GB2135127A - Transformer tap-changer switch - Google Patents

Abstract

In an on-load tap-changer switch including moving and fixed arcing contacts (21 min , 22 min , and 14), the moving arcing contacts (21 min and 22 min ) are cylindrical rollers of such length as to cooperate with the fixed arcing contacts 14A, 14B etc. which are off-set in the direction of the longitudinal axis of the moving arcing contact rollers (21 min , 22 min ) so that erosion of the moving arcing contacts is spread along the length of each roller giving prolonged life or permitting softer materials to be used for the moving arcing contacts but not requiring more material to be used for the fixed arcing contacts 14. <IMAGE>

Description

SPECIFICATION Transformer tap-changer switch This invention relates to transformer tap-changer switches and in particularto contact arrangements for switches.

Transformer tap-changer constructions are known in which a plurality of fixed contacts are arrayed around a central spindle of a wiper arrangement in which a wiper contact is mounted for positioning in contact with any one of the fixed contacts.

Also associated with the main fixed and wiper contacts are so-called arcing contacts which make and break before and after the main contacts so that any arcing causes erosion of these sacrificial contacts rather than the main contacts. The arcing contacts associated with the wiper are conveniently called moving or transitional arcing contacts and those associated with the fixed contacts as the fixed arcing contacts.

Either or both of the moving and fixed arcing contacts are frequently formed as cylindrical rollers which make rolling contact in order to spread erosion of the contacts around the periphery thereof, thereby reducing the frequency with which such arcing contacts have to be inspected and replaced.

The extent of erosion of the contacts is a function of the energy transferred, that is, a function of the arcing current and the time of current flow so that a reduction in arcing current flow or time of current flow so that a reduction in arcing current flow or time of arcing current flow across any area of contact surface over a specific time period serves to reduce the long-term rate of erosion.

In this respect the prolongation of arcing contact life may also be considered a function of the contact dimensions and geometry.

Many features, such as contact spacing and roller diameters are fixed by other controlling factors, but others are not and it is an object of the presnt invention to provide a transformer tap-changer switch arrangement which provides longer operating life for the arcing contacts thereof.

According to the present invention a transformer tap-changer switch arrangement comprises at least one moving arcing contact in the form of a cylindrical roller of length greater than cooperating areas of any one of the fixed arcing contacts, said fixed arcing contacts being staggered in position with respect to the longitudinal axis of the, of each, moving contact such that the contact areas between fixed and moving contacts are not all at the same axial position on the, or each, moving contact.

Preferably adjacent fixed arcing contacts are displaced from each other with respect to the longitudinal axis of the or each, moving contact. Such longitudinal displacement of adjacent fixed arcing contacts may be such that they each contact a different longitudinal region of the, or each, moving contact.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which Figure lisa partly schematic plan view of one known form of tap-changer switch showing the functional relationship between arcing contacts, Figure 2 is an elevation of the contacts of the switch of Figure 1, Figure 3 is an elevation of the arcing contact arrangement according to the present invention applied to a tap-changer switch of the form of Figure 1, Figure 4 is an elevation similar to Figure 3 but showing a different disposition of arcing contacts, and Figure 5 is a perspective view of the arrangement of contacts of a different form of tap-changer switch according to the present invention.

Referring to Figure 1 a tap-changer switching arrangement comprises a plurality of fixed main contacts 10 each connected to a tapping point of a transformer winding (not shown). The switch also comprises a wiper assembly 11 rotatable about a shaft 12 and including a main wiper contact 13 which is spring loaded so as to make contact with each of the fixed main contacts 10 as the wiper assembly is rotated between tap positions.

Each fixed contact 10 is in the form of a post atop which is a rotatable arcing contact 14 formed as a cylinder of tungsten-copper or any other suitable arcing material. From its association with the fixed contact each rotatable arcing contact is called hereinafter a fixed arcing contact despite its construction as a roller.

The wiper assembly 11 also includes arms 15, 16 at the remote ends of which are pivots 17 and 18 respectively on which are mounted cranked arms 19 and 20. The arms 19 and 20 each carry at an end thereof a cylindrical roller forming a moving arcing contact 21,22. The other arm of the crank is connected by a compression spring 23, 24to the wiper assembly such that the moving arcing contacts are biased towards the centre of the wiper assembly. The cranked arms are arranged to pass over the fixed contact posts and the moving arcing contacts are located so as to be biased against, and roll across, the fixed arcing contacts as the wiper assembly is rotated.

An insulated lever 25 extends between the two moving ocntacts so that when, as shown, one moving arcing contact 21 is against a fixed arcing contact the other is held from being biased into contact with an adjacent fixed arcing contact.

The moving arcing contact 21 is connected electrically to the main wiper contact 13 and to form one transformer output by lines 26,27 and by way of a resistor 28 to the moving arcing resistor 22.

