GONZALEZ, Patricia (Bellas Artes 4, Casa 75, Zaragoza, E-50012, ES)
ARAUZO, Francisco (c/Menendez Pidal No.3, Zaragoza, E-50009, ES)
SCHAAL, Stéphane (15 rue des Pêcheurs, Lipsheim, Lipsheim, F-67640, FR)
GONZALEZ, Patricia (Bellas Artes 4, Casa 75, Zaragoza, E-50012, ES)
ARAUZO, Francisco (c/Menendez Pidal No.3, Zaragoza, E-50009, ES)
| P A T E N T C L A I M S
1. A low voltage coil for a transformer, with a winding made from a conductive foil (1) and an insulating foil (2) forming a plurality of turns where two subsequent turns of conductive foil are in each case separated by at least one turn of insulating foil, characterized in that, the circumferential surface of the winding is at least partially covered by a fire resistant protective, layer.
2. A low voltage coil according to claim 1, characterized in that the protective layer comprises an electrically insulating non-combustible material.
3. A low voltage coil according to claim 1 or 2, characterized in that the protective layer is in the shape of a wrap (7) comprising at least one turn of a protective foil (8) with a layer comprising glass fibers impregnated with silicone resin, preferably with an admixture of mica.
4. A low voltage coil according to claim 3, characterized in that the number of turns of protective foil (8) is between two and six and preferably equals four.
5. A low voltage coil according to one of claims 1 to 4, characterized in that the low voltage coil comprises at least one spacer means inserted between subsequent turns, in particular extending over the length of the winding in an axial direction, and which spacer means separates an inner part winding (3) from a subsequent part winding by a gap (5) surrounding the said inner part winding (3) .
6. A low voltage coil according to claim 5, characterized in that the at least one spacer means comprises several spacers (6), which are arranged parallel to each other in winding direction, each spacer (6) in particular extending over the length of the coil in the axial direction.
7. A low voltage coil according to claim 6, characterized in that the each of the spacers (6) is either in the shape of a tube or of a profile, in particular, a U-profile or T-profile.
8. A low voltage coil according to claim 5, characterized in that the at least one spacer means comprises a corrugated sheet, the corrugated sheet in particular extending over the length of the coil in the axial direction, with creases parallel to the axial direction.
9. A low voltage coil according to one of claims 5 to 8, characterized in that the spacer means comprises inorganic material, in particular metal, e.g., aluminium or steel, and/or ceramic and/or glass.
10. A low voltage coil according to one of claims 1 to 9, characterized in that the winding is at least partially sealed at one end by a protective cover (9) of epoxy resin at least partially extending over an end face of winding.
11. A dry type transformer comprising a low voltage coil according to one of claims 1 to 10, with a core with at least one leg surrounded by the said low voltage coil and a high voltage coil surrounding the said at least one leg and the said low voltage coil. |
D E S C R I P T I O N
LOW VOLTAGE COIL AND TRANSFORMER
Field of the invention
The invention concerns a low voltage coil for a transformer as well as a dry type transformer comprising such low voltage coil, as used for power distribution in a range of 50 kVA to 25'0OO kVA.
Prior art
A coil according to the generic part of claim 1 is known from WO 03/107 364 Al. " The coil is resin-encapsulated and has spacer means comprising a plurality of tubes made from a thermoplastic material and distributed over the entire cross section of the coil.
Summary of the invention
It is the object of the invention to improve the known coil of generic type with respect to its fire resistance without impairing its mechanical properties. This object is attained by the features of the characterising portion of claim 1.
It has been found that the measures proposed according to the invention significantly improve the fire resistance of a dry type transformer. Temperatures reached during a standard test were significantly lower and the time to self- extinguish was shortened by more than half. Mechanical properties were essentially equal to those of known generic type low voltage coils.
Brief description of the drawings
The invention will be explained with reference to the following figures which show only an embodiment.
Fig. 1 shows part of a cross section through a low voltage coil according to the invention,
Fig. 2 shows a radial section through an end portion of the low voltage coil of Fig. 1, along II-II, and
Fig. 3 shows a radial section similar to Fig. 2, along III-III in Fig. 1.
Description of the preferred embodiments
Fig. 1 shows a cross section through a low voltage coil according to the invention. The low voltage coil comprises a single winding essentially axially symmetric with respect to a vertical axis and made from a conductive foil 1 and an insulating foil 2 in such a way that two subsequent turns of conductive foil are separated by a turn of insulating foil. The conductive foil 1 may comprise aluminium and/or copper. α first lead-out of the low voltage coil is connected to its inner end margin and a second lead-out to its outer end margin (not shown) . The insulating foil 2 may comprise a polymer, e.g., polyimide, polyester, polyamide and/or a mixture of polymers and/or aramide paper and/or some other material known to be adequate by those skilled in the art. It may comprise several layers each commprising such material. DMD (laminate of Dacron, Mylar, Dacron) , NMN (laminate of Nomex, Mylar, Noitiex) , TMT (laminate of (tufQuin, Mylar, TufQuin) , Nomex, Mylar and Kapton have been found to be particularly suitable. The insulating foil 2 is
broader than the conductive foil 1 by about 30 mm. so as to extend beyond the lateral margins of the latter.
