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AULETTA, Tommaso (Skepparbacken 4, Västerås, S-722 11, SE)
JAKSTS, Albert (Ekorrvägen 13, Västerås, S-722 43, SE)
LIU, Rongsheng (Mastvägen 40, Västerås, S-723 48, SE)
PETTERSSON, Leif (Snöflingegatan 13, Västerås, S-723 50, SE)
WALFRIDSSON, Lars (Infanterigatan 52, Västerås, S-723 50, SE)
HJORTSTAM, Olof (Tortuna Kolsta 10, Västerås, S-725 96, SE)
AULETTA, Tommaso (Skepparbacken 4, Västerås, S-722 11, SE)
JAKSTS, Albert (Ekorrvägen 13, Västerås, S-722 43, SE)
LIU, Rongsheng (Mastvägen 40, Västerås, S-723 48, SE)
PETTERSSON, Leif (Snöflingegatan 13, Västerås, S-723 50, SE)
WALFRIDSSON, Lars (Infanterigatan 52, Västerås, S-723 50, SE)
CLAIMS
1. An insulation liquid for electrical or electromagnetic devices, the liquid comprises a carrier liquid and nano- particles, wherein the particles are non-magnetic and inorganic, characterized in that the insulation liquid is transparent .
2. An insulation liquid according to claim 1, wherein the insulation liquid has a transparency, k, greater or equal to 0.005 m, preferably grater or equal to 0.3 m, and most preferably greater or equal to 1.5 m.
3. An insulation liquid according to claim 1 or 2, wherein the particles have an average size of 1-80 nm.
4. An insulation liquid according to any of the preceding claims, wherein the particles have an average size of 1-30 nm.
5. An insulation liquid according to any of the preceding claims, wherein the particles are conductive or semi- conductive .
6. An insulation liquid according to any of the preceding claims, wherein the particles are at least partly covered with a surfactant having a thickness in the interval 0.5-20 nm.
7. An insulation liquid according to claim 6, wherein the surfactant is one of the following: a silane, a fatty acid, a polymer, or an alkanol amine.
8. An insulation liquid according to any of the preceding claims, wherein the particles are dehydrated.
9. An insulation liquid according to any of claims 6-8, wherein the nanoparticles comprises a hydrophobic surfactant.
10. An insulation liquid according to any of the preceding claims, wherein the nanoparticles are metal oxides.
11. An insulation liquid according to claim 10, wherein the metal oxides are one of the following: ZnO, MgO, SiO 2 , Al 2 O 3 , Fe 3 O 4 or TiO 2 .
12. An insulation liquid according to any of the preceding claims wherein the carrier liquid comprises one of the following: an oil, a transformer oil, a high-temperature liquid such as a silicone oil, or an ester.
13. An insulation liquid according to any of the preceding claims, wherein the insulation liquid comprises 0.01-0.5 % by volume of nano-particles and 99.5-99.99 by volume of the carrier liquid.
14. An insulation liquid according to any of the preceding claims, wherein the insulating liquid comprises nanoparticles that are adapted to absorb UV-light.
15. An insulation system comprising an insulation liquid according to any of claims 1-14.
16. An insulation system according to claim 15, wherein the system comprises a solid insulation.
17. An insulation system according to claim 16, wherein the solid insulation is cellulose based.
18. An insulation system, according to claim 16, wherein the solid insulation is polymeric.
19. A method for manufacturing an insulation liquid comprising a carrier liquid and nano-particles, comprising mixing the carrier liquid with the nano-particles being nonmagnetic and inorganic.
20. A method according to claim 19, comprising dehydrating the particles before they are mixed with the oil.
21. A method according to claim 20, comprising dehydrating the particles at a temperature in the interval of 100-600 0 C, preferably 185-600 0 C during 24-48 hours.
22. A method according to any of claims 19-21, comprising coating at least part of the surface of the nanoparticles with a surfactant.
23. Use of an insulation liquid according to any of claims 1 to 14 in an electrical device such as: a transformer, cables, bushings, capacitors, cable joints and cable terminations.
24. Use of an insulation system according to any of claims 15-18 in an electrical device such as: a transformer, cables, bushings, capacitors, cable joints and cable terminations. |
Insulation liquid
TECHNICAL FIELD
The present invention relates to an insulation liquid for electrical or electromagnetic devices. The insulation liquid comprises a carrier liquid and nano-particles . The present invention also relates to a method for manufacturing an insulation liquid comprising nano-particles and to the use of the insulation liquid.
