Experimental Research and Analysis of AC Critical Current in HTS Tapes

High-temperature superconductor (HTS) has found its way into both ac and dc transmission; yet HTS technology was not specially designed for either transmission type. Now critical current of HTS in dc circumstance is defined, while research on ac current-carrying capability of HTS is still scarce. A new critical current criterion of the ac condition should be established for further studies. In the paper, different dc critical current measurement methods were applied to analyze ac critical current. The E-I curves were obtained by the four-probe method and the frequency ranges from 30 to 540 Hz. The ac current carrying capacity of the superconductors is discussed and analyzed by the RMS method, virtual n value method, and the loss concept method. A criterion of ac critical current was proposed. The RMS method and the virtual n value method are no longer applicable in broad frequency band. While the loss concept method is still applied to explain the theory of critical current which varies with frequency; therefore, an ac critical current calculation method is obtained and the calculative equation at different frequencies is established.