AU4444799A

AU4444799A - Immersed transformer self-protected by a device including a circuit breaker and fuses

Info

Publication number: AU4444799A
Application number: AU44447/99A
Authority: AU
Inventors: Didier Fulchiron; Christophe Preve; Jacques Wild
Original assignee: Schneider Electric Industries SAS
Current assignee: Schneider Electric Industries SAS
Priority date: 1998-08-14
Filing date: 1999-08-13
Publication date: 2000-03-09
Anticipated expiration: 2019-08-13
Also published as: ID23712A; NO320723B1; EP0981140B1; AU753147B2; AR021200A1; NO993766D0; EA002377B1; EP0981140A1; EA199900657A3; FR2782409B1; ES2213999T3; CN1206670C; EA199900657A2; FR2782409A1; DE69914420D1; NO993766L; DE69914420T2; CN1245342A; BR9903587A

Description

rIUUIU I I 28/5/B Regulation 3.2(2)

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: r r r Invention Title: IMMERSED TRANSFORMER SELF-PROTECTED BY A DEVICE INCLUDING A CIRCUIT BREAKER AND FUSES The following statement is a full description of this Invention, Including the best method of performing It known to us IMMERSED TRANSFORMER SELF-PROTECTED BY A DEVICE INCLUDING A CIRCUIT BREAKER AND FUSES The invention relates to an electrical transformer whose windings and magnetic circuit are immersed in a dielectric liquid or gas contained in a tank, and which is provided with a protection device, designed to limit the effects of an internal malfunction of the transformer.

The document FR-A-2,712,730 describes a three-phase transformer of this type bathing in an oil tank, whose protection device comprises per phase at least one protective fuse located between each electrical power supply phase and the corresponding primary windings of the transformer, and a three-phase breaking switch located between the fuses and the corresponding phases of the transformer. Opening of the breaking switch is performed by a 15 trip device in response to a signal from a sensor sensitive to the variations of at least one parameter representative of the characteristics of the dielectric liquid. The measured parameter is the pressure of the dielectric, which can constitute an indication of the local •malfunctions internal to the transformer circuits for example a pin-point failure of the electrical insulation of a winding, which could degenerate into a hot spot which is manifested only by a very low overcurrent of the primary winding current supply. The fuses are designed to palliate collapses of the internal impedance of the transformer, inducing a large ohmic drop. To provide optimum protection, at least two fuses are fitted in series on each phase, with staggered breaking thresholds: one of the fuses has a very short time constant and is sensitive to very high currents exceeding six times the rated current of 25 the transformer; the other fuse is sensitive to overloads corresponding to three to five times the rated current, with a greater time constant. The assembly is designed in such a way that, if it operates to interrupt the power supply to the transformer primary windings, this power supply cannot be re-established without a special tank dismantling operation being performed by a specialist. This is the reason why the breaking switch is not resettable and is immersed in the tank, along with the fuses. This deliberate arrangement stems from the notion that any tripping of the protection device necessarily corresponds to an internal transformer malfunction, and in no case to an external incident. It would indeed be quite inadmissible to impose a repair operation on a sound transformer, due to the occurrence of an external fault, in particular on the load-side installation. Means therefore have to be provided performing selectivity with respect to electrical faults external to the transformer.

Experience has shown that there is in practice a risk of undesirable melting of the fuses of the protection device, and in particular of the fuses operating in the low overcurrent range.

Moreover, in case of a small overcurrent, slightly less than three times the rated intensity of the transformer for example, the immersed low melting current fuses of the protection device may cause heat rises inadmissible for their immediate environment, without however melting and interrupting the current. Serious failures can then be generated which are contrary to the objective sought for with fitting of the fuse. Moreover, heating of the fuses causes premature ageing thereof, and therefore an increasing risk of malfunctioning with time.

