EP1166297B1 - Device for preventing explosions in electrical transformers - Google Patents

Description

The present invention relates to the field of prevention against the explosion of electric transformers cooled by a large volume of combustible fluid.

Electric transformers suffer losses both in the windings only in the iron part, which require dissipation of the heat produced. So the high power transformers are usually cooled by a fluid such as oil. The oils used are dielectric and are liable to catch fire beyond with a temperature of the order of 140 ° C. The transformers being very expensive elements, their protection requires attention special.

An insulation fault causes, at first, a significant electric arc which causes an action of the electrical protection which triggers the supply cell of the transformer (circuit breaker). The electric arc also causes a consequent diffusion of energy which generates a release of decomposition gas of dielectric oil, especially hydrogen and acetylene.

Following the evolution of gas, the pressure inside the transformer tank increases very quickly, resulting in often very violent deflagration. Deflagration results in a significant tear in the mechanical connections of the tank (bolts, welding) of the transformer which puts the said gases in contact with the oxygen in the ambient air. Acetylene being auto-flammable in presence of oxygen, a fire starts immediately and spreads the fire to other equipment on the site which may contain also large quantities of combustible products.

Explosions are due to short circuits caused by overloads, overvoltages, a gradual deterioration of insulation, insufficient oil level, appearance of water or mold or failure of an insulating component.

We know, in the prior art, protection systems fire for electrical transformers which are operated by fire or fire detectors. But these systems are works with significant inertia, when the transformer oil is already in flames. So we just limit the fire to the equipment concerned so as not to spread fire to the installations neighbors.

To slow the decomposition of the dielectric fluid due to a electric arc, we can use silicone oils instead of conventional mineral oils. However, the explosion of the tank of the transformer due to the increase in internal pressure is delayed only by an extremely short duration, of the order of a few milliseconds. This duration does not allow the implementation of means to prevent the explosion.

Document WO-A-97/12379 discloses a method of explosion and fire prevention in a transformer electric with a tank filled with coolant fuel, by detecting a break in the electrical insulation of the transformer by pressure sensor, fluid depressurization cooling contained in the tank, by means of a valve, and cooling the hot parts of the coolant by injection of an inert gas under pressure into the bottom of the tank in order to stir said fluid and prevent oxygen from entering the tank of the transformer. This process gives satisfaction and makes it possible to avoid the explosion of the transformer tank.

The object of the present invention is to provide a device improved allowing extremely rapid decompression of the tank to further increase the probability of safeguarding the integrity of the transformer, on-load tap-changers and bushings.

The explosion prevention device, according to the invention is intended for an electrical transformer comprising a tank filled with combustible coolant, and a means of decompression of the transformer tank. Way to decompression includes a rupture element provided with a part of reservoir including first zones of reduced thickness compared to rest of the retaining part and capable of tearing without fragmentation during the rupture of said element, and of the second thickness zones reduced compared to the rest of the retaining part and able to bend without tearing when said element breaks. The said element of rupture is capable of breaking when the pressure inside the tank exceeds a predetermined ceiling.

Preferably, the rupture element is provided with a member sealing on the side of the fluid and capable of sealing holes small diameter formed in the retaining part. The holes can form tear caps and be adjacent to the first areas of reduced thickness.

In one embodiment of the invention, the member sealing is in the form of a coating on the part of retained, said coating preferably being based on polytetrafluoroethylene.

Preferably, the retaining part is domed in shape. outward convexity, opposite to the fluid.

In one embodiment of the invention, the part of retainer is metallic, stainless steel, aluminum, or aluminum alloy.

Preferably, the device comprises a detection means of rupture integrated into the rupture element, which allows detection pressure in the tank relative to the predetermined ceiling.

In one embodiment of the invention, the means of rupture detection comprises an electric wire capable of breaking in same time as the rupture element.

In one embodiment of the invention, the electric wire is stuck on the breaking element.

Advantageously, the electric wire is placed on the side of the retaining part opposite the fluid.

In one embodiment of the invention, the electric wire is covered by a protective film.

The invention also relates to a prevention system against the explosion of an electric transformer including a tank filled with combustible coolant, and a means of decompression of the transformer tank. The system includes several devices as described above, including one or more on a main tank containing the windings and one on each on-load changer.

