Cable fitting for HVDC cables

Abstract

A cable fitting for cables that can be used for high-voltage direct-current (HVDC) energy transmission, the cable fitting having an electrically insulating layer. A process for the production of an electrically insulating layer of such a cable fitting, and also the use thereof.

Claims

1. A cable fitting for HVDC cables, comprising a molding which has multilayer structure and which has an electrically insulating layer that comprises an elastomer and that comprises a field-controlling particulate filler, wherein the field-controlling particulate filler is metal-oxide-containing core-shell particles, the core of the core-shell particles consists of a dielectric material and the shell comprises at least one electrically conductive or electrically semiconductive metal oxide.

2. The cable fitting according to claim 1, that is a cable sleeve, a cable end seal, a cable plug or a cable bushing.

3. The cable fitting according to claim 1, wherein the elastomer is a silicone rubber.

4. The cable fitting according to claim 3, wherein the silicone rubber is a RTV2 silicone rubber, a HTV silicone rubber or a LSR silicone rubber.

5. The cable fitting according to claim 1, having a density of the core-shell particles of <5 g/cm.sup.3.

6. The cable fitting according to claim 1, wherein the electrically conductive or electrically semiconductive metal oxide is a doped metal oxide, a metal suboxide or an oxygen-deficient metal oxide.

7. The cable fitting according to claim 6, wherein the metal oxide has been doped with one or more of the elements antimony, indium, tungsten, molybdenum, chromium, cobalt, manganese, iron, cadmium, gallium, germanium, tin, vanadium, niobium, tantalum, cerium, scandium, lanthanum, yttrium, bismuth, titanium, copper, calcium, strontium, barium, aluminum, arsenic, phosphorus, nitrogen, boron, fluorine or chlorine.

8. The cable fitting according to claim 1, wherein the metal oxide is an oxide, mixed oxide or oxide mixture of zinc, tin, germanium, titanium, gallium, indium, antimony, silicon, tungsten, molybdenum, lead, cadmium, calcium, strontium, barium, copper or rhenium.

9. The cable fitting according to claim 1, wherein the electrically insulating layer comprises a quantity of from 0.1 to 25%, based on the volume of the electrically insulating layer, of the field-controlling particulate filler.

10. The cable fitting according to claim 1, wherein the volume resistivity of the electrically insulating layer is in the range from 10.sup.8 to 10.sup.13 ohm*cm.

11. A process for the production of an electrically insulating layer of a cable fitting according to claim 1, comprising homogeneously mixing with one another an unhardened elastomer-precursor composition, a crosslinking agent and a field-controlling particulate filler which comprises metal-oxide-containing core-shell particles, the core of the core-shell particles consisting of a dielectric material and the shell comprising at least one electrically conductive or electrically semiconductive metal oxide, and also optionally further additives, to give an insulation-layer-precursor composition, introducing the insulation-layer-precursor composition into a hollow body which has a cavity with an exterior shape corresponding to the shape of the electrically insulating layer of a cable fitting, and hardening the insulation-layer-precursor composition in a crosslinking manner by passage of time or introduction of heat and/or of high-energy radiation, and removing the resultant insulation layer from the hollow body.

12. The process according to claim 11, wherein the insulation-layer-precursor composition is introduced into the hollow body by means of an injection molding process.

13. The process according to claim 11, wherein the unhardened elastomer-precursor composition is a RTV2 silicone composition, a HTV silicone composition or a LSR silicone composition.

14. The process according to claim 11, wherein, based on the volume of the insulation-layer-precursor composition, the quantity present therein of the field-controlling particulate filler is 0.1 to 25%.

15. The process according to claim 11, wherein the density of the field-controlling particulate filler is <5 g/cm.sup.3.

16. An electrically insulating layer on a substrate, comprising an elastomer and a field-controlling particulate filler, where the field-controlling particulate filler is metal-oxide-containing core-shell particles, the core of the core-shell particles comprises a dielectric material and the shell comprises at least one electrically conductive or electrically semiconductive metal oxide, and the elastomer is a silicone rubber.

17. The electrically insulating layer according to claim 16, having a density of the metal-oxide-containing core-shell particles<5 g/cm.sup.3, and the distribution of these in the elastomer is homogeneous.

18. The electrically insulating layer according to claim 16, having a volume resistivity of 10.sup.8 to 10.sup.13 ohm*cm.

19. The electrically insulating layer according to claim 16, wherein the substrate is an HVDC cable having, at least partially, an electrically conductive surface.

