EP2221838B1 - Method, device and system for protecting an electrical appliance, as well as a transformer and a substation comprising such a device or system - Google Patents

Description

The invention relates to a method for protecting electrical appliances against the effects resulting from overloads, and more particularly to multi-phase transformers for the distribution of medium-voltage / low-voltage electrical energy, with a view to providing early protection. excessive overloads in current operating situations. The invention also relates to a device for implementing this method, as well as to a protection system comprising the above protection device and a protection device against the effects of internal defects of said devices. The invention also relates to a transformer and a transformer station having such a device or system.

It is now common practice to protect medium-voltage / low-voltage distribution transformers against the effects of internal faults, in order to avoid the secondary effects following a transformer failure. Such solutions are described for example in patent documents EP0817346 , EP 0 800 251 or EP 0 653 765 .

The described devices have the essential function of eliminating the secondary effects due to transformer failure. Their object is not to prevent the occurrence of damage in the device, but to prevent it from becoming a manifestation dangerous to the environment, such as a fire or explosion.

The document EP0817346 In particular, there is a solution which additionally provides a disconnection of all the phases supplying the transformer, thus avoiding the persistence of a fault current which would be detrimental to the maintenance of operation of the medium-voltage network.

But a power of the transformer in its initial configuration on the site of operation, by means of an actuator, accessible by example of the outside should be discarded. This solution would indeed be dangerous for the intervener who would like to turn the transformer back on without any prior repair which can only be done in the workshop.

Experience shows that the origin of damage to operating transformers is frequently linked to an overload situation. Such cases appear particularly in areas where electricity needs are highly evolving, in developing countries for example. The increase in energy needs, following the extension of an agglomeration or the local increase of economic activity, generally takes place in an unplanned context, which does not allow an anticipation of the adaptation of the transformer power demanded. It is then gradually overloaded, to the point of falling into damage due to excessive heating experienced by its electrical windings.

A transformer may also be overloaded due to an insulation fault appearing on the low-voltage lines that it feeds. This defect, more or less distant, then results in an impedant short circuit, generating an abnormal current comparable to an overload.

It is also common to protect distribution transformers against overloads, as well as short-circuits, by low-voltage circuit breakers located outdoors, or directly integrated in the oil bath of the device to be protected, or on the medium voltage circuit of the transformer. Advantageously, with this latter solution, the intensity to be interrupted is lower and thus the cost of the function reduced.

However, the efficiency of this type of circuit breaker solution is not sufficient: it appears that the failure rate related to overload cases is greater than 5% per year of operation on certain networks, causing serious damage to the distributor. energy. This lack of efficiency is explained by the conditions of use of these circuit breakers, whether they are outdoors or indoors (harsh outdoor environment, triggered by the intensity and its intensity). duration; temperature taken into account different from that of the elements sensitive to transformer overload).

Independently of the aspects related to the reliability, these solutions also present a risk for the operator: they are planned to allow a maneuver of closure, following a triggering. However this trigger can also be concomitant or consecutive to a transformer fault, which causes the heating at the origin of the operation of the protection. In such a case, the operator can carry out a maneuver under load or under tension of the damaged transformer and is then exposed to the occurrence of touching voltages or dangerous steps, or even fires or explosions.

• assurer une protection efficace et fiable contre les surcharges du transformateur et donc réduire les cas d'avarie, tout en permettant une exploitation optimale,
• être sécurisant en interdisant tout risque de remise sous tension d'un transformateur en situation de défaut,
• réduire les cas de défaillance pour réaliser une protection complète contre les effets des défauts internes des transformateurs ou équivalents.

The objects of the invention are to overcome the disadvantages of the state of the art and, more particularly, to achieve at least one of the following objectives:

• provide effective and reliable protection against transformer overload and thus reduce damage, while allowing optimum operation,
• be safe by prohibiting any risk of powering up a transformer in a fault situation,
• reduce failure events to achieve complete protection against the effects of internal transformer or equivalent faults.

