GB2148598A - Transformer on-load tap selector - Google Patents

Abstract

A transformer on-load tap selector comprises fixed contacts 301, 302, 303 and a wiper arm 33 carrying a main contact 34 movable between them. The wiper arm supports a pair of contact carrying arms which are separable during a tap changing movement, each arm carrying a pair of arcing contacts 37, 38, the first contact 39, 41 of each pair being connected to a common wiper arm terminal 36 and by way of a bridging impedance 43, 44 to the second contact 40, 42 of the pair. The second contacts 40, 42 engage with a fixed arcing contact 312 and each first contact 39, 41 engages with the second contact 40, 42 of the other pair so that a direct current path exists between the fixed arcing contact 312 and the common terminal 36. To change taps, the main contact 34 is broken and one, (leading), pair of contacts is moved to an adjacent tap. Transitional impedance paths from the adjacent fixed contacts to the common terminal 36 exist by way of the second contacts 40 and 42 which act as conventional transitional arcing contacts until the four- contact combination is re- established. <IMAGE>

Description

SPECIFICATION Transformer on-load tap selector This invention relates to transformer on-load tap selectors of the resistor transition type and in particular to such selectors suitable for operating in the flag cycle switching mode.

On-load tap selectors are well known including an arrangement in which main and transitional, or arcing, contacts are moved between adjacent fixed contacts (connected to transformer tapping points) in a predetermined sequence to prevent arcing between the fixed and moving main current carrying contacts.

As an example of the functional relationship between the various contacts for one such sequence, also called a flag cycle, reference is made to Figs. 1 (a) to (e).

A tap selector mechanism comprises a plurality of fixed contacts 11, 12, 1 3 etc. arranged in a circular manner about a rotatable axle 1 4 which carries a movable member 1 5 also rotatable about the axle between tap positions. The movable member 1 5 carries a main current carrying movable contact 1 6 of high electrical conductivity such as copper which connects the common terminal 1 7 at the axle to the fixed contact.The main contact 1 6 also has associated therewith and electrically connected a main arcing contact 1 8 of tungsten-copper which, when the main contact leaves a tap position, is arranged to trail slightly and carry all the main current for a short time prior to breaking from the fixed contact.

The main arcing contact has adjacent each side thereof a transitional contact 1 9, 20 also of tungsten-copper and connected by way of a resistor 21, 22 to the common terminal 17.

When an adjacent tap is selected the movable member 1 5 is rotated about axle 1 4.

Initially the (leading) transitional contact 1 9 and the main contact 1 6 break with the fixed contact leaving the main arcing contact 1 8 and (trailing) transitional contact 20, Fig. 1(b).

Further rotation causes the main arcing contact to break from the fixed contact and support any arcing between them until the arc resistance grows to the same order of magnitude as the resistor 22 when current flow transfers to that path. Continued rotation causes the leading transitional contact 1 9 to make with fixed contact 1 3 and transitional current paths now exist between both fixed contacts and the common terminal 17, Fig.

1(c).

Further rotation causes the trailing transitional contact 20 to break from fixed contact 12, transitional current being maintained between fixed contact 1 3 and common terminal 19 by way of resistor 21. The main contact 1 6 then contacts the fixed contact 1 3 led by main arcing contact 1 8 to establish a direct current path between the fixed contact and the common terminal by-passing resistor 21, Fig. 1(d).

It will be appreciated that the fixed contacts have the portions thereof to which arcing from the transitional and main arcing contacts occur also formed of tungsten-copper alloy.

Finally, as the moving contacts pass across the fixed contact to their final position they are joined by trailing transistional contact 20, Fig. 1(e).

It will be appreciated that the arrangement is symmetrical in that the same flag cycle occurs whichever the direction of rotation.

This arrangement of tap selector does however have some disadvantages. In Fig. 1 it will be seen that the spread of the moving contacts 1 9 and 20 must be substantially equal to the separation of the fixed contacts.

It will be appreciated that to prevent electrical breakdown between adjacent contacts there is a minimum separation to be maintained between, say, fixed contact 1 3 and the transitional contact 1 9 when adjacent fixed contact 12, or between transitional contact 20 and fixed contact 11. This factor determines the minimum separation between adjacent fixed contacts and as there may be between 1 5 and 30 fixed contacts in the circular array the structure may be bulky.

Proposals have been made to make a more compact structure by varying the size and separation of fixed contacts.

