AU2009208790A1 - On-load tap-changer - Google Patents

Description

WO 2009/095686 PCT/GB2009/000267 ON-LOAD TAP-CHANGER The invention relates to on-load tap-changer mechanisms for use in electrical transformers, and in particular to apparatus and methods for controlling the timing of operations in on-load tap-changing. 5 High voltage transformers, used for example in electrical substations, are subject to varying electrical loads depending upon how much power is being drawn downstream from the transformer. On-load tap-changers allow for selection of different turns ratios on a transformer without the need to interrupt the load 10 current. This makes on-load tap-changers useful for power transformers where interruptions in load current would be undesirable. Providing a number of tap positions on a transformer winding allows the number of turns of the transformer to be selected, producing a transformer with effectively 15 a variable turns ratio. This enables voltage regulation of the secondary (output) side of the transformer to suit different loads. A typical circuit design for an on-load tap-changer is shown schematically in figure 1. The on-load tap-changer 100 comprises a transformer coil 160 attached 20 to a first electrical terminal 180, the transformer coil 160 having a number of tap position switches 110, 112 for selecting different turns ratios on the coil 180. A diverter switch 130, which includes a rotary switch arm 170, and diverter impedances 140, 142, connect the tap position switches 110, 112 to a second electrical terminal 190, completing the circuit. 25 Tap position switch 110 is shown closed, connecting the associated tap position through to terminal 190 via the rotary switch arm 170, which is shown short circuiting the diverter impedance 140. 30 The tap changer follows a series of steps to complete an on-load tap-change between the tap positions associated with the switches 110, 112, outlined as follows: WO 2009/095686 PCT/GB2009/000267 e tap switch 112 is closed; e rotary switch arm 170 of the diverter switch 130 rotates anti-clockwise to a position where current passes through diverter impedance 140 alone to terminal 190; 5 * rotary switch arm 170 turns further to a position where load passes through diverter impedances/ 140 and 142 simultaneously; * rotary switch arm 170 turns further to a position where load passes through diverter impedance 142 alone to terminal 190 e rotary switch arm 170 of the diverter switch 130 turns further to a position 10 where diverter impedance 142 is shorted and the load is supplied through tap position 112; and " tap switch 110 is opened. The above illustrates only one of a number of possible sequences for tap 15 changing, but shows the general principles involved. A more detailed tap changing sequence, but with a similar sequence of event to that given above, is disclosed in US 4,081,741. This document also shows the use of vacuum switches (also known as vacuum interrupters), used when load is transferred from one tap to another, the vacuum switch avoiding undesirable electrical discharges. 20 Since a complex series of steps is required during a tap-switching operation, typically involving high voltages, on-load tap-changer assemblies tend to be large and mechanically complex. In high voltage transformers, tap-changers tend to be mounted within an oil-filled cavity within the transformer, requiring the tap 25 changer to be mounted and removed vertically in relation to the transformer. Both of these requirements tend to make installation and removal of tap-changer difficult and potentially hazardous, particularly if carried out within an operational electricity substation. 30 It is an object of the invention to address one or more of the above mentioned problems. 2 WO 2009/095686 PCT/GB2009/000267 According to a first aspect of the invention there is provided an on-load tap changer switching mechanism comprising: an annular cam defining a plurality of raised portions; and a switch assembly disposed at least partly within and rotatable relative to 5 the annular cam, the switch assembly including an electrical switch and a cam follower engaged with the inner surface of the annular cam, wherein the plurality of raised portions are configured to actuate the cam follower on relative rotation of the switch assembly and the annular cam, actuation of the cam follower causing operation of the electrical switch. 10 According to a second aspect of the invention there is provided a method of operating an on-load tap-changer switching mechanism comprising an annular cam defining a plurality of raised portions and a switch assembly disposed at least partly within and rotatable relative to the annular cam, the switch assembly 15 including an electrical switch and a cam follower engaged with the inner surface of the annular cam, the method comprising actuating the cam 'follower by engagement with one or more of the plurality of raised portions on relative rotation of the annular cam and switch assembly, actuation of the cam follower causing operation of the electrical switch. 