Characteristics of NbTi mechanical persistent current switch and mechanism of superconducting connection at contact

Abstract

A Persistent Current Switch (PCS) in a SMES system requires not only low ON resistance but also high OFF resistance in order to efficiently perform a load-leveling operation. Because a mechanical switch has the advantage of infinite OFF resistance, it can operate without generating Joule losses during the coil excitation and energy transfer process. However, it will cause an energy storage loss as long as the ON resistance, which mainly depends on contact resistance, is not decreased to zero. In this study, firstly case studies on energy losses due to the ON resistance of a mechanical switch and OFF resistance of a superconducting switch are carried out so as to show the necessity of a mechanical switch as a PCS in a large-scale SMES system. Secondly the contact resistance characteristics of a mechanical switch with NbTi bulk contact pieces are experimentally investigated as a function of switch driving force, switch pressing force, and ON–OFF operation number. Additionally, the effects of contact piece shape and its surface roughness on the contact resistance are examined. It was found that a superconducting connection at contact can be realized even if the switch is repeatedly opened and closed many times. The developed mechanical PCS, which has zero contact resistance, supports a superconducting current flow of more than 200 A. The formation mechanism of the superconducting connection at contact is modeled and analyzed based on theoretical considerations.