Sample cables including a few types of defects (such as voids, contaminants, and protrusions) with more volume than usual, were prepared intentionally for clarifying how these defects separately affect insulation characteristics, under the presence/non-presence of moisture. In the case of void inclusion, insulation characteristics is even more susceptible to AC voltage than impulse voltage, and the distribution of lifetimes is classified into a worn-out failure type due to discharge deterioration. While protrusions and some other contaminants can greatly deteriorate original dielectric strength, these defects have a relatively slight effect on long-term endurance in the atmosphere. Therefore, the distribution of lifetimes is classified into an early failure type. With refernce to influence of moisture, in strong electric field (exceeding 15kV/mm) and in a region of short-term, there is an increase in harmfulness around defects with water absorbed, thereby inducing electrical trees and resulting in a breakdown. In light electric field (no more than 15kV/mm) and in a region of a long-term, harmfulness of defects will not deepen to a considerable extent, and growth of water trees is influential very possibly.

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A new evaluation method of submicron morphological changes of the insulating polymer near the semiconducting interface in an XLPE power cable was proposed based on TEM observations. The method provides higher sensitivity on material degradation due to aging than the conventional methods. A long term aging test was carried out on an XLPE cable specimen with insulation thickness of 9mm. The specimen was aged under AC voltage stress of 63kV at 1kHz and heat-cycling thermal stress by the conductor current with real time of 13, 164h.
The experimental results show that the dielectric breakdown strengths did not decrease or rather slightly increased in spite of generation of bow-tie trees after the aging. Material analysis of the insulation was done and the result indicated that there were submicron morphological changes such as spherulite diameter and lamella thickness. It was suggested that these changes were brought by the thermal stress and might cause the slight increase of the breakdown strengths. Therefore it may be concluded that the new method is useful to detect fine material changes.

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In extra-high-voltage cable lines, a sheath surge voltage arises between insulation sleeves of GIS-CH and cross-bonded insulation joints (IJs) due to the high frequency and high voltage surge that arises from the operation of GIS-CB. To protect the cable sheath from the sheath surge voltage, surge by-pass capacitor and ZnO arresters are used with GIS-CH, and ZnO arresters are used with IJs. The slight impedance of the lead wires on these protection devices are also a problem with respect to high-voltage surge. Through experiments on extra-high-voltage cable lines we compared the effectiveness of these various protection methods. We found that all the protection methods for GIS-CH are effective, and that for IJs care must be taken to minimize the lead wire impedance of the protection devices, bacause a high-frequency surge might arise extending as far as the second crossbonded section.
In simulating the sheath surge voltage by EMTP, we studied methods for conviniently attenuating the high frequency component of the surge voltage in the cable and obtained results that agree closely with the actually measured sheath surge voltage.

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Optical ZCT (Zero-phase Current Transformer) which combines a optical magnetic field sensor with a magnetic ring core of silicon steel plates has been developed. Bi-doped YIG _??_Yttrium Iron Garnet, (BiYbGd)₃Fe₅O₁₂_??_ having a large verdet constant is used as a Faraday element. Optical ZCT detects the conductor current of 0 to 2, 000 A at high resolution and has been applied to the fault section detection system for 66kV underground branch transmission lines which monitors a multi-terminal line having Y-branch joint box in on-line state. Optical ZCTs and a section detector/indicator are connected with optical fibers which are capable of non-repeatedly transmitting signal over 6km, allow to monitor a long-distance underground power transmission lines. Developed system is now operating on JR Kasai line (Shinkawa pump line) of Tokyo Electric Power Company.

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This paper describes a new partial-discharge detection method for an XLPE-insulated copperwire-shielded cable line using a “Yoke coil”. Partial discharges are measured with the Yoke coil which detects the azimuthal component of the current flowing through the copper wires wound on an outer semi-conducting layer with a given angle against the axial direction of the cable.
The highest detection sensitivity of 2 pC have been achieved at a frequency range of 5-20 MHz using a 77kV 100mm² model cable line (200m in length). It was confirmed that the partial discharge caused by an artificial defect has successfully detected using the proposed method.
In the PD measurement for an actual 77kV XLPE insulated live line, the detection sensitivity was 10 pC.

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The present paper describes detection by the AE (acoustic emission) sensor of partial discharge in the prefabricated joint, one of the accessories for XLPE cables, which is composed of factory-formed parts. Since the prefabricated joint can be installed in a short time, an increasing number of such joints have been used on power transmission line. However, the interface between the parts of prefabricated joint tend to exhibit weak points because of the structure peculiar to this type of joint.
We provided the AE sensor for practicability in comparison with conventional electrical defect detection methods. In coure of the present study, a method for locating a defect using multiple signals from the AE sensors was developed and its validity was evaluated by the dielectric breakdown test.