20 There now follows a description of preferred embodiments of the invention, by way of non-limiting example, with reference being made to the accompanying drawings in which: figure 1 shows a circuit diagram of a typical design of an on-load tap 25 changer; figure 2a shows a schematic plan view of a tap changer in a first configuration; figure 2b shows a schematic plan view of an alternative embodiment of a tap changer in the first configuration; 30 figures 3a to 3c show detailed sectional views of the tap changer of figure 2a; figures 3d and 3e show detailed sectional views of the tap changer of figure 2b; 3 WO 2009/095686 PCT/GB2009/000267 figure 4 shows a perspective view of a pair of slidable contacts around a perimeter edge of an annular cam; figure 5 shows a schematic partial cutaway plan view of an annular cam within a mounting frame having electrical contacts; 5 figure 6 shows a perspective view of a contact carrier plate having a pair of slidable contacts; figure 7 shows a sectional view of a part of the contact carrier plate and a spring-loaded contact pin; figure 8 shows a schematic plan view of the tap changer of figure 2 in a 10 second configuration; figure 9 shows a schematic plan view of the tap changer of figure 2 in a third configuration; figure 10 shows a schematic plan view of an alternative tap changer arrangement; 15 figure 11 shows a schematic sectional view of a slidable contact arrangement for the alternative tap chafiger arrangement of figure 8; figure 12 shows a schematic perspective view of an exemplary embodiment for one phase of a tap-changer figure 13 shows an electrical connection diagram for a tap changer; 20 figures 14a and 14b illustrate a switching sequence for an exemplary tap changer; figure 15a shows a perspective view of an exemplary tap changer assembly; figure 15b shows an alternative perspective view of selected portions of 25 the exemplary tap changer of figure 15a; figure 16 shows a perspective view of a rotatable contact arrangement; and figures 17a to d show sectional views of the rotatable contact arrangement of figure 16 in different configurations. 30 Figure 1 has already been discussed in relation to the background to the invention, described above. Figure 2a illustrates a tap changer mechanism 200 according to an aspect of the invention, in which a switching assembly 220 is disposed within an annular cam 4 WO 2009/095686 PCT/GB2009/000267 210. The switching assembly 220 comprises a first and a second vacuum switch 224a, 224b, each of which are connected to a respective cam follower 222a, 222b via connecting linkages 226a, 226b. The vacuum switches 224a, 224b provide diverter switch contacts for the on-load tap changer, each vacuum switch 224a, 5 224b being actuated by the respective cam follower 222a, 222b and connecting linkages 226a, 226b. Figure 2b illustrates an alternative embodiment of a tap changer mechanism 200', in which identified parts 210', 220', 222a', 222b', 224a', 224b', 226a', 226b' 10 correspond to similarly numbered parts identified in the embodiment in figure 2a. The connecting linkages 226a', 226b', however, are arranged differently to those 226a, 226b in figure 2a, in that the biasing means for the cam followers, in the form of return springs 228a', 228b', are provided in line with the cam followers 222a', 222b', rather than in line with the vacuum switches 224a, 224b in figure 2a. 15 This biased return mechanism is shown in more detail in figures 3d and 3e, described below. A more detailed sectional view of the vacuum switch 224a, connecting linkage 226a, cam follower 222a and annular can 210 of figure 2a is shown in figures 3a 20 to 3c, together with other associated components. Upon relative rotation of the annular cam 210 and the switching assembly 220, the cam follower 222 follows the profile 310 of the inner surface of the annular cam 210 to produce radial linear motion of the cam follower 222a, i.e. in a direction 25 substantially orthogonal to the axis of rotation of the switching assembly 220 relative to the annular cam 210. This radial linear motion serves to open and close the vacuum switch 224a. Figures 3a to 3c illustrate a sequence showing how this is achieved, with the vacuum switch 224a being closed in figures 3a and 3b, and open in figure 3c. Only one vacuum switch 224a of the pair of switches 224a, 30 224b is shown in figures 3a to 3c. With reference to figure 3a, the cam follower 222a, which may for example comprise a rotatable wheel to reduce wear, follows the profile 310 of the annular cam 210 as the annular cam 210 rotates relative to the switching assembly 220. 5 WO 2009/095686 PCT/GB2009/000267 The cam follower 222a is biased against the annular cam 210 by means of a first, or opening, spring 320 disposed between the connecting linkage 226a and the vacuum switch 224a. 5 As shown in figure 3a, both the first and second springs 320, 330 are disposed coaxially around a vacuum switch actuator comprising a plunger 340 and a connecting rod 350, the vacuum switch 224a being actuated (i.e. opened) by sliding the plunger 340 in the direction indicated by arrow 370. A locknut 360 located on the connecting rod 350 provides an end stop for the connecting linkage 10 226a, which is able to slide along the connecting rod against the bias provided by the opening spring 320. The profile 310 of the annular cam 210 comprises a plurality of raised portions 311, these raised portions 311 being circumferentially spaced around the annular 15 cam 210, each of the raised portions 311 corresponding to a substantially common diameter 315. When the cam follower 222a