US2694102A - Electrostatic shielding - Google Patents

Description

1954 R. B. KAUFMAN ELECTROSTATIC SHIELDING Filed Aug. 1, 1952 Inventor: Richard B; KauFrnan,

H'i-s Attorney United rates Patent C) ELECTROSTATIC SHIELDING R ch r B- Kaufman, Pittsfi ld, Mass, a ignor t enlal E ct ic Company, a corporation .of New York Application August 1, {1952, Serial No. 302,288 '2 Claims. or. 174- 140) Th s i nt n ae e to el ctro t tic shielding an more particularly to a new and improved corona shield.

It is well known that when two spaced electrodes or electrical conductors having a dielectri or electrical insulating medium therebetween are subjected to a difference of potential, an electrostatic stress is set up in the insulating medium. It is also well known that if one of the electrodes is smaller than the other one or if it has relatively sharp corners, edges, or points, which amounts to the same thing, that the stress concentrations in the insulating medium will be highest in the vicinity of the smaller area electrode. It is not .so ,well known that if the insulating medium is partly solid and partly fluid with the solid insulation adjacent the larger electrode and the fluid insulation adjacent th Smaller electrode that the stress concentrations adjacent the smaller electrode are usually still further increased. This is because the dielectric constant of most solid insulating materials is higher than the dielectric constant of most gaseous or liquid insulating materials so that the specific capacitance effect is lower in the fluid insulation than it is in the solid insulation and consequently, as the voltage across series capacitors divides in inverse proportion to their capacities, the stress (or voltage drop) will be highest in the vicinity of the smaller electrode which is immersed in the fluid insulating means.

There are a number of well-known ways of reducing such high stress concentrations. For example, the size of the electrodes or their spacing or both may be increased, or any sharp edges or corners or points on the effectively smaller electrode may be rounded off or bare metal shields may be connected to the electrode where high stress concentrations exist so as in effect to round off and increase its effective area. However, all of such means are usually relatively expensive and often diflicult to apply because of space limitations.

In accordance with this invention, there is provided a novel shield which is characterized by a solid insulated third or auxiliary electrode or conductor which is directly connected to the smaller main electrode or the electrode adjacent to which there are high stress concentrations in a fluid insulating means and which solid insulated conductor extends into the region of such high stress concentrations. In this manner, not only is the effective electrode configuration so changed as greatly to reduce the stress concentrations but also solid insulation, which can better withstand high stress concentrations than fluid insulation, is placed in the region where the highest stress concentrations occur. A simple and inexpensive form of insulated shielding electrode is an ordinary insulated wire or cable which, being relatively flexible, can easily be shaped to a configuration which will be most effective for shielding.

The invention is particularly well adapted to situations where a high potential lead or conductor is to be passed through a wall or cover of a metallic housing for electrical apparatus. This is usually done by providing a hole in the housing through which either an insulated cable is led or else an insulating bushing is inserted in the hole and electrical connections made to the ends of its center or through conductor. Very often in such situations, the stress concentrations at the edge of the opening or hole in the casing are sufliciently high to cause corona, and I have found that this can easily and inexpensively be prevented by wrapping a length of ordinary insulated wire or cable around the lead or bushing adjacent the hole in the casing and connecting 2,694,102 Patented Nov. 9, 1 954 2 directly the ends of the length of Wire or cable to the metal casing.

An .Object of the invention is to provide a new andimproved electrostatic shield.

Another object of the invention is to provide a novel,- simple, and inexpensive highvoltage bushing substitute.-

A further obje f t invention is to provi 1. proved corona shielding arrangement for electrical lead-" incondll i tS- The nven ion will be be ter un e s od f om th 15 lowing description taken in pany ng vdrtw ng and th appende la m In th draw the single ur is .a s d l ati n view par ly in sect n of a prefe d em im n o th invention.

Re er now to h d aw n th r is s ow h in a metal plate w c may co stitu a par o n s closing casing for electrical apparatus (not shown). la is p ov ded w th h 2 whic ord n il .W b rou d a d ha e s a edg R sins thr u h th ol i a len th o .qo duct .4 w sh Ord na i w l b str h an s ci s a r ss sestiqn n h h i extend through the hole 2 axially thereof. Solid insulation 5 of any suitable type surrounds or covers the conductor 4. ;For example, .the conductor 4 and the insulation 5 may constitute an ordinary high voltage electricalcableor the insulation 5 may ,be considered as being'a high'voltage insulating bushing interposed -between ,cohhctor [stand the plate 1 which then would in ,effect' be the equivalent .of'the ordinary ,ground flange on such a bushing." The whole'de'vice is shown 'immersed in air but it will, of course, be understood that any other well-known insulating fluid either liquid or gaseous may be substituted for air and in some cases there may be gaseous insulation on one side of the plate 1 and liquid insulation on the other side of the plate 1 as, for example, in the case of ordinary liquid-filled transformers or circuit breakers.

