AU2013285296B2

AU2013285296B2 - Differential protection device

Info

Publication number: AU2013285296B2
Application number: AU2013285296A
Authority: AU
Inventors: Hichem CHETOUAN
Original assignee: Hager Electro SAS
Current assignee: Hager Electro SAS
Priority date: 2012-07-05
Filing date: 2013-06-28
Publication date: 2016-06-16
Anticipated expiration: 2033-06-28
Also published as: PT2870670T; EP2870670B1; FR2993058A1; CN104521088A; WO2014006308A1; EP2870670A1; CN104521088B; PL2870670T3; AU2013285296A1; FR2993058B1; ES2589307T3

Abstract

The invention relates to a differential protection device with I

Description

DIFFERENTIAL PROTECTION DEVICE

The present invention relates to a differential protection device consequently primarily making it possible to detect ground-fault currents. Such a device is in particular applicable in electric line protection apparatuses called Residual Current Devices (RCD), such as switches or differential circuit breakers, used in household or service industry electric facilities and that protect circuit trees which in turn are for example distinguished based on the type of load connected downstream or the receivers connected in the circuit that they protect.

These electric facilities are powered by an alternating electric grid, and the device to which the invention pertains is primarily intended to provide third-party safety by detecting any possible ground leak in one of the circuits to be protected; the source of any such leak must be located as quickly as possible to disconnect that circuit from the electric grid and thus stop the risk. Such leaks are for example caused by direct third-party contacts with an uninsulated conductor and indirect contacts due to ground faults. These electrical incidents can cause accidents that may sometimes be fatal.

In the aforementioned context, differential protection is provided in a fairly standardized manner in most of these cases: the conductors of the circuit to be protected pass in a core made from a ferromagnetic material, said conductors making up the primary of the transformer whereof one or more winding(s) constitute the secondary. In such a configuration, the core is a magnetic flux concentrator, and in the event of a leak resulting in any imbalance in the input and output currents in the conductors of the lines to be protected, the flux created in the core creates a voltage in the secondary winding, used to command directly or through the electronics dedicated to an actuator that may be of the relay type, which in turn actuates a mechanism triggering the apparatus in question and/or those of other protection apparatuses of the facility.

These differential protection electric apparatuses can have varying characteristics. Thus, they can be distinguished by their sensitivity, which in particular depends on the type of electrical facility in which they are used. Furthermore, some of these apparatuses can only detect leaks for alternating currents, while other, more complete apparatuses offer solutions also applicable to direct current. That is the case for the protection device according to the invention, which is particularly better adapted to detecting leaks made up of direct currents or variable currents comprising a direct component. The particular problem that the existence of a direct signal poses is as follows: in that case, there is no variation of the flux in the core made from ferromagnetic material and the voltage across the terminals of the secondary is therefore zero. The detection therefore cannot be based on the expected consequences of quantifiable magnetic flux variations in the core.

However, the existence of a relatively high direct fault of the primary in particular results in saturating the magnetic material making up the core, characteristic which can then be used in that detection: this is the principle of the present invention. In this respect, the device according to the invention, with sensitivity IDn and which allows the detection of leaks in an electric facility powered by at least one phase conductor and, in some cases, a neutral conductor of the grid, therefore making up the primary of a core made from ferromagnetic material, additionally comprises a first secondary winding, wound around said core and connected to a current or excitation voltage generator emitting an alternating current with constant amplitude and without a direct component. The core is also provided with a second secondary winding connected to processing stages for the signals induced by the leak currents appearing in the primary conductors and by the current passing through the first secondary winding, stages provided to control an actuator of the relay type provided to actuate a mechanical lock for triggering an electric protection apparatus.

In this scenario, the core makes up a magnetic circuit for the magnetic fluxes coming from the magnetic fields radiated by the primary conductors on the one hand, and produced by the first secondary winding on the other hand. The latter is passed through by an alternating current with no direct component and the working domain of the core, relative to the curve B(H) and consequently centered around the point H=B=0 on the corresponding characteristic curve of the material making up the core.

