Current sensing system

Abstract

A current sensing system (1) for sensing current in a multicore cable (3) is disclosed. The system comprises a current sensor (4) comprising a sensor body (11) having an aperture (12) which has a centre line (13) and at least one set (17) of magnetic field sensors (18) and apparatus (5) for processing signals from the magnetic field sensors (18). Each set of magnetic field sensors comprises at least five magnetic field sensors arranged around the aperture in a respective plane perpendicular to the centre line and configured to measure at least in plane magnetic fields. Each magnetic field sensor has a respective in-plane axis of sensitivity (19) which may be inherent or which may be established by processing signals from the magnetic field sensor that correspond to in-plane orthogonal fields. In each set of magnetic field sensors, the axes of sensitivity rotate in a given sense an integer number of times with angular position of the magnetic field sensors in a given sense around the centre line so as to measure a multipole magnetic field distribution.

Description

[0001] Current sensing system

[0002] Field

[0003] The present invention relates to a current sensing system.

[0004] Background

[0005] A Rogowski coil can be used to measure an alternating current in a wire. The coil is toroidally wound around an aperture through which the current-carrying wire passes. The current in the wire generates a voltage across the coil proportional to the time- derivative of the current. Due to the symmetry of the coil, currents flowing outside the toroid do not generate a voltage in the current sensor, and so the arrangement provides excellent immunity to stray fields from other closely-located currents.

[0006] Sentec (TM) SlimSensor (TM) is an example of a sensor which is based on the same principles as the Rogowski coil. In this case, the toroidal coil is formed from a number of separate, bobbin-wound solenoid coils arranged in a toroidal geometry.

[0007] A Rogowski coil arrangement is sensitive only to cylindrically-symmetric magnetic fields generated by net currents flowing through the aperture. In some cases, however, it may be useful to sense magnetic fields generated by other current arrangements in multi-core cables. For example, two parallel wires in which the currents flow in antiparallel directions can be viewed as a current dipole resulting in a dipole-like magnetic field distribution. Similarly, an arrangement of four parallel wires can generate a quadrupole-like magnetic field distribution. Any fields from arrangements of parallel currents can be expanded in terms of these quantities.

[0008] Sensors capable of measuring currents in multi-core conductors tend to be based on an arrangement of tangentially- and radially-oriented sensors. For example, WO 2004 / 086063 Al describes a four-coil arrangement. First and second coils are arranged to have substantially parallel axes and are located on the circumference of a notional circle with the first coil means having its axis tangential to the circle and the second coil means having its axis extending radially of the circle. Third and fourth coils have substantially parallel axes and are located on the circumference of the notional circle close to the first and second coil. The third coil has its axis extending radially of the circle and the fourth coil has its axis tangential to the circle such that the first and third coil form a close, adjacent first pair of coils with substantially orthogonal axes. The second and fourth coils form a close, adjacent second pair of coil with substantially orthogonal axes.

[0009] 182174PCT1 Summary

[0010] According to a first aspect of the present invention there is provided a current sensing system for sensing current in a multicore cable. The system comprises a current sensor comprising a sensor body having an aperture which has a centre line and at least one set of magnetic field sensors and apparatus for processing signals from the magnetic field sensors. Each set of magnetic field sensors comprises at least five magnetic field sensors arranged around the aperture in a respective plane perpendicular to the centre line and configured to measure at least in plane magnetic fields. Each magnetic field sensor has a respective in-plane axis of sensitivity which may be inherent or which may be established by processing signals from the magnetic field sensor that correspond to in-plane orthogonal fields. In each set of magnetic field sensors, the axes of sensitivity rotate in a given sense an integer number of times with angular position of the magnetic field sensors in a given sense around the centre line so as to measure a multipole magnetic field distribution.

[0011] Rotation of the axes of sensitivity by an integer number of times can improve immunity or even achieve substantial immunity from magnetic fields generated by conductors outside of the aperture.

[0012] Expressed differently, adding the angles of the axes of sensitivity for magnetic field sensors in one set results in an integer number of rotations (i.e., integer multiple of 2n in radians or integer multiple of 360° in degrees).

[0013] The given sense may be clockwise or anticlockwise.

[0014] The aperture may be elongate, for example, tubular. The aperture may comprise a plurality of discrete sections. Each discrete section may comprise a printed circuit board.

[0015] Each magnetic field sensor may have at least two axes of sensitivity, at least one of the axes of sensitivity not being radial or tangential with respect to the centre line.

[0016] The magnetic field sensors are preferably angularly equally spaced.

[0017] Each magnetic field sensor may be rotated dependent on its angular position so as to rotate its axis of sensitivity. Thus, the axes of sensitivity may be inherently de may be physically configured. The apparatus may include a sensitivity axis selector for each magnetic field sensor, wherein the sensitivity axis selector is configured to receive a target axis of sensitivity and first and second orthogonal magnetic field values and to output a magnetic field value corresponding to the target axis of sensitivity.

[0018] The axis of sensitivity, MJ, may depend on its angular position 0 around the centre line (13) according to: J = N 0 + qjo where N is a positive, non-zero integer and ipo is a constant. The current sensor may comprise a pair of sets of magnetic field sensors which have the same value of N and values of ipo which differ by n / 2.

[0019] The current sensor may comprise a pair of sets of magnetic field sensors which have the same value of N and values of ipo which differ by n / 2.

[0020] The at least one set of magnetic field sensors may comprise at least two sets of magnetic field sensors. At least two sets of magnetic field sensors may be arranged concentrically. At least two sets of magnetic field sensors may be arranged on first and second faces of a board of the sensor body, the second face being opposite to the first face. At least two sets of magnetic field sensors may be arranged on first and second different boards of the sensor body. The current sensing system may comprise a first set of magnetic field sensors lying in a first plane perpendicular to the centre line, and a second set of magnetic field sensors lying in a second plane perpendicular to the centre line which is different to the first plane.

[0021] Each set of magnetic field sensors may have a geometric centre and at least two sets of magnetic field sensors may be interleaved in an axis parallel to the centre line such that the geometric centre of the magnetic sensors of two or more sensor arrangements are collocated in a plane perpendicular to the axis of the aperture.

[0022] Magnetic field sensors in a set of magnetic field sensors may be connected in series between a pair of terminals. Thus, the output of the set of magnetic field sensors can be the sum of the outputs of the magnetic field sensors. Magnetic field sensors in a set of magnetic field sensors may be connected in parallel between a pair of terminals. Thus, the output of the set of magnetic field sensors can be a parallel combination of the outputs of the magnetic field sensors.

