AU2007327432B2

AU2007327432B2 - Electrical energy meter comprising at least one inductive type electricity measuring sensor, and associated sensor

Info

Publication number: AU2007327432B2
Application number: AU2007327432A
Authority: AU
Inventors: Yvon Fichen; Jean-Francois Tissier
Original assignee: Itron Global SARL
Current assignee: Itron Global SARL
Priority date: 2006-11-30
Filing date: 2007-11-19
Publication date: 2010-09-09
Anticipated expiration: 2027-11-19
Also published as: PL2089724T3; ES2348345T3; AU2007327432A1; FR2909452B1; WO2008065291A2; FR2909452A1; EP2089724B1; BRPI0719731A2; EP2089724A2; ATE472738T1; ZA200903492B; DE602007007513D1; WO2008065291A3

Abstract

The invention relates to an electrical energy meter comprising at least one sensor (1; 1') for measuring inductive type electric current with a primary circuit (3; 3') traversed by an alternating current (I) to be measured, a secondary circuit (40) magnetically coupled in air with the primary circuit (3; 3'), a shielding element (6) of the unit formed by the primary circuit (3; 3') and the secondary circuit (40), and means (7, 10,17,26, 90) for measuring the saturation state of the shielding element (6), comprising in accordance to the invention a winding forming a tertiary circuit (7; 7') between the secondary circuit (40) and the shielding element (6), through which flows an injection current (I1NJ) with an alternating frequency (fi), fixed and different from the fundamental and harmonic frequencies of the current (I) to be measured; generating means (10, 90) for the injection current (IINJ), and means (7, 10,17,26, 90) of measuring a representative value of the mutual inductance (M2) between the secondary and tertiary circuits, and a comparison means (25) of the measured value with at least one calibrating value (M2max), a state of partial or total saturation of the shielding element which can be deducted when the measured value is less than the calibration value.

