WO1998035362A1

WO1998035362A1 - Method of impregnating an electrical insulation system with a dielectric fluid

Info

Publication number: WO1998035362A1
Application number: PCT/SE1997/002203
Authority: WO
Inventors: Anna Kornfeldt; Arne Gustafsson; Robert Leandersson
Original assignee: Abb Research Ltd.
Priority date: 1997-02-06
Filing date: 1997-12-22
Publication date: 1998-08-13
Also published as: AU5353798A; SE511380C2; SE9700414D0; SE9700414L

Abstract

The present invention relates to a method for impregnating a porous material, which method comprises impregnating the insulation material with an impregnating fluid comprising an oil-like hydrocarbon mixture. The method is characterised in that the impregnation is performed in the presence of a tenside. The tenside is preferably a non-ionic tenside. The invention also relates to the using of the method for impregnating the insulation in an electric device, preferably a direct current cable.

Description

METHOD OF IMPREGNATING AN ELECTRICAL INSULATION SYSTEM WITH A DIELECTRIC FLUID

Field of the invention

The present invention relates to a method of impregnating a porous material, such as an insulation material containing cellulose, which method comprises treating the porous material with an impregnating fluid, which comprises an oil-like hydro-carbon mixture. The impregnation is preceded by degassing of the fluid. The invention also relates to a use of the method for impregnating a paper insulation in a cable.

Prior art

Insulated cables are, at present, used in a great number of areas, such as power transmission and distribution, control systems as well as optical fibre systems. One of the first materials to be used as insulation material was paper. Even though there is a constant progress in developing new insulation materials, the techniques for manufacturing the paper insulated cables also continues to progress.

High voltages place specific demands on the cable type and, above all, on the insulation. Lately, extruded dielectric materials have been put to use as insulation in high voltage cables, but the paper insulation cables still hold a significant position on the market. A reason for this is that electric malfunctions are very rare in conventional oilfilled paper insulation cables, which means that it is to a higher extent possible to avoid costly operation disturbances. Another reason is their good performance in terms of longevity and maintenance.

An oilfilled paper insulation cable has the following structure. An electrical conductor of e.g. copper or aluminium is sheathed by an insulation of highly pure cellulose containing paper and carbon containing paper, which in turn are sheathed by a reinforcing metal casing, outside of which is arranged the outer casing of the cable, which is a metal casing. The conductor may be segmented, solid or have a configuration which comprises a channel for oil filling.

The properties of the impregnating fluid have shown to be very important for the final usability and performance, since a poorly insulated cable is a cable with unsatisfactory function. The impregnating process is very time-consuming and the financial cost during this manufacturing phase is high, and the result, i.e. the degree of impregnation, will, as was mentioned above, be crucial for the electrical properties of the cable. There are demands for certain special characteristics of the impregnating fluid, concerning among other things viscosity. The impregnating process is favoured by low viscosity at the lowest possible temperature, so that the process may be carried out with the lowest possible consumption of energy. At the same time it is desirable for the impregnating fluid, once inside the cable, to exhibit the highest possible viscosity.

The efficiency of the impregnating process has, however, proven to be the most crucial factor concerning the final properties of the cable. Insulating paper material and impregnating fluid must interact in such a way as to prevent cavities from forming inside the cable, should the fluid at a later stage start to move or expand, due to differences in temperature and pressure which may arise during operation. It is also important that the impregnating process is not in any way incomplete, leaving pockets of gas in the paper material, since such pockets may cause electrical discharges during operation, which may seriously damage the cable, and, consequently, give rise to high costs. The speed and degree of impregnation are determined by differences in pressure, porosity of the paper, oil viscosity and the surface energies and wettability of the fibre surfaces.

The prior art impregnation is very slow, leading to unnecessary costs, especially due to the energy which has to be provided during the process.

