M. Shiotsu

Improvement of Power System Stability by Use of Superconducting Fault Current Limiter With ZnO Device and Resistor in Parallel

Superconducting fault current limiters (SCFCLs) are expected to improve the reliability and stability of power systems. SCFCLs can be classified into R-types (resistive) and L-types (inductive) by the fault current limiting impedance. An L-type FCL is more effective in suppressing the voltage drop during a fault. On the other hand, a R-type FCL is more effective in consuming the acceleration energy of generators at the fault. Both functions lead to the improvement of the transient stability of the power system. We have proposed and fabricated the FCL unit, which is expected to have both functions. It consists of an inductive superconducting fault current limiter with a resistor and a ZnO device in parallel. In this paper, the simulation analysis on one machine and an infinite bus transmission system including the proposed FCL unit was carried out. EMTP (electro-magnetic transients program) was used in order to analyze the power system characteristics of the FCL unit. A rotor angle of the generator and a critical fault clearing time were analyzed to evaluate the effects of the FCL unit on the improvement of the transient stability of the model power system. A contribution share of the voltage drop suppression and the acceleration energy consumption to the improvement of the power system stability were clarified with various conditions, such as, generator output and fault clearing time.

Study on recovery time of a superconducting fault current limiter with adjustable trigger current level

The recovery time of a transformer type SCFCL (superconducting fault current limiter) with adjustable trigger current level, is studied experimentally The recovery time is defined as the required time of zero current period of SCFCL for recovery from its current limiting mode to its waiting mode. A trial SCFCL, which was designed and made, is tested to measure the recovery time with various fault time. The experimental results show that the recovery time depends on the fault time. When the fault time is longer than 200 ms, the recovery time becomes shorter and approaches a certain value (a few ten ms) as the fault time is longer.

Boiling heat transfer characteristics for heat inputs with various increasing rates in liquid nitrogen

Abstract The transition processes from non-boiling (single-phase) to film boiling or nucleate boiling on a horizontal cyclinder of 1.2 mm in diameter in liquid nitrogen caused by exponential, rampwise, and stepwise heat inputs with various increasing rates, were made clear at pressures ranging from 20.7 to 582.4 kPa for the saturated liquid, and at pressures of 1016 and 2047 kPa for the subcooled liquid (the temperature of which is 77.4 K). The direct transition from non-boiling (the heat transfer mechanism of which is quasi-steadily increasing natural convection) to film boiling was observed even for exponential heat input with the long period of around 100 s at atmosphere pressure. The transient heat transfer processes after the vapour film collapse at minimum film boiling in liquid nitrogen were also measured at pressures ranging from 101 to 392 kPa.

Photographic study on transitions from non-boiling and nucleate boiling regime to film boiling due to increasing heat inputs in liquid nitrogen and water

The behavior of vapor bubbles and vapor film during the transition from non-boiling regime such as natural convection or transient conduction regime to film boiling regime on a 1.2-mm diameter platinum horizontal cylinder in liquid nitrogen and in water due to exponentially increasing heat inputs, ranging from a quasi-steady state heat input to a very rapidly increasing one, were examined by photographs taken by a high-speed video camera. The experiments for water were performed for the two cases without and with pre-pressurization before each experimental run. It was confirmed by the observation of vapor behavior that the direct transitions in liquid nitrogen and in water which is pre-pressurized before each run occur due to the explosive-like heterogeneous spontaneous nucleation (HSN) in originally flooded cavities not only in the transient conduction regime, but also in quasi-steadily increasing natural convection regime without the vapor bubbles from active cavities entraining vapor. It was also confirmed that the semi-direct transition from conduction regime to film boiling with nucleate boiling due to the rapidly increasing heat inputs in water occurs due to the HSN with nucleate boiling at around the lower limit of HSN surface superheat in subcooled water even for the non-prepressure case. The lower limit of HSN surface superheat was measured as an initial boiling surface superheat caused by a quasi-steadily increasing heat input for the case with pre-pressurization before each experimental run.

Film boiling heat transfer from a vertical cylinder in forced flow of liquids under saturated and subcooled conditions at pressures

Forced convection film boiling heat transfer on a vertical 3-mm diameter and 180-mm length platinum test cylinder located in the center of the 40-mm inner diameter test channel was measured. Saturated water, and saturated and subcooled R113 were used as the test liquids that flowed upward along the cylinder in the test channel. Flow velocities ranged from 0 to 3 m s−1, pressures from 102 to 490 kPa, and liquid subcoolings for R113 from 0 to 60 K. The heat transfer coefficients for a certain pressure and liquid subcooling are almost independent of flow velocity and of a vertical position on the cylinder for the flow velocities lower than ≈1 m s−1 (the first range), and they become higher for the velocities higher than ≈1 m s−1 (the second range). Slight dependence on a vertical position being nearly proportional to z−1/4, where z is the height from the leading edge of the test cylinder, exists for the flow velocities in the second range. The heat transfer coefficients at each velocity in the first and second ranges are higher for higher pressure and liquid subcooling. Correlation for the forced convection film boiling heat transfer with radiation contribution on a vertical cylinder was derived by modifying an approximate analytical solution for a two-phase laminar boundary layer model to agree better with the experimental data. It was confirmed that the experimental data of film boiling heat transfer coefficients in water and R113 were described by the correlation within ±20% difference.