Description

AN ELECTRICAL ENERGY METER INCLUDING AT LEAST ONE INDUCTIVE TYPE CURRENT MEASUREMENT SENSOR, AND AN ASSOCIATED SENSOR The present invention relates to an electrical 5 energy meter including at least one inductive type alternating current measurement sensor, and it also relates to an associated inductive type current measurement sensor. Alternating current measurement sensors are 10 conventionally used in electricity consumption meters for industrial or residual use. More precisely, one sensor is associated with each of the phases of the meter. Thus, for a single-phase meter, a single sensor is used, whereas for a three-phase meter, three measurement 15 sensors are used to enable current to be measured in each phase. Such sensors are difficult to make because they require great immunity to environmental constraints, whether of the thermodynamic type (temperature, pressure, 20 humidity) or of the electromagnetic type (electric and magnetic fields). Furthermore, the current measurement sensor must present linearity and accuracy over a large dynamic range of currents and over a large frequency passband, since 25 the wave of the measured current may contain numerous harmonics (fundamental = 50 hertz (Hz) or 60 Hz, with harmonics up to the 21st). At present, the conventional solutions for such sensors can be organized in four large families: 30 - passive sensors of the resistive type (shunt); - current transformers; - inductive sensors; and - active sensors of the Hall effect type, or magnetic field sensors. 35 The physical principle used by the current sensor determines its immunity to environmental parameters, and in particular determines the fraudulent techniques that 2 can be applied to the meter in order to falsify its measurement of current, and consequently falsify the account kept of electrical energy consumption. For example, a shunt type resistive sensor, based on 5 Ohm's law, is in principle insensitive to continuous magnetic fields, and is only slightly sensitive to alternating magnetic fields. However it presents poor temperature stability. Document FR 2 800 167 discloses an inductive type 10 current measurement sensor comprising a U-shaped linear primary conductor made of copper for conveying the alternating current to be measured, and a coil disposed inside the U-shape, forming a secondary circuit. The current measurement principle is thus based on magnetic 15 coupling in air between the primary circuit constituting one phase of the network in which the electricity meter is connected, and the secondary circuit that is electrically isolated from the primary circuit. A shielding element of material having high magnetic 20 permeability (of the nickel-iron alloy type) surrounds the assembly. Because of the shielding element presenting high magnetic permeability (greater than 100,000), such an inductive sensor is highly immune to external alternating 25 magnetic fields, and in a multi-phase electricity meter requiring one sensor per phase, it presents very good rejection of magnetic influences between phases internal to the meter. In contrast, the high magnetic permeability materials used saturate at field levels that 30 are quite low, typically of the order of 0.7 teslas (T) for a nickel-iron type alloy having a nickel content of 80% (of the mumetal type). Consequently, such an inductive sensor presents low immunity to continuous magnetic fields, in particular those produced by powerful 35 permanent magnets that could be used by a fraudulent person. Saturation of the shielding by means of a permanent magnet will always give rise to a reduction in 3 the mutual inductance between the primary circuit and the coil that forms the secondary circuit, thereby leading to the measured current being smaller than the current actually consumed. 5 One known solution for making the above inductive type current measurement sensor immune consists in using additional shielding surrounding the shielding element, with the additional shielding being made of a ferromagnetic material that presents a high value for its 10 saturation field (typically greater than 2 T). For a three-phase meter, this shielding is constituted by a single piece of soft iron completely surrounding the three current measurement sensors relating to the three phases. 15 The dimensions (size and thickness) of such a part can be very large when it is necessary to counter fraud by certain types of magnet that present pole areas of large dimensions (50 millimeters (mm) x 50 mm) and that present remanent fields of the order of 1.2 T. 20 Nevertheless, that anti-fraud technique is incompatible with the present trend that consists in reducing the size of meters. Reducing the volumes of electricity meters leads to a reduction in the distances between the outside faces of such meters and the sensor 25 shielding elements. Typically, the minimum distances are of the order of 15 mm on the side faces and in the range 10 mm to 15 mm at the rear of the meter. Unfortunately, these small distances require the thickness of the soft iron shielding to be increased. The size and the weight 30 of the soft iron shield then become too great for mass production use in electricity meters. Document GB 2 409 528 discloses an electricity energy meter including at least one inductive type current measurement sensor comprising a primary circuit 35 for conveying an alternating current to be measured, a secondary circuit magnetically coupled in air with the primary circuit, a shielding element to provide the 4 assembly formed by the primary circuit and the secondary circuit with magnetic isolation, and measurement means for measuring a saturation state of the shielding element while the energy meter is in use so as to detect possible 5 fraud by using a permanent magnet. In that document, the measurement means are constituted by a winding around a portion of the shielding element. An object of the present invention is to provide a solution that is compact, making it possible to detect 10 any attempt at permanent magnet fraud on a meter having an inductive type current measurement sensor. According to the invention, the measurement means comprise: - in the inductive sensor, a winding forming a 15 tertiary circuit inserted between the secondary circuit and the shielding element, and designed to convey an alternating injection current of fixed frequency that is distinct from the fundamental and harmonic frequencies of the current to be measured; and 20 - in the meter, generator means for delivering said injection current, measurement means for measuring a value representative of the mutual inductance between the tertiary circuit and the secondary circuit, and comparator means for comparing the measured value with at 25 least one calibration value, with it then being possible to deduce that the shielding element is in a partially or totally saturated state in the event that the measured value is less than the calibration value. The injection current used preferably presents a 30 triangular waveform. This current waveform induces a squarewave voltage in the secondary circuit, thus making it possible to extract the fundamental frequency of the injection current by synchronous detection. Advantageously, the means for comparing the measured 35 value compares said value M 2 of the mutual inductance with two limit values stored in the meter, a first value corresponding to the calibration value representative of 5 a non-saturated state of the shielding element, and a second value corresponding to a totally saturated state of the shielding element. The comparator means can then generate a warning 5 signal once the measured value M 2 of the mutual inductance is situated in a predefined range situated between said first and second values. In a preferred embodiment, the meter also includes a correction module making it possible once a fraud has 10 been detected, for the total energy actually consumed by the meter to be recalculated on the basis of the measured value representative of the mutual inductance between the tertiary circuit and the secondary circuit. In a second aspect, the invention provides an 15 inductive type current measurement sensor comprising a primary circuit for conveying an alternating current to be measured, a secondary circuit magnetically coupled in air with the primary circuit, a shielding element for providing the assembly formed by the primary circuit and 20 the secondary circuit with magnetic isolation, and measurement means for measuring a saturation state of the shielding element so as to detect possible fraud by using a permanent magnet, the sensor being characterized in that said measurement means include a winding forming a 25 tertiary circuit inserted between the secondary circuit and the shielding element, and serving to convey an injection current at a fixed frequency different from the fundamental and harmonics frequencies of the current to be measured. 30 Advantageously, the tertiary circuit is in a form similar to that of the primary circuit so as to obtain firstly mutual inductance between the tertiary circuit and the secondary circuit that is very close to the mutual inductance between the primary circuit and the 35 secondary circuit, and secondly mutual inductance between the primary circuit and the tertiary circuit that is negligible. Thus, when the sensor is used in a meter, 6 this mutual inductance is negligible concerning its influence on measurement at mains frequency. In a first possible embodiment of the sensor, the primary circuit is a generally U-shaped linear conductor, 5 the secondary circuit is a multi-turn coil centered inside the U-shape, and the tertiary circuit is an open loop formed by a generally U-shaped continuous conductive track, certain segments of the track corresponding to an inlet and an outlet of the tertiary circuit extending in 10 a first plane parallel to the plane of the primary circuit, and at least one segment extending in a second plane parallel to the first plane and disposed symmetrically relative to the plane of the primary circuit and symmetrically inside the shielding element. 15 In a variant, the primary circuit is a multi-turn coil, and the tertiary circuit is a continuous conductive track, certain segments of the track corresponding to an inlet and to an outlet of the tertiary circuit extending in a first plane parallel to the plane of the primary 20 circuit, at least one segment extending in a second plane parallel to the first plane and disposed symmetrically relative to the plane of the primary circuit and symmetrically inside the shielding element, the segments placed facing the multi-turn coil also being circularly 25 arcuate in shape. Either way, this particularly optimized configuration in two symmetrical planes relative to the primary circuit enables fraud to be detected regardless of the side or the face of the sensor approached by the 30 magnet and partially saturated thereby. Advantageously, said tertiary circuit is arranged inside the sensor so that the inlet and the outlet of the tertiary circuit are remote from the inlet and the outlet of the primary circuit. 35 In the preferred embodiment, the conductive track forming the tertiary circuit is etched on a flexible printed circuit support.