Summary of the invention

The present invention solves, or eliminates to a substantial degree, the problems which were defined above, and contributes to satisfying the new higher demands by providing a method of impregnation of a porous material, which method comprises treating the porous material with impregnating fluid, said fluid being a oil-like hydro-carbon mixture. The impregnation is preceded by degassing of the impregnating fluid. The inventive method is characterised in that said impregnation and or degassing is performed in the presence of a tenside. The presence of a tenside has proven to give the impregnating fluid surprisingly good properties. partly for optimising the degree of impregnation, i.e. filling the porous matrix with oil. and partly for degassing of the impregnation fluid before the impregnation.

The invention also relates to use of a method in which impregnation and/or degassing is performed in the presence of a tenside in order to speed up and improve the degassing of the impregnating fluid in oil-filled cables, e.g. high voltage cables. Detailed description of the invention

According to a first aspect, the present invention relates to a method of impregnating a porous material, such as a insulating material containing cellulose, e.g. a paper insulation in an oil- filled cable. The inventive method comprises treating the porous material with an impregnating fluid comprising an oil-like hydro-carbon mixture, said impregnating fluid being degassed before the impregnation, and is characterised in that it is performed in the presence of a tenside. Said tenside is, according to a preferred embodiment, present in an amount of 0.01 to 1.5 % by weight, preferably 0.05 to 0.5 % by weight. In the present application, the word "tenside" refers to one or more types, according to what is considered suitable, of surface active agents, or surfactants, known as tensides.

According to a preferred embodiment, the invention also relates to a method in which the tenside which is present during the impregnation and/or the degassing is a non-ionic tenside or a tenside mixture.

More specifically, the present invention comprises embodiments of the above method, in which the tenside present is chosen from the group of sugar-based, fluorosilicone-based, fluor-based, ethoxylated or silicone-based tensides, or any mixture thereof.

According to an alternative embodiment, the invention is an impregnating method, as described above, in which the tenside present is an anionic or cationic tenside or a tenside mixture. These tensides may advantageously be used in cases where they do not influence the electric performance of the cable in a negative way.

According to an embodiment of the inventive method the tenside is added to the impregnating fluid in order to improve the degassing process before said impregnating fluid is applied to the porous material. The degassing may be effected by means of one or more of the methods below: enlarging the surface of the impregnating fluid, heating and/or at the same time exposing the impregnating fluid to low pressure. The gas bubbles formed may then advantageously be sucked from the surface. The degassing may advantageously be performed at a temperature of 60-80°C.

According to a specific embodiment of the inventive method, the tenside contents in the fluid is reduced after completion of the degassing. Possibly, said tenside contents may be eliminated completely. The reduction, or atmospheric degassing, is performed after the degassing, but before the impregnation, and is carried out by means of another heating and/or increasing of the temperature of the degassed impregnating fluid, and is especially well suited for cases in which the presence of a specific tenside in the finished cable is undesirable. The choice of tenside is thus determinative for the necessity of a later elimination of said tenside. This embodiment is thus advantageous in cases where only an improved degassing is needed.

The presence of a tenside during the inventive method is thus aimed at improving the efficiency of the impregnating process. This efficiency improvement is based upon two different technical backgrounds. Firstly, it has, according to the invention, been shown that the efficiency is drastically improved by the presence of a tenside in the impregnating fluid during the impregnating process. Secondly, the impregnating efficiency according to the invention may be improved further by improving the degassing of the impregnating fluid, since an impregnating process is less efficient if the impregnating fluid contains air or gases. Such degassing has, according to the present invention, been proven to be drastically favoured by the presence of certain tensides. The skilled artisan is able to judge, from case to case, which tenside or tensides is/are most suited for the purpose. The tenside is, however, chosen in such a way as to avoid unnecessary foaming. It should further be chosen so that it, to a satisfactory degree, is soluble in oil. Factors which are important for the choice of tenside is discussed in further detail below in connection to the experimental part of the present application, for instance under the heading "Discussion".

More specifically the above mentioned embodiment of the inventive method may be performed in such a way as to reduce the tenside contents in the impregnating fluid after completed degassing. Such a reduction is achieved, in a simple manner, by heating the impregnating fluid, and is preferably used with tensides which might deteriorate the properties of the finished, impregnated cellulose material. It might for, example, be advantageous, regarding the electric properties of the insulation, to avoid certain tensides in the finished product.

