Detection of open circuit of external transformer

Abstract

The present application relates to the detection of an open circuit of an external transformer. An electric meter (1) comprises: a front circuit (5) connected to an external transformer (CT), and a conductor (7) connected to the external transformer via the front circuit; a test circuit (14) comprising a test generation chain (15) and a test measurement chain (16) connected to the conductor, and a processing component (11); the test generation chain is arranged to apply a test voltage (Vt) to the conductor, the test voltage (Vt) generating a test current (It) circulating in the test measurement chain, the test voltage having a level depending on the impedance of the external transformer, and the processing component (11) is arranged to detect the disconnection of the external transformer when the level of the test voltage is greater than a predefined detection threshold.

Description

Technical Field

[0001] The present invention relates to the field of electricity meters and in particular to meters intended for industrial applications. Background Art

[0002] Meters intended for industrial use can measure very high currents, typically up to 2000A.

[0003] Such meters, for example three-phase meters, are usually connected to the phases of the distribution network via external transformers mounted on the phases. These external transformers have a transformation ratio typically equal to 2000.

[0004] It would seem very interesting to be able to detect an open circuit of an external transformer circuit without intervention of an operator in order to prevent malfunctions of the customer and / or the energy distributor quickly and at low cost.

[0005] The present invention is directed to detecting an open circuit in an external transformer connected to a meter simply, inexpensively, reliably and without operator intervention. Summary of the invention

[0006] To achieve this purpose, an electric meter is proposed, comprising:

[0007] a front circuit arranged to be connected to an external current transformer located outside the meter and mounted on a phase of the distribution network;

[0008] a conductor arranged to be connected to an external current transformer via a front circuit;

[0009] a test circuit comprising a test generation chain having an output connected to the conductor, a test measurement chain having an input connected to the conductor, and a processing component;

[0010] The test generation chain comprises a voltage generator and is arranged for applying a test voltage on the conductor, the test voltage generating a test current circulating in the test measurement chain and having a level depending on the equivalent impedance of the front circuit and the external current transformer, the test measurement chain being arranged to measure the level of the test voltage, the processing component being arranged to acquire the level of the test voltage and to detect the disconnection of the external current transformer when the level of the test voltage is greater than a predefined detection threshold.

[0011] Thus, by applying a test voltage on a conductor connected to the external current transformer via the front circuit, a test current is generated that circulates in the test measurement chain. The level of the test voltage depends on the impedance of the external current transformer. The comparison of the level of the test voltage with a predefined detection threshold thus makes it possible to detect whether the external current transformer is open circuited.

[0012] The invention thus provides a very reliable and very robust detection criterion, which makes it possible to detect a cut-out of an external transformer without intervention of an operator. The invention is not very complex and inexpensive to implement, since it requires only a relatively simple test circuit to be integrated in the meter.

[0013] Furthermore, an electricity meter such as described above is proposed, wherein the front circuit comprises an internal transformer arranged to be connected to an external current transformer.

[0014] Furthermore, an electricity meter such as described above is proposed, the electricity meter further comprising a main measurement chain having an input connected to the conductor, the main measurement chain being arranged such that a main measurement current, being an image of the phase current circulating on the phase, circulates in the main measurement chain, the processing component being arranged to acquire the level of the main measurement current, to compare it with a predefined test threshold, and to control the test generation chain such that a test voltage is applied to the conductor when the level of the main measurement current is less than said predefined test threshold.

[0015] Furthermore, an electricity meter such as described above is proposed, the processing assembly comprising a test input belonging to a test measurement chain and a main input belonging to a main measurement chain, the test input and the main input being connected to the same analog-to-digital converter integrated in the processing assembly.

[0016] Furthermore, an electric meter such as described above is proposed, the test generation chain comprising a first switch between the conductor and the voltage generator, the processing assembly being arranged to close the first switch such that the test generation chain applies the test voltage to the conductor.

[0017] Furthermore, an electric meter such as described above is proposed, the test measurement chain comprising a second switch connected to the conductor and a detection chain downstream of the second switch, the processing assembly being arranged to close the second switch such that a test current circulates in the test measurement chain.

