EP0144347A1

EP0144347A1 - Active current transformer.

Info

Publication number: EP0144347A1
Application number: EP84901762A
Authority: EP
Inventors: Richard Friedl
Original assignee: Landis and Gyr AG; LGZ Landis and Gyr Zug AG
Current assignee: Siemens Building Technologies AG; Landis and Gyr AG
Priority date: 1983-05-24
Filing date: 1984-04-27
Publication date: 1985-06-19
Also published as: DE3318749C2; DE3461233D1; EP0144347B1; WO1984004849A1; JPH0426530B2; JPS60501434A; DE3318749A1; US4629974A

Abstract

Le transformateur de courant doté d'un enroulement primaire (1), d'un enroulement secondaire (2) et d'un enroulement détecteur (3) possède un couplage rigide entre l'enroulement secondaire (2) et l'enroulement détecteur (3); mais le coefficient de couplage entre ces enroulements (2, 3) et l'enroulement primaire (1) est pourtant bien inférieur à 1, puisque seule une partie de l'écoulement magnétique O/1 produit par le courant I1 à mesurer dans l'enroulement primaire (1) est détectée par l'enroulement détecteur (3). Cet écoulement partiel O/13 est compensé par l'écoulement produit au moyen d'un amplificateur de réglage placé dans l'enroulement secondaire (2).The current transformer provided with a primary winding (1), a secondary winding (2) and a detector winding (3) has a rigid coupling between the secondary winding (2) and the detector winding (3 ); but the coupling coefficient between these windings (2, 3) and the primary winding (1) is however much less than 1, since only part of the magnetic flow O / 1 produced by the current I1 to be measured in the primary winding (1) is detected by the detector winding (3). This partial flow O / 13 is compensated by the flow produced by means of a control amplifier placed in the secondary winding (2).

Description

Active current transformer

The invention relates to an active power converter of the type specified in the preamble of claim 1. ^'

Current transformers with electronic error compensation are known (DE-OS 2330048). In such current transformers, the coupling factor K, which is determined solely by the magnetic conductivities, is practically always the same. This is achieved by the closed magnetic circuit, so that the translation ratio of the transducer is determined by the ratio of the number of turns of the primary and secondary winding. A remaining small current error is eliminated by the compensation of the magnetic flux. In the case of large transmission ratios with correspondingly high flooding, the current transformer described above requires a considerable amount of material and space, since the entire primary flow has to be compensated.

Particularly when measuring the current strengths of large alternating currents, the primary winding generates significantly larger floodings than converters equipped with a ferromagnetic core require for the magnetic core to function properly. To reduce the resulting expenditure on magnetic core material and the space and cost requirements, transducers are known whose magnetic core comprises only a part of the conductor cross section carrying the measuring current. Special problems arise with regard to the relatively large temperature dependency and the need to eliminate principle-related phase errors between the primary and secondary currents by means of special measures.

Furthermore, an active current transformer is known in which the current to be measured is divided into two separate conductors which are wound in opposite directions to form the primary winding and whose resistance values differ from one another (DE-OS 31 40544).

./.

OMPI

^' In addition, active current transformers are known which completely dispense with ferromagnetic cores (DE-OS 28 12303) and in which the secondary winding is designed as a toroid through which the primary conductor carrying the measuring current passes. Although such transducers work relatively precisely, their mechanical structure is too complex for mass production.

In the known current converters, the coupling factor K = 1 is always aimed for between the primary and the secondary winding, the transmission ratio being determined solely by the ratio of the number of turns of these windings.

The invention is based on the object of providing a current transformer which, compared to the known arrangements, can be constructed more simply and at least have the same metrological properties and can be produced more cost-effectively.

The problem is solved by the features specified in claim 1. Appropriate embodiments of the invention are characterized by the features of the subclaims.

In the current transformer according to the invention, in contrast to the known arrangements, the current transmission ratio is not exclusively determined by the ratio of the number of turns of the primary and secondary windings, but also to a considerable extent by the coupling factor, which indicates the ratio of partial flow to total flow . By choosing a coupling factor that is substantially below one, the transmission ratio can be set within wide limits without changing the number of turns, so that the secondary and detector windings can be made comparatively small. While a coupling factor K = 1 is aimed at in the known converters, in the case of the invention there is a significant deviation from this value in order to set the desired high transmission ratios with the aid of the coupling factor, that is to say the partial flow detection on the secondary side. The current error is also compensated in a known manner for the new converter

./. by choosing a suitable reinforcement.

