FR2782409A1 - UNDERWATER TRANSFORMER SELF-PROTECTED BY A DEVICE INCLUDING A CIRCUIT BREAKER AND FUSES - Google Patents

Abstract

The electrical transformer 1 comprises a tank 4 containing a liquid or gaseous dielectric 5, and in which are immersed: a primary circuit and a secondary circuit, and a protection device connected to the primary circuit and comprising a circuit breaker 15 controlled by tripping means and one or more fuses 11. The fuses are arranged between a passage 6 of the tank and the circuit breaker 15. The fuses 11, the circuit breaker 15 and the tripping means are combined in such a way that under conditions corresponding to a short circuit at the terminals of the secondary circuit, the circuit breaker contact members separate and interrupt the current without a fuse blowing, and there is a threshold value of current intensity passing through the fuse, lower than the breaking capacity of the corresponding pole of the circuit breaker and beyond which the blowing of the fuse is completed before an order to separate the components of c ontact given by the tripping means could have caused the separation of the contacts.

Description

UNDERWATER TRANSFORMER SELF-PROTECTED BY A DEVICE

  INCLUDING A CIRCUIT BREAKER AND FUSES

  The invention relates to an electric transformer whose windings and magnetic circuit are immersed in a liquid or dielectric gas contained in a tank, and which is provided with an integrated protection device, intended to limit the effects of a failure.

internal of the transformer.

  Document FR-A-2 712 730 describes a three-phase transformer of this type immersed in an oil tank, the protection device of which comprises at least one protective fuse per phase disposed between each phase of electrical supply and the primary windings transformer, and a three-phase breaker located between the fuses and the corresponding transformer phases. The breaker is opened by a trigger, in response to a signal from a sensor sensitive to variations in at least one parameter representative of the characteristics of the dielectric liquid. The parameter measured is the pressure of the dielectric, which can constitute an index of local faults internal to the transformer circuits - for example a point fault in the electrical insulation of a winding, liable to degenerate into a hot spot - not manifested. only by a very low overcurrent of the primary supply current. The fuses are intended to compensate for collapses of the internal impedance of the transformer, inducing a significant ohmic drop. To ensure optimum protection, at least two fuses are connected in series on each phase, with stepped cut-off thresholds: one of the fuses has a very short time constant and is sensitive to very large currents exceeding six times the rated current the transformer; the other is sensitive to overloads corresponding to three to five times the nominal intensity, with a higher time constant. The assembly is designed so that, if it intervenes to interrupt the supply of the primary windings of the transformer, it is not possible to restore this supply without special intervention to disassemble the tank by a specialist. This is the reason why the breaker is not

  resettable and submerged in the tank, as well as the fuses. This deliberate provision under-

  suggests that any tripping of the protection device necessarily corresponds to an internal fault in the transformer, and in no case to an external incident. Indeed, it would be completely inadmissible to impose an intervention on a healthy transformer, due to an external fault, especially on the downstream installation. It is therefore essential that provision be made for

  means ensuring selectivity with respect to electrical faults external to the transformer.

  Experience shows that in practice there remains a risk of unwanted melting of the fuses of the protection device, and in particular of fuses working in the field of low overcurrents.

2 2782409

  Furthermore, in the event of a low overcurrent, slightly less than three times the nominal current of the transformer for example, the fuses with low melting current submerged in the protection device can give rise to unacceptable heating for their immediate environment, without however melting and interrupt the current. We can then generate serious failures that go against the objective sought with the installation of the fuse. In addition, the heating of the fuses causes their premature aging, and therefore

  an increasing risk of dysfunction over time.

  Document EP-A-0 468 299 also describes a three-phase transformer immersed in oil, with which is associated, for each primary phase, a protection device comprising, connected in series with the windings of the primary phase concerned, a single-phase circuit breaker controlled by an overcurrent release sensitive to the current flowing through the circuit breaker, and a fuse whose nominal current intensity is at least 5 times greater than the nominal phase current of the transformer, and! 5 preferably 10 or 20 times greater. Such sizing makes it possible to limit the field of intervention of fuses to the primary short-circuits of the transformer, other electrical incidents, including the primary over-currents due to short-circuits on the secondary side of the transformer, being taken into account by the circuit breaker. The presence of

  fuses in turn allow you to choose a circuit breaker of low performance, therefore inexpensive.

  notably having a relatively low breaking capacity.

