Degradation of XLPE cable by water trees gives rise to harmonics in the loss current. The authors are engaged in development of diagnostic methods for water tree degradation utilizing these harmonic components. As a result of this development, the following was discovered. The harmonic components occurring in the loss current provide an extremely good indication of the state of degradation. Harmonic components arise as a result of the nonlinear voltage-current characteristic of water trees. An equivalent circuit which incorporates this nonlinearity of water trees faithfully reproduces the loss current waveforms obtained in experiments. When a voltage at a frequency differing from commercial frequencies is superposed, a degradation signal occurs at a characteristic frequency.

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Partial discharge (PD) measurement is a promising technique that can confirm the insulation performance of CIS. In order to establish highly sensitive diagnosis technique of GIS insulation, frequency characteristics of electromagnetic wave which are determined by electrical aperture such as spacers should be clarified, and it is inevitable to elucidate how the frequency characteristics of aperture affect the electromagnetic wave spectrum received by antennas.
In this paper, we measured the electromagnetic wave characteristics invading into a model GIS, with changing the aperture condition of GIS. Next, the electromagnetic wave radiated from the spacer aperture with and without bolts was also measured and analyzed by using an equivalent circuit analysis program “PSpice”. Comparing these measurements with analysis, we investigated the relationship between the spacer aperture configuration and the radiated electromagnetic wave spectrum from the aperture, and discussed how the aperture condition contributed to the radiation characteristics of electromagnetic wave out of GIS.

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We have developed a fiber-optic gas pressure monitoring system with new gas density sensors for detecting the leakage of insulation gas of the Gas Insulated Transmission Line (GIL). In this system gas pressure of over one hundred gas compartments can be monitored with several optical fibers and without electric power supply to the sensors. This simple and highly reliable system enables speedy gas leakage detection, allowing us to take appropriate measures before reaching its critical pressure. Developed sensor makes mechanical bend of an optical fiber when the gas leakage, and causes an optical transmission loss as the detected signal. Using a conventional optical time domain reflectmeter with these sensors, a multi point sensing system can be constructed. In this paper we report the principle of this sensor and the design of the monitoring system for the Shinmeika-Tokai GIL.

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This paper describes the development of insulation diagnosis system of GIS using UHF (ultra high frequency) method which is detected by electromagnetic wave signal in UHF region. A new UHF partial discharge sensor with high sensitivity is developed based on the principle of dipole antenna. Its sensitivity is better than 0.3 pC. Identification of defects and discrimination from noise signal are developed by phase spectrum method of applied voltage. Moreover, the accuracy of diagnosis advances by a new method of neural network combined with frequency spectrum and phase spectrum.

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An arc occurred between a pair of parallel conductors is driven by the electromagnetic force. Such traveling phenomena of arcs are found in low voltage circuit breakers, switch boxes or panel boards at accidental condition. Until now, these study have been reported only for relatively low current level, and there are few study for short circuit large current level. Also, the traveling velocity of the arc reported was an average value through whole arcing time. This paper describes the experimental results on the traveling arc between a pair of parallel conductors for alternating current from 600 to 1, 100 A(rms.) with a frequency of 60Hz. The material of conductors is copper and the gap of the two conductors is 15mm. The arc velocity and the arc extinguishing properties are studied. Measurements were made on the average velocity through traveling period and also the instantaneous velocity at each phases of the sinusoidal current. The instantaneous velocity ranged from 2 to 65 m/s and showed a proportional expression to the power of current, same as the average velocity case. Influences of shape of the electrode especially its width upon the arc length, traveling velocity and extinguishing properties were clarified.

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A fault current limiter (FCL) is extensively needed to suppress fault currents and is particularly required for trunk power system heavily connecting to high-voltage transmission lines, such as 5OOkV. lines which constitute the nucleus of the system.
In this paper, we have proposed a principle of a new FCL, consisting of solid-state diodes, a DC coil and a bypass AC coil. To evaluate the performance of this FCL, the Electromagnetic Transients Program (EMTP) analysis has heen executed on a model system. The stability of the new FCL connected with trunk power system is also analyzed by a power system simulator. These analyses show that the new FCL expectantly suppresses the fault current in the power system, without reducing its stability.

