EP0653765B1 - 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 electrical transformers self-protected immersed polyphase.

Remember that an "immersed" transformer is a transformer of which the electrical windings and the circuit are cooled by immersion in a dielectric liquid (usually mineral oil) contained in a hermetic tank. Depending on whether the liquid dielectric fills any or part of the tank. we will be in the presence of a transformer submerged "full fill" or "partial fill".

We call "protected submerged transformer", a transformer of the aforementioned type comprising equipment intended for protect it from the possible consequences of damage internal, such as damage or insulation faults and the extent or persistence of which can lead to bursting of the tank.

According to available information, in France for example. close to 1 ° / _°° of the fleet of submersible distribution transformers. more than 500 devices, explode every year. thus endangering the safety of people, immediately or in the more or less short term (fires, spreading of dielectric in floors 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 (i.e. generally not exceeding 160 kVA of total power), to use instead of an assembled sheet steel tank, a molded aluminum tank made up of two half-shells joined by waterproof welding.

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

We know from document USP 4,192,174 a device for protection of transformers against explosions of the tank consisting of a detector of variations in characteristics dielectric liquid (such as overpressure or excessive temperature, or its level) mounted on the tank and capable of emitting an alarm signal to the outside a monitoring station.

We also know from documents EP 0093076 or USP 4,223,364, devices for automatic disconnection from the network of power supply which are on board the transformers to protect. By means of a resettable breaker which, when triggered, 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 preset security threshold.

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

In addition, they take into account that damage degenerative interns at relatively fast rate of evolution slow. Processors therefore remain exposed to risks explosion resulting for example from sudden overloads and massive current drawn on the network by the transformer. These explosion risks, which arise from damage internal, can lead to destruction of the device extremely quickly, well in any case before the probe detection of dielectric liquid anomalies could react, if its location is not in the immediate vicinity from the place in the tank where the damage is born.

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

To this end, the invention relates to a transformer submersible polyphase electric heater, placed in an airtight tank filled with dielectric liquid and provided with equipment self-protection against the risk of explosion of the tank by disconnection from 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 resides essentially in the design of a rupture device automatic, autonomous current flow, without intervention from outside the transformer and not resettable by the user (for security reasons). This action device very quick break to avoid arcing phenomena electric when opening circuits, is sensitive to both untimely current overloads and anomalies dielectric liquid reflecting the birth of a local internal defect with relatively slow evolution, at less at its beginning.

Whether of internal origin, or caused by external phenomena, dangerous faults which may occur in a submerged transformer differentiate into done in two categories depending on whether their degeneration up the explosion of the tank is a rapid process (some hundredths, or even a few thousandths of a second), or a rather long process (usually a few seconds, but sometimes reaching several hours or even several days).

Rapidly developing damage is characterized by large variation in the impedance of the conductor on which they appear, or act. As a rule, they lie rather at the free ends of the windings (inputs-outputs).

As they lead to a massive and brutal call from current on the network, protecting the transformer at their against is ensured by operating range fuses suitable for overcurrents to counter.

As elsewhere, such current overloads can occur over a wide range of intensity, (i.e. to fix ideas from 20 to more than 10,000 A. for a transformer distribution of 160 kVA of power for example), we has the advantage of opting for a battery of at least two fuses cascaded by phase and having differentiated cutoff thresholds, one being assigned to rather weak currents (by example from 20 to 200 A.), the other having in charge the currents stronger (greater than 200 A, in the example considered).

Slow-onset degenerative damage is often occasional faults located inside the windings, such as insulation faults in the conductors or layers of conductors between them constituting these windings.

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

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

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

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

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

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

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

Conversely, the temperature of the dielectric liquid does not can be used as a characteristic whose variations are monitor by the sensor. The speed of diffusion of the heat within the liquid mass, (in addition to capacity high heat to ensure good cooling), is far too weak indeed in view of the possible speed evolution of the defects to be detected.

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

• la figure 1 montre une vue d'ensemble du dessous d'un équipement de protection d'un transformateur en position de fermeture ;
• 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 in the closed position;
• Figures 2 show, in side view in 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 by a unit sensitive to changes in the pressure of the dielectric liquid associated with a unit sensitive to brutal current overloads called massively 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 mounting plate 3 is here of the mechanical membrane type (not shown), of which the deformation, transmitted by the rod 4, is proportional at the pressure of the dielectric coolant where the membrane is submerged.