BACKGROUND ART
An insulation liquid for electrical or electromagnetic devices, such as a power transformer, is subject to different types of voltages, such as AC voltages having a wide range of amplitudes and frequencies, and impulse voltages. The insulation liquid should have a high ability to withstand stresses imposed by electrical fields of a particular voltage, i.e. they should have a high lightning impulse withstand. The impulse voltage is normally the highest voltage stressing the insulation liquid. When an insulation liquid in a power transformer is subject to a critical dielectric stress, such as an impulse voltage, an insulation breakdown may occur.
When the efficiency of electrical energy transmission and distribution is maximized, the current densities and voltages inside the electrical or electromagnetic device are increased. A higher voltage increase the level of electric stress applied to the insulation components of the electrical or electromagnetic device. To compensate for higher stresses in, for example, a power transformer it is often necessary to
increase the space between winding turns filled with insulation liquid, such as transformer oil. By increasing the distance the dimensions of the transformer are increased and thereby the cost for the transformer.
It is known from, for example, US 5,863,455 that addition of magnetic particles to a conventional transformer oil, also called a ferrofluid, can enhance the lightening impulse withstand of the transformer oil. The enhanced lightning impulse withstand, due to the addition of, for example, iron oxide to the transformer oil, seems to be a direct effect of a reduced propagation speed of streamers. A streamer is a discharge in the insulation liquid that starts at an electrode. Streamers in the insulation liquid may have different speeds and structures depending on, for example, at which electrode they have started.
A ferrofluids is very dark or black, giving the fluid an appearance of being used or oxidized even when it is new. Thereby, there is no possibility to see through the ferrofluid to, for example, inspect a winding in a transformer .
US 5,863,455 also disclose a nanofluid comprising non- magnetic nano-particles, for example, organic materials, such as a polymeric material and non-organic aerosils.
There is a need to improve the existing insulation liquids for electrical or electromagnetic devices to improve the ability to withstand stresses imposed by electrical fields in the insulation liquid, as compared to known insulation liquids, also the new insulation liquid should be cost effective to use.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an insulation liquid for an electrical or electromagnetic device with similar or improved properties as compared to known insulation liquids.
According to a first aspect of the invention this object is achieved by an insulation liquid having the features of claim 1. Advantageous embodiments of the invention will be clear from the description below and in the dependent claims.
An insulation liquid for an electrical or electromagnetic device according to the invention comprises a carrier liquid and nano-particles wherein the particles are non-magnetic and inorganic particles. By adding nanoparticles being nonmagnetic and inorganic to a carrier liquid the dielectrical properties, the lightning impulse withstand and the thermal conductivity of the insulation liquid are improved as compared to known insulation liquids. Also, the degradation of the insulation liquid under breakdown and pre-breakdown events in the insulation liquid is reduced.
In this description and subsequent claims, the term "nanoparticles" refers to particles having at least one dimension, i.e. width, length and/or height between 0.1 and 100 nm. The nano-particles may be of any regular or irregular shape and have several or all dimensions within the interval given above. The filler particles may for example be in the form of spheres, hollow spheres, a polyhedral, tubes or fibres.
In the description and subsequent claims, the term "carrier liquid" refers to the medium surrounding the nano-particles, and could also be referred to as a dispersion medium wherein the nano-particles are dispersed.
According to one embodiment of the invention the insulation liquid is transparent. With an insulation liquid that is transparent it will be possible to, for example, inspect windings in a transformer through an inspection window without having to empty, at least partly, the transformer of insulation oil.
In this description and subsequent claims, the term "transparent" can be derived from I = I 0 exp (-d/k) , where "k" is a value of the transparency of the insulation liquid, "k" could also be compared to penetration depth, i.e. the greatest distance in the insulation liquid on which it is possible to see an object. "I" refers to the light intensity after the light has passed a distance "d" in the liquid. "I 0 " is the original light intensity before the light has passed the distance "d".
According to an embodiment of the invention the insulation liquid has a transparency "k" greater or equal to 0.005 m, preferably greater or equal to 0.3 m, and most preferably greater or equal to 1.5 m.
According to an embodiment of the invention the particles have an average particle size of 1-80 nm, and most preferably an average particle size of 1-30 nm.