The system can include at least one device such as described above, on at least one electrical crossing.

Simultaneously, the rupture of the rupture element takes place where the tank decompression, and the wire break where the detection excessive and abnormal pressure.

Of course, terms such as "on the fluid side" or "to the opposite of the fluid "means before rupture.

The explosion prevention device is suitable for the main tank of a transformer, for the tank of the on-load changers, and for the bushing tank electric, this last tank being also called oil box. The electrical bushings have the role of isolating the main tank from a transformer of high and low voltage lines to which are connected transformer windings via rods exit. Each outlet rod is surrounded by an oil box containing a certain amount of isolation fluid. The fluid for isolation of bushings and / or oil boxes is a different oil that of the transformer.

One can provide a nitrogen injection means connected to a upper part of an oil can and capable of triggering when detection of a fault. Nitrogen injection can help evacuation of the fluid downstream of the rupture element. The nitrogen injection may especially avoid the entry of air into the oil box, an air inlet being likely to promote fire.

Explosion prevention device can be fitted a means of detecting the triggering of the supply cell of the transformer and of a control unit which receives the signals emitted by the sensor means of the transformer and which is capable to issue control signals.

The explosion prevention device can include a means of cooling the hot parts of the fluid, by injection of inert gas at the bottom of the main tank, controlled by a control signal from a control unit. In effect, certain parts of the coolant undergo a heating capable of igniting it. Injection of an inert gas into the bottom of the main tank causes mixing of the cooling which homogenizes the temperature and reduces the gas evolution.

la figure 1a est une vue en coupe transversale du dispositif de prévention selon l'invention;

la figure 1b est une vue partielle agrandie de la figure 1a ;

la figure 2 est une vue de dessus correspondant à la figure 1;

la figure 3 est une vue générale d'un transformateur équipé d'un dispositif de prévention selon l'invention;

la figure 4 est une vue générale d'un transformateur équipé de plusieurs dispositifs de prévention destinés à partager la cuve, les changeurs de prise en charge et traversées selon l'invention.

la figure 5 est une vue schématique représentant la logique de fonctionnement du dispositif représenté en figure 4, selon l'invention; et

la figure 6 est une vue en coupe transversale d'une traversée équipée d'un dispositif de prévention selon l'invention.

The invention will be better understood on studying the detailed description of a few particular embodiments taken by way of non-limiting examples and illustrated by the appended drawings, in which:

Figure 1a is a cross-sectional view of the prevention device according to the invention;

Figure 1b is an enlarged partial view of Figure 1a;

Figure 2 is a top view corresponding to Figure 1;

Figure 3 is a general view of a transformer equipped with a prevention device according to the invention;

Figure 4 is a general view of a transformer equipped with several prevention devices intended to share the tank, the load changers and crossings according to the invention.

Figure 5 is a schematic view showing the operating logic of the device shown in Figure 4, according to the invention; and

Figure 6 is a cross-sectional view of a crossing fitted with a prevention device according to the invention.

As can be seen in Figures 1a, 1b and 2, the element of rupture 1 is circular convex convex on the downstream side and is designed to be mounted on an outlet orifice, not shown, of a tank containing a dielectric fluid. The breaking element 1 includes a retaining part 4 in the form of a metallic veil thin, for example stainless steel, aluminum, or aluminum alloy. The retaining part 4 is kept tight between two flanges 2, 3 in the form of discs. The breaking element 1 comprises, in addition to the retaining part 4, a sealing coating 9 arranged on the upstream side, in other words covering the concave side of the retaining part. For example, the coating 9 is based on polytetrafluoroethylene.

The retaining part 4 is provided with radial grooves 5 la dividing into six portions. Radial ridges 5 are hollowed out in a fraction of the thickness of the retaining part 4 so that a rupture is made by tearing of the retaining part 4 the along the said streaks 5. without fragmentation to prevent fragments of the retainer 1 is not torn off and moved by the fluid passing through the retaining element 1 and risk damaging a downstream pipe.