One proposed construction is described in Patent Application No. 2,078,445, in which transitional contact rollers make rolling contact with cylindrical fixed arcing contacts and are mounted on arms of a parallel linkage such that at a tap location they are located one each side of the fixed contact and upon subsequent rotation the trailing transitional contact bears against the fixed contact which acts as a cam to displace the trailing transitional contact radially, a movement which is converted by the linkage into an increased circumferential separation of the transitional contacts to effect stepping between adjacent fixed contacts.

The linkage is arranged so that the adjacent fixed contacts are bridged by resistances associated with the arms during the transfer of the main contact.

Thus at a tap position the minimum lateral extent of the moving contact assembly can be reduced so that the minimum separation between 'parked' transitional contacts and adjacent fixed contacts is correspondingly reduced.

However, the arrangement is somewhat mechanically complex in that main and main arcing contacts also have to be provided and despite the linkage permitting the transitional contacts to 'close-up' the requirement for them to straddle the fixed contact laterally, in order that the camming action takes place to expand the linkage, results in a construction in which the overall contact width at a tap location is not minimal.

It is an object of the present invention to provide a transformer on-load tap selector of the resistor transition type suitable for operating in the flag cycle switching mode capable of a more compact construction than hitherto known.

According to the present invention a transformer on-load tap selector comprises a plurality of fixed contacts each having a fixed arcing contact spaced apart from each other and a wiper arm, movable between predetermined locations adjacent the fixed arcing contacts, carrying two pairs of movable contacts a first movable contact of each pair being connected to a common wiper arm terminal and by way of a bridging impedance element to the second movable contact, and control means operable to move the contact pairs separately with movement of the wiper arm so that at the predetermined locations the second contact of each pair contacts the fixed arcing contact at that location and the first contact of each pair contacts the second contact of the other pair, by-passing the impedance elements and providing current paths between the fixed arcing contact and the common wiper arm terminal, and as the wiper arm is moved between adjacent old and new locations one, leading, pair of movable contacts is moved before the other, trailing, pair, the second contact of the leading pair breaking contact with the old fixed arcing contact and moving into contact with the new fixed arcing contact so that both fixed arcing contacts are temporarily coupled to the common wiper arm terminal by way of the bridging impedance elements, followed by movement of the trailing pair, the second contact of the trailing pair breaking contact with the old fixed arcing contact and moving into contact with both the new fixed arcing contact and the second contact of the leading pair and the first contact of the trailing pair moving into contact with the second contact of the leading pair.

It will be appreciated that in most tap changer constructions the fixed contacts are located in an arc or circle at a constant distance from an axle about which the moving contacts rotate. Whilst the following description concentrates on such rotary motion the present invention is applicable to other constructions, for example, employing rectilinear motion.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure l(a) is a schematic plan view of a known form of tap selector described above, Figs. 1(b) to (1e) illustrating the disposition of contacts during a tap selecting step, Figure 2 is a schematic circuit depiction of the tap selecting arrangement according to the present invention at one selected tap location, Figure 3(a) and 3(b) illustrate the positions taken up by the various selector components during selection of an adjacent tap, Figure 4 is a perspective view of one construction of selector illustrating the relationship between moving and fixed contacts at one selector position, Figures 5(a) and (b) are plan and elevational views, respectively, of an alternative construction of selector illustrating the relationship between moving and fixed contacts at one selector position, and Figure 6 shows a plan view a tap selector of Fig. 5 between taps illustrating control means for imparting relative movement between the moving contacts.

Referring to Fig. 2 a plurality of transformer tapping points are brought out to a corresponding number of fixed contacts 30,, 302.

303. . The fixed contacts comprise a stem of copper, or like high conductivity material, on which is supported a fixed arcing contact 31 in the form of a roller of tungsten copper alloy. The fixed contacts are arranged in a circular arc about a centre axle 32 on which is rotatable a selector wiper arm 33.