  However, this device is located outside the transformer. This results in a large footprint and a complexity of assembly on site. In particular, a large bypass distance in the air must be preserved between the upstream and downstream connections of the circuit breaker, due to the very high currents likely to pass through during a primary short circuit. The large number of connections outside the transformer tank and of which

  isolation is difficult to control, induces an additional risk of failure.

  This problem is further accentuated by the fact that the device is not active with respect to short circuits upstream of the fuse, in particular those generated by the electrical connection between the circuit breaker and the fuse. Indeed, the fuse is connected in series between the circuit breaker and the transformer and the current sensor controlling the tripping of the circuit breaker is downstream of the fuse. As a result, the fault current in the event of a short circuit between the circuit breaker and the fuse crosses the circuit breaker with an intensity which exceeds its breaking capacity, so that the circuit breaker is not able to break the circuit. The fault then leads to a

  outage at an upstream node of the network, to the detriment of overall availability.

3 2782409

  In addition, the device does not provide for the simultaneous tripping of the three circuit breakers each associated with a phase of the transformer primary. As a result, we can witness, under certain conditions, a partial tripping due to a fault, leaving in an unopened phase a leakage current which remains lower than the tripping current of the circuit breaker or which induces a significant delay in breaking, with all harmful consequences for the primary circuit (risk of explosion) and / or for the entire installation (non-selectivity by

source station report).

  Document US-A-4 323 871 describes an electrical circuit protection apparatus, not specifically dedicated to the protection of a transformer, and comprising a circuit breaker and fuses, all immersed in a tank filled with oil. The circuit breaker is intended to cut low fault currents while fuses are intended to cut large fault currents. The circuit breaker comprises, in its single-phase version, a conventional vacuum interrupter with a fixed contact member and a movable contact member in translation! 5 in the cylindrical body of the bulb. The movable contact member comprises a rod kinematically connected to an opening and closing mechanism with an opening spring. This mechanism includes an operating lever allowing manual opening and closing through the tank. It also has a cam-shaped opening lock which is resiliently returned to a rest position. A U-shaped bimetal trigger device cooperates with the cam and retains it in an active position where it blocks the mechanism in the closed position. The bimetallic strip is traversed by the current passing through the bulb. In the event of a fault current, it curves and releases the cam, which in turn releases an opening pawl acting on the opening spring and causing the opening of the mechanism. A three-phase circuit breaker is derived from the previous one by juxtaposition of three single-phase circuit breakers. The single-phase opening and closing mechanisms are mechanically linked together, on the one hand at the level of the opening pawls, and on the other hand at the level of the operating levers, so as to form a single mechanism allowing opening and simultaneous or almost simultaneous closing of the three poles of the circuit breaker, both on electrical fault and on manual command. In these devices, the bimetallic strip of each phase is connected in series between the bushing and the corresponding vacuum interrupter and is traversed by the phase current. It therefore introduces by its presence an additional risk of electrical fault, especially since it constitutes a mobile and unprotected conductor directly immersed in the oil. In addition, the bimetallic strip is placed, for constructive reasons, directly upstream or downstream of the circuit breaker that it controls. It cannot therefore take into account

  short circuits at the bushings, or between the bushing and the circuit breaker.

  The invention therefore aims to overcome the drawbacks of the state of the art and in particular to achieve, at low cost and in the volume of a submerged transformer tank

4 2782409

  conventional, high-security integrated protection, which is active for low overcurrents while avoiding the untimely risks of fusion encountered in the devices

with fuses.