is located against one of the raised portions 311, the vacuum switch 224a is closed. Referring to figure 3b, the annular cam 210 is shown having rotated clockwise 20 relative to the switching assembly 220 (indicated by arrow 380), causing the cam follower 222a to follow the profile 310 and lift the connecting linkage 226a away from the vacuum switch 224a. In an alternative arrangement, the same effect is achieved by rotating the switching assembly 220 relative to a stationary annular cam 210 in the opposite direction. In this intermediate configuration, the 25 connecting linkage 226a meets the end stop provided by the locknut 360, and the vacuum switch remains closed due to pressure exerted by the contact pressure spring 330. The distance Dl between the connecting linkage 226a and the vacuum switch 224a, and the distance D2 between the connecting linkage 226a and the plunger 340 have both increased by the same amount, in comparison with 30 the distances DI, D2 shown in figure 3a. Referring now to figure 3c, the annular cam 210 is shown having rotated further clockwise relative to the switching assembly 220 (indicated by arrow 380), causing the cam follower 222a to follow the profile 310 and lift the connecting 6 WO 2009/095686 PCT/GB2009/000267 linkage 226a further away from the vacuum switch 224a, the cam follower 222a resting in a trough 312 between two adjacent raised portions 311. In this configuration, the cam follower 222a is at its furthest extent away from the vacuum switch 224a. The distance Dl between the connecting linkage 226a and 5 the vacuum switch 224a has increased further compared with that shown in figure 3a, while the distance D2 between the connecting linkage 226a and the plunger 340 remains the same as in figure 3b, since the plunger 340 has lifted out of the vacuum switch 224a. The vacuum switch 224a is consequently now open. 10 Shown in figures 3d and 3e are views of the alternative embodiment of figure 2b in corresponding sectional view arrangements to those in figures 3a and 3c, i.e. with the vacuum switch closed (figure 3d) and open (figure 3e). Similarly labelled components have functions corresponding to those shown in figures 3a to 3c. A first spring 320' biases the cam follower 222a' against the inner perimeter of the 15 annular cam 210', and a second spring 330' functions to open the vacuum switch 224a' by actuating the linkage 226a' when the cam folloiver 222a' moves into a trough 312' on the cam 210'. Operation of the switching assembly 220' is therefore similar to the embodiment of figures 2a and 3a to 3c, but with the springs 320', 330' operating on a slidable plunger 351 actuating the cam follower 20 222a' rather than on the plunger 350' linked to the vacuum switch 224a'. This arrangement simplifies construction and reduces the overall size of the switching assembly 220'. The arrangement also provides for increased stability for the actuating mechanism, reducing the possibility of the mechanism sticking when in use, because the biasing mechanism linked directly to linear motion of the cam 25 follower 222a' minimises or eliminates any rotational moment generated on the vacuum switch plunger 350' during actuation. Referring now to figure 4, electrically conductive slidable contacts 41 0a, 41 0b are disposed on an outer curved surface 420 of a rotatable annular cam 210, which is 30 preferably made of an electrically insulative material such as a polymer. In the embodiment shown, the conductive slidable contacts 410a, 410b are also curved. For mounting and rotatable engagement within a frame (see figure 5), the annular cam 210 may further comprise an outer planar flanged portion 450. 7 WO 2009/095686 PCT/GB2009/000267 The slidable contacts 41 0a, 41 Gb are shown in figure 4 in a staggered relationship relative to one another with respect to the direction of rotation 430 of the annular cam 210. The slidable contacts 410a, 410b are arranged in specific positions around the outer surface 420 of the annular cam 210 relative to the raised portions 5 311 on the inner surface 440 of the annular cam 210. The contacts 410a, 410b move at the same rate as the annular cam profile 310, and so timing of actuation of the switching assembly 220 will be in synchronisation with movement of the contacts 410a, 410b during a tap-changing operation through rotation of the annular cam 210. 10 Referring to figure 5, an annular cam 210 of the type shown in figure 4 is shown in plan view when rotatably mounted within a mounting frame 510, the mounting frame 510 comprising a plurality of electrical contacts in the form of terminals 520a, 520b. The slidable contacts 410a, 410b force spring-loaded pins 530a, 530b 15 (the springs for which are not shown in figure 5) to make contact with respective electrical terminals 520a, 520b. Each pin 530a, 530b breaks contact when the slidable contacts 410a, 410b move around to a subsequent terminal 520b. By precisely locating the slidable contacts 410 in relation to the raised portions 20 311 of the annular cam 210, a sequence of switching events can be accurately determined for a given tap-changing operation. In an alternative embodiment, in which the switching assembly 220 is rotatable relative to a fixed annular cam 210, a contact carrier plate carries the electrically 25 conductive slidable contacts, as shown in figure 6. The