JPS6229451B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to insulators, particularly materials for reducing stress (pressure due to high electric field fields) on the surface of insulated conductors at the terminals of power cables.

It is known that in electrical conductors shielded with insulators, such as high-voltage power cables, there is a discontinuity, or stress (high electric field field), in the electric field of the shield terminal. When this discontinuity, or electrostatic potential gradient, exceeds a certain value on the surface of an electrical insulator surrounded by a gas such as air, a gas discharge occurs.
That is, a corona effect occurs, producing ozone and other gases that degrade electrically insulating surfaces, create pores, and destroy the insulation system.

This phenomenon occurs, for example, where the coil of a high-voltage generator coil emerges from a grounded stator, the edges of the foil of foil-wound capacitor bushings, and especially in high-voltage cables. As a result, the corona discharge induced by the high electric field punctures and destroys the shielded electrical components.

The corona effect can be prevented by equalizing the potential gradient on the surface of the insulated conductor. Virsberg, US Pat. No. 3,066,180, uses a semiconductor or material with voltage dependent resistivity, such as silicon carbide, in a solid resin-like thermoset or air-drying binder such as an epoxy resin or an oil-modified alkyd resin. However, this Virsberg patented composition requires lengthy curing cycles, and the resulting coating is essentially a thermoset, relatively inflexible, and once cured, it can be reused. It is not possible to flow and therefore to repair discontinuities in the treated insulated cable.

Suelmann, U.S. Pat. No. 3,210,460, discloses the use of a semiconductor coating or paint to terminate the ground shield in an attempt to eliminate the problems caused by the inherent rough surfaces obtained when using air pockets and conventional stress cone constructions. It is stated that. The coating is a film-forming liquid varnish composition, such as an epoxy-modified urea resin filled with particulate non-linear silicon carbide, an epoxy-modified melamine-formaldehyde resin, a modified phenol-formaldehyde resin.
Since the varnish is applied dissolved in a solvent such as toluene, residual solvent will create weak spots in the coating unless the solvent is removed in a separate drying step. Additionally, applying a coating in the form of a varnish limits the thickness of the coating that can be easily applied. These resins also require long curing cycles and are irreparable if discontinuities occur during coating.

U.S. Pat. No. 3,210,461 to Berg teaches the use of mixtures of silicon carbide and carbon in resinous coatings, such as chlorofluorocarbon resins, and also uses nonlinear silicon carbide as the only semiconducting material in the resin. for,
It states that rapid erosion occurs due to aging and corona attack.

Oatess, U.S. Pat. No. 3,317,655, teaches the use of heat-shrinkable stress relief cones to provide conductive discontinuities with an insulating increased wall or increased cross-section, thereby distributing lines of electric force. . Graphite and acetylene black have been disclosed as semiconductor materials suitable for this use. Shrinkable cones do not necessarily tighten tightly around the insulation, but are hand-built cones.
While clearly superior to molded or cast stress relief cones, the Oatess cone, once retracted around the terminal, may not be able to be readjusted if the cone is eroded by corona discharge or otherwise. it is obvious.

Salahshourian U.S. Patent No. 3,631,519;
3644662 and Anderson US Patent 3396231
The patents teach applying a semiconductor layer, such as a nylon tape impregnated with carbon black or polyethylene or butyl rubber compounded with carbon black, to the surface of the shield conductor. However, the carbon-filled rubber material must be extruded onto the conductor, while the impregnated nylon tape does not necessarily prevent discontinuities in the semiconductor layer around the insulated cable. Furthermore, the butyl rubber used in this US patent has the fatal defect of not having low temperature fluidity and low viscosity and therefore not being self-supporting.

U.S. Pat. No. 2,446,387 to Peterson also teaches the application of resin coatings containing semiconductor materials, particularly graphite, acetylene black, and metal powders to the surfaces of insulated conductors. However, when resin coatings are applied in the form of dispersions or varnishes, the problem can arise that residual solvent limits the thickness of the coating obtained.

In many of the above documents there are further layers of semiconductor and resin between the conductor and the cable. but,
It will be clear that this layer is ineffective in preventing stress and corona effects at the terminals of such shielded cables. U.S. Pat. No. 2,090,510 to Bower and U.S. Pat. No. 3,749,817 to Shiga disclose systems and compositions suitable for semiconductor layers between conductors and insulators.

One of the objects of the present invention is to provide a material and apparatus that is easily applied to the terminals of shielded conductors, provides a flexible long-term protective coating to the shielded conductors, is self-repairing, and is well resistant to stress-induced failure. It is to be.

According to the present invention, A. (a) an ethylene/propylene copolymer that satisfies all of the above requirements; (b) an isobutylene/isoprene copolymer containing about 99% of isobutylene monomer units and 1% of isoprene units and having a molecular weight of about 300,000 or more; (c) Polybutene, (d)
B. A cold-flowing, low-viscosity, corona-resistant elastomer selected from the group consisting of chlorosulfonated polyethylene, (e) ethylene/propylene/diene terpolymers, and (f) mixtures thereof; B. about 350 parts per 100 parts of the elastomer. ~600 parts of a semiconductor material; and C. a material for relieving electrical stress under pressure around the terminals of a conductor shielded with a packaging material consisting essentially of up to about 10 parts of a processing aid. provided.

