The travelling of an arc between parallel electrodes can be used in order W suppress the fault current by inserting the electrode, resistance to the fault circuit, where the arc velocity plays the most important role. In order to increase the arc velocity a special piling-up type has been theoretically recommended, the arc velocity of which is expected to become about 22% faster than that in the ordinary parallel type.

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A new analytical approach to the fmite element calculation of torque in motors including eccentric rotor, based on the Maxwell Stress Method, is proposed in this paper, where the effects due to eccentricity on the cogging torque and the torque characteristics with regard to the speed are confirmed. A brushless DC motor is employed as an analytical model for two kinds of eccentricity. Consequently, in the case of eccentric motors, the unbalanced radial force functions as the repulsive force against rotation which causes the reduction of average torque, and two kinds of eccentric motors have same tendency, concerning the effects on the torque. This proposed approach is applicable to any kind of motors including eccentric rotor.

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For a reliable insulation design of HV (high voltage) power apparatus, potimization of electric field distribution has been an important technique. We ave been developing highly efficient algorithm for an automatic electric field optimization using a personal computer. However, to make the optimization technique more powerful and more accurate for electrical insulation design of HV power apparatus, we should consider not only the electric field distribution but also dielectric breakdown characteristics. Iin this paper, we describe a new optimization technique which enables us to calculate an optimum electrode contour based on volume effect of dielectric breakdown characteriatics. Using a calulation example, the optimum electrode contour could be calculated with dependency of dielectric breakdown characteriatics on stressed volume. consequently, we could confirm that the optimization algorithm on dielectric insulation performance has much higher efficiency for insulation design of HV power apparatus compared with previous optimization algorithm for electric field.

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The gas insulated transformers have been developed and applied to substations widely, because of non-flammability and compactness. In this type of transformer, the coolant gas is forced to flow in the gap between coils to cool them. This cooling performance is influenced by the gas flow pattern in windings. Therefore, it is important to know flow distribution to develop coil structures for cooling. In this paper, the appropriate flow path configurations were determined on the basis of numerical analysis. These analytical results are verified in comparison with the measured data.

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We have been developing optimal design methods for high-field and highly homogeneous superconducting magnets for NMR study using mathematical programing method. It is hoped that the error of produced magnetic field is very small, however, the error field is caused by winding errors, warp of coils, ferromagnetic objects around a magnet and so on. It is needed to reduce the error field in the design stage. The presented design technique is combination of the simulated annealing, which is one of most effective methods for nonlinear optimization problems, and the sensitivity analysis. In the design stage of the high-field and highly 4emoge-neous superconducting magnets, this method reduces the error coefficient of the magnetic field homogeneity around the center region, which is caused by winding errors. The detail of algorithm and the examples of its application to the high-field and highly homogeneous superconducting magnets are shown.

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Recently the request for the compactness of rotating machines such as traction motors has been increasing and thus the rotating machines should be of high power density. This makes the magnetic saturation of the rotor-yoke more excessive and the heat generation per unit volume increasing To prevent the temperature rise, ventilation holes are provided in the rotor-yoke. Ventilation holes have not only the direct radiating effect but also the ventilation effect in the air-gap by reducing air-flow resistance. Moreover, ventilation holes have the role that lightens the weight of rotating machines. If we make the cross-area of ventilation hole larger for cooling however, the effective magnetic area becomes smaller, which causes in the further magnetic saturation. The magnetic saturation gives significant influence on the fundamental component of the air-gap flux density, which increases the magnetizing current and thus temperature. Hence, it can be said that the compactness of motors makes the problem of the magnetic saturation more serious.
With this background, this paper describes the design optimization of ventilation holes in induction motors. By investigating the relationship between the rotor-yoke configuration with ventilation holes and the magnetic flux density in the air-gap, we determine the desirable ventilation holes arrangement. Some numerical results, which demonstrate the validity of the proposed approach, are also presented.

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Isolated phase bus (IPB)is available for large power plant such as thermal power plant and nuclear power plant to connect between the generator and transformers. Each of three phase bus has an enclosure which is connected electricaly together at the both terminals with a short circuit plate, through which induced current by bus current flows in enclosures.
Some efforts has been made to estimate the current distribution in the short circuit plate by two dimentional analysis of Finite Element Method, but more detail information about the current distribution is needed to develop the larger capacity and improved IPB structures.
This paper presents the results of three dimentional FEM analysis of eddy currents and heat conduction applied to IPB short circuit plates and its modified structure.