The continuous winding comprises an inner part winding 3 and a coaxial outer part winding 4 separated by a gap 5 which . completely surrounds the inner part winding 3. The gap 5 is established by spacer means which comprises twelve individual spacers 6 in the shape of straight aluminium tubes of rectangular cross section which are distributed over the circumferential surface of the inner part winding 3, each of them being arranged parallel to the axis of the winding. In a preferred embodiment, the spacers extend over the whole length of the coil. Of course, there can be more or less spacers than indicated, preferably between ten and thirty and they may differ in cross section and other properties from the ones described. For instance, they can be profiles, in particular, U-profiles or T-profiles. Any material with adequate mechanical properties can be used, however, inorganic materials which are fire resistant are preferred, in particular aluminium, steel and/or some other metal and/or alloy and/or even ceramic and/or glass as the use 'of such materials has been found to have a positive effect on the fire resistance properties of the low voltage coil. Instead of individual spacers other spacer means may be employed, e.g., a corrugated metal sheet, in particular, an aluminium sheet with creases parallel to the axis. There may also be several gaps, subdividing the low voltage coils into three or more coaxial part windings. On the other hand, there may also be no gap at all. At the upper end of the low voltage coil the inner part winding 3 and the outer part winding 4 are filled with DMD saturated laminate.
The circumferential surface of outer part winding 4 is surrounded by several turns of the insulating foil 2 and at
least partially by a fire resistant layer, in particular a wrap 7 which comprises several, in particular, four, turns of an insulating non-combustible protective foil 8. The protective foil 8 preferably comprises a layer of woven glass fibers impregnated with silicon resin, preferably with an admixture of mica.. Such foils are also known as mica paper and are available from various sources. Cablosam ® foils, e.g., Cablosam ® 366.21-10, DURAPOL FR-HA 2, RAL 7001 and S 2000 E available from VonRoll Isola and CABLEFLEX 1/2A and.2/2 from jsi have been found to be suitable. However, other materials may be used as well as long as they have the required fire resistance and mechanical properties. In particular, the material has to be non-combustible and electrically insulating and be suitable for partially or completely covering the circumferential surface of the winding.
Fig. 2 shows a radial section through an end portion of the low voltage coil of Fig. 1, along the cut H-II, and Fig. 3 shows a radial section similar to Fig. 2, along the cut IH-III in Fig. 1. In these figures it is shown that at the lower end the low voltage coil is sealed by a protective cover 9 which extends ' at least partially or, in a preferred embodiment completely, over the bottom end face, of the inner winding 3 and the outer winding 4 and into their lower parts while leaving out the gap 5 in order to allow air circulation for cooling and which comprises epoxy resin, e.g., EPR-M4-1291-SP with EPH 105 added as a hardener as available from Hexion. The upper end of the low voltage coil is sealed in the same way.
The low voltage coil is produced in the manner described below:
A winding machine is loaded with rolls of conductive foil 1 and insulating foil 2 and a narrower roll of DMD saturated laminate. The first lead-out is attached to the conductive foil by tungsten inert gas welding. Then the inner part winding 3 of the low voltage coil is wound and its upper part filled with the DMD saturated laminate wound up together with the conductive foil 1 and the insulating foil 2. After that the spacers 6 are put into place. The winding process is then continued and the outer part winding 4 applied in the same way as the inner part winding 3 before. The second lead-out is then attached to the conductive foil, again by tungsten inert gas welding. Now, the wrap 7 is applied to the circumferential surface of the outer part winding 4 by winding four turns of protective foil 8 on it. Finally, to apply the protective cover 9 the lower part of the low voltage coil is sealed with a liquid mass comprising epoxy resin and, finally, the low voltage coil is cured in an oven for 2h at 120 0 C.
The low voltage coil as described and a reference coil without a wrap and a protective layer and with polyester- filled glass fiber spacers were subjected to fire tests according to the IEC 60076-11 standard. In each case, a fire was lighted with 7.7 1 of alcohol and the temperature above the fire was measured as prescribed by the said standard.
The results were as follows:
temperature in temperature in time in min to 0 C after 45 min P C after 60 min . self-extinguish reference 206.7 165.3 157 embodiment 86.9 59.8 62
Obviously, . the fire resistance properties of the low. voltage coil according to the above-described embodiment of the invention shows a very significant improvement over the respective properties ■ of the conventional reference coil.
The invention also concerns a transformer comprising at least one. inventive low voltage coil. The transformer may otherwise be of 'known construction, e.g., as shown in WO 00/31 761 Al, with a core with three legs connected by an upper yoke and a lower yoke, each leg being surrounded by a low voltage coil and a high voltage coil surrounding the leg and the low voltage coil. Such dry type transformers are employed in particular as distribution transformers for a power range of between 50 kVA and 25'00O kVA.
List of reference symbols
1 conductive foil
2 insulating foil
3 inner part winding
4 outer part winding
5 gap 6 spacer
7 wrap
8 protective foil
9 protective cover