SINGLE-GATE LOOP MEASURING METHOD AND METERS WITH DC LEAKAGE CURRENT COMPENSATION

Description

Technical Field

[0001] The present invention relates to the field of electrical installation control and more particularly concerns the measurement of earth resistances using loop ohmmeters. Previous technique

[0002] Loop ohmmeters are devices designed to check the conformity of earth connections in electrical installations or buildings, such as the one illustrated in the figure 1 They are particularly suitable when the electrical installation has multiple parallel earthings forming several successive earth loops because the measurement does not require driving auxiliary stakes into the ground or opening an earth connection (e.g., earth bar) to isolate the electrical installation.

[0003] Traditionally, loop ohmmeters use two magnetically isolated transformers that enclose the grounding conductor. One transformer injects a voltage by induction, and the other measures the current flowing through the loop. The impedance of the ground connection is then deduced from the ratio between this voltage and the measured current.

[0004] However, and as the figure 2 , it is known by patent EP1 566 644 B1 filed in the name of the applicant a single-core loop ohmmeter which makes it possible to do without the current transformer and the magnetic isolation between the two transformers by using only one transformer consisting of a toroid with a primary winding Np, used simultaneously for the injection of the voltage and the measurement of the current flowing in the earth loop, and a secondary winding Ns with a single turn formed by the earth connection conductor and whose loop impedance Zx is to be measured.Applying a setpoint voltage Vp to the terminals of the transformer's primary causes a measurement current Ip to flow through this primary, which induces, in the enclosed conductor forming the secondary, the secondary voltage Vs generating the secondary current Is = Vs / Zx flowing in the loop, m being the ratio of the number of turns of the secondary winding Ns and the primary winding Np, i.e. m = Ns / Np.

[0005] Since the voltage applied across the primary winding, Vp, is a known quantity, if the transformer were perfect, it would suffice to measure the primary current, Ip, to determine the impedance, Zx. However, in reality, the transformer is not perfect and has magnetic flux losses, iron and copper losses, and a finite magnetic circuit permeability.

[0006] Also, the figure 3 illustrates the equivalent electrical diagram of the real transformer referred to the primary on which, Rf represents the equivalent resistance to the iron losses of the transformer, Lµ the magnetizing inductance of the transformer, i.e. the image of the non-infinite permeability of the magnetic circuit, If and Iµ the components of the magnetizing current of the transformer, Rp the resistance of the primary winding, i.e. the image of the copper losses, Ip the leakage inductance of the primary, i.e. the image of the magnetic flux losses, Rs the resistance of the secondary winding, Is the leakage inductance of the secondary and Ep the real voltage generating the magnetic flux of the transformer.

[0007] This equivalent scheme can be simplified given that the winding Ns is constituted by the loop whose impedance Zx we want to measure, it follows that Ns = 1 and Is becomes negligible and can be considered as being equal to zero, as is Rs, and that the value of Rp is negligible compared to ZxNp 2< hence Rp ≈ 0.

[0008] The equivalent diagram therefore reduces to the simplified form illustrated on the figure 4 , which allows us to write: Ip / Vp = 1 / Z = 1 / Rf + 1 / jLµω + 1 / ZxNp 2<

[0009] Where ω = 2nf, f being the frequency of the setpoint voltage Vp.

[0010] The loop impedance measurement method described above therefore requires constant knowledge of the values ​​of Rf and Lµ to determine Zx. These two values ​​are obtained under no-load conditions (open loop) before the transformer core is installed around the loop whose impedance Zx is to be measured.

[0011] However, these values ​​vary depending on the level of induction in the toroid, which depends on the presence of parasitic AC and DC currents circulating in the measurement loop and which must therefore be compensated so that the values ​​remain usable during the measurement phase and guarantee the desired measurement accuracy over the desired measurement range. Description of the invention

[0012] The main purpose of the present invention is to reduce the influence of induction in the toroid (and mainly on iron losses of which Rf is the electrical model) due to the presence of parasitic DC current in the measurement loop.

