Electric Switching Device Comprising Insulating Parts Reinforced With Polyvinyl Alcohol Fibres

Abstract

An electric switching device with insulating parts of mouldings, particularly extinguishing chamber plates in an arc extinguishing chamber, such mouldings being formed of resin binder reinforced with fiber material which is at least primarily fibers of polyvinyl alcohol or acetalised polyvinyl alcohol. The binder is an unsaturated resin converted into polymerised form.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electric switching device and especially a high voltage circuit breaker in which the insulating parts are subjected to the influence of electric arcs during operation.

2. The Prior Art

Insulating parts which during operation come into contact with electric arcs are subjected to high mechanical, electrical and thermal stresses. It is of decisive importance for the speration of the switching means that the material in said insulating parts is sufficiently resistant to arcs so that the material is not burnt away to too great an extent. The material must also have such properties that the influence of the arc does not give rise to polluted surfaces, primarily as a result of the formation of soot, and thus to creepage sparking. It has proved extremely difficult at the same time to achieve sufficient mechanical strengh and also sufficient resistance to electric arcs. Examples of switching devices where problems of the type described arise are circuit breakers and contactors and also isolators having extingushing chutes.

Furthermore, the requirements are permanently increasing for short breaking times for high voltage circuit breakers, i.e. that time from the moment when the breaker receives an opening impulse signal until the moment when the current is cut off. The increased requirements for short breaking times are due to the expansion of electric power transmission networks and thus the increasing short-circuit powers. In order to achieve short breaking times it is important that the voltage strength in the extinguishing chamber is built up so quickly that so called dielectric re-ignition does not occur. The material in the insulating parts which comes into contact with the arc is therefore very important since a flash-over is often caused by creepage currents along the surfaces of the insulating parts.

Previously a great number of inorganic and organic insulating materials have been used for said insulating parts. Recently, however, moulded material with binders of synthetic resin have been used to an increasing extent, such as polyester resins and epoxy resins with powdered and fibrous fillers. In order to satisfy the requrements of mechanical and thermal strength glass fibres have been principally used as fibrous filler and reinforcing material. It is also known to use mouldings of acrylate resins such as polymethyl methacrylate without filler.

However, it has been found that there are problems in using glass fibres as filler and reinforcing material. The problem have probably something to do with the considerable tendency to creepage current which heated glass fibres have and with the fact that the glass is altered and roughened at the high temperatures of the arc, which results in the glass retaining soot particles so that the dielectric strength of the insulation will be very poor along the surface.

SUMMARY OF THE INVENTION

According to the present invention it has been found possible while retaining the good mechanical and thermal properties of the insulating parts which can be achieved with glass fibres as reinforcement, to considerably improve the resistance of these insulating parts to arcs. The improvement is achieved by the use of a fibre material in the insulating parts which consists at least substantially of polyvinyl alcohol fibres or acetalized polyvinyl alcohol fibres. A likely explanation of the improvement achieved is that polyvinyl alcohol disintegrates at low temperatures to produce gases, for instance water vapour, which effectively keeps the arc at a distance from the surface of the insulating parts and thus saves the surface from the direct influence of the arc. At the same time, the gas production contributes to extinguishing the arc and thus to shortening the duration of the arc.

The present invention relates more specifically to a switching device having insulating parts of mouldings reinforced with fibre material, which are subjected to the influence of electric arcs and is characterised in that the fibre material consists at least substantially of polyvinyl alcohol fibres or acetalized polyvinyl alcohol fibres.

Polyvinyl alcohol fibre has been a commerical product at least since 1950. In the following description the term polyvinyl alcohol fibres includes not only polyvinyl alcohol fibres as such but also acetalized polyvinyl alcohol fibres, i.e. polyvinyl alcohol fibres which have been acetalized.

When manufacturing mouldings according to the present invention, polyvinyl alcohol fibres can be used in the manner conventional for glass fibres, i.e. in the form of wool, short fibres or fibre strings, cloth and webs, together with a resinous binder. The reinforced products can be produced by compression moulding compounds and injection moulding compounds, by moulding impregnated webs to laminates, by hand lay-up, fibre winding and centrifugal casting.

The length of the fibres in moulding compounds is suitable 1 - 100 mm, preferably 3 - 50 mm and in injection moulding compounds suitably 1 - 50 mm, preferably 1 - 10 mm. Their diameter is suitably 1 - 50 mm.

