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Title:
COMPOSITE FORMULATIONS FOR DIRECT CURRENT INSULATION
Document Type and Number:
WIPO Patent Application WO/2016/101988
Kind Code:
A1
Abstract:
The present invention relates to an insulation material for a DC electrical component suitable for insulating High Voltage Direct Current (HVDC). Further, the present invention refers to a DC electrical component such as a cable, a bushing or a cable joint comprising the inventive insulation material. Also, the present invention is concerned with the use of the insulation material for DC electrical current.

Inventors:
QI LEJUN (CN)
YU TAO (CN)
CHEN JIANSHENG (CN)
ZHANG YIBO (CN)
MENG DELUN (CN)
HO CHAU-HON (CH)
ROCKS JENS (CH)
Application Number:
PCT/EP2014/079075
Publication Date:
June 30, 2016
Filing Date:
December 22, 2014
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ABB TECHNOLOGY AG (CH)
International Classes:
H01B3/44; C08K3/36
Domestic Patent References:
WO2011128147A12011-10-20
WO2006008422A12006-01-26
Foreign References:
US20120012362A12012-01-19
EP0255657A11988-02-10
Attorney, Agent or Firm:
ZIMMERMANN & PARTNER (München, DE)
Download PDF:
Claims:
Claims

1. Insulation material for a DC electrical component, the insulation material being a polymer composition comprising

- a polymer matrix and

- an inorganic filler,

wherein the inorganic filler comprises group 13 and/or 14 oxide particles, and wherein the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 100 °C by less than or equal to 50%.

2. Insulation material according to claim 1, wherein the insulation material has a

temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 120 °C by less than or equal to 65%

3. Insulation material according to claim 1, wherein the insulation material has a

temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 25 °C deviates from the DC conductivity at 140 °C by less than or equal to 65%.

4. Insulation material according to any one of the preceding claims, wherein the conductivity is essentially constant over a period of at least over 24 hours.

5. Insulation material according to any one of the preceding claims, wherein the inorganic filler comprises AI2O3 and/or S1O2 as the group 13 or 14 oxide particles.

6. Insulation material according to any one of the preceding claims, wherein the inorganic filler has an average diameter of up to 10 μπι, preferably between 10 nm and 5 μπι, more preferably between 20 nm and 3 μπι and most preferably between 100 nm and 2 μιη.

7. Insulation material according to any one of the preceding claims, wherein the inorganic filler is a powder having a BET surface area of at least 3 m2/g, preferably of between 3 m2/g and 150 m2/g.

8. Insulation material according to any one of the preceding claims, wherein the amount of inorganic filler is between 0.1 and 10 wt% of the polymer composition.

9. Insulation material according to any one of the preceding claims, wherein the polymer matrix is polypropylene based and/or comprises at least one polypropylene-based copolymer.

10. Insulation material according to one of claims 1 to 8, wherein the polymer matrix is cross-linked polyethylene (XLPE).

11. Insulation material according to any one of the preceding claims, wherein the insulation material has a DC conductivity when measured at an electrical field strength of 30 kV/mm and at a temperature of 70 °C of less than 8xl0"15 S/m, preferably of less than 5xl0"15 S/m, more preferably of less than 2xl0~15 S/m.

12. Insulation material according to any one of the preceding claims, wherein the insulation material further contains one or more optional additives selected from wetting/dispersing agent, plasticizer, antioxidant, light absorber, further additive(s), preferably less than 5% by weight for all above stated ingredients together based on the total weight of the insulation material.

13. Method for producing an insulation material, comprising the steps of

a) combining at least one polymer material and one inorganic filler, wherein the inorganic filler comprises group 13 and/or 14 oxide particles,

and

b) compounding the at least two ingredients in a rheomixer or a extruder at temperatures above 140 °C.

