Impedance, admittance and surge impedance formulas of a vertical conductor with a circular cross-section above an imperfectly conducting earth have been derived applying a complex penetration depth of the earth and Neumann's formula.
It becomes clear that the frequency- and time-dependence of the characteristic impedance due to an imperfectly conducting earth is not significant, and thus it can be neglected, i. e. a perfectly conducting earth can be assumed in an evaluation of a tower surge impendance. The propagation velocity is nearly equal to the light velocity in free space, or about 90% of the light velocity. However, the frequency-dependence of the resistance and the attenuation constant of a tower can not be neglected. The surge impendance formula developed in the paper agrees well with various measured results, and is expected to be very useful in a surge analysis.

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This paper presents an approach to obtain the optimal generation dispatch in real power system operation by solving a multi-objective optimization problem. In deciding the optimal system operation, various objectives, such as economy, quality and environmental impact, should simultaneously be attained. However, these objectives are contradictory to each other and are in trade-off relations, thus making it difficult to handle this class of problem. In the recent study, we were proposed the algorithm to treat the multi-objective optimal dispatch problems using the fuzzy coordination technique.
In the foregoing approach, however, optimization is successively conducted by nonlinear programming and the computation time is extensively increases as the size of the target power system is expanded. Thus, for the practical application of this approach to a real-scale system, we need to resort to a high-speed approximation for the reduction of computation time and storage requirement.
In this study, we propose the approach decomposed into static dispatch and dynamical adjustment so that we can apply it to a relatively large scale system. In this approach we can obtain an optimal solution by satisfying some predetermined optimality conditions, thus enabling rapid convergence and application to a real-scale system.
The effectiveness of the proposed approach is verified through numerical analysis on the test system approximating the system of Kansai Electric Power Company.

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Knowledge acquisition and extension of expert systems for electric power systems are becoming difficult, and this paper proposes a solution by symbolic machine learning. The authors have developed a learning method HCL (Hierarchical Concept based Learning), which enables us to acquire knowledge to make switching sequences of power system operation.
HCL builds up knowledge base of an expert system from operational experiences in the past. The learning process of HCL is divided into organization phase and generalization phase. In the organization phase, HCL analyzes each experience and structures it by recognizing several aims of switching operations, and individual knowledge is produced. In the generalization phase, HCL integrates similar experiences into common operational rules by translating their individual names to variables.
HCL features the use of some conceptual hierarchies for organization, for instance the hierarchy of domain model which expresses the structural relationships between devices or equipment, such as ‘a_part_of’ or ‘connected_to’ relations. Based on the conceptual hierarchy, HCL enrolls all the operations in a tree structure, of which the nodes express their several aims.
As application examples of HCL, knowledge acquisition of operational sequences for bus-bar switching is shown.

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Short circuit study is one of the most fundamental planning analysis procedures in electric power utilities. Countermeasures for increasing transfer capabilities such as installation of additional facilities are originally implemented for the purpose of system stability improvement. From the viewpoint of short circuit currents, however, the reinforcement of transmission networks is commonly the major cause of increasing short circuit currents. Therefore, the problem related to short circuit studies should be paid greater attention to in the future, especially in terms of coordination of power system availability and reliability.
In order to effectively solve the problem, it is highly required to develop an advanced algorithm by which short circuit currents are easily computed at a time for a large number of alternative network configurations corresponding to operational change in bus connection at main substations. This report proposes a new calculation method to efficiently modify the Z-bus matrix elements relevant to short circuit studies of large power networks, and also presents its sufficient practicality by applying the proposed method to the calculation of short circuit currents in a couple of model network.

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In modern industrialized societies, it is mandatory to supply uninterruptedly high quality electric energy with modest cost while preserving better environments. This can only be realized through very sophisticated power system operations that compromise several contradictory factors stemming from economy, security and environment. Since these factors are in trade-off relationships to each other, they should be treated as such. A class of problems that simultaneously satisfy several criteria in trade-off relationships are called “multi-objective optimization problems”. Optimal power flow (OPF) has been regarded as the most powerful means to obtain such system operation plans, it only optimizes a single objective function.
This paper proposes an efficient solution methodology for a class of multi-objective optimal power flow problems which makes use of a heuristic search method. An optimal solution can be found in the proposed heuristic search method based on local information on a preference index which is arbitrarily chosen out of a given set of objectives. This circumvents the exhaustive evaluation of all non-inferior solutions which is needed in the existing multi-objective OPF algorithm proposed by the authors, thus dramatically reducing the solution time. The proposed method has been coded and applied to the IEEE 57 node test system. Simulation results have demonstrated the possibility of utilizing this method in on-line environments.

