GB2224884A - Surge arresters - Google Patents

Abstract

A gas discharge voltage surge arrester comprises a cylindrical tubular housing (1) of a semi-transparent ceramic material in the opposite ends of which are located two electrodes (2) and (3) having peripheral flanges secured to the ceramic housing (1) by brazing washers (4). The oppositely disposed end faces of the electrodes (2) and (3) define a spark gap (9) and the interior of the surge arrester is filled with an inert gas or mixture of inert gases. The surge arrester includes means in the form of an electroluminescent medium, such as a phosphor layer (6), to cause the surge arrester to emit light upon the occurrence of an overvoltage pulse across the electrodes and thereby facilitate the switching of the surge arrester at a substantially constant level.

Description

SURGE ARRESTERS The present invention relates to gas discharge surge voltage arresters such as are in common use to protect electrical and electronic equipment from high voltage transients and surge voltages.

Generally such surge voltage arresters comprise two or more spaced electrodes which are maintained in their spaced relationship by a housing of insulating material containing an inert gas or gas mixture. The housing is made of a ceramic or glass and hence is least partially transparent to light. It may be of tubular or cylindrical form having two electrodes arranged at and closing its ends. An apertured inner mediate electrode may be disposed between the end electrodes.

It is known that the ignition voltage or switching time of gas filled surge arresters can e reduced and made consistent by the inclusion of a ver small amount of radioactive material (tritium, Ni63, thorium, etc.). When radioactive material is not present in such devices, it is found that if the tubes are kept in darkness for several hours, gas breakdown requires significantly higher voltages compared to when radioactive nuclei are present. Subsequent discharges, if initiated shortly afterwards, occur at voltages typical for the gas discharge tubes containing radioactive material. However there is a gradual return to the higher ignition voltages if subsequent discharges are delayed for more than few minutes and tubes are again kept in darkness.Thus, the switching time performance of radioactive-free gas discharge tubes is generally inconsistent.

However, the presence of visible or ultra violet light on the arrester restores in radioactive-free gas discharge tubes the desirable switching time performance which is inherent in devices containing radioactive nuclei. Moreover the illumination of the surge arrester reduces its breakdown voltage when a rapidly rising surge voltage (impulse sparkover voltage) is applied. This is strong evidence for photo-triggering of the discharge phenomenon in gas filled surge arresters. Therefore it is apparent that the ignition or breakdown voltage can be stabilised and reduced not only by radioactive decay but also by photons.

It is an object of the present invention to provide a gas discharge surge voltage arrester which does not include any radioactive material but which has a reproducible and consistent switching time performance even when in darkness.

From one aspect the invention provides a cas discharge surge voltage arrester having a substan t - ially constant switching time performance which is achieved by including a light emitting means to illuminate the interior of said arrester in response to an applied overvoltage pulse.

From another aspect the invention provides a gas discharge surge voltage arrester comprising a housing of insulating material containing spaced electrode surfaces in an atmosphere of an inert gas and includ ing means responsive to an electrical overvoltage pulse applied to said arrester to emit light such as to facilitate the switching of said surge arrester at a substantially consistent level.

The invention further provides a method of operating a non-radioactive gas discharge surge voltage arrester so as to achieve a practically consistent switching time performance which consists in illuminating the interior of said arrester with light generated upon the application of an overvoltage pulse to said arrester.

The light emitted may be visible light or ultra viclet light.

According to a preferred form of the invention the illumination of the interior of the surge arrester is achieved with the aid of an electrcluminescent medium. More specifically, a phosphor material such as a zinc or cadmium oxide, sulphide or selenide is energised to emit light by an overvoltage pulse applied to the surge arrester. The luminescent material may be applied as a layer or coatIng either externally or internally to the wall ov the housing c= the arrester.

The principle empioyed in the present invention is that the energy of a surge voltage pulse applied to the arrester excites the luminescent material which in turn emits light into the interior of the arrester, either directly or through the at least semi-transparent material of the housing. The light initiates an ionisation process in the interelectrode region or at the electrode surface which results in a prompt gas breakdown of the arrester.

The results achieved give a reproduceable and consistent relatively fast switching time characteristic to the surge arrester without the use of any radioactive material within the surge arrester.