14. A DC electrical component for high voltage DC power transmission or distribution, such as a cable, a bushing, or a cable joint, the DC electrical component comprising a conductor surrounded at least partially by an insulation layer of the insulation material according to any of the preceding claims 1 to 12.

15. The DC electrical component according claim 14, being a cable, a bushing, or a cable joint and further comprising at least one of the following (ai) to (a3): (ai) a conductive layer arranged between the conductor and the inner surface of the insulation layer,

(a2) a semi-conductive layer arranged between the conductor and the inner surface of the insulation layer or covering an outer surface of the insulation layer,

(a3) a jacketing layer covering an outer surface of at least one of the conductive layer, insulation layer and/or semi-conductive layer.

16. A DC electrical component according to claims 14 or 15, wherein the insulation layer has a thickness of more than 10 mm.

17. Use of the insulation material according to any one of the claims 1 to 12 for a DC electrical component.

Description:
Composite Formulations for Direct Current Insulation

[0001] The present invention relates to an insulation material for a DC electrical component suitable for insulating High Voltage Direct Current (HVDC). Further, the present invention refers to a DC electrical component such as a cable, a bushing or a cable joint comprising the inventive insulation material. Also, the present invention is concerned with the use of the insulation material for DC electrical current.

[0002] With the global growth of technology a continuously increasing demand of HV DC equipment (especially cables, bushings and cable joints) is observed. For such HVDC equipment capable DC specific insulation materials are desirable. For example High Voltage Direct Current (HVDC) power cables are a huge field of application for insulation materials, in particular DC insulation materials since many AC insulation materials cannot be used for DC. Unless appropriate insulation is provided, significant leakage currents can flow from the DC equipment such as a cable into the surrounding environment. Such leakage currents give rise to significant power loss, especially when the conductivity is too high within the insulation material, as well as to heating of the electrical insulation. The heating of the insulation can increase the leakage current due to the reduction of the resistance with increasing temperature and can further lead to significant increase of conductivity of the insulation material. As these power losses and possible thermal runaways are predominant in HVDC power systems and do rather less occur with AC the insulation material should provide specific properties that fit for the requirements for application in DC systems.

[0003] It is therefore the object of the present invention to provide improved insulation materials that are capable of fulfilling the specific requirements for application and use in DC and HVDC systems. In view of the above an insulation material of claim 1 is suggested, as well as a method for producing the same of claim 13, a DC electrical component cable of claim 14 and use of the insulation material claim 17. In this invention, specific inorganic fillers (metal oxide particles) are added into a matrix.

[0004] According to one aspect of the present invention an insulation material for a DC electrical component is suggested, the insulation material being a polymer composition comprising

- a polymer matrix and - an inorganic filler.

[0005] The inorganic filler comprises group 13 and/or 14 oxide particles. The inorganic filler herein comprises at least one of group 13 and 14 oxide particles, preferably of the general formula M x O n , wherein x is 1, 2 or 3 and n is 1, 2, 3 or 4. The at least one of group 13 and 14 element for the oxide particles can in principle be selected from the group consisting of B, Al, Ga, In, Th, Si, Ge, Sn and Pb, representing M in the above stated general formula M x O n . In one embodiment the inorganic filler is or substantially consists of such group 13 or 14 oxide particles. In a preferred embodiment the oxide particles are selected from AI2O 3 and/or S1O2 or at least one of AI2O 3 and Si0 2 . S1O2 can be suitably used in different polymorphic forms such as quartz, seifertite, stishovite, and coesite. In a most preferred embodiment the inorganic filler is AI2O 3 . Alumina can be suitably used in all three modifications α-, β- and γ-Α1 2 θ3. The inorganic filler can further be suitably used with an average diameter of up to 10 μιη, preferably between 10 nm and 5 μιη, more preferably between 20 nm and 3 μπι and most preferably between 100 nm and 2 μπι. In other words, the inorganic filler can comprise group 13 and/or 14 oxide micro- and nanoparticles, such as micro- and nano-alumina and/or micro- and nano-silica. The inventors have particularly found that temperature stable DC conductivity of the herein suggested insulation material can be observed over a wide range of average diameters as specified above. The skilled person is aware that when using powders the particles will have irregular shapes. The skilled person therefore will further understand that "average diameters" mean that the diameters are measured on an irregular bulk and are then commonly averaged. The diameter is typically measured through the center of the particle where the shortest distance between two points on opposite sides of the particle can be assigned. Additionally, the average diameter was taken as correctly measured when particles were purchased with a specific given diameter in the meaning of particle size.