20. An insulating layer in a cable sleeve, a cable end seal, a cable plug or a cable bushing for HVDC cables, comprising in said insulating layer, end seal, plug or bushing an electrically insulating layer according to claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a diagram of a cable sleeve of the present invention with an outer conductor layer (1), insulation layers (2) and (3) of the invention, and also an HVDC cable duct (4) with conductive inner layer as connecting element (5).

(2) The present invention will be illustrated below with reference to Examples, but not restricted thereto.

EXAMPLES

Examples 1 to 3

(3) Production of Core-Shell Particles as Fillers

(4) 100 g of ground and classified natural mica are suspended in 1900 ml of deionized water. The following are admixed dropwise under acidic conditions with the suspension at 75° C., with stirring: a 50% by weight aqueous SnCl.sub.4 solution, HCl and a 35% by weight aqueous SbCl.sub.3 solution. The pH is kept constant by simultaneous controlled metering of aqueous sodium hydroxide solution into the mixture. After addition of the entire quantity of the solution, stirring is continued at 75° C. for 30 min, and then a 50% by weight solution of titanium oxychloride is added uniformly at constant pH, with metering of aqueous sodium hydroxide solution into the mixture (addition of titanium oxychloride only in Example 3). Then, with stirring, the mixture is cooled to room temperature and the reaction mixture is neutralized. The resulting pigment is isolated by filtration by way of a suction funnel, washed with water, dried at 140° C. and annealed at 800° C. for about 30 min. This gives a pigment powder of colour from pale grey to yellow-ochre, depending on antimony content. In Examples 1-3 the powder resistivities of the pigments are varied, as shown in Table 1, by varying the antimony content in the tin oxide. The resultant pigment particles have a core made of natural, lamellar mica (particle size<15 μm) and have a shell, precipitated thereon and adhering firmly thereto, made of antimony-doped tin oxide, and also optionally titanium oxide.

(5) The resistivity of a pigment powder is measured as already described above.

(6) TABLE-US-00001 TABLE 1 Example mol % Sb mol % Ti p [ohm .Math. cm] 1 8 0 28 2 1.0 0 2.5 × 10.sup.6 3 1.0 8 3.0 × 10.sup.9

Examples 4 to 8

(7) Production of Silicone Test Samples

(8) Production of Silicone Plaques Made of Room-Temperature-Crosslinking Silicone (RTV2)

(9) The quantities stated in Table 2 of core-shell particles from Examples 1 to 3 are premixed in a container with the respective proportions of component A of a commercially available RTV2 silicone resin (materials data from manufacturer: A:B=9:1, viscosity of mixture 3500 mPa*s at 23° C., Shore A hardness 45°), and homogenized in a high-speed vacuum mixer (Hauschild) at pressure reduced to 4 mbar and 1600 revolutions per minute for at least 2 minutes. The respective quantities of component B of the same RTV2 silicone resin are then added, and the components are again premixed and homogenized in the high-speed vacuum mixer for at least 1 minute at 4 mbar and 1600 revolutions per minute. The viscous composition is now quickly, within the time permitted by the pot life, poured into a mould which prescribes the geometric dimensions of the test sample. The silicone resin is hardened in the mould for at least 30 minutes at 70° C. After cooling of the mould, the mould is opened and the test samples removed and stored in a dust-free environment.

(10) The silicone test samples are configured as plaques with dimensions 100 mm×100 mm and thickness 5 mm, 2 mm and 1 mm for mechanical testing and, respectively, electrical tests. The quantity and nature of the starting materials is varied according to Table 2.

(11) TABLE-US-00002 TABLE 2 RTV2, RTV2, Filler Exam- component component Filler quantity PCV* ples A [g] B [g] Type [g] [%] 4 108.0 12.0 — 0.0 0.0 5 97.2 10.8 Example 2 12.0 3.1 6 75.6 8.4 Example 2 36.0 11.1 7 97.2 10.8 Example 1 12.0 2.8 8 97.2 10.8 Example 3 36.0 11.9 *PCV = pigment concentration volume

Examples 9-11

(12) Production of Silicone Plaques Made of Liquid Reactive Silicone Compounds (LSR)