[0013] This goal is achieved by a method for measuring a loop impedance Zx in a single-core ohmmeter comprising a single transformer having a primary winding of Np turns and a secondary winding of a single turn forming a measurement loop of impedance Zx, characterized in that, in order to compensate for a DC leakage current flowing in the measurement loop without canceling a measurement current Ip generating an alternating induction of constant average value for the measurement of the loop impedance Zx, said measurement current Ip is added to a constant current opposite and proportional to the parasitic induction produced by the DC leakage current, to be reinjected into the primary winding.

[0014] This parasitic induction can be measured by various methods such as a hall effect or fluxgate sensor or a Rogowski loop, but according to an advantageous embodiment, the measurement of the parasitic induction is derived from an output voltage Ub delivered across the terminals of a second secondary winding of Na turns of the single transformer and first integrated and then successively compared with a determined threshold and subjected to low-pass filtering.

[0015] Thus, by measuring, by whatever means and preferably by means of a second secondary winding, a quantity whose amplitude is correlated to a continuous component of the induction resulting from a DC leakage current, and by compensating the associated magnetic fields by injecting a DC current into the primary, the influence of this DC leakage current circulating in the measurement loop on the iron losses is reduced.

[0016] Advantageously, the comparison consists of determining the duration for which the output voltage Ub is positive and the duration for which it is negative.

[0017] The invention also relates to a single-core loop ohmmeter implementing the aforementioned method.

[0018] Depending on the embodiment envisaged, the output voltage processing module of the second secondary winding may consist of an integrator followed by a comparator with a determined threshold and a low-pass filter, the voltage-current converter and the value of the shunt resistance being chosen so that the number of ampere-turns supplied by the primary winding to a single transformer magnetic core is equal to the number of ampere-turns supplied to this magnetic core by the single-turn secondary winding, or alternatively, the output voltage processing module of the second secondary winding may consist of an integrator followed by a comparator with a determined threshold and a low-pass filter whose output voltage is corrected in a digital correction module.

[0019] Advantageously, the digital correction module consists of a microcontroller preceded by an analog-to-digital converter and followed by a digital-to-analog converter.

[0020] Preferably, the comparator is configured to determine the duration for which the output voltage Ub is positive and the duration for which it is negative.

[0021] Advantageously, the ratio between the number of turns Np of the primary winding and the number of turns Na of the second secondary winding is equal to one.

[0022] Preferably, the transformer is made in the form of a non-opening magnetic sensor left permanently on a grounding conductor and the loop impedance Zx is periodically transmitted to a remote server via at least one communication network. Brief description of the drawings

[0023] Other features and advantages of the present invention will become apparent from the description given below, with reference to the accompanying drawings which illustrate an example of an embodiment without being limiting in any way and on which: [ Fig. 1 ] there figure 1 shows a set of electrical lines with grounding points connecting various buildings, [ Fig. 2 ] there figure 2 schematically shows a single-core loop ohmmeter, [ Fig. 3 ] there figure 3 shows the equivalent electrical circuit of a real transformer referred to the primary, [ Fig. 4 ] there figure 4 shows the simplified diagram of the transformer of the figure 3 , [ Fig.5 ] there figure 5 shows two voltage curves across the second secondary winding of the transformer, [ Fig.6 ] there figure 6 is a schematic diagram of a single-core DC leakage flux compensated loop ohmmeter according to the invention, and [ Fig.7 ] there figure 7 is a schematic diagram of a single-core loop ohmmeter with DC leakage flux compensation in the case of purely analog processing. Description of the implementation methods

[0024] The principle of the invention is based on measuring the induction using a second secondary winding to compensate for its influence on Rf and Lµ. By using this second secondary winding, whose output voltage is the derivative of the magnetic flux, as an indicator of the DC flux, a proportional current is reinjected into the primary winding to compensate for this disturbing DC flux created by the current to be measured flowing in the measurement loop, without having to interrupt the measurement.