[0006] The inorganic filler can further be provided as particles in the form of a powder having a BET surface area of at least 3 m 2 /g, preferably having a BET surface area in the range of 3 m 2 /g to 50 m 2 /g, preferably in the range of 3 m 2 /g to 100 m 2 /g, and more preferably in the range of 3 m 2 /g to 150 m 2 /g.

[0007] The amount of inorganic filler particles can advantageously lie in the range of 0.1 wt% to 10 wt%, in particular of 0.25 to 3 wt%. [0008] Also, the insulation material as suggested herein can comprise more than one sort of group 13 and group 14 oxide particles. According to the herein suggested insulation material the polymeric composition can comprise only one sort of group 13 and 14 particles, but also a combination of two, three, four or more sorts of group 13 and group 14 oxide particles.

[0009] It was surprisingly found that the addition of inorganic fillers comprising group 13 and/or 14 oxide particles can lead to a temperature stable reduction in conductivity, particularly stable at elevated temperatures. This temperature stable reduction in conductivity is particularly achieved when using alumina and/or silica, particularly micro- and nano- alumina and/or micro- and nano-silica. However, also other group 13 and/or 14 oxide particles can lead to this beneficial temperature stable reduction in conductivity. Such behavior could not have been observed for MgO particles. Such thermally stable conductivity as specified herein is particularly advantageous as for example in HVDC cables the generation of heat during current flow will not deteriorate the insulation properties of the insulation material. Moreover, using the insulation material as suggested herein minimizes power loss by leakage currents and possible thermal runaways since the conductivity remain stable upon heat exposure to the insulation material.

[0010] Moreover, it was also found that the addition of inorganic fillers as defined herein leads to a significant reduction of the DC conductivity without sacrificing other properties. In particular, the addition of the fillers not only significantly decreases the electrical conductivity in comparison to the matrix without the addition of the inorganic filler material, but also surprisingly stabilizes the DC conduction current. It was found that specifically the addition of inorganic fillers as defined herein such as micro- and nano-alumina results in a decrease of DC conductivity when measured at an electrical field strength of 30 kV/mm and at a temperature of 70 °C by at least one order of magnitude. Such drastic reduction in the DC conductivity makes the material suitable for application in HVDC insulation products. Meanwhile, the addition of such fillers does not deteriorate other properties of the original matrix in that the addition does not affect the melting and crystallization temperature of the polymer material. Also, the elongation at break and the tensile strength are not at all influenced by the addition of an inorganic filler material, while the material may tend to show only a slightly higher rigidity. All deviations observed for the properties above fall within standard deviation. [0011] Without being bound to the following theory it is believed that the filler comprising group 13 and/or 14 oxide particles attract charges within the matrix and trap them. Thereby, the charges are hindered to further move through the matrix and additionally reject and repel following charges once trapped. As a result current flow is hampered. Moreover, it is believed that a synergetic effect arise from the use of the filler as defined herein and a matrix in such way that the matrix preserves a certain flowability and flexibility even when stored below the melting point and that the filler particle can slightly move on a nanoscale level towards charges and can thereby more effectively serve as "lightning arrestor". However, unless heated over the melting point of the material the effect remains stable.

[0012] The insulation material as specified above has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 100 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%.

