EP0653765A1 - Self-protected immersed polyphase electrical transformer - Google Patents

Abstract

This transformer, with a sealed tank filled with a dielectric cooling liquid, is provided with equipment for self protection against the risk of explosion of the tank, consisting of: * an immersed unit for protection against internal breakdowns degenerating relatively slowly and comprising: - an immersed sensor 1, sensitive to changes of a characteristic of the dielectric liquid which surrounds it, chosen from among those, such as pressure, which react at the site of the sensor practically upon the appearance, at any site in the liquid mass, of a degenerative defect, this sensor delivering an "on or off" signal in response to this alteration. - a non-rearmable polyphase contact breaker 5 between the phases of the power supply and the corresponding primary winding of the transformer. - and a trigger 6 for control of the opening of the contact breaker 5 driven by the signal delivered by the sensor 1, when the amplitude of the said signal exceeds a preestablished safety threshold value: * and a unit for protection against sudden current overloads drawn by the transformer from the power supply means, comprising, on each of the phases of the power supply, an array of fuses 26, 27 mounted in cascade and including staged cutoff thresholds. <IMAGE>

Description

The present invention relates to self-protected immersed polyphase electrical transformers.

Recall that an "immersed" transformer is a transformer whose electrical windings and magnetic circuit are cooled by immersion in a dielectric liquid (generally a mineral oil) contained in an airtight tank. Depending on whether the liquid dielectric fills all or part of the tank, we will be in the presence of a submerged transformer with "full filling" or "partial filling".

The term “protected submerged transformer” is used to mean a transformer of the aforementioned type comprising equipment intended to protect it against the possible consequences of internal damage, such as deterioration or defects in the insulation, the extent or persistence of which can lead to the bursting of the tank.

According to available information, in France for example, nearly 1 ‰ of the submerged distribution transformer fleet, or more than 500 devices, explode each year. thus endangering the safety of people, immediately or in the more or less short term (fires, spreading of dielectric in soils, etc ...). The apparent cause is either an abnormal drop in the level of the dielectric liquid in the tank, or an untimely rise in the internal temperature which leads to the bursting of the tank following too high a rise in the pressure of the dielectric liquid.

In order to avoid these unfortunate consequences, it has already been proposed, for small power transformers (ie not exceeding 160 kVA of total power generally), to use instead of a tank made of assembled sheet steel , a molded aluminum tank made up of two half-shells joined by waterproof welding.

Such a tank strengthening solution actually attaches to the effects of a rise in temperature. In addition, it results in a significant additional cost of the transformer, and its application remains limited to devices of relatively low power.

Document USP 4,192,174 discloses a device for protecting transformers against tank explosions constituted by a detector for variations in the characteristics of the dielectric liquid (such as excess pressure or excessive temperature, or its level) mounted on the tank and capable of '' send an outside alarm signal to a monitoring station.

Also known, from documents EP 0093076 or USP 4,223,364, are devices for automatic disconnection from the power supply network which are installed on the transformers to be protected. By means of a resettable switch which, activated by a trip device, opens the electrical circuit, these devices respond as soon as the temperature of the dielectric liquid at the location of a thermal probe exceeds a pre-established safety threshold.

This type of device, applied seems it to single-phase transformers only, appears specifically designed for the self-protection of submerged transformers with partial filling.

In addition, they take into account only internal degenerative damage at a relatively slow rate of evolution. The transformers therefore remain exposed to explosion risks resulting for example from sudden and massive overloads of current called on the network by the transformer. These risks of explosion, which originate from internal damage, can lead to the destruction of the device extremely quickly, well in any case before the dielectric liquid anomaly detection probe has been able to react, if its location d he implantation is not in the immediate vicinity of the place in the tank where the damage is born.

The object of the invention is to propose a global solution to the self-protection of submerged transformers using equipment with autonomous and automatic operation, protecting them from the risk of explosion, whatever the nature of the internal defect. which is the cause.

To this end, the subject of the invention is a submerged polyphase electric transformer, placed in an airtight tank filled with a dielectric liquid and provided with self-protection equipment against the risks of explosion of the tank by disconnection of the power supply network.

The transformer according to the invention is as defined in the attached claims.

As will be seen in more detail, the invention essentially resides in the design of a device for breaking the passage of the automatic current, autonomous, without intervention from outside the transformer and not resettable by the user (for reasons of security). This device, with a very rapid breaking action to avoid the phenomena of electrical arcing during the opening of the circuits, is sensitive both to untimely current overloads and to dielectric liquid anomalies reflecting the birth of an internal local defect. with relatively slow evolution, at least at its beginning.