[0018] In addition, an electric meter such as the one described above is proposed, wherein the detection chain comprises, from upstream to downstream, a high-pass filter, an envelope detector, an analog-to-digital converter and a digital low-pass filter.

[0019] Furthermore, an electricity meter such as described above is proposed, which is a multiphase meter comprising a plurality of front circuits, each front circuit being arranged to be connected to an external current transformer located outside the meter and mounted on one phase of the multiphase distribution network.

[0020] Furthermore, an electric meter such as described above is proposed, the test generation chain comprising a single voltage generator and a number of first switches, each first switch being connected to one of the preceding circuits.

[0021] Furthermore, an electric meter such as described above is proposed, the test measurement chain comprising a single detection chain and a number of second switches, each second switch being connected to one of the preceding circuits.

[0022] In addition, a test method is proposed, which is implemented in a processing component of an electric meter such as the one described above and comprises the following steps:

[0023] The test voltage is applied to the conductor by the test generation chain;

[0024] Acquiring the level of the test voltage via the test measurement chain;

[0025] When the level of the test voltage is greater than a predefined detection threshold, the switching off of the external current transformer is detected.

[0026] In addition, a computer program is proposed, comprising instructions for driving a processing component of an electric meter such as described above to perform the steps of a testing method such as described above.

[0027] Furthermore, a recording support readable by a computer is proposed, on which a computer program such as described above is recorded.

[0028] The invention may be better understood from the following description of certain non-limiting embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Reference will be made to the accompanying drawings, in which:

[0030] [ Figure 1 ] Figure 1 Represents a three-phase electricity meter according to the invention connected in a four-wire configuration to an external transformer installed on a phase of a distribution network;

[0031] [ Figure 2 ] Figure 2 An electrical diagram of a meter according to the invention is shown;

[0032] [ Figure 3 ] Figure 3 is with Figure 1 A similar diagram of a three-wire configuration. DETAILED DESCRIPTION

[0033] See also Figure 1 , the electricity meter 1 according to the invention is here a three-phase meter intended to measure the electrical energy supplied to an installation 2 of consumers via a distribution network 3. The installation 2 is an industrial installation.

[0034] The distribution network 3 here comprises three phases Ph1, Ph2, Ph3 and a neutral N (four-wire configuration). A circuit breaker 4 is positioned "at the boundary" between the distribution network 3 and the installation 2. The three phases Ph1, Ph2, Ph3 and the neutral point N enter the circuit breaker 4 and emerge from it towards the installation 2. The circuit breaker 4 makes it possible to cut off the distribution of electrical energy.

[0035] The instrument 1 includes four voltage input terminals V1, V2, V3, VN and three current input terminals I1, I2, I3 (each current input terminal includes two ports).

[0036] The voltage input terminal V1 is connected to the phase Ph1 , the voltage input terminal V2 is connected to the phase Ph2 , the voltage input terminal V3 is connected to the phase Ph3 , and the voltage input terminal VN is connected to the neutral point N.

[0037] The current input I1 is connected to an external current transformer CT1 mounted on phase Ph1, the current input I2 is connected to an external current transformer CT2 mounted on phase Ph2, and the current input I3 is connected to an external current transformer CT3 mounted on phase Ph3. The external current transformers are located outside the meter 1.

[0038] In this case, each external current transformer has a transformation ratio equal to 2000.

[0039] The meter 1 recovers the image of the phase current circulating on the phases Ph1, Ph2 and Ph3 through the external transformers CT1, CT2 and CT3 and the voltages on the voltage input terminals V1, V2 and V3 for metering measurement.

[0040] See also Figure 2 , the meter 1 comprises, for each external current transformer CT (and therefore for each current input terminal comprising two ports I+ and I−), a front circuit 5 connected to said external transformer CT.

[0041] Here, the front circuit 5 includes an internal current transformer T connected to an external current transformer CT. More specifically, two terminals of the primary of the internal transformer T are connected to two terminals of the secondary of the external transformer CT. The internal transformer T is a ferromagnetic toroidal transformer.