[0013] The skilled person will understand the any given deviation in percent is referred to the conductivity value measured at 100 °C. In a preferred embodiment the temperature stable conductivity is less than 3.5xl0 "15 S/m. Further, the skilled person will understand that a temperature stable conductivity also implies that the conductivity is upheld over a longer period of time without significant increase. Temperature stable preferably also implies that the conductivity is essentially constant over a period of at least over 24 hours, more preferably at least over 100 hours, even more preferably at least 240 hours and most preferably for at least 21 days. Essentially constant means that the measured conductivity at a pre-determined temperature and electric field does not change either in the form of an increase or in the form of a reduction. Essentially constant further preferably means that the conductivity upon set condition changes by <±30%.

[0014] In a preferred embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 120 °C by less than or equal to 65%, preferably 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%. In a more preferred embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 25 °C deviates from the DC conductivity at 105 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%. In a further preferred embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 25 °C deviates from the DC conductivity at 140 °C by less than or equal to 65%, preferably 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%.

[0015] In a more preferred embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength in the range from 10 to 40 kV/mm, preferably from 5 to 80 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 100 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%.

[0016] In a specific embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 100 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10% with a filler content in the range of 0.1 to 10 wt%, preferably in the range of 0.25 to 3 wt%, more preferably of 1 wt%.

[0017] In a further specific embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 120 °C by less than or equal to 65%, preferably 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10% with a filler content in the range of 0.1 to 10 wt%, preferably in the range of 0.25 to 3 wt%, more preferably of 1 wt%.

[0018] In a further specific embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 25 °C deviates from the DC conductivity at 105 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10% with a filler content in the range of 0.1 to 10 wt%, preferably in the range of 0.25 to 3 wt%, more preferably of 1 wt%. [0019] In a further specific embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 25 °C deviates from the DC conductivity at 140 °C by less than or equal to 65%, preferably 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10% with a filler content in the range of 0.1 to 10 wt%, preferably in the range of 0.25 to 3 wt%, more preferably of 1 wt%.

[0020] In a further specific embodiment the insulation material has a temperature stable DC conductivity when measured at an electrical field strength in the range from 10 to 40 kV/mm, preferably from 5 to 80 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 100 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10% with a filler content in the range of 0.1 to 10 wt%, preferably in the range of 0.25 to 3 wt%, more preferably of 1 wt%.

[0021] Further, the insulation material exhibits a temperature stable DC conductivity in that the DC conductivity is at 50 °C more than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10% and less than or equal to 150%, preferably 140%, more preferably 130 %, even more preferably 120% and most preferably 110% of the conductivity at 100 °C with a filler content in the range of 0.1 to 10 wt%, preferably in the range of 0.25 to 3 wt%, more preferably of 1 wt%. Also, the insulation material exhibits a temperature stable DC conductivity in that the DC conductivity is constantly less than 4xl0 "15 S/m, preferably 3xl0 "15 S/m, at an electrical field strength of 30 kV/mm over the whole temperature range of 50 to 100 °C with a filler content in the range of 0.1 to 10 wt%, preferably in the range of 0.25 to 3 wt%, more preferably of 1 wt%, when measured on plates of the suggested insulation material having a thickness in the range of 0.1 to 5 mm, e.g. 0.5 to 1 mm over at least 24 hours.

[0022] The DC conductivity as referred to herein was measured according common procedures generally known by the skilled person. In particular the DC conductivity was measured as the steady state conductivity at 30 kV/mm on 1 mm thick plates for at least 24 hours at the specified temperature. If the temperature is not particularly specified, the DC conductivity is measured at 70 °C.

[0023] The herein suggested insulation material further has a DC conductivity when measured at an electrical field strength of 30 kV/mm and at a temperature of 70 °C of less than 8x10 " S/m, preferably of less than 5x10 " S/m, and more preferably of less than 3.5xl0 "15 S/m and even more preferably of less than 2xl0 "15 S/m. Such conductivities can be detected over a long period of time, at least over 24 hours, preferably over at least 100 hours, preferably over at least 240 hours .