The document EP-A-0,468,299 further describes a three-phase transformer immersed in oil, to which there 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 overload trip device sensitive to the current flowing in the circuit breaker, and a fuse whose rated current intensity is at least 5 times greater than the intensity of the rated phase current of the transformer, and preferably 10 or 20 times greater. Such a S 15 dimensioning enables the scope of operation of the fuses to be limited to primary shortcircuits of the transformer, other electrical incidents, including primary overcurrents due to short-circuits of the secondary side of the transformer, being taken into account by the circuit breaker. The presence of the fuses in return enables a low-performance, and therefore inexpensive, circuit breaker to be chosen, notably having a relatively low breaking capacity.

However, this device is situated outside the transformer. This results in large overall dimensions and a complexity of assembly on site. In particular, a large bypass distance in air has to be preserved between the line-side and load-side connections of the circuit breaker, due to the very strong currents liable to flow through the circuit breaker when a primary short-circuit occurs. The large number of connections outside the transformer tank whose isolation is difficult to master gives rise to an additional risk of failure.

This problem is further accentuated by the fact that the device is not active with respect to short-circuits line-side of the fuse, in particular those generated by the electrical connection between the circuit breaker and fuse. The fuse is in fact connected in series between the circuit breaker and transformer and the current sensor controlling tripping of the circuit breaker is located load-side from the fuse. The fault current in the event of a short-circuit between the circuit breaker and fuse therefore flows through the circuit breaker with an intensity which exceeds its breaking capacity, so that the circuit breaker is not able to interrupt the circuit. The fault then leads to a break at the level of a line-side node of the power system, to the detriment of the global availability.

Furthermore, the device does not provide for simultaneous tripping of the three circuit breakers each associated to a phase of the transformer primary. Partial tripping due to a fault can therefore take place under certain conditions, leaving a leakage current in an unopened phase which remains lower than the circuit breaker tripping current or which gives rise to a large delay in breaking, with all the harmful consequences for the primary circuit (risks of explosion) and/or for the whole installation (non-selectivity with respect to the source sub-station).

The document US-A-4,323,871 describes an electrical circuit protection apparatus, not specifically dedicated to protection of a transformer and comprising a circuit breaker and fuses, the whole system being immersed in a tank filled with oil. The circuit breaker is designed to interrupt low fault currents whereas the fuses are designed to break large fault 'currents. The circuit breaker comprises, in its single-phase version, a conventional vacuum *cartridge with a stationary contact means and a movable contact means movable in 15 translation in the cylindrical body of the cartridge. The movable contact means comprises a 'rod kinematically linked to an opening and closing mechanism with an opening spring. This mechanism comprises an operating lever enabling manual opening and closing through the tank. It also comprises an opening lock in the form of a cam which is flexibly urged to a rest position. A U-shaped trip device with a bimetal strip operates in conjunction with the cam and keeps it in an active position where it blocks the mechanism in the closed position.

The current flowing through the cartridge flows through the bimetal strip. Should a fault o current occur, it curves and releases the cam, which in turn releases an opening ratchet acting on the opening spring and causing opening of the mechanism. A three-phase circuit S "breaker is derived from the above by juxtaposition of three single-phase circuit breakers.

25 The single-phase opening and closing mechanisms are mechanically connected to one another, on the one hand at the level of the opening ratchets, and on the other hand at the level of the operating levers, so as to form a single mechanism enabling simultaneous or almost simultaneous opening and closing of the three poles of the circuit breaker, both on an electrical fault and by a manual control operation. In these devices, the bimetal strip of each phase is connected in series between the bushing and the corresponding vacuum cartridge and the phase current flows through this bimetal strip. The bimetal strip therefore introduces by its presence an additional risk of electrical fault, as it constitutes an unprotected movable conductor directly immersed in oil. Furthermore, the bimetal strip is, for construction reasons, placed directly up-line or down-line from the circuit breaker it commands. It can therefore not take into account short-circuits at the level of the bushings, or between the bushing and circuit breaker.