According to an embodiment of the invention the particles of the insulation liquid comprises a coating of a surfactant. The thickness of the surfactant is preferably in the interval 0.5-20 nm. The surfactant is, for example, one of the following: a silane, a fatty acid, a polymer or an alkanol amine. The silane may be deposited via wet chemistry or vapour deposition. By arranging a surfactant of the above described type on the nano-particles the dispersion of the nano-particles in the insulation liquid will be promoted. The
insulation liquid will thereby have an improved transparency compared to an insulation liquid with nano-particles without a surfactant of the above described type.
According to an embodiment of the invention the nano- particles are conductive or semi-conductive. The nano- particles preferably have a conductivity of 10 ~5 to 10 5 S/cm. This will further improve the dielectric properties of the insulation liquid because the streamer speed of a positive streamer in the insulation liquid is reduced. One possible explanation is that it is because the free moving electrons are trapped at the conducting nanoparticles . Conductive or semi-conductive particles trap free electrons in a liquid much quicker than non-conductive particles. The conversion of the negative charges from high mobility electrons to low mobility ions improves the dielectrical properties of the insulation liquid. An example of non-magnetic, inorganic and conducting nano-particles is ZnO (zinc oxide) .
According to an embodiment the nano-particles are one of the following: doped insulating materials, doped semiconductors, undoped semiconductors or metals. Doped insulating materials are, for example, SiO 2 or Al 2 O 3 . Doped or undoped semiconductors are, for example, or Fe 3 O 4 , TiO 2 , ZnO, MgO, BN, Si or SiC. Examples of metallic nano-particles are: Ag or Cu.
According to an embodiment of the invention the nano- particles are dehydrated. As produced the nano-particles are hydrophilic and at least partly covered with water. To improve the properties of the insulation liquid, such as the life time and dielectric properties, the nano-particles that are mixed into the carrying liquid are free from water or have lower water content than the as produced nano-particles. Generally presence of water has detrimental effect on the dielectrical properties of the insulation liquid.
According to an embodiment of the invention the nanoparticles comprises a hydrophobic surfactant. The hydrophobic surfactant prevents water from being absorbed on the nanoparticles in the insulation liquid. Thereby the water content of the insulation liquid is decreased with improved dielectric properties as a result.
According to an embodiment of the invention the insulation liquid comprises 0.01-30 % by volume of nano-particles and
70-99.99 % by volume of the carrier liquid, preferably 0.01- 0.5 % by volume of nano-particles and 99.5-99.99 % by volume of the carrier liquid. The carrier liquid comprises, for example, transformer oil, a high-temperature liquid, such as silicone oil or an ester, such as Biotemp™.
According to an embodiment of the invention the insulation liquid comprise nano-particles that are adapted to absorb UV- light in the insulation liquid. The UV absorbing nano- particles absorb a significant fraction of the UV light generated at an electrical breakdown or pre-breakdown event. This will, at least partly, prevent UV light from promoting the ongoing breakdown or pre-breakdown process. For example, the number of secondary electrons generated by UV adsorption of the molecules in the insulation liquid will be suppressed. This reduces the generation of electrons and ions in the insulating liquid which suppresses the streamer propagation during the pre-breakdown phase. Thereby the dielectric properties of the insulating liquid are improved.
The insulation liquid has preferably an electrical resistivity from 10 14 ohm-m down to 10 7 ohm-m.
According to a second aspect of the invention the object of the invention is achieve by an insulation system according to claim 15.
According to an embodiment of the invention the insulation system comprises a solid insulation and insulation liquid according to any of the above embodiments of the invention. The solid insulation is, for example, cellulose based or polymeric .
According to a third aspect of the invention the object of the invention is achieve by a method of manufacturing an insulation liquid according to claim 19. The method comprises mixing a carrier liquid with nano-particles being non- magnetic and inorganic.
According to an embodiment of the invention the method comprises dehydrating the particles before they are mixed with the carrier liquid. This is for example done by drying the particles at a temperature in the interval of 100-600 0 C, preferably 185-600 0 C, for 24-48 hours.
According to an embodiment of the invention the method comprises coating at least part of the surface of the nanoparticles with a surfactant.
The insulation liquid or insulation system is preferably used in electrical or electromagnetic devices, such as a transformer for high voltages, cables, bushings, capacitors, cable joints and cable terminations.
The insulation liquid with non-magnetic and inorganic nanoparticles restores and may even improve the electrical insulation properties of insulation liquids under a variety of voltage shapes including AC and impulse shape. The
detrimental effects of high moisture content in the insulation liquids and/or the detrimental effects of ageing by-products and/or particulate contaminants in the insulation liquid may be reduced by an insulation liquid according to the invention. The statistical spread of the breakdown voltages in the insulation liquid is reduced as compared to transformer oils according to prior art.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described in greater detail by description of preferred embodiment and with reference to the accompanying drawing, wherein
Figure 1 is a Weibull plot showing the breakdown voltage for an insulation liquid according to the invention.
