GB2566996A - On-load tap-changer - Google Patents

Abstract

An on-load tap-changer switching mechanism 20 comprises an annular cam plate 21 defining a radially outward-facing cam surface 24 having a plurality of raised portions 25, and a switching assembly 10 disposed at least partly within the outward-facing cam surface and being rotatable relative to it. The switching assembly comprises an electrical switch and a cam follower 30-1,2 that engages with the cam surface and is actuated by the raised portions on rotation of the switch assembly, causing operation of the switch. The outward-facing cam surface may be provided within a cam track 22 recessed from a front surface of the cam plate that includes a radially inward-facing cam surface having a plurality of raised portions 29 which are aligned with the raised portions 25 of the outward-facing cam surface. A second switch and cam follower may be provided, which may be actuated by the raised portions at a different time to the first switch and cam follower. Sliding contacts 15,16 may be mounted on the switch carriers 34-1,2, and have surfaces extending along the direction of rotation. The sliding contacts may engage with tapping contacts 50 coupled to the cam plate and may be biased into an extended position.

Description

ON-LOAD TAP-CHANGER

The invention relates to on-load tap-changer mechanisms such as those suitable for use in electrical transformers, and in particular to apparatus and methods for controlling the timing of switching operations in on-load tap-changing.

High voltage transformers, used for example in electrical substations, are subject to varying electrical loads depending upon how much power is being drawn downstream of the transformer. On-load tap-changers allow for selection of different turns ratios on a transformer without the need to interrupt the load current. This makes on-load tapchangers 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 a variable turns ratio. This enables voltage regulation of the secondary (output) side of the transformer to suit different loads.

WO 2009/095686 describes a number of arrangements for an on-load tap changer, the principles of which are briefly summarised below, with reference to figures 1 and 5.

An on-load tap-changer 100 comprises a transformer coil 160 attached 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, connects the tap position switches 110, 112 to a second electrical terminal 190, completing the circuit.

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 a diverter impedance 140.

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:

• tap switch 112 is closed;

• rotary switch arm 170 of the diverter switch 130 rotates anti-clockwise to a position where current passes through the diverter impedance 140 alone to terminal 190;

• rotary switch arm 170 turns further to a position where load current passes through diverter impedances 140 and 142 simultaneously, i.e. in parallel;

• rotary switch arm 170 turns further to a position where load current passes through diverter impedance 142 alone to terminal 190;

• rotary switch arm 170 of the diverter switch 130 turns further to a position where diverter impedance 142 is shorted and the load current 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-changing, but shows the general principles involved.

Figure 2 illustrates an annular tap changer mechanism 200 as seen in WO 2009/095686, in which a switching assembly 220 is disposed within an annular cam 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 the contacts of the diverter switch 130 for the on-load tap changer, each vacuum switch 224a, 224b being actuated by the respective cam follower 222a, 222b and connecting linkages 226a, 226b.

A more detailed sectional view of the vacuum switch 224a, connecting linkage 226a, cam follower 222a and annular can 210 of figure 2 is shown in figures 3a to 3c, together with other associated components.

Upon relative rotation of the annular cam 210 and the switching assembly 220, the cam follower 222a 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 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, 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. 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.

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 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 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 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 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 D1 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 the distances D1, 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 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 D1 between the connecting linkage 226a and 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.

Referring now to figure 4, electrically conductive slidable contacts 410a, 410b are disposed on an outer curved surface 420 of the rotatable annular cam 210, which is preferably made of an electrically insulative material such as a polymer. 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.

The slidable contacts 410a, 410b 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 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.

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 (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 311 of the annular cam 210, a sequence of switching events can be accurately determined for a given tap-changing operation.

It is an object of the invention to provide improvements to certain aspects of the design of the tap changer described in WO 2009/095686. These improvements may include improving the performance and reducing the complexity of the selected parts of the mechanism.

According to one aspect, the present invention provides an on-load tap-changer switching mechanism comprising:

an annular cam plate defining a radially-outward-facing cam surface having a plurality of raised portions;

a switching assembly disposed at least partly within, and rotatable relative to, the radially-outward-facing cam surface of the annular cam plate, the switching assembly including an electrical switch, and a cam follower engaged with the radially-outward-facing cam surface;

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 the cam follower causing operation of the electrical switch.

