WO1994028429A1

WO1994028429A1 - Method and device for measuring the dynamic components of an electric current in the presence of a continuous component

Info

Publication number: WO1994028429A1
Application number: PCT/CH1994/000100
Authority: WO
Inventors: François COSTA; Eric Laboure
Original assignee: Liaisons Electroniques-Mecaniques Lem S.A.
Priority date: 1993-06-01
Filing date: 1994-05-31
Publication date: 1994-12-08
Also published as: CH689765A5; DE4493433T1; FR2706040B1; FR2706040A1

Abstract

The method comprises a first step for converting by magnetic coupling the current (I(t)) to be measured into a first induced current (I1(t)), and a second magnetic coupling current conversion step, in which the input current is the induced current from the first conversion step, and the induced current from the second conversion step (Is(t)) is applied to a measurement impedance (8) to provide an image of the dynamic component of the current (I(t)) to be measured.

Description

METHOD AND DEVICE FOR MEASURING COMPONENTS

DYNAMICS OF AN ELECTRIC CURRENT IN THE PRESENCE

OF A CONTINUOUS COMPONENT

The present invention relates to a method for measuring dynamic components, at frequencies higher than megahertz, in an electric current (I (t)) flowing in a conductor, this method comprising a first step of transformation by magnetic coupling of the current to be measured ( I / t)) into a first induced current (Iι (t)), and at least a second current transformation step by magnetic coupling in which the input current is the induced current from the preceding transformation step, the current induced from the last transformation step (Is (t)) being measured to obtain an image of the dynamic components of the current to be measured (I (t)).

It also relates to a measurement device implementing this method.

Current measurement represents at present a _(link essential in many control devices, automation and industrial metrology. It is expected that more and more a measure of current that it is operable in a wide frequency band and in the presence or not of a DC component. This is due to the considerable development of high frequency power electronic equipment, in particular switching power supplies, where the bandwidths of the signals to be observed reach several hundreds of MHz. , oscillations created by the existence of parasitic capacitances and inductances can reach frequencies of several tens of MHz, justifying a bandwidth of several hundreds of MHz to correctly observe these phenomena. In addition, there is an increasing need for measurement equipment intended for electromagnetic compatibility (EMC) studies.

Measuring methods are known which generally use a current transformer comprising a toroid at the center of which is placed the conductor traversed by the current to be measured and which is provided with a secondary winding with a large number of turns generating an output voltage which can be integrated to provide an image of the current to be measured. The probes produced on this principle have good performance at low frequencies but are not suitable for high frequencies due to the number of turns being too high.

There are also known efficient Hall effect current measurement methods at low frequency but having an insufficient bandwidth limited by the performance of the internal current control implemented. There are also measurement probes combining a Hall effect probe and a current transformer, which have a continuous bandwidth up to several tens of MHz but are very sensitive to the electromagnetic environment.

There is also a measurement method using an optical fiber as a current-sensitive element. This method, based on the Faraday effect, allows the measurement of very high currents under excellent isolation conditions, with a large bandwidth and good precision and is currently used in high voltage and current applications.

Also known is a method comprising measuring the voltage across a resistor through which the current to be measured in an ^'electronic device. The bandwidth corresponding to this process is only limited by the skin effect in the resistive element. However, this method has the drawback of not ensuring isolation between the power part and the measuring device. A main aim of the present invention is to remedy these drawbacks by proposing a method for measuring dynamic components of an electric current flowing in a conductor, which makes it possible to take account of the presence of a direct component and offers a very high bandwidth.

According to the invention, the method is characterized in that, in the first transformation step, the magnetic coupling is obtained by using a relatively high reluctance magnetic circuit to limit the incidence of a direct component of the current to be measured (I (t)) on the measurement of the dynamic components, and that, in each of the successive stages of transformation, the magnetic coupling is obtained by the use of a respective magnetic circuit having a significantly higher reluctance low to minimize the cutoff frequency, low measurement.

