坂本 織江

Electromagnetic transient (EMT) simulations of relatively large power systems have become quite common, for instance, in the case where simulations of HVDC converters are carried out with large ac power systems connected to the converters. Thus, the increase in computation time is a serious concern. To reduce computation time, this paper proposes a method to reduce computational demand of a remote power system which is located far from the source of a transient event to be simulated. In the proposed method, the remote power system, which is supposed to be represented by a three-phase EMT-based model, is reduced to a single-phase phasor-based model, and the size of the circuit to be simulated is thus reduced and the dynamics calculations of inductors and capacitors included in the remote power system are neglected. The calculation algorithm of generator models included in the remote power system is also simplified. The proposed method has been applied to EMT simulations of the WEST 10 benchmark power system prepared by the IEEJ, and it has been shown that the computation time is remarkably reduced without significant loss of accuracy if the portion assumed to be the remote power system is sufficiently far from the source of a transient event.

Introduction of wind power generation is promoted all over the world so as to utilize renewable energies. Among various types of wind power generation, two-speed switching type squirrel-cage induction generators are used in remote sites and island power systems due to its low cost and ease of maintenance. However, outputs of these generators tend to fluctuate and may deteriorate power quality. Especially, direct interconnection of the wind generator causes an instantaneous voltage sag in the power system. In addition, there are some large induction motor loads such as pumps in water desalination plants and motors in factories and these motors loads are also considered to be affected by the operation of the wind generators. In this study, effect of voltage-stabilizing control of a flywheel energy storage system on stable operation of these induction machines is investigated with developed induction machine models and island power system model

Superconducting generators (SCGs) are considered to bring many advantages to electric power grids such as high generation efficiency, improvement of power quality, capability to enhance power system stability, and so on. In order to investigate these benefits in the conditions similar to those of a real power system, electromagnetic transient (EMT) analysis in three-phase instantaneous values is very useful. In this study, a new SCG model for power system transient analysis has been developed with an EMT software named Expandable Transient Analysis Program. Novelty of the newly developed model is in the calculation procedure using a two-stage diagonally implicit Runge-Kutta method and a second-order Runge-Kutta method (RK2). The proposed method enabled us to interface the SCG model with the outer power system without prediction or delay. The method is considered to enhance the numerical stability of transient analyses including SCGs by explicitly limiting the integral calculation errors through the use of RK2. It was confirmed that the developed model is useful to EMT analysis including SCGs.

We prepared two kinds of ionic liquids (DEME BF4, DEME TFSI), and measured the thermal conductivity of the ionic liquids and the ac loss of the Bi-2223 tape using the ionic liquid impregnation under dc magnetic field in liquid nitrogen. As a result, the thermal conductivity of DEME BF4 is approximately twice that of the epoxy. When the mechanical loss is maximum value, the loss can be reduced about 98.5% by using the ionic liquid compared with no impregnation. We consider that the tape impregnated into the ionic liquid is effective for reducing the ac loss. These results produce fundamental data to reduce the ac loss of conduction-cooled high-temperature superconducting coils.

In this paper, the authors proposed the loop microgrid system composed of loads, Superconducting Magnetic Energy Storage (SMES), and renewable energies such as solar and wind generators. The system adjusts the balance of supply and demand in the microgrid by compensating the power fluctuation of renewable energy sources with SMES. The SMES is connected with two independent Current Source Inverters (CSIs) in order to control power flow in the microgrid as well as at the interconnection with the main power system. The aim of this paper is to control power flow at the point of interconnection with the main system and the microgrid. The system was simulated by using PSIM and MATLAB/Simulink. The authors proved that the system can control the power flow as expected..

Owing to the recent requirement for smart grids, the efficient utilization of electric power including motor control with power electronics apparatus has become important. To verify control effect under conditions similar to those of a real power system, electromagnetic transient (EMT) analysis software is very useful. In the EMT analysis of power systems, the modeling method used for rotating machines has a large impact on the accuracy and efficiency of the analysis. In this paper, a new squirrel-cage induction machine model for XTAP as a voltage source behind the primary winding leakage reactance (VBR) model is proposed. The noteworthy feature of the method is the calculation procedure using a two-stage diagonally implicit Runge-Kutta (2S-DIRK) method, second-order Runge-Kutta method (RK2), and and dq transformations without any prediction. This approach is considered to enhance the numerical stability of transient analyses including those of rotating machines by explicitly limiting the calculation errors through the use of RK2.