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The difference of arc parameters for each breaking point causes the occrence of non-simultaneous eurrent chopping when the parallel capacitor applied for each breaking point has larger values. The voltage between inductive load terminals is determined by the average value of the chopped currents for individual breaking points, and is not increased obviously by the non-simultaneous occurrence of chopping, unless large values of parallel capacitor are applied.

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Flow velocity and pressure distribution were measured in the downstream region without arc by using laser fluid flow velocimeter system and pressure transducer system, respectively. Flow pattern was also observed by using Schlielen optical system. By these experiments, the flow characteristic was clarified in detail. Flow characteristic was calculated by using the compressible fluid analytical program of FLIC method. Pressure distribution obtained in the experiment was compared with the calculated one. Two data show good agreement in general.
Based on the calculated pressure distribution, the breakdown voltage was estimated from E/p where E is electric field strength and p is pressure. It was found that estimated breakdown voltage is very close to the breakdown voltage measured during contact opening operation.

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An expert system is presented that produces the operation sequence for disconnecting apparatuses from a power system under normal operating conditions.
To keep the reliablity of power supply on the network during the operation, the operation sequence must be produced in due consideration of network states, such as load factors and routes of transmission line. Besides, since the network which system manages, includes many high-voltage consumers, the operation sequence for each consumer should be divided into detailed switch operation sequence.
So as to produce detailed switching operation sequence simultaneously with macro network operation sequence, we introduce the multi-layered object-oriented network model for the knowledge base of this expert system.
Each layer of this model represents network configuration in the different degree of detail, and the consistency between states of layers is maintained by inter-layer message-passing mechanism. On the layer which represents apparatuses and their connections in detail, the inference engine can generate the switching operation sequence. On another layer which represents control centers and consumers, the inference engine can search for routes of power supply and can generate the sequence of changing over the network. Therefore, by using each layers properly, the inference engine can produce various kinds of operation sequences.
Furthermore the object-oriented network model makes easy maintenance and extension of the system.

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A 50 MVA Self-Commutated Static Var Compensator (SVC) using world largest GTO thyristors rated 6kV-2, 500A has been developed for voltage control of electric power system. The rated DC voltage is 16.8kV which is four times higher than the highest one manufactured so far. To achieve the high DC voltage, 8 GTO thyristors are connected in series. Voltage unbalance between GTOs during switching transient was investigated. A new overcurrent limiting method is proposed, since conventional fuse protection can not be applied due to such high voltage. Newly developed technologies for this large and high voltage self-commutated SVC are described.
Although simple star-delta connection transformers are used, harmonics generated from SVC are almost equivalent to 24 pulse converter by shifting the pulse timing appropriately. The PWM control based on 150Hz is applied and the SVC is designed to continue operation during system faults. The performance of controller is successfully tested by a simulator, and typical test results are introduced.

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Electric power is in great demand, and it is transmitted to distant places for consumers with high efficiency, where various insulators are being used for power transmission and distribution line for insulation.
Insulators contaminated by salt particles have been causes for flashovers on the surface under high voltage in humid climate and light rain. Rapid salt contamination of the insulator surface by typhoons has been investigated on the campus of University of the Ryukyus for several years.
The results obtained from the exposure tests are:
(1) Accumulative contamination of the insulator surface occurred always on the bottom surface of the insulators, and on the upper surface of insulators during the period without rainfall.
(2) The extent of contamination of the pin-type insulators with the saltproof dishes was 1/2_??_1/6 in comparison to that of pin-type insulators without ones.
(3) The salt contamination of the insulator surfaces is almost always influenced by wind speed and precipitation.
(4) Flashover voltage of salt contminated insulators by typhoons drops to approximately 1/10 of that for clean and dry insulators.

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