EP0536019B1 - High/low tension non-explosive immersed transformer being under permanent overload conditions - Google Patents

Abstract

The invention relates to a non-explosive medium/low tension transformer under permanent overload conditions. The transformer comprises a metal case 1 containing the insulated primary and secondary transformation windings. The transformation windings 2 are immersed in an insulating fluid 20. The insulation of the primary and secondary windings 2 is achieved per pair of synthetic insulating materials (10, 20). Application to transformers such as transformers suspended on poles or fixed transformers. <IMAGE>

Description

The invention relates to a high voltage - low voltage submersible transformer for an electrical energy distribution network.

Currently, users of the electrical energy distribution network are in the majority of cases supplied with this low voltage energy, 220 V or 380 V, from a high voltage transformer 20,000 V / low voltage.

These customers impose a low average annual load on the distribution network, but a significant peak load.

The high / low voltage transformers currently in operation on the distribution network typically include a metal tank containing the primary and secondary transformation windings. These electromagnetically coupled windings are electrically isolated, by means of a paper insulator and are immersed in a mineral oil, contained in the tank and ensuring the role of dielectric insulator.

The physicochemical properties of this insulator do not allow optimal use of these transformers. This type of insulation does not accept a high crest factor, the crest factor being defined as the ratio between the peak value of the load and the average load. Thus, to any argument of 6 ° K of the hot spot temperature of the transformer, corresponds a decrease by a factor of two in the life of the insulator, and, ultimately, of the transformer.

The use of a silicone oil in which the primary and secondary windings of an electric transformer are immersed was described in French patent application No. 2 326 016 published on 04.22.1977.

The object of the present invention is to remedy to the aforementioned drawbacks, and in particular to allow the use of a high / low voltage submerged transformer allowing a large increase in the possibilities of operation under permanent overload conditions, and in particular of a non-explosive transformer.

Another object of the present invention is the implementation of a high / low voltage submerged transformer allowing a large reduction in size compared to the transformers of the prior art, at equal power.

Another object of the present invention is finally the implementation of a high / low voltage submerged transformer in which the hot spot temperature is higher, which also allows a reduction in mass, at equal power.

The high / low voltage submerged transformer object of the present invention is defined by the characteristics of claim 1. It comprises a metal tank containing the primary and secondary isolated windings of transformation immersed in an insulating fluid.

It is first of all remarkable in that it uses solid and liquid insulators such that the assembly has high thermal stability, characterized by a thermal index equal to or greater than 180 ° C., and an aging law such as exceeding temperature compared to this thermal index must be at least 19 K to double the speed of consumption of the transformer, these characteristics making the device more suitable for the type of load specific to public distribution. The thermal index is defined by the temperature class of the insulation, itself defined by IEC publication 85: Recommendations relating to the classification of materials intended for the insulation of electrical machines and apparatus according to their stability thermal in service.

It is also remarkable in that it uses solid and liquid insulators such that the assembly has a high stability in the event of an electric arc internal to the device, and generates significantly less decomposition gas than the dielectric systems usually used.

It is also remarkable in that the insulation of the primary and secondary windings is carried out by a pair of synthetic insulating materials.

The invention finds application in high / low voltage transformers of overhead electrical distribution networks, in particular in pole top transformers.

• la figure 1 représente une vue d'un transformateur haute/basse tension objet de l'invention,
• la figure 2 représente une vue d'un transformateur haute/basse tension objet de l'invention, appliqué à un transformateur de haut de poteau,
• la figure 3a représente une vue plus générale d'un mode de réalisation particulier d'un transformateur fixe objet de l'invention,
• la figure 3b représente une vue en coupe longitudinale de la figure 3a selon le mode de réalisation précité,
• la figure 3c représente un schéma électrique de câblage, côté haute tension, du transformateur et des enroulements tels que représentés en figure 3b.

It will be better understood on reading the description and observing the drawings included for illustrative purposes in which:

• FIG. 1 represents a view of a high / low voltage transformer object of the invention,
• FIG. 2 represents a view of a high / low voltage transformer object of the invention, applied to a pole top transformer,
• FIG. 3a represents a more general view of a particular embodiment of a fixed transformer object of the invention,
• FIG. 3b represents a view in longitudinal section of FIG. 3a according to the above-mentioned embodiment,
• FIG. 3c represents an electrical wiring diagram, on the high voltage side, of the transformer and the windings as shown in FIG. 3b.

The non-explosive high / low voltage transformer, object of the present invention, used in permanent overloading regime will now be described in conjunction with FIGS. 1 and 2.