The wiper arm carries a main contact 34 of copper which is slidable axially of arm and biased by spring 35 into contact with the copper stem of the fixed contact. Only part of the main contact is shown for clarity. The main contact is connected electrically to a common wiper arm terminal 36. The wiper arm also carries two pairs 37, 38 of movable contacts 39, 40 and 41, 42 respectively. A first movable contact 39, 41 of each pair is electrically connected to the common wiper arm terminal 36 and by way of a bridging impedance element, a transition resistor 43, 44 to the second movable contact 40, 42 respectively. The movable contacts 39-42 are carried by contact carrying arms (shown in Figs. 4 and 5) which are separable circumferentially as the wiper arm is rotated between tap locations by control means (such as illustrated in Fig. 6).Rotational movement of the wiper arm causes withdrawal of the main contact from the fixed contact and then differential movement of the contact pairs causes firstly one, leading, pair of contacts followed by the other, trailing, pair of contacts to take up a similarly disposed location in relation to an adjacent fixed contact, the motion being completed by engagement of the main and fixed contacts. It will be seen from Fig. 2 that when the wiper arm 33 is located adjacent a fixed contact 302 the disposition of movable contacts 39-42 in relation to each other is such that the second contact of each pair 40, 42, contacts the fixed arcing contact and the first contact of each pair 39, 41 contacts the second contact of the other pair, by-passing the resistors 43 and 44 and providing a low resistance current paths between the fixed contact 302 and the wiper arm terminal 36.

Referring now to Fig. 3(a), when the wiper arm 33 is moved from an old location adjacent contact 302 to a new location adjacent fixed contact 303 the pair 38 of movable contacts becomes the leading pair. During a first part of the motion the main contact 34 breaks from the fixed contact but because of the low resistance paths by way of the roller pairs there is no arcing. Then the leading pair of contact rollers moves to the new location, the second contact 42 breaking with the fixed arcing contact 312 and making contact with the fixed arcing contact 313. The first contact 41 of the leading pair also moves adjacent the fixed contact 303 but without making contact.

The leading pair of contacts effectively acts as a main arcing contact when the contacts break the direct current path and between the old fixed contact 302 and the wiper terminal 29 leaving a transitional path only by way of the second movable contact 40 and resistor 43. A transitional current path is also established between the new fixed arcing contact 313 and the wiper terminal 33 by way of second movable contact 42 and the bridging resistor 44.

Having estabished two transitional current paths by way of both fixed arcing contacts a second part of the wiper arm motion causes the trailing pair of contacts 37 to move from the old fixed arcing contact 312 to the nw fixed arcing contact 31 3 taking up the position shown in Fig. 3(b) (corresponding to Fig. 2) with the second trailing contact 40 between the fixed arcing contact and first leading contact to by-pass bridging resistor 44 and with the first trailing contact 39 in contact with the second leading contact 42 also by-passing the bridging resistor 43.

It will be appreciated that when a tap selection is made requiring the wiper arm to be moved in the opposite direction, e.g. from a location adjacent fixed contact 303 to a location adjacent fixed contact 302, then the contact pair 37 becomes the leading pair and the contact pair 38 the trailing pair.

It will be seen from the above that whilst the present invention requires four movable arcing contacts rather than the three required for known arrangements, it is not essential that those movable contacts extend laterally of the fixed arcing contact enabling a construction to be employed in which the movable contacts are arrayed substantially radially with respect to the axle 32 and minimising the separation required of adjacent fixed contacts.

It will be appreciated that there are numerous arrangements for implementing the above described four-arcing contact configuration and two of these will be described briefly.

Referring now to Fig. 4 this shows a perspective view of a tap selector switch in which the actual disposition of movable contacts corresponds closely to the schematic arrangement of Fig. 2 and corresponding items are given like reference numerals.

The fixed contact 302 comprises a stem 30'2 of low resistivity copper or the like, as described above, atop which is a fixed arcing contact 312 in the form of a roller of tungstencopper alloy.

The wiper arm 33 carries a main contact 34 in the form of a slab of low resistivity copper or the like. The main contact 34 is supported in a blind recess 33' in the arm 33 also containing a bias spring 35 which urges the main contact into engagement with the fixed contact. The main contact 34 has limited radial travel to prevent it emerging from the recess during movement between tap locations.

The arm 33 also supports two contact (carrying) arms 45 and 46 preferably of electrically insulating material which supports the movable contact pairs 37 and 38 respectively.

The contact arms 45 and 46 are pivotable relative to each other about axle 32 and move with arm 33 about the axle. The relative pivotal motion provided by control means (not shown) causes the arms to be brought together as shown at a tap location with the movable and fixed arcing contacts in a radially extending line or separated by an angular displacement, equal to the angular separation of adjacent fixed contacts, when between tap locations.

The movable contacts 39-42 are also rollers of tungsten copper alloy and are supported in the arms 45, 46 by spindles 47 which to ensure proper contact are each biased by spring means (not shown) towards the position of the fixed arcing contact i.e. the contacts 39 and 42 are biased radially outwards and the contacts 40, 41 are biased radially inwards. The travel of the movable contacts is limited to prevent the first contacts 39 and 41 from engaging the fixed arcing contact directly.