Electrostatically, metal plate 1 and conductor 4 constitute two spaced electrodes between which there is solid insulation 5 which surrounds the electrode 4 and fluid insulation in which the electrode 1 is immersed. Ordinarily, the plate 1 will be at substantially ground potential whereas the electrode or conductor 4 will have a potential very much different than ground potential. For example, in the case of a high voltage alternating current system, the potential of conductor 4 will vary sinusoidally between peak values very much above and below the ground potential of plate 1. Also, conductor 4 will ordinarily be subjected to still higher occasional transient voltage peaks, such as are caused by lightning or switching surges. Under these conditions, the regions of highest stress concentration would ordinarily be adjacent the edges 3 of the hole 2 because these present small electrode areas relative to the electrode area of the conductor 4 and the edges 3 are in a dielectric medium having a lower dielectric constant than the solid insulation 5 which surrounds the conductor 4. In many arrangements of this kind, the stress concentration in the regions adjacent the edges 3 are sufiiciently high to produce corona and even flash-over with their attendant objectionable results, which not only include losses but also destructive effects and objectionable radiation effects.

I have found that a very simple and inexpensive way of entirely eliminating such objectionable high stress concentrations is to wrap a length of ordinary insulated wire or cable 6 around the solid insulation 5 where it passes through the hole 2, and preferably for an appreciable distance beyond the hole on both sides as shown in the drawing. At least one end, and preferably both ends of the insulated conductor 6 are solidly connected to the plate 1 as shown in the drawing at 7. Such connections may be made in any well-known manner such as by soldering, brazing or bolting.

By means of the connections 7, the edges 3 of the hole 2 are completely shielded because the conductor 6 is at the same potential as the plate 1, so that there is no stress in the region between the hole 2 and the wound insulated wire 6. At the same time, high stress ice its scop Will b poi e ou in c nn t on wi h the acwm regions in the air or fluid dielectric at the boundaries of the solid insulation 5 are eliminated, because of the presence of the solid insulation on the insulated wire 6 which surrounds and closelyembraces the insulation 5. In other words, solid insulation is placed in the regions where highest stress concentrations are most likely to occur. Stating the matter another way, the arrangement shown in the drawing shields the regions, near the edges 3, of maximum electrical stress by moving the electric field from those regions into regions of higher dielectric strength, namely, the solid insulation of the member 6.

Another advantage of the arrangement is that there is solid insulation in intimate contact with both metal electrodes, where it is most eifective. In other words the wire 6 becomes the electrode, which heretofore had been the edges 3 of the hole 2 in the plate 1, and solid insulation surrounds the Wire 6. The invention also eliminates all insulating fiuid, except thin films which are strong, from the electric field. This is a decided advantage because, as heretofore explained, fluids are the weak link in composite insulation structures composed of solid and fluid in series.

While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not Wish to be limited thereto since many modifications may be made, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a flat plate of metal provided with a sharp-edged round hole therethrough, a straight length of cylindrical electrical conductor extending through and beyond said hole axially thereof, cylindrical solid insulating means surrounding said conductor where it passes through said hole and in the vicinity of both sides of said hole, and an electrostatic shield for the region adjacent the edges of said hole comprising a length of flexible electrical conductor covered by a substantially equal length sheath of flexible electrical insulation coiled around said cylindrical solid insulating means in said hole and in the vicinity of said hole and having both ends of said flexible conductor electrically connected directly to said metal plate.

2. In combination, a fiat plate of metal providedwith a sharp-edged round hole therethrough, a straight length of insulated electric cable extending through and beyond both sides of said hole axially thereof, and a corona shield for the region adjacent the edges of said hole comprising a length of flexible wire covered by a substantially equal length sheath of flexible electrical insulation coiled around said length of cable at and beyond both sides of where the latter passes through said hole and having both ends of said wire electrically connected directly to said metal plate, adjacent turns of said coiled insulation sheath of said wire being in contact with each other.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,933,008 Emanueli Oct. 31, 1933 1,972,590 Higgins Sept. 4, 1934 FOREIGN PATENTS Number Country Date 254,401 Great Britain July 5, 1926 474,703 Great Britain Nov. 5, 1937

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