[0042] The internal transformer T has here a transformation ratio equal to 1. The meter 1 comprises a conductor 7 which is connected to the external transformer CT via a front circuit 5. The conductor 7 is connected to a first terminal B1 of the secondary of the internal transformer T.

[0043] The meter 1 also comprises a main measuring chain 8 .

[0044] The input of the main measuring chain 8 is connected to the conductor 7 .

[0045] The main measurement current Ip circulates in the main measurement chain 8. The main measurement current Ip is a representative image of the phase current I circulating on the phase Ph where the external transformer CT is installed. In operation, the main measurement chain 8 is intended to measure the main measurement current Ip to evaluate the phase current I.

[0046] The main measurement chain 8 comprises from downstream to upstream a low pass filter 9, a first analogue to digital converter (ADC) 10 and a processing component 11. Here, by "upstream" this means on the distribution network 3 side and by "downstream" this means on the measurement side.

[0047] The processing component 11 is a microcontroller in this case.

[0048] In this case, the first ADC 10 is integrated into the microcontroller 11 .

[0049] The low-pass filter 9 is an active filter (for example, with a cut-off frequency equal to 5 kHz) integrating a gain (for example, equal to 1). The low-pass filter 9 makes it possible to remove interfering noises inside the meter 1 or coming from the distribution network 3, which could degrade the measurement of the main measurement current Ip (and therefore the evaluation of the phase current I).

[0050] The microcontroller 11 is suitable for executing the instructions of a program to implement the test method to be described. The program is stored in a memory 12 which is integrated in or connected to the microcontroller 11 .

[0051] The low pass filter 9 has an input connected to the main measurement chain 8 (and therefore to the conductor 7 and to the first terminal B1 of the secondary of the internal transformer T) and an output connected to the main input Ep of the microcontroller 11. The main input Ep is internally connected to the first ADC 10.

[0052] The low-pass filter 9 integrates a measuring resistor (not shown) having a terminal connected to the conductor 7 and a terminal connected to ground.

[0053] This measuring resistor makes it possible to transform the main measuring current Ip into a mirror voltage of the main measuring current Ip, which is filtered by the low-pass filter 9 and is applied to the input Ep.

[0054] The first ADC 10 of the microcontroller 11 thus generates a digital measurement of the mirror voltage and therefore of the main measurement current Ip circulating in the main measurement chain 8 .

[0055] A second terminal B2 of the secondary of the internal transformer T is connected to ground and to a ground input Em of the microcontroller 11 .

[0056] Furthermore, the meter 1 comprises a test circuit 14, which comprises a test generation chain 15, a test measurement chain 16 and a microcontroller 11. The microcontroller 11 therefore belongs here both to the main measurement chain 8 and to the test circuit 14.

[0057] The output of the test generation chain 15 is connected to the connector 7 .

[0058] The test generation chain 15 comprises, from upstream to downstream, a first switch S, a resistor R and a voltage generator 17. The first switch S is therefore situated between the conductor 7 and the voltage generator 17.

[0059] The voltage generator 17 generates an output voltage Vs, for example a 2V sinusoidal voltage. The voltage generator 17 generates an output voltage Vs, for example a 2V peak-to-peak sinusoidal voltage with a frequency of 1 kHz. The resistor R has a resistance value equal to 500 kΩ.

[0060] The output of the voltage generator 17 is connected to a first terminal of a resistor R. A second terminal of the resistor R is connected to a terminal of a first switch S that is not connected to the output of the test generation chain 15 .

[0061] The input of the test measurement chain 16 is connected to the conductor 7 and thus to the output of the test generation chain 15 .

[0062] The test measurement chain 16 comprises, from upstream to downstream, a second switch S′ and a detection chain 19 . The second switch S′ has a terminal connected to an input of the test measurement chain 16 and a terminal connected to the detection chain 19 .

[0063] The detection chain 19 comprises, from upstream to downstream, a high-pass filter 20, an envelope detector 21, a second ADC 22 and a digital low-pass filter 23. The second ADC 22 is integrated in the microcontroller 11. The digital low-pass filter 23 is programmed in the microcontroller 11.