In the case of an arrangement without ferromagnetic material, the coupling factor is achieved by a sufficiently large geometric distance between the primary and detector winding or secondary winding. When using ferromagnetic materials, the coupling factor is also influenced by the permeability of these materials.

The current converter according to the invention with a high current ratio is distinguished from the known arrangements primarily by further considerable simplifications, by insensitivity to temperature changes and by small phase errors. Further advantages are the possibility of simply assembling the primary conductor with the other necessary components by introducing (inserting) these parts into the magnetic field of the current flowing in the primary conductor, so that, for example, these components can also be replaced without opening the primary circuit the unrestricted function when DC components are present in the primary and secondary circuits. In addition, the use of the arrangement according to the invention with the known circuits for digital flux compensation in the secondary circuit and the known blanking of the magnetic flux with sawtooth signals are particularly suitable. Because of the low impedance of the secondary winding, the frequency of the measuring cycles can be chosen to be relatively high. Depending on the application, the new current transformer can be equipped with or without ferromagnetics. The new transformer is particularly suitable for use in Electricity meters for single and multi-phase alternating current.

The principle of the invention and some exemplary embodiments are explained in more detail below with reference to drawings.

1a shows a basic illustration of the inventive method

Current transformer, 1b shows the basic circuit diagram of a known compensated current transformer,

./.

OMPI 2 shows a current transformer with a flat conductor,

3a shows an end view of the converter according to FIG. 2 with a device for setting the coupling factor,

3b shows a longitudinal section through the converter according to FIG. 3a,

FIG. 4 shows an end view of the converter according to FIG. 2 with a device for setting the coupling factor that is modified compared to FIGS. 3a and 3b,

5 shows an end view of the converter according to FIG. 2 with two further devices for setting the coupling factor,

6 an end view of the transducer with a device for reducing the influence of external external fields,

7 shows a section through the converter according to FIG. 6 along the line VII-VII,

8a is a side view of a shielding pot,

8b the shielding pot according to FIG. 8a in the end view, partly in section,

9a shows a side view of a further shielding pot and

9b shows a section through the shielding pot according to FIG. 9a along the line A - A.

FIG. 1 a represents the invention in the most general way. It concerns the arrangement of a mutual inductor, consisting of a primary winding 1 carrying the alternating current I to be measured with the number of turns W .., through whose surface the magnetic flux JET. occurs, and a detector winding 3 which is separated from the partial flow "K", i.e. is penetrated by part of the river ^, while the other Te l ^ .. as a leakage flux does not penetrate the winding 3. A secondary winding 2 with the number of turns W ~ is relatively firmly coupled to the winding 3.

./.

OMPI If the partial flow ~~ is present, as is known in the case of compensated current transformers (FIG. 1b), the voltage induced in the detector winding 3 is fed to an amplifier arrangement V which builds up a current I ₂ in the secondary winding 2 such that the partial flow j3T ₁₃ , which passes through the detector winding 3, is almost completely compensated for with usually high amplification, the coupling factor not being influenced by the amplification. The secondary measuring current I ~ is then very precisely the current I to be measured. in the winding 1 proportional. The current transformer according to the invention uses a coupling factor K, which is considerably smaller than one, to increase the transmission ratio. According to FIG. 1a, it results in K = yes, /. and leads to a gear ratio

1 ^w 2 I / I = - • - ^• ^l 1 ^7l 2 KW ₁

2 shows an exemplary embodiment of the current transformer according to the invention with ferromagnetic material. The transformer has a flat conductor 1a carrying the current to be measured, which is provided with current leads 4 and 5 and forms the primary winding 1 with only one turn W. The flat conductor 1a surrounds a tube section 6 made of ferromagnetic material, which in turn concentrically comprises a cylindrical ferromagnetic core rod 7 also made of ferromagnetic material, on which the windings 3 and 2 are applied. An electrically insulating layer 9 is located between the two legs of the flat conductor 1 a in the area of the current supply lines 4 and 5 and between the cylindrical primary winding 1 formed by the flat conductor 1 a and the ferromagnetic tube section 6. The ferromagnetic core rod 7 with the windings 3 and 2 is opposite the pipe section 6 in the annular space 8 either filled with insulation or fixed by other mechanical means.

The arrangement works as follows: The magnetic field generated by the current I_ to be measured is defined in the cylindrical primary winding 1 by the ferromagnetic tube section 6 and the ferromagnetic core rod 7 in two magnetic fluxes.