To do this, the invention starts from the observation that a transformer immersed in a liquid dielectric, such as a mineral oil, has a greatly accelerated aging as soon as the temperature within its electrical windings exceeds a certain value. For example, for cellulosic insulators impregnated with a mineral oil, this value is 98 ° C. In addition, if the temperature of the device exceeds 140 ° C in a hot spot, the transformer will suffer a short-term damage (a few tens of hours of operation).

It was then found critical that, in order to obtain effective protection of the transformer against overloads, this protection should be based on a measurement the actual temperature of the electrical windings. This approach results in taking into account the thermal inertia of the transformer resulting from the superposition of two time constants: a time constant of the heat exchange between the oil bath and the ambient air (in general of the order of 2 hours) and a time constant of the heat exchange between the electric windings and the oil bath (generally of the order of 10 minutes).

Or interrupt a service intensity, even in overload conditions, on the medium voltage circuit of the transformer can be achieved with a limited breaking capacity, for example of the order of ten amperes more simply and economically if it is to act on the low-voltage circuit sized for operating intensities of several hundred amperes.

More specifically, the invention, as defined in claim 1, relates to a method of protection against the effects of overloads of a medium-voltage / low-voltage electrical transformer or transformer having an active part, the apparatus being immersed in a liquid dielectric. The method consists in interrupting the medium-voltage supply current as a function of a thermal detection of the variation in temperature of the active part of the device to be protected, the interruption being carried out directly in the liquid dielectric.

In the case of a device in polyphase mode, the method consists in producing, for each phase, an interruption of the rotating average-voltage electrical current by fixed and mobile contacts, the movable contacts of all the phases being integral in rotation and able to create, depending on the rotational position, a galvanic connection between the fixed contacts specific to each phase.

The invention also relates to a device as defined in claim 3.

Such a device for protecting against the effects of overloading of said electrical transformer or voltage transforming apparatus comprises, inter alia, a medium-voltage supply current switch, a control mechanism, and a thermal probe able to trigger the control mechanism. The The probe is positioned to be exposed to the temperature of the active part of the device to be protected. The switch and its control are integrated in the liquid dielectric volume of the device.

• l'interrupteur du courant d'alimentation en moyenne-tension est un interrupteur rotatif polyphasé comprenant, pour chaque phase, deux contacts fixes et deux contacts mobiles reliés électriquement entre eux, les contacts mobiles de toutes les phases étant solidaires d'un axe rotatif et aptes, selon la position de l'ensemble rotatif, à créer une liaison galvanique entre les deux contacts fixes propres à chaque phase ;
• la commande actionnant l'interrupteur comporte un ressort de rappel agissant sur un levier solidaire d'un axe de rotation, formant une fonction « tumbler » (c'est-à-dire à bascule) apte à donner deux positions de stabilité pour lesquelles le courant d'alimentation est soit passant soit interrompu;
• le levier sur lequel agit le ressort de rappel présente un degré de liberté en rotation par rapport à l'axe de la commande, ce degré de liberté étant limité angulairement par une goupille solidaire de l'axe de l'interrupteur et d'une lumière dans le moyeu du levier sur lequel agit le ressort de rappel, de manière à obtenir une vitesse d'ouverture des contacts de l'interrupteur indépendante de celle du mécanisme de commande ;
• la commande à l'ouverture de l'interrupteur est opérée par un électroaimant ;
• une commande manuelle situé à l'extérieur de l'appareil permet de fermer l'interrupteur en entraînant le levier sur lequel agit le ressort de rappel au-delà du point d'équilibre du tumbler, celui-ci assurant ensuite la rotation de l'axe avec une vitesse indépendante de la vitesse d'entraînement de la commande manuelle ;
• la commande manuelle n'est active qu'en fermeture de l'interrupteur;
• l'électroaimant peut être alimenté - en cas de besoin - par une tension prélevée aux bornes d'un des enroulements basse-tension du transformateur à protéger ;
• l'électroaimant est commandé par une sonde thermique constituée d'un bilame provoquant la fermeture du circuit d'alimentation du solénoïde de l'électroaimant en cas de dépassement d'un seuil de température prédéfini ;
• la sonde thermique est positionnée en partie supérieure d'un des canaux de refroidissement de l'enroulement basse-tension du transformateur ;

