In recent years, the harmonics and EMI noise sent out from an electric power conversion system are getting a great deal of attention in electromagnetic environment problems. So, the considerations for mentioned above have been getting important. Therefore, there is need to do a detail frequency analysis such as transient phenomena at the switching. So, we have proposed the analytic technique using Wavelet-transform which is possible to catch a phenomenon from the both sides of the time and the frequency. In this paper, we performed the analysis using Wavelet-transform against the actual measured date at the switching interval. Then, based on obtained result, we describe the high validity of Wavelet-transform and the application to the power electronics field.

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In this paper, we carried out a case study on fundamental insulation design for the application of N₂/SF₆ gas mixtures to 275kV gas-insulated busbar. Firstly, we investigated the impulse ratio in N₂/SF₆ gas mixtures and pointed out that the insulation design might be based on the negative switching impulse voltage in the case of lower SF₆ content. Secondly, we estimated the tank diameter and SF₆ gas amount with consideration of the electric field strength on conductor surface and temperature rise in gas mixtures. We clarified that the tank diameter at the lower SF₆ content is mainly determined by the electric field strength on the conductor surface and quantified the optimum gas pressure and SF₆ content for a target reduction of SF₆ gas amount.

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From the viewpoint of mitigating global warming by SF₆ gas, many studies have been carried out to develop insulation gases or gas mixtures alternative to SF₆ gas. However, few studies have so far focused on creepage discharge characteristics on solid dielectrics in the alternative gases, which is very important for electrical insulation design of gas insulated power equipment. In this paper, we investigated the creepage discharge propagation characteristics on solid dielectrics in N₂/SF₆ gas mixtures under positive lightning impulse voltage application. We simultaneously measured streak images and current waveforms of the creepage discharge for different mixture rates of SF₆ gas in N₂ gas. Experimental results revealed that (1) discharge propagation length dramatically decreased by a smalladdition of SF₆ gas, (2) highly time-resolved observation using streak camera clarified that the creepage discharge propagated stepwise, (3) step length of creepage discharge propagation decreased drastically at a small mixture rate of SF₆ gas, (4) prominent enhancement of ion drift velocity in streamer channel at a small mixture rate of SF₆ gas activated the streamer-to-leader transition process, and brought about the steep increase in the step propagation velocity of creepage discharge.

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SF₆ gas with excellent dielectric properties is commonly used as an insulation gas for electric power apparatus. However, SF₆ gas is nowadays considered to be a greenhouse gas, because of its high global warming potential (GWP). Thus, the development of new insulation gases or gas mixtures alternative to SF₆ gas is strongly needed.
N₂/O₂ gas mixtures can be promising substitutes with no GWP. In this paper, we investigated partial discharge (PD) and breakdown characteristics of N₂/O₂ gas mixtures under non-uniform electric field, and compared with those in N₂/SF₆ gas mixtures. As a result, PD inception voltage (PDIV) in N₂/O₂ gas mixtures increased linearly with increasing the gas pressure, and decreased slightly with increasing the mixture rate of O₂ gas, while PDIV in N₂/SF₆ gas mixtures increased remarkably at a small addition of SF₆ gas. This could be due to the difference in the critical electric field strength between O₂ gas and SF₆ gas. Breakdown voltage (BDV) in N₂/O₂ gas mixtures as well as N₂/SF₆ gas mixtures exhibited nonlinear characteristics against the gas pressure, as was in the case with pure SF₆ gas. The gas pressure at which the BDV reaches the peak value shifted to the lower gas pressure side with increasing the mixture rate of O₂ gas and SF₆ gas in N₂ gas. However, the leader-type PD, which is peculiar to N₂/SF₆ gas mixtures, was not detected in N₂/O₂ gas mixtures. These results suggested that the mechanism in discharge development from PD inception to breakdown in N₂/O₂ gas mixtures might be different from those in N₂/SF₆ gas mixtures.

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In order to discuss alternative insulation gases to SF₆ for gas-insulated equipment such as a GIS, in relation to environmental concerns regarding the global greenhouse effect, we investigate basic insulation characteristics of N₂ gas in the high-pressure range up to 2.0MPa. This paper describes the lightning impulse breakdown characteristics of high-pressure N₂ under a quasi-uniform field gap and a non-uniform field gap with presence of a metallic particle. Our experimental results show that the withstand electric field level of N₂ at a gas pressure of 1.0MPa is higher as the practical allowable electric field on the conductor used in GIS design. Based on the results, we point out the possibility of using high-pressure N₂ as an alternative insulation gas for practical gas insulated equipment. However, positive breakdown voltage in high-pressure N₂ with presence of a metallic particle is extremely lower than that of quasi-uniform field gap. Therefore it is necessary to consider a complete elimination of metallic particles or a method to weaken their harmful effects.