The contact arms 45, 46 also carry transitional bridging resistors 43 and 44. The electrical connections between the movable contacts, bridging resistors and common terminal 36 in accordance with the circuit of Fig. 2 are by way of conventional flexible cables but have been omitted from the Figure for clarity.

The contact arms 45 and 46 may be in the same rotational plane as they only move to and from each other but are located above the level of the fixed arcing contact 312 so that they can pass over the fixed contacts.

It will be appreciated that whereas in the arrangement of Fig. 4 the circumferential distance required between adjacent tap locations is reduced the radial extension of the wiper arm by movable contacts 40 and 41 beyond the fixed contact counters the magnitude of reduction.

Figs. 5(a) and 5(b) show in plan and elevation respectively an alternative construction in which all the movable contact rollers are lo cated radially inboard of the fixed contact 302, like references again being given to parts corresponding to those shown in Fig. 2 or 4.

The fixed arcing contact 31 t2 corresponding to the fixed arcing contact 312 of Figs. 2 and 4 is formed by a stacked pair of tungsten copper rollers. The wiper arm 33 carries a main contact 34 biased by spring 35 towards the fixed contact stem 30'2.

The wiper arm also support contact carrying arms 50 and 51 similar in disposition and relative seperability to the above described contact carrying arms 45 and 46.

The contact carrying arm 50 is rotatable about axle 32 and carries shafts 52, 53 supporting movable contacts 41 and 42, the shaft 52 being of such length that the movable contact 42 aligns with and abuts the upper fixed contact roller and the shaft 53 being of such length that the movable contact aligns with, but is displaced from the lower fixed arcing contact roller.

The contact carrying arm 51 is also rotatable about selector axle 32 and supports a shaft 55 carrying the movable contact 40 in alignment with, and abutting, the lower fixed arcing contact roller and a shaft 54 on which is carried the movable contact 39 in alignment with, and abutting the movable contact 42.

The movable arcing contact and/or their supporting axles are resiliently mounted with respect to the arms 51 and 50 and biased towards the fixed arcing contact rollers so that in taking up the contacting positions shown each is displaced against the bias force to ensure a good electrical connection.

The contact carrying arms are circumferentially bowed away from each other to permit the shafts 53 and 54 to extend towards the other arm whilst clearing the roller carried by that arm. However the shafts 52 and 55 do not interfere and are arranged to be coaxially with the contact rollrs 40 and 42 in a radial line with the fixed arcing contact 31 '2. Thus even though the contacts 39 and 41 are slightly laterally displaced with respect to the fixed contact they are sufficiently distant from any adjcent fixed contact not to reduce the effective separation of adjacent fixed contacts.

A slight lateral displacement of the movable contact rollers 40 and 42 may also be beneficial in that when a new tap position is selected and the trailing contact arm delivers the trailing contacts these engage with the leading movable contact or new fixed arcing contact as appropriate with a wedging action, ensuring good electrical contact, without relying upon the positional accuracy required when the roller axles are radially aligned.

It will be appreciated that other constructions of contact carrying arms may be employed in which only one of the movable contacts is displaced from the radial line or which enable the movable contact rollers 39 and 41 to be located coaxially with each other on the radial line. Furthermore the movable and/or fixed arcing contacts may comprise other than the rollers rotatable about shaft extending perpendicular to the plane of rotations as described.

In order to function the contact carrying arms have to be separated circumferentially by the arm control means as the wiper arm rotates. Any suitable mechanism for doing this may be employed but desirably it involves a minimum of separately applied motions.

One form of such an arm control means is illustrated in plan view in Fig. 6 and is based upon the form of such mechanism disclosed in British Patent No. 1,026,846.

The disposition of movable contacts is that of the embodiment described above in relation to Fig. 5.

In addition to the component parts shown in Fig. 5 the base board of the tap selector contains a circular wavy groove 60 the wave length of which is equal to the pitch of the fixed contacts. A peg 61 is located in the groove and extends through a radially extending slot 62 in the wiper arm 33 to form a pivot pin for a pair of linking arms 63. 64, one each of which is pivoted to the contact carrying arms 50 and 51 respectively.

The linking arms and/or peg 61 are coupled by a further linking member 65 of electrically insulating material to the main contact 34 such that radial movement of the peg relative to the wiper arm also imparts a corresponding radial movement to the main contact 34.