In the case where the sum of the currents conveyed by the primary conductors is zero, which means that there is no leak, the magnetic flux in the core, resulting from the sum of their respective magnetic fluxes, is also zero. Thus, the only magnetic flux generated in the core is that which results from the first secondary winding passed through by the alternating current with no direct component. The voltage across the terminals of the second secondary winding then results only from the variable magnetic flux produced by the first secondary winding.

In the case of a ground fault, the time sum of the currents conveyed by the primary conductors is no longer zero. The magnetic flux that results from the magnetic fields radiated by these primary conductors in the core is then no longer zero either.

The magnetic flux that flows in the core is currently the sum of two magnetic fluxes, namely that which results from the time imbalance of the currents in the primary conductors, and that which is produced continuously by the first secondary winding.

Likewise, the voltage induced in the second secondary winding of the core results from the two aforementioned fluxes. The idea at the base of the invention is to create the conditions for a noticeable variation of the signal in the second secondary winding, in case of leak detected at the primary conductors, in order to generate triggering of the differential electric apparatus under good conditions.

To that end, the invention is characterized in that: - the material making up the core has a maximum permeability μΓ for a magnetic excitation H corresponding to a maximum leak current that is equal to twice Idn; - the slope of the curve providing the permeability μΓ according to H is at least equal to -15m/mA in the descending portion of said curve.

These characteristics reflect a particular choice of the magnetic material of the core, guaranteeing the obtainment in the second secondary winding of the core of the voltage whose main rectified value is, in the presence of a ground fault, substantially lower than that which would be obtained without such a fault. Furthermore, the above characteristics mean that this voltage, which is in particular rectified by the electronic stages situated downstream from the winding, decreases significantly based on the intensity of the unbalanced fault currents of the primary conductors.

Concretely, to choose the material, the characteristic B(H) and consequently μΓ(Η) is such that the maximum permeability is in practice reached for the lowest possible magnetic field value H. Consequently, an unbalanced current, even below twice the current lDN, is sufficient to offset the working field of the core in the saturation zone of the characteristic B(H), and consequently in the zone of the characteristic μΓ(Η) where the relative permeability not only becomes weaker, since it corresponds to the magnetic saturation of the materials making up the core, but also decreases significantly.

Furthermore, this makes it possible to not necessarily saturate the magnetic material with the excitation signal from the first secondary winding, which has the advantage of significantly reducing the consumption of the device. It is in fact necessary to situate oneself in the saturation zone only when the leak flux is added to the flux created by the permanent alternating current, which will be described as excitation current.

The second aforementioned condition on the slope of the μΓ(Η) indicates a rapidly decreasing permeability according to H after reaching the maximum permeability. However, H constitutes the image of the fault current. This characteristic guarantees that an offset of the working field in that zone causes a very significant decrease of the main rectified voltage across the terminals of the second secondary winding due to the rapid decrease of the permeability and consequently of the voltage across the terminals of the second secondary winding.

The permeability is in fact no longer constant, in the event the second secondary winding produces a signal that depends both on the excitation current due to the first secondary winding and the direct fault current, because the operation of the core becomes offset toward the saturated region of the curve B(H), where the permeability—which is the slope of that curve—varies in the direction of a decrease.

Other parameters make it possible to further optimize the decrease of said voltage, for example the sizing of the core. The inner and outer diameters thereof have been adjusted to values that make it possible to minimize the current emitted by the generator to the first secondary winding.

Thus, according to the invention, the dimensions of the core obey the following relationship: (Dext+Dint) x H (prmax) — (4xN1xIdn)/|"1 where Dext is the outer diameter of the core, Dint is the inner diameter, N1 is the number of primary turns, H(prmax) is the value of the magnetic excitation for the maximum permeability, and Idn is the sensitivity of the differential device.

The material of the core is of course important, since the selection of the material is the main factor making it possible to provide these characteristics.

Preferably, the current emitted by the generator has a frequency higher than the frequency of the grid. As previously mentioned, the saturation of the core by that current is not required. In fact, the idea is to reduce the excitation current to reduce the consumption of the device while trying to obtain, by adding fluxes due to the excitation and the currents, a high enough voltage at the output of the second secondary winding, such that the signal can be used by the electronic stages placed downstream.