In accordance with the aforementioned figures, the transformer which is the subject of the invention comprises a metal tank denoted 1 containing the primary and secondary isolated transformation windings denoted 2 in FIG. 1. These windings are, conventionally, immersed in an insulating fluid. The high-voltage terminals are designated by BHTA and the low-voltage terminals by BBT.

According to a particularly advantageous aspect of the present invention, the insulation of the primary and secondary windings is carried out by a pair of synthetic insulating materials noted (10,20).

According to a particularly advantageous aspect of the present invention, the insulation of the primary and secondary windings can be carried out by means of a paper made of polyaramide fibers which is for example marketed by the Société du Pont de Nemours under the trade name NOMEX, this aramid paper forming one of the materials 10 of the pair of insulating materials, this aramid paper can also be used as covering material for the winding conductors.

According to an advantageous characteristic of the transformer which is the subject of the invention, the other of the materials 20 of the pair of insulating materials is formed by a silicone oil in which the primary and respectively secondary windings constituting the active part of the transformer are immersed. An enameling varnish compatible with silicone oil and with a high thermal index, greater than or equal to 180 ° C., can also be used in place of the aforementioned covering.

It will be noted in particular that the use of a NOMEX aramid paper and a silicone oil makes it possible to divide by a minimum of a factor of 10, the quantity of gas formed in the event of an internal electric arc in the tank of the transformer, which thus limits the rise in pressure of the tank and the risk of explosion.

It will also be noted that the pair of insulating materials used also makes it possible to produce a transformer with high admissible overloading, thanks to a high thermal index and to a temperature class of the resulting insulation higher than that accepted in conventional embodiments.

In fact, electricity distributors must supply low-voltage customers whose average annual load is low and whose peak load is high, in particular in rural areas where second homes are located, for example.

Currently, high voltage / low voltage transformers used on the electrical distribution network, use a couple of dielectric insulators such as cellulose paper / mineral oils, which due to its physicochemical properties does not allow to exploit the best high voltage / low voltage transformers. Indeed, this type of insulation does not accept a high crest factor, the crest factor being defined as the ratio between the peak value of the load and the average load applied to the transformer. Indeed, to any increase of 6 K in the hot spot temperature of the transformer compared to the temperature resulting from the sizing recommended by the standards in force for a rated load, corresponds in fact a reduction by a factor 2 of the duration of life of this transformer.

The use of the couple of insulating materials in accordance with the object of the present invention makes it possible to remedy the aforementioned drawback. Consequently, it is possible, thanks to this use, to obtain on the one hand a higher hot spot temperature, which makes it possible to reduce the size and the mass of the apparatus, and on the other hand a large increase in possibilities overloading of transformers conforming to the object of the present invention. The use of the aforementioned pair of insulating materials makes it possible to obtain high operating temperatures, and above all, allows permanent overheating of large amplitudes. Thus, for comparison, for an overload of 20% above the rated speed, the transformer of the prior art ages 25 times faster compared to its rated operating speed, while under identical conditions, a transformer produced in accordance with the object of the present invention will age only eight times faster, an average value having been retained for the latter.

According to another particularly advantageous aspect of the high voltage / low voltage transformer object of the present invention, it allows the possibility of a high temperature operating regime, and allows for example to use the active part of a transformer 100 kVA to make a 160 kVA transformer.

Thus, to make a power transformer P kVA, the transformer object of the present invention comprises as active part, the active part of a power transformer p kVA with p <P. The transformer object of the present invention is thus brought into operation in permanent overload regime in the power ratio.

A particularly advantageous consequence of the technical characteristic of the transformer object of the invention cited above, immediately appears according to which for a nominal power equal to P, the transformer according to the present invention has a reduction in volume of the order of 50% compared to the transformer of the prior art.

In order to improve the service life of the high voltage / low voltage transformers which are the subject of the invention, and in accordance with a characteristic advantageous without limitation thereof, in order to reduce the risk of explosion of these transformers, the tank 1 of the transformer consists of an aluminum alloy in place of conventional materials such as steel sheets. Indeed, these conventional materials do not allow a pressure of 1 bar to be exceeded inside the current tanks, and although the quantity of gas formed in the event of an electric arc inside the tank is reduced by a factor of 10 due to the use of the couple of insulating materials mentioned above, the pressures reached for a fault current of 1000 amperes, in the case of a fault lasting 0.5 seconds, are generally between 2 and 5 bars.

Consequently, and in accordance with a new advantageous aspect of the transformer which is the subject of the present invention, the tank thereof can be produced in the form of two half-shells made of aluminum alloy injected under pressure. The aluminum alloy used may for example be the aluminum alloy bearing the reference (AS7G). Preferably, the tank will be produced by two injected half-shells, the shells being assembled by argon welding.

