c t e r - i s e d in that the outer surfaces of at least one of the yokes (18, 78) are completely or partially shielded by a shield (50) of a ferro or ferrimagnetic material neither covering the secondary winding nor portions thereof.

9. Current transformer as claimed in claim 8, c h a r- a c t e r i s e d in that the core comprises two circular, opposing, wound portions and two almost rectilinear yokes interconnecting the two circular, wound portions, and where the rectilinear yokes are provided with a shield as claimed in one or more of the preceding claims 3 to 5.

10. Current transformer as claimed in claim 8 or 9, c h a r a c t e r i s e d in that a ferro or ferrimag- netic, shielding material forms an integrated part of the enclosure of the current transformer.