Power system is a dynamic system which includes several kinds of nonlinear elements. According to nonlinear analyses using the Hopf bifurcation theory, it is detected that a limit cycle exists around an operating point, which may affect the stability of power system significantly. This paper presents a numerical method to analyze the nonlinear characteristics in power systems by observing the power swing after some perturbation where the coefficients of nonlinear terms are determined by using the least mean square method. The effectiveness of the proposed method is confirmed through some numerical analyses. The transient stability is evaluated by using the method. The result shows that the region of stability is occasionally narrower than that is evaluated by the classical method.

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This paper proposes a new method for discriminating partial discharges (PD) from external noises by using the first peak height and cumulative wave parameter of internal electromagnetic waves in GIS. The first peak height and cumulative wave parameter in PD signals are new parameters that are related to the PD charge and the PD-sensor point range. The first peak height-cumulative wave parameter distribution consisting of many PD signals has two characteristics, (1) the distribution shape is on a proportional line and (2) slope of the line is related to the PD-sensor point range. These characteristics were confirmed from measured data of a model GIS. This paper proposes a discrimination method using these characteristics. Simulation results show that this method is effective for discriminating PD from noise signals that originate from various points.

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A high speed phase shifter (HSPS) is an effective device to stabilize power systems by regulating phase angle of line voltages. The HSPS has two types of primitive hardware construction, inverter-type and tap-changer type. The former, which is called a unified power flow controller (UPFC), has various functions regarding power system stability and it has been the subject of many studies. The latter is called a thyristor controlled phase shifter (TCPS). The TCPS controls the phase angle of line voltages by step regulation and has limited functions compared with the UPFC which controls it continuously. But the TCPS is a competitive device because of its simpler hardware and higher cost performance. At present there are only a few TCPS studies which deal with its functions and control method, taking into consideration its hardware.
This paper deals with the essential control pattern of the TCPS for power system stabilization and phase regulating performance, considering hardware; i.e. the effectiveness of the TCPS using step-regulation in power system stabilization by simulation analysis. Furthermore, the possibility of phase control performance using step regulation is evaluated by experiments. Results shows that the TCPS using 9-step regulation is an effective device for power system stabilization.

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This paper presents a new method of designing nonlinear observers for squirrel-cage induction generators in the windmill power systems. The control law which globally stabilizes the currents, angular velocity and pitch angle under full state measurement is determined by Lyapunov's direct method. Exponentially stable observers for the induction generator subsystem in the windmill power system, which is modeled as a bilinear system, is proposed. The design of the proposed observer is based on Lyapunov's direct method, and its estimation error decays to zero exponentially irrespective of the input. It is also shown that the observer-based extended system is globally stable.

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In order to take effective countermeasures for harmonics suppression and harmonic disturbances in distribution systems, precise simulation on harmonic power flow is required. Precise equivalent circuit on harmonics is required for the simulation. However, it is impossible to obtain such equivalent circuit by dividing harmonic voltage by harmonic current simply. Therefore, we have investigated the load characleristics, which can be separated as load admittance and harmonic current source, by opening and closing static capacitors in distribution systems.
As a resull, we have made a proposal for forecast model of harmonic impedance on load side varying with load characteristics.

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We have developed a novel diagnosis of oil filled middle-sized power transformers by optical analysis. The color of insulating oil gradually turns to yellowish in the long term use. The coloring is caused by the increase of electronic transition absorption due to thermal oxidation of insulating oil.
We derived a relationship between the aging degree of deteriorated oil and the optical absorption loss. The aging degree of deteriorated insulating oil can be estimated using the diagnostic master curve, which is obtained with an accelerated heating experiment. Furthermore, the optical absorption loss was correlated with the average value of polymerization degree of the insulating paper. Therefore, this optical technique is theoretically applicable to diagnosis for power transformers.