According to a further embodiment of the invention, a thickener, in the form of a substance or a mixture which gives the impregnating fluid a higher viscosity, in contact with the impregnating fluid, the fluid being thickened after the impregnation in presence of the thickener. Such a thickener may comprise a polymer or a polymer mixture. Preferably the thickener is added, completely or partly, to the fluid before the impregnation. As an alternative the thickener is added, completely or partly, to the insulation before the impregnation. The thickener may, of course, be added partly to the fluid, and partly to the insulation.

According to a preferred embodiment the fluid is brought into contact with a gelling agent, in the form of a substance or a mixture, which gives the fluid a thermo-reversible liquid-gel transformation where the impregnating fluid, at high temperatures, such as the temperatures used during impregnation, is in the form of a liquid, and where the impregnating fluid, at low temperatures, such as typical operation temperatures for a DC cable, is in the form a elastic highly viscous gel, is brought into contact with the impregnating fluid and that the impregnating fluid is gelled in contact with the gelling agent. Preferably, the impregnating fluid is transformed, after being in contact with the gelling agent, into a gelling impregnating fluid, with a thermo-reversible liquid-gel transformation. For a fluid in the form of an oil the gelling agent typically comprises a polymer, a co-polymer or a polymer mixture. This gelling fluid with a thermoreversible liquid-gel transformation, may also contain fine particles with a grain size smaller than 100 nm. The gelling agent may, like the thickener, be added to the fluid completely or partly, before the impregnation. As an alternative, the gelling agent is added, completely or partly, to the insulation before the impregnation. The gelling agent may, of course, be added partly to the fluid and partly to the insulation.

A second aspect of the invention is the use of a method according to the invention for impregnating a porous, fibrous or laminated electrically insulating body with a dielectric impregnating fluid. The body is comprised in an electrical device, and the impregnation of the body with impregnating fluid is, according to the invention, carried out in the presence of a tenside. Such a device may be a capacitor, comprising a laminated electrically insulating body between two electrodes, which body, according to the invention, is impregnated with a dielectric impregnating fluid in the presence of a tenside. Preferably, the inventive method is used for impregnating a spun insulation with oil. in an oil-filled cable of a conventional type, preferably a high voltage cable, said cable comprising a conductor, sheathed by the spun insulation, and a casing wrapped around the insulation, and the impregnation of the cable insulation with oil is. according to the invention, performed in the presence of a tenside. Preferably the cable comprises a spun paper insulation of a highly purified cellulose containing paper and carbon containing paper. Such cables are preferably used as high voltage cables, for example for the purpose of electric power transmission by means of high voltage direct current.

Short description of the drawings

Figure 1 shows the surface tension of oil without additive, and with a tenside additive of

0.1 % by weight, according to the experiment.

Figure 2 through 5 are diagrams describing the wetting of paper by impregnating fluids, with different tenside additives. In the diagrams, the contact angle of a drop of fluid is a function of time.

Detailed description of the drawings

Figure 1 shows the surface tension of oil without additives, and with a tenside additive of 0.1 % by weight, according to the experiment. It shows clearly the surface tension decreasing by adding the tensides, which is especially clear for FC.

Figure 2 is a diagram in which the contact angle of a fluid drop is a function of time. A drop of oil without additives is compared to impregnating fluids containing different tensides (see explanation of reference characters in the figure). The contact angle is low already from the start for the fluids containing tensides, and declines clearly thereafter.

Figure 3 is analogous to figure 2, but showing results for impregnating fluids comprising other tensides.

Figure 4 is analogous with figures 2 and 3, but showing results for impregnating fluids comprising other tensides.

Figure 5 is analogous with figures 2-4, but showing results for the impregnating fluid FS (fluorosilicone) in different concentrations.