contact carrier plate 600, mounted for rotation with the switching assembly 220 about a mounting point 605, comprises electrically conductive slidable contacts 610a, 610b, disposed on the underside of the contact carrier plate 600, which is preferably made of an electrically insulative material such as a polymer. The conductive slidable 30 contacts 610a, 610b are preferably spring mounted on the carrier plate 600, as shown in more detail in figure 7. As for the rotatable annular cam 210 shown in figures 4 and 5, the slidable electrical contacts 610a, 610b are arranged in a staggered relationship relative to one another with respect to the direction of rotation 630 of the contact carrier plate 600, to allow for synchronisation of 8 WO 2009/095686 PCT/GB2009/000267 switching during a tap-changing operation as the switching assembly 220 rotates with the contact carrier plate 600. The contact carrier plate 600 is shown in cross-section in more detail in figure 7. 5 One of the slidable electrical contacts 61 0a is shown, being spring-mounted to the contact carrier plate 600. As the contact carrier plate 600 rotates, the contact 61 Ga travels towards a pin 710, which may also be spring-mounted. Either or both of the contact 61 Ga and pin 710 preferably have a tapered or rounded leading edge, to allow lateral movement of the contact 610a to cause the pin to make electrical 10 contact with an electrical terminal 720. Figures 8 and 9 show the mechanism of figure 2 in different configurations corresponding to different rotational orientations of the annular cam 210 relative to the switching assembly 220. As the annular cam 210 rotates from the 15 orientation shown in figure 2, where both vacuum switches 224a, 224b are closed, the configuration changes to that of figure 8, where the first vacuum switch 224a opens as the first cam follower 222a follows the cam profile towards a trough 312 between two adjacent raised portions 311, while the second vacuum switch 224b remains closed. As the annular cam 210 rotates further, the positions of the 20 vacuum switches are reversed, as shown in figure 9, where the first vacuum switch 224a is closed and the second vacuum switch 224b is opened. As the annular cam 210 continues to rotate, this switching sequence repeats, and is synchronised with the sequence of electrical contacts being made by the slidable electrical contacts around the outer surface of the annular cam 210 (or on the underside of the contact 25 carrier plate 600), as described above. An arrangement where the switching assembly is rotatable relative to the annular cam is shown in figure 10. The switching assembly 820 is rotatable about a mounting point 850, while the annular cam 810 and mounting frame 830 are kept 30 stationary. Since the annular cam 810 does not rotate relative to the mounting frame 830, the cam profile 310 can be formed on the inner surface of the mounting frame 830 itself. The electrical contacts 720 (figure 7) are shown on a planar surface of the mounting frame 830. Slidable contacts on the contact carrier plate (not shown), being rotatable with the switching assembly 820, slide along a planar 9 WO 2009/095686 PCT/GB2009/000267 surface of the mounting frame 830, as shown in more detail in figure 7. This is shown in further detail in figure 11, where a slidable contact 910 on the rotatable contact carrier plate 600 is shown approaching a contact pin 840 in the direction indicated by arrow 920. An arrangement of slidable contacts 910 can be made on 5 the rotatable contact carrier plate 600 to provide the required tap-switching ability, using the vacuum switches when required and in synchronisation with electrical contacts being made and broken by movement of the slidable contacts 910 relative to the pins 840 in the stationary mounting frame 830. 10 A typical implementation of a tap-changer mechanism according to an aspect of the invention would involve the use of two vacuum switches (also known as vacuum bottles) for each phase, amounting to a total of six vacuum bottles for a complete tap changer. Figure 12 shows a schematic perspective view of an exemplary embodiment for one phase of such a tap-changer. A driving cog 1010 15 is linked via a connecting shaft 1015 to the switching assembly 1020 comprising two vacuum switches,"the switching assembly 1020 being mounted on an aniular cam 1030, which itself is mounted within a barrier board 1040. Rotation of the driving cog 1010 causes the switching assembly 1020, comprising the arrangement shown in figure 8, to rotate relative to the annular cam 1030 and 20 actuate the required sequence of switching operations to perform the tap-changing operation. A basic electrical diagram illustrating the required electrical connections for a tap changer according to the invention is shown in figure 13. A transformer coil 1320 25 comprises a common terminal 1319 and a plurality of electrical taps 1302-1318 arranged to allow for differing output loads. The switching assembly 220 is arranged to rotate, e.g. in the direction indicated by the arrow 1310, so that successive electrical connections are made as the assembly 220 rotates, connecting a terminal 1301 to a chosen electrical tap terminal 1302-1318. 