7 The invention and the advantages it provides can be better understood on reading the following description made with reference to the accompanying figures, in which: 5 - Figures la to 1c are perspective views, two assembled and one exploded, showing a sensor in a first possible embodiment in accordance with the invention; - Figure 2 shows the tertiary circuit of the sensor in the first embodiment; 10 - Figure 3 is a diagrammatic plan view showing how the primary circuit and the tertiary circuit of the sensor in the first embodiment are associated with each other; - Figures 4a to 4c are perspective views, two 15 assembled and one exploded, showing a sensor in a second possible embodiment in accordance with the invention; - Figure 5 shows the tertiary circuit of the sensor of the second embodiment; - Figure 6 is a diagram showing how the primary 20 circuit and the tertiary circuit of the sensor in the second embodiment are associated; - Figure 7 is a block diagram showing the operation of a preferred embodiment of a meter fitted with a sensor of the inductive type of the invention; and 25 - Figure 8 is a diagram showing an example of an internal circuit for a three-phase meter, in which the same injection current is caused to flow in each of the tertiary circuits of the three current sensors. With reference to Figures la-1c, 2, and 3, there 30 follows a description of a first embodiment of a sensor 1 enabling the invention to be implemented. Figure 1c is an exploded view of the sensor, whereas Figures la and lb are two perspective views of the assembled sensor, one seen from above and the other from below, showing the 35 sensor with all of the elements shown in Figure 1c, with the exception of the shielding element, for manifest reasons of clarity.

8 The sensor 1 comprises a base 2 of electrically insulating material serving as a housing to receive a portion of a first linear conductor 3 constituting a "primary" circuit of the sensor. The first conductor 3 5 is substantially U-shaped, with the base 2 being molded onto the base of the U-shape, for example, so as to provide electrical insulation for the primary circuit. The ends of the U-shape constitute the connection terminals of the sensor. When the sensor is connected, 10 alternating current I for measuring is thus conveyed by the primary circuit 3. The base 2 is also formed in such a manner as to receive and co-operate with a sensing element support 4 made of material that is likewise insulating. 15 As can be seen more particularly in Figure 1c, the base 2, generally in the form of a rectangular parallelepiped, presents a plane portion 20 defined on either side by rims 21. The plane portion 20 has a receptacle-forming recess 22 centered on a central axis 20 C. The sensing element support 4 is generally in the form of a plate having, on one face, a mandrel around which there is positioned coaxially a multi-turn coil 40 forming the secondary circuit of the sensor. In the example shown here, the receptacle-forming recess 22 of 25 the base 2 is cylindrical and suitable for co-operating with the rims of the coil 40 to center the secondary circuit inside the U of the primary circuit 3. A part 5 placed in the recess 22 forms an insulator element between the primary circuit 3 and the secondary circuit 30 40. The above-described set of elements 2 to 5 is assembled along the axis C and is held in place by a shielding element 6 (Figure lc) of rectangular parallelepiped shape, placed slidably on the rims 21 of 35 the base 2. The shielding element 6 is made of a ferromagnetic alloy of the nickel iron type having a 9 nickel content lying in the range 50% to 80% so as to guarantee high magnetic permeability. Up to this point, the sensor 1 is entirely similar to the inductive sensor described in document 5 FR 2 800 167 in the name of the Applicant. The difference contributed by the invention comes from adding, in the preferred embodiment of the invention, an additional element in the sensor that serves, as explained below, to measure a partially or 10 totally saturated state of the shielding element 6, which state is characteristic of an attempted fraud using a permanent magnet. The additional element consists in a winding forming a tertiary circuit 7, which is inserted between the 15 secondary circuit 40 and the shielding element 6. When the sensor 1 is associated in particular with one of the phases of a meter that is in use in accordance with the invention, the tertiary circuit 7 serves to carry an "injection" alternating current IINJ of small 20 amplitude (typically of the order of 10 milliamps (mA)), and of frequency fi that is fixed and different from the fundamental and harmonic frequencies of the current to be measured. The choice of frequency fi is important in order to make it possible in the secondary circuit 40 to 25 distinguish between signals induced by the passage of the current I to be measured in the primary circuit 3 and signals induced by the passage of the injection current IINJ in the tertiary circuit 7. When selecting the frequency fi, it an also be appropriate to exclude other 30 frequencies, typically frequencies used in applications such as centralized remote control, which frequencies are themselves inter-harmonic frequencies. The choice of frequency fi might also be constrained by the need to accommodate potential variations in the fundamental 35 frequency (50 Hz or 60 Hz) and the associated harmonics. The configuration and the position of the tertiary circuit 7 relative firstly to the primary and secondary 10 circuits 3 and 40, and secondly to the shielding element 6, are also preferably optimized in order to satisfy the following three requirements: - the mutual inductance between the tertiary circuit 5 7 and the secondary circuit 40 must be very close to the mutual inductance between the primary circuit 3 and the secondary circuit 40. This requirement is particularly necessary in the preferred embodiment of the invention as a meter in which it is necessary not only to detect 10 attempted fraud, but also to measure correctly the amount of energy that has really been consumed; - the primary and tertiary circuits 3 and 7 respectively must have as little influence as possible on each other, which in other words means that the mutual 15 inductance between these two circuits should be as small as possible so that it can be considered as being negligible. This serves in particular to ensure that the presence of the tertiary circuit does not disturb the measurement characteristics of the meter by reducing the 20 sensitivity of the sensor 1; and - the tertiary circuit 7 needs to be spread over the confined space inside the shielding element 6 along the axis C of the sensor so as to be capable of detecting when the shielding is in a partially saturated state 25 regardless of which face or side of the shielding element 6 has actually been approached by a permanent magnet during an attempted fraud. The tertiary circuit 7 satisfying the above three requirements is shown in Figures lc, 2, and 3. As can be 30 seen in Figures 2 and 3, the tertiary circuit 7 is a conductive track forming an open loop. Certain segments of the track correspond to an inlet E (segments 70 in Figure 2) and to an outlet S (segments 71 in Figure 2) of the tertiary circuit 7 that extend in a first plane 80 35 parallel to the plane of the primary circuit 3. At least one other track segment 72 extends in a second plane 81 parallel to the first plane 80, and placed opposite to 11 the first plane, preferably symmetrically to said first plane 80 about the primary circuit 3. These two planes 80 and 81 are thus located symmetrically within the confined spaced inside the shielding element 6. Finally, 5 lateral segments 73, 74 interconnect the segments in the two planes 80 and 81 so as to form a track that is continuous. In order to facilitate understanding, it should be observed that the track segments 70, 71, and 74 are visible in Figure 2 as if the elements were 10 transparent. It should also be observed that the segment 72 presents a rounded shape suitably selected to be similar to the shape of the base of the U-shape of the primary circuit 3 (see Figure 3). Thus, the tertiary circuit 7, 15 when projected onto a plane, presents overall the same shape as the primary circuit 3, thus making it possible to satisfy the first two above-mentioned requirements relating to the various mutual inductances. The third above-mentioned requirement is satisfied by the way the 20 active segments of the tertiary circuit 7 are distributed in the two planes 80 and 81. In the preferred embodiment, the tertiary circuit 7 is arranged in the sensor 1 in such a manner that its inlet E and outlet S are remote from the inlet and outlet 25 of the primary circuit (see Figure 3). This arrangement serves not only to make it easier to connect the sensor inside the meter, but also to guarantee good electrical isolation between the two circuits. The tertiary circuit 7 is preferably made in the 30 form of a conductive track etched on a support in the form of a flexible printed circuit 8. Thus, when assembling the various elements of the sensor, it is very easy to fold the flexible support 8 around the assembly formed by the base 2, the sensing element support 4, and 35 the secondary coil 40, so as to have the shape shown in Figure 2. The shielding element 6 is then positioned by sliding and holds together the elements of the sensor 1.

