Current Imbalance and AC Losses of Long-Distance DC HTS Cable

Despite intensive research in the field of applied superconductivity, we only now start to use this phenomenon for the most obvious application, namely, power transmission. At present, there is an explicit shift from ac to dc high-temperature superconducting (HTS) cable systems due to the many advantages of dc. However, even with dc, a transmission line is subjected to current fluctuations and, consequently, ac losses. One more factor that reduces the efficiency of HTS lines is imbalance in the current. It is associated with the presence of electrical resistance at the soldered connections of the superconducting tapes. These resistances are very low but against the background of zero resistance of the rest of the superconductor they determine the individual currents in tapes and introduce essential nonlinearity into the dependence of these currents on the total current. A large national project on dc HTS power transmission was launched in Japan in the fiscal year 2013. Within the framework of the project, two HTS cables were laid in Ishikari (Hokkaido) for commercial and research purposes. Since the novel design of the thermal insulation is used, it becomes important to carefully consider the contribution of internal heat generation. In order to estimate ac losses, the frequency characteristics of the ripple current were calculated, taking into account current imbalance in the cable.