THE CAM SLOT

Technical Field

The present invention relates generally to tap changers for transformers, and in particular to a selector switch mechanism with compensation for the position of a cam slot in a tap changer of a transformer.

Background

In power transformers, on-load tap changers (OLTC) are used to change tapping connections of transformer windings while the transformer is energised. To satisfy the needs of increased service life and reduced maintenance of on-load tap changers, tap changers of newer type often uses vacuum interrupters to quench the arc instead of in oil as in traditional tap

changers .

To control the tap changer in a transformer, several vacuum interrupters which in turn are controlled by rotating

mechanisms are used. To affect the vacuum interrupters and thus the tap changer in a predetermined certain pattern, the rotating mechanism can be provided with one or several cam followers following cam slots or grooves provided with cams located on the inside of a cylinder.

A common problem when cam slots and cam followers are used is that the mechanism to be activated by the cam movement might be unintentionally affected by irregularities in the position of the cam slot. Even small irregularities might affect the vacuum interrupters since they are operated with very small movements, often only a few mm. A cam slot position diverging from its original position with 1 mm thus can cause not wanted affection of the interrupter mechanism.

DE 198 21 775 and corresponding CA 2 311 781 (Dl) describes a selector switch of the type mentioned above. The selector switch is developed for tapped transformers to enable

continuous tap switching with respect to a tapped winding, whereby fixed stepped contacts are arranged inside a cylinder made of insulating material on a horizontal plane for each phase. A rotating actuating or switching shaft is located inside the insulating cylinder. The actuating shaft has a moveable hinged contact carrier for each phase that is to be switched and vacuum switching cells are respectively and vertically arranged on said contact carrier. In more detail, Dl illustrates (see figure 1, and page 5, line 16 - page 6 line 14) a cylindrical tap changer with a switching shaft (2) that carries a contact support (3) mounted on a pivot block (4) on the shaft. Two vacuum interrupters or switches (27, 29) (claim 1) are mounted vertically (upright) on the contact support (3) and their actuating rods (stems 35, 36) extend vertically (axially) . In order to operate the actuating rods (35, 36) of the vacuum switches (27 and 28), there are two levers (37, 38) that each have a free end carrying a

respective roller bolt (41, 42) with a respective control roller (39, 40) . Both levers (37, 38) are pivotal on

respective pivots (43, 44) . The control rollers (39, 40) of the levers (37, 38) themselves ride on a control ring or cam ring (45) provided with cams. The two control surfaces (46, 47) serve to actuate the vacuum switches (27, 28) in that as the respective control rollers (39, 40) ride onto a cam or bump the respective levers (37, 38) will pivot about their pivots (43, 44) and thus operate the actuating rod (35 or 36) of the respective vacuum switch (27 or 28) . The contact support (3) in Dl can handle and compensate for the problem with irregularities in the position of the cam slot, as described above (see Dl page 9, line 9-18), since the contact support (3) has two guide rollers (55, 56) that roll on opposite faces of the output contact ring (48) and thus guide the entire contact support (3) . The contact supports (3) is pivotal on the switching shaft (2) and are guided by the ring (48) so that there is a proper control of the rollers (39, 40) and thus actuation of the vacuum switches (27, 28) .

A problem with having the entire contact support pivotally arranged and guided is that also the vertically arranged vacuum switches are carried and also pivotally mounted to the shaft. When mounting the vacuum interrupters in the cylinder, the cuffs (33, 34 in Dl) of the vacuum interrupters are filled with oil, and there is a risk that air might be trapped if the vacuum interrupters are not vertically arranged during the filling process. Thus, pivotal arrangement of the vacuum interrupters makes the manufacturing of the tap changer more difficult and also risks deteriorating the function of the vacuum interrupters .

Summary of invention

It is an object of the invention to provide a selector switch mechanism for a tap changer in a transformer overcoming the above mentioned problem.

The tap changer comprises a cylinder and a vertical switching shaft inside the cylinder, wherein contacts carried by the shaft engage and disengage corresponding contacts on the inside of the cylinder, and comprising axially arranged vacuum interrupters actuated by a respective actuating lever

following a ring structure with cams arranged on the inside of the cylinder. The tap changer also includes at least one guide that follows the ring structure, each actuating lever being pivotally connected to the guide. The tap changer is

characterised in that the guide is pivotally arranged in relation to the vacuum switches, and the vacuum switches is fixedly secured to the switching shaft.