The quantity of polyvinyl alcohol fibres in the finished mouldings is suitably 5 - 40, preferably 10 - 25 percent of the weight of the mouldings.

As examples of suitable binders for the moulding may be mentioned ethylenically unsaturated polyester resins with methyl methacrylate, other acrylates or methacrylates, such as the monomers mentioned below having several arcylate groups, as well as diallylphthalate or tirallyl cyanurate as ethylenically unsaturated monomers in a quantity of 15 - 45 percent of the weight of the polyester resin including the monomer, acrylate resins such as a mixture of on the one hand a linear acrylate polymer, for example a polymer of methyl methacrylate, methyl acrylate, ethyl methacrylate or ethyl acrylate or a copolymer of two or more of these substances and, on the other hand, a monomer having several acrylate groups, for example ethylene glycol dimethacrylate, 1,4-butanedisldimethacrylate, neopenthyl glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, pentaethylene glycol dimethacrylate, hexaethylene glycol dimethacrylate and trimethylol propane trimethacrylate or corresponding compounds in which the methacrylate groups have been replaced by acrylate groups and mixtures of two or more such monomers containing methcrylate or acrylate groups, at least partially polymerised diallylphthalate, epoxy resins with curing agents of anhydride type such as hexahydrophthalic acid anhydride, carbamide resins and melamine resins. Particularly preferred are the acrylate resins mentioned and especially a mixture of a linear acrylate polymer of the type mentioned and a monomer in the form of a monomeric acrylate compound consisting of a polyethylene glycol dimethacrylate and/or diacrylate such as a dimetacrylate or diacrylate of diethylene glycol, triethylene glycol, tetraethylene glycol, pentaethylene glycol, hexaethylene glycol and heptaethylene glycol, possibly with an additive resin containing ethylenically unsaturated groups which are copolymerisable with the unsaturated groups in the polyethylene glycol dimethacrylate or diacrylate. As examples of such additive resins may be mentioned unsaturated polyester resins, partially polymerised allyl esters, for example diallylphthalate and diallylmaleate, acryl-modified, preferably cycloaliphatic epoxy resins and unsaturated polybutadiene resins, for example having a molecular weight of 1,000 - 3,000 and containing approximately 85 percent of the unsaturated groups in 1,2 position. The quantity of linear acrylate polymer should preferably be 10 - 25 percent and the quantity of monomeric acrylate compound about 65 - 90 percent of the total weight of these substances. The quantity of additive resin should be at the most 25 percent, i.e., 0 - 25 percent of the total weight of acrylate polymer, monomeric acrylate compound and additive resin. The quantity of binder is suitable 15 - 70 percent, preferably 20 - 50 percent of the weight of the moulding.

The unsaturated polyester resin may be of conventional type. It can be produced in the normal manner by esterifying ethylenically unsaturated and saturated, preferably aliphatio dicarboxylic acids or corresponding anhydrides with an equivalent quantity or a slight excess of a bifunctional alcohol. As examples of unsaturated acids which can be used for this purpose may be mentioned maleic acid, fumaric acid, itaconic acid as such, or in the form of anhydrides. Examples of saturated acids which can be used are primarily adipic acid, sebacic acid, succinic acid as such or in the form of anhydrides. As examples of suitable bifunctional alcohols may be mentioned glycols such as ethylene glycol, propylene glycol and butylene glycol as well as polyglycols such as diethylene glycol, triethylene glycol and dipropylene glycol.

The mouldings may also contain inorganic and organic powered filler, such as chalk, kaolin, dolomite, bauxite, mica powder, silica powder, zirconium dioxide, zirconium silicate, silicon carbide, aluminum trihydrate, cellulose powder, polyacetal powder (polyoxymethylene), etc. The quantity of powder filler may be at the most 70, suitably 1 - 70 and preferably 15 - 60 percent of the weight of the moulding, i.e. the total weight of the binder, fibrous material and powdered filler.