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The application of SF₆ gas insulated transformer is increasing instead of oil immersed transformer, for simplification of maintenance and nonflamability of substations, however the heat capacity of SF6 gas is smaller than that of the insulation oil.
To improve the cooling performance of SF6 gas insulated transformers, it is necessary to understand the detail of gas flows and the temperature rise particularly near the windings. In order to attempt cooling optimization of disk windings of SF₆ gas insulated transformers, we have carried out numerical calculation and experiments. This paper describes validity of the numerical calculation to compare with the results of flow visualization by PTV (Particle Tracking Velocimetry) and heat-run tests of the real transformer.

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When very fast transient overvoltages (VFTO) enter a transformer, it happens that they excite large interturn voltage in the windings due to resonance phenomenon. We simulated the transient interturn voltage by the method described bellow.
First, the transfer function H(ω), which represents the ratio of intertum voltage to input voltage, of a transformer is obtained by using the multiconductor transmission-line theory. Secondly, VFTO in GIS, v_in, (ω), is analyzed by Electro-Magnetic Transients Program (EMTP), and then V_in (ω) in the frequency domain is simulated using Discrete Fourier Transform. Lastly, applying the Inverse Discrete Fourier Transform to V_in(ω)x H(ω), we obtain the response of intertum voltage to the VFTO in the time domain.
We simulated transient intertum voltage in a 500 kV class model transformer for two kinds of VFTO waveforms. Large intertum voltage was generated when the frequency of oscillation in VFTO coincided with the resonant frequency of transformer windings.

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We developed a compact SR magnet which is operated at 3. 5T(350A) at persistent current mode. This SR magnet is used for the light source of X ray lithography. If the SR magnet is located at the semiconductor factory, the magnet needs to be highly reliable for a long term persistential operation. The coil should be protected from a large terminal voltage due to its high inductance in case of quenching even if it is a rare accident. We evaluate the coil voltage and developed an active coil protection system for long term persistent operation. This system is composed of the quench detector, external resistive heaters and current power supplies for those heaters. Each components are connected to UPS (uninterruptive power supply) that can supply power for 15 hours with batteries. We can lower the maximum quench voltage to the sufficient safety level that is about 1, 000V by using this active coil protection system. We have obtained total 40, 000hours of operational experiences of this superconducting magnet at persistent current mode without any trouble. At present, this magnet is under successive operation for nearly one year. Through these operations, we have confirmed the high reliability of this superconducting magnet.

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A. C. superconducting machines such as superconducting generators, transformers or resistive current limiters need large current ca-pacity conductors. These conductors are generally fabricated as multi-strand cables with multifilamentaly NbTi superconductors whose current capacity is a few tens of Ampere. AC current degradation has been often observed in a. c. use of such multi-strand cables. Several reasons for this degradation have been pointed out; non-uniformity of strand currents, mechanical disturbances, thermomagnetic in-stability and a. c. losses. However, it has not been overcome yet.
Since the angle between the strand axis and the cable axis changes along the cable axis in multi-strand cables, the strands are exposed to a spatially distributed magnetic field, which has longitudinal and transverse components changing periodically due to multiple cabling. This paper discussed mainly the thermomagnetic instability due to the distributed transverse magnetic field, which was compared with the self-field instability and the longitudinal field instability experimentally and theoretically. It was confirmed that the a. c. current degradation due to the distributed transverse field could be induced for four reasons; non-linear E-j characteristic, strong dependence of E-j characteristic on magnetic field in low field region, uniformity of the axial current profile in the superconductor with the high resistive matrix and the poor thermal diffusion of CuNi/NbTi composites.

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Mayr equation like numerical arc model is applied to simulation of transient phenomena on vacuum switch operation. Additionally, a simple numerical method we call “Input Masking” is proposed and introduced into this calculation method. The following features of arc voltage and current are easily described by this method, semi-constancy of arc voltage, burst of spike voltage, small current chopping. Calculated and measured wave forms of arc voltage and current show similar characteristics.

