EP0923785A1

EP0923785A1 - High voltage/low voltage transformer with thermoplastic air-core insulation

Info

Publication number: EP0923785A1
Application number: EP96930212A
Authority: EP
Inventors: Bruno Guilbert; Sylvain Laureote; Dominique Feldmann; Brigitte Louyot; Michel Sacotte; Lana Sheer; Lloyd Fox; Jean-Michel Falguiere; Albert P. Walrave
Original assignee: Electricite de France SA; Schneider Electric SE; EI Du Pont de Nemours and Co
Current assignee: Electricite de France SA; Schneider Electric Industries SAS
Priority date: 1996-09-04
Filing date: 1996-09-04
Publication date: 1999-06-23
Anticipated expiration: 2016-09-04
Also published as: DE69629318T2; WO1998010447A1; AU6934196A; EP0923785B1; DE69629318D1

Abstract

The invention concerns a transformer with air-core insulation comprising at least one high voltage coil and at least one low voltage coil mounted on at least one column of a magnetic circuit. characterised in that its high voltage coil (10) is contained in a thermoplastic resin insulating encapsulation (12) coated with a surface layer (18) made of an electroconductive thermoplastic resin compatible with the insulating encapsulation and its low voltage coil (1) is coated with an electroconductive resin (7).

Description

High voltage / low voltage transformer with dry thermoplastic insulation.

The present invention relates to transformers and relates more particularly to dry transformers.

Transformers used in energy distribution to the public have a rated power between 5 VA and 2500 kVA.

They are used to lower the network voltage level from a "medium voltage" level between 5 and 36 kV to a low voltage level below 500 V for the supply of electrical energy to domestic customers.

Immersed type transformers, insulated in a dielectric liquid (mainly oil) represent the vast majority of current use and supply. In fact, mineral oil is an inexpensive material, a good insulator and a good coolant for cooling windings. However, mineral oil has the disadvantage of being a good fuel, which is harmful in the event of an internal arc in the transformer. To overcome the risk of fire inherent in mineral oil, dielectric liquids with a high fire point have been developed. Polychlorobenzene was the most used, but it was then banned because of its non-biodegradability and the harmfulness of certain decomposition products on fire. Esters or silicone oils would be the best contenders for replacing polychlorobenzene in transformers. However, these products are not yet unanimous, and their use is very marginal. Their commercial future is certainly compromised by the increasing severity of the laws regulating the use of liquid dielectrics because of the risks of cold pollution.

Solid insulation devices or dry devices developed strongly after the ban on the use of polychlorobenzene. They are mostly used in establishments open to the public (buildings, workshops), to reduce the risk of fire. Their use in distribution remains marginal because of the significant additional cost compared to mineral oil devices.

Current dry transformers have large air knives ensuring an insulation function between windings and between windings and mass, which increases their bulk, their mass and therefore their cost. To overcome this problem, some manufacturers use a surface metallization of the windings, which allows them to reduce the bulk. This technique has the disadvantage of being expensive and difficult to implement. Today there are two main families of dry transformers: coated and impregnated. The manufacturing methods of these devices are certainly different, but the insulation materials used have in common that they are thermosetting and require hot polymerization, which induces a high manufacturing cost.

The invention aims to remedy the aforementioned drawbacks of dry transformers by creating a transformer of this type which, while being compact and of a reduced cost price compared with known dry transformers, has similar or even improved performance.

It therefore relates to a transformer with dry insulation comprising at least one high voltage winding and at least one low voltage winding mounted on at least one column of a magnetic circuit, characterized in that its high voltage winding is contained in a coating d insulation in thermoplastic resin coated with a surface layer in an electroconductive thermoplastic resin for fixing the potential, compatible with the insulating coating and its low voltage winding is coated in an electrically conductive resin.

