Influence of Films on the Surface of Electrical Contacts at Cryogenic Temperatures

Superconducting devices in power generation, transmission and distribution imply the use of electrical contacts in liquid nitrogen LN2 atmosphere. In electrical power engineering, reliable operation of electrical contacts is essential for the system availability. This paper presents the results of experimental work and calculations of films' influence on the surface of electrolytic tough pitch copper conductors on the contact resistance in LN2. The contact resistance was measured at a crossed rod arrangement of different diameters at low and high contact forces in respective experiments. For small diameters, the rods were brought into contact either directly in LN2 or in ambient air before being immersed in a cryovessel. For bolted busbar joints the joint resistance was measured in LN2 at very high joint forces. In order to determine the influence of the film, film resistivities for the contact resistance were applied and evaluated. The interaction between constriction resistance and film resistance is further discussed. It was found that the influence of films on electrical contacts at cryogenic temperatures is significant and cannot be neglected. The separate consideration of constriction resistance and film resistance of contacts formed in LN2 is not appropriate. Applying very high forces on electrical joints in power devices in LN2 cannot ensure a stable contact resistance for long-term operations.