Title of Invention

A DIFFERENTIAL PROTECTION DEVICE

Abstract

ABSTRACT A DIFFERENTIAL PROTECTION DEVICE SENSITIVE TO PULSED CURRENTS The present invention relates to a differential protection device of an electrical installation. This device comprises a current transformer (1) comprising a toroid (2) formed by a magnetic core, primary windings formed by the active conductors of the installation and a secondary winding (3) at the terminals of which a differential fault signal is established when a differential fault occurs in the primary windings, a tripping relay (4) of a current breaking apparatus, and a secondary connection circuit (5) connecting the relay (4) to the terminals of the secondary winding said circuit (5) being arranged to bring about tripping when said signal exceeds a preset threshold. The above-mentioned secondary circuit (5) comprises a full-wave rectifier (6, 7) and a capacitor (8), connected in parallel with the secondary winding (3) of the toroid (2), and the magnetic core is made of a nanocrystalline material.

Full Text

The present invention relates to a differential protection device of an electrical installation, of the kind comprising a differential transformer comprising a toroid formed by a magnetic core, primary windings formed by the active conductors of the installation and a secondary winding at the terminals of which a differential fault signal is established when a differential fault occurs in the primary windings a tripping relay of a consent breaking apparatus, and a secondary connection circuit connecting the relay to the terminals of the above-mentioned secondary winding, said circuit being arranged to bring about tripping when said signal exceeds a preset threshold. Trip devices by fault current have been used for many years for protection of machines and people. For protection of people, the tripping current can be about 30 mA, whereas it is in a range of about 300 to 500 mA for protection of machines. However, during the last few years, more and more electronic devices with current rectifier effect have been incorporated in numerous electrical apparatuses. These rectifier effects may generate a DC component liable to influence the operation of the differential device. The increased use of electronic trip devices, notably in household appliances, also requires the latter to respond in complete safety not only to alternating currents but also to pulsed DC fault currents. The limit values defined for trip devices of this kind have been set by the standard/VDE 0664) Trip devices of , the previously mentioned kind are known, which meet this specific requirement, wherein the magnetic core of the transformer toroid is made of a crystalline material designed for this use. The main quantities characteristic of these materials are the induction amplitude ',B for a sinusoidal excitation current, the static induction elevation AB stat for a half-wave rectified sinusoidal excitation current, and the dynamic induction elevation AB dyne for a full-wave rectified sinusoidal excitation current. In these trip devices, it is known to fit a capacitor between the transformer secondary winding and the relay tripping winding in order to increase the sensitivity of the trip device by increasing the power at the level of the relay. An oscillating circuit is thus formed by the secondary winding and the capacitor. The resonance frequency of this oscillating circuit then has to be tuned with the frequency of the voltage in the secondary winding due to the fault current. Tuning of this resonant circuit is performed by defining the number of turns of the secondary winding based on the capacitance values prescribed for the capacitor, and the tripping conditions of the trip device are determined. However, the number of turns finally set simply represents a compromise between the different forms of fault current. European Patent application (EP-0,563,606 describes a current transformer for a trip device, enabling safe interruption of a user circuit subjected to pulsed currents to be obtained, tripping being performed in a manner practically independent from the form of the fault current. These results are obtained due to the use of a magnetic core made of a nanocrystalline material presenting the following magnetic characteristics : Br/Bs 0.6T, for a field intensity amplitude of 100mA/cm, ABdyn max > 0.7T and ABdyn/',B > 0.7, these magnetic cores having been achieved in two stages. The magnetic quantities presented in this patent are in fact interesting for a trip device setup using a capacitor. The presence of this capacitor and the fact of obtaining ABdyn/',B > 0.7 enable a more symmetrical current form to be obtained on the secondary. In this case, the polarised electromagnetic trip device operates for two more symmetrical thresholds, which results in a greater ease of setting adjustment when manufacturing. However, although they are advantageous, these features are nevertheless constraining, as they require a particular treatment of the magnetic core materials. Furthermore, in this patent, the trip device does not enable an efficient