[0023] The magnetic field sensors in the set of magnetic field sensors may be connected in the digital domain. Thus, analogue signals from the magnetic field sensors may be digitised into digital signals, and the digitised signals can be combined digitally, for example using a software application, to connect the magnetic field sensors in series or in parallel achieve a series.

[0024] Magnetic field sensors in a first set of magnetic field sensors may be connected in series or parallel between a first pair of terminals and magnetic field sensors in a second set of magnetic field sensors are connected in series or parallel between a second pair of terminals, wherein one of the first pair of terminals and one of the second pair of terminals are connected by a wired connection around the aperture without the first and second set of magnetic terminals forming a closed loop.

[0025] Magnetic field sensors in a set of magnetic field sensors may be connected in series between first and second terminals and has a tap between an adjacent pair of magnetic field sensors which is connected or connectable to a voltage reference. The voltage reference may be ground.

[0026] Magnetic field sensors in a set of magnetic field sensors may be connected in series such that every odd-numbered sensor in the set are connected in series between a first terminal and an intermediate terminal and have a first polarity to form a first subset of magnetic field sensors and every even-numbered sensor in the set may be connected in series between the intermediate terminal and a second terminal and have second, opposite polarity and form a second subset of magnetic field sensors, wherein the intermediate terminal may be connected or connectable to a voltage reference.

[0027] The sensor body may comprise first and second parts which divide the sensor body through the aperture and a set of magnetic field sensors may be arranged around the aperture with a constant angular pitch.

[0028] The sensor body may comprise first and second parts which divide the sensor body through the aperture for allowing the sensor body to be assembled around a cable. The sensor body may have a slit between the aperture and an outer edge. The slit may be sufficiently wide to allow a cable outside of the sensor body to be positioned within the aperture of the sensor body. Alternatively, the sensor body may be sufficiently flexible to allow a cable outside of the sensor body to be positioned within the aperture of the sensor body.

[0029] There may be at least one pair of sets of magnetic field sensors, wherein the magnetic field sensors in the pair of sensor arrangements may mounted on both sides of a printed circuit board such that alternate sensors in the first set are mounted on either side of the printed circuit board, and alternate sensors in the second set are mounted on either side of the printed circuit board, with the magnetic field sensors in the first and second sets at each position around the aperture mounted opposite each other.

[0030] Each magnetic field sensor may comprise at least one inductor. The at least one inductor is preferably suitable for mounting on a printed circuit board.

[0031] Each magnetic field sensor may comprise at least one magnetoresistive device. The at least one magnetoresistive device is preferably suitable for mounting on a printed circuit board.

[0032] The magnetic field sensors may be wound coils on a bobbin structure configured to provide coils having a predetermined axis of magnetic field sensitivity.

[0033] The magnetic field sensors may be surface mount inductors.

[0034] Each set of magnetic field sensors may be mounted on one side of a printed circuit board.

[0035] Each set of magnetic field sensors may be divided between at least two printed circuit board such that bringing the at least two printed circuit boards together forms the sets of magnetic field sensors.

[0036] The current sensor may comprise a plurality of printed circuit boards, wherein each printed circuit board support a set of magnetic field sensors on each side of the printed circuit board at the same radial distance from the centre line [claim 32]

[0037] The current sensor may include at least one pair of sets of magnetic field sensors in which the axis of sensitivity of the magnetic field sensors rotates five times around the sensor aperture with respect to a centre line, with the amplitude of the measurements of the signals from this pair compared to the amplitude of the measurements in at least one other pair of sensors in which the axis of sensitivity of the magnetic field sensors rotates less than five times around the sensor aperture with respect to a fixed axis.

[0038] The current sensor may include at least one pair of sets of magnetic field sensors in which the axis of sensitivity of the magnetic field sensors rotates four times around the sensor aperture with respect to a centre line, with the amplitude of the measurements of the signals from this pair compared to the amplitude of the measurements in at least one other pair of sensors in which the axis of sensitivity of the magnetic field sensors rotates less than four times around the sensor aperture with respect to a fixed axis.

[0039] The apparatus for processing signals may comprise a processor configured to determine a position a wire and current through the wire. The apparatus for processing signals may comprise a microcontroller or system-on-a-chip (SoC) which includes the processor. The microcontroller or SoC may include an analogue to digital converter (ADC). The apparatus for processing signals may comprise a discrete ADC.

[0040] The processor maybe configured to determine the position a wire and current through the wire based on a round wire or segmented wire.

[0041] According to a second aspect of the present invention there is provided a system comprising a cable comprising at least two wires and a current sensing system of the first aspect, wherein the cable is disposed in the aperture .

[0042] The system may have two, three, four, five or more wires.

[0043] According to a third aspect of the present invention there is provided a computer- implemented method of reconstructing current(s) in at least one wire in a cable using magnetic fields values. The method comprises providing an estimate of position of a wire and current through the wire for each of at least one wire, calculating a set of magnetic field values dependent on the estimate of position and estimate of current for each of the at least one wire, comparing the measured magnetic field values and the calculated magnetic field values to determine an error, determining whether the error falls below a given threshold. The method comprises, upon a negative determination, adjusting the estimate of position and / or the estimate of current for one or more wires and to repeat the calculation of a set of magnetic field values, the comparison of the measured magnetic fields and the calculated magnetic field and the determination whether the error falls below a given threshold, and upon a positive determination, outputting the estimate of position and the estimate of current for one or more wires as the position and currents for the one or more wires.

[0044] According to a fourth aspect of the present invention there is provided a current sensor comprising a body with an aperture with an axis and a centre, wherein the aperture is sized to accommodate at least one conductor capable of carrying current; at least two arrangements of at least five magnetic field sensors arranged around the aperture, wherein the magnetic field sensors are sensitive to magnetic fields in one or more directions.

[0045] The magnetic field sensors may be sensitive to magnetic fields in one or more directions, wherein a least one of the directions may be not radial or tangential with respect to the centre.

[0046] The aperture may accommodate one conductor capable of carrying current, two conductors capable of carrying currents, three conductors capable of carrying currents, four conductors capable of carrying currents, or five conductors capable of carrying currents, wherein each conductor has a location within the aperture and a current.

[0047] The body may have at least two sub-bodies that may be moved relative to each other in a manner to enclose the at least one conductor capable of carrying current to form the aperture for, and / or wherein the body may be at least in some part flexible to provide the capability to enclose around the at least one conductor capable of carrying current to form the aperture.