According to one possible configuration, the current generator emitting an excitation current to the address of the first secondary winding is powered by a first AC/DC converter.

As mentioned, a certain number of processing stages for the signal appearing across the terminals of the second secondary winding make it possible to obtain a control signal for a relay capable of actuating a mechanical lock. The device according to the invention is such that the second secondary winding is in particular connected to at least one amplifier stage of the signal and/or one time delay stage, at the output of which a comparison stage between a reference voltage and the output voltage is found, then a power control stage to control a relay.

In fact, preferably, the time delay stage is a voltage multiplier AC/DC converter stage or a rectifier stage with a capacitance across its terminals.

This converter must be able to generate a time delay between its input and output, that delay in practice making it possible to have a delayed reaction facilitating the management of spikes of the EMC type and strong balanced currents, and to manage issues that occur upon powering on the device.

The reference voltage at the input of the comparison stage preferably comes from the first AC/DC converter. According to one possibility, a time delay stage may additionally be connected between the first AC/DC converter and the comparison stage.

Furthermore, the use of a comparator stage, which is well suited to an alternating current solution, is less suitable for a direct current solution: this comparator stage may then be made up of a subtracter stage followed by a threshold voltage detection stage.

The invention will now be described using the appended figures, in which: - figure 1 shows a B(H) characteristic adapted to the device according to the invention, - figure 2 shows a pr(H) characteristic that corresponds to the previous one, - figures 3 and 4 respectively show the B(H) and μΓ(Η) characteristics in the case where no leak current is present, - figure 5 shows an example of curve B(t) that can be obtained with no leak current, with the corresponding alternating current Uc(t), - figures 6 and 7 show the characteristics B(H) and pr(H) in the event a leak current is present, - figure 8 shows an example of curve B(t) that can be obtained with leak current lDc, which appears in the figure as the sum of the currents lDc+lAc(t), - figure 9 diagrammatically illustrates the device according to the invention in a block diagram, and - figure 10 shows one possible alternative of the invention.

In reference to figure 1, the appropriate characteristic B(H) for a core suitable for a differential protection device according to the invention is such that on the curve corresponding to the drift, a characteristic pr(H) with apices obtained for the lowest possible value of H in absolute value, as shown in figure 2, is obtained. Once the second secondary winding (L2) shown in figures 9 and 10 is no longer only excited by the alternating excitation current Uc(t) traveling through the first secondary winding (L1), for operation centered on B=H=0 in characteristic B(H) as shown in figure 3, but a leak current at the primary appears, a shift of the working zone around a point BO occurs. This is shown in figure 6.

In the first case, shown in figure 3, the working point is in the linear region of the curve B(H), resulting in a quasi-constant permeability μΓ between μπ and μΓ2, as shown in figure 4, and a voltage U2 at the secondary (L2) that can be expressed as follows:

BAc and HAc are the values of B and H when there is only one alternating excitation signal AC, in other words a single excitation current lAc(t). In the case of an excitation current lAC(t) and a direct fault currents Idc, as illustrated in figure 6, the working point is offset toward the nonlinear saturation region of the curve B(H) and the permeability μΓ is no longer constant and becomes dependent on H (see figure 7), which also depends on the direct fault current. The voltage U2 is then expressed:

Bdc and HDC are the values of B and H due to the leak current DC. In the latter case, the median oscillation point of the permeability between its min (B2) & max (B1) values is not situated at B=0 as in figure 3, but at a point BO greater than 0 (or less than 0 depending on the sign of HDc), which is shown in figure 6.

If Hoc increases sufficiently, in the case under consideration BO moves toward the right of the curve, which means that the operating point of the core is offset toward the saturated region of the curve of B(H) and, in that zone, the permeability is lower (see figure 7) and the voltage experiences a correlated decrease.

Figure 8 shows the signals that appear, both for the currents creating magnetic fluxes and for the resulting magnetic induction, as a function of time.

