Reignition of vacuum interrupters causes hf currents of several hundred of kHz. Careful observation has led to the conclusion that failure to interrupt such hf current is caused by a combination of post-arc current and TRV. Post-arc current of several A during several 100 ns was measured and analysed, leading to an adequate description of the decaying plasma. It is concluded that for larger values of gap and di/dt post-arc current is responsible for interruption failure, while small gaps usually fail to interrupt due to TRV.

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The complexity of power systems and wide range of operating conditions are beyond limitations of model-ling techniques used in the conventional controller designs. Thus in order to provide reliable system operation, it will be better that controllers be designed considering various sources of uncertainties in power systems, e. g., uncertainties in modelling, measurements and fault occurrences. Furthermore for many dynamical systems whose state equations include time varying uncertain parameters it is desireable to design a stabilizing state feedback controller which takes into account uncertainties [1]-[6]. This paper introduces a Riccati equation approach to synthesize a full state robust observer and a state feedback robust controller for generator control. The structure of the controller is fixed and in the design procedure only bounds of the system parameters are needed. The variation bounds of the power system parameter uncertainties are obtained by changing the parameters from their typical values. In this paper one circuit line opening is modeled as the main source of uncertainty. The effectiveness of the proposed control is demonstrated on a one machine to infinite bus system. Nonlinear simulations are carried out in the presence of faults followed by one circuit line opening with limiter consideration and with time varying uncertainties entering the output matrix. The result is compared with LQG control.

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This paper first gives a brief survey over four existing evaluation techniques, i. e., recursive convolution technique, fast Fourier tra. nsfornl, Fourier expansions approximation and Gram-Charlier expansions for reliability indices, and then makes a fair comparison by using both test systems and the practical power systems for the evaluation of the loss of load probability (LOLP), expected unserved energy (EUE) from the viewpoint of both accuracy and computation time.

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To avoid the miss-operation by magnetizing inrush current, the second harmonic component is commonly used for blocking the operation of percentage differential relays. However, we have plans to introduce 1, 000kV transmission lines and underground 500kV lines and forecast that the second harmonic component in fault current is increased together with the capacitance in power system. This paper describes a new method to discriminate internal faults from inrush current by the sum of active power flowing into transformers from each terminal. Inrush current flows by the saturation of iron core and instantaneous power flows in and out according to the big change of energy in magnetic field. But the average power is almost zero for energizing. On the other hand, an internal fault with arc discharge consumes large power. Therefore, by watching the average power or the integral of instantaneous power during one period, internal faults can be discriminated from inrush current. To check the performance of this method, actual inrush current and voltage waveforms of 500/154kV transformer are accurately measured by digital equipment. The usefulness of this method is confirmed by applying it for these data.

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500kV XLPE insulated cable with an insulation thickness of 27mm has been developed for long distance transmission lines. Basic studies on 500kV XLPE cable have been shown that contaminants in the insulation may be the factor determing electrical performance. This hypothesis is justified by the good correlation obtained between statistical estimations of the size of largest contaminant in the insulation and electrical characteristics of full-sized cables. Voltage-withstand and long-term tests have confirmed the design values for minimum breakdown stress, for AC and impulse voltage, and for the degradation coefficients.

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In recent yesrs, the demand of electric power in urban area has been increasing more and more in Japan. In order to realize long-distance underground power transmission lines, we need cable joints with enough electrical performance. We carried out basic research from 1989 to 1992 and determined a joint type and the design values.
This paper reports on the electric performance of fabricated 500kV joints based on the design. We carried out various breakdown tests for the purpose of evaluating the performance. And it was found that design values, degradation coefficient, temperature coefficient, and EL were appropriate. Also it was found that the approaches to design EL, factors governing the performance, thickness dependence of breakdown stress, and the improved ratio of the perfoprmance at treatment portion of cable screen, were appropriate.
Through evaluation we found that it should be possible to apply the Extrusion Molded Joints to 500kV XLPE cable lines.

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In order to develop SF₆ gas-insulated switches with long service life and high reliability for medium voltage ratings, it is necessary to decrease arc erosion of contacts in SF₆ gas. In this paper the effects of SK gas pressure and magnetic field for are blow-out on the erosion were studied experimentally in detail using sintered Cu-W contacts. The test current, SF₆ pressure and magnetic field were 100 to 400 A, 0.1 to 0.6 MPa and 0 to 50mT, respectively. The results show that the arc erosion rate increased linearly with the SF₆ pressure and the magnetic field was effective for the reduction of anode erosion (i.e. the application of a mere 50mT lowered the erosion rate to 1/3 to 1/4). These phenomena may be attributed respectively to the increase of the evaporation rate according to the shrinkage of arcing spot areas by the SF₆ pressure and the decrease of the evaporation rate according to high speed movement of the arcing spot on the anode by the magnetic field. But the cathode erosion was insensitive to the magnetic field. The influence of W grain size and composition ratio of Cu to W on the erosion rate was also studied and the smaller W grain at Cu content of 50 to 70 wt % was found to he best for the suppression of erosion under the present test conditions. Furthermore the increase of N₂ mixture ratio to SF₆ under constant total gas pressure greatly decreased the contact erosion.