  More generally, the invention aims to propose, at low cost and in a reduced volume, a transformer with integrated protection device for safe operation, which is able to deal with both internal faults and faults at the secondary terminals. Recall that the requirement for the dependability of a system implies that the following must be taken into account simultaneously: system security, ie its ability to avoid a catastrophic event; its reliability, ie its probability of not failing over a given period; its availability, that is to say its probability of operating at a given instant, which itself depends on the reliability and maintainability of the system, which is its probability of being repaired in a given time interval. In this case, the safety requirement implies that all internal faults of the transformer which are likely to cause the explosion of the tank, cause the transformer to be switched off. The reliability requirement implies in particular that the presence of the protection device itself does not create the risk of additional failure. The availability and maintainability requirements imply good selectivity in the treatment of internal faults requiring a heavy intervention on the transformer and the treatment of faults on the secondary circuit outside the transformer for which the interventions must be reduced to minimum. According to a first aspect of the invention, these objectives are achieved by means of a three-phase electric transformer comprising: e a tank containing a liquid or gaseous dielectric, * a three-phase primary circuit and a three-phase secondary circuit, each comprising windings immersed in the dielectric , each phase of the primary circuit entering the tank through an insulating bushing, * a protection device comprising: 30. at least two fuses immersed in the dielectric, the fuses being connected in series each on a phase of the primary circuit of the transformer between the insulating bushing and the windings of the primary circuit and having a breaking capacity sufficient to ensure the breaking of the corresponding phase in the event of a three-phase short circuit of the primary circuit, * a multipolar circuit breaker comprising, for each phase provided with one of the fuses, a pole immersed in the dielectric and connected in series on said phase between the corresponding fuse and the windings, each pole comprising contact members capable of being taken in relation to each other

2782409

  a contact position in which they ensure the passage of current and a separation position, the circuit breaker further comprising an opening mechanism common to all of the poles, capable of passing the contact members from their contact position to their position of separation, s. tripping means associated with the circuit breaker opening mechanism and comprising an overcurrent tripping device, the fuses, the circuit breaker and the tripping means being associated in such a way that under conditions corresponding to a short circuit at the terminals of the secondary circuit, the circuit breaker contact members separate and interrupt the current without the fuses starting, and there is a threshold value for the intensity of the current flowing through each fuse, lower than the breaking capacity of the corresponding pole of the circuit breaker and beyond which the blowing of the fuse is completed before an order for separation of the contact members given by the tripping device at a maximum of

  current could have caused the contacts to separate.

  According to a second aspect of the invention, these objectives are achieved by means of a single-phase electric transformer comprising: * a tank containing a liquid or gaseous dielectric, a primary circuit and a secondary circuit, each comprising one or more windings immersed in the dielectric, the phase of the primary circuit entering the tank via an insulating bushing, a protection device comprising: at least one fuse immersed in the dielectric, connected in series with the phase of the primary circuit of the transformer between the insulating bushing and the circuit winding (s) primary and having a breaking capacity

  sufficient to ensure the breaking of the corresponding phase in the event of a short-

  circuit of the primary circuit, a circuit breaker comprising a pole immersed in the dielectric, connected in series between the fuse and the windings, and comprising contact members capable of taking one relative to the other in a contact position in which they ensure the passage of current and a separation position, the circuit breaker further comprising an opening mechanism capable of passing the contact members from their contact position to their separation position, to tripping means associated with the opening of the circuit breaker and comprising an overcurrent tripping device, the fuse, the circuit breaker and the tripping means being associated in such a way that under conditions corresponding to a short circuit at the terminals of the secondary circuit,

6 2782409

  the circuit breaker contact members separate and interrupt the current without a fuse blowing, and there is a threshold value for the intensity of the current flowing through the fuse, lower than the breaking capacity of the corresponding pole of the circuit breaker and beyond which the blowing of the fuse is completed before an order for separation of the contact members given by the overcurrent tripping device has been able to

  lead to separation of the contacts.

  The following developments, more focused on the three-phase assembly, also apply

  to the single-phase transformer, unless otherwise specified.

  The presence of a circuit breaker in the protection device makes it possible to limit unavailability in the event of a fault on the secondary circuit. It is then sufficient, after the cause of the fault has disappeared, to close the circuit breaker, an operation which does not require major intervention on site, and can even be carried out remotely if necessary, if the circuit breaker is fitted with a remote control for electric lock. The opening of all the poles of the circuit breaker being simultaneous, any risk of partial interruption of the transformer, i.e. of interruption of an insufficient number of phases, leaving one of the circuit's current loops closed

primary, is discarded.