[0024] The polymer matrix of the suggested insulation material preferably is a thermoplastic polymer matrix. The polymer matrix, preferably the thermoplastic polymer matrix, can comprise or consist of any suitable material such as polyethylene, polypropylene, ethylene propylene rubber, poly-4-methylpentene, polyvinylchloride, polymethylmethacrylate, polystyrene, polyoxymethylene, polyethylenterephthalate or bisphenol-A-polycarbonate. But also other materials such as resins and rubber materials can be used.

[0025] In one embodiment of the present invention the polymer matrix, preferably the thermoplastic polymer matrix, is polypropylene based and/or comprises at least one polypropylene-based co-polymer. Polypropylene based as used herein means the matrix consists of or comprises to a larger degree polypropylene or polypropylene-co-polymers, for instance at least 40% based on the total amount of the polymer matrix. Polypropylene based further implies that the properties of the matrix are significantly influenced by the amount of polypropylene or polypropylene-co-polymers.

[0026] In another embodiment of the present invention the polymer matrix, preferably the thermoplastic polymer matrix, is polyethylene based and/or comprises at least one polyethylene-based co-polymer. Polyethylene based as used herein means the matrix consists of or comprises to a larger degree polyethylene or polyethylene-co-polymers, for instance at least 40% based on the total amount of the polymer matrix. Polyethylene based further implies that the properties of the matrix are significantly influenced by the amount of polyethylene or polyethylene-co-polymers.

[0027] The insulating matrix material may or may not be cross-linked. Cross-linking could be achieved physically, for example by means of electron beaming, X-ray, or y- radiation; or achieved chemically. When chemically cross-linked, the cross-linking agent could for example be dicumyl peroxide (DCP), a silane or another peroxide, and the cross- linking agent could for example occur in a concentration within the range of 0.1-2.2 wt%. A cross-linked material is typically more mechanically stable at high temperatures than a material which has not been cross-linked. In a preferred embodiment the polymer matrix, preferably the thermoplastic polymer matrix, comprises or consists of crosslinked polyethylene material (XLPE).

[0028] According to one embodiment the polymer matrix may also be a blend of polymers and/or copolymers. In a preferred embodiment the polymer matrix comprises

- a first copolymer being an ethylene-propylene copolymer comprising from 40 to 99% by weight propylene and from 1 to 60% by weight ethylene, based on the total weight of the first copolymer, and

- a second copolymer being a butylene-propylene copolymer, wherein the second copolymer comprises from 5 to 50% by weight 1-butylene and from 20 to 95% by weight propylene, based on the total weight of the second copolymer, wherein the weight ratio of the first copolymer to the second copolymer ranges from 99:1 to 1:99, preferably 10:90 to 90: 10, more preferably 75:25 to 25:75 and most preferably 50:50.

[0029] According to said preferred embodiment the first copolymer has a suitable weight average molecular weight M w of 30000 g/mol to 1000000 g/mol. Accordingly, the first copolymer has a number average molecular weight M n of 5000 g/mol to 300000 g/mol. According to an aspect, the copolymer blend is understood to be a blend of at least one copolymer and one further component, e.g. a polymer.

[0030] According to an alternative embodiment the polymer blend comprises a first and a second copolymer with the same first polymer as specified above, but the second copolymer further comprises from 5 to 50% by weight by weight ethylene, based on the total weight of the second copolymer In a further preferred embodiment of the present invention, the second copolymer consists of 1-butylene, propylene and ethylene, preferably from 5 to 50% by weight of 1-butylene, preferably from 20 to 85% by weight propylene, and preferably from 5 to 55% by weight ethylene, based on the total weight of the second copolymer.