./. divided, the relationship of which is largely determined by the magnetic conductivities of the two ferromagnetic parts 6 and 7. Consequently, with the same ferromagnetic material (for example ferrite material) and radially symmetrical arrangement of the core rod 7 in the tube section 5 with the same length of the parts 6 and 7, the division ratio of the magnetic fluxes and thus the coupling factor K largely depends on the ratio of the perpendicular to the tube or the longitudinal axis of the radial cut surfaces. The coupling factor K can be further reduced by shortening the core rod 7 while maintaining the length of the pipe section 6. The coupling factor can also be changed, among other things, by shifting the core rod 7 in its longitudinal direction.

A reduction in the coupling factor according to FIGS. 3a and 3b can also be achieved by rotating the core rod 7 with the windings 3 and 2 into an angular position relative to the pipe section 6, so that the winding 3 only part of the maximum detectable magnetic flux.

4 shows one of many possibilities of how the coupling factor for an arrangement according to FIG. 2 can be finely adjusted. An annular ferromagnetic sheet 10 with a radially inward nose 10a is placed on an end face of the tube section 6. On the corresponding end face of the ferromagnetic core rod 7, a ferromagnetic sheet metal flag 11 is pivotally attached, which increases the coupling factor when approaching the nose 10a of the sheet 10 and decreases when turning away. The same effect is achieved with an embodiment for fine adjustment according to FIG. 5, in that a ferromagnetic sheet metal flag 12 is rotatably mounted eccentrically to the pipe section 6 on or near the pipe section 6 and via the field that emerges from the pipe section 6. is rotated until the desired translation is available. In addition, there is the possibility of using ferromagnetic screws 13 to deliberately influence the field profile and thus the flow distribution between the tube section 6 and the core rod 7 (FIG. 5).

./. Residual phase errors can be compensated for by suitable loading of the partial flows with metal parts in which eddy currents arise.

In order to concentrate the magnetic field outside the primary winding 1 (FIG. 2), a ferrite cylinder or pot can be pushed over this winding, which is slotted on the jacket side for the passage of the current leads 4 and 5.

For very precise measurements it is expedient to external stray fields, particularly those that overlay the useful field in the axial direction of the tube arrangement ^'shield. According to Fig.

6 and 7 make the lengths of the tube section 6 and the core rod 7 with the windings 3 and 2 shorter than the axial length of the cylindrical primary winding 1 formed by the flat conductor 1a and to close the front ends of the primary winding 1 with circular ferromagnetic covers 16 and 17, these covers are held at a distance from the ferromagnetic parts 5 and 7 via the non-magnetic disks 14 and 15. As a result, all the parts located within the ring section of the flat conductor 1 a - possibly including electronic elements - can be pushed into the conductor tube as a common block.

In the case of high external fields, it is advisable to use the primary winding 1, windings 2 and 3, the pipe section 6 and the rod core

7 to be arranged in a largely closed, advantageously cylindrical shielding pot made of ferromagnetic material according to FIGS. 8a and 8b. The shielding pot advantageously consists of two shell halves 18 and 19, the contact surfaces 20 of the two shell halves 18 and 19, the longitudinal axis of the shielding pot 18, 19 and thus also the longitudinal axis of the tube section 6 (FIG. 2) lying in a common plane. The two shell halves 18 and 19 are provided with cutouts 21 and 22 for carrying out the current leads 4, 5 (FIG. 2) of the flat conductor 1a and the connections of the windings 2 and 3. During assembly, the shell halves 18 and 19 are folded over the current transformer described. In this way, the inner converter part can also be connected to a current

./.

OMPI Flat conductors 1a must be replaced at the operating location.

9a and 9b show an embodiment of a cylindrical shielding pot consisting of two shell parts 23 and 24, in which the contact surfaces 25 of the two shell parts 23 and 24 in one to the longitudinal axis of the shielding pot 23, 24 and thus to the longitudinal axis of the tube section 6 (Fig. 2) vertical plane. In the example shown, the shell part 23 is designed as a pot and the shell part 24 as a lid. A tubular pin 26 or 27 is formed on the inside of the circular end face of the shell part 23 or 24. These pipe journals 26 and 27 together form the pipe section 6 (FIG. 2) for the purpose of flow division, an air gap 28 between the opposite end faces of the pipe journals 26, 27 ensuring shear of the magnetic circuit and thus preventing magnetic short-circuiting. The hollow cylinder-shaped space 29 between the jacket of the shielding pot 23, 24 and the tubular pin 26, 27 serves to receive the primary winding 1 or the flat conductor 1a and the space 30 inside the tubular pin 26, 27 to accommodate the core rod 7, the Secondary winding 2 and the Detektorwic development 3. To carry out the power supplies 4, 5 of the flat conductor 1 a is a breakthrough in the shielding pot 23, 24

31 and a breakthrough for the passage of the coil wires to the room 30

32 provided.