  Due to the immersion of the circuit breaker poles and fuses in the circuit breaker tank, it is possible to offer a factory-mounted assembly, for which installation on site is reduced, which significantly reduces the risks of failure caused by the protection device. The arrangement of fuses upstream of the circuit breaker makes it possible to deal with the risks of

  electrical faults induced by the presence of the circuit breaker.

  The immersion of the opening mechanism offers corrosion protection and lubrication which are favorable from the point of view of aging. The overall reliability is increased. It is possible to provide very small poles due on the one hand to the low breaking performance required of the circuit breaker, and on the other hand to the immersion in the dielectric, which makes it possible to reduce the bypass distance of the circuit breaker, that is to say

  the distance between its upstream and downstream connections.

  With regard specifically to the three-phase transformer, it should be noted that a fuse and a pole of the circuit breaker can be fitted ideally on each phase, but that it is also possible to provide only two of the three phases with a fuse and d '' a pole of the circuit breaker, which is sufficient to interrupt the current in the whole circuit

7 2782409

  primary and also makes it possible to reduce costs, provided that the loss of selectivity vis-à-vis the network protection means upstream of the transformer in the

earth fault.

  Advantageously, the overcurrent triggering device comprises at least one measuring means for measuring the intensity of the current flowing in a phase of the primary circuit of the transformer. In practice, the sensor is a current transformer. Such an arrangement ensures that a short circuit in the tank between a phase and the earth at a point located between the fuse and the corresponding pole of the circuit breaker, which gives rise to this fuse, is seen by the tripping means of so as to ensure complete separation of the transformer. It is particularly advantageous for the measuring means to measure the current flowing at a point located upstream of the fuse, preferably outside the tank, upstream of the crossing. This ensures that all faults inside the tank, including faults at the bushing or its connection to the fuse will be seen by the sensor and will cause the circuit breaker to open. In order to avoid triggering cases for faults external to the tank, it is then advisable to choose a measurement point as close as possible to the crossing. A particularly advantageous choice is to have a transformer toroid of

  current on the part of the crossing external to the tank, or in the immediate vicinity thereof.

  According to an advantageous embodiment, the fuse or fuses have an elongated shape with two conductive ends intended for their connection upstream on the side of the bushing and downstream on the side of the circuit breaker, located on either side of a median part whose external surface is insulated, and in that, for each phase provided with a fuse, the conductive parts situated between the insulating bushing and the external surface isolated from the middle part of the fuse are surrounded by a solid insulator. Indeed, the electrical junction between the crossing and the

  fuse is the only point in the installation not protected by the fuse and circuit breaker combination.

  It is therefore a vulnerable point in terms of installation security. The solid insulation therefore offers a good guarantee that there are no faults. Alternatively, and preferably, the fuse (s) have an elongated shape with two conductive ends intended for their connection upstream on the side of the bushing and downstream on the side of the circuit breaker, located on either side of a median part whose external surface consists of an insulator, and

  in that for each phase provided with a fuse, the fuse constitutes with the crossing a

  monobloc assembly and in that the external surface of this sub-assembly, in its part internal to the tank, comprising the bushing, the upstream conductive end and the central part of the fuse, consists of one or more solid insulators forming a solid insulation without interruption. This eliminates any risk of primary short circuit inside the tank by

  upstream of the fuse, therefore any risk of fault leading to the explosion of the tank.

8 2782409

  To refine the protection against internal faults, it is advantageous to provide that the tripping means further comprise a tripping device controlling the opening of the circuit breaker when at least one parameter representative of the state of said dielectric exceeds a threshold determined. In practice, various physical parameters of the state of the dielectric are accessible: the pressure of the liquid, its temperature, the level of the liquid in the tank, but also the presence of gas in the tank which makes it possible inter alia to detect a gaseous decomposition of the dielectric or solid insulator, for example by a low power arc between turns on one of the transformer windings. It is also advantageous to provide for tripping of the circuit breaker as a function of certain characteristics of the secondary circuit (secondary current) or primary circuit (melting of one or

multiple fuses).

