FR2793599A1 - MV / LV TRANSFORMER WITH DRY INSULATION, WITH A LINEARLY DISTRIBUTED ELECTRIC FIELD, FOR THE DISTRIBUTION OF ELECTRICAL ENERGY IN RURAL AREAS - 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

I 2793599

  MV / LV TRANSFORMER, DRY ISOLATION, FIELD

  LINEARLY DISTRIBUTED ELECTRIC FOR THE

  DISTRIBUTION OF ELECTRICAL ENERGY IN THE ENVIRONMENT

  The present invention relates to a single-phase or multi-phase dry-insulated MV / LV electrical transformer intended for indoor or outdoor installation

  for the distribution of electrical energy.

  Unlike other devices generally used for the distribution of electrical energy outside, dry-insulated transformers allow the operator to overcome the constraints related to environmental protection and imposed by the presence of a dielectric liquid such as a mineral oil which, although generally not presenting any particular toxicity, nevertheless implies a means of retention in the event of leakage to avoid any accidental spreading which could pollute runoff water, ground water, etc. However, this retention means is difficult to envisage for a transformer intended for an outdoor installation, as is the case of distribution transformers installed at the top of

post for example.

  MV / LV dry-insulated transformers are widely used in the distribution of electrical energy. However, the technologies generally used, on the one hand do not allow devices to be produced at costs comparable to those of liquid dielectric transformers, in particular in powers less than or equal to 250kVA, and, on the other hand, do not allow outdoor installation. Consequently, the user of a dry type transformer is forced to place the device in a protective envelope against the weather, which accentuates the additional cost of the installation. In addition, known dry isolation MV / LV transformers generally include encapsulation of the MV winding using a thermosetting resin, such as an epoxy resin. Other solutions can also be envisaged: patent No. 88 13 180 (TRANSFIX patent) presents one of them, particularly suitable for the production 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 atmospheres.

  The object of the invention is to overcome the drawbacks 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 situated between the LV winding and the MT winding, and in that this cylinder is coated on

  its inner surface with a conductive or semi-layer

  conductive carried to the potential of the earth, and on its outer surface of a layer of conductive material linear distributor of voltage RLT, on which is

  locally wound MT winding.

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

radiates into the ambient air.

  Preferably, the MT winding of the transformer according to the invention has a structure of small width pancakes, consisting of a superposition of layers of turns, and connected in series, so that the distribution of the potential

  in said winding is of axial type.

  Other features and benefits of

  the invention will emerge from the description which follows,

  taken by way of nonlimiting example, with reference to the appended figures in which: - Figure 1 represents a vertical section of a first embodiment of a transformer according to the invention, - Figure 2 represents a vertical section of a second embodiment of a transformer according to the invention, - Figure 3 shows an example of coupling the windings of a three-phase transformer according to the invention, - Figure 4 shows schematically a

  variant of the transformer shown in Figure 1.

  - Figure 5 shows schematically a second variant of the transformer shown in

figure 1.

  FIG. 1 represents a three-phase MV / LV transformer with dry insulation intended for indoor or outdoor installation for the distribution of

electrical energy.

  As can be seen in this figure, around a magnetic circuit 1, a LV winding 2 is placed and, positioned concentrically with this winding 2, an insulating cylinder 3 having on its inner surface a conductive or semi-conductive layer 4 connected to the earth potential, and on its outer surface, a layer of semiconductor material 5 designated RLT, whose function is to ensure a linear distribution of the voltage over the entire length of the insulating cylinder. An MT 6 winding is positioned directly on the layer of RLT material and covers the central part. This winding has the particularity of being produced so that the evolution of the potential between the various turns which constitute it takes place in a privileged manner according to the

axial direction.

  The insulating cylinder 3 is made of a material with high dielectric characteristics and obtained by molding, extrusion or any other process making it possible to obtain a cylinder free of vacuoles and perfectly homogeneous. This material can 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 metallic powder onto the inside diameter of the insulating tube 3. Semiconductor materials whose permittivity and resistivity vary depending on the field are known and used in electrical engineering for the production of high voltage cable accessories, and more particularly in the manufacture of retractable ends which equip these cables. These so-called linear voltage distributors (RLT) materials generally consist of one or more oxides, such as silicon carbide, mixed with a resin or putty. In the present application, the layer 5 of RLT material can be obtained by winding a sheet, winding a ribbon, or shrinking a tube pre-molded onto the diameter

outside of the insulating cylinder 3.-

  According to a preferred embodiment of the invention illustrated in FIG. 1, the MT winding 6 is made up of small width wafers 7, connected together in series. Each of these wafers consists of layers of superposed turns 9 made from a wire of conductive material 8 of coated circular section

an insulating varnish.

  According to another embodiment presented in FIG. 2, the MT winding is obtained using a strip of bare conductive material of rectangular section, the material used being chosen for its ductility. This conductive tape 10 is wound on the field and associated with a film

insulator 11 of small thickness.

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

electric arc.

  To improve the resistance of the transformer to climatic aggressions, the latter is entirely coated with a layer 12 of synthetic material having good resistance to ultraviolet rays, to electrical pathway, and to transverse electrical stresses which could lead to perforation of said coating. This can be obtained for example by soaking the transformer in a bath, or by spraying with a pneumatic gun, a material

  synthetic with the desired characteristics.

  In this way, the mechanical parts are also protected against corrosion and the gaps between the various parts of the device are filled. Figure 4 shows the transformer described in Figure 1 and equipped with its

coating 12.

  FIG. 5 presents a variant, the advantage of which is to lengthen the path distance in the zones where the electric field radiating in the air is the densest, 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 distributed regularly over the insulating cylinder (13). The coating 12 applied to the whole of the transformer seals the contact zone between the rings 13 and the layer 5 of RLT material and thus produces skirts identical to

  those that would include an electrical insulator.

Claims (5)

  1. MV or LV single or multi-phase transformer, dry type insulation, intended for 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 MT winding (6), and in that this cylinder (3) is coated on its   inner surface of a conductive or semi-layer   conductive (4) brought to earth potential, and on   its outer surface with a layer of semi-material   linear voltage distributor conductor (5) on   which is locally wound the MT winding (2).   2. Transformer according to claim 1, characterized in that the MT winding (6) has a structure of small width wafers, consisting of a superposition 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 the axial type. 3. Transformer according to claim 1, characterized in that the MT winding (6) consists of the field winding of a ribbon (10) of conductor in association with a film of insulating material (11), such that the distribution of   potential in said winding is of axial type.   4. Transformer according to claim 1, characterized in that it comprises a uniform coating (12) consisting of a layer of synthetic material having electrical and physical characteristics suitable for outdoor use, this coating (12) can be obtained by dipping or spraying. 5. Transformer according to claim 4, characterized in that it comprises, on either side of the MT winding, rings (13) made of insulating material constituting skirts intended to lengthen the distances of electrical path between the MV winding (6) and the parts linked to the ground potential of said transformer, said skirts being located on the insulating cylinder (3) coated with the layer of linear voltage distributing semiconductor material and being covered with the insulating coating (12 ) applied to the whole transformer.