GB1571064A - Apparatus for testing the performance of insulators - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO APPARATUS FOR TESTING THE PERFORMANCE OF INSULATORS (71) We, CENTRAL ELECTRICITY GENERATING BOARD, a British Body Corporate, of Sudbury House, 15 Newgate Street, London, EC1A 7AU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement : - This invention relates to the testing of the performance of insulators.

In many localities, pollution is a limiting criterion for determining the length of outdoor insulators in high voltage power systems.

Pollution problems arise from a number of causes and the extent of pollution varies widely between different localities. Common examples of pollution are saline pollution in the neighbourhood of the sea and industrial pollution from chimney discharges. A requirement exists therefore to assess the local effect of pollution. As described in the paper by P. J. Lambeth entitled "Insulators for 1000 to 1500 kV Systems" Phil. Trans. R.Soc.

Lond. A275 pages 153-163 (1973), a string of insulator units may be energised at a constant voltage from an auto-reclose circuit breaker, the junctions between successive units of the string being connected through separate fuses to earth. These fuses thus effectively short-circuit all the insulator units in the string except one. When pollution and wetting conditions have become sufficiently severe to cause this single insulator unit to flash over, the fuse connected to the end of that insulator unit blows and the circuit breaker trips. When the circuit breaker recloses, the stressed insulator length is now increased by the addition of the next insulator unit in the string. This process is repeated whenever the pollution severity and wetting combination builds up to a level such as to cause flashover, until the stressed insulator length becomes just long enough to withstand the maximum severity at the site. By having a number of insulators of different types all energised at the same voltage and at the same site, it is possible to determine by this means the best type, that is the shortest insulator, which will withstand local conditions without flashover. Conversely by using the same types in different localities or different heights, it is possible to compare pollution severities in terms of insulator pollution performance. Provision may be made for automatically recording when the various fuses blow.

For this technique to be successful, a suitable fuse must be used which will be sufficiently robust to remain intact over a period of years in a corrosive atmosphere in an exposed condition, with peak leakage currents typically up to one or more amperes flowing. The fuse must part when the short circuit current flows through it and allow the connection from the fuse to earth to fall or be pulled away. Fuses incorporating an explosive element have been found satisfactory from the point of view of meeting these requirements.

These fuses however have to be connected electrically to the insulator string and here tofore this has been done by means of a clamp on the insulator, the fuse being attached to this clamp. With such a construction, part of the fuse is left on the clamp and thus connected where it may impair clearance between adjacent insulators of the string and produce electrical interference. Furthermore both the clamp and the blown fuse may introduce corrosion problems. The fuses have been connected to earth by means of a flexible lead and, when the fuse blows, this lead with part of the fuse has been allowed to fall away under gravity. It has been found that this lead with the part of the fuse may blow in the wind and interfere with air clearances between high voltage points and earth.

The present invention is directed to an improved form of test apparatus of this kind for establishing the length of an insulator necessary to withstand the local pollution.

According to this invention in test apparatus of the kind having a string of insulator units arranged to be energised with a constant voltage from a circuit breaker and having the junction between the units of the string separately connected through fuses to ground, each fuse is connected to a junction between insulator units by means of a releasable clamp, each fuse being arranged, when blown, mechanically to release the clamp. The circuit breaker, in the known way, may be an auto-reclose circuit breaker. In many installations e.g. with horizontal insulators, the released clamp with its fuse can be allowed to fall away from the insulator. In other cases however, tension means may be provided operative when a clamp is released automatically to pull the clamp away from the insulator units.

The releasable clamp may have jaws springloaded into an open position. The jaws mightnormally be held in the closed position against the spring force by the fuse. Preferably however a latch is provided for holding the jaws closed, the latch being mechanically operable by fuse rupture to release the jaws; the fuse in this case is not mechanically stressed by the jaw opening spring. The fuse may conveniently be an explosive fuse which, when ruptured, releases the latch to open jaws of the clamp.