12 The sensor 1 in the first embodiment is particularly suitable for measuring currents of large amplitude (typically in the range 10 mA to 100 amps (A)). Such sensors are used on electricity meters located in low 5 voltage networks with the network phases passing through the meters. Figures 4a-4c, 5, and 6 are views similar to those of the above-described figures, except that they show a second embodiment of a sensor 1' in accordance with the 10 invention, which second embodiment is particularly adapted to measuring currents of small amplitude (typically 1 mA to 10 A). These sensors are used on electricity meters placed on high-power low voltage networks or on high voltage networks, i.e. networks in 15 which the current sensors are connected to current reducing transformers. The sensor 1' differs from the sensor 1 only in the form of the primary and tertiary circuits. The other elements are unchanged, and as a result they have 20 references that are identical to those described above. The primary circuit 3' of the sensor 1' uses an electrical conductor of smaller diameter than the primary circuit 3 of the sensor 1, typically having a diameter of the order of 1.2 mm. Consequently, instead of having a 25 simple U-shape like the primary circuit 3 of the sensor 1, the primary circuit 3' is here in the form of a multi turn coil. The shape of the tertiary circuit 7' is thus also different, in particular as a result of the presence of three track segments 70', 71', and 72' (Figure 5) that 30 are placed facing the multi-turn coil of circularly arcuate shape. Once more, the segments 70' and 71' are made visible in Figure 5 as if the elements shown were transparent. Thus, when projected onto a plane, the three segments 70', 71', and 72' form a circular loop 35 placed facing the primary circuit. Once more, the similarity in the shape of the primary and tertiary circuits 3' and 7' serves to ensure firstly that the 13 mutual inductance between the tertiary circuit 7' and the secondary circuit 40 is very close to the mutual inductance between the primary circuit 3 and the secondary circuit 40, and secondly that the mutual 5 inductance between the primary circuit 3' and the tertiary circuit 7' is negligible. Regardless of which sensor 1 or 1' is used, the description below relates to the principle whereby an attempt at fraud by using a permanent magnet is detected. 10 Below, the following notation is used: - M 1 , the value of the mutual inductance between the primary circuit 3 or 3' and the secondary circuit 40; - M 2 , the value of the mutual inductance between the tertiary circuit 7 or 7' and the secondary circuit 40; 15 and - M 3 , the value of the mutual inductance between the primary circuit 3 or 3' and the tertiary circuit 7 or 7'. Furthermore, for each of the mutual inductances Mi (for i having values 1 or 2), the following notation is 20 used: - Mima, the maximum value of the mutual inductance Mi corresponding to a shielding element 6 in a normal state, i.e. not saturated, not even partially. This value, which thus constitutes a reference value, is obtained 25 when calibrating the meter that is fitted with the sensor, and it is stored in the meter; - Misat, the value of the mutual inductance Mi when the shielding element 6 is totally saturated. This value is estimated by construction as corresponding to the 30 value of the mutual inductance when there is no shielding element 6, and it is likewise stored in the meter; and - Mi(x), any value of the mutual inductance Mi corresponding to the shielding element 6 being in a state of greater or lesser saturation. 35 The following relationship thus applies: Misat < Mi (x) < mi.