I M The object of the invention is therefore to overcome the drawbacks of the state of the technique and in particular to achieve, at low cost and in the volume of a conventional immersed transformer tank, an integrated protection with a high degree of safety, which is active for low overcurrents while preventing the risks of nuisance melting encountered in fuse devices.

More generally, the object of the invention is to propose, at low cost and in a small volume, a transformer with an integrated protection-device of reliable operation, which is able to deal both with internal faults and faults on the secondary terminals. Let us recall that the operating safety requirement of a system implies that the following be taken into account simultaneously the safety of the system, i.e. its ability to prevent a catastrophic event its reliability, i.e. its probability of not failing over a given time; its availability, .e.

its probability of operating at a given moment, which itself depends on the reliability and maintainability of the system, which is its probability of being repaired within a given time 15 interval. In this particular instance, the safety requirement implies that all the internal transformer faults which are liable to give rise to explosion of the tank result in disconnection of the transformer from the circuit. The reliability requirement implies in S* particular that the presence of the protection device itself does not generate any additional risk of failure. The availability and maintainability requirements imply a good selectivity in dealing with internal faults requiring a heavy maintenance operation on the transformer and in dealing with faults on the secondary circuit outside the transformer for which o.

maintenance operations must be reduced to the minimum.

S According to a first feature of the invention, these objectives are achieved by means of a three-phase electrical transformer comprising a tank containing a liquid or gas dielectric, a three-phase primary circuit and a three-phase secondary circuit, both comprising windings immersed in the dielectric, each phase of the primary circuit entering the tank via an insulating bushing, a protection device comprising 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 primary circuit windings and having a sufficient breaking capacity to perform breaking of the corresponding phase in case of a three-phase short-circuit of the primary circuit, a multipole circuit breaker comprising, for each phase equipped 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 means able to take with respect to one another a contact position in which they ensure current flow and a separation position, the circuit breaker comprising in addition an opening mechanism common to all the poles, designed to move the contact means from their contact position to their separation position, Stripping means associated to the opening mechanism of the circuit breaker and comprising an overload trip device, the fuses, circuit breaker and tripping means being associated in such a way that under conditions corresponding to a short-circuit at the terminals of the secondary circuit, the contact means of the circuit breaker separate and interrupt the current without the fuses starting to melt, and that there exists a threshold value of the current intensity flowing in each fuse, this threshold being lower than the breaking capacity of the corresponding pole of the circuit breaker and above which threshold melting of the fuse has taken place before a separation order of the contact means given by the overload trip device has been S 15 able to cause separation of the contacts.

According to a second feature of the invention, these objectives are achieved by means of a single-phase electrical transformer comprising a tank containing a liquid or gas dielectric, a primary circuit and a secondary circuit, both 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 S at least one fuse immersed in the dielectric, being connected in series on 25 the phase of the primary circuit of the transformer between the insulating bushing and the winding or windings of the primary circuit and having a sufficient breaking capacity to perform breaking of the corresponding phase in case 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 means able to take with respect to one another a contact position in which they ensure current flow and a separation position, the circuit breaker comprising in addition an opening mechanism designed to move the contact means from their contact position to their separation position, tripping means associated to the opening mechanism of the circuit breaker and comprising a overload trip device, the fuse, circuit breaker and tripping means being associated in such a way that under conditions corresponding to a short-circuit at the terminals of the secondary circuit, the contact means of the circuit breaker separate and interrupt the current without the fuse starting to melt, and that there exists a threshold value of the current intensity flowing in the fuse, this threshold being lower than the breaking capacity of the corresponding pole of the circuit breaker and above which threshold melting of the fuse has taken place before a separation order of the contact means given by the overload trip device has been able to cause separation of the contacts.

The developments below, focussed more particularly on the three-phase assembly, also apply to the single-phase transformer, unless specified otherwise.