  It can also be provided, as an alternative or in conjunction with the current sensor at the transformer primary, that the overcurrent triggering device includes at least one measuring means for measuring the intensity of the current flowing in a phase of the secondary circuit of the transformer. Due to the potential saturation of the magnetic circuit of the transformer, it is not always satisfactory to take the trip information from the circuit breaker on the primary circuit. In practice it is possible to equip one or more branches of the secondary circuit, preferably each of the phases and if necessary the neutral, with a current sensor. According to one embodiment, the two means for measuring the current are also associated in a differential protection device of the transformer. A device of this type is described for example in the document FR-A-2 14 771 integrated

on this point here by reference.

  Preferably, the fuse (s), the circuit breaker and the tripping means are associated in such a way that if the fuse or one of the fuses blows, the tripping means give an order for contact separation. There exists a range of values of the current intensity - namely: the values exceeding the threshold value of intensity beyond which the blowing of the fuse is completed before an order of separation of the organs of

  contact given by the tripping means could have resulted in the separation of the contacts -

  for which the separation of the contacts of the poles of the circuit breaker is subsequent to the melting of at least one of the fuses. There is also a range of current intensity values for

  primary phase - namely: the values between the value corresponding to a short-

  secondary circuit and the threshold value defined previously - for which the order in which the blowing of the fuse (s) succeeds and the separation of the contacts is indifferent. In this field, in fact, the fault current is less than the breaking capacity of the circuit breaker and has

  especially with the breaking capacity of fuses.

9 2782409

  Preferably, the fuse or fuses are limiting. This ensures that the current flowing

  in the primary circuit during blown fuses remains at an acceptable level.

  In a particularly advantageous manner, each pole of the circuit breaker comprises a vacuum interrupter immersed in the dielectric. Due to the presence of fuses providing protection in the event of high short-circuit current, it is possible to use vacuum bulbs of low performance, very compact. The short bypass distance of the vacuum interrupter, that is to say the short distance between the upstream and downstream connections of the ampoule, does not cause any additional problem, since the connections are immersed in the oil which provides good electrical insulation. The vacuum interrupter therefore offers an entirely favorable compromise in terms of space, performance and

of cost.

  Preferably, the circuit breaker includes a closing mechanism capable of passing the contact members from their separate position to their contact position, comprising an operating member accessible from outside the tank, the opening mechanism being suitable for cause the breaker (s) to open, regardless of the state of the closing mechanism. It is then possible, in the event that the trip originates from outside the transformer, to put it back into service without opening the tank. Priority in opening prevents deterioration of the transformer in the event of maintenance of the

fault at the time of closing.

  Other advantages and characteristics of the invention will emerge from the description which follows

  follow different embodiments of the invention, given by way of nonlimiting examples and represented in the appended drawings in which: FIG. 1 represents an electrical diagram of a transformer according to a first embodiment of the invention; * Figure 2 shows a section of an arrangement corresponding to the first embodiment of the invention; 30. Figure 3 shows a section in a plane perpendicular to the plane of Figure 1; FIG. 4 represents a diagram where the current time characteristics of the circuit breaker and the fuses of the transformer are shown; FIG. 5 schematically represents the elements of a transformer according to a second embodiment of the invention; 35. FIG. 6 represents an electrical diagram of a third embodiment of the invention; in FIG. 7 schematically represents the elements of a transformer according to a

  fourth embodiment of the invention.

2782409

  With reference to FIGS. 1 to 3, a three-phase low-voltage medium voltage transformer I comprises primary 2 and secondary 3 windings immersed in a tank 4 containing a dielectric liquid 5, in practice oil. Each phase of the primary circuit of the transformer enters the tank 4 via a multifunctional part 6

  with a medium voltage crossing 7.

  The multifunctional part 6 has in its external part to the tank 4 a specific racking-in area 8 and in its internal part to the tank a plug-in area 9 for a head 10 of a fuse 11. The fuse 11 is a conventional limiting fuse, with a central part forming a body 12 whose external wall is cylindrical and insulating, and two ends the head 10 and the foot 13 - made of metal. The insertion zone 9 comprises a tubular external wall of electrically insulating elastomeric material, the lower end of which cooperates with the

  body 12 of fuse 11 so as to seal between the head 10 of the fuse and the oil 5.