[0048] Each magnetic field sensor may have a least one axis of sensitivity, an output, and at least two connections. The axis, or axes, of sensitivity of the, or each, magnetic field sensor in an arrangement may be located in one plane perpendicular to the axis of the aperture. The axis of sensitivity of each magnetic field sensor in an arrangement may be rotated with respect to the adjacent magnetic field sensor in the arrangement by an amount proportional to the angular distance between the sensors with respect to the centre of the aperture such the axis of sensitivity of the magnetic field sensors in an arrangement rotates an integer number of times. The axis of sensitivity of each magnetic field sensors in an arrangement may be orientated in the plane, wherein adjacent magnetic field sensors have differing rotated orientations proportional to the angular distance between the sensors with respect to the centre of the aperture; wherein the summed differences in the differing rotated orientations of the at least five magnetic field sensors in an arrangement equals an integer number of turns.

[0049] There may be at least one pair of sensor arrangements in which the rotation of the differing rotated orientations of the axes of sensitivity of the magnetic field sensors may be the same, but the absolute rotational difference between the axes of sensitivity of any two sensors in the different sensor arrangements in the pair at the same angular position about the sensor aperture may be 90 degrees.

[0050] The at least five sensors in an arrangement may be arranged in a circle with a circumference and a centre coincident with the centre of the aperture. Each sensor of the at least five sensors in an arrangement may be positioned at a specific position along the circumference, wherein each sensor may be oriented in the plane at an angle relative to the previous sensor. Each sensor of the at least five sensors in an arrangement may be equi-spaced on the circumference. Each sensor of the at least five sensors in an arrangement may be orientated around an axis parallel to the axis of the aperture wherein each subsequent sensor may be turned slightly more, creating a pattern of evenly spaced, rotated sensors around the circle.

[0051] The output of each arrangement may be directly or indirectly connected to a computer capable of analysing the output of each arrangement to determine the location of at least one current carrying conductor and at least one current.

[0052] The sensor arrangements may be arranged in two or more planes each perpendicular to the axis of the aperture. The sensor arrangements may be arranged radially around the aperture. Each sensor arrangement may have a geometric centre and two or more sensor arrangements are interleaved in an axis parallel to the axis of the aperture, such that the geometric centre of the magnetic sensors of two or more sensor arrangements are collocated in a plane perpendicular to the axis of the aperture.

[0053] The outputs of each sensor in each arrangement may be connected in series to provide a differential arrangement output derived from all the sensors in the arrangement. The outputs of each sensor in each arrangement may be connected in parallel to provide a differential arrangement output derived from all the sensors in the arrangement. For each differential arrangement output, one end of the differential arrangement output may be traced back around the aperture to a point close to the other differential arrangement output such that the total wiring of the arrangement does not form a closed loop around the aperture.

[0054] A sensor arrangement connected in series may have a centre within the arrangement and the centre of the connected arrangement of sensors may be grounded.

[0055] The series connections between the sensors in each arrangement may be made by connecting every second sensor in each arrangement with one polarity to form a subset of sensors, and between every other sensor with the opposite polarity to form a subset , wherein the two subsets are connected at one ground connection; wherein the arrangements provide three connections to form a differential output of the sensor arrangement.

[0056] The body may have at least two sub-bodies that abut to form a split line and the magnetic sensors of an arrangement have a constant pitch that may be maintained across the split line.

[0057] The magnetic field sensors may be inductor suitable for mounting on a PCB. The magnetic field sensors may be magneto-resistance magnetic field sensors. The magnetic field sensors may be magneto-resistance magnetic field sensors suitable for mounting on a PCB. The magnetic sensors may be wound coils on a bobbin structure designed to result in the coils having a predetermined axis of magnetic field sensitivity. The magnetic sensors may be surface mount inductors. The magnetic field sensors may be Hall effect sensors, flux gate sensors or SQUIDs. Each arrangement of sensors may be mounted on one side of a PCB. Each arrangement of sensors may be split among at least two PCBs such that when brought together the sensor arrangement. There may be several PCBs with an arrangement of sensors on each side of the PCB at the same radius or different radius from the aperture axis mounted in the body of the sensor.

[0058] There may be at least one pair of sensor arrangements, and the sensors in the pair of sensor arrangements may be mounted on both sides of a PCB, such that alternate sensors in the first sensor arrangement in the pair are mounted on either side of the PCB, and alternate sensors in the second sensor arrangement in the pair are mounted on either side of the PCB, with the sensors in the first and second sensor arrangements at each position around the aperture mounted opposite each other.

[0059] The aperture may be of any predetermined shape including circular.

[0060] There may be at least one pair of sensor arrangements in which the axis of sensitivity of the magnetic field sensors rotates five times around the sensor aperture with respect to a fixed axis, with the amplitude of the measurements of the signals from this pair compared to the amplitude of the measurements in at least one other pair of sensors in which the axis of sensitivity of the magnetic field sensors rotates less than five times around the sensor aperture with respect to a fixed axis.

[0061] There may be at least one pair of sensor arrangements in which the axis of sensitivity of the magnetic field sensors rotates five times around the sensor aperture with respect to a fixed axis, with the amplitude of the measurements of the signals from this pair compared to the amplitude of the measurements in at least one other pair of sensors in which the axis of sensitivity of the magnetic field sensors rotates less than five times around the sensor aperture with respect to a fixed axis.

[0062] More than one axis of magnetic sensitivity may be incorporated into a single magnetic field sensor that may be surface mount and / or has a ceramic core and / or is a wire wound inductor. Two orthogonal axis of magnetic sensitivity may be incorporated into a single magnetic field sensor that is surface mount and / or has a ceramic core and / or may be a wire wound inductor.

[0063] The magnetic field sensors may be used to determine currents being carried within a multi core cable.

[0064] The analogue outputs of the arrangements of magnetic field sensors may be used to determine currents being carried within a multi core cable.

[0065] The magnetic field sensors may be arranged to measure a dipole magnetic field within the aperture and / or a quadrupole magnetic field within the aperture and / or a hexapole (or "sextupole") magnetic field within the aperture and / or an octupole magnetic field within the aperture and / or a decupole (of "ten pole") magnetic field within the aperture and / or a dodecapole (or "12 pole") magnetic field within the aperture. The analogue magnetic field sensors may be combined in arrangements of a magnetic field sensors, wherein: a "dipole" arrangement of magnetic field sensors may be configured to measure a dipole magnetic field within the aperture and / or a "quadrupole" arrangement of magnetic field sensors may be configured to measure a quadrupole magnetic field within the aperture and / or a "hexapole" arrangement of magnetic field sensors may be configured to measure a hexapole (or "sextupole") magnetic field within the aperture and / or an "octupole" arrangement of magnetic field sensors may be configured to measure an octupole magnetic field within the aperture and / or a "decupole" arrangement of magnetic field sensors may be configured to measure a decupole magnetic field within the aperture and / or a "dodecapole" arrangement of magnetic field sensors may be configured to measure a dodecapole magnetic field within the aperture.