14 representing the fact that any attempt at fraud reduces to some extent the mutual inductances M, and M 2 as a result of the increase in the reluctance of the magnetic circuit as seen by the primary and tertiary magnetic 5 fields. With the configuration precautions taken for the tertiary circuit, the mutual inductance M 3 has a value that is very small (less than 5 nanohenries (nH), i.e. 1/ 1 5 0 0 th of the mutual inductance M). 10 Linear interpolation can be used to connect variation in the mutual inductance M, with variation in the mutual inductance M 2 . It can thus be shown that the relationship for variation between the mutual inductances Mi and M 2 is as follows: 15 Mi(x) /Mimax = (1 + a) (M 2 (x) /M 2 max) - a (1) where: a = ( (M 2 sat/M 2 max) - (Misat/Mimax) ) / (1 - (M 2 sat/M 2 max) This linear relationship represents the facts that if M 2 (x) = M2max, then Mj(x) = Mimax, which corresponds to 20 the standard (calibration) state of the shielding element, and that if M 2 (x) = M2sat, then M,(x) = Misat, which corresponds to the totally saturated state of the shielding element. Knowledge of variation in the mutual inductance M 2 (X) 25 relative to its reference value M 2 max makes it possible at least to detect when the shielding element is in a partially or totally saturated state, characteristic of permanent magnet fraud, and possibly also to perform calculation to correct the value of the mutual inductance 30 Mj(x) compared with its reference value, and consequently to compensate the values obtained for the measured current. The principle of performing detection and correction is explained below with reference to Figure 7 that is in 35 the form of a simplified block diagram showing the various components of a meter in accordance with the invention.

