WO1979000737A1

WO1979000737A1 - Utilization of a glass-ceramic material as electrical insulator

Info

Publication number: WO1979000737A1
Application number: PCT/SE1979/000050
Authority: WO
Inventors: V Orpana; O Waerulf; M Wisur
Original assignee: Ssab Svenskt Stal Ab; V Orpana; O Waerulf; M Wisur
Priority date: 1978-03-10
Filing date: 1979-03-09
Publication date: 1979-10-04
Also published as: SE7802801L; JPS55500150A; EP0012772A1; DK448579A

Abstract

Utilization as electrical insulator (16) of a glass-ceramic material made from blast furnace slag. The material has a content of a CaO which exceeds or is equal to 10 per cent by weight preferably amounting to approximately 22 per cent by weight, and contains also substances with the approximative values in per cent by weight stated hereinafter: 57 % SiO 6 % Al2O3 2,6% MgO 0,2% FeO 0,05% MnO 0,1% TiO2 0,2% S 0,01% P2O5 1,7% Na2O 5,0% K2O The electrical insulator constitutes a pin insulator (16) in a gas-insulated tube cable or tube capsule (10, 12) intended for high voltage.

Description

Utilization of a glass-ceramic material as electrical insulator .

The present invention relates to utilization as electrical insulator of a glass-ceramic material made from blast furnace slag.

It is desirable that an underground tube cable generally has an effective service length of at least 50 to 100 years, and to realize this aim v e ry high demands must be put on all details forming part of the cable, i.a. the required insulators which normally will not be replaced in the cable during the said period of time because of the highly complicated working moments associated with such a replacement. Therefore, in order to satisfy the high demands the insulators must have the following properties:

- excellent electrical insulating capacity,

- excellent mechanical strength,

- excellent shape permanence,

- excellent corrosion resistance,

- excellent resistance against ageing, and

- excellent resistance against carbonizing.

One of the reasons why underground tube cables have not yet been used to any greater extent as an alternative to conventional overhead power transmission lines is that the costs of manufacture are relatively high, which i.a. resides therein that there are no cheap insulators which to a desirable extent satisfy all abovestated demands.

Hitherto, the insulators have been made mainly from ceramic materials such as porcelain, of glass and of various epoxy compositions. The insulators consisting of epoxy material have proved to be those which are best suited for utilization in tube cables and tube capsules. Such insulators of epoxy material are manufactured by moulding, but the manufacturing process is highly complicated.

The high costs are also the sole overshading reason why insulators of conventional glass-ceramic material have not at all been utilized in connection with underground tube cables and tube capsules in spite thereof that this material has proved to possess the desired material pro- perties.

The main object of the invention is to provide an electrical insulator which satisfies the demands set forth in the introductory part of this description, especial regard being taken to the requirement of resistance against ageing, simultaneously as the insulator is cheap to manufacture. This is rendered possible, according to the invention, by utilization as electrical insulator of a glass- ceramic material obtained from blast furnace slag, said glass-ceramic material having a content of CaO exceeding or equalling 10 per cent by weight, and preferably amounting to approximately 22 per cent by weight, the electrical insulator forming a pin insulator in a gas-insulated tube cable or tube capsule intended for high voltage. Glass- ceramic material or so-called glass-ceramics of the stated kind have proved to satisfy the aforestated demands on insulator material equally well as conventional glass- ceramic materials having a lower content of CaO and . produced from molten glass. As a starting material for glass-ceramic materials intended for insulators, there is thus utilized extremely cheap so-called low-grade blast furnace slag which is intermixed with a suitable quantity, determinable in advance, of silica, Si0₂, and treated in a controlled process utilizing high temperatur and high pressure. A material produced in this way and known under the denomination "Slaggsital" has the fol lowing particu lars:

- weight by volume 2.6 - 2.75 gs/cm³

- modulus of elasticity 0.75- 10⁶ - 1.1-10⁶

- bending transverse strength 650 - 1200 kgs/cm²

- compressive strength 4500 - 6000 kgs/cm²

- specific impact value (V-notch value) 2.8 4.0 kgcms/cm²

- microhardness 600 - 800 kgs/mm² longitudinal expansion coefficient 72-10 -- 95-10^{- 7} water absorption 0 %