The retaining part 4 is provided with through holes 6 of very small diameter located one in the center of the retaining part 4 and the others distributed one by streak 5 near the center. In other words, seven holes 6 are arranged, six in hexagon and one in the center. Holes 6 form tear primers with even lower resistance than streaks 5 and ensure that the tear begins in the center of the retaining part 4 and spreads outwards. The formation of minus one hole 6 per streak 5 ensures that streaks 5 will tear simultaneously by offering the strongest possible cross-section, the holes 6 other than the central hole being disposed at equal distance from the center. Alternatively, one could consider a number of streaks 5 different from six, and / or several holes 6 per streak 5. The coating seal 9 is able to seal the holes 6.

The burst pressure of the retainer 1 is determined, in particular, by the diameter and the position of the holes 6, the 5 streak depth, material thickness and composition forming the retaining part 4.

As seen in Figure 2, the retaining portion 4 is provided with grooves 7, each groove 7 being formed on a segment right joining the intersection of a streak 6 and the circular edge of the retaining part 4 and the intersection of a streak 6 adjacent to the previous and the circular edge of the retaining part 4. However, the Figure 2 is a top view and the retaining part 4 is curved. We will therefore understand that the grooves 7 follow the curvature of the part of retain 4 and would be in side view of the elliptical arcs. A groove 7 and two adjacent streaks 6 form a triangle 8 which when broken goes separate from neighboring triangles by tearing the material in streaks 6 and deform downstream by folding along the groove 7. The grooves 7 ensure the folding of the triangles 8 without tearing to avoid tearing off of the so-called triangles 8 liable to damage a downstream line or obstruct flow in the downstream line thus increasing the pressure drop and slowing depressurization upstream side. The pressure drop due to the retaining element 1 after breakage is reduced when the number of streaks 5 and grooves 7 The number of grooves 5 and grooves 7 also depends on the diameter of the retaining element 1.

The flange 3 disposed downstream of the flange 2 is pierced with a hole radial in which a protective tube 10 is arranged. rupture comprises an electric wire 11 fixed on the retaining part 4 of the downstream side and arranged in a loop. The electric wire 11 extends into the protective tube 10 to a connection box 12. The wire electric 11 extends over almost the entire diameter of the element of retainer 1, with a portion of wire 11a disposed on one side of a streak 5 parallel to said streak 5 and the other portion of wire 11b disposed radially on the other side of the same streak 5 parallel to the said streak 5. The distance between the two portions of wire 11a, 11b is small. This distance may be less than the maximum distance between two holes 6 so that the wire 11 passes between the holes 6.

The electric wire 11 is covered by a protective film 12 which serves both to prevent corrosion and to bond it to the downstream face of the retaining part 4. The composition of this film 12 will also be chosen to avoid changing the rupture pressure of the rupture element 1. The film 12 can be made of embrittled polyamide. The outbreak of the breaking element necessarily leads to the cutting of the wire electric 11. This cut can be detected extremely simple and reliable by interrupting the flow of a passing current by wire 11 or by voltage difference between the two ends wire 11.

As illustrated in FIG. 3, the transformer 13 comprises a main tank 14 resting on the ground by means of feet 15 and is supplied with electrical energy by wires 16 surrounded by insulators 17. The main tank 14 is filled with cooling, for example, of dielectric oil and is generally designed to withstand a relative internal pressure of 1 bar.

The main tank 14 is provided with a compensating sleeve elastic 18 downstream of which is mounted a breaking element 1 of which the burst allows the pressure variation due to be detected without delay deflagration caused by the breakdown of the electrical insulation of the transformer. The rupture element 1 is supported by a reservoir 19 intended to collect the oil coming from the main tank 14 after bursting of the rupture element 1. The reservoir 19 is equipped with a piping 20 for evacuating the gases from the oil to the open air. If the transformer is installed in an enclosed space, piping 20 will emerge outside the said enclosed space. The main tank 14 is thus immediately depressurized and partially emptied into the reservoir 19. The rupture element 1 may be provided to burst at a determined pressure of less than 1 bar, for example between 0.2 and 0.9 bar, preferably between 0.5 and 0.8 bar.

An air isolation flap 20a is arranged in the piping 20 to prevent the entry of oxygen from the air which could supply the combustion of gases which can be explosive and that of oil in the tank 19 and in the main tank 14.

The transformer 13 is supplied via a supply cell, not shown, which comprises means for power cut such as circuit breakers to protect the transformer 13 and which is fitted with tripping sensors.