[0066] Brief Description of the Drawings

[0067] Certain embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

[0068] Figure 1 is a schematic block diagram of a current sensing system which comprises a current sensor which includes at least two sets of magnetic field sensors;

[0069] Figure 2A illustrates two sets of magnetic field sensors;

[0070] Figure 2B illustrates four axially-offset sets of magnetic field sensors;

[0071] Figure 2C illustrates two axially-offset pairs magnetic field sensors;

[0072] Figure 3A illustrates position and orientation of a magnetic field sensor defined with respect to a radial axis;

[0073] Figure 3B illustrates position and orientation of a magnetic field sensor defined with respect to a fixed axis;

[0074] Figure 4A illustrates a simulation of a magnetic field pattern arising from a single wire which can be measured using a Rogowski coil-like arrangement of magnetic field sensors;

[0075] Figure 4B illustrates a simulation of a dipole magnetic field pattern arising from two wires which can be measured using a dipole arrangement of magnetic field sensors;

[0076] Figure 4C illustrates a simulation of a quadrupole magnetic field pattern arising from four wires which can be measured using a quadrupole arrangement of magnetic field sensors;

[0077] Figure 4D illustrates a simulation of a hexapole magnetic field pattern arising from six wires which can be measured using a hexapole arrangement of magnetic field sensors;

[0078] Figures 5A to 5G illustrates different arrangements of magnetic field sensors;

[0079] Figure 6 are greyscale plots of numerically-calculated mutual inductance for different sets of magnetic field sensors;

[0080] Figure 7A is a front perspective view of a current sensing system;

[0081] Figure 7B is a rear perspective view of a current sensing system;

[0082] Figure 8 is a perspective view of an experimental set up;

[0083] Figure 9 is a plot of measured external current rejection for the current system shown in Figure 8;

[0084] Figure 10 illustrates a hinged current sensor;

[0085] Figure 11 illustrates a part of a two-part current sensor;

[0086] Figure 12 is process flow diagram of a method of processing signals from the arrangement of magnetic field sensors;

[0087] Figure 13 illustrates a magnetic field sensor having a selectable axis of sensitivity;

[0088] Figure 14 illustrates an arrangement of magnetic field sensors having respective selectable axes of sensitivity; Figure 15 is process flow diagram of a method of processing signals from the arrangement of magnetic field sensors shown in Figure 14;

[0089] Figure 16 schematically illustrates an interleaved configuration of first and second sets of magnetic field sensors;

[0090] Figure 17 schematically illustrates a concentric configuration of first and second sets of magnetic field sensors;

[0091] Figure 18 schematically illustrates a set of magnetic field sensors in which magnetic field sensors are connected in series between first and second terminals;

[0092] Figure 19 schematically illustrates a set of magnetic field sensors in which magnetic field sensors are connected in parallel between first and second terminals;

[0093] Figure 20 schematically illustrates a set of magnetic field sensors in which magnetic field sensors are connected in series between first and second terminals and which include a centre tap; and

[0094] Figure 21 schematically illustrates a set of magnetic field sensors in which alternate magnetic field sensors are connected.

[0095] Detailed Description of Certain Embodiments

[0096] Introduction

[0097] Arrangements of magnetic sensors are herein described which can be considered to be adaptations of Rogowski-type current sensors which allow measurement of higher- order current distributions.

[0098] These may comprise arrangements of, for instance, connected, discrete sensors orientated at specific angles and having specific polarities, or continuously-wound coils having specific properties. The sensors may take the form of coils which may have an air-core, a dielectric core or a magnetic core (such as, a ferrite core) or semiconductor sensors which have an analog output (such as, a bridge-type magneto-resistance sensor).

[0099] The sensor arrangement is chosen so that it is substantially immune to currents flowing outside the aperture of the coil arrangement, similar to a traditional Rogowski or current transformer coil, which arises due to cancellation of magnetic fields. Immunity may be defined as a value of less than 5% for a ratio of first and second signal magnitudes, the first signal magnitude taken when a conductor is placed at 1.5 R (in other words, outside the ring of sensors) and a second signal magnitude taken when the conductor when the conductor is placed a at 0.5 R (in other words, inside the ring of sensors), where R is the radius of a set of magnetic field sensors. The output does not require digitisation in order to extract useful information. The signals from multiple coil arrangements can be mixed using an analog mixer to yield the currents in the individual conductors. If digitisation is used, however, then this can be achieved using fewer ADC input channels in the case that the summing is performed digitally. This is because the number of inputs depends on the number of current poles that to be detected, not the number of sensors.

[0100] The magnetic field sensors can be physically arranged so that their axis (or axes) of sensitivity are fixed, in other words, using the inherent axis (or axes) of sensitivity of the magnetic field sensor(s). If, however, magnetic field sensors have at least two orthogonal axes of sensitivity, then the outputs of the sensors can be processed in a way that their axes of sensitives are selectable, effectively providing a universal set of sensors whose axis (or axes) of sensitivity are variable (or "programmable").

[0101] Expressed differently, the axis (or axes) of sensitivity can be established by processing signals from the magnetic field sensor.

[0102] Current sensor system

[0103] Referring to Figure 1, a current sensing system 1 for measuring current in one or more wires 2 (or "conductors") is shown. The wire(s) 2 may be part of a cable 3 and may be arranged to be multicore or segmented. The cable 3 may take the form of a high-voltage (e.g., > IkV) three-phase cable.

[0104] The current sensing system 1 comprises a current sensor 4 and signal processing apparatus 5 comprising a front-end circuit 6 and a processor 7. The front-end circuit 6 receives and processes signals from the current sensor 4 and outputs a processed signal to the processor 7 for further current reconstruction.

[0105] The processor 7 may output a result, such as a value, to an output device 8, such as a display. The processor 7 may take the form of a processor in a microcontroller unit (MCU) (not shown).