According to one form of implementation of this method, in the first transformation step, the magnetic coupling is obtained by the use of a magnetic circuit made of a material with high magnetic permeability, comprising at least one air gap. In this case, the magnetic field in said air gap can be measured to obtain a measurement of the DC component of the current to be measured.

The measurement device, implementing the method according to the invention, comprises a set of at least two intensity transformers arranged in cascade comprising a first intensity transformer having the conductor traversed by the current to be measured as its primary, and a last current transformer having a secondary winding closed on a measurement impedance, each transformer following the first having for primary a conductor electrically shorting the secondary winding of the preceding transformer, and it is characterized, according to a form of embodiment, in that the magnetic circuit of the first intensity transformer (Tl) is made of a ferromagnetic material having a sufficiently low relative permeability to limit to a desired level the impact of a continuous component of the current to be measured on the measurement dynamic components, and that the magnetic circuits of the intensity transformers of rank greater than one (T2) are made of a ferromagnetic material having a significantly higher relative permeability.

According to another embodiment, the device is characterized in that the magnetic circuits of the intensity transformers are made of a ferromagnetic material having a relatively high permeability and that the magnetic circuit of the first intensity transformer (Tl) comprises at minus an air gap. It can then include means for measuring the magnetic field in said air gap.

Other features and advantages of the invention will appear in the description given below of exemplary embodiments illustrated in the appended drawings in which:

- The figure shows schematically the general arrangement of a device according to the invention;

- fig. lb shows, also schematically, an alternative embodiment of the device of Figure la;

- Figure 2 is a sectional view of a measuring device implementing the method according to the invention;

- Figure 3 is a perspective view of a measuring device according to the invention; - Figure 4 is a sectional view of a housing arranged to contain the components of the measuring device according to the invention;

- Figure 5 illustrates an example of internal mounting of a device according to the invention;

FIG. 6 represents an experimental validation arrangement of a measurement device according to the invention; and

FIG. 7 shows two current waveforms corresponding respectively to a measurement by inductive probe and to a measurement carried out with the device according to the invention.

We will now explain the measurement method according to the invention by describing a particular embodiment of a device according to the invention, with reference to Figures la to 5.

The method used in the invention consists in cascading at least two intensity transformers, the magnetic circuits of which consist of ferrites of different performance. Thus, with reference to FIG. 1a, a measuring device 1 according to the invention comprises a first transformer T1 constituted for example by a magnetic circuit 4 of toroidal shape, having for primary a conductor 2 crossed by the current I (t) that one wishes to measure and for secondary a winding 5 comprising several turns, for example ten, and a second transformer T2 comprising a magnetic circuit 3, for example also of toroidal shape. This second transformer T2, placed in a plane orthogonal to the plane of the first transformer Tl, has for primary a single conductor 9 passing through the center of the torus 3, connected on the one hand, to a first terminal of the secondary winding 5 of the first transformer Tl, and on the other hand, to the second terminal of said secondary winding 5 through a short circuit 6. The second transformer T2 also has a secondary winding 7 wound around the toroid 3 and connected to a measurement impedance 8 traversed by an output current Is (t) which is the image of the current I (t) to be measured.

The ferrite constituting the magnetic circuit 4 of the first transformer Tl has a low relative permeability, in particular to avoid, in the targeted measurement range, saturation by the DC component of the current to be measured I (t), and too strong influence of this component on the low cut-off frequency, while the ferrite constituting the following toroid 3, or more generally all of the following tori in cascade, has a much higher permeability to ensure the lowest possible low cut-off frequency.

The current to be measured I (t) is applied to the primary 2 of the first intensity transformer Tl whose secondary 5, traversed by a current Il (t), is placed in short circuit through the primary 9, 6 of the second transformer of intensity T2.

This arrangement is repeated when several intensity transformers are cascaded. The secondary winding of the last current transformer is closed on a measurement impedance.