The tension means to remove the clamp from the insulator conveniently are either gravity-operated or spring-operated.

With the above-described construction, the clamp as a whole together with the fuse is withdrawn away from the insulator units so avoiding the problems mentioned above. The aforementioned tensioning means may incorporate the electrical connector to earth or a separate flexible connector may be used.

The tensioning means may comprise a wire tension spring or a gravity-operated device having a cable extending from the fuse and clamp unit over a pulley and carrying a weight.

The clamp conveniently comprises a pair of jaws shaped to fit around the structure on which the clamp is to be secured. The shaping of the jaws will therefore depend on the form of the insulator structure which is to be tested.

In some cases, the clamp may be secured around an insulating element, for example around the periphery of a porcelain rod insulator; many rod insulators are so long that they need to be sub-divided and in this case the jaws may be arranged to fit around the insulating rod. Commonly however the insulator units would be suitably sub-divided and may have metal elements for example metal caps around which the jaws can be clamped. The jaws may be covered externally with a soft material to avoid damage to the insulator as the clamp is pulled away. For example a soft sheath may be secured around each jaw. Additionally a plastic shroud may cover the clamp or sheet may be clipped under the clamp to avoid damage from metal parts other than the jaws if they should contact the insulator; this has the advantage that it also prevents the possibility of a flashover rooting on the body of the clamp rather than on the insulator cap or the jaw of the clamp.

The jaws conveniently comprise a pair of members pivotally secured together and operated by spring-loaded levers. The spring, which is arranged to open the jaws, may be tensioned or compressed so that the jaws are in the closed position with the aforementioned latch arranged to hold a sliding member retaining the jaws in the closed position, a latch-withdrawing release arm being held in the retaining position mechanically by the fuse so that the jaw members are opened by the spring on rupture of the fuse. In such a construction, preferably provision is made for loading the operating spring and hence closing the jaws, after the release arm has been secured in position to facilitate clamping of the jaws on an insulator.

In many cases, it may be desirable that opening of the clamp is delayed until current ceases to flow.

In the following description, reference will be made to the accompanying drawings in which: Figure 1 illustrates diagrammatically a test insulator with a plurality of fuses; Figure 2 shows a string of insulators with one fuse element and clamp, illustrating one construction of tensioning means for with- drawing the fuse element and clamp; Figure 3 is a plan view of the tensioning means of Figure 2; Figure 4 illustrates an alternative construction of tensioning means; Figure 5 is a diagram illustrating one construction of clamp and fuse element; Figure 6 is a diagram to an enlarged scale showing in elevation a pivot used in the clamp of Figure 5; Figure 7 is a diagram showing in greater detail the construction of sliding nut lock mechanism used in the clamp of Figure 5; Figures 8 and 9 are diagrams illustrating two cross-sections of jaws for resilient clamping; Figure 10 is a diagrammatic view of another construction of clamp; Figures 11 and 12 are respectiveIy a side elevation and a section along the line 12-12 of Figure 11 showing a clamp release; and Figure 13 illustrates another arrangement of tensioning means.

Referring to Figure 1 there is shown diagrammatically a test insulator comprising a first insulating unit 10 with a number of separate and conveniently similar insulating units 11. A high voltage source 12 is connected to the lower end of insulating unit 10, with the units 11 between the other end of unit 10 and earh 13. The junctions between the units 10 and 11 and between the successive units 11 are separately connected through fuses 15 to earth at 16. Current responsive resistors 17 may be provided in circuit together with recording means (not shown). Such a construction of testing circuit is known in itself and is described in the aforementioned paper by P. J. Lambeth. The present invention is concerned more particularly with the mechanical construction of the fuse units and their securing onto the insulator string.

Referring to Figure 2 there is shown diagrammatically a chain of insulator units 20.

These drawings, for simplicity, show only one clamp and fuse unit 21 clamped between two of the insulator units. This clamp and fuse unit is electrically connected by a flexible cable 22 to an earth terminal 23 on a fixed structure 24. A tension spring 25 extends between a spring anchor point 26 on the structure 24 and the damp unit and is tensioned so that, when the clamp is released from the insulator, the spring pulls the clamp and fuse unit away from the insulator string.

