• Energy Research

The offshore wind power system is expected to grow rapidly during the upcoming years. Simultaneously, the capacity of wind generators can surpass the rating of 15 MVA, necessitating step-up transformers to match this increased capacity. However, the main challenge is maintaining the compactness and lightness of the transformers while meeting high power demands. With this contribution, we explore the feasibility of a superconducting transformer with very high current density. This study delves into the mitigation of AC losses in a 15 MVA HTS transformer for wind energy applications by means of numerical simulations based on the finite-element method. Employing a 2D axisymmetric T-A formulation coupled with an electrical circuit, we estimate AC losses across hundreds of turns of REBCO tapes within a full transformer at reasonable computational time. The T-A formulation considers the superconducting layer of REBCO tapes as infinitely thin, thus allowing to overcome the problem of simulating a superconductor with an extremely high width-to-thickness ratio. To address the substantial transport current value on the high-voltage side, we utilized several parallel Roebel cables, optimizing parameters such as the number of strands in each cable, inter-strand gaps, and vertical separations between cables. Our findings suggest that altering the winding voltage and hence conductor length is not an effective strategy for reducing AC losses. Furthermore, for mitigating the high AC losses at winding ends, we integrate magnetic flux diverters into the transformer structure. Eventually, we propose two distinct designs operating at temperatures of 20 K and 70 K. This research introduces valuable insights and approaches for designing, optimizing and developing HTS transformers. 

Commutation failures represent a prevalent issue encountered in line-commutated-converter high voltage direct current (LCC-HVDC) systems. As the widespread deployment of HVDC systems continues, the risk associated with commutation failures increases, posing a growing threat to power grids due to their potential to trigger severe consequences, including cascading failures and widespread blackouts. This research paper aims to address the significant issue of commutation failure within direct current (DC) systems through advocating for the use of resistive-type Superconducting Fault Current Limiters (R-SFCLs). To substantiate the efficacy of this proposed strategy, an array of simulations are executed using the PSCAD/EMTDC software. This comprehensive study investigates the performance characteristics of R-SFCLs configured with varying resistance values, scrutinizing their response under diverse fault resistance scenarios and distinct fault initiation times within the LCC-HVDC system. The outcomes of these simulations are that SFCLs confer significant advantages for mitigating commutation failures, surpassing traditional mitigation methods in terms of effectiveness. Consequently, SFCLs emerge as an optimal solution to prevent commutation failures in the HVDC systems.

Abstract Along with advancements in superconducting technology, especially in high-temperature superconductors (HTSs), the use of these materials in power system applications is gaining outstanding attention. Due to the lower weight, capability of carrying higher currents, and the lower loss characteristic of HTS cables, compared to conventional counterparts, they are among the most focused large-scale applications of superconductors in power systems and transportation units. In near future, these cables will be installed as key elements not only in power systems but also in cryo-electrified transportation units, that take advantage of both cryogenics and superconducting technology simultaneously, e.g., hydrogen-powered aircraft. Given the sensitivity of the reliable and continuous performance of HTS cables, any failures, caused by faults, could be catastrophic, if they are not designed appropriately. Thus, fault analysis of superconducting cables is crucial for ensuring their safety, reliability, and stability, and also for characterising the behaviour of HTS cables under fault currents at the design stage. Many investigations have been conducted on the fault characterisation and analysis of HTS cables in the last few years. This paper aims to provide a topical review on all of these conducted studies, and will discuss the current challenges of HTS cables and after that current developments of fault behaviour of HTS cables will be presented, and then we will discuss the future trends and future challenges of superconducting cables regarding their fault performance.

Superconducting fault current limiters (SFCLs) offer an efficient means of limiting fault currents and supporting system reliability. However, they weaken the fault characteristics and reduce the sensitivity of traditional relay protection. The seamless integration of SFCLs with protective relays remains a complex and under-explored area, impeding their widespread industrial adoption. In parallel, current differential protective (CDP) relays are almost the primary protection for all high-voltage electrical equipment and are the cornerstone of global power system security. This paper fills a critical knowledge gap by researching the intricate interaction between resistive superconducting fault current limiters (R-SFCLs) and current differential protective relays. Our investigation commences with a comprehensive mathematical analysis, while researching the influence of R-SFCLs on CDP operation. Subsequently, we conduct a series of comparative experiments using the Matlab Simulink software platform. These tests evaluate the sensitivity, dependability, and security of CDPs in scenarios with and without R-SFCLs. The simulation results not only confirm the accuracy of our analytical framework but also shed light on the multifaceted relationship between R-SFCLs and CDPs. This research contributes to a deeper understanding of how R-SFCLs can be effectively integrated into power systems, offering a roadmap for enhancing grid protection.