The method according to the invention thus makes it possible to obtain:

- a very wide bandwidth (from a few KHz to a few 100 MHz),

- a wide effective current range (up to 1000 A effective),

- a very low sensitivity of the passband to the DC component of the measured current,

- a very low electromagnetic susceptibility. In one. practical example of embodiment of the invention using two intensity transformers T1, T2 according to the arrangement described in FIG. la, a compact measuring device 20 comprises a first measuring module 35 containing in particular the two intensity transformers T1 , T2 and a second module 34 containing in particular the load resistor 8 and connection means 33 of the measurement device to external equipment (not shown), with reference to FIGS. 2 to 5. The first module 35 comprises, with reference to the sectional view of FIG. 2, a first housing 25 designed to receive the first intensity transformer Tl and a second housing 26 designed to receive the second transformer T2 and a common mode filtering torus Tm. These two housings 25, 26 are made in a non-magnetic conductive part 49 which provides the electromagnetic shielding of the measuring device. In the first housing 25, the first intensity transformer Tl is placed around a ring 21 of non-magnetic material with a radius substantially equal to or less than the radius of the interior passage of the wound toroid 5 and of length less than the height of the housing 25 of so that a predetermined spacing e separates the free end of this ring 21 from the wall of the first housing 25 which is opposite. This spacing, equal for example to 1/10 mm, is provided to ensure that the ring 21, which is electrically connected to the shield 49, is not in electrical short circuit with this shield, which would have the effect of creating a loop surrounding the magnetic circuit 4. The non-magnetic ring 21 makes it possible to eliminate the primary / secondary capacitance and to decrease the primary leakage inductance at high frequency (HF). The conductor through which the current to be measured is placed in the housing 31 formed by the inner part of the non-magnetic ring. The second housing 26 communicates with the first housing 25 through a narrow passage 27 sufficient to allow the passage of the conductors 9, 6 connected to the terminals of the secondary winding 5. In this second housing 26, the aforementioned conductors pass through the central hole of the torus Tm of common mode filtering intended to reduce the circulation currents in the shielding part 49 of the measuring device. The secondary current 5 of the first intensity transformer T1 constitutes the primary current of the second intensity transformer T2. This is placed in the second housing in such a way that the axis of symmetry of the torus of this second intensity transformer T2 is orthogonal to the axis of symmetry of the torus of the first intensity transformer Tl. The first conductor 9 , which constitutes the primary conductor of the second intensity transformer T2, is arranged coaxially and so as to produce a short circuit through the part 28 of the shielding piece 49 which surrounds the second intensity transformer T2 and which is connected electrically to the second conductor 6.

The measuring device 20 produced in practice is presented externally, with reference to FIG. 3, in the form of a parallelepipedic box consisting of a first box 35 and a second box 34 separated from the first by a copper plate 30 ; these two boxes having an identical section and being made of the same non-magnetic conductive material, for example Durai. The first box 35, traversed by a cylindrical housing 31, intended to receive the conductor 2 which is the object of the current measurement, is closed by a flat cover 38 comprising a cylindrical hole 39 coinciding with the cylindrical housing 31 and four holes 37a, 37b, 37c, 37d placed at the four corners of said cover 38 and arranged to allow the screwing of this cover relative to the first housing 35. The copper plate 30, having a surface greater than the common section of the first and second housings 35, 34 constitutes for the measuring device according to the invention a mass reference. The second housing 34, provided in particular for receiving the measurement resistor 8, comprises two holes 61, 62 allowing the mechanical connection of the assembly constituted by the first housing 35, the copper plate 30 and the second housing 34, and a connector 33 of coaxial type comprising an external armature 57 electrically connected via the body of the second housing 34 to a first terminal of the measurement resistor 8, and an internal coaxial core 36 electrically connected to the second terminal of said resistance 8. Thus, the voltage observed at the level of the connector 33 is an image of the current passing through the secondary winding of the intensity transformer T2.

