Yoshishige Murakami

Application of superconducting magnet energy storage to improve power system dynamic performance

The application of superconducting magnet energy storage (SMES) to the stabilization of a power system with long-distance bulk power transmission lines which has the problem of poorly damped power oscillations is presented. Control schemes for stabilization using SMES capable of controlling active and reactive power simultaneously in four quadrant ranges are proposed. The effective locations and the necessary capacities of SMES for power-system-stabilizing control are discussed in detail. Results of numerical analysis and experiments in an artificial power-transmission system demonstrate the significant effect of the control by SMES on the improvement of power-system oscillatory performance. >

Ultrashort Electromagnetic Pulse Radiation from YBCO Thin Films Excited by Femtosecond Optical Pulse

We have observed ultrashort electromagnetic pulse radiation from YBa2Cu3O7-δ thin-film dipole antennas. The supercurrent transient is created by the excitation of the supercarriers into quasiparticles with a femtosecond laser pulse, and freely propagated electromagnetic pulses are measured and characterized. A pulse with 0.5 ps full width at half-maximum was obtained, containing frequency components up to 2.0 THz. A femtosecond time-resolved characterization of the spectra revealed that they strongly depend on the excitation conditions, and the quasiparticle recombination time becomes longer with increase in the excitation intensity. It is also observed that the radiation power increases in proportion to the square of both the bias current and the laser power in the region of weak excitation, which is consistent with the classical theory based on a two-fluid model. In the region of strong excitation, deviation from the classical theory was observed.

Terahertz radiation from superconducting YBa2Cu3O7−δ thin films excited by femtosecond optical pulses

Ultrashort electromagnetic waves (600 fs width) from superconducting YBCO thin films have been observed by irradiating current‐biased samples with femtosecond optical laser pulses (80 fs width). The Fourier component of the pulse extends up to ∼2 THz. The characteristics of the radiation are studied and the radiation mechanism is ascribed to the ultrafast supercurrent modulation by the laser pulses, which induce the nonequilibrium superconductivity.

Simultaneous Active and Reactive Power Control of Superconducting Magnet Energy Storage Using GTO Converter

By using GTOs (Gate Turn-Off Thyristor) in place of ordinary thyristors in a conventional six pulse Greatz bridge converter, forced commutation is possible, so in the usage of ac-dc power conversion the reactive power of not only lagging but also leading phase can be absorbed. This feature enables superconducting magnet energy storage (SMES) to control reactive power in the wide range between lagging and leading phases. By virtue of this control ability , SMES will be applicable for a power system stabilyzer. A GTO converter which mainly consists of six GTOs was developed and tested. In this GTO converter, the energy stored in leakage inductances of the transformer is handled by voltage clipper circuit. Then, by using two sets of these GTO converters, we carried out some experiments of simultaneous active and reactive power control of SMES. The experiments on the developed control system were successfully performed and the ability of controlling reactive power in the wide range between lagging phase and leading phase was verified.

Enhanced THz radiation from YBCO thin film bow-tie antennas with hyper-hemispherical MgO lens

We demonstrate power enhancement of terahertz (THz) radiation from YBCO thin films. An ultra-wide-band electromagnetic pulse, which contains frequency components over 3 THz, was excited by ultrafast supercurrent modulation with femtosecond optical pulse irradiation. The maximum THz radiation power emitted into free space was achieved up to 0.5 /spl mu/W with the combination of a bow-tie antenna and an MgO hyper-hemispherical lens. We discuss the radiation power dependence on temperature, excitation power, and bias supercurrent.

Experimental study on stabilization of model power transmission system by using four quadrant active and reactive power control by SMES

This paper presents the experimental results of stabilization of a model power transmission system by using a Superconducting Magnet Energy Storage (SMES). The SMES, which was composed of two sets of GTO (Gate Turn Off thyristor) power converters and a superconducting coil, is capable of controlling active power (P) and reactive power (Q) simultaneously in four quadrants by changing the firing angles of power converters. The model power transmission system was designed to simulate the behavior of a real scale long distance bulk power transmission system with voltage of 500 kV, capacity of 2000 MVA and length of 280 km. In this study, we have experimented power system stabilizing control by applying P-Q simultaneous control ability of SMES. From the results of experiment it was demonstrated that stabilizing effect by means of SMES is very significant.

Control of a superconducting coil by a MOSFET power converter operating at near liquid nitrogen temperature

The on-resistance of a power MOSFET has a minimum value at around 80 K. This characteristic was utilized in order to operate a power converter circuit using power MOSFETs in a cryostat with a superconducting coil. As a result, it was confirmed that power losses of the chopper circuit and the size of the current leads can be greatly reduced. The power loss of the circuit was half of that at room temperature, and the loss can be reduced further by using diodes with lower on-voltages at low temperatures. The size of the current leads was reduced by using the chopper circuit as a DC transformer.

Charging and discharging characteristics of SMES with active filter in transmission system

For SMES (Superconducting Magnet Energy Storage) composed of Superconducting Magnet (SM) and thyristor converters, the harmonic currents which are generated by SMES converter should be compensated. We have developed active filter, the output of which is adaptively controlled such that the deviation of the waveform of ac line current from the sinusoidal one can be cancelled out. SMES system composed of two sets of GTO (Gate Turn-Off) thyristor converter, 0.5 MJ pulsed SM which are controlled by a direct digital controller, and the active filter, has been developed. The SMES system has been connected to a model power transmission system together with a 10 kVA generator. The controlled characteristics of SMES in the model power transmission system and compensated characteristics of active filter have been experimentally studied. The experimental results show that harmonic currents of SMES can be successfully compensated by active filter, and simultaneous active and reactive power control of SMES can be carried out without the influence of harmonic distortion.

Fundamental analysis of dynamic stability in superconductive power systems

The features of instability in a long-distance bulk power transmission model system with superconductive power transmission lines or with superconductive synchronous generators are analyzed numerically. It is shown that self-excited oscillation is easily induced in superconductive AC power transmission systems. Torsional oscillation and electromechanical power swing in a model power system with a superconductive turbine generator power plant are analyzed for various system conditions, and it is confirmed in a digital simulation study that adequate energy control of the field winding of a superconductive generator can stabilize the power system.

THz emission from high-T/sub c/ superconductors by optical pulse excitation

Emission of ultrashort electromagnetic pulses from current-biased superconducting YBCO films has been observed by exciting with femtosecond laser pulses. The emission device is made of YBCO strip lines with a bridge at their center on an MgO substrate. The electric field of the pulse is detected by a photoconductive switch made of LT-GaAs. The emitted pulse width is /spl sim/0.5 psec and its Fourier component extends up to 2 THz. Characteristics of the THz emission such as bias current, pump power and temperature dependence are investigated in detail. The emission mechanism is ascribed to the ultrafast supercurrent modulation caused by the photoexcitation.