US3207624A

US3207624A - Lightning arrester valve element and method for making the same

Info

Publication number: US3207624A
Authority: US
Inventors: Lawrence M Burrage; Darrel D Mcstrack
Original assignee: McGraw Edison Co
Current assignee: McGraw Edison Co
Priority date: 1961-07-26
Filing date: 1961-07-26
Publication date: 1965-09-21
Anticipated expiration: 1982-09-21

Description

Sept- 1965 1.. M. BURRAGE ETAL 3,207,624

LIGHTNING ARRESTER VALVE ELEMENT AND METHOD FOR MAKING THE SAME Filed July 26. 1961 ELECTRICAL INSULATING COATING com msmc:

POTASSIUM SILICATE ZINC OXIDE powoznso MICA KAOLIN CLAY LIGHTNING ARRESTER VALVE BLOCK INVENTORS. znwezwc: M. BUR/@965 DHRREL a. NcST/EOCK BY mii f! 1 o a 3 3 J & J

United States Patent "ice 3,207,624 LIGHTNING ARRESTER VALVE ELEMENT AND METHOD FOR MAKING THE SAME Lawrence M. Bur-rage, South Milwaukee, and Darrel D.

McStrack, New Berlin, Wis., assignors to McGraw- Edison Company, Milwaukee, Wis., a corporation of Delaware Filed July 26, 1961, Ser. No. 128,314 9 Claims. (Cl. 117-201) This invention relates generally to negative resistance valve blocks and more particularly to an insulating coating which is applied to portions of the blocks.

Conventional valve type lightning arresters include a series gap which normally insulates the arrester from the line but which sparks over when a predetermined voltage, generally of a relatively large magnitude, is impressed across the gap. In general most present day valve type lightning arresters include a number of gaps in series with one another. Another function that the gap(s) must perform is to interrupt the follow current subsequent to a drop in the impressed voltage, thereby removing again, the lightning arrester from the line.

The series gaps, however, are generally incapable of interrupting currents of the magnitude involved in system faults. As a result some means must be provided to limit follow currents to a magnitude which the series gap(s) can interrupt. This then is the primary function of the valve block.

The valve block must have the capacity to discharge high currents at excessive voltages while increasing its resistance to discharges at normal voltages. A high apparent resistance at low voltages and a low apparent resistance upon high current flow is, then, aprime characteristic of a successful valve block.

One type of block utilized successfully is made up of silicon-carbide particles which are held together by a suitable binder and/or cement such as water glass, clay or the like. Since present day lightning arrester bodies are in general cylindrical in shape these valve blocks also take a similar shape.

Due to the fact that the blocks are composed of silicon carbide crystals which have relatively sharp corners the projecting crystals are subject, during surges, to high voltages stresses which may cause ionization of the air in contact with these surfaces. If we take an imaginary pair of conducting projecting crystals we can well see that if the insulation strength of the air between the crystals is exceeded an arc discharge will occur. Subsequent to this initial breakdown an increase in stress across the remaining crystals and intervening air occurs. Further ionization takes place initiating additional arcs until the cumulative conclusion of the process occurs, viz, complete block flashover.

Once the block .has flashed over there is no way to limit the follow current so that the series gap(s) can interrupt the flow. The result is known as line lockout and is indicative of lightning arrester failure.

Thru the use of a circuit interrupting device of the type described in a patent issued to H. O. Stoelting et al. 2,860,210, November 11, 1958 and assigned to the assignee of the instant application, line lockout can be effectively prevented. However, after the arrester is removed from the line the lightning protection is also lost.

To prevent valve flashover various insulating coatings have been applied to the sides of the blocks. While a few of these insulating coatings have been moderately successful in preventing flashover many of them have offered little or no solution to the problem.

It is therefore our thought that a much improved insulating coating is needed. Such an improved insulating coating must have a number of characteristics which have either singly or jointly been lacking in prior art coatings.

3,207,624 Patented Sept. 21, 1965 Among these characteristics are:

(1) High thermal resistance (2) Good mechanical strength (3) High electrical breakdown strength (4) Simplicity in producing and applying the coating (5) High homogeneity of structure void of pin holes (6) High resistance to cracking.

It is therefore an object of our invention to provide an improved insulating coating which is particularly applicable to lightning arrester valve blocks.

Another object of our invention is to provide an insulating coating which when applied to a lightning arrester valve block, will positively prevent block flashover.

Another object of our invention is to provide an insulating substance which has high thermal resistance.

Another object of our invention is to provide an insulating substance which has high mechanical strength.

A further object of our invention is to provide an insulating substance which has high electrical breakdown strength.

A further object of our invention is to provide an insulating substance which has a high resistance to cracking.

A still further object of our invention is to provide an insulating substance which when set is substantially void of pin holes.

Another object of our invention is to provide an insulating substance which is simple and relatively inexpensive to produce and which may be applied to the surface to be insulated with a great deal of facility.

Other objects and advantages of our invention will be apparent from the following description of the preferred embodiments of the invention taken in connection with the accompanying drawings in which:

FIG. 1 is a perspective view of a valve element to which the improvement described herein has been applied.

Reference numeral 10 indicates the negative resistance valve element which may be composed of fine or coarse particles of silicon carbide, boron carbide or any similar substance which has the characteristics of a good valve element. The insulating coating 11 hereinafter to be described is applied to the curvate side walls of the block as by brushing, spraying, rolling or the like.

