Yasunobu Yokomizu

Development of flux-lock-type fault current limiter with high-T/sub c/ superconducting element

The present paper proposes a new the fault current limiter (FCL), which consists of a high T/sub c/ superconducting (HTS) element, three coils wound on the same core, and a magnetic field coil covering the HTS element. In this FCL, the initial limiting current level can be controlled by adjusting the inductances of the coils. Furthermore, an external AC magnetic field is applied to the HTS element to get higher resistance in the current-limiting phase. A current-limiting experiment by a model FCL was carried out, and the limiting performance of the FCL was observed. Both the initial limiting current level and the limiting impedance could be increased in this FCL compared with the corresponding values of the HTS element itself.

Total voltage drops in electrode fall regions of , argon and air arcs in current range from 10 to 20 000 A

Measurements were made of the sum of cathode fall and anode fall voltages, i.e. electrode fall voltages for , argon and air arcs at a pressure of 0.1 MPa in wide current range from 10 to 20 000 A. For each arc, copper - tungsten, copper, iron and tungsten were used as electrode materials. The arcs proved to have electrode fall voltages of 17.5, 17.5, 17.5 and 13.8 V at the Cu - W, Cu, Fe and W electrodes respectively. The electrode fall voltage of each arc was affected by the electrode material. It was also found that the electrode fall voltage of each arc at a given electrode was independent of current in the above range. Further, the electric field strength at cathode surface and thickness of cathode fall region were estimated on the basis of Temperature-and-Field emission theory.

A study on required volume of superconducting element for flux flow resistance type fault current limiter

We have proposed a fault current limiter (FCL) suppressing an overcurrent only by a flux flow resistance of a high temperature superconductor (HTS). If the fault current is interrupted within an allowable time t/sub a/ after the fault occurs, the flux flow resistance type FCL can instantly recover to the superconducting state and pass a load current. In this paper, the volume V/sub r/ of the HTS in the FCL required to satisfy the specified limiting effect and t/sub a/ was investigated theoretically. The volume V/sub r/ depends on the critical current density and the flux flow resistivity of the HTS. As the flow resistivity and/or critical current density increase, V/sub r/ can be reduced to obtain a certain current limiting effect. When t/sub a/ is specified, V/sub r/ has a maximum value at a certain flux flow resistance. If both the limiting effect and t/sub a/ are given, the required volume is constant and independent of critical current density or flux flow resistance.

Power system stability improvement by energy storage type STATCOM

This paper describes an approach to design a damping controller of an energy storage type STATCOM. The energy storage type STATCOM (ESTATCOM) is an advanced flexible AC transmission system (FACTS) device, which controls both reactive and active power injection/absorption to the power system. It also provides a better power swing damping. Using a linearized block diagram proposed by the authors, the present study examines the design of the ESTATCOM damping controller. Several case studies have been performed to evaluate the power swing damping effect of the ESTATCOM on one-machine infinite bus system. The results of the study show that an ESTATCOM, which controls both reactive and active power injection/absorption to power system, has a more significant effect on power swing damping than that controlling the reactive power alone.

Evaluation of LFC capacity for output fluctuation of photovoltaic power generation systems based on multi-point observation of insolation

The output of a photovoltaic power generation system (PV system) fluctuates depending on weather conditions. Large-scale introduction of PV systems may cause some difficulties in the operation of an electric power system. The evaluation of the influence of PV systems on the power system operation should consider the smoothing effect of the outputs by the dispersed introduction of PV systems. This paper discusses the LFC (load frequency control) capacity for output fluctuation of PV systems based on the insolation data simultaneously observed at 5 points around Nagoya, Japan. As a result, when the weather was clear but clouds moved frequently, the capacity. for LFC based on the regional averaged insolation was evaluated less than a half of that evaluated by the insolation at one point. Furthermore, LFC capacity might be dominated by the speed factor rather than the magnitude factor of load fluctuation including PV systems output when a large capacity of PV systems is installed.