The radially-outward-facing cam surface may be provided within a cam track recessed from a front surface of the annular cam plate. The cam track may further include a radiallyinward-facing cam surface having a plurality of raised portions. The plurality of raised portions of the radially-outward-facing cam surface may be in circumferential alignment with the plurality of raised portions of the radially-inward-facing cam surface. The cam follower may comprise a component pivotally attached to a carrier of the electrical switch at a first pivot point and pivotally attached to an actuator shaft of the electrical switch at a second pivot point, and a pin extending axially from the cam follower component into the recessed cam track such that the pin travels within the cam track during rotation of a switching assembly relative to the annular cam plate. The second pivot point may be a slotted pivot configured to facilitate rotation of the cam follower relative to the actuator shaft and limited lateral motion transverse to the axis of the actuator shaft. The electrical switch may be mounted on a carrier and coupled to the cam follower by an actuator shaft. The electrical switch and actuator shaft may have a displacement axis for the actuator disposed at an oblique angle to a radius of the annular cam plate passing through the point of contact of the cam follower with the radially-outward-facing cam surface. The plurality of raised portions may be configured to displace the cam follower in a radially outward direction thereby driving an actuator of the electrical switch against the bias of a closing spring to open contacts of the electrical switch. The mechanism may further include a closing spring configured to bias contacts of the electrical switch towards a closed position as the cam follower travels around the radially-outward-facing cam surface, the bias being overcome by displacement of the cam follower by the raised portions without the assistance of an opening spring.

The mechanism may further comprise:

a slidable contact mounted on a common carrier with the electrical switch; and at least one tapping contact coupled to the annular cam plate and configured to engage a contact surface of the slidable contact during relative rotation of the switching assembly and the annular cam plate, the contact surface extending along a direction of the relative rotation, wherein the slidable contact is biased towards an extended position for contact with the at least one tapping contact and has a limited degree of rotational freedom about an axis transverse to the direction of relative rotation for rocking motion during periods of contact with the at least one tapping contact.

The limited degree of rotational freedom may comprise between 2.5 and 10 degrees in at least one direction from an undeflected position.

The switching assembly may further comprise a second one of said electrical switches and a second cam follower coupled to the second switch and engaged with the radiallyoutward-facing cam surface, wherein the plurality of raised portions are configured to actuate the second cam follower on relative rotation of the annular cam and switch assembly, actuation of the second cam follower causing operation of the second electrical switch at different times to the operation of the first electrical switch.

The mechanism may further include a second slidable contact mounted on a common carrier with the second electrical switch;

the at least one tapping contact configured to engage a contact surface of the second slidable contact during relative rotation of the switching assembly and the annular cam plate, the contact surface of the second slidable contact extending along a direction of the relative rotation, wherein the second slidable contact is configured to make and break contact with the at least one tapping contact at different times to those of the first slidable contact.

According to another aspect, the present invention provides an on-load tap-changer comprising the mechanism as defined above.

According to another aspect, the invention provides an electrical transformer comprising the on-load tap-changer as defined above.

According to another aspect, the invention provides a method of changing a transformer tapping using a tap-changer switching mechanism comprising:

rotating a switching assembly disposed at least partly within, and rotatable relative to, a radially-outward-facing cam surface of an annular cam plate, the annular cam plate defining a plurality of raised portions in the radially-outward-facing cam surface, the switching assembly including an electrical switch, and a cam follower engaged with the radially-outward-facing cam surface;

actuating the cam follower by the plurality of raised portions on the relative rotation of the annular cam and switch assembly, to cause operation of the electrical switch.

According to another aspect, the invention provides an on-load tap-changer switching mechanism comprising:

an annular cam plate defining a cam track defining a plurality of raised portions;

a switching assembly disposed at least partly within, and rotatable relative to, the cam track, the switching assembly including an electrical switch, and a cam follower engaged with the cam track;

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 the cam follower causing operation of the electrical switch, and wherein the cam follower comprises a component pivotally attached to a carrier of the electrical switch at a first pivot point and pivotally attached to an actuator shaft of the electrical switch at a second pivot point, and a pin extending axially from the cam follower component into the cam track such that the pin travels within the cam track during rotation of a switching assembly relative to the annular cam plate, the second pivot point being a slotted pivot configured to facilitate rotation of the cam follower relative to the actuator shaft and limited lateral motion transverse to the axis of the actuator shaft.

