GB2457079A

GB2457079A - On-load tap changer

Info

Publication number: GB2457079A
Application number: GB0801927A
Authority: GB
Inventors: Christopher Mark Nichol
Original assignee: Brush Transformers Ltd
Current assignee: Brush Transformers Ltd
Priority date: 2008-02-01
Filing date: 2008-02-01
Publication date: 2009-08-05
Also published as: GB0801927D0; JP2011512028A; WO2009095686A1; ZA201006103B; CN101990693A; AU2009208790A1

Abstract

An on-load tap-changer switching mechanism (200) comprising: an annular cam (210) defining a plurality of raised portions (311); and a switch assembly (220) disposed at least partly within and rotatable relative to the annular cam, the switch assembly including an electrical switch (224a) and a cam follower (222a) 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 the cam follower causing operation of the electrical switch.

Description

ON-LOAD TAP-CHANGER

Field of the Invention

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.

Background

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 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 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 1 60 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, connect 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 the diverter impedance 140. p

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 diverter impedance 140 alone to terminal 190; * 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 * rotary switch arm 170 of the diverter switch 130 turns further to a position 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- 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.

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-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.

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It is an object of the invention to address one or more of the above mentioned problems.

Summary of the invention

According to 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 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.

Detailed Description

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-changer; figure 2 shows a schematic plan view of a tap changer in a first configuration; figures 3a to 3c show detailed sectional views of the 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; figure 5 shows a schematic partial cutaway plan view of an annular cam within a mounting frame having electrical contacts; 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;

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figure 8 shows a schematic plan view of the tap changer of figure 2 in a 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; figure 11 shows a schematic sectional view of a slidable contact arrangement for the alternative tap changer 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; and figures 1 4a and I 4b illustrate a switching sequence for an exemplary tap-changer.

Figure 1 has already been discussed in relation to the background to the invention, described above.

Figure 2 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 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, 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 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 222 follows the profile 310 of the inner surface of the annular cam 210 to produce radial linear motion of the cam follower 222a. This radial linear motion serves to open and close the vacuum switch 224a. Figures 3a to 3c

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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 cain 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 Dl between the connecting linkage 226a and the

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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 Dl, 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 cain follower 222a is at its furthest extent away from the vacuum switch 224a. The distance Dl 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 41 Oa, 41 Ob are disposed on an outer curved surface 420 of a rotatable annular cam 210, which is preferably made of an electrically insulative material such as a polymer. In the embodiment shown, the conductive slidable contacts 41 Oa, 41 Ob 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, 41 Ob are shown in figure 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 41 Oa, 41 Ob during a tap-changing operation through rotation of the annular cam 210.

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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 shdable contacts 4lOa, 41 Ob 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.

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 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 contacts 61 Oa, 61 Ob 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, 6lOb 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 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.

One of the slidable electrical contacts 6lOa is shown, being spring-mounted to the contact earner plate 600. As the contact carrier plate 600 rotates, the contact 61 Oa travels towards a pin 710, which may also be spring-mounted. Either or both of the contact 61 Oa and pin 710 preferably have a tapered or rounded leading edge, to allow lateral movement of the contact 61 Oa to cause the pin to make electrical contact with an electrical terminal 720.

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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 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 3 11, while the second vacuum switch 224b remains closed. As the annular cam 210 rotates further, the positions of the 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 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 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 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 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.

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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 is linked via a connecting shaft 1015 to the switching assembly 1020 comprising two vacuum switches, the switching assembly 1020 being mounted on an annular 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 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 comprises a common terminal 1319 and a plurality of electrical taps 1302-13 18 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 1302-13 18.

Figures 1 4a and I 4b 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 14b. At position A, both vacuum switches VM and VR are closed, the terminal switch M is closed and the bypass resistor switch R1 is open. As the switching assembly moves away from position A, the bypass resistor switch R1 closes, followed by the vacuum switch VM opening and the terminal switch M opening.

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 R1 opening and the vacuum switch VR closing again to complete the switching operation. Each of the switching operations is

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synchronised according to the relative positions of the slidable contacts, pins and raised portions of the annular cam, as described above.

Other embodiments are intentionally within the scope of the invention as defined by the appended claims. f

Claims

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

3. The mechanism of claim I 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 respect to the mounting frame.

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

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.

7. The mechanism of claim 6 wherein the slidable contact is disposed on an outer perimeter of the annular cam.

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.

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. 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.

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

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

14. The mechanism of claim 12 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. The mechanism of claim 2 wherein the cam follower is biased against the inner surface of the annular cam.

16. The mechanism of claim 15 wherein the switch assembly comprises a spring configured to bias the cam follower against the inner surface of the annular cam.

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

18. An electrical transformer comprising the on-load tap-changer of claim 17.

19. An on-load tap-changer switching mechanism generally as herein described, with reference to the accompanying drawings of figures 2 to 12.