The presence of a circuit breaker in the protection device enables unavailability to be i limited in the event of a fault on the secondary circuit. After the cause of the fault has been cleared, the circuit breaker simply has to reclosed, an operation which only requires a simple operation on site, and can even be performed remotely if the circuit breaker is :i 15 equipped with an electrical closing remote control. Opening of all the circuit breaker poles being simultaneous, any risk of partial breaking of the transformer, i.e. of breaking of an insufficient number of phases, leaving one of the primary circuit current loops closed, is S° •avoided.

Due to the fact that the circuit breaker poles and the fuses are immersed in the circuit :"breaker tank, a factory-assembled unit can be proposed, for which on-site fitting is reduced, which notably reduces the risks of failure generated by the protection device. Arrangement of the fuses line-side of the circuit breaker enables the risks of electrical faults induced by the presence of the circuit breaker to be dealt with.

Immersion of the opening mechanism for its part provides an anticorrosion protection and lubrication which are favourable from the ageing point of view. The overall reliability is thereby increased.

Poles of very small dimensions can be fitted due on the one hand to the low breaking performances required from the circuit breaker, and on the other hand to immersion in the dielectric, which makes it possible to reduce the circuit breaker bypass distance, i.e. the distance between its line-side and load-side connections.

Specifically concerning the three-phase transformer, it should be noted that a fuse and a pole of the circuit breaker can ideally be fitted on each phase, but that it can also be provided to equip two only of the three phases with a fuse and a pole of the circuit breaker, which is sufficient to interrupt the current in the whole of the primary circuit and moreover enables costs to be reduced, provided that the loss of selectivity with respect to the power supply protection means line-side of the transformer in case of an earth fault is admitted.

Advantageously, the overload trip device comprises at least one means for measuring the current intensity 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 earth at a point situated between the fuse and the corresponding pole of the circuit breaker, which results in melting of this fuse, is seen by the tripping means so as to achieve total separation of the transformer. It is particularly advantageous for the measuring means to measure the current flowing at a point situated line-side from the fuse, preferably outside the tank, line-side from the bushing. It is thus ensured that all the faults inside the tank, including the faults at the level of the bushing or of its connection to the fuse will be seen by the sensor and result in opening of the circuit breaker. In order S:to prevent cases of tripping for faults external to the tank, it is then indicated to choose a 15 measurement point as close as possible to the bushing. A particularly advantageous choice is to dispose a current transformer toroid on the part of the bushing external to the tank, or in the immediate proximity thereof.

According to an advantageous embodiment, the fuse or fuses have an elongate shape with two conducting ends designed for their line-side connection on the bushing side and loadside connection on the circuit breaker side, situated on each side of a middle part whose o* o external surface is insulated, and for each phase equipped with a fuse, the conducting parts situated between the insulating bushing and the insulated external surface of the middle part of the fuse are enveloped in a solid insulator. The electrical junction between the bushing 25 and the fuse is in fact the only point of the installation not protected by the fuse and circuit breaker association. It is therefore a vulnerable point as far as safety of the installation is concerned. The solid insulation thereby provides a good guarantee of absence of faults.

Alternatively, and preferably, the fuse or fuses have an elongate shape with two conducting ends designed for their line-side connection on the bushing side and load-side connection on the circuit breaker side, situated on each side of a middle part whose external surface is formed by an insulator, and for each phase equipped with a fuse, the fuse constitutes a monoblock sub-assembly with the bushing, and the external surface of this sub-assembly, in its part internal to the tank, comprising the bushing, the line-side conducting end and the middle part of the fuse, is formed by one or more solid insulators forming a solid insulation without interruption. Any risk of a primary short-circuit inside the tank line-side of the fuse, and therefore any risk of a fault leading to explosion of the tank, is then eliminated.

