US2296621A - Lightning arrester - Google Patents

Description

E. J. WADE Sept. 22, 1942.

LIGHTNING ARRESTERS Filed Aug. 18. 1939 nvento Elmer J. Wade Hl 5 Att OPT-1e S by )Van/17 Patented Sept. 22, 19.42v

LIGHTNING ABRESTEB Elmer J. Wade, Pittsfield, Mass., assignor to General Electric Company, a corporation -of New York y Application August 18, 1939, Serial No. 290,855

Claims.

My invention relates to lightning arresters and more particularly to arresters of the arc expulsion type. In this type of arrester a hollow insulating tube is provided'with a conductive metal electrode at each end, one of the electrodes being grounded and the other electrode being spaced by a gap from an electrical conductor whichis to be'protected. One or both of the electrodes extend into the tube to provide an internal gap short enough to compel any discharge from the protected conductor to the grounded electrode to follow a path across the internal gap instead of a path between the electrodes along the outer surface of the tube. The insulating tube is composed of some suitable material such as hard fiber or is lined with some suitable material to give off quantities of gas under the influence of the heat of an are and the evolution of this gas is so rapid that a high pressure is created inside the tube by the arc to blow it out through one of the electrodes which is provided with an opening for this purpose. The line electrode nearest the .protected high voltage conductor tends to asume a voltage determined by the capacity between this electrode and the high voltage circuit and the capacity 'between thiselectrode and the grounded electrode and any other nearby grounded objects. The line electrode is usually much nearer the protected high voltage line than it is to the grounded electrode or any other grounded object, so that it tends to assume a voltage veiy much above the grounded electrode. The distribution of this high voltage along the insulating tube is also not uniform. 'I'his non-uniform distribution of voltage tends to cause a discharge along the outer surface of the tube rather than across the internal gap even though the internal gap provides a shorter path for the discharge. The high voltage on the inner end of the line electrode also tends to produce corona at this point and this corona may in time attack the adjacent inner surface of the tube and result in a deterioration of the tube and a shortening of its useful life.

' The general object of the invention is to provide an impruved lightning arrester of the arc expulsion type including means for reducing the voltage of the line electrode to eliminate corona within the tube and to improve the distribution ofthe voltage along the outer surface of the tube to make it more certain that any discharge between the electrodes will follow a path across the internal gap instead of a path outside of the tube.

It is also an object of my invention to provide a new and improved lightning arrester which will be reliable in operation and which will have a relatively long life in service.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which Fig. 1 is a view of a short section of a three-phase high voltage electrical power circuit equipped with arc expulsion arresters constructed in accordance with the invention; Fig. 2 is a longitudinal sectional view of one of the arc expulsion arresters; vand Figs. 3 and 4 are enlarged sectional detail views of the two ends of the arrester tube with their respective electrodes.

In the arrangement shown in Fig. l, a supporting pole I0 with a cross-arm II supports the three conductors I2 of a three-phase high voltage electrical circuit. Brackets I3 secured to the pole I0 and cross-arm II and connected to a ground Il support three arc expulsion arresters I5, one for each of the three high voltage conductors I2. Each arrester tube I! includes an insulating tube IU provided with a grounded electrode I1 secured to its supporting bracket I3 and a line electrode I8 spaced by a suitable gap from the corresponding high voltage conductor I2. The insulating tube includes two tightly telescoped tubes I9 of hard fiber or any suitable material which will give off a quantity of gas when heated by an arc. The line electrode It is in the form of a conductive cap of aluminum or other suitable metal and has a flange threaded onto the line end of the insulating tube. The line electrode IB also includes a portion or metal pin 20 extending some distance into the opening 2| of the insulating tube and which may be spaced from the inside walls of the tube, as is illustrated in Figs. 2 and 3. The ground electrode I1 is also in the form of a cap with a flange threaded onto the adjacent end of the insulating tube, this ground electrode I'l having an opening 22 communicating with the opening 2I in the insulating tube. The ground electrode I1 may also include a metal tube 23 extending into an enlarged portion of the adjacent end of the opening 2| within the insulating tube. The opening 22 in the electrode I1 and the opening in the metal tube 23 should be as large in cross-section as the opening 2| in the insulating tube so that the path of the gases expelled from the insulating tube may not be restricted.

In order to reduce the normal voltage between the electrodes I'I and I8 resulting from the proximity of the high voltage conductor I2, the outer surface of the insulating tube has a layer or coating 2l of high resistance material which is extended along the threaded outer surfaces at each end of the tube so as to make good contact with the two electrodes I1 and I8. Thus deterioration of the inner tube walls adjacent the electrodes, and particularly adjacent the line electrode, due to corona formation at the electrodes, or within the tube, during normal voltage conditions is substantially precluded so that the reliability of operation of the resulting device is improved and its life made longr. It has been found possible to secure better and more uniform resistance characteristics if a sizing coating 25 is first applied to the insulating tube and then the resistance coating itself applied along the surface of the tube over this sizing coating. 'I'he sizing coating 25 may be a film of Bakelite varnish or it may be a few wraps of thin paper impregnated with Bakelite varnish. This sizing coating provides a suitable surface to which the resistance coating 24 will adhere satisfactorily and a surface to which a more uniform resistance coating may be applied than if it were applied directly to the outer insulating tube I9 which may be more or less porous. The sizing coating 25 further protects the resistance coating 24 from any moisture which may be present or which mayvreach the insulating tubes I9. After the resistance coating 24 has been applied, a protective coating 26 is applied over it to protect it from external.