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We have proposed a new type fault current limiter, which consists of a Aux-lock reactor with a high Tc superconducting (HTS) element and ac magnetic field coil (Flux-Lock Type Fault Current Limiter: FLT-FCL). The FLT-FCL can increase both the current capacity and limiting impedance by a transformer action and ac magnetic field applying mechanism. This paper reports conceptual design of FLT-FCL to apply to 6.6 kV/200 A distribution system. The theoretical expressions were given for the current limiting behavior and a new concept of “quench power” was proposed to estimate required number of HTS elements for two types of FLT-FCL and for basic type FCL consisting of only HTS elements. From the calculation results, a design guideline of FLT-FCL was presented.

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A testing method which makes it possible to measure 10 kA_rms class AC quench current (Iq) and AC losses in AC magnetic field (50-60 Hz), has been investigated. A testing apparatus was developed and the specification was determined in consideration of the testing method for 10 kA_rms class Nb-Ti stranded cables, expected conductor size, permissible bending strain and so on. The AC superconducting magnet generating the maximum magnetic field of 1 T_peak in the bore of 150mm and a 10 kA_rms class power supply were manufactured. For the Iq measurement, the AC current is directly applied to the specimen in the field of 0.5 T_peak (50-60 Hz). As regards the AC losses measurement, the magnetization method using pick-up coils was adopted and its sensitivity has been improved by reducing unnecessary output signal. By some trial tests, it was confirmed that the apparatus using the testing method can be adopted to estimations of basic AC characteristics. Moreover, improvements of conductor structure based on measurement results brought about low AC losses 10 kA_rms class Nb-Ti cables and 5 kA_rms class Nb₃Sn cables.

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It becomes more important to evaluate the visual impact of new overhead transmission towers, when designing a route of the transmission lines. This paper proposes a system for designing a route of overhead transmission lines taking into account the visual impact of the towers. The system calculates candidate areas suitable for constructing new towers using visibility simulation techniques. The towers constructed in the candidate areas will be invisible from the specific viewpoint, or have not serious visual impact upon the environment. The system also computes how long the towers exceed the skyline. After determining the position and the height of towers, the user can check the visual impact of the towers using computer generated images. The system can also generate both route maps and section views, and greatly reduce the time to design a route of transmission lines.

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Thermomagnetic instability in superconducting wires composing multi-strand cables is a problem in development of cables with large current capacity. This paper clarifies quench property of AC superconducting wires in distributed magnetic field which is applied to strands in the cable, and stabilization of the AC superconducting wires considering the effect of the longitudinal magnetic field or fraction of copper embedded in the strand.
First, the degradation of quench current of CuNi/NbTi superconducting wires in distributed magnetic field is exhibited with simple test samples. Second, the quench property of the strand in a (6+1)³ cable and the optimal twist pitch of the cable for high stabilization are discussed. Last, the effect of copper on the quench property of the strand and the appropriate fraction of the copper for the suppression of the quench current degradation in the distributed magnetic field are discussed.

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The ZnO surge arrester with a series gap has been generally applied to the 6 kV power distribution systems in Japan, and recently also, line-insulator including ZnO elements has been applied with the improvement of these elements. However, the performance characteristics of them have still not been sufficiently clarified for a long period of usage and so the deterioration diagnostics have not been established in the hot-line. Therefore, it is desired to present the changes in the characteristics for usage over a long period, and then establish the diagnostic method of their deterioration.
We have investigated the leakage current, lmA operation voltage and residual voltage.
In this paper, for investigating the deterioration of the arrester, the changes in the leakage current, the divided voltage of ZnO elements were measured with elapsed time after each set of 50 operations of the arrester operated repeatedly by a large impulse current.
As a result, it was found that the leakage current and divided voltage of ZnO elements increased with the number of operation times. Consequently, it was found that the moisture of ZnO elements affects the characteristics of the leakage currents

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The paper presents measured results of corona wave-deformation on an 1/4. 4 scaled-down test line of a 77 kV system with various boundary conditions corresponding to a substation. The measured results have shown an increase of the phasewire voltage at the receiving-end under a certain boundary condition, while the ground-wire voltage is always decreased by the corona wave-deformation. The phenomenon is theoretically investigated, and it has been made clear that the increase is, in principle, caused by heavier attenuation of a traveling-wave on a ground wire than that on a phase wire, and negative reflection of the traveling-wave from the ground wire to the phase wire. The condition of the voltage increase has been theoretically derived.

