EP1052659B1 - MT/LT dry transformer with linear distributed electric field for distribution of electric energy in rural networks - Google Patents

Abstract

The system provides dry insulation of the transformer with semiconductor coatings controlling voltage distribution. The transformer comprises an isolating cylinder (3) for each of the phases, situated between the low tension winding (2) an the medium tension winding (6). The cylinder (3) is coated on its inner surface with a conductive or semi-conductive layer (4) which held at earth potential. The cylinder is coated on its outer surface with a semi-conductive layer (5) which provides a linear distribution of voltage. The medium tension winding is locally wound around this cylinder outer surface. The medium tension wind (6) has a structure comprising thin discs consisting of the superposition of layers of coils (9) wound in series, so that there an axial distribution of potential.

Description

The present invention relates to a single phase or polyphase electrically insulated MV / LV transformer with dry insulation for indoor or outdoor installation for the distribution of electrical energy.

Unlike other devices generally used for the distribution of electrical energy outdoors, dry isolation transformers allow the operator to overcome the constraints related to the protection of the environment and imposed by the presence of a dielectric liquid such as a mineral oil which, although generally not particularly toxic, nevertheless implies a means of retention in the event of a leak to prevent any accidental spreading which could pollute the runoff water, the water table, etc. However, this retention means is difficult to envisage for a transformer intended for an outdoor installation, as is the case for distribution transformers installed at the top of the pole for example.

Dry-insulated MV / LV transformers are widely used in the distribution of electrical energy. Nevertheless, the technologies generally used, on the one hand, do not make it possible to produce devices at comparable costs to those of liquid dielectric transformers, in particular in powers of less than or equal to 250 kVA, and, on the other hand, do not allow a outdoor installation. As a result, the user of a dry type transformer is forced to place the device in a weather protection envelope, which increases the cost of the installation.

In addition, known dry insulated MV / LV transformers generally include encapsulation of the MV winding with a thermosetting resin, such as an epoxy resin. Other solutions may also be considered: the FR-A-2,637,729 (No. 88 13 180, patent TRANSFIX) has one of them, particularly suitable for the realization of power transformers used indoors. However, for all these technologies, the electric field is not directed, which on the one hand, leads to relatively large dielectric distances, and on the other hand, is incompatible with external and polluted environments.

The object of the invention is to overcome the disadvantages of the cited prior art by means of a dry isolation transformer of simple design and low cost.

The transformer according to the invention is characterized in that it comprises for each of its phases an insulating cylinder located between the winding BT and the winding MT, and in that this cylinder is coated on its inner surface with a conductive or semiconductor layer brought to the potential of the earth, and on its outer surface of a layer of RLT linear voltage distribution conductor material, on which is wound locally the MV winding.

Thanks to this structure, an insulation having an open electric field but nevertheless homogeneously distributed in the sensitive areas where it radiates in the ambient air is produced.

Preferably, the MV winding of the transformer according to the invention has a wafer structure of small width, consisting of a superposition of layers of turns, and connected in series, so that the distribution of the potential in said winding is of type axial.

• La figure 1 représente une coupe verticale d'un premier mode de réalisation d'un transformateur conforme à l'invention,
• la figure 2 représente une coupe verticale d'un deuxième mode de réalisation d'un transformateur conforme à l'invention,
• la figure 3 représente un exemple de couplage des enroulements d'un transformateur triphasé conforme à l'invention,
• la figure 4 représente schématiquement une variante du transformateur représenté à la figure 1.
• la figure 5 représente schématiquement une deuxième variante du transformateur représenté à la figure 1.

Other features and advantages of the invention will emerge from the description which follows, taken by way of non-limiting example, with reference to the appended figures in which:

• The figure 1 represents a vertical section of a first embodiment of a transformer according to the invention,
• the figure 2 represents a vertical section of a second embodiment of a transformer according to the invention,
• the figure 3 represents an example of coupling of the windings of a three-phase transformer according to the invention,
• the figure 4 schematically represents a variant of the transformer shown in FIG. figure 1 .
• the figure 5 schematically represents a second variant of the transformer shown in FIG. figure 1 .

The figure 1 represents a three-phase dry-insulated MV / LV electric transformer for indoor or outdoor installation for the distribution of electrical energy.

As can be seen in this figure, around a magnetic circuit 1, is placed a winding BT 2 and, positioned concentrically to this winding 2, an insulating cylinder 3 having on its surface a conductive or semiconducting layer 4 connected to the earth potential, and on its outer surface, a layer of semiconductor material designated RLT, the function of which is to ensure a linear distribution of the voltage over the entire length. of the insulating cylinder. An MT winding 6 is positioned directly on the layer of material RLT and covers the central portion. This winding has the particularity of being realized so that the evolution of the potential between the various turns that constitute it is done in a preferred way in the axial direction.