15 In the block diagram, there can be seen a sensor 1 or 1' as described above for measuring the alternating current I associated with a phase of the network to which the meter is connected. The meter also has a 5 microprocessor 9 controlling the various elements of the meter. Finally, in accordance with the invention, the meter includes means for generating an alternating injection current IINJ at the fixed frequency fi that is different from the frequencies used by the electricity 10 network. By way of example, a frequency fi is selected that is equal to 327.23 Hz. Since the sensor is of the differentiating inductive type, the waveform of the injection current is preferably selected to be triangular. Thus, the resultant waveform 15 at the outlet from the sensor is a squarewave signal, and the fundamental component at the frequency fi can advantageously be measured by synchronous detection. Such a waveform for the injection current can be obtained by a frequency generator 90 operating at the frequency fi, 20 internal to the microprocessor 9 and controlled to deliver a squarewave of frequency fi to a signal integrator circuit 10 that delivers a triangular voltage signal at the frequency fi. The injection current is then set by the ratio between the voltage delivered by the 25 integrator circuit 10 and the load resistance. In a variant, it would be possible to use a local oscillator to deliver the squarewave signal of frequency fi to the integrator circuit. The tertiary circuit of the sensor, represented by 30 its inlet E and its outlet S, is placed in a current loop connected in series between the outlet of the shaper circuit 10 and two resistors Rl and R2 connected to ground so that the tertiary circuit does indeed convey the injection current IINJ at the frequency fi. 35 In conventional manner, the meter further comprises means 11 for picking up the inlet voltage U of the sensor 1, 1', corresponding to the phase voltage, and means 12 16 for picking up from the outlet voltage of the sensor, a voltage that is representative of the current I measured by the sensor 1, 1'. The signals output by the means 11 and 12 are then conventionally sampled by an analog-to 5 digital converter 13. Both of the means 11 and 12 are constituted by resistive divider bridges that also serve to lower voltage levels to values compatible with the input levels that are acceptable to the analog-to-digital converter 13. 10 The samples are then subjected to two parallel processing paths. The various processes are represented inside the microprocessor 9 since, in the preferred implementation, these processes are performed purely in software form. 15 The first processing path, to the left in Figure 7, corresponds to conventional extraction of the energy consumed by the meter on the corresponding phase. It comprises firstly a digital integrator 14 serving to produce samples that are images of the signals U and I. 20 This integration is made necessary by using a sensor that is inductive, which by its very nature differentiates signals. The voltage and current samples output by the digital integrator 14 are then multiplied together in pairs in the main measuring module, and the mean consumed 25 energy W is calculated from the various products obtained over a given duration, e.g. one second. It should be observed that the current samples at the frequency fi do not contribute to calculating this mean energy since there is no voltage signal at the frequency fi. The mean 30 power W is then stored in a register or a memory 16 of the meter. In the preferred embodiment of the invention, the second processing path is used to measure variations in the mutual inductance M 2 as a result of synchronous 35 detection, in order to deduce therefrom a permanent magnet fraud, and to correct the mutual inductance M, so 17 as to be able to correct the value for the energy actually consumed. This is described below. Firstly, it should be recalled that the output signal from the sensor 1, 1' is a superposition of 5 signals at mains frequency (50 Hz or 60 Hz) and at harmonics thereof, together with signals at the frequency fi and at harmonics thereof. A synchronous detector 17 takes the samples delivered by the analog-to-digital converter 13, and multiplies the output signal Vs from the 10 sensor by a first or sine signal and by a second or cosine signal at the frequency fi, the frequency fi being provided by the oscillator 90. These two multiplications provide two signals Vdetl and Vdet2 at the output from the synchronous detector 17, given by: 15 Vdeti = Vs sin(27Efjt) Vdet2 = Vs cos(27cfit) It should be observed that when a local oscillator is used instead of the frequency generator 90, it is necessary to recreate signals at the frequency fi. 20 These two signals Vdetl and Vdet2 are then filtered by lowpass filters 18 with a low cutoff frequency so as to be in a position to eliminate the ripple amplitude created by modulation between the frequency of the current generated on the tertiary circuit and the 25 frequency components of the current on the primary circuit, so as to deliver only the continuous values for the signals Vdet, and Vdet2' The frequency component output by the sensor 1, 1' at the frequency fi can also be expressed as follows: 30 Vs(fi) = M 2 1127nf. cos(27tfit - p) = 2 V e + V~et 2 cos(27nfit - (pi) in which equation Ii is the amplitude of the injection current IIN in the tertiary circuit. Thus, knowing the continuous values of the signals 35 Vdetl and Vdet2, the frequency fi of the injection current, and the amplitude Ii, it is possible to determine the mutual inductance M 2 between the secondary circuit and the 18 tertiary circuit of the sensor in application of the following relationship: M = 2 Vdetl + Vdet2 _ A (2) 27Iifi 2'nIifi with: 2 2 5 A = 2 Vdetl + Vdet2 The value A is extracted from a calculation module 19 that receives the outputs from the lowpass filters 18. The amplitude Ii may be a fixed parameter stored in the meter. Under such circumstances, the mutual 10 inductance M 2 can be deduced directly by applying the above relationship (2). In a variant, and in the preferred embodiment shown in the block diagram, the microprocessor 9 of the meter acts in real time to measure the amplitude of the voltage 15 developed by the injection current IIJw conveyed by the tertiary circuit passing through a reference resistor, here the resistor R 2 . Thus, as shown in the block diagram of Figure 7, the meter has means 20 for measuring the injection current by picking up the voltage across the 20 terminals of the reference resistor R 2 , and means 21 for extracting the amplitude B such that: B = R2I1NJ Under such circumstances, the value of the mutual inductance M 2 is calculated by an extractor module 22 by 25 applying the following relationship:

M

2 = (A/B) . (R 2 /27Efi) (2b) The advantage of measuring the amplitude of the voltage developed by the injection current IINJ in real time lies in the fact that the real presence of an 30 injection current is monitored at all times, and if this were not so it would not be possible to apply the principle on which fraud is detected and the associated correction is made. Thus, in the preferred embodiment, the meter also has comparator means 23 for verifying that 19 the extracted value B does indeed lie between known upper and lower limits Bmin and Bma previously stored in the meter. Otherwise, the meter delivers an error signal SE1 indicating in particular that the value extracted for M 2 5 should not be taken into account. This error signal may be stored in an error journal 24, for example. Once the mutual inductance M 2 has been extracted and validated as described above, the meter can verify where this value lies between the value M 2 ma characteristic of a 10 non-saturated shielding element, and the value M2sat characteristic of a shielding element that is totally saturated. It is recalled that these two extreme values result from the calibration of the meter and have previously been stored in the meter. 15 For this purpose, the meter has comparator means 25 enabling the value M 2 to be compared with a first or upper limit, less than M 2 m (e.g. equal to 0.97 M 2 ma) and a second or lower limit, greater than M2sat (e.g. equal to 1.05 M2sat). The meter can thus deduce that there has been 20 an attempt at fraud whenever the measured value for M2 lies within the range situated between the lower limit and the upper limit. Preferably, no comparison is made unless the measured value for M 2 has remained stable over a certain duration, e.g. of the order of several seconds, 25 so as to take account of the response times of the lowpass filters 18. At this stage, the comparator means 25 of the meter can also verify that the value extracted for M 2 does not lie below the value M2sat or above the value M 2 ma 30 Otherwise it may generate an error signal SER 2 stored in the error journal 24. When an attempt at permanent magnet fraud is detected by the comparator means 25, they generate a warning signal SWmN delivered to a journal 26 for 35 recording events and/or to a device (not shown) of the LED or display screen type serving to display the warning signal to the outside of the meter.