[0025] However, as the figure 5 The passage of a DC current through a toroid (the ferromagnetic element of the transformer) with high permeability biases it and distorts the hysteresis loop. As a result, a sinusoidal signal circulating in a primary winding is not perfectly reproduced in a secondary winding; in particular, the duration for which the signal in the secondary winding is positive differs from the duration for which it is negative. Thus, in the figure showing the signal observed across the terminals of this secondary winding, the dashed curve corresponds to the case where the DC leakage current is zero and shows a perfectly sinusoidal secondary winding voltage, while the solid curve corresponds to the presence of a DC leakage current through the toroid and shows a secondary winding voltage with a distorted amplitude that does not pass through the center point of the measurement.In particular, the duration of the positive part of the voltage is shorter than the duration of its negative part.

[0026] Based on this observation, the invention proposes to compare the duration of the positive alternation to the duration of the negative alternation of the signal taken from the second secondary winding to ensure compensation of the parasitic direct current flowing in the measurement loop (hereinafter DC leakage current).

[0027] There figure 6 illustrates a first schematic diagram of this compensation carried out in the single-core loop ohmmeter of the invention.

[0028] The transformer 10 is illustrated by its torus (the magnetic core 12), the primary winding 14 with Np turns, the single-turn secondary winding 16 Ns formed by the impedance measurement loop Zx, and the second secondary winding 18 with Na turns. Preferably, the ratio between the number of turns in the primary winding Np and the second secondary winding Na is equal to one, although this ratio is not limiting (it can be greater or less than 1). The current injected into the primary winding 14, the amplitude of which is fixed by a shunt resistor 19, comes from a voltage-to-current converter 20, typically an operational amplifier.receiving the output of an adder 22 intended to add a compensation signal (intended to create an alternating induction of constant average value in the toroid) to the measurement signal and therefore receiving on the one hand the alternating measurement voltage Vp corresponding to the voltage useful for measuring Zx and on the other hand a voltage Ucomp which corresponds to the control system put in place to compensate for the DC leakage current, and which is taken from the output of a digital-to-analog converter (DAC 30) controlled from a microcontroller 32 preceded by an analog-to-digital converter (ADC 34) whose input is taken from the output of a low-pass filter 36 preceded by a comparator 38 whose input is connected to an integrator 40 necessary to obtain a signal representing the induction in the toroid,that is to say, to find a constant current opposite to the DC leakage current and representing the magnetic field resulting from this DC leakage current and taken from the terminals of the second secondary winding 18. The comparator with a determined threshold allows comparison of the duration during which the output voltage Ub is positive and the duration during which it is negative, that is to say the periods when the induction in the magnetic core (the torus) is greater than 0 and those when it is less than 0.

[0029] The set of DAC 30, microcontroller 32 and ADC 34 form a digital correction module which, associated with the integrator 40, comparator 38 and low-pass filter 36, constitutes a processing module for the output voltage Ub.

[0030] The microcontroller 32 records and stores one (or advantageously several) values ​​of the integrated measurement (a monotonic value not directly proportional to the DC induction) of the comparator output in the absence of leakage current: for an open loop, and advantageously for one or more closed loops with a known impedance. These values ​​allow the definition of a control setpoint that modifies the generated current so that the average of the measured values ​​converges towards the setpoint.

[0031] There figure 7 illustrates a second schematic diagram of the compensation performed in the single-core loop ohmmeter in the case of using purely analog processing instead of mixed processing that is both analog and digital.

[0032] In this configuration, the adder directly receives the analog compensation current from the low-pass filter, and the voltage-to-current converter 20 and the value of the shunt resistor 19 are then chosen such that the number of ampere-turns supplied by the winding 14 to the magnetic core 12 is equal to the number of ampere-turns supplied to this magnetic core by the single winding 16. Thus, by ensuring that the direction of current flow in windings 14 and 16 is opposite to each other, the continuous magnetic flux coupled to the magnetic core 12 by the single winding 16 is canceled by the continuous magnetic flux coupled to the magnetic core 12 by the winding 14.

[0033] Left permanently on a ground connection conductor, i.e. with a transformer forming a non-opening magnetic sensor, the loop ohmmeter according to the invention can continuously monitor the loop impedance Zx, and therefore the ground impedance synonymous with the quality of protection at the point of installation of this sensor.

