Cellulose material having impregnation and use of the cellulose material

Abstract

Cellulose fibers are impregnated with polyethyleneimine so that the impregnation forms a type of network, which can reduce the specific resistance of the cellulose material owing to the electrical conductivity of the network. The cellulose material can thereby be advantageously adapted to use as electrical insulation of transformers, the cellulose material in this case being soaked in transformer oil. An adaptation of the specific resistance of the cellulose material to the specific resistance of the oil lead to improved dielectric strength of the transformer insulation. A method for impregnation of the cellulose material is described.

Claims

1. A cellulose material, comprising: multiple stacked plies, each ply comprising cellulose fibers with an impregnation of polyethyleneimine, ionomers of the polyethyleneimine impregnation in each ply cross-linked in polymerized form to adjacent plies, and the cellulose material having an electrical conductivity in a same order of magnitude as transformer oil.

2. The cellulose material as claimed in claim 1, wherein the cellulose fibers are also impregnated with particles of titanium oxide.

3. The cellulose material as claimed in claim 2, wherein the polyethyleneimine is appended to the particles of titanium oxide.

4. The cellulose material as claimed in claim 3, wherein the cellulose material has a specific resistivity of 10.sup.12 Ωm.

5. The cellulose material as claimed in claim 4, wherein the cellulose material is flat and has a uniform concentration of the polyethyleneimine throughout.

6. The cellulose material as claimed in claim 3, wherein the cellulose material is flat and has a uniform concentration of the polyethyleneimine throughout.

7. The cellulose material as claimed in claim 2, wherein the cellulose material has a specific resistivity of 10.sup.12 Ωm.

8. The cellulose material as claimed in claim 7, wherein the cellulose material is flat and has a uniform concentration of the polyethyleneimine throughout.

9. The cellulose material as claimed in claim 2, wherein the cellulose material is flat and has a uniform concentration of the polyethyleneimine throughout.

10. The cellulose material as claimed in claim 1, wherein the cellulose material has a specific resistivity of 10.sup.12 Ωm.

11. The cellulose material as claimed in claim 10, wherein the cellulose material is flat and has a uniform concentration of the polyethyleneimine throughout.

12. The cellulose material as claimed in claim 1, wherein the cellulose material is flat and has a uniform concentration of the polyethyleneimine throughout.

13. Insulation for a transformer containing transformer oil, comprising: multiple stacked plies, each ply comprising cellulose fibers impregnated with polyethyleneimine, ionomers of the polyethyleneimine impregnation in each ply cross-linked in polymerized form to adjacent plies, and the insulation having an electrical conductivity in a same order of magnitude as the transformer oil in the transformer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects and advantages will become more apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which:

(2) FIG. 1 is a perspective view of cellulose fibers surrounded by a network of polymers (or else alternatively with particles of titanium oxide), according to one working example of the cellulose material,

(3) FIG. 2 is a schematic section through a multi-ply cellulose material according to another working example of the cellulose material,

(4) FIG. 3A is a section of a working example of the cellulose material, as used as insulation in a transformer,

(5) FIGS. 3B and 3C are schematic illustrations of the voltages across the insulation material illustrated in FIG. 3A, and

(6) FIG. 4 is schematic side view of a working example of a manufacturing plant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout.

(8) According to FIG. 1, a cellulose material is represented by two cellulose fibers 12, in which polyethyleneimine 13 forms a network 14, of which a section is shown. This network 14 is obtained by conducting a polymerization of the polymer only after impregnation of the cellulose fibers 12 and production of the cellulose material. The network 14 permeates the cellulose material, so as to assure an electrically conductive connection of the electrically conductive polymer. Therefore, the network 14 in the polyethyleneimine 13 reduces the specific resistivity of the cellulose material in the manner described above.

(9) In addition, FIG. 1 shows, in schematic form, particles 13a of titanium oxide held within the tissue formed by the cellulose fibers 12. The cellulose fibers 12 result in interstices in which the particles 13a can be fixed. An additional fixing effect is also brought about by the polyethyleneimine 13 which also covers the surface of the particles 13a and thus brings about a mechanically stabilizing effect in the region of these particles 13a.

(10) In FIG. 2, it is apparent that a cellulose material may also be formed from several plies 13. In this case, the plies have been impregnated only after they have been produced. It is therefore apparent that the network 14 of the polymers is present in each case only in the vicinity of the surface of the plies 13, because the electrolyte with which the plies 13 have been impregnated has penetrated only into the surface of the individual plies. However, polymerization of the polymer has only occurred once the plies 13 have already been joined to form the cellulose material, and so the networks that form extend across the plies and thus contribute firstly to better cohesion of the cellulose material and secondly to a decrease in the specific resistivity of the cellulose material.

(11) An electrical insulation material 18 according to FIG. 3A has several plies of paper 19 as cellulose material with oil layers 20 therebetween. The papers 19 have also been impregnated with oil, which is not shown specifically in FIG. 3A. What FIG. 3A does show is the impregnation with polyethyleneimine 13 between the papers. The insulation shown in FIG. 3A surrounds, for example, the windings used in a transformer, which have to be electrically insulated from the outside and from one another.