According to another aspect, the invention provides an on-load tap-changer switching mechanism comprising:

an annular cam plate defining a cam track defining a plurality of raised portions;

a switching assembly disposed at least partly within, and rotatable relative to, the cam track, the switching assembly including an electrical switch, and a cam follower engaged with the cam track;

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 the cam follower causing operation of the electrical switch, the mechanism further comprising a slidable contact mounted on a common carrier with the electrical switch and at least one tapping contact coupled to the annular cam plate and configured to engage a contact surface of the slidable contact during relative rotation of the switching assembly and the annular cam plate, the contact surface extending along a direction of the relative rotation, wherein the slidable contact is biased towards an extended position for contact with the at least one tapping contact and has a limited degree of rotational freedom about an axis transverse to the direction of relative rotation for rocking motion during periods of contact with the at least one tapping contact

Embodiments of the present invention will now be described byway of example and with reference to the accompanying drawings in which:

Figure 1 shows a circuit diagram of a prior art design of an on-load tap-changer;

Figure 2 shows a schematic plan view of a prior art tap changer in a first configuration;

Figures 3a to 3c show detailed sectional views of parts of the prior art tap changer of figure 2;

Figure 4 shows a perspective view of a pair of slidable contacts around a perimeter edge of an annular cam according to the prior art tap changer;

Figure 5 shows a schematic partial cutaway plan view of an annular cam within a mounting frame having electrical contacts according to the prior art tap changer;

Figure 6 shows a schematic diagram illustrating a switching operation sequence of an on-load tap changer deploying two vacuum interrupters (also referred to as vacuum switches) and two sliding contact switches;

Figure 7 is a front and side perspective view of selected components of an on-load tap changer, with certain components omitted for visibility of the selected components;

Figure 8 is a front view of selected components of the on-load tap changer of figure

7, with vacuum interrupters shown in sectional view to reveal internal features;

Figures 9a and 9b are front views of a vacuum interrupter, carrier and cam follower assembly of the on-load tap changer of figure 7;

Figure 10 is a perspective rear view of selected components of the on-load tap changer of figures 7-9 including carrier, sliding switches and tapping contact pins;

Figure 11 is a detailed rear view of sliding switches used in the tap changer of figure 10;

Figure 12 is a timing diagram showing the relative timing of operation of various switches of the tap changer of figures 6 to 11;

Figure 13 is a perspective view of a three-phase on-load tap changer using the mechanisms of figures 7 to 11.

Figures 1 to 5 have already been described in detail in relation to the background of the invention.

Throughout the present specification, the descriptors relating to relative orientation and position, such as “top”, “bottom”, “horizontal”, “vertical”, “left”, “right”, “up”, “down”, “front”, “back”, as well as any adjective and adverb derivatives thereof, are used in the sense of the orientation of features as presented in the drawings. However, such descriptors are not intended to be in any way limiting to an intended use of the described or claimed invention.

Figure 6 shows a sequence of operations 1 to 9 for the on-load tap changer. Components are the same for each figure and reference numerals have not been repeated across all the diagrams. On-load tap changer 1 comprises a transformer coil 2 having at least two tappings 3, 4 each providing a tapping contact 5. A switch assembly 10 is electrically coupled to an external connector 6. The switch assembly 10 comprises (i) a first vacuum interrupter 11 defining a first branch of the switch assembly configured to switchably connect and disconnect the connector 6 to successive tapping contacts 5 by way of a first sliding contact 15, and (ii) a second vacuum interrupter 12 in series with a resistor 13 together defining a second branch of the switch assembly 10 configured to switchably connect and disconnect the connector 6 to successive tapping contacts 5 by way of a second sliding contact 16. The first branch of the switch assembly 10 may also be referred to as the main current path M and the second branch of the switch assembly 10 may also be referred to as the transition current path T.

As seen in figure 6-1, external connector 6 is electrically connected to the first tapping 3 via both the first branch of the switch assembly 10 (vacuum interrupter 11 and sliding contact 15) and the second branch of the switch assembly (vacuum interrupter 12, resistor 13 and second sliding contact 16).

As seen in figure 6-2, the second vacuum interrupter 12 opens, thereby ensuring that any current flow between the tapping 3 and the connector 6 is via the first branch of the switching assembly 10 only, i.e. taking the second sliding contact 16 off-load.

In figure 6-3, the switching assembly 10 has moved relative to the tapping contacts 5 such that second sliding contact 16 breaks contact from the tapping contact 5 of tapping 3. This breaking of contact occurs under no load and contact arcing and damage is prevented.

In figure 6-4, the switching assembly 10 has moved relative to the tapping contacts 5 such that the second sliding contact 16 now makes contact with the tapping contact 5 of tapping 4. This making of contact occurs under no load and contact arcing and damage is prevented.

In figure 6-5, the second vacuum interrupter 12 closes thereby providing a current path between the connector 6 and each of tapping 3 and tapping 4, respectively via the first branch and second branch of the switching assembly 10. Resistor 13 limits any circulating current around the first and second branches of the switching assembly.

