EP0186688A1

EP0186688A1 - High voltage switching device

Info

Publication number: EP0186688A1
Application number: EP19850903030
Authority: EP
Inventors: Thomas Mark White; Robert Thomas Frederick Samwell
Original assignee: Electricity Council
Current assignee: Electricity Council
Priority date: 1984-06-21
Filing date: 1985-06-20
Publication date: 1986-07-09
Also published as: WO1986000464A1; AU4440085A

Abstract

Un commutateur à haute tension comporte une enveloppe extérieure électriquement conductrice, un commutateur sous vide et un mécanisme d'actionnement situé dans l'enveloppe. L'enveloppe est remplie d'un matériau isolant solide d'encapsulage. Une ouverture et une niche conique pratiquées dans l'enveloppe forment une partie de réception femelle d'une connexion à haute tension, si bien que le commutateur peut être monté directement sur une douille de connexion mâle. Le commutateur peut être un détecteur de fuite ou un coupe-circuit.A high voltage switch has an electrically conductive outer shell, a vacuum switch, and an actuation mechanism located in the shell. The envelope is filled with a solid insulating encapsulating material. An opening and a conical niche formed in the casing form a female receiving part of a high voltage connection, so that the switch can be mounted directly on a male connection socket. The switch can be a leak detector or a circuit breaker.

Description

HIGH VOLTAGE SWITCHING DEVICE

The present invention is concerned with high voltage switching devices , particularly as might be used in electric power distribution systems, eg for the control of cable circuits and the protecti on of transformers provided at local sub-stations .

It is common practice in power distribution systems for a number of local sub-stations to be connected to a single high voltage distribution supply cable from a primary sub-stati m. Typically the high voltage supply cable operates at 11 V in the United Kingdom and the local sub-station has a distribution transformer providing low voltage supplies to local users at the usual mains voltage . It is a required practice that protection be provided at each o f the distribution trans formers in the local sub-stations . Hi therto this protection has normally taken the form of High Breaking Capacity (KBC) fuses connecting the transformer to the high voltage system, together with a switch for isolating the transformer - rcm the system. Normally , the high voltage cables are arranged as an cpen ring e.g. from a primary sub-station interconnecting various

< ^• local sub-stations . The suppl to each sub-station distribution transformer is spurred off the ring via an HBC fuse and switch arrangement. A fault in the transformer resulting in fault current being drawn through the fuse operates the fuse . The connection to- the distribution transformer is made at a point on the ring main between two series connected oil immersed switches. The switch associated with the fuses is arranged to open auto¬ matically in response to the operation of the HBC fuse . Thus the transformer is isolated.

However, provision of such automatic switch/fuse combinations at each local sub-station is expensive especially bearing in mind that the number of failures of such transformers is relatively low so that most such protection arrangements are not required to operate throughout their lifetime . Furthermore, the above described fuse protection is known only to provide re liable protection against faults on the high voltage cable to the distribution transformer and in the transformer high voltage winding. Some low voltage transformer winding faults and low voltage busbar faults may result in insufficient high voltage current to operate the HBC f ses .

In the above described normal arrangement, a fault in a particular transformer has the effect of isolating only that one transformer from the ring main. Other sub-stations on the ring are still supplied.

Nevertheless, there has been substantial investigation into alternative methods of protecting distribution transformers which might be ^'cheaper yet still at least as effective and compliant with necessary safety requirements. A most useful element in an alternative protection arrangement would be a high voltage switching device which is relatively inexpensive, yet reliable and capable of operating safely at the voltages in question, typically 11 kV. Such ^'switches could also have a number of- other applications in the local distribution network, such as circuit breaking and sectionalising.

High voltage switchgear used hitherto in these applications has been expensive and bulky. Typical known switches are oil filled with mechanically operating toggle mechanisms for opening and closing the contacts. Automatic operation is achieved by trip mechanisms responsive to the blowing of fuses or otherprotective electricalrelays. An attempt at a more compact switch is decr.ibed in US patent No.3471669. This -shows a vacuum switch encapsulated in a metal housing. The switch has a mechanically operated toggle mechanism to open and close the contacts of the vacuum switch, with an operating lever extending out of the housing. The shaft of the lever must be able to rotate where it passes through the encapsula- tion. There is thus a danger of insulation breakdown by tracking along the interface between the shaft and the encapsulation, especially if moisture is able to penetrate this interface.

In the description that follows reference is made to a form of high voltage connection which is commonly used in the industry. Connection is made between a first male portion, which is formed with a frustoconical insulating bushing, and a second female receptacle portion, which has an internally frustoconical aperture into which the male portion fits. A central connecting element, typically a screw threaded plug at the tip* of the male portion makes electrical connection with a connecting element, typically a screw threaded hole, set at the base of the female portion. The frustoconical surfaces mate together to form a tight seal. A connection of this kind will be referred to hereinafter as a high voltage connection as hereinbefore defined.

