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Title: Study of superconducting fault current limiters in power systems
Author: Dong, Qihuan
ISNI:       0000 0004 7968 5056
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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Power systems have increased both in scale and complexity as a result of rapid growth in the demand for power. The most common causes of failures in power systems are short-circuit faults. Short- circuit currents can grow thousands of times larger than normal system operating currents, i.e., within milliseconds. This places enormous thermal and mechanical stress on existing transmission infrastructure and may even cause blackouts. With the expansion and modernisation of power systems, fault current levels have also increased. The contributing factors include: the installation of new transmission facilities, transformers and generators, the upgrading of existing generators, and the integration of renewable energy sources. Conventional protection devices are unable to handle high fault current levels. Therefore a novel approach is required to limit the fault current level and to improve the resilience and reliability of power systems. The superconducting fault current limiter (SFCL) is one of the most significant and useful applications for high-temperature superconductors. It has many unique features which limit fault currents in power systems. Using the electrical characteristics of superconductors for current limitation is not a new idea. SFCL was first proposed in 1966 and has since been further developed. However, there is still much research needed on the SFCL device itself and its coordination with the traditional power grid. This thesis provides comprehensive and in-depth research on SFCLs and there are four new findings in this work. Firstly, we use four-probe methods to identify the best hierarchical structure for the 2G HTS YBCO tapes used for SFCL device in order to improve the fault current limiting effect. Three commercially available YBCO tapes are compared and the characteristics of one of the tapes have never been previously published. The work forms the foundation for the proper selection of YBCO tapes for SFCL devices, to support varying development and operational engineering requirements. Secondly, the study of SFCL integrated with a real AC power distribution network in Germany is accomplished using Finite Element Modeling (FEM) on Matlab Simulink. It provides the complete research procedures of SFCL integrated with a grid network and the underlying impact of this integration is thoroughly analyzed. The real data and sophisticated models employed here assure the simulation results are reliable and instructive for practical operation. Thirdly, we use PSCAD to simulate the SFCL installed in a Direct Current (DC) grid, and the SFCL has proved to be effective in preventing commutation failure in DC grid. Prior research done on the influence of SFCL on commutation failure has shown that it can mitigate commutation failure to some extent, but here we demonstrate R-SFCLs with correct resistance values can entirely solve the problem of commutation failures. It also shows that the SFCL has unsurpassed advantages compared to traditional mitigation methods. Fourthly, the coordination of SFCL with the Incremental Power Frequency Relay (IPFR) are studied by using PSCAD. The influence of SFCLs on grid protection is a critical area for SFCL engineering applications. The work examines the influences of R-SFCLs on IPFR of transmission lines, and proposes corresponding compensation methods. The theoretical analysis is innovative, and valuable for engineering applications.
Supervisor: Coombs, Tim Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: superconductivity ; power system ; superconducting fault current limiters