In figure 6-6, the first vacuum interrupter 11 opens thereby ensuring that any current flow between the tapping 3 and the connector 6 via the first branch of the switching assembly 10 ceases, i.e. taking the first sliding contact 15 off-load.

In figure 6-7, the switching assembly 10 has moved relative to the tapping contacts 5 such that first sliding contact 15 breaks contact from the tapping contact 5 of tapping 3. This breaking of contact occurs under no load and contact arcing and damage is prevented.

In figure 6-8, the switching assembly 10 has moved relative to the tapping contacts 5 such that the second sliding contact 15 now makes contact with the tapping contact 5 of tapping

4. This making of contact occurs under no load and contact arcing and damage is prevented.

In figure 6-9, the first vacuum interrupter 11 closes thereby providing a current path between the connector 6 and tapping 3 via both the first branch and second branch of the switching assembly 10.

This completes a full tap change between tapping 3 and tapping 4. The procedure is completely reversible to return the tap changer to tapping 3 from tapping 4. There are preferably further tappings 3 to the left and right of each diagram to which the switching assembly can move in the same fashion.

Precise control of the timing of the opening and closing of the vacuum interrupters 11, 12 coordinated with movement of the sliding contacts 15, 16 of the switching assembly 10 is required to ensure correct operation of the tap changer.

Figure 7 shows modifications and improvements to the on-load tap changer of figures 2 to 5 described above. With reference to figure 7, on-load tap changer 20 comprises an annular cam plate 21 defining a cam track 22 extending around the cam plate 21. The cam track 22 preferably comprises a recessed track 22 in the front surface 23 of the cam plate 21 which defines a circumferential radially-outward-facing cam surface 24, i.e. having its surface parallel to a central axis of rotation 41 passing through the centre of the annular cam plate. The radially-outward-facing cam surface 24 has low portions 26 having a common principal radius π measured from the central axis of rotation 41. The cam surface 24 further defines a plurality of raised profile portions 25 interspersed between the low portions 26, each raised profile portion 25 projecting radially outward from the common principal radius n.

The cam track 22 may also include a circumferential radially-inward-facing cam surface 27 having low portions 28 with common principal radius Γ2 measured from the central axis of rotation 41 and raised profile portions 29 interspersed between the low portions 28. The raised portions 29 of the radially-inward-facing cam surface 27 are preferably in circumferential alignment with the raised portions of the radially-outward-facing cam surface 24.

A pair of cam followers 30-1, 30-2 each includes a pin 31-1, 31-2 extending axially from the respective cam follower 30 into the recessed cam track 22 such that the pins 31 run within the cam track 22 during rotation of a switching assembly 10 relative to the annular cam plate 21. The cam followers 30 may be broadly triangular in shape and each comprises a first pivot point 32-1,32-2 by which the cam follower is coupled to a respective vacuum interrupter carrier 34-1, 34-2, and a second pivot point 33-1, 33-2 by which the cam follower is coupled to a respective vacuum interrupter 11,12 (best seen in figure 8).

As seen in figure 8, each vacuum interrupter 11,12 comprises a first contact 17 (e.g. a fixed contact), a second contact 18 (e.g. a moveable contact) and an actuator shaft 19 facilitating axial displacement of the second contact 18 relative to the first contact 17. The actuator shaft 19 is driven by the respective cam follower 30 via the second pivot point 33. A housing 35 contains a bias spring 36 which is axially compressed against a seat 37 within the housing 35 and a shoulder 38 on the actuator shaft 19. (The spring is omitted from the diagram for the vacuum interrupter 11 for revealing clearer detail of the component parts.) The bias spring 36 thereby acts as a closing spring providing the necessary closing forces on the vacuum interrupter second contact 18, against the first contact 17.

As best seen in figures 9a and 9b, the opposite end of the vacuum interrupter 12 is coupled to the carrier 34 (figure 9b) and provides a terminal 39 (figure 9a) electrically connected to the fixed contact 17. A corresponding construction applies to the carrier for vacuum interrupter 11.

Further detail of the cam follower 30 is seen in figure 9b. The first pivot point 32 facilitates rotation of the cam follower 30 relative to the carrier 34 and the second pivot point 33 includes a slotted hole arrangement which facilitates not only rotation of the cam follower 30 relative to the actuator shaft 19 but also a limited lateral motion transverse to the axis of the actuator shaft 19 and in the plane of the drawing.