  The upper end of this tubular wall forms a seal with the racking-in zone 8. The multifunctional part 6 therefore constitutes, with the fuse 11 mounted, a monobloc assembly whose external walls form a solid insulation without interruption between the bushing 7 and the middle part 12 of the fuse 11. It is thus ensured that any electrical fault upstream of the fuse 11 is in fact external to the tank 4, and consequently does not generate any

risk of explosion.

  The other end of the fuse 11 is connected to a connection side on the source side 14 of a circuit breaker module 15, via an electrical connection 16 comprising a

sheathed electrical conductor.

  The circuit breaker module 15 comprises, for each phase, a vacuum interrupter 17 of conventional structure, with a body 18 forming a cylindrical chamber containing a fixed contact member 19 and a movable contact member 20 guided axially in the bulb and extended by a control rod 21. A bulb of this type is described for example in document US-A-4 323 871 whose description is incorporated here on this point by reference. An opening and closing mechanism 22 of this circuit breaker is also of the type described in document US-A-4 323 871, the description of which is incorporated here by reference. This mechanism 22 is provided with an operating lever 23 accessible from the outside of the tank 4,

  which allows manual opening and closing.

  The connection area on the load side 24 of the circuit breaker module 15 is connected to the primary windings 2 of the transformer 1 via a tap changer 25. The tap changer is of the type described for example in document US-A-4 504 811, the description of which is there 2782409 incorporated on this point here by reference. It has a fixed connection bar ensuring departures to different points of the primary windings, and a mobile bar carrying the connection points to the downstream areas of the circuit breaker. The displacement of the movable bar allows, simultaneously for each phase of the primary, the connection of the load side of the circuit breaker selectively at one of the starting points towards the corresponding primary winding. The tank 4 is sealed or almost sealed (the latter term covering the case of tanks comprising, for example, a small air drying pipe) and the oil level is such that the insertion zone 9, the fuses 11 and the vacuum bulbs 17, as well as the moving parts of the mechanism 22 of the circuit breaker 15, other than the operating lever

23, are immersed.

  Near the medium-voltage crossing 7, outside the tank 4, is arranged a toroid 26 of a measuring transformer 27 giving a measurement of the intensity of the current flowing in the crossing. An overcurrent release device 28 receives the signal and

  controls the opening and closing mechanism 22 of the circuit breaker module 15.

  The current time diagram in FIG. 4 makes it possible to visualize the behavior of the device. Have been plotted on this diagram: on the ordinate the time and on the abscissa the intensity of the phase current. On the one hand, the tripping curve 40 of the circuit breaker is shown and

  on the other hand the melting curve 41 of the fuse of the corresponding phase.

  Compared to the nominal current IN. the circuit breaker admits an Is overload without tripping. The tripping curve 40 of the circuit breaker bends beyond Is so that at a threshold value ICCBT corresponding to the intensity of the current flowing in a phase of the primary during a three-phase short circuit in the secondary of the transformer, the circuit breaker has a short tripping time, in this example 0.1 seconds. The fuse blowing curve 41 is, in this zone, well above the circuit breaker breaking curve, since the blowing would only take place after 3 seconds of exposure to the current. In practice this results in the fact that in the event of a short circuit in the transformer secondary, the circuit breaker controlled by its overcurrent relay cuts the current in the transformer primary before

  the fuses do not have time to heat up significantly.

  Beyond the threshold value ICCBT, there is a threshold value IF, lower than the breaking capacity Ic of the circuit breaker, at which the tripping curves 40 of the circuit breaker and of blowing 41 of the fuses intersect. This means that if a short circuit on the primary circuit of transformer 1 causes a current of intensity greater than IF on at least one phase, the

12 2782409

  blowing of the fuse (s) 11 exposed to this current will be completed before an order of

  trip was not transmitted to the circuit breaker 15 by the trip device 28.

  However, one of the current sensors 27 will have seen the short-circuit current pass and the tripping device 28 will subsequently cause the circuit breaker to open 15. This arrangement makes it possible to ensure that the entire primary circuit is isolated even if the short

  circuit caused only one fuse to blow 11.

