This article describes the power system reformation in Japan, and the future perspectives to realize a carbon-neutral society by 2050 in the viewpoint of electric power system. Further, some possibilities to apply superconducting power technologies to support a carbon-neutral society have been investigated as well.

This paper describes the measurement of eigenvalues for stability of operating electric power systems by changing power of a superconducting magnetic energy storage. The concept of the electric power system stability is introduced for cryogenic and superconducting researchers. The method of measuring the eigenvalues for stability of operating electric power systems is described. Some examples of the measurements by use of simulators and in a real power system are shown.

This paper discusses the feasibility of SMES as a measurement apparatus for electric power system stability. The system stability is evaluated by eigenvalues that express the oscillation modes of the electric power system. To measure the eigenvalues, the storage system requires controllability of charge/discharge with a rapid cycle of less than 1 s. Because the oscillation modes of the power system vary in real time and their distribution is complex, the mobility of the SMES system is the most important parameter for system stability measurement. In this work, the author carried out a design study on a 1-MJ-class mobile SMES system using MgB2 Rutherford cables. The results showed that 1) the 1-MJ-class mobile SMES components can be installed in a 40-feet dry container; 2) because of the effect of the force-balanced coil (FBC) design, the SMES coil can be excited up to 2.0 T or 3.0 T without reinforcements for the MgB2 Rutherford cables; and 3) the 1-MJ-class mobile SMES coil can be cooled using 3 or 4 sets of conventional cryocoolers, including the cooling system for a 80 K thermal shield at a cooling temperature of 20 K or 10 K. Compared to the conventional lithium-ion battery energy storage, the SMES system has a current supply capability with a minimum influence on the power system and a design flexibility of the stored energy achieved by selecting an optimal cooling temperature depending on the power system conditions without an increase in the total weight of the SMES coils. These features show the technical advantages of SMES as a measurement apparatus for electric power system stability.

SMES (Superconducting Magnetic Energy Storage System) is a power storage technology whose realization has been expected for a long time. If the efficient power storage becomes possible, that will allow to store temporarily the power generation output of renewable energy such as solar power and wind power generation, whose usage expands more and more, and supply it according to the load. It is expected that it can contribute to the stabilization of the energy supply. A major feature of SMES is that compared to other power storage technologies such as secondary batteries and capacitors, the superconducting coil, which is an energy storage unit, does not deteriorate with repeated charging and discharging, even in a short time. A compact system can be realized because the capacity of the storage unit can be designed without the excess or deficiency common for applications that require high output. From this point of view, since 2003, we have developed SMES that compensates for the instantaneous voltage drop, and after field tests, since 2007, we have been conducting a commercial operation of SMES that can output 10 MW for up to 1 second with a track record of continuous operation of 100,000 hours or more. The history of research and development of SMES as an instantaneous voltage drop compensation device, the results of research and development, and the operation results are summarized in this article.

The superconducting Magnetic Energy Storage (SEMS) application still has a great potential to stabilize the utility grid when the uncontrollable power generation from renewable sources increases and power flows change rapidly due to the broad introduction of high-speed response semiconductor switching devices. Along with the development of liquid hydrogen supply chain, the SMES system using MgB₂ conductors also attracts great attention at this point. Although the MgB₂ wires which have critical temperature of around 39 K have been commercially available with more affordable prices, their bending strain sensitivity is an issue to be solved for fabricating large-scale conductors and coils. The experience of constructing a 10-kJ SMES system using Bi2223 tapes and the successful demonstration of compensating very fast electric power fluctuations in the previous project will help us to develop a larger-scale MgB₂ SMES system by investigating conductor and coil design while considering its bending strain sensitivity and mechanism of critical current deterioration to maximize its performance as one of the most promising energy storage devices, following the movement toward a CO₂-free environment.

Microscopic theory of the depairing current density (j_d) is briefly reviewed for the applied-superconductivity communities. The goal of this article is to introduce the Kupriyanov-Lukichev-Maki theory of j_d for s-wave superconductors in the diffusive limit, which is the most reliable calculation of j_d based on the BCS theory and is relevant to various superconducting devices. The effect of subgap states on j_d is also discussed.