TECHNICAL FIELD

The present invention refers to a selector switch arrangement for a high voltage ( > 1 kV ) device, such as a tap-changer, comprising a cylindrical current collector made of electrically conductive material and having a first and a second end, a movable contact arranged movable along the perimeter of said first end of said contact device and in contact with said first end and at least two contacts fixedly connected to said contact device

BACKGROUND ART

Tap changers are used to change the tapping connection of a transformer winding while the transformer is energized. The tap changer can be designed as a single unit for single and three-phase applications with one common neutral point. Depending on the three-phase rating, this might require three separate units, each having its own insulated phases. Tap changers can be located either inside the transformer main tank or outside in its own

compartment. Switching from one position to another has to be performed through an impedance to avoid a short circuit between two steps of the regulating winding. Tap changers can be on-load or off-load, and on-load tap changers generally includes a diverter switch operating to effect transfer current from one voltage tap to the next. The diverter switch does the entire on load making and breaking of currents, whereas the selector switch pre-selects the tap to which the diverter switch will transfer the load current. The selector switch also contains at least two fixed contacts arranged in the housing of the tap-changer, a current collector, vacuum interrupters, movable contacts and a mechanism arranged for controlling (open/close) the vacuum interrupters. The current collector is arranged to transfer current between the movable contact of the selector switch and the fixed contacts arranged in the housing of the tap- changer.

Today, current collectors are typically made completely from copper (Cu). The reason for this is the need for having the current collectors made from a material with low conduction point resistance and high mechanical streght. However, the use of copper makes it expensive to manufacture the current collectors. Aluminium is also efficient as a conductor but inefficient when used as, or together with a moving contact. The reason for this is that aluminium has low surface hardness and therefore has a build-up of aluminium oxide on its surface. Too much build-up of aluminium oxide will increase contact resistance, since the electrical conductivity for aluminium oxide is poor, and hence when the tap-changer is in operation the temperature will increase. Therefore use of additional cooling of the tap-changer might be required and this must be avoided.

SUMMARY OF THE INVENTION

The object of the invention is to provide a selector switch arrangement, which is less expensive to manufacture compared to previous solutions, and has low conduction point resistance and high mechanical strength.

This is object is achieved by the selector switch arrangement initially defined, which is characterised in that the cylindrical current collector comprises a first and a second cylindrical part arranged joined together wherein the first part comprises the first end substantially made of copper or an copper alloy, and the second part comprises the second end substantially made of aluminium or an aluminium alloy. By such arrangement, and compared to previously known solutions, the selector switch arrangement will be less expensive to

manufacture since the total amount of copper in the current collector will be significantly reduced. Also, by such arrangement low conduction point resistance between the movable contact, which is substantially made of pure copper, and the first end of the cylindrical current collector is achieved as the copper in the first end of the cylindrical current collector will not suffer from build-up of oxides. Thereby, although the material in the second cylindrical part of the current collector in unstable regarding surface oxidation and thereby easily can increase its resistance, the selector switch arrangement will not suffer from build-up of aluminum oxides and thus it will have at least equal electrical and mechanical properties compared to previously known selector switch arrangements.

According to an embodiment of the invention the height of the first part of the current collector is less, or even significantly smaller, than the height of the second part of said current collector. Thereby, content of copper in the current collector will be less than the content of aluminium. Thus the manufacturing cost will be reduced compared to previously known solutions. Another advantage is that the current collector will have less weight compared to previously known solutions. According to a further embodiment of the invention the composition of the current collector consists of 10-15 % by weight of copper and the rest of aluminium.

According to a preferred embodiment of the invention the selector switch arrangement is a part of a tap-changer.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 a shows a perspective view, as seen from above, of a selector switch arrangement according to an embodiment of the invention.

Figure 1 b shows a perspective view, as seen from below, of the selector switch arrangement seen in figure 1 a. Figure 2 shows a perspective view of the cylindrical current collector with a fixed contact attached

DETAILED DESCRIPTION

Figure 1 a and 1 b shows a perspective view of a selector switch arrangement 1 for a high voltage device comprising a cylindrical current collector 2 made of electrically conductive material and having a first 3 and a second end 4, a movable contact 5 arranged movable along the perimeter of the first end 3 of the cylindrical current collector 2 and in contact with the first end 3 and at least two contacts (not shown) fixedly connected to the cylindrical current collector 2. The cylindrical current collector 2 comprises a first 6 and a second 7 cylindrical part arranged joined together wherein the first part 6 comprises the first end 3 and is substantially made of copper, a material with high conductive and mechanical properties, or a copper alloy. The second part 7 comprises the second end 4 and is substantially made of aluminium, a material with low conduction point resistance, or an aluminium alloy. Preferably, the first part 6 of the cylindrical current collector 2 is made of pure copper. However, it is also possible to use a copper alloy or to cover the first part 6 of the cylindrical current collector 2 with a precious metal, e.g. silver (Ag). Preferably, the second part 7 of the cylindrical current collector 2 is made of pure aluminium. However, it is also possible to use an aluminium alloy. By such arrangement low conduction point resistance between the movable contact 5 and the first end 3 of the cylindrical current collector 2 is achieved since copper does not suffer from build-up of oxides. Thereby, the selector switch arrangement 1 will not suffer from build-up of aluminum oxides and thus it will have at least equal electrical and mechanical properties compared to previously known selector switch arrangements.

After that current has been transferred from the movable contact 5 to the first part 3 of the cylindrical current collector 2, the copper in the first part 6 of the cylindrical current collector 2 will work as contact conduction point material and the aluminium in the second part 7 of the cylindrical current collector 2 will work as the main conductor. Thus, current will be transferred from the first part 6 to the second part 7. Further the selector switch arrangement 1 comprises a mechanism 10 which is arranged to open or close vacuum interrupters (not shown). The movable contact 5, the mechanism 10 and the vacuum interrupters are arranged attached to a selector switch shaft 1 1 . The cylindrical current collector 2 is secured by fastening means (not shown), on the inside perimeter of an elongated housing (not shown). At least two fixed contacts (not shown) are fastened in place in the housing.

The first part 6 of the cylindrical current collector 2 is arranged to receive the movable contact 5 which is arranged to transfer current to the cylindrical current collector 2. The second part 7 of the cylindrical current collector 2 comprises at least two static contacts 8 arranged to transfer current to fixed contacts (not shown) arranged in place in the elongated housing.

The first 6 and the second 7 part of the cylindrical current collector 2 are arranged jointly together, for example by means of friction stir welding, or similar methods such as laser welding. Thereby aluminium and copper will integrate and thus stable resistance, despite the build-up of oxidations on the material, will be achieved. One major advantage is that using friction stir welding, or similar methods such as laser welding, means that no other material is involved except copper and aluminium. Figure 2 shows the cylindrical current collector 2 with a fixed contact 9 attached thereto.