111 11111110 0 To enhance protection against internal faults, it is advantageous to provide for the tripping means to comprise in addition a trip device commanding opening of the circuit breaker when at least one parameter representative of the state of said dielectric exceeds a set threshold. In practice, different 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 enables among other things a gaseous breakdown of the dielectric or of a solid insulator to be detected, for example by an arc of low power between turns on one of the transformer windings. It is also advantageous to provide tripping of the circuit breaker according to certain characteristics of the secondary circuit (secondary current) or primary circuit (melting of one or more fuses).

It can also be provided, as an alternative to or jointly with the current sensor on the S'transformer primary, for the overload trip device to comprise 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 circuit breaker tripping information from the primary circuit. In practice it is possible to equip one or more branches of the secondary circuit of a current sensor, preferably each of the phases and the neutral if present. According to one •embodiment, the two current measuring means are moreover associated in a differential protection device of the transformer. A device of this type is described for example in the document FR-A-214,771 integrated on this point here by reference.

oooo Preferably, the fuse or fuses, circuit breaker and tripping means are associated in such a S way that if the fuse or one of the fuses melts, the tripping means give a separation order of 25 the contact means. There is a range of current intensity values i.e. the values exceeding the intensity threshold value above which the fuse has completely melted before a separation order of the contact means given by the tripping means has been able to cause separation of the contacts for which separation of the contacts of the circuit breaker poles takes place subsequent to melting of at least one of the fuses. There is also a range of primary phase current intensity values i.e. the values comprised between the value corresponding to a secondary short-circuit and the previously defined threshold value for which the order in which melting of the fuse or fuses and separation of the contacts take place is of no importance. In this range, the fault current is in fact lower than the breaking capacity of the circuit breaker and even more so than the breaking capacity of the fuses.

Preferably, the fuse or fuses are limiting. It is thus ensured that the current flowing in the primary circuit during melting of the fuses remains at an admissible level.

In particularly advantageous manner, each circuit breaker pole comprises a vacuum cartridge immersed in the dielectric. On account of the presence of the fuses performing protection in the case of a short-circuit current of high intensity, it is possible to use lowperformance vacuum cartridges of very small dimensions. The small bypass distance of the vacuum cartridge, i.e. the short distance between the line-side and load-side connections of the cartridge, then does not give rise to any additional problem, as the connections are immersed in oil which provides a good electrical insulation. The vacuum cartridge therefore provides a very favourable compromise in terms of size, performance and cost.

Preferably, the circuit breaker comprises a closing mechanism designed to move the contact means from their separated position to their contact position, comprising an operating means accessible from outside the tank, the opening mechanism being designed to cause i opening of the circuit breaker pole or poles whatever the state of the closing mechanism. It is then possible, should tripping occur due to a cause external to the transformer, to put the 15 transformer back into operation without opening the tank. The fact that opening takes priority prevents any damage to the transformer should the fault persist when closing takes place.

Other advantages et and features of the invention will become more clearly apparent from the following description of different embodiments of the invention, given as non-restrictive examples only and represented in the accompanying drawings in which: *oo.

figure 1 represents a wiring diagram of a transformer according to a first embodiment of the invention figure 2 represents a cross section of an arrangement corresponding to the first embodiment of the invention e figure 3 represents a cross section in a plane perpendicular to the plane of figure 1 e figure 4 represents a diagram where the time current characteristics of the circuit breaker and fuses of the transformer are indicated e figure 5 schematically represents the elements of a transformer according to a second embodiment of the invention figure 6 represents a wiring diagram of a third embodiment of the invention; figure 7 schematically represents the elements of a transformer according to a fourth embodiment of the invention.

With reference to figures 1 to 3, a three-phase medium-voltage low-voltage transformer 1 comprises primary windings 2 and secondary windings 3 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 multifunction part 6 comprising a medium-voltage bushing 7.

The multifunction part 6 comprises in its part external to the tank 4 a specific insertion zone 8 and in its part internal to the tank an engagement zone 9 of a head 10 of a fuse 11. The fuse 11 is a conventional limiting fuse, with a middle part forming the body 12 whose external wall is cylindrical and insulating, and two metal ends the head 10 and the foot 13.