As shown in Figure 3, alternate tension springs 25 are arranged to extend radially at different angles from; the insulator string to avoid any possibility of a clamp and fuse unit interfering with the next adjacent unit as it is pulled dear. In the construction shown, the bottommost fuse ruptures first. If the conditions are reversed so that the insulator is energised from the top and the topmost fuse blows first, then the various tension springs can be all arranged in different radial directions so as to prevent a falling damp interfering with a lower unit.

Figure 4 shows an alternative form of tensioning making use of a pulley and weight system. In Figure 4, the clamp and fuse unit is shown at 30 and is connected electrically by a flexible cable 31 to an earth clamp 32 and fixed structure 33. The cable is arranged over pulleys 35, 36, the lower pulley 36 carrying a weight 37 to tension the flexible cable. Thus when the clamp is released, the flexible cable pulls the clamp and fuse unit away from the insulator string 38. A stop 39 prevents the clamp and fuse unit from being pulled too far.

Figure 13 shows in plan a preferred arrangement of tensioning means in which an electric cable 130 extends from a clamp and fuse unit 131 on an insulator 132 to a fixed point 133, conveniently one corner of a supporting tower forming an electrical earth connection. A flexible rope 134 forming a mechanical but not an electrical connection, also extends from the clamp and fuse unit 131 to a pulley 135 at another fixed point 136, conveniently another corner of the tower. The rope 134 is clamped to the cable, as shown at 137, between the tower and the insulator.

The rope 134 is tensioned by a weight so that, when the clamp unit 131 is released, it pulls the unit 131 and the cable 130 away from the insulator. This arrangement avoids any necessity to have the electric cable passing over a pulley and there is no risk of the earth current flowing through the pulley.

Figure 5 illustrates one construction of fuse and clamp unit. This unit has a pair of arcuate jaws 40 shaped to fit around and engage the insulator assembly at the required point on the insulator string. The size and shape of these jaws will be determined by the construction of insulator employed. The two jaws are conveniently formed of phosphor bronze or other corrosion-resistant material and will be further described later. The jaws 40 are bolted on the ends of lever arms 42 of insulating material by means of metal bolts 43 which bolts serve also as electrical connectors to respective cables 44, one for each jaw. As shown in more detail in Figure 6 the insulated lever arms 42 are pivoted on a pin 46 which carries a rod 47 formed at least partly of an insulating material. This rod 47 passes through a further pivot 48 (Figure 5) carrying insulating arms 49 connected to the ends of the arms 42 to form a lazy-tongs structure.

The further pivot 48 is mounted on a housing 50 containing a helical compression spring 51 arranged between the end of the housing 50 and a nut 52 on a threaded portion 53 of the rod 47. A knurled operating knob 54 is provided for rotating the rod 47 within the housing 50. When the spring 51 is compressed, it may be secured in this compressed condition by means of a pawl 60 (shown in further detail in Figure 7) on a release arm 61 which is pivoted at 62 on the aforementioned housing 50. The release arm 61 is springloaded by a spring 63 (Figure 7) so as tQ be urged in a direction to withdraw the pawl 60 and release the aforementioned nut 52. The release arm 61 is held in the locking position by means of a split pin 64 on an element 65 around an explosive fuse 66 carried on insulating fuse carrier 67. This fuse carrier is mounted on the aforementioned housing 50.

The two cables 44 from the respective jaws 40 are connected to a terminal 68 carrying one end of the fuse 66. A terminal 69 at the other end of the fuse 66 is connected by the aforementioned flexible cable 22 (Figure 2) or 31 (Figure 4) to earth. It will be seen that, when the fuse 66 ruptures, the arm 61 is released so removing the stop 60 from the nut 52. This nut moves in the housing 50 under the action of the compression spring 51 so causing the jaws 40 to open. As shown in Figure 6, a second spring 67' may be provided between the rod 47 and pivot pin 46 to take up any play. When the jaws 40 open, the whole clamp and fuse assembly is pulled away from the insulator string by means of the tensioning device described with reference to Figures 2 and 3 or with reference to Figure 4.