In the practical embodiment described with reference to FIG. 4, the first and second housings 35, 34 are produced by machining homogeneous parallelepipedal blocks of the same material, for example Durai, in which recesses are made to receive the constituent elements of the measuring device according to the invention. FIG. 4 represents a top view of the bare housings 35, 34 which constitute both the structures for receiving the components and an electromagnetic shielding. Thus, the first housing 35 comprises a first annular housing 41 designed to receive the first intensity transformer Tl and having in its internal part the non-magnetic ring 21, a second housing 42, of substantially parallelepiped shape, intended to receive the torus Tm common mode filtering and a device for coaxial connection of the secondary winding of the first current transformer Tl to the primary conductor of the second current transformer T2, and a third housing 43, of cylindrical shape, intended to receive the second transformer of intensity T2. The second housing 42 communicates with the first housing by a small hollowed-out passage of depth substantially less than the depth of the first and second housing and designed to allow the passage of the conductors connected to the secondary winding of the first current transformer Tl. third recess 43 communicates with the second housing 42 by a small circular hole 60 for receiving a threaded rod (not shown in Figure 4) acting between the coaxial conductor 1 ^* secondary winding of the first current transformer Tl and the primary of the second intensity transformer T2. The first housing 35 also includes four threaded holes 37a, 37b, 37c, 37d designed to receive four screws (not shown) intended for fixing the cover 38 (cf. FIG. 3) relative to said first housing 35, and two threaded holes 47 , 48 designed to receive two screws (not shown) for fixing the second housing 34 and the copper plate 30 to the first housing 35. The copper plate 30 comprises two fixing holes 63, 64 respectively opposite the holes 47, 48 formed in the first housing and holes 62, 61 made in the second housing 34, a main hole 45 provided for the passage of a threaded rod and an auxiliary hole provided for the passage of the output conductors of the secondary winding of the second current transformer T2 . The second housing 34 comprises a housing 46 designed to receive the measurement resistor 8, a hole 44 made on its end wall and situated in the axis of the main hole 45 of the copper plate 30, and of the communication hole 60 between the second and third housings 42, 43, and holes 62, 61 respectively opposite the holes 63, 64 of the copper plate and the holes 47, 48 of the first housing 35, these holes 62, 61 being provided for receiving screw.

We will now describe the assembly of the various elements of the measuring device according to the invention within the housings, with reference to FIG. 5.

The first intensity transformer Tl is placed in the housing 41. A first insulating element 51, for example a thin insulating film, is placed between the outer periphery of the first intensity transformer Tl and the wall 71 of the cylindrical housing 41 The secondary winding 5 is preferably made up of a small number of turns, for example ten, so as to limit the value of the parasitic inductances and capacitances. These turns are distributed uniformly over the toroidal magnetic circuit 4 and preferably produced by placing several elementary conductors, for example four, in parallel, in order to reduce the leakage inductance of the intensity transformer T1 and the resistance by skin effect. At the outlet of the secondary winding 5, the parallel conductors are combined to respectively constitute the conductor 9 intended to be connected to the conductive rod of a screw 56 and the conductor 6 intended to be connected to the coaxial shield constituted by a part of the first housing 35. These two conductors 9, 6 pass into the internal orifice of the filtering torus Tm in common mode and then are connected respectively to a first washer 52 and to a second washer 54 separated from each other by an insulating washer 53, all of these three washers 52, 54, 53 being held in contact against a wall of the second housing 42 by a fixing system comprising the screw 56, a nut 55 located in the second housing 42 and at contact of the first washer 52, and the head 70 of the screw 56, located in the housing 46 of the second housing 34 and in contact with the copper plate 30. The second intensity transformer T2 is placed in the third housing 43 of the first housing 35 and galvanically isolated from the internal walls of the third housing by an insulating film 51 '. Its secondary winding 7 is arranged in substantially the same way as the secondary winding of the first transformer T1: low number of turns, conductors placed in parallel and uniform distribution over the toroid. The output conductors of the secondary winding 7 are passed through the auxiliary hole 65 of the copper plate 30 and terminate in the housing 46 of the second housing 34 to be connected to the terminals of the measurement resistor 8, the value of which can for example be chosen equal to 50 ohms. A first terminal 72 of the measurement resistor 8 is electrically connected to the base 58 of the connector 33, while the second terminal 73 of the resistance 8 is electrically connected to the core 36 of said connector 33. The latter is fixed to the side wall of the second housing 34 by a nut 74 and its base 58 is in electrical contact with the second housing 34 and the copper plate 30. It is thus possible to provide a mounting with an insulating washer 59 on the outside and a nut 74 made of material non-conductive, for example nylon. Many other arrangements of the output connector can be envisaged for delivering the voltage across the measurement resistor to the outside of the measurement device.

