Experimental and Analytical Study on Contact Resistance and Current Distribution of NI HTS Coils in Low-Frequency AC Method

A low-frequency alternating current (LFAC) method was proposed to measure and evaluate the contact resistance between turns of NI HTS coils and its accuracy was investigated using REBCO test coils. In order to accurately evaluate the contact resistance using the LFAC method, it is necessary that the current flows uniformly in the radial direction of the NI coil. The radial current distribution of the NI coil was experimentally evaluated using eight pickup coils and non-uniform radial currents were confirmed in a previous study. The locally different radial current distribution in the NI coil indicates the spatial non-uniformity of contact resistance. In this study, the frequency characteristics of the ac current distribution and the effect of the spatial non-uniformity of contact resistance in the NI coil were investigated using a numerical analysis based on the 2D PEEC model. The two-dimensional modeling of the spatial non-uniform contact resistance was able to explain the distribution of the inhomogeneous contact resistance in the NI coil measured by the pickup coils. Therefore, the validity of the experimental results using the pickup coils was demonstrated by PEEC models.