Whether of internal origin, or caused by external phenomena, the dangerous faults which can occur in an immersed transformer differ in fact in two categories according to whether their degeneration until the explosion of the tank is a fast process (a few hundredths , or even a few thousandths of a second), or a rather long process (generally a few seconds, but sometimes reaching several hours or even days).

Fast-evolving damage is characterized by a large variation in the impedance of the conductor on which it appears, or acts. As a general rule, they are rather located at the free ends of the windings (inputs-outputs).

As they lead to a massive and brutal current demand on the network, the protection of the transformer against them is ensured by fuses with an operating range adapted to the overcurrents to be thwarted.

As in addition, such current overloads can occur over a wide intensity range, (i.e. to fix ideas from 20 to more than 10,000 A. for a distribution transformer of 160 kVA of power for example), we have advantage to opt for a battery of at least two fuses in cascade per phase and having different cut-off thresholds, one being assigned to rather weak currents (for example from 20 to 200 A.), the other having in charge stronger currents (greater than 200 A, in the example considered).

Slow-onset degenerative damage is often a point fault located inside the windings, such as insulation faults in the conductors or the layers of conductors between them making up these windings.

These local degradations can, more or less slowly, degenerate into "hot spots" by creating very high current intensities flowing in tight loops very localized constituting real mini short-circuits. As these degenerative phenomena often evolve rather slowly at least at the beginning, they therefore manifest themselves only very discreetly, by an over-intensity which can be much lower than the intensity of entry into the primary, and which therefore passes easily unnoticed, or even be undetectable.

It is precisely phenomena of this type that are taken into account by the protection unit using the liquid dielectric medium as mediator of the damage, between the place where it occurs and the place where the detection sensor is installed.

This delay in transmitting the presence of a defect must be compatible with the speed of its degeneration process, namely of the order of a second or a few seconds, not beyond, for defects with the slowest progress.

It is for this reason that the choice of the characteristic of the dielectric liquid to be monitored by the sensor is decisive. This characteristic must indeed be such that its measurement at the location of immersion of the sensor is representative almost at the same instant (ie not more than one second of difference approximately) of the appearance of a local degenerative damage does anywhere else in the tank.

It will advantageously be the pressure of the dielectric liquid, the variations of which are transmitted at the speed of propagation of the sound in this incompressible medium.

It may also be, for similar reasons, the noise that certain damages make and that the dielectric liquid will transmit to the sensor at the speed of sound.

It may also be the physical nature of the dielectric liquid itself and whose modifications, such as the appearance of a gaseous phase due to local boiling, would reflect a sharp increase in temperature at the place of birth of a degenerative defect.

Conversely, the temperature of the dielectric liquid cannot be used as a characteristic whose variations are to be monitored by the sensor. The speed of diffusion of heat within the liquid mass, (moreover with a high calorific capacity to ensure good cooling), is far too low indeed in view of the possible rapidity of evolution of the defects to be detected.

According to the invention, the sensor used will therefore be a pressure sensor preferably, or a sound level meter, or a gas bubble detector for example, but not a temperature probe, since the temperature of the liquid located at the location except in extreme cases, the probe would not be instantly representative of the temperature at another location within the tank.

• la figure 1 montre une vue d'ensemble du dessous d'un équipement de protection d'un transformateur;
• les figures 2 montrent, en vue latérale selon la direction A-A de la figure 1, l'équipement de protection d'abord en position de fermeture (figure 2a), puis en position d'ouverture (fig. 2b);
• la figure 3 donne les caractéristiques de fonctionnement de la batterie de fusibles de protection à l'égard des surcharges massives de courant.

The invention will be well understood, and other aspects and advantages will appear clearly in the light of the description which follows, given by way of example with reference to the plates of attached drawings in which:

• Figure 1 shows an overview of the underside of a transformer protection equipment;
• Figures 2 show, in side view along the direction AA of Figure 1, the protective equipment first in the closed position (Figure 2a), then in the open position (Fig. 2b);
• FIG. 3 gives the operating characteristics of the battery of protective fuses with regard to massive current overloads.

In all the figures, the same elements are designated by identical references.

As can be seen, the protective equipment consists of a unit sensitive to changes in the pressure of the dielectric liquid associated with a unit sensitive to sudden current overloads massively called up by the transformer on the supply network.