[0031] Further, the basic principle of the polymeric matrix has been described in PCT/EP2013/066790, filed August 12, 2013, which can be utilized with the inventive concept described herein and is therefore incorporated herein by reference. [0032] The skilled person is aware that the insulation material as suggested herein can further contain one or more optional additives selected from wetting/dispersing agent, plasticizer, antioxidant, light absorber, further additive(s), preferably less than 5% by weight for all above stated ingredients together based on the total weight of the insulation material. In particular, in order to improve the dispersion of filler particles in the polymer matrix the insulation material may comprise a dispersing agent to an amount of less than 5%, preferably less than 2% and most preferably less than 1% based on the total weight of the insulation material. Suitable dispersing agents are stearate esters, phosphoric acid esters such as phosphoric acid dimethyl esters, glycerol trioleat, dibutyl amine, sulfanates, polyethylene glycol, stearic acid, citric acid and fish oils. Such dispersing agents can be directly added into the formulation. Also, surface treatment in addition to dispersing chemicals can be applied in the formulation to further improve the dispersion the filler particle. A suitable surface treating agent is for example hexamethyldisilazane (HMDS). Such surface treating agents can be used to couple the -OH polar groups on the particle surface. Suitable antioxidant agent are selected from the group comprising or consisting of phenols, phosphites, B-blends, hydroxylamines, aromatic amines, thioethers, lactones, metal deactivators and hindered amines. In particular antioxidants are suitably selected from the group comprising or consisting of Irganox 1010 ® , Irganox B 225 ® , Irganox B215 ® , Irganox B 561 ® , Irgafos 38 ® , Irgafos 126 ® , Irgafos 168 ® , Irgastab FS 042 ® , Irganox PS 800 ® , Irganox PS 802 ® , Irganox MD 1024 ® , Santonox R ® and distearyl thiodipropionate. Also, the dispersion of filler particles in the polymer matrix the insulation material may comprise an antioxidant agent to an amount of less than 5%, preferably less than 2% and most preferably less than 1% based on the total weight of the insulation material.

[0033] According to a further embodiment of the invention, the insulation material may additionally comprise a fiber reinforcing material, preferably continuous mineral or organic fibers, more preferably glass fibers and ceramic fibers, most preferably mineral fibers from aluminum oxide fibers, basalt fibers and glass fibers or aramide fibers and polyester fibers. These fibers of one or different kinds may be present in the inventive insulation material in an amount of up to 5% by weight, preferably, up to 3% by weight, more preferably up to 1% by weight based on the total weight of the insulation material.

[0034] According to another aspect, the herein suggested insulation material can be produced by a method comprising the steps of a) combining at least one polymer material and one inorganic filler, wherein the inorganic filler comprises at least one of group 13 and 14 oxide particles, and

b) compounding the at least two ingredients in a rheomixer or a extruder at temperatures above 140 °C.

Suitable temperatures for polypropylene based matrix materials are in the range of 150 to 200 °C, preferably 165-190 °C. Compounding usually takes about 3 to 10 minutes, however it dependent on the temperature, the amount of materials to be compounded as well as on the nature on the material to be compounded. Further, the skilled person understands that the method also comprises a step of cooling the compounded composition to room temperature, preferably slowly by a regulated, e.g. stepwise decrease of the ambient temperature.

[0035] The method as described above is highly advantageous for the manufacture of insulation materials for DC electrical components as it is possible to provide a very homogeneous polymeric composite with filler particles which are well distributed within the polymeric matrix enabling efficient trapping of charges when DC current is applied on the insulation material. Moreover, by providing a very homogeneous distribution of fillers and possible additives the risk of local over-heating and so-called hot spots with damaging the material can be avoided as the heat distribution is likewise improved with the distribution of fillers and additives. Therefore, one aspect of the present invention is directed to an insulation material produced by the herein suggested method.