Instead of employing an electroluminescent material, other means may be used to generate light within the surge arrester upon the application of an overvoltage pulse. For example a photodiode arrangement may be employed.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view through one embodi ment of surge arrester according to the invention, Figure 2a is a side view of a second embodiment and Figure 2b is a larger scale scrap sectional view in the region II-II of Figure 2a, Figure 3a is a side view of a third embodiment and FIgure 3b is a larger scale scrap sectional view in the region III-III of Figure 3a, Figure A is a sectional view through a =urtner emcaiment of surge arrester according to the invent- ion, Figure 5 is a perspective view of a cylindrical ceramic member forming part of a further embodiment of surge arrester according to the invention; and Figure 6 is a similar perspective view of another embodiment of cylindrical ceramic member.

In all of the drawings, the surge arresters have been drawn much larger than their actual size and moreover the various layers shown have been drawn with exaggerated thickness for the sake of clarity.

Referring to Figure 1, the surge arrester illustrated is of well known basic form, and comprises a cylindrical tubular housing 1 of a semi-transparent ceramic material in the opposite ends of which are located two electrodes 2 and 3 having peripheral flanges secured to the ceramic housing 1 by brazing washers 4. The oppositely disposed end faces of the electrodes 2 and 3 define a spark gap 9 and the interior of the surge arrester is filled with an inert gas or mixture of inert gases. As is well known in the art, the surge arrester electrodes may also be coated with an activating or emissive material but in the devices of the invention no radioactive material is present.

According to the present invention, means are provided -to cause the surge arrester to emit light upon the occurrence of an overvoltage pulse across the electrodes and thereby facilitate the switoning cf the surge arrester at a substantiall consistent To this end, in =e present emboaiment, at east tart of the outer surface of the ceramic housing is coated with a transparent conducting film 5 for example by spraying with a tin salt at about 500-600"C. "is conducting film 5 is then coated with a layer 6 of an electroluminescent phoso9?r in an organic binder of high dielectric constant. When the layer 6 is drv it is in turn coated with a layer 7 of barium titanate also in an organic binder and finally this layer 7 is coated with an electrically conducting paint 8, such as graphite. As can be seen the conducting film 5 is connected to the electrode 2 and the electrically conducting paint 8 is connected to the electrode 3.

When a surge voltage pulse is applied across the electrodes, the phosphor layer 6 emits light thereb triggering the gas discharge in the surge arrester.

It will be understood that in practice the layers 5 to 8 are very thin, e.g. some tens of microns thick.

Instead of forming the transparent conducting film 5 on the cylinder 1 as described above, this film may be formed by a piece of transparent electrically conducting plastics film which is wrapped around the cylinder.

In the embodiment of Figures 2a and 2b the light emitting means is formed by engraving a thin tin oxide conducting film applied to the ceramic cylinder 1 so as to form two concentric spirals 11, 12 with a gap of about 30 to 40 microns between them. The spirals are coated with a phosphor 13as is shown in Figure 2b and each spiral forms one of the electrodes of a luminous capacitor. One end of spiral 11 is in ohmic contact wit electrode 2 and one end of spiral 12 is in comic contact with electrode 3, so that the phosphor emits lIght into the interior of the surge arrester upon the application of an overvoltage pulse and thereby triggers the gas discharge. In this embodiment the barium titanate and graphite layers are not required.

Figures3a and 3b show a modification of the embodiment in Figures 2a and 25 in which two fine insulated wires lla, 12a are wound spirally around the ceramic cylinder 1 instead of using an engraved tin oxide film. Each wire serves as one of the electrodes of a capacitor having the phosphor 13 as its dielectric and the device operates in a similar manner to the previous embodiment.

Figure 4 illustrates an embodiment which is a modification to the embodiment of Figure 1 and wherein the various layers forming the light emitting means are applied to the interior surface of the ceramic cylinder 1. Thus layer 8a of electrically conducting paint, such as graphite is coated on the inner wall of the cylinder and is connected to electrode 2. This layer is covered by barium titanate layer 7a on which is deposited the electroluminescent phosphor layer 6a.

Finally, the transparent conducting film Sa, e.g. of a tin salt, is appliedsto the layer 6a and is connected to the electrode 3. As stated above the layers are very thin e.g. some 30-40 microns thick. The device operates in the same manner as previously described.

