CuCo2Be- and CuCr1Zr-helical springs are used as contact elements for static plug-in power connectors in gas insulated substations (GIS) to achieve a long service time with a low and stable joint resistance.
The long-time behaviour of plug-in power connectors with silver coaled contact surfaces is affected by different ageing mechanism during the rated service time of more than 40 years.
Stress relaxation is one important ageing mechanism, because the helical spring is a mechanical and thermal highly stressed part of the plug-in power connector. Due to stress relaxation in the spring of the connector the contact force is decreased continuously. The joint resistance depends on the contact force. A high contact force generates a low contact resistance.
In long-term experiments up to 34.580 hours the changing of the contact force and joint resistance for CuCo2Be and CuCr1 Zr springs were measured. Although the contact force decreased during the test, the changing of the joint resistance were negligible. The reduction of the contact force can be calculated for service time up to 40 years with a verified finite element model of the helical spring. Due to the results of this work, it can be expected that the influence of stress relaxation on the long term behaviour of this type of plug-in power connector up to 40 years is acceptable low.
Slow reciprocate sliding with displacement amplitudes in the range of mm between the helical spring and the conductor is a further possible ageing mechanism for the plug-in power connector. The sliding will be generated by thermal expansions of the conductors due to daily load changes. Sliding leads to damage and wear in the contact area.
To simulate the sliding in the joint up to 30.000 cycles a test setup has been developed and builds up. The influence in sliding in ambient air on the long term behaviour depends on joint temperature. Due to strong adhesion by higher temperatures the silver coating on the contacts can be damaged and weared down to the ignoble basic material. In this case, fretting corrosion leads to a higher contact resistance. It can be expected that sliding in GIS with SFs atmosphere doesn't affect the long term behaviour of the plug-in connector, because corrosion process in SFs isn't known.