The unit sensitive to changes in the pressure of the dielectric liquid essentially comprises a sensor 1 and an actuator 2. The sensor, mounted on a fixing base 3, is here of the mechanical membrane type (not shown), the deformation of which is transmitted by the rod 4, is proportional to the pressure presented by the dielectric coolant at the place where the membrane is immersed.

The actuator comprises a breaker 5 and a trigger 6 linked to the sensor 1 and urging the breaker.

The breaker is formed by two connection supports made of electrically insulating plastic, one, the support 7, carrying, by means of bases 29 in bakelite distributed over the length, the terminals 8, 8 ', 8' ' output of phases U, V, W of the three-phase power supply network supplying the transformer, the other support 9, being equipped with the corresponding terminals 10, 10 ', 10' 'for input into the windings (not shown) of the primary of the transformer.

As can be seen, the output terminals 8 ... are arranged opposite the input terminals 10 ... on their respective support and cooperate with each other to ensure the electrical contacts in the closed position of the breaker (fig. 2a ). In this case, the input terminals 10 ... are formed by copper studs which are encircled by contacts with a copper blade in the form of elastic forks forming the output terminals 8 ...

• une position, dite de fermeture du rupteur, visible sur les fig. 1 et 2a, et dans laquelle les contacts entre les bornes de sortie 8... et d'entrée 10... sont assurés;
• une position décalée angulairement de la précédente avec un débattement suffisant pour éloigner les bornes d'entrée des bornes de sortie correspondantes sans risque d'arcages électriques entre-elles. Il s'agit de la position d'ouverture du rupteur, montrée sur la figure 2b.

In this embodiment, the support 7 is mounted movable in rotation about its longitudinal axis so as to be able to take two angular positions:

• a position, known as closing the breaker, visible in FIGS. 1 and 2a, and in which the contacts between the output terminals 8 ... and input 10 ... are ensured;
• a position angularly offset from the previous one with sufficient clearance to move the input terminals away from the corresponding output terminals without risk of electrical arcing between them. This is the opening position of the breaker, shown in Figure 2b.

Thanks to its design in two elongated bodies arranged parallel to one another, one being fixed, the other movable in rotation about its axis, and each carrying, distributed throughout, cooperating electrical terminals with the paired terminals of each other to close the electrical circuits, the circuit breaker thus designed is perfectly suited to polyphase transformers to also serve as a switch, tap changer.

For this purpose, in fact, and according to an embodiment not shown in the figures, one of the supports, for example the fixed support 9, may include, in the immediate vicinity of each of its input terminals 10 ... plates of electrical copper connections connected to the primary winding corresponding to said input terminal according to different entry points of the current in this winding. These different entry points correspond to different voltage values applied to the phase supplying the winding.

Thus, it suffices to provide translatable jumpers for bridging between the input terminal and one or the other of these plates to ensure the adjustment of the transformer to the input voltage imposed on the phases by the supply network. . These jumpers can advantageously be carried by a translatable rod parallel to the fixed support 9 and driven by a "rack and pinion" system controlled manually by an operator.

The breaker 5 also comprises a means with elastic antagonistic action, here a leaf spring 11, one end of which is fixed on the movable support 7 and the other end bears freely on a vertical surface 28 of a fixed base 12. As it will be understood, this spring has the role of constantly tending to place the mobile support 7 in the open position

The movable support 7 also includes a rotation locking stop 13 which ensures the locking of the support in the closed position by cooperating with the trigger 6, as will be seen more clearly below. Here, the stop 13 is formed by a finger fixed perpendicular to the movable support 7 and whose free end 14 is shaped as a hook.

The trigger 6 is constituted, in the example considered, by a rigid movable element 15 acting as a lever and by a trigger 16.

The movable element 15 has its fixed point 17 at one end. The other end 18, left free, cooperates with the trigger 16. The mediating point 19 of the lever 15 is fixed to the end of the rod 4 of the membrane 3 of the sensor 1.

The trigger 16 is pivotally mounted on a spring axis 20. A seat 21 of the trigger is urged by the free end 18 of the element 15, and its other seat 22, in opposition to the first, is shaped to maintain the hook end 14 of finger 13.

The mobile support 7 is, as can be seen, mounted in bearings 23 provided on the fixed base 12 and which themselves also carry the fixed support 9. At the ends of the latter, complementary bearings 23 'are provided to maintain the movable support 7 at its ends. These bearings as well as the fixed support 9 are, like the mobile support 7, made of an electrically insulating material, for example bakelized paper.

