The SMES is characterized by several superior features such as allowing highly efficient, high-speed, input and output of electric energy. The application of a SMES to an electric power system contributes to stabilizing the power system and maintaining power quality, and it is anticipated that there will be extensive beneficial effects, such as upgraded controllability of power systems. Previous national development projects established the basic technology necessary to make the SMES feasible; however, challenges related to reducing system cost have remained. In the project phase from FY1999 to FY2003, the focus of development was to produce an SMES dedicated to power system stabilization, while placing a particular emphasis on finding ways to reduce the costs of the superconducting coil and peripheral devices. As a result, we now expect to be able to develop a low-cost SMES that is competitive with other power-control devices including SVC.
High-Tc superconducting technology is thought to provide many merits for SMES systems. For example, a cryo-cooled system can be used as a cooling system for high-Tc superconducting coils, indicating that a wide range of operation temperatures can be selected. As a result, the heat capacity of the coil system becomes much larger than that at 4.2 K. If we could absorb transient heat generation with the heat capacity of the coil, SMES systems can be designed under the over-current state of critical current for a short duration. As the cooling capacity for an average heat load will be enough to cool the High-Tc superconducting coil system for an SMES, it is expected that refrigeration system cost can be reduced. Moreover, we are developing a high critical-current superconducting wire for the SMES system. The Bi2212 Rutherford conductors can carry 4 kA at 26 K under cryo-cooling. We also estimate the superconducting wire cost of the whole coil system, which is designed to minimize the superconductor volume. The conclusion is that the cost of a high-Tc SMES system can be reduced remarkably by using low-cost YBCO superconducting wires in the future.
Since 1999, a Japanese national project to realize commercially feasible SMES for designated applications has been promoted. Model coils of cost-reduction SMES for load fluctuation compensation and frequency control have been fabricated and tested. Performance testing was carried out at Chubu Electric Superconductivity Test Center for the multipole solenoid system, model coil utilizing forced cooling conductor of NbTi stabilizing copper and separation CIC. Compared with the original design value, coupling loss was reduced to 1/50 or lower, thereby realizing extremely low current loss for a high-capacity large-scale conductor. A total of 10,000 repetitive energization tests were executed to confirm coil reliability. Effectiveness of the multipole was confirmed by measuring the leakage magnetic field. In addition, high-speed energization of 2 kA/s was achieved, confirming the possibility of expanding applicability to system stabilization. As a result, we verified that the cost reduction technologies including a conductor/coil manufacturing technology achieved the initial goal.
The authors have developed a solenoid model coil to stabilize the power system in superconducting magnetic energy storage (SMES) system with the aim of drastically reducing system cost. The single solenoid model coil is designed with the rated current, maximum magnetic field and coil charge rates equivalent to those of a practical 100 MW/15 kWh-class SMES using multi-pole solenoid coils. In addition, the coil is characterized by the use of a stabilized Al-NbTi CIC conductor that has a graded structure, layered winding and a high magnetic field with a compact design. To verify coil performance, several tests such as a the measurement of rated current and AC loss, high-speed pulse operation, coil quenching, and insulation characteristics were conducted using a model coil at Kyushu Electric's Imajuku testing center. Test results proved that the SMES model coil performs as well as the initially designed coil, while doing so at a drastically reduced cost.