In use, the switching assembly 10 rotates relative to the annular cam plate 21. In a preferred arrangement, the annular cam plate 21 is static relative to a housing of the tap changer 20 and the switching assembly rotates within it about the central axis 41. In alternative arrangements, the switching assembly 10 could be static and the annular cam plate 21 rotates around it, or both rotate relative to one another around the common axis

41.

As the switching assembly 10 rotates, each of the pins 31-1,31-2 running in the cam track 22 causes a respective one of the cam followers 30-1, 30-2 to be deflected radially outward, against the bias of the respective closing spring, by raised portions 25. As a result, the actuator shaft 19 is axially displaced to open the respective vacuum interrupter 11,12 against the bias of the closing spring 36. At other periods of the rotation, the pins 31 ride along the low portions 26 of the cam track 22 such that the cam followers 30 allow the respective actuator shafts 19 to return the respective vacuum interrupter 11, 12 to the closed position.

By contrast, the earlier design discussed in relation to figures 2 to 5 required both an opening spring (e.g. 320) and a closing contact pressure spring (e.g. 330) to actuate each vacuum interrupter. Vacuum interrupter contacts 17,18 can be subject to contact welding caused by arcing or high currents in use and in the prior art arrangement a strong opening spring 320 is required to ensure sufficient force to open the vacuum interrupter against any contact welding. A further conflicting requirement is that high current vacuum interrupter contacts require a strong closing force to ensure proper contact is maintained during high current flows.

In the improved design of figures 7 to 9, only a single spring 36 is required for providing adequate closing forces on the contacts 17, 18, and this contact spring can be optimised to ensure sufficient contact pressure to avoid problems with very high currents, contact welding, contact bounce etc. All opening forces required for overcoming any opening resistance caused, e.g. by contact welding etc, can be overcome by use of the radiallyoutward-facing cam surface 24 acting on the cam followers 30. This can substantially simplify the design of the carrier 34 and spring arrangement, reduce the component count and total mass of the switching assembly 10, reducing cost, and potentially improving reliability of the apparatus not least by virtue of a reduced number of components that could fail. Adequate opening force can be assured by an appropriate gearing ratio of a drive motor used to rotate the switching assembly via toothed gear wheel 40. This may be further assisted by operation of the gearing with a flywheel which may also be desired for other purposes.

The design described in connection with figures 7 to 9 also provides a safety feature in that it is assured that the vacuum interrupter contact 18 must have opened by the radiallyoutward displacement of the cam follower 30 for the rotating switching assembly to be able to continue with rotation of the switching assembly to a position in which sliding contacts 15,16 are switched. This may prevent any incorrect operation of a sliding contact 15 or 16 under an electrical load condition (i.e. when the respective vacuum interrupter had not opened), for example which could otherwise be caused by opening spring failure in the design of figures 2 to 5.

As shown in figures 7 and 9, the carriers 34 are configured such that the vacuum interrupter axes are somewhat oblique to the radius of the annular cam plate 21. This enables a more compact disposition of the switching assembly and avoids the requirement for laterally-extending connecting linkages 226a in the prior art device of figures 3a-3c. This avoids or substantially reduces side-loading forces on the vacuum interrupters 11, 12 and their actuator shafts 19 which can improve reliability. The somewhat oblique alignment of the carriers 34 may also provide greater design freedom for improved mechanical alignment in the precise circumferential positioning of the pins 31-1, 31-2 of the cam followers 30-1, 30-2 which determines the precise relative timing of operation of the vacuum interrupters 11, 12 to one another and to the sliding contacts 15, 16 to be described further below. In a general aspect, therefore, the vacuum interrupters 11, 12 each have an actuator shaft 19 with a displacement axis disposed at an oblique angle to a radius of the annular cam plate approximately passing through the point of contact of the cam follower with the radially-outward-facing cam surface, e.g. at pin 31. In a preferred arrangement, the oblique angle may lie within the range of 20 to 30 degrees. The pair of vacuum interrupters 11,12 may therefore converge at an angle of 40 to 60 degrees and in the illustrated arrangement at approximately 45 degrees. The carriers 34 for each of the vacuum interrupters 11,12 may form a unitary carrier structure as shown.

As seen particularly in figure 9b, the slotted second pivot point 33 provides a degree of freedom to accommodate the rotation of the cam follower about the first pivot point and this ensures that the vacuum interrupter 11,12 can be rigidly mounted in the carrier 34 without imposing significant lateral or side-loading forces on the vacuum interrupter actuator shaft 19 thereby improving reliability. The vacuum interrupters 11,12 thereby do not require pivotal mounting in the carriers 34 further simplifying the design and component count. In a general aspect, the slotted second pivot point 33 facilitates rotation of the cam follower relative to the actuator shaft 19 and limited lateral motion transverse to the axis of the actuator shaft.

