GB2050719A - Surge arrester - Google Patents

Abstract

An electrical surge arrester comprises a porcelain insulator housing (1) having a bore (15) formed with integral spacer ribs (16) which extend helically of the bore axis. Within the bore (15) there is mounted a voltage dependent resistor arrangement comprising, for example, a series array of spark gaps and voltage dependent resistor elements. A pressure relief diaphragm (22) is provided at one end of the bore (15) and is arranged to rupture on fingers (23) in the event of an overpressure occurring within the bore. A gas flow deflector (not shown) is arranged to deflect gas venting from the bore when the diaphragm (22) ruptures down along the length of the insulator housing (1). An end cap (20) supports the diaphragm (22) and the fingers (23) by virtue of having portions (26) turned over a shoulder (21) formed on the insulator housing (1), and also provides support for a terminal post (25). <IMAGE>

Description

SPECIFICATION Improvements in electrical surge diverters This invention concerns improvements in electrical surge diverters (also known as surge arresters) as used particularly, though not necessarily exclusively, in electrical power distribution and transmission systems for the safe handling of atmospherically induced surges (lightning strikes for instance) and overvoltages caused by switching operations.

As is well known, surge arresters comprise a series arrangement of spark gaps and non-linear resistors contained in the bore of a glazed porcelain insulator housing. In the event of a voltage surge on a line connected to the arrester, the spark gaps flash over and put an earth fault on the line system for the duration of the surge. The power follow current which can flow through the arrester is limited by the series non-linear resistors to a value which can be cleared by the spark gaps.

Magnetic spark gap arrangements are known which serve to rotate and expand the struck arc for the purpose of ensuring uniform distribution of arc energy over the whole spark gap surface -- this reduces deterioration of the spark gaps with the passage of time and enables high performance of the arrester on both power frequency and impulse sparkovers even after years of onerous service.

The series arrangement of spark gaps and nonlinear resistors is conventionally sealed in the porcelain housing which is commonly vacuum dried and then filled with a specially dried inert gas prior to final sealing. More especially in the case of larger sized, so-cailed station class diverters which are available in both light and heavy duty classifications for all system voltages from say 3kV to 245kV for example, but also in the case of low voltage and distribution diverters generally used as a protection from damage due to lightning strikes for equipment connected to overhead distribution lines up to about 33kV, it is known to provide a pressure relief diaphragm at one or both ends of the insulator housing bore adapted to rupture on fingers provided for the purpose in the event of an overpressure occurring in the gas filling of the diverter as a result of the expansion of the gas filling occurring when the diverter operates to divert a current surge to earth. The diaphragm protects the porcelain housing of the diverters against explosive shattering as otherwise might occur, the diaphragms being designed to rupture before pressure within the housing reaches a level at which the housing might shatter.

Conventionaily the series spark gaps/non-linear resistors arrangement was sized to be a fairly close fit within the bore of the insulator housing. It was then found that an improved performance under gas expansion conditions was obtained by oversizing the bore relative to the spark gaps/nonlinear resistors arrangement to provide easier gas glow conditions within the insulator housing when the diverter is under stress. Spacer elements were provided to locate the spark gaps/non-linear resistors arrangement within the oversized insulator bore, the spacer elements being formed of synthetic plastics material and being formed so as to permit gas flow axially of the bore through the annular space between the centrally located spark gaps/non-linear resistors arrangement and the surrounding bore wall.

According to one apsect of the present invention, the porcelain insulator housing has integrally formed spacer ribs within its bore for locating the spark gaps non-linear resistors arrangement, the spacer ribs preferably extending continuously throughout the length of the bore and exhibiting a generally helical twist in their extent from one end of the bore to the other.

Improved pressure relief diaphragm arrangements are provided at one end at least of the bore, and a gas flow deflector is provided at such end or ends of the bore as are provided with a pressure relief diaphragm, the gas flow deflector being adapted and arranged for deflecting gases venting from said bore when said pressure relief diaphragm is ruptured around the outside of the insulator housing towards the opposite end thereof.

It is currently believed, though we have not been able to establish the correctness of this belief, that the "rifling" of the bore of the insulator housing provided by the helical spacer ribs formed therein imparts a rotation about the bore axis to gas venting under pressure from the ruptured pressure relief diaphragm in consequence of an excessive overpressure occurring within the bore, This rotation of the venting gas, it is believed, results in the action of the gas flow deflector being better defined so as to produce, in such blow-out situations, a more compact and better defined deflected gas shroud around the outside of the insulator housing which, by virtue of plasma effects, is capable of sustaining the discharge to earth which is the prime function of the diverter.