According to other features of the invention: the coating of the high-voltage winding has in its lateral surface orifices for the passage of the connection terminals of the winding;

- Said orifices for the passage of the connection terminals of the high-voltage winding are formed in a lateral strip of electrically insulating thermoplastic resin compatible with the thermoplastic resin of the insulation coating and in excess thickness with respect to this- this; the transformer comprising high-voltage coupling bars, each coupling bar comprises a conductive core coated with an insulating thermoplastic resin, coated with an electroconductive thermoplastic resin; - Between the conductive core of the coupling bar and the insulating coating thereof is inserted a precoat of electroconductive thermoplastic resin intended to avoid partial discharges which risk appearing between the metal of the core and the insulator the coating if the metal-insulator interface is not perfect or if there is a risk of deterioration thereof due to thermal or mechanical stresses;

each coupling bar comprises at each of its ends a member for connecting the core of the bar with a connection terminal for a high-voltage winding, one of the connection members forming a connection terminal with the outside, said connecting members each being surrounded by a head of thermoplastic resin molded in one piece with the insulating coating of the bar and having dimensions adapted to the orifices for the passage of the connection terminals of the high-voltage winding formed in the insulating coating of said winding, the head further comprising a coating of electroconductive thermoplastic resin molded in one piece with the electroconductive coating of the whole of the closed off;

- Said at least one low-voltage winding is wound directly on a sleeve of electroconductive resin coating the corresponding column of the magnetic circuit;

- The magnetic circuit further comprises yokes bringing together the columns of the magnetic circuit, said yokes also being coated with a resin coating; - the resin coating of at least one of the yokes of the magnetic circuit includes extra thickness zones for fixing the connection conductors of the low voltage windings and for the output of the low voltage connection terminals of the transformer; - Between the high voltage and low voltage windings, are formed circulation channels for a cooling fluid defined by projections made of one material with at least one of the coatings of said windings; - additional orifices for the output of adjustment terminals are provided in the lateral band of the coating of the high-voltage winding, these orifices being each closed by an insulating plug coated with a layer of electrically conductive material for fixing the potential in contact with the electroconductive coating layer of the high voltage winding to ensure the continuity of the electroconductive layer for fixing the potential.

The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the accompanying drawings, in which:

- Figs. 1A to 1E are schematic perspective views showing the assembly phases of a dry three-phase transformer according to the invention;

- Fig.2 is a sectional view on a larger scale of the dry transformer of Fig.lE;

- Fig.3 is a section along line 3-3 of Fig.2; - Fig.4 is a sectional view along the line

4-4 of Fig.2;

- Fig.5 is a perspective view of the upper yoke of the transformer of Fig.lE provided with an insulating coating; - Fig.6 is a perspective view of the lower yoke of the transformer of Fig.lE provided with an insulating coating;

- Fig .7 is a sectional view of a straight bar for coupling between high-voltage winding of the transformers of Fig.l; and

- Fig .8 is a sectional view of a bent bar of construction similar to that of Fig.7.

In Fig.lA to 1E, there is shown the various stages of mounting a three-phase transformer with dry insulation according to the invention.

In FIG. 1A, a low voltage winding 1 has been shown placed on a column 2 of the magnetic circuit of a three-phase transformer shown more fully in FIGS. 1D and 1E. To carry out the arrangement of FIG. 1A, one begins by assembling according to a usual method the laminated column 2 of the magnetic circuit of the transformer by stacking of precut sheets of conventional silicon steel. On the central part of column 2, i.e. outside the end parts 2A and 2B shaped bevel, for the subsequent meeting of the column 2 with yokes for closing the magnetic circuit, a sleeve 3 of polymer resin is molded.

This is done by injection, or by hot strip compression, if the polymer used is thermoplastic.

However, the material used can also be a thermosetting resin. It is important that the material is conductive of heat and this as much as possible. To this end, it may contain an adequate proportion of carbon, for example, which has the additional advantage of also making it weakly conductive of electricity, in accordance with a preferred variant of the invention which will be explained by the following. After fitting compression rings 4

(Fig. 4) at the ends of the sleeve 3 of polymer resin, a metallic electrical conductor 5 is wound on the surface of the sleeve, which will form the low voltage winding of the transformer as shown in particular in FIG. 1D and also on the Figures 2,3 and 4.

This conductor 5 can be in the form of large strips or flat aluminum or copper. It is conventionally wrapped in oiled insulating paper. Each of its ends is extended outside by conventional connection bars 6.