Effects of system pressure on minimum film boiling temperature for various liquids

Abstract The minimum temperatures of saturated pool film boiling on horizontal cylinders of various diameters in various liquids such as water, Freon-11, Freon-113, isopropanol, ethanol, liquid nitrogen, and liquid argon were measured for a wide range of system pressures. A correlation for minimum film boiling temperature on solid surfaces with various thermal physical properties in various liquids is presented based on the experimental results. The experimental minimum film boiling temperatures and the lower limits of heterogeneous spontaneous nucleation temperatures measured on the same cylinder in liquid nitrogen at various pressures agreed well with each other. The vapor film collapse in film boiling seems to occur at the lower limit of heterogeneous spontaneous nucleation temperature. Minimum film boiling temperatures on pure and oxidized Zircaloy-4 in water are estimated by using the correlation at system pressures ranging from 101 kPa to 7 MPa.

Simulation study on operating characteristics of superconducting fault current limiter in one-machine infinite bus power system

A simulation model of a Superconducting Fault Current Limiter (SCFCL) based on the experimental results was proposed. Simulation studies were performed in one-machine infinite bus system with parallel transmission lines, which corresponds to the experimental system. The simulation results agree well with the experimental ones. Power system operating characteristics of the SCFCL were investigated and discussed by simulation results Effects of the SCFCL to the power system are verified from the simulation results. It is shown that the SCFCLs make critical clearing time longer and improve the system stability.

Quasi-steady nucleate boiling and its life caused by large stepwise heat input in saturated pool liquid He I☆

Abstract Quasi-steady nucleate boiling on a horizontal cylinder in liquid He I caused by large stepwise heat input, whose height was far beyond the value corresponding to the steady critical heat flux, was investigated. It was confirmed that quasi-steady nucleate boiling exists with a certain life at the state corresponding to the point on the extension of the steady nucleate boiling curve. The correlation to express the life for the step heat input was given as a function of quasi-steady nucleate boiling heat flux. Another correlation to express the total energy transferred to the liquid from the cylinder surface before the end of quasi-steady nucleate boiling for a step heat input was also derived. It was made clear that this correlation is applicable not only to step heat input but also to the heat input which increases exponentially at first and then keeps constant (exponential step heat input). Namely, it is possible by using the correlation to predict the time when the quasi-steady nucleate boiling terminates after the addition of step and exponential step heat inputs.

Correlations for subcooled pool film boiling heat transfer from large surfaces with different configurations

Abstract Experimental saturated and subcooled-pool film-boiling heat-transfer data from horizontal cylinders in liquid nitrogen for wide ranges of surface superheat, liquid subcooling and system pressure showed little dependence on the diameter for relatively large diameters. A correlation for subcooled pool film boiling heat transfer from horizontal cylinders with relatively large diameters was derived by slightly modifying a theoretical solution based on laminar boundary layer theory to accord better with experimental data. A similar correlation applicable to saturated and subcooled local film-boiling heat-transfer on vertical plates and vertical cylinders was similarly derived using existing data. Local pool film-boiling heat-transfer coefficients at the midpoint of a light water PWR-type active fuel rod of a pulsed reactor agreed within experimental error with the values calculated from the correlation for the subcooling ranging from 10 to 80 K.

Heterogeneous Spontaneous Nucleation Temperature on Solid Surface in Liquid Nitrogen

Direct transitions from natural convection to film boiling were observed for quasi-steadily increasing heat inputs to a horizontal cylinder in a pool of saturated liquid nitrogen at the pressures from 50 to 207 kPa, and for ramp heat inputs to the cylinder at higher pressures. The heat flux at the direct transition for quasi-steady heat input under atmospheric pressure was about 14% of the steady critical heat flux of nucleate boiling obtained experimentally, and the surface temperature at the direct transition point was about 10 K lower than the theoretical value of the homogeneous nucleation temperature. The direct transition was recognized based on these facts to be due to the heterogeneous spontaneous nucleation at the direct transition point. The heterogeneous spontaneous nucleation temperature (which is the surface temperature at the direct transition point) is clearly dependent on the increasing rate of the heat input, and it increases from its lower limit for a quasi-steady heat input to near the value of homogeneous nucleation temperature for a rapid heat input with the increase of the increasing rate. The heterogeneous spontaneous nucleation temperature was measured for ramp heat inputs with various increasing rates to horizontal test heaters with different surface finishes in liquid nitrogen at pressures ranging from 50 kPa to 580 kPa. The heterogeneous spontaneous nucleation caused by the quasi-steady heat input was observed on the mirror finished surface at the pressure higher than that on the commercial one.