Besides polyvinyl alcohol fibres the mouldings may also contain other fibrous material such as cotton, sisal fibres and asbestos to a total weight of less than the weight of the polyvinyl alcohol fibres.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained by describing a number of embodiments by way of example with reference to the accompanying drawing which shows schematically a high voltage circuit breaker of minimum liquid type with transverse extinguishing chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The circuit breaker shown has a cylindrical extinguishing chamber 1 and a cylinder 2 concentric with this, both of glass-fibre reinforced plastic. At the upper end of these parts is a metallic cover 3 provided with a current terminal 4 connected to the stationary contact 5 of the circuit breaker. The movable contact 6 of the breaker consists of a contact plug which can be displaced within the current collector 7 and through an arcing chamber 8 in relation to the stationary contact 5 arranged in the upper part of the arcing chamber. A number of extinguishing chamber plates 9 are arranged horizontally in the arching chamber. The lower surface of each plate is flat over the entire area of the plate. Each plate has a central hole 12 for the movable contact and a substantially radial groove at the top which extends from the hole 12 to an exhaust channel 11 inside the wall of the extinguishing chamber. Said groove thus forms a horizontal gap 10 (transverse blowing gap) which leads from the arcing chamber 8 and opens out into the exhaust channel 11. The thickness of the material of each plate is greater towards the exhaust channel 11, i.e. in those parts which surround and are higher than the radial groove than towards the opposite end from the hole 12, so that there are open spaces between the plates towards the latter direction, as can be seen from the drawing. The plates are stacked on each other, in close contact with each other where the thickness of the plates is greatest (behind the plane shown in the drawing). In order to protect the inner wall of the extinguishing chamber against the hot gases blown out through the gaps 10, a cylindrical screen 17 of moulded material reinforced with polyvinyl alcohol fibres may be arranged inside the cylinder 1 along a part of its circumference. The binder in this material may be of the same sort as that mentioned for the insulating parts subjected to the influence of electric arcs, as for the plates 9. The inner wall of the extinguishing chamber cylinder 1 may also be protected by giving the plates 9 of the extinguishing chamber the same diameter as the inner diameter of the extinguishing chamber cylinder and shaping each plate with a hole running in axial direction spaced from the periphery and thus from the wall of the extinguishing chamber. The holes in the plates stacked on top of each other thus together form an exhaust channel corresponding to the exhaust channel 11 in the drawing.

The circuit breaker is filled with liquid, for example oil, to the level 13 in the cover. A porcelain insulator 14 is arranged around the cylinder 2. During a breaking process the oil in the arc zone is vaporized due to the influence of the arc and an extremely high pressure is thus built up in the arcing chamber 8. As the transverse blasting gaps 10 are exposed by the movable contact 6, the arc is subjected to a powerful flow of oil and gas which cools and deionises the arc. In order to ensure that small currents are broken, a number of liquid pockets 15 may be arranged in the lower part of the extinguishing chamber.

In the arrangement shown the extinguishing chamber plates 9 are subjected to the influence of an arc. According to the present invention these plates are made of mouldings reinforced with polyvinyl alcohol fibres. In the following examples will be given for their manufacture.

Example 1

A moulding compound is manufactured of the following components:

30 parts by weight of an unsaturated polyester resin manufactured in conventional manner from 2 mols maleic acid anhydride, 1 mol succinic acid and 3.3 mols ethylene glycol and mixing 70 percent by weight of the reaction product thus obtained with 30 percent by weight methyl methacrylate. Esterifying is carried out at 165 -190.degree.C to an acid number of 35.

0.5 parts by weight benzoyl peroxide

1.5 parts by weight zinc stearate

20 parts by weight polyvinyl alcohol fibres having a length of 15 mm

95 parts by weight zirconium dioxide

The components are kneaded together to a homogenous pulp similar to cotton waste. This is compression moulded and cured in the mould at a pressure of 100 kg/cm.sup.2 and a temperature of 140.degree.C for 4 minutes.

Example 2

A moulding compound is manufactured of the following components:

30 parts by weight of an acrylate resin consisting of 75 parts by weight triethylene glycol dimethacrylate and 25 percent by weight polymerised methyl methacrylate having a molecular weight of 140,000

0.5 parts by weight butyl perbenzoate

15 parts by weight polyvinyl alcohol fibres having a length of 15 mm

5 parts by weight cellulose powder

1.5 parts by weight zinc stearate

48 parts by weight kaolin

The moulding compound is manufactured and moulded in the same way as the compound described in Example 1.

Example 3

An injection moulding compound is manufactured of the following components:

36 parts by weight pre-polymerized diallylphthalate with a melting point of 95.degree.C

4 parts by weight monomeric diallylphthalate

1 parts by weight butyl perbenzoate

10 parts by weight polyvinyl alcohol fibres having a length of 3 mm

48 parts by weight kaolin

The components are mixed well and rolled together in a rolling mill at 80 - 90.degree.C. After cooling the rolled product is ground to particles with a size of approximately 3 mm. The compound can be transfer moulded and injection moulded at a temperature of 160.degree.C. A suitable pressure is 100 kg/cm.sup.2, suitable transfer pressure 300 kg/cm.sup.2 suitable curing time 150 s and suitable injection time 15 s.