The on4oad tap changer 20 as described in relation to figures 7 to 9 further includes an improved configuration of sliding contacts 15, 16 (figure 6).

With reference to figures 7 and 10, the switching assembly 10 further includes a pair of sliding contacts 15, 16 mounted on the carrier 34. Each sliding contact 15, 16 is displaceable to a depressed condition (i.e. displaceable in a radially inward direction) and spring-biased to an extended condition (i.e. based in a radially-outward direction). A plurality of tapping contact pins 50 each corresponding to one of the tapping contacts 5 of figure 6 are disposed circumferentially around the back face of the annular cam plate 21 (only two of which are shown in figure 10, the others being omitted for clarity). In the embodiments shown in the drawings, the tapping contacts 5, 50 are positioned at 20 degree intervals. The tapping contact pins 50 may be disposed on the annular cam plate 21 or on a separate structure coupled to the annular cam plate. As readily seen in figure 10, the pair of sliding contacts 15, 16 each have a contact surface 54 of circumferential length slightly shorter than the separation of adjacent tapping contact pins 50 such that the sliding contact conditions required in the various stages of figure 6 can all be achieved.

Specifically, sliding contacts 15, 16 are positioned and configured such that there is a rotation position in which both contacts 15 and 16 are in contact with the same tapping contact pin 50-1 (figure 6-1); a rotation position in which one contact 16 has disconnected from the tapping contact pin 50-1 while the other contact 15 is still connected to the tapping contact pin 50-1 (figure 6-3); a rotation position in which one contact 16 has connected to the next tapping contact pin 50-2 while the other contact 15 is still connected to the tapping contact pin 50-1 (figure 6-4); a rotation position in which one contact 16 remains connected to the next tapping contact pin 50-2 while the other contact 15 has disconnected from the tapping contact pin 50-1 (figure 6-7); and a rotation position in which both contacts 15 and 16 are in contact with the next tapping contact pin 50-2 (figure 6-8).

With reference to figure 11, each sliding contact 15, 16 is biased into the extended (upward) condition by a pair of spring members 51-1,51-2. The sliding contact preferably comprises a generally trapezium-shaped body 55 having tapering (non-parallel) sides 52 and an approximately parallel base 53 and contact surface 54. Maintaining good contact pressure between the contact surface 54 and the relevant tapping contact pin 50 is important for optimal operation throughout the period that the sliding contact is in moving contact with a tapping contact pin 50. It has been found that providing at least two bias points on the body 55 close to leading and trailing sides 52-1, 52-2 of the base 53 using springs 51-1, 51-2 provides an even upward pressure, while allowing a small rotational displacement of the contact body 55 in its housing 56 about an axis close to a leading edge 57 and / or trailing edge 57 by virtue of the tapering, non-parallel sides. (The edge 57 that is leading or trailing will, of course, depend on the tap switching direction as previously discussed.) This small rotational degree of freedom or rocking contact ensures better contact engagement and disengagement as a leading edge 57 and trailing edge 57 engages / disengages with the contact pin 50, and ensures a more consistent and balanced contact pressure as the contact body is in sliding engagement with a tapping contact pin. In an example as shown, the tapering, non-parallel trapezium sides converge at a relative angle in the range 5 to 20 degrees. For example, each of the sides 52-1, 522 may be disposed at an angle of between 2.5 and 10 degrees from the perpendicular to the contact surface 54. In the example shown, each of the sides is disposed at an angle of approximately 5 degrees to the perpendicular, thereby giving a convergence of the nonparallel trapezium sides of 10 degrees.

Figure 12 shows an example timing diagram 80 for the tap changer of figures 6 to 11. Each of the switch positions corresponding to figures 6-1 to 6-9 is indicated along the xaxis of the graph. The tap changer 20 is fully reversible in operation, and the specific timing diagram 80 represents operation in the reverse sequence of figures 6-9 to 6-1 (i.e. transitioning from tapping 4 to tapping 3), the period or duration of each condition of figures 6-9 to 6-1 being indicated below the traces. Trace 81 represents the operation of the first sliding contact 15 in the main current path. Trace 82 represents the operation of the first vacuum interrupter 11 in the main current path. Trace 83 represents the operation of the second sliding contact 16 in the transition current path. Trace 84 represents the operation of the second vacuum interrupter 12 in the transition current path. In each trace, high level corresponds to switch closed and low level corresponds to switch open. Maintaining the timing of each portion of the switching cycle is important.