30 Figures 14a and 14b illustrate a typical sequence of switching operations for an on-load tap-changer according to an aspect of the invention. As the switching assembly 220 rotates from position A to position B, indicated by the arrow 1410, a sequence of switching operations is made, as illustrated by the diagram of figure 10 WO 2009/095686 PCT/GB2009/000267 14b. At position A, both vacuum switches Vm and VR are closed, the terminal switch M is closed and the bypass resistor switch R 1 is open. As the switching assembly moves away from position A, the bypass resistor switch R, closes, followed by the vacuum switch VM opening and the terminal switch M opening. 5 As the switching assembly 220 rotates further, the terminal switch M closes, followed by the vacuum switch VM. The vacuum switch VR then opens, followed by the bypass resistor switch Ri opening and the vacuum switch Vp closing again to complete the switching operation. Each of the switching operations is synchronised according to the relative positions of the slidable contacts, pins and 10 raised portions of the annular cam, as described above. Figure 15a shows a perspective view of an alternative exemplary embodiment of a tap changer assembly 150 according to the invention, in which an annular cam 210' is fixed to a mounting frame or main board 153, and a switch assembly 220' 15 is rotatably mounted on the main board 153 within the annular cam 210'. The switching assembly 220' is rotatable about a central shaft 151, which preferably also acts as a central electrical contact for the tap changer assembly 150, i.e. electrically connectable to terminal 1301 shown in figure 13, as well as defining an axis about which the switching assembly 220' rotates. The switching assembly 20 220' comprises the vacuum switches 224a', 224b', which are operated by actuation of cam followers 222a', 222b', as well as a further contact arrangement (described below with reference to figures 15b and 16), all of which is rotatable together with respect to the annular cam 210'. As the switching assembly 220' rotates with respect to the annular cam 210', the sequence of switching operations occurs as 25 described above, and electrical contact changes over between adjacent contact shafts 152 arranged around the periphery of the annular cam 210'. The contact shafts 152 provide electrical connections to each tap 1302-13 18 (figure 13) on the transformer to which the tap changer assembly 150 is to be connected. 30 The embodiment shown in figure 15a illustrates an advantageous feature of the invention, in that the overall assembly 150 has a reduced size compared with conventional tap changers, due to the switching assembly 220' being provided within an internal space defined by the annular cam 210' and being rotatable within a plane defined by the cam 210'. The arrangement of cam followers 222a', 11 WO 2009/095686 PCT/GB2009/000267 222b' biased against the inner surface of the cam 210', allows for actuation of the vacuum switches 224a', 224b' within the plane of the cam 210', , i.e. orthogonal to the axis of rotation, thereby reducing the overall height of the tap changer assembly 150. This arrangement also substantially simplifies the mechanical 5 design of the overall tap changer assembly 150. A further perspective view of the tap changer assembly 150 of figure 15a is shown in figure 15b, in which the underside of the annular cam 210' and switching assembly 220' is shown, the main board 153 having being removed. In this view, 10 the contact arrangement 154 mentioned above can be seen. The contact arrangement 154, also shown in isolation in figure 16, comprises two spring biased slidable contacts 155a, 155b, a first contact 155a being a main electrical contact, i.e. for carrying electrical current during normal use, and a second contact 155b being a resistor contact for connecting through to a bypass resistor during a 15 tap changing operation. The two contacts 155a, 155b will typically be of different sizes due to the' different current carrying requirements, but operate in similar ways, both being resiliently biased against one of the contact shafts 152 arranged around the annular cam 210' during use. Similarly to the other embodiments described above, the contacts 155a, 155b are provided in a staggered arrangement 20 on the switching assembly so that electrical connections are made in the proper sequence and synchronise with operation of the vacuum switches during a tap changing operation. Figure 17 illustrates an exemplary sequence of operations where the contact 25 arrangement 154 effects a changeover in connection from a first tap position at a first contact shaft 152a to a second tap position at a second contact shaft 152b. The contact 155 (i.e. either of the main or resistor contacts 155a, 155b) is initially biased against the first contact shaft 152a. After rotation of the contact assembly 154, the contact 155 disconnects from the first contact shaft 152a and connects to 30 the second contact shaft 152b. With the contact 155 being the main contact 155a, during the brief interval between breaking contact from the first contact shaft and making contact with the second contact shaft 152b, the resistor contact 155b, being offset relative to the main contact 155a, carries electrical current via the second contact shaft 152b. 12 WO 2009/095686 PCT/GB2009/000267 Other embodiments are intentionally within the scope of the invention as defined by the appended claims. 13