Further in accordance with the invention, a stress relief laminate for use in shielded conductor terminals comprises a high voltage insulating tape capable of applying a compressive force to a terminal to which a stress relief material as described above is bonded to a portion of one side. It was discovered that.

Thus, the method of the present invention for preventing corona-induced failure at the terminals of a shielded conductor is to apply the stress-reducing side of the laminate described above around the insulation of the shielded conductor toward its insulating surface until there is no stress-reducing material left. The method consists of wrapping the high-voltage insulating tape in the laminate inside out and wrapping the tape over itself and the insulating surface.

Further in accordance with the present invention, corona-induced failure at the terminals of the shield conductor is prevented by applying a pad of cold-flowable, corona-resistant stress relief material around the insulation of the shield conductor.

In one aspect, the present invention provides a corona-induced fracture system comprising an electrical conductor, an electrical insulator disposed around the electrical conductor, and a stress relief material that flows at room temperature around the insulator due to the compressive force of a high voltage insulating tape. Concerning shielded electrical conductor terminals that withstand.

Other objects, advantages, and features of the present invention will become apparent from the description herein and the accompanying drawings.

In the accompanying drawings, FIG. 1 is a perspective view of the stress relief pad of the present invention.

FIG. 2 is a partial cross-sectional view illustrating a method for preventing corona-induced failure in a terminal of a shielded cable.

FIG. 3 is a perspective view of a stress-reducing laminate for application to a terminal of a shielded conductor.

FIG. 4 is a partial cross-sectional view showing an alternative method using a stress-reducing laminate.

In FIG. 1, pad 11 consists of a flat strip, which can vary in thickness up to about 100 mils, and is constructed of a cold-flowing, corona-resistant elastomer containing a semiconductor material and processing aids. The use of pads to prevent corona-induced fractures in the terminals of shielded electrical cables is illustrated in FIG. In FIG. 2, a pad 11 is wrapped around an insulating surface 12 of a shield conductor, and a compression insulating member 13 such as a high-voltage insulating tape is placed thereon. In FIG. 2, the cable insulation shield is indicated by 14.
A copper or other conductive tape shield is designated by 15 and a cable jacket by 16. FIG. 2 also illustrates a shielded electrical conductor that resists corona-induced breakdown, with 12 representing an electrical insulator placed around the conductor, and 11 a compressive force of a high-voltage insulating tape member 13 to insulate the insulator. The stress-reducing material flowed at room temperature is shown in the box 12.

The stress-reducing laminate of the present invention is shown in FIG. In FIG. 3, 21 represents a high voltage insulating tape member capable of applying a compressive force to the terminal of an insulated cable that is coupled to a portion of one side of the stress relief material 22. It is clear that the embodiment shown in FIG. 3 is one in which the stress relieving material is bonded to the entire surface of the high voltage insulating tape member to which the stress relieving member is bonded.

FIG. 4 shows the use of the stress-reducing laminate shown in FIG. 3. In FIG. 4, the insulation 31 of the shielded electrical cable 32 is wrapped around the stress relief surface 33 of the laminate adjacent to the insulation 32 of the laminate of FIG. 3 until the stress relief material 34 is exhausted. Then, the compression insulating tape member 35 is turned over, wrapping itself and the insulating surface. Fourth
Also shown in the figure are a cable insulation shield 36, a copper tape shield 37 and a cable jacket 38. FIG. 4 further illustrates the shielded electrical conductor of the present invention in which corona-induced breakdown is prevented, where 31 represents an electrical insulator disposed around the electrical conductor, and 33 represents a high-voltage insulating tape member. 35 represents a stress relieving material that flows around this insulator at room temperature under a compressive force of 35.

As used herein, the term "room-temperature flowable, corona-resistant elastomer" refers to A. (a) ethylene/propylene copolymer, (b) containing about 99% isobutylene monomer units and 1% isoprene units and having a molecular weight of about
300,000 or more isobutylene/isoprene copolymer, (c) polybutene, (di) chlorosulfonated polyethylene, (v) ethylene/propylene/diene terpolymer, and (f) mixtures thereof.

A representative example of an ethylene-propylene copolymer useful in the practice of this invention is “Vistalon”.
404” [Exxon product, specific gravity 0.86; Mooney viscosity
MLI8 (212〓), 35-45〕.

An example of an isobutylene-isoprene copolymer useful in the practice of this invention is Exxon's product "Butyl 065." This material consists of approximately 99% isobutylene monomer units and 1% isoprene units and has a molecular weight of 350,000.