It is a common practice to provide terminal bushings on electrical equipment, such as distribution transformers, which form the male portions of high voltage connections as hereinbefore defined.

According to the present invention, a high voltage switching device comprises an outer covering of an electrically conductive material, a vacuum switch and a switch operating mechanism mounted within the covering. solid electrical insulation material encapsulating the switch and operating mechanism and insulating them from the covering, an aperture in the covering, said solid insulation material being shaped in association with said aperture to form a female receptacle portion for receiving through said aperture the insulated bushing of the male portion of a high voltage connection as hereinbefore defined, and a conductive connecting element for the female receptacle portion, encapsulated in the insulation material and connecting to one terminal of the vacuum switch.

The use of a vacuum switch encapsulated by solid insulation material provides a compact, relatively cheap, but nevertheless highly serviceable device readily meeting the required standards for such devices. Further, by forming the switching device with a female receptacle portion of a high voltage connection as hereinbefore defined, great advantage can be made of the compactness and attendent lightness of the switching device. The female receptacle portion can be connected directly on to the terminal bushing of a distribution transformer or other item of electrical equipment. Thus, no special provision need be made for mounting the switching device as has always been necessary hitherto. The mounting is effected by connecting the female receptacle portion of the device on to an existing, bushing of the electrical equipment. It will be appreciated that the switching device need not be in fact connected directly to the terminal bushing of the electrical equipment. It is a known practice for several connections to be made to a bushing using successive high voltage connections as hereinbefore defined in line. T connectors are known for this purpose whereby a first T connector, eg carrying one cable end of a ring- main, can be connected directly to the transformer bushing, and then the switching device can be connected by means of an intermediate bushing to the other connection port of the T member.. However, in applications where the switching device is intended for protecting the distribution transformer, the switching device or devices will normally be connected first to the transformer bushing.. Preferably, electrically conductive elements are provided within said covering moulded in contact with high voltage carrying metallic components to reduce electric field concentrations resulting from angularity of the metallic components, said elements being themselves insulated from the covering by the solid insulation material.

In a preferred embodiment, said operating mechanism is wholly encapsulated within said covering and non mechanical actuating means are provided to initiate operation of the mechanism from outside the covering. This arrangement has no mechanical operating lever extending from the covering with its attendant insulation breakdown problems. All mechanical elements are entirely housed within the covering and actuation is provided by non mechanical means which may be solidly sealed to prevent ingress of moisture.

In another aspect of the present invention, a high voltage switching device comprises an outer covering of an electrically conductive material, a vacuum switch and a switch operating mechanism mounted within the covering, solid electrical insulation material encapsu¬ lating the switch and operating mechanism and insulating them from the covering, and at least one conductive connecting element encapsulated in the insulation material and connected to one terminal of the vacuumm switch to enable an external high voltage connection to be made to said one terminal, wherein said operating mechanism is wholly encapsulated within said covering and non mechanical actuating means are provided to initiate operation of the mechanism from outside the covering.

In one arrangement, hydraulic means within said covering are provided and are conveniently employed with a hydraulic-pneumatic interface mounted -exterior to said covering. This permits the hydraulic means to be completely sealed and the hydraulic pressure to be produced from a source of pneumatic pressure.

The pneumatic pressure may be derived from an chemical charge detonated when operation of the switch is required. The switch may be independently operable to open and to close or may be normally in one state and irreversibly operable to change to the other state.

Having been operated, such an irreversible switch must then be replaced by a new one. A chemical actuator within the covering may be employed to trigger the irreversible operating switch. The switching device may be configured as an in line switch between high voltage cables connected by means of two said female receptacl'e portions, eg a circuit breaker or isolating switch, or a sectionalising switch. Instead the switching device may be formed with a protruding bushing forming a male portion, for the connection thereto of one of the cables.

Alternatively, the switching device may be configured as a fault throwing switch with the second contact of the vacuum switch being connected to an earth conductor. -The switching device may then have two said female receptacle portions with said connecting means providing through connection means enabling respective high voltage connections ^'to be made in series by means of- the two female portions and connecting the first contact of the vacuum- switch to the through connection means. Conveniently, the covering is T-shaped ^'having said female receptacle portions in opposite ends of the cross arm of the T and the vacuum switch and the operating mechanism housed in the upright of the T with the earth conductor extending from the bottom end of said upright.