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A UHF interferometer system for detecting progression of lightning discharges has been accomplished and applied in the observation of winter thuderstroms occurred in Hokuriku area of Japan. This system, operating at center frequency of 327MHz with a bandwidth of 6MHz and the time resolution of 1μs, consists of five antennas which produce long (5λ/2) and short (1λ/2) baselines in orthogonal directions in a horizontal plane. We confirmed its performance by both experimental tests in a Laboratory and some theoretical simulations. We observed many lightning discharges occurred in winter thunderstorms. The preliminary analysis shows that the system can give us lightning discharges occurred in winter thunderstorms. The preliminary analysis shows that the system can give us the image of the progression of lightning discharges.

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Toward understanding seasonal characteristics of the lightning channel and its relationship to transmission line faults, lightning channels were observed through the year with many still cameras equipped with liquid-crystal shutters. The observation was carried out at Tonami Area in Hokuriku. A liquid-crystal shutter camera was developed for this purpose, which fully opened its shutter 3ms after a light detector sensed a light pulse. The delay time of 3ms is much shorter than that of about 30ms with a mechanical shutter. The period of the observation was from November 1992 until October 1994. 165 incidents were successfully photographed which included photographs showing several lightning channels in a frame. Based on these data, the relationship between the frequency of transmission line faults and that of lightning flashes striking the ground and transmission towers is known. Furthermore, seasonal dependence of the aspects of lightning channels such as progressing direction and the existence of the horizontal part of a channel are discussed.

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A probe method is described for measuring conductance of hot gas ejected from a molded case circuit breaker during arc interruption process. Two steel rods of lmm diameter were adopted as probing electrodes, then being located near to an exhaust hole of the hot gas. Ratio of a current passing through gap between the probing electrodes to a voltage across them gives an apparent conductance of the hot gas. Net conductance was estimated from the apparent value with consideration of cool gas layer near to the probing electrode surface. The net conductance gnet proved to increase markedly from 0.6 to 2.3mS by a factor of 4 with a rise of a peak value Ipeak of an arc current from 6.5 to 8.8kA. The electrical conductivity a was then estimated from gnet by taking into consideration of the effective cross-section of the measurement area of the probe method. The electrical conductivity σ was found to grow from 0.071 to 0.27 S/m with increasing Ipeak. Above results suggest that the probe method is much more useful as means of assessing the ejected hot gas.

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In most cases a tilt angle of PV array on a ground is adjusted to the angle that can gain a maximum irradiation. When PV module was extreme expensive compare with other components of PV system, it was optimum angle. But it is not always an optimum tilt angle from a economical point of view now and in the future. Because the price of PV module has reduced, and a wind load and snow load on PV array change according to a change of the tilt angle besides irradiation, and it affects a construction cost of a support structure and foundation.
From the economical point of view, I studied the optimum angle of stndard PV array on the ground at ten sites except heavy snow area in Japan. The construction cost was analyzed, and it was made clear that the optimum economical angle, the angle that made a cost for generation of electric power cheapest, was smaller than that can gain the maximum irradiation by about 20 degrees at most sites.

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Design and detailed numerical analyses of high performance nonequilibrium disk MHD generator for large scale experimental facility are performed considering supersonic nozzle and boundary layer. The present study treats a disk generator using cesium seeded helium plasma with 100MW thermal input, of which channel section has already been conceptually designed by the authors. At first, two-dimensional time-dependent calculations are carried out for the supersonic nozzle section. Then trial and error processes determine specifications of the nozzle which can not only accelerate the working plasma to the desired condition but also realize the fully ionized seed plasma. This indicates the feasibility of design of the supersonic nozzle with excellent performance. Next, two-dimensional time-dependent calculations of the whole generator considering the supersonic nozzle and the boundary layer are carried out to modify the generator height distribution using the displacement thickness of the boundary layer. The performance characteristics of the modified generator are then examined in detail. As a result, it is confirmed that the generator has the same excellent performance as expected in the conceptual design.

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The present paper proposes a method of linear stability analysis for subsonic disk MHD generators considering boundary conditions and load conditions. The flow in the MHD channel is described by the quasi-one-dimensional equations, whose variational equations describe the behavior of perturbations of the flow. The growth rate of perturbations is determined so that the boundary conditions and the load condition will be satisfied. Whether the perturbations grow or decay is judged diagrammatically in a way similar to the Nyquist's method. The stability is analyzed of a coal-fired inflow subsonic disk MHD generator of commercial scale. The linear stability analysis and time-dependent calculations show that both the inlet condition and the load condition much affect the stability of the flow. While the flow tends to be unstable when the inlet swirl ratio is fixed, it is stable under the constant current load condition or under the ohmic load condition when the inlet azimuthal velocity is held constant.

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