Hiroaki Tsutsui

Wind farms linked by SMES systems

The objective of this paper is to introduce the concept of wind farms linked by SMES systems. In this work, the SMES system is applied to a wind farm that is interconnected with a grid through a back-to-back DC link for the variable speed operation of the wind turbines. This system enables the output power leveling of the wind farm depending on the power demand and can reduce the capacity of the converter system by selecting an optimal discharge/charge rate of the SMES. By using the stored energy of the SMES, this system can also compensate the inertia of the blades so that the wind turbine speed can be rapidly controlled depending on the wind condition. This paper describes the design condition of the SMES for the output power leveling of the wind farm and discusses the SMES configuration for a 100-MW class wind farm.

Helically wound coils for high field magnets

The winding current density of a superconducting coil is one of the key parameters to realize high field magnet systems with smaller sized superconducting coil. Force-balanced coil (FBC) which is a helically wound toroidal coil can control the distribution of working stresses and minimize the structure requirements by selecting an optimal number of poloidal turns. The winding current density of a superconducting coil is estimated from the relationship between ampere-meters of conductor and structure requirements based on the virial theorem. In this case, the FBC can obtain the stored energy for the same winding current density 20 times larger than that in the toroidal field coils case and about 120 times larger than that in the solenoid case. By applying the FBC concept, superconducting magnets will be realized in smaller size.

Demonstration of the stress-minimized force-balanced coil concept for SMES

Strong electromagnetic force caused by high magnetic field and coil current is a serious problem in SMES systems. In facing this problem, we proposed the concept of Force-Balanced Coil (FBC) which is a helically wound toroidal coil. Based on the virial theorem, the FBC can minimize structure requirements for energy storage by selecting an optimal number of poloidal turns. We designed and fabricated a small experimental device which is composed of inner and outer helical coils mutually wound in opposite toroidal directions using NbTi superconductors. The distribution of the working stresses in this device can be changed by selecting the optimal current ratios between inner and outer coil currents. From the experimental results, we demonstrated the validity of the FBC concept.

Design and fabrication of a 7-T force-balanced helical coil

Force-balanced coil (FBC) is a helically wound toroidal coil. The FBC can minimize working stresses by selecting an optimal number of poloidal turns. In order to demonstrate the validity of the FBC concept for high field superconducting magnets, a 7-T force-balanced coil (7-T FBC) was designed. The 7-T FBC will have 270-kJ stored magnetic energy with a major radius of 0.20 m at maximum magnetic field of 7.0 T. This coil is a hand-made coil using NbTi superconductor. The 7-T FBC is designed within an allowable radius whose maximum working stress within an allowable stress of the stabilizer of NbTi strand. Then the winding of the 7-T FBC will be carried out without reinforcing materials for NbTi strand. The experiments will be conducted with pool boiling liquid helium cooling.

Voltage Rating Reduction of Magnet Power Supplies Using a Magnetic Energy Recovery Switch

A new concept of magnet power supplies that can reduce voltage ratings of the power supplies is proposed. Circuit diagram and operation principles of magnetic energy recovery switch (MERS) are described. MERS consists of a capacitor and four semi-conductor devices such as MOSFETs and IGBTs. It is connected in series to a power supply and a coil. MERS is a switch module and it has no power supply in itself. Because MERS generates a voltage required for the inductance of the coil, the power supply only has to supply a voltage required for the resistance of the coil. Therefore, using MERS can reduce voltage rating and capacity of the power supply. Two types of power supply using MERS and voltage rating reduction are discussed. Comparatively small power supplies for high-repetition pulsed magnets and alternating magnetic field coils can be designed. Some experiments were carried out and confirmed that MERS can reduce voltage ratings of power supplies

Development of a One Tenth Sized Model Coil for 100-MJ Class SMES Using Force-Balanced Coil Concept

A one tenth sized model coil based on the force-balanced coil (FBC) design for 100-MJ class SMES has been designed. This coil is a hand-made coil using NbTi superconductors, and the winding of the model FBC is now in progress. The FBC is a helically wound hybrid coil of toroidal field coils and a solenoid. This coil can minimize the mechanical stress and the required mass of the structure for induced electromagnetic forces. The model FBC with an outer diameter of 0.53 m will have the stored energy of 270 kJ at the maximum magnetic field of 7.0 T. The winding form is made of aluminum alloy and slots are cut on its surface with the shape of the helical winding. From the results of the cooling test with liquid nitrogen, the significant deformations of the winding form have not been observed. The winding of the model coil is carried out without reinforcing materials for the NbTi strand. The in-plane curvature of helical coils causes the winding errors. Then after a slight winding tension is applied to the NbTi strand, each winding is adjusted to be fixed in its place. The experiments will be conducted with liquid helium cooling. The DC power supply for the model FBC can simulate various SMES operations. As a first step, the ramp-rate dependence of the superconducting properties of the model coil will be investigated in order to evaluate the mechanical stress in the helical windings without the reinforcing materials for the NbTi strand.

Construction of a 7-T force-balanced helical coil

Force-Balanced Coil (FBC) is a helically wound toroidal coil that can minimize the required mass of its structure by selecting an optimal number of poloidal turns per toroidal turn. A 7-T Force-Balanced Coil (7-T FBC) was designed in order to demonstrate the feasibility of the FBC concept for high field superconducting magnets. The 7-T FBC with an outer diameter of 0.53 m will have 270-kJ stored magnetic energy at maximum magnetic field of 7.0 T. This coil is a hand-made coil using NbTi superconductor. The maximum working stress of the 7-T FBC is lower than the elastic limit of the Cu matrix so that the 7-T FBC can be excited up to the rated magnetic field of 7.0 T without reinforcing materials for the NbTi strand. The winding form is made of aluminum alloy and slots are cut on its surface with the shape of the helical winding with a numerically controlled (NC) lathe. The experiments will be conducted with pool boiling liquid helium cooling in order to measure the quench properties of the 7-T FBC and evaluate the working stresses in the helical windings.