Figure 5 illustrates diagrammatically a variat- ion of the embodiments shown in Figures 2a, 2 and 3a, 3b in which the engraved tin salt or wire spirals 11, ila and 12, 12a are formed on tne inner wall c= tne ceramic cylinder 1 instead of on tne outer surface The electrodes 2 and 3 are omitted for the sake cf clarity but the respective spirals are ohmically connected to the respective electrodes as previously.

The inner wall of the ceramic cylinder is coated with the electroluminescent phosphor material over te spirals.

Figure 6 illustrates another arrangement cf capacitor electrodes formed on the inner wall of the ceramic cylinder 1. These capacitor electrodes 14, 15 are arranged in pairs in the axial direction of the cylinder with the capacitor electrodes 14 extending to one edge and able to make ohmic contact with the electrode 2 of the gas discharge tube and the capacitor electrodes 15 extending to the opposite edge and able to make ohmic contact with the other electrode 3 of the gas discharge tube. The capacitor electrodes may be formed of thin wire, an engraved tin salt or a graphite line (pencil stripe) and the inner surface of the ceramic cylinder 1 is then coated with the electroluminescent material. As in the previous embodiment, the electrodes 2 and 3 of the gas discharge tube have been omitted for the sake of clarit.

An engraved tin salt or wire electrode may have a thickness of some 40 microns and a pencil stripe may have a thickness of up to 200 microns. The distance between two electrodes of a pair may be about 70 microns and the spacing of the unconnected end cf any capacitor electrode from the edae of the cylinder may be about 0.5mm.

whilst particular embodiments have been descr- ibed it will be understood that various modifications may be made without departing from the scope of his invention.

Claims (1)

1. (1) A gas discharge surge voltage arrester having a substantially constant switching time performance wherein said arrester includes a light emitting means to illuminate the interior of said arrester in response to an applied overvoltage pulse.

   (2) A gas discharge surge voltage arrester comprising a housing of insulating material containing spaced electrode surfaces in an atmosphere of an inert gas and including means responsive to an electrical overvoltage pulse applied to said arrester to emit light such as to facilitate the switching of said surge arrester at a substantially consistent level.

   (3) A gas discharge surge voltage arrester as claimed in claim 1 or 2, wherein the light emitted is visible light or ultra violet light.

   (4) A gas discharge surge voltage arrester as claimed in claim 1, 2 or 3 wherein the illumination cf the interior of the surge arrester is achieved with the aid of an electroluminescent medium.

   (5) A gas discharge surge voltage arrester as claimed in claim 4, wherein the elec,roLuminescent medium is a phosphor material, such as zic or cadmium oxide, sulphide or selenide, energisable te emit light by an overvoltage pulse applied to the surge arrester.

   (5) A gas discharge surge voltage arrester as claimed in claim 4 or 5, wherein the luminescent material is applied as a layer or coating either externally or internally to the wall cf the housing cf the arrester.

   (7) A gas discharge surge voltage arrester as claimed in claim 4, 5 or 6, containing two spaced electrode surfaces, and also including electrical conductors which are electrically connected to the electrode surfaces, respectively, and between which the electroluminescent material is located.

   (8) A gas discharge surge voltage arrester as claimed in claim 7, wherein the electrical conductors consist of two electrically conductive coatings.

   (9) A gas discharge surge voltage arrester as claimed in claim 8, wherein the electrically conductive coatings form two concentric spaced-apart spirals.

   (10) A gas discharge surge voltage arrester as claimed in claim 7, wherein the electrical conductors form the electrodes of a capacitor and the electroluminescent medium forms the dielectric of the capacitor.

   (11) A method of operating a non-radioactive gas discharge surge voltage arrester so as to achieve a practically consistent switching time performance which consists in illuminating the interior of said arrester with light generated upon the application of an overvoltage pulse to said arrester.

   (12) A method as claimed in claim 11, wherein the light emitted is visible light or ultra violet light.

   (13) A method as claimed in claim 11 or 12, wherein the illumination of the interior of the surge arrester is achieved with the aid of an electroluminescent medium.

   (14) A gas discharge surge voltage arrester substantially as herein described with reference to Figures 1 or 2a and 2b or 3a and 3b or 4 or 5 or 6 of the accompanying drawings.

   (15) A method of operating a non-radioactive gas discharge surge voltage arrester substantially as herein described with reference to the accompanying drawings.