[0036] According to one aspect of the present invention a DC electrical component for high voltage DC power transmission or distribution, such as a cable, a bushing, or a cable joint, is suggested, the DC electrical component comprising a conductor surrounded at least partially by an insulation layer of the insulation material as suggested herein. In particular, a DC electrical component for high voltage DC power transmission or distribution is suggested, the DC electrical component comprising a conductor surrounded at least partially by an insulation layer of the insulation material comprising

a polymer matrix and

an inorganic filler, wherein the inorganic filler comprises at least one of group 13 and 14 oxide particles, and wherein the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 100 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%, and preferably wherein the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 120 °C by less than or equal to 65%, preferably 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10% .

More preferably, the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 25 °C deviates from the DC conductivity at 105 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%.

Even more preferably, the insulation material has a temperature stable DC conductivity when measured at an electrical field strength of 30 kV/mm, so that the DC conductivity at 25 °C deviates from the DC conductivity at 140 °C by less than or equal to 65%, preferably 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%.

Further preferably, the insulation material has a temperature stable DC conductivity when measured at an electrical field strength in the range from 10 to 40 kV/mm, preferably from 5 to 80 kV/mm, so that the DC conductivity at 50 °C deviates from the DC conductivity at 100 °C by less than or equal to 50%, preferably 40%, more preferably 30%, even more preferably 20% and most preferably 10%

[0037] The herein suggested DC electrical component being a cable, a bushing, or a cable joint further comprises at least one of the following (al) to (a3):

(al) a conductive layer arranged between the conductor and the inner surface of the insulation layer,

(a2) a semi-conductive layer arranged between the conductor and the inner surface of the insulation layer or covering an outer surface of the insulation layer,

(a3) a jacketing layer covering an outer surface of at least one of the conductive layer, insulation layer and/or semi-conductive layer. [0038] In one embodiment of a DC electrical component as described above the insulation layer has a thickness of more than 10 mm, preferably more than 20 mm and more preferably more than 30 mm.

[0039] A cable comprising the insulation material as herein suggested can be rated for high DC voltages, for example voltages of 150 kV, or higher. The design of an HVDC cable product in terms of insulation thickness and rated voltage can be such that the radial electric field in the insulation layer tolerates an electrical field exceeding 10 MV/m at the rated voltage of the cable product, and for many applications, such that the radial electric field in the insulation layer tolerates an electrical field exceeding 25 MV/m at the rated voltage.

[0040] According to a further aspect of the present invention the use of the herein suggested insulation material for a DC electrical component is suggested. In particular, the use of the insulation material for insulating of a cable, a bushing or a cable joint is suggested.

[0041] The present invention shall be described in more detail in the following Examples. One skilled in the art will appreciate that the technology presented herein is not limited to the embodiments disclosed in the accompanying examples and the foregoing description, which are presented for purposes of illustration only, but it can be implemented in a number of different ways, and it is defined by the later on following claims.

Examples

[0042] The below Examples are given merely for illustrating purposes and should not be seen in any way of limiting the scope of the invention to these specific embodiments. The skilled person is aware that the invention can be implemented in a number of different ways and not only in the way as presented in the Examples.

[0043] The fillers and matrix (TP) were firstly compounded by a torque rheomixer. The filler content was 1 wt%. The compounding was completed at 185-190 °C for 5 minutes. Then the mixtures were hot pressed into thin plaques (0.1 mm and 0.5 mm in thickness) for dielectric measurements (DC conductivity and breakdown strength). Some of the mixture were also inject molded into dumbbell samples for tensile tests. TP can typically be any thermoplastic polymer which suitably melts at elevated temperatures, generally higher than 110 °C, and can be extruded, processes and well blended with fillers as suggested herein. Typically suitable polymers are polyethylene, polypropylene, polybutylene, cross-linked or not, co-polymers thereof and blends of such polymers or co-polymers. For the Examples below different thermoplastic polymer blends were used consisting of a first copolymer being a common ethylene-propylene-copolymer, which was mixed with a common butylene-propylene copolymer blend. Different ratios in the range of 20:80 to 80:20 of first and second copolymer were used for which similar results were obtained. Suitable polymers which could be used for the experiments below are Borclear RB707CF ® , Borclear RB709CF ® , Bormed SC876CF ® , Bormed RE816CF ® and Bormed RD808CF ® derivable from Borealis; Versify 4000 ® , Engage 8540 ® , Engage ENR 7256 ® , Elite 5960G ® , Infuse 9100 ® derivable from Dow Chemicals; Purell EP274P ® , Adflex Q100F ® , Koattro KT AR05 ® derivable from Lyondell Basell; Mosten MP 720 ® , Mosten EP 501 ® derivable from Unipetrol.