For example, a period of time between a vacuum interrupter being opened and the corresponding sliding contact being opened (e.g. periods 6-8 and 6-4) should be sufficiently long for zero current to be achieved. This may depend on the current frequency of the tap changer. In one embodiment, this period should be a minimum of 10 ms, ideally 15 ms to include a safety margin. The period of time during which both main and transition current paths are active (i.e. period 6-5) should be kept as short as possible to minimise circulating currents around the two tapping contacts 3, 4, which will be limited by the resistor 13. Trace 85 shows the opening and closing displacement of the vacuum interrupter moving contact in the main current path. Trace 86 shows the opening and closing displacement of the vacuum interrupter moving contact in the transition current path. These traces show there is sufficient contact opening gap in each vacuum interrupter prior to the corresponding sliding contact operation.

The respective durations of the periods 6-9 to 6-1 will be determined in part by the geometry of the circumferential cam track 22 and the cam surface features, the number of tapping contact pins 50 distributed around the annular cam plate, the spacing / separation of the sliding contacts 15, 16 and the lengths of their contact surfaces 54. In addition, the speed of relative rotation of the switching assembly 10 and the annular cam plate 21.

With reference to figure 13, a three-phase implementation of an on-load tap changer 70 is shown. Each phase has one tap changer 20 as seen in figures 6 to 11, and three such tap changers 20-1, 20-2, 20-3 are configured with a common drive mechanism 71. The centre phase tap changer 20-2 may be driven directly by the drive mechanism 70 and the two outer phase tap changers 20-1,20-3 may be driven through two idler gears 72-1, 72-

2. Synchronisation of the three tap changers is maintained through the gear mechanism.

The cam track 22 and circumferential radially-outward-facing cam surface preferably extend fully around 360 degrees of the annular cam plate. This enables the switching assembly to complete more than one rotation in the same direction, which can be utilised in conjunction with a changeover lever which brings a second set of tapping contact pins into position, e.g. by axial displacement thereof. This doubles the number of tappings that can be served by the tap changer. In an alternative arrangement, where fewer tapping contacts are required, the cam track and circumferential radially-outward-facing cam surface need not extend a full 360 degrees around the cam plate.

For optimal performance, it is desirable to enable the drive mechanism to start rotating the gear 40 as quickly as possible to commence a tap changing operation and to stop quickly on completion of the tap changing operation, to prevent over-run of the drive mechanism to a subsequent tapping contact. Fast stop and start capability thereby allows a greater number of tapping contacts to be disposed around the annular cam plate. In the example, there are 17 contacts and a changeover position each occupying 20 degrees of rotation. The drive mechanism must be able to start and stop quickly enough to be able to resolve each 20 degree segment, while running at the correct speed through the 20 degree segment. The gearing ratio must also be adequate to drive the rotation of the switching assembly 20 with enough mechanical advantage to ensure that smooth operation against the mechanical resistances of the cam followers 30 driving the vacuum interrupter 11,12 opening forces. To provide optimal gear ratios for accurate speed control, adequate drive strength, and fast start-stop over a limited arc of rotation (e.g. 20 degrees), a motor assisted by a spring-loaded flywheel with mechanical brake may be used to drive the rotation of the switching assembly 20.

The described arrangements use vacuum interrupters for the switches on the main and transition current paths for improved electrical performance and reduced arcing, but it will be understood that other axially actuated switches could be used.

The arrangement of cam track 22 shown in the figures comprises a channel defining both radially-outward-facing cam surface 24 and radially-inward-facing cam surface 27 in which the raised portions 29 of the radially-inward-facing cam surface 27 are preferably in circumferential alignment with the raised portions of the radially-outward-facing cam surface 24. This can provide additional guidance to the pins 31 of the cam followers 30, potentially offering a further fail-safe feature in that in the event of a failure of contact spring 36, the vacuum interrupter may be forced to close by the action of the radiallyinward-facing cam surface 27, closure being then maintained at least by atmospheric pressure if not spring pressure. However, only the radially-outward-facing cam surface 24 is essential and the radially-inward-facing cam surface 27 could be dispensed with, e.g. in a cam plate with a single circumferential step, or a single circumferential step combined with a tapered return to the level of the front surface 23.