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A Superconducting generator (SCG) is expected to be used as the next generation machines because it has many special merits compared to a conventional machine, e. g. improvement of power system stability, high efficiency, and extension of operable range in leading phase. Super-GM has been promoting R & D of 70, 000 kW class SCG (Model Machine) to establish key technologies required for the design and fabrication of a 200, 000 kW class SCG (Pilot Machine). We commenced the verification test in March 1997, and successfully finished consecutive tests for slow response type machines in September 1998, to verify the outstanding special features. Following are main results;
(1) The synchronous reactance is confirmed to be 0.406 pu, which gives us the estimate that X_d can be made 1/3-1/5 that of conventional machines.
(2) The efficiency of Model Machine is 98.15%, Which leads to the estimation of that of Pilot Machine (99.08%). This figure enable us to obtain approximately 0.6% improvement of the efficiency in generator unit compared to the conventional (the refrigeration cost is not included).
(3) It is verified that leading phase operation of nominal rating is possible, which implies that the leading phase operation capacity of SCG can be doubled.

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We have developed semiconductor lightning-surge-protection devices for communications equipment. To improve the surge-handling capability of the device, distributed emitter structure and thin substrate were introduced based on transient thermal design and low energy dissipation design. High break-over voltage and high surge-handling capability device techniques were developed for both planer and mesa structures. These devices provide highly reliable communications equipment against over-voltages and over-currents caused by both nearby and far-end lightning return strokes.

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Fault analyses are carried out for the interconnecting system of commercial-scale subsonic diagonal type MHD generator and AC power system through line-commutated inverter. The behavior of the interconnecting system is first examined when single misfiring of one thyristor occurs in the inverter. In this case, the load current increases because the inverter system is short-circuited. Following the recovery of inverter commutation, the load current decreases to the rated value and the MHD generator is restored to the nominal condition. Next, the cases of single-line ground fault and of three-phase short circuit fault are investigated. The line voltage decreases and thus the load currents increase after the fault. This increase of load currents destroys the design-point flow of the MHD generator. The phase-control angle control of inverters is required to recover the nominal operation of the MHD generator.

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In near future, it expects that the PV systems will be interconnected to the utility distribution lines with high density beyond the range dealt with by the current Japanese guideline for grid interconnection. As for islanding prevention under the high density interconnection, not only more reliable detection of islanding but also prevention of unnecessary detection in normality are strongly required to keep safety and stability of the grid. A new isanding prevention method of ‘Active/Passive Series Method’ was designed in the study. The sequence of the method is; (a) Inverter current is made decrease a little when 1^st passive detective function operates., (b) Inverter operation is stopped when 2^nd passive detective function detects remarkable change of grid voltage, cased by a positive feed back effect of the current control inverter in islanding. As the results of a simulation using EMTP and an experiment, it is clear that islanding can be detected by enhancing the sensitivity of the 1^st passive detection and setting the current drop rate for 15% at most. The result also indicates that inverter current drop is restricted to less than 15% and the current can be recovered within 1 sec. by combination with the MPT control of inverter even when unnecessary operation of the 1^st passive detective function occurs in normal condition. From above, applicability of the designed method to the high density interconnection is confirmed.

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A two-dimensional analysis of two-phase flow using a particle model is carried out for a pulsed MHD channel of which working body consists of the combustion gas of a solid propellant and the liquid particles of Al₂O₃. The analysis reveals that a boundary-layer separation becomes small when the diameter of liquid particles is large because the growth of the boundary-layer separation is prevented by the interaction between the gas phase and the liquid particles in the separated boundary-layer. The fluctuations of the velocity and the temperature behind the shock wave is caused by the separated boundary-layer extending to the duct center. Liquid particles do not collide with the downstream region of channel walls behind the shock wave, because of the deceleration of liquid particles by the shock wave and of the strong interaction by the velocity difference between the two phases. The boundary-layer separation decreases the power output partly because the separated boundary-layer stores much energy and accordingly the heat loss on the walls becomes large.