The whole is therefore carried by the base 12, which is fixed under a plate 24 which is just fixed to the upper part of the transformer before immersion in the tank.

Note the advantageous presence of lugs 25 forming extensions on the terminals 8, ... of the movable support 7 and which serve as an end stop for rotation of this support in the open position coming to bear against the fixed support 9 .

The device which has just been described operates in the following manner:

In normal operation of the transformer, the breaker 5 is in the closed position, as shown in FIGS. 1 and 2a.

If the pressure within the dielectric liquid increases, following an abnormal rise in temperature caused for example by a local defect in the electrical insulation of the windings of the transformer, the deformation of the membrane 3 which results therefrom is transmitted, by the rod 4, to the lever 15, which communicates it, by amplifying it at its free end 18, to the trigger 16.

It will be noted that a clearance is advantageously provided for mounting between the end 18 and the trigger. This clearance is set to be filled when the overpressure limit is reached.

Thus, it also fulfills the function of timer, by making it possible to discriminate the phenomena too fleeting to really threaten the behavior of the transformer.

At this time, the end 18 urges the trigger 16, which by pivoting about its axis 20, will release the retaining finger 13 at its other end 22. Under the effect of the spring 11, the movable support 7 thus unlocked, pivots about its axis in order to disconnect the output terminals 8 ... from the corresponding input terminals 10 ..., thus putting the transformer off-circuit. The breaker is then in the irreversible opening position, as shown in FIG. 2b.

The entire device is placed inside the transformer tank, so that once the electrical contacts have been broken, it is no longer possible to reset the device without special dismantling of the tank. .

Furthermore, being non-resettable, it is therefore important that the breaker can act within extremely short time periods so that the simultaneous opening of the circuits on the phases is not accompanied, or as little as possible, by electrical arcing phenomena when the paired connection terminals 8 ... and 10 ... move away from each other.

This great speed of opening of the circuits as well as their simultaneity are precisely obtained thanks to the particular design of the breaker in two supports 7 and 9 elongated and parallel and one of which is driven by a rotational movement around its axis. This allows, by providing one of them with bases such as 29 acting as an arm carrying the terminals 8 at their end, to achieve very high opening speeds, namely proportional, for an angular speed of rotation given, to the length of the support arm, which can therefore be dimensioned as desired.

This protection device is completed by an additional protection unit which is sensitive to high currents which can be suddenly called up on the supply network by the transformer, for example due to a collapse of the internal impedance caused by a short - local circuit, but with significant ohmic drop.

This protection unit is constituted by one, or preferably two fuses, with stepped cut-off thresholds, and connected in cascade on each of the phases of the supply, in series with the electrical connections 8 ...- 10 ..., either downstream, or, as shown in the figures, upstream of these connections with respect to the supply network.

As can be seen, these fuses, designated by the references 26 and 27, are also fixed on the base 24. They are in the form of cylindrical bars, electrically segmented into three independent parts between them and each relating to a phase of three-phase power.

Their respective ranges of fusion with temperature as a function of time are visible in the diagram in FIG. 3.

Fuse 27 for example is intended to operate in a very high current intensity range, identified by the letter (c), going beyond 200 A.

Fuse 26, mounted in series with the previous one, has a lower operating range (marked with the letter b), ranging from 15 to 200 A, for example. The lower threshold of 15 A corresponds approximately to 3 to 5 times the nominal current of a conventional distribution transformer of 160 kVA of three-phase power. Fuse 26 is intended to protect the transformer against more or less lasting uncontrolled fluctuations in the input impedance of the primary. This can indeed drop drastically and thus make a sudden call of current of intensity, relatively more moderate than that taken into account by the fuse 27, but nevertheless completely detrimental to the behavior of the transformer.

The range marked (a) in the figure is the action zone of the breaker 5 described above. Its field of intervention, as already explained, is that of low intensities, that is to say that of the nominal current of operation of the transformer (for example 3 A.), in which the internal anomalies detrimental to the transformer can manifest themselves as very low overcurrents, which therefore easily go unnoticed.

It goes without saying that the invention cannot be limited to the embodiment described above, but extends to multiple variants and equivalents insofar as its definition given in the claims below is respected.

Thus, for example, the retaining finger 13 can be replaced by a hollow imprint made in the movable support 7 itself.

Similarly, the trigger 16, although advantageous, is not an essential intermediary between the lever 15 and the finger 13, which can be put in direct contact with one another.