Figure 7 shows in further detail one construction of sliding nut lock making use of a pawl 60 spring-loaded by a spring 70 around the pawl pivot 71. This pawl 60 is carried on the release arm 61 which is spring-loaded by the spring 63 around the arm pivot 62. The pawl 60 and pawl spring 70 are arranged so that the nut can be moved upwardly (in the drawing) past the pawl and will then be retained.

In one form of construction, the housing 50 and nut 52 are splined or, in some other way, shaped to prevent rotation of the nut in the housing. With this construction, the spring 51 can be loaded in a number of ways.

For example, the jaws 40 may be forced together so pulling the nut 52 past the pawl 60. When the jaws are released, the nut will be forced back by the spring 51 against the pawl 60. Another way of loading the spring 51 is by firmly clamping the jaws a fixed distance apart and then either putting a clip around the rod 47 between the knurled knob 54 and the housing 50 or by turning the knurled knob 54 to drive the nut 52 beyond (i.e. below in the drawing) the pawl 60. After the spring 51 has been loaded in any of the above ways, jaw adjustment to clamp the device onto an insulator can be achieved by rotating the knurled knob 54.

In another form of construction, the nut 52 and the inside of the housing 50 are cylindrical so that the nut normally can rotate freely in the housing. In this case, the spring 51 is loaded by forcing the jaws together so that the nut 52 is pulled past the pawl 60. The nut has a recess 73 (Figure 7) to engage the pawl and so to prevent rotation of the nut in the housing when the nut is so engaged.

The jaw adjustment can then be achieved, as previously described, by rotation of knob 54.

Preferably a resistive shunt (not shown) is permanently connected across the fuse, e.g.

between the terminals 68 and 69, in each clamp and fuse unit. This shunt has a resistance which is as low as possible commen- surate with not preventing the blowing of the fuse on short circuit. The shunt serves to carry the current after the fuse blows and so prevents any possible arcing damage occurring before the current is interrupted by the operation of the circuit breaker.

Figure 8 illustrates, in transverse section, a jaw arm 40 for engaging on the shed of a porcelain insulator 85. In this construction, the metal jaw arm is a gun-metal element 86 which, in the region adjacent the insulator, is shaped to embrace a pair of neoprene elements 87 of generally cylindrical form so that the clamp will not slip off the shed, each element 87 being wrapped with aphosphor bronze tape 88 so that these tapes form the surfaces contacting the insulator 85 and are in electrical contact with the metal part 86 of the jaw arm 40. A foam plastic outer coating 90 is pro vided around the external surface of the jaw arm 40 to prevent any possible damage to the porcelain insulator as the clamp is pulled away after rupture of a fuse.

Figure 9 illustrates, in transverse section, a jaw arm for engaging a metal cap 95 of an insulator unit of a string of insulators. In this construction, the jaw arm 96 is of gun metal and, in section, is bifurcate to embrace a resilient core 97 covered with a phosphor bronze tape 98. The jaw arm externally has a resilient outer coating 99 similar to that of Figure 8.

In the constructions described above, it may be preferred to introduce a short delay before opening the jaws of the clamp in order to allow the current to fall to zero and thereby to prevent any possibility of welding of the jaws to the metalwork of the insulator. This may readily be achieved in the previously described constructions by providing a damper to delay the start of the release of the jaws until the auto reclose breaker has cleared.