The provision of the above fault throwing ^"switch in a T-shaped covering provides a very convenient arrangement. T-shaped and elbow shaped connectors are becoming established as a convenient way of connecting cable tails to transformers and a fault throwing switch provided in a T-shaped covering similar to a T-shaped connector housing can most conveniently be mounted directly to the transformer so that the high voltage supply to the transformer passes through the cross arm of the T via the through connection means. The earth conductor emerging from the upright of the T is solidly connected to earth to provide for the earth fault current on closing of the fault throwing switch.

Other covering shapes are also useful. An elbow shaped covering may be used for a fault throwing switch where through connection of the high voltage line is not required.

For other applications, eg circuit breakers or sectionalising switches, an in line covering shape is appropriate. It has been discovered that a vacuum switch has ideal characteristics for use as a fault throwing switch in that when held open it has an adequate breakdown voltage and can be operated with high reliability. Furthermore the vacuum switch is extremely compact for its purpose.

According to another aspect of the present invention, apparatus for protecting a ground mounted electrical power distribution transformer in a distri¬ bution system comprises, at the transformer installation, a fault throwing switch, which is normally open circuit, connected between one phase of the high voltage. supply, to the transformer and earth and arranged to close in response to a control signal to make a closed circuit resulting in earth fault current and fault detection means to generate said Control signal on detecting a fault current in any phase of the high voltage supply to the transformer, and, at a remote source installation supplying a plurality of said transformers on a common high voltage supply line, a circuit breaker responsive to an earth fault current in any phase of the line to .isolate the line.

The above arrangements can provide secure protection for distribution transformers by detecting fault currents locally at the transformer and in response thereto initiating an earth fault current which can be detected remotely at a source installation, typically the primary sub-station. The installation at each of the distribution transformers of the necessary fault throwing switch and fault detection means can be very much cheaper than the existing practice of installing HBC fuse and switch combinations. It is recognised that the arrangement proposed in the present invention has the effect of temporarily isolating an entire high voltage supply line, including several distribution transformers (local sub-stations), but this is considered an entirely acceptable penalty in view of the low number of transformer faults expected.

In one arrangement, the circuit breaker at the source installation is an auto-reclosing type arranged to reclose after a predetermined time delay and there is provided at the transformer installation for each ^'phase of the high voltage supply, a respective sectiona¬ lising switch which is normally closed to connect the respective phase of the high voltage supply line to the transformer, said sectionalising switch for said one phase being located to carry said earth fault current through the fault throwing switch, and control means responsive to said fault signal and subsequent cessation of the earth fault current on isolation of the line by the circuit breaker to open the sectionalising switches, to isolate the transformer from the line before reclosing of the circuit breaker. This arrangement provides automatic isolation of the faulted distribution transformer.

In the above, fault detection means has been described as arranged to detect fault currents in any phase of the high voltage supply to the transformer. The fault detection means may additionally be arranged to generate said control signal on detecting faults in the low voltage connections to the transformer and the low voltage winding of the transformer. Ideally, any imbalance of energy entering or leaving the transformer is detected. In this way .full protection of the transformer can be provided.

Examples of the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of an electrical power distribution system incorporating distribution transformer detection apparatus embodying the present invention;

Figure 2 is a schematic diagram illustrating the installation of a fault throwing switch at a distribution transformer installation; Figure 3 is a schematic diagram illustrating additionally the provision of a sectionalising switch at the transformer installation;

Figure 4 is a more detailed cross-sectional view in elevation of a fault throwing switch provided in a T-shaped covering;

Figure 5 is a cross-sectional view of part of the switch of Figure 4 showing the actuating mechanism 'of the switch; Figure 6, 7 and 8 are plan views of parts of the mechanism shown in Figure 5 and

Figure 9 is a cross sectional view of a circuit breaker provided in an "in-line" covering shape together with a block schematic diagram of control circuitry.

Referring to Figure 1, an 11 kV ring main 10 is shown looping from a primary sub-station 11. A number of local sub-stations 12, 13, 14, 15 etc, are shown connected into the ring main 10. The main is normally held open at one point 25. At each sub-station the ring main is connected to a distribution transformer 16 to step the voltage down to the local mains voltage. A fault throwing switch 17 is provided ( rmecting to earth at least ene phase of the high voltage input lines to the transformer 16. The fault throwing switch is normally open as shewn in the drawing and is designed to withstand the line voltage, particularly 11 kV, of the ring main 10.

A fault detector 18 is provided associated with each transformer 16 to detect fault currents arising at least in the high voltage feeds to the transformer. Such fault currents may arise for example frαn a breakdown in the high voltage windings of the transformer.