Likewise again, as will be understood, the solution described above takes direct account of the pressure of the dielectric liquid as a quantity indicative of a rise in temperature.

According to a variant already mentioned, the pressure can be monitored indirectly. For this purpose, the pressure sensor 1 can be replaced by a gas detector. In fact, when the temperature rises, there is in fact no serious risk of explosion of the transformer due to internal overpressure as long as there is no boiling of the dielectric liquid.

Such a gas formation detector can advantageously be housed in the highest point of the tank. If necessary, due to the presence of baffles which can form pockets, several detectors will be used distributed at the high points where such pockets are more easily liable to form.

Although initially designed for transformers, the invention applies equally to any other similar electrical device, that is to say to any device comprising electrical windings immersed in a dielectric liquid and which is suitable, for example. for the sake of simplification, also to denote by the general term "transformer" in the present specification.

Claims (10)

Polyphase submersible electric transformer placed in an airtight tank filled with a dielectric liquid and provided with self-protection equipment against the risk of explosion of the tank by disconnection from the electrical supply network, characterized in that said equipment consists of: * on the one hand, a protection unit reacting to internal faults at a slow degeneration speed and immersed in the dielectric liquid within the tank and comprising: - a sensor (1) sensitive to variations in a characteristic of said dielectric liquid chosen from those, such as pressure, the measurement of which at any location is representative of the appearance of a local defect anywhere else within the tank, said sensor delivering a signal in response to these variations; - a non-resettable multi-phase contact breaker (5) between the phases of the electrical supply (u, v, w) and the corresponding primary windings of the transformer; - And a trigger (6) for controlling the opening of the switch (5) solicited by the signal delivered by the sensor (1), when the amplitude of said signal exceeds a preset safety threshold value; * and on the other hand, a protection unit against current overloads called on the supply network by the transformer and comprising at least one fuse (27) on each of the phases (u, v, w) of food. Transformer according to claim 1, characterized in that said protection unit against current overloads is constituted, on each of the phases of the electrical supply, by a battery of at least two fuses (26, 27) having staged and cascaded cutoff thresholds. Transformer according to claim 1 or 2 characterized in that said protection unit against current overloads is also immersed in the dielectric liquid within the tank. Transformer according to claim 1, characterized in that the sensor (1) is of the mechanical type, and delivers a signal in the form of a force applied to the trigger (6) to activate the latter when said safety threshold is exceeded. Transformer according to claim 1 characterized in that the breaker (5) consists of: - two supports (7, 9) for electrical connections, made of electrically insulating material, one (9) carrying the terminals (10, 10 ', 10'') for current input in the primary windings of the transformer, the other (7) carrying the corresponding terminals (8, 8 ', 8'') of output of the phases of the power supply, said input and output terminals being arranged opposite one another on their respective support and in contact with each other to ensure the electrical connection of the transformer in the closed position of the breaker, one (9) of the supports being fixed and the other (7) being mounted so as to be able to rotate about its axis between a position closing and an open position of the breaker; - means with elastic antagonistic action (11) permanently tending to place the breaker (5) in the open position by moving the output terminals (8, ...) away from those of the input ones (10, .. .) by rotation of the mobile support (7); - And a member (13) for locking in rotation of the mobile support (7) ensuring the maintenance of the breaker (5) in the closed position while being engaged in the trigger (2). Transformer according to claim 5, characterized in that on the mobile support (7) the terminals (8) are located on carrier bases (29) which move them away from the axis of rotation of the support. Transformer according to claim 5, characterized in that said movable support (7) is rotatably mounted in bearings (23) which also hold the fixed support (9), said bearings being carried by a base (12) fixed to a plate base (24) and carrying the trigger (6) and providing a bearing surface (28) to the means with elastic antagonistic action (11). Transformer according to claim 1, characterized in that the trigger (6) is formed by a movable element (15) urged by the sensor (1), and by a pivoting trigger (16) pivoting with return spring (20), one end of which (22) cooperates with the member (13) for locking the rotation of the mobile support (7) to keep the breaker (5) in the closed position, and the other end (21) cooperates with said mobile element (15) . Transformer according to claim 1 or 2, characterized in that said protection unit against current overloads is mounted upstream of the protection unit sensitive to the characteristics of the dielectric liquid, with respect to the power supply. Transformer according to claim 5 characterized in that the breaker comprises switching means making it possible to simultaneously adapt the points of entry of the current into the windings of the primary to the value of the voltage applied by the electrical network on each phase of the food.

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