FR2915632A1 - Low voltage front link protecting method, involves setting electric screen on low voltage link, and connecting electric screen with three-phase disconnection and independent stiker - Google Patents

Abstract

The method involves setting an electric screen (19) on a low voltage link(15) that is constituted of an insulated cable and bar. The electric screen is connected with a three-phase disconnection (12) and an independent stiker (13). The stiker is sensitive to the current passed to a ground (14). The link is connected in downstream to a low voltage apparatus (17), which protects a distribution network (18) and in upstream connected to an electric bushing (16). Independent claims are also included for the following: (1) a device for electrically protecting the low voltage front connection of a transformer (2) a voltage transformer station.

Description

METHOD AND DEVICE FOR ELECTRICAL PROTECTION OF THE AVALK LINKS OF A

  The invention relates to a device for detecting and eliminating any electrical fault occurring at the level of the electrical connections located downstream of a medium voltage / low voltage transformer, and upstream. the low voltage protection generally provided for this type of installation. The invention typically applies to transformer stations for distribution of electrical energy, but can also relate to any type of installation using a medium voltage / low voltage transformer. It applies more particularly to installations comprising one or more transformers equipped with a protection device against the effects of internal faults as described in the patents cited below.

  In a conventional installation, the single or multiphase electrical connection, generally of reduced length, is not protected against a possible single or polyphase electrical fault which, if it arises, would be particularly intensive and could be maintained for a period of time. important time. This results in a danger for the equipment as well as for the personnel likely to be in the environment of the installation.

  To overcome this risk, certain national regulations have imposed instead of medium voltage protection, the use of fuses whose melting characteristic ensures elimination in less than 0.1 s of a possible single or multi-phase fault occurring on this low voltage link (hereinafter BT), a link of greater intensity.

  However, experience has shown the drawbacks of such a solution: a medium-voltage fuse (hereinafter MT) reacting in less than 0.1 s on a short-circuit BT, and more particularly on a single-phase LV short-circuit, must have a low rating that exposes the fuse to the transformer's over-current interrupts; - The coordination of the same fuse MT with the LV protections itself becomes difficult and the selectivity can no longer be reliably provided, given the tolerances on the operating characteristics, especially for high currents; - In addition, the variability of the short-circuit impedance of the network supplying the transformer, as well as the impedance of the LV circuit between the transformer and the fault, can lead to failure to comply with the maximum theoretical delay of 0.1 s. because of an effective fault current less than the presumed current.

  In conventional installations, it is therefore necessary, to respect the melting time of less than 0.1 s, to choose a fuse which will in fact be sensitive to the switching overcurrent and not coordinated with the 15 LV fuses, an unacceptable situation in the field. functional. The regulation then becomes inapplicable (see comments below with reference to Figure 2).

  Patent documents EP 1 122 848 and EP 0 817 346 describe the use of a striker sensitive to fault currents flowing upstream, between the mass of the active part of the transformer and the grounding of the transformer. . Such currents generally result from an insulation fault between a medium voltage phase of the transformer and the mass thereof.

  The object of the present invention is to ensure the protection of the downstream connection to the transformer by ensuring a power off in case of a fault in less than 0.1 seconds while overcoming the aforementioned drawbacks. To do this, the invention provides for isolating the low voltage connection and connecting this connection thus isolated to a striker type protection function as mentioned above.

  More specifically, the subject of the invention is a method for the electrical protection of the downstream connections of a transformer comprising a function for detecting isolation fault and dedicated disconnection of the transformer, consisting in forming a screen function on the links. low voltage located downstream of the power transformer and to connect the screen function to the detection and disconnection function.

  In one embodiment, the detection and disconnection function is an integral part of a protection function of the transformer itself.

  The invention also relates to a device for implementing the method. In this device for protecting the electrical connections of a transformer, connected to an upstream supply network and comprising an active part, each low-voltage downstream link integrates, between the transformer to which it is connected by an electrical bushing and an apparatus. low-voltage protection of a downstream distribution network, an electric screen connected to a disconnection means can isolate the active part of the transformer of the medium voltage supply network.