In addition, and according to a particularly advantageous aspect of the object of the present invention, the geometry of the tank perfectly matches the contours of the active part of the transformer, reducing the distances between parts under high voltage and parts at ground potential at minimum required. This distance d is between 10 and 30 mm, 10 < d <30 mm.

Therefore, in the event of a dielectric fault between phase and earth, the length of the arc and the energy that it develops are reduced compared to the case of a conventional device. In addition, a significant reduction in the amount of dielectric liquid is obtained.

The tanks thus produced make it possible to withstand an internal tank pressure of at least 5 bars. This allows the device to withstand an internal fault, with a short-circuit current of up to 1000 A, of duration corresponding to the reaction time of the protections usually installed on high-voltage, MV, distribution networks, this without manifestation outside.

In addition, the use of aluminum makes it possible to produce a tank requiring a small volume of oil and, consequently, a reduction in the mass of synthetic liquid dielectric and a corresponding saving, a reduction in the quantity of liquid capable of '' generate decomposition gases, spread in the natural environment or support combustion.

In order to compensate for the expansion of the silicone oil contained in the tank during the creation of faults by electric arc, in accordance with an advantageous aspect of the transformer of the present invention, the tank itself is partially filled with electrically insulating dielectric fluid, the upper part of the aforementioned free fluid surface constituted by the silicone oil being filled with a neutral gas such as nitrogen for example. FIG. 1 shows the free surface of the oil level denoted NH, the upper part above the aforementioned free surface being filled with the aforementioned gas. Such an oil is a product clearly less harmful to the environment than the mineral oils used conventionally. This quality, associated with the fact that the volume of liquid required for a given power is reduced by a factor of 2, gives the transformer according to the invention the advantage of a device which is not very dangerous for the environment in the event of leakage of the dielectric.

According to an advantageous embodiment, the silicone oil used was an oil sold by the company RHONE-POULENC under the name Rhodorsil Oil 604 V 50, this oil belonging to the family of dimethylpolysiloxanes.

Among the insulating materials 10 capable of being used for the implementation of the transformer which is the subject of the present invention, there may be mentioned in addition to the aramid paper NOMEX previously mentioned, the aramid paper with mica, and the insulating material sold by the same company of Pont de Nemours International SA under the Kapton® brand. These materials are high quality materials capable of working at temperatures between 200 to 220 ° for 20 years without significant change in their dielectric or insulating characteristics.

In Figure 3a, there is shown a front view, a side view and a top view of the transformer object of the present invention, wherein the tank 1 was made of aluminum alloy as mentioned above.

Although this embodiment gives complete satisfaction for fault currents of less than 1000 amperes and of duration frequently encountered on overhead networks, taking into account the response times of current protection systems, the aforementioned tanks being then able to withstand the corresponding increase in internal pressures, in order to allow protection against explosion against fault currents by electric arc whose intensity is greater than 1000 amperes and which can reach 6000 amperes, a modification of the transformer object of the invention further comprises on each high voltage supply, as shown in FIG. 3b in particular, a fuse denoted 30, this fuse being placed inside the tank and submerged or not in the silicone oil.

Preferably, the fuses 30 could be constituted by fuses with current limitation, which make it possible to ensure extinction of the electric fault arc. According to another particularly advantageous aspect, expulsion fuses may be used. In these two cases, these fuses may have a breaking capacity greater than one hundred amperes.

According to another advantageous characteristic, and according to an advantageous non-limiting embodiment, the fuses 30 can be arranged in each of the high-voltage incoming terminals.

It will in fact be understood that this modification of the transformer which is the subject of the present invention achieves a synergistic effect between the protection against bursting or explosion, due to the use of insulation between primary and secondary windings produced by a pair of insulating materials. synthetic, covering faults of reduced intensity, of intensity less than 1000 amps for example, and the protection due to fuses covering faults of high intensity.

Thanks to the above combination or synergy, it is possible to considerably reduce the requirements applied to fuses for which, as is well known to those skilled in the art, the main difficulty posed is the breaking of low currents, a such breaking capacity in the context of the object of the present invention is no longer necessary, which thus makes possible the use of economical fuses.

Consequently, a modification of the transformer according to the invention comprises three high-voltage fuses located between the supply phases and the active part of the transformer, of simplified design since it should only protect the transformer for fault currents located beyond 1000 A, as a complement to the measures mentioned above to form a combination of protections which economically cover all the short-circuit intensities encountered on a HTA distribution network.

In FIG. 3c is shown on the one hand the high voltage incoming terminals, and the fuses 30 in their electrical wiring diagram with the high voltage windings, which conventionally are connected in a triangle or in a star.