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Recently the hydrogen energy system is proposed as a countermeasure for the depletion of energy resources and the global warming. Polymer electrolyte electrolysis cell (PEEC) can produce efficiently pure hydrogen under high current density by electric power from renewable energy. We estimated the performance of large PEEC set at central position of stack with the adiabatic boundary condition by using the two-dimensional simulation code in our previous report. The overpotential is assumed to be the difference between the measured cell voltage and the Nernst potential in our calculation. The distributions of temperature and current density are estimated not to be influenced directly by cell size, and the cell temperature increases a little along the flow direction, but the current density is uniform in most of our calculation conditions due to the small change of overpotentials and Nernst potential. The heat sink by water evaporation decreases with increase of PEEC operating pressure showing the larger temperature increase than that for an atmospheric operation. To keep the cell temperature under 100°C, it is desirable to operate PEEC with the water electrolyzing ratio less than 1%.

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It is very difficult to analyze the behavior of a free arc column from a series of continuous images, e. g. the images obtained using a high-speed video camera. This paper presents a useful method for estimating the three dimensional path of the long gap free arc column using image processing technique without making any subjective judgments. In this method, first of all, a set of two orthogonal digital images of the arc column is entered as the first data. Next, this set of images is processed using a new extraction scheme in order to obtain automatically a linear shape as the extraction area, which is especially useful for arc images including indistinct or low contrast sections. Finally, a set of two linear shapes in the extraction images is transformed into the three dimensional path in the coordinate system at actual size. This paper also describes the results ofapplying this method to continuous images of the long gap free arcs obtained using high-speed video cameras. The change with time of the visualized path, the length and expansion for the arc column with the direct current of 100 and 2, 000A and the gap length of 1. 6m, were derived quantitatively.

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An Improvement Method of breakdown strength at the interface between an insulation layer and a semiconducting layer in a XLPE power cable was proposed. The Method was named as “Interface Diffusion Method” and is based on an additive mixed into semicon layer of an XLPE power cable. The authors found that tha additive diffused into insulation layer during cross-linking proces in XLPE cable manufacturing and the additive improves the lamellar orientation at the interface and forms a Diffusion layer the additive at the interface resulting increase of the breakdown strength. The method was applied to an XLPE power cable with 9mm insulation with successful results. The cable passed withstand voltage test and a long-term insulation performance test as 154kV cable. In addition to these cable test, material analysis was done on the interface region of the cable such as TEM analysis and FT-IR analysis. The material analysis showed that lamellar orientation in the XLPE at the interface was improved and diffusion layer of the additive was formed as expected.

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To compactize EHV XLPE power cables and joints has been very important requirements among utilitiy companies and so we developed a new method to improve the breakdown strength at the semicon-interface in a mold-type cable joints in order to fulfil the requirements. The method was named as “Interface Diffusion Method” and is based on an additive mixed into semicon layer of an XLPE power cable joint. We found that the additive, conventional surfctant, diffused into insulation layer during heating process at cable jointing. The additive improves the lamellar orientation at the interface and forms a diffusion layer near the interface, and consequently increase the breakdown strength at the interface. The method was applied to XLPE power cable joints models with insulation thickness up to 9 mm and the model passed a voltage endurance test for 154kV cable joint.
This paper describes some experimental results in terms of the breakdown strength and material analysis of XLPE insulation at the interface in the mold with a transmission electron microscope (TEM). TEM analysis showed that the lamellar orientation in the XLPE insulation at the interface is improved as well as that in the XLPE insulation in the cable part.

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We have researched the bubble formation of transformer winding under overload condition, especially critical temperature of bubble formations varying water contents in dielectric materials. Bubble formation strongly depends on water contents in dielectric material. Critical temperature of bubble formation increases according to decreasing water contents in solid dielectric materials. Comparing from temperature rising rate, bubble formation of actual transformer seem to be higher temperature than model test results.

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When the heat deterioration of the insulation paper in the range of 120-180°C was evaluated in the average degree of polymerization, it was found out that it declined to less than 800 in the early stages of deterioration rapidly, and the slow deterioration continued the next. The deterioration characteristics of the average degree of polymerization can be explained in a unified manner, by using “the life consumption rate” which was defined in this paper. The over-load examination of the model transformer was done, and it was found out that the actual measured value of the average degree of polymerization, corresponded to the calculated value based on the heating history. Therefore, the life consumption rate and the decreasing degree of a average degree of polymerization caused by short time overload operation, can be obtained by the proposed calculation. On the other hand, deterioration characteristics of a transformer, which operated in a usual operation for a long time, should be investigated in future.