We have found that a particularly good insulating substance may be prepared by properly combining the following ingredients in the indicated proportions within the limits described:

The above ingredients are thoroughly mixed, with a small amount of water if necessary, applied to the valve blocks in a uniform thickness, and the coated blocks baked in an oven at temperatures between 225 F. and 450 F. until the coating has set and all of the moisture has been removed. The baking process may take from 1-4 hours depending upon the temperature, the coating thickness, and the amount of water originally added to the mixture.

While it is not absolutely clear why the coated substance functions as well as it does to prevent valve flashover, it is thought that the mica provides high electrical breakdown and mechanical strength while at the same time being somewhat resilient due to its hardened plate like structure. The mica in turn is bonded together The ingredients so far described if used in combination would form a substantially impenetrable film on the surface of the block thereby making it impossible for water to be driven from the block during the baking step. The result would be blistering and cracking of the block sufficient to render it unfit for service. It should be noted that this blistering and cracking will occur irrespective of the amount of water added to the mixture since the constitute ingredients can occlude a certain amount of moisture from the air.

To prevent the above from occurring we add kaolin clay which has for its function the creation of extremely minute channels in the coating, thru which water may exit to the exterior of the coating.

We have found that a coating prepared in the manner above described provides an insulating, punctureproof coating which will not fiashover in operation and which will not crack, craze, spall, or blister.

A very successful coating was prepared with the following percentage by weight combination of the indicated substances The ingredients were thoroughly mixed by ball milling for several hours. The mixture was then sprayed on the blocks to a thickness of approximately .005 inch and the blocks were placed in a drying oven at a temperature of 450 F. for a period of 2 hours. Upon cooling the coated block was tested in a variety of manners including being subjected to surges of 65,000 and 100,000 amperes having respectively a 10 x 20 microsecond wave shape and a 5 x microsecond wave shape.

The same block was also tested at 1500 amperes (10 X 20,18). In all cases the valve block adequately performed its function. In no case did the block flashover or was there puncturing of the valve block coating. The coating was observed to be completely clear of any cracking, spalling, or crazing even after repeated high surge tests. I

While the particular example indicated, proved to be highly successful for our enunciated purposes variations of the ingredients within the limits specified is entirely possible.

While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that various changes and modifications can be made therefrom without significantly departing from the invention and, therefore, it is intended for the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. A method of producing an electrically insulating coating on a block of resistance material including the steps of, applying to said block a composition, of matter comprising by weight substantially:

60-80% of potassium silicate 5-10% of zinc oxide 5-15% of powdered mica 515% of kaolin clay and baking the so coated block for a predetermined time at a predetermined temperature sufiicient to harden said coating.

2. A method of producing an electrically insulating coating on a lightning arrester valve block including the steps of, applying to portions of said block a composition of matter comprising by weight substantially:

71.5% of potassium silicate 7.1% of zinc oxide 10.7% of powdered mica 10.7% of kaolin clay, and subjecting the so coated block to a temperature of 450 F. for a period of 2 hours.

3. A method of producing an electrically insulating coating on a block of resistance material including the steps of applying a substance comprising by weight substantially:

60-80% of potassium silicate 5-10% of zinc oxide 5-15 of mica 5-15% of kaolin clay to said block, and baking the coated block for a period of from l-4 hours at a temperature between 225 F. and 450 F.

4. An electrically insulating composition for coating portions of a lightning arrester valve block consisting essentially of substantially:

71.5% by weight of potassium silicate, 7.1% by weight of zinc oxide,

10.7% by weight of powdered mica, and 10.7% by weight of kaolin clay.

5. An electrically insulating homogeneously united composition of matter composed of approximately:

60-80% by weight of potassium silicate, 5-10% by weight of zinc oxide,

5-15% by weight of powdered mica, and 5-15% by weight of kaolin clay.

6. A valve block for use in a lightning arrester, said valve block having a substantial axial dimension and being characterized by an electrically insulating coating intimately adhered to the outer axially extending surface of said valve block, said coating comprising a major amount of potassium silicate and lesser amounts of zinc oxide, mica and kaolin clay, said zinc oxide cooperating with said potassium silicate to form a refractory cement, the amount of mica being sufficient to provide predetermined electrical breakdown strength, and the quantity of kaolin clay being sufficient to provide exit for moisture contained in said coating.

7. The valve block of claim 6 wherein said coating comprises, by weight, substantially:

60-80% of potassium silicate 5-10% of zinc oxide 5-15% of powdered mica 5-15% of kaolin clay.

8. The valve block of claim 6 wherein said coating comprises, by weight, substantially:

71.5% of potassium silicate 7.1% of zinc oxide 10.7% of powdered mica 10.7% of kaolin clay.

References Cited by the Examiner UNITED STATES PATENTS 8/41 Berkey 338-21 8/61 Munger et al. 106-84 RICHARD D. NEVIUS, Primary Examiner.

JOSEPH REBOLD, Examiner.

Claims

1. A METHOD OF PRODUCING AN ELECTRICALLY INSULATING COATING ON A BLOCK OF RESISTANCE MATERIAL INCLUDING THE STEPS OF, APPLYING TO SAID BLOCK A COMPOSITION OF MATTER COMPRISING BY WEIGHT SUBSTANTIALLY: