Design considerations for SMES systems applied to HVDC links

The objective of this work is to discuss the concept of HVDC link with energy storage, especially with superconducting magnetic energy storage (SMES). The SMES will be one of the candidates as a power storage system for load leveling and power stabilization. Using superconducting coils instead of DC reactors, the SMES can be incorporated into the HVDC link. However, in large-scale SMES, the superconducting coils require special considerations for induced electromagnetic forces to limit allowable tensile stresses. To overcome this problem, the authors developed the concept of the force-balanced coil (FBC) which is a helically wound coil, and can significantly reduce the required mass of the structure for the same magnetic energy compared with the conventional coil configurations. In order to demonstrate the feasibility of the FBC concept, the authors have developed a superconducting model coil using NbTi/Cu composite strands. The hand-made windings of the model coil were neither impregnated with epoxy resin nor reinforced with stainless steel wires. From the results of the excitation tests with liquid helium cooling, the model coil was successfully excited up to 6.1 T, corresponding to 86% of the theoretical limit of the superconductivity. Based on achievement and status of large superconducting coils for high energy physics and nuclear fusion, a road map of large-scale SMES is developed. From the results, it is expected that the first 1 GWh class SMES system for daily load leveling will be installed in the period of 2030-40. However, in the case of HVDC links, the design condition of the SMES system involves many technical challenges from the view point of the superconducting engineering. For instance, the coil current is ordinary selected to at least over 10 kA due to the coil protection. On the other hand, the rated voltage of the HVDC will be over 100 kV, and the direct current is only about 1 kA. As a further step of this work, the power conversion system and th- e insulation method for SMES should be optimized.