According to particular embodiments:

• the switch of the medium-voltage supply current is a polyphase rotary switch comprising, for each phase, two fixed contacts and two movable contacts electrically connected to each other, the movable contacts of all the phases being integral with a rotary axis and adapted, according to the position of the rotary assembly, to create a galvanic connection between the two fixed contacts specific to each phase;
• the control actuating the switch comprises a return spring acting on a lever secured to an axis of rotation, forming a "tumbler" function (that is to say, rocking) capable of giving two stability positions for which the supply current is either passing or interrupted;
• the lever on which the return spring acts has a degree of freedom in rotation with respect to the axis of the control, this degree of freedom being limited angularly by a pin integral with the axis of the switch and a light in the hub of the lever on which the return spring acts, so as to obtain a contact opening speed of the switch independent of that of the control mechanism;
• the control at the opening of the switch is operated by an electromagnet;
• a manual control located outside the device closes the switch by driving the lever on which the return spring acts beyond the equilibrium point of the tumbler, which then ensures the rotation of the axis with a speed independent of the driving speed of the manual control;
• the manual control is active only when the switch is closed;
• the electromagnet can be powered - if necessary - by a voltage taken at the terminals of one of the low-voltage windings of the transformer to be protected;
• the electromagnet is controlled by a thermal probe consisting of a bimetallic strip causing the solenoid supply circuit of the electromagnet to close if a predetermined temperature threshold is exceeded;
• the thermal probe is positioned in the upper part of one of the cooling channels of the low-voltage winding of the transformer;

According to a particular aspect, the invention also relates to a protection system against the overload effects of a medium / low-voltage transformer comprising the overload protection device defined above coupled to a protection device against the effects of internal faults of the transformer. Such a device is described for example in patent documents EP0817346 or EP1122848 .

• l'interrupteur du dispositif de protection contre les surcharges est mis en commun aux deux dispositifs pour être apte à constituer un moyen de déconnexion pour chacun des deux dispositifs ;
• la commande manuelle comporte un fusible mécanique apte à empêcher la fermeture de l'interrupteur actionné à l'ouverture par le dispositif de protection contre les effets des défauts internes du transformateur.

According to particular embodiments:

• the switch of the overload protection device is shared by the two devices to be able to constitute a means of disconnection for each of the two devices;
• the manual control comprises a mechanical fuse capable of preventing the closing of the switch actuated when opened by the protection device against the effects of the internal faults of the transformer.

The invention also relates to a medium-voltage / low-voltage electrical power distribution transformer including the overload protection device or system above, as well as the transformer station comprising such a transformer. of distribution.

• la figure 1, un schéma de principe d'un transformateur équipé d'un dispositif selon l'invention ;
• la figure 2, une vue perspective de l'interrupteur rotatif d'un exemple de dispositif selon l'invention ;
• les figures 3a et 3b, un exemple de tumbler de la commande d'interrupteur dans ses deux positions bistables ;
• la figure 4, une vue perspective éclatée d'un exemple de commande manuelle de remise sous tension du transformateur ;
• la figure 5, un schéma général de système de protection selon l'invention intégrant une fonction de protection contre les effets des défauts internes du transformateur ; et
• la figure 6, une vue perspective d'un exemple du système de protection de la figure 5.

Other characteristics and advantages as well as a better understanding of the invention will become apparent on reading the detailed description which follows, referring to exemplary embodiments with reference to the appended drawings which represent, respectively:

• the figure 1 a schematic diagram of a transformer equipped with a device according to the invention;
• the figure 2 a perspective view of the rotary switch of an exemplary device according to the invention;
• the Figures 3a and 3b , an example of tumbler of the switch control in its two bistable positions;
• the figure 4 , an exploded perspective view of an example of manual control of powering up the transformer;
• the figure 5 , a general scheme of protection system according to the invention incorporating a protection function against the effects of internal defects of the transformer; and
• the figure 6 , a perspective view of an example of the protection system of the figure 5 .

