Transition Criteria for Critical Current Measurement of High Temperature Superconductor in AC Circumstance

There are a number of AC power devices using high temperature superconductor (HTS) such as Superconducting Fault Current Limiter (SFCL), Superconducting Transformer, AC Superconducting cable and so on. However, the operating current of HTS power devices at alternating current (AC) circumstances is almost decided by critical current which is measured in direct current (DC) circumstances. Applying DC critical current criterion to AC power devices cannot be ideal. So, for AC power devices, a new AC critical current criterion must be needed. Commercial AC frequencies of current grid are 50 and 60 Hz. Therefore, in AC circumstance, the voltage induced by inductance is not negligible. For this paper, AC conducting experiments on variable super conducting tape samples were executed. The voltage induced by inductance is removed by filtering process and the voltage produced by resistance remains and is considered as a parameter (or guide line) of quench. With filtering process, amplitude of voltage generated by resistance can be calculated. As a result of those experiments, the `Amplitude of input current'-`amplitude of voltage induced by resistance' curve was acquired and alternating critical current criteria were inferred.

I. Introduction

To decide the operating current of the superconducting power devices such as AC HTS cables and SFCLs and superconducting transformers in AC circumstance, DC critical current criterion has been applied to the devices. However, most of those devices have inductive components due to indispensable winding parts within them, and thus applying AC to those devices could produce not only resistive voltage components due to superconducting materials, but also inductive voltage components by alternating current. So, applying DC critical current criterion to AC power devices can not reflect the influence of the inductive voltage components.