The magnetic circuit of the first intensity transformer Tl is, in the present example, made of ferrite material having a low relative permeability (for example, Ni-Z having a μ _R of approximately 300) and therefore not very sensitive to a direct component of the primary current. The circuit of the second intensity transformer T2, and more generally of any other cascade transformers, is made of Zn-Mn ferrite material having a much higher permeability so that the cut-off frequency is as low as possible.

It should be noted that instead of using, in the first transformer, a continuous magnetic circuit with low permeability, it is possible to use a circuit made of a material with high permeability, but having an air gap. In such a case, it is also possible to house, in this air gap, a magnetic field detector such as a Hall cell, to allow the measurement of the DC component of the current as described in US Pat. No. 5,146,156 .

FIG. 1B shows such an alternative embodiment of the device in FIG. 1a, according to which the magnetic circuit 4 'of the first transformer T' _λ has an air gap 10. A Hall cell 11 is placed in the air gap 10 and is connected, by means of conductors represented by line 12, to a measuring device 13 for measuring the DC component or components at low frequencies.

We will now describe the operation of the measuring device according to the invention with reference to FIGS. 1a, 3 and 5 and present an experimental validation mode with reference to FIGS. 6 and 7.

The presence of a current I (t) in the conductor 2 has the effect of generating a magnetic induction in the magnetic circuit 4 of the first transformer Tl. If the current I (t) varies temporally, the variation of the magnetic induction causes the creation of an induced current Il (t) in the secondary winding. This induced current flows through the conductors * 9 and 6 passing through the center of the filter core in common mode Tm and contributes to magnetize the magnetic circuit 3 of the second intensity transformer T2. A variable magnetic induction appears in the magnetic circuit 3 in response to the flow of current in the conductors 6 and 9, and contributes to the creation of an induced current i _s (t) in the secondary winding 7 of the second transformer of intensity T2 which is closed on the measurement resistor 8. The voltage observed at the terminals of the measurement resistor 8 is then a homothetic image of the current since the different physical quantities: current, magnetic induction, voltage are linked by linear relations. Indeed, the materials used for the magnetic circuits of the intensity transformers are chosen so that a quasi-perfect linearity is observed between the primary current and the secondary current. The low relative permeability of the material used for the magnetic circuit of the first current transformer makes it possible to avoid saturation due to the existence of a continuous component of the current to be measured. The use of cascade current transformers, two in the example described, makes it possible to use secondary windings with a small number of turns and consequently to significantly reduce the value of the parasitic components appearing in the equivalent assembly of the device.