Heat resistance according to

GO ST 11103-64 test piece

30 x 30 x 4 m s 100 150 °C temperature of deformation under load 900 - 1000 °C ther al conductiv ty at

20°C Cal/m, t, °C 0.9

- melting point 1230 - 1270 °C

- acid-resistance in 96% HO 99.15 - 99.98 %

- dielectric strength in electric field at 50 c.p.s. 40 50 kVs/mm

- tangential angle for dielectric losses at 50 c.p.s 0.07 - 0.0029

- specific electric capacity at 50 c.p.s. 7 -7.7

- electrical resistivity 1.7-10 12 Ohm/cm

Hereinafter two examples are given for a chemical analysis (values in per cent) relating to firstly the said "Slaggsital" material and secondly a glass-ceramic material of conventional type which at present is utilized for insulators.

In the Chalmers University of Technology the dielectric strength and the running spark limit have been compared for the "Slaggsital" material and epoxy materials containing various fillers. The tests were carried out in an atmosphere of sulfur hexafluoride, SF₆, and evidenced that the "Slaggsital" material was a material equivalent to the epoxy material with regard to utilization in insulators. In endurance tests insulators made of porcelain, epoxy material and glass-ceramic material were, in addition, while kept under voltage, subjected to an aggressive coastal atmosphere. The glass-ceramic insulator solely remained entirely unattacked by corrosion and showed to possess extremely high resistance against ageing, which indicates that this insulator is especially well suited for utilization in underground tube cables and tube capsules.

The above-mentioned "Slaggsital" material has earlier been produced mainly in the shape of plates and discs and has been utilized as covering material in the building industry and as grinding material in the mining industry and the iron and steel industry and in the coal industry and also as acid-proof lining in the chemical industry.

With reference to the accompanying drawing, there will now be described a tube cable containing insulators of the type envisaged by the invention.

Figure 1 is a side view of a tube cable section which is partly cut up for the purpose of improved illustration

Figure 2 is a sectional view on an enlarged scale of the tube cable shown in Figure 1.

The tube cable consists of a sleeve which may be composed of an exterior part 10 and a shielding interior part 12. Disposed within the sleeve are three tubular conductors 14 of aluminium for three-phase operation which are applied symmetrically with a spacing from one another and the encasing sleeve, pin insulators 16 constructed in accordance with the principles of the invention and disposed in spaced relationship along the conductor units serving as spacing members. The ends of the cable section are closed by insulators also made according to the invention in the shape of end discs 18. The prefabricated tube cable sections can already in connection with the fabrication be filled with insulating medium up to operation pressure. When joining the sections together in the ground a minor quantity only of insulating medium need to be added. Alternately, all insulating medium may, of course, be supplied in one operation in connection with the mounting of the tube cable.

The pin insulators 16 are directly secured onto the cable sleeve or (not shown), they may be mounted on a ring member which is inserted into the said sleeve. The insulating medium between the conductors 14 an t e conductors and the sleeve 10, 12 may consist of e.g. sulfur hexafloride, SF₆, or gaseous nitrogen, N₂, or a mixture of said gases. Oil may also be used in some cases.

Claims

1. Utilization as electrical insulator of a glass-cerami material made from blast furnace slag, c h a r a c t e r i z e d by the glass-ceramic material having a content of CaO exceeding or equalling 10 per cent by weight, preferably 22 per cent by weight, and in addition containing the following substances with the approximative values in per cent by weight stated hereinafter:

and the electrical insulator forming a pin insulator in a gas-insulated tube cable or tube capsule intended for high voltage.

2. Utilization according to claim 1, c h a r a c t e r i z e d by the insulating gas consisting of sulfur hexafluoride, SF₆.

3. Utilization according to claim l, c h a r a c t e r i z e d by the insulating gas consisting of a gas mixture containing nitrogen and sulfur hexafluoride.