moisture and from mechanical injury. This outside protective coating 26 may be a coating of Bakelite varnish or a few wraps of paper or cloth or both, impregnated withBakelite varnish. The sizing coating 25 and the protective coating 26, as well as the resistance coating 24, preferably extend slightly under the flanges of both electrodes I'I and I8 and the edges of these iianges are preferably spun down tightly onto the outer surface of the protective coating 26 to exclude moisture. Each of the coatings or layers 24, 25 and 26 may be cured by the application of heat to make their characteristics more permanent. Various materials may be used in the high resistance coating 24, excellent results having been obtained, however, with Bakelite varnish in which finely divided carbon black is distributed. In place of the carbon black, various other conductive and semi-conductive materials may be used, such as lamp black, graphite, tungsten sulphide, chromite, tin oxide, molybdenite, copper oxide, lead peroxide and silicon carbide. The resistance material forming the high resistance coating 24 may be applied to the surface of the sizing coating 25 by means of a spray gun. Very pulses which is extremely reliable and which has a very long life in service.

y The invention has been explained by describing and illustrating a particular construction of an arc expulsion tube and various modifications of details but it is obvious that other'changes may be made without departing from the spirit of the invention and the scope of the appended claims.

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

1. A lightning arrester including an insulating tube of gas emitting material, an electrode at each end of the tube. at least one of the electrodes extending into the tube tol provide an internal gap shorter than the external distance between the electrodes, and a high resistance coating along the outer surface of the tube and connecting said electrodes to increase the uniformity of distribution of the voltage between the electrodes whereby deterioration of the inner tube walls adjacent the electrodes due to corona formation at the electrodes during normal voltage conditions is substantially precluded.

uniform results have been secured by rotating the tube in a lathe and applying the resistance coating from a spray gun mounted to slide at a fixed distance along a path parallel with the tube.

It` has been found that elimination of corona at the inner end of the line electrode I8 and a good distribution of the voltage between the electrodes I'I and I8 along the surface of the tube are secured when the resistance of the resistance coating 24 is of the order of from 5 to 20 megohms in a tube of such size that the length of the resistance coating was about 22 inches. This is a resistance of about one quarter to one megohm per inch of the resistance coating. With a tube of this character, the normal voltage of the line electrode I8 was reduced from about 25,000 volts to approximaetly 1000 volts and the uniformity of distribution of', the voltage along the surface of the tube was much improved. The comparatively low voltage of the line electrode I 8 due to the resistance coating 24' eliminated all corona within the tube and the uniformity of the voltage distribution along the outer surface of the tube made the appearance of an external arc between the electrodes extremely unlikely.

'Ihe invention provides an improved lightning arrester of the arc expulsion type for discharging tube, at least one of the tween the electrodes, means for preventing buri i-Jv or interrupting high voltage, high frequency im- 2. A lightning arrester including an insulating tube of gas emitting material, an electrode at each end of the tube, at least one of the electrodes extending into the tube to provide an internal gap shorter than the external distance between the electrodes, a sizing coating on the outer surface of said tube, and a high resistance coating on said sizing coating and connecting said electrodes to increase the uniformity of distribution of the voltage between the electrodesl 3. A lightning arrester including an insulating tube of gas emitting material, an electrode at each end of the tube, at least one of the electrodes extending into the tube to provide an internal gap shorter than the external distance between the electrodes, a sizing coating including a varnish along the outer surface of said tube, and a high resistance coating on said sizing coating and connecting said electrodes to increase the 'uniformity of distribution of the voltage between the electrodes, said resistance coating including a varnish and a finely divided and at least semiconductive material distributed in the varnish.

4. A lightning arrester including an insulating electrodes extending into the tube to` provide an internal gap shorter than the external distance between said electrodes, means for preventing burning of the inner walls of said tube by corona forming on said one electrode during normal line voltage conditions comprising a high resistance coating along the outer surface of the tube and connecting said electrodes, a protective coating of insulating material provided over said high resistance coating so as substantially to preclude the alteration of the electrical characteristics of` said coating during use of said arrester.

5. A lightning arrester including an insulating tube of gas evolving material, an electrode at each end of the tube, at least one of the electrodes extending into the tube to provide an internal gap shorter than the external distance be-f;

ing of the inner walls of said tube by corona' forming on said one electrode during normal linevoltage conditions comprising a high resistance coating provided along the outer surface of said tube, a coating of weather-proof material provided over said high resistance coating so as s ubstantially to preserve its electrical characteristics.

ELIVIER J. WADE.

Images