The engagement zone 9 comprises a tubular external wall made of electrically insulating elastomer material, whose bottom end cooperates with the body corps 12 of the fuse 11 so as to achieve tightness between the head 10 of the fuse and the oil 5. The top end of this tubular wall achieve tightness with the insertion zone 8. The multifunction part 6 thereby constitutes, with the fuse 11 fitted, a monoblock assembly whose external walls form a solid insulation without interruption between the bushing 7 and the middle part 12 of the i fuse 11. It is thus ensured that an electrical fault occurring line-side of the fuse 11 is in fact external to the tank 4, and consequently does not give rise to any risk of explosion of the tank.

The other end of the fuse 11 is connected to a source side connection strip 14 of a circuit breaker module 15 by means of an electrical connection 16 comprising a sheathed electrical S- •conductor.

The circuit breaker module 15 comprises, for each phase, a vacuum cartridge 17 of *"conventional structure, with a body corps 18 forming a cylindrical chamber containing a stationary contact means 19 and a movable contact means 20 guided axially in the cartridge and extended by a control rod 21. A cartridge of this type is described for example in the document US-A-4,323,871 the description whereof is incorporated here on this point by 25 reference. An opening and closing mechanism 22 of this circuit breaker is also of the type described in the document US-A-4,323,871 the description whereof is incorporated here on this point by reference. This mechanism 22 is equipped with an operating lever 23 accessible from outside the tank 4, which enables manual opening and closing.

The load-side connection terminal 24 of the circuit breaker module 15 is connected to the primary windings 2 of the transformer 1 by means of a tap changer 25. The tap changer is of the type described for example in the document US-A-4,504,811, the description whereof is incorporated here on this point by reference. It has a fixed connection strip providing departures to different points of the primary windings, and a movable strip supporting the connection points to the load-side connection terminals of the circuit breaker.

Movement of the movable strip enables, simultaneously for each phase of the primary, connection of the load-side connection terminal of the circuit breaker selectively to one of the departure points to the corresponding primary winding.

The tank 4 is airtight or almost airtight (the latter term covering the case of tanks comprising for example an air drying pipe of small dimensions) and the oil level is such that the engagement zone 9, fuses 11 and vacuum cartridges 17, and also the movable parts of the mechanism 22 of the circuit breaker 15, other than the operating lever 23, are immersed.

Near to the medium-voltage bushing 7, outside the tank 4, there is arranged a toroid 26 of a measuring transformer 27 giving a measurement of the current intensity flowing in the bushing. An overload trip device 28 receives the signal and controls the opening and closing mechanism 22 of the circuit breaker module The behaviour of the device can be seen on the time current diagram of figure 4. The following have been plotted on this diagram on the y-axis the time and on the x-axis the phase current intensity. The circuit breaker tripping curve 40 and the melting curve 41 of the fuse of the corresponding phase have been represented.

With respect to the rated current IN, the circuit breaker admits an overload I s without tripping. The circuit breaker tripping curve 40 deviates as from I s so that at a threshold value ICCBT corresponding to the current intensity flowing in a phase of the primary when a *"three-phase short-circuit occurs on the transformer secondary, the circuit breaker has a short tripping time, in this example 0.1 seconds. The fuse melting curve 41 is, in this zone, well above the circuit breaker breaking curve, as melting would only take place after 3 seconds exposition to the current. In practice this is expressed by the fact that in the event 25 of a short-circuit on the transformer secondary, the circuit breaker commanded by its overload relay breaks the current in the transformer primary before the fuses have time to heat notably.