Additicrtally severe faults in the low voltage winding of the transformer and the low voltage cables may also produce sufficient fault current in the high voltage side to be detected by the unit 18. In respense to the detection of the fault, the unit 18 triggers the fault thrcwing switch 17 to close thereby ccnnecting the respec¬ tive -phase of the supply line to earth and enabling an earth fault current to flow.

The ring main 10 is protected at the primary sub-station 11 by circuit breakers 19 provided in both arms of the main. The circuit breakers are capable when actuated of interrupting a fault current in the main and thereby isolate the main frαn the electrical supply . The circuit breakers 19 are actuated in response to the detection of a^' fault current in the main by means of detectors 20. The equipment constituted by circuit breakers 19 and detectors 20 may comprise existing kinds of source circuit breaker arranged to trip disconnecting the mains in response to. fault currents.

It will be appreciated that in order to protect the transformers 16 at each of the sub-stations, it is necessary only for the circuit breaker installation at the primarysub-station 11 to respond to an earth fault current as caused by the fault throwing switch 17. The local fault detector 18 at each transformer can be much more readily arranged to detect fault currents at the transformer 16 which could not be detected easily by equipment at the primary sub-station 11.

In. a preferred arrangement, the detector 18 comprises simply current transformers provided on each of the high voltage input phases to the transformer 16. Time Limit Fuses provided in the detector 18operate in" response to currents flowing in the current transformers above a predetermined maximum for more than a predet- termined length of time. On operation of the fuses, a control signal is generated to operate the fault throwing switch 17.

It will be appreciated that the protection afforded the transformers 16 by the above described arrangement has the effect of isolating the entire ring main 10 in response to a fault at any one of the transformers connected to the ring main. Normally, manually operable isolation switches are provided at each-of the ^" sub-stations to enable the faulty euipment to be taken off line so that the ring main can then be re-energised to provide power to the other sub-stations. Conveniently, however, the circuit breaker provided at the area sub-station 11 is made to be of the auto-reclosing type, ie automatically re-closing to re-energise the.supply after a predetermined delay.

The sub-station illustrated generally at 15 is ' an example of a modified form which automatically isolates a faulty transformer from the line. In the modified form of sub-station illustrated at 15, a sectionalising switch 21 is additionally provided in series between the connection to the ring main 10 and the point 22 to which the fault throwing switch 17 is connected. The sectionalising switch 21 is operated by a control unit 23 -arranged to open the sectionalising switch 21 only after the cessation of the earth fault current caused by the fault throwing switch 17. The control unit 23 is arranged then to open the sectional¬ ising switch 21 during the dead time when the ring main 10 is isolated by the circuit breaker at the primary sub-station and before the circuit breaker recloses automatically. In this way on re-closing, the faulted equipment has been isolated from the line so that the remaining sub-stations can be re-energised.

The above described fault detectors 18 at each sub-station sense only fault currents in the high voltage lines to the transformer. Some faults in the low voltage side of the transformer may not produce sufficient fault current on the high voltage side to actuate the fault throwing switch. Accordingly, in an alternative arrangement, the detector 18 is arranged also to monitor the low voltage side of the transformer directly, eg by further current transformers sensing currents on the low voltage lines to the transformer, including the low voltage neutral line. In this way protection can be provided to the transformer ensuring detection of transformer faults and thereby actuating the fault throwing switch to signal the fault to the primary sub-station.

Referring to Figure 2, a very convenient way of installing the fault throwing switch 17 at a distribu- tion transformer is illustrated. The fault throwing switch takes the form of a vacuum switch 30 which is held open and provided in the housing of a T shaped connector 31. T connectors are known and useful for making multiple spur connections to a line and for connecting the line to plant bushings such as the. bushing 32 of^" a transformer. The housing 31 can be provided with a through connection between the ports 33 and 34 of the connector so that the connector can be interposed between the transformer bushing 32 fønd two further ordinary T connectors 35 and 36 by which the high voltage line cables are directly connected to the transformer. The vacuum switch 30 has one contact solidly connected to the through connection line between ports ^'33 and 34. The other contact of the switch is solidly connected via the third port of the housing to earth as at 37.

The details of the detection unit for detecting a fault current in the supply to the transformer and actuating the switch 30 to connect the supply line to earth are not shown in this drawing. Referring now to Figure 3, an additional element 40 is illustrated interposed between the housing 31 containing the fault throwing switch 30 and the first of the T connectors 35 connecting the supply line cable to the transformer. The element 40 incorporates a further vacuum switch 41 to operate as the sectional¬ ising switch 21 of Figure 1.

Itwill be appreciated in Figures 2 and 3 that the T connectors 35 and 36 together with the housing 31 are shown separated for convenience only. Furthermore, it will be appreciated that the two high voltage cables connected to* the single bushing 32 of the transformer by means of T connectors 35 and 36 constitute the incoming and outgoing elements of one phase of the ring main 10.

