The paper demonstrates that normal power system operating data may be used to determine the power-frequency characteristics in power system. Data are processed as random signals using spectral analysis techniques and the results estimate the generator and load power-frequency characteristics as transfer functions. The generator characteristics K_g is determined as a transfer function of the system in which input signals are frequency fluctuation ΔF and generator power setting signal ΔP_g and output signal is generator power output ΔP_e. For estimating the load characteristics K_l, there is a difficulty. There is a relation of ΔP_l=ΔL_o+K_lΔF, between actual load ΔP_l, ideal load under the state of nominal frequency ΔL_o and fluctuation of system frequency ΔF. While, because it is impossible to measure ΔL_o directly, ΔL_o and K_lΔF can not be separated. As to a partial system, when its capacity is small enough compared with the total system capacity, the contribution of ΔL_o in a partial system to ΔF of the total system may be ignored, that is, ΔF and ΔL_o are considered as independent phenomena. So, the cross-spectrum of ΔF and ΔL_o decreases to zero according to the increase of data in number and K_l of the partial system can be estimated.

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Dynamic stability of power system is essentially determined by the following two types of inherent oscillation modes.
(1) Voltage oscillation mode which is mainly related to the state variables of generator excitation circuit and system voltage controller.
(2) Power swing mode which is mainly related to the generator rotor phase angle and angular velocity (swing equation).
AC Voltage instability caused by the voltage oscillation mode may limit DC transmission power when DC system is interconnected with the weak AC system which has low SCR (Short Circuit Ratio). Damping and frequency of the voltage oscillation mode are affected by both AC and DC system voltage control parameters. This implies that the AC/DC system volttage stability should be analyzed with system models representing dynamic voltage-reactive power characteristics adequately.
This paper presents (i) definition of the so-called voltage oscillation mode in a typical AC/DC model system using an eigenvalue analysis technique, (ii) its verification by CRIEPI's AC • DC Power System Simulator, and (iii) converter control scheme to improve whole AC/DC system voltage stability.

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After the study of phenomena and condition on both loop current interruption by disconnector (DS) and magnetic induced current interruption by earthing switch (ES), the optimum interruption methods were researched. The conclusions are summarized.
(1) Since the arc is rotated at the high speed in magnetic driving type interrupting chamber, loop current in GIS is interrupted by this type chamber with approximately 1/3 shorter arcing time comparing with that by free burning type interrupting chamber. Therefore, even after 200 time loop current interruption, insulation performance on insulator was not deteriorated. Magnetic driving type interrupting chamber is most appropriate for DS with loop current interrupting duty due to the small driving energy required.
(2) The most effective configuration, including the number of coil turn, was found for the magnetic driving type chamber of full scale disconnector.
(3) The suction type interrupting chamber which can produce gas flow with small-sized chamber and low driving energy is most suitable for magnetic induced current interruption of ES. The interruption performance of magnetic induced current in 550 kV 12, 000 A transmission line was confirmed on practical opening speed.

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New technologies such as synchronized switching and power electronics will make it possible to change the configuration of transmission network in the future electric power systems. It is called VIPS (Variable Impedance Power System) by the authors. The authors have been studying the effects of those technologies on power system stabilities. This paper deals with the improvement of transient stability by means of switching-over control of transmission lines. It is made clear that in some power systems the simultaneous energization of the line de-energized in normal state with the fault clearing better improves first swing stability than the pre-and post-fault energization of the line. The energy function based index to select such a transmission line is derived. In addition, this paper shows multiple switching of transmission line improves damping of post-fault conditions, where minimization of time derivative of energy functions is used. The switching-over control is applied to multi-machine power systems and its effectiveness is demonstrated.

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Construction of dynamic equivalents plays an important role in the effort to reduce the computational burden of the power system stability studies. There are various approaches for the construction of power system dynamic equivalents. Among them, coherency based equivalents attracted the attention of power engineers due to it's accuracy and the simplicity in practical implementation. In actual practice, a pair of machines is usually said to be coherent if their angular difference remains nearly constant after a disturbance. However affinity of this definition with the final goal; i. e., model reduction, is questionable because it does not give explicit quantitative assessment of the effects of the equivalencing. Therefore this definition of coherency is necessary to be reconsidered.
In this paper, the authors propose a new approach to group machines in the external system for the equivalencing. Unlike conventional methods, we set the approximate maximum error in the rotor position of any machine in the study system due to the merger of two machines in the external system as an index to describe the eligibility of these two machines to be merged. Numerical examples are provided in order to prove the effectiveness of the proposed method.

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Disk type CCMHD generator with high enthalpy extraction ratio has been discussed from experiments and numerical simulations. The pressure and heat losses of the fluid are found to reduce the output power for the high load resistances. It is also found that the wrong inlet plasma condition affects on the plasma to present the instabilities in the inlet region and results in the reduction of output power. The generator with high enthalpy extraction ratio should match with the load resistance under the situation where the working gas properties change with the power generations. The simulations can interprete the power generation experiments with the FUJI-1 and the shock tunnel facilities.

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The paper describes the investigation of the electric equivalent circuit in redox flow battery. The electric circuit is mathematically analyzed with parameters which are cell resistance, manifold resistance, slit resistance, cell number, charge or discharge current and inner electromotive force. In the results, it is found that shunt current and power losses are able to be analized in the case of the large cell number. It is also clarified that effective cell number N_s exists and is shown by N_s=N/R^1/2_s, where N is cell number and R_s is slit resistance. The normalized terminal voltage, the current efficiency and the power efficiency are approximation of arc-tangent function with variable N_s. These values are mainly influenced by the cell resistance when N_s is lower than about 1, and are dependent on the manifold resistance in the larger N_s region. It is considered that this approximate function is effective and helpful for total system simulation.

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We have already developed a 6, 600V/210V, 100kVA hybrid-type superconducting transformer. In its overcurrent test, the superconducting winding in the low-voltage side was partially overheated and burned due to excessive ohmic heating by post-quenching current during 1s. In this paper, the progress of the temperature distribution along the superconducting winding is revealed with the lapse of time after quenching. The estimated temperature rises support the actual experimental data of the overheated superconducting winding as follows: (1) The wire temperature of the quench initiating point in 1s after the quenching has reached as high as 1, 970K. This temperature is much higher than the melting temperature of the copper stabilizer, 1, 356K, and nearly equal to that of NbTi, 2, 173 K. (2) The temperature within 150 mm from the quench initiating point has exceeded the melting temperature of copper. (3) If the maximum permissible temperature of the superconducting transformer winding is 300K, it is necessary to break the post-quenching current within 0.25s after the quencing.

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Gate turn-off thyristors (GTO) were used for control of transients in the accelerator power supplies of the JT-60 neutral beam injection system. The GTO switches were designed to clip the initial voltage overshoot which was produced at the previous turn-off. Greater than 20-kV initial voltage overshoot was expected in the JT-60 neutral beam injection system. Such a high voltage overshoot would reduce the maximum operating voltage. The output voltage was controlled by using nonlinear resistors connected in parallel with the GTO elements. Using this design, the output voltage was regulated to within 1.2kV.

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