Beyond the threshold value ICCBT, there is a threshold value IF, lower than the circuit breaker breaking capacity, at which the circuit breaker tripping curve 40 and fuse melting curve 41 cross. This means that if a short-circuit on the primary circuit of the transformer 1 gives rise to a current of an intensity greater than I F on one phase at least, melting of the fuse or fuses 11 exposed to this current will be completed before a tripping order has been transmitted to the circuit breaker 15 by the tripping device 28. However, one of the current sensors 27 will have seen the short-circuit current flow and the tripping device 28 will subsequently cause opening of the circuit breaker 15. This arrangement ensures that the whole of the primary circuit is insulated, even if the short-circuit has only caused melting of a single fuse 11.

Other tripping modes of the circuit breaker are scheduled. 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, these data comprise for example measurements of the level of the liquid, its temperature, its pressure, or of the presence of a gas in the tank.

According to a second embodiment, represented in figure 5, a monoblock multifunction sub-assembly 50 comprises a medium-voltage connection part 51, a flexible electrical connection 52, a bushing 53 and a fuse head fitting part 54. The flexible connection 52 comprises a core 55 comprising a metal braid or a fine cable. The sub-assembly comprises a moulded external coating 56 made of insulating elastomer material. A flange 57 is :i provided for fixing to the tank 4. Electrical insulation is thus obtained without interruption 1' between the middle part of the fuse 11 and the medium-voltage connection 51 external to 15 the tank. This device has the additional advantage with respect to the previous one of eliminating a connection part with the medium-voltage switchgear apparatus on the source side.

The same principles apply in single-phase, as indicated by figure 6 representing the wiring 20 diagram of a third embodiment of the invention. A single current sensor is then sufficient to perform control of the circuit breaker.

.9.o o9oe A fourth embodiment, illustrated in figure 7, differs from the first embodiment essentially by the structure of the bushing and of the line-side 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 formed by a sheathed conductor. This arrangement, of lower performance, is however also less costly since it reduces the number of specific parts.

The above examples of embodiments have been taken in the field of medium-voltage lowvoltage transformers. The invention is however also applicable to other types of transformers, in particular to medium-voltage source transformers. The transformer primary can be either delta-connected or star-connected. In the latter case, it may be useful to fit a current sensor on the neutral bushing for control of the overload device.

Furthermore, other types of circuit breaker than vacuum circuit breakers can be envisaged.

The invention can also be applied with a gas dielectric, in particular SF6.

"Comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or ccmponents but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

Claims

1. A three-phase electrical transformer comprising a tank containing a liquid or gas dielectric e a three-phase primary circuit and a three-phase secondary circuit, both comprising windings 3) immersed in the dielectric, each phase of the primary circuit entering the tank via an insulating bushing a protection device comprising e at least two fuses (11) 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 primary circuit windings and having a sufficient breaking capacity to perform breaking of the corresponding phase in case of a three-phase short-circuit of the '"primary circuit, 15 a multipole circuit breaker (15) comprising, for each phase equipped with one of the fuses a pole immersed in the dielectric and connected in series on said phase between the corresponding fuse (11) and the windings each pole comprising contact means (19, 20) able to take with respect to one another a contact position in which they ensure current flow and a separation position, the circuit breaker (15) comprising in addition an opening mechanism (22) common to all the poles, designed to oooo move the contact means (19, 20) from their contact position to their separation position, 9 tripping means (28, 30) associated to the opening mechanism (22) of the 25 circuit breaker (15) and comprising a overload trip device (28), the fuses circuit breaker (15) and 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 means (19, 20) of the circuit breaker (15) separate and interrupt the current without the fuses (11) starting to melt, and that there exists a threshold value of the current intensity flowing in each fuse, this threshold being lower than the breaking capacity of the corresponding pole of the circuit breaker (15) and above which threshold melting of the fuse (11) has taken place before a separation order of the contact means (19, 20) given by the overload trip device (28) has been able to cause separation of the contact means (19,