Conveniently an air damper is used, as shown in Figure 10. In that figure, there are shown a pair of jaws 100, 101 formed of berylliwm copper or phosphor bronze which are pivotally mounted respectively on arms 102, 103 hingedly connected by a spring 104 which tends to move the jaws apart. Connecting cables 105, 106 are secured to the jaws. The arms 102, 103 are held together by a threaded rod 107 passing through an internally threaded element 108 pivotally mounted on arm 103, the rod 107 having a head 110 (Figure 11) held between two arms 111, 112 (Figure 12) of a spring 113. These spring arms 111, 112 are kept in engagement with a neck 114 on the rod 107 under the head 110 by means of two parallel plates 115, 116 on an arm 120. This arm 120 is hingedly mounted by means of a spring 124 on the aforementioned arm 102 and is retained against the spring force by means of a fuse 122. Rupture of this fuse releases the arm 120 to move away from arm 102. The movement of the plates 115, 116 releases the spring arms 111, 112 and these move apart thereby releasing the threaded rod 107. The jaws can now open.

In the construction of Figure 10, a delay is introduced by an air damper 125 arranged between the arms 120, 102. This damper com- prises a chamber formed of two flexible dished elements sealed together around their periphery and secured to the two arms 102, 120. A small bleed hole 126 allows air slowly to leak into the chamber thereby delaying the movement of the arm 120.

WHAT WE CLAIM IS:- 1. Test apparatus of the kind having a string of insulator units arranged to be energised with a constant voltage from a circuit breaker and having the junction between the units of the string separately connected through fuses to ground, wherein each fuse is connected to a junction between insulator units by means of a releasable clamp,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   the release arm 61 which is spring-loaded by the spring 63 around the arm pivot 62. The pawl 60 and pawl spring 70 are arranged so that the nut can be moved upwardly (in the drawing) past the pawl and will then be retained.

   In one form of construction, the housing 50 and nut 52 are splined or, in some other way, shaped to prevent rotation of the nut in the housing. With this construction, the spring 51 can be loaded in a number of ways.

   For example, the jaws 40 may be forced together so pulling the nut 52 past the pawl 60. When the jaws are released, the nut will be forced back by the spring 51 against the pawl 60. Another way of loading the spring 51 is by firmly clamping the jaws a fixed distance apart and then either putting a clip around the rod 47 between the knurled knob 54 and the housing 50 or by turning the knurled knob 54 to drive the nut 52 beyond (i.e. below in the drawing) the pawl 60. After the spring 51 has been loaded in any of the above ways, jaw adjustment to clamp the device onto an insulator can be achieved by rotating the knurled knob 54.

   In another form of construction, the nut 52 and the inside of the housing 50 are cylindrical so that the nut normally can rotate freely in the housing. In this case, the spring 51 is loaded by forcing the jaws together so that the nut 52 is pulled past the pawl 60. The nut has a recess 73 (Figure 7) to engage the pawl and so to prevent rotation of the nut in the housing when the nut is so engaged.

   The jaw adjustment can then be achieved, as previously described, by rotation of knob 54.

   Preferably a resistive shunt (not shown) is permanently connected across the fuse, e.g.

   between the terminals 68 and 69, in each clamp and fuse unit. This shunt has a resistance which is as low as possible commen- surate with not preventing the blowing of the fuse on short circuit. The shunt serves to carry the current after the fuse blows and so prevents any possible arcing damage occurring before the current is interrupted by the operation of the circuit breaker.

   Figure 8 illustrates, in transverse section, a jaw arm 40 for engaging on the shed of a porcelain insulator 85. In this construction, the metal jaw arm is a gun-metal element 86 which, in the region adjacent the insulator, is shaped to embrace a pair of neoprene elements

   87 of generally cylindrical form so that the clamp will not slip off the shed, each element

   87 being wrapped with aphosphor bronze tape 88 so that these tapes form the surfaces contacting the insulator 85 and are in electrical contact with the metal part 86 of the jaw arm 40. A foam plastic outer coating 90 is pro vided around the external surface of the jaw arm 40 to prevent any possible damage to the porcelain insulator as the clamp is pulled away after rupture of a fuse.

