Isolated DC current and voltage sensor with low crosstalk

Abstract

A device for magnetic measurement that includes a DC magnetic field sensor having at least four discrete elements, each of the discrete elements being constituted by at least one coil and a magnetic material without remanence, where the at least four discrete elements are substantially identical. The magnetic field sensor includes a first discrete element orientated in a given direction, and a second discrete element associated therewith to constitute a first differential pair. The second discrete element being orientated in parallel but in an opposite direction relative to the first discrete element. The device further including two other discrete elements constituting a second differential pair substantially identical to the first differential pair. The two other discrete elements being parallel to the orientation of the first differential pair but are respectively orientated in an opposite direction to the first and second discrete elements of the first differential pair.

Claims

1. A device for magnetic measurement comprising: a DC magnetic field sensor constituted by at least four discrete elements, each of said discrete elements being constituted by at least one coil and a magnetic material without remanence, the at least four discrete elements being substantially identical, wherein the magnetic field sensor comprises: a first discrete element orientated in a given direction, and a second discrete element associated therewith to constitute a first differential pair, the second discrete element is orientated in parallel but in an opposite direction relative to the first discrete element; two other discrete elements constituting a second differential pair substantially identical to the first differential pair, but the two other discrete elements being parallel to the orientation of the first differential pair but the two other discrete elements of the second differential pair being respectively orientated in an opposite direction to the first and second discrete elements of the first differential pair; and a conversion component configured for converting a quantity to be measured to a magnetic field, wherein the magnetic field has a projection along a path constituted by the first discrete element, the second discrete element and the two other discrete elements, wherein said projection is non-zero.

2. The device according to claim 1, wherein the conversion component is constituted by a conductor composed of at least one element of two bars forming a U shape.

3. The device according to claim 2, wherein the conversion component is constituted by at least two conductive elements placed above and below a measurement cell.

4. The device according to claim 2, wherein the conversion component is constituted by two substantially identical additional bars generating a substantially zero field at a level of the discrete elements.

5. The device according to claim 2, wherein the conversion component is constituted by several layers of conductors that are isolated from each other.

6. The device according to claim 2, wherein the conversion component is placed in bypass on a primary conductor so that the primary conductor generates a magnetic field component perpendicular to the discrete elements.

7. The device according to claim 6, wherein the conversion component and the primary conductor are constituted by substantially identical conductive materials.

8. The device according to claim 1, wherein the conversion component is constituted by at least one multi-turn coil connected in parallel to a primary conductor via a resistor with a high impedance.

9. The device according to claim 1, wherein the discrete elements are transducers each constituted by at least one coil and a superparamagnetic core.

10. The device according to claim 1, wherein the discrete elements are substantially placed in the same plane.

11. A method for measuring high current consisting of placing the device of claim 1, in bypass of a primary conductor to capture a fraction of primary current.

12. The device according to claim 1, wherein the conversion component is constituted by a conductor composed by two elements, each of said two elements having two bars forming a U shape.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics will become apparent on examination of the detailed description of an embodiment which is in no way limitative, and the attached drawings, in which:

(2) FIG. 1 shows several examples of dual differential topologies according to the invention,

(3) FIG. 2 shows several examples of forms of primary conductors for generating the field to be measured,

(4) FIG. 3 shows several examples of measurement in two planes with to the right a structure with several stages for the uniformization of the to primary field,

(5) FIG. 4 shows a measurement example with several stages for the uniformization of the primary field with a measurement in a single plane,

(6) FIG. 5 shows an example of a transducer with a conversion component IpHp constituted by two parts situated above and below the measurement cell and two busbars,

(7) FIG. 6 shows a example of a transducer for measuring current, and

(8) FIG. 7 shows an example of a wound current transducer for measuring small currents and/or voltages, to the left, the IpHp converter with the 4 coils arranged for example on a PCB support as well as the direction of the primary fields; to the right the orientation of the 4 discrete measurement elements in side view.

DETAILED DESCRIPTION

(9) In FIG. 1 several examples of dual differential topologies for the production of a magnetic field sensor are shown. A compact sensor is obtained by using a measurement cell based on the use of elongate discrete elements allowing the local measurement of an orientated magnetic field while still seeking to optimize immunity to crosstalk due to external conductors. For this purpose at least four discrete elongate elements are used, substantially identical and distributed in space according to a dual differential topology as shown in FIG. 1. Starting from a first discrete element orientated in a given direction, the second element which is associated with it in order to constitute a first differential pair orientated in a substantially identical direction but opposite way. Thus this first pair will be insensitive to uniform external fields whatever their directions. It should be noted that this differential pair will only be sensitive to the gradients of the external fields in a favoured direction. The second differential pair will be substantially identical to the first but once more orientated in a direction substantially identical, but opposite way, to the first pair. Thus this measurement cell will be insensitive to the uniform fields but also to the uniform gradients of an external field. A sensitivity to the gradient of the field gradient will remain and in this regard it is preferable to seek to bring the elements together to the maximum extent in order to reduce this residual sensitivity.

(10) In order to allow the measurement of a variable other than a magnetic field, this measurement cell is associated with a component allowing the conversion of the measurand to a magnetic field, called primary magnetic field Hp, the projection of which along the path constituted by the 4 discrete elements is non-zero. Preferentially in order to improve the linearity, the primary field will be substantially uniform at the level of each of the four discrete elements.

