As a very attractive application of superconductivity, Superconducting Magnetic Energy Storage (SMES) has been studied for 30 years, and with the development of superconductor technology, it has now just come to the stage of construction for practical SMES systems. The first systematic study by R. W. Boom aimed at having a very large-scale SMES for diurnal load leveling in electric power line systems were followed by many other design studies and projects. The representative projects at present are the USA ETM 20MWh project and 500kWh one by Babcock & Wilcox. The former is the first project concerning a practical scale system, which, however, had to be closed in 1982 before construction. The latter is now in progress and is to be installed in Anchorage ML & P in 1997 as the first commercial SMES system. The main points of SMES design are economic design of coils, support structures for magnetic force, and power conditioning systems. The basic idea for these are explained first. Seen from the side of application, it should be characterized by the stored energy (W) and the power (P) from it, and wide variety of applications ranging from SMES's for transmission line stabilization to those for diurnal load leveling can be discussed in P-W map. The practical region and what scaling steps in the development have taken place are shown in the map. Next, points of magnet technology varies with the scale of SMES. They are characterized by the average current density (Jav) and W, and are discussed in Jav-W map which has an allowable domain restricted by some limitations. The above two maps may help to figure out the whole feature of SMES systems. Finally, the present status of SMES development and problems are stated with regard to ETM design, B & W project and Micro-SMES, as well as on the state of the art of Japanese SMES research, in some detail.
To compare stability in AC use of NbTi and Nb3Sn superconductors, a critical quench margin of the superconductors has been defined. The comparison has been done based on the critical quench margin obtained by numerical calculations on conditions of AC operation in the AC field. The comparison has also been done on other conditions of DC operation in DC fields superposed by AC fields. Through these comparisons, the advantages of Nb3Sn superconductors over NbTi ones have been found as follows: As for AC operation in AC field (commercial frequency, amplitude: 0.1-0.5T) in power apparatuses, such as fault current limiters and shunt reactors, the high current density type Nb3Sn (Jc: around 4, 000A/mm2 at 0.5T, total loss: around 20kJ/m3/cycle in a peak field of 0.5T, 50Hz) is more stable than NbTi. As for AC operation in an AC field (commercial frequency, amplitude: 0.5-1.5T) in power apparatuses, such as armature windings of superconducting generators and power transformers, the low AC loss type Nb3Sn (Jc: around 2, 000A/mm2 at 0.5T, total loss: around 2kJ/m3/cycle in a peak field of 0.5T, 50Hz) is more stable than NbTi. Furthermore, as for DC operation in a DC field (5T) superposed by AC field (amplitude: peak field of 0.1-1.5T, frequency: 2.5-350Hz), the high current density tpe Nb3Sn is more stable than NbTi.