The parameters of the groove wave are chosen such that the wave has a cycle or pitch equal to the separation of adjacent fixed contacts, say an angle of 28 subtended at the centre axle 32. The peak-to-peak amplitude in the radial direction is chosen with the dimensions and pivot points of the linking arms such that when the peg is at the radially outward peak, adjacent a fixed contact, the angle between the arms is a minimum that is, the contacts 40 and 42 are coaxial, whereas when the peg is at the radially inward peak, between fixed contacts as shown, the contact arms are separated by a further angle of 28.

Considering a tap changing operation, the wiper arm 33 is rotated from a position adjacent fixed contact 302 and initially the main contact is removed from the fixed contact by a combination of this circumferential displacement and the radial displacement resulting from the camming action of the groove on the peg 61.

As the wiper arm rotates through an angle 8 to the opposite groove wave peak each contact arm 50, 51 is rotated with respect to the wiper arm by an angle 8. Thus the trailing contact arm 50 remains substantially stationary with respect to the old fixed contact 302 whereas the leading contact arm 51 moves to the new fixed contact 303 to engage fixed arcing contact 313. During the continued rotation of the wiper arm through the remaining angle 8 camming action of the groove on the peg 61 is reversed so that the contact arm 51 remains substantially stationary relative to the fixed contact 303 and the trailing contact arm is drawn through the angle 28 to take up its new position alongside it, the final part of the motion being the engagement of main contact with the new fixed contact.

It will be appreciated that this represents only an exemplary scheme for the arm control means and that any other arrangement for separately stepping the contact carrying arms 50 and 51, while the main contact 34 is in transit, may be employed.

Further detailed changes may be made in accordance with known practices. For instance the impedance elements, described in the illustrative embodiments as resistors, may be formed by inductors where appropriate.

Claims (9)

1. A transformer on-load tap selector comprising a plurality of fixed contacts each having a fixed arcing contact spaced apart from each other and a wiper arm, movable between predetermined locations adjacent the fixed contacts, carrying two pairs of movable contacts a first movable contact of each pair being connected to a common wiper arm terminal and by way of a bridging impedance element to the second movable contact, and control means operable to move the contact pairs separately with movement of the wiper arm so that at the predetermined locations the second contact of each pair contacts the fixed arcing contact at that location and the first contact of each pair contacts the second contact of the other pair, by-passing the impedance elements and providing current paths between the fixed arcing contact and the common wiper arm terminal, and as the wiper arm is moved between adjacent old and new locations one, leading, pair of movable contacts is moved before the other, trailng, pair, the second contact of the leading pair breaking contact with the old fixed arcing contact and moving into contact with the new fixed arcing contact so that both fixed contacts are temporarily coupled to the common wiper arm terminal by way of the bridging impedance elements, followed by movement of the trailing pair, the second contact of the trailing pair breaking contact with the old fixed contact and moving into contact with both the new fixed contact and the second contact of the leading pair and the first contact of the trailing pair moving into contact with the second contact of the leading pair.

2. A tap selector as claimed in claim 1 in which the wiper arm moves in a plane transversely to which extend the fixed contacts and in which the second movable contacts are disposed at each of said predetermined locations substantially in a line extending transversely to the direction of motion of the movable contacts.

3. A tap selector as claimed in claim 2 in which the first contacts are also disposed substantially along said line.

4. A tap selector as claimed in any one of claims 1 to 3 in which the second contacts of each pair are arranged to contact a fixed arcing contact on opposite sides thereof.

5. A tap selector as claimed in claim 4 in which the movable and fixed arcing contacts are substantially coplanar with respect to the plane of motion of the wiper arm.

6. A tap selector as claimed in any one of claims 1 to 3 in which the second contacts of each pair are arranged to contact the fixed arcing contact at the same side thereof.

7. A tap selector as claimed in claim 6 except when dependent from claim 3 in which at least one of the first contacts is displaced from the line extending transversely to the direction of motion of the movable contacts.

8. A tap selector as claimed in any one of the preceding claims in which the movable contacts are resiliently mounted and biased towards the fixed arcing contact position so as to be displaced against said bias upon contact with the fixed arcing contact or other movable contact to cause contact pressure.

9. A tap selector as claimed in any one of the preceding claims in which the fixed arcing contacts and movable contacts are cylindrical rollers rotatable about axles extending perpendicularly to the plane motion of the wiper arm.

1 0. A transformer on-load tap selector substantially as herein described with reference to and as shown in, Figs. 2 to 4 or Figs.

5 to 6 of the accompanying drawings.