DESCRIPTION OF PREFERRED EMBODIMENT
An insulation liquid for use as transformer oil has been manufactured by mixing a 99.90-99.97 % by volume of regular transformer oil (NytrolOX) with 0.03-0.1 % by volume of Zinc oxide (ZnO) nano-particles having an average size in the interval of 1-80 nm. The lightning impulse (LI) breakdown probability was studied for the transformer oil with ZnO and compared to lightning impulse breakdown for regular transformer oil (NytrolOX) without nano-particles. The lightning impulse breakdown probability of the transformer oil with ZnO was also compared to a ferrofluid comprising magnetic nanoparticles and having a saturation magnetization of 2 Gauss. Also, the LI breakdown for a nanofluid comprising 99.90-99.97 % by volume of a regular transformer oil (NytrolOX) mixed with 0.03-0.1 % by volume of Al 2 O 3 was studied.
Figure 1 is a Weibull plot illustrating the Weibull probability distribution for the lightning impulse breakdown voltage of the four insulation liquids described above. When designing a transformer, a low probability value for electrical breakdown, for example 0.1 %, is often desired. When the design criterion for the insulation liquid in a transformer is increased higher low probability electrical breakdown values are required. This means that the slope in a Weibull plot should be steeper for an insulation liquid with improved lightening impulse breakdown. In figure 1 it can be seen that the line representing the nanofluid with ZnO, line a, is very steep. Also it can be seen that the line for ferrofluid, line c, is steeper than the line for regularly used transformer oil (NytrolOX) , line b, and the nanofluid mixed with Al 2 O 3 , line d.
By using a nanofluid with improved electrical breakdown voltages for low probabilities, i.e. an insulation liquid having a steeper Weibull distribution curve, compared to regular transformer oil (NytrolOX), insulation distances in for example a transformer can be reduced. This leads to reduced manufacturing costs of the transformer.
Also the higher electrical breakdown voltages lead to an increased reliability and life-time of a transformer insulation system.
It could therefore be concluded that a small amount of a nonmagnetic inorganic nanoparticles, such as ZnO is improving the breakdown properties of an insulation liquid for electrical or electromagnetic devices compared to regular insulation oil.
From lightning impulse (LI) studies of a regular transformer oil (NytrolOX) it is found that the breakdown voltage for the
oil in general are reduced successively after repeated breakdown tests. One possible explanation is that the oil is degraded at each breakdown event and thus is showing a decrease in breakdown voltages for successive LI breakdown tests. In studies of transformer oil with dispersed nano particles, such as iron oxide, ZnO, Al 2 O 3 , no clear correlation between repeated LI breakdown tests and the measured breakdown voltage is observed.
The mechanism for the reduced degradation in an insulation liquid comprising nano-particles compared to an insulation liquid without nano-particles could be explained by:
- Harmful remains or entities from previous breakdown or pre- breakdown events are absorbed on the nanoparticles . This inactivates their harmful effects.
- UV light is absorbed on the nano-particles during breakdown and pre-breakdown events. This prevents the UV light from degrading the insulation liquid.
According to an embodiment of the invention the insulation liquid is manufactured by dehydrating ZnO-particles of a size in the interval of 1-80 nm, preferably 1-30 nm, before the particles are mixed with a regular transformer oil, such as NytrolOX. The dehydration is done by drying the particles at a temperature in the interval of 100-600 0 C, preferably 185- 600 0 C, for 24-48 hours.
According to an embodiment of the invention the nano- particles of the insulation liquid comprises a coating of a surfactant. The surfactant is, for example, a silane deposited via wet chemistry or vapour deposition on the surface of the nano-particles. The thickness of the surfactant is preferably in the interval 0.5-20 nm.
The invention is of course not in any way limited to the preferred embodiments described above; several possibilities to modifications thereof should on the contrary be evident to a person skilled in the art, without deviating from the basic idea of the invention as defined in the appended claims.
For example, non-magnetic insulation liquid comprising inorganic nano-particles can be used as insulating liquid for any electrical or electromagnetic devices, such as transformers, cables, bushings, capacitors, cable joints and cable terminations. Also the non-magnetic insulation liquid comprising inorganic nanoparticles can be used for impregnating porous insulation material in any of the above mentioned electrical or electromagnetic devices.
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