differential protection to be obtained for common applications such as graduators which can have very brief opening angles ranging from 135° to 180°. The present invention overcomes these problems and proposes a differential protection device whose sensitivity to pulsed currents is improved without constraining magnetic characteristics for the magnetic core of the transformer toroid. The invention, in a particular embodiment, in addition enables a wider protection to be achieved, in pulsed currents, notably for angles a > 135° concerning certain graduators. For this purpose, the object of the present invention is to achieve a differential protection device of the kind previously mentioned, this device being characterized in that the above-mentioned secondary circuit comprises a rectifier and that the magnetic core of the toroid is formed by a nanocrystalline material. According to a particular embodiment of the invention, the magnetic core is made from a soft magnetic iron based alloy made up of at least 50% of fine crystal grains of a size smaller than 100 nm and containing, in addition to an iron content greater than 60 % (at), 0.5 to 2 % of copper, 2 to 5 % at least of one of the following metals, niobium, tungsten, tantalum, zirconium, hafnium, titanium and/or molybdenum, 5 to 14 % of boron and 14 to 17 % of silicon. According to a particular embodiment, the above-mentioned rectifier comprises two diodes respectively connecting the two ends of the secondary winding to one of the ends of the coil of the relay whereas the other end of the coil is connected to a mid¬point of the secondary winding. Advantageously, the two diodes are Zener diodes. According to another embodiment, the above-mentioned rectifier comprises a diode bridge connecting the two ends of the secondary winding to the terminals of the coil of the relay. According to a particular feature, the secondary circuit comprises in addition a capacitor connected in parallel with the secondary winding of the toroid. According to another feature, the voltage surge factor of the secondary circuit is about 3.5. According to another feature, the resonance frequency of the circuit comprising the secondary winding of the toroid and the capacitor is about 120 Hz. According to a particular feature, the secondary circuit comprises in addition a storage capacitor and means for comparison connected to the storage capacitor and comprising a monitoring output connected to the control means of the relay, so as to supply a tripping signal to the relay if the value of the voltage of the capacitor is greater than a preset threshold. Advantageously, the means for comparison comprise a comparator or a voltage threshold diode. Preferably, the control means of the relay comprise a thyristor. Accordingly the present invention provides a differential protection device of an electrical installation comprising a differential transformer comprising a toroid formed by a magnetic core, primary windings formed by the active conductors of the installation and a secondary winding at the terminals of which a differential fault signal is established when a differential fault occurs in the primary windings, a tripping relay of a current breaking apparatus, and a secondary connection circuit connecting the relay to the terminals of the above-mentioned secondary winding, said circuit being arranged to bring about tripping when said signal exceeds a preset threshold, characterized in that the above-mentioned secondary circuit comprises a rectifier and the magnetic core of the toroid is made of nanocrystalline material. But other advantages and features of the invention will become more cieariy apparent from the following description, refining to the accompanying drawings given as examples only and in which : Figure 1 is a curve representative of the variation of the tripping thresholds for a crystalline material and a nanocrystalline material. Figures 2, 3 and 6 respectively illustrate three embodiments of a trip, device according to the invention. Figure 4 illustrates two curves representing on the y-axis the voltage surge factor and on the X-axis the ratio C/C(50Hz), respectively for a crystalline material and a nanocrystalline material. Figure 5 illustrates the relative reduction of the thresholds in class A, achieved by the nanocrystalline compared to the crystalline according to the opening angle. In figures 2, 3 and 6, three embodiments can be seen respectively of a differential trip device D according to the invention designed to be incorporated in or associated for example to an electrical circuit breaker (not represented) for breaking the active conductors supplying an electrical installation. This device D comprises commonly to the three embodiments, a differential transformer 1 formed by a magnetic core toroid 2 comprising a primary winding (not represented) formed by the active conductors of the installation passing through the toroid 2, and a secondary winding 3 connected to the coil of a tripping relay 4 of the polarised type, by a secondary connecting circuit 5. The magnetic core of the toroid 2 is made of