[0064] Here, the high-pass filter 20 is an active filter (with a cutoff frequency of, for example, 500 Hz) that integrates a gain (e.g., equal to 1).

[0065] The high pass filter 20 and the envelope detector 21 are mounted in series. The output of the enclosure detector 21 is connected to a test input Et of the microcontroller 11. The test input Et is connected to a second ADC 22 internally.

[0066] The low-pass filter 23 acquires the digital measurement generated by the second ADC 22. The low-pass filter 23 has here a gain equal to 1 and a cut-off frequency of 10 Hz.

[0067] The first switch S and the second switch S′ are controlled by the microcontroller 11 .

[0068] The first switch S makes it possible to control the application of the test voltage Vt on the conductor 7 .

[0069] By default, the first switch S is in an open state, so that the test generation chain 15 does not apply a voltage to the conductor 7 and thus to the secondary of the internal transformer T.

[0070] The second switch S' makes it possible to selectively connect or disconnect the test measurement chain 16 of the conductor 7 and, therefore, of the secondary and main measurement chain 8 of the internal transformer T. The second switch S' is in an open state by default.

[0071] To detect whether the external transformer CT is disconnected (cut off), the microcontroller 11 first acquires the level Np of the main measurement current Ip (measured by the main measurement chain 8) and compares the level Np with a predefined test threshold St. The level Np is, for example, the effective value of the main measurement current Ip.

[0072] When the level Np is less than the predefined test threshold, that is, when: Np<St,

[0073] The microcontroller 11 closes the first switch S and the second switch S'. The first switch S and the second switch S' may be closed simultaneously or one after another. The two switches remain closed during the measurement time.

[0074] By closing the first switch S, the microcontroller 11 controls the test generation chain 15 such that it generates a test voltage Vt on the conductor 7. The test voltage Vt is generated from the output voltage Vs.

[0075] The application of the test voltage Vt generates a current that circulates in the different branches connected to the conductor 7 . The application of the test voltage Vt produces in particular a test current It that circulates in the test measurement chain 16 .

[0076] The level of the test voltage Vt depends on the equivalent impedances connected to the different branches of the conductor 7 and therefore in particular on the equivalent impedances of the front circuit 5 and of the external transformer CT.

[0077] When the external transformer CT is switched off, the equivalent impedance of the front circuit 5 and the external transformer CT is larger: the test voltage Vt increases.

[0078] Therefore, the test measurement chain 16 measures the level Nt of the test voltage Vt.

[0079] The microcontroller 11 acquires the level Nt of the test voltage Vt (leaving the digital low-pass filter 23) and compares it with a predetermined detection threshold Sd.

[0080] If the level Nt of the test voltage Vt is greater than the predefined detection threshold, that is, if: Nt>Sd,

[0081] The microcontroller 11 then detects the disconnection of the external transformer CT.

[0082] The predefined detection threshold Sd is typically equal to 0.75V.

[0083] Otherwise, the microcontroller 11 detects that the external transformer CT is not switched off.

[0084] The high-pass filter 20 and the envelope detector 21 make it possible to remove the 50 Hz and possible interruptions from the distribution network 3 on the one hand and to have a quasi-continuous signal at the second ADC 22 (digitally) filtered by the low-pass filter 23 on the other hand.

[0085] A test voltage Vt is generated for a determined duration D, where, for example:

[0086] D=1s.

[0087] The measurements are repeated at predetermined regular intervals T, for example:

[0088] T = 10mn,

[0089] And this only requires Np<St.

[0090] When the meter 1 detects a cut-off of the external transformer CT, the meter 1 generates an alarm and transmits it to the distributor and / or the customer via conventional communication means: wired or wireless, radio, through an online carrier current, etc.

[0091] It should be noted that it is advantageous to calibrate the output voltage Vs of 2 V peak-to-peak at 1 kHz generated by the voltage generator 17 in the factory in order to adjust the permanent steady-state measurement so as to have adequate performance relative to the level of the predetermined detection threshold Sd.