The tap changer as described above has particular usefulness as a bolt-on or retrofit tap changer for use with an existing transformer system in that it is small and compact and does not need to be contained within a common oil tank shared with the transformer. It 5 can be mounted external to the transformer oil tank thereby making maintenance and servicing much easier and less costly not least since it is not necessary to lift the tap changer module vertically out of a main oil tank of the transformer containment structure.

Other embodiments are intentionally within the scope of the appended claims.

Claims (16)

1. An on-load tap-changer switching mechanism comprising:

an annular cam plate defining a radially-outward-facing cam surface having a plurality of raised portions;

a switching assembly disposed at least partly within, and rotatable relative to, the radially-outward-facing cam surface of the annular cam plate, the switching assembly including an electrical switch, and a cam follower engaged with the radially-outward-facing cam surface;

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 the cam follower causing operation of the electrical switch.

2. The mechanism of claim 1 in which the radially-outward-facing cam surface is provided within a cam track recessed from a front surface of the annular cam plate.

3. The mechanism of claim 2 in which the cam track further includes a radially-inwardfacing cam surface having a plurality of raised portions.

4. The mechanism of claim 3 in which the plurality of raised portions of the radiallyoutward-facing cam surface are in circumferential alignment with the plurality of raised portions of the radially-inward-facing cam surface.

5. The mechanism of claim 2 in which the cam follower comprises a component pivotally attached to a carrier of the electrical switch at a first pivot point and pivotally attached to an actuator shaft of the electrical switch at a second pivot point, and a pin extending axially from the cam follower component into the recessed cam track such that the pin travels within the cam track during rotation of a switching assembly relative to the annular cam plate.

6. The mechanism of claim 5 in which the second pivot point is a slotted pivot configured to facilitate rotation of the cam follower relative to the actuator shaft and limited lateral motion transverse to the axis of the actuator shaft.

7. The mechanism of claim 2 in which the electrical switch is mounted on a carrier and coupled to the cam follower by an actuator shaft, the electrical switch and actuator shaft having a displacement axis for the actuator being disposed at an oblique angle to a radius of the annular cam plate passing through the point of contact of the cam follower with the radially-outward-facing cam surface.

8. The mechanism of claim 1 in which the plurality of raised portions are configured to displace the cam follower in a radially outward direction thereby driving an actuator of the electrical switch against the bias of a closing spring to open contacts of the electrical switch.

9. The mechanism of claim 1 further including a closing spring configured to bias contacts of the electrical switch towards a closed position as the cam follower travels around the radially-outward-facing cam surface, the bias being overcome by displacement of the cam follower by the raised portions without the assistance of an opening spring.

10. The mechanism of claim 1 further comprising:

a slidable contact mounted on a common carrier with the electrical switch; and at least one tapping contact coupled to the annular cam plate and configured to engage a contact surface of the slidable contact during relative rotation of the switching assembly and the annular cam plate, the contact surface extending along a direction of the relative rotation, wherein the slidable contact is biased towards an extended position for contact with the at least one tapping contact and has a limited degree of rotational freedom about an axis transverse to the direction of relative rotation for rocking motion during periods of contact with the at least one tapping contact.

11. The mechanism of claim 10 in which the limited degree of rotational freedom comprises between 2.5 and 10 degrees in at least one direction from an undeflected position.

12. The mechanism of any preceding claim in which the switching assembly further comprises a second one of said electrical switches and a second cam follower coupled to the second switch and engaged with the radially-outward-facing cam surface, wherein the plurality of raised portions are configured to actuate the second cam follower on relative rotation of the annular cam and switch assembly, actuation of the second cam follower causing operation of the second electrical switch at different times to the operation of the first electrical switch.

13. The mechanism of claim 12 further including a second slidable contact mounted on a common carrier with the second electrical switch;

the at least one tapping contact configured to engage a contact surface of the second slidable contact during relative rotation of the switching assembly and the annular cam plate, the contact surface of the second slidable contact extending along a direction of the relative rotation, wherein the second slidable contact is configured to make and break contact with the at least one tapping contact at different times to those of the first slidable contact.

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

15. An electrical transformer comprising the on-load tap-changer of claim 14.

16. A method of changing a transformer tapping using a tap-changer switching mechanism comprising:

rotating a switching assembly disposed at least partly within, and rotatable relative to, a radially-outward-facing cam surface of an annular cam plate, the annular cam plate defining a plurality of raised portions in the radially-outward-facing cam surface, the switching assembly including an electrical switch, and a cam follower engaged with the radially-outward-facing cam surface;

actuating the cam follower by the plurality of raised portions on the relative rotation of the annular cam and switch assembly, to cause operation of the electrical switch.