The high voltage ceramic capacitor of unit type has been developed for gas insulated switchgears. The capacitor has good advantages such as complete dry type and the easy adjustment of the series connection for the different rated voltage. There were main two problems to be solved.
One was the improvement of the electric field concentration around the edge of the electrodes which were composed with thin silver layer. The reduction of gap around the flat surface of a capacitor leads to the significant increase in the breakdown voltage of the full wave lightning impulse voltage.
The other was the improvement of the reverse piezo electric effect which greatly affected mechanically and electrically combined breakdown for the chopped wave. The application of the metal reinforced terminals on the both flat surface improves the breakdown characteristics for the steep wave.

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The transformer insulated with SF₆ gas has recently applied to the hazard free substation. The gas insulated transformer has different insulating material from the oil insulated transformer. But long time reliability of insulation to gas insulation is insufficient in comparsion with oil insulation.
This paper describes the experimental result of V-t characteristics of ac voltage in a long period of time and V-N characteristics of impulse voltage in SFe gas.
V-t characteristics has the threshold value that is nearly equal to partial discharge disappearance voltage and V-N characteristics is related to weak point of insulating paper.

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The deterioration products formed due to partial discharge in a void filled with sulfur hexafluoride (SF₆) were analyzed. The SF₆ content in the void decreased with ageing; after about 15h of ageing, the SF₆ content was about 15%. Then, after 24h of ageing, spots of powder formed on the void surface. When the powder was exposed to air, it changed into a colorless liquid. It was determined that this liquid was sulfuric acid (H₂SO₄) and thus, the powder was sulfur trioxide (SO₃). According to ESCA analysis of the void surface after 100h of ageing at 21kV_peak, S_2s and S_2p spectra were observed. However, they were not detected in the virgin sample. This result shows S atom exists on the void surface, but the process is still not clear.

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Breakdown Voltages in uniform and quasi-uniform field gaps are sensitive to the presence of small protrusion on the electrode surface in SF₆ at high pressures. The aim of the present work is to study direct breakdown and corona stabilized breakdown for the transitive region from uniform to non-uniform gap in Dry Air and SF₆ at low pressures up to a critical pressure when direct breakdown takes place by a leader discharge acrossing the gap in SF₆. In a parallel-plane gap with a variable-height protrusion subjected to the DC voltage, corona onset voltage is remarkably controlled by the protrusion height. The present electrode arrangement has the advantage of directly measuring the minimum critical guiding field strength for the propagation of a streamer discharge at corona onset.
The experimental observations have been explained qualitaively on the basis of a streamer model and precise electric field calculations of gap.

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This paper deals with the ion wind of SF₆ gas which is generated by partial discharges at a tip of a needle electrode at dc fields. The velocity measurements of the ion wind are performed by a hot wire annemometer, using a needle(φ0.2, length 10) -plate(φ120) set in compressed SF₆ gas (3_??_5 atm). The velocity of the wind increases with partial discharge currents and reaches 1_??_2m/s. The wind velocity is combined with the partial discharge current using the theoretical equation. The caluculated values agree fairly well with the experimental results. The test results conducted in pressurised wind tunnel show that the wind velocity which makes aluminium particles move ranges from 0.5 to 1.3m/s, depending on the shape of the particle. We compare the force given to the particle by the ion wind ofSF₆ gas with the force caused by the electric field and the weight of the particle, selecting an aluminium sphere of diameter 2mm. The calculations show that the force from the ion wind is of the order of 1×10^-4N and is comparable with the force of the electric field and the weight of the particle.

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A high-current pulsed discharge in SF₆ has been used as a closing switch for a pulsed power generator. The characteristics of a high-current pulsed discharge are not clear, since it is difficult to measure the time-dependent arc voltage accurately.
In the present paper, the voltage-current characteristics and the time-dependent arc resistance of the high-current pulsed discharge in SF₆ are obtained accurately from the arc current and the breakdown voltage by solving the circuit equation. The electrode separation and the pressure are changed from 0.1 to 1cm and from 200 to 760Torr, respectively. The current rise time is about 380ns. The minimum arc resistance occurs at the maximum arc current, and is from 0.04 to 0.1 Ω under the experimental conditions. The voltage-current characteristics in SF₆, which are characterized by five phases, are similar to the characteristics not in Ar but in Air.

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The influence due to metallic-particle attached on the supporting insulator is remarkable in the decreasing of dielectric strength in SF₆ insulated apparatus. In relation with this problem, the authors have conducted a series of studies on improvement method for particle initiated breakdown performance.
In this work, DC breakdown voltage characteristics and breakdown mechanism are investigated under metallic-particle contaminated conditions in SF₆ gas by varying the particle condition and surface shape of acrylic-plate with a rod-plane electrode. The main results show that a rib increases breakdown voltage by a corona stabilization effect and the elongation of discharge path but a groove decreases drastically its effect at lower gas pressures. The breakdown voltage at higher pressures than 4 atm may be estimated on the basis of discharge maintaining field and discharge length which depend on gas pressure and surface shape.

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The charge density produced by streamers on an insulator surface in SF₆ has been investigated by using a probe method with a high speed temporal resolution. Concentric circular probes, which also act the role of a plane electrode, are used in this probe method. Probe signals are observed oscilloscopically and converted into the charge densities through a numerical calculation. This method reveals the charge distribution before a disturbance caused by the “back discharge”. The charge density so obtained ranges from several nC/cm² up to about 60 nC/cm². The density depends on the pressure, voltage height and the position of the streamer.
The electric field on the insulator is numerically analyzed taking the surface charge into account. The internal electric field of the streamer is found to be 40_??_50kV/cm•atm when the streamer ceases its propagation, however it partly exceeds the critical one (89kV/cm•atm) during the propagation.