The low voltage winding thus produced is then encapsulated in a layer 7 of dielectric polymer resin a few millimeters thick, for example 3 mm. This resin envelope 7 is produced by hot compression of superimposed sheets of thermoplastic material, which is also thermally conductive in order to promote heat exchange with the surrounding air. Preferably, this resin is also slightly conductive of electricity, which improves the overall safety of the device to be produced, by putting the earth at potential at point 7A represented in FIG. 1A.

This completes the manufacture of a low voltage winding of the transformer according to the invention.

The sleeve 3, although being an intermediate entity which will disappear as such in the completed embodiment shown in Figure 1A, nevertheless fulfills several essential roles. First, thanks to its rigidity, it provides the necessary mechanical maintenance of the stacking of the sheets forming the column 2 whereas, conventionally, this cohesion is ensured by means of bolts and nuts.

Then, the sleeve 3 fulfills a major role of heat exchanger which promotes the cooling of the column 2 or of the winding 5 by conduction of heat through the thermally conductive material of which said sleeve is made.

In addition, if it is also made slightly conductive of electricity, it is possible to improve the safety of use of the final device by simple grounding of the sleeve. This arrangement is facilitated if, as is understood, provision is made to make the sleeve 3 protrude from the end of the winding 5 so as to then have a winding 5 completely coated.

Furthermore, as already indicated, it serves as a winding support for the metallic electrical conductor 5 intended to be energized. Thus, among other advantages, the oiled paper insulation which envelops the conductor 5 no longer risks being injured during winding. Up to now, in fact, we were led to avoid this risk by interposing between the columns of the magnetic circuit and the low-voltage windings wound around these columns, strips of wooden spacers. which removed the conductor from the sometimes hurtful sharp angles of the magnetic circuit.

As shown in FIG. 1A, on the outer surface of the electroconductive coating polymer resin 7 are provided projections 8 in the form of longitudinal ribs forming spacers for the high-voltage coil shown in FIG. 1B and which will be placed around the 'low voltage stage of Figure 1A.

These ribs, from 3 to 50 mm in height for example, also serve to define between them channels 9 for circulation of a cooling fluid by convection such as air between the low voltage stage and the high voltage stage as can be seen for example in Figure 1C. In the present example, the outer surface of the dielectric polymer resin coating 7 has four ribs 8 distributed at 90 ° relative to each other along the periphery of the coating envelope 7.

As already indicated above, the coating or encapsulation resin envelope 7 preferably has a certain capacity for conducting electricity, in particular when it is formed from a polymer resin charged with electrically conductive particles or with a naturally conductive polymer resin.

Thanks to such an arrangement, an antistatic effect is obtained to prevent dust, inevitably present in the air from being deposited on the wall of the cooling channels 9. It is also possible to ground the resin casing 7 by electrically connecting it to earth, as shown in 7A, which improves the safety of the transformer in the event of failure or local degradation of the oiled insulation paper of the metallic conductor 5 under voltage. In fact, all of the polymer resin parts comprising the sleeve 3 and the protective envelope 7 can safely be slightly conductive, since the electrical voltage of the conductor 5 at the low voltage stage of a transformer does not exceed usually not 400 to 500 volts.

The compression rings 4 are in fact end flanges intended to laterally contain the metal conductor wound 5 in multi-layer turns and are shaped to take account of the helical winding of the latter. It is necessary for the end turns of each layer of the preformed winding of the conductor 5 to bear continuously against these flanges over their entire length. This implies an inward or outward ramp shape of the bearing face of each ring 4 according to the orientation of the winding helix of the turns.

This is the reason why there are two concentric rings with inverted ramps for the two-layer windings usually used. Such rings have here a major advantage, namely to form a stuffing mass at the ends of the sleeve 3 which allows compression molding of thermoplastic resin sheets 7 satisfactorily. Otherwise, the volume to be filled with the molded resin would make an injection operation necessary which is not generally intended for solid parts and which is much less economical than compression.

The low-voltage stage shown in FIG. 1A is of course produced in as many copies as the transformer to be built has phases.

Figures 1B to 1E schematically show the stages of construction of a three-phase dry transformer.

In FIG. 1B, a high voltage winding intended to cooperate with the low voltage winding in FIG. 1A is shown. To produce this high-voltage winding 10, one begins by winding an electrical conductor coated with an insulating sheath to form a winding formed of coaxial turns of circular, oval or polygonal shape on one or more layers. The metallic conductor 11 (FIG. 1D) is generally made of copper or aluminum and the insulating sheath is generally an enameling or covering in an insulator generally made of paper, to isolate between them the contiguous turns. The winding thus obtained is coated by injection or by compression in a layer of electrically insulating thermoplastic resin 12 (FIG. 2).