[0106] The current sensor 4 comprises a sensor body 11 having an aperture 12 (or "passage") having a centre line 13 (which can run along an axis 14) and a periphery

[0107] 15 which may take the form of a wall. The aperture 12 is generally circular in a plane

[0108] 16 perpendicular to the centre line 13 (expressed differently, as viewed along the centre line 13) although it may be other shapes such as elliptical or polygonal. In the case of an irregular polygon, the centre of the polygon (through which the central line runs) can be taken to be its centroid. The aperture may be elongate, for instance, tubular. As will be shown in more detail, the aperture can be formed from discrete parts (such as printed circuit boards). The wires 2 and the cable 3 need not run through the centre of the aperture 12, but can be offset from the centre line 13.

[0109] The sensor body 11 may comprise at least two parts, for example first and second parts joined by a hinge or two parts which can be assembled, so as to allow the current sensor 4 to be placed around the wire(s) 2 so that the wire runs through the aperture 12.

[0110] The current sensor 4 may be encapsulated in an elastomer or other suitable material (not shown) or contained in a housing (not shown).

[0111] Referring also to Figures 2A to 2C, the current sensor 4 comprises at least one set 17 (or "arrangement") of magnetic field sensors 18 arranged around the aperture 12.

[0112] Figure 2A schematically shows a pair of ( / .e., two) magnetic field sensor arrangements 18. Each magnetic field sensor arrangement 18 may be referred to as an aggregated magnetic field sensor arrangement 18 (or "aggregated magnetic field sensor set").

[0113] Figure 2B shows an assembly of (aggregated) magnetic field sensor arrangements 18.

[0114] Figure 2C shows an assembly of pairs of (aggregated) magnetic field sensor arrangements 18.

[0115] The magnetic field sensors 18 may have a specific, physically-defined orientation which fixes their axis or axes of sensitivity 19. In this case, the current sensor 4 comprises at least two sets of magnetic field sensors 18, each arranged to measure magnetic fields for a respective, specific magnetic field pattern.

[0116] If the magnetic field sensors 18 may have at least two orthogonal axes of sensitivity, then it is possible to resolve a magnetic field into at least orthogonal components. In that case, only one set of magnetic field sensors 18 is required (although additional sets can be used) and the arrangement of magnetic field sensors 18 can effectively be changed to measure different magnetic field patterns.

[0117] Each set 17 consists of at least five magnetic field sensors 18. Each magnetic field sensor 18 has at least one magnetic field sensing axis 19. Each magnetic field sensor 18 may have two, three or more magnetic field sensing axes 19 which may be orthogonally orientated. Each set 17 of magnetic field sensors 18 are arranged in the plane 16 perpendicular to the centre line 16 and configured to measure magnetic fields in the plane 16.

[0118] A magnetic field sensor 18 may take the form of a coil (not shown) wound on a respective bobbin (not shown) having a core (not shown) which may comprise a dielectric material (such as air) or a magnetic material (such as ferrite). A magnetic field sensor 18 may comprise one or more magnetoresitive devices, such as an anisotropic magnetoresistive (AMR) sensor or a tunnel magnetoresistive (TMR) sensor, Hall device, flux gate or SQUID. A magnetic field sensor 18 may be formed in a semiconductor chip and may comprise more than two magnetic field sensing devices which may be arranged and orientated to allow more magnetic field components in two or more axes to be measured.

[0119] The sets 17 of magnetic field sensors 18 may be arranged in rings 20 around the aperture 12. The magnetic field sensors may be arranged at a radius R from the centre (in other words, centreline 13) of the aperture 13. Additionally or alternatively, the sets 17 of magnetic field sensors 18 may be lengthwise (or "longitudinally" or "axially") offset. More than one set 17 of magnetic field sensors 18 may be arranged in one ring 20 wherein different magnetic field sensors 18 below to different sets 17.

[0120] As will be explained in more detail hereinafter, the current sensor system 1 can be used to measure currents in a cable 3 comprising one or more wires 2.

[0121] Sensor angles

[0122] The magnetic field sensor 4 is made up of a number of arrangements 17 of magnetic field sensors 18 in which the sensors 18 in each arrangement have a specific distribution of angles of the angle of sensitivity around the measurement aperture 12.

[0123] In each sensor arrangement 17, the angle and polarity of the sensors 18 is configured such that the difference between the relative angle of the sensitivity axis 19 of the two sensors to the centre of the sensor arrangement is an integer times their relative angle to the centre of the sensor arrangement.

[0124] Referring also to Figure 3A, each sensor 18 has an angular position e relative to the centre of the sensor arrangement, and an orientation 4> with respect to the direction to the centre of the sensor arrangement. The orientation of a sensor and its axis of sensitivity in an arrangement is given by < > = Nd + < >o (for example, in radians).

[0125] For N = 0 and <po = n / 2, the sensor array is a Rogowski current sensor.

[0126] For N = 1, there are two dipole sensor arrangements with <po = 0, TT / 2, labelled dipole x and dipole y.

[0127] For N = 2, there are two quadrupole sensor arrangements with <po = 0, TT / 2 labelled quadrupole x and quadrupole y.

[0128] In Figure 3A, the orientation of the sensor is defined with respect to a radial axis. The orientation of the sensor may, however, be defined with respect to a fixed reference axis.

[0129] Referring also to Figure 3B, the same sensor 18 is shown. In this case, the orientation of a sensor in an arrangement is given by v = Nd + 0o.

[0130] For N = 1 and 0o = TT / 2, the sensor array is a Rogowski current sensor.

[0131] For N = 2, there are two dipole sensor arrangements with 0o differing by TT / 2 and labelled dipole x and dipole y.

[0132] For N = 3, there are two quadrupole sensor arrangements with 0o differing by TT / 2 labelled quadrupole x and quadrupole y.

[0133] Referring to Figures 4A to 4D, first, second, third and fourth magnetic field patterns 20 are shown generated by a single wire, two wires, four wires and six wire respectively.

[0134] The first, second, third and fourth magnetic field patterns 20i, 20z, 2O3, 204 can be measured using a Rogowski coil arrangement of magnetic field sensors, a dipole arrangement of magnetic field sensors, a quadrupole arrangement of magnetic field sensors and a hexapole arrangement of magnetic field sensors, respectively.

[0135] Referring also to Figures 5A to 5G, a number of different arrangements 17 are shown. In Figures 2A to 2C, examples of sensor arrangements 17 for sixteen magnetic field sensors 18 (in this case, in the form of coils) are shown. There may be fewer or more magnetic field sensors 18.

[0136] A benefit of these types of sensor arrangement is that, similar to a Rogowski current sensor, the sensor arrangements are insensitive to currents flowing outside the sensor arrangement. This insensitivity increases with the number of magnetic field sensors 18.