[0034] As shown by figure 1 , it may include its own means of communication in connection via a communication network (advantageously wireless such as 3G-5G or Wifi in particular) or wired (Ethernet in particular) with the corresponding means of communication of a local measurement box 50 to regularly transmit its impedance measurements to a remote server 52 via the Internet network 54 for example, thus avoiding the need for an operator to travel to ensure the control of the impedance Zx.

[0035] With this invention, the DC leakage current is compensated without canceling the measurement current injected for measuring the loop impedance Zx. Indeed, the measurement current Ip intended to be injected into the primary winding Np must not be affected by the DC leakage current control; hence the presence of low-pass filtering in the DC leakage current compensation signal to prevent this compensation from canceling the measurement current.

Claims

1. A method for measuring a loop impedance Zx in a single-torus ohmmeter including a single transformer having a primary winding of Np coils (14) and a secondary winding of a single coil (16) forming a measurement loop of impedance Zx, characterized in that, to compensate for a stray induction produced by a DC leakage current circulating in the measurement loop without canceling a measurement current Ip generating an alternating induction of constant average value for the measurement of the loop impedance Zx, said measurement current Ip is added to a constant current opposite and proportional to the stray induction produced by the DC leakage current, to be re-injected into the primary winding.

2. The method for measuring a loop impedance Zx according to claim 1, wherein the measurement of the stray induction is derived from an output voltage Ub delivered across a second secondary winding of Na coils of the single transformer and first integrated then successively subject to a comparison with a determined threshold and to a low-pass filtering.

3. The method for measuring a loop impedance Zx according to claim 1 or 2, wherein the comparison consists in determining the duration during which the output voltage Ub is positive and the duration during which it is negative.

4. A single-torus ohmmeter for measuring a loop impedance Zx, including a single transformer having a primary winding of Np coils (14) and a secondary winding of a single coil (16) forming a measurement loop of impedance Zx, characterized in that, to compensate for a stray induction produced by a DC leakage current circulating in the measurement loop without canceling a measurement current Ip generating an alternating induction of constant average value for the measurement of the loop impedance Zx, the transformer includes a second secondary winding of Na coils (18) whose output voltage Ub is delivered to a processing module (30 - 40) providing a voltage intended to be added to an alternating measurement voltage Vp in an adder (22) delivering the current to be injected into the primary winding (14) after passage through a voltage-current converter (20) supplying current to the primary winding via a shunt resistance (19).

5. The single-torus ohmmeter according to claim 4, wherein the module for processing the output voltage of the second secondary winding (18) consists of an integrator (40) followed by a comparator with a determined threshold (38) and by a low-pass filter (36), the voltage-current converter (20) and the value of the shunt resistance (19) being chosen so that the number of ampere-turns provided by the primary winding (14) to a magnetic core (12) of the single transformer is equal to the number of ampere-turns provided to this magnetic core by the single-coil secondary winding (16).

6. The single-torus ohmmeter according to claim 4, wherein the module for processing the output voltage of the second secondary winding (18) consists of an integrator (40) followed by a comparator with a determined threshold (38) and by a low-pass filter (36) whose output voltage is corrected in a digital correction module (30, 32, 34).

7. The single-torus ohmmeter according to claim 6, wherein the digital correction module consists of a microcontroller (32) preceded by an analog-to-digital converter (ADC 34) and followed by a digital-to-analog converter (DAC 30).

8. The single-torus ohmmeter according to claim 5 or 6, wherein the comparator is configured to determine the duration during which the output voltage Ub is positive and the duration during which it is negative.

9. The single-torus ohmmeter according to any one of claims 4 to 8, wherein the ratio between the number of coils Np of the primary winding (14) and the number of coils Na of the second secondary winding (18) is equal to one.

10. The single-torus ohmmeter according to any one of claims 4 to 9, wherein the transformer is made in the form of a non-opening magnetic sensor left permanently on a ground connection conductor and the loop impedance Zx is transmitted periodically to a remote server (54) via at least one communication network (52).