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The radiation intensity in a cross section of a gas blasted arc column is distributed asymmetrically due to violent gas flow. An observed radiation intensity is an integrated one along the optical path. Therefore, to obtain the distribution in the cross section, it is necessary to solve the integral equation. This paper describes an approximate numerical method for deriving the asymmetrical distribution of radiation intensity from the distributions observed from two directions perpendicular to each other.
In this method, the radiation distribution in a cross section is assumed to be represented by a product of the two terms: one being an elliptic distribution, the other being two polynomials that represent the degree of asymmetry concerned in x-direction and y-direction. The unknown coefficients in those polynomials are numerically determined by an iteration process.
Three asymmetrical distributions are actually derived by this method. However, it is essentially difficult to derive the asymmetrical distribution from only two observed data. Thus, this method gives an approximate distribution. An average error is estimated to be less than 10% if the eccentricity of ellipse is less than 0.5. As an eccentricity of arc cross section approximated by an ellipse is always near to zero, this method is applicable to the gas blasted arc.

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We have been developing GILs for use as large capacity transmission lines since 1969. In 1979, GIL was applied to a 154kV in-yard link line of The Tokyo Electric Power Co., Inc. for the first time in Japan. In 1980, we began the development of a compact GIL to improve cost efficiency and applicability, it was applied to a 500kV incoming line of The Tokyo Electric Power Co., Inc. in 1985. This project planned as 275kV getaway lines connected to overhead line (810mm²×4) in Shin Noda Substation of The Tokyo Electric Co., Inc. The 8, 000A class 275kV GILs are the largest current underground transmission lines in the world. The paper describes the design, installation and tests of the project.

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Experimental studies have been carried out on triggerring characteristics of SF₆ discharge gap switch by use of XeCl excimer laser (wavelength=308nm).
First, laser irradiation characteristics are studied on a pure SF₆ in the pressure range of p=160_??_3, 800 Torr. Using a lens of f (focusing distance)=133mm, the laser is irradiated into the gas, where the energy absorption is studied. If the laser is injected into the gas with the energy above a certain threshold for the breakdown, the rate of energy absorption is found to be_??_17% of the incident energy at p>760 Torr.
Injecting the laser into the SF₆-filled gap switch (gap length=7mm, p=760 Torr.), we have studied the triggering characteristics. Excellent triggering characteristics were obtained; delay time for the discharge_??_20 ns, and the jitter_??_260 ps when the gap voltage is operated at 99% of the self breakdown voltage. In addition, the triggering characteristics are studied by changing the focusing point axially. It is found that both the delay time and the jitter decrease when the focusing point tends to approach the high voltage electrodes.

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Feasibility of a disk type non-equilibrium MHD generator with subsonic flow is studied numeri cally. The subsonic generator is found to have a high adiabatic efficiency under the high magnetic flux density (_??_10T) as well as a generator with supersonic flow. The subsonic generator is not so large compared with the supersonic generator for a given enthalpy extraction ratio. The calculation also reveals that the Hall electric field becomes very high for the high magnetic flux density to produce a fully ionized seed plasma. The breakdown phenomena on the insulator wall should be tested experimentally.

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To reinforce the reliability for stable power supply, several maintenance and monitoring systems for the underground power transmission lines have been already applied in main ultra high voltage lines. As one of the monitoring systems, the measuring of exact conductor temperature in loading cables is necessary to smoothly operate the system. Though it is very difficult to directly measure the conductor temperature in the loading cable, conductor temperature is estimated by measuring heat flows radiated from cable surface, cable surface temperatures and load current.
In Japan, 275kV aluminium sheathed oil filled (OFAZV) cables are mainly used as the main ultra high voltage underground power transmission lines, while 275kV aluminium sheathed XLPE (CAZV) cable and wire shielded XLPE (WS/CV) cable are recently adopted due to maintenancefree and non-inflammability. But, it is very difficult to estimate the conductor temperature in case that the eddy current losses in cable sheath are relatively large or the heat flows radiated from cable are not uniform such as the trefoil configuration of the aluminium sheathed cables. Therefore, two methods to estimate the conductor temperature in loading cables in arbitrary configurations are proposed, namely by measuring the cable surface temperature and the heat flows radiated from cable, and by measuring the cable surface temperatures and cable load current. The applicability of those methods is verified by using 275kV 1×2, 000mm² CAZV and 154kV 1×2, 000mm² WS/CV cables. One of those methods was experimentally applied in acutual underground cable lines, 275kV OFAZV cable for summer in 1989.

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This paper proposes a new fault point identification which objective is to realize the region protection of the power system. The proposed approach makes use of the global data and then identifies the fault point according to the pattern recognition, where the global data is the negative sequence voltages of power system on fault. The identification of fault point is executed by two level hierachical recognition structure. The fault point is distinguished by the index of Mahalanobis generalized distance. In the upper level, that is the global identification level, we search the subsystem in which the fault occurred, according to the measured voltage pattern. In the second, that is the local identification level, the fault point in the subsystem is inferred by using the index of vector difference between the neighboring buses. The model of 5 generators and 15 lines system is applied to verify the proposed methodology. The results show that our approach is suitable and effective for identifying the fault point.

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