Figure 4 to 8 illustrate the T-shaped connector housing 31 with the vacuum contactor 30 providing the fault throwing switch in greater detail.

The T connector has an outer covering 50 typically formed of an electrically semi-ccnducting rubber or plastics material. All current transmitting elements within the connector are insulated from the housing 50.

Opposed ports 51 and 52 of the T connector enable through connection to be made eg between the connecting bushing 32 ofthe transformer connected in port 51 and an interconnecting bushing connected in port 52 to connect the T to an adjacent T connector 35. The through connection is provided via a through connecting element 53 centrally located between the ports 51 and 52. Each of the ports 51 and ^' 52 is formed as a female receptacle shaped to receive the frusto-conical bushing of the male portion of a standard high voltage interconnection.

The vacuum switch 54 is mounted within the upright ^' of the T. The switch 54 has a fixed contact 55 at one end and a moving contact 56 at the other end 26. The fixed contact 55 is connected by means of a metal component 57 to the through connecting element 53. The moving contact 56 takes the form of a plunger extending through a gas seal in the body of the switch 54. To hold the switch 54 open, the plunger 56 is held pulled out of the body in the position shown in Figure 5. Atmospheric pressure urges the plunger 56 into the body towards the closing position of the switch and further a compression spring 58 is compressed between a thrust washer 59 forming part of the plunger and a seat 60 to enhance the closing force applied to the plunger. ^~~~ e plunger 56 with an extension shaft 27 and compression spring 58 extends through a metal tubular element 61 which is firmly attached to the lower end 26 of the switch body by fixing screws 28. The shaft 27 is a sliding fit in a bore 62 through a disc element 29 which is secured in the tube 61. The disc element 29, best seen in Figure 7, has radial bores containing compression springs 80 urging restraining balls 63 radially inwards to seat in a circumferential groove 81 around the shaft 27. Adjustment screws 82 are provided in the radial bores permitting adjustment of the force with which the balls 63 are urged into the groove 81. The plunger 56 is thereby restrained against the closing force provided by atmospheric pressure and the compression spring 58 to hold the switch open. However the screws 82 are set so that the retaining effect of the balls 63 can be defeated by applying excess closing -effort to the shaft over and above that exerted by atmospheric pressure and the compression spring 58.

A chemical actuator 64 is mounted co-axially with the shaft 27 to be fired by a trigger signal supplied on lines 65 to apply the required excess force to the end of the shaft 27 to defeat the restraining balls 63 and thereby permit the shaft and plunger to move rapidly to the closed position, thereby closing the switch.

Three metal wire braid connectors 83 are electrically connected by connecting screws 84 to the thrust washer 59 of the plunger, and are fed down through the tube 61 through holes 85 in the disc element 29, and out of the end cap 86 of the tube through holes 87. The braid connectors 83 are connected to earth outside the housing 50.

The lines 65 to the chemical actuator extend out through the bottom end of the upright of the T housing along with the braid connectors 83. The lines 65 are connected to fault detection circuitry associated with the current transformers sensing the current in the high voltage supply to the transformer.

Along the entire length of the upright of the T connector there is an annular layer 71 of insulation material insulating the current carrying elements within the housing from the outer semi-conducting wall of the housing. The cavities at the lower end of the upright of the housing from which extend the earthingconductor braids 83 and the lines 65 are also 5 completely filled with an insulating sealing compound. Screens 72 and 73 of semi-conducting material may be provided immediately adjacent the high voltage conducting elements of the unit to smooth out electric field concentrations which might otherwise impair the

10 insulation^'. The screens 72 and 73 may be made from a conducting rubber material.

The entire fault throwing switch unit illustrated, in Figures 4 to 8 is very cαipact and can be made relatively cheaply. It provides a reliable open circuit via the

15 vacuum switch 54 until operated in response to fault detection. The unit is then capable of conducting current to earth to allow earth fault current to flow, hence signalling the primary sub-station to open the relevant circuit breakers.

20 Once a transformer has faulted, operating the fault throwing switch unit, the unit as illustrated in Figure 4 must be replaced on curing the fault. No provision is made to re-set the vacuum switch 54 which is encapsulated within the housing. Because of

25 the cheapness of the unit and the relative rarity of transformer faults, the cost of- replacing the unit after a single operation is minimal. Because the fault throwing switch is mounted directly on a transformer terminal bushing, no additional mounting arrangements need be made.