  According to particular embodiments: the disconnecting means comprises a striker responsive to currents flowing to the ground which acts on the three-phase disconnection internal to the transformer; each electrical connection to be protected consists of an insulated cable, the electric screen consisting of a braid or strip covering the insulation of the cables, of an independent or non-insulating sheath 25 the screen of the environment of the cable ; each electrical connection to be protected consists of a polyphase bar sheath, the sheath being metallic to electrically partition each phase individually, and electrically isolated from the environment of the links to be protected. The invention also relates to a medium voltage / low voltage transformer station comprising such a protection device.

  Other characteristics and advantages of the invention will emerge from the detailed reading which follows and which refers to the appended figures, representing respectively: in FIG. 1, the diagram of a conventional installation; in FIG. 2, the characteristic curves for breaking the fuses on a conventional installation; - In Figure 3, a circuit diagram of a transformer station incorporating an exemplary device according to the invention; in FIG. 4a, a first example of low-voltage connections with isolated cables of the device according to FIG. 3; in FIG. 4b, a second example of low-voltage bar links of the device according to FIG. 3; and - in FIG. 5, the characteristic curves of the various protections of a substation according to the invention. FIG. 1 shows an example of a simplified single-line diagram of conventional installation, comprising an upstream supply line MT 1, equipped with protection fuses 1a, connected to a star delta transformer 2 delivering a current BT to an electrical connection line. swallows 4 mono- or polyphase depending on the equipment powered by this line. This downstream line is also provided with low-voltage protection fuses 5. The lines MT and BT are respectively connected to the transformer 2 by electrical feedthroughs 3 arranged at the interface 2a (in dashed mixed lines) of the transformer, the interface 2a integrating, in an oil bath, the structures specific to the transformer.

  The diagram of FIG. 2 illustrates the characteristics of the fuses of a concrete example of an installation such as that represented in FIG. 1, the transformer having a power of 100 kVA and MV and LV voltages respectively equal to 22 kV and 420 V .

  On this diagram, established in logarithmic scales, the abscissa axis corresponds to the scale of intensities I, that of ordinates 30 to that of time t. The shaded area (a) corresponds to the melting curve of the MV A fuses (FIG. 1) of 10 A with a manufacturing tolerance equal, in this example, to 10% of intensity for a known melting time. The 10 A MV caliber is the highest permissible value which nevertheless allows the interruption of a single-phase short-circuit current BT 35 in less than 0.1 s. The single-phase short-circuit current is taken into account in the example since it is less than the current of this three-phase short circuit. In the same way, the curve (b) corresponds to the melting characteristic of the BT 5 fuse of 160 A caliber, this rating being imposed by the rated current BT of the transformer amounting to 137 A. The melting curve ( b) is brought to voltage equal to that of the melting curve (a) in order to directly compare the characteristics.

  In addition, it is reported in this same diagram, the theoretical value of a short-circuit current of short-circuit LV Icc brought back to the primary voltage of the transformer, here 38 A, for a coupling to the transformer-type triangle-star and a short-circuit voltage of 4%, as well as the Zend tripping overcurrent occurring when the transformer is energized for a period of 0.1 s. This overcurrent Ienc, considered 15 is equal to twelve times the rated intensity MT, ie 32 A.

  Thus, compliance with the melting time of less than 0.1 s for an intensity of 38 A implies choosing a fuse responsive to the overcurrent and non-coordinated to the LV fuses, since selectivity is no longer ensured in the zone. Zr of the two characteristics (a) and (b), at the level of currents close to the short-circuit current. The diagram of Figure 2 will be compared below that of Figure 5 established within the scope of the invention illustrated below.