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In recent years, electric power transmission lines and power apparatus tend to be operated under higher voltage and larger capacity. These trends cause changes in the electromagnetic environment around them, and thus the precise measurement and analysis are required. Magnetic field distribution around power transmission line is very complicated because of the three-phase current condition, the three-dimensional configuration of conductors, the existence of ferromagnetic structure, and so on. Moreover, little attempt has been ever tried to measure the relationship between power-line current condition and the magnetic field distribution; thus, the effects of current condition, for example harmonic component have not been fully understood.
From these backgrounds, we firstly measured the magnetic field distribution using reduced-scale transmission line model under the current condition including harmonics. Next, at the site of actual power transmission line, we simultaneously measured the magnetic field distribution and the current of power lines. As a result, the effect of harmonic component on the magnetic field distribution was quantitatively clarified.

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Converter-excitation of cage induction generator has emerged as a suitable candidate for low cost variable speed Wind Energy Conversion System, because the converter can also transfer output energy from the generator to electrical load. If a suitable control method is applied to the variable voltage and variable frequency excitation of the generator, highly efficient energy conversion will be achieved. The present paper deals with static characteristics of the induction generator, where its output power is controlled using Voltage/Frequency ratio control scheme. The output power is also controlled in accordance with maximum power characteristics of fixed blade pitch wind turbines. High efficiency is achieved by the control over wide shaft speed variation. The paper also develops an analytical method to know these characteristics.

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Over the last five to ten years, significant progress has been made in high-power semiconductor devices and in their practical applications to power systems. This comes not only from sophisticated semiconductor technology but also from the demand for a higher degree of frequency and voltage stability, and for greater reliability in power systems. This paper deals with an adjustable speed rotary condenser capable of not only reactive power control but also active power control based on a flywheel effect of the rotor. The behavior of a power system consisting of the adjustable speed rotary condenser, a synchronous generator and a transmission line is subjected to a set of nonlinear differential equations. The set of nonlinear equations can be linearized by limiting attention to small perturbation around a reference state, thus leading to the so-called Heffron-Phillips model of the power system. The Heffron-Phillips model derived is effective in analyzing effects of the adjustable speed rotary condenser on power system stabilization. The validity of the analysis is confirmed by computer simulation based on EMTDC. Finally, it is discussed how well power system stabilization is achieved by the rotary condenser. As a result, the rotary condenser has the function of decoupling reactive power control from active power control, thus producing a good effect on power system stabilization which would not be achieved by a conventional inverter-based static var compensator.

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A new approach to predict a swing of generators against the remaining generators in a power system is proposed. Dynamic behavior of the generators is supposed to be represented by a set of state equations for the equivalent power system model composed of two generators. The values of parameters of equations of equivalent generators are estimated so that the simulated values using the equivalent power system model coincide with the data measured at the actual network for the past short period. Then the swing during the short period in the future is predicted by solving the equations defined on the equivalent.

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This paper describes a control method and system configuration of UPFC(Unified Power Flow Controller). UPFC is one of the promising power electronics equipments because it is an extremely flexible power controller capable of controlling all of the transmission line valiables(voltage, impedance and voltage phase angle). Due to the control flexibility, however, it is important to select a proper contol scheme corresponding to the purpose.
Under the reserch project for the reinforcement technology of electric power interconnection, a miniature model of UPFC for the real time analog simulator was developed. Series side of UPFC has three control modes (perpendicular voltage control, impedance control and voltage angle control). It is used for power flow control and/or power system stabilizing control. Shunt side is used for voltage control or reactive power control in addition to compensating the power required from series side. Various tests were performed to compare control modes and the perpendicular voltage control is proposed as the simplest and most practical method because the capacity of the shunt compensator is minimized and power flow control sensitivity is maintained over a wide range of angle.

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In 1989, trace of PCB (less than 50 mg PCB/kg insulation oil) was found in reclaimed oil used in some pole transformers. It was keenly required to remove the detrimental PCB in a reliable method.
Elimination demonstration tests of PCB and the like were carried out by a solvent cleaning technique using some local transformers operating in site. They were broken down into classified metals by way of chilled crushing, and were subjected to preliminary cleaning for five minutes and then secondary cleaning for ten minutes, thus obtaining harmless iron, coppers and porcelains which could reduce their residual PCB lower than the lowest determination limit (0. 05 mg of PCB/kg of parts). Similarly, papers and woods were subjected to preliminary cleaning for sixty minutes and then secondary cleaning for six hundreds minutes, thus reducing their residual PCB at lower then the lowest determination limit, depending on the types of transformer. Further these tests demonstrated that the solvent cleaning system adapted for this test could be reliably operated, thus securing the ambient and work-environmental safety.

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