The main tank 14 includes a cooling means fluid by injecting an inert gas such as nitrogen at the bottom of the main tank. This cooling reduces the amount of dangerous gases from the decomposition of the fluid and reduce the proportion of hydrogen in said quantity of dangerous gas. The gas inert is stored in at least one pressurized bottle 21 provided a pyrotechnic valve 22, a regulator 23 and a pipe 24 bringing the inert gas to the bottom of the main tank 14. The opening of the valve 22 is controlled by a break signal in origin of the rupture detector integrated in the rupture element 1, in coincidence with a trigger signal from one of the protections transformer 13. The injection of inert gas causes a slight rise in the level of dielectric fluid in the tank main 14 and a flow in the tank 19.

Such a protection system is economical, autonomous by compared to neighboring installations, compact and without maintenance.

The transformer 13, illustrated in FIG. 4, is of a power range greater than that of Figure 3 and is equipped with one or more load and bushing changers high and low voltages.

In order to guarantee a constant level of cooling in the main tank 14, the transformer 13 is provided with an auxiliary tank 25 in communication with the tank main 14 by line 26.

Line 26 is provided with an automatic valve 27 which closes the pipe 26 as soon as it detects a rapid movement of the fluid. Thus, during an explosion of the main tank 14, the pressure in pipe 26 suddenly drops which causes a start of fluid flow which is quickly stopped by plugging the automatic valve 27. This prevents the fluid contained in the booster tank 25 to supply the transformer fire 13.

The main tank 14 includes a sensor for the presence of coolant vapor also called buchholz 28 mounted at a high point of the main tank, generally on the driving 26. Deflagration due to a break in electrical insulation quickly releases vapor from the fluid in the tank main 14. A vapor sensor 28 is therefore efficient for detect a break in the electrical insulation.

The transformer 13 includes a valve 29 arranged between its tank 14 and the elastic compensating sleeve 18. The valve 29 is constantly open when the transformer 13 is energized, and can be closed during maintenance operations carried out on transformer 13 being de-energized. Downstream of the rupture element 1, there is mounted a depressurization pipe 30 provided with a flap 31 air isolation. The depressurization line 30 leads to a non-hazardous sump or flow.

The transformer 13 can be equipped with one or more on-load changers 32 serving as interfaces between said transformer 13 and the electrical network to which it is connected for ensure constant voltage despite variations in the current supplied to the network. The on-load tap-changer 32 is equipped with a tank 33 connected by a depressurization line 34 to the line of depressurization 30. In fact, the on-load changer 32 is also cooled by a flammable coolant. In due to its reduced volume, the explosion of a tap changer charge 32 is extremely violent and can be accompanied by projection of flaming coolant jets. The driving depressurization 34 is provided with a rupture element 35 capable tear in the event of a short circuit and therefore overpressure inside the on-load tap-changer 32. The breaking element 35 is similar to that referenced 1 and adapted sizing. We thus avoids the explosion of the tank 33 of said on-load changer 32.

The transformer 13 includes several crossings 36 to connect it to a high power network voltage. Figure 6 shows an example of crossing electric. The electrical bushing 36 includes a tank or oil 37 of generally cylindrical shape with a lower end mounted on the main tank 14 and the free upper end. A outlet rod 38 from the main tank 14 passes through the oil can 37 from one end to the other. A waterproof electrical insulator 39 is disposed between the outlet rod 38 and the wall of the main tank 14. Similarly, an electrical insulator 40 is disposed between the outlet rod 38 and the free upper end of the oil box 37 which is almost completely filled with oil in normal operation.

A line 41 connects the bottom of the oil box 37 and the depressurization line 34 of the on-load tap-changer 32. A rupture element 42 is disposed in and closes the pipe 41 in normal conditions. The breaking element 42 is similar to that referenced 1, and adapted sizing.

An inert gas injection pipe 43 opens into the top of the oil box 37 and is connected to one or more bottles 21 (figure 4).

It was found that the short circuits of the crossings most often from insulation 39 which ages or crack under the effect of the vibrations of the main tank 14 on which it is fixed. The electric arc due to the short circuit gives off a considerable energy from where a rise in the temperature of the oil, the gas evolution and a sudden increase in pressure in the oil box 37. The increase in pressure causes the rupture of the insulator 39 or the oil box 37. In contact with air, gases ignites and oil spills onto transformer 13. A major fire ensued.