The measuring device 20 according to the invention has been used in an experimental validation setup 80 comprising a continuous power source 81, a filtering capacitor 82, a load 85, in particular an inductive load such as an electric motor. direct current, a freewheeling diode 84 mounted in antiparallel on the load 85, and a power transistor 86, for example a MOSFET transistor. The drain of transistor 86 is connected to a terminal of the load 85 connected to the anode of the freewheeling diode 84, its gate is connected to a gate control circuit 83 and its source is connected to the ground of the assembly. The source current I (t) is measured on the one hand by the measuring device 20 according to the invention, and on the other hand, by observation of the voltage drop across a calibrated inductance L present between the source and mass and very low value. The voltage Vs (t) generated by the measuring device 20 is multiplied by a coefficient (1 /) to deliver a dynamic image I _τ (t) of the current to be measured I (t).

The voltage drop across the inductance L is observed by means of a probe 87, possibly attenuating if the peak voltage level justifies it, and is then integrated to deliver an image I _R (t) of the current I (t ) to measure. This measurement method, which does not however offer any galvanic isolation, can provide, because of its simplicity of implementation, a reference measurement making it possible to validate the method according to the invention.

If we study the current response to a rising edge applied to the gate of transistor 86 cause the conduction of the latter, we obtain the waveforms 7.a and 7.b corresponding respectively to the currents I _R (t) and I _τ (t) obtained with the inductive integration probe and with the measuring device according to the invention. There is a very good correspondence between the reference waveform I _R (t) and the waveform I _τ (t) from the device according to the invention, which highlights the performance of the device according to the invention in terms of bandwidth and accuracy.

It is thus possible to produce measuring devices having a low sensitivity to electromagnetic disturbances and high performance. By way of example, the following characteristics have been obtained for a measuring device actually produced:

a sensitivity of 100 mV / A on a load of 50 Ohms;

a bandwidth (at -3 dB): from 4 kHz to 300 MHz; low sensitivity to direct current: a direct current of 20 A raises the low cut-off frequency by a factor of 2; and a nominal true effective current of 50 A.

Claims

1. Method for measuring dynamic components, at frequencies higher than megahertz, in an electric current (I (t)) flowing in a conductor (2), this method comprising a first step of transformation by magnetic coupling of the current to be measured ( I (t)) into a first induced current dι (t)), and at least a second current transformation step by magnetic coupling in which the input current is the induced current from the previous transformation step, the current armature of the last transformation step (Is (t)) being measured to obtain an image of the dynamic components of the current to be measured (I (t)), characterized in that, in the first transformation step, the magnetic coupling is obtained by implementing a relatively high reluctance magnetic circuit (4) to limit to a desired level the incidence of a direct component of the current to be measured (I (t)) on the

^• measurement of the dynamic components, and that, in each of the successive transformation steps, the magnetic coupling is obtained by implementing a respective magnetic circuit (3) having a significantly lower reluctance to minimize the low cut-off frequency of the measured.

2. Method according to claim 1, characterized in that, in the first transformation step, the magnetic coupling is obtained by the use of a magnetic circuit made of a material with high magnetic permeability, including at least one air gap.

3. Method according to claim 2, characterized in that the magnetic field is measured in said air gap to obtain a measurement of the DC component of the current to be measured.

4. Device for measuring dynamic components, frequencies higher than Megahertz, in an electric current (I (t)) flowing in a conductor (2), characterized in that i comprises a set of at least two transformers of intensifies (Tl, T2) of current arranged in cascade, a first intensity transformer (Tl) having for its primary the conductor (2) ^* traversed by the current to be measured, and a last intensity transformer (T2) having a secondary winding (7) closed on a measurement impedance (8), each transformer following the first having for primary a conductor electrically shorting the secondary winding of the preceding transformer, characterized in that the magnetic circuit (4) of the first transformer intensity (Tl) is made of a ferromagnetic material having a sufficiently low relative permeability to limit to a desired level the impact of a continuous component of the current to be measured on the measurement of the dynamic components, and that the magnetic circuits (3) intensity transformers of rank greater than one (T2) are made of a ferromagnetic material having a significantly higher relative permeability lifted.