Assuming the fixed contact to which the main wiper contact is connected is 10A and a tap change is to be made to make contact with an adjacent main current 10s, the wiper assembly is rotated in a clockwise direction. Initially as the moving arcing contact 21 rolls about the fixed arcing contact 14A the main wiper contact 13 breaks with fixed contact 10A but connection 26 between the moving arcing contact 21 and main wiper contact 13 obviates any arcing between these main contacts.

Further rotation brings moving arcing contact 22 into contact with fixed arcing contact 14B causing d path to exist by way of resistor 28 between the fixed contacts 14B and 1 4A the main wiper contact 13 and line 27.

Yet further rotation causes the moving arcing contact 22 to break with contact 1 4A between which contacts arcing may occur.

Final rotation brings the main wiper contact into contact with 1 Os followed by the moving arcing contact 21 making contact with 14B and moving contact 22 taking up a position adjacent fixed arcing contact l4c.

In a reverse sequence moving arcing contact 22 would move back into contact with fixed arcing contact 14B before the moving arcing contact 21 leaves it and a switching arc would develop between the fixed arcing contact 14B and the moving arcing contact 22 as it breaks contact. It will be appreciated thatthetap-change may take place on-load with the current making and breaking occuring only between the rolling arcing contacts.

The arrangement thus far described is well known and the elevation of Figure 2 shows the relationship in the orthogonal plane between the main and arcing contacts.

Referring now to Figure 3 this shows an elevation similar two that of Figure 2 but with the contacts arranged in accordance with the present invention.

Like components are given the same reference numbers and corresponding ones have the refer ence numbers primed. The cylindrical rollers com prising the moving arcing contacts 21' and 22' are of twice the length of those shown in Figure 2. The fixed contacts are constructed simiiarly to those shown in Figure 2 but the fixed arcing contacts 14 on adjacent contacts are displaced from each other in the direction of the longitudinal axis of the moving arcing contacts by the length of the fixed arcing contact roller so that different regions of each moving arcing contact align with adjacent fixed arcing contacts. It will be seen that for each pair of tap changes any arcing is spread between a different areas of each moving arcing contact surface.

It will be appreciated that adjacent fixed arcing contacts may be mounted at any number of different positions, e.g. as shown for three in Figure 4, but preferably several spaced fixed contacts are at the same height in orderto limit the length of each moving arcing contact to a practicable value and to minimise any other changes required in the structure of the wiper assembly. Furthermore in practice it is usual for a plurality of consecutive tap changes to take place only between any two adjacent taps so that it is of less significance that any relationship between adjacent fixed arcing contact positons repeats itself than that the relationship exists for adjacent fixed arcing contacts.

It will be understood that the spacing of adjacent .fixed arcing contacts along the longitudinal axis of the moving arcing contats need not be equal to the length of each fixed arcing contact roller as shown in Figures 3 and 4. The separation may be greater, but is preferably less so that erosion does not result in 'high spots' at areas of the moving arcing contacts where no contact occurs, any spreading of the contact area in the direction of the longitudinal axis of the moving arcing contacts serving to extend their lives.

It wil!further be appreciated that by altering the positions only of the fixed arcing contacts the long term increase in arcing contact area, that is, over many operations, is achieved at the comparatively low cost of the extra length moving arcing contact, of which there are usually only two.

The present invention is not limited to the above exemplary construction. The contact arrangement of another form of tap-changer is shown in Figure 5. In this construction the fixed arcing contacts are truely fixed comprising tungsten-copper alloy edges or other suitable material 50 to copper blades 51 forming the main contacts. The wiper assembly 52 carries pairs of rollers 53, 54, comprising moving arcing contacts and 55 comprising the moving main contact.

Again it will be seen that by having the fixed contacts of the same length and construction but extending into the path of the roller contacts to different degrees, and by having roller contacts longerthan in known switches of this type, the erosion of the roller contacts is spread over their greater length and gives prolonged operating life.

As a corollary to prolonged operating life of a very expensive material, such as the tungsten-copper normally employed, the present invention also enables inferior materials such as copper-chromium alloy, mild steel or even copper two approach what is presently considered to be a normal operating life for tungsten-copper.

Claims (4)

1. A transformer tap-changer switch arrangement comprising at least one moving arcing contact in the form of a cylindrical roller of length greater than cooperating areas of any one of the fixed arcing contacts, said fixed arcing contacts being staggered in position with respect to the longitudinal axis of the, or each, moving contact such that the contact areas between fixed and moving contacts are not all at the same axial position on the, or each, moving contact,

2. A transformer tap changing switch as claimed in Claim 1 in which adjacent fixed arcing contacts are displaced from each other with respect to the longitudinal axis of the, or each, moving arcing contact.

3. Atransformertap changing switch as claimed in Claim 2 in which the longitudinal displacement between said adjacent fixed arcing contacts is such that they each contact a different longitudinal region of the, or each, moving arcing contact.

4. Atransformertap changing switch substantially as herein described with reference to, and as shown in, Figures 3 or 4, or Figure 5 of the accompanying drawings.