EXAMPLE

The impregnating oil consists of mineral oil, with paraffines, naphtenes and aromatic components, and an additive of a viscosity increasing polymer. The surface tension gradient is paraffmes < aromatics < polymer additive (E.I. Darwish et al.; J. Brandrup & E.H. Immergut). Said gradient could be from 30 mN/m, depending on the chain length of the paraffmes, up to approximately 40 mN/m for the polymer at 20°C.

For the cellulose fibres, the surface tension gradients are areas with resin < lignin < cellulose (G. Carlsson). The wetting thus differs, depending on the properties of the paper and the composition of the impregnating fluid.

Material

Paper: DC paper with a thickness of 70 μm (of the same type that was used for the "Baltic

Cable" manufactured by ABB) (batch no. 78250-1) was dried at 120°C during 96 h, and was kept in a dessicator before measurement. During drying, paper strips were attached to a hard surface.

Oil: For the study, a base oil of the type which is used for cable manufacturing, was used, with the exception that it did not contain a polymer thickener additive. The reason for not using the complete impregnating oil was that the measurements had to take place at room temperature, and it is, then, not possible to form distinct drops of the oil for graphic analysis.

Tensides: The tensides chosen were, with one exception, non-ionic, and at least partly soluble in oil. Foam-stabilising tensides were avoided, one product excepted. The tested products are shown below.

Experiment

Dynamic contact angle and absorption

Fibro 1100 DAT is a compact contact angle testing instrument with an integral CCD camera, equipped with a computer and graphic analysis. The instrument determines the dynamics of wettability, absorption and adsorption of a fluid applied on a surface. The fibro-DAT instrument was in this case used for measuring the dynamic contact angle (wettability) and absorption of an oil in a strip of paper. These measurements made possible a comparison of the efficiency of different tensides. The high viscosity of the oil made an automatic application of the oil impossible. Manual application was therefore necessary, which to a certain extent reduced the reproducibility.

It was sometimes difficult to perform measurements during the first seconds of a test. The reason for this delay was an unexpected switching of the delay set function between "off and "on". This resulted in some test runs which were delayed with approximately three seconds. In order to achieve high reproducibility, it was necessary to perform a great number of test runs.

Surface tension

The surface tension at the oil-air interface was determined by the Du Nouy-ring method. This method measures the maximum weight of a fluid lifted by a platinum ring, which is lifted up from the fluid surface. The force which is necessary in order to lift the ring is correlated with the surface tension.

Sample manufacturing

Oil samples containing 1.0 % and 0.1 % by weight of tensides were manufactured. The tensides were diluted in either hexane or ethanol, depending on their solubility in said solvents. The dilution gave a better precision in weighing the amount which was necessary to achieve the desired concentration.

Manufacturing of 0.1 % solution: 70 g of oil was mixed with 5g of tenside in solvent, with a concentration of 0.014 g tenside/g solvent. The percentage of the active substance in the commercial products and solvents used, are shown below.

RESULTS

Surface tension

The surface tension of the oil was 38.6 nM/m. Addition of 0.1 % of tenside reduced the surface tension, see fig. 1. Further addition of FC and Q2 up to 1.0 % by weight reduced the surface tension to 31 mN/m for both, while the surface tension for EM and FS remained the same as for 0.1 %.

Wettability

The dynamic contact angle which was measured at the start, approximately one second before applying the drop, was reduced by adding 0.1 % by weight of tenside to the oil. The angle did, however, reach the same level as for pure oil after approximately 200 seconds.

The results show that the wetting of the paper surface may be improved. The spontaneous spreading of the oil over the surface is faster during the first 20 s, and is thereafter slower, as the contact angle approaches 30° (figure 4).

The best wetting was obtained with AG 6202. This is a sugar-based tenside, which, according to these tests, seems to be the most suitable to comply with the demands at the gas/cellulose interface, but not at the gas/oil interface. The molecule is similar to both the oil and the cellulose, it is non-ionic, and very interesting according to the present invention.

The Fluorad product, FC 740, seemed promising, but was not subjected to further study, due to its tendency to give rise to a stable foam. No other Fluorad product with reasonable solubility in oil was available.