It will also be noted that the transformer according to the invention can be produced in such a way that the low-voltage windings are dimensioned for adiabatic heating at a given current lower than that of the high-voltage windings, this measure guaranteeing in the event of a maintained short circuit downstream of the transformer, the appearance of an electrical fault on the high-voltage side, resulting in a large fault current because it is not limited by the leakage inductance between the MV winding and the LV winding, and therefore a fusion better and faster high-voltage fuses.

A non-explosive high / low voltage transformer has thus been described which is particularly advantageous insofar as it also allows operation in a permanent overload regime.

• quasi absence de risque d'explosion,
• diminution des pertes fer et des courants magnétisants pour une puissance P, cette diminution correspondant à celle d'un transformateur de puissance inférieure p,
• réduction de poids de 30% environ par rapport au transformateur de l'art antérieur d'une part par utilisation d'une cuve en alliage d'aliminium de densité plus légère, et d'autre part par l'utilisation de la partie active de transformateur de puissance plus faible pour une puissance P supérieure, ce qui permet bien entendu, ainsi que représentée en figure 1b, la réalisation de transformateurs de haut de poteau particulièrement avantageux, une très forte réduction de volume d'environ 50% dans les conditions indiquées précédemment,
• diminution du risque de corrosion,
• diminution du risque de propagation d'incendie externe du fait de la limitation du risque d'explosion,
• suppression de la protection basse tension.
• diminution du risque de pollution froide en cas de fuite grâce au volume réduit de diélectrique et aux qualités intrinsèques de ce dernier.

In addition to the characteristics mentioned above, the transformer which is the subject of the invention has the following advantages compared to the transformer of the prior art:

• almost no risk of explosion,
• reduction in iron losses and magnetizing currents for a power P, this reduction corresponding to that of a transformer with a lower power p,
• weight reduction of about 30% compared to the transformer of the prior art of a on the one hand by the use of a lighter density aluminum alloy tank, and on the other hand by the use of the active part of the lower power transformer for a higher power P, which of course allows, as well as shown in FIG. 1b, the realization of particularly advantageous pole top transformers, a very large volume reduction of around 50% under the conditions indicated above,
• reduced risk of corrosion,
• reduction of the risk of external fire propagation due to the limitation of the risk of explosion,
• removal of low voltage protection.
• reduction in the risk of cold pollution in the event of a leak thanks to the reduced volume of dielectric and the intrinsic qualities of the latter.

In addition, the transformer of the present invention makes it possible to take into account new requirements such as the reduction of voltage drops internal to the transformer, the connection of surge arresters or the connection under voltage.

It also allows, thanks to all of the aforementioned qualities, the use of very simplified fuses for overall protection covering all types of possible faults. Similarly, the use of a foundry aluminum tank providing the advantages of a pressure-resistant container and a small volume of oil, thus provides the system with additional protection against the risk of explosion. and improves the technical and economic performance of the whole.

Finally, it should be noted that, although an increase in electrical losses under load can be brought about Obviously, such an increase is of little importance however because on the one hand the average load of the rural transformers is very weak, approximately 15% of their nominal power, and on the other hand this additional cost is however compensated by the reduction of the losses mentioned .

Claims (7)

1. An immersed high voltage - low voltage transformer comprising a metal tank (1) containing a liquid dielectric (20) in which the primary and secondary windings (2) are immersed, said windings in turn being insulated by a solid insulant (10), characterised in that:

   a) the liquid dielectric (20) and the solid insulant (10) have a thermal index of at least 180°C and at this temperature ensure, on the one hand, the absence of any emission of decomposition gas and, on the other hand, substantially the absence of any emission of gas as a result of electric arcs, the thermal index being defined by the temperature class of the insulant as defined by publication 85 of the I.E.C: Recommendations relative à la classification des matières destines à l'isolement des machines et appareils électriques en fonction de leur stabilité thermique en service (= "Recommendations relating to the classification of materials intended for the insulation of electrical machines and apparatus in dependence on their thermal stability in operation"),

   b) the tank withstands an internal pressure of at least 5 bar,

   c) the distance between the live parts inside the tank and the wall of the tank is substantially constant and is between 10 and 30 mm.
2. A transformer according to claim 1, characterised in that the liquid dielectric is silicone oil.
3. A transformer according to claim 1, characterised in that the solid insulant is polyaramid fibre paper.
4. A transformer according to claim 1, characterised in that the solid insulant covering the conductors of the high-voltage windings is enamel.
5. A transformer according to any one of claims 1 to 4, characterised in that the tank is of cast aluminium alloy.
6. A transformer according to any one of claims 1 to 5, characterised in that the volume above the oil level comprises a neutral gas.
7. A transformer according to any one of claims 1 to 6, characterised in that current-limitation or expulsion fuses are provided between the windings and the current input terminals.

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