2. A single-phase electrical transformer comprising a tank containing a liquid or gas dielectric a primary circuit and a secondary circuit, both comprising one or more windings 3) 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 (11) immersed in the dielectric connected in series on the phase of the primary circuit of the transformer between the insulating bushing and the winding or windings of the primary circuit and having a sufficient breaking capacity to perform breaking of the corresponding phase in case of a short-circuit of the primary circuit, a circuit breaker (15) comprising a pole immersed in the dielectric connected in series between the fuse (11) and the windings of the primary circuit and comprising contact means (19, 20) able to take with respect to one another a contact position in which they ensure current flow and a separation position, the circuit breaker (15) comprising in addition 15 an opening mechanism (22) designed to move the contact means (19, from their contact position to their separation position, tripping means (28, 30) associated to the opening mechanism (22) of the circuit breaker (15) and comprising an overload trip device (28), the fuse circuit breaker (15) and 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 means (19, 20) of the circuit breaker (15) separate and interrupt the current without the fuse (11) starting to melt, and that there exists a threshold value of the current intensity flowing in the fuse this threshold being lower than the breaking capacity of the corresponding pole of the circuit breaker 25 and above which threshold melting of the fuse (11) has taken place before a separation order of the contact means (19, 20) given by the overload trip device (28) has been able to cause separation of the contact means.

3. The transformer according to either one of the claims 1 and 2, characterized in that the overload trip device (28) comprises at least one measuring means (27) for measuring the intensity of the current flowing in a phase of the primary circuit of the transformer

4. The transformer according to claim 3, characterized in that the measuring means (27) measures the current flowing at a point situated line-side from the fuse or fuses in particular outside the tank line-side from the bushing The transformer according to any one of the foregoing claims, characterized in that the fuse or fuses (11) have an elongate shape with two conducting ends (10, 13) designed for their line-side connection on the bushing side and load-side connection on the circuit breaker (15) side, situated on each side of a middle part (12) whose external surface is insulated, and that for each phase equipped with a fuse the conducting parts situated between the insulating bushing and the insulated external surface of the middle part (12) of the fuse are enveloped in a solid insulator.

6. The transformer according to any one of the foregoing claims, characterized in that the fuse or fuses (11) have an elongate shape with two conducting ends (10, 13) designed for their line-side connection on the bushing side and load-side connection on the circuit breaker (15) side, situated on each side of a middle part (12) whose external surface is formed by an insulator, and that for each phase equipped with a fuse the fuse (11) constitutes a monoblock sub-assembly with the bushing and that the external surface of this sub-assembly, in its part internal to the tank comprising the bushing the :i line-side conducting end (10) and the middle part (12) of the fuse is formed by one or more solid insulators forming a solid insulation without interruption. *6*6

7. The transformer according to any one of the foregoing claims, characterized in that the tripping means (28, 30) comprise in addition a trip device (30) commanding opening of the circuit breaker when at least one parameter representative of the state of said dielectric exceeds a set threshold. *o6 6

8. The transformer according to any one of the foregoing claims, characterized in that the overload trip 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. S•

9. The transformer according to any one of the foregoing claims, characterized in that the fuse or fuses circuit breaker (15) and tripping means (28, 30) are associated in such a way that if the fuse or one of the fuses melts, the tripping means (28, 30) give a separation order of the contact means. transformer according to any one of the foregoing claims, characterized in that the fuse or fuses (11) are limiting.

11.The transformer according to any one of the foregoing claims, characterized in that each pole of the circuit breaker comprises a vacuum cartridge (17) immersed in the dielectric

12.The transformer according to any one of the foregoing claims, characterized in that the circuit breaker (15) comprises a closing mechanism (22) designed to move the contact means (19, 20) from their separated position to their contact position, comprising an operating means (23) accessible from outside the tank the opening mechanism (22) being designed to cause opening of the circuit breaker pole or poles whatever the state of the closing mechanism (22). DATED this 13th day of August 1999. SCHNEIDER ELECTRIC INDUSTRIES SA WATERMARK PATENT TRADEMARK ATTORNEYS 290 BURWOOD ROAD HAWTHORN. VIC. 3122. *00 .:9000 0 00 6 *oo