   Figure 9 illustrates, in transverse section, a jaw arm for engaging a metal cap 95 of an insulator unit of a string of insulators. In this construction, the jaw arm 96 is of gun metal and, in section, is bifurcate to embrace a resilient core 97 covered with a phosphor bronze tape 98. The jaw arm externally has a resilient outer coating 99 similar to that of Figure 8.

   In the constructions described above, it may be preferred to introduce a short delay before opening the jaws of the clamp in order to allow the current to fall to zero and thereby to prevent any possibility of welding of the jaws to the metalwork of the insulator. This may readily be achieved in the previously described constructions by providing a damper to delay the start of the release of the jaws until the auto reclose breaker has cleared.

   Conveniently an air damper is used, as shown in Figure 10. In that figure, there are shown a pair of jaws 100, 101 formed of berylliwm copper or phosphor bronze which are pivotally mounted respectively on arms 102, 103 hingedly connected by a spring 104 which tends to move the jaws apart. Connecting cables 105, 106 are secured to the jaws. The arms 102, 103 are held together by a threaded rod 107 passing through an internally threaded element 108 pivotally mounted on arm 103, the rod 107 having a head 110 (Figure 11) held between two arms 111, 112 (Figure 12) of a spring 113. These spring arms 111, 112 are kept in engagement with a neck 114 on the rod 107 under the head 110 by means of two parallel plates 115, 116 on an arm 120. This arm 120 is hingedly mounted by means of a spring 124 on the aforementioned arm 102 and is retained against the spring force by means of a fuse 122. Rupture of this fuse releases the arm 120 to move away from arm 102. The movement of the plates 115, 116 releases the spring arms 111, 112 and these move apart thereby releasing the threaded rod 107. The jaws can now open.

   In the construction of Figure 10, a delay is introduced by an air damper 125 arranged between the arms 120, 102. This damper com- prises a chamber formed of two flexible dished elements sealed together around their periphery and secured to the two arms 102, 120. A small bleed hole 126 allows air slowly to leak into the chamber thereby delaying the movement of the arm 120.

   WHAT WE CLAIM IS:- 1. Test apparatus of the kind having a string of insulator units arranged to be energised with a constant voltage from a circuit breaker and having the junction between the units of the string separately connected through fuses to ground, wherein each fuse is connected to a junction between insulator units by means of a releasable clamp,

   each fuse being arranged, when blown, mechanically to release the clamp.
2. 2. Apparatus as claimed in claim 1 wherein the circuit breaker is an auto-reclose circuit breaker.
3. 3. Apparatus as claimed in either claim 1 or claim 2 wherein tension means are provided operative when a clamp is released automatically to pull the clamp away from the insulator unites.
4. 4. Apparatus as claimed in claim 3 wherein the tension means are gravity-operated.
5. 5. Apparatus as claimed in claim 3 wherein the tension means are spring-operated.
6. 6. Apparatus as claimed in any of claims 3 to 5 wherein the tension means incorporate an electrical connector to earth.
7. 7. Apparatus as claimed in any of the preceding claims wherein the releasable clamp has jaws spring-loaded into an open position.
8. 8. Apparatus as claimed in claim 7 wherein the jaws are normally held in the closed position against the spring force by the fuse.
9. 9. Apparatus as claimed in claim 7 wherein a latch is provided for holding the jaws closed, the latch being mechanically operable by fuse rupture to release the jaws.
10. 10. Apparatus as claimed in claim 9 wherein the fuse is an explosive fuse which, when ruptured, releases the latch to open jaws of the clamp.
11. 11. Apparatus as claimed in any of claims 7 to 10 wherein said jaws are covered externally with a soft material to avoid damage to the insulator as the clamp is pulled away.
12. 12. Apparatus as claimed in any of claims 7 to 11 wherein the jaws comprise a pair of members hingedly secured together and operated by spring-loaded levers.
13. 13. Apparatus as claimed in any of the preceding claims wherein a damper is provided to introduce a delay after the fuse ruptures such that the current flow ceases before the damp opens.
14. 14. Apparatus for testing the performance of insulators substantially as hereinbefore described with reference to the accompanying drawings.