(11) For a current sensor, this involves for example an electrical conductor through which a current passes that is an image of the primary current, a component called IpHp. Preferentially this conductor IpHp is constituted by bar elements forming at least one U as shown for example in FIG. 2.

(12) For a voltage sensor or for a sensor of low intensity-current, this involves for example a conductor through which a current passes that is an image of the primary voltage and the component will be called UpHp. Preferentially this conductor UpHp is a multi-turn coil connected either in parallel with a primary voltage via a resistor of very high impedance, or directly in series with a low-intensity current. An embodiment example is shown in FIG. 7.

(13) It is possible to imagine that the sensitive elements are Hall effect transducers, or Flux Gate, AMR/GMR, or Nel Effect type transducers. The demerit of the FluxGate or AMR/GMR transducers is that they are very sensitive to external fields perpendicular to the direction of measurement. Thus the benefit of the dual differential form will be nullified by the non-linear effects introduced by the external fields.

(14) Preferentially, the discrete elements will be Nel Effect transducers each constituted by at least one coil and a superparamagnetic core.

(15) The device according to the invention makes it possible to ensure accurate measurement despite a stressed environment and in particular in the presence of low thermal resistivity. It is important to operate at zero flux with a field feedback type system, in order to eliminate the effects of the drifts of the transducer materials (Hall or Nel Effect). Therefore a field compensation coil must be used and in this respect a discrete element of the coil type with Nel Effect is preferentially used.

(16) In order to improve the performance of the sensor in terms of linearity, in the case of current sensors a solution is used making it possible to uniformize to the maximum the primary field and to orientate to the maximum in the direction of the measurement coils. Then preferentially a conversion component IpHp will be used, constituted by at least two conductive elements placed above and below the measurement cell as shown in FIGS. 3 and 4.

(17) In order to reduce the heat dissipation in the sensor, the conversion component of the primary current to primary field may be connected in bypass of the primary conductor. Thus, only a fraction of current will be bypassed, thus making it possible to reduce the power necessary for the feedback in order to operate at zero flux. Preferentially this bypass will be placed between two substantially identical busbars so that the magnetic field generated at the level of the measurement cells by the current which is not bypassed in the conversion component IpHp is minimal, or even zero. An embodiment example is described in FIG. 5. As regards cost, a busbar with simple grooves can then be used to constitute the conversion component IpHp.

(18) In order to ensure that the ratio between the bypassed current and the total primary current is indeed independent of the temperature, an identical conductive material will preferentially be chosen for all the parts of the conversion component IpHp. This is one of the reasons why one cannot usefully carry out this measurement in bypass with shunt technology.

(19) The device according to the invention makes it possible to withstand the stresses of primary voltages. Preferentially, a measurement cell is used, following a topology making it possible to place the discrete elements in one and the same compact volume. Thus, it will be possible in a restricted space to ensure an isolation distance (minimum distance between the conversion elements and the primary conductor or conductors) that is as large as possible. Similarly, it will be easier to place the measurement elements in a Faraday cage connected to a fixed potential. Thus preferentially, the measurement cell will be surrounded by a conductive electrostatic screen. It is possible to produce the assembly of the discrete elements by using a technology of the PCB (Printed Circuit Board) type, which has the advantage of allowing a high degree of control over the symmetry of the four coil elements. The magnetic circuit can be fitted into the PCB according to a known embedded component technique, or directly by filling the isolating composite (for example epoxy) with magnetic particles (for example with superparamagnetic nanoparticles). Similarly, the electrostatic screen can be constituted by copper layers on both sides of the PCB, closed at the ends by specific metallizations.

(20) The device according to the invention allows a DC and AC measurement at high frequency and despite large variations in current dI/dt. In order to allow DC and AC measurement at high frequency, the conversion component IpHp must not be very sensitive to the frequency. For this purpose, care is taken to use a thickness for the conversion components IpHp which is substantially less than the skin thickness calculated at the maximum frequency of use. If for thermal reasons for example and/or space requirement it is not possible to respect this constraint, then the conversion components IpHp will preferentially be constituted by several layers of conductors that are isolated from each other. It will then be possible to measure both the DC and AC components at the level of the measurement cell.

(21) The device according to the invention allows a measurement in bypass on a busbar. In order to carry out a retrofit measurement of the current circulating in a conductor of busbar type, the sensor described above can be used and simply fixed on said busbar in order to bypass a fraction of its primary current. The magnetic field at the level of the discrete measurement elements will then be composed of the primary field to be measured, offset from the field originating from the busbar. In order to limit to the maximum the nonlinearities that the busbar could introduce, preferentially a choice will be made to orientate the discrete elements in a direction substantially perpendicular to that of the primary field generated by the busbar where there is no bypass. Thus, only the bypass current will generate a field in the axis of orientation of the discrete elements.

(22) FIG. 6 shows an example of a transducer for measuring current. In the figure on the left, a device is shown which can be identical to that of FIG. 5, the figure on the right illustrating more specifically a busbar in the shape of a U.

(23) Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without exceeding the scope of the invention.