We have proposed a Dyneema fiber reinforced plastic (DFRP) as a coil bobbin material. The DFRP has some properties such as high thermal conduction, easy mechanical processing, and expansion with cool down. We fabricated superconducting coils having an YBCO tape, cooled the coils with a refrigerator, and supplied a dc current to the conduction cooled coils. Thermal stability of the coils was also estimated. The DFRPs, glass fiber reinforced plastics, and AlN blocks were used as the bobbin materials for the coils. From the experimental results, the thermal stability of the coils increased with increasing of the winding tension of the coils, and the coil having the DFRP bobbin showed better performance than the coils with the glass fiber reinforced plastic and the AlN bobbins. We think that contact force between the superconducting tape and the bobbin became large due to the thermal expansion of the DFRP bobbin. These results showed that DFRP can represent a viable opportunity as bobbin material for conduction cooled high-temperature superconducting coils.

We fabricated nonimpregnated HTS superconducting coils with a BSCCO tape. Bobbin materials in the coils are a Dyneema(R) fiber reinforced plastic (DFRP) and a glass fiber reinforced plastic (GFRP). We excited those coils with ac currents, and estimated mechanical losses. According to the measured data, the mechanical loss decreased with increase of winding tension of the coils, because strong winding tension fixed the coils tightly. Also, the mechanical loss occurred in the DFRP coil was smaller than that occurred in the GFRP coil. A thermal expansion coefficient of the DFRP is a negative value, that is, the DFRP expands with cooling down from room temperature to cryogenic temperature. The expansion of the DFRP bobbin made the winding of the coil fix tightly, and the mechanical loss decreased. From those experimental results, we think that the DFRP bobbin is useful for decreasing of mechanical losses of the ac coils.

Nowadays some nuclear power plant in Japan is shut down due to problem of security in earthquake disaster. Consequently, Japan will encounter a lack of electrical energy in the future if new electrical generation is not constructed. Solar energy is one of appropriate renewable energy for Japan. Solar panel produces electrical energy by using the natural solar radiation from sun. PV Solar farm consists of multiple components, including the photovoltaic modules, mechanical and electrical connections. PV solar farm with grid-connected can directly connect to the existing electricity grid by using inverter circuit toward transformer. In this paper, PV solar farm is utilized as a power generation which injects active power into a Japan east power system. An optimal location and sizing of solar farm by using multiobjective Bees optimization (MBO) is proposed to minimize fuel and emission costs of overall system with considering solar radiation energy in each area. The results show that the proposed method found the optimal position of solar farm with minimum cost of fuel and environmental pollution. © 2013 IEEE.

In this paper, the authors proposed a new circuit topology of AC loop microgrid with a Superconducting Magnetic Energy Storage (SMES) and solar and wind generators. The SMES is connected with two converters controlled independently to supply power to different parts of the microgrid. The balance of supply and demand in the microgrid is adjusted by controlling the converters. The aim of this study is to control power flow between the main power system and the microgrid so as to reduce reverse power flow. The authors proved that SMES can control the power flow in the proposed microgrid through the numerical simulation by use of PSIM and MATLAB/Simulink.

Integration of wind power generation into small islands has been one of the demonstration projects in Okinawa Prefecture. Since such integration could degrade power quality including frequency in an island grid, a frequency stabilizing system using flywheels has been installed into a small island. In order to establish a proper frequency stabilizing scheme for the small island, an accurate model of a diesel generator including governor is vital. Therefore, the model was developed based on the measured values of generator dump tests. A new frequency stabilizing scheme was also developed through time-domain simulation of the island grid model, which consists of the above-mentioned diesel generator model and an equivalent load change representing wind power variation. The proper parameters of the scheme were derived considering role sharing between the diesel generators and the flywheels. The developed stabilizing scheme was applied to the flywheels in the island grid and revealed excellent performance in mitigating frequency variation. (c) 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 180(1): 2435, 2012; Published online in Wiley Online Library (). DOI 10.1002/eej.21277

The use of photovoltaics (PV) has increased in some small islands of Okinawa Prefecture, Japan in response to various renewable energy demonstration projects. Since this increased usage could deteriorate the power quality, including frequency, of an isolated grid in a small island, a frequencystabilizing system using lithium-ion capacitors (LiC) has been integrated into the grid. Because a frequency-stabilizing scheme is the heart of the frequency-stabilizing system, a scheme which employs deviation of PV output (Delta P) and system frequency (Delta F) as inputs was developed. The parameters of the Delta P input part were derived for mitigating stepwise changes of system frequency caused by a sudden fluctuation in PV output. The parameters of the Delta F part were set so as to alleviate the high-frequency components of the system frequency deviation caused by load fluctuation. The developed scheme was applied to the stabilizing system using LiC in the island grid and was found to greatly alleviate frequency fluctuation.