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

The breaker is formed by two connection supports in electrically insulating plastic material, one, support 7, bearing via bakelite bases 29 distributed over the length, terminals 8, 8 ', 8' 'of output phases U, V, W of three-phase power supply network supplying the transformer, the other support 9, being fitted with terminals corresponding 10, 10 ', 10' 'entry in the windings not represented from the transformer primary.

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 electrical contacts in the closed position of the switch (fig. 2a). In this case, the input terminals 10 ... consist of copper studs that are encircled by contacts with shaped copper blades 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 each other, one being fixed, the other mobile in rotation around its axis, and each bearing, distributed throughout, cooperating electrical terminals with the paired terminals of each other to close the electrical circuits, the breaker thus designed happens to be perfectly suited to polyphase transformers to serve also of switch, tap changer.

For this purpose indeed, 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 ... copper electrical connections connected to the winding of the primary corresponding to said input terminal according to different entry points of the current in this winding. These different entry points correspond to values of different voltages applied to the phase supplying the winding.

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

The breaker 5 also includes an antagonistic action means elastic, here a leaf spring 11, one end of which is fixed on the mobile support 7 and the other end takes free support on a vertical surface 28 of a fixed base 12. As will be understood, this spring has the role of tending in constantly placing the mobile support7 in the open position

The movable support 7 also includes a stop for rotation lock 13 which ensures the blocking of the support in the closed position by cooperating with the trigger 6, as we will see better later. Here, the stop 13 is formed by a finger fixed perpendicular to the movable support 7 and whose free end 14 is hook shaped.

The trigger 6 consists, in the example considered, by a rigid movable element 15 leveraging 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 median point 19 of the lever 15 is fixed at 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 range 21 of the trigger is biased by the end free 18 of element 15, and its other bearing 22. in opposition at the first, is shaped to maintain the end crochet 14 with finger 13.

The mobile support 7 is, as can be seen, mounted in bearings 23 provided on the fixed base 12 and which carry themselves also the fixed support 9. At the ends of this last, additional bearings 23 'are provided for hold the movable support 7 at its ends. These bearings as well as the fixed support 9 are, like the mobile support 7, in an electrically insulating material, for example in 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 terminals 8, ... of mobile support 7 and which serve as the end of rotation stop for this support in position by coming to bear against the fixed support 9.

The device which has just been described operates from the as follows:

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

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

Note that a game is advantageously provided for mounting between the end 18 and the trigger. This game is set for be filled when the overpressure limit is reached.

So it also fulfills the timer function, by allowing to discriminate the phenomena too fleeting for really threaten the transformer.

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

The entire device is placed inside of the transformer tank, so that the breaking of electrical contacts once made, it is no longer possible to reset the device without special disassembly intervention of the tank.

Furthermore, being non-resettable, it is therefore important that the breaker can act in an extremely short time so that simultaneous opening of the circuits on the phases is not accompanied no, or as little as possible, electrical arcing phenomena when the paired connection terminals 8 ... and 10 ... move away from each other.

This rapid opening of circuits as well as their simultaneity is precisely obtained thanks to the particular design of the breaker in two supports 7 and 9 elongated and parallel and one of which is animated by a movement of rotation around its axis. This allows, by endowing one of them of bases such as 29 playing the role of arm carrying their ends the terminals 8, to achieve speeds very high, namely proportional, for a given angular speed of rotation, at arm length carrier, which can therefore be sized as desired.

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

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

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

Their respective ranges of fusion with the temperature in as a function of time are visible in the diagram in Figure 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 by the letter b), ranging from 15 to 200 A, for example. The threshold 15 A lower, approximately 3 to 5 times the nominal current of a distribution transformer classic 160 kVA three-phase power. Fuse 26 is intended to protect the transformer against fluctuations more or less lasting uncontrolled impedance primary school entrance. This can indeed drop drastically and thus make a brutal call of intensity current, relatively more moderate than that taken into account by the fuse 27 but nevertheless completely detrimental to the transformer outfit.

The range marked (a) in the figure is the action area of the switch 5 described above. His area of intervention, as we have already explained, is that of low intensities, i.e. that of the nominal operating current of the transformer (for example 3 A.), in which the anomalies internal detrimental to the transformer may occur by very low overcurrents, which therefore pass easily overlooked.