[0092] Thus, the injection of a calibrated test voltage Vt of significant level between the two terminals of the secondary of the internal transformer T makes it possible to detect the change in the equivalent impedance between said two terminals. Thus, a very robust criterion for detecting the disconnection of the external transformer CT is constructed. The frequency of 1 kHz is sufficiently far from the frequency of the network 3 to ensure that the detection is not contaminated by said frequency of the network 3.

[0093] It is noted that the active filter has a first order and the static gain of the filter is adapted so as to avoid saturation at the ADC and that a signal with a significant level (and corresponding to the input range of the ADC) is applied at the input of the ADC.

[0094] As has been seen, the meter 1 is a multiphase meter, in this case a three-phase meter. The principles of the invention just described apply to the three phases of the meter 1 .

[0095] The meter 1 thus comprises three front circuits 5 (one for each phase), each front circuit being arranged to be connected to one of the external current transformers CT1 , CT2 , CT3 .

[0096] The test circuit 14 includes a single processing component (microcontroller 11 ).

[0097] The test generation chain 15 comprises a single voltage generator 17 and several first switches S1 , S2 , S3 (one for each phase), each first switch being connected to one of the preceding circuits 5 .

[0098] The three first switches S1 , S2 , S3 are controlled by the microcontroller 11 .

[0099] Likewise, the test measurement chain 16 comprises a single detection chain 19 and several second switches S′ 1 , S′ 2 , S′ 3 (one per phase), each second switch being connected to one of the preceding circuits 5 .

[0100] Only one of the three first switches can be closed at a given time to inject a test voltage Vt on a single phase. Likewise, only one of the three second switches can be closed at a given time to select the phase on which the measurement must be made. For a given phase Phi (i from 1 to 3), the measurement is made when both switches Si, S'i associated with said phase are closed and all other switches are open.

[0101] It is noted that, as an option, an external transformer may be installed on the neutral conductor N and connected to the fourth current input IN of the meter 1. The invention is thus implemented in the same way to detect the disconnection of an external current transformer on the neutral conductor.

[0102] exist Figure 3 In the embodiment of the present invention, the electric meter 1' is also a three-phase meter intended to measure the electric energy supplied to a user's installation 2' through a distribution network 3' (via a circuit breaker 4'). This installation 2' is also an industrial installation.

[0103] In this case, the distribution network 3' comprises only three phases Ph1, Ph2, Ph3 (without neutral), and the meter comprises only three voltage inputs V1, V2, V3 (and three current inputs I1, I2, I3). External transformers CT1, CT2, CT3 are mounted on the phases Ph1, Ph2, Ph3. The meter 1' comprises a test circuit (integrated test generation chain and test measurement chain) similar in structure to the meter 1 and operating in the same way.

[0104] Of course, the invention is not limited to the embodiments described but encompasses any variant coming within the scope of the invention.

[0105] The present invention can be implemented with a single-phase or multi-phase meter connected to any number of phases.

[0106] Of course, the architecture of the instrument may differ from that described here.

[0107] Here it has been described that the microcontroller 11 belongs to both the main measurement chain and the test circuit. However, it is possible to have a first microcontroller integrating a first ADC and dedicated to the main measurement chain, and a second microcontroller integrating a second ADC and dedicated to the test circuit.

[0108] Likewise, it is possible to use a single ADC that is integrated in a microcontroller and belongs to both the main measurement chain and the test measurement chain. The microcontroller thus comprises a main input belonging to the main measurement chain and a test input belonging to the test measurement chain. The test input and the main input are connected to an ADC that integrates a multiplexer having (at least) two inputs connected to the test input and the main input, respectively, and an output connected to a conversion core of the ADC.

[0109] The ADC(s) may also be one or more ADCs external to the microcontroller.

[0110] Just like the high-pass filter and the envelope detector of the test measurement chain, the low-pass filter of the main measurement chain can be a software component programmed in the microcontroller; in this case, the ADC(s) are located upstream of these modules.

[0111] The processing component need not be a microcontroller, but may be a different component and for example a conventional processor, a DSP (digital signal processor), or a programmable logic circuit such as an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit).