Proposal of flux flow resistance type fault current limiter using Bi2223 high T/sub c/ superconducting bulk

A flux flow resistance type fault current limiter (FCL) suppressing an overcurrent only by the flux flow resistance of a high temperature superconductor (HTS) was proposed. This type of FCL may recover to superconducting state so that a load current is passed immediately after the fault clearing because the flux flow resistance disappears at the moment that the current becomes lower than the critical current. That is an infinitely short recovery may be achieved. In this paper, we carry out a feasibility study on the flow resistive FCL using Bi2223 bulk installed at the outgoing feeder in a 6.6 kV distribution substation. It is pointed out the flux flow resistance type FCL may be realized by using the bulk of about 10/sup -2/ to 10/sup -1/ m/sup 3/.

Particle composition of high-pressure SF/sub 6/ plasma with electron temperature greater than gas temperature

Numerical computation was performed to derive particle compositions of high-pressure SF/sub 6/ plasmas in which the electron temperature T/sub e/ is greater than the gas temperature T/sub g/. The effective excitation temperature T/sup ex/ for each of particles was proposed to express the distribution of particles in excited states. Furthermore, the relation equations for chemical equilibrium in a two-temperature plasma were developed on the basis of the second law of thermodynamics. At fixed T/sub g/ in the range of 6000-9000 K, the increasing T/sub e/ from 9000-20 000 K encourages the dissociation and ionization of particles, causing the composition of SF/sub 6/ plasma at 0.1 MPa to differ markedly from the composition under the condition of thermal equilibrium.

On the determination of the multi-temperature SF6 plasma composition

We have calculated the multi-temperature SF6 plasma composition by two different methods. One method is based on the mass action law whereas the other method uses a kinetic chemical reaction scheme. The first approach is furthermore split up in two different calculations. One is based on the multi-temperature Saha equation and the other is based on the excitation temperature of the involved species. With respect to the electron density we conclude that the multi-temperature Saha equation results in appreciable higher number densities compared to both the kinetic scheme and the excitation-temperature-based calculations. The difference increases with decreasing gas temperature and increasing electron temperature (i.e. with increasing degree of kinetic non-equilibrium). The kinetic scheme and the excitation-temperature-based calculation yield comparable results for higher gas temperatures. In this region the excitation-temperature-based method is preferable over the kinetic method. However, for the dielectric case (i.e. lower gas temperature) we find different results at higher electron temperatures. The excitation-temperature-based calculation method requires additional modelling at this point.

Inrush-current-limiting with high T/sub c/ Superconductor

We propose an inrush-current-limiting element as a new application of high-temperature superconductor (HTS). The inrush-current-limiting element is required to recover automatically to superconducting state without any current interruption after the current limiting operation. The limiting element proposed suppresses the inrush current using the flux-flow resistance generated in the HTS. Since the flux-flow resistance is small as to be neglected when the instantaneous value of the current in the HTS is below its critical-current, the limiting element may satisfy the above requirement for the inrush-current-limitation. To verify the performance of the inrush-current-limiting element, we performed the current-limiting tests by a limiting element made of bulk Bi2223. As a result, it was confirmed that the limiting element suppressed the magnetizing inrush current of a transformer and self-recovered to superconducting state.

PD characteristics as a stochastic process and its integral equation under sinusoidal voltage

This paper proposes an integral equation to describe the stochastic fluctuation of partial discharge (PD) occurrence under sinusoidal waveform voltage conditions based on a simple PD model (an equivalent circuit model). The stochastic behavior of PD fluctuation is assumed to arise from the fluctuation of PD delay time after the discharge inception voltage is built up across a discharge gap. The distribution of delay time is assumed to obey an exponential distribution and the dependence of the delay time on the overvoltage is assumed to be expressed by an exponential function of the overvoltage. The integral equation has been solved numerically under the low voltage condition in which only one PD can occur in a half AC voltage cycle. It is found that the proposed integral equation can provide the basic characteristics such as PD pulse repetition rate vs. applied voltage phase angle.