nanocrystalline material. The secondary circuit 5 of the trip device D represented in figure 2 is formed by two diodes 5, 7 connected on input respectively to the two ends of the secondary winding 3 and on output to the positive pole of the tripping relay 4, whereas the negative pole of the relay 4 is connected to a mid-point 3a of the secondary winding 3 of the toroid 2. In the embodiment illustrated in figure 3, the secondary circuit 5 comprises in addition a capacitor 8 connected in parallel to the secondary winding 3 of the toroid 2. It should be noted that sneer diodes or other equivalent devices will be advantageously used to prevent spurious tripping. In the embodiment illustrated in figure 6, the secondary circuit 5 comprises a tuning capacitor 13 connected in parallel with the secondary winding 3 of the toroid 2 and with a rectifier bridge P whose outputs are connected in parallel to a storage capacitor 14, which is connected in parallel with the relay 4 and a thyristor 15 mounted in series, and with a threshold circuit 16 comprising a control output connected to the thyristor 15. For the first two embodiments, the tripping relay 4 commands tripping of the circuit breaker when the fault signal delivered to the coil of the relay 4, when a fault current appears in the primary windings, exceeds a preset tripping threshold. When the voltage obtained on the secondary 3 does not present two symmetrical half-waves, the rectifier 6, 7 performs symmetrisation of the current on the secondary, necessary for the polarised relay 4 to operate for two symmetrical thresholds. In the third embodiment, the threshold circuit 16 supplies a tripping signal to the relay 14 via the thyristor 15, when the voltage at the terminals of the storage capacitor 14 exceeds a certain threshold. In the embodiment illustrated in figure 2, operation of the trip device D with a good sensitivity to pulsed currents can be obtained for example by using a nanocrystalline presenting the following magnetic characteristics : ABdyn (100 mA t/cm) In figure 1, curve (a) represents the standard boundary establishing the tripping thresholds, in terms of the opening angle, according to the standard VDE 0664. Curve (b) represents the variation of the thresholds in terms of (a) when a crystalline material (comprising 78 % Nl) is used for the magnetic core of the toroid (2) in a trip device (D) whose secondary circuit comprises a rectifier, whereas curve (c) represents the same variation for use of a nanocrystalline material in the same type of trip device. Referring to figure 1, it can be seen that the use of a nanocrystalline material for the magnetic core of the toroid 2 in a secondary circuit 5 with rectifier enables differential tripping thresholds (S) to be obtained (curve c) in terms of the opening angle (a) which are appreciably lower than those established by the standard boundary (curve a), for opening angles up to 135°. The reduced losses proper to nanocrystallines able to be highlighted by measurement of AB dyn in terms of the frequency enable a differential protection to be obtained with a widened opening angle (a), as illustrated in figure 5. This curve represents on the y-axis the reduction of the thresholds (R) in %, in pulsed currents, achieved by use of nanocrystalline compared with a crystalline comprising 55 % Ni. In the embodiment illustrated in figures 3 and 6, the capacitor 8 enables the power supplied to the relay 4 to be increased. The use of the nanocrystalline material enables sufficient energy to trip the relay 4 to be picked up and transmitted to the secondary 3. The secondary winding 3 and capacitor 8 form a resonant circuit whose resonance frequency is chosen in such a way that the voltage surge created by the capacitor 8 is about 3.5 times greater than that occurring without a capacitor. It can be noted that the voltage surge factor (f) is defined as being the ratio between the voltage of the capacitor 8 at a given frequency over the voltage at the terminals of the secondary winding 3, without a capacitor, for the same frequency. The resonance frequency is also chosen so that the pass-band of the filter comprising the capacitor does not chop the harmonics of frequency higher than 50 Hz present in the pulsed current signals too much, notably for a = 135°. When a crystalline material is used, this frequency is in general about 75 Hz. When the material constituting the magnetic core is a nanocrystalline material, due to its low losses it generates a voltage surge twice that of a traditional crystalline material, as can be seen in figure 4. Curves d, e of this figure 4 represent on the y-axis the voltage surge factor f, and on the x-axis the ratio between the capacitance c of the capacitor and a capacitance value for a resonance of 50 Hz (C, 50 Hz), respectively for a nanocrystalline material (d) and for a traditional crystalline material (e). It can thus be seen on these curves that for a voltage surge coefficient value of 3.5, the corresponding quantity C/C (50 Hz) for a nanocrystalline B is lower than that A of the crystalline, which enables a resonance frequency