[0112] The front circuit does not necessarily have to include an internal current transformer, but may include any component, even just a conductive element connecting the conductor 7 and the ground to the ports I+ and I−, respectively.

Claims

1. An electric meter (1, 1'), comprising: a front circuit (5) arranged to be connected to an external current transformer (CT) located outside the meter and installed on a phase (Ph) of the distribution network (3, 3'); a conductor (7) arranged to be connected to the external current transformer via the front circuit; A test circuit (14) comprising a test generation chain (15) having an output connected to the conductor (7), a test measurement chain (16) having an input connected to the conductor, and a processing component (11); The test generation chain comprises a voltage generator (17) and is arranged to apply a test voltage (Vt) on the conductor, the test voltage generating a test current (It) circulating in the test measurement chain and having a level depending on the equivalent impedance of the front circuit and the external current transformer, the test measurement chain being arranged to measure the level of the test voltage, the processing component (11) being arranged to acquire the level of the test voltage and to detect the disconnection of the external current transformer when the level of the test voltage is greater than a predefined detection threshold.

2. The electric meter according to claim 1, characterized in that The front circuit (5) comprises an internal current transformer (T) arranged to be connected to the external current transformer (CT).

3. An electric meter according to any one of the preceding claims, characterized in that It also comprises a main measurement chain (8) having an input connected to the conductor (7), the main measurement chain being arranged so that a main measurement current (Ip) as an image of the phase current (I) circulating on the phase (Ph) circulates in the main measurement chain, the processing component (11) being arranged to acquire the level of the main measurement current in order to compare it with a predefined test threshold and to control the test generation chain (15) so that the test voltage (Vt) is applied to the conductor when the level of the main measurement current is less than the predefined test threshold.

4. The electric meter according to claim 3, characterized in that: The processing component comprises a test input belonging to the test measurement chain and a main input belonging to the main measurement chain, the test input and the main input being connected to a first analog-to-digital converter integrated in the processing component.

5. The electric meter according to claim 1 or 2, characterized in that: The test generation chain (15) comprises a first switch (S) between the conductor (7) and the voltage generator (17), the processing component (11) being arranged to close the first switch (S) so that the test generation chain applies the test voltage (Vt) to the conductor (7).

6. An electric meter as claimed in claim 1 or 2, wherein the test measurement chain (16) comprises a second switch (S') connected to the conductor (7) and a detection chain (19) located downstream of the second switch, and the processing component is arranged to close the second switch (S') so that the test current circulates in the test measurement chain.

7. The electric meter according to claim 6, characterized in that The detection chain (19) comprises, from upstream to downstream, a high-pass filter (20), an envelope detector (21), a second analog-to-digital converter (22) and a digital low-pass filter (23).

8. The electric meter according to claim 1 or 2, characterized in that: The electricity meter is a multiphase meter comprising a plurality of front circuits, each front circuit (5) being arranged to be connected to an external current transformer (CT) located outside the meter and mounted on one phase of a multiphase distribution network.

9. The electric meter according to claim 5, characterized in that: The test generation chain (15) comprises a single voltage generator (17) and a plurality of first switches (S), each first switch (S) being connected to one of the preceding circuits.

10. The electric meter according to claim 6, characterized in that The test measurement chain (16) comprises a single detection chain (19) and a number of second switches (S'), each second switch (S') being connected to one of the preceding circuits.

11. A testing method implemented in a processing component (11) of an electricity meter (1, 1') as claimed in one of the preceding claims and comprising the following steps: The test voltage (Vt) is applied to the conductor (7) by the test generation chain (15); Acquiring the level of the test voltage via the test measurement chain (16); When the level of the test voltage is greater than a predefined detection threshold, a cut-out of the external current transformer (CT) is detected.

12. A recording medium readable by a computer, on which a computer program including instructions is recorded, wherein the instructions drive the processing component (11) of the electric meter according to any one of claims 1 to 10 to execute the steps of the test method according to claim 11.

13. A computer program comprising instructions for driving a processing component (11) of an electric meter as claimed in any one of claims 1 to 10 to perform the steps of the test method as claimed in claim 11.

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