It will be appreciated that different forms and 5 configurations of high voltage supply switches can be made embodying the principles of the invention. Instead of a T-shaped connector housing as illustrated an elbow shaped housing may be used if no through connection is required. An in-line switch, eg a sectionalising switch or a

10 • circuit breaker, may be made using an in-line two port housing shape with the ports in opposite ends of the housing enabling connections to be made to respective contacts of a vacuum switch.

Figure 9 illustrates an "in-line" form of

15 switch embodying the present invention. In figure 9, a vacuum switch 90 is located axially inside a cylindrical covering 91. The covering 91 is formed of an electrically conducting plastics material providing an outer screen to the switch assembly. The vacuum switch 90 comprises* in - 20 the normal way a ceramic tubular body 92 with axially

.mounted contacts 93 and 94. Contact 93 is mounted fixed relative to one end face of the vacuum switch 90 and is solidly connected, eg by welding, to a connecting member 95 in which there is an axial screw threaded bore 96.

25 The vacuum switch 90 together with connecting member 95 are located within the covering 91 by means of solid encapsulation material 97. The encapsulation material 97 is electrically insulating and serves to insulate all the current carrying and conductive elements located within the covering 91 from the covering itself. The insulating material 97 is formed adjacent an open end 98 of the cylindrical cover 91 to form a frusto-conical inwardly tapered receptacle 99. The receptacle 99 is shaped to form the female receptacle portion of a high voltage connection of the type herein- before defined. Thus, the receptacle 99 is sized to receive the frusto-conically shaped bushing 100 of the male portion of such a connection. A screw threaded conducting stud 101 extending from the head of the bushing 100 locates and screws in to the bore 96 in the connecting member 95 at the base of the receptacle 99. •

The bushing 100 of the male portion is made of an insulating epoxy resin and is relatively hard, whereas the solid encapsulation material 97 comprises an insulating elastomeric material. Accordingly, the complementarily shaped frusto-conical surfaces of the bushing 100 -and the receptacle 99 make an interference fit w,ith each other when the stud 101 is received in the bore 96, to make a secure moisture seal.

The second contact -94 of the vacuum switch 90 is formed as a plunger iθ2 extending from the other end of the .vacuum switch through a gas seal formed by a bellows 103. The plunger 102 extendsaxially along a cylindrical bore provided in a conducting rod member 104 extending coaxially from the end of the vacuum switch. The rod member 104 extends outwards through the opposite end opening of the cylindrical covering 91 and is terminated in a screw threaded connecting stud 105. An end portion of the rod 104 has moulded around it a frusto-conical outwardly tapering bushing 106, so that the combination of the bushing 106 and connecting stud 105 themselves form the male portion of a high voltage connection of the kind hereinbefore defined. The bushing 106 is formed of an epoxy resin insulating material similar to that normally used for connection bushings such as bushing 100. Electrical connection is made between the rod 104 and the plunger 102 of the movable contact 94 by means of a Multilam contact 107. The Multilam contact 107 is formed of a pack of washer like elements solidly located within the bore of the rod 104 and providing a sliding fit with the plunger 102. The multiplicity of contacts provided by this arrangement ensure low resistance contact at all times.

The operating mechanism^'for the vacuum switch 90 is generally located within the bore in the rod 104. A compression spring 108 seated against a base recess of the bore in the rod 104 bears against an hydraulic piston 109 mounted on the plunger 102, tending to urge the plunger to close the contacts 93 and 94. The piston 109 can travel axially in a cylinder portion 110 of the bore in the rod 104 between a first position in which the contacts 93 and 94 are closed and a second position in which the contacts are open, as shown in the drawing. An hydraulic packing seal enables the plunger 102 to move relative to the end face of the cylinder 110 whilst preventing leakage of hydraulic fluid in the cylinder.

Hydraulic fluid to actuate the piston 109 in the cylinder^', to move the plunger and contact 94 from the closed to the open position as shown in the drawing, is supplied along a pipe 112 from a pneumatic/hydraulic interface unit 113 mounted on the outside of the covering 91.

The plunger 102 is retained in the open position as shown, on actuation of the piston 109 by a pulse of hydraulic fluid along the pipe 112, by means of an hydraulically released latch 114. The latch 114 is urged by a compression spring 115 radially inwards relative to the plunger 102 so as to locate into a suitable gr.oove or recess provided in the plunger 102 when the contacts of the vacuum switch are opened as shown. - ^τhe latch 114 then prevents the contacts from being closed again under the influence of the compression spring 108, even if the hydraulic , pressure applied to the piston 109 is no longer maintained.