The coating 12 has on its lateral surface orifices 14 for the passage of the connection terminals 15 of the winding.

The orifices 14 for the passage of the connection terminals of the high-voltage winding are formed in a lateral strip 16 of thermoplastic resin insulating from electricity, compatible with the thermoplastic resin of the insulation coating 12 and in excess thickness relative to this one.

Advantageously, the lateral strip 16 is produced by molding in one piece with the coating 12. It can also be produced by hot compression on the coating 12.

On the coating layer 12 is further pressed by hot compression, a layer 18 a few millimeters thick of an electrically conductive thermoplastic resin covering all the surfaces of the high-voltage winding as well as outside than inside of it.

This resin forming a surface layer 18 (FIG. 2) is charged with carbon particles to make it electrically conductive and it is of the same nature as the insulating polymer forming the coating 12. Thus, the interfaces with the insulators can be made free of vacuoles.

The high voltage coil is then finished. It remains to be specified that the thicknesses of insulation on the winding are sufficient to withstand the maximum dielectric stresses that can be applied between the winding and the electrically conductive layer 18 connected to earth, or between the internal connections of the winding and the electrically conductive layer 18 connected to the ground, between the terminals 15 of the winding and the electrically conductive layer 18 connected to the ground.

The lateral strip 16 of thermoplastic resin formed of a stack of insulating thermoplastic material similar or identical to that of the insulating coating 12 and of an electrically conductive surface layer 18, covers the internal connections and the terminals 15 of the winding. The orifices 14 formed around the output conductors 15 are intended to allow the connection of the high voltage coil to the network, or to connect the high voltage coil to other high voltage coils so as to achieve an electrical coupling such as 'It will be described with reference to Figure 1E, and to connect to the high voltage coil, an adjustable connector such as the set of conductors 20 appearing in Figure 1B in a hole 22 of square shape formed in the side strip 16 between the orifices 14 for passage of the connection conductors of the high-voltage winding.

The high voltage winding 10 is then placed on the low voltage winding 1 as shown in FIG. 1C.

For this purpose, the low voltage winding 1 mounted on its column 2 is introduced into the high voltage winding 10. The ribs 8 present on the periphery of the conductive coating 7 of the low voltage winding, allow try to center the high voltage stage on the axis of the magnetic column 2 of the low voltage winding and to provide a channel 9 for cooling by ambient air between the low voltage 1 and high voltage windings 10, the ribs 8 thus forming spacers between the two windings.

A coupled high voltage - low voltage stage was thus created.

Such stages are used to make a three-phase transformer such as that shown in Figures 1D and 1E.

To this end, three low voltage - high voltage stages are assembled and the columns 2 of these stages are assembled by upper and lower yokes 24 and 26 as shown clearly in FIG. 2.

The cylinder heads 24 and 26 are also made of laminated magnetic sheet.

The yokes 24 and 26 being placed so as to cooperate with vertical columns 2 each carrying a low voltage winding 1 and a high voltage winding 10, form a closed magnetic circuit comprising a central column and two lateral columns.

In view of their assembly, the lateral columns 2 have bevelled ends 2A, 2B, while the central column has ends 2C, 2D in symmetrical points. The cylinder heads 24 and 26 are shaped in a complementary manner to the ends of the columns 2 with which they are assembled. As shown in Figure 1E, each of the yokes 24 and 26 has a coating 28, 30 of thermoplastic resin. The coatings 28 and 30 which are shown in FIG. 1E and which also appear in FIGS. 2.5 and 6, have the main function of ensuring the mechanical maintenance between them of the sheets forming the cylinder heads 24 and 26 and the assembly of these cylinder heads with the corresponding ends of the columns 2. The resin forming the coatings 28 and 30 preferably has the same chemical base as that used for the coating of the columns 2 so as to obtain good weld between the two materials.

The coating of cylinder heads has the second function of protecting them against external aggressions.

The coating of cylinder heads 24 and 26 also has the following additional functions.