  Other circuit breaker trip modes are provided. In particular, one or more sensors 29 of data representative of the state of the dielectric are provided, connected to a tripping device 30. In the present case of a liquid dielectric, this data includes, for example, measurements of the level of the liquid, temperature, pressure, or

gas in the tank.

  According to a second embodiment, shown in FIG. 5, a one-piece multifunctional sub-assembly 50 comprises a medium-voltage connection part 51, a

  flexible electrical connection 52, a bushing 53 and a plug-in part 54 of the fuse head.

  The flexible link 52 comprises a core 55 comprising a metal braid or a thin cable.

  The sub-assembly comprises an outer covering 56 molded from insulating elastomeric material.

  A flange 57 allows attachment to the tank 4. This provides electrical insulation without interruption between the middle part of the fuse 11 and the medium voltage connection 51 outside the tank. This device has the additional advantage compared to the previous one

  eliminate a connection piece with the medium-voltage switchgear on the source side.

  The same principles apply in single phase, as shown in Figure 6 showing the electrical diagram of a third embodiment of the invention. A single sensor

  current is then sufficient to control the circuit breaker.

  A fourth embodiment, illustrated in FIG. 7, differs from the first embodiment essentially by the structure of the bushing and of the upstream connection of the fuse of each phase. The bushing is connected to the head connection of the limiting fuse 8, by means of an electrical connection 58 consisting of a sheathed conductor. This arrangement, which is less efficient, is however also less expensive since it reduces the number of specific parts. The examples of embodiments above have been taken in the field of low voltage medium voltage transformers. The invention is however also applicable to other types of transformers, in particular to medium voltage source transformers. The

  transformer primary connection can be either triangle or star.

  In the latter case, it may be useful to have a sensor on the neutral bushing.

  current capable of driving the device at overcurrent.

  Furthermore, it is possible to envisage other types of circuit breaker than vacuum circuit breakers. The invention also finds application with a gaseous dielectric, in particular SF6.

14 2782409

Claims (11)

