Optimisation of contact materials for vacuum circuit breaker

As in many other technical areas, in electric power engineering a continuous increase of miniaturization and thus an increase of power density can be observed. The combination of demands on switchgears in modern power grids, such as a high resistance to erosion, a high breaking capacity, a low restrike probability and a low chopping current vary depending on the particular application an can even be contradictory. To optimize a contact material for a specific switching property the basic understanding of the contact material needs to be extended. Besides an electrical evaluation of the switching properties – the electrical measurements can be carried out externally – a metallurgical investigation of the contacts is required. That usually implies the destruction of the vacuum chamber.

Instead usage of a commercial, hard-soldered vacuum chamber a UHV chamber, based on CF components, was designed. The chamber can be fully disassembled and allows metallurgical investigations without destruction. Thus, the cause-effect-relationship can be derived directly.

In a first project phase the model vacuum circuit breaker was designed, implemented and commissioned.

With the help of a pumping station – a combination of rotary vane pump and turbomolecular pump – the chamber reaches UHV pressure levels within pump-down times of 2 hours. To provide a realistic contact opening and a high reliability the vacuum chamber was combined with a spring operated switch drive with high energy content.

In a second project phase the current breaking capacity of a standard contact material was evaluated and statistically verified under the given circumstances of the model circuit breaker and the test circuit. The results define a reference breaking capacity that is used for comparison with braking capacities of all modified contact materials.

Currently investigated metallurgical parameters are for example

• gas content,
• microstructure and
• morphology

of the contact material. First results indicate partly strong effects on the breaking capacity and other switching properties.

The project is organised as interdisciplinary working group. The overall project goal is the generation of an fundamental understanding for the correlation between the properties of the contact material and their electrical effects on the switching behavior. The increase of the current braking capacity, a special demand on circuit breaker, is a straight research goal.