Example 4

A compound is manufactured of the following components:

50 parts by weight of a melamin-formaldehyde resin (2 mols formaldehyde per mol melamine)

30 parts by weight calcium carbonate

2 parts by weight zinc stearate

18 parts by weight polyvinyl alcohol fibres having a length of 15 mm

The components are kneaded together in a sigma kneader. When the water has evaporated in the furnace at approximately 50.degree.C, the product obtained is ground to particles having a size of approximately 3 mm. The compound is suitable for compression moulding, injection moulding and transfer moulding. It can be handled under the same conditions as the compound described in Example 3.

Example 5

A moulding compound is manufactured of the following components:

35 parts by weight of an epoxy resin containing of a mixture of 51 percent by weight of a cycloaliphatic epoxy resin part with an epoxy content of 6.2 equ./kg and a viscosity of 1.5 . 10.sup.5 cP at 25.degree.C, 48.5 percent by weight of a curing agent part consisting of a hexahydrophthalic acid anhydride and 0.5 percent by weight of an acceleration part consisting of benzyl dimethylamine

10 parts by weight polyvinyl alcohol fibres having a length of 10 mm

2 parts by weight zinc stearate

30 parts by weight aluminum trihydrate

23 parts by weight zirconium silicate

The components are kneaded together at around 75.degree.C. The doughy compound thus obtained is cured at a temperature of 170.degree.C and a pressure of 100 kg/cm.sup.2.

Example 6

A moulding compound is manufactured of the following components:

30 parts by weight of a resin consisting of 60 parts by weight of percent tetraethylene glycol dimetacrylate, 25 percent by weight of a copolymerisate of equal parts methyl acrylate and methyl methacrylate having a molecular weight of 130,000 and 15 percent by weight of an unsaturated polyester resin produced in conventional manner from diethylene glycol and maleic acid anhydride using 1.1 mol diethylene glycol per mol maleic acid, by boiling at 165 - 190.degree.C to an acid number of 40.

0.5 parts by weight benzoyl peroxide

12 parts by weight polyvinyl alcohol fibres having a length of 25 mm

20 parts by weight cellulose powder having a particle size of around 100 microns

The moulding compound is manufactured and moulded in the same was as the compound described in Example 1.

Example 7

A moulding compound is manufactured and moulded in the manner stated in Example 6 with the exception that the unsaturated polyester resin stated there is replaced by an equal amount of prepolymerised diallylphthalate (for example DAPON from Food Machinery Corporation, USA).

The compounds described in Example 1 - 7 can be used not only for manufacturing extinguishing chamber plates in high voltage circuit breakers, but also for manufacturing other insulating parts which are subjected to the influence of electric arcs in switching devices of different types, such as extinguishing chutes in contactors and isolators.

Claims

I claim:

1. Electric switching device having at least one insulating part of mouldings of a resin binder reinforced with fibre material, which is subjected to influence of electric arcs, in which the fibre material consists essentially of fibres of polyvinyl alcohol fibres or acetalized polyvinyl alcohol.

2. Electric switching device according to claim 1, consisting of a high voltage circuit breaker comprising an extinguishing chamber, in which the insulating part constitutes an extinguishing chamber plate in the extinguishing chamber.

3. Electric switching device according to claim 2, in which the binder consists essentially of an unsaturated polyester resin converted into cured form and containing an acrylate or methacrylate as monomer.

4. Electric switching device according to claim 2, in which the binder consists essentially of a mixture of a linear acrylate polymer and at least one monomer comprising a polyethylene glycol dimethacrylate or polyethylene glycol diacrylate, converted into polymerised form.

5. Electric switching device according to claim 4, in which the content of linear acrylate polymer constitutes 10 - 35 percent and the content of monomer 65 - 90 percent of the total weight of these substances.

6. Electric switching device according to claim 4, in which the binder contains an additive resin containing ethylenically unsaturated groups which are copolymerisable with the unsaturated groups in the polyethylene glycol dimethacrylate or diacrylate, the quantity of additive resin being at the most 25 percent of the total weight of acrylate polymer, monomeric acrylate compound and additive resin.