WO1997010631A1

WO1997010631A1 - Improvements relating to electrical power distribution

Info

Publication number: WO1997010631A1
Application number: PCT/GB1996/002242
Authority: WO
Inventors: Rodney Meredith Doone
Original assignee: Bowthorpe Industries Limited
Priority date: 1995-09-13
Filing date: 1996-09-11
Publication date: 1997-03-20
Also published as: GB2305310A; AU6936796A; GB9518693D0

Abstract

For enabling ready location of failed polymeric surge arresters in electrical power distribution systems, such surge arrestors commonly being visually intact even when failed, a disconnect device (known per se) is coupled in series with the surge arrester. The operation of the disconnect device can readily be seen. To overcome the problem that equipment coupled to the line is then left unprotected, a spark gap or solid state varistor device is connected in parallel with the disconnect device which has such electrical characteristics as not to prejudice normal operation of the disconnect device in response to surge arrester failure. The spark gap or solid state varistor device provides a path to ground in the event that the line is again struck by lightning.

Description

IMPROVEMENTS RELATING TO ELECTRICAL POWER DISTRIBUTION

Field of the Invention:

This invention concerns improvements relating to electrical power distribution and more particularly concerns the defection of fault conditions in electrical power distribution systems incorporating polymeric surge arresters.

Background of the Invention:

Surge arresters are commonly employed in electrical power distribution systems for protecting power lines against the effects of lightning strikes. The surge arrester is a normally open circuit or high resistance device which goes closed circuit or low resistance under application of a high voltage such as might be experienced when a power line is struck by lightning. Porcelain housed surge arresters used to be the norm for protecting distribution class systems, but have nowadays largely been superseded by polymeric type surge arresters which have superior electrical and other properties. The original proposal for a polymeric surge arrester is described in our British Patent No. 2073965 and the current market leader polymeric surge arrester is described in our British Patent No. 2 133 199. other polymeric surge arresters have ben proposed and are described in US-A-4 656 555, US-A-4 833 433, US-A-4 864 456, EP-A-0 335 480, EP-A-0 372 106, EP-A-0 393 854, EP-A-0 397 163 and EP-A-0 410 644. The present invention has been made with the polymeric surge arrester of our British Patent No. 2 188 199 in mind, but is prima facie applicable to other polymeric type surge arresters including those described in the patent documents listed above. One rationale for the development of the polymeric surge arrester was the disadvantageous behaviour of porcelain housed arresters which have a tendency towards explosive self-destruction under fault conditions with the result that the surrounding environment is showered with hot fragments which can cause fires. Except in the most excessive of fault conditions, polymeric surge arresters tend to maintain their structural integrity under fault conditions and often cannot readily be seen to have failed. A linesman searching for a faulty arrester in a system employing porcelain housed arresters has no difficulty in locating the failed arrester, but this, more often than not, is not the case with power distribution systems employing polymeric arresters. The present invention addresses this problem, which is a known problem. Some utility companies approach this problem by use of complex electronics systems designed to monitor normal and fault currents in polymeric type arresters, but this is a high cost solution and is not favoured on this account. Another proposal which we have employed with some, though limited success, is to retain an indicator, a reflective disc for example, to a member in heat transfer relation to the arrester by means of a fusible material, the arrangement being such that when the arrester fails the heat that is generated causes the fusible material to melt and release the indicator.

Objects & Summary of the invention

It is thus the principal object of the present invention to overcome or at least substantially reduce the aforementioned problem of locating faulty polymeric arresters in an electrical power distribution system. Another object of the present invention is to achieve this aim at affordable cost. According to the present invention, a disconnect device is used with a polymeric arrester in an electrical power distribution system, the disconnect device being connected between the arrester and ground potential, and there is further provided a spark gap or solid state varistor device coupled to provide a circuit to ground for fault current flowing through the arrester after the disconnect device has operated, the impedance characteristics of the spark gap or solid state varistor device being selected in comparison with those of the disconnect device such that the presence of the spark gap or solid state varistor device does not prejudice the operation of the disconnect device.

Disconnect devices are known in the art and have been widely used with distribution class surge arresters, though their use has been somewhat discredited on account of the fact that operation of a disconnect device to remove a ground fault resulting from a failed arrester leaves the power line and, more particularly, the transformer at the end of the line unprotected. An arrester failure caused by a lightning strike on a power line and the resulting operation of an associated disconnect device thus leaves the line and associated equipment exposed to a following lightning strike and, contrary to popular belief, lightning does not uncommonly strike twice in the same place. The preferred disconnect device for use in the practice of the present invention is the kind of device which is currently manufactured by Bowthorpe EMP Limited of Stevenson Road, Brighton,

Sussex (GB) and which incorporates an explosive charge arranged to be detonated when an excessive fault current flows to ground through the device in consequence of failure of the associated arrester. Other disconnect devices are known which incorporate springs retained by fusible material with the spring arranged to release and effect disconnection when the fusible material melts under ground fault current. Such other disconnect devices, though not preferred, could be utilized in the practice of the present invention.