  1. Three-phase electric transformer (1) comprising: ò a tank (4) containing a liquid or gaseous dielectric (5), has a three-phase primary circuit and a three-phase secondary circuit, each comprising windings (2,3) immersed in the dielectric , each phase of the primary circuit entering the tank (4) via an insulating bushing (7), To a protection device comprising: ò at least two fuses (11) immersed in the dielectric (5), the fuses being connected in series each on a phase of the primary circuit of the transformer (1) between the insulating bushing (7) and the windings (2) of the primary circuit and having a breaking capacity sufficient to ensure the breaking of the corresponding phase in the event of a three-phase short circuit of the primary circuit, e a multipole circuit breaker (15) comprising, for each phase provided with one of the fuses (11), a pole immersed in the dielectric (5) and connected in series on said phase between the fuse (11) co corresponding and the windings (2), each pole comprising contact members (19, 20) capable of taking, relative to each other, a contact position in which they ensure the passage of current and a separation position, the circuit breaker (15) further comprising an opening mechanism (22) common to all of the poles, capable of passing the contact members (19, 20) from their contact position to their separation position, means tripping (28, 30) associated with the opening mechanism (22) of the circuit breaker (15) and comprising an overcurrent tripping device (28), the fuses (11), the circuit breaker (15) and the tripping means (28, 30) being associated in such a way that under conditions corresponding to a short circuit at the terminals of the secondary circuit, the contact members (19, 20) of the circuit breaker (15) separate and interrupt the current without starting blown fuses (11), and there is a v their current intensity threshold passing through each fuse, lower than the breaking capacity of the corresponding pole of the circuit breaker (15) and beyond which the blowing of the fuse (11) is completed before an order to separate the contact members ( 19, 20) given by the overcurrent release device (28) could have caused the   separation of the contact members (19, 20).   2. Single-phase electric transformer (1) comprising: * a tank (4) containing a liquid or gaseous dielectric (5); 2782409   ò a primary circuit and a secondary circuit, each comprising one or more windings (2, 3) immersed in the dielectric (5), the phase of the primary circuit entering the tank (4) via an insulating bushing (7), protection device comprising: at least one fuse (11) immersed in the dielectric (5), connected in series with the phase of the primary circuit of the transformer (1) between the insulating bushing (7) and the winding (s) of the primary circuit ( 2) and having a breaking capacity sufficient to ensure the breaking of the corresponding phase in the event of a short circuit of the primary circuit, 10. a circuit breaker (15) comprising a pole immersed in the dielectric (5), connected in series between the fuse (11) and the windings of the primary circuit (2), and comprising contact members (19, 20) capable of taking relative to each other a contact position in which they ensure the passage of current and a position of separation, the disjo actuator (15) further comprising an opening mechanism (22) capable of passing the contact members (19, 20) from their contact position to their separation position, trigger means (28, 30) associated with the opening mechanism (22) of the circuit breaker (15) and comprising an overcurrent tripping device (28), the fuse (11), the circuit breaker (15) and the tripping means (28, 30) being associated in such a way so that under conditions corresponding to a short circuit at the terminals of the secondary circuit, the contact members (19, 20) of the circuit breaker (15) separate and interrupt the current without the fuse (11) blowing up, and that there is a threshold value for the intensity of the current flowing through the fuse (11), lower than the breaking capacity of the corresponding pole of the circuit breaker (15) and beyond which the fusing of the fuse (11) is completed before a order of separation of the contact members (19, 20) given by the device itif overcurrent release (28) could have caused the separation of the contact members.   3. Transformer according to either of Claims 1 and 2, characterized in that the   overcurrent release device (28) comprises at least one measuring means (27) for measuring the intensity of the current flowing in a phase of the circuit transformer primary (1).   4. Transformer according to claim 3, characterized in that the measuring means (27) measures the current flowing at a point located upstream of the fuse (s) (11),   particular outside the tank (4), upstream of the bushing (7). 16 2782409   5. Transformer according to any one of the preceding claims, characterized in that   that the fuse (s) (II) have an elongated shape with two conductive ends (10, 13) intended for their connection upstream on the side of the bushing (7) and downstream on the side of the circuit breaker (15), located on either side other of a middle part (12) whose external surface is insulated, and in that, for each phase provided with a fuse (11), the conductive parts situated between the insulating bushing (7) and the insulated external surface of the middle part (12)   of the fuse are surrounded by solid insulation.   6. Transformer according to any one of the preceding claims, characterized in that   that the fuse (s) (11) have an elongated shape with two conductive ends (10, 13) intended for their connection upstream on the side of the bushing (7) and downstream on the side of the circuit breaker (15), located on either side other of a middle part (12) whose external surface consists of an insulator, and in that for each phase provided with a fuse (11), the fuse (11) constitutes with the crossing (7) a sub -one-piece assembly and in that the outer surface of this sub-assembly, in its part internal to the tank (4), comprising the bushing (7), the upstream conductive end (10) and the middle part (12) of the fuse (11), consists of one or more solid insulators forming solid insulation without interruption.   7. Transformer according to any one of the preceding claims, characterized in that   that the tripping means (28, 30) further comprise a tripping device (30) controlling the opening of the circuit breaker when at least one parameter representative of   the state of said dielectric exceeds a determined threshold.   8. Transformer according to any one of the preceding claims, characterized in that   that the overcurrent triggering device (28) comprises at least one measuring means for measuring the intensity of the current flowing in a phase of the secondary circuit of the transformer.   9. Transformer according to any one of the preceding claims, characterized in that   that the fuse (s) (11), the circuit breaker (15) and the tripping means (28, 30) are combined in such a way that if the fuse or one of the fuses blows, the means of   release (28, 30) give an order to separate the contact members.   1 0.Transformer according to any one of the preceding claims, characterized in   that the fuse (s) (11) are limiting. 17 2782409   1. Transformer according to any one of the preceding claims, characterized in that   that each pole of the circuit breaker has a vacuum interrupter (17) immersed in the dielectric (5).   12.Transformer according to any one of the preceding claims, characterized in that   that the circuit breaker (15) comprises a closing mechanism (22) capable of passing the contact members (19, 20) from their separate position to their contact position, comprising an operating member (23) accessible from the outside of the tank (4), the opening mechanism (22) being capable of causing the opening of the pole (s) of the circuit breaker whatever   or the state of the closing mechanism (22).