[0137] Figure 5 is a greyscale plot of numerically-calculated mutual inductance between coils and a straight wire perpendicular to the coil aperture at variable positions within the plane. The Rogowski coil is uniformly sensitive to currents flowing in the wire. For all coil arrangements, the mutual inductance is small outside the aperture.

[0138] Referring to Figures 7A and 7B, the sensor body 11 takes the form of a printed circuit board (PCB) having a circular aperture 12. The PCB 11 supports three sets 18 of magnetic field sensors 17 on a front face 21 of the board and two sets of 18 of magnetic field sensors on a rear face 22. Each set 18 of magnetic field sensors 17 consists of 24 sensors 17 in the form of surface mount ceramic core wire wound inductors. The three sets of magnetic field sensors 17 are arranged in concentric rings around the aperture 12. The PCB 11 also supports circuit 3 and processing 4.

[0139] Referring to Figure 8, an experimental set up is shown in which three central unpowered conductors 2u,i, 2u,2, 2u,3 pass through the aperture 12 and two powered external conductors 2p,i, 2P,2 run generally parallel to the unpowered conductors 2u,i, 2U,2, 2U,3, next to the PCB 11, outside the aperture 12. Three sensor sets 17 are shown simply in chain for clarity.

[0140] Figure 9 is a plot of rms current through a wire against sensor output current for first, second and third phases for an experimental arrangement shown in Figure 8. The results show less than 0.3% coupling between the current in the external conductors 2U,2, 2U,3 and the measurement output.

[0141] Current sensor configuration for installation

[0142] Current sensors are often installed with the cable already in place and so the sensors are configured to allow the sensors to be attached or assembled around the cable. Referring to Figure 10, a current sensor 4 is shown comprising a generally tubular sleeve 30 having first and second parts 31i, 312 ("sides" of "halves") joined by a living hinge 32.

[0143] In a central portion 33 of the sleeve 30, the outside 33 of the sleeve 30 is provided with two circumferential ridges 34 between which annular one or more C-shaped PCBs 11 are disposed on each side 31i, 312 of the sleeve. The current sensor 4 also includes distally-disposed permanent magnet 35 for applying a high de bias magnetic field inside the sleeve 30 when the two halves 31i, 312 are closed.

[0144] Thus, a series of PCBs 11 can be held in a structure that can be clamped around a multicore cable 3 (Figure 1). Each PCB 11 hold an arrangement of sensors 17 (Figure 1) on each side.

[0145] Referring to Figure 11, a PCB sub-assembly 36 is shown which can provide half of the magnetic field sensors 18 of a current sensor. The sub-assembly 36 comprises a base plate 37 which supports a row of C-shape PCBs 11 which project away from the base plate 37.

[0146] Thus, two PCB sub-assembly 36 can form a complete set of magnetic field sensors 18.

[0147] Current reconstruction

[0148] Referring to Figures 1 and 12, a method of current reconstruction performed by the instrumentation 5 will now be described.

[0149] The magnetic field due to any current distribution through the aperture can be expanded in a multipole expansion of cylindrical multipole moments, where the origin of the cylindrical co-ordinate system is the centre of the aperture. The moments can be measured directly by the assemblies herein described.

[0150] The multipole moment outputs Mmeas are compared to multipole moment values Mcai calculated using a Biot-Savart calculation of the magnetic fields generated by a set of more than one conductors flowing through the aperture and the sensor geometry. By using an iterative algorithm, the geometry of the conductors and currents is varied to match to the multipole moment outputs by minimising the square of the differences in measured outputs and calculated outputs, that is, an error value E. The magnetic field sensors 18 output multipole moment values Mmeas (that is, magnetic field moment strength values) which are captured by instrumentation 5 (step 1A). The processor 7 calculates initial positions Po of the wires 2) which is used as an initial position estimate P (step S2).

[0151] The processor 7 calculates currents I in the wires 2 based on the measured multipole values M meas and the position estimates P (step S3). This can be achieved using:

[0152] I — P Mmeas (1)

[0153] The processor 7 also calculates multipolar moment values Mcaibased on estimated currents and position estimates.

[0154] The processor 7 calculates the square of the differences in measured outputs and calculated outputs (step S4) :

[0155] £ — I I Mmeas—Meal I I2(2)

[0156] The processor 7 compares the error value £ with a threshold value A (step S5).

[0157] If the error value £ falls below the threshold value A, then the estimated currents and position are used. If the error value £ does not fall below the threshold value A, then a new position P' is randomly estimated (step S6) and the process of calculating currents based on the new position P' (step S3), calculating a new error (step S4) and comparing the new error with the threshold value (step S5) is repeated.

[0158] Further information about current reconstruction can be found in

[0159] - R. Luo et al. : "A TMR Sensor Array for Non-Invasive Current Measurement in Three- Phase Cable and Error Analysis" IEEE Access, volume 13, pages 80272-80282 (2025),

[0160] - G. Geng et al. : "Noninvasive Current Sensor for Multicore Cables", IEEE Transactions on Power Delivery, volume 33, no. 5, pages 2335-2343 (2018) and

[0161] - K. Zhu et al. '. "On-Site Non-Invasive Current Monitoring of Multi-Core Underground Power Cables With a Magnetic-Field Sensing Platform at a Substation", IEEE Sensors Journal, volume 17, no. 6, pages 1837-1848 (2017), which are incorporated herein by reference.

[0162] Configurable magnetic field sensor approach In the current sensors hereinbefore described, the magnetic field sensors 18 (Figures 7A and 7B) are physically arranged so that their axis (or axes) of sensitivity are fixed.

[0163] If, however, magnetic field sensors have at least two orthogonal axes of sensitivity, then the outputs of the sensors can be processed in a way that their axes of sensitives are selectable, effectively providing a universal set of sensors whose axis (or axes) of sensitivity are configurable (or "programmable").

[0164] Referring to Figure 13, a multi-axis magnetic field sensor 18 and a sensitivity axis selector 40 (or "filter") are shown.

[0165] The multi-axis magnetic field sensor 18 is capable of sensing magnetic field strength in at least two orthogonal directions, for instance, perpendicular directions. The sensitivity axis selector 40 receives at least two values of magnetic field strength and a target direction 0 (or "target angle") and outputs magnetic field strength for the target direction. The sensitivity axis selector 40 can be implemented in the digital domain. For example, the instrumentation 5 (Figure 1) may receive raw analogue values, digitise the values and process the digital values to obtain the magnetic field strength for the target direction.

[0166] Referring to Figure 14, a configurable magnetic field sensor 4 is shown.

