Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
1999-07-09
2001-05-22
Riley, Shawn (Department: 2838)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
C174S11000P, C174S044000
Reexamination Certificate
active
06235992
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric device for medium and high voltage transmission and/or distribution lines containing as insulating filler a silicone-based composition having improved chemical-physical characteristics, in particular, dielectric strength and compressibility.
2. Description of the Related Art
It is known that electric devices for medium and high voltage transmission and/or distribution lines, such as circuit breakers, insulated switch-gears, power and distribution transformers, instrument transformers, surge arresters, cable terminations, pole heads, bushings, insulators and similar, generally contain suitable free volumes filled with an insulating filler. In particular, said insulating filler, which can be solid or semisolid, liquid or gaseous, must basically guarantee a desired dielectric strength among the parts of the device having different voltage due to the normal electrical functioning.
At the present state of the art, use of well known insulating fillers for different applications and/or specific needs presents some drawbacks.
For example, when liquid insulating fillers are used, typically dielectric oils, compensating volumes must be employed to counteract effects caused by thermal expansions of insulating liquids; in the case of gaseous insulating fillers, such as sulphur hexafluoride (SF
6
) or nitrogen and/or their mixtures, systems monitoring the gas pressure as well as gas filling up devices are necessary to keep insulating properties unchanged.
In addition, in both case of gaseous and liquid insulating fillers, special devices and security systems must be used to prevent and/or to detect filler leakages which could cause misfunctioning of the electric device as well as environmental pollution.
This affects the electric devices in terms of structural complexity and also overall reliability of the insulating system.
In some applications, solid or semisolid materials, such as polyurethane, silicone foams or rubbers, are also used as insulating fillers. In particular, such solid fillers give the advantage to prevent leakages from the electric device as well as to eliminate special devices and security systems; but, on the other hand, they present the main problem of requiring accurate process controls to reach a good quality of adhesion to the surrounding parts of the device. As a matter of fact, in the case of defective and non-homogeneous adhesion to the electric device surrounding parts, inceptions of destructive electric discharges may happen due to air filtering; these electric discharges could cause the electric device breakdown.
In order to avoid this drawback, adhesion promoters have to be used and all surfaces must be treated with primers so as to obtain a good and homogeneous filler adhesion; such treatments are expensive and complicated especially in the case of electric devices having complex geometry and in particular when functional elements such as cables, mechanical rods, connections under voltage are present.
Another problem related to the use of known solid or semisolid insulating fillers is due to the intrinsic properties of such materials generally presenting high coefficients of thermal expansion associated to a negligible, like a liquid, compressibility; this fact represents a limit to the applications.
For instance, the use of cross-linked silicone elastomers for electric applications as insulating fillers, is already known. These materials have a thermal expansion coefficient of about 10
3
C
−1
and a low compressibility, comparable to the typical value of a liquid; since in these applications the silicon elastomer is inserted in closed spaces and expands because of a possible heating, particurarly in the case of high-voltage applications, damages to the electric device could happen.
SUMMARY OF THE INVENTION
The main task of the present invention is to overcome the above described drawbacks by realising an electric device for medium and high voltage transmission and/or distribution lines, containing as insulating filler a material which, when inserted in a free volume to be filled, could saturate said volume thereby achieving dielectric strength levels necessary to the electric device functioning.
Within the scope of the aforesaid task, another aim of the present invention is to realise an electric device for medium and high voltage transmission and/or distribution lines, comprising an insulating filler having improved compressibility in order to be applicable within a wide functioning temperature range without requiring compensating volumes.
A further aim of the present invention is to realise an electric device for medium and high voltage transmission and/or distribution lines, comprising an insulating filler having an optimum adhesion on the internal surfaces without requiring surface treatments and adhesion promoters.
Yet another aim of the present invention is to realise an electric device for medium and high voltage transmission and/or distribution lines, having an insulating filler material with no filler leakage problems, so as to eliminate all related monitoring and signalling systems normally used in already known devices.
This task, as well as these and other aims which will become apparent in greater detail hereinafter, are achieved by an electric device for medium and high voltage transmission and/or distribution lines having a free volume V undergoing to electrical stress, said volume V being filled with an insulating filler, characterised in that said insulating filler include a compressible silicone-based composition having a volume under normal conditions ranging from 1.01 to 1.2 V, at the temperature of 25° C. Electric devices for medium and high voltage transmission and/or distribution lines are to be considered for voltages of above 1000 Volts. In practice, the free volume V of the electrical device is filled with an extra-volume of a compressible silicon-based composition, said extra-volume ranging from 1.01 and 1.2 V when measured at 25° C.
In particular, such silicone-based composition has a good compressibility thus enabling to compensate possible thermal shrinkage and/or expansions over a wide functioning temperature range without requiring additional compensating volumes. Said compressibility can be advantageously obtained by mean of hollow compressible micro-spheres. Generally said micro-spheres have diameters ranging from 5 &mgr;m to 100 &mgr;m and are made of plastic material; in particular said microspheres are based on organic polymeric material, such as, polyacrylonitrile, polyvinyl chlorides, polyvinyl acetates, polyesters, polycarbonates, polyethylenes, polystyrenes, polymethyl methacrylates, polyvinyl alcohols, ethylcellulose, nitrocellulose, benzylcelloluse, epoxy resins, hydroxypropylmethylcellulose phthalate, copolymers of vinyl chloride and vinyl acetate, copolymers of vinyl acetate and cellulose acetobutyrate, copolymers of styrene and maleic acid, copolymers of acrylonitrile and styrene, copolymers of vinylidene chloride and acrylonitrile and similar substances.
Examples of suitable micro-spheres are those commercially available under the trade mark Dualite® by Pierce and Stephens Corp., and those sold under the trade mark Expancel® by Akzo Nobel.
The silicone-based composition generally comprises the following constituents:
a) a crosslinkable polyorganosiloxane; and,
b) an organosilicon crosslinker.
An example of crosslinkable polyorganosiloxane is given by compounds having general formula:
in which R is a monovalent hydrocarbon group having up to 18 carbon atoms, R′ is a monovalent hydrocarbon or hydrocarbonoxy group, a hydrogen atom or a hydroxyl group, and x is an integer having a value of from 10 to 1500.
Examples of organosilicon crosslinker can be selected from silanes, low molecular weight organosilicon resins and short chain organosiloxane polymers.
Further characterising features and advantages of the invention will be more clearly from the description of preferred, but not exclusive embodiments of
Abisso Alberto
Signorelli Manuela
ABB Research Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Riley Shawn
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