The insulating cylinder 3 is made of a material with high dielectric characteristics and obtained by molding, extrusion or any other method making it possible to obtain a cylinder that is free of vacuoles and perfectly homogeneous. This material may be a thermoplastic material, such as polyethylene used in the manufacture of high voltage cables, or an elastomer, silicone or EPDM for example. The conductive or semi-conductive layer 4 can be obtained by spraying a synthetic material loaded with a metal powder onto the inside diameter of the insulating tube 3. The semiconductor materials whose permittivity and resistivity vary according to the field Electrical are known and used in electrical engineering for the realization of high voltage cable accessories, and more particularly in the manufacture of retractable ends that equip these cables. These so-called linear voltage divider (RLT) materials generally consist of one or more oxides, such as silicon carbide, mixed with a resin or putty. In this application, the layer 5 of material RLT can be obtained by the winding of a sheet, the winding of a ribbon, or the shrinkage of a pre-molded tube on the outside diameter of the insulating cylinder 3.

According to a preferred embodiment of the invention illustrated by the figure 1 , the winding MT 6 consists of wafers 7 of small width, interconnected in series. Each of these slabs consists of superimposed layers of turns 9, made from a wire of conductive material 8 of circular section coated with an insulating varnish.

According to another embodiment presented to the figure 2 , the winding MT is obtained using a ribbon 10 of bare conductive material of rectangular section, the material used being chosen for its ductility. This conductive strip 10 is wound on the field and associated with an insulating film 11 of small thickness.

As can be seen on the figure 1 , the shape of the electric field resulting from this design mode corresponds to the case of a three-phase transformer whose MT windings 6 are coupled in a triangle, as is presented on the figure 3 . Thus, it can be seen that the radial electric field situated between the winding MT 6 and the winding BT 2 is completely contained in the insulating cylinder 3, between the layer 4 connected to the earth potential and the RLT layer 5. The axial field is distributed on the one hand over the entire axial length of the winding MT 6, and secondly at the ends of the insulating cylinder 3 not covered by the winding MT 6, but nevertheless covered with the layer 5 of material RLT . In these three zones, the field electrical radiating in the air is distributed homogeneously and remains small enough to exclude any risk of partial discharges or bypassing by an electric arc.

To improve the resistance of the transformer against climatic aggressions, the latter is entirely coated with a layer 12 of synthetic material having good resistance to ultraviolet radiation, electrical pathways, and transverse electrical constraints that could lead to perforation of said coating. This can be obtained for example by dipping the transformer in a bath, or by spraying with a pneumatic gun, a synthetic material having the desired characteristics. In this way is also provided the protection against corrosion of the mechanical parts and the filling of existing gaps between the different parts of the device. The figure 4 presents the transformer described in figure 1 and provided with its coating 12.

The figure 5 presents a variant whose interest is to lengthen the tracking distance in the areas where the electric field radiating in the air is the most dense, and which correspond to the ends of the insulating cylinder not covered by the winding MT. For this purpose, rings 13 of insulating material are evenly distributed on the insulating cylinder (13). The coating 12 applied to the entire transformer tightens the contact area between the rings 13 and the layer 5 of material RLT and thus performs skirts identical to those that would comprise an electrical insulator.

Claims (5)

1. Single or multiple-phase MV/LV electrical transformer, with dry insulation, intended for an indoor or outdoor installation, characterized in that it comprises, for each of its phases, an insulating cylinder (3) located between the LV winding (2) and the MV winding (6), and in that this cylinder (3) is coated on its inner surface with a conductive or semi-conductive layer (4) raised to the earth potential, and on its outer surface with a layer of linear voltage distributing semi-conductive material (5) on which is locally wound the MV winding (2).
2. Transformer according to Claim 1, characterized in that the MV winding (6) has a narrow slab structure, constituted of an overlay of layers of turns (9), and connected in series, in such a way that the distribution of the potential in said winding (6) is of axial type.
3. Transformer according to Claim 1, characterized in that the MV winding (6) is constituted of the field winding of a strip (10) of conductor in association with a film of insulating material (11), so that the distribution of the potential in said winding is of axial type.
4. Transformer according to Claim 1, characterized in that it comprises a uniform coating (12) constituted of a layer of synthetic material having appropriate electrical and physical characteristics for outdoor use, this coating (12) being obtainable by immersion coating or by spraying.
5. Transformer according to Claim 4, characterized in that it comprises, on either side of the MV winding, rings (13) of insulating material constituting skirts intended to extend the electrical path distances between the MV winding (6) and the parts linked to the earth potential of said transformer, said skirts being located on the insulating cylinder (3) coated with the layer of linear voltage distributing semi-conductive material and being covered with the insulating coating (12) applied to the whole of the transformer.

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