of about 120 Hz to be chosen. This results in a widening of the differential pass-band, which enables a wider protection range to be achieved in class A (pulsed currents), i.e. for angles 180° (these angles being present in certain graduators). It can be noted that the magnetic core could advantageously be formed by a soft magnetic iron based alloy made up of at least 50% of crystallites of a size smaller than 100 nm and containing the following atomic percentage : in addition to an iron content greater than 60 %, 0.5 to 2 % of copper, 2 to 5 % at least of one of the following metals, niobium, tungsten, tantalum, zirconium, hafnium, titanium and/or molybdenum, 5 to 14 % of boron and 14 to 17 % of silicon. A differential protection device has thus been achieved enabling lowering of the pulsed current tripping thresholds to be achieved notably for opening angles a > 135°, due on the one hand to the low losses characterising nanocrystailine and on the other hand to a widening of the pass-band of the assembly formed by the toroid and tuning capacitor. Naturally, the invention is not limited to the embodiments described and illustrated which have been given as examples only. On the contrary, the invention also comprises all the technical equivalents of the means described as well as their combinations if these are made according to the spirit of the invention. WE CLAIM: 1. A differential protection device of an electrical installation comprising a differential transformer comprising a toroid formed by a magnetic core, primary windings formed by the active conductors of the installation and a secondary winding at the terminals of which a differential fault signal is established when a differential fault occurs in the primary windings, a tripping relay of a current breaking apparatus and a secondary connection circuit connecting the relay to the terminals of the above-mentioned secondary winding, said circuit being arranged to bring about tripping when said signal exceeds a preset threshold, characterized in that the above-mentioned secondary circuit (5) comprises a rectifier (6, 7) and the magnetic core of the toroid (2) is made of nanocrystalline material. 2. The device as claimed in claim 1, wherein the magnetic core is made from a soft magnetic iron based alloy, made up of more than 50% of fine crystal grains of a size smaller than 100 nm and containing, in addition to an iron content greater than 60% (at), 0.5 to 2% of copper, 2 to 5% at least of one of the following metals, niobium, tungsten, tantalum, zirconium, hafnium, titanium and/or molybdenum, 5 to 14%o of baron and 14 to 17% of silicon. 3. The device as claimed in any of the claims 1 or 2, wherein the above mentioned rectifier comprises two diodes (6, 7) respectively connecting the two ends of the secondary winding (3) to one of the ends of the coil of the relay (4) whereas the other end of the coil is connected to a mid-point (3a) of the secondary winding 4. The device as claimed in claim 3, wherein the two diodes (6,7) are two Zener diodes. 5. The device as claimed in claims 1 or 2, wherein the above mentioned rectifier comprises a diode bridge connecting the terminals of the secondary winding (3) to the terminals of the coil of the relay (4). 6. The device as claimed in any one of the preceding claims, wherein the secondary circuit (5) comprises in addition a capacitor (8) connected in parallel with the secondary winding (3) of the toroid (2). 7. The device as claimed in claim 6,wherein the voltage surge factor (f) of the secondary circuit (5) is about 3.5. 8. The device as claimed in claim 7, wherein the resonance frequency of the circuit comprising the secondary winding (3) of the toroid (2) and the capacitor (8) is about 120 Hz. 9. The device as claimed in any one of the preceding claims, wherein the secondary circuit (5) has in addition a storage capacitor (14) and means for comparison (16), said means for comparison being connected to the storage capacitor (14) and comprising a monitoring output connected to the control means (15) of the relay (4) so as to supply a tripping signal to the relay (4) if the value of the voltage of the capacitor (14) is greater than a present threshold. 10. The device as claimed in claim 9, wherein the means for comparison (16) comprise a comparator. 11. The device as claimed in claim 9, wherein the means for comparison (16) comprise a voltage threshold diode. 12. The device as claimed in any of the claims of 9 to 12, wherein the control means of the relay (4) comprise a thyristor (15). 13. A differential protection device of an electrical installation substantially as herein described with reference to the accompanying drawings.

Documents:

614-mas-1996 abstract duplicate.pdf

614-mas-1996 abstract.pdf

614-mas-1996 claims duplicate.pdf

614-mas-1996 claims.pdf

614-mas-1996 correspondence others.pdf

614-mas-1996 correspondence po.pdf

614-mas-1996 description (complete) duplicate.pdf

614-mas-1996 description (complete).pdf

614-mas-1996 drawings.pdf

614-mas-1996 form-1.pdf

614-mas-1996 form-26.pdf

614-mas-1996 form-4.pdf

614-mas-1996 petition.pdf