[0167] The magnetic field sensor 4 comprises an arrangement 17 of magnetic field sensor 18 each having at least two, orthogonal axes 19 of sensitivity. In this case, the arrangement 17 of magnetic field sensors 18 takes the form of a circular array which are arranged around the aperture 12. In this case, there are eight magnetic field sensors 18. There may, however, be more sensors 18.

[0168] The output of each sensor 18 passes through a respective sensitivity axis selector 40 having a respective target direction 0. Together, the arrangement 17 of magnetic field sensor 18 and the target directions 0 can be used to measure a specific multipole magnetic field, e.g., Rogowski, dipole x, dipole y, etc.

[0169] There may be more than one arrangement 17 of magnetic field sensor 18, each configured to measure a respective multipole magnetic field at the same time.

[0170] A single arrangement 17, however, can be used to measure different multipole magnetic fields. This can be achieved in different ways. It can be achieved by multiplexing taking a first measurement for a first set of target directions 0 to measure a first multipole magnetic field and taking a second measurement sufficiently soon (for example, less than 10 ps or less than Ips) with a second set of target directions 0 to measure a second, different multipole magnetic field.

[0171] It can be achieved by using more than one sensitivity axis selector 40 for each magnetic field sensor 18. Thus, two or more sensitivity axis selector 40 each having a different target direction 0 receives the same output from one magnetic field sensor 18.

[0172] As with the sensor arrangements hereinbefore described, the arrangement of magnetic field sensor is chosen so that it is immune to currents flowing outside the aperture of the coil arrangement because of cancellation of the magnetic fields in the coil arrangements.

[0173] Current reconstruction

[0174] Referring to Figures 14 and 15, a method of current reconstruction performed by the instrumentation 5 will now be described.

[0175] Current is reconstructed in substantially the same way as the method hereinbefore described. In this case, however, the signals are pre-processed to obtain multipole moment values.

[0176] The magnetic field sensors 18 output magnetic field vector Bmeas which are captured by instrumentation 5 (step Sl.B. l). The processor 7 converts this to a multipole moment value M meas — (dx.meas, dy.meas) (Step S1.B.2).

[0177] The process continues as hereinbefore described (steps S2 to S6).

[0178] Sensor wiring

[0179] The magnetic field sensors 18 can be arranged and / or wired physically or logically (in other words, in the digital domain) in different ways.

[0180] Figure 16 and 17 illustrate interleaved and concentric configurations of magnetic field sensors, respectively. Referring to Figure 16, an interleaved configuration 51 of first and second sets 17i, 172 of magnetic field sensors 18 are shown. In this case, the first and second sets 17i, 172 of magnetic field sensors are configured (either physically or logically) to measure quadrupole x and quadrupole y moment values respectively.

[0181] Each set 17i, 172 of magnetic field sensors 18 is circular when viewed along the centre axis 13, but are alternately displaced in a forward direction and in a backward direction parallel to the centre axis 13. Each set 17 of magnetic field sensors 18 has a geometric centre 52i, 522. The two sets 17 of magnetic field sensors 18 are interleaved in a direction parallel to the centre line 13 such that the geometric centres 52i, 522 of the magnetic field sensors are collocated in a plane perpendicular centre line.

[0182] Referring to Figure 17, an interleaved configuration 61 of third and fourth sets 173, 174 of magnetic field sensors 18 are shown. In this case, the third and fourth sets 173, 174 of magnetic field sensors are configured (either physically or logically) to measure dipole x and quadrupole x moment values respectively.

[0183] The third set 173 of magnetic field sensors 18 are arranged in a first ring 62i having a first radius ri and the fourth set 174 of magnetic field sensors 18 are arranged in a second ring 622 having a second radius r2, where r2 > ri.

[0184] Figures 18 and 19 illustrate configurations in which magnetic field sensors are connected in series and parallel, respectively.

[0185] Referring to Figure 18, a series configuration 71 of a fifth set 17s of magnetic field sensors 18 is shown.

[0186] The magnetic field sensors 18 are connected in series between a pair of terminals 72, 73. The signal appearing across the terminals 72, 73 can be fed into an amplifier or comparator 74.

[0187] Referring to Figure 19, a parallel configuration 81 of a sixth set 17e of magnetic field sensors 18 is shown.

[0188] The magnetic field sensors 18 are connected in parallel between a pair of terminals 82, 83. The signal appearing across the terminals 82, 83 can be fed into an amplifier or comparator 84. Figures 20 and 21 illustrate configurations which have a centre tap.

[0189] Referring to Figure 20, a series configuration 91 of a seventh set 17? of magnetic field sensors 18 is shown.

[0190] The magnetic field sensors 18 are connected in series between a pair of terminals 92, 93. A tap 95 is taken between adjacent middle sensors 18MI, 18M2 which can be connected to a reference line 95, which in this case is ground.

[0191] Referring to Figure 21, a series configuration 101 of an eighth set 17s of magnetic field sensors 18 is shown.

[0192] Magnetic field sensors 18 are connected, in series, to every other sensor in the loop (in other others, not to its nearest neighbour, but to its next nearest neighbour and form two subsets of sensors. The first subset is connected in series between a first terminal 102 and a node 103, and the second subset is connected in series between the node 103 and a second terminal 104. The node 105 can be connected to a reference line 95, which in this case is ground

[0193] Conclusion

[0194] Embodiments of a current sensor capable of measuring currents in individual cores of a multicore cable are described. The sensor is based on arrangements of magnetic field sensors whose outputs can be combined, resulting in a series of effective sensors which are insensitive to currents flowing outside of the sensor aperture. The outputs of the effective sensors can be analysed to determine the geometry of the multicore cable, and the currents flowing in the individual cores.

[0195] Embodiments herein described can have one or more advantages. For example, the current sensor can enable measurement of currents in different cores of a multicore cable. The current sensor can exhibit external conductor immunity similar to a Rogowski coil or current transformer. If the current sensor output is digitised, then fewer ADC channels are required.

[0196] External current immunity can be improved by adding more sensors. If, however, all the sensors are to be measured separately, this can increase the cost of a device. Conversely, by choosing to measure the necessary sensor arrangements needed for the application, an arbitrary number of sensors can be added to the arrangement while the number of measurement channels stays the same. Direct measurement of signal-of-interest increases sensitivity. Direct measurement of signal-of-interest increases sensitivity and precision by nulling uniform magnetic fields and other spurious signals before the ADC.

[0197] Modification

[0198] It will be appreciated that many modifications may be made to the embodiments herein described.