It ensures the positioning of the high voltage and low voltage windings 1 and 10 relative to the yokes 24 and 26.

It also provides mechanical maintenance of the low voltage conductors 6 between the outputs of the low voltage windings and the low voltage terminals of the transformer.

For this purpose, the coating 28 of the upper yoke comprises zones 32 in excess thickness for fixing the low voltage connection conductors 6 and for the output of the corresponding low voltage connection terminals of the transformer.

The coating 28 of the upper yoke 24 also forms a base for the low voltage terminals of the transformer.

It of course also provides mechanical maintenance of the low-voltage coupling bars which are also protected by the above-mentioned extra thickness zones 32. This coating finally ensures an increase in the heat exchange surface of the cylinder head beneficial for cooling if the resin used is thermally conductive.

As can be seen in Figure 1E, the assembly comprising the magnetic circuit including the yokes upper and lower 24 and 26 are coated with a coating and whose columns 2 each carry a low voltage winding 1 and a high voltage winding 10, is then mounted on a chassis 34. The main function of the chassis 34 is positioning of the transformer with respect to the ground. The axis of the transformer windings is preferably close to vertical to improve the cooling of the transformer by natural convection of the ambient air. The frame 34 can also be used to reinforce the timing of the windings. This setting is necessary to oppose the electro-dynamic forces exerted on the windings, in particular during network short-circuits. It is then possible to mount the coupling bars 35 and 36 of the high-voltage windings as shown in FIG. 1E.

In Figure 7, there is shown a straight coupling bar such as the bar 35 of Figure 1E. This high-voltage coupling bar comprises a conductive core 37 coated with an insulating thermoplastic resin 38, coated with an electroconductive thermoplastic resin 39.

Between the conductive core 37 of the coupling bar and the insulating coating 38 thereof, is inserted a pre-layer of electrically conductive thermoplastic resin 40 intended to avoid partial discharges which may appear between the metal of the core 37 and the insulation of the coating 38 if the metal-insulator interface is not perfect or if there is a risk of deterioration of the latter due to thermal or mechanical stresses.

The conductive bar 36 shown in FIG. 8 is a bar comprising elbows 42 intended for it allow to pass over the straight bars 35 as clearly shown in Figure 1E.

The constitution of the coupling bar 36 is moreover the same as that of the coupling bars 35. Each coupling bar comprises at each of its ends, a connecting member 43, 44 of the core 37 of the bar with a terminal. for connecting a high voltage winding. The connection member 43, 44 is surrounded by a head 45, 46 made of a thermoplastic resin molded in one piece with the insulating coating 38 and having dimensions adapted to the orifices 14 for the passage of the terminals 15 for connection of a high voltage winding formed in the coating 12 of said winding. Each of the heads 45,46 further comprises a coating 47,48 of an electrically conductive thermoplastic resin molded in one piece with the electrically conductive coating 39 of the assembly of the bar.

Thanks to the arrangement which has just been described, the accessible surface of the coupling bars 35, 36 has a potential referenced to ground provided that the assembly makes it possible to ensure electrical continuity between the electrically conductive layers 39, 47 , 48 of the bars and the electrically conductive layers 18 of the high-voltage windings 10. Furthermore, each bar comprises at one of its ends, a connection member 50 with the outside consisting of a junction plug and at its two ends a connection member 52 with a terminal of a high-voltage winding, this member comprising a spring socket.

Considering more particularly FIGS. 6 and 7, it can be seen that the coating 28.30 of each of the upper and lower yokes 24.26 of the three-phase transformer according to the invention has on its faces turned towards the winding assemblies constituting the transformer, studs 54 for positioning the windings relative to the cylinder heads.

Furthermore, the coating 28, 30 of the yokes 24 and 26 covers over a few millimeters in length, the sleeve 3 molding the magnetic columns 2.