The device shown in figures 9 and 10 is primarily made up of the following elements: - A power supply (C1) allowing an AC/DC conversion of the voltage of the grid. - A core (1) passed through by the currents (2) of the conductors of the electrical facility to be protected: phase conductors (2) and optionally a neutral conductor. - The core (1) is provided with at least two secondary windings L1 and L2. - An auto-frequency generator (G) continuously injecting an alternating excitation signal (voltage or current) Uc(t) with a fixed amplitude and frequency higher than that of the grid, via the first secondary winding (L1). The saturation of the core (1) by the excitation field H produced by the excitation coil (L1) is not necessary. - A second secondary winding (L2)—or more—to measure the voltage induced by the excitation winding (L1) and its variation following the presence of a fault current. - A stage (C2) allowing an AC/DC conversion of the voltage delivered by the second secondary winding (L2) of the core. Preamplification of this voltage may be necessary to ensure an optimal conversion via a first amplifier stage (A1). This stage (C2) also allows, with the help of the presence of at least one capacitance, the creation of a delay between the input signal (U1 or U2) coming from the second secondary winding (L2) and its output voltage (U3). Such a delay makes it possible to have a delayed reaction facilitating spike management: EMC and strong currents.

Alternatively, the delay may be produced by a voltage multiplier. - A second (optional) amplification stage (A2). - A subtracter (S) (figure 9) or comparator (C) (figure 10) makes it possible to compare the voltage (U4) to a reference voltage (U5). - A time delay circuit (R) makes it possible to delay the arrival of the reference voltage at the subtracter (S) or the comparator (C) to avoid the issues that occur following powering on of the device. - The output of the subtracter (S) may be connected to the input of a voltage detector (D). In the case where a comparator (C) is used, the output can serve as a control signal attacking the input of the control stage (P). - The output of the voltage detector (D) or that of the comparator (C) drives a control stage (P) in power electronics, making it possible to give the order to an electromagnetic relay (or an actuator) (10) to unlock a mechanical lock (11) and open the contacts (12) placed on the conductors (2) of the grid, which provides protection for people with respect to the lines in question.

The circuits shown in figures 9 and 10 are only possible examples, which may comprise modifications, some of which have been mentioned (adding an amplifier stage, different time delay possibilities, etc.) without going beyond the scope of the invention.

Claims

1. A differential protection device with sensitivity Idn allowing the detection of leaks in an electric facility powered by at least one phase conductor and, in some cases, a neutral conductor of the grid, said conductor making up the primary of a core made from ferromagnetic material provided with a first secondary winding connected to a current or excitation voltage generator emitting a current with constant amplitude without a direct component, and a second secondary winding connected to processing stages for the signals induced by the leak currents appearing in the primary conductors and by the current passing through the first secondary winding in order to control an electromagnetic actuator or a relay provided to actuate a mechanical lock, wherein: the material making up the core has a maximum permeability μΓ for a magnetic excitation H corresponding to a maximum leak current that is equal to twice Idn; - the slope of the curve providing the permeability μΓ according to H is at least equal to -15 m/mA in the descending portion of said curve.

2. The differential device according to the preceding claim, wherein the dimensions of the core (1) obey the following relationship:

where Dext is the outer diameter of the core, Dint is the inner diameter, N1 is the number of primary turns, H(prmax) is the value of the magnetic excitation for the maximum permeability, and Idn is the sensitivity of the differential device.

3. The differential protection device according to any one of the preceding claims, wherein the current emitted by the generator has a frequency higher than the frequency of the grid.

4. The differential protection device according to the preceding claim, wherein the generator is powered by a first AC/DC converter.

5. The differential protection device according to any one of the preceding claims, wherein the second secondary winding is connected to at least one amplifier stage of the signal and/or one time delay stage, at the output of which a comparison stage between a reference voltage and the output voltage is found, then a power control stage to control a relay.

6. The differential protection device according to the preceding claim, wherein the time delay stage is a voltage multiplier AC/DC converter stage or a rectifier stage with a capacitance across its terminals.

7. The differential protection device according to one of claims 5 and 6, wherein the reference voltage comes from the first AC/DC converter.

8. The differential protection device according to the preceding claim, wherein a time delay stage is connected between the first AC/DC converter and the comparison stage.

9. The differential protection device according to one of claims 5 to 8, wherein the comparator stage is made up of a subtracter stage followed by a threshold voltage detection stage.