Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-05-27
2001-02-13
Young, Lee (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C361S117000, C338S021000
Reexamination Certificate
active
06185813
ABSTRACT:
The present invention relates to the field of lightning arresters.
It is particularly applicable to lightning arresters for high voltages, typically in electricity networks having a nominal r.m.s. voltage between phases that is greater than 1 kV.
Lightning arresters are devices designed to be connected between ground and an electricity line, in particular a medium-voltage line or a high-voltage line, so as to limit the amplitude and the duration of any voltage surges that appear on the line.
Such voltage surges may, for example, be due to atmospheric phenomena such as lightning or induction in the conductors.
Such voltage surges may also be due to operations being performed on the line while it is live.
In general, a lightning arrester is formed by a stack of various varistors, the stack usually nowadays being a stack of disks based on zinc oxide, whose resistivity is strongly non-linear as a function of the applied voltage.
More precisely, such varistors allow almost no current to pass so long as the voltage across their terminals is less than a triggering threshold, and they allow very high currents of as much as several tens of kA to pass when the voltage applied across their terminals exceeds the above-mentioned triggering threshold.
The number of varistors used in the lightning arrester is such that the nominal operating voltage on the electricity line is lower than the triggering threshold across the terminals of the stack of varistors.
Thus, the lightning arrester can withstand the nominal operating voltage on a continuous basis without there being any leakage current, while also making it possible to drain off very high discharge currents that can appear temporarily over the line in the event of an accidental voltage surge occurring.
Numerous types of lightning arrester have already been proposed.
Indeed, the field of lightning arresters has generated very abundant literature.
Today, a known lightning arrester generally comprises:
a stack of varistors;
two contact parts made of an electrically conductive material and placed at respective ends of the stack of varistors; and
a cover made of an electrically insulating material surrounding the stack of varistors.
The above-mentioned cover made of an electrically insulating material has itself been the subject of very abundant literature.
For example, Document GB-A-2 073 965 proposes making the cover from a heat-shrinkable material.
Documents U.S. Pat. No. 4,298,900, DE-A-3 001 934, DE-A-3 002 014 propose also placing an outer housing made of porcelain over the heat-shrinkable cover.
Documents U.S. Pat. No. 4,092,694 and U.S. Pat. No. 4,100,588 propose placing each varistor in a silicone-based ring, and disposing the resulting stack of varistors surrounded in this way inside a porcelain housing.
Document U.S. Pat. No. 2,050,334 proposes placing a stack of varistors in a porcelain housing and filling the space formed between the porcelain housing and the stack of varistors with a filler material formed, for example, of a halogenated compound based on wax.
Documents EP-A-0 008 181, EP-A-0 274 674, EP-A-0 231 245, and U.S. Pat. No. 4,456,942 propose making the cover around the varistors from an elastomer material, the cover being formed in particular by overmolding.
More precisely, Document EP-A-0 274 674 proposes molding a cover over a stack of varistors, which cover is made of a composite material based on elastomer, EPDM, silicone, or some other resin that may optionally be filled.
Document U.S. Pat. No. 4,161,012 also proposes disposing a cover made of elastomer over the varistors. That document proposes making the cover by depositing elastomer over the outside surfaces of the varistors, or by molding the cover over the varistors, or else by preforming the elastomer cover, and then inserting the varistors into said cover.
As early as 1958, Document U.S. Pat. No. 3,018,406 proposed making the cover in the form of two preformed complementary shells, and an outer cover of a plastics material injection molded over the varistors.
Document U.S. Pat. No. 3,586,934 proposes making the cover from a synthetic resin, e.g. a resin based on epoxy or on polyester, or even a silicone or polyester varnish.
Document EP-A-0 196 370 proposes making the cover over a varistor body by casting a synthetic resin formed, for example, of epoxy resin, of polymer concrete, of silicone resin, or of an elastomer, or by covering the varistor body with a shrinkable tube made of a plastics material, or else by providing the stack with a layer of synthetic resin.
Furthermore, Documents U.S. Pat. Nos. 4,656,555, 4,905,113, 4,404,614, EP-A-0 304 690, EP-A-0 335 479, EP-A-0 335 480, EP-A-0 397 163, EP-A-0 233 022, EP-A-0 443 286, and DE-A-0 898 603 propose making the cover surrounding the stack of varistors from composite materials made up of fibers, generally glass fibers, impregnated with resin.
More precisely, Document U.S. Pat. No. 4,656,555 proposes firstly forming a winding of fibers based on a plastics material, such as polyethylene, or based on glass, or even on ceramic, optionally impregnated with resin, e.g. epoxy resin, and then forming a housing over the outside of the winding, which housing is made of a weather-resistant polymer material, e.g. based on elastomer polymers, synthetic rubber, thermoplastic elastomers, or EPDM.
That document proposes more precisely either preforming the weather-resistant polymer housing, and then engaging the stack of varistors as provided with the fiber winding into the housing, or else firstly forming the fiber winding over the stack of varistors, and then making the weather-resistant polymer housing by molding it over the winding, by spraying polymer over the winding, or by inserting the stack of varistors as provided with the winding into a bath of polymer.
Document U.S. Pat. No. 4,404,614 proposes disposing the following successively over a stack of varistors: a first cover based on glass fibers impregnated with resin, e.g. epoxy resin, then a second cover based on glass flakes and on epoxy resin, and finally a resilient outer cover based on EPDM rubber or on butyl rubber.
That document indicates that the first cover, the second cover, and the outer cover may be put in place successively over the stack of varistors, or the covers may be formed in reverse order.
The document also mentions the possibility of molding the outer cover over the second cover based on glass flakes and on epoxy resin.
Document EP-A-0 233 022 proposes forming the following over a stack of varistors: a shell based on glass fibers reinforced with epoxy resin, then an elastomer-based cover that is heat-shrinkable or that can be released by equivalent mechanical means on said shell.
In a variant, the cover can be molded in situ and can be based on a synthetic resin or on a polymer material.
That document indicates that the shell may be preformed. That document also proposes to use a sheet of pre-impregnated fibers.
Document EP-A-0 304 690 proposes firstly making a filamentary winding of glass fibers impregnated with resin, then to form a coating of an elastomer material of the EPDM type over the outside of the winding by injection.
Document EP-A-0 355 479 proposes placing the following in succession on the stack of varistors: firstly a barrier formed of a film of plastic, e.g. based on polypropylene, then a winding of non-conductive filaments, and finally a housing made of weather-resistant elastomer.
Document EP-A-0 397 163 proposes placing the following in succession over the stack of varistors: a filamentary winding impregnated with resin, followed by a coating of elastomer, e.g. EPDM, over the winding, the coating being formed by injection.
The technique of using a composite material is very old.
Indeed, as early as 1946, Document DE-A-0 898 603 proposed using glass fibers impregnated with resin for varistor covers.
More recently, Document FR-A-2 698 736 has proposed a method of manufacturing a lightning arrester, which method comprises steps consisting in making a stack of varistors, in forming a first cover of a composite material over
Blakely & Sokoloff, Taylor & Zafman
Smith Sean
Soule Materiel Electrique
Young Lee
LandOfFree
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