(12) The electrical insulation of a transformer, in the case of operation, has to prevent electrical breakdowns on application of an AC voltage. In this case, the insulation characteristics of the insulation are dependent on the permittivity of the components of the insulation. For oil the permittivity figure ∈.sub.o is about 2, and that for paper ∈.sub.p is 4. When the insulation is subjected to AC voltage stress, therefore, with respect to the stress on the individual insulation components, the voltage U.sub.o across the oil is about twice as high as the voltage U.sub.p across the paper. If the nanocomposite in which the paper 19 has been impregnated with polyethyleneimine in the manner shown in FIG. 3A is used, the polyethyleneimine does not affect the voltage distribution in the insulation, since the permittivity figure of the polyethyleneimine is likewise roughly within this order of magnitude and therefore the permittivity ∈.sub.comp of the impregnated paper is also roughly 4. Thus, in the insulation, the voltage U.sub.o across the oil is also roughly twice as high as the voltage U.sub.comp across the nanocomposite (paper).

(13) If faults occur in the transformer, the dielectric strength of the insulation may also be important in the presence of DC voltages. In that case, however, distribution of the voltage present between the individual insulation constituents is dependent not on the permittivity but on the specific resistivity of the individual components. The specific resistivity ρ.sub.o of oil is 10.sup.12 Ωm. In contrast, ρ.sub.p of paper is three orders of magnitude higher and is 10.sup.15 Ωm. The effect of this is that, in the presence of a DC voltage, the voltage across the oil U.sub.o is one thousand times the voltage across the paper U.sub.p. In the case that the insulation is subjected to a DC voltage, this imbalance harbors the risk of breakdown in the oil and of failure of the electrical insulation.

(14) The network of polyethyleneimine introduced into the paper 19 may be adjusted via choice of the concentration of polyethyleneimine (between 0.1 and 1000 Ωcm) in such a way that the specific resistivity of the paper ρ.sub.p is lowered. This allows establishment of a specific conductivity ρ.sub.comp for the composite which approaches the specific resistivity ρ.sub.o and in the ideal case corresponds roughly thereto. In the case of a specific resistivity ρ.sub.comp of roughly 10.sup.12 Ωm, the voltage U.sub.o across the oil is in the region of the voltage U.sub.comp across the composite, so as to establish a balanced voltage profile in the insulation.

(15) This advantageously improves the dielectric strength of the insulation, since the stress on the oil is perceptibly reduced.

(16) These considerations can also be made analogously for other devices operated with DC current or components thereof. The required specific resistivity can be adjusted via the density of the network formed by the polyethyleneimine. In this way, it is especially possible to adjust the electrical properties of electrical insulation components to the particular application.

(17) FIG. 4 shows a manufacturing plant for a cellulose material in the form of a paper web 22 suitable for conducting a working example of the method. This plant has a first vessel 23 for an electrolyte 24, with ionomers of polyethyleneimine present in the electrolyte. In addition, cellulose fibers 12 or a mixture of cellulose fibers 12 and nanoparticles of titanium oxide (not shown) are trickled into the electrolyte 24 from a reservoir vessel 25. In this way, in a manner which is known per se and therefore not shown in detail, a pulp is produced in the electrolyte 24, which is separated out on a conveyor belt 26 in the form of a sieve. This conveyor belt leads into a second vessel 27 where the electrolyte 24 can drip off, resulting in formation of an already partly dewatered mat from the cellulose fibers. The electrolyte is fed by a pump 28 to a reprocessing system 29, where the required concentration of the ionomers is reestablished. The processed electrolyte can be fed to the first vessel 23 via a feed 30.

(18) At a later stage in the process, the paper web 22 is produced from the cellulose material obtained. First of all, there is a further dewatering operation by a roller pair 31, with collection of the electrolyte released in this dewatering in the vessel 27. Subsequently, the paper web 22 passes through a next roller pair 32, with achievement of a comparatively high entwinement angle through the S-shaped guiding of the paper web around the roller pair. This is because the roller pair is heated by the heating devices 33a indicated, such that heat transfer to the paper web is possible. For this purpose, additional heating devices 33b may also be used in support. The heating devices 33a, 33b bring the paper web to the polymerization temperature, such that the ionomers polymerize to give polyethyleneimine and the network already described above forms. In the course of this treatment, there is also further dewatering.

(19) After polymerizing the ionomers, electrolyte can be applied to the paper web once again by a further feed apparatus 34, the now substantially dewatered paper web being absorptive enough for electrolyte impregnation of the cellulose fibers to be possible. Subsequently, the paper web 22 passes through a further roller pair 35 and is dewatered again as a result. A further dewatering and polymerization of the additionally introduced ionomers is achieved by a roller pair 36, the latter being heatable in the manner described for the roller pair 32 by heating devices 33a, 33b.

(20) As soon as the paper web 22 leaves the roller pair 36, the paper web has been substantially dewatered. However, it still contains a residual water content and is consequently fed to a drying unit 37 and can be dried if required in this drying unit.

(21) In this regard, it should be noted that the specific resistivity p of the paper web 22 produced is dependent not just on the content of polyethyleneimine but also on the residual water content. If the paper web is to be used, for example, as electrical insulation in a transformer, it has to be impregnated with oil and consequently must contain an absolute minimum level of water. This can be ensured through the subsequent drying in the drying unit 37. The drying unit 37 may be configured, for example, as an oven.

(22) A description has been provided with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 358 F3d 870, 69 USPQ2d 1865 (Fed. Cir. 2004).