To the right of this covering, there is a sealed weld between the coating 28.30 and the sleeve 3 during the molding of the coating 28.30 on the sleeve 3. This welding allows: - the mechanical retention of the cylinder heads 24.26 on columns 2,

- Protection against external aggressions of the magnetic circuit consisting of the yokes 24, 26 and the columns 2. Referring again to FIG. 1E, it can be seen that the orifices 22 formed in the side strips 16 of the coatings of the high voltage windings 10 carried by each transformer column are intended for the output of the adjustment terminals 20 of the corresponding high voltage windings. These orifices are advantageously closed each by an insulating plug 64 coated with a layer 65 of electrically conductive material for fixing the potential in contact with the electrically conductive layer 18 for coating the corresponding high voltage winding to ensure the continuity of the layer. electrically conductive for fixing the potential.

Although in the example which has just been described the invention is applied to a three-phase transformer, it is of course possible to apply it to the production of a single-phase transformer comprising at least one set of low windings -tension, high-voltage carried by a column of a closed magnetic circuit.

Claims

1. Dry-insulated transformer comprising at least one primary winding and at least one secondary winding mounted on at least one column of a magnetic circuit, characterized in that its high-voltage winding (10) is contained in a coating (12) d insulation in thermoplastic resin coated with a surface layer (18) in an electroconductive thermoplastic resin for fixing the potential compatible with the insulating coating and its low voltage winding (1) is coated in an electroconductive resin (7).

2. Transformer according to claim 1, characterized in that the coating of the high-voltage winding has in its lateral surface orifices (14) for the passage of the terminals (15) for connection of the winding.

3. Transformer according to claim 2, characterized in that said orifices (14) for the passage of the terminals (15) for connection of the high-voltage winding are formed in a lateral strip (16) of thermoplastic resin insulating from electricity compatible with the thermoplastic resin of the insulation coating (12) and in excess thickness with respect thereto and serving to cover the internal connections of the high voltage winding.

4. Transformer according to one of claims 1 to 3, comprising high voltage coupling bars (35,36), characterized in that each coupling bar comprises a conductive core (37) coated with an insulating thermoplastic resin (38 ), coated with an electroconductive thermoplastic resin (39).

5. Transformer according to claim 4, characterized in that between the conductive core (37) of the coupling bar and the insulating coating (38) thereof is inserted a pre-layer (40) of thermoplastic resin electrically conductive intended to avoid partial discharges which may appear between the metal of the core and the insulation of the coating if the metal-insulation interface is not perfect or if there is a risk of deterioration of that - this due to thermal or mechanical constraints.

6. Transformer according to one of claims 4 and 5, characterized in that each coupling bar (35,36) comprises at each of its ends a member (43,44) for connecting the core (37) of the bar with a connection terminal of a high-voltage winding, said connection member being surrounded by a head (45, 46) of thermoplastic resin molded in one piece with the insulating coating (38) of the bar and having dimensions adapted to the orifices (14) for the passage of the terminals (15) for connection of the high-voltage winding formed in the coating of said winding, the head (45,46) further comprising a coating (47,48) made of resin electroconductive thermoplastic molded in one piece with the electroconductive coating (39) of the entire bar.

7. Transformer according to one of claims 1 to 6, characterized in that said at least one low-voltage winding (1) is wound directly on a sleeve (3) of electroconductive resin coating the column (2) corresponding to the magnetic circuit.

8. Transformer according to one of claims 1 to 7, characterized in that the magnetic circuit further comprises yokes (24,26) joining the columns (2) of the magnetic circuit, said yokes also being coated with a coating of resin (28.30).

9. Transformer according to claim 8, characterized in that the resin coating (28) of at least one of the yokes (24) of the magnetic circuit comprises zones (32) in excess thickness for fixing the conductors for connection of the windings and for the output of the low voltage connection terminals (6) of the transformer.

10. Transformer according to one of claims 8 and 9, characterized in that the cylinder head coating resin (24,26) is a thermally conductive resin.

11. Transformer according to one of claims 1 to 10, characterized in that between the high voltage and low voltage windings, are formed channels (9) for circulating a cooling fluid defined by projections (8) of material with at least one of the coatings (7 or 12) of said windings.

12. Transformer according to any one of claims 3 to 11, characterized in that additional orifices (22) for the output of adjustment terminals (20) are formed in the lateral strip of the coating (16) of the high voltage winding, these orifices each being closed by an insulating plug (64) coated with a layer (65) of electrically conductive material for fixing the potential in contact with the electrically conductive layer (18) for coating the high voltage winding (10), to ensure the continuity of the electrically conductive layer for fixing the potential.