Electrical characterizing of superconducting power cable consisted of Second-Generation High-temperature superconducting tapes

Abstract

With the continuous decline in the price of second-generation (2G) high temperature superconducting (HTS) tapes, the 2G HTS cables are a promising candidate to significantly improve the electrical power transmission capacity and efficiency. In order to make the HTS cable competitive to its counterparts in the power market, much ongoing research work have made considerable contributions to the HTS cable design. In this thesis, the challenges of electrical issues of superconducting power cable using 2G HTS tapes have been addressed. The specific contributions of the thesis include: the influence of anisotropic characteristics of 2G HTS is investigated in order to increase critical current of HTS cable; For improvement of transmission efficiency and safety, the homogenization of HTS cable current distribution is achieved considering the influence of contact resistances and HTS layer inductances; AC loss of HTS cable is obtained through experimental measurement for cooling system design; and the impact of HTS cable on power grids is analysed for safe integration of HTS cable into grids.This thesis starts with a literature review of superconductivity and developments of 2G HTS cable. Following the literature review is the critical current investigation of HTS cable considering the anisotropy of 2G HTS tape. 2G HTS tapes were placed in a highly uniform electromagnetic field and the in-field critical currents were measured with various magnitudes and orientations of the magnetic field. The anisotropic characteristics of 2G HTS tape were determined by non-linear curve fitting using measured in-field critical currents and further implemented into the HTS cable finite element method (FEM) modelling. The modelling results indicate that the gap distances among the tapes in the HTS cable affect the critical current of the HTS cable due to the anisotropic characteristics. In order to investigate the critical current of HTS cable with respect to gap distances, an HTS cable circuit model with adjustable gap distances among the parallel placed HTS tapes was designed and built. Extensive experimental and FEM modelling were performed and the results indicate that the minimized gap distance among the neighboring HTS tapes can be beneficial to increase the overall cable critical current.With DC transporting current, the homogenization of current distribution of HTS cable is achieved by controlling the contact resistances. A 1.5 m long prototype HTS cable consisted of two HTS layers was fabricated and tested as a further investigation of the HTS cable circuit model. The magnitude of the contact resistance related to each HTS layer was measured to quantitatively calculate the current distribution. It is found that only a few micro-ohms difference of contract resistances can still cause severe imbalanced current distribution. The FEM modelling work was carried out to obtain the balanced current distribution by varying the contact resistances. With AC transporting current, the inductances of HTS layers in the cable also pose a significant influence on current distribution issues. An optimal algorithm was developed to achieve homogeneous current distribution by optimal design of the cable diameter, pitch angle and winding direction. Another short prototype cable wound with two HTS layers was built according to the optimal design and the current distribution was experimentally measured between the two layers. It is found out that the optimal algorithm is effective to homogenize the AC current distribution.A reliable AC loss measurement was carried out on the 1.5 m long prototype HTS cable in order to quantify the AC loss of the HTS cable for cooling system design. The experimental measurement method is based on the electrical four probe method adopting a compensation coil to cancel the large inductive component of the cable. The HTS cable with long geometry is easily influenced by the surrounding electromagnetic field so that the measured AC loss signal can be influenced. In order to overcome this problem, a symmetrical current return path was utilized in order to eliminate the electromagnetic interface surrounding the HTS cable. The AC loss measurement results are stable and low-noise for a set of AC frequencies, which proves the accuracy of the measurement technique.Finally, a new superconductor component in PSCAD/EMTDC (Power System Computer Aided Design/Electromagnetic Transients including DC) was developed in order to investigate the impact of the HTS cable integrated into the meshed power network. The superconductor component developed in PSCAS/EMTDC takes into account the heat exchange with the HTS cable cryogenic envelope and the detailed configuration of YBCO HTS tape so that HTS cable model is able to accurately predict the power flow, fault current level and grid losses of the power grid with HTS cables.

Date of Award	25 Aug 2016
Original language	English
Awarding Institution	University of Bath
Supervisor	Weijia Yuan (Supervisor)