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A new Fault Current Limiter (FCL) is proposed which consists of a conventional Current Limiting Reactor (CLR) and a Thyristor Controlled Series Compensator (TCSC) in series. In normal operation, it works to compensate for impedance of the CLR and a line, while it limits short circuit currents during system faults. After the fault clearance, it damps power oscillations. Effectiveness of fault current suppression and damping effects on power system oscillations were studied by digital simulations and by an analogue simulator. It was found that the proposed FCL could decrease impedance of a conventional CLR reactor due to inductive impedance of the TCSC and have good damping characteristics. From investigation of cooperative protection system for the FCL, it showed that fast bypasses of the thyristors in the TCSC were useful for overvoltage protection, fault current limit and decrease in capacity of protective arresters connected in parallel with the TCSC.

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Since a new method has developed on partial discharge (PD) detection from insulation joints (IJs) in long-distance XLPE cable lines, the PD measuring tests using this technique have come into common use for confirming the soundness of the lines. This method we call S. P. O. M enables us not only to detect PD safely and easily but also to calibrate the detection sensitivity using the indirect calibration from metallic-foil electrodes attached on both insulated surfaces of an IJ. Although PDs from normal joint (NJ) cannot be detected at NJ, attenuated signals transmitted from NJ can be detected at adjacent IJs at which calibration of detection sensitivity is also possible considering the attenuation constant of the cable line. The paper describes the principle of this method in IJ, and some applications to NJ and terminations considering the correction factor K which we defined for indirect calibration.

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This paper presents an adaptive control technique for the variable series capacitor (VSrC) using a Recurrent Neural Network (RNN). Since the parameters of the controller determined by Genetic Algorithm (GA) which is one of optimization algorithms are optimum for certain operating point, it is not possible to realize good control performance against variations of operating point and fault point in power systems. Then the adaptive controller proposed in this paper consists of the optimized controller using GA and the RNNs. As the RNN is learned on-line, robust control performance can be realized in various conditions. The effectiveness of this control method is verified by simulation results of a multi-machine power system.

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This paper proposes a tabu-search method with the priority list of generator forthe unit commitment in power systems. Embedding the priority list into tabu search works to reduce the solution space efficiently. The unit commitment is formulated as a mixed-integer problem. In this paper, it may be decomposed into problems to determine on-off conditions and output of generator. The former is solvedby tabu search while the latter is done by the equal-lambda method. This paper focuses on a fast and rigorous method for determining the on-off conditions in binary number. Tabu search is effective for solving a combinatorial optimization problem. However, it is inclined to time-consuming due to the increase of solution candidates in larger problems. Therefore, this paper proposes a tabu search technique that considers the priority list in the unit commitment. It allows to reduce computational effort drastically. The effectiveness of the proposed methodis tested in sample systems.

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As the electric power demand increases, power systems become larger and more complex. The dependence of industries on electric power is increasing. And the influence of outages in power systems is becoming more crucial. Therefore preventive control is desired in power system operations. In this paper, we propose a generation rescheduling method to solve the problem. First, when the output of the j-th generator is increased we compute the kinetic energy change at the fault clearing time through Taylor series expansion. Using the information we decide the ratio of the rescheduling. Second, we reschedule generation using the linearity relationship between the critical clearing time and the generator output on the faulted bus for the critical contingency. Then the generation rescheduling is implemented. The proposed method is demonstrated using a sample 9 machine 20 bus system.

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CGS (cogeneration system) is a typical example of the distributed power and heat source, and will be increasingly more important for the future. By connecting several CGSs with both electricity and heat distribution grids, an advanced district energy network, which is economically more efficient and environmentally more benign, can be established. But considering some cogenerators who behave to maximize their own profits, the district energy network is not always established because of each cogenerator's motivation to acquire its own profit as much as possible. The key issue for participants to the network is related with the question of what type of partner they should cooperate together to maximize their profits.
In this paper, we attempt to analyze the coalition behavior in the district energy network taking advantage of the cooperative game theory and to estimate the future feasibility of group management for CGS. As a major result, it is revealed that with the share of residence type demand increased in the district energy network, all players have incentive to participate in the network. And we provide for the analysis of the effect of the construction cost assignment of heat grids for the players.

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We study the voltage stiffness effect for cascading tripping and its protection in power systems.
This paper describes a new selective method of proper system separating point to prevent from system failure extension to wide area. We define “the voltage stiffness” by short circuit capacity and line impedance, and explain an operation to select proper separation point.
In addition, we consider the security after system separations. It is dominated by both increase of reactive power transmission losses and margin of reactive power supply of generator.
The basic 4 power system structures are also considered in this paper. The topological analysis of power system can show the best system structure in both power interchange and cascading trip protection. We apply the proposed method to practical power systems and simulate power system faults. Simulation results show that separating system at the selected point can defend cascading tripping.

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