  FIG. 3 illustrates the electrical diagram of an example of a three-phase transformer station according to the invention. The transformer 6 comprises a protection with three fuses 10, installed in the oil bath 7 of the transformer. These fuses 10 incorporate strikers 11 which act, during the passage of an abnormally high current, on a three-phase disconnection 12 causing isolation between the active part 8 of the transformer and the medium voltage network 9 which feeds it from A three-phase network 1. Moreover, an independent striker 13 causes the opening of the three-phase disconnection 12 when a current is established between the mass M of the active part 8 and the ground 14 of the transformer. The LV phase connections 15 are connected, upstream, to the electrical feedthroughs 16 of the transformer 6 which feeds the links and, downstream, to a LV switchgear 17 which protects the distribution network 18. A neutral 2N connection line connected to earth T completes the distribution circuit. The links 15 are generally constituted either of insulated cables 15a, as illustrated in FIG. 4a, or of bars 15b, visualized in FIG. 4b and installed in a sheath. According to the invention, these links 15 integrate an electrical screen 19. With reference to FIG. 4a, the connections BT15 being insulated cables 15a, the electric screen 19a is disposed on the insulation 20 of the cable 15a, itself Isolated from the environment of the cable by an insulation 22. According to different embodiments, the electric screen 19a may consist of a simple wire mesh of small section copper wire, a braid or a strip wrapped around the wire. cable 15a. The insulation 22 may be a simple insulating sheath reported. The cable can also consist generally of a shielded cable trade. According to FIG. 4b, the LV links 15 are constituted by a busbar 15b with a partitioned sheath 19b. To serve as an electric screen according to the invention, this sheath 19b is conductive, metallic in the example. It partitions each phase individually and is electrically insulated from the environment by insulating spacers 25. The electric screens 19, as illustrated in FIG. 3, are electrically connected to the detection striker 13 of the transformer by a connection 26, an electrical bushing 27 and an internal connection to the transformer 28. The occurrence of an insulation fault of a phase of the link 15 then immediately implies the flow of a current to the earth 14 of the transformer via the links 26 and 28, then by the striker 13 which instantly causes the power off.

  The diagram shown in FIG. 5 shows the characteristics (a) and (b) of cutoff BT and MT of the protections of the transformer station, in a manner equivalent to that of FIG. 2 but with the disconnection characteristic linked to the striker when this post is made according to the invention.

  Curve (a) shows this cutoff characteristic of the firing pin. It has the particularity that, at the onset of weak currents of the order of ten amperes (and therefore very much of this, BT fault currents which are of the order of a thousand amperes), the delay of reaction is limited to a mechanical operating time of the disconnection function 12 (Figure 3), to which is added a time of interruption of the electric current. However, the total of the reaction time remains of the order of approximately 0.05 s, which makes it possible to have an almost instantaneous break for fault currents much lower than the fault currents that can appear on the links 15. The fuse MT can then be chosen with a higher caliber since it no longer has to provide the cut in less than 0.1 s. Thus, in the above example, the gauge is 16 A, shielding the system from unwanted fusions.

  The invention is not limited to the embodiments described and shown. It can be applied for any type of installation, mono or polyphase and whatever the type of transformer.

Claims (8)

  A method of electrically protecting the downstream connections (15, 15a, 15b) of a transformer (2, 6) having an isolating and disconnecting fault detection function (12-13) dedicated to the transformer, characterized in that it consists in forming a screen function (19, 19a, 19b) on the low voltage links (15, 15a, 15b) located downstream of the supply transformer and connecting the screen function to the function of detection and disconnection. io   2. Electrical protection method according to claim 1, characterized in that the detection and disconnection function (12-13) is an integral part of a protection function (10 to 13) of the transformer itself.   3. Device for protecting the downstream connections of a transformer for carrying out the method according to claim 1 or 2, this transformer being mounted on an upstream supply network (9) and comprising an active part (8), the device being characterized in that each low-voltage downstream link (15) integrates, between the transformer (6) to which it is connected in supply by an electrical bushing (16) and a low-voltage switchgear (17) for protection of a downstream application network (18), an electric screen (19) connected to a protection means (10 to 13) able to isolate the active part (8) of the transformer from the medium voltage supply network (9).   4. Protective device according to the preceding claim, characterized in that the protection means comprises a striker (13) sensitive to currents flowing to ground (14), and acts on the three-phase disconnection function (12) internal to the transformer (6).   5. Protective device according to claim 3 or 4, characterized in that, each electrical connection to be protected being constituted by an insulated cable (19), the electric screen (19a) is formed of a braid or a strip covering the insulation of the cables, a sheath (22) being independent of the screen (19a) insulating it from the external environment of the cable.   6. Protective device according to claim 3 or 4, each electrical connection to be protected being integrated into a bushing (23), the device being characterized in that the sheath (19b) is metallic and electricallycloisonne each bar individually, and in that the sheath is electrically isolated from the environment.   7. Protection device according to claim 6, characterized in that the electrical insulation of the sheath (19b) is made by insulating spacers (25).   8. medium voltage transformer station / low voltage, characterized in that it comprises a protection device according to any one of claims 3 to 7.