5. Device for measuring dynamic components, at frequencies higher than megahertz, in an electric current (I (t)) flowing in a conductor (2), characterized in that it comprises a set of at least two transformers d intensity of current arranged in cascade, a first current transformer having primary to the conductor through which the current to be measured, and a final processor ^'intensity having a closed secondary winding on a measuring impedance, each processor according to the first having for primary a conductor electrically shorting the secondary winding of the preceding transformer, characterized in that the magnetic circuits of the intensity transformers are made of a ferromagnetic material having a relatively high permeability and that the magnetic circuit of the first transformer intensity (Tl) comprises at least one air gap.

6. Device according to claim 5, characterized in that it comprises means for measuring the magnetic field in said air gap.

7. Device according to any one of claims 4 to

6, characterized in that the secondary winding (5) of the first intensity transformer (Tl) comprises a predetermined number of turns constituted by the parallel connection of a predetermined number of elementary conductors and distributed regularly over the magnetic circuit ( 4) of said first transformer (Tl).

8. Device according to any one of claims 4 to

7, characterized in that it further comprises, at each connection between the secondary winding (5) of an intensity transformer (Tl) and the primary of the following intensity transformer (T2) produced by means two conductors (9, 6), a common mode filtering torus (Tm) crossed in its center by these two conductors (9, 6).

9. Device according to any one of claims 4 to

8, characterized in that it further comprises means (35) for shielding at least part of the components of said device and in particular the current transformers.

10. Device according to claim 9, characterized in that it comprises a box (35) made of non-conductive non-magnetic material and designed to contain part of the components (Tl, T2, Tm) of said device (1), this box (35 ) being provided with a hole (31) for receiving the conductor (2) through which the current to be measured (I (t)) passes.

11. Device according to claim 8, characterized in that the magnetic circuit (4) of the first intensity transformer (Tl) is a toroid having a circular window substantially in correspondence with the hole (31) of said housing (35), and in that a conductive non-magnetic ring (21) is provided inside said window to receive the conductor (2) through which the current to be measured (I (t)) passes, this ring (21) having one of its ends in mechanical and electrical contact with a first portion of the internal wall (49) of the housing (35).

12. Device according to claim 11, characterized in that the other end of the ring is spaced from a second portion of the inner wall of the housing (35).

13. Device according to any one of claims 4 to 12, characterized in that it comprises a first housing (35) for containing the two intensity transformers (T1, T2), and a second housing (34) containing the measurement impedance (8) and output means (33) for acquiring the voltage across said measurement impedance (8), these two tin boxes (35, 34) made of a non-conductive magnetic material and separated by a conductive part intermediate (30), in particular made of copper, provided for a ground connection.

14. Device according to claim 13, characterized in that the first housing (35) comprises a first housing (41) of substantially cylindrical shape for receiving the first intensity transformer (Tl), and a second housing (42) for receiving intermediate connection means (52, 53, 54, 55) between the first intensity transformer (T1) and the second intensity transformer (T2), this second housing (42 communicating with the first housing (41) by a first pass (27).

15. Device according to claim 14, characterized in that the first housing (35) further comprises a third housing (43), having substantially the shape of an axis cylinder perpendicular to the axis of the first housing (41) , to receive the second intensity transformer (T2), this third housing (43) communicating with the second housing (42) through a hole (60 formed in the wall which separates them and having for wall the intermediate conductive part (30) .

16. Device according to claim 15, 'characterized in that the intermediate connection means comprise a first central part (56) connected by a first conductor (9) to a first terminal of the secondary winding (5) and placed in the window of the second current transformer (T2), and a second part (54) providing, in cooperation with a portion of the first housing (35) and the conductive part intermediate (30), a coaxial shorting around ^'the first central part (56) so as to form a current loop around the magnetic circuit (3) of the second current transformer (T2), this second part (54) being connected by a second conductor (6) to the second terminal of. the secondary winding (5) of the first current transformer (Tl).