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Boundary element method is applied to calculation of the electric field intensity caused by the thunder-clouds. The field is calculated from the source or cloud charge under the given boundary condition, and the pound surface where the electric potential is equal to zero is chosen to be the boundary. The boundary expands to infinite area, therefore a special device, or a new type semi-infinite boundary element, is used for the effective field calculation. To examine the validity of the approach, some numerical calculations are carried out for the problem in which the exact solution is known. It is found that the approach is more effective compared with the conventinal method. The numerical simulations using the topographic data in Hokuriku area are also shown.

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AC superposition method we investigated is a new technique for hot line diagnosis of 6.6kV XLPE power cables, and mechanisms for the diagnosis are based on unique phenomenon of water trees under AC electric field. For the phenomenon we have already reported that the slight current at Δ f [Hz] was observed and recognized as a deterioration signal from water trees when AC voltage, whose frequency was selected at commercial frequency multiplied by an even number and added Δ f [Hz], was superposed on to the shielding layer of power cable with water trees. This phenomebon is probably caused by a kind of modulation effect due to some non-linear characteristics of water trees because the frequency Δ f [Hz] is not included in all frequency components of applied and superposed voltage. However, there is no theoretical explanation.
So we analyzed theoretically the time dependence of non-linear conductance of water tree under AC electric field and indicated that the conductance was able to be expanded to Fourier series composed of commercial frequency multiplied by even numbers. Also we formulated the current at Δ f [Hz] and the AC loss current including harmonic waves by using the Fourier series expansion and equivalent circuit of water tree. From the results, the characteristics of these currents confirmed by various experiments were successfully explained and effectiveness of the AC superposition method became apparent.

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It is desirable to establish the accelerate evaluation method for phosphoric acid fuel cell durability in order to promote the PAFC's utilization. It is necessary to clarify the mechanism of voltage loss and investigate the accelerate factors for the establishment of test method. The accelerate factors of voltage loss has been defined by the subscale cell tests and it was found that the temperature is the key factor which accelerate the cell voltage loss by fullscale and subscale test. This paper focused the voltage loss by the gas diffusion which appeared after long time run and investigated the voltage loss mode change and the relation between the loss mode and an accelerate factor by the result of fullscale cell test of 20, 000 hour run. It was obtained that the accelerate test by temperature as an accelerate factor could adapt to the prediction of voltage loss till 40, 000 hour run.

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Although a closest bifurcation point (CBP) is known to be a superior security measure against voltage collapse, no fast computation methods exist presently. In such a circumstance, the authors have proposed an approximation method of CBP in⁽¹¹⁾ where two major problems are left unsolved. They are: (1) the method cannot take into Q limits of generators and (2) it utilizes a low volt-age power flow solution, which is not easily obtained unless a suitable initial estimate is available.
This paper presents a series of techniques to solve these problems. It is shown that the proposed method can provide fairly accurate CBP without suffering from any major problems. The total computation time is not more than that of ten power flows even when no information of low voltage solution is available in advance. The proposed method will make it possible a reliable on-line monitoring and control of voltage stability.

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Mid and Western area 60Hz system of Japan forms a typical longitudinal power system. A weakly-damped low-frequency oscillation mode exists. There is a possibility that autoparametric resonance occurs for large disturbances like system faults because of interactions among oscillation modes. In this paper, we confirm the autoparametric resonance with a large simulator called APSA. The simulator accurately models generators and transmission lines. The confirmation is important to show the existence of the autoparametric resonance. First, we observe with the simulator that generator swings decay for short faults while they diverge for long faults. It is difficult to verify that this phenomenon is the autoparametric resonance with swing curves only. As its aids, we use eigenvalue analyses and digital simulations. We identify oscillation modes contained in the swing curves to extract two main oscillation modes. Then, we show that the force applied on the low-frequency oscillation mode draws the same trace as that of the autoparametric resonance under the action of the other mode. This proves that the phenomenon observed with the simulator is truly the autoparametric resonance. Lastly, we execute simulations under various system conditions, and show the autoparametric resonance occurs under normal conditions.

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A great ammount of SF₆ gas with the potent greenhouse effects has usually been filled in the high-voltage power equipment. To reduce ammount of SF₆ released into the atmosphere, we need to constract rapidly the SF₆ recycle system.
The aim of this paper is to measure directly gaseous filling and refilling characteristics of the Zeolum-absorbent by a new weighing machine and take design parameters used in the recycle system.

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This paper proposes the use of a gas mixture containing c-C₄F₈ (perfluorocyclobutan) which may substitute conventional SF₆ in gas insulated systems. It discusses the gas mixtures from the standpoint of boiling temperature, dielectric strength and greenhouse effect.

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