Claims

Claims1. A current sensing system (1) for sensing current in a multicore cable (3), the system comprising:■ a current sensor (4) comprising a sensor body (11) having an aperture (12) which has a centre line (13) and at least one set (17) of magnetic field sensors (18); and■ apparatus (5) for processing signals from the magnetic field sensors (18); wherein each set of magnetic field sensors comprises at least five magnetic field sensors arranged around the aperture in a respective plane perpendicular to the centre line and configured to measure at least in plane magnetic fields, wherein each magnetic field sensor has a respective in-plane axis of sensitivity which is inherent or which is established by processing signals from the magnetic field sensor that correspond to in-plane orthogonal fields; wherein, in each set of magnetic field sensors, the axes of sensitivity rotate in a given sense an integer number of times greater than one with angular position of the magnetic field sensors in the given sense around the centre line so as to measure a magnetic field distribution.

2. The current sensing system of claim 1, wherein each magnetic field sensor (18) is rotated dependent on its angular position so as to rotate its axis of sensitivity (19).

3. The current sensing system of claim 1, wherein the apparatus includes a sensitivity axis selector (40) for each magnetic field sensor (18), wherein the sensitivity axis selector is configured to receive a target axis of sensitivity and first and second orthogonal magnetic field values and to output a magnetic field value corresponding to the target axis of sensitivity.

4. The current sensing system of claim 1, 2 or 3, wherein the axis of sensitivity, MJ, depends on its angular position 0 around the centre line (13) according to: J = N 0 + qjo where N is a positive, non-zero integer and ipo is a constant.

5. The current sensing system of claim 1 or any one of claims 2 to 4, wherein the at least one set (17) of magnetic field sensors (18) comprises at least two sets of magnetic field sensors.

6. The current sensing system of claim 5, wherein at least two sets of magnetic field sensors are arranged concentrically.

7. The current sensing system of claim 5 or 6, wherein at least two sets of magnetic field sensors are arranged on first and second faces of a board of the sensor body, the second face being opposite to the first face.

8. The current sensing system of claim 5, 6 or 7, wherein at least two sets of magnetic field sensors are arranged on first and second different boards of the sensor body.

9. The current sensing system of claim 5 or any one of claims 6 to 8, comprising: a first set of magnetic field sensors lying in a first plane perpendicular to the centre line; and a second set of magnetic field sensors lying in a second plane perpendicular to the centre line which is different to the first plane.

10. The current sensing system of claim 1 or any one of claims 2 to 9, wherein each set of magnetic field sensors has a geometric centre and wherein at least two sets of magnetic field sensors are interleaved in an axis parallel to the centre line such that the geometric centre of the magnetic sensors of two or more sensor arrangements are collocated in a plane perpendicular to the axis of the aperture.

11. The current sensing system of claim 1 or any one of claims 2 to 10, wherein magnetic field sensors in a set of magnetic field sensors are connected in series between a pair of terminals.

12. The current sensing system of claim 1 or any one of claims 2 to 11, wherein magnetic field sensors in a set of magnetic field sensors are connected in parallel between a pair of terminals.

13. The current sensing system of claim 11 or 12, wherein the magnetic field sensors in the set of magnetic field sensors are connected in the digital domain.

14. The current sensing system of claim 1 or any one of claims 2 to 13, wherein magnetic field sensors in a first set of magnetic field sensors are connected in series or parallel between a first pair of terminals and magnetic field sensors in a second set of magnetic field sensors are connected in series or parallel between a second pair of terminals, wherein one of the first pair of terminals and one of the second pair of terminals are connected by a wired connection around the aperture without the first and second set of magnetic terminals forming a closed loop.

15. The current sensing system of claim 1 or any one of claims 2 to 14, wherein magnetic field sensors in a set of magnetic field sensors are connected in series between first and second terminals and has a tap between an adjacent pair of magnetic field sensors which is connected or connectable to a voltage reference.

16. The current sensing system of claim 1 or any one of claims 2 to 15, wherein magnetic field sensors in a set of magnetic field sensors are connected in series such that every odd-numbered sensor in the set are connected in series between a first terminal and an intermediate terminal and have a first polarity to form a first subset of magnetic field sensors and every even-numbered sensor in the set are connected in series between the intermediate terminal and a second terminal and have second, opposite polarity and form a second subset of magnetic field sensors, wherein the intermediate terminal is connected or connectable to a voltage reference.

17. The current sensing system of claim 1 or any one of claims 2 to 16, wherein sensor body (11) comprises first and second parts which divide the sensor body through the aperture and a set of magnetic field sensors are arranged around the aperture with a constant angular pitch.

18. The current sensing system of claim 1 or any one of claims 2 to 17, wherein sensor body (11) comprises first and second parts which divide the sensor body through the aperture for allowing the sensor body to be assembled around a cable.

19. The current sensing system of claim 1 or any one of claims 2 to 18, wherein there is at least one pair of sets of magnetic field sensors, wherein magnetic field sensors in the pair of sensor arrangements are mounted on both sides of a printed circuit board such that alternate sensors in the first set are mounted on either side of the printed circuit board, and alternate sensors in the second set are mounted on either side of the printed circuit board, with the magnetic field sensors in the first and second sets at each position around the aperture mounted opposite each other.

20. The current sensing system of claim 1 or any one of claims 2 to 19, wherein each magnetic field sensor (18) comprises at least one inductor.

21. The current sensing system of claim 1 or any one of claims 2 to 20, wherein each magnetic field sensor (18) comprises at least one magnetoresistive device.

22. The current sensing system of claim 1 or any one of claims 2 to 21, wherein the apparatus (5) for processing signals comprises a processor (7) configured to determine a position a wire and current through the wire,

14. 23. A system comprising : a cable comprising at least two wires; and a current sensing system of claim 1 or any one of claims 2 to 22, wherein the cable is disposed in the aperture .

24. A computer-implemented method of reconstructing current(s) in at least one wire in a cable using magnetic fields values measured using the current sensor of claim 1 or any one of claims 2 to 22, the method comprising : providing an estimate of position of a wire and current through the wire for each of at least one wire; calculating a set of magnetic field values dependent on the estimate of position and estimate of current for each of the at least one wire; comparing the measured magnetic field values and the calculated magnetic field values to determine an error; determining whether the error falls below a given threshold; and upon a negative determination, adjusting the estimate of position and / or the estimate of current for one or more wires and to repeat the calculation of a set of magnetic field values, the comparison of the measured magnetic fields and the calculated magnetic field and the determination whether the error falls below a given threshold; and upon a positive determination, outputing the estimate of position and the estimate of current for one or more wires as the position and currents for the one or more wires.

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