(13) The quantity stated in Table 3 of component A of a commercially available silicone resin of type LSR (materials data from manufacturer: A:B=1:1, viscosity of components 100 Pa*s at 20° C., Shore A hardness 40°) is weighed into a container with the respective quantity of core-shell particles from Examples 2 and 3 and the quantity stated in Table 3 of component B of the same LSR silicone resin, and premixed. Care must be taken to minimize introduction of air. The container is placed in a high-speed vacuum mixer (Hauschild), and the mixture is homogenized at pressure reduced to 4 mbar and 2000 revolutions per minute for at least 3 minutes. The highly viscous composition is now quickly, within the time permitted by the pot life, poured into a mould, preheated to 60° C., which prescribes the geometric dimensions of the test sample. The silicone resin is crosslinked in the mould for at least 5 hours at 125° C. After cooling of the mould, the test sample is removed and hardened for a further 14 hours at 125° C. on a glass sheet in an oven.

(14) The silicone test samples are configured as plaques with dimensions 100 mm×100 mm and thickness 5 mm, 2 mm and 1 mm for mechanical testing and, respectively, electrical tests.

(15) Table 3 shows the variation of quantities of silicone components and filler.

(16) TABLE-US-00003 TABLE 3 LSR silicone, LSR silicone, Filler Exam- component A component B Filler quantity PCV ple [g] [g] type [g] [%] 9 60.0 60.0 — 0.0 0.0 10 53.9 53.9 Example 2 12.2 3.1 11 54.3 54.3 Example 2 11.5 3.3 12 54.3 54.3 Example 3 11.3 3.3 13 43.2 43.2 Example 3 33.6 11.3

(17) Measurement of Shore A Hardness:

(18) The hardness of elastomers is determined in accordance with DIN ISO 7619-1 by using force provided by a spring for impact-free impression of a steel indenter into the test sample for 15 seconds. The indenter for Shore A determination here has the shape of a conical frustum.

(19) Measurement of Elongation Properties:

(20) Elongation at break and tensile strength are measured on a dumbbell specimen of the type conventionally used for elastomers (DIN 53504 S2), thickness 1 mm, in accordance with ISO 37 by an Instron 5967 test system with traverse velocity 200 mm/min. Because the RTV2 material has a lower degree of crosslinking, it soon deviates from elastic behaviour and, starting at about 30% elongation, begins to exhibit irreversible behaviour. The LSR material, with its higher degree of crosslinking, mostly exhibits elastic behaviour until shortly prior to fracture.

(21) Measurement of Dielectric Strength:

(22) Dielectric strength is measured on test specimens of thickness 2 mm (about 30 mm×40 mm) by a DTA 100 insulation material tester from Baur. The sample is clamped tightly between two disc-shaped electrodes in accordance with ASTM D877, and the test cell is filled with silicone oil (AP 100, Aldrich) so that the test specimen is completely covered, in order to avoid prior discharges through the air. The voltage is increased in steps of 2 kV/s, and the voltage preceding discharge is recorded.

(23) Table 4 shows the corresponding test results.

(24) TABLE-US-00004 TABLE 4 Example 4 5 6 7 8 Filler from Example — 2 2 1 3 Silicone type RTV2 RTV2 RTV2 RTV2 RTV2 PCV [%] 0.0 3.1 11.1 2.9 11.9 Shore A hardness 35 48 69 48 69 Elongation at break 115 120 128 126 142 [%] (5.3) (5.8) (5.6) (5.4) (6.1) (at tensile stress in MPa) Dielectric strength 42.2 48.9 50.7 48.5 51.2 [kV] Example 9 10 11 12 13 Filler from Example — 1 2 3 3 Silicone type LSR LSR LSR LSR LSR PCV [%] 0.0 3.1 3.3 3.3 11.3 Shore A hardness 33 47 53 55 70 Elongation at break 325 324 326 335 340 [%] (2.9) (3.0) (3.0) (3.3) (3.9) (at tensile stress in MPa) Dielectric strength 31.2 39.8 41.2 43.4 52.8 [kV]

(25) The dielectric strength, in the form of the breakdown voltage determined, increases significantly with the filler loading in the polymer composites shown, with no resultant adverse effect on elastic extensibility. Example 8 in particular, with a filler loading of about 12% by volume, also exhibits substantially higher mechanical strength than the unfilled silicone material. In the case of the LSR silicone, dielectric strength increases by up to 69% with increasing filler loading, and the elastic extensibility of the filled material also increases. In the case of the RTV2 material with its smaller degree of crosslinking, a higher filler loading also leads to up to 21% improvement in dielectric strength. The best electrical properties can be achieved by using the semiconductive filler of Example 3.