With reference to the schematic diagram of the figure 1 a transformer 1 equipped with a protection device according to the invention adopts the following principle.

Such a device 3 comprises a rotary switch 20 medium-voltage, placed in the tank 2 of the transformer 1, for each phase A, B, C supplying the transformer 1, an actuator 8, and a control mechanism 6. This switch 20, located upstream of the active part 4 of the transformer 1, can be closed manually, for example by a lever lever 100 accessible from outside the tank 2 of the transformer. The mechanism tumbler function 6, that is to say, toggles, provides a contact speed at closing of the switch independent of the operating speed of the handle 100. Such a mechanism is described in detail below, with reference to Figures 3a, 3b and 4 .

This mechanism also allows, thanks to an accumulation of energy in a spring 7 that it integrates, the opening of the switch 20 following the operation of an actuator 8 such as an electromagnet. This electromagnet 8 causes the opening when it is supplied with a voltage under the effect of the closure of the contacts of a thermal probe 9 of bimetallic type. The supply voltage required for the electromagnet 8 is directly taken from the low-voltage winding 10 of the transformer 1 for the supply of the low-voltage phases "a", "B" and "c", accompanied by the neutral "n". This probe is located in the low-voltage winding 10 included in the active part 4 of the transformer 1.

The operation of the overload protection device is then as follows. Following an abnormal current and duration load applied to the transformer in service, the transformer is de-energized by the intervention of the device described above. The distribution of energy is then interrupted and the distributor is informed by his customers of the absence of tension. An operator goes on site to note the interruption due to overload, indicated by the position of the joystick 100. The operator then carries out a verification of the low-voltage network and, if necessary, a load shedding of the transformer or a repair on the low-voltage network. In the meantime, the transformer cooled down and went down to a temperature allowing it to be powered up again. In the opposite case, the powering on of the transformer causes the electromagnet 8 to be energized and, as soon as the manual action of the operator on the handle 100 has been completed, the switch 20 of the protection device automatically open, preventing the transformer from being switched back on sustainably.

The figures 2 , 3a , 3b and 4 illustrate, in detail, the constituent elements of a non-limiting embodiment of a device 3 according to the invention.

With reference to the figure 2 , the rotary switch of insulating material 20, guided by two bearings 21a and 21b, comprises a link 22 for each phase supplying the active part of the transformer. Each link 22 is capable of being interposed between two contacts 23 and 24, respectively by upstream and downstream rotation. A rotary switch 20 is thus constituted capable of interrupting a transformer operating current, even in an overload situation. The contacts 23 and 24 are positioned on parallel insulating walls 25, completing the bearings 21a and 21b to form a housing 200. The switch 20 is shown here in the open position and the control mechanism 6 is shown in connection with the hub 2A extending the rotary switch 20, beyond the bearing 21b, on the shaft 26 integral with the rotary switch 20.

On the Figures 3a and 3b , which detail the control mechanism 6 tumbler function, the axis 26 comprises a pin 27 may be driven by a ring 28 mounted on the hub of the axis. The ring 28 is itself secured to a lever 29 mounted on the hub 2A ( figure 2 ). The lever 29 is connected to a rod 30 by a hinge pivot 3A, the rod 30 having a longitudinal slot 3B along which slides a stud 31 mounted on the bearing 21b. The spring 7, mounted on the rod 30 and end-linked to the stud 31, can act in pressure on the lever 29.

• à la position fermée de l'interrupteur (figure 3a), dans laquelle les liaisons 22 sont couplées aux contacts 23 et 24, et
• à la position ouverte de l'interrupteur (figure 3b) dans laquelle les liaisons 22 sont totalement découplées des contacts 23 et 24.

Depending on the angular position of the rod 30, the spring 7 will be more or less compressed. The angular position of the ring 28 which corresponds to the vertical alignment of the rod 30 and the lever 29, represents a hard point K on the pivot path C between two stable extreme angular positions S1 and S2, respectively corresponding to:

• at the closed position of the switch ( figure 3a ), in which the links 22 are coupled to the contacts 23 and 24, and
• at the open position of the switch ( figure 3b ) in which the links 22 are completely decoupled from the contacts 23 and 24.