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As research and development of central solenoids for fusion reactors, a 13 T, 640 MJ superconducting pulsed coil has been constructed. The coil has a 1.6m inner diameter, a 3.6m outer diameter and a 1.7m height, which is composed of a inner module and an outer one. The coil has been fabricated with layer winding method in order to realize compact machine. A forced cooled Nb₃Sn cable-in-conduit conductor with a rectangular incoloy conduit with a circular hole has been used for the coil windings. The coil is designed to be charged up to 46 kA corresponding to 13 T with a ramp rate of 0.4 T/s and discharged to 0 kA with a time constant of 10.8 s, which corresponds to the maximum ramp rate of 1.2 T/s. The ac loss in the coil is expected to be about 10 kW during operation with a ramp rate of 0.4 T/s.

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It has been developed the method to improve a fleet availability for PAFC power plant as on-site repairing operation. The detour operation from a defect cell in the stack has been investigated. The method of this is used a carbon block with some adhesive on the contact interface to form a electrical by-pass, thus the defect cell could be detoured from stack current path and low resistant path by the carbon block would be secured. Some improvement on the contact or fixation have been examined and the methods have been confirmed their life performances with bench scale cell tests. The results of these are discussed

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A number of study on the transport and thermodynamic properties have been reported on the SF₆ gas arcs for high voltage, high current circuit breakers with including the impurity contamination of Cu vapor, Al vapor or PTFE (-C₂F₄-) vapor. Until now, there are few study for the general plastic vapor contamination. This paper describes the transport, thermodynamic and wall-stabilized arc properties of air contaminated with plastic vapors found in arc-chamber of low-voltage but considerably high-current circuit breakers. Total 31 chemical species were took into account in the calculation of plastic vapor contaminated air, which were produced by chemical interactions between mixed particles such as OH, CO, NH, CN, etc.. The plastic material considered here is polymethylpentene (-C₃H₅-), polybutylene (-C₄H₈-), polymethylmethacrylate (-C₅H₈O₂-), polybutyleneterephthalate (-C₁₂H₁₄O₄-) and polycarbonate (-C₁₆H₁₈O₃-). Calculation results showed that the electrical conductivity σ below 7, 000 K is decreased by the plastic vapor contamination compared to the pure air, especially, in the case of plastic material having no oxygen atom. The thermal conductivity κ is, on the contrary, increased by the plastic vapor contamination, especially at around 3, 600K, where κ takes three-times larger value compared to the pure air due to H₂ and OH dissociation reactions. Based on the transport and the thermodynamic properties thus obtained, transient characteristics of the wall-stabilized air arc were calculated for the current decreasing ratio di/dt of the alternating current 1, 000 A (rms.) around the current zero. The time-dependent decaying properties of the temperature distribution and the arc conductance were disenssnd for plastic contaminated conditions.

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Hydro-power Generator have many Resistive Thermal Detectors (RTD) which is used to detect over heating of insulation materials for generator stator windings.
By using RTD as the coupling capacitors, In-service Partial Discharge Detection for the insulation diagnosis was studied. For the quantitative treatment of the detected partial discharge, calibration method was developed to utilize calibration data obtained at the periodical insulation diagnosis under off-service condition.

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The windage loss gives some contribution to the total losses of high-speed rotary machine such as motor/generator with flywheel or turbines. In order to overcome this problem, the rotor can be operated in a low pressure/vacuum chamber. However, this method finds difficulties in removing heat from the rotor, to seal and to design suspension system. In addition, its breakdown voltage is very low. This paper presents experimental results of windage loss reduction using helium and air mixture that also has the property of cooling gases. The results show significant windage loss reduction for Helium (50%) and air (50%) mixture with a breakdown voltage of about 88% that of the air. The breakdown voltage and thermal property of the gases have also been measured.

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