It was found that the coupling factor K and the linearity of the current transformer described are mainly determined by the size relationships Bp: Lm, Lm: L "s and D" p: D _m m. Mean

B Width of the flat conductor 1a P

L length of the pipe section 6 m ^a

L length of tubular windings 2 and 3

D inside diameter of the tube formed by the flat conductor 1a and

D „m outside diameter of the pipe section 6. ./. The best compromise between the small coupling factor K at the same time a large winding space on the one hand and good linearity anderer¬ hand ^ck ibt at Bp: Lm = 2: 1, Lm = Ls "and Dp: Dm ^ 2: 1.

The arrangement according to the invention is also suitable because of the small proportion of ferromagnetic material (iron powder, ferrite) for feeding a sawtooth signal of relatively high frequency into the winding 2, the zero crossings of the voltage at the detector winding in electronic multiplier arrangements using the so-called time division method 3 are immediately available as a duty cycle for controlling the second measurement variable.

/.

Claims

PATENT CLAIM

1. Active current transformer with a primary winding to be measured, the primary winding with the number of turns W, a secondary winding carrying the measuring current I ₂ with the number of turns W, and a detector winding which is practically firmly coupled to the secondary winding and whose induced winding Voltage generates a current in the secondary winding by means of a control amplifier, the magnetic flux of which compensates for the magnetic flux passing through the detector winding, characterized in that the detector winding (3) generates a large current transmission ratio

I ~ KW ₁ a partial magnetic flux yes' ..- des of

of the primary winding (1) generated magnetic flux £ f-, which is determined by the ratio of the partial flux to the total flux: J0 determined coupling factor K is significantly less than one.

2. Current transformer according to claim 1 with ferromagnetic material and with a current I_ to be measured. Leading flat tube shaped to a tube with radially outward Stromzufüh¬ leads, characterized in that a ferromagnetic tube section (6; 26, 27) is arranged within the conductor tube, which also the tubular secondary winding (2) and detector winding ( 3) carrying ferromagnetic rod core (7).

3. Current transformer according to claim 2, characterized in that for the fine adjustment of the transmission ratio, both the ferromagnetic pipe section (6) and the core rod (7) directly or via magnetic conductive additional parts have geometric and thus magnetic asymmetries, which by their shape (10 , 11) or, for example, by sheet metal flags (12) or screws (13) (Fig. 4 and 5).

4. Current transformer according to one of the preceding claims, characterized in that for further reduction of the coupling factor K, the winding surfaces of the detector winding (3) and the secondary winding tion (2) relative to the direction of the magnetic field of the current l _* in the primary winding (1) can be rotated by an angle (α).

5 5. Current transformer according to claim 2 or 3, characterized in that to reduce the influence of external external fields, the ferromagnetic tube section (6) and the core rod (7) with the windings (3; 2) shorter than the axial length of the flat conductor ( 1a) is formed primary winding (1), and cover (16; 17)

10 magnetically conductive shielding material, each end of the pipe section (6).

6. Current transformer according to one of claims 2 to 4, characterized gekenn¬ characterized in that the primary winding (1), the secondary winding (2), 5, the detector winding (3), the tube section (6; 26, 27) and the rod core (7 ) are arranged in a shielding pot (18, 19; 23, 24) made of ferromagnetic material.

7. Current transformer according to claim 6, characterized in that the 2 _Q shielding pot (18, 19) consists of two shell halves (18; 19) and that the contact surfaces (20) of the two shell halves (18; 19) and the longitudinal axis of the pipe section ( 6) lie in a common plane.

25 8. Current transformer according to claim 6, characterized in that the shielding pot (23, 24) consists of two shell parts (23, 24) whose contact surfaces (25) lie in a plane perpendicular to the longitudinal axis of the tube section (6; 26, 27) .

30 9. Current transformer according to claim 8, characterized in that the pipe section consists of two pipe journals (26; 27) which are formed on each of the shell parts (23; 24) and are separated from one another by an air gap (28).

35 10. Current transformer according to one of claims 2 to 5, characterized gekenn¬ characterized in that the ratio of the width of the flat conductor (1a) to the length of the tube section (6) is approximately 2: 1.

OMPI

11. Current transformer according to claim 10, characterized in that the length of the tube section (6) is approximately equal to the length of the tubular secondary winding (2) and detector winding (3).

12. A current converter according to claim 10 or 11, characterized in that the ratio of the inner diameter of the tube formed by the flat conductor (1a) to the outer diameter of the tube section (6; 26, 27) is equal to or less than 2: 1.

, /.