In the present power transmission system, electric power devices are equipped under a conception of the insulation coordination and are protected from the over-voltage of the lightning and switching surge by the arrester. However, a future superconducting power transmission system is characterized by the heavy current transmission capability, the lower transmission loss and lightning-surge-free circumstances. Thus, in the superconducting power transmission system, superconducting devices should be protected rather from quenching by the over-current of a short-circuit fault than from the breakdown by over-voltage.
In this paper, a new conception of the coordination for the superconducting transmission system is proposed as a “quenching current level coordination” instead of the insulation coordination. Furthermore, it is pointed out that the quenching current level coordination has the duality relationship with the present insulation coordination. A superconducting fault currant limiter should play an important part of the quenching current level coordination to prevent the other superconducting devices from quenching.

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SAFIR (Systeme de Surveillance et d' Alerte Foudre par Interferometrie Radioelectrique) has been equipped and operated since June 12th, 1992 as a cooperative project among Osaka University, Kansai Electric Power Co., INC. and a French manufacturer DIMENSIONS. The operational coverage includes Northern Kin-ki District, Wakasa District and Hokuriku District. To evaluate the SAFIR system performance, we chose both the summer thunderstorm active area and the winter thunderstorm area. That meant we intended to evaluate the system performance through the summer thunderstorm activity and to apply the system to winter thunderstorm monitoring. As a method for evaluation, the cross correlation between the radar echo pattern and the distribution pattern of lightning discharge was calculated. We obtained the high cross correlation and got to the conclusion that the accurate location of SAFIR was proved statistically. We also show the case study of the occurrence of a ground flash to demonstrate the function of lightning warning by SAFIR.

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The possibility of laser-induced lightning has been investigated for the future application of protecting electric power systems from lightning strokes. This technique induces lightning strokes artificially along a laser-produced breakdown plasma channel and conducts them to a harmless place on the ground.
To clarify the plasma formation mechanism and the discharge (sparkover) induction process, we have performed sparkover induction experiments for two types of long-gap electrode configurations, plate-to-rod and plate-to-plate, with a 50-J TEA CO₂ laser. In the present paper, the sparkover voltage characteristics and interferometric measurement on laser-produced plasma are mainly described, together with sparkover induction for a series of metal beads arranged equidistantly which simulate a plasma channel.

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This paper investigates the voltage stability in a single OLTC (On-load Tap Changer) system, which consists of a single OLTC and multiple generators as well as loads. It is shown that the sensitibity of load voltage to tap position, dV/dn=(I₁=exact value), plays an important roll, independent of the existense of the equilibrium of the tap position. That is, when the equilibrium exists, the region where I₁>0 is in the stabity region; if the equilibrium disappears, the stability region aslo disappears but the voltage is still recovering in the region where the I₁>0. Based on these results, the region of reverse control action of OLTC is defined as the region where I₁<0. It is proved that another sensitibity, I₂, which is defined as dV/dn when constant power characteristics are assumed for loads, can be used to estimate the region of reverse control action when the exact load charecteristics are not available.

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Ferroresonant overvoltages can occur on an isolated feeder connected to Distribution System Generators (DSGs). This paper analyzed the overvoltages and the following results are obtained.
(1) The overvoltages occur on a three-phase four-line wye-grounded system such as 400V class distribution lines but do not occur on a three-phase three-line ungrounded system such as 6.6kV distribution lines.
(2) The overvoltages can occur when DSGs are separated from the utility source not at the primary side but at the secondary side of a delta-wye grounded transformer connected to the feeder.
(3) The saturated reactances of transformer excitation circuit and the phase-to-neutral capacitances on the isolated feeder eatablish the ferroresonance circuit.
(4) The ferroresonance generates the phase-to-neutral voltages of 2.6 pu. The voltages are suppressed to 1.8 pu with installation of surge-absorbers but continue. The DSGs have to be disconnected from the islanding system by overfrequency or overvoltage relays to drop the continuing overvoltages.

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This paper describes the characteristics of a lightning surge which propagates on an incoming line between a first tower and a substation. Experimental results by the use of a 1/10 scale model considering an actual line configration, which is a double circuit vertical line at the tower and a horizontal line at the substation, show that line voltages of an applied phase are not affected by the line configration, however, the other phases are affected. Calculated results by the EMTP, dividing into several parallel conductors, and approximate formulas agree well with the experimental results. Since mutual impedances between the applied phase and the other phases change on the incoming line, the line voltages of the other phases increase or/and decrease due to reflections.

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The tower surge impedance derived from the electromagnetic field theory doesn't always coincide with the measured values satisfactorily. The theory derived by Lundholm is the most famous one, and believed to have been established, but it doesn't coincide with the measured values. We investigated his theory precisely, and found his theory was incorrect. He derived the loop voltage method and skillfully used vector potential, electric and magnetic field. Especially he combined the vector potential with the electric field, however we clarified that, in this point the errors came in. The vector potential is the quantity from which the magnetic field is derived, therefore the electric field must be derived from the magnetic field coiled around. In most cases, undoubtedly the electric field can be calculated from the vector potential.
In this case, however, the magnetic field is propagating, therefore the vector potential is also propagating, so that the electric field derived from the vector potential is the circulating local field. The electric field, therefore, must be calculated, considering the propagation phenomena and the simultaneity. We derived a new theoretical equation of the surge impedance; and found the theoretical values comparatively well coincide with the measured ones.

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Ultra high voltage transmission lines must be protected from lightning surges and the following large-current fault arcs. Horns which are installed at the jumpers on the strain towers have a role to protect the insulators from the fault arcs by coordinating insulation strength and holding large-current fault arcs between the horns. Although a flat surface of the electrode tip with a small diameter effectively meets these requirements, it results in the deterioration of corona discharge depressing capabilities in a steady state on power transmission lines. In order to improve corona discharge depressing capabilities and to reduce deflection of arc jets, we finally developed two types of hemispherical shaped arc horn electrodes with slits. The arc jet control performance of those horns was experimentally disclosed as follows:
(1) Two types of arc horn electrodes with four or six slits show remarkable are jet control performance in the current of larger than 5 kA.
(2) The addition of a rod type central electrode at the center of the hemispherical shaped electrodes results in the considerable extension of arc jet controllability in the wider current zone by containing the arc spot within the tip area of the central electrodes.
Based on the above results, arc horn electrodes with slits have been designed and utilized for practical application.

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A series of experimental trials have been carried out to realize the high efficiency a-Si//poly-Si tandem solar cell. Employing p type μc-SiC as a wide gap heterojunction window, a-SiC as an interface buffer layer and n type μc-Si as a back ohmic contact layer, 17.2% conversion efficiency has been achieved with the structure of ITO/p μc-SiC/n poly-Si/n μc-Si heterojunction. Utilizing an optically transparent a-Si p-i-n cell as a top cell and inserting an optical coupler between the top and the poly-Si bottom cell, a high total efficiency of 21.0% has been obtained so far on the four-terminal tandem cell. This conversion efficiency value represents a world record for a-Si basis tandem solar cells.

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Effects of plasma structure on the performance of closed cycle disk MHD generator under high enthalpy extraction condition have been examined with r-θ two dimensional numerical simulation based on two temperature model MHD equation. It is found that higher load resistance and seed fraction as compared to the design value decrease the nonuniformity of plasma structure and improve the generator performance under the inlet condition of the low electron temperature. It is also shown that slight pre-ionization at the inlet of MHD channel is effective for the improvement of the performance of generator.

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