The fluorosilicone-based product, FS 1265, has the same ability as FC 740 to give a low surface tension for oil/paper, and is sold as a defoamer. This component did thus seem to be very interesting for the purposes of the present invention.

Further experiments were conducted for some products at a higher (1.0 % by weight) and lower (0.01 % by weight) tenside content. The results are shown below. Contact angle, start Contact angle, 200 s

Tenside 0,01% 0,1% 1,0% 0,01% 0,1% 1,0%

AG 6202 79 60 - 21 24

Empimin - 79 81 30 30

FS 1265 74 71 81 22 22 25

Q2-5211 - 75 73 24 28

Footnote: For oil without additive, the contact angle was at the start 86 and after 200 s 23.

According to these results, a concentration of approximately 0.1 % by weight seemed to be the best for wettability. This does not necessarily have to be the case for absorption and optimum impregnability, however. The relatively large contact angle with 1.0 % FS 1265 compared to the lower concentrations of this additive were confirmed in a separate experiment, but they are yet to be explained (see figure 5).

Absorption

The absorption of the oil into the paper was studied by the decrease of drop volume during 200 s. These results are. however, difficult to interpret, since they relate to the spontaneous fluid penetration in the paper, and not, which is the case in e.g. cable impregnation, the forced penetration. The applied volume, and thus the obtained changes, are also very small. The volume changes obtained after 200 s are shown below.

Footnote: Decrease for oil without added tenside was 1,1 μl.

It is important to note, that a higher concentration resulted in a lower absorption, with the exception of FS 1265. This is, at least for Q2-5211. congruent with theory, and can be ₁₂

explained by a too low surface tension to air, at a higher concentration.

DISCUSSION

The results from the experiments described above show, that the initial contact angle of the oil against the paper is between 80 and 90°. In theory, said angle should be even bigger, as the speed of the liquid front increases. The initial wetting of the oil is thus quite low. The wetting can, however, by means of the present invention, be improved, by adding tenside. Six different types of tenside were tested, and it was found that the initial contact angle could be decreased to approximately 60°.

The additives considered to be the most interesting according to the invention, are sugar- based tenside and fluorosilicone tenside. At present, the first seems to be the most advantageous, probably because of its likeness to both the oil and the cellulose.

The initial contact angle is, even when tensides have been added, reasonably big, and since the contact angle increases as the fluid front advances, it is thought to be of considerable importance to further decrease the angle. A person skilled in the art should be able to optimise the tenside additives, by changing the hydrophobic/hydrophilic balance of the molecule, and by combining different tensides. Such modified tensides and combinations of tensides also fall within the scope of the present invention, even though they are not described in detail in this specification.

The most advantageous additives for lowering the surface tension of the oil are fluoroaliphatic compounds or fluorosilicone compounds. Solid materials with these components are known to have low surface energies, and they are, in this case, located at the oil surface. These compounds also provided a good wettability against paper surfaces. Two products, polymethylsiloxane modified with a long-chain alkyl (Tegopren 3130) and alkyl and polyether (Tegopren 7008), from Th Goldschmidt AG, were not available until after the present experiments were conducted, and are expected to give a low surface tension.

When choosing a tenside for the method according to the present invention, a low concentration dependency is important, since the concentration is decreased during the process of impregnation, due to the adsorption of the tenside at the cellulose surface.

Claims

1. Method for impregnating, in relation to production of an electrical device, a body of a porous, fibrous and/or laminated material, said body being impregnated with an impregnating fluid which comprises a hydrocarbon mixture with oil-like properties, characterised in that a tenside is added to the impregnating fluid before the impregnation, and that the impregnation is performed in the presence of the tenside.

2. Method according to claim 1 , characterised in that the impregnating fluid is degassed before the impregnation, and that the degassing is performed with the tenside present.

3. Method according to claim 1 or 2, characterised in that the porous material is a material containing cellulose, preferably an insulation material containing cellulose, such as a paper insulation in an oilfilled cable.

4. Method according to claim 1, 2 or 3, characterised in that the amount of the tenside present is approximately 0.01 - 1.5 % by weight.