The invention, together with additional features and advantages thereof, will be best understood from consideration of the following detailed description of exemplary embodiments thereof which are illustrated, partially, in the accompanying drawings wherein: Figure 1 is an axial sectional view showing a prior art form of surge arrester; Figure 2 is an axial sectional view showing the insulator housing of a surge arrester according to the invention provided with helical spacer ribs within its bore; Figure 3 is an end elevation of the insulator housing shown in Figure 2; and Figures 4 to 8 illustrate different end seal and pressure relief diaphragm arrangements which are usable with the insulator housing of Figures 2 and 3.

Referring first to Figure 1 , the surge arrester shown therein is the EMP 5000 amp duty class valve type distribution lightning arrester available from Bowthorpe-EMP and described in their leaflet DBA/8. This arrester is well known in the art and will be described herein only to such an extent as is necessary for the differences between the known arrester and the hereinafter described arrester according to the invention to be appreciated.

As shown, the arrester of Figure 1 comprises a shedded porcelain insulator body 1 having a central bore 2 within which is packed a series arrangement of schematically illustrated spark gaps 3, voltage dependent resistors 4 and spacers 5 all well known in the art and not to be described herein. An upper end terminal 6 is sealed to the end face of insulator body 1 by means of a high quality compression seal 7, and is protected by means of a metal shroud 8 secured to the insulator body 1 by virtue of having a turned edge 9 engaged with a slightly outwardly diverging upper end portion of the body 1.A lower end terminal 10 is constituted by a metal plate 11 turned to embrace the lower end of the insulator body 1 with a compression seal 12 interposed between the body 1 and plate 11, and a terminal connector part 13 sealed to the central part of the plate 11 which, as shown, projects into the end of the bore 2. The arrester as illustrated is intended for bracket mounting, a waist portion 14 of the insulator body 1 providing a seat for a mounting clamp. As is also known, the lower end terminal 10 can incorporate an earth lead disconnect device, for example incorporating a small explosive charge, such that in the unlikely event of arrester failure, from whatever cause, the arrester will automatically disconnect itself from earth, thereby preventing a continuing earth fault on the system.

Referring now to Figures 2 and 3, a shedded porcelain insulator of generally conventional external configuration is shown having an internal bore 1 5 with generally rectangular, internal, integrally formed spacer ribs 1 6 extending longitudinally thereof with a helical twist; that is to say, the ribs 1 6 extend generally helically within the bore 1 5. As shown most clearly in Figure 3, four ribs 1 6 are provided and each rib occupies about 90 of the total circular extent of the bore in its extent from one end of the insulator to the other; that is to say, each rib extends from a starting point at one end of the bore to a point at the other end of the bore which is spaced from the starting point by an angular dimension, relative to the bore axis, of 900.

It will be seen that the longitudinal curvature of the ribs 1 6 has not been shown in Figure 2 for convenience, and that Figures 2 and 3 are to slightly different scales.

Referring now to Figure 4, there is shown therein an exemplary end seal and pressure relief diaphragm arrangement for an arrester/diverter constructed with the insulator body of Figures 2 and 3. For convenience, the ribs 1 6 are shown in Figure 4 as extending parallel to the bore axis, but it will be appreciated that they will preferably be helical as in Figures 2 and 3. A rubber sealing ring 17 is seated on the ground end face of the insulator body 1, and seated on the sealing ring 1 7 is a metal plate 1 8 having a central portion depressed into the end of the insulator bore 1 5 as shown, the ends of the ribs 1 6 being cut away or otherwise stopped short of the end face of the insulator as shown to permit the central portion of plate 1 8 to enter the end of the bore 1 5.A number of gas escape apertures 1 9 are formed through the plate 18, only one of these being shown in Figure 4. A capping element 20 formed of spun metal is seated on top of plate 1 8 as shown and has an edge turned about the shoulder 21 formed on insulator body 1 so as to secure the capping elernent to the insulator. A pressure relief diaphragm 22 is captured around its edges between the plate 1 8 and the capping element 20, being sealed thereto for example by means of silicon rubber cement. Sharp ended fingers 23 are released from the capping element 20 as shown arid are turned towards the diaphragm 22 such that in response to an overpressure within bore 15, the diaphragm 22 will balloon outwardly and be ruptured by the fingers 23.The release of the fingers 23 from the capping element 20 provides gas escape openings 24 in the capping element 20. A terminal post 25 is secured to the capping element 20. An annular metal retaining element 26 completes the task of retaining the capping element 20 on the insulator body. In use, a shroud similar to the shroud 8 shown in Figure 1 could be provided to protect the assembly of Figure 4 from the elements, but preferably a concave gas diverter is provided to deflect gases venting via apertures 24 down round the outside of the insulator body 1.