The latch 114 is itself-released by means of a hydraulic piston and cylinder arrangement illustrated at 115, whereby hydraulic fluid is delivered to the cylinder along a pipe 116 from another pneumatic/hydraulic interface unit 117 also mounted on the outside of the cover 91. Delivery of a charge of hydraulic fluid along the conduit 116, depresses the piston of the latch 114 against the compression spring, withdrawing the latch from the groove or recess in the plunger 102 so that the plunger 102 can then be moved under the influence of the compression spring 108 to close the contacts. Delivery of a charge of hydraulic pressure along the conduit 112 can reopen the switch, with the latch 114 again locating in the recess in the plunger under the influence of the compression spring in the arrangement 115.

The chargesof the hydraulic" luid delivered to open or close the vacuum switch 90 come from the pneumatic /hydraulic interface units 113 and 117 as mentioned before. Each of these comprises a movable member able to communicate pneumatic pressure to the hydraulic fluid whilst sealing one from the other. The interface units are -shown in the drawing as pistons moving in cylinders, but may alternatively be formed by means of flexible diaphrams. A 'charge of pneumatic pressure can be delivered to each of the interface units from respective pressure boxes 118 and 119. The pressure boxes 118 and 119, the interface units 113 and 117, and the inter¬ connecting conduits are normally at substantially atmospheric pressure, with the vacuum switch remaining in its previous state. Pneumatic pressure to operate the switch to change its state is produced by detonating a small explosive charge in the respective one of the pressure boxes 118 and 119. Explosive charges are shown mounted in each of the pressure boxes at 120 and 121 respectively. These can be detonated electrically from respective selectors 122 and 123. Detonation of a charge in one of the pressure boxes 118 produces a pneumatic pressure pulse along the conduit to the respective pneumatic/hydraulic interface unit 113 and 117, which in turn produces the necessary hydraulic fluid charge to operate the respective opening or closing mechanism of the switch.

It will be appreciated that in high voltage distribution applications, there are normally three supply phases which woμld be controlled simultaneously by means of three of the switch assemblies illustrated in figure 9. The three switches may be controlled simultaneously from the pressure boxes 118 and 119 by means of respective additional pneumatic conduits 24 and 25. In the switching device illustrated in figure 9, all electrically conducting elements of the switch are insulated from the outer covering shield 91 by means of the solid encapsu¬ lation material 97. The hydraulic fluid is of course also insulating and arranged to have a suitably high dielectric strength. Conveniently, the interface units 113 and 117 are mounted on a conducting shield band 126 extending around the assembly at the base of the bushing 106 and forming an earthing point for the outer shield covering 91. Conductive plastic or elastomeric elements 127 and 128 may be provided moulded around the outside of metallic portions of the switch mechanism so as to provide electrical stress relief.

It can be seen that the entire switching device may be mounted by means of the female receptacle 99 on the existing connecting bushing, eg of a distribution transformer. Additional connections to the transformer can then be made by means of the bushing 106 of the switching device.

Claims

CLAIMS :

1. A high voltage switching device comprising an outer covering of an electrically conductive material, - a vacuum switch and a switch operating mechanism mounted within the covering, solid electrical insulation material encapsulating the switch and operating mechanism and insulating them from the covering, an aperture in the covering, said solid insulation material being shaped in association with, said aperture to form a- female receptacle portion for receiving through said aperture the insulated bushing of the male portion of a high voltage connection as hereinbefore defined, and a conductive connecting element for the female receptacle portion, encapsulated in the insulation material and connected to one terminal of the vacuum switch.

2. A switching device as claimed in claim 1 wherein electrically conductive elements are provided within said covering moulded in contact with high voltage carrying metallic components to reduce electric field concentrations resulting from angularity of the metallic components, said elements being themselves insulated from the covering by the solid insulation material.

3. A switching device as claimed in claim 1 or claim 2 wherein said operating mechanism is wholly encapsulated within said covering.and non-mechanical - 33 -

actuating means are provided to initiate operation of the mechanism from outside the covering.

4. A switching device as claimed in claim 3 wherein said non-mechanical actuating means includes hydraulic means within said covering.

5. A switching device as claimed in claim 4 and including hydraulic/pneumatic interface means mounted exterior to said covering, completely sealing the hydraulic fluid and enabling hydraulic pressure for the hydraulic means to be selectively applied by selective application of pneumatic pressure to the interface means.

6. A switching device as claimed in claim 5 and including a pneumatic enclosure, a pressure conduit connecting the enclosure to the interface means and means for selectively detonating a chemical charge into the enclosure to generate said pneumatic pressure applied to the interface means.

7. A switching device as claimed in any of claims 4, 5 or 6, wherein the switch operating mechanism and the actuating means are arranged for opening and closing the contacts of the vacuum switch.