Ethylene-propylene useful for the purpose of the present invention
A typical example of a diene terpolymer is Dupont (EI
Dupont de Nemours &Co.'s product, "Nordel 1320". This substance has a specific gravity of 0.85. Other representative examples of ethylene-propylene-diene terpolymers useful in the practice of this invention are "Nordel 2722" and "Nordel 2522", also products of EIDupont de Nemours & Co. Ethylene-propylene-diene terpolymers are preferred elastomers that may contain high proportions of semiconducting material, ie, about 350 to 600 parts per 100 parts of elastomer.

A typical polybutene (polyisobutylene) useful for making the stress-reducing material of the present invention is "Indopol", which has a molecular weight of about 1500 to 2000.
H1500” [Amoco Chemical
Co) products] and “Oronite”
32E” [Chevron Chemical
Co) product]. Polybutenes in the above molecular weight ranges are often used in combination with one of the aforementioned polymers to improve the "tackiness" and "viscosity" of the blend.

Preferred blends for purposes of the present invention consist of an isobutylene-isoprene copolymer containing at least about 99% isobutylene monomer units and having a molecular weight of about 300,000 or greater and a polybutene having a molecular weight of about 1,000 to 2,000. This isobutylene-isoprene copolymer and polybutene are 1:0.8 to 1:1
used in a weight ratio of

As used herein, "semiconductor material" refers to nonlinear silicon carbide and other n-type semiconductor materials with similar nonlinear resistivity. The nonlinear resistivity of silicon carbide and other semiconductor powders is determined by measuring the voltage for various currents in a cylindrical column 2.5 cm (1 inch) in diameter. The powder is packed between a pair of electrodes in the column at a pressure of 400 psi. Then the formula I=
KV ⁿ increases the current proportional to the voltage. To obtain a satisfactory stress-reducing material, the value of n should not be greater than about 2. Materials with n values as high as 7 are commercially available.

In other words, the volume resistivity of the n-type semiconductor material used in the present invention can vary from a few million ohm-cm to several million ohm-cm.

The value of n is related to particle size,
Any particle size from about 100 to about 1000 meshes is satisfactory for the practice of this invention. However, particles of 400 to 600 meshes are preferred.

A representative silicon carbide product useful in the practice of this invention is "Crystalon", a product of Norton Co.

"Silicon carbide", which has the essential properties as a material of the present invention, is actually silicon carbide containing some impurities such as CPA1, and pure silicon carbide is not a semiconductor for the purposes of the present invention.

The amount of semiconductor used in the stress relief materials of the present invention ranges from about 350 to about 100 parts per 100 parts of elastomer.
600 copies. However, about 400 to about 500 parts is preferred.

Among processing aids, isopropyl tri-isostearic titanate is
Amounts ranging from 0 to 10 parts per part are preferred stabilizers. This material is called “Ken React”.
Kenrich petrochemical Inc.;
Bayonne, New Jersey).

As used herein, the terms "compression member" and "high voltage insulating tape member" are synonymous and are of sufficient strength to apply the necessary compressive force to the stress relief material wrapped around the shield conductor. It is an electrically insulating material with A typical example of a material used for this purpose is “Vistalon 3708” (trade name from Exxon Corporation, with a specific gravity of
0.86 ethylene-propylene-dienter polymer*) with a thickness of 30 mm. The tape composition also contains clay-silicate filler and carbon black.

EXAMPLE As an example, the following ingredients were ground together to create a stress relief material of the present invention.

Parts by weight 100 Isobutylene-isoprene copolymer with 99% isobutylene units and 1% isoprene units (molecular weight 350,000) 6 Isopropyl tri-isostearic titanate 400 Silicon carbide (600 mesh) 80 Polybutene [molecular weight 1500; Chevron Chemical Co. ) The manufactured product well prevents corona effects in the terminals of shielded power cables.

Numerous changes and modifications have been made without departing from the principles of the invention.
Replacement is possible.

[Brief explanation of the drawing]

FIG. 1 is a perspective view of the stress reducing material pad of the present invention. FIG. 2 shows a method for preventing corona-induced failures at the ends of shielded cables. FIG. 3 is a diagram partially showing a cross section.
FIG. 3 is a perspective view of a stress relief laminate for use at the end of a shielded electrical conductor.
FIG. 4 is a partial cross-sectional view showing an alternative method of using stress-reducing laminates. 11,22,34...Stress reducing material of the present invention, 12,31...Electric insulator, 13,21,3
5... Insulating member for stress reduction material compression.

Claims (1)

[Claims] 1 A (a) ethylene/propylene copolymer,
(b) Approximately 99% of isobutylene monomer units and 1
% of isoprene units and has a molecular weight of approx.
300,000 or more isobutylene/isoprene copolymer, (c) polybutene, (di) chlorosulfonated polyethylene, (v) ethylene/propylene/diene terpolymer, and (f) mixtures thereof. a viscous, corona-resistant elastomer; B. about 350 to 600 parts of a semiconductor material per 100 parts of the elastomer; A material for reducing electrical stress under pressure.