20 Furthermore, in the preferred embodiment, the meter can itself correct the energy value W stored in the register 16, where the value is below the real value as a result of the fraud. 5 To do this, and given the above relationship (1) enabling the value M, to be deduced from the measurement M2, a correction module 27 calculates a corrected energy value Wcor by applying the following relationship: Wcor = W (Mimax/Mi (X) ) 10 This energy is the sum of the energy W calculated by the measuring system of the meter having added thereto additional energy deduced from the correction Wadd = W (Mimax/Mi(x) - 1) I.e.: 15 Wcor = W + Wadd The block diagram of Figure 7 includes only one sensor 1 or 1'. Naturally, the invention applies in the same manner for a polyphase meter having a plurality of sensors 1, 1', each associated with one phase of the 20 meter. Thus, it is possible to detect and even to correct permanent magnet fraud independently on each phase. Nevertheless, under such circumstances, the meter may advantageously be simplified by providing only one 25 injection current loop for the meter, with the tertiary circuits in each sensor being connected in series in this current loop. This configuration ensures that the same value of current is injected into each of the tertiary circuits, thereby simplifying calculation of the mutual 30 inductances M 2 for each of the phase sensors. Figure 8 is a diagram showing three sensors of the type 1 or 1' used in a three-phase meter, with the respective tertiary circuits thereof connected in series in the loop for injecting the current IINJ* 35 The sensor and the meter of the invention serve to conserve immunity from external alternating magnetic fields as provided by the shielding element, while also 21 enabling the partially or totally saturated state of said shielding element to be monitored continuously, thereby enabling an attempted permanent magnet fraud to be detected. In its preferred version (block diagram of 5 Figure 7), the meter also has means for compensating the current measurement errors that result from such a permanent magnet fraud. The configuration and the arrangement of the tertiary circuit can also be optimized for guaranteeing fraud detection regardless of the way in 10 which the permanent magnet is approached.

Claims

1. An electricity energy meter including at least one inductive type current measurement sensor (1; 1') comprising a primary circuit (3; 3') for conveying an 5 alternating current (I) to be measured, a secondary circuit (40) magnetically coupled in air with the primary circuit (3; 3'), a shielding element (6) to provide the assembly formed by the primary circuit (3; 3') and the secondary circuit (40) with magnetic isolation, and 10 measurement means (7, 10, 17-26, 90) for measuring a saturation state of the shielding element (6) while the energy meter is in use so as to detect possible fraud by using a permanent magnet, the meter being characterized in that said measurement means comprise: 15 - in the inductive sensor (1; 1'), a winding forming a tertiary circuit (7; 7') inserted between the secondary circuit (40) and the shielding element (6), and designed to convey an alternating injection current (IINJ) of fixed frequency (fi) that is distinct form the fundamental and 20 harmonic frequencies of the current (I) to be measured; and - in the meter, generator means (10, 90) for delivering said injection current (IINJ), measurement means (17-19, 21, 22) for measuring a value 25 representative of the mutual inductance (M 2 ) between the tertiary circuit (7; 7') and the secondary circuit (40), and comparator means (25) for comparing the measured value with at least one calibration value (M 2 ma), with it then being possible to deduce that the shielding element 30 is in a partially or totally saturated state in the event that the measured value is less than the calibration value.

2. A meter according to claim 1, characterized in that 35 the generator means (10, 90) deliver an injection current (IINJ) with a triangular waveform, and in that the measurement means (17-19, 21, 22) for measuring a value 23 representative of the mutual inductance (M 2 ) between the tertiary circuit (7; 7') and the secondary circuit (40) comprise a synchronous detector (17) delivering a first signal (Vdeti) and a second signal (Vdet2) resulting from 5 multiplying the output signal (Vs) from the sensor (1; 1') respectively by a sine, first signal and by a cosine, second signal at the frequency (fi) of the injection current (IINJ), and filter means (18) delivering continuous values for said first and second signals (Vdet1 10 and Vaet 2 )

3. A meter according to claim 2, characterized in that said measurement means (17-19, 21, 22) for measuring a value representative of the mutual inductance (M 2 ) between 15 the tertiary circuit (7; 7') and the secondary circuit (40) further include an extractor module (22) for calculating the value M 2 of said mutual inductance using the relationship: A M2 = 7If 20 in which: - fi is the frequency of the injection current; - A is a first variable such that: A = 2 Vd2et1 + Vd2et2 Vdet1 and Vdet2 being the continuous values of said first 25 and second signals; and - IT is a second variable representing the amplitude of the injection current in the tertiary circuit (7; 7').

4. A meter according to claim 3, characterized in that 30 the second variable Ii is predetermined and stored in the meter.

5. A meter according to claim 3, characterized in that it includes measurement means (20) for measuring the 35 amplitude of the injection current in the circuit and 24 delivering a value B that is representative of the amplitude of a voltage across the terminals of a resistor (R 2 ) of known resistance conveying the injection current (IINJ) 5

6. A meter according to claim 5, characterized in that it includes comparator means (23) for comparing the value B with two limit values known to the meter, respectively a lower limit (Bmin) and an upper limit (B,.), said 10 comparator means (23) delivering an error signal (SERRO if the value B does not lie between these two limit values.