The provision of a disconnect device enables the condition of a polymeric surge arrester readily to be determined by visual inspection, since the operation of the disconnect device, as is well known, leaves the disconnect device cabling trailing from the arrester mounting. To overcome the abovementioned problem that operation of the disconnect device leaves the line and its associated equipment disadvantageously exposed to a following lightning strike for example, the present invention provides a spark gap or solid state varistor device which provides a path to ground for fault current flowing through the failed arrester notwithstanding the operation of the disconnect device. This path for ground fault current effectively protects the line and associated equipment against the effect of a following lightning strike. The existence of the ground fault is readily detected by equipment conventionally provided in the power distribution system, and commonly this or other equipment provides an indication of the area within which the failed arrester is located, and it is then a simple matter for a linesman to identify the actual arrester that has failed and effect its replacement.

The present invention thus provides for the return of disconnect devices, hitherto substantially discredited, to the power distribution field and provides the spark gap or solid state varistor device to overcome the disadvantage which has discredited the use of disconnect devices. This provides a simple and attractive solution to the problem of detecting failed polymeric type surge arresters.

Polymeric surge arresters as described in our British Patent No. 2 188 199, these being the preferred kind of polymeric surge arresters for use in the practice of the present invention, comprise a rigid core formed of zinc oxide varistor blocks and aluminium spacer blocks encased within a shell of fiberglass which s adhered to the surfaces of the respective blocks, and a shedded outer housing of polymeric material which is heat shrunk or mechanically released onto the rigid core or is formed in situ on the core, the core itself and the interface between the core and the outer housing being substantially free of voids and gaseous entrapments. Such polymeric surge arresters have great physical strength and this is an advantageous and attractive feature. In keeping with this form of surge arrester construction, the spark gap or solid state varistor device is preferably of similar high strength construction and may for example comprise a reinforced epoxy resin body within which there is encased a spark gap or varistor device, the body incorporating provision for coupling to the end termination of a polymeric surge arrester at one end thereof and for coupling to a mounting at the other end thereof.

As aforementioned, the impedance characteristics of the spark gap or solid state varistor device have to be selected in comparison to those of the disconnect device such that the presence of the spark gap or solid state varistor device does not prejudice the operation of the disconnect device. In the practice of the present invention therefore, the disconnect device must operate in response to failure of the polymeric surge arrester under lightning strike conditions for example before the spark gap or solid state varistor device begins to pass a substantial ground fault current. This is an essential requirement, since otherwise the spark gap or solid state varistor device could shunt ground fault current away from the disconnect device and thereby inhibit its operation which would defeat the purpose of the present invention. It is within the normal skill and knowledge of the addressee of this specification to construct the disconnect device and the spark gap or solid state varistor device so that these requirements are satisfied. For example, a disconnect device for use with a distribution class surge arrester having a voltage rating of ll to 34 kV might be arranged to withstand 2.5 kV before operating whereas the spark gap or solid state varistor device might be arranged not to operate below say 5 kV.

The above and further features of the present invention are set forth with particularity in the appended claims and will be explained in the following detailed description given with reference to the accompanying drawings.

Brief Description of the Drawings: Figure 1 shows an exemplary arrangement of a polymeric surge arrester, a disconnect device and a spark gap or solid state varistor device in accordance with the teachings of the present invention;

Figure 2 shows an enlarged cross-sectional view of the disconnect device of the arrangement shown in Figure 1 ; and

Figure 3 shows an enlarged cross-sectional view of a spark gap device suitable for use in the arrangement shown in Figure 1.

Detailed Description of the Embodiment:

Referring to Figure 1, shown therein is an exemplary assembly according to the teachings of the present invention. As shown, a polymeric surge arrester 1 which, in use, will have its upper terminal end coupled to an overhead power line for example, has its lower terminal end coupled to ground through a disconnect device 2 and a parallel-connected spark gap device 3, the construction of the last-mentioned two devices being shown in Figures 2 and 3. More particularly, an externally screw-threaded stud that is provided at the lower terminal end of the surge arrester 1 is received within an internally screw¬ threaded boss forming part of the spark gap device 3 and the two are thus engaged with each other with a galvanized steel strap 4 engaged therebetween. The disconnect device 2 is secured at its upper end to the strap 4 and is coupled at its lower end via a grounding cable 5 to the lower end of the spark gap device 3 whereat there is provided an earth terminal assembly comprising an externally screw-threaded stud and plural lock nuts and washers. An earthed mounting strap 6 completes the assembly and provides for the mounting of the assembly to a line pole for example. The polymeric surge arrester 1 is preferably constructed in accordance with the teachings of our British Patent No. 2 188 199 but, as previously mentioned herein, is not restricted to such. As recited in claim 1 of our British Patent No. 2 188 199 such a surge arrester comprises an elongate rigid core constituted by a stack of varistor blocks held in face to face contact between first and second terminal blocks by virtue of said varistor blocks and terminal blocks being encased within a rigid shell of reinforced rigid plastics material bonded to the peripheral surfaces of the terminal blocks, and a shedded outer housing for said core comprising a preformed sleeve of polymeric heat-shrink material or elastomeric material shrunk or fitted tightly onto said core, the interfaces between the varistor and terminal blocks and their encasing shell and between the shell and the outer housing being voidless and free of gaseous entrapments. More particularly, as described in detail in the specification of our British Patent No. 2 188 199, a preferred form of polymeric surge arrester comprises metal (zinc) oxide varistor blocks, aluminium alloy heat sink/spacer blocks and terminal blocks structurally combined within a glass reinforced plastics shell which is bonded to the outer cylindrical surfaces of the blocks. The varistor blocks, heat sink/spacer blocks, terminal blocks and the glass reinforced plastics shell constitute a unitary structural arrester core of great physical strength wherein the facing surfaces of the respective blocks are held in face to face physical and electrical contact without air entrapment or bleed of plastics material. A heat-shrink sleeve with integral sheds of alternating greater and lesser diameter is shrunk about the arrester core with inter- positioning of a fluid mastic material to ensure that the interface between the heat-shrink sleeve and the outer surface of the arrester core is free of voids or air entrapment and cannot be ingressed by moisture. Stainless steel end caps are fitted to each end of the arrester with a silicone rubber or like sealant filling the spaces between the interior of the end caps and the arrester core, and are retained by stainless steel terminal assemblies which are screw- threadedly engaged with the terminal blocks with seals provided to prevent moisture ingress into the mated screw threads. As compared to an equivalent porcelain housed surge arrester, a surge arrester constructed as described above has the significant advantage of displaying a non-explosive failure mode and affords yet further advantages in that it is light weight and yet is very strong and robust and is resistant to damage through vandalism and improper handling and is unaffected by atmospheric pollutants and impervious to moisture ingress. Polymeric surge arresters constructed in accordance with these teachings are available from Bowthorpe EMP Ltd. , Stevenson Road, Brighton, East Sussex BN2 2DF, England (GB) .

Referring now to Figure 2, shown therein is a cross-sectional view of an XP10C disconnect device that is available from Bowthorpe EMP Ltd. aforementioned. As shown, the device comprises upper and lower case mouldings 7 and 8 formed of synthetic plastics material which are ultrasonically welded together. The case mouldings enclose upper and lower arcing inserts 9 and 10 which are formed of brass and which define between them a spark gap 11. The upper brass insert 9 is tapped at 12 to accept a mounting bolt, employed in Figure 1 for retaining the disconnect device 2 to the strap 14 , and the lower brass insert 10 is tapped at 13 to accept a cable connector 14, employed in Figure 1 for connecting the grounding cable 5 to the disconnect device 2. The upper end of the cable connector 14 is recessed at 15 to define a carrier for a blank 5.6mm (0.22 inch) rifle cartridge. Within the body of the disconnect device there is a central chamber which contains a linear resistor 16, a brass retaining disc 17, a phosphor bronze spring 18 and a tubular ceramic (porcelain) spacer 19. The construction of the disconnect device as thus described places the spark gap 11 in parallel with the series circuit constituted by the upper brass insert 9, the spring 18, the resistor 16, the brass disc 17 and the lower brass insert 10 with its coupled cable connector 14. Under normal transient conditions arising for example through normal breakdown of the varistor elements in the surge arrester 1 as the result of a lightning strike, for example, the spark gap 11 in the disconnect device passes the transient current safely to ground until the varistor elements reset, but under fault conditions where the surge arrester passes a significant ground fault current virtually continuously the resistor 16 is rapidly heated to such an extent that the cartridge retained in the recess 15 explodes and shatters the plastics body of the disconnect device. The grounding cable 5 then falls free of the strap 4. The disconnection of the grounding cable 5 provides a readily visible indication that the surge arrester 1 has failed, since otherwise the disconnect device would not have operated. Were it not for the provision of the spark gap or solid state varistor device 3, the surge arrester 1 would now be disconnected from ground and equipment coupled to the power line would no longer be protected against, say a following lightning strike. The provision of the spark gap or solid state varistor device 3 however ensures that in the event of a following lightning strike on the line a path to ground continues to be provided for ground fault current, thereby ensuring that equipment connected to the line continues to be protected.