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This paper describes the developmental research of DC XLPE cable. Through a series of material investigations, XLPE compound containing highly purified special filler was developed.
To check the DC insulation performance of the cable insulated with this new material, a prototype cable with 9mm insulation thickness was manufactured. It was confirmed that the performance of the prototype cable was excellent. Based on the study of the prototype cable, a 250 kV DC cable with 20mm insulation thickness was designed and manufactured. Through a series of voltage tests, excellent DC insulation performance of the developed cable was verified.

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In a previous report, we described the process for the selection of modified HDPE, as a new insulating material for HV DC cable. The effect of insulation thickness and the long-term V-t characteristics for DC breakdown of the modified HDPE cable were continuosly investigated and promising results for an application to HV DC cable were obtained.
In order to clarify the mechanism of the excellent DC characterittics of modified HDPE cable, we examined the quantitative space charge characteristics of modified HDPE compared with XLPE, by using a pulsed electro-acoustic method. XLPE gives hetero space charge distribution, although modified HDPE gives homo space charge distribution. In addition, in modified HDPE, the small dependence of space charge amount on DC applied time and DC applied stress are thought to be the reason for such excellent DC characteristics. The advantageous space charge distribution in modified HDPE is found to be the complex effect of a modified group and additives which are included in modified HDPE.

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Submarine cables have more possibility of serious mechanical troubles by ship's anchor or equipments for fishing, more than conventional underground cables. Detecting these troubles in submarine cable before electrical breakdown is very useful technology to improve the reliability of submarine transmission line, considering these laying circumstance. We have developed the mechanical-damage-detectable-sensor applied by optical fiber. Actual 66kV XLPE submarine cables composite mechanical-damage-detectable-sensor were manufactured and were subjected to various mechanical tests. Press down test for the sensors by optical fiber units showed a good detectability to mechanical deformation by sample tests. The actual cable by 66kV XLPE cable were subjected to mechanical test by CIGRE recommendation, press down test where the cables were deformed by mechanical pressure and electrical breakdown test under the mechanical deformation. These data provided that the developed 66kV XLPE cable has the mechanical-damage-detectability and the required properties.
The cable will be installed in actual field on April in 1992.

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This paper presents a diagnostic method of power system using logical constraints and a diagnostic system based on this idea. Logical constraints represent causal relations and hierarchy relations among states. These logical relations are derived from initial causal relations and primitive relations among basic state elements. First step of this method is to generate additional causal relations for efficient inference in case that both causal relations and hierarchy relations exist. Next, definite and possible states are generated. Information which means direction of inference is attached to states and used in this process. Also, inference processes to find out explanation roots between states and to verify a possible cause are applied. Finally, preference solution is selected. If preference solution is considered as a possible cause, next preference solution is infered after negating the prefarence solution.

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The idea of unbalanced load flow calculation was proposed many years ago. At that time, however, the needs for such techniques was not an urgent one. Modern power system networks are comprised of long untransposed transmission lines. Therefore, for some kinds of analysis, it is now almost impossible to treat a system as though it were a symmetrical network. The aims of most previous studies were oriented to solve voltage/current imbalance in local or small systems, as local imbalance was a serious concern. This is still an important issue, but more recently our needs have become concentrated on practical bulk power systems, since principal EHV lines are entirely untransposed. In this paper, the development of practical unbalanced load program and practical experience with it are reported. This program was developed for steady state analysis of large-scale practical networks under many possible unbalanced condition. The Newton-Raphson method in polar coordinates is used, since fast and moderate convergent characteristics are suitable for solving solutions. The program was applied to practical case studies. The models contain all of the 500 kV and most of the 275 kV overhead transmission lines in TEPCO. Since the density of the Jacobian matrix is higher than in single phase load flows, the number of non zero elements is extremely large. Convergent characteristics are excellent in all cases. The fear of numerical instability has been allayed.

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It has not been performed to theoretically analyze the earth resisteance where the electrodes are obliquely set up in the earth. This paper discribes the calculation results of the earth resistance and the earth surface potential when one or more rod electrodes are obliquely driven in the earth. For the earth resistance of obliquely and radiailly driven short rods, the experimental results correspond approximately with the calculated results. Furthermore, at the grid grounding systems with rod electrodes, it is confirmed to decrease a potential gradient on the surface when the rods are obliquely driven.

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