In a preferred embodiment the invention provides a tap changer arrangement, which tap changer comprises a vertically arranged insulated cylinder having contact elements projecting inwards, a switching shaft extending in the axial direction of the cylinder, and being rotatably arranged in the cylinder and a contact carrying structure attached to the shaft, and rotating with the shaft and provided to effect tap changes upon

rotation of the shaft inside the cylinder. The tap changer also comprises vacuum interrupters arranged vertically on the contact carrying structure and the vacuum interrupters are operated by activating levers following a cam ring arranged on the inside of the cylinder. The tap changer also includes a reference guide carried by the contact carrier structure, which reference guide upon rotation of the shaft, follows a reference surface of the cam ring, the activating lever being pivotally connected to the reference guide and thereby the reference guide keep the actuating lever in position in relation to the reference surface of the cam ring. The tap changer and the arrangement is characterised in that the reference guide is pivotally arranged in relation to the shaft and in that each vacuum interrupter is fixedly secured to the shaft. In this way the reference guide is free to follow the cam ring up and down if the vertical position of the cam ring varies in relation to the shaft and/or contact carrier, whereas the vacuum interrupters are vertically and axially oriented and do not pivot. The vacuum interrupter are arranged mechanically between the shaft and the reference guide, the vacuum interrupters are on the fixed structure of the contact carrier whereas the

reference guide is arranged between a pivot on the contact carrier and the cam ring.

The actuating lever is preferably pivotally connected to the reference guide, having its pivot point on the guide.

Preferably, the actuating lever is rotatably connected by means of a hinge to an actuating rod of the vacuum

interrupter, the actuating rod extending in the axial

direction of the vacuum interrupter. The axial direction of the vacuum interrupter is the axial, and vertical, direction of the shaft. Preferably the reference guide is pivotally arranged and has a pivot axis in the same direction as the axis of the hinge connection between the actuating lever and the actuating rod. Preferably the pivot axis of the reference guide and the axis of hinge between actuating lever and actuating rod are the same axis of rotation, i.e. they share rotation axis.

In an embodiment the reference guide is provided with two guide rollers for running against the reference surface of the cam ring.

In an embodiment the actuating lever is provided with a roller for running against the cam ring, and the roller follows the cams of the ring.

In a preferred embodiment is the lever arm provided with a second cam follower arranged to follow a second cam slot arranged on the inside surface of said cylinder. Brief Description of the Drawings

Figure 1 illustrates a side view of an interior of a tap changer cylinder in accordance with the invention.

Figure 2 illustrates details of the arrangement of figure 1 from above .

Figure 3 is a simplified perspective view of an arrangement of a selector switch according to the invention, compensating for the cam slot position.

Figure 4 illustrates the inside surface of a cylinder provided with two cam slots and a reference surface.

Detailed Description

Figure 1 illustrates a contact arrangement inside a

cylindrical tap changer, which contact arrangement is provided in accordance with the invention. Figure 1 is a side view of a vertically arranged cylindrical tap changer. In more detail figure 1 illustrates a cylindrical tap changer comprising a cylinder 2 and a rotatable switching shaft 3 arranged inside the cylinder, in the centre of the cylinder 2. The cylinder 2 is provided with tap contacts 12 extending into the cylinder. These tap contacts 12 are adapted to mate with interior contacts 22 for providing a tap change. The interior tap contacts 22 are secured to the shaft so that they rotate with the shaft. To provide good electrical connection the interior contacts 22 are spring loaded. The arrangement also includes vacuum interrupters, one is illustrated 5, and a mechanism for operating the vacuum interrupter. The mechanism and the vacuum interrupter are fastened to the shaft and rotate with the shaft. The mechanism includes an actuating lever 6 pivotally arranged on a pivot 9. One end of the lever 6 is provided with a cam follower 16 and is arranged to run in a cam ring 7 arranged inside the cylinder. The other end of the actuating lever is connected by a hinge 10 to an actuating rod 13 of the vacuum interrupter 5. When the tap changer is operated during a tap change the lever 6 rotates with the shaft around the shaft axis, and the cam follower 16 follows the cam ring 7, and upon meeting a cam (not illustrated in figure 1, see figure 4) the actuating lever 6 pivot around pivot 9 and lifts the actuating rod 13 up (vertically) , and actuates the vacuum interrupter 5. The pivot 9 of the actuating lever 6 is

arranged on a reference guide 8. The reference guide 8 is, in one end, connected via a horizontal pivot 11 to the switching shaft 2 and it rotates with the shaft 2. The reference guide 8 is in its other end arranged to follow, a reference surface 17 of, the cam ring 7, and for this purpose this other end includes a ring follower provided with rollers 18a, 18b. Thus, the reference guide 8 is arranged to rotate together with the switching shaft, and if the altitude of the contact

arrangement varies with the altitude of the cam ring 7, the reference guide 8 is free to follow the cam ring 7, up and down, by rotating slightly around its pivot 11 of the

arrangement. Note that, the pivot 11 is arranged so that the reference guide can rotate in relation to the vacuum

interrupter 5, since the vacuum interrupter are fixed to the shaft and do not rotate around in relation to the switching shaft 2 (and also not around the pivot of the reference guide 8) . Mechanically speaking, the pivot 11 of the reference guide is arranged between the vacuum interrupter 5 and the cam ring 7, so that the vacuum interrupter 7 does not follow the variations of the cam ring 7 in the vertical direction, whereas the reference guide 8 follows the vertical variations of, the reference surface 17 of, the cam ring 7.