Figure 3 is an enlarged cross-sectional showing of an exemplary spark gap device 3. The device is constructed to exhibit high physical strength since it serves in the arrangement of Figure 1 for supporting the surge arrester 1. The device 3 comprises a reinforced synthetic resin material body portion 20 within which there is captured an upper spark gap electrode 21, a lower spark gap electrode 22 and a ceramics material spacer 23. As shown, the upper electrode 21 is formed with an internally screw¬ threaded bore 24 for engagement with the lower end termination of the surge arrester 1 in the assembly of Figure 1, and the lower electrode 22 has an externally screw-threaded spigot 25 for coupling to the grounding cable 5 and the mounting strap 6. The spark gap is defined between the facing ends of annular portions 26 and 27 of the electrodes 21 and 22, and a spring- loaded cylindrical metal slug 28 is soldered into an accommodating bore in the upper electrode. The slug 28 will normally be retained out of the path of ground fault current which will be passed by the spark gap, but in the event of an excessive ground fault current flowing for too long, the solder will melt and release the slug 28 which, under the action of spring 29, will move to bridge the gap between the two electrodes 21 and 22.

As mentioned hereinbefore, the spark gap defined in the spark gap device 3 of Figure 3 is so dimensioned in relation to the spark gap of the disconnect device 2 of Figure 2 that the operation of the disconnect device 2 is not prejudiced by the presence of the spark gap device 3. For a distribution class system wherein the surge arrester 1 is rated at 11 kV to 34 kV for example, the spark gap in the connect device might be set to arc over at around 2.5 kV and that in the spark gap device might be set so that it will not arc over below, say, 5 kV. The present invention thus provides a ready means of detecting short circuit failures in power distribution systems employing polymeric surge arresters without exposing connected line equipment to damage from subsequent lightning strikes. The invention is subject to modification and variation without departure from the ambit of the invention as set forth in the appended claims. Thus, whilst the surge arrester 1 is preferably constructed in accordance with our British Patent No. 2 188 199 alternative solid state, polymer housed surge arresters are known which could be utilized in the practice of the invention. Likewise, alternative disconnect devices are known and could be used in the practice of the invention. The spark gap device of Figure 3 is in all respects exemplary and could be replaced by an equivalent solid state varistor device employing silicon carbide or metal (zinc) oxide varistor elements for example.

Claims

CLAIMS :

1. In or for an electrical power distribution system, the combination of a polymeric surge arrester, a disconnect device for connection between the ground terminal of the surge arrester and ground potential, and a spark gap or solid state varistor device for connection in parallel with the disconnect device, the electrical characteristics of the disconnect device and of the spark gap or solid state varistor device being such that the presence of the latter in circuit with the former and with the surge arrester does not prejudice the operation of the former.

2. The combination of claim 1 wherein the polymeric surge arrester comprises an elongate rigid core constituted by a stack of varistor blocks held in face to face contact between first and second terminal blocks by virtue of said varistor blocks and terminal blocks being encased within a rigid shell of reinforced rigid plastics material bonded to the peripheral surfaces of the terminal blocks, and a shedded polymeric outer housing for said core comprising a preformed sleeve of heat-shrink material or elastomeric material shrunk or fitted tightly onto said core or formed in situ on the core, the interfaces between the varistor and terminal blocks and their encasing shell and between the shell and the outer housing being voidless and free of gaseous entrapments.

3. The combination of claim 2 wherein the varistor blocks of the surge arrester are zinc oxide varistor blocks.

4. The combination of any preceding claim wherein the disconnect device incorporates an explosive charge.

5. The combination of claim 4 wherein the disconnect device is substantially as herein described with reference to Figure 2 of the accompanying drawings.

6. The combination of any preceding claim wherein the surge arrester and the spark gap or solid state varistor device are physically interconnected with an electrically conductive strap provided therebetween, the disconnect device is mounted to said strap at one end, and a grounding cable is coupled to the other end of the disconnect device. 8. The combination of claim 6 wherein the grounding cable is coupled at its other end to the ground end of the spark gap or solid state varistor device.

9. The combination of any preceding claim wherein the spark gap device comprises first and second electrodes defining a spark gap between them, the electrodes being secure din a body formed of reinforced synthetic resin material.

10. The combination of claim 9 wherein one of said electrodes is provided with a spring-loaded slug of electrically conductive material arranged to be moved so as to bridge the spark gap in the event of an arc running in said gap for an excessive time period.

11. The combination of any of claims 1 to 8 wherein the solid state varistor device comprises a silicon carbide varistor.

12. The combination of any of claims 1 to 8 wherein the solid state varistor device comprises a metal oxide varistor, e.g. a zinc oxide varistor.

13. In or for an electrical power distribution system, a combination as claimed in claim 1 and substantially as herein described with reference to Figure 1 of the accompanying drawings.

14. An electrical power distribution system incorporating at least one combination as claimed in any of the preceding claims.

15. A spark gap or solid state varistor device for use in the combination of any of claims 1 to 13.