During the explosion, the deterioration of the insulation 39 creates often an oil leak from the main tank 14 which feeds the fire and promotes its extension to transformer 13, its accessories and neighboring installations.

On the contrary, according to the present invention, the element of rupture 42 is chosen with a rupture pressure lower than the test pressure of the oil box 37. The increase in pressure causes the rupture element 42 to burst, whence immediate depressurization of the oil box 37 and flow of oil. The rupture detection thanks to the integrated wire allows control the injection of inert gas through piping 43 to avoid introducing oxygen from the ambient air into the oil can 37 and promote the flow of oil. The electrical protections from transformer 13 are used to trigger transformer 13 to put it out of service. Only the damaged electrical bushing must then be repaired, resulting in a reduction in costs and the downtime of the transformer 13.

The transformer 13 will also include a unit of control, not shown, connected to each rupture detector of the rupture elements 1, 35 and 42. Any rupture of one of the elements 1, 35 or 42 detected in coincidence with the triggering of protections electrical transformer will inject inert gas into the main tank 14, the on-load changers 32 and the electrical bushings 36 because a short circuit in one of these elements often leads to deterioration of others (Figure 5). The transformer 13 is also taken out of service by the protections electric only. As seen in Figure 5, the trigger one of the electrical protections of the transformer (Buchholz, overcurrent detector, earth fault detector, protection differential) and one of the rupture elements causes the injection inert gas in all elements containing combustible fluid.

The control unit can also be connected to the sensors accessories such as fire detector, steam sensor 28 (buchholz) and to the trigger sensor of the power cell to start a fire extinguishing in case of failure of explosion prevention.

Thanks to the invention, there is thus a device for explosion prevention in a transformer that requires little modifications to the transformer elements, which detects the insulation breaks extremely quickly and act simultaneously so as to limit the resulting consequences. it helps prevent oil capacity explosions and fires resulting in reducing damage from short circuits on the transformer as well as on-load changers and crossings.

Claims (10)

1. Device for prevention against explosion of an electrical transformer (13) comprising an enclosure filled with combustible coolant, and a means for decompressing the enclosure of the transformer, characterized in that the decompression means comprises a rupture element (1) provided with a retention part (4) including first zones which have a reduced thickness in comparison with the rest of the retention part and are capable of tearing without fragmenting when the said element ruptures, and second zones which have reduced thickness in comparison with the rest of the retention part and are capable of folding without tearing when the said element ruptures, the said rupture element being capable of breaking when the pressure inside the enclosure (14) exceeds a predetermined ceiling.
2. Device according to Claim 1, characterized in that the rupture element (1) is provided with a sealing component which is arranged on the coolant side and is capable of closing off small-diameter holes (6) formed in the retention part.
3. Device according to Claim 2, characterized in that the sealing component is in the form of a lining (9) on the retention part, the said lining being preferably based on polytetrafluoroethylene.
4. Device according to any one of the preceding claims, characterized in that the retention part has a domed shape with convexity outwards, on the opposite side to the coolant.
5. Device according to any one of the preceding claims, characterized in that the retention part is metallic, made of stainless steel, aluminium or aluminium alloy.
6. Device according to any one of the preceding claims, characterized in that it comprises a rupture-detection means integrated with the rupture element.
7. Device according to Claim 6, characterized in that the rupture-detection element comprises an electrical wire (11) capable of breaking at the same time as the rupture element (1), the electrical wire being adhesively bonded on the rupture element.
8. Device according to Claim 7, characterized in that the electrical wire is arranged on the opposite side of the retention part to the coolant, the electrical wire being covered with a protective film (12).
9. System for prevention against explosion of an electrical transformer (13) comprising an enclosure (14) filled with combustible coolant, and a means for decompressing the enclosure of the transformer, characterized in that it comprises a plurality of devices according to any one of the preceding claims, including one on a main enclosure (14) containing the windings and one on each on-load tap changer (32).
10. System according to Claim 9, characterized in that it comprises at least one device according to any one of the preceding claims, on at least one electrical feed-through (36).

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