7. A meter according to any one of claims 3 to 6, characterized in that said comparator means (25) compares 15 said value M 2 of the mutual inductance with two limit values stored in the meter, a first limit value (M2,,) corresponding to the calibration value representative of a non-saturated state of the shielding element (6), and a second value (M2,,at) corresponding to a totally saturated 20 state of the shielding element (6).

8. A meter according to claim 7, characterized in that the comparator means (25) generate a warning signal (SwA) as soon as said value M 2 for the mutual inductance lies in 25 a predefined range situated between said first and second values.

9. A meter according to claim 8, characterized in that the comparator means (25) do not generate said warning 30 signal (SWAN) unless said value M 2 is stable for a period of several seconds.

10. A meter according to claim 8 or claim 9, characterized in that said warning signal (SWA) is 35 delivered to an event-recording journal (26) of the meter, and/or to a display device of the meter enabling the warning to be seen from outside the meter. 25

11. A meter according to any one of claims 7 to 10, characterized in that said comparator means (25) also generate an error signal (SER2) in the event of said value 5 M 2 of the mutual inductance not lying between said first and second values.

12. A meter according to any preceding claim, characterized in that it further includes a corrector 10 module (27) for calculating the total energy actually consumed by the meter in the event of a fraud being detected, on the basis of the measured value representative of the mutual inductance (M 2 ) between the tertiary circuit (7; 7') and the secondary circuit (40). 15

13. A meter according to any preceding claim, characterized in that it includes a plurality of inductive measurement sensors (1; 1'), each associated with one phase of the meter, and in that the tertiary 20 circuits corresponding to all of the sensors are connected in series in a single current loop conveying said injection current (IINJ) *

14. An inductive type current measurement sensor (1; 1') 25 comprising a primary circuit (3; 3') for conveying an alternating current to be measured, a secondary circuit (40) magnetically coupled in air with the primary circuit (3; 3'), a shielding element (6) for providing the assembly formed by the primary circuit (3; 3') and the 30 secondary circuit (40) with magnetic isolation, and measurement means (7; 7') for measuring a saturation state of the shielding element (6) so as to detect possible fraud by using a permanent magnet, the sensor being characterized in that said measurement means 35 include a winding forming a tertiary circuit (7; 7') inserted between the secondary circuit (40) and the shielding element (6), and serving to convey an injection 26 current (IINJ) at a fixed frequency (fi) different from the fundamental and harmonics frequencies of the current to be measured. 5 15. A sensor (1; 1') according to claim 14, characterized in that the tertiary circuit (7; 7') is in a form similar to that of the primary circuit (3; 3') so as to obtain firstly mutual inductance (M 2 ) between the tertiary circuit (7; 7') and the secondary circuit (40) that is 10 very close to the mutual inductance (Ml) between the primary circuit (3; 3') and the secondary circuit (40), and secondly mutual inductance (M 3 ) between the primary circuit (3; 3') and the tertiary circuit (7; 7') that is negligible.

15

16. A sensor (1) according to claim 15, characterized in that the primary circuit (3) is a generally U-shaped linear conductor, in that the secondary circuit (40) is a multi-turn coil centered inside the U-shape, and in that 20 the tertiary circuit (7) is an open loop formed by a generally U-shaped continuous conductive track, certain segments (70, 71) of the track corresponding to an inlet (E) and an outlet (S) of the tertiary circuit (7) extending in a first plane (80) parallel to the plane of 25 the primary circuit (3), and at least one segment (72) extending in a second plane (81) parallel to the first plane and disposed symmetrically relative to the plane of the primary circuit and symmetrically inside the shielding element (6). 30

17. A sensor (1') according to claim 15, characterized in that the primary circuit (3') is a multi-turn coil, and in that the tertiary circuit (7') is a continuous conductive track, certain segments (70', 72') of the 35 track corresponding to an inlet (E) and to an outlet (S) of the tertiary circuit (7') extending in a first plane (80) parallel to the plane of the primary circuit (3'), 27 at least one segment (71') extending in a second plane (81) parallel to the first plane and disposed symmetrically relative to the plane of the primary circuit and symmetrically inside the shielding element 5 (6), the segments (70', 71', 72') placed facing the multi-turn coil also being circularly arcuate in shape.

18. A sensor (1; 1') according to claim 16 or claim 17, characterized in that said tertiary circuit (7; 7') is 10 arranged inside the sensor (1; 1') in such a manner that the inlet (E) and the outlet (S) of the tertiary circuit (7; 7') are remote from the inlet and the outlet of the primary circuit (3; 3'). 15

19. A sensor (1; 1') according to any one of claims 16 to 18, characterized in that the conductive track forming the tertiary circuit (7; 7') is etched on a flexible printed circuit support (8).

20 20. A sensor (1; 1') according to any one of claims 14 to 19, characterized in that the shielding element (6) serves to support the primary, secondary, and tertiary circuits of the sensor.