In order to demonstrate the large-scale integration of wind power generation into small isolated island power systems, a test facility composed of two wind turbine generator systems (WTs) and a flywheel energy storage system (FESS) was introduced in a remote island in Okinawa prefecture, Japan in 2009. Since large-scale integration of wind energy may cause fluctuations in the frequency and voltage of the power system, the FESS was equipped with both frequency-and voltage-stabilizing control systems. The new frequency-stabilizing control scheme developed for the FESS has been in operation since March 2010. This paper describes improvement of the voltage-stabilizing control system of the FESS. The proposed system was examined in the actual power system and proved to reduce voltage fluctuation successfully. In addition, numerical simulations revealed that the improvement of the voltage-stabilizing control system also helped to stabilize frequency.

In recent years, installation of renewable energy into load supply systems has drastically increased. Load modeling considering dispersed generators is indispensable to improve the accuracy of power system analysis. Most of the installed capacity of dispersed generators currently consist of synchronous generators equipped with excitation controllers with Automatic Q Regulator (AQR) or Automatic Power Factor Regulator (APFR). These excitation controllers are thought to give large influence on dynamic behavior of load supply systems, because the behavior of load bus voltage is strongly related to load characteristics. The post-fault dynamic behavior of load supply systems including generators with AQR was investigated using CRIEPI's AC and DC power system simulator. In this paper, load drop characteristics and features of transient and steady state responses of the load bus voltage are experimentally demonstrated. Some key issues for developing the load model including generators are also discussed.

Superconducting generators (SCGs) have many features such as high generation efficiency, small size, capability to enhance power system stability, and so on. Introduction of SCGs will offer great benefits to electric power systems. Appropriate design is necessary for SCGs to demonstrate these beneficial effects. There are some former studies on explicit expressions on relations between electrical or mechanical characteristics and structure-parameters of SCGs. Obtained expressions gave good view of generator characteristics. In this study, approximations used in the expressions are modified for recent design of damper cylinders. Some consideration on design of damper cylinders for SCGs is discussed as well.

Superconducting Generators (SCG's) are considered to have high tolerance for harmonic currents. To confirm the effects, some experiments on characteristics of a slow response excitation type SCG for harmonic armature currents were carried out by use of a real-time power system simulator. Furthermore, to analyze the results, analytical expressions on the eddy currents of the slow response type SCG were derived based on two-dimensional electromagnetic analysis. The expressions give simple relation between the eddy currents and the SCG's structural parameters such as radiuses and electric conductivity of damper cylinders. This paper describes the results of the experiments and some discussion on relation between harmonic current absorption and structural parameters of SCG's.

Superconducting Generator (SCG) has many advantages such as small size, high generation efficiency, low impedance, and so on. An improved power system with many potential good properties may be obtained by introduction of SCG. In order to study the behaviors of SCG in power systems, a digital type SCG model for an existing analog type real-time power system simulator is developed in this work, which is suitable for real-time simulations when the generator constants and control systems are changed frequently. By use of the above-mentioned equipments, real-time simulation in cases of SCG with different generator constants and in case of SCG in multi-machine power system has been conducted. The real-time simulation results verify the effects of SCG on power system stability enhancement and show the influence of generator constants on its stability-improving effects.

This paper describes how the power system ability can be improved by introducing a superconducting generator (SCG) into power system networks. This is of particular interest in the case of power system network with IPP (independent Power Producer). In this work, a digital type SCG model for an existing analog real-time power system simulator was developed, and experimental studies by use of the above-mentioned equipments have been conducted to investigate the effects of SCG on available generator power output and power transmission capacity. The influence of generator constants on SCG's behavior is studied in a signal machine infinite bus power system model. A comparison of available generator power output and ATC (Available Transmission Capacity) is made between SCGs and conventional generator (CG) in a five-machine infinite bus power system model. The experimental results have shown the advantages of SCG over CG in terms of power system stabilities.