Numerical modelling of superconducting power cables with second generation high temperature superconductors
The advancements made in the manufacturing technology have made the second-generation high-temperature superconducting (2G HTS) applications increasingly appealing for the power transmission industry. The 2G HTS materials, with their inherent characteristics of extremely low resistance and immense power carrying capacity, now have the true potential of transforming the entire power network and bringing about an exponentially more powerful and virtually ‘loss free’ future for the power industry. The topic of numerical modelling of superconducting applications, being an effective method of understanding the electromagnetic properties of superconductors, has since received much attention from the research community. Amongst the broad range of existing utilities, the HTS power cable with 2G superconducting wires is one of the most attracting and widely adopted HTS applications. Over the years, the effort of developing comprehensive 3D numerical models for 2G HTS cables has yielded many outstanding research outcomes. However, with more advanced cable designs being developed and commercialised, their geometrical features are becoming increasingly complicated. This brings about a new challenge as the complex structure of HTS power cables significantly increases the computation power needed to perform simulations. This thesis aims to address this issue by adopting innovative approximation method to build a compact 3D numerical model for HTS cables designed with YBCO wires whilst maintaining good accuracy. In doing so, this project hopes to contribute to improving the efficiency of modelling HTS cables. This thesis starts by providing a literature review of the superconductors and their current applications on power transmission cables. Following the literature review, the methodologies applied to build this compact HTS cable model are then presented. The progressive model building process of this HTS cable model and the corresponding simulation results are placed in the succeeding chapters. Consequently, the summary and conclusions are reached at the end chapter of this thesis. This compact HTS cable model is developed based on the electromagnetic equation system known as the T formulation. In order to investigate the anisotropic characteristics of the superconducting materials, the finite element method (FEM) is adopted to discretize the equation system in simulated region. Based on solving the current vector potential within the simulated region, a comprehensive profile of the 2G HTS conductor (i.e. current density, perpendicular magnetic field, conductivity, critical current and loss distribution) can be obtained by this model. This project starts by initially building a 2D single HTS wire model. The single wire model is then validated by the experiment data provided by the research collaborators as well as the calculation results derived by analytical equations. Following the validation of the single wire model, a study of HTS wire dynamic loss under various external magnetic field settings is performed using the 2D single wire model and its variants. Since the ultimate target of this project is to build a compact 3D HTS cable model, an anisotropic homogenous-medium approximation is adopted to expand the 2D model into 3D to reduce the workload of calculation while maintaining good accuracy. Upon the completion of a functioning 3D single layer cable model, multi-layered cable structure is then built with the same strategy. In order to fully explore the computation resources on hand, a parallel computing structure is also developed at this stage to employ multicore operation to unravel the large-scale dense matrix imposed by a 3D problem. Finally multiple case studies were carried out with the 3D cable models to identify scenarios where 2D models can be used to approximate the 3D cable structure and where accurate 3D models have to be built in order to reflect the realistic properties of HTS cables brought about by different designs. The difference in results and the root causes were analysed. The outcome of this research will be used to help identify more efficient strategies of building superconducting cable models.