The pivot 9 of the actuating lever 5 moves with the reference guide 8, so that the pivot 9 also follows the reference surfaces of the cam ring 7. Thus, the function of the actuating lever 6 is not affected by height variations between the shaft 2 and the cam ring 7, such variations are

compensated for by the reference guide 8 that carries the pivot 9 of the actuating lever 6.

The actuating lever 6 is operatively connected to the

actuating rod 13 of the vacuum interrupter 5 by means of a hinge, so that the pivotal actuating movement of the actuating lever 6 is transformed into an essentially vertical actuating movement of the rod 13 inside the vacuum interrupter 5. Thus, the hinge allows a rotation of the lever in the horizontal plane, around an axis 10 that is parallel with the pivot axis 9. The pivot 11 of the reference guide 8 is arranged in the same axis 10, 11 as the hinge 10, so that the reference guide rotate around the hinge axis 10 of the lever 6 and rod 13 connection. To let the reference 8 pivot around the rotatable connection of the actuating lever 6 and vacuum interrupter actuating rod 13 is a preferred feature of the invention. This axis lies at an altitude or vertically between, preferably in the middle between, the contact surface of the actuator 6 and cam ring 7 and the contact surfaces 17 of the reference guide and the cam ring 7.

Also, the interior contacts 22 are preferably fixed to the reference guide 8, as illustrated in the figure, so that these contacts follow the cam ring 7 reference surfaces 17 in the vertical direction.

The axes of rotation for the lever pivot 9, lever hinge 10 and guide pivot 11 are located at a respective radial distance from the shaft 3 and each of these axes extend in a

circumferential direction. Figure 2 illustrates a contact arrangement in accordance with the invention from above. It more clearly illustrates the pivot 11 of the reference guide 8 and the hinge 10 of the connection between the actuating lever 6 and the actuating rod 13 of the vacuum interrupter 5. As can be seen the hinge 10 and pivot 11 have a common axis of rotation (illustrated by the line through hinge 10 and pivot 11) . Figure 2 illustrates the contact arrangement between the inner surface of the insulated cylinder 2, which is provided with the cam ring 7, and the switching shaft (not illustrated) . It shows an

arrangement of two vertical vacuum interrupters 5, each of which is provided with an actuating rod below the joint 10 connecting each rod to a corresponding actuating lever 6, at the innermost end of the actuating lever 6. The actuating lever 6 is arranged in a groove provided with cams in the cam ring 7. Each actuating lever 6 is rotatable around its respective pivot 9, where it is connected to the reference guide 8. The reference guide 8 is pivotally arranged by a pivot 11, for example by means of a bearing on a shaft, to the contact arrangement so that it can rotate relative the vacuum interrupters 5. Thus, the guide 8 provides a foundation for the pivot 9 of the actuation lever 6, which foundation

automatically adjusts to height variations between the cam ring 7 and the vacuum interrupters 5, and thereby also the switching shaft 2.

Figure 3 illustrates the arrangement of the vacuum interrupter 5 on the contact carrier in accordance with the invention in a simplified view in perspective. The view illustrates the pivot 10 of the reference guide and its cam following rollers 18 running on the reference surfaces 17 thereby following the cam ring, so that the vertical position of the pivot 9 for the actuating lever 6 follows the cam ring. The hinge 10 of the actuating lever 6 is not affected by a possible vertical variation of the cam ring 7, so the actuating lever 6 will only operate the vacuum interrupter 5, i.e. only lift the actuating rod of the vacuum interrupter, when the cam follower 16 meets and interacts with a cam in the cam groove of the cam ring 7.

Figure 4 illustrates an example of a cam ring 7 of the

invention, which is arranged on the interior surface of the insulated cylinder of the tap changer (which cylinder 2 meets the outer surface of the cam ring) . The cam ring 7 is provided with two grooves provided with cams 9, 11 for actuating a respective vacuum interrupter (corresponding to the two interrupters 5 of figure 2) via a respective actuating lever (6 of figures 1-3) running in the cam groove, or cam race. Thus, the cam grooves 9, 11 are provided in the cam rings 7 and extend annularly in relation to the cylinder. The cam ring 7 is also provided with a reference surface 17 provided as a rail for the reference guide, a rail having no cams.