The following materials were used in the examples (Table 1):

Function Type Material details Supplier

Fumed aluminum oxide with a

alumina Evonik, Germany

BET surface area of 130 m 2 /g

alumina Average diameter 1 μπι particle Alfa Aeser, USA

Filler

silica Average diameter 1 μπι particle Sibelco, USA

99%, 20 nm, coated with

MgO 0.14%-0.17% Nanoamor, USA hexamefhyldisilazane (HMDS)

Stearate ester of

Dispersant poly(12- Stearate ester without solvent Lubrizol, USA hydroxystearic acid) [0044] Formulations (Table 2)

[0045] DC conductivity tests

The sample thickness is around 0.5 mm. The applied voltage was lOkV or 15 kV, meaning an electric field of 20 kV/mm or 30 kV/mm. The temperature was 70 °C. The current through the plaques was recorded continuously by a picoammeter. The experiments last for 100 hours. The currents after 100 hours were used to calculate the conductivity.

[0046] Electrical field dependent DC conductivity test (Table 3)

Temperature Conductivity

Formulation E [kV/mm] [°C]

[x It) -14 S/m]

No. 1 (TP, Comparative 70

20 1.1

Example)

No. 1 (TP, Comparative 70

30 1.3

Example)

No. 3 (TP+Nano-alumina) 20 70 0.1

No. 3 (TP+Nano-alumina) 30 70 0.2

No. 4 (TP+Micro-alumina) 20 70 0.2

No. 4 (TP+Micro-alumina) 30 70 0.2

No. 5 (TP+Micro-silica) 20 70 0.3

No. 5 (TP+Micro-silica) 30 70 0.3 [0047] Temperature dependence of DC conductivity (Table 4)

[0048] Breakdown tests (Table 5)

Breakdown tests were performed on 0.1 mm plaques by using 20 mm/75 mm sphere -plate electrode in oil (25# transformer oil) with a voltage ramp of 500 V/s. Formulation Average BD strength (V/μηι)

No. 1 (TP, Comparative

409.9 + 50.3

Example)

No. 3 (TP+Nano-

416.8 + 35.9

alumina)

No. 4 (TP+Micro-

418.3 + 45.7

alumina)

No. 5 (TP+Micro-

420.4 ± 35.5

silica)

[0049] Thermal analysis (Table 6)

Melting temperature (Tm) and crystallization temperature (Tc) of the samples were measured by using a differential scanning calorimetry (DSC). DSC measurement (5-10 mg sample) was carried out according to ISO 3146, part 3 method C2 with a heat/cool/heat cycle. The scan rate was 10 °C/min with the temperature ranging from 40 to 210 °C.

[0050] Tensile properties (Table 7)

Tensile properties were measured at room temperature according to ISO 527-.2. The sample thickness is 4 mm. The gauge length is 20 mm and the speed of testing is 50 mm/min. Young's Tensile Elongation

Formulation Modulus Strength at Break

(MPa) (MPa) (%)

No. 1 (TP, Comparative

117 22 327 Example)

No. 3 (TP+Nano-alumina) 132 22 323

No. 4 (TP+Micro-alumina) 122 22 333

No. 5 (TP+Micro- silica) N/D 24 330