8. A switching device as claimed in claim 7 wherein a first contact of the vacuum switch is fixed and the second contact is formed as a plunger extending through a gas seal out from the switch body, and the - 32 -

operating mechanism comprises means resiliently urging the plunger into the body to close the switch, an hydraulic cylinder, a piston on the plunger movable in the cylinder by hydraulic pressure against said resilient urging means to open the switch, and latching means releasably holding the plunger in an open position against the effort of the urging means, whereby the latching means can be released to close the switch.

9. A switching device as claimed in claim 8, wherein the latching means is hydraulically operable to release the plunger to close the switch.

10. A^' switching device as claimed in claim 3, wherein the operating mechanism is arranged to hold the vacuum switch normally in one state and includes triggering means actuatable to change the switch irreversibly to the other state.

11. A switching device as claimed in claim 10, wherein a first contact of the vacuum switch is fixed and the second contact .is formed as a plunger extending through a gas seal out from the switch body, and the

^•operating mechanism comprises means urging the plunger into the body to close .the switch, restraining means releasably holding the plunger in an open position against the effort of the urging means and said triggering means actuatable to release the plunger to close the switch.

12. A switching device as claimed in claim 11, wherein the restraining means is arranged to be defeated, releasing the plunger, by a load on the plunger in excess of that exerted by the urging means, and the triggering means is arranged to exert said excess load to release the plunger.

13. A switching device as claimed- in claim 12, wherein the restraining means comprises clip means resiliently seating in a transverse groove in the plunger and located in a recess fixed relative to the switch body, said excess load on the plunger forcing the clip out of the groove to release the plunger.

14. A switching device as claimed in claim 13, wherein the dip means comprises a plurality of restraining balls fitting in respective radial bores and resiliently urged inwards to engage in a circumferential groove around the plunger.

15. A switching device as claimed in any one of claims 11 to 14, wherein said triggering means is a chemical actuator.

16. A switching device as claimed in any of claims 10 to 15, wherein the actuating means includes fault sensing circuitry in the covering to generate a trigger signal to actuate the triggering means.

17. A switching device as claimed in claim 16, wherein the fault sensing circuitry has sensing lines passing out of the covering for connection to external sensing means.

18. A switching device as claimed in any preceding claim having two of said female receptacle portions and connecting elements enabling high voltage connections to be made to each contact of the vacuum switch by means of respective, said receptacle portions.

19. A switching device as claimed in any of claims 1 to 17, wherein the outer covering has a second aperture and a bushing of insulating material is provided protruding from each said second aperture to form in association therewith a male portion of a high voltage connection as hereinbefore defined, a second conductive connecting element being provided for said male portion, insulated from the outer covering and connected to the other terminal of the vacuum switch.

20. A switching device as claimed in any of claims 1 to 17, and configured as a fault throwing switch, the second contact of the vacuum switch being connected to an earth conductor.

21. A switching device as claimed in claim 20, and having at least two of said female receptacle portions and said connecting means^*providing through connection means enabling respective high voltage connections to be made in series by means of the two female portions and connecting the first contact of the vacuum switch to the through connection means.

22. A switching device as claimed in claim 21, wherein the covering is T-shaped having said female receptacle portions in opposite ends of the cross-arm of the T and the vacuum switch and the operating mechanism housed in the upright of the T with the earth conductor extending from the bottom end of said upright.

23. Apparatus for protecting a ground mounted electrical power distribution transformer in a distri¬ bution system, comprising, at the transformer installation, a fault throwing switch, which is normally open circuit, connected between one phase of the high voltage supply to the transformer and earth and arranged to close in response to a control signal to make a closed circuit resulting in earth fault current and fault detection means to generate said control signal on detecting a fault current in any phase of the high voltage supply to the transformer, and, at a remote source installation supplying a plurality of said transformers on a common high voltage supply line, a circuit breaker responsive to an earth fault current in any phase of the line to isolate the line. - 36 -

24. Apparatus as claimed in claim 23, wherein the circuit breaker at the source installation is an auto-reclosing type arranged to re-close after a pre¬ determined time delay and there is provided at the transformer installation for each phase of the high voltage supply, a respective sectionalising switch which is normally closed to connect the respective phase of the high voltage supply line to the transformer, said sectionalising switch for said one phase being located to carry said earth fault current through the fault throwing switch, and control means responsive to said fault signal and subsequent cessation of the earth fault current on isolation of the line by the circuit breaker to open the sectionalising switches, to isolate the transformer from the line before re-closing of the circuit breaker.

25. Apparatus as claimed in claim 23 or claim 24, wherein the fault detection means is arranged to generate said control signal on detecting additionally faults in the low voltage connections to the transformer and the low voltage winding of the transformer.

26. A switching device substantially as hereinbefore described with respect to and illustrated_" in figures 4 to 8 or figure 9 of the accompanying - drawings.