Numerical electromagnetic modeling of HTS power transmission cables

This Ph.D. thesis consists of two successive phases. The objective of the first phase was to develop a fast and accurate numerical model to solve the electromagnetic problem of helically wound thin tape conductors. This method must be applicable to find current distribution and AC losses in High Temperature Superconducting (HTS) power transmission cables made of coated tapes. In the second phase of the project, the developed model was used to perform parametric analysis of the AC loss behavior of single layer HTS cables with different design schemes and design parameters. The main objective of this phase was focused on the minimization of AC losses in HTS cables either by searching for optimal designs parameters or alternative design schemes.;In the latest generation of HTS power cables, superconducting coated tapes are the conductors of choice. The thickness of the superconducting layer of these tapes is around 1 to 2 µm, and depending on the application, their width varies from 4 to 12 mm. In the cable design, such tapes are helically wound around cylindrical formers in single or multi-layer arrangements. The complicated geometry of the tapes as well as the non-linear resistivity of their superconducting layer make the accurate solution of the electromagnetic problem of HTS cables quite challenging.;During the first phase of this thesis, we introduced a numerical method to compute current and field distribution in helically wound thin tape conductors when one or many of them are arranged in a symmetrical manner. According to the symmetry arguments associated with the geometry of the problem, and neglecting the thickness of the tapes, the real 3-D problem of helically wound tapes could be reduced to a computationally small 1-D problem whose domain lies along the half-width of any of the constituting tapes. The low frequency version of the eddy current equation, as the governing equation of the problem, is discretized over this reduced dimension study domain. To establish a direct relationship between the current density and the vector potential in the problem formulation, the solution of the Biot-Savart integral to find the magnetic vector potential of helically wound current sheets is used.;As a consequence of considering the real 3-D geometry of the tapes in problem formulation, the proposed model is more accurate than many previous 2-D methods that cannot take into account the helical configuration of the tapes. On the other hand, because of using symmetry arguments to reduce the size of the study domain, the method is very efficient in terms of computational time. To verify the validity of the proposed method, we performed experimental measurements of AC losses in solenoid-type cables made of a sample of YBCO-coated conductor tape. Excellent agreement was observed between the experimental data and the simulation results.;In the second phase of this thesis work, we used the numerical technique developed in the previous phase to study the AC loss behavior of single layer HTS cables. In accordance with previous studies, simulation results revealed that, the main loss mechanism in these cables arises from the presence of gaps between the adjacent tapes. These small gaps disturb the field distribution near the edges of the tapes so that in these region the magnetic field experienced by the tapes has large components perpendicular to the wide face of the tapes. With smaller gaps, cables show lower AC losses. But due to inevitable mechanical considerations, there is always a minimum limit for the gap size.;Aimed at undermining the gap effects, we investigated the AC loss behavior of three different design schemes for single layer HTS cables, in which the gap regions are covered by the overlap of the adjacent tapes or by inserting narrow tapes below the main tapes. Simulation results showed that these designs are quite effective in reducing AC losses of HTS power cables.