An electromagnet 8 is positioned on a shoulder 21e of the bearing 21b so as to act directly on the lever 29 when its core 3C exits under the effect of a supply voltage.

• l'interrupteur rotatif 20 est entraîné à l'ouverture uniquement après le passage du point dur K, la vitesse de séparation des contacts 23 et 24 ne dépendant plus alors que de l'action du ressort 7 et non de celle de l'électroaimant 8;
• l'interrupteur rotatif 20 est entraîné à la fermeture après le passage de ce même point dur selon une vitesse d'approche des contacts 23 et 24, dépendant là encore du ressort 7 et non de l'action manuelle visant la fermeture de l'interrupteur.

In order to ensure the independence of the speed of rotation of the rotary switch 20 relative to the control means, the ring 28 comprises two symmetrical slots 32 into which the ends of the pin 27 are defined angularly so that:

• the rotary switch 20 is driven to the opening only after the passage of the hard point K, the speed of separation of the contacts 23 and 24 no longer depending on the action of the spring 7 and not that of the electromagnet 8 ;
• the rotary switch 20 is driven to closure after the passage of the same hard point according to a speed of approach of the contacts 23 and 24, again depending on the spring 7 and not the manual action to close the switch .

The figure 4 shows, in exploded view, the tumbler function mechanism 6 coupled to an external manual control 11 adapted, from a handle 100, to turn the transformer back on, following triggering from the open position ( figure 3b ). Since it is undesirable to switch off the transformer manually - regardless of any overload situation - the rotary switch is designed not to allow power off from the closed position. It has no disconnection power, and is therefore not considered here as an actuator for isolating the transformer from the network.

Thus, this control 11 makes it possible, from the lever 100, to rotate only in one direction, in the clockwise direction (arrow F) in the example in an exploded view, the lever 29 and the ring 28 in order to closing the switch.

The control 11 consists of an axis 40 terminated by the pin 27, the axis 40 passing through a sealed bearing 41 integral with a wall 5 of the transformer tank, and an eccentric drive finger 42. A return spring 43 mounted in the bearing 41 allows the lever 100 to have an angular position representative of the position of the lever 29: by means of a suitable registration on the wall 5 - "0" and "1" - It is then possible to remotely see the respectively open or closed state of the protective device. Thus, this control does not allow a manual opening, an action in the opposite direction of F not allowing the drive lever 29 to be driven by the drive finger 42.

As described above, the overload protection device is advantageously connected in series with a protection device against the effects of internal defects as described for example in patent documents. EP0817346 and EP1122848 . In this way, in case of a fault in the transformer, a disconnection is performed at the device for protection against the effects of internal faults and a closing operation of the overload protection device has no effect. The protection of the operator is so always assured. There can not be power on or under load of a transformer in a fault situation.
It is therefore advantageous to couple the overload protection function of the device 3 above with a protection function of the transformer against the effects of internal faults. The assembly then forms a protection system incorporating this overload protection device coupled to a protection device against the effects of internal transformer faults. Such a device is described for example in patent documents EP0817346 or EP1122848 .

The coupling between the protective devices can be a connection or a combination in order to make the whole more economical (reduction in the number of parts) and more compact.

Advantageously, the combination of the two devices can be achieved by pooling the switch of the overload protection device, this switch 20 then constituting a disconnection means for each of the two devices. Thus, the rotary switch 20 also provides the three-phase disconnection function.

The figure 5 shows the schematic diagram of such a combination. With reference to the patent document EP 0817346 , the function of protection against internal faults includes, for each phase A, B, C supplying the transformer 1, a medium-voltage fuse 50 sealed to mineral oil. Alternatively, one of the three phases may not have a fuse.