Figure 5 shows an end seal and pressure relief arrangement similar to that of Figure 4 but with a slightly different, concentrically corrugated, diaphragm and with the sharp ended fingers (serving for rupturing the diaphragm in the event of an overpressure occurring) formed separately of the capping element. The same reference numerals are used in Figure 5 as are used for similar and identical parts in Figure 4.

Figure 6 shows an end seal pressure relief arrangement similar to that of Figure 5 but with the fingers 23 for rupturing the diaphragm 22 supported from the terminal post 25.

Figures 7 and 8 show end seal and pressure relief arrangements similar to those of Figures 5 and 6 respectively, except that the plate 18, which serves as a locator for one end of the series spark gaps/voltage dependent resistors combination of the diverter/arrester, is replaced in the Figures 7 and 8 arrangements by a generally equivalent combination of a member 27 which rests upon the spark gaps/voltage dependent resistors combination and has a head portion 28 with a spherically domed surface which mates with a complementarily shaped depression in a member 29 affixed to the capping member 20, the diaphragm 22 being trapped between the complementarily shaped surfaces of the members 28 and 29 as shown. These arrangements permit a certain amount of self-adjusting movement of the member spark gaps/non-linear resistors which ensures that it is soundly seated, atop the combination.

The aforedescribed arrangements are in all respects merely examples of constructions falling within the scope of the invention and it will be appreciated that many variations and modifications can be made without departure therefrom.

Claims (14)

1. An electrical surge arrester comprising a porcelain insulator housing formed with an internal bore having one or more spacer ribs formed integrally with the bore wall and extending generally helically within the bore relative to the axis thereof.

2. An electrical surge arrester as claimed in claim 1 wherein the one or more spacer ribs extend continuously throughout the axial length of the bore.

3. An electrical surge arrester as claimed in claim 1 or 2 wherein a pressure relief diaphragm is provided at least at one end of the bore, said diaphragm being rupturable in the event of an over-pressure occurring within the bore, and a gas flow deflector is provided associated with the or each such diaphragm and adapted and arranged for deflecting gases venting from the bore when the associated diaphragm ruptures around the outside of the insulator housing and towards the opposite end thereof.

4. An electrical surge arrester as claimed in claim 3 including at least one finger located adjacent said diaphragm and adapted and arranged for assisting the rupture thereof in the event of such an overpressure occurring within the bore as to deform the diaphragm into contact with the fingers and thereby effect rupture thereof.

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5. An electrical surge arrester as claimed in claim 3 or 4 wherein the diaphragm is concentrically corrugated for ease of deformation in the event of an overpressure occurring within the bore.

6. An electrical surge arrester as claimed in any of claims 3 to 5 wherein the diaphragm is retained by means of a capping element having an edge thereof turned over a shoulderformed on the external surface of the insulator housing.

7. An electrical surge arrester as claimed in claim 6 as dependent upon claim 4 wherein the said at least one finger is released from the capping element.

8. An electrical surge arrester as claimed in claim 6 or 7 wherein the capping element serves as a support for a terminal post which has an inner end juxtaposed with the diaphragm and spaced by the diaphragm from a member which serves to locate the voltage dependent resistor arrangement of the arrester axially within the bore.

9. An electrical surge arrester as claimed in claim 8 wherein the inner end of the terminal post, the juxtaposed portion of the diaphragm, and the contacting surface of said locating member are formed to permit a degree of self-adjustment of the locating member to ensure it a sound seating atop the voltage dependent resistor arrangement.

10. An electrical surge arrester as claimed in -any of the preceding claims wherein the housing is substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings.

11. An electrical surge arrester as claimed in any of the preceding claims and having a pressure relief diaphragm and end capping arrangement substantially as herein described with reference to any one of Figures 4 to 8 of the accompanying drawings.

12. An electrical surge arrester comprising a voltage dependent resistor combination located within a bore of a porcelain insulator housing having a pressure relief diaphragm and end capping arrangment substantially as herein described with reference to any one of Figures 4 to 8 of the accompanying drawings.

1 3. An electrical surge arrester as claimed in claim 12 wherein the bore oF the insulator housing has one or more integrally formed spacer ribs extending generally helically within the bore relative to the axis thereof.

14. An electrical surge arrester comprising a porcelain insulator housing formed with a bore, voltage dependent resistor elements housed within said bore, a pressure relief diaphragm at at least one end of said bore, one or more rupture fingers mounted proximate said diaphragm for rupturing the same in the event of deformation thereof due to an overpressure within the bore, and an end capping member securing the diaphragm and the rupture finger(s) and serving as a mounting for a terminal post.

1 5. An electrical surge arrester as claimed in claim 1 4 wherein spacer ribs are formed integrally with the walls of said bore and extend generally helically relative to the bore axis.