5. Method according claim 4, characterised in that the amount of the tenside present is approximately 0.05 - 0.5 % by weight.

6. Method according to any of the above claims, characterised in that the tenside is a non- ionic tenside or tenside mixture.

7. Method according to claim 6, characterised in that the tenside is chosen from the group of sugar-based, fluorosilicone-based, fluor-based, silicone-based or ethoxylated tensides, or any combination thereof.

8. Method according to any one of claims 1-5, characterised in that the tenside which is present is an anionic tenside or tenside mixture.

9. Method according to any of claims 1-5, characterised in that the tenside which is present is a cationic tenside or tenside mixture.

10. Method according to any of the above claims, characterised in that the tenside contents of the impregnating fluid are reduced after degassing.

11. Method according to claim 10, characterised in that the impregnating fluid is heated further after degassing, and/or by further raising the temperature of the impregnating fluid and that the tenside contents of the fluid is reduced.

12. Method according to any one of the above claims, characterised in that a thickener, in the form of a substance or a mixture which gives the impregnating fluid a higher viscosity, is brought into contact with the impregnating fluid and that the fluid, after impregnation, is gelled with the thickener and the tenside present.

13. Method according to claim 12, characterised in that the fluid is an oil and that the gelling agent comprises a polymer or a polymer mixture.

14. Method according to any one of claims 12 or 13, characterised in that the thickener is added to the fluid, at least partly, before the impregnation.

15. Method according to any one of claims 12 or 13, characterised in that the thickener is added to the body, at least partly, before the impregnation.

16. Method according to any of the above claims, characterised in that a gelling agent, in the form a substance or a mixture, which gives the fluid a thermo-reversible liquid-gel transformation where the impregnating fluid at high temperatures, such as the temperatures used for impregnation, is in a fluid condition, and that the impregnating fluid at low temperatures, such as the normal operating temperatures of a DC cable, is in the form of a highly viscous elastic gel, is brought into contact with the impregnating fluid, and that the impregnating fluid is gelled with the gelling agent present.

17. Method according to claim 16, characterised in that the impregnating fluid, after being brought into contact with the gelling agent, and after the partial dissolution of the gelling agent in the impregnating fluid, is transformed into a gelling impregnating fluid with a thermo-reversible liquid-gel transformation.

18. Method according to claim 16 or 17, characterised in that the fluid is an oil, and that the gelling agent comprises a polymer, a co-polymer or a polymer mixture.

19. Method according to claim 18, characterised in that the gelling agent comprises fine particles with a grain size of less than 100 nm.

20. Method according to any one of claims 16 to 19, characterised in that the gelling agent is, at least partly, added to the fluid before the impregnation.

21. Method according to any one of claims 16 to 19, characterised in that the gelling agent is, at least partly, added to the body before the impregnation.

22. Use of a method according to any one of the claims 1-21 for impregnating a porous, fibrous or laminated electrical insulating body with a dielectric impregnating fluid, in an electrical device comprising an impregnated dielectric body, characterised in that the impregnation of the body is performed in the presence of tenside.

23. Use of a method according to any one of the claims 1-21 for impregnating a laminated electrically insulating body with a dielectric impregnating fluid, in a capacitor with a laminated dielectric body between two electrodes, characterised in that the impregnation of the body is performed in the presence of tenside.

24. Use of a method according to any one of the claims 1-21 for impregnating a spun insulation with oil, in an oil-filled cable of a conventional type, preferably a high voltage cable, the cable comprising a conductor which is wrapped in the spun insulation and a sheathing surrounding the insulation, characterised in that said impregnation of the cable insulation with oils is performed in the presence of tenside.

25. Use of a method according to claim 24 for impregnating a spun paper insulation with oil in an oil-filled cable of a conventional type, preferably a DC cable, the cable comprising a conductor surrounded by the insulation which comprises highly pure cellulose paper and carbon containing paper, and a sheathing surrounding the insulation, characterised in that said impregnation of the cable insulation with oil is performed in the presence of tenside.