In accordance with the scheme of the figure 5 each fuse 50 is inserted between the point of connection to the corresponding phase, A-B-C of the medium-voltage network, and the rotary switch 20 to which the fuse, which includes a firing pin 51, is mechanically connected by a connection 56 on an axis 54. Similarly, a ground striker 52 is connected between the mass M of the active part of the transformer and the grounding T of the apparatus. The strikers 51 and 52 are connected to a rotating axis 54 mounted on the axis 26 of the switch 20 by a link 55. The other references 4, 6, 8, 9, 100 and the phases "a - b - c - n "refer to the figure 1 and the description attached to it.

The figure 6 details an embodiment of the combination of functions described with reference to the figure 5 . The strikers 51 and 52 are positioned facing the triggers 53, interconnected by the axis 54 which is rotated during the operation of at least one of the strikers 51 and / or 52. The rotational connection 55 is established between this axis 54 and the overload protection device with axis 26 integral with the rotary switch 20 protruding from the bearing 21 a. The fuses 50 are connected by the links 56 on the axis 54.

The figure 6 presents the assembly while the switch is open following the operation of the striker 51 belonging to the fuse 50 of the phase C. Thus, the action of at least one striker rotates the axes 54 and 26, the rotating assembly 20 associated with the axis of rotation 26. The tumbler function mechanism 6 (shown more specifically in FIGS. Figures 3a, 3b and 4 ) is also rotated to an angular position clearly exceeding the hard point, corresponding to the maximum compression of the spring 7, then the tumbler prolongs the opening movement of the switch 20 by releasing its energy.

An advantageous function, for the safety of people and the effectiveness of the function against internal faults, is to prohibit any attempt to reset the overload protection following opening on internal fault.

• il est apte à entraîner le levier 29 et à comprimer le ressort 7, lorsque le fonctionnement du transformateur est rétabli suite à intervention sur surcharge,
• il se rompt sous l'effet de l'addition du couple résistant dû au ressort 7 et du couple résistant dû à l'un ou plusieurs des percuteurs 51 ou 52, lequel est environ cinq fois supérieur au couple généré par le ressort 7.

According to an exemplary embodiment, this function is obtained, with reference to figures 4 and 6 from the drive finger 42 which constitutes a mechanical fuse. In this variant, this finger 42 has at its base an embrittlement, for example by reduction of section, so that:

• it is able to drive the lever 29 and to compress the spring 7, when the operation of the transformer is restored following intervention overload,
• it breaks under the effect of the addition of the resisting torque due to the spring 7 and the resisting torque due to one or more strikers 51 or 52, which is about five times greater than the torque generated by the spring 7.

In this second case of operation due to an internal fault in the transformer, requiring repair in the workshop, the replacement of this part should not be a disadvantage in itself.

The invention is not limited to the embodiments described and shown. It is for example possible to arrange the triggers 53 directly on the rotary switch 20, the fuses 50 and the striker 52 being then included in the plane of the housing 200. The distances between the fuses 50 and the striker 52 , and the links 22 of the rotary switch 20 must then be chosen so as to ensure sufficient isolation.

Claims (17)

1. Method of protection against the effects of overloads of a medium-voltage/low-voltage transformation electrical apparatus or transformer (1) having an active portion (4), the apparatus being submerged in a liquid dielectric, characterized in that it consists in interrupting (20) the medium-voltage supply current as a function of a thermal detection (9) of the variation in temperature of the active portion (4) of the apparatus to be protected (1) triggering in a protection device the operation of an actuator (8) capable of acting directly on a lever (29) connected, via a pivot-based articulation, with a connecting rod (30) on which a spring (7) is mounted so that the interruption is carried out directly in the liquid dielectric.
2. Method of protection against the effects of overload according to Claim 1, characterized in that, in the case of an apparatus in polyphase mode, it consists in carrying out, for each phase (A, B, C), a rotary interruption (20) of the medium-voltage electric current by fixed contacts (23, 24) and movable contacts (22), the movable contacts (22) of all the phases being secured in rotation and capable of creating, as a function of the rotation position, a galvanic connection between the fixed contacts (23, 24) specific to each phase.
3. Device for protection against overloads of an electric apparatus submerged in a liquid dielectric, such as a medium-voltage low-voltage polyphase transformer (1) comprising:

   - a switch of the medium-voltage supply current (20),

   characterized in that the device comprises:

   - a control mechanism (6) comprising a lever (29) connected, by a pivot-based articulation (3A), with a connecting rod (30) on which a spring (7) is mounted;

   - an actuator (8) capable of acting directly on the said lever (29) and capable of actuating this switch (20), and

   - a thermal probe (9) that can trigger the control mechanism (6), the probe (9) being positioned so as to be subjected to the temperature of the active portion (4) of the apparatus (1) to be protected, the switch (20) and its control mechanism (6) being incorporated into the volume of liquid dielectric of the apparatus.
4. Device for protection against overloads according to the preceding claim, characterized in that the control mechanism (6) actuating the switch (20) comprises a return spring (7) acting on a lever (29) secured to the rotation shaft (26) of the switch (20), giving the latter two positions of stability (S1, S2) for which the supply current is respectively flowing or interrupted, and forming a tumbler function.
5. Device for protection against overloads according to the preceding claim, in which the lever (29) on which the return spring (7) acts has a degree of freedom in rotation relative to the shaft (26) of the switch (20), this degree of freedom being limited angularly by means of a pin (27) secured to the shaft (26) of the switch and of an opening (32) of defined dimension in the hub (2A) of the lever (29) on which the return spring (7) acts, so as to obtain a speed of opening of the contacts (22) of the switch (20) that is independent of that of the control mechanism (6).
6. Device for protection against overloads according to any one of Claims 3 to 5, in which the switch (20) of the medium-voltage supply current is a polyphase rotary switch comprising, for each phase (A, B, C), two fixed contacts (23, 24) and two movable contacts (22) electrically connected together, the movable contacts (22) of all the phases being secured to one and the same rotary assembly (20) and being capable, according to the position of the rotary assembly, of creating a galvanic connection between the two fixed contacts (23, 24) specific to each phase.
7. Device for protection against overloads according to any one of Claims 3 to 6, in which the switch (20) is controlled on the opening of the contacts (22, 23, 24) by an electromagnet (8).
8. Device for protection against overloads according to any one of Claims 3 to 7, in which a manual control (11) situated outside the apparatus (1) is used to close the switch (20) by operating the lever (29) on which the return spring (7) acts beyond the point of equilibrium (K) of the tumbler-function mechanism (6), this mechanism then being capable of rotating the shaft (26) of the switch (20) with a speed independent of the speed of operation of the manual control (11).
9. Device for protection against overloads according to the preceding claim, in which the manual control (11) is active only on closing of the contacts (22, 23, 24) of the switch (20).
10. Device for protection against overloads according to any one of Claims 7 to 9, in which the electromagnet (8) is supplied by a voltage drawn from the terminals of one of the low-voltage windings (10) of the transformer (1) to be protected.
11. Device for protection against overloads according to any one of Claims 7 to 10 in combination with Claim 3, in which the electromagnet is controlled by the thermal probe (9) consisting of a bimetallic strip causing the closure of the supply circuit of a solenoid in the event of a predefined temperature threshold being exceeded.
12. Device for protection against overloads according to any one of Claims 3 to 11, in which the thermal probe (9) is positioned in the top portion of a cooling pipe of the low-voltage winding (10) of the transformer (1).
13. System for protection against the effects of overload of a medium/low-voltage transformer comprising the device for protection against overloads according to any one of Claims 3 to 12, coupled with a fused protection device (50) against the effects of internal defects of the transformer (1).
14. Protection system according to Claim 13, in which the switch (20) of the device for protection against overloads is placed in common with both the devices in order to be capable of forming a disconnection means for each of the two devices.
15. Protection system according to the preceding claim, in which the manual control mechanism (11) comprises a mechanical fuse (42) capable of preventing the closure of the switch (20) actuated on opening of the contacts (22, 23, 24) by the device for protection against the effects of internal defects of the transformer.
16. Transformer for the distribution of medium-voltage/low-voltage electric power including the protection device or system according to any one of Claims 3 to 15.
17. Transformer station comprising a distribution transformer according to Claim 16.

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