Claims (22)

1. An on-load tap-changer switching mechanism comprising: an annular cam defining a plurality of raised portions; and 5 a switch assembly disposed at least partly within and rotatable relative to the annular cam, the switch assembly including an electrical switch and a cam follower engaged with the inner surface of the annular cam, wherein the plurality of raised portions are configured to actuate the cam follower on relative rotation of the annular cam and switch assembly, actuation of 10 the cam follower causing operation of the electrical switch.

2. The mechanism of claim 1 wherein the plurality of raised portions are provided on an inner surface of the annular cam. 15

3. The mechanism of claim 1 or claim 2 further comprising a mounting frame having a plurality of contact terminals, the annular cam and switch assembly being mounted at least partly within the mounting frame.

4. The mechanism of claim 3 wherein the annular cam is rotatable with 20 respect to the mounting frame.

5. The mechanism of claim 3 wherein the switch assembly is rotatable with respect to the mounting frame. 25

6. The mechanism of claim 4 wherein the switch assembly and mounting frame are fixed relative to one another, and the annular cam comprises a slidable contact configured to be sequentially engaged with the plurality of contact terminals upon rotation of the annular cam. 30

7. The mechanism of claim 6 wherein the slidable contact is disposed on an outer perimeter of the annular cam. 14 WO 2009/095686 PCT/GB2009/000267

8. The mechanism of claim 7 comprising a plurality of said slidable contacts arranged in a staggered arrangement around the outer perimeter of the annular cam. 5

9. The mechanism of claim 5 wherein the annular cam and the mounting frame are fixed relative to one another, the switch assembly comprising a slidable contact configured to be sequentially engaged with the plurality of contact terminals upon rotation of the switch assembly relative to the mounting frame.

10 10. The mechanism of claim 9 wherein the slidable contact is slidable along a planar face of the mounting frame.

11. The mechanism of claim 9 or claim 10 comprising a plurality of said slidable contacts arranged in a staggered arrangement on the switch assembly. 15

12. The mechanism of claim 11 wherein the plurality of contact terminals are in the form of a plurality of electrical contact shafts arranged around the periphery of the annular cam. 20

13. The mechanism of claim 1 wherein the cam follower is configured to actuate the electrical switch through actuation in a direction substantially orthogonal to the axis of rotation of the switch assembly relative to the annular cam. 25

14. The mechanism of any preceding claim wherein the electrical switch comprises a vacuum switch.

15. The mechanism of any preceding claim wherein the switch assembly comprises a pair of said cam followers, each cam follower configured to cause 30 operation of a corresponding one of a pair of said electrical switches.

16. The mechanism of claim 14 wherein the pair of said cam followers is configured to cause sequential operation of the pair of said electrical switches upon relative rotation of the annular cam and the switch assembly. 15 WO 2009/095686 PCT/GB2009/000267

17. The mechanism of claim 2 wherein the cam follower is biased against the inner surface of the annular cam. 5

18. The mechanism of claim 17 wherein the switch assembly comprises a spring configured to bias the cam follower against the inner surface of the annular cam.

19. An on-load tap-changer comprising the mechanism of any preceding 10 claim.

20. An electrical transformer comprising the on-load tap-changer of claim 19.

21. A method of operating an on-load tap-changer switching mechanism 15 comprising an annular cam defining a plurality of raised portions and a switch assembly disposed af least partly within and rotatable relative to the annular cam, the switch assembly including an electrical switch and a cam follower engaged with the inner surface of the annular cam, the method comprising actuating the cam follower by engagement with one or more of the plurality of raised portions 20 on relative rotation of the annular cam and switch assembly, actuation of the cam follower causing operation of the electrical switch.

22. An on-load tap-changer switching mechanism generally as herein described, with reference to the accompanying drawings of figures 2a to 17. 25 16