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

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

Similarly, the trigger 16, although advantageous, is not an essential intermediary between the lever 15 and the finger 13, which can be linked directly to each other.

Likewise again, as we will have understood, the solution described above directly takes into account the pressure of the dielectric liquid as an indicator of an elevation of temperature.

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

Such a gas formation detector can be accommodated advantageously in the highest point of the tank. Yes necessary, due to the presence of baffles which may form pockets, we will use several distributed detectors at high points where such pockets are more easily susceptible to train.

Although originally designed for processors, the invention also applies to any other electrical appliance similar, that is to say any device comprising electric windings immersed in a dielectric liquid and which we agree, for the sake of simplification, to name also by the general term "transformer" in the present brief.

Claims (10)

1. Immersed polyphase electrical transformer placed in a sealed tank filled with a dielectric liquid and provided with a piece of equipment for self-protection against the risks of the tank exploding by disconnection from the electrical supply mains, characterized in that the said piece of equipment consists of:

   on the one hand, a protection unit which reacts to internal defects having a slow rate of degeneration and which is immersed in the dielectric liquid within the tank and comprises:

   a sensor (1) which is sensitive to the variations in a characteristic of the said dielectric liquid chosen from those such as the pressure, the measurement of which at any point is representative of the appearance of a local defect anywhere else within the tank, the said sensor delivering a signal in response to these variations;

   a non-resettable polyphase contact breaker (5) between the phases (u, v, w) of the electrical supply and the corresponding primary windings of the transformer; and

   a trip (6) for causing the breaker (5) to open, supplied with the signal delivered by the sensor (1), when the amplitude of the said signal exceeds a pre-established safety threshold value;

   and, on the other hand, a unit for protection against current overloads called on the supply mains by the transformer and comprising at least one fuse (27) on each of the phases (u, v, w) of the supply.
2. Transformer according to Claim 1, characterized in that the said unit for protection against current overloads consists, on each of the phases of the electrical supply, of a bank of at least two fuses (26, 27) which have staged cutoff thresholds and are connected in series.
3. Transformer according to Claim 1 or 2, characterized in that the said unit for protection against current overloads is also immersed in the dielectric liquid within the tank.
4. 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 trip (6) in order to activate the latter when the said safety threshold is exceeded.
5. Transformer according to Claim 1, characterized in that the breaker (5) consists of:

   two electrical connection supports (7, 9) made of electrically insulating material, one support (9) carrying the terminals (10, 10', 10") for input of the current into the windings of the primary of the transformer and the other support (7) carrying the corresponding output terminals (8, 8', 8") for the phases of the electrical supply, the said input and output terminals being arranged opposite each other on their respective support and in contact with each other in order to provide the electrical connection for 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 move in rotation about its axis between a closed position and an open position of the breaker;

   resilient counteracting means (11) tending permanently to place the breaker (5) in the open position, mutually moving the output terminals (8...) away from the input terminals (10...) by rotation of the movable support (7); and

   a device (13) for rotationally locking the movable support (7), ensuring that the breaker (5) is kept in the closed position by being engaged in the trip (2).
6. Transformer according to Claim 5, characterized in that the terminals (8) on the movable support (7) are located on supporting bases (29) which move them away from the axis of rotation of the support.
7. Transformer according to Claim 5, characterized in that the said movable support (7) is mounted so as to rotate in bearings (23) which also keep the fixed support (9) in position, the said bearings being carried by a base (12) which is fastened to a baseplate (24), carries the trip (6) and presents the resilient counteracting means (11) with a bearing surface (28).
8. Transformer according to Claim 1, characterized in that the trip (6) is formed from a movable element (15), stressed by the sensor (1), and from a pivoting trigger (16) with a return spring (20), one end (22) of which interacts with the device (13) for rotationally locking the movable support (7), in order to keep the breaker (5) in the closed position, and the other end (21) of which interacts with the said movable element (15).
9. Transformer according to Claim 1 or 2, characterized in that the said unit for protection against current overloads is mounted upstream of the protection unit sensitive to the characteristics of the dielectric liquid with respect to the electrical supply.
10. Transformer according to Claim 5, characterized in that the breaker